Progesterone regulates osteopontin expression in human trophoblasts: a model of paracrine control in the placenta?

Osteopontin (OPN), a matrix glycosylated phosphoprotein, has been proposed to play a role(s) in basic cellular processes, such as neovascularization and tissue remodeling, which are essential to placental morphogenesis and embryo implantation. We have shown OPN to be expressed by cytotrophoblasts of the chorionic villus, and a putative progesterone regulatory element in the OPN promoter suggests hormonal regulatory control. This led us to test the hypothesis that progesterone regulates OPN expression in human cytotrophoblasts. Cytotrophoblasts isolated from human placentas were treated with combinations of progesterone, RU486, and/or aminoglutethimide, and their expression of OPN was assessed by Northern hybridization and immunocytochemistry. The expression of OPN messenger RNA (mRNA) declined as trophoblasts aggregated, but rebounded at later times when syncytia and mononuclear cytotrophoblasts coexisted in culture. Progesterone increased OPN mRNA expression by aggregating mononuclear cytotrophoblasts. Aminoglutethimide suppression of endogenous steroidogenesis by syncytiotrophoblasts inhibited OPN expression, whereas the addition of exogenous progesterone to cells treated with aminoglutethimide reversed this inhibitory effect. These observations were confirmed at the protein level by immunocytochemistry. Treatment of cytotrophoblasts with both progesterone and RU486 inhibited the up-regulatory effect on OPN mRNA associated with exposure to progesterone alone, further confirming a direct effect of progesterone. We conclude that progesterone up-regulates OPN expression in human cytotrophoblasts, and we propose that in vivo, progesterone secretion by syncytiotrophoblasts regulates the expression of OPN by the underlying cytotrophoblasts. As the receptors for OPN, alpha(v) integrins, are expressed by syncytiotrophoblasts, we postulate that these paracrine regulatory mechanisms contribute to the adhesive and/or signaling events between the two trophoblast cell types of the chorionic villus.