Previous studies have shown that ovotoxicity induced in rats by dosing with 4-vinylcyclohexene diepoxide (VCD) is likely via acceleration of the normal rate of atresia (apoptosis). The present study was designed to investigate the apoptosis-related caspase cascades as a component of this phenomenon in isolated ovarian small follicles. Female F344 rats were given a single dose of VCD (80 mg/kg, i.p., on Day 1; a time when ovotoxicity has not been initiated), or dosed daily for 15 days (80 mg/kg, i.p., on Day 15; a time when significant ovotoxicity is underway). Ovaries were collected after the final dose. Small preantral follicles (25–100 μm in diameter) were isolated, cellular fractions were prepared, and cleavage activity or protein expression levels of caspases-3, -8, and -9 were measured. Cytosolic caspase-3 activity was increased in small follicles (P < 0.01) by VCD treatment (Day 1, 2.86 ± 0.23; Day 15, 3.25 ± 0.64, VCD/control, n = 3). This activation was not seen in large or antral follicles (not targeted by VCD). Procaspase-3 protein was increased(P < 0.05) by VCD treatment 212% over controls in small ovarian follicles in Day 15, but not Day 1-dosed rats. Immunofluorescence staining intensity was evaluated by confocal microscopy. Caspase-3 protein, located in the cytosolic compartment of oocytes and granulosa cells of preantral follicles in various stages of development, was selectively increased (P < 0.05) in primordial and small primary follicles from Day 15 VCD-dosed rats. Caspase-8 activity was increased in small follicles in Day 15, but not in Day 1-treated rats; whereas caspase-9 activity was increased by VCD on Day 1 in the mitochondrial fraction. Thus, these data provide evidence that accelerated atresia induced in small ovarian follicles in rats by VCD is associated with activation of a caspase-mediated cascade.
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