Cushing’s Disease—Monthly Injection Is Good Alternative to Surgery

Written by Kathleen Doheny with Maria Fleseriu, MD, FACE, and Vivien Herman-Bonert, MD

Cushing’s disease, an uncommon but hard to treat endocrine disorder, occurs when a tumor on the pituitary gland, called an adenoma—that is almost always benign—leads to an overproduction of ACTH (adrenocorticotropic hormone), which is responsible for stimulating the release of cortisol, also known as the stress hormone.

Until now, surgery to remove the non-cancerous but problematic tumor has been the only effective treatment. Still, many patients will require medication to help control their serum cortisol levels, and others cannot have surgery or would prefer to avoid it.

Finally, a drug proves effective as added on or alternative to surgery in managing Cushing’s disease. Photo; 123rf

New Drug Offers Alternative to Surgery for Cushing’s Disease

Now, there is good news about long-term positive results achieved with pasireotide (Signifor)—the first medication to demonstrate effectiveness in both normalizing serum cortisol levels and either shrinking or slowing growth of tumors over the long term.1,2  These findings appear in the journal, Clinical Endocrinology, showing that patients followed for 36 months as part of an ongoing study had improved patient outcomes for Cushing’s disease.2

“What we knew before this extension study was—the drug will work in approximately half of the patients with mild Cushing’s disease,” says study author Maria Fleseriu, MD, FACE, director of the Northwest Pituitary Center and professor of neurological surgery and medicine in the division of endocrinology, diabetes and clinical nutrition at the Oregon Health and Sciences University School of Medicine.

“Pasireotide also offers good clinical benefits,” says Dr. Fleseriu who is also the president of the Pituitary Society, “which includes improvements in blood pressure, other signs and symptoms of Cushing’s symptom], and quality of life.”2

What Symptoms Are Helped by Drug for Cushing’s Disease?  

Among the signs and symptoms of Cushing’s disease that are lessened with treatment are:3

  • Changes in physical appearance such as wide, purple stretch marks on the skin (eg, chest, armpits, abdomen, thighs)
  • Rapid and unexplained weight gain
  • A more full, rounder face
  • Protruding abdomen from fat deposits
  • Increased fat deposits around the neck area

The accumulation of adipose tissue raises the risk of heart disease, which adds to the urgency of effective treatment. In addition, many individuals who have Cushing’s disease also complain of quality of life issues such as fatigue, depression, mood and behavioral problems, as well as poor memory.2

As good as the results appear following the longer term use of pasireotide,2 Dr. Fleseriu admits that in any extension study in which patients are asked to continue on, there are some built-in limitations, which may influence the findings. For example, patients who agree to stay on do so because they are good responders, meaning they feel better, so they’re happy to stick with the study.

“Fortunately, for the patients who have responded to pasireotide initially, this is a drug that can be  continued as there are no new safety signals with longer use,” Dr. Fleseriu tells EndocrineWeb, “and when the response at the start is good, very few patients will lose control of their urinary free cortisol over time. That’s a frequent marker used to monitor patient’s status. For those patients with large tumors, almost half of them had a significant shrinkage, and all the others had a stable tumor size.”

What Are the Reasons to Consider Drug Treatment to Manage Cushing’s Symptoms

The extension study ”was important because we didn’t have any long-term data regarding patient response to this once-a-month treatment to manage Cushing’s disease,” she says.

While selective surgical removal of the tumor is the preferred treatment choice, the success rate in patients varies, and Cushing’s symptoms persist in up to 35% of patients after surgery. In addition, recurrent rates (ie, return of disease) range from 13% to 66% after individuals experience different durations remaining in remission.1

Therefore, the availability of an effective, long-lasting drug will change the course of therapy for many patients with Cushing’s disease going forward. Not only will pasireotide benefit patients who have persistent and recurrent disease after undergoing surgery, but also this medication will be beneficial for those who are not candidates for surgery or just wish to avoid having this procedure, he said.

Examining the Safety and Tolerability of Pasireotide  

This long-acting therapy, pasireotide, which is given by injection, was approved in the US after reviewing results of a 12-month Phase 3 trial.1  In the initial study, participants had a confirmed pituitary cause of the Cushing’s disease. After that, the researchers added the optional 12-month open-label, extension study, and now patients can continue on in a separate long-term safety study.

Those eligible for the 12-month extension had to have mean urinary free cortisol not exceeding the upper limit of normal (166.5 nanomoles per 24 hour) and/or be considered by the investigator to be getting substantial clinical benefit from treatment with long-action pasireotide, and to demonstrate tolerability of pasireotide during the core study.1

Of the 150 in the initial trial, 81 participants, or 54% of the patients, entered the extension study. Of those, 39 completed the next phase, and most also enrolled in another long-term safety study—these results not yet available).2

During the core study, 1 participants were randomly assigned to 10 or 30 mg of the drug every 28 days, with doses based on effectiveness and tolerability. When they entered the extension, patients were given the same dose they received at month.1,2

Study Outcomes Offer Advantages in Cushing’s Disease

Of those who received 36 months of treatment with pasireotide, nearly three in four (72.2%) had controlled levels of urinary free cortisol at this time point.2 Equally good news for this drug was that tumors either shrank or did not grow. Of those individuals who started the trial with a measurable tumor (adenoma) as well as those with an adenoma at the two year mark (35 people), 85.7% of them experienced a reduction of 20% or more or less than a 20% change in tumor volume. No  macroadenomas present at the start of the study showed a change of more than 20% at either month 24 or 36.2

Improvements in blood pressure, body mass index (BMI) and waist circumference continued throughout the extension study.1  Those factors influence CVD risk, the leading cause of death in those with Cushing’s.4

As for adverse events, most of the study participants, 91.4%, did report one or more complaint during the extension study—most commonly, it was high blood sugar, which was reported by nearly 40% of participants.2. This is not surprising when you consider that most (81.5%) of the individuals participating in the extension trial entered with a diagnosis of diabetes or use of antidiabetic medication, and even more of them (88.9%) had diabetes at the last evaluation.1  

This complication indicates the need for people with Cushing’s disease to check their blood glucose, as appropriate.

Do You Have Cushing’s Disese [sic]? Here’s What You Need to Know 

Women typically develop Cushing’s disease more often than men.

What else should you be aware of if you and your doctor decide this medication will help you? Monitoring is crucial, says Dr. Fleseriu, as you will need to have your cortisol levels checked, and you should be on alert for any diabetes signals, which will require close monitoring and regular follow-up for disease management.

Another understanding gained from the results of this drug study: “This medication works on the tumor level,” she says. “If the patient has a macroadenoma (large tumor), this would be the preferred treatment.” However, it should be used with caution in those with diabetes given the increased risk of experiencing high blood sugar.

The researchers conclude that “the long-term safety profile of pasireotide was very favorable and consistent with that reported during the first 12 months of treatment. These data support the use of long-acting pasireotide as an effective long-term treatment option for some patients with Cushing’s Disease.”1

Understanding Benefits of New Drug to Treat Cushing’s Diseease [sic]

Vivien S. Herman-Bonert, MD, an endocrinologist and clinical director of the Pituitary Center at Cedars-Sinai Medical Center in Los Angeles, agreed to discuss the study findings, after agreeing to review the research for EndocrineWeb.

As to who might benefit most from monthly pasireotide injections? Dr. Herman-Bonert says, “any patient with Cushing’s disease that requires long-term medical therapy, which includes patients with persistent or recurrent disease after surgery.” Certainly, anyone who has had poor response to any other medical therapies for Cushing’s disease either because they didn’t work well enough or because the side effects were too much, will likely benefit a well, she adds.

Among the pluses that came out of the study, she says, is that nearly half of the patients had controlled average urinary free cortisol levels after two full years, and 72% of the participants who continued on with the drug for 36 months were able to remain in good urinary cortisol control .1

As the authors stated, tumor shrinkage was another clear benefit of taking long-term pasireotide. That makes the drug a potentially good choice for those even with large tumors or with progressive tumor growth, she says. It’s always good for anyone with Cushing’s disease to have an alterative [sic] to surgery, or a back-up option when surgery isn’t quite enough, says Dr. Herman-Bonert.

The best news for patients is that quality of life scores improved,she adds.

Dr Herman-Bonert did add a note of caution: Although the treatment in this study is described as ”long-term, patients will need to be on this for far longer than 2 to 3 years,” she says. So, the data reported in this study may or may not persist, and we don’t yet know what the impact will be 10 or 25 years out.

Also, the issue of hyperglycemia-related adverse events raises a concern, given the vast majority (81%) of patients who have both Cushing’s disease and diabetes. Most of those taking this drug had a dual diagnosis—having diabetes, a history of diabetes, or taking antidiabetic medicine.

If you are under care for diabetes and you require treatment for Cushing’s disease, you must be ver mindful that taking pasireotide will likely lead to high blood sugar spikes, so you should plan to address this with your healthcare provider.


Dr. Fleseriu reports research support paid to Oregon Health & Science University from Novartis and other 0companies and consultancy fees from Novartis and Strongbridge Biopharma. Dr. Herman-Bonert has no relevant disclosures.

The study was underwritten by Novartis Pharma AG, the drug maker. 


Cortisol Levels Predict Remission in Cushing’s Patients Undergoing Transsphenoidal Surgery

In patients with Cushing’s disease, removing the pituitary tumor via an endoscopic transsphenoidal surgery (TSS) leads to better remission rates than microscopic TSS, according to new research.

But regardless of surgical approach, plasma cortisol levels one day after surgery are predictive of remission, researchers found.

The study, “Management of Cushing’s disease: Changing trend from microscopic to endoscopic surgery,” was published in the journal World Neurosurgery.

Because it improves visualization and accessibility, endoscopic TSS has been gaining popularity over microscopic TSS to remove pituitary tumors in Cushing’s disease patients. Yet, although this surgery has been associated with high remission rates, whether it outperforms microscopic surgery and determining the factors affecting long-term outcomes may further ease disease recurrence after TSS.

A team with the All India Institute of Medical Sciences addressed this topic in 104 patients who underwent surgery from January 2009 to June 2017. Among these patients, 47 underwent microscopic surgery and 55 endoscopic surgery. At presentation, their ages ranged from 9 to 55 (mean age of 28). Also, patients had been experiencing Cushing’s symptoms over a mean duration of 24 months.

Eighty-seven patients showed weight gain. Hypertension (high blood pressure) and diabetes mellitus were among the most common co-morbidities, found in 76 and 33 patients, respectively. Nineteen patients had osteoporosis and 12 osteopenia, which refers to lower-than-normal bone mineral density.

As assessed with magnetic resonance imaging, 68 patients had a microadenoma (a tumor diameter smaller than one centimeter) and 27 had a macroadenoma (a tumor one centimeter or larger). Only two patients had an invasive pituitary adenoma.

Two patients with larger tumors were operated on transcranially (through the skull). The surgery resulted in total tumor removal in 90 cases (86.5%). A blood loss greater than 100 milliliter was more common with endoscopic than with microscopic TSS.

Ten patients developed transient diabetes inspidus, two experienced seizures after surgery, and six of nine patients with macroadenoma and visual deterioration experienced vision improvements after TSS.

The incidence of intraoperative leak of cerebrospinal fluid — the liquid surrounding the brain and spinal cord — was 23.2%, while that of post-operative leak was 7.7% and was more common in microadenoma than macroadenoma surgery (9.8% vs. 5.0%).

Seventeen patients were lost to follow-up and two died due to metabolic complications and infections. The average follow-up was shorter for endoscopic than with microscopic surgery (18 months vs. 35 months).

Among the remaining 85 cases, 65 (76.5%) experienced remission, as defined by a morning cortisol level under 5.0 μg/dL, restored circadian rhythm (the body’s internal clock, typically impaired in Cushing’s patients), and suppression of serum cortisol to below 2 μg/dl after overnight dexamethasone suppression test.

The remission rate was 54.5% in pediatric patients and was higher with endoscopic than with microscopic TSS (88.2% vs. 56.6%). Also, patients with microadenoma showed a trend toward more frequent remission than those with macroadenoma (73.2% vs. 64.3%).

Ten of the remaining 20 patients experienced disease recurrence up to 28 months after surgery. Sixteen cases revealed signs of hypopituitarism, or pituitary insufficiency, which were managed with replacement therapy.

A subsequent analysis found that morning cortisol level on day one after surgery was the only significant predictor of remission. Specifically, a one-unit increase in cortisol lowered the likelihood of remission by 7%. A cortisol level lower than 10.7 μgm/dl was calculated as predicting remission.

Overall, the study showed that “postoperative plasma cortisol level is a strong independent predictor of remission,” the researchers wrote, and that “remission provided by endoscopy is significantly better than microscopic approach.”


Long-acting pasireotide provides ‘sustained biochemical improvements’ in Cushing’s disease

For patients with persistent or recurring Cushing’s disease, monthly pasireotide therapy was safe and effective, leading to normal urinary free cortisol levels in 47% of patients after 2 years, according to findings published in Clinical Endocrinology.

Maria Fleseriu headshot 2019

Maria Fleseriu

“The management of Cushing’s syndrome, and particularly Cushing’s disease, remains challenging,” Maria Fleseriu, MD, FACE, professor of neurological surgery and professor of medicine in the division of endocrinology, diabetes and clinical nutrition in the School of Medicine at Oregon Health & Science University and director of the OHSU Northwest Pituitary Center, told Endocrine Today. “Long-acting pasireotide provided sustained biochemical improvements and clinical benefit in a significant proportion of patients with Cushing’s disease who elected to continue in this extension study. There were many adverse events reported overall, but no new safety signals emerging over long-term treatment.”

In the last decade, medical treatment for Cushing’s disease has progressed from a few steroidogenesis inhibitors to three novel drug groups: new inhibitors for steroidogenic enzymes with possibly fewer adverse effects, pituitary-directed drugs that aim to inhibit the pathophysiological pathways of Cushing’s disease, and glucocorticoid receptor antagonists that block cortisol’s action, Fleseriu, who is also an Endocrine Today Editorial Board member, said.

In an open-label extension study, Fleseriu and colleagues analyzed data from 81 adults with confirmed Cushing’s disease with mean urinary free cortisol not exceeding the upper limit of normal, who transitioned from a 12-month, randomized controlled trial where they were assigned 10 mg or 30 mg once-monthly intramuscular pasireotide (Signifor LAR, Novartis). During the main study, researchers recruited participants with mean urinary free cortisol level concentration 1.5 to five times the upper limit of normal, normal or greater than normal plasma and confirmed pituitary source of Cushing’s disease. Participants who elected to continue in the extension were considered biochemical responders or benefited from the study drug per the clinical investigator, Fleseriu said.

“As in all extension studies, the bias is inherent that patients deemed responders tend to continue, but for any type of treatment for pituitary tumors, and particularly Cushing’s disease, long-term, robust data on efficacy and safety parameters is essential,” Fleseriu said.

Median overall exposure to pasireotide at the end of the extension study was 23.9 months, with nearly half of patients receiving at least 1 year of treatment during the extension phase. Researchers found that improvements in clinical signs of hypercortisolism were sustained throughout the study and median urinary free cortisol remained within normal range. Overall, 38 participants (47%) had controlled urinary free cortisol at month 24 (after 12 months of treatment during the extension phase), with researchers noting that the proportion of participants with controlled or partially controlled urinary free cortisol was stable throughout the extension phase.

“Interestingly, the median salivary cortisol level decreased but remained above normal (1.3 times upper limit of normal) at 3 years,” Fleseriu said.

As seen in other pasireotide studies, and expected based on the mechanism of action, researchers observed hyperglycemia-related adverse events in 39.5% of participants, with diabetes medications initiated or escalated in some patients, Fleseriu said. However, mean fasting glucose and HbA1c were stable during the extension phase, after increasing in the main study. Within the cohort, 81.5% had type 2 diabetes at baseline (entering extension phase) and 88.9% patients had type 2 diabetes at last assessment.

“Pasireotide acts at the tumor level, and tumor shrinkage is seen in many patients,” Fleseriu said. “In this study, 42% and 32.1% had a measurable microadenoma or macroadenoma, respectively, on MRI at the start of pasireotide treatment; an adenoma was not visible in almost a quarter of patients at 2 years.”

Among patients with a measurable adenoma at baseline and at month 24 (n = 35), 85.7% experienced a reduction of at least 20% or a 20% change in tumor volume between the two time points. Improvements in median systolic and diastolic blood pressure, BMI and waist circumference were sustained during the extension, Fleseriu said.

“The long-term safety profile of pasireotide was favorable and consistent with that reported during the first 12 months of treatment,” the researchers wrote. “These data support the use of long-acting pasireotide as an effective long-term treatment option for some patients with [Cushing’s disease].”

Fleseriu said individualized treatment selecting patients who will derive benefit from therapy will be crucial, balancing both efficacy and the potential risks and costs. – by Regina Schaffer

Disclosures: Fleseriu reports she has received consultant fees and her institution has received research support from Novo Nordisk and Pfizer. Please see the study for all other authors’ relevant financial disclosures.


Cushing’s syndrome driver mutation disrupts protein kinase A allosteric network, altering both regulation and substrate specificity

 See all authors and affiliations

Science Advances  28 Aug 2019:
Vol. 5, no. 8, eaaw9298
DOI: 10.1126/sciadv.aaw9298


Genetic alterations in the PRKACA gene coding for the catalytic α subunit of the cAMP-dependent protein kinase A (PKA-C) are linked to cortisol-secreting adrenocortical adenomas, resulting in Cushing’s syndrome. Among those, a single mutation (L205R) has been found in up to 67% of patients. Because the x-ray structures of the wild-type and mutant kinases are essentially identical, the mechanism explaining aberrant function of this mutant remains under active debate. Using NMR spectroscopy, thermodynamics, kinetic assays, and molecular dynamics simulations, we found that this single mutation causes global changes in the enzyme, disrupting the intramolecular allosteric network and eliciting losses in nucleotide/pseudo-substrate binding cooperativity. Remarkably, by rewiring its internal allosteric network, PKA-CL205R is able to bind and phosphorylate non-canonical substrates, explaining its changes in substrate specificity. Both the lack of regulation and change in substrate specificity reveal the complex role of this mutated kinase in the formation of cortisol-secreting adrenocortical adenomas.


The cyclic adenosine monophosphate (cAMP)–dependent protein kinase A (PKA) plays a fundamental role in the function and replication of endocrine cells (1), and aberrant cAMP signaling has been linked to several endocrine diseases (2). The first mutations in PKA have been found in the regulatory (R) subunits, which have been associated with Carney complex, a multiple neoplasia syndrome manifesting via adrenocortical adenomas, cutaneous and neuronal tumors, cardiac myxomas, and pigmented lesions of the skin and mucosae (2). Only recently have somatic mutations been identified in the PRKACA gene coding for the catalytic α subunit of PKA (PKA-C) and discovered in cortisol-secreting adrenocortical adenomas responsible for Cushing’s syndrome (2) (Fig. 1A). Nearly all identified mutations lie adjacent to the active site cleft and the regulatory/catalytic subunit interface (Fig. 1, B and C). Among these mutations, the most frequently found (PKA-CL205R) is a point mutation leading to substitution of Leu at position 205 (or 206, depending on the convention used for numbering) with Arg and has been found in up to 67% of Cushing’s syndrome patients (37).

Fig. 1 Architecture of PKA-C and locations of the Cushing’s syndrome mutations.

(A) Structure of PKA-C [Protein Data Bank (PDB) ID: 1ATP] in complex with pseudo-substrate PKI depicting the location of Cushing’s mutations (yellow spheres) in relation to structural elements of the kinase. The E248Q mutation includes an additional deletion (del243-247), and the S212R mutation includes an insertion (insIILR) not depicted. (B) X-ray structure of PKA-CL205R (PDB ID: 4WB6) with the overlay of PKI5–24 (PKIWT from PDB ID: 1ATP) describing the architecture of the peptide binding site and steric clash between kinase (PKA-CL205R) and pseudo-substrate. (C) Structure of the R/C complex (PDB ID: 2QCS) depicting locations of Cushing’s mutations in relation to the pseudo-substrate inhibitory sequence of the R-subunit. (D) Primary sequence comparison of common regulators (RIα, RIIβ, and PKI) and peptide substrates of the catalytic subunit (Kemptide and VPS36).

Inactive PKA exists as an inactive holoenzyme (R2C2) containing an R-subunit dimer bound to two catalytic (C) subunits. The inhibitory sequence of the R-subunit occupies the active site of the enzyme (Fig. 1D). Upon stimulation of membrane receptors coupled to the stimulatory Gs protein, produced cAMP binds to the R-subunits, unleashing active C-subunits (8). The spatiotemporal regulation of the kinase is provided by ancillary proteins such as A-kinase anchoring proteins that, via interactions with R-subunits, localize PKA-C in close proximity to its substrates (9). In addition, PKA-C is regulated by an endogenous inhibitor (PKI), whose function is to block access to substrates and recruit PKA-C to the nuclear export complex (CRM1 and RanGTP) (8). When this spatiotemporal regulation fails, PKA-C hyperphosphorylates its targets, leading to disease.

Structurally, PKA-C consists of two lobes. The N-lobe, smaller and more dynamic, comprises mostly β strands, as well as the αC helix, and harbors the adenosine 5′-triphosphate (ATP)–binding site (10). The C-lobe, larger and made up of α helices, is more rigid and harbors the substrate binding groove. PKA-C toggles between three major conformational states: open (apo), intermediate (nucleotide-bound), and closed (ternary complex with nucleotide/substrate-bound) (11). The L205R mutation is located at the interface of the N- and C-lobes of PKA-C in the P+1 loop, a highly conserved region of the enzyme that creates a hydrophobic pocket for substrate docking. On the basis of its positioning, the L205R mutation has been proposed and shown by independent laboratories to disrupt the binding of R-subunits and render the enzyme constitutively active (1213). In vivo studies revealed that the catalytic activity of PKA-CL205R is comparable to that of wild type (12). However, recent phosphoproteomic mapping showed a drastic change in the phosphorylation profile, suggesting that the mutation preserves the ability to phosphorylate downstream substrates, although with a different selectivity (1415). On the basis of these data, it has been proposed that disruption of the signaling network leading to the phosphorylation of non-canonical substrates may contribute to tumorigenesis. Notably, the x-ray crystal structures of the wild-type and mutant kinases in complex with nucleotide and pseudo-substrate, PKI5–24, are nearly superimposable [root mean square deviation (RMSD) = 0.49 Å] (16) and do not thoroughly explain the weaker binding for R-subunit, the lack of regulation by PKI, and the changes in the phosphoproteomic profile.

To gain mechanistic insights into the multifarious effects of this single mutation, we carried out solution nuclear magnetic resonance (NMR) spectroscopy in concert with binding thermodynamics, kinetic assays, and molecular dynamics (MD) simulations. We found that this single mutation, in addition to changing the protein-protein interface for substrate recognition, causes a disruption of the internal allosteric communication, reducing its affinity for nucleotide. The partial ablation of the intramolecular allosteric communication prevents the kinase from reaching a completely closed state, reducing the binding affinity for canonical substrates and pseudo-substrates, thus hindering endogenous regulation. Notably, kinetic assays using a substrate identified by phosphoproteomic analyses as a preferred substrate of PKA-CL205R (14) show a higher catalytic efficiency and a rearrangement of the internal structural dynamics and allosteric communication. Together, our results indicate that rewiring of the internal allosteric network and global changes in dynamics contribute to the complex dysregulation of the signaling network of PKA-CL205R within tumor cells.


PKA-CL205R shows a marked loss of binding cooperativity between ATP and PKI

To determine the thermodynamics of nucleotide binding (ATPγN) to both PKA-CWT and PKA-CL205R, we used isothermal titration calorimetry (ITC). We found that PKA-CL205R exhibits a threefold lower binding affinity compared to PKA-CWT [Kd (dissociation constant) of 215 ± 15 μM versus 83 ± 8 μM, respectively]. Fitting of the titration curves shows that the binding of ATPγN to PKA-CL205R is enthalpically favorable and entropically unfavorable, whereas both contributions are favorable for ATPγN binding to PKA-CWT. To evaluate the binding cooperativity between nucleotide and substrate (1718), we analyzed the binding thermodynamics of the pseudo-substrate peptide inhibitor, PKI5–24, to both PKA-CWT and PKA-CL205R in the presence and absence of nucleotide. ΔH, −TΔS, ΔG, and Kd, as well as the cooperativity coefficients (σ) derived from ITC titrations, are summarized in table S1. The binding thermodynamics of PKI5–24 to the apo forms of the two kinases is comparable, with similar enthalpy changes (favorable). When saturated with ATPγN, PKA-CWT binds PKI5–24 with higher affinity (Kd = 0.16 ± 0.02 μM) (18). In contrast, PKA-CL205R displays a 62-fold reduction in binding affinity (Kd = 10 ± 3 μM), with a net decrease in both ΔH and ΔS. The binding of PKI5–24 to the PKA-CWT/ATPγN complex is highly cooperative (σ = 106 ± 18), whereas its binding cooperativity to the PKA-CL205R/ATPγN complex decreases 18-fold (σ = 6 ± 2). Therefore, this single mutation affects both the affinity for ligands and the binding cooperativity between nucleotide and pseudo-substrate.

L205R mutation causes a reduction in phosphorylation kinetics

To evaluate the catalytic efficiency of PKA-CL205R, we carried out steady-state coupled assays using the standard substrate, Kemptide. As expected from the binding thermodynamics and previous kinetic assays, the Leu-to-Arg substitution at the P+1 loop increases the KM (Michaelis constant) 11-fold (table S2). The kcat for the mutant increases from 19 to 41 s−1, which results in an overall decrease in catalytic efficiency (kcat/KM) by over fivefold compared to PKA-CWT. These results are in quantitative agreement with experiments carried out by two other independent groups (1619).

PKI binding to PKA-CL205R falls short in shifting the enzyme to a fully closed state

To analyze the structural changes of PKA-C upon binding PKI and the effects of the L205R mutation, we mapped the amide backbone fingerprint of the enzyme using [1H, 15N]–TROSY (transverse relaxation-optimized spectroscopy)–HSQC (heteronuclear single-quantum coherence) experiments (20). The amide signatures of the kinases in different ligated forms are displayed in fig. S1. The binding of ATPγN to both PKA-CWT and PKA-CL205R gives rise to similar chemical shift perturbations (CSPs), with the exception of the residues in the Gly-rich, catalytic, and Mg2+positioning loops that show larger chemical shift changes for PKA-CWT (fig. S2). To evaluate the global response to ligand binding for the kinase and its mutant, we used CONCISE (coordinated chemical shifts behavior), which performs a statistical analysis on linear chemical shift trajectories of each amide resonance to identify the position of each state along the conformational equilibrium (21). Following ligand binding, the amide resonances of PKA-C display linear chemical shift trajectories, with the amide resonances corresponding to the apo enzyme populating one extreme and those of the closed state (ternary complex with PKI) populating the opposite extreme of the linear correlations. This behavior exemplifies a conformational equilibrium along the three major conformational states of the kinase (open, intermediate, and closed), whose relative populations are modulated by ligand binding. A total of 55 residues exhibit linear chemical shift trajectories for both PKA-CWT and PKA-CL205R, indicating a fast exchange limit in the NMR chemical shift time scale. These residues are distributed throughout the kinase. We found that the binding of ATPγN and PKI shifts the conformational equilibrium of PKA-CWT toward a fully closed state. In contrast, these ligands drive the mutant to a partially closed state, which is located between the intermediate and closed conformation (table S3). This is supported by thermostability data obtained by circular dichroism (CD) measurements (table S4). CD melting curves show that the PKA-CWT/ATPγN/PKI complex has a higher melting temperature (Tm) than the corresponding complex with PKA-CL205R, further suggesting that this complex may adopt a more open and unstable conformation. Since the extent of the closed state is correlated with binding affinity and cooperativity (18), these results are consistent with the loss in allosteric cooperativity revealed by thermocalorimetric data.

L205R disrupts the allosteric network of the kinase upon PKI binding

To determine a possible correlation between the binding cooperativity and the intramolecular allosteric network, we analyzed the correlated CSPs for both PKA-CWT and PKA-CL205R using chemical shift covariance analysis (CHESCA) (22). This method identifies the allosteric networks of residues that are involved in concerted responses to ligand binding (23). The [1H, 15N]-TROSY-HSQC spectra of the three forms of wild-type and PKA-C mutant (apo, ATPγN bound, and ATPγN/PKI bound) were used for the analysis. The CHESCA matrices for PKA-CWT and PKA-CL205R reported in Fig. 2 (A and B) reveal remarkable differences in their allosteric networks. There is an overall reduction in the number of correlations throughout PKA-CL205R (Fig. 2, C and D), with specific domains of the enzyme more affected by this single mutation. In particular, residues belonging to the Gly-rich, activation loop and peptide-positioning loop experience the most substantial reductions in both number and extent of chemical shift covariation. In addition, several interresidue correlations within the activation loop (W196), αF helix (K217, V219, D220, G225, V226, and A233), and αG helix (Q245) are also ablated. The loss of interresidue correlations for the αF helix is especially important as this motif spans the hydrophobic core of the C-lobe and is conserved throughout the AGC kinase family. The function of the αF helix is to anchor all the hydrophobic motifs within the kinase core and to orchestrate catalysis (2425). As highlighted before, all Cushing’s driver mutations, with the exception of E31V, are located near L205R and are likely to affect the enzyme in a similar manner, disrupting these important allosteric nodes.

Fig. 2 Allosteric network of interactions observed upon pseudo-substrate binding.

The CHESCA correlation matrix for (A) PKA-CWT upon binding PKI and (B) PKA-CL205R upon binding PKI. (C) Correlations corresponding to the binding of PKI to PKA-CWT plotted on the structure of PKA-C. Residues that are commonly correlated for both PKA-CWT and PKA-CL205R are highlighted. (D) Correlations corresponding to the binding of PKI to PKA-CL205R plotted on the structure of PKA-C. Specific residues that are correlated for only PKA-CL205R are highlighted. Only correlations with rij > 0.98 are shown throughout.

Binding of VPS36 substrate to PKA-CL205R rewires the intramolecular allosteric network and reestablishes binding cooperativity

In addition to affecting the regulation of PKA-C by the R-subunits, L205R changes the kinase’s downstream substrate specificity (1415). Specifically, it was found that in Escherichia coli, PKA-CL205R favors noncanonical substrates with negatively charged residues (Glu or Asp) at positions P+1, P+2, and P+3. From our own phosphoproteomic studies on endogenous substrates in human cells (14), we identified several substrates that are preferentially phosphorylated by the mutated kinase. Among those substrates, we selected the top target hyperphosphorylated by PKA-CL205R: the vacuolar protein-sorting–associated protein 36 (VPS36), a protein that plays a role in endosomal sorting of ubiquinated cargo proteins by the endosomal sorting complex. The recognition sequence of VPS36 (RRLSEEM) for PKA-C has two Glu residues in the P+1 and P+2 positions. Using steady-state coupled enzyme assays with Kemptide and a VPS36-derived peptide (residues 115 to 130 encompassing the PKA-C recognition sequence and denoted VPS36 from hereon), we found that PKA-CL205R reduces the kinase’s catalytic efficiency for Kemptide, which contains a hydrophobic residue at the P+1 position. In contrast, the mutation causes an increase in kcat (from 5 to ~18 s−1) for VPS36, leading to a threefold increase in catalytic efficiency (Fig. 3Aand table S2). In addition, the ITC analysis carried out with VPS36 indicates that PKA-CL205R/ATPγN has an affinity typical of other substrates (Kd = 3.5 ± 0.1 μM), while the binding to wild type cannot be detected reliably, suggesting a notably lower binding affinity (table S1). A possible explanation is that the acidic residues in positions P+1 and P+2 of VPS36 hamper the intermolecular interactions between the peptide and residues lining the binding pocket of PKA-CWT, while they might favor the interactions with PKA-CL205R. These results support the proposed altered substrate specificity for PKA-CL205R detected in intact cells.

Fig. 3 Rewiring of the allosteric network of PKA-CL205R upon binding VPS36.

(A) Steady-state phosphorylation kinetics of Kemptide and VPS36 peptides for PKA-CWT and PKA-CL205R. Corresponding values can be found in table S2. (B) CONCISE analysis on the apo, ATPγN, ATPγN/PKI, and ATPγN/VPS36 states of PKA-CWT and PKA-CL205R. (C) The CHESCA correlation matrix for PKA-CWTupon binding VPS36. (D) The CHESCA correlation matrix for PKA-CL205R upon binding VPS36.

To further investigate the underlying mechanism for the change in substrate specificity of PKA-CL205R, we analyzed the trajectories of the amide resonances for the wild-type and mutated kinase upon binding VPS36. Although VPS36 binding causes linear chemical shift changes similar to PKI binding, the number of residues involved in the binding response is substantially higher for PKA-CL205R. In addition, VPS36 binding causes an overall attenuation of the peak intensities throughout the entire fingerprint of the enzyme, with some resonances broadened beyond detection (fig. S3). For instance, resonances associated with the αF helix and Gly-rich loop are noticeably broadened, suggesting an increase in protein dynamics in the microsecond to millisecond time scale. The CONCISE analysis shows that the conformation of the PKA-CWT/ATPγN/VPS36 complex only reaches the intermediate state. In contrast, the PKA-CL205R/ATPγN/VPS36 complex reaches a state lying between the intermediate and closed states (Fig. 3B). This position in between the intermediate and closed states was also observed in the crystal structure of PKA-CWT bound to a substrate peptide (26), suggesting that the ternary complex of PKA-CL205R with VPS36 adopts a catalytically committed state.

The CHESCA maps for VPS36 binding to PKA-CWT and PKA-CL205R are radically different (Fig. 3, C and D). While the PKA-CWT/ATPγN/PKI complex shows an intense cross-talk between the small and large lobes, the corresponding map for PKA-CWT/ATPγN/VPS36 reveals a complete loss of allosteric interactions. Notably, correlations between the nucleotide binding site and the substrate binding hub are completely missing. In particular, the interresidue correlations for the activation loop, peptide-positioning loop, and the αF and αG helices are either reduced in their number or completely missing. In contrast, a number of correlations between the N- and C-lobes for PKA-CL205R, which are absent for the complex with PKI, are regained upon VPS36 binding. Note that 10 of the resonances corresponding to residues located in and around the Gly-rich loop are broadened beyond detection and could not be included in the CHESCA analysis; therefore, the extent of coordinated changes in chemical shifts is largely underestimated. Nonetheless, the higher density of correlations found in the PKA-CL205R/ATPγN/VPS36 complex underscores an extensive rewiring of the internal allosteric network, supporting the reestablished binding cooperativity.

PKA-CWT and PKA-CL205R explore different conformational states

To determine the effects of the L205R mutation on the conformational energy landscape of PKA-C, we carried out parallel MD simulations of the ternary complexes in explicit water environments starting from their respective x-ray coordinates [Protein Data Bank (PDB) IDs: 4WB6 and 1ATP] (16). The dominant motion of the kinase core was imaged using principal components analysis (PCA) of the Cα atoms encompassing residues 50 to 300. PC1 depicts the global opening-closing motion of the small and large lobes (fig. S4A), while PC2 describes the shearing motion of the two lobes that can be envisioned as an “asymmetric bite.” To describe the opening-closing motion of the active site, we monitored the time dependence of the distance between the Cα atoms of S53 in the Gly-rich loop and G186 (fig. S4, B and C). The binary and ternary complexes of PKA-CWT and PKA-CL205R sample similar conformational energy basins; however, the apo form of PKA-CL205Rexplores a considerably broader basin than the wild type. The Gly-rich loop of PKA-CWT toggles between the open and closed conformations, featuring a transient hydrogen bond between S53 and the γ-phosphate of ATP, whereas the Gly-rich loop of PKA-CL205R exhibits larger motions with the S53-G186 distance ranging from 5 to 18 Å and, more frequently, samples a conformation that hinders nucleotide binding, which might explain its weaker binding affinity (Fig. 4A). R205 causes steric hindrance with the C-terminal tail of PKI, disrupting the hydrophobic pocket required to anchor the Ile residue in the P+1 position, which likely contributes to the loss of PKI binding affinity.

Fig. 4 Conformational dynamics of the activation loop of PKA-CWT and PKA-CL205R upon binding substrate.

(A) Distinct opening-closing motions of the Gly-rich loop highlighting the hindered conformation that occludes the entering of ATP in PKA-CL205R. (B) Probability density describing the conformation of the activation loop in response to different substrates and pseudo-substrates. (C) [1H, 15N]-TROSY-HSQC spectra showing the backbone amide chemical shift changes of the W196 indole amide (located on the activation loop) in response to binding PKI or VPS36. (D) X-ray structure of PKA-CWT(gray) (PDB ID: 1ATP) with the overlay PKA-CL205R in complex with PKI (light blue) (PDB ID: 4WB6) describing the architecture of the peptide binding site and activation loop flip. (E) X-ray structure of PKA-CWT (gray) (PDB ID: 1ATP) with the overlay PKA-CL205Rin complex with VPS36 (light blue) (MD simulations) describing the architecture of the peptide binding site and activation loop flip.

To visualize the alterations of the protein-protein interaction interface that may influence binding cooperativity, we analyzed the probability distribution of the inter-residue contacts between the substrate and the kinase binding pocket. Overall, PKA-CL205R shows a contact map with PKI similar to the wild-type enzyme (fig. S5, A and B). The high-affinity binding region (HAR) and the consensus binding sequences remain bound throughout the entire MD trajectories with a probability of contact greater than 0.8. However, the L205R mutation reduces the probability of interactions between the Ile (at position P+1 in PKI) and the P+1 loop from 0.5 to 0.2. In contrast, VPS36 shows stable interactions with the P+1 loop with a probability greater than 0.8 for Glu at the P+1 position and R205 (fig. S5C). Moreover, the N terminus of VPS36 forms transient interactions with the C-terminal tail located in the small lobe of PKA-CL205R, which are absent in the corresponding complex with PKI. As a result, the peptide is more dynamic, a feature that might explain the dramatic exchange-broadening in the NMR spectrum of the PKA-CL205R/ATPγN/VPS36 complex.

As observed previously (27), the backbone of the tertiary complex PKA-CWT/ATP/PKI is mostly rigid (fig. S5, D and F). The activation loop adopts a stable conformation as in the x-ray structure (PDB ID: 1ATP) (28) throughout the entire MD simulation with an RMSD of ~1.5 Å. In contrast, the activation loop and the adjacent loop that is anchored to the αF helix (residues 210 to 220) in the corresponding PKA-CL205R/ATP/PKI complex are more flexible, adopting two distinct conformations in which the most populated is similar to the x-ray structure (PDB ID: 1ATP) and a second minor flipped conformation (Fig. 4B). In this conformation, the activation loop no longer interacts with the C-lobe and forms a salt bridge with E86 of the αC helix (Fig. 4, D and E). For the PKA-CL205R/ATP/VPS36 complex, the activation loop becomes more ordered and the flipped configuration becomes dominant. The existence of these two conformations is corroborated by chemical shift trajectories observed for the indole resonance of W196 in the activation loop (Fig. 4C). Upon ligand binding, the indole resonance of W196 follows a linear chemical shift trajectory from the apo to the ternary form. This conformational change is highly correlated with the other major allosteric nodes (CHESCA plots), indicating that it follows the cooperative structural changes of the enzyme. In contrast, the CHESCA plots for PKA-CL205R show no correlations for these loops/residues, suggesting that the L205R mutation disrupts the internal allosteric network and may dislodge the activation loop from the large lobe.

The disruption of the allosteric network is mirrored by the pairwise mutual information plots (29) obtained from the analysis of the MD trajectories. For the wild-type enzyme, there are strong inter-residue correlations throughout the entire core of the kinase, especially among key catalytic motifs such as the Gly-rich loop (S53), the activation loop (R194, W196), and the αF (Y215), αG (D241), and αH (N283) helices, as well as the PIF motif at the C-terminal tail (F347) (fig. S6A). In contrast, PKA-CL205R lacks a number of allosteric interactions within the N-lobe, with only a few correlations between the activation loop and the C-lobe (fig. S6B). As for the CHESCA correlation maps, the mutual information plots reveal that the allosteric network between the N-lobe and C-terminal tail of PKA-CL205R is partially recovered upon VPS36 binding (fig. S6C) with the engagement of E91 in the αC helix and S53 of the Gly-rich loop.


The L205R mutation was discovered independently in our laboratory and in other laboratories as the dominant genetic alteration in cortisol-producing adrenocortical adenomas responsible for Cushing’s syndrome (37). Although it has a direct link with changes in cAMP/PKA signaling, adrenal Cushing’s syndrome has different molecular etiologies (2). Initially, rare germline mutations were found in the PRKAR1A gene encoding the regulatory Iα subunit of PKA in patients with Carney complex (30). Most of these mutations occur at the interface between the R- and C-subunits or near the cAMP-binding site in cyclic nucleotide binding (CNB) domain A and are thought to cause aberrant regulation of the kinase (31). Only recently has the focus shifted toward somatic mutations occurring in the C-subunit of the enzyme. Even in this case, most mutated residues reside at the interface of the R/C complex. However, the molecular mechanisms for the aberrant regulation of cAMP signaling by these mutations have been a matter of debate. Specifically, two independent studies suggested that the L205R mutation interferes with the formation of the holoenzyme, rendering the C-subunit constitutively active (36). Sato et al. (4), on the other hand, proposed that this mutation does not interfere with the R-subunit regulation and that the development of the pathology is linked to intrinsic higher phosphoryl transfer activity of the mutant kinase. Recent studies both in vitro and in intact cells have put forward a possible new mechanism, suggesting that the mutated enzyme has altered substrate specificity (141519). Notably, this arginine preceding the APE motif (residues 206-208) is characteristic in the CMGC kinase family and assumes a key role in substrate recognition and kinase activation (32). Perhaps PKA-CL205R might lead to promiscuity toward substrates of CMGC kinases.

Our studies reveal that the dysfunction of PKA-CL205R is multifaceted, whereby allosteric cooperativity is reduced, substrate specificity is altered, and canonical regulation is ablated. The L205R mutation disrupts the hydrophobic interactions between the enzyme and substrate, causing a dramatic decrease in PKI binding affinity. This is in line with the impaired binding of the R-subunits found in our previous study (12) as PKI and R-subunits share similar consensus sequences. Our CHESCA analyses reveal that L205R perturbs the allosteric network of PKA-C and disrupts specific allosteric nodes that connect the small and large lobes. The allosteric nodes are defined as the hotspots in the network with the highest number of CHESCA correlations (33). In particular, the allosteric communication between the node including L205 and encompassing the activation loop and αF and αG helices and the node surrounding the Gly-rich loop, β2, and β3 is ablated. Closer analysis of the CHESCA map of PKA-CWT reveals that each allosteric node harbors Cushing’s mutations located in the activation/αF/αG node except E31V, where the latter resides at the N-terminal αA helix, a unique regulatory motif in PKA-C (Fig. 5A) (34). Although spatially distinct, these allosteric nodes are highly correlated and coupled to one another to facilitate intramolecular communication and control binding cooperativity. Perturbation to one node, such as the case of L205R, has long-range effects on the other node. The latter suggests that Cushing’s mutations including E31V may disrupt the allosteric communication eliciting similar global responses in terms of binding cooperativity and regulation.

Fig. 5 Cushing’s mutations are located in allosteric nodes identified via CHESCA.

(A) Correlation matrix of PKA-CWT when bound to PKI emphasizing locations of Cushing’s mutation in relation to allosteric nodes. (B) Missing correlations for PKA-CL205R upon binding PKI colored according to the community map of PKA-CWT. (C) CHESCA correlation matrices for PKA-CWT + PKI, PKA-CL205R + PKI, and PKA-CL205R + VPS36, respectively, plotted on the structure of PKA-C with colors specific to the community map analyses completed previously for PKA-CWT (35). Specific communities are emphasized to highlight elements that experience the most dramatic changes in the number and extent of chemical shift covariance for PKA-CL205R upon binding PKI and VPS36.

The CHESCA analyses of the chemical shifts parallel the predictions obtained from MD simulations using the community analysis. Analyzing the different forms of the kinase, McClendon et al. (35) identified allosterically linked communities within PKA-C, each associated with a particular function or regulatory mechanism. Plotting correlated residues on PKA-C’s community maps reveals that the mutation dramatically reduces the allosteric communication between communities A, D, and F with respect to the wild-type enzyme (Fig. 5, B and C). In contrast, binding of VPS36 to PKA-CL205R reestablishes the intercommunity communication and the allosteric network.

The lack of regulation only partially explains the aberrant function of PKA-CL205R. Our thermodynamic and kinetic analyses reveal that this mutation shifts the specificity of PKA-CL205Rtoward substrates containing acidic residues in the P+1 and P+2 positions, with a dramatic effect on binding cooperativity. While the cooperativity is greatly abolished with the classical consensus sequence, the mutated enzyme regains cooperativity with complementary substrates such as VPS36, among others. The reason for this behavior is apparent from the atomic mapping of the cooperative global response of the enzyme to ligand binding. While the allosteric communication between the N-lobe, harboring the ATP binding site, and the C-lobe, with the substrate binding site, is abolished with the classical consensus sequence, the binding of the complementary substrate (VPS36) coincides with an extensive rewiring of the allosteric network of communication between the two lobes, altering substrate specificity. It should be noted that VPS36 lacks the HAR, which might affect its binding kinetics and thermodynamics. As with previous studies (17), VPS36 does not drive the enzyme to a completely closed state, perhaps facilitating product release. The reduction of the binding affinity for the nucleotide we observed under our experimental conditions was also detected by Luzi et al. (19). Taken with our NMR and thermodynamic studies, MD simulations suggest that the mutation changes the energy landscape of the kinase (Fig. 6). First, the mutation causes a reduction in binding affinity for nucleotide, which may be due to an increase in the population of the enzyme with the Gly-loop partially occluded. This conformation has been detected in other crystal structures and might prevent the nucleotide to intercalate in the C-spine of the enzyme (36). Moreover, a significant population of the enzyme spans a conformational space featuring a flipped conformation of the activation loop and a salt bridge formed between R194 and E86 in the αC helix. Notably, an increased plasticity of the activation loop, highlighted by W196, contributes to disruptions in the docking surface with the R-subunit (37). At the same time, the R194-E86 salt bridge suggests a different dynamic coupling between the activation loop and the αC helix. It is possible that this specific conformational state might dictate the selectivity of the substrate binding and the aberrant profile observed in phosphoproteomic assays.

Fig. 6 Schematic of the energy landscape for PKA-CWT and PKA-CL205R, combining thermodynamics and MD simulations data.

Relative free energy for the binding of ATPγN, PKI, and VPS36 to PKA-CWT and PKA-CL205R derived from ITC data. (1) Apo PKA-CL205R samples mostly uncommitted states, with the Gly-rich loop partially occluded and the αC helix turned outward. (2) Binary PKA-CL205R features a wired allosteric network for substrate binding, i.e., committed state. (3) Ternary PKA-CL205R complex with lower affinity for PKI (i.e., higher free energy relative to PKA-CWT). The conformation of the activation loop of PKA-CL205R is in equilibrium between an unflipped and a sparsely populated flipped conformation. (4) PKA-CL205R/ATPγN/VPS36 ternary complex features a flipped conformation of the activation loop with the electrostatic interactions between E86 and R194.

Given the ubiquitous nature of PKA and its involvement in numerous cell signaling events, its implication in specific diseases has been overlooked. Only recently, the attention of researchers has been directed to the PRKACA gene and its role in pathological lesions. A few years ago, Simon and co-workers discovered the presence of a chimeric construct of the PKA-C subunit that is the main driver of fibrolamellar hepatocellular carcinoma (38). Later on, our group and others discovered single mutations or insertions at the R/C interface that are linked to adrenal Cushing’s syndrome (37). More recently, Tseng et al. (39) reported the discovery of a new insertion in a similar region that is involved in the development of myxoma, a sporadic form of tumor in the atrium of the heart. It is possible that other defects in the PRKACA gene might be found to be implicated in the development of other tumors.

The analysis of the allosteric network carried out in this work reveals highly coupled allosteric nodes that harbor all Cushing’s mutations found in patients and may help to explain why mutations located in two spatially distinct regions of the enzyme result in the same phenotype. Our findings suggest that analysis of allosteric networks using CHESCA may prove useful in predicting mutations that perturb catalytic function for enzymes beyond PKA-C. Furthermore, our work reveals that rewiring of the intra- and intermolecular interactions leads to changes in cooperativity and selectivity. This suggests the opportunity to exploit small molecules or peptides that can change the internal communication and modulate the activity of this aberrant mutant selectively, i.e., without affecting the function of the wild type.


Sample preparation

The recombinant human Cα subunit of cAMP-dependent PKA cDNA (PKA-CWT and PKA-CL205R) was cloned into a pET-28a vector. A tobacco etch virus (TEV) cleavage site was incorporated via mutagenesis into the vector between the cDNA coding for the kinase and a thrombin cleavage site. The kinase was expressed in E. coli BL21 (DE3) pLysS cells in M9 minimal media supplemented with 15NH4Cl. Protein overexpression was induced with 0.4 mM isopropyl β-D-thiogalactopyranoside and carried out overnight at 20°C. PKA-C purification was carried out using Ni2+ affinity chromatography. Cells were lysed using a French press in 50 mM tris-HCl, 30 mM KH2PO4, 200 mM NaCl, 200 μM ATP, 5 mM 2-mercaptoethanol, and lysozyme [lysis buffer (15 mg/100 ml)] (pH 8.0). After removing cell debris by centrifugation at 18,000 rpm for 45 min, the supernatant was incubated with Ni2+ nitrilotriacetic acid resin (Thermo Fisher Scientific) at 4°C overnight. The resin was washed with 50 mM tris-HCl, 30 mM KH2PO4, 200 mM NaCl, 10 mM imidazole, and 5 mM 2-mercaptoethanol (pH 8.0) and eluted with 50 mM tris-HCl, 30 mM KH2PO4, 200 mM NaCl, 250 mM imidazole, and 5 mM 2-mercaptoethanol (pH 8.0). Fractions containing PKA-C were cleaved overnight at 4°C with a sufficient amount of recombinant TEV while being dialyzed into 20 mM KH2PO4, 25 mM KCl, 0.1 mM phenylmethylsulfonyl fluoride, and 5 mM 2-mercaptoethanol (pH 6.5). The three isoforms of PKA-C (corresponding to the three phosphorylation states with identical catalytic parameters) were separated by chromatography on a HiTrap SP column (GE Healthcare Life Sciences) using a linear gradient from buffer A [20 mM KH2PO4 (pH 6.5)] to 30% buffer B [20 mM KH2PO4 and 1 M KCl (pH 6.5)] at a flow rate of 2 ml/min. Isoform II with pThr197, pSer338, and pSer10 was used for all NMR experiments. Peptides (Kemptide/PKI5–24/VPS36) were synthesized using standard Fmoc chemistry on a CEM Liberty Blue microwave synthesizer, cleaved with Reagent K (82.5% trifluoroacetic acid, 5% phenol, 5% thioanisole, 2.5% ethanedithiol, and 5% water) for 3 hours, and purified using a semipreparative Supelco C18 reverse-phase HPLC column at 3 ml/min. The VPS36 peptide sequence used for all studies is as follows: QIEFYRRLSEEMTQR. The molecular weight and quantity of the peptides were verified by liquid chromatography–mass spectrometry and/or amino acid analysis (Texas Tech Protein Chemistry Laboratory).

ITC measurements

ITC measurements were performed with a low-volume Nano ITC (TA Instruments). PKA-CWT and PKA-CL205R were dialyzed into 20 mM Mops, 90 mM KCl, 10 mM dithiothreitol (DTT), 10 mM MgCl2, and 1 mM NaN3 (pH 6.5). PKA-C concentrations for ITC measurements were between 100 and 130 μM as confirmed by A280 = 53,860 M−1 cm−1. All measurements with ATPγN-saturated PKA-CWT and PKA-CL205R were performed at 2 mM ATPγN and 4 mM ATPγN, respectively. ITC measurements were performed at 300 K in triplicate. Approximately 300 μl of PKA-C was used for each experiment, as well as 50 μl of 2 to 4 mM ATPγN, 0.6 to 4 mM PKI, or 2 mM VPS36 in the titrant syringe. The heat of dilution of the ligand into the buffer was taken into account for all experiments and subtracted accordingly. Binding was assumed to be 1:1, and curves were analyzed with the NanoAnalyze software (TA Instruments) using the Wiseman isotherm (40)


(1)where d[MX] is the change in total complex with respect to change in total protein concentration and d[Xtot] is dependent on r (the ratio of Kd with respect to the total protein concentration) and RM (the ratio between total ligand and total protein concentration). The free energy of binding was determined using the following


where R is the universal gas constant and T is the temperature at measurement (300 K). The entropic contribution to binding was calculated using the following


Calculations for the cooperativity constant (σ) were calculated as follows

σ=Kd ApoKd Nucleotide

where Kd Apo is the Kd of PKI5–24 binding to the apoenzyme and Kd Nucleotide is the Kd of PKI5–24 binding to the nucleotide-bound enzyme.

Enzyme assays

Steady-state activity assays with Kemptide and VPS36 were performed under saturating ATP concentrations and spectrophotometrically at 298 K, as described by Cook et al. (41). The values of Vmax and KM were obtained from a nonlinear fit of the initial velocities to the Michaelis-Menten equation.

Circular dichroism

A change in ellipticity was measured at 220 nm on a Jasco J-815 spectrometer. A temperature scan between 20° and 70°C at a rate of 1°C/min with an equilibration time of 15 s was performed to unfold the protein. Spectra Manager Pro software was used to fit a two-state sigmoidal function. The inflection point was taken as the Tm (in °C), the point at which 50% of the protein is folded.

NMR spectroscopy

Uniformly 15N-labeled PKA-CWT and PKA-CL205R were overexpressed and purified as described above. NMR experiments were performed in 90 mM KCl, 20 mM KH2PO4, 10 mM DTT, 10 mM MgCl2, and 1 mM NaN3 at pH 6.5. Standard [1H, 15N]-TROSY-HSQC experiments were carried out for PKA-CL205R and PKA-CWT on 600-MHz and 900-MHz Bruker Avance III spectrometers equipped with TCI cryoprobes, respectively. Concentrations for samples were 0.2 to 0.3 mM as determined by A280 measurements, 12 mM ATPγN was added for the nucleotide-bound form, and 0.2 to 1.2 mM PKI or 0.4 to 1.8 mM VPS36 was added for the ternary complex. Full-length PKI was used for all NMR experiments and subsequent analyses. Spectra were collected at 300 K, processed using NMRPipe (42), and visualized using Sparky (43).

All [1H, 15N]-TROSY-HSQC experiments were acquired with 2048 (proton) and 256 (nitrogen) complex points. Spectra acquired in complex with VPS36 were acquired with substantially more scans (NS = 96) compared to other spectra [number of scans (NS) = 32 to 64]. Combined CSPs were calculated using 1H and 15N chemical shifts according to the following



Changes in CSPs were calculated according to the following for both nucleotide binding and pseudo-substrate binding



Chemical shift analyses

Coordinated chemical shifts behaviorCONCISE was used to monitor chemical shift trajectories and measure the change in equilibrium position using each PKA-C construct (apo, ATPγN, ATPγN/PKI, and ATPγN/VPS36). Using PCA, this method identifies sets of residues whose chemical shifts respond linearly to the conformational transition. Each residue provides a measure of the equilibrium position for every PKA-C construct in the form of scores along the first principal component (PC1). The equilibrium position for a given construct is given by the average of all principal component scores over all linear residues. To identify the residues whose chemical shifts follow a linear trajectory, a threshold of 3.0 for the ratio of the SDs of PC1 over PC2 was used, and residues not exhibiting a significant chemical shift were excluded based on linewidth. After this threshold was applied, a total of 55 residues formed the subset that was used to trace the equilibrium position of each state for PKA-CWT and PKA-CL205R.

Chemical shift covariance analysisCHESCA was used to identify and functionally characterize allosteric networks of residues eliciting concerted responses to, in this case, nucleotide, pseudo-substrate, and substrate. A total of three states were used to identify inter-residue correlations: apo, ATPγN-bound, and either ATPγN/PKI- or ATPγN/VPS36-bound. Identification of inter-residue correlations by CHESCA relies on agglomerative clustering and singular value decomposition. Pairwise correlations between chemical shift variations experienced by different residues are analyzed to identify networks of coupled residues and, when plotted on a correlation matrix, allow for the identification of regions that are correlated to one another. A correlation coefficient cutoff of 0.98 was used to filter nonlinear residues. Residues not exhibiting a significant chemical shift (small shifts in parts per million) were excluded based on linewidth. Linewidth was calculated for each resonance in each of the three forms (apo, ATPγN, and ATPγN/PKI or ATPγN/VPS36). For each residue, the maximum change in chemical shift was calculated in both the 1H (x) and 15N (y) dimension (Δδx,y). Residues were included in CHESCA analysis only if they satisfied the following: Δδx,y > ½ ΔνxA,yA + ½ Δν xB,yB, where A and B correspond to two different forms analyzed (note that there is no dependence on which two forms satisfied this statement).

MD simulations

System setup on wild type and L205RWe used the crystal structure of PKA-CWT (PDB ID: 1ATP) (28) and PKA-CL205R (PDB ID: 4WB6) (16) as the template and chose a monomer (chain A, protein; chain I, PKI5–24) from the dimer. We further aligned the current structure with the full-length PKA-CWT and added the missing residues 1 to 12 at the N terminus. The protonation state of histidine residues followed our previous settings (27). The protein was solvated in a rhombic dodecahedron solvent box with a TIP3P water molecule layer extended approximately 10 Å away from the surface of the proteins. Counter ions (K+ and Cl) were added to ensure electrostatic neutrality corresponding to an ionic concentration of ~150 mM. All protein covalent hydrogen bonds were constrained with the LINCS (linear constraint solver) algorithm, and long-range electrostatic interactions were treated with the particle-mesh Ewald method with a real-space cutoff of 10 Å. Parallel simulations on the apo form, the binary form with one Mg2+ ion and one ATP, and the ternary form with two Mg2+ ions, one ATP, and one PKI5–24 were performed simultaneously using GROMACS 4.6 (44) in CHARMM36a1 force fields (45). Each system was minimized using the steepest decent algorithm to remove the bad contacts and then gradually heated to 300 K at a constant volume over 1 ns, using harmonic restraints with a force constant 1000 kJ/(mol·Å2) on heavy atoms of both proteins and nucleotides. Over the following 12 ns of simulations at constant pressure (1 atm) and temperature (300 K), the restraints were gradually released. The systems were equilibrated for an additional 20 ns without positional restraints. A Parrinello-Rahman barostat was used to keep the pressure constant, while a V-rescale thermostat with a time step of 2 fs was used to keep the temperature constant. Each system was simulated for 1.05 μs, with snapshots recorded every 20 ps. A total of 3.15 μs and 157,500 conformations were used for the analyses.

Energy landscapes using PCACartesian principal components of the backbone atoms were calculated using the GROMACS modules g_covar and g_anaeig to identify the large-scale, low-frequency conformational dynamics of the catalytic core. All the trajectories were aligned with the starting structure (minimization of the crystal structure to remove bad contacts) using helices E (residues 140 to 160) and F (residues 217 to 233) as a reference frame. Dominant principal components were computed from each resulting ensemble from the individual simulations. Moreover, the distance between the Cα atom of Ser53 and Gly186 was measured to characterize the opening and closing motions of the Gly-rich loop. Different trajectories were mapped onto the 2D projection along PC1 and S53-G186 distances.

Docking and simulation of the ternary complexes bound with VPS36 peptide and ATPWe used the unwound conformation of VPS36 and used HADDOCK (46) server for docking into the binding cleft of PKA-C for both wild type and L205R. Specifically, we used the easy interface and selected the active residues of PKA-C, i.e., 133, 168, 202, 198, 204, 205, 207, 230, and 330, as well as the active residues for VPS36, i.e., 3, 6, 7, 9, and 10. The passive residues were set automatically around the active residues by the server. The top-scored structures were further solvated for MD simulations following the same protocol as the ternary complexes with PKI.

Mutual information analysis and mapping of the allosteric networkTo monitor the allosteric differences of the wild type and L205R, MutInf (29) was used to compute mutual information between all residues. MutInf is a python package that translates the distribution of dihedral angles of residues into their conformational entropy and identifies the correlated motions between residues. The time series of dihedral angles in the MD ensemble was computed using g_chi and divided into six overlapped blocks, and then, correlations of local motions were computed as the mutual information between selected residue pairs in each block. The results were averaged over these blocks to filter out the correlations that were not statistically significant. These matrices of mutual information and their differences were further mapped onto the crystal structure. Graph analysis was applied to detect hubs in the networks and key allosteric communication pathways in PKA-C.


Supplementary material for this article is available at

Fig. S1. [1H, 15N]-TROSY-HSQC spectra for PKA-CWT and PKA-CL205R in apo, ATPγN, and ATPγN/PKI-bound and ATPγN/VPS36-bound forms.

Fig. S2. CSPs observed upon ligand binding for PKA-CWT and PKA-CL205R.

Fig. S3. Intensity plot for the binding of VPS36 to ATPγN-saturated PKA-CL205R.

Fig. S4. PCA of the catalytic lobes in PKA-CWT and PKA-CL205R.

Fig. S5. Probability of the formation of inter-residue contact and ΔRMSF of PKA-C upon forming ternary complexes with PKI5–24 or VPS36.

Fig. S6. Allosteric changes upon peptide binding revealed by MD simulation and mutual information (MutInf) analysis.

Table S1. Changes in enthalpy, entropy, free energy, and dissociation constant of binding ATPγN, PKI5–24, and VPS36 for PKA-CWT and PKA-CL205R.

Table S2. Kinetic parameters of Kemptide and VPS36 phosphorylation by PKA-CWT and PKA-CL205R.

Table S3. PCA and SD of the CONCISE analysis of the structural states analyzed.

Table S4. Tm as determined using CD.

This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial license, which permits use, distribution, and reproduction in any medium, so long as the resultant use is not for commercial advantage and provided the original work is properly cited.


Acknowledgments: NMR experiments were carried out at the Minnesota NMR Center, and MD calculations were performed at the Minnesota Supercomputing Institute. Funding: This work was supported by the National Institutes of Health (GM100310 and S10 OD021536 to G.V.) and the CRC/Transregio 166 (Project C1 to D.C.), as well the IZKF Würzburg (grant B-281 to D.C.). K.B. was partially supported by a fellowship through a grant of the German Excellence Initiative to the Graduate School of Life Sciences, University of Würzburg. Author contributions: C.W. prepared the PKA-C samples; designed, executed, and analyzed all NMR experiments; and contributed to the writing of the manuscript. Y.W. carried out all MD simulations and contributed to the writing of the manuscript. C.O. carried out triple-resonance NMR experiments for backbone assignment of PKA-C. A.K. carried out the initial NMR, thermodynamics, and kinetic experiments. J.C. assisted in the preparation of PKA-C samples. K.B. and D.C. contributed to the critical analysis of the data and writing of the manuscript. J.G. assisted Y.W. with setting up the MD and Markov model calculations. D.A.B. contributed to the critical analysis of the data and writing of the manuscript. S.S.T. contributed to the writing of the manuscript. G.V. designed the experiments and contributed to the writing of the manuscript. Competing interests: The authors declare that they have no competing interests. Data and materials availability: All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

View Abstract

Corcept Therapeutics Announces Allowance of Additional Patent Covering the Use of Korlym to Treat Patients With Cushing’s Syndrome

MENLO PARK, Calif., Aug. 28, 2019 (GLOBE NEWSWIRE) — Corcept Therapeutics Incorporated (NASDAQ: CORT) announced today that the United States Patent and Trademark Office has issued a Notice of Allowance for a patent covering the administration of Korlym® with food.  The patent will expire in November 2032.

“This patent covers an important finding of our research – that for optimal effect, Korlym must be taken with food,” said Joseph K. Belanoff, MD, Corcept’s Chief Executive Officer. “Korlym’s label instructs doctors that ‘Korlym must always be taken with a meal.’”

Upon issuance, Corcept plans to list the patent, entitled “Optimizing Mifepristone Absorption” (U.S. Pat. App. 13/677,465), in the U.S. Food and Drug Administration’s Approved Drug Products with Therapeutic Equivalence Evaluations (the “Orange Book”).  Korlym is currently protected by ten patents listed in the Orange Book.


Hypercortisolism, often referred to as Cushing’s syndrome, is caused by excessive activity of the hormone cortisol. Endogenous Cushing’s syndrome is an orphan disease that most often affects adults aged 20-50. In the United States, an estimated 20,000 patients have Cushing’s syndrome, with about 3,000 new patients diagnosed each year. Symptoms vary, but most people with Cushing’s syndrome experience one or more of the following manifestations: high blood sugar, diabetes, high blood pressure, upper-body obesity, rounded face, increased fat around the neck, thinning arms and legs, severe fatigue and weak muscles. Irritability, anxiety, cognitive disturbances and depression are also common. Hypercortisolism can affect every organ system in the body and can be lethal if not treated effectively.

About Corcept Therapeutics Incorporated

Corcept is a commercial-stage company engaged in the discovery and development of drugs that treat severe metabolic, oncologic and psychiatric disorders by modulating the effects of the stress hormone cortisol.  Korlym® (mifepristone) was the first treatment approved by the U.S. Food and Drug Administration for patients with Cushing’s syndrome.  Corcept has discovered a large portfolio of proprietary compounds, including relacorilant, exicorilant and miricorilant, that selectively modulate the effects of cortisol but not progesterone. Corcept owns extensive United States and foreign intellectual property covering the composition of its selective cortisol modulators and the use of cortisol modulators, including mifepristone, to treat a variety of serious disorders.

Forward-Looking Statements

Statements in this press release, other than statements of historical fact, are forward-looking statements, which are based on Corcept’s current plans and expectations and are subject to risks and uncertainties that might cause actual results to differ materially from those such statements express or imply. These risks and uncertainties include, but are not limited to, Corcept’s ability to generate sufficient revenue to fund its commercial operations and development programs; the availability of competing treatments, including generic versions of Korlym; Corcept’s ability to obtain acceptable prices or adequate insurance coverage and reimbursement for Korlym; and risks related to the development of Corcept’s product candidates, including regulatory approvals, mandates, oversight and other requirements. These and other risks are set forth in Corcept’s SEC filings, which are available at Corcept’s website and the SEC’s website. In this press release, forward-looking statements include those concerning Corcept’s plans to list the patent “Optimizing Mifepristone Absorption” in the Orange Book; Korlym’s current protection by ten patents listed in the Orange Book; and the scope and protective power of Corcept’s intellectual property. Corcept disclaims any intention or duty to update forward-looking statements made in this press release.

Christopher S. James, MD
Director, Investor Relations
Corcept Therapeutics

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