Down-regulation of the INK4 family of cyclin-dependent kinase inhibitors by tax protein of HTLV-1 through two distinct mechanisms.
Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV-1) affects multiple regulatory processes of infected cells through activation and repression of specific transcription and also through modulation of functions of cell cycle regulators. Previously, we found that Tax binds to p16ink4a, a member of the INK4 family of cyclin-dependent kinase inhibitors, and counteracts its inhibitory activity, resulting in cell cycle progression. In this study, we examined the effects of Tax on other members of the INK4 family and found that Tax can bind to p15ink4b similarly to p16ink4a, but not to p18ink4c and p19ink4d. Tax binding to p15ink4b inactivated its function and restored CDK4 kinase activity. Accordingly, Tax-expressing cells became resistant to p15ink4b-mediated growth arrest induced by TGFbeta. On the other hand, expression of p18ink4c was transcriptionally repressed by Tax through the E-box element of the promoter, which may contribute to the marked reduction of p18ink4c mRNA in HTLV-1-infected T-cells. These observations indicate that Tax suppresses the inhibitory activities of INK4 family members through two independent mechanisms: functional inhibition of two INK4 proteins and repression of expression of another INK4 protein. These effects may play roles in HTLV-1-induced deregulation of the cell cycle, possibly promoting cellular transformation.
Suzuki
T
T
Department of Cellular and Molecular Biology, Institute of Medical Science, The University of Tokyo, Shirokanedai, Minato-ku, Tokyo, 108-8639, Japan.
Narita
T
T
Uchida-Toita
M
M
Yoshida
M
M
D002352:Carrier Proteins / Q000378:metabolism*; D018797:Cell Cycle Proteins; D002455:Cell Division; D002460:Cell Line; D050762:Cyclin-Dependent Kinase Inhibitor p15; D019941:Cyclin-Dependent Kinase Inhibitor p16 / Q000378:metabolism*; D050763:Cyclin-Dependent Kinase Inhibitor p18; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism; D015536:Down-Regulation; D004791:Enzyme Inhibitors / Q000378:metabolism; D016356:Gene Products, tax / Q000378:metabolism*; D005814:Genes, Viral; D015368:Human T-lymphotropic virus 1 / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D019169:Jurkat Cells; D011233:Precipitin Tests; D011401:Promoter Regions, Genetic; D014158:Transcription, Genetic; D016212:Transforming Growth Factor beta / Q000494:pharmacology; D025521:Tumor Suppressor Proteins
C493966:CDKN2B protein, human; C493971:CDKN2C protein, human; D002352:Carrier Proteins; D018797:Cell Cycle Proteins; D050762:Cyclin-Dependent Kinase Inhibitor p15; D019941:Cyclin-Dependent Kinase Inhibitor p16; D050763:Cyclin-Dependent Kinase Inhibitor p18; D004791:Enzyme Inhibitors; D016356:Gene Products, tax; D016212:Transforming Growth Factor beta; D025521:Tumor Suppressor Proteins; D018844:Cyclin-Dependent Kinases