Changes in expression and subcellular localization of nuclear retinoic acid receptors in human endometrial epithelium during the menstrual cycle

The endometrium is a uniquely dynamic tissue in that it undergoes monthly cycles of proliferation and secretory activity, and is regulated by ovarian steroid hormones. In this study, we focused on retinoic acid receptors (RAR and RXR) which are ligand-dependent transcription factors belonging to the large family of steroid hormones and are expected to affect to cell growth and differentiation in the endometrium. We analysed the expression and subcellular localization of the RA receptors in 57 samples of human endometrium by immunohistochemistry and Western blotting. In the nuclei of the endometrial epithelium, the RA receptors were expressed strongly in the proliferative phase. However, RAR were drastically reduced in the epithelial nuclei during the secretory phase in association with changes in serum oestradiol and in the expression of the oestrogen receptor. The expression of RXR was localized in the epithelial nuclei throughout the menstrual cycle. Confocal laser scanning microscopical observation clearly showed the difference in the localization between RAR and RXR in the secretory phase. Furthermore the findings of immuno-electron microscopy showed pooled RAR around the rough endoplasmic reticulum, suggesting that transport of these receptors to the nuclei is inhibited. These findings suggest that RAR and RXR work mainly in the proliferative phase and that in the endometrium RXR may play a different role to RAR during the secretory phase.