The expression of c-myc during follicular growth and luteal formation in the rat ovary in vivo

Abstract

The processes of folliculogenesis and formation of corpora lutea involve proliferation and differentiation of the follicular cells. The expression of several oncogenes is associated with the proliferative phase in many cell types. The present study examined the expression and hormonal regulation of the c-myc proto-oncogene during follicular development and the luteal phase of pseudopregnancy. Follicular development was initiated by pregnant mare's serum gonadotropin (PMSG) in immature rats followed two days later by the injection of human chorionic gonadotropin (hCG) to induce ovulation and luteal formation. Ovaries were collected at different time points and the content and distribution of c-myc mRNA/protein were examined. C-myc increased rapidly after the administration of both PMSG and hCG, but the effect of PMSG was less pronounced. The increase after PMSG was transient and localized primarily to the granulosa cells of developing follicles. The ovulatory dose of hCG resulted in a rapid and substantial increase of c-myc mRNA and protein with maximal levels at 1 h and 2-4 respectively. At this stage, the c-myc protein was localized to the follicular cells, the surface epithelium and, to some extent, to the interstitial tissue. There was a subsequent decrease prior to ovulation. The luteal phase was characterized by decreasing levels of c-myc with increasing luteal age. In order to examine the involvement of specific hormones in the regulation of c-myc, hypophysectomized, immature rats were injected sequentially with estradiol (E2) and follicle-stimulating hormone (FSH). Hypophysectomy resulted in a decrease of c-myc compared with intact animals. The administration of E2 resulted in an increase of c-myc mRNA and protein. The subsequent treatment with FSH did not result in a further increase and the levels remained at the same level as with E2 only. However, an ovulatory dose of hCG to E2 and FSH primed animals resulted in an additional increase of c-myc mRNA and protein. The levels after E2 and FSH were considerably lower compared with those of untreated ovaries of intact, immature animals, suggesting the involvement of other endocrine and paracrine factors. The presence of proliferating cell nuclear antigen in cell extracts indicated that the expression of c-myc was associated with phases of increased proliferation of follicular cells after hormonal stimulation. The results demonstrate that c-myc is regulated by hormones (E2, gonadotropins) in the rat ovary during follicular development to the preovulatory stage. The pronounced increase prior to ovulation also suggests a role for c-myc in the regulation of proliferative events involved in luteal formation.