Induction of early growth response protein-1 gene expression in the rat ovary in response to an ovulatory dose of human chorionic gonadotropin

Granulosa cells in a mature ovarian follicle have an abundance of LH/hCG re ceptors that respond rapidly to an ovulatory surge in gonadotropins. Within minutes, membrane signal transduction sets in motion metabolic changes tha t lead to follicular rupture. This study provides evidence that the initial ovarian response to such an ovulatory stimulus includes induction of the i mmediate-early transcription factor gene for early growth response protein- 1 (Egr-1). Immature Wistar rats were primed with 10 IU equine CG (eCG), sc, and 48 h later the 12-h ovulatory process was initiated by 10 IU hCG, sc. Ovarian RNA was extracted at 0, 0.5, 1, 2, 4, 8, 12, and 24 h after the pri med animals were injected with hCG. The RNA extracts were used for RT-PCR d ifferential display for random detection of gene expression in the stimulat ed ovarian tissue. Northern analysis of one of the differentially amplified complementary DNAs confirmed that it was part of a gene that was significa ntly up-regulated within 1 h after the ovaries had been stimulated by hCG. Maximum transcription was at 4 h after hCG, and expression declined to 0 h control levels by 24 h after hCG. Subcloning and sequence analysis revealed that the complementary DNA matched the gene for Egr-1. In situ hybridizati on indicated that the Egr-1 messenger RNA was in the granulosa layer of mat ure follicles. Western blotting confirmed the temporal pattern of Egr-1 exp ression detected by differential display, Northern analysis and in situ hyb ridization. The Egr-1 protein is approximately 84 kDa. In conclusion, the d ata show that expression of the zinc finger transcription factor Egr-1 is a n early event in the cascade of inflammatory-like changes that occur in an ovulatory follicle in response to a trophic hormone.