Inhibition of the integrin alpha-V beta-3 reverts the paradoxical effect of levothyroxine replacement during bexarotene therapy in cutaneous T-cell lymphoma

Bexarotene is a specific RXR agonist that has been used for the treatment of cutaneous T-cell lymphoma (CTCL). The mechanism of action is pleiotropic but impacts directly on CTCL cell proliferation, chemotaxis and apoptosis, and indirectly on lymphoma immunity. Bexarotene causes hypothyroidism in more than 90% of patients thus requiring the concomitant administration of levothyroxine. Hereon we investigated the consequence of levothyroxine administration to the antineoplastic effect of bexarotene in CTCL. We found that bexarotene induces transcriptional and biological changes in CTCL cells related to decreased cell proliferation and chemotaxis, as well as increased proliferation and interferon response. Although lack of levothyroxine supplementation during bexarotene treatment increased apoptosis and decreased cell proliferation of CTCL cells in vitro and in vivo, it also decreased the lymphoma immunity. Since levothyroxine activates both the ubiquitous TRA nuclear receptor and the more restricted integrin V3 membrane receptor that is overexpressed in CTCL cells, we investigated their role in bexarotene treatment. We demonstrated that genetic and pharmacologic inhibition of the integrin V3 receptors resulted in improved bexarotene-induced effects on apoptosis, cell proliferation and chemotaxis while maintaining the lymphoma immunity. Our results provide a mechanistic rationale for evaluating the addition of the integrin V3 inhibitor cilengitide to CTCL therapeutic regimens that are based on bexarotene and levothyroxine supplementation. RNA was isolated from HuT 78 cells transfected with siRNA for integrins (si-ITGAV/B3); TRA (si-TR) or non-target sequences (si-CT) followed by treatment with bexarotene in the presence of physiological concentrations of TH, for 6 hours using TRI-REAGENT