Liana Asatryan

Lipolysis of triglyceride-rich lipoproteins generates PPAR ligands: Evidence for an antiinflammatory role for lipoprotein lipase

Increased levels of triglyceride-rich lipoproteins provoke lipid accumulation in the artery wall, triggering early inflammatory responses central to atherosclerosis like endothelial adhesion molecule expression. The endogenous mechanisms limiting such reactions remain poorly defined. Lipoprotein lipase (LPL) plays a central role in lipid metabolism by hydrolyzing triglyceride rich lipoproteins and releasing fatty acids. We found that LPL treatment reversed tumor necrosis factor α and very low-density lipoprotein (VLDL)-stimulated endothelial vascular cell adhesion molecule 1 (VCAM1) induction and VCAM1 promoter responses, thus recapitulating effects reported with synthetic peroxisome proliferator-activated receptor (PPAR) agonists. In fact, these LPL effects on VCAM1 were absent in endothelial cells isolated from PPARα-deficient mice. This finding suggests a novel antiinflammatory role for LPL. Further studies reveal specificity for PPAR activation through lipolysis in regards to lipoprotein substrate (VLDL ≫ LDL > HDL), PPAR isoform (PPARα ≫ PPARδ > PPARγ), and among fatty acid-releasing lipases. These PPAR responses required intact LPL catalytic activity. In vivo, transgenic mice overexpressing LPL had increased peroxisome proliferation, but not in the genetic absence of PPARα. Although human plasma possesses minimal PPARα activation despite containing abundant free fatty acids, marked PPARα activation is seen with human plasma after LPL is added in vitro or systemically released in vivo. These data suggest a previously uncharacterized pathway in which the key lipolytic enzyme LPL can act on circulating lipoproteins to generate PPARα ligands, providing a potentially important link between lipoprotein metabolism and distal PPARα transcriptional effects.

High-Density Lipoprotein Hydrolysis by Endothelial Lipase Activates PPARα. 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)&agr;-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 TNF&agr;-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-&agr;≫-&ggr;>-&dgr;. Moreover, HDL stimulation induced expression of the canonical PPAR&agr;-target gene acyl-CoA-oxidase (ACO) in a PPAR&agr;-dependent manner in ECs. Conditioned media from EL-adenovirus transfected cells but not control media exposed to HDL also activated PPAR&agr;. PPAR&agr; activation by EL was most potent with HDL as a substrate, with lesser effects on LDL and VLDL. Finally, HDL inhibited leukocyte adhesion to TNF&agr;-stimulated ECs isolated from wild-type but not PPAR&agr;-deficient mice. This data establishes HDL hydrolysis by EL as a novel, distinct natural pathway for PPAR&agr; 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.

Dual Roles for Lipolysis and Oxidation in Peroxisome Proliferation-Activator Receptor Responses to Electronegative Low Density Lipoprotein

Low density lipoprotein (LDL) exists in various forms that possess unique characteristics, including particle content and metabolism. One circulating subfraction, electronegative LDL (LDL(–)), which is increased in familial hypercholesterolemia and diabetes, is implicated in accelerated atherosclerosis. Cellular responses to LDL(–) remain poorly described. Here we demonstrate that LDL(–) increases tumor necrosis factor α (TNFα)–induced inflammatory responses through NFκB and AP-1 activation with corresponding increases in vascular cell adhesion molecule-1 (VCAM1) expression. LDL receptor overexpression increased these effects. In contrast, exposing LDL(–) to the key lipolytic enzyme lipoprotein lipase (LPL) reversed these responses, inhibiting VCAM1 below levels seen with TNFα alone. LPL is known to act on lipoproteins to generate endogenous peroxisomal proliferator-activated receptor α (PPARα) ligand, thus limiting inflammation. These responses varied according to the lipoprotein substrate triglyceride content (very low density lipoprotein ≫ LDL > high density lipoprotein). The PPARα activation seen with LDL, however, was disproportionately high. We show here that MUT LDL activates PPARα to an extent proportional to its LDL(–) content. As compared with LDL(–) alone, LPL-treated LDL(–) increased PPARα activation 20-fold in either cell-based transfection or radioligand displacement assays. LPL-treated LDL(–) suppressed NFκB and AP-1 activation, increasing expression of the PPARα target gene IκBα, although only in the genetic presence of PPARα and with intact LPL hydrolysis. Mass spectrometry reveals that LPL-treatment of either LDL or LDL(–) releases hydroxy-octadecadienoic acids (HODEs), potent PPARα activators. These findings suggest LPL-mediated PPARα activation as an alternative catabolic pathway that may limit inflammatory responses to LDL(–).

Low Density Lipoprotein (LDL) Modification: Basic Concepts and Relationship to Atherosclerosis

A large number of clinical studies support the hypothesis that the risk for atherosclerosis is associated with the proportion of different LDL subfractions in blood. Electronegatively modified forms of LDL (LDL–) isolated using different chromatographic techniques are characterised by significant differences in the protein and lipid content as compared to the native LDL subfraction. LDL– composition appears to influence its atherogenic properties as well as its high susceptibility to oxidation and impaired metabolism. Increased LDL– levels are found in subjects with coronary artery disease, particularly in diabetics and patients undergoing haemodialysis (HD). Whether elevated LDL– levels are due to the LDL oxidation in blood remains disputed despite the oxidative character of LDL– modification. Plausible means for LDL– formation in blood include glycation and protein-radical interactions with ApoB 100. The latter can prevail during HD as observed in in vitro studies using a model HD system. The rapid and progressive formation of LDL– during standard HD can be significantly reduced employing haemolipodialysis (HLD), which provides local delivery of specific antioxidants (vitamin E and C) to blood at concentrations above normal physiologic levels. This procedure appears to be more effective than oral supplementation with antioxidants and may be a promising approach to reducing the rapid progression of atherosclerosis in HD patients.

Loop 2 Structure in Glycine and GABAA Receptors Plays a Key Role in Determining Ethanol Sensitivity

The present study tests the hypothesis that the structure of extracellular domain Loop 2 can markedly affect ethanol sensitivity in glycine receptors (GlyRs) and γ-aminobutyric acid type A receptors (GABAARs). To test this, we mutated Loop 2 in the α1 subunit of GlyRs and in the γ subunit of α1β2γ2GABAARs and measured the sensitivity of wild type and mutant receptors expressed in Xenopus oocytes to agonist, ethanol, and other agents using two-electrode voltage clamp. Replacing Loop 2 of α1GlyR subunits with Loop 2 from the δGABAAR (δL2), but not the γGABAAR subunit, reduced ethanol threshold and increased the degree of ethanol potentiation without altering general receptor function. Similarly, replacing Loop 2 of the γ subunit of GABAARs with δL2 shifted the ethanol threshold from 50 mm in WT to 1 mm in the GABAA γ-δL2 mutant. These findings indicate that the structure of Loop 2 can profoundly affect ethanol sensitivity in GlyRs and GABAARs. The δL2 mutations did not affect GlyR or GABAAR sensitivity, respectively, to Zn2+ or diazepam, which suggests that these δL2-induced changes in ethanol sensitivity do not extend to all allosteric modulators and may be specific for ethanol or ethanol-like agents. To explore molecular mechanisms underlying these results, we threaded the WT and δL2 GlyR sequences onto the x-ray structure of the bacterial Gloeobacter violaceus pentameric ligand-gated ion channel homologue (GLIC). In addition to being the first GlyR model threaded on GLIC, the juxtaposition of the two structures led to a possible mechanistic explanation for the effects of ethanol on GlyR-based on changes in Loop 2 structure.

Ivermectin Antagonizes Ethanol Inhibition in Purinergic P2X4 Receptors

ATP-gated purinergic P2X4 receptors (P2X4Rs) are expressed in the central nervous system and are sensitive to ethanol at intoxicating concentrations. P2XRs are trimeric; each subunit consists of two transmembrane (TM) α-helical segments, a large extracellular domain, and intracellular amino and carboxyl terminals. Recent work indicates that position 336 (Met336) in the TM2 segment is critical for ethanol modulation of P2X4Rs. The anthelmintic medication ivermectin (IVM) positively modulates P2X4Rs and is believed to act in the same region as ethanol. The present study tested the hypothesis that IVM can antagonize ethanol action. We investigated IVM and ethanol effects in wild-type and mutant P2X4Rs expressed in Xenopus oocytes by using a two-electrode voltage clamp. IVM antagonized ethanol-induced inhibition of P2X4Rs in a concentration-dependent manner. The size and charge of substitutions at position 336 affected P2X4R sensitivity to both ethanol and IVM. The first molecular model of the rat P2X4R, built onto the X-ray crystal structure of zebrafish P2X4R, revealed a pocket formed by Asp331, Met336, Trp46, and Trp50 that may play a role in the actions of ethanol and IVM. These findings provide the first evidence for IVM antagonism of ethanol effects in P2X4Rs and suggest that the antagonism results from the ability of IVM to interfere with ethanol action on the putative pocket at or near position 336. Taken with the building evidence supporting a role for P2X4Rs in ethanol intake, the present findings suggest that the newly identified alcohol pocket is a potential site for development of medication for alcohol use disorders.

Ivermectin reduces alcohol intake and preference in mice.

The high rate of therapeutic failure in the management of alcohol use disorders (AUDs) underscores the urgent need for novel and effective strategies that can deter ethanol consumption. Recent findings from our group showed that ivermectin (IVM), a broad-spectrum anthelmintic with high tolerability and optimal safety profile in humans and animals, antagonized ethanol-mediated inhibition of P2X4 receptors (P2X4Rs) expressed in Xenopus oocytes. This finding prompted us to hypothesize that IVM may reduce alcohol consumption; thus, in the present study we investigated the effects of this agent on several models of alcohol self-administration in male and female C57BL/6 mice. Overall, IVM (1.25-10 mg/kg, intraperitoneal) significantly reduced 24-h alcohol consumption and intermittent limited access (4-h) binge drinking, and operant alcohol self-administration (1-h). The effects on alcohol intake were dose-dependent with the significant reduction in intake at 9 h after administration corresponding to peak IVM concentrations (C(max)) in the brain. IVM also produced a significant reduction in 24-h saccharin consumption, but did not alter operant sucrose self-administration. Taken together, the findings indicate that IVM reduces alcohol intake across several different models of self-administration and suggest that IVM may be useful in the treatment of AUDs.

A point mutation in the ectodomain‐transmembrane 2 interface eliminates the inhibitory effects of ethanol in P2X4 receptors

J. Neurochem. (2010) 112, 307–317.

Oxidative stress during ex vivo hemodialysis of blood is decreased by a novel hemolipodialysis procedure utilizing antioxidants.

The high cardiovascular mortality in patients receiving hemodialysis (HD) has been attributed, in part, to oxidative stress. Here we examined the effectiveness of antioxidants introduced by means of a novel hemolipodialysis (HLD) procedure in terms of reducing oxidative stress during ex vivo blood circulation. Oxidative stress was studied in a model HD system resembling the extracorporeal circulation of blood during clinical HD. Blood circulation produced an increase of up to 280% in free hemoglobin levels and an increase of 320% in electronegative LDL (LDL(-)) subfraction. A significant correlation between LDL(-) and free hemoglobin levels confirmed previous findings that LDL(-) formation during ex vivo circulation of blood can be mediated by the oxidative activity of free hemoglobin. These effects were significantly attenuated during HLD using a dialysis circuit containing vitamin E with or without vitamin C. By contrast, HLD with vitamin C alone had a marked pro-oxidant effect. TBARS, lipid hydroperoxides, vitamin E and beta-carotene content in LDL were not significantly altered by the HD procedure. These findings demonstrate the occurrence of oxidative stress in human plasma where lipoproteins are a target and indicate antioxidant-HLD treatment as a specific new approach to decreasing the adverse oxidative stress frequently associated with cardiovascular complications in high-risk populations of uremic patients.

LDL protein nitration : Implication for LDL protein unfolding

Oxidatively- or enzymatically-modified low-density lipoprotein (LDL) is intimately involved in the initiation and progression of atherosclerosis. The in vivo modified LDL is electro-negative (LDL(-)) and consists of peroxidized lipid and unfolded apoB-100 protein. This study was aimed at establishing specific protein modifications and conformational changes in LDL(-) assessed by liquid chromatography/tandem mass spectrometry (LC/MS/MS) and circular dichroism analyses, respectively. The functional significance of these chemical modifications and structural changes were validated with binding and uptake experiments to- and by bovine aortic endothelial cells (BAEC). The plasma LDL(-) fraction showed increased nitrotyrosine and lipid peroxide content as well as a greater cysteine oxidation as compared with native- and total-LDL. LC/MS/MS analyses of LDL(-) revealed specific modifications in the apoB-100 moiety, largely involving nitration of tyrosines in the alpha-helical structures and beta(2) sheet as well as cysteine oxidation to cysteic acid in beta(1) sheet. Circular dichroism analyses showed that the alpha-helical content of LDL(-) was substantially lower ( approximately 25%) than that of native LDL ( approximately 90%); conversely, LDL(-) showed greater content of beta-sheet and random coil structure, in agreement with unfolding of the protein. These results were mimicked by treatment of LDL subfractions with peroxynitrite (ONOO(-)) or SIN-1: similar amino acid modifications as well as conformational changes (loss of alpha-helical structure and gain in beta-sheet structure) were observed. Both LDL(-) and ONOO(-)-treated LDL showed a statistically significant increase in binding and uptake to- and by BAEC compared to native LDL. We further found that most binding and uptake in control-LDL was through LDL-R with minimal oxLDL-R-dependent uptake. ONOO(-)-treated LDL was significantly bound and endocytosed by LOX-1, CD36, and SR-A with minimal contribution from LDL-R. It is suggested that lipid peroxidation and protein nitration may account for the mechanisms leading to apoB-100 protein unfolding and consequential increase in modified LDL binding and uptake to and by endothelial cells that is dependent on oxLDL scavenger receptors.