title: Differential localization of A-Raf regulates MST2-mediated apoptosis during epithelial differentiation.
abstract: A-Raf belongs to the family of oncogenic Raf kinases that are involved in mitogenic signaling by activating the MEK-ERK pathway. Low kinase activity of A-Raf toward MEK suggested that A-Raf might have alternative functions. We recently identified A-Raf as a potent inhibitor of the proapoptotic mammalian sterile 20-like kinase (MST2) tumor suppressor pathway in several cancer entities including head and neck, colon, and breast. Independent of kinase activity, A-Raf binds to MST2 thereby efficiently inhibiting apoptosis. Here, we show that the interaction of A-Raf with the MST2 pathway is regulated by subcellular compartmentalization. Although in proliferating normal cells and tumor cells A-Raf localizes to the mitochondria, differentiated non-carcinogenic cells of head and neck epithelia, which express A-Raf at the plasma membrane. The constitutive or induced re-localization of A-Raf to the plasma membrane compromises its ability to efficiently sequester and inactivate MST2, thus rendering cells susceptible to apoptosis. Physiologically, A-Raf re-localizes to the plasma membrane upon epithelial differentiation in vivo. This re-distribution is regulated by the scaffold protein kinase suppressor of Ras 2 (KSR2). Downregulation of KSR2 during mammary epithelial cell differentiation or siRNA-mediated knockdown re-localizes A-Raf to the plasma membrane causing the release of MST2. By using the MCF7 cell differentiation system, we could demonstrate that overexpression of A-Raf in MCF7 cells, which induces differentiation. Our findings offer a new paradigm to understand how differential localization of Raf complexes affects diverse signaling functions in normal cells and carcinomas.
authors.affiliation: Systems Biology Ireland, University College Dublin, Dublin, Ireland.
authors.forename: J
authors.initials: J
authors.lastname: Rauch
authors.affiliation: Systems Biology Ireland, University College Dublin, Dublin, Ireland.
authors.forename: D
authors.initials: D
authors.lastname: Vandamme
authors.affiliation: Clinical Cooperation Group Molecular Oncology, Department of Head and Neck Research, Ludwig-Maximilians-Universität München, Helmholtz Zentrum München, German Research Center for Environmental Health, Munich, Germany.
authors.forename: B
authors.initials: B
authors.lastname: Mack
authors.affiliation: Systems Biology Ireland, University College Dublin, Dublin, Ireland.
authors.forename: B
authors.initials: B
authors.lastname: McCann
authors.affiliation: Systems Biology Ireland, University College Dublin, Dublin, Ireland.
authors.forename: N
authors.initials: N
authors.lastname: Volinsky
authors.affiliation: Conway Institute of Biomolecular & Biomedical Research, University College Dublin, Dublin, Ireland.
authors.forename: A
authors.initials: A
authors.lastname: Blanco
authors.affiliation: Clinical Cooperation Group Molecular Oncology, Department of Head and Neck Research, Ludwig-Maximilians-Universität München, Helmholtz Zentrum München, German Research Center for Environmental Health, Munich, Germany.
authors.forename: O
authors.initials: O
authors.lastname: Gires
authors.affiliation: Systems Biology Ireland, University College Dublin, Dublin, Ireland.|Conway Institute of Biomolecular & Biomedical Research, University College Dublin, Dublin, Ireland.|School of Medicine and Medical Science, University College Dublin, Belfield, Dublin 4, Ireland.
authors.forename: W
authors.identifier: 0000-0001-5777-5016
authors.initials: W
authors.lastname: Kolch
mesh_terms: D017209:Apoptosis; D053181:Caspase 8 / Q000378:metabolism; D002454:Cell Differentiation / Q000187:drug effects; D002462:Cell Membrane / Q000378:metabolism; D004847:Epithelial Cells / Q000166:cytology / Q000378:metabolism; D057809:HEK293 Cells; D006367:HeLa Cells; D006801:Humans; D061986:MCF-7 Cells; D008856:Microscopy, Fluorescence; D008928:Mitochondria / Q000378:metabolism; D019950:Mitogen-Activated Protein Kinase 1 / Q000378:metabolism; D048052:Mitogen-Activated Protein Kinase 3 / Q000378:metabolism; D020890:Neuregulin-1 / Q000494:pharmacology; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D048491:Proto-Oncogene Proteins A-raf / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000378:metabolism; D000090742:Serine-Threonine Kinase 3; D000099317:Hippo Kinases
chemical_list: D053181:Caspase 8; D019950:Mitogen-Activated Protein Kinase 1; D048052:Mitogen-Activated Protein Kinase 3; D020890:Neuregulin-1; D017346:Protein Serine-Threonine Kinases; D048491:Proto-Oncogene Proteins A-raf; D034741:RNA, Small Interfering; D000090742:Serine-Threonine Kinase 3; C557409:KSR2 protein, human; C097835:STK3 protein, human; D000099317:Hippo Kinases