Estrogen Rapidly Activates the PI3K/AKT Pathway and Hypoxia-Inducible Factor 1 and Induces Vascular Endothelial Growth Factor A Expression in Luminal Epithelial Cells of the Rat Uterus

We have previously shown that 17beta-estradiol (E-2) increases vascular endothelial growth factor A (Vegfa) gene expression in the rat uterus, resulting in increased microvascular permeability, and that this involves the simultaneous recruitment of hypoxia-inducible factor 1 (HIF1) and estrogen receptor alpha (ESR1) to the Vegfa gene promoter. Both events require the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (AKT) pathway. However, those studies were carried out using whole uterine tissue, and while most evidence indicates that the likely site of E-2-induced Vegfa expression is luminal epithelial (LE) cells, other studies have identified stromal cells as the site of that expression. To address this question, the pathway regulating Vegfa expression was reexamined using LE cells rapidly isolated after E-2 treatment. In addition, we further characterized the nature of the receptor through which E-2 triggers the signaling events that lead to Vegfa expression using the specific ESR1 antagonist ICI 182,780. In agreement with previous results in the whole uterus, E-2 stimulated Vegfa mRNA expression in LE cells, peaking at 1 h (4- to 14-fold) and returning to basal levels by 4 h. Treatment with E-2 also increased phosphorylation of AKT in LE cells, as well as of the downstream mediators FRAP1 (mTOR), GSK3B, and MDM2. The alpha subunit of HIF1 (HIF1A) was present in LE cells before E-2 treatment, was unchanged 1 h after E-2, but was >2-fold higher by 4 h. Chromatin immunoprecipitation analysis showed that HIF1A was recruited to the Vegfa promoter by 1 h and was absent again by 4 h. The E-2 activation of the PI3K/AKT pathway, HIF1A recruitment to the Vegfa promoter, and Vegfa expression were all blocked by ICI 182,780. In summary, the rapid E-2-induced signaling events that lead to the expression of Vegfa observed previously using the whole uterus occur in LE cells and appear to be initiated via a membrane form of ESR1.