title: Stimulation of cyclin-dependent kinase activity and G1- to S-phase transition in human lymphocytes by the human T-cell leukemia/lymphotropic virus type 1 Tax protein.
abstract: The human T-cell leukemia/lymphotropic virus type 1 (HTLV-1) induces a malignant lymphocytic disease. The HTLV-1 transactivator protein, Tax, is believed to be crucial for the development of the disease since it is transforming in vitro and induces tumors in transgenic animals. Although the transcriptional modulation of viral and cellular gene expression by Tax has been analyzed thoroughly, it has remained unclear how the Tax functions act on the cell cycle of primary T cells. To investigate the mechanism of Tax-mediated T-cell stimulation, we transduced primary human cord blood T cells with a conditional, tetracycline repressor-based tax expression system. Permanent Tax expression results in an abnormal proliferation of T cells which closely resemble HTLV-1-infected lymphocytes. Suppression of Tax synthesis stopped lymphocyte growth and caused cell cycle arrest in the G1 phase. Upon reinduction of tax expression, the arrested cells entered the S phase. This showed that Tax has mitogenic activity, which is required for stimulating the G1- to S-phase transition of immortalized lymphocytes. In mammalian cells, the G1-phase progression is controlled by the serial activation of several cyclin-dependent kinases (Cdks), starting with Cdk4 and Cdk6. In the presence of Tax, both Cdk4 and Cdk6 were activated. The suppression of Tax synthesis, however, resulted in a significant reduction of the Cdk4 and Cdk6 activities but did not influence the expression of Cdk4, Cdk6, or cognate D-type cyclin proteins. These data suggest that Tax induces Cdk4 and Cdk6 activity in primary human T lymphocytes; this Cdk activation is likely to account for the mitogenic Tax effect and for the abnormal T-cell proliferation of HTLV-1-infected lymphocytes.
authors.affiliation: Institut für Klinische und Molekulare Virologie der Friedrich-Alexander Universität Erlangen-Nürnberg, Germany.
authors.forename: I
authors.initials: I
authors.lastname: Schmitt
authors.forename: O
authors.initials: O
authors.lastname: Rosin
authors.forename: P
authors.initials: P
authors.lastname: Rohwer
authors.forename: M
authors.initials: M
authors.lastname: Gossen
authors.forename: R
authors.initials: R
authors.lastname: Grassmann
mesh_terms: D000818:Animals; D002453:Cell Cycle / Q000187:drug effects*; D002471:Cell Transformation, Neoplastic; D002478:Cells, Cultured; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016193:G1 Phase / Q000187:drug effects; D015870:Gene Expression; D016356:Gene Products, tax / Q000235:genetics / Q000502:physiology / Q000633:toxicity*; D015368:Human T-lymphotropic virus 1 / Q000235:genetics / Q000472:pathogenicity* / Q000502:physiology; D006801:Humans; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D011518:Proto-Oncogene Proteins; D011995:Recombination, Genetic; D018144:Rhadinovirus / Q000235:genetics; D016196:S Phase / Q000187:drug effects; D013601:T-Lymphocytes / Q000166:cytology / Q000187:drug effects / Q000201:enzymology; D013752:Tetracycline / Q000494:pharmacology
chemical_list: D016356:Gene Products, tax; D011518:Proto-Oncogene Proteins; D017346:Protein Serine-Threonine Kinases; C495900:CDK4 protein, human; C495911:CDK6 protein, human; D051358:Cyclin-Dependent Kinase 4; D051361:Cyclin-Dependent Kinase 6; D018844:Cyclin-Dependent Kinases; D013752:Tetracycline