title: Mammalian Ste20-like kinase (Mst2) indirectly supports Raf-1/ERK pathway activity via maintenance of protein phosphatase-2A catalytic subunit levels and consequent suppression of inhibitory Raf-1 phosphorylation.
abstract: Many tumor suppressor proteins act to blunt the effects of mitogenic signaling pathways. Loss of function mutations in the merlin tumor suppressor underlie neurofibromatosis type 2 (NF2), a familial autosomal dominant cancer syndrome. Studies of Drosophila suggest that Hippo (hpo) is required for inhibition of cell proliferation mediated by dMer, the orthologue of human merlin. Mammalian sterile 20-like kinase-2 (Mst2) is a mammalian Hpo orthologue, and numerous studies implicate Mst2 as a tumor suppressor. Mst2 is negatively regulated by the proto-oncoprotein Raf-1 in a manner independent of the kinase activity of Raf-1. We sought to determine whether, in mammalian cells, merlin could positively regulate Mst2. We also sought to determine whether Mst2, in addition to being negatively regulated by Raf-1, might itself reciprocally regulate Raf-1. In contrast to findings from Drosophila, we find no evidence that mammalian merlin positively regulates mammalian Mst2. Instead, surprisingly, RNA interference silencing of Mst2 leads to elevated inhibitory phosphorylation of Raf-1 at Ser-259 and impaired Raf-1 kinase activity. Consequent to this, ERK pathway activation and cell proliferation are attenuated. Phosphatase-2A (PP2A) dephosphorylates Raf-1 Ser-259 in response to mitogens. Interestingly RNA interference silencing of Mst2 triggers a striking proteasome-dependent decrease in the levels of the catalytic subunit of PP2A (PP2A-C). A similar effect is achieved upon silencing of large tumor suppressor (LATS)-1 and LATS2, direct substrates of Mst2. Our studies reveal a more complex role for Mst2 than previously thought. The Mst2 --> LATS1/2 pathway, by maintaining PP2A-C levels, may, in some situations, positively affect mitogenic signaling.
authors.affiliation: Molecular Cardiology Research Institute, Tufts Medical Center, and the Department of Biochemistry, Tufts University Sackler School of Graduate Biomedical Sciences, Boston, Massachusetts 02111.
authors.forename: Geoffrey K
authors.initials: GK
authors.lastname: Kilili
authors.affiliation: Molecular Cardiology Research Institute, Tufts Medical Center, and the Department of Biochemistry, Tufts University Sackler School of Graduate Biomedical Sciences, Boston, Massachusetts 02111. Electronic address: jkyriakis@tuftsmedicalcenter.org.
authors.forename: John M
authors.initials: JM
authors.lastname: Kyriakis
mesh_terms: D016475:3T3 Cells; D000818:Animals; D020134:Catalytic Domain; D045744:Cell Line, Tumor; D004789:Enzyme Activation; D006801:Humans; D047468:Immunoprecipitation; D051379:Mice; D025581:Neurofibromin 2 / Q000378:metabolism; D010766:Phosphorylation; D054648:Protein Phosphatase 2 / Q000737:chemistry / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*; D000090742:Serine-Threonine Kinase 3; D000099317:Hippo Kinases
chemical_list: D025581:Neurofibromin 2; D054648:Protein Phosphatase 2; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D000090742:Serine-Threonine Kinase 3; C097835:STK3 protein, human; D000099317:Hippo Kinases