Abstract 629: Sensing Nutrient Overload: The Role of the Endothelium

Abstract

Recent data suggest that leukocyte accumulation into visceral fat depots occurs within 72 following high fat feeding. At this early stage, neutrophils have been found to be the predominant leukocyte population to infiltrate the visceral fat. Other studies have shown that deletion of the high affinity counter-receptor for P-selectin, P-selectin glycoprotein ligand-1 (PSGL-1), attenuate immune cell infiltration into fat depots of chronically high fat fed mice. Accordingly, we tested the hypothesis that endothelial, expressed P-selectin serves as a molecular sensor for initiating inflammatory responses in the microcirculation of the visceral fat following nutrient overload. Wild-type (WT) and P-sel-/- C57BL/6 mice were given either a low fat (10% fat) or a high fat (60% fat) meal by gavage. All mice were studied 1, 2, 3, 4 and 24 hours post gavage by intravital microscopy (IVM) to measure postprandial kinetics of leukocyte rolling (LR) and leukocyte adhesion (LA) in the microcirculation of the mesenteric fat pads. In WT mice, administration of high fat meals acutely and transiently increased LR and LA in the visceral fat microcirculation (p<0.001 vs low fat meal). In contrast, high fat meals failed to increase LR and LA in P-sel-/- mice (NS vs low fat meal WT). Immunofluorescence studies demonstrated that fat meals upregulate cell surface expression of P-selectin in the visceral fat microcirculation, but not in the subcutaneous one. Flow cytometry studies demonstrated that neutrophils are the main leukocyte population to acutely infiltrate visceral fat depots after lipid overload. Mechanistic studies were also undertaken using palmitate, a postprandially elevated, unsaturated free fatty acid. Superfusion of (dose) palmitate during IVM acutely increased LR and LA in the microcirculation of the mesenteric fat pads of WT mice (p<0.01 vs control), a response that was prevented in vivo by P-selectin antibody blockade. Taken together, these data demonstrate that endothelial P-selectin plays a key role in the early inflammatory responses to fat overload by promoting neutrophil trafficking in the microcirculation of the visceral fat. These data first uncover a novel nutrient-sensory role of the vascular endothelium, which warrants further investigation.