Endothelial Surface N-Glycans Mediate Monocyte Adhesion and Are Targets for Anti-inflammatory Effects of Peroxisome Proliferator-activated Receptor γ Ligands

Endothelial-monocyte interactions are regulated by adhesion molecules and key in the development of vascular inflammatory disease. Peroxisome proliferator-activated receptor (PPAR) gamma activation in endothelial cells is recognized to mediate anti-inflammatory effects that inhibit monocyte rolling and adhesion. Herein, evidence is provided for a novel mechanism for the anti-inflammatory effects of PPAR gamma ligand action that involves inhibition of proinflammatory cytokine-dependent up-regulation of endothelial N-glycans. TNF alpha treatment of human umbilical vein endothelial cells increased surface expression of high mannose/hybrid N-glycans. A role for these sugars in mediating THP-1 or primary human monocyte rolling and adhesion was indicated by competition studies in which addition of alpha-methylmannose, but not alpha-methylglucose, inhibited monocyte rolling and adhesion during flow, but not under static conditions. This result supports the notion that adhesion molecules provide scaffolds for sugar epitopes to mediate adhesion with cognate receptors. A panel of structurally distinct PPAR gamma agonists all decreased TNF alpha-dependent expression of endothelial high mannose/hybrid N-glycans. Using rosiglitazone as a model PPAR gamma agonist, which decreased TNF alpha-induced high mannose N-glycan expression, we demonstrate a role for these carbohydrate residues in THP-1 rolling and adhesion that is independent of endothelial surface adhesion molecule expression (ICAM-1 and E-selectin). Data from N-glycan processing gene arrays identified alpha-mannosidases (MAN1A2 and MAN1C1) as targets for down-regulation by TNF alpha, which was reversed by rosiglitazone, a result consistent with altered high mannose/hybrid N-glycan epitopes. Taken together we propose a novel anti-inflammatory mechanism of endothelial PPAR gamma activation that involves targeting protein post-translational modification of adhesion molecules, specifically N-glycosylation.