Abstract
AHA1 (activator of HSP90 ATPase) is a cochaperone of the ATP-dependent molecular chaperone, HSP90, which is involved in the maturation, stabilization/degradation, and function of oncogenic proteins. HSP90 operates in a multimeric complex driven by the binding and hydrolysis of ATP. Treatment of cells with the HSP90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) results in the degradation of client proteins via the ubiquitin-proteasome pathway. As AHA1 increases the ATPase activity of HSP90, we hypothesized that modulation of AHA1 expression could influence the activity of client proteins and/or the cellular response to 17-AAG. We show that the basal expression of AHA1 is different across a panel of human cancer cell lines, and that treatment with 17-AAG resulted in sustained AHA1 up-regulation. Increasing the expression of AHA1 did not affect the sensitivity to 17-AAG, but did increase C-RAF activity and the levels of phosphorylated MEK1/2 and ERK1/2 without affecting total levels of these proteins or of client proteins C-RAF, ERBB2, or CDK4. Conversely, small interfering RNA-selective knockdown of >80% of AHA1 expression decreased C-RAF activity and reduced the levels of MEK1/2 and ERK1/2 phosphorylation. Moreover, the AHA1 knockdown resulted in a significant (P < 0.05) increase in sensitivity to 17-AAG, due in part to a 2- to 3-fold increase in apoptosis. These results show that the reduction of AHA1 levels could decrease the phosphorylation of key signal transduction proteins, and for the first time, separate the activation and stabilization functions of HSP90. Furthermore, AHA1 knockdown could sensitize cancer cells to 17-AAG. We conclude that modulation of AHA1 might be a potential therapeutic strategy to increase sensitivity to HSP90 inhibitors.
Highlights
HSP90 is a unique and essential molecular chaperone in all eukaryotic cells [1]
We show that modulation of AHA1 levels could alter the activity of representative HSP90 client proteins and the phosphorylation status of key signaling proteins MEK1/2, ERK1/2, and AKT without affecting the total levels of these proteins or of the client proteins C-RAF, ERBB2, or CDK4
The results suggest that AHA1 is involved in regulating the activation status, but not the stability, of several signal transduction proteins and indicate that AHA1 levels may play a role in cellular response to HSP90 inhibitors
Summary
HSP90 is a unique and essential molecular chaperone in all eukaryotic cells [1]. It forms the core component of a multimeric chaperone complex that regulates the balance of stabilization, activation, and degradation of client proteins including numerous transcription factors and kinases. Note: Supplementary data for this article are available at Cancer Research Online (http://cancerres.aacrjournals.org/). In cancer cells, HSP90 allows proteins with activating but potentially destabilizing mutations to continue to function rather than being degraded, and supports malignant transformation and progression [8,9,10,11,12,13]
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