Microsomal epoxide hydrolase expression in the endometrial uterine corpus is regulated by progesterone during the menstrual cycle.

We have shown previously that high expression levels of microsomal epoxide hydrolase (mEH) correlate with a poor prognosis of breast cancer patients receiving tamoxifen, suggesting that enhanced mEH expression could lead to antiestrogen resistance (Fritz et al. in J Clin Oncol 19:3-9, 2001). Thus, the purpose of this study was to gain insights into the role of mEH in hormone-responsive tissues. We analyzed biopsy samples of the endometrium by immunohistochemical staining, pointing to a regulation of mEH during the menstrual cycle: during the first half mEH expression was low, increased during the second half and reached highest levels during pregnancy. Additionally, the progesterone receptor (PR) positive human endometrial cell lines IKPRAB-36 (estrogene receptor alpha [ERalpha] negative) and ECC1-PRAB72 (ERalpha positive) were chosen to further investigate the hormonal regulation of mEH expression. Western Blot and quantitative RT-PCR analysis revealed an increase of mEH expression after treatment with medroxy-progesterone 17-acetate (MPA) in the ERalpha containing ECC1-PRAB72 cells. In contrast our results suggest that MPA had no influence on the mEH protein level in the ERalpha- IKPRAB-36 cells. In conclusion, mEH expression is regulated by progesterone in the presence of both PRs and ERalpha.