THROMBOSPONDIN-1, AN INHIBITOR OF ANGIOGENESIS, IS REGULATED BY PROGESTERONE IN THE HUMAN ENDOMETRIUM

Thrombospondin-1 (TSP1), a multifunctional extracellular matrix glycop rotein, has been shown to suppress the angiogenic response in vivo and in vitro. We hypothesized that TSP1 might play a role in the inhibiti on of capillary morphogenesis during the endometrial cycle and examine d its expression in 46 human endometrial specimens. Our results show t hat the expression of TSP1 in the endometrium is (a) cycle-dependent, (b) associated with periods of low capillary growth, and (c) regulated by progesterone. TSP1 protein was identified in the basement membrane of capillaries of the functional endometrium during the secretory pha se. Abundant expression of TSP1 mRNA in the secretory phase was also d etected by in situ hybridization, in contrast to the low levels seen i n the proliferative phase. These findings were confirmed by Northern a nalysis of proliferative and secretory endometrium. Transcripts for TS P1 were observed predominantly in stromal cells, but signal was also d etected in some endothelial and smooth muscle cells. Since the prolife ration of endometrial tissue is regulated by steroid hormones, we test ed the effects of estrogen and progesterone on TSP1 expression by stro mal cells isolated from human endometrium. We found that levels of TSP 1 mRNA and protein were increased after incubation with progesterone. Maximal stimulation of mRNA was observed after 8 h of treatment with 1 0-50 mu M progesterone, and the effect was suppressed by the progester one antagonist RU-486. Induction by progesterone was cell-specific and equivalent to the stimulation mediated by PDGF. Finally, the levels o f TSP1 present in progesterone-stimulated cultures were sufficient to inhibit the migration of endothelial cells in vitro; this effect was n ullified by anti-TSP antibodies. We therefore propose that the product ion of TSP1 at later stages of the endometrial cycle is linked to the inhibition of vessel formation and that TSP1 expression is progesteron e-dependent in this tissue.