Mouse Ascites Golgi (MAG) Mucin Expression and Regulation by Progesterone in the Rat Uterus

Objective: To study the regulation of the blood group A-related high-molecular weight mucin glycoprotein epitope (mouse ascites golgi, MAG)—a menstrual cycle-dependent marker of endometrial receptivity—in a non-human endometrium model. Methods: Immature Sprague-Dawley rats were injected with 1 μg of estradiol, 100 μg of testosterone, 100 μg of dexamethasone, 2.5 mg of progesterone (P), 0.325 mg of RU486, P and RU486, 100 μg of tamoxifen, or vehicle for 3 days, sacrificed, and the uteri were stained for MAG. Immunohistochemistry and blood analysis were the measurements used to compare the specimens from the exogenous hormonal and endogenous hormonal groups. Electron microscopy was used to locate the MAG epitope in one pseudopregnant adult Sprague-Dawley rat. Results: The MAG epitope was present in endometral glands of Sprague-Dawley rats, with maximal expression during proestrus and diestrus. Electron microscopy confirmed the Golgi location of this MAG epitope. In the untreated group, less than 0.5% of endometrial glands stained for MAG. The MAG was seen only in the glands of the P-treated rats and RU486 blunted this stimulatory effect by more than 95%. As little as 0.1 mg of P promoted MAG expression, with maximal response at 2.5 mg. Staining was seen 24 hours after P treatment, peaked at 72 hours, then declined. Induction of endogenous P by superovulation with pregnant mare serum gonadotropin (PMSG) and hCG (pseudopregnancy) also resulted in strong MAG glandular staining. Conclusion: Our results suggest that the MAG epitope is cyclically expressed and induced by P in rat endometrial glands.