title: Myc downregulation by transforming growth factor beta required for activation of the p15(Ink4b) G(1) arrest pathway.
abstract: The antimitogenic action of transforming growth factor beta (TGF-beta) in epithelial cells involves cyclin-dependent kinase (cdk) inhibitory gene responses and downregulation of c-Myc expression. Although the cdk inhibitory responses are sufficient for G(1) arrest, enforced expression of c-Myc prevents G(1) arrest by TGF-beta. We investigated the basis of this antagonism by using Mv1Lu lung epithelial cell lines that conditionally express levels of human c-Myc. We show that c-Myc prevents induction of the cdk4 inhibitor p15(Ink4b) and the subsequent inhibition of G(1) cdks by TGF-beta. We assessed the significance of this effect by analyzing the oligomeric state of cdk4 in these cells. In proliferating cells, endogenous cdk4 is distributed among three populations: an abundant high-molecular-mass (>400-kDa) pool of latent cdk4 that serves as a source of cdk4 for cyclin D, a low-abundance pool containing active cyclin D-cdk4 complexes, and an inactive population of monomeric cdk4. Cell stimulation with TGF-beta converts the latent and active cdk4 pools into inactive cdk4, an effect that is specifically mimicked by overexpression of p15 but not by other forms of G(1) arrest. This process of TGF-beta-induced cdk4 inactivation is completely blocked by expression of c-Myc, even though the latent and active cdk4 complexes from c-Myc-expressing cells remain sensitive to dissociation by p15 in vitro. c-Myc causes a small increase in cyclin D levels, but this effect contributes little to the loss of TGF-beta responses in these cells. The evidence suggests that c-Myc interferes with TGF-beta activation of the p15 G(1) arrest pathway. TGF-beta must therefore downregulate c-Myc in order to activate this pathway.
authors.affiliation: Cell Biology Program and Howard Hughes Medical Institute, Memorial Sloan-Kettering Cancer Center, New York, New York 10021, USA.
authors.forename: B J
authors.initials: BJ
authors.lastname: Warner
authors.forename: S W
authors.initials: SW
authors.lastname: Blain
authors.forename: J
authors.initials: J
authors.lastname: Seoane
authors.forename: J
authors.initials: J
authors.lastname: Massagué
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002352:Carrier Proteins / Q000235:genetics / Q000378:metabolism*; D018797:Cell Cycle Proteins; D002460:Cell Line; D056741:Cyclin D; D051358:Cyclin-Dependent Kinase 4; D050762:Cyclin-Dependent Kinase Inhibitor p15; D019941:Cyclin-Dependent Kinase Inhibitor p16; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D016213:Cyclins / Q000378:metabolism; D004247:DNA / Q000235:genetics; D015536:Down-Regulation / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000378:metabolism; D016193:G1 Phase / Q000187:drug effects* / Q000235:genetics* / Q000502:physiology; D005786:Gene Expression Regulation; D016259:Genes, myc / Q000187:drug effects*; D006801:Humans; D008907:Mink; D008969:Molecular Sequence Data; D011401:Promoter Regions, Genetic; D011518:Proto-Oncogene Proteins; D016212:Transforming Growth Factor beta / Q000494:pharmacology*; D025521:Tumor Suppressor Proteins
chemical_list: C493966:CDKN2B protein, human; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D056741:Cyclin D; D050762:Cyclin-Dependent Kinase Inhibitor p15; D019941:Cyclin-Dependent Kinase Inhibitor p16; D016213:Cyclins; D004791:Enzyme Inhibitors; D011518:Proto-Oncogene Proteins; D016212:Transforming Growth Factor beta; D025521:Tumor Suppressor Proteins; D004247:DNA; C495900:CDK4 protein, human; D051358:Cyclin-Dependent Kinase 4; D018844:Cyclin-Dependent Kinases