Upregulation of connexin 43 by retinoids but not by non-provitamin A carotenoids requires RARs

Expression of connexin 43 (Cx43) is correlated with reduced indexes of neoplasia and is upregulated by cancer-preventive retinoids and carotenoids in nontransformed human and murine fibroblasts and keratinocytes. The molecular mechanism of upregulation, however, is poorly understood. Three retinoic acid receptor (RAR) antagonists (Ro 41-5253, BMS453, and BMS493) were capable of suppressing retinoid-induced Cx43 protein expression in 10T1/2 cells. However, Ro 41-5253 did not suppress protein expression by the non-provitamin A carotenoids astaxanthin or lycopene. In contrast, Cx43 induction by astaxanthin but not by a RAR-specific retinoid was inhibited by GW9662, an antagonist of peroxisome proliferator activated receptor-gamma activation. Simultaneous treatment with the maximally effective concentration of a retinoid and with beta-carotene or the non-provitamin A carotenoid astaxanthin resulted in supraadditive upregulation of Cx43 expression, again indicating separate mechanisms of gene regulation by these two cancer preventive agents.