Retinoic acid dramatically enhances the arsenic trioxide-induced cell cycle arrest and apoptosis in retinoic acid receptor alpha-positive human T-cell lymphotropic virus type-I-transformed cells

Adult T-cell leukemia/lymphoma, caused by the human T-cell lymphotropic virus type I, is an aggressive neoplasm of mature activated T cells that is generally resistant to conventional therapy. While arsenic trioxide (As) inhibits the growth and induces apoptosis in HTLV-I-infected T cells, synergistically, when combined with interferon-alpha, variable effects on growth with all trans retinoic acid treatment have been reported in ATL-derived cell lines and fresh ATL cells. In this study, we investigate the effects of ATRA alone or in combination with As in HTLV-I-transformed cells.Four HTLV-I-transformed cell lines (HuT-102, MT2, C8166 and C91PL) were treated with different doses of ATRA alone or in combination with As for one to three days. Cell growth was assessed by cell count with 3H-thymidine incorporation. Cell cycle distribution was assessed by propidium iodine-labeled DNA content by flow cytometry. Apoptosis was evaluated by Hoechst nuclear staining and annexin-V binding assays. Expression of retinoid receptors, the viral transactivator Tax, and the proteins bcl-2 and IkappaB-alpha proteins, was analysed by Western blot.Only C8166 cells were sensitive to the ATRA-induced growth inhibitory effect while HuT-102, MT2, and C91PL were resistant to ATRA treatment (up to 10(-5) M). The retinoid X receptor alpha and the retinoic acid receptor gamma (RARgamma) proteins were expressed in all four cell lines, while RARalpha protein was only detected in the HuT-102 and C8166 cells. The combination ATRA/As showed a highly synergistic effect on HuT-102 cells, and, to a lesser extent, on C8166 cells and resulted in a dramatic inhibition of cell proliferation and induction of massive apoptosis in HuT-102 cells, associated with caspase activation. While ATRA alone had no effect on Tax and IkappaB-alpha protein levels, ATRA increased the As-induced Tax degradation and the up-regulation of IkappaB-alpha protein. In contrast, the expression of bcl-2 protein was not significantly affected by any of the treatments.Our data provide a rationale for combined ATRA and As-therapies in ATL patients refractory to conventional therapy and expressing RARalpha in their leukemic cells.