IFNγ sensitizes for apoptosis by upregulating caspase-8 expression through the Stat1 pathway

Abstract

Resistance of tumors to cytotoxic therapy may be due to disrupted apoptosis programs and remains a major obstacle in cancer treatment. Here, we report that IFNγ sensitizes resistant tumor cells with absent or low caspase-8 expression for apoptosis induced by death-inducing ligands or cytotoxic drugs by upregulating caspase-8 through a Stat1/IRF1 dependent pathway. Combined treatment using IFNγ with TRAIL, APO1, TNFα or cytotoxic drugs cooperated to trigger apoptosis in various resistant tumor cell lines derived from Ewing tumor, neuroblastoma or medulloblastoma, while single agents exerted only a minimal effect. Importantly, IFNγ induced caspase-8 expression also in cells with inactivation of the caspase-8 gene by hypermethylation, although no direct effect of IFNγ on the methylation status of regulatory sequences of the caspase-8 gene was found. IFNγ-mediated facilitation of apoptosis was inhibited by the caspase-8 specific inhibitor zIETD.fmk or in caspase-8 mutant Jurkat cells implying a prominent role of caspase-8 in mediating sensitization by IFNγ. Upregulation of caspase-8 and sensitization for apoptosis by IFNγ was blocked by overexpression of dominant-negative mutants of Stat1 or in Stat1-deficient U3A cells, while complementation of Stat1-deficient U3A cells with wild-type Stat1 restored the IFNγ effect. Moreover, ectopic expression of IRF1 induced caspase-8 expression thereby sensitizing cells for TRAIL-, APO1- or doxorubicin-induced apoptosis. These findings provide evidence that the Stat1/IRF1 pathway is involved in induction of caspase-8 expression and apoptosis initiated by IFNγ and indicate that IFNγ might be an effective strategy to sensitize various resistant tumor cells with deficient caspase-8 expression for chemotherapy- or death receptor-induced apoptosis.