Akt shows variable sensitivity to an Hsp90 inhibitor depending on cell context.

Hsp90 inhibitors are currently in clinical trials for cancer therapy based on their ability to promote proteasomal degradation of oncogenic protein kinases and nuclear receptors. Results from recent studies suggest that cancer cells are more sensitive to these inhibitors than cells from healthy tissues. We analyzed an immortalized cell line Ba/F3 for sensitivity to the Hsp90 inhibitor geldanamycin in the absence and presence of the oncogenic tyrosine fusion kinase NPM-ALK expressed from a retroviral vector. Our results showed that NPM-ALK expression makes Akt and Cdk4 more resistant to degradation in the presence of geldanamycin, and there was a slightly reduced amount of apoptosis. The mechanism underlying the effect of NPM-ALK on Akt stability was probed by comparison of the turnover of the kinase after translation inhibition and geldanamycin treatment. We observed that Akt was degraded more rapidly in the presence of GA than upon translation inhibition without NPM-ALK expression. This suggests that NPM-ALK protects the mature kinase. Furthermore, Akt failed to bind to the Cdc37 chaperone in cells expressing NPM-ALK, which also correlates with increased Akt stability.