HTLV-1 Tax oncoprotein inhibits the estrogen-induced-ER α-Mediated BRCA1 expression by interaction with CBP/p300 cofactors.

BRCA1 is a multifunctional tumor suppressor, whose expression is activated by the estrogen (E2)-liganded ERα receptor and regulated by certain recruited transcriptional co-activators. Interference with BRCA1 expression and/or functions leads to high risk of breast or/and ovarian cancer. Another multifunctional protein, HTLV-1Tax oncoprotein, is widely regarded as crucial for developing adult T-cell leukemia and other clinical disorders.

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.

Role of caspase 9 in activation of HTLV-1 LTR expression by DNA damaging agents.

Adult T-cell leukemia (ATL) is caused by HTLV-I. The viral Tax oncoprotein plays a central role in initiating the process to ATL. However, after infection HTLV-1 enters into latency, during which virus gene expression is very low, so that the level of Tax is likely insufficient for exerting its oncogenic activities.

The mechanism of HTLV-1 LTR activation by TPA varies in different human T-cell lines: role of specific PKC isoforms.

We demonstrate here that TPA activates HTLV-1 LTR expression in Jurkat and H9 T-cell lines, by strictly different mechanisms. In Jurkat cells this activation is exerted by a PKCalpha- and PKCvarepsilon-antagonized mechanism which operates through an Sp1 binding site residing within the Est responsive region 1 of the LTR. On the other hand, in H9 cells TPA activates the LTR by two consecutive mechanisms; the first depends on PKCeta activity and is exerted through the 21 bp repeats of the LTR, whereas the second is analogous to that observed in Jurkat cells, except that it is antagonized by PKCdelta.