Endothelial lipase is a major determinant of HDL level
For the past three decades, epidemiologic studies have consistently demonstrated an inverse relationship between plasma HDL cholesterol (HDL-C) concentrations and coronary heart disease (CHD). Population-based studies have provided compelling evidence that low HDL-C levels are a risk factor for CHD, and several clinical interventions that increased plasma levels of HDL-C were associated with a reduction in CHD risk. These findings have stimulated extensive investigation into the determinants of plasma HDL-C levels. Turnover studies using radiolabeled apolipoprotein A-I, the major protein component of HDL, suggest that plasma HDL-C concentrations are highly correlated with the rate of clearance of apolipoprotein AI. However, the metabolic mechanisms by which HDL are catabolized have not been fully defined. Previous studies in humans with genetic deficiency of cholesteryl ester transfer protein, and in mice lacking the scavenger receptor BI (SR-BI), have demonstrated that these proteins participate in the removal of cholesterol from HDL, while observations in individuals with mutations in hepatic lipase indicate that this enzyme hydrolyzes HDL triglycerides. In this issue of the JCI, reports from laboratories of Tom Quertermous and Dan Rader now indicate that endothelial lipase (LIPG), a newly identified member of the lipase family, catalyzes the hydrolysis of HDL phospholipids and facilitates the clearance of HDL from the circulation. Endothelial lipase was initially cloned by both of these laboratories using entirely different strategies. Quertermous and his colleagues identified endothelial lipase as a transcript that was upregulated in cultured human umbilical vein endothelial cells undergoing tube formation, whereas the Rader group cloned endothelial lipase as a transcript that was upregulated in the human macrophage-like cell line THP-1 exposed to oxidized LDL. Database searches revealed that endothelial lipase shows strong sequence similarity to lipoprotein lipase (44 percent identity) and hepatic lipase (41 percent identity), two well-characterized lipases that function at vascular endothelial surfaces. Critical motifs associated with lipase activity (GXSXG and the catalytic triad S169, D193, H274), and with heparin binding were strongly conserved. Interestingly, in contrast to both lipoprotein lipase and hepatic lipase, endothelial lipase has little triglyceride hydrolase activity in vitro but instead cleaves fatty acids from the sn-1 position of phosphatidylcho-line. In in vitro assays the enzyme is most active on lipids presented in HDL, although it will release fatty acids from all classes of lipoproteins. Consistent with this finding, adenovirus-mediated overexpression of endothelial lipase in LDL receptor-deficient mice reduced plasma concentrations of VLDL and LDL cholesterol by about 50 percent, whereas HDL-C decreased to almost zero in these animals. These data suggested that endothelial lipase may play a role in HDL catabolism.
- Research Organization:
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Director, Office of Science. Office of Biological and Environmental Research. Life Sciences Division (US)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 812460
- Report Number(s):
- LBNL-52875; JCINAO; R&D Project: 864T1A; TRN: US200314%%85
- Journal Information:
- Journal of Clinical Investigation, Vol. 111, Issue 3; Other Information: Journal Publication Date: February 2003; PBD: 30 Jan 2003; ISSN 0021-9738
- Country of Publication:
- United States
- Language:
- English
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