A thrombospondin-mimetic peptide, ABT-898, suppresses angiogenesis and promotes follicular atresia in pre- and early-antral follicles in vivo.

Using a novel in vitro angiogenesis assay, we previously showed that thrombospondin (TSP)-1 has antiangiogenic effects on rat follicles and induces apoptosis in granulosa cells in vitro. ABT-898 is an octapeptide mimetic of TSP-1 closely related to ABT-510. Here, we demonstrate the inhibitory effects of ABT-898 on follicular angiogenesis and its proapoptotic effect on granulosa cells. To investigate the potential of this peptide to inhibit follicular angiogenesis in vivo, marmoset monkeys were treated with 2.5 mg/kg ABT-898 twice daily throughout the follicular phase of the cycle. Although treatment did not block emergence of dominant follicles, angiogenesis was reduced in preantral and early-antral follicles. Furthermore, the incidence of atresia at these follicle stages was increased. To investigate whether treatment with ABT-898 would interfere with the timing or duration of the normal ovulatory rise in plasma progesterone, marmosets were treated with a depot formulation containing 25 mg ABT-898 at the start of the follicular phase, with a second injection after 2 wk. Despite active concentrations of peptide being maintained in the circulation, no apparent effects on the ovulatory cycle were observed. Taken together, these results indicate that ABT-898 is capable of having a dual effect by inhibiting follicular angiogenesis and promoting atresia of antral follicles in vivo but does not prevent ovulation or induce luteolysis, as has been observed with direct vascular endothelial growth factor inhibitors. These results suggest that ABT-898 could be a novel therapeutic to inhibit abnormal angiogenesis and induce atresia of accumulated follicles in polycystic ovary syndrome.

Thrombospondin-1 inhibits angiogenesis and promotes follicular atresia in a novel in vitro angiogenesis assay.

Thrombospondin-1 (TSP-1) is a putative antiangiogenic factor, but its role in regulating physiological angiogenesis is unclear. We have developed a novel in vitro angiogenesis assay to study the effect of TSP-1 on follicular angiogenesis and development. Intact preantral/early antral follicles dissected from 21-d-old rat ovaries were cultured for 6 d in the presence or absence of TSP-1. At the end of the culture period, angiogenic sprouting from the follicles was quantified using image analysis. Follicles were fixed and sectioned, and follicular apoptosis was assessed by immunohistochemistry for activated caspase-3 in granulosa cells. The results showed that TSP-1 inhibited follicular angiogenesis (P < 0.01) and promoted follicular apoptosis (P < 0.001) in a dose-dependent manner. To determine whether the proapoptotic activity of TSP-1 is mediated by direct effects on granulosa cells, isolated granulosa cells were cultured with TSP-1 (0, 10, 100, and 1000 ng/ml) for 48 h. Apoptosis was quantified using a luminescent caspase-3/7 assay. TSP-1 promoted apoptosis of granulosa cells in a dose-dependent manner (P < 0.05), suggesting that TSP-1 can act independently of the angiogenesis pathway to promote follicular apoptosis. These results show that TSP-1 can both inhibit follicular angiogenesis and directly induce apoptosis of granulosa cells. As such, it may have potential as a therapeutic for abnormal ovarian angiogenesis and could facilitate the destruction of abnormal follicles observed in polycystic ovary syndrome.