Docetaxel-Induced Apoptosis of Human Melanoma Is Mediated by Activation of c-Jun NH2-Terminal Kinase and Inhibited by the Mitogen-Activated Protein Kinase Extracellular Signal-Regulated Kinase 1/2 Pathway

Abstract

Our studies have shown variable sensitivity of cultured melanoma cells to docetaxel. To better understand this response, we studied the role of signal transduction pathways in modulating docetaxel-induced melanoma killing. Involvement of c-Jun NH(2)-terminal kinase (JNK), extracellular signal-regulated kinase 1/2 (ERK1/2), p38 mitogen-activated protein kinase, and Akt signaling was studied by evaluating their extent of activation in melanoma cells after treatment with docetaxel. The effect of their activation on docetaxel-induced apoptosis was assessed using biochemical inhibitors of the pathways and Western blot analysis of proteins involved. Docetaxel induced activation of both JNK and ERK1/2 but not p38 mitogen-activated protein kinase or Akt kinases. Apoptosis was dependent on activation of JNK and mediated through activation of caspase-2 and caspase-dependent changes in Bax and Bak. The levels of activated JNK in individual lines showed a close correlation with the levels of apoptosis. In contrast, activation of ERK1/2 by docetaxel inhibited apoptosis and the levels of activation in individual lines were inversely correlated to the degree of apoptosis. Studies on the Bcl-2 family proteins seemed to reflect changes induced by activation of JNK and ERK1/2 pathways. Docetaxel-induced JNK activation was required for Bcl-2 phosphorylation as well as caspase-2-dependent activation of Bax and Bak and subsequent mitochondrial release of apoptosis-inducing factor and cytochrome c. In contrast, activation of ERK1/2 resulted in degradation of BH3-only protein Bim and phosphorylation of Bad. These studies provide further insights into sensitivity of melanoma cells to taxanes and provide a basis for the current rationale of combining taxanes with inhibitors of the Raf-ERK1/2 pathway.