Tamoxifen, a selective estrogen modulator (SERM) that has found clinical utility in the treatment of breast cancer, is an antagonist in the breast and an agonist in the uterus. These agonist actions in the uterus lead to an increased risk of endometrial cancer. In this study in mice we have analyzed the mechanism of action of tamoxifen in inducing cell proliferation in the uterine luminal epithelia. Tamoxifen induces a wave of DNA synthesis in these epithelial cells with kinetics similar to those seen after 17β-estradiol (E₂) treatment. However, by these criteria of mitogenicity, it is much less potent and never achieves full estrogenicity. This uterine epithelial cell proliferation is preceded by the mobilization of cyclin D1 from the cytoplasm to the nucleus which, together with CDK4, phosphorylates members of the Rb-retinoblastoma family of proteins, pRb and p107. Subsequent to this initial nuclear accumulation of cyclin D1, cyclin E and then cyclin A are induced that, together with the activation of CDK2, results in enhanced cyclin E- and cyclin A-dependent CDK2 kinase activity and further phosphorylation of pRb and p107. These actions of tamoxifen parallel those of E₂. Tamoxifen also induced the classical estrogen water imbibition response. However, in this it was more potent, producing a maximal response at doses that do not affect DNA synthesis. This suggests that the uterotropic response is not an accurate predictor of the compound’s hyperplasia responses. We can conclude that, in its effects on proliferation, tamoxifen acts as a classical impeded estrogen and this suggests that the AF-1 transcription activation domain of the estrogen receptor that is activated upon both E₂ and tamoxifen binding to this receptor regulates these responses in the uterus.

Although Gja1 has been proved to play an important role in uterine decidualization, its regulatory mechanism remains largely unknown. Here, we showed that Gja1 was highly expressed in the decidual cells and promoted the proliferation of uterine stromal cells and expression of Prl8a2 and Prl3c1, which were two well-known differentiation markers for decidualization. Further analysis revealed that Gja1 might act downstream of Acvr1 and cAMP to regulate the differentiation of uterine stromal cells. Administration of cAMP analog 8-Br-cAMP to Acvr1 siRNA-transfected stromal cells resulted in an obvious increase of Gja1 expression, whereas PKA inhibitor H89 impeded the induction of Gja1 elicited by Acvr1 overexpression, indicating that cAMP–PKA signal mediates the regulation of Acvr1 on Gja1 expression. In uterine stromal cells, knockdown of Gja1 blocked the cAMP induction of Hand2. Moreover, siRNA-mediated downregulation of Hand2 impaired the stimulatory effects of Gja1 overexpression on the expression of Prl8a2 and Prl3c1, whereas constitutive expression of Hand2 reversed the inhibitory effects of Gja1 siRNA on stromal differentiation. Meanwhile, Gja1 might play a vital role in the crosstalk between Acvr1 and Hand2. Collectively, Gja1 may act downstream of cAMP–PKA signal to mediate the effects of Acvr1 on the differentiation of uterine stromal cells through targeting Hand2.