Granulosa cell luteinization involves the attenuation of gonadotropin-induced proliferation. Although recent evidence indicates that primate granulosa cells stop dividing within 12 h of an ovulatory stimulus, early events in cell cycle arrest remain unknown. In the current study an in vitro model of primate granulosa cell luteinization is established that allows assessment of early events in terminal differentiation. A luteinizing dose of human chorionic gonadotropin (hCG) results in a secondary rise in proliferation before cell cycle arrest that is paralleled by a transient increase in the expression of c-Myc. In contrast, the c-Myc antagonists Mad1, Mad4, and Mxi1 are transiently repressed by hCG. Max, the common dimerization partner for Myc and Mad, is similarly repressed by hCG, suggesting that changes in the expression of this gene may further regulate the activity of Myc and Mad. To determine whether other cell cycle regulatory families are involved in luteinization, the expression of p53 and the wild-type p53-inducible phosphatase (wip1) was examined. Similar to Mad and Max, p53 and wip1 are transiently repressed by hCG, suggesting that the p53 and Mad pathways have either parallel or cooperative roles in luteinization. Thus, luteinization of primate granulosa cells is preceded by a burst of proliferation that is regulated by changes in the relative levels of c-Myc, Max, and Mad as well as p53.
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