Differential localization of A-Raf regulates MST2-mediated apoptosis during epithelial differentiation

• DOI: 10.1038/cdd.2016.2
• Publication Date: 2016-02-19T13:01:17Z
• Authors: J Rauch; D Vandamme; B Mack; B McCann; N Volinsky; A Blanco; O Gires; W Kolch
• Keywords: 0301 basic medicine; 571; Neuregulin-1; Apoptosis; Protein Serine-Threonine Kinases; HNSCC; Proto-Oncogene Proteins A-raf; Serine-Threonine Kinase 3; 03 medical and health sciences; Breast cancer; A-Raf; Humans; RNA, Small Interfering; Mitogen-Activated Protein Kinase 1; Caspase 8; Mitogen-Activated Protein Kinase 3; KSR2; Cell Membrane; Cell Differentiation; Epithelial Cells; Mitochondria; HEK293 Cells; Microscopy, Fluorescence; Differentiation; MCF-7 Cells; RNA Interference; HeLa Cells

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.