Thrombospondin-1 expression is increased during follicular atresia in the primate ovary

Thrombospondin (TSP)-1 is an antiangiogenic extracellular matrix glycoprotein that modulates several aspects of cellular function. The aim of this study was to determine the pattern of TSP-1 mRNA and protein expression as well as expression of its receptor CD36 in the marmoset ovary and to investigate the effects of inhibition of gonadotropins or VEGF activity on TSP-1 and CD36 expression in vivo. GnRH antagonist or VEGF Trap, a soluble decoy receptor, was administered on d 0 of the follicular phase of the cycle, and ovaries were collected at the end of the follicular phase (d 10). TSP-1 mRNA and protein were present in granulosa cells of preantral and antral follicles, with the highest staining at the late secondary and tertiary stages. Moreover, expression of TSP-1 mRNA and protein was significantly increased in tertiary follicles undergoing atresia. CD36 protein was detected in granulosa cells of preantral and antral follicles as well as in endothelial cells of large vessels. Inhibition of gonadotropin secretion or VEGF activity had no effect on TSP-1 expression; however, expression of CD36 protein was inhibited by the VEGF Trap. In conclusion, TSP-1 may be involved in the cessation of angiogenesis in follicles undergoing atresia; alternatively, TSP-1 may act on granulosa and/or endothelial cells to promote follicular atresia in the ovary. Angiogenesis is likely to involve a balance between pro- and antiangiogenic factors. Our results suggest that loss of VEGF activity does not regulate TSP-1 expression directly but may influence TSP-1 activity via down-regulation of the CD36 receptor.