Membrane-bound progesterone receptor expression in human aortic endothelial cells

Observational studies demonstrate that estradiol and progesterone affect vasoreactivity. In animal studies, progesterone treatment causes immediate relaxation of pre-contracted arteries with inhibition of calcium influx in vascular endothelial and smooth muscle cells, suggesting a non-genomic mechanism of action. In this study we investigated the presence of novel membrane-bound progesterone receptors in human aortic endothelial cells and correlated the expression with cell-cycle stage. Western blotting analysis with an antibody directed to the hormone-binding domain of the classic progesterone receptors shows predominant bands at 100 and 60 kD, whereas analysis with an antibody to the DNA-binding region shows only the 100-kD band. In contrast, classic nuclear progesterone receptors B and A are identified at 116 and 94 kD in similarly processed T47D cells. Both novel bands localize to the membrane fraction after differential centrifugation. Plasma membrane-bound progesterone receptor was further shown with immunofluorescent antibody and ligand-binding studies in a small percentage of human aortic endothelial cells. Fluorescent activated cell sorting demonstrated that approximately 8% of the human aortic endothelial cells expressed a plasma membrane progesterone receptor and that a greater percentage of the expressing cells were in the G2/M-phase of the cell cycle. Treatment with progesterone conjugated to BSA did not show any significant cell-cycle changes. Plasma membrane-bound progesterone receptor in vascular endothelial cells may regulate the non-genomic actions of progesterone, and expression of the receptor appears to vary with cell cycle stage.