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 μM bexarotene for 24, 48, 72, and 96 h. Apoptosis was determined by flow-cytometry analysis of sub-G₁ 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 μM 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 α and retinoic acid receptor α 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 α, retinoic acid receptor α, and survivin. These findings support apoptosis as a mechanism for bexarotene therapy in CTCL.