title: Cooperative regulation of the cell division cycle by the protein kinases RAF and AKT.
abstract: The RAS-activated RAF-->MEK-->extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3'-kinase (PI3'-kinase)-->PDK1-->AKT signaling pathways are believed to cooperate to promote the proliferation of normal cells and the aberrant proliferation of cancer cells. To explore the mechanisms that underlie such cooperation, we have derived cells harboring conditionally active, steroid hormone-regulated forms of RAF and AKT. These cells permit the assessment of the biological and biochemical effects of activation of these protein kinases either alone or in combination with one another. Under conditions where activation of neither RAF nor AKT alone promoted S-phase progression, coactivation of both kinases elicited a robust proliferative response. Moreover, under conditions where high-level activation of RAF induced G(1) cell cycle arrest, activation of AKT bypassed the arrest and promoted S-phase progression. At the level of the cell cycle machinery, RAF and AKT cooperated to induce cyclin D1 and repress p27(Kip1) expression. Repression of p27(Kip1) was accompanied by a dramatic reduction in KIP1 mRNA and was observed in primary mouse embryo fibroblasts derived from mice either lacking SKP2 or expressing a T187A mutated form of p27(Kip1). Consistent with these observations, pharmacological inhibition of MEK or PI3'-kinase inhibited the effects of activated RAS on the expression of p27(Kip1) in NIH 3T3 fibroblasts and in a panel of bona fide human pancreatic cancer cell lines. Furthermore, we demonstrated that AKT activation led to sustained activation of cyclin/cdk2 complexes that occurred concomitantly with the removal of RAF-induced p21(Cip1) from cyclin E/cdk2 complexes. Cumulatively, these data strongly suggest that the RAF-->MEK-->ERK and PI3'K-->PDK-->AKT signaling pathways can cooperate to promote G(0)-->G(1)-->S-phase cell cycle progression in both normal and cancer cells.
authors.affiliation: Cancer Research Institute and Department of Cellular and Molecular Pharmacology, UCSF Comprehensive Cancer Center, 2420 Sutter St., Box 0128, San Francisco, CA 94143-0128, USA.
authors.forename: Amer M
authors.initials: AM
authors.lastname: Mirza
authors.forename: Stephan
authors.initials: S
authors.lastname: Gysin
authors.forename: Nisar
authors.initials: N
authors.lastname: Malek
authors.forename: Kei-ichi
authors.initials: K
authors.lastname: Nakayama
authors.forename: James M
authors.initials: JM
authors.lastname: Roberts
authors.forename: Martin
authors.initials: M
authors.lastname: McMahon
mesh_terms: D000818:Animals; D001549:Benzamides / Q000494:pharmacology; D015153:Blotting, Western; D002070:Butadienes / Q000494:pharmacology; D002453:Cell Cycle; D018797:Cell Cycle Proteins / Q000378:metabolism; D002455:Cell Division; D002457:Cell Extracts; D045744:Cell Line, Tumor; D002478:Cells, Cultured; D002867:Chromones / Q000494:pharmacology; D019938:Cyclin D1 / Q000378:metabolism; D050760:Cyclin-Dependent Kinase Inhibitor p27; D004622:Embryo, Mammalian / Q000166:cytology; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005347:Fibroblasts / Q000187:drug effects / Q000378:metabolism; D006801:Humans; D051379:Mice; D008856:Microscopy, Fluorescence; D009025:Morpholines / Q000494:pharmacology; D041681:NIH 3T3 Cells; D009570:Nitriles / Q000494:pharmacology; D010190:Pancreatic Neoplasms / Q000473:pathology; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000378:metabolism*; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt; D012333:RNA, Messenger / Q000378:metabolism; D012190:Retroviridae / Q000235:genetics; D025521:Tumor Suppressor Proteins / Q000378:metabolism; D048490:raf Kinases / Q000235:genetics / Q000378:metabolism*
chemical_list: C120227:2-(2-chloro-4-iodophenylamino)-N-cyclopropylmethoxy-3,4-difluorobenzamide; D001549:Benzamides; D002070:Butadienes; C493961:Cdkn1b protein, mouse; D018797:Cell Cycle Proteins; D002457:Cell Extracts; D002867:Chromones; D004791:Enzyme Inhibitors; D009025:Morpholines; D009570:Nitriles; D011518:Proto-Oncogene Proteins; D012333:RNA, Messenger; D025521:Tumor Suppressor Proteins; C113580:U 0126; D019938:Cyclin D1; D050760:Cyclin-Dependent Kinase Inhibitor p27; C085911:2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one; C494918:AKT1 protein, human; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D048490:raf Kinases