title: Mitogen-activated protein kinase pathway-dependent tumor-specific survival signaling in melanoma cells through inactivation of the proapoptotic protein bad.
abstract: 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 signal-regulated kinase (ERK)/RSK MAPK signaling module is constitutively hyperactivated, and Bad is maintained in its inactive state by phosphorylation at Ser(75) in a MEK/ERK/RSK-dependent manner in melanoma cells. In contrast, in normal melanocytes, Bad is highly phosphorylated at multiple residues (Ser(75), Ser(99), and Ser(118)) 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.
authors.affiliation: Laboratory of Cancer Pharmacogenetics, Van Andel Research Institute, Grand Rapids, Michigan 49503, USA.
authors.forename: Kathryn M
authors.initials: KM
authors.lastname: Eisenmann
authors.forename: Matthew W
authors.initials: MW
authors.lastname: VanBrocklin
authors.forename: Nancy A
authors.initials: NA
authors.lastname: Staffend
authors.forename: Susan M
authors.initials: SM
authors.lastname: Kitchen
authors.forename: Han-Mo
authors.initials: HM
authors.lastname: Koo
mesh_terms: D002352:Carrier Proteins / Q000037:antagonists & inhibitors* / Q000378:metabolism; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000502:physiology; D004789:Enzyme Activation; D006801:Humans; D020935:MAP Kinase Signaling System / Q000502:physiology*; D008544:Melanocytes / Q000201:enzymology / Q000378:metabolism; D008545:Melanoma / Q000201:enzymology* / Q000473:pathology; D020929:Mitogen-Activated Protein Kinase Kinases / Q000378:metabolism; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D038744:Ribosomal Protein S6 Kinases, 90-kDa / Q000378:metabolism; D051025:bcl-Associated Death Protein
chemical_list: C494819:BAD protein, human; D002352:Carrier Proteins; D051025:bcl-Associated Death Protein; C097276:CILK1 protein, human; D017346:Protein Serine-Threonine Kinases; C069329:RPS6KA1 protein, human; D038744:Ribosomal Protein S6 Kinases, 90-kDa; D020929:Mitogen-Activated Protein Kinase Kinases