title: Plexin B1 is repressed by oncogenic B-Raf signaling and functions as a tumor suppressor in melanoma cells.
abstract: Human melanomas show oncogenic B-Raf mutations, which activate the B-Raf/MKK/ERK cascade. We screened microarrays to identify cellular targets of this pathway, and found that genes upregulated by B-Raf/MKK/ERK showed highest association with cell-cycle regulators, whereas genes downregulated were most highly associated with axon guidance genes, including plexin-semaphorin family members. Plexin B1 was strongly inhibited by mitogen-activated protein kinase signaling in melanoma cells and melanocytes. In primary melanoma cells, plexin B1 blocked tumorigenesis as measured by growth of colonies in soft agar, spheroids in extracellular matrix and xenograft tumors. Tumor suppression depended on residues in the C-terminal domain of plexin B1, which mediate receptor GTPase activating protein activity, and also correlated with AKT inhibition. Interestingly, the inhibitory response to plexin B1 was reduced or absent in cells from a matched metastatic tumor, suggesting that changes occur in metastatic cells which bypass the tumor-suppressor mechanisms. Plexin B1 also inhibited cell migration, but this was seen in metastatic cells and not in matched primary cells. Thus, plexin B1 has tumor-suppressor function in early-stage cells, although suppressing migration in late-stage cells. Our findings suggest that B-Raf/MKK/ERK provides a permissive environment for melanoma genesis by modulating plexin B1.
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309-0215, USA.
authors.forename: G M
authors.initials: GM
authors.lastname: Argast
authors.forename: C H
authors.initials: CH
authors.lastname: Croy
authors.forename: K L
authors.initials: KL
authors.lastname: Couts
authors.forename: Z
authors.initials: Z
authors.lastname: Zhang
authors.forename: E
authors.initials: E
authors.lastname: Litman
authors.forename: D C
authors.initials: DC
authors.lastname: Chan
authors.forename: N G
authors.initials: NG
authors.lastname: Ahn
mesh_terms: D000818:Animals; D045744:Cell Line, Tumor; D002465:Cell Movement; D048049:Extracellular Signal-Regulated MAP Kinases / Q000502:physiology; D056385:Group II Chaperonins; D006801:Humans; D008545:Melanoma / Q000473:pathology / Q000517:prevention & control*; D008546:Melanoma, Experimental / Q000517:prevention & control; D051379:Mice; D018832:Molecular Chaperones / Q000502:physiology; D009419:Nerve Tissue Proteins / Q000502:physiology*; D048493:Proto-Oncogene Proteins B-raf / Q000502:physiology*; D051057:Proto-Oncogene Proteins c-akt / Q000037:antagonists & inhibitors; D011956:Receptors, Cell Surface / Q000502:physiology*; D015398:Signal Transduction / Q000502:physiology*; D018874:Spheroids, Cellular; D025521:Tumor Suppressor Proteins / Q000502:physiology*
chemical_list: C414726:MKKS protein, human; D018832:Molecular Chaperones; D009419:Nerve Tissue Proteins; C492119:PLXNB1 protein, human; D011956:Receptors, Cell Surface; D025521:Tumor Suppressor Proteins; C482119:BRAF protein, human; C482120:Braf protein, mouse; D048493:Proto-Oncogene Proteins B-raf; D051057:Proto-Oncogene Proteins c-akt; D048049:Extracellular Signal-Regulated MAP Kinases; D056385:Group II Chaperonins