The cochaperone CDC37 promotes association of HSP90 with the protein kinase subset of client proteins to maintain their stability and signalling functions. HSP90 inhibitors induce depletion of clients, which include several oncogenic kinases. We hypothesised that the targeting of CDC37 using siRNAs would compromise the maturation of these clients and increase the sensitivity of cancer cells to HSP90 inhibitors. Here we show that silencing of CDC37 in human colon cancer cells diminished association of kinase clients with HSP90 and reduced levels of the clients ERBB2, CRAF, CDK4 and CDK6, as well as phosphorylated AKT. CDC37 silencing promoted the proteasome-mediated degradation of kinase clients, suggesting a degradation pathway independent from HSP90 binding. Decreased cell signalling through kinase clients was also demonstrated by reduced phosphorylation of downstream substrates and colon cancer cell proliferation was subsequently reduced by inhibition of the G1/S-phase transition. Furthermore, combining CDC37 silencing with the HSP90 inhibitor 17-AAG induced more extensive and sustained depletion of kinase clients and potentiated cell cycle arrest and apoptosis. These results support an essential role for CDC37 in concert with HSP90 in maintaining oncogenic protein kinase clients and endorse the therapeutic potential of targeting CDC37 in cancer.

Phenotypic screens, which focus on measuring and quantifying discrete cellular changes rather than affinity for individual recombinant proteins, have recently attracted renewed interest as an efficient strategy for drug discovery. In this article, we describe the discovery of a new chemical probe, bisamide (CCT251236), identified using an unbiased phenotypic screen to detect inhibitors of the HSF1 stress pathway. The chemical probe is orally bioavailable and displays efficacy in a human ovarian carcinoma xenograft model. By developing cell-based SAR and using chemical proteomics, we identified pirin as a high affinity molecular target, which was confirmed by SPR and crystallography.

The HSP90 molecular chaperone plays a key role in the maturation, stability and activation of its clients, including many oncogenic proteins. Kinases are a substantial and important subset of clients requiring the key cochaperone CDC37. We sought an improved understanding of protein kinase chaperoning by CDC37 in cancer cells. CDC37 overexpression in human colon cancer cells increased CDK4 protein levels, which was negated upon CDC37 knockdown. Overexpressing CDC37 increased CDK4 protein half-life and enhanced binding of HSP90 to CDK4, consistent with CDC37 promoting kinase loading onto chaperone complexes. Against expectation, expression of C-terminus truncated CDC37 (ΔC-CDC37) that lacks HSP90 binding capacity did not affect kinase client expression or activity; moreover, as with wildtype CDC37 overexpression, it augmented CDK4-HSP90 complex formation. However, although truncation blocked binding to HSP90 in cells, ΔC-CDC37 also showed diminished client protein binding and was relatively unstable. CDC37 mutants with single and double point mutations at residues M164 and L205 showed greatly reduced binding to HSP90, but retained association with client kinases. Surprisingly, these mutants phenocopied wildtype CDC37 overexpression by increasing CDK4-HSP90 association and CDK4 protein levels in cells. Furthermore, expression of the mutants was sufficient to protect kinase clients CDK4, CDK6, CRAF and ERBB2 from depletion induced by silencing endogenous CDC37, indicating that CDC37’s client stabilising function cannot be inactivated by substantially reducing its direct interaction with HSP90. However, CDC37 could not compensate for loss of HSP90 function, showing that CDC37 and HSP90 have their own distinct and non-redundant roles in maintaining kinase clients. Our data substantiate the important function of CDC37 in chaperoning protein kinases. Furthermore, we demonstrate that CDC37 can stabilise kinase clients by a mechanism that is not dependent on a substantial direct interaction between CDC37 and HSP90, but nevertheless requires HSP90 activity. These results have significant implications for therapeutic targeting of CDC37.