Conjugated estrogens in the endometrium during the estrous cycle in pigs

Estrogen metabolism results in the formation of inactive estrogen sulphates and glucuronides. Despite the lack of receptor binding, circulating conjugated estrogens might serve as a reservoir for the active form through the involvement of specific cleaving enzymes. In order to elucidate the potential role that estrogen conjugates play in the regulation of the estrous cycle, we determined the concentration of progesterone, estrogen and estrogen conjugates in serum and endometrial homogenates of cycling gilts. In addition, we determined the mRNA expression changes of enzymes (UDP glucuronosyltransferase (UGT), β-glucuronidase (GUSB), sulphotransferases (SULT) and steroid sulphatase (STS)) and transporters (multidrug resistance-associated protein (MRP), organic anion-transporting polypeptide (OATPs)) involved in the estrogen metabolism in the endometrium across the estrous cycle. GUSB displayed highest expression at estrous (day 0), decreasing expression during metestrus (day 3 and 6), minimal expression on day 10 and 12, and increasing expression towards proestrus (day 18), suggesting either a stimulation by estrogens or a negative impact of progesterone. The mRNA expression of the influx-transporter OATP1A2 significantly increased from day 0 to 6 and decreased again by day 10, while the efflux-transporters (MRP1, MRP2, and MDR1) displayed minimal expression at day 3 and 6. The mRNA expression of the UDP-glucuronsyltransferases followed a similar pattern, with minimal expression found at day 6. The analyses of the concentration of local and circulating steroid hormones points towards an interaction of the analyzed transporters and enzymes with steroid hormones, thereby possibly regulating the reservoir of active steroids contributing to the endometrial function.