Role of Tax protein in human T-cell leukemia virus type-I leukemogenicity

HTLV-1 is the etiological agent of adult T-cell leukemia (ATL), the neurological syndrome TSP/HAM and certain other clinical disorders. The viral Tax protein is considered to play a central role in the process leading to ATL. Tax modulates the expression of many viral and cellular genes through the CREB/ATF-, SRF- and NF-κB-associated pathways. In addition, Tax employs the CBP/p300 and p/CAF co-activators for implementing the full transcriptional activation competence of each of these pathways. Tax also affects the function of various other regulatory proteins by direct protein-protein interaction. Through these activities Tax sets the infected T-cells into continuous uncontrolled replication and destabilizes their genome by interfering with the function of telomerase and topoisomerase-I and by inhibiting DNA repair. Furthermore, Tax prevents cell cycle arrest and apoptosis that would otherwise be induced by the unrepaired DNA damage and enables, thereby, accumulation of mutations that can contribute to the leukemogenic process. Together, these capacities render Tax highly oncogenic as reflected by its ability to transform rodent fibroblasts and primary human T-cells and to induce tumors in transgenic mice. In this article we discuss these effects of Tax and their apparent contribution to the HTLV-1 associated leukemogenic process. Notably, however, shortly after infection the virus enters into a latent state, in which viral gene expression is low in most of the HTLV-1 carriers' infected T-cells and so is the level of Tax protein, although rare infected cells may still display high viral RNA. This low Tax level is evidently insufficient for exerting its multiple oncogenic effects. Therefore, we propose that the latent virus must be activated, at least temporarily, in order to elevate Tax to its effective level and that during this transient activation state the infected cells may acquire some oncogenic mutations which can enable them to further progress towards ATL even if the activated virus is re-suppressed after a while. We conclude this review by outlining an hypothetical flow of events from the initial virus infection up to the ultimate ATL development and comment on the risk factors leading to ATL development in some people and to TSP/HAM in others

Differential Role of PKC-Induced c-Jun in HTLV-1 LTR Activation by 12-O-Tetradecanoylphorbol-13-acetate in Different Human T-cell Lines

We have previously shown that TPA activates HTLV-1 LTR in Jurkat T-cells by inducing the binding of Sp1-p53 complex to the Sp1 site residing within the Ets responsive region 1 (ERR-1) of the LTR and that this activation is inhibited by PKCalpha and PKCepsilon. However, in H9 T-cells TPA has been noted to activate the LTR in two consecutive stages. The first stage is activation is mediated by PKCetta and requires the three 21 bp TRE repeats. The second activation mode resembles that of Jurkat cells, except that it is inhibited by PKCdelta. The present study revealed that the first LTR activation in H9 cells resulted from PKCetta-induced elevation of non-phosphorylated c-Jun which bound to the AP-1 site residing within each TRE. In contrast, this TRE-dependent activation did not occur in Jurkat cells, since there was no elevation of non-phosphorylated c-Jun in these cells. However, we found that PKCalpha and PKCepsilon, in Jurkat cells, and PKCetta and PKCdelta, in H9 cells, increased the level of phosphorylated c-Jun that interacted with the Sp1-p53 complex. This interaction prevented the Sp1-p53 binding to ERR-1 and blocked, thereby, the ERR-1-mediated LTR activation. Therefore, this PKC-inhibited LTR activation started in both cell types after depletion of the relevant PKCs by their downregulation. In view of these variable activating mechanisms we assume that there might be additional undiscovered yet modes of HTLV-1 LTR activation which vary in different cell types. Moreover, in line with this presumption we speculate that in HTLV-1 carriers the LTR of the latent provirus may also be reactivated by different mechanisms that vary between its different host T-lymphocyte subclones. Since this reactivation may initiate the ATL process, understanding of these mechanisms is essential for establishing strategies to block the possibility of reactivating the latent virus as preventive means for ATL development in carriers

The first LTR activation phase in TPA-treated H9 cells is mediated by the non-phosphorylated c-Jun.

<p>H9 cells and their subclones stably transfected with c-Jun shRNA (H9/c-Jun shRNA) or with p53 shRNA (H9/p53 shRNA) were treated with TPA for 36 hr (the peak time-point of the first phase). (A) Whole cell extracts were prepared from part of the treated cells for measuring the level of the non-phosphorylated c-Jun (c-Jun) by Western blot analysis with anti c-Jun antibody. Equal sample loading was assessed by re-processing the blotted filter with anti actin antibody. TPA-untreated H9 cells without shRNA served as negative control, whereas TPA-treated H9 cells without shRNAs served as positive control. (B) The remaining cells were transfected with the LTR-Luc reporter. TPA-untreated cells without shRNAs served as negative control and TPA-treated cells without shRNAs served as positive control. The pRL-renilla plasmid was included in this and all the subsequent transient transfection experiments as internal control for assessing variation in the transfection efficiency. The enzymatic activities were measured at 24 hr post transfection and the Luc activity was normalized to that of renilla and plotted as fold of the respective control. The documented results presented the average of triplicate transfections ± SE. (C) H9 cells without (left panels) and with (right panels) anti c-Jun shRNA were transfected with plasmid expressing non-phosphorylated c-Jun (ectopic c-Jun) or with plasmid expressing constitutively active PKCδ which elevates only phospho-c-Jun. Cells without these ectopic plasmids served as control. At 24 hr after transfection the whole cell extracts of the transfected and non-transfect cells were subjected to Western blot analysis with anti-c-Jun antibodies (top panels) and with antibodies detecting only phosphorylated c-Jun (middle panels). Equal sample loading was assessed with anti actin antibody (bottom panels). (D) H9 cells (left panel) and their subclone stably transfected with anti c-Jun shRNA (H9/c-Jun shRNA, right panel) were transiently transfected with LTR-Luc alone (control) or together with ectopic c-Jun- or ectopic PKCδ- expressing plasmids. Calculation of the enzymatic activities and their presentation were as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0029934#pone-0029934-g004" target="_blank">Figure 4B</a>.</p

Schematic summary of the TPA-induced LTR activation pathways in (A) H9 and (B) Jurkat cells.

<p>Schematic summary of the TPA-induced LTR activation pathways in (A) H9 and (B) Jurkat cells.</p