title: RKIP downregulates B-Raf kinase activity in melanoma cancer cells.
abstract: The Raf-MEK-ERK protein kinase cascade is a highly conserved signaling pathway that is pivotal in relaying environmental cues from the cell surface to the nucleus. Three Raf isoforms, which share great sequence and structure similarities, have been identified in mammalian cells. We have previously identified Raf kinase inhibitor protein (RKIP) as a negative regulator of the Raf-MEK-ERK signaling pathway by specifically binding to the Raf-1 isoform. We show here that RKIP also antagonizes kinase activity of the B-Raf isoform. Yeast two-hybrid and coimmunoprecipitation experiments indicated that RKIP specifically interacted with B-Raf. Ectopic expression of RKIP antagonized the kinase activity of B-Raf. We showed that the effects of RKIP on B-Raf functions were independent of its known inhibitory action on Raf-1. The expression levels of RKIP in melanoma cancer cell lines are low relative to primary melanocytes. Forced expression of RKIP partially reverted the oncogenic B-Raf kinase-transformed melanoma cancer cell line SK-Mel-28. The low expression of RKIP and its antagonistic action on B-Raf suggests that RKIP may play an important role in melanoma turmorgenesis.
authors.affiliation: Medical College of Ohio, Department of Biochemistry & Cancer Biology, 3035 Arlington Avenue, Toledo, OH 43614-5804, USA.
authors.forename: Sungdae
authors.initials: S
authors.lastname: Park
authors.forename: Miranda L
authors.initials: ML
authors.lastname: Yeung
authors.forename: Sandy
authors.initials: S
authors.lastname: Beach
authors.forename: Janiel M
authors.initials: JM
authors.lastname: Shields
authors.forename: Kam C
authors.initials: KC
authors.lastname: Yeung
mesh_terms: D000727:Androgen-Binding Protein / Q000494:pharmacology*; D015536:Down-Regulation; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D008544:Melanocytes / Q000502:physiology; D008545:Melanoma / Q000235:genetics* / Q000473:pathology*; D049488:Phosphatidylethanolamine Binding Protein; D048493:Proto-Oncogene Proteins B-raf / Q000096:biosynthesis / Q000494:pharmacology*; D012441:Saccharomyces cerevisiae / Q000235:genetics; D015398:Signal Transduction; D012878:Skin Neoplasms / Q000235:genetics* / Q000473:pathology*; D014407:Tumor Cells, Cultured
chemical_list: D000727:Androgen-Binding Protein; C511482:PEBP1 protein, human; D049488:Phosphatidylethanolamine Binding Protein; C482119:BRAF protein, human; D048493:Proto-Oncogene Proteins B-raf