INHIBITORS OF OXIDATIVE STRESS MIMIC THE ABILITY OF FOLLICLE-STIMULATING-HORMONE TO SUPPRESS APOPTOSIS IN CULTURED RAT OVARIAN FOLLICLES

We have reported that members of the bcl-2 gene family are expressed and gonadotropin regulated in ovarian granulosa cells during follicular maturation and atresia. Because Bcl-2, a protein that prevents apoptosis in several cell types, is reported to function as an antioxidant or free radical scavenger, the present studies were designed to investigate if oxidative stress plays a role in granulosa cell apoptosis during follicular atresia in the immature rat ovary. In the first series of experiments, the role of oxidative stress in the induction of granulosa cell apoptosis was directly tested using a defined in vitro follicle culture system. Healthy antral follicles obtained from equine CG (eCG)-primed immature (27 day old) rats were incubated in serum-free medium for 24 h in the absence or presence of FSH (100 ng/ml; a control for inhibiting apoptosis), superoxide dismutase (SOD; 10-1000 U/ml), ascorbic acid (0.01-1 mM; a free radical scavenger), N-acetyl-L-cysteine (25-100 mM; a free radical scavenger and stimulator of endogenous glutathione peroxidase activity), or catalase (10-1000 U/ml). Granulosa cells within follicles incubated in medium alone exhibited extensive apoptosis after 24 h of incubation, and this onset of apoptosis was blocked by treatment with FSH (29 +/- 4% of controls; P < 0.001, n = 3). Moreover, apoptosis in follicles was also inhibited by treatment with SOD (44 +/- 4% of controls at 1000 U/ml; P < 0.01, n = 3), ascorbic acid (55 +/- 9% of controls at 1 mM; P < 0.05, n = 3), N-acetyl-L-cysteine (24 +/- 7% of controls at 100 mM; P < 0.001, n = 3), or catalase (35 +/- 6% of controls at 1000 U/ml; P < 0.001, n = 3). In the second series of experiments, complementary DNAs corresponding to secreted (SEC-SOD), copper/zinc-containing (Cu/Zn-SOD), and manganese-containing (Mn-SOD) forms of rat SOD, rat seleno-cysteine glutathione peroxidase (GSHPx), and rat catalase were isolated and used to synthesize antisense RNA probes for Northern and slot blot analysis of changes in SOD, GSHPx, and catalase gene expression during follicular maturation. In vivo priming of 25-day-old female rats for 2 days with 10 IU eCG, which promoted antral follicular growth and survival, increased levels of messenger RNA encoding SEC-SOD (216 +/- 9% of saline-treated controls, P < 0.05, n = 3) and Mn-SOD (222 +/- 14% of saline-treated controls, P < 0.05, n = 3) vs. saline-treated controls. However, gonadotropin priming did not alter expression of Cu/Zn-SOD, GSHPx, or catalase messenger RNA in the ovary. Nevertheless, the induction of SEC-SOD and Mn-SOD expression by eCG provided further evidence that gonadotropins may promote granulosa cell survival in developing antral follicles via activation of an oxidative stress response. Collectively, these data suggest that the gonadotropin-mediated inhibition of follicular atresia involves enhanced expression of oxidative stress response genes whose products may then function to protect granulosa cells from the damaging effects of reactive oxygen species.