Human T-cell leukemia virus type 1 tax releases cell cycle arrest induced by p16(INK4a)

The human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein causes cellular transformation by deregulating important cellular processes such as DNA repair, transcription, signal transduction, proliferation, and growth. Although it is clear that normal cell cycle control is deregulated during HTLV-1-induced cellular transformation, the effects of Tax on cell cycle control are not well understood. Flow cytometric analyses of human T cells indicate that cell cycle arrest in late G(1), at or before the G(1)/S restriction point, by p16(INK4a) is relieved by Tax. Furthermore, Tax-dependent stimulation of 5-bromo-2'-deoxyuridine incorporation and transcriptional activation is inhibited by p16(INK4a) This result suggests that p16(INK4a) is able to block Tax-dependent stimulation of DNA synthesis and cell cycle progression into S phase, In vitro binding assays with recombinant glutathione S-transferase fusion proteins and [S-35] methionine-labeled proteins indicate that Tax binds specifically with p16(INK4a) but not with either p21(cip1) or p27(kip1), Furthermore, sequential immunoprecipitation assays with specific antisera and [S-35] methionine-labeled cell lysates subsequent to coexpression with Tax and p16(INK4a) indicate that the mo proteins form complexes in vivo. Immunocomplex kinase assays with cyclin-dependent kinase 4 antiserum indicate that Tax blocks the inhibition of cdk4 kinase activity by p16(INK4a). This study identifies p16(INK4a) as a novel cellular target for Tax and suggests that the inactivation of p16(INK4a) function is a mechanism of cell cycle deregulation by Tax.