The uterotrophic effects of relaxin: Activation of ligand-independent estrogen receptor-mediated transcription

Abstract

The peptide hormone/growth factor relaxin has numerous and diverse effects in the cardiovascular system, the nervous system, and throughout the female reproductive tract. In females, relaxin is involved with the processes of cervical ripening and relaxation of the pubic symphysis prior to parturition, mammary gland and nipple development, and uterine growth and metabolism. The time course and nature of relaxin's effects on the uterus closely mirror those of the steroid hormone estradiol (E2), suggesting that the signaling pathways of relaxin and E2 may share some common components. This dissertation examines the relationship between relaxin and E2-mediated signaling in the uterus and in breast cancer cells. In immature, ovariectornized rats, relaxin and E2 both induced increases (75% and 100%, respectively) in the uterine wet weight/body weight ratio versus controls 6 hours following treatment. These increases were completely inhibited by pre-treatment with ICI 182 780, a potent and specific estrogen receptor (ER) antagonist. Histological analysis of uterine cross sections revealed that the uterine edema was localized to the endometrium; ICI 182 780 pre-treatment blocked this edema. From these data, we concluded that relaxin's uterotrophic effects involved ligand-independent activation of ERs. RT/PCR examination of uterine mRNA from ovariectomized rats treated with relaxin or E2 showed that both significantly and rapidly up-regulated vascular endothelial growth factor (VEGF) and progesterone receptor expression. These experiments were followed by RT/PCR analysis of ERbeta regulation, which revealed that both relaxin and E2 down-regulated uterine ERbeta1 and ERbeta2 in vivo. This is the first report of E2 regulation of ERbeta2 and of relaxin regulation of ERbeta regulation. To confirm that relaxin activates ERs and ER-mediated transcription, we transfected relaxin and E2 responsive MCF-7 cells with an ERE-Luc reporter gene vector and treated them with relaxin and E2. Luciferase expression increased significantly in response to both relaxin and E 2; these increases were blocked by co-treatment with ICI 182 780. From these data, we concluded that relaxin increased reporter gene expression through ligand-independent stimulation of ER-ERE-mediated transcription. In summary, relaxin's trophic effects on the uterus involve, to a large part, activation of ERs.