Spatiotemporal expression patterns of ER-α and ER-α protein and mRNA in hamster ovarian cells during the estrous cycle and following hypophysectomy and selective hormone replacement were evaluated by immunofluorescence, immunoblotting and in situ hybridization analyses. Whereas ER-β mRNA and protein expression predominated in granulosa cells and ER-α expression was in interstitial and thecal cells, overlap in receptor subtype expression across cell types was evident. Both ER subtypes were present from primordial follicle stage onward. ER-α mRNA levels and immunoreactivity started increasing from D3:0900 h in intersitial and granulosa cells and peaked on the proestrous (D4:0900 h). Regionalized higher expression of ER-α in granulosa cells in and around the forming antrum was evident. Surface epithelial cells were also positive. ER-β mRNA and protein expression increased markedly in granulosa and interstitial cells on D2:0900 h, reached a peak on D3:0900 h, and then declined sharply on D4:0900 h. No change in ER expression occurred following the preovulatory gonadotropin surge. Whereas FSH or human CG stimulated ER-α mRNA and protein expression in hypophysectomized hamsters, only FSH could stimulate ER-β mRNA and protein, and the effect was significantly attenuated by human CG. ER expression was stimulated by estrogen, but progesterone strongly inhibited estrogen action. These results indicate that ER expression is cell type specific to the larger extent and is critically regulated by reproductive hormones.
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