Induction of apoptosis in lymphoid leukemia cells: differential effects of rar and rxr retinoids with dexamethasone

Retinoids have been shown to regulate a number of cellular processes, including cell growth and differentiation . The identification of subfamilies of retinoid receptors (RARa.β.yand RXR a,β,v) has led to the development of ligands with specific binding affinities. Bexarotene (Targretin), is a selective RXR ligand which has been demonstrated to inhibit growth and induce apoptosis in HL60 cells and in epithelial cancer cell lines. Both oral and topical bexarotene have shown clinical efficacy in patients with cutaneous T-cell lymphoma, but the mechanism has not been elucidated. We examined the effects of bexarotene and the RAR-specific retinoid, ATRA, on cell growth and induction of apoptosis in the T-leukemia cell line, HUT78, NALM-6 pre-B cells, and fresh leukemia cells from patients with CLL. At concentrations of 105 to 1010 M, we found growth inhibition but no apoptosis, as measured by change in Annexin V immunostaining or the expression of apoptosis-associated proteins, including PARP, Bad, Bcl-2 and BclXL. Because previous studies have demonstrated that the combination of steroids with ATRA induced apoptosis in myeloma cell lines, we investigated the effects of the addition of dexamethasone to cell lines exposed to either bexarotene or ATRA. A 3-fold enhancement in cytotoxicity was demonstrated by Annexin V immunostaining with the combination of2x10SM bexarotene+ 10!M dexamethasone in the HUT78 T-cells compared to either drug alone, with no significant difference in cytotoxicity in the presence of 2xIO5M ATRA+ dexamethasone, whereas in the pre-B NALM-6 cells, there was a 5-fold enhancement in cytotoxicity with 2x10's M ATRA+ dexamethasone but not bexarotene+ dexamethasone. In fresh CLL cells, 2xl05M bexarotene alone induced apoptosis in 30-50% of the cells, whereas the combination of 105M dexamethasone and bexarotene induced apoptosis in 70-75% after 48 hours of exposure with the combination of bexarotene+ dexamethasone. These results suggest that RXR and RAR retinoid receptor ligands display differential effects in T and B-leukemia cells, perhaps dependent on their state of differentiation, and that the combination of retinoids and dexamethasone induce apoptosis to a more signficant degree than retinoids alone. The combination of bexarotene and dexamethasone would be worthy of further investigation in the clinic.