The Effects of Estradiol and Catecholestrogens on Glycogen Catabolism and Glucose Mobilization in the Uterus of the Mink (Neovison vison).

Abstract

Embryonic development prior to formation of the placenta depends in part on uterine glandular secretions (histotroph) that are rich in carbohydrates, including glycogen. In mink, total uterine glycogen concentrations decrease 55% between estrus and the peri-implantation period, presumably to meet the metabolic demands of developing embryos. Estradiol-17beta (E2) stimulates uterine glycogen production and circulating E2 levels peak during proestrous and decline thereafter. We have shown that the effects of E2 are mediated in part through the catecholestrogens: 2-hydroxycatecholestradiol (2-OHE2) and 4-hydroxycatecholestradiol (4-OHE2) produced by the endometrium. It is however, unknown what is responsible for glycogen catabolism as the time of implantation draws near. While progesterone concentrations are increasing at this time, exogenous progesterone stimulated uterine glycogen accumulation in mink with intact ovaries. Although E2 levels are significantly reduced during most of the peri-implantation period, a transitory peak in E2 secretion occurs near the expected time of implantation. It is therefore possible that E2 and/or catecholestrogens facilitate both glycogen synthesis and breakdown. Therefore, to investigate the effects of these steroids on glycogen catabolism and glucose mobilization, our objectives were to determine the effects of E2, 2-OHE2 and 4-OHE2 on (1): uterine glycogen content and (2): gene expression levels for glycogen-synthase, glycogen-phosphorylase, and glucose-6-phosphatase. Mink (N=24) were ovariectomized and assigned at random to one of four groups: 1 = Controls, 2 = E2, 3 = 2-OHE2, and 4 = 4-OHE2. Animals were injected sc once daily with hormone (40ug/kg BW) in sesame seed oil, or oil only (controls) for 3 consecutive days. On day four, mink were sacrificed and uteri collected. Total uterine glycogen concentrations (mg/g dry wt) were increased 8-fold by E2 and 4-OHE2 and 4-fold by 2-OHE2, compared to controls (P<0.05). Interestingly, the percent glycogen content of the glandular epithelium was increased 4 to 5 fold by 4-OHE2 and E2 (P<0.05), but was not different between 2-OHE2 and controls. Expression of glycogen-synthase mRNA did not differ between E2, 4-OHE2 and controls, but was reduced 5-fold in response to 2-OHE2 (P<0.01). Uterine glycogen-phosphorylase mRNA expression was increased 3-fold by E2 and 4-OHE2, and 10-fold by 2-OHE2 (P<0.01). The expression of glucose-6-phosphatase mRNA was greatest in response to 2-OHE2, followed by 4-OHE2 and then E2 (P<0.05). We conclude that E2 and/or catecholestrogens influence both glycogen synthesis and breakdown, perhaps as a result of different concentrations or exposure time. Although 4-OHE2 is considered to be a more potent estrogen than 2-OHE2, our data show that 2-OHE2 had a greater effect than E2 and 4-OHE2 on the expression of all glycogen metabolizing genes investigated here. It is possible that as the time of implantation approaches, the production and/or sensitivity of the uterus to 2-OHE2 predominates over both 4-OHE2 and E2, promoting glycogen catabolism and glucose mobilization. Such a mechanism, could serve to regulate the release of glucose from uterine glycogen reserves, providing a source of nutrition to the developing embryos, which may be essential for implantation and reproductive success. (Funded by Fur Commission USA and NIH INBRE P20RR016454) (poster)