Hsp90 inhibition in adrenocortical carcinoma: Limited drug synergism with mitotane.

90 kDa heat shock proteins (Hsp90) act as protein chaperones and play a role in modulating endoplasmic reticulum (ER) stress. Hsp90 inhibitors are under clinical investigation as cancer treatment. Mitotane therapy of adrenocortical carcinoma (ACC) has been shown to act through lipid-induced ER-stress. To explore the potential of Hsp90 inhibitors in ACC as a single agent and in combination with mitotane, we analyzed two independent gene expression data sets of adrenal tumors in silico and treated the ACC cell line model NCI-H295 with Hsp90 inhibitors BIIB021 (B) and CCT18159 (C) alone and in combination with mitotane. ER-stress markers were monitored by immunoblotting. Drug synergism was quantified using the median effect model with cell viability as read-out. Cytosolic Hsp90 isoforms AA1 and AB1 were significantly overexpressed in ACC. Viability of H295 cells was impaired by B and C as single agents with an EC50 of 5.7 × 10-6 M and 12.1 × 10-6 M. B but not C dose-dependently increased XBP1 splicing and CHOP expression indicative of ER-stress activation. ER-stress marker expression was enhanced by co-incubation of B with 10  μM but not 5  μM mitotane. Maximal CHOP expression was induced by 25 μM mitotane alone with no additional effect of B. Combination indices (CI) of B and C with mitotane ranged from 0.64 to 1.38 and 0.68 to 1.30, respectively where CI values < 0.5 support clinically-relevant drug synergism. In conclusion, Hsp90 paralogs are differentially expressed in ACC and B but not C activates ER-stress in ACC cells. No meaningful drug synergism of Hsp90 inhibitors with mitotane was observed.