Involvement of endoplasmic reticulum stress in Docetaxel-induced JNK-dependent apoptosis of human melanoma

Our previous studies revealed that Docetaxel-induced apoptosis of melanoma cells is entirely dependent on activation of the JNK signalling pathway. Here, we show that Docetaxel-induced apoptosis is mediated by induction of ER stress. This was shown by Docetaxel-induced activation of proteins involved in ER stress signalling namely GRP78, ATF6, IRE1alpha, and PERK/eIF2alpha. Knockdown of IRE1alpha by siRNA markedly inhibited Docetaxel-induced JNK activation and downstream targets of JNK indicating that activation of IRE1alpha was upstream of activation of the JNK. Co-immunoprecipitation experiments showed that activation of JNK is due to activation of ASK1 through formation of an IRE1alpha-TRAF2-ASK1 complex. ER stress mediated activation of the JNK pathway is downstream of activation of PKCdelta in that downregulation of PKCdelta expression using specific PKCdelta siRNA significantly inhibited Docetaxel-induced activation of IRE1alpha and the JNK pathway. These findings provide new insights to understand the mode of action of taxanes in treatment of human melanoma.