title: Regulation of the protein kinase Raf-1 by oncogenic Ras through phosphatidylinositol 3-kinase, Cdc42/Rac and Pak.
abstract: Activation of the protein kinase Raf-1 is a complex process involving association with the GTP-bound form of Ras (Ras-GTP), membrane translocation and both serine/threonine and tyrosine phosphorylation (reviewed in [1]). We have reported previously that p21-activated kinase 3 (Pak3) upregulates Raf-1 through direct phosphorylation on Ser338 [2]. Here, we investigated the origin of the signal for Pak-mediated Raf-1 activation by examining the role of the small GTPase Cdc42, Rac and Ras, and of phosphatidylinositol (PI) 3-kinase. Pak3 acted synergistically with either Cdc42V12 or Rac1V12 to stimulate the activities of Raf-1, Raf-CX, a membrane-localized Raf-1 mutant, and Raf-1 mutants defective in Ras binding. Raf-1 mutants defective in Ras binding were also readily activated by RasV12. This indirect activation of Raf-1 by Ras was blocked by a dominant-negative mutant of Pak, implicating an alternative Ras effector pathway in Pak-mediated Raf-1 activation. Subsequently, we show that Pak-mediated Raf-1 activation is upregulated by both RasV12C40, a selective activator of PI 3-kinase, and p110-CX, a constitutively active PI 3-kinase. In addition, p85Delta, a mutant of the PI 3-kinase regulatory subunit, inhibited the stimulated activity of Raf-1. Pharmacological inhibitors of PI 3-kinase also blocked both activation and Ser338 phosphorylation of Raf-1 induced by epidermal growth factor (EGF). Thus, Raf-1 activation by Ras is achieved through a combination of both physical interaction and indirect mechanisms involving the activation of a second Ras effector, PI 3-kinase, which directs Pak-mediated regulatory phosphorylation of Raf-1.
authors.affiliation: Department of Biochemistry and Molecular Biology, Walther Oncology Center, Indiana University School of Medicine, Indianapolis, 46202, USA.
authors.forename: H
authors.initials: H
authors.lastname: Sun
authors.forename: A J
authors.initials: AJ
authors.lastname: King
authors.forename: H B
authors.initials: HB
authors.lastname: Diaz
authors.forename: M S
authors.initials: MS
authors.lastname: Marshall
mesh_terms: D000818:Animals; D019556:COS Cells; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D020558:GTP Phosphohydrolases / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism; D009154:Mutation; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism*; D010766:Phosphorylation; D011233:Precipitin Tests; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000235:genetics / Q000378:metabolism*; D015398:Signal Transduction; D014162:Transfection; D020764:cdc42 GTP-Binding Protein / Q000378:metabolism*; D054462:p21-Activated Kinases; D020744:rac GTP-Binding Proteins / Q000378:metabolism*; D018631:ras Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D004791:Enzyme Inhibitors; D020558:GTP Phosphohydrolases; D006160:Guanosine Triphosphate; D020928:Mitogen-Activated Protein Kinases; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D020764:cdc42 GTP-Binding Protein; D054462:p21-Activated Kinases; D020744:rac GTP-Binding Proteins; D018631:ras Proteins