Quantitative effects on c-Jun N-terminal protein kinase signaling determine synergistic interaction of cisplatin and 17-allylamino-17-demethoxygeldanamycin in colon cancer cell lines

We investigated the effects of cisplatin and the hsp90 inhibitor 17-allylamino-17-demethoxygeldanamycin (17-AAG) in combination in a panel of human colon adenocarcinoma cell lines that differ in their p53 and mismatch repair status. Analysis of cytotoxicity after combined treatment revealed additive effects of cisplatin and 17-AAG in the HCT 116, DLD1, and SW480 cell lines and antagonism in HT-29 cells. Clonogenic assays demonstrated antagonism in HT-29, an additive effect in SW480, and synergism in HCT 116 and DLD1 cell lines. Analysis of signaling pathways revealed that cisplatin-induced activation of c-Jun N-terminal kinase (JNK) was fully blocked by 17-AAG in HT-29 and SW480 cells, whereas in HCT 116 and DLD1 cells it was inhibited only partially. The activation of caspases was also more pronounced in DLD1 and HCT 116 cell lines. These data suggested that a minimal level of apoptotic signaling through JNK was required for synergism with this combination. To test this hypothesis, we used the specific JNK inhibitor SP600125; when JNK was inhibited pharmacologically in HCT 116 and DLD1 cells, they demonstrated increased survival in clonogenic assays. Alternatively, sustained activation of JNK pathway led to an increase of the cytotoxicity of the cisplatin/17-AAG combination in HT-29 cells. Taken together, these data suggest that the synergistic interaction of this combination in colon cancer cell lines depends on the effect exerted by 17-AAG on cisplatin-induced signaling through JNK and associated pathways leading to cell death. An implication of that finding is that quantitative effects of signaling inhibitors may be critical for their ability to reverse cisplatin resistance.