High-density lipoprotein hydrolysis by endothelial lipase activates PPARalpha: a candidate mechanism for high-density lipoprotein-mediated repression of leukocyte adhesion.
Although high-density lipoprotein (HDL) is known to inhibit endothelial adhesion molecule expression, the mechanism for this anti-inflammatory effect remains obscure. Surprisingly, we observed that HDL no longer decreased adhesion of U937 monocytoid cells to tumor necrosis factor (TNF)alpha-stimulated human endothelial cells (EC) in the presence of the general lipase inhibitor tetrahydrolipstatin. In considering endothelial mechanisms responsible for this effect, we found that endothelial lipase ( EL) overexpression in both EC and non- EL- expressing NIH/3T3 mouse embryonic fibroblasts cells significantly decreased TNFalpha- induced VCAM1 expression and promoter activity in a manner dependent on HDL concentration and intact EL activity. Given recent evidence for lipolytic activation of peroxisome proliferator-activated receptors (PPARs)-nuclear receptors implicated in metabolism, atherosclerosis, and inflammation-we hypothesized HDL hydrolysis by EL is an endogenous endothelial mechanism for PPAR activation. In both EL-transfected NIH cells and bovine EC, HDL significantly increased PPAR ligand binding domain activation in the order PPAR-alpha> >-gamma>-delta. Moreover, HDL stimulation induced expression of the canonical PPARalpha- target gene acyl-CoA-oxidase (ACO) in a PPARalpha-dependent manner in ECs. Conditioned media from EL-adenovirus transfected cells but not control media exposed to HDL also activated PPARalpha. PPARalpha activation by EL was most potent with HDL as a substrate, with lesser effects on LDL and VLDL. Finally, HDL inhibited leukocyte adhesion to TNFalpha- stimulated ECs isolated from wild-type but not PPARalpha-deficient mice. This data establishes HDL hydrolysis by EL as a novel, distinct natural pathway for PPARalpha activation and identifies a potential mechanism for HDL- mediated repression of VCAM1 expression, with significant implications for both EL and PPARs in inflammation and vascular biology.[1]References
- High-density lipoprotein hydrolysis by endothelial lipase activates PPARalpha: a candidate mechanism for high-density lipoprotein-mediated repression of leukocyte adhesion. Ahmed, W., Orasanu, G., Nehra, V., Asatryan, L., Rader, D.J., Ziouzenkova, O., Plutzky, J. Circ. Res. (2006) [Pubmed]
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