The ARP-1 orphan receptor represses steroid-mediated stimulation of human placental lactogen gene expression.

Studies were performed to determine whether ARP-1, which is an orphan receptor of the steroid receptor superfamily, inhibits basal activity of the human placental lactogen (hPL) promoter and the increase in hPL promoter activity in response to the receptors for thyroid hormone (TR) and retinoic acid (RAR). Co-transfection of an ARP-1 expression vector into BeWo choriocarcinoma cells, along with an expression vector containing 1.2 kb of the hPL promoter coupled to a CAT reporter gene, resulted in a dose-dependent inhibition of basal CAT activity. In addition, ARP-1 inhibited the stimulation of CAT activity by RAR alpha and TR beta expression vectors. Mobility shift assays demonstrated that ARP-1 binds specifically to a composite steroid response element on the hPL promoter that confers retinoic acid and T3 responsiveness. The results implicate an inhibitory role for ARP-1 in the regulation of hPL gene expression and strongly suggest that hPL gene expression is regulated, at least in part, by the interaction of stimulatory and inhibitory members of the steroid receptor superfamily.