Alterations of endometrial epithelial–mesenchymal transition and MAPK signalling components in women with PCOS are partially modulated by metformin in vitro

Abstract Growing evidence suggests that epithelial–mesenchymal transition (EMT) and its regulator mitogen-activated protein kinase (MAPK) contribute to endometria-related reproductive disorders. However, the regulation of EMT and MAPK signalling components in the endometrium from polycystic ovary syndrome (PCOS) patients has not been systematically investigated and remains elusive. In humans, how metformin induces molecular alterations in the endometrial tissues under PCOS conditions is not completely clear. Here, we recruited 7 non-PCOS patients during the proliferative phase (nPCOS), 7 non-PCOS patients with endometrial hyperplasia (nPCOSEH), 14 PCOS patients during the proliferative phase (PCOS) and 3 PCOS patients with endometrial hyperplasia (PCOSEH). Our studies demonstrated that compared with nPCOS, PCOS patients showed decreased Claudin 1 and increased Vimentin and Slug proteins. Similar to increased Slug protein, nPCOSEH and PCOSEH patients showed increased N-cadherin protein. Western blot and immunostaining revealed increased epithelial phosphorylated Cytokeratin 8 (p-CK 8) expression and an increased p-CK 8:CK 8 ratio in PCOS, nPCOSEH and PCOSEH patients compared to nPCOS patients. Although nPCOSEH and PCOSEH patients showed increased p-ERK1/2 and/or p38 protein levels, the significant increase in p-ERK1/2 expression and p-ERK1/2:ERK1/2 ratio was only found in PCOS patients compared to nPCOS patients. A significant induction of the membrane ERβ immunostaining was observed in the epithelial cells of PCOS and PCOSEH patients compared to nPCOS and nPCOSEH patients. While in vitro treatment with metformin alone increased Snail and decreased Claudin 1, N-cadherin and α-SMA proteins, concomitant treatment with metformin and E2 increased the expression of CK 8 and Snail proteins and decreased the expression of Claudin 1, ZO-1, Slug and α-SMA proteins. Our findings suggest that the EMT contributes to the switch from a healthy state to a PCOS state in the endometrium, which might subsequently drive endometrial injury and dysfunction. We also provide evidence that metformin differentially modulates EMT protein expression in PCOS patients depending on oestrogenic stimulation.