Redefining the Phenotype of Heat Shock Protein 90 (Hsp90) Inhibitors

The phenotypes produced when cells are treated with the heat shock protein 90 (Hsp90) inhibitors AUY922 or 17‐AAG (classical inhibitors) are different to those produced when cells are knocked down with Hsp90α. Pull‐down assays using classical inhibitors suggest that these molecules bind to multiple targets other than Hsp90. Classical inhibitors also induce similar protein markers as other anti‐cancer therapies cisplatin and bortezomib that do not target Hsp90. Together these data suggest that AUY922 and 17‐AAG acts on multiple targets and likely kills cells through multiple mechanisms. Comparing these classical inhibitors to the effects seen when treating cells with C‐terminal Hsp90 modulators reveals that C‐terminal modulators effectively bind to Hsp90, and induce phenotypic markers consistent with the Hsp90α CRISPR knockdown data. Our findings challenge the current interpretation of Hsp90 inhibitors and suggest that a large body of literature that describes the Hsp90 phenotype and inhibitors is re‐examined in this context.