Hydroxyestrogens inhibit angiogenesis in swine ovarian follicles

The rapid, controlled, and cyclical nature of angiogenesis in the ovarian follicle suggests the potential for sex steroids to influence neovascularization. Angiogenesis is regulated by a local balance between the levels of endogenous stimulators and inhibitors. Multiple lines of evidence suggest that estrogens stimulate angiogenesis via effects on endothelial cells. However, it is of outstanding value to investigate the negative control of this process. Since the main estrogen metabolites, 2-hydroxyestradiol and 4-hydroxyestradiol (4-OHE2) have been demonstrated to function as anti-estrogen in several estrogen-dependent organs; the aim of this study was to investigate their potential involvement in the modulation of follicular angiogenesis. Hydroxyestrogens were quantified in swine follicular fluid and their effects were studied on granulosa cell vascular endothelial growth factor (VEGFA) production and tested in an angiogenesis bioassay. Our study documents that these molecules are physiologically present in swine follicular fluid and their concentrations significantly (P<0.001) increase during follicle development. Moreover, angiogenesis bioassay revealed that both hydroxyestrogens significantly (P<0.001) inhibited new vessel growth. We evidenced that the most potent negative effect is mediated by 4-OHE2. The anti-angiogenic potential of this molecule is also supported by its ability to inhibit (P<0.001) VEGFA production by granulosa cells. Increased knowledge in this area is of utmost importance for future therapeutic options to contrast infertility disorders associated with aberrant angiogenesis; this would be also very useful for the treatment of diseases characterized by disregulated angiogenesis and vascular regression.