Vascular endothelial growth factor and angiopoietin-1 regulate leukocyte adhesion to endothelial cells through the nuclear receptor Nur77

Vascular endothelial growth factor (VEGF) and angiopoietin 1 (Ang-1) are critical regulators of angiogenesis. Additionally, both have been found to participate in inflammatory processes: VEGF as a pro-inflammatory and Ang-1 as an anti-inflammatory mediator. Nur77 is a member of a family of orphan receptors (NR4A) that includes Nurr1 and Nor1 and plays a role in regulating vascular inflammation; however, this has yet to be fully understood. The aim of this study was to evaluate whether Nur77 expression in endothelial cells (ECs) serves as a negative feedback mechanism designed to inhibit NFkappaB induction, dampen VEGF-induced E-selectin and VCAM1 expression and enhanced leukocyte adhesion to ECs, and mediate the suppression of EC activation induced by Ang-1 treatment.Treatment of human umbilical vein endothelial cells (HUVECs) with either VEGF or Ang-1 significantly and transiently induced Nur77 expression and enhanced PKD-dependent HDAC7 phosphorylation and mobilization from the nucleus to the cytosol. HUVECs transduced with adenoviruses expressing mutated HDAC7 or a dominant-negative PKD1 inhibited VEGF-, but not Ang-1-, induced Nur77 expression. Inhibition of PI3K and ERK1/2 resulted in the suppression of Ang-1-induced Nur77 expression whereas the inhibition of JNK resulted in significantly greater induction of Nur77 by Ang-1. NFκB binding activity and gel shift assays revealed that Nur77 inhibits VEGF-induced NFκB activity. Overexpression of Nur77 showed titre-dependent upregulation of IκBα mRNA and protein expressions that was not evident in HUVECs transduced with viruses expressing a dominant-negative form of Nur77 (Ad-dnNur77). Functionally, Nur77 was found to suppress VEGF-induced mRNA and protein expressions of the adhesion molecules E-selectin and VCAM1. Importantly, the role of Nur77 in cytokine-induced leukocyte adhesion to ECs was examined. Adherence of U937 cells to HUVECs activated by VEGF was suppressed by overexpressing Nur77 whereas the loss of Nur77 by siRNA interference resulted in augmentation of adhesion. Interestingly, Ang1 was able to dampen VEGF-induced monocyte adhesion to HUVEC monolayers. I conclude that Nur77 plays an important role in providing a negative feedback mechanism designed to attenuate VEGF-induced pro-inflammatory responses through selective inhibition of NFκB activation. Furthermore, Nur77, in part, may be vital to the Ang-1 anti-inflammatory response.