Female sex steroid hormonal control of cell cycle regulatory molecules and genetic evidence for their interactions in mice
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The uterine epithelium in response to female sex steroid hormones provides an excellent model to study cell proliferation in vivo. When administered to ovariectomized adult mice estradiol-17beta (E2) stimulates a synchronized wave of cell proliferation in the epithelial cells, while pretreatment with progesterone (P4) completely inhibits it. Using a simple method to isolate the uterine epithelium with high purity, we have shown that P4 pre-treatment prohibited the E2-induced nuclear localization of cyclin D1 and cdk4 without inhibiting their levels or kinase activities, abrogated the E2-induced cyclin E, A/cdk2 activation and inhibited hyperphosphorylation of Rb and p107. P 4 is used therapeutically to oppose the effect of E2 during hormone replacement therapy and in the treatment of uterine adenocarcinoma. This study showing a novel mechanism of cell cycle inhibition by P4 may provide the basis for the development of novel anti-estrogens.;P27Kip1 is expressed highly in the uterine epithelium. This, together with the fact that the p27-/- females are infertile due to an implantation failure, prompted us to examine the cell proliferation response to hormones in p27-deficient uteri. The results showed that p27 is not required for E2 and P4 action in the uterus. However, we pinpointed at least one defect to the ovary, and found that the lack of the nidatory estrogen surge was the cause of the implantation defect in p27-/- mice. Furthermore, the luteal cells failed to exhibit a timely withdrawal from the cell cycle.;Expression of cyclin D1 in the corpus luteal cells made it possible that this is the functional D type cyclin in these cells and that the lack of p27 resulted in its hyperactivity. To reveal epistatic relationship between these two cell cycle regulators in vivo, we generated mice nullizygous for both p27 and cyclin D1 genes. As hypothesized, the embryos of p27-/-D1-/- females implanted. However, p27/D1-deficient ovaries still exhibited prolonged cell proliferation in the corpus luteal cells. The double nullizygous mice also showed rescued body size and retinal structure and cellularity. These data show an epistatic relationship between p27 and cyclin D1 in some but not all cell types. Cyclin D1 and p27 are central to cell cycle regulation. Analysis of the mechanism of the opposing effect of these two genes in mice may contribute to the further understanding of the hormonal control of cell cycle and their roles in tumorigenesis.