Serine 26 in Early Growth Response-1 Is Critical for Endothelial Proliferation, Migration, and Network Formation

Background Vascular endothelial cell proliferation, migration, and network formation are key proangiogenic processes involving the prototypic immediate early gene product, Egr-1 (early growth response-1). Egr-1 undergoes phosphorylation at a conserved Ser26 but its function is completely unknown in endothelial cells or any other cell type. Methods and Results A CRISPR/Cas9 strategy was used to introduce a homozygous Ser26>Ala mutation into endogenous Egr-1 in human microvascular endothelial cells. In the course of generating mutant cells, we produced cells with homozygous deletion in Egr-1 caused by frameshift and premature termination. We found that Ser26 mutation in Egr-1, or Egr-1 deletion, perturbed endothelial cell proliferation in models of cell counting or real-time growth using the xCELLigence System. We found that Ser26 mutation or Egr-1 deletion ameliorated endothelial cell migration toward VEGF-A₁₆₅ (vascular endothelial growth factor-A) in a dual-chamber model. On solubilized basement membrane preparations, Ser26 mutation or Egr-1 deletion prevented endothelial network (or tubule) formation, an in vitro model of angiogenesis. Flow cytometry further revealed that Ser26 mutation or Egr-1 deletion elevated early and late apoptosis. Finally, we demonstrated that Ser26 mutation or Egr-1 deletion increased VE-cadherin (vascular endothelial cadherin) expression, a regulator of endothelial adhesion and signaling, permeability, and angiogenesis. Conclusions These findings not only indicate that Egr-1 is essential for endothelial cell proliferation, migration, and network formation, but also show that point mutation in Ser26 is sufficient to impair each of these processes and trigger apoptosis as effectively as the absence of Egr-1. This highlights the importance of Ser26 in Egr-1 for a range of proangiogenic processes.