The extracellular signal-regulated kinase 1/2 triggers angiogenesis in human ectopic endometrial implants by inducing angioblast differentiation and proliferation

Problem: The role of extracellular signal-regulated kinase (ERK) 1/2-mediated angiogenesis during endometriotic nidation is unknown. We posit that ERK1/2-induced angioblast differentiation and proliferation promotes ectopic endometrial angiogenesis. Methods of study: Human eutopic and ectopic endometria were immunostained for total-(T-) or phosphorylated-(P-) ERK1/2 or double-immunostained for P-ERK1/2-CD34 and PCNA-CD34. Estradiol (E-2), cytokines, normal peritoneal fluid (NPF) or endometriotic peritoneal fluid (EPF) +/- PD98059, an ERK1/2 inhibitor, treaded primary human endometrial endothelial cells (HEECs) were evaluated by T-/P-ERK1/2 immunoblotting, MTT viability and tube formation assays. Results: HEECs exhibited higher endothelial P-ERK1/2 immunoreactivity in ectopic vs eutopic endometria. Double--immunostained ectopic endometria displayed abundant CD34-positive angioblasts exhibiting strong P-ERK1/2 and PCNA immunoreactivity. EPF and vascular growth factor (VEGF)-A significantly increased HEEC proliferation and P-ERK1/2 levels. PD98059 reduced basal, EPF, and VEGF-induced HEEC proliferation and promoted vascular stabilization following tube formation. Conclusion: Enhanced ERK1/2 activity in angioblasts by such peritoneal factors as VEGF, E2 induces proliferation to trigger ectopic endometrial angiogenesis.