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 G 1 phase. Upon reinduction of tax expression, the arrested cells entered the S phase. Thisshowed that Tax has mitogenic activity, which is required for stimulating the G 1 -to S-phase transition of immortalized lymphocytes. In mammalian cells, the G 1 -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.

Mechanisms of T-Cell Activation by Human T-Cell Lymphotropic Virus Type I

“…Although Tax induced CD95 ligand expression (41,42) and the CD95-CD95 ligand interaction is known to activate IL-1␤-converting enzyme-proteases, blocking experiments failed to implicate this pathway in Tax-mediated induction of apoptosis (42). In contrast to these observations, a tetracycline repressor-based Tax expression system failed to detect apoptosis in lymphocytes expressing Tax (254).…”

“…HTLV-I-mediated interference with cell cycle-regulating proteins was initially demonstrated in T-cell clones from patients with HAM/TSP; in contrast to uninfected T-cell clones, pRb was constitutively hyperphosphorylated in HTLV-I-infected T-cell clones (119). The hyperphosphorylation of pRb correlates with Tax expression in a tetracycline repressor-based Tax expression system (254). Importantly, although transforming growth factor ␤ (TGF-␤) completely abolished hyperphosphorylation of pRb in CD3-TCR-stimulated, uninfected T-cell clones, it did not prevent pRb phosphorylation in HTLV-Iinfected T-cell clones (119).…”

“…It remains to be determined whether Tax also inhibits p15 INK4b , a mediator of TGF-␤ inhibition, which is 97% homologous to p16 INK4a in the last three of its four ankyrin motifs (264). Inhibition of p16 INK4a may explain the Tax-induced activity of CDK4 and CDK6 and thus the ability of Tax to induce G 1 -to S-phase progression in lymphocytes (254), although Tax can also activate E2F-mediated transcription independently of p16 INK4a (175). Tax may also enhance cyclin D-CDK4 activity by decreasing the expression of p18 INK4c (7).…”

“…The significance of this is unknown. Tax does not appear to switch the cyclin D isotype from D2 to D3 (7,254). The CDK inhibitor p27 KIP1 is a critical regulator of the G 1 restriction point, since (i) IL-2R signaling eliminates p27 KIP1 (72,165,226) through a rapamycin-sensitive pathway (226); (ii) rapamycin-sensitive cells become rapamycin resistant if p27 KIP1 synthesis is inhibited by antisense oligonucleotides (150); (iii) antisense inhibition of p27 KIP1 synthesis prevents the cells from becoming quiescent (47,249); and (iv) p27 KIP1 links TGF-␤ to cell cycle arrest in mink epithelial cells (238).…”

In Vivo Rescue of a Silent tax -Deficient Bovine Leukemia Virus from a Tumor-Derived Ovine B-Cell Line by Recombination with a Retrovirally Transduced Wild-Type tax Gene

“…Interestingly, both HTLV-1 and BLV Tax exhibit oncogenic potential in cell culture, since these proteins have been shown to immortalize primary rat fibroblasts and cooperate with Ha-Ras in their complete transformation (51,62,69,70,78). HTLV-I Tax has also been found to stimulate the G 1 -to S-phase transition in immortalized human T lymphocytes (57,80).…”