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.