Cell type-specific E2F activation and cell cycle progression induced by the oncogene product Tax of human T-cell leukemia virus type I

The transactivator protein Tax of human T-cell leukemia virus type I plays an important role in the development of adult T-cell leukemia probably through modulation of growth regulatory molecules including p16(INK4a). The molecular mechanism of leukemogenesis induced by Tax has yet to be elucidated. We analyzed Tax function in the cell cycle using an interleukin-2 (IL-2)-dependent human T-cell line (Kit 225) that can undergo cell cycle arrest at G(0)/G(1) phase by deprivation of IL-2. Tax activated endogenous E2F activity in IL-2-starved Kit 225 cells, resulting in activation of E2F site-carrying promoters of genes involved in G(1) to S phase transition in a cell type-dependent and p16(INK4a)-independent manner. The ability of Tax mutants to activate E2F coincided with that to activate nuclear factors kappaB and AT, sole expression of which, however, did not activate E2F, suggesting involvement of another pathway in activation of E2F. Introduction of Tax by a recombinant adenovirus induced cell cycle progression to G(2)/M phase in resting Kit 225 cells accompanied by endogenous cyclin D2 gene expression. Similarly, Tax-induced cell cycle progression was seen with peripheral blood lymphocytes prestimulated with phytohemagglutinin. Analyses with Tax mutants did not allow Tax-induced cell cycle progression to be differentiated from Tax-dependent activation of E2F, suggesting that Tax induces cell cycle progression presumably through activation of E2F. Nevertheless, infection with an E2F1-expressing virus, which is sufficient for induction of S phase in serum-starved fibroblasts, was not sufficient for either E2F activation or cell cycle progression in IL-2-starved Kit 225 cells, implying differential regulation of E2F activation and cell cycle progression in T-cells that is activated by Tax.     

An E2F site in the 5'-promoter region contributes to serum-dependent up-regulation of the human proliferating cell nuclear antigen gene

The synthesis of proliferating cell nuclear antigen (PCNA) is strictly regulated during the cell cycle. To investigate the contribution of the promoter region to the up-regulation of human PCNA expression at the onset of S phase, we have examined 17 putative elements with reporter assays in quiescent L-O2 cells and following serum stimulation. The E2F-like sequence 5'-TTCCCCGCAA-3' located at -84 to -75 is required for the serum-induced transactivation. In electrophoretic mobility shift assays, nuclear extracts from asynchronous L-O2 cells exhibit two binding activities toward the -75 E2F oligonucleotide, and the minor band, whose formation could be interfered with by E2F-1 antibody, represents an S phase-specific complex. This is the first demonstration of the E2F site in the human PCNA 5' promoter as a serum-responsive element.