title: Mutant B-RAF mediates resistance to anoikis via Bad and Bim.
abstract: Normal cells undergo anoikis when they lose adhesion to or encounter an inappropriate extracellular matrix. By contrast, oncogenic signaling in tumor cells enables resistance to anoikis, a trait that contributes to tumor progression. The B-RAF serine-threonine kinase is mutated in multiple cancers and functions as an oncogene in melanoma. Previously, we demonstrated that B-RAF and downstream mitogen-activated protein/extracellular signal-regulated kinase kinase (MEK) signaling are necessary for protection from anoikis in mutant B-RAF-expressing melanoma cells. Regulation of Bcl-2 family members in melanoma and their role in B-RAF-mediated survival is poorly defined. Here, we provide evidence that B-RAF-MEK signaling protects against anoikis through alterations in two proapoptotic Bcl-2 family proteins: Bcl-xL/Bcl-2-associated death promoter (Bad) and Bcl-2-interacting mediator of cell death (Bim). B-RAF-MEK signaling regulates phosphorylation of the inhibitory serine-75 residue of Bad, and decreases Bad mRNA expression. RNA interference and overexpression experiments demonstrate that Bad contributes to the susceptibility of B-RAF-depleted cells to anoikis. Additionally, B-RAF-MEK signaling regulates the expression of Bim(EL), mainly through control of protein turnover. Increased Bim(EL) levels induce apoptosis in suspended cells and are required for anoikis in B-RAF-depleted cells. Depletion of Bim together with Bad has an additive effect on protecting B-RAF knockdown cells from anoikis. Together, our data show that Bad and Bim are major B-RAF responsive proteins regulating apoptosis in melanoma cells.
authors.affiliation: Center for Cell Biology and Cancer Research, Albany Medical College, Albany, NY 12208, USA.
authors.forename: K
authors.initials: K
authors.lastname: Boisvert-Adamo
authors.forename: A E
authors.initials: AE
authors.lastname: Aplin
mesh_terms: D023102:Anoikis / Q000235:genetics*; D051017:Apoptosis Regulatory Proteins / Q000378:metabolism / Q000502:physiology*; D000072224:Bcl-2-Like Protein 11; D002448:Cell Adhesion / Q000235:genetics / Q000502:physiology; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D020930:MAP Kinase Kinase Kinases / Q000502:physiology; D008545:Melanoma / Q000235:genetics; D008565:Membrane Proteins / Q000378:metabolism / Q000502:physiology*; D050505:Mutant Proteins / Q000502:physiology; D010766:Phosphorylation; D046988:Proteasome Endopeptidase Complex / Q000378:metabolism; D011518:Proto-Oncogene Proteins / Q000378:metabolism / Q000502:physiology*; D048493:Proto-Oncogene Proteins B-raf / Q000037:antagonists & inhibitors / Q000235:genetics / Q000502:physiology*; D034741:RNA, Small Interfering / Q000494:pharmacology; D015398:Signal Transduction / Q000502:physiology; D014407:Tumor Cells, Cultured; D051025:bcl-Associated Death Protein / Q000235:genetics / Q000378:metabolism / Q000502:physiology*
chemical_list: D051017:Apoptosis Regulatory Proteins; C000606775:BCL2L11 protein, human; D000072224:Bcl-2-Like Protein 11; D008565:Membrane Proteins; D050505:Mutant Proteins; D011518:Proto-Oncogene Proteins; D034741:RNA, Small Interfering; D051025:bcl-Associated Death Protein; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf; D020930:MAP Kinase Kinase Kinases; D046988:Proteasome Endopeptidase Complex