Inhibition of hypoxia-induced stress signaling through JNK potentiates the effects of oxaliplatin. The JNK pathway plays a role in both autophagy and apoptosis; therefore, it was determined how much of the effect of JNK inhibition on oxaliplatin sensitivity is dependent on its effect on autophagy. We studied the impact of JNK isoform down-regulation in the HT29 colon adenocarcinoma cell line on hypoxia- and oxaliplatin-induced responses. Electron microscopic analyses demonstrated that both oxaliplatin- and hypoxia-induced formation of autophagosomes were reduced significantly in HT29 cells treated with the JNK inhibitor SP600125. The role of specific JNK isoforms was defined using HT29-derived cell lines stably expressing dominant negative constructs for JNK1 and JNK2 (HTJ1.3 and HTJ2.2, respectively). These cell lines demonstrated that functional JNK1 is required for hypoxia-induced autophagy and that JNK2 does not substitute for it. Inhibition of autophagy in HTJ1.3 cells also coincided with enhancement of intrinsic apoptosis. Analysis of Bcl2-family proteins revealed hyper-phosphorylation of Bcl-XL in the HTJ1.3 cell line, but this did not lead to the expected dissociation from Beclin-1. Consistent with this, knockdown of Bcl-XL in HT29 cells did not significantly affect the induction of autophagy, but abrogated hypoxic resistance to oxaliplatin due to the faster and more robust activation of apoptosis.ImplicationsThese data suggest that balance between autophagy and apoptosis are shifted toward apoptosis by down-regulation of JNK1, contributing to oxaliplatin sensitization. These findings further support the investigation of JNK inhibition in colorectal cancer treatment.
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