Cisplatin induces the proapoptotic conformation of Bak in a Delta MEKK1-dependent manner

In a panel of four human melanoma cell lines, equitoxic doses of cisplatin induced the proapoptotic conformation of the Bcl-2 family protein Bak prior to the execution phase of apoptosis, Because cisplatin-induced modulation of the related Bar protein was seen in only one cell line, a degree of spec ificity in the signal to Bak is indicated. Little is known about upstream r egulation of Bak activity. In this study, we examined whether the apoptosis -specific pathway mediated by a kinase fragment of MEKK1 (Delta MEKKL) is i nvolved in the observed Bak modulation. We report that expression of a kina se-inactive fragment of MEKK1 (dominant negative MEKK [dnMEKK]) efficiently blocked cisplatin-induced modulation of Bak and cytochrome c release and c onsequently also reduced DEVDase activation and nuclear fragmentation. Acco rdingly, expression of a kinase-active MEKK1 fragment (dominant positive ME KK) was sufficient to induce modulation of Bak in three cell lines and to i nduce apoptosis in two of these. dnMEKK did not block cisplatin-induced c-J un N-terminal kinase (JNK) activation, in agreement with a specifically pro apoptotic role for the Delta MEKK1 pathway. Finally, we show that reduction of Bak expression by antisense Bak reduced cisplatin-induced loss of mitoc hondrial integrity and caspase cleavage activity in breast cancer cell line s. In summary, we have identified Bak as a cisplatin-regulated component do wnstream in a proapoptotic, JNK-independent Delta MEKK1 pathway.