Inactivation of the Proapoptotic Protein Bad Tumor-Specific Survival Signaling in Melanoma Cells Mitogen-Activated Protein Kinase Pathway-Dependent

Mitogen-activated protein kinase (MAPK) signaling regulates fundamental cellular functions including proliferation, differentiation, and survival. We have demonstrated previously that inhibiting MAPK signaling induces apoptosis in melanoma cells but not in normal melanocytes, suggesting that the MAPK pathway propagates essential survival signals in melanoma cells. Here, we report that the 90-kDa ribosomal S6 kinase (RSK), a downstream effector in the MAPK signaling cascade, phosphorylates and inactivates the Bcl-2 homology 3-only proapoptotic protein Bad, thereby mediating a MAPK-dependent tumor-specific survival signal in melanoma cells. The MAPK kinase (MEK)/extracellular signalregulated kinase (ERK)/RSK MAPK signaling module is constitutively hyperactivated, and Bad is maintained in its inactive state by phosphorylation at Ser in a MEK/ERK/RSK-dependent manner in melanoma cells. In contrast, in normal melanocytes, Bad is highly phosphorylated at multiple residues (Ser, Ser, and Ser) in a MAPK pathway-independent manner. Importantly, ectopic expression of a constitutively activated RSK mutant abrogates Bad activation and renders melanoma cells resistant to apoptosis induced by a MEK inhibitor. Furthermore, overexpressing alanine-substituted (S75A) Bad further sensitizes melanoma cells to MEK inhibitor-induced apoptosis. Our results suggest that the MAPK pathway mediates melanoma-specific survival signaling by differentially regulating RSK-mediated phosphorylation of the proapoptotic protein Bad and may present potentially selective therapeutic targets for the treatment of melanomas.