Expression of endometrial glycogen synthase kinase-3β protein throughout the menstrual cycle and its regulation by progesterone

Glycogen synthase kinase-3 beta (GSK-3 beta) is a serine/threonine kinase that plays a role in glycogen synthesis by inhibiting glycogen synthase (GS) through phosphorylation. We hypothesized that GSK-3 beta by virtue of its role in glycogen synthesis through the inhibition of GS will play a role in the preparation of the endometrium for blastocyst implantation. Immunohistochemical (IHC) analysis and Western blot analysis (WBA) detected GSK-3 beta in the endometrium, myometrium, Fallopian tube and ovary. WBA showed more than 5-fold higher endometrial expression of the phosphorylated GSK-3 beta (pGSK-3 beta) isoform (inactive) in the secretory phase as compared with the proliferative phase (P < 0.001), whereas no differences in total GSK-3 beta expression were detected. IHC analysis confirmed the WBA and showed marked expression of pGSK-3 beta predominantly in glandular epithelial cells in early and mid secretory endometrium with scant expression during the proliferative phase. In in vitro experiments using human endometrial-derived epithelial cell line (HES), progesterone did not alter total GSK mRNA or protein expression. However, progesterone induced a dose-dependent increase in the expression of pGSK-3 beta, which could be blocked by RU486. Cyclic expression of GSK-3 beta's active and inactive forms in the endometrium suggests that sex hormones regulate the expression of this enzyme. In vitro experiments demonstrate that progesterone through receptor-mediated mechanisms induces phosphorylation of endometrial GSK-3 beta.