The tumor suppressors Tsc1 and Tsc2 form the tuberous sclerosis complex (TSC), a regulator of mTOR activity. Tsc1 stabilizes Tsc2; however, the precise mechanism involved remains elusive. The molecular chaperone heat‐shock protein 90 (Hsp90) is an essential component of the cellular homeostatic machinery in eukaryotes. Here, we show that Tsc1 is a new co‐chaperone for Hsp90 that inhibits its ATPase activity. The C‐terminal domain of Tsc1 (998–1,164 aa) forms a homodimer and binds to both protomers of the Hsp90 middle domain. This ensures inhibition of both subunits of the Hsp90 dimer and prevents the activating co‐chaperone Aha1 from binding the middle domain of Hsp90. Conversely, phosphorylation of Aha1‐Y223 increases its affinity for Hsp90 and displaces Tsc1, thereby providing a mechanism for equilibrium between binding of these two co‐chaperones to Hsp90. Our findings establish an active role for Tsc1 as a facilitator of Hsp90‐mediated folding of kinase and non‐kinase clients—including Tsc2—thereby preventing their ubiquitination and proteasomal degradation.

SUMMARYThe molecular chaperone Hsp90 protects deregulated signaling proteins that are vital for tumor growth and survival. Tumors generally display sensitivity and selectivity toward Hsp90 inhibitors; however, the molecular mechanism underlying this phenotype remains undefined. We report that the mitotic checkpoint kinase Mps1 phosphorylates a conserved threonine residue in the amino-domain of Hsp90. This, in turn, regulates chaperone function by reducing Hsp90 ATPase activity while fostering Hsp90 association with kinase clients, including Mps1. Phosphorylation of Hsp90 is also essential for the mitotic checkpoint because it confers Mps1 stability and activity. We identified Cdc14 as the phosphatase that dephosphorylates Hsp90 and disrupts its interaction with Mps1. This causes Mps1 degradation, thus providing a mechanism for its inactivation. Finally, Hsp90 phosphorylation sensitizes cells to its inhibitors, and elevated Mps1 levels confer renal cell carcinoma selectivity to Hsp90 drugs. Mps1 expression level can potentially serve as a predictive indicator of tumor response to Hsp90 inhibitors.

The practising clinician treating a patient with metastatic clear cell renal cell carcinoma (CCRCC) faces a difficult task of choosing the most appropriate therapeutic regimen in a rapidly developing field with recommendations derived from clinical trials. NCCN guidelines for kidney cancer initiated a major shift in risk categorization and now include emerging treatments in the neoadjuvant setting. Updates of European Association of Urology clinical guidelines also include immune checkpoint inhibition as the first-line treatment. Randomized trials have demonstrated a survival benefit for ipilimumab and nivolumab combination in the intermediate and poor-risk group, while pembrolizumab plus axitinib combination is recommended not only for unfavorable disease but also for patients who fit the favorable risk category. Currently vascular endothelial growth factor (VEGF) targeted therapy based on tyrosine kinase inhibitors (TKI), sunitinib and pazopanib is the alternative regimen for patients who cannot tolerate immune checkpoint inhibitors (ICI). Cabozantinib remains a valid alternative option for the intermediate and high-risk group. For previously treated patients with TKI with progression, nivolumab, cabozantinib, axitinib, or the combination of ipilimumab and nivolumab appear the most plausible alternatives. For patients previously treated with ICI, any VEGF-targeted therapy, not previously used in combination with ICI therapy, seems to be a valid option, although the strength of this recommendation is weak. The indication for cytoreductive nephrectomy (CN) is also changing. Neoadjuvant systemic therapy does not add perioperative morbidity and can help identify non-responders, avoiding unnecessary surgery. However, the role of CN should be investigated under the light of new immunotherapeutic interventions. Also, markers of response to ICI need to be identified before the optimal selection of therapy could be determined for a particular patient.

No abstract