Heat-shock protein 90 inhibitors synergistically enhance melanoma differentiation-associated gene-7-mediated cell killing of human pancreatic carcinoma

Pancreatic cancer is one of the intractable diseases and an effective therapeutic strategy is required to improve the prognosis. We examined possible antitumor effects of adenoviruses expressing melanoma differentiation-associated gene-7/interleukin-24 (Admda-7) and a heat-shock protein 90 (Hsp90) inhibitor to human pancreatic carcinoma cells. Ad-mda-7 and an Hsp90 inhibitor, geldanamycin (GA), produced cytotoxic effects, and a combinatory use of Ad-mda-7 and GA further achieved synergistic effects. Administration of N-acetyl-L-cysteine, an inhibitor of reactive oxygen species, eliminated Ad-mda-7- and GA-mediated cytotoxicity. Ad-mda-7 augmented phosphorylated AKT levels but GA did not influence the phosphorylation. GA-treated cells showed cleavage of poly-(ADP-ribose) polymerase but not caspase-3, and upregulated Hsp70 and LC3A/B II levels, whereas Ad-mda-7-treated cells did not. GA treatments augmented ubiquitination and markedly increased melanoma differentiation-associated gene-7 (MDA-7) expression levels. These findings suggest that Ad-mda-7-mediated cytotoxicity is dependent on reactive oxygen species but independent of apoptosis or autophagy, and that GA-mediated cytotoxicity was linked with caspase-independent apoptosis and/or autophagy. A mechanism underlying the combinatory effects of Ad-mda-7 and GA remained complex and the synergism is attributable to multiple factors including increased MDA-7 protein stability by GA.