Endothelial lipase and HDL function

Cardiovascular diseases are the major cause of death worldwide. The maintenance of vascular homeostasis and vascular health is largely dependent on preserved endothelial function. The major task of vascular endothelium is the maintenance of the normal vascular tone, largely dependent on the balance between endothelium-derived relaxing and contracting factor. It is well established that high- density lipoprotein (HDL) exerts direct beneficial effect on the vascular endothelium, primarily by stimulation of nitric oxide (NO) and inhibition of oxidation and inflammation. Although decreased HDL plasma levels are a major risk factor for coronary artery disease (CAD), accumulating evidence suggest that the functionality of HDL, which is markedly impaired in various pathologies, is an important predictor of HDL atheroprotective capacity. Among various cellular and plasma factors, endothelial lipase (EL) is a strong negative regulator of HDL plasma levels and a modulator of HDL lipid composition. Interestingly, EL plasma levels and activities are increased in the pathologies associated with impaired HDL functionality such as type 2 diabetes, subclinical atherosclerosis, end-stage renal disease or acute inflammation. Although the role of EL in HDL metabolism and vascular inflammatory response is well established, nothing is known whether and how EL modification of HDL impacts the endothelial function of HDL, in particular the capacity to promote NO and vasorelaxation. Based on our preliminary work showing markedly diminished vasorelaxing capacity of HDL isolated from EL-overexpressing mice, we hypothesize that EL by altering the lipid and protein composition of HDL generates dysfunctional HDL with markedly diminished endothelial function. To test our hypothesis the following aims are defined: 1. To assess lipid and protein composition of EL-modified HDL (EL-HDL). 2. To study the capacity of EL-HDL to induce eNOS activating pathway, NO production and vasorelaxation. 3. To examine whether scavenger receptor B type I (SR-BI)- or sphingosine-1-phosphate (S1P) receptor-dependent endothelial activities of HDL are affected by EL modification. 4. To assess the capacity of EL-HDL to prevent cholesterol/oxysterol-induced endothelial dysfunction. 5. To examine antioxidant and anti-inflammatory capacity of EL-HDL. 6. To investigate relationship between EL serum levels and HDL functionality in CAD patients. We believe that the outcome of the proposed work will largely contribute to our understanding whether and how EL affects endothelial-protective activities of HDL. The discovery of novel mechanisms that promote endothelial dysfunction might provide new targets for therapeutic intervention in prevention of atherosclerosis.

We found that a serum protein, endothelial lipase (EL), by altering composition of a serum constituent, called high-density lipoprotein (HDL), better known as good cholesterol, modulates the blood vessel protective function of HDL. Blood vessel health is largely dependent on the preserved function of vascular endothelial cells. These cells serve as a barrier between the blood and blood vessel wall and are principal regulator of blood vessel function. Endothelial dysfunction, caused primarily by high blood pressure, high plasma sugar or lipid levels, or smoking, is the earliest sign of impaired blood vessel function. If not recognized and treated, endothelial dysfunction progresses into atherosclerosis and clinical manifestations such as myocardial infarction, heart failure or stroke. Healthy HDL exerts beneficial effects on vascular endothelial cells. To study how EL by altering HDL composition alters HDL function we isolated HDL from human plasma and modified isolated HDL with EL. We then examined size, composition, and functional features of EL-modified HDL. We found that EL decreased HDL size and markedly altered lipid and protein composition of HDL. Interestingly, different tests revealed that these EL-induced changes in size and composition translated into both deterioration and augmentation of the protective functions of HDL. Namely, EL decreased the capacity of HDL to collect cholesterol from cells and EL- increased concomitantly the capacity of HDL to act as an antioxidant and to induce endothelial production of nitric oxide, the activities crucial for the maintenance of blood vessel health. Accordingly, our results show that EL negatively and positively affects HDL function and in turn vascular health. Furthermore, we examined the role of EL and HDL in acute heart failure (AHF), a leading cause of hospitalization and mortality worldwide. Adequate risk estimation in AHF is crucial for proper in-hospital and post-discharge treatment, which largely determines the rate of mortality and re-hospitalization. Despite established clinical signs and serum biomarkers, the estimation of risk in AHF is difficult, inaccurate and poorly applicable in daily clinical practice. Therefore, the identification of new biomarkers may improve the risk assessment and thus help physicians in initiating appropriate therapeutic interventions. We collected blood from 152 hospitalized AHF patients at admission and performed detailed analyses of serum constituents including EL and HDL. We found a positive association of EL with serum levels of small HDL particles. Importantly, we also found that low serum concentrations of small HDL-particles in serum obtained at admission i.e. immediately after onset of AHF, strongly predict 3-month mortality in AHF patients. Inclusion of small HDL particles, which most probably are generated by EL, into routine biochemical diagnostics of AHF may improve recognizing patients at risk, thus help initiating adequate therapy and decreasing mortality.