Inflammatory Cytokine-Induced HIF-1 Activation Promotes Epithelial–Mesenchymal Transition in Endometrial Epithelial Cells

The endometrium undergoes repeated proliferation and shedding during the menstrual cycle. Significant changes to this environment include fluctuations in the partial pressure of oxygen, exposure to a high-cytokine environment associated with intrauterine infection, and inflammation. Chronic endometritis is a condition wherein mild inflammation persists in the endometrium and is one of the causes of implantation failure and miscarriage in early pregnancy. It is thought that the invasion of embryos into the endometrium requires epithelial-mesenchymal transition (EMT)-associated changes in the endometrial epithelium. However, the effects of inflammation on the endometrium remain poorly understood. In this study, we investigated the effects of the intrauterine oxygen environment, hypoxia-inducible factor (HIF), and inflammation on the differentiation and function of endometrial epithelial cells. We elucidated the ways in which inflammatory cytokines affect HIF activity and EMT in an immortalized cell line (EM-E6/E7/TERT) derived from endometrial epithelium. Pro-inflammatory cytokines caused significant accumulation of HIF-1α protein, increased HIF-1α mRNA levels, and enhanced hypoxia-induced accumulation of HIF-1α protein. The combined effect of inflammatory cytokines and hypoxia increased the expression of EMT-inducing factors and upregulated cell migration. Our findings indicate that pro-inflammatory factors, including cytokines and LPS, work synergistically with hypoxia to activate HIF-1 and promote EMT in endometrial epithelial cells.