Modulation of AHA1: A novel therapeutic strategy to increase cellular sensitivity to the HSP90 inhibitor 17-allylamino-17- demethoxygeldanamycin (17-AAG)

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AHA1 (Activator of HSP90 ATPase) is a co-chaperone 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 first-in-class HSP90 ATPase-inhibitor 17-AAG results in degradation of client proteins via the ubiquitin-proteasome pathway. Co-chaperones such as AHA1 and HSP72 have been shown to be upregulated with 17-AAG treatment. As AHA1 is known to significantly increase the intrinsic ATPase activity of the human HSP90 (Panaretou et. al. Mol Cell 10:1307-1318), we hypothesised that modulation of AHA1 expression could influence HSP90 activity and the cellular response to treatment with 17-AAG. We show that the induction of AHA1 is a result of stress induced transcription mediated by HSF1, as previously reported for other heat shock proteins. When AHA1 was knocked down (~80%) using RNA interference, there is a significant (P<0.05) increase in sensitivity to 17-AAG, as demonstrated by a 2-3 fold decrease in IC50 value and a similar fold induction in apoptotic detached cells. The role of AHA1 on HSP90 client protein activity was also been investigated. When AHA1 protein expression was knocked down by siRNA oligonucleotides there was no effect on the total levels of HSP90 client proteins C-RAF, ERBB2 or CDK4. Similar results were obtained when AHA1 was overexpressed. Interestingly, however, MEK1/2 and ERK1/2 phosphorylation were decreased when AHA1 was knocked down and increased when AHA1 was overexpressed. These changes in phosphorylation of MEK1/2 and ERK1/2 were caused by changes in the activity of the HSP90 client protein C-RAF. Changes in AKT phosphorylation with AHA1 overepxression were also observed, which was shown to be due to increased AKT activity. These results suggest that AHA1 has a role in client protein activation, and that reduction of AHA1 levels can sensitise cancer cells to 17-AAG. Thus, upregulation of AHA1 in response to the HPS90 inhibitor may be a protective or resistance mechanism and modulation of AHA1 could be a therapeutic strategy to increase sensitivity to HSP90 inhibitors.