Effects of chronic heat stress on granulosa cell apoptosis and follicular atresia in mouse ovary

BACKGROUND: Heat stress is known to alter follicular dynamics and granulosa cell function and may contribute to the diminished reproductive efficiency commonly observed in mammals during the summer. Although several investigators have studied heat-induced ovarian injury, few reports have focused on the effects of chronic heat stress on ovarian function and the molecular mechanisms through which it induces ovarian injury. METHODS: In Exp. 1, 48 female mice were assigned to a control or heat-stressed treatment. After exposure to a constant temperature of 25 °C for 7, 14, 21 or 28 d (n = 6) or to 42 °C for 3 h per d for 7, 14, 21 or 28 d (n = 6), the mice were euthanized and their ovaries were analyzed for follicular atresia, granulosa cell apoptosis, changes in the abundance of HSP70 protein and serum concentrations of estradiol. In Exp. 2, the expression of HSP70 and aromatase was quantified in antral follicles cultured in vitro at 37 or 42 °C for 24 h. In Exp. 3, granulosa cells from ovaries maintained at 37 or 41 °C for 2 h were analyzed for their expression of HSP70, Bim, caspase-3 and cleaved caspase-3. RESULTS: In Exp. 1, body weight and food intake of heat-stressed mice decreased (P < 0.05) compared with control mice while the concentration of estradiol in serum was lower (P < 0.05) in heat-stressed mice than in control mice. Compared with control mice, the percentage of atretic follicles and the number of antral follicles with severe apoptotic signals were increased (P < 0.05) after 21 d of heat-stressed treatment. HSP70 protein was more abundant (P < 0.05) in heat-stressed mice than control mice. In Exp. 2, heat stress increased HSP70 and decreased aromatase proteins (P < 0.05) in antral follicles. In Exp. 3, TUNEL-positive granulosa cells from heat-stressed ovaries were observed concomitant with a significant increase in HSP70, Bim and cleaved caspase-3 protein. CONCLUSION: Heat-stress in mice decrease estradiol in serum and aromatase in antral follicles but increased number of atretic follicles and granulosa cell undergoing apoptosis which may explain the decreased fertility commonly observed in heat-stressed animals.