Abstract
PURPOSE Retinoic acid (RA) treatment has been used for redifferentiation of metastatic thyroid cancer with loss of radioiodine uptake. The aim of this study was to improve the understanding of RA resistance and investigate the role of bexarotene in thyroid cancer cells. EXPERIMENTAL DESIGN A model of thyroid cancer cell lines with differential response to RA was used to evaluate the biological effects of retinoid and rexinoid and to correlate this with RA receptor levels. Subsequently, thyroid cancer patients were treated with 13-cis RA and bexarotene and response evaluated on radioiodine uptake reinduction on posttherapy scan and conventional imaging. RESULTS In thyroid cancer patients, 13-cis RA resistance can be bypassed in some tumors by bexarotene. A decreased tumor growth without differentiation was observed confirming our in vitro data. Indeed, we show that ligands of RARs or RXRs exert different effects in thyroid cancer cell lines through either differentiation or inhibition of cell growth and invasion. These effects are associated with restoration of RARβ and RXRγ levels and downregulation of NF-κB targets genes. We show that bexarotene inhibits the transactivation potential of NF-κB in an RXR-dependent manner through decreased promoter permissiveness without interfering with NF-κB nuclear translocation and binding to its responsive elements. Inhibition of transcription results from the release of p300 coactivator from NF-κB target gene promoters and subsequent histone deacetylation. CONCLUSION This study highlights dual mechanisms by which retinoids and rexinoids may target cell tumorigenicity, not only via RARs and RXRs, as expected, but also via NF-κB pathway.
