Induction of apoptosis by bexarotene in cutaneous T-cell lymphoma cells: relevance to mechanism of therapeutic action.

PURPOSE Bexarotene is the first synthetic rexinoid approved for the treatment of all stages of cutaneous T-cell lymphoma (CTCL) however the mechanism of bexarotene action is unknown. We examined the effects of bexarotene on induction of apoptosis and expression of its cognate receptors in well-established CTCL cell lines (MJ, Hut78, and HH). EXPERIMENTAL DESIGN CTCL cells were treated with 0.1, 1, and 10 microM bexarotene for 24, 48, 72, and 96 h. Apoptosis was determined by flow-cytometry analysis of sub-G(1) hypodiploid nuclei and annexin V binding populations. Apoptosis-associated proteins and retinoid receptors were detected by Western blots. RESULTS Bexarotene treatment at 1 and 10 microM for 96 h increased the number of cells with sub-G1 populations and annexin V binding in a dose-dependent manner compared with vehicle controls (DMSO) in all three cell lines, respectively. Bexarotene treatment suppressed the expression of retinoid X receptor alpha and retinoic acid receptor alpha proteins in all three lines compared with untreated controls. Bexarotene treatment decreased the protein levels of survivin, activated caspase-3, and cleaved poly(ADP-Ribose) polymerase, but had no obvious effect on expression of Fas/Fas ligand and bcl-2 proteins in all three CTCL lines. CONCLUSIONS Bexarotene treatment at clinically relevant concentrations causes apoptosis of CTCL cell lines in association with activation of caspase-3 and cleavage of poly(ADP-Ribose) polymerase, as well as down-regulation of retinoid X receptor alpha, retinoic acid receptor alpha, and survivin. These findings support apoptosis as a mechanism for bexarotene therapy in CTCL.