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S Parthasarathy

Publications and source records attributed to S Parthasarathy.

At least 19 recordsLinked to original sources
PubMed1992

Evidence for a concerted reaction between lipid hydroperoxides and polypeptides.

The events accompanying oxidative modification of low density lipoprotein (LDL) are multiple and complex, and the precise mechanisms remain to be determined. In the present studies, we examined a simple system in which we first prepared large amounts of lipid hydroperoxides (from linoleic acid or from phospholipids containing linoleic acid) by using soybean lipoxygenase (linoleate:oxygen oxidoreductase, EC 1.13.11.12). Linoleoyl hydroperoxide was then incubated with polypeptides in the absence of metal ions. We observed the generation of fluorescent products with a spectrum like that of oxidized LDL. The generation of fluorescent products from incubation of polypeptides with linoleoyl hydroperoxide was manyfold greater than that generated on incubation with preformed 4-hydroxynonenal at the same concentration. Superoxide dismutase (superoxide: superoxide oxidoreductase, EC 1.15.1.1) had no effect on the generation of fluorescent products. Incubation of linoleoyl hydroperoxide with cytochrome c (cyt c) under the same conditions led to progressive reduction of cyt c at a rate determined by the initial linoleoyl hydroperoxide concentration. This reduction was not significantly inhibited by probucol but was inhibited, although never completely, by superoxide dismutase. Even at 100 micrograms/ml, superoxide dismutase inhibited by only 65%. From these results, we are led to suggest a concerted reaction between the peroxy radical and free amino groups of polypeptides or phosphatidylethanolamine to generate fluorescent adducts. During oxidation of LDL or of cell membranes, this mechanism may occur side by side with the conventional Schiff base mechanism.

Cytochrome c Group↗
PubMed1992

Regulated expression of the human acetylated low density lipoprotein receptor gene and isolation of promoter sequences.

The acetylated low density lipoprotein (AcLDL) receptor is expressed on tissue macrophages after their differentiation from monocyte precursors and has been proposed to play a role in the generation of foam cells in atherosclerotic lesions. In the present studies, THP-1 human monocytic leukemia cells were used to investigate mechanisms responsible for expression of the AcLDL receptor gene after treatment with phorbol 12-myristate 13-acetate (TPA). TPA-dependent accumulation of AcLDL receptor mRNA was not detected until after a lag phase of 12 hr and was blocked by concurrent treatment with cycloheximide. In addition, the TPA-dependent induction of AcLDL receptor activity and mRNA levels was inhibited by retinoic acid and dexamethasone treatment. Isolation and sequence analysis of the promoter regions for the human and bovine AcLDL receptor genes indicated high sequence similarity. Binding sites for AP-1 proteins or other known transcription factors were not conserved between the two species, suggesting that novel factors are required for AcLDL receptor expression.

Base Sequence↗
PubMed1992

Aminoguanidine inhibits oxidative modification of low density lipoprotein protein and the subsequent increase in uptake by macrophage scavenger receptors.

Aminoguanidine decreases the formation of advanced glycosylation end products that occurs during chronic hyperglycemia. Presumably this occurs because early glycosylation products preferentially bind to aminoguanidine rather than to lysine groups of adjacent proteins. Because oxidative modification of low density lipoprotein (LDL) also involves derivatization of lysine residues of apolipoprotein (apo) B by reactive aldehydes formed during the decomposition of oxidized fatty acids, we postulated that aminoguanidine might also inhibit the oxidatively induced modification of LDL protein. To test this hypothesis we oxidized LDL by incubation with Cu2+ or with endothelial cells in the absence or presence of aminoguanidine. Aminoguanidine prevented apo B lysine modification, as measured by fluorescence spectroscopy, and inhibited in a dose-dependent manner the oxidatively induced increase in subsequent macrophage uptake. At concentrations that inhibited apo B modification (5-10 mM), aminoguanidine increased the lag time in diene conjugation but did not affect the plateau value reached. These data indicate that aminoguanidine inhibits oxidative modification of LDL protein in large part by binding reactive aldehydes formed during lipid peroxidation and preventing their subsequent conjugation to apo B. Thus, aminoguanidine (and related compounds) may be of dual benefit in inhibiting atherosclerosis, both by inhibiting formation of advanced glycosylation end products and by inhibiting the modification of LDL apo B that makes it a ligand for scavenger receptors.

Animals↗
PubMed1992

Direct modification of low density lipoprotein by the spin trap 3,5-dibromo-4-nitrosobenzenesulfonic acid.

We have previously reported that the spin trap alpha-phenyl-tert-butyl nitrone (PBN) inhibited the oxidative modification of low density lipoprotein (LDL) (Kalyanaraman, B., Antholine, W.E. and Parthasarathy, S. (1990) Biochim. Biophys. Acta 1035, 286-292). In the present study, we report that 3,5-dibromo-4-nitrosobenzenesulfonic acid (DBNBS), a water-soluble spin trap, also inhibited the oxidation of LDL as measured by the formation of thiobarbituric acid reactive substances (TBARS). However, when compared with LDL incubated without DBNBS, the DBNBS-incubated LDL showed increased negative charge on agarose gel electrophoresis and was avidly degraded by mouse peritoneal macrophages. Despite the suggestion of biological modification, there was no decrease in lysine-amino groups in DBNBS-incubated LDL. Furthermore, reductively methylated LDL in which more than 85% of the amino group of lysines was blocked, was also modified by DBNBS. A sulfonic acid analog of PBN failed to modify LDL in a similar manner, suggesting that the presence of sulfonic acid alone does not ensure modification. When LDL was incubated with DBNBS, radical adducts associated with both lipid and protein were detected by electron paramagnetic resonance (EPR) technique. It is suggested that DBNBS may bind to the apoprotein B100 and lipids of LDL by a lysine-independent mechanism resulting in increased recognition and degradation by macrophages. The present work offers a novel approach for rapid modification of LDL.

Benzenesulfonates↗
PubMed1992

Synergistic interaction between the probucol phenoxyl radical and ascorbic acid in inhibiting the oxidation of low density lipoprotein.

Chain-breaking antioxidants such as butylated hydroxytoluene, alpha-tocopherol, and probucol have been shown to decrease markedly the oxidative modification of low density lipoprotein (LDL). Their mechanism of action appears to involve scavenging of LDL-lipid peroxyl radicals. The purpose of this study was to investigate the occurrence of radical reactions produced during oxidation of LDL and LDL-containing probucol initiated by lipoxygenase or copper. In addition, we have investigated the possibility of a synergistic interaction between ascorbate and probucol in inhibiting the oxidation of LDL. Incubation of LDL-containing probucol and lipoxygenase produced a composite electron spin resonance (ESR) spectrum due to the endogenous alpha-tocopheroxyl radical and probucol-derived phenoxyl radical. The spectral assignment was further verified by chemical oxidation of alpha-tocopherol and probucol. In the presence of ascorbic acid, these radicals in the LDL particle were reduced to their parent compounds with concomitant formation of the ascorbate radical. In both the peroxidation of linoleic acid and the copper-initiated peroxidation of LDL, the antioxidant activity of probucol was significantly increased by low (3-6 microM) concentrations of ascorbate. The probucol-dependent inhibition of LDL oxidation was enhanced in the presence of ascorbic acid. We conclude that the reaction between the phenoxyl radical of probucol and ascorbate results in a synergistic enhancement of the antioxidant capacity of these two compounds and speculate that such reactions could play a role in maintaining the antioxidant status of LDL during oxidative stress in vivo.

Analysis of Variance↗
PubMed1992

The role of oxidized low-density lipoproteins in the pathogenesis of atherosclerosis.

Hypercholesterolemia is a major risk for atherogenesis. Recent evidence suggests that oxidative modification of the major cholesterol-carrying lipoprotein, low-density lipoprotein (LDL), renders it more atherogenic. Not only does oxidized LDL (Ox-LDL) have enhanced uptake by macrophages, which contributes directly to foam cell formation, it may also adversely affect many other aspects of arterial wall metabolism and thus contribute further to the atherogenic process. Inhibition of the oxidation of LDL may be another approach to inhibiting atherogenesis, additive to or even synergistic with lowering of plasma LDL levels.

Animals↗
PubMed1992

Effect of probucol dosage on plasma lipid and lipoprotein levels and on protection of low density lipoprotein against in vitro oxidation in humans.

To determine whether probucol's ability to confer antioxidant protection to low density lipoprotein (LDL) could be dissociated from its ability to lower high density lipoprotein (HDL) cholesterol, 17 hypercholesterolemic patients were treated with either a standard dose, 1 g/day (4 tablets), or a low dose, 250 mg/day (1 tablet), of probucol for a 6-month period. Effects of therapy on lipoprotein levels and on susceptibility of LDL to in vitro oxidation were measured at frequent intervals. Probucol levels in plasma LDL rose less rapidly in the 1-tablet group but were nearly 50% of levels in the 4-tablet group after 6 months. HDL cholesterol and apolipoprotein A-1 decreased 17.6% and 27.9%, respectively, in the 1-tablet group compared with 28.0% and 38.3%, respectively, in the 4-tablet group (p = 0.07 and p = 0.06). In the 4-tablet group, LDL was protected from copper and endothelial cell-mediated oxidation after 2 months of therapy. In the 1-tablet group, equal degrees of protection occurred, but only after 6 months of therapy. In the whole study group, the decrease in LDL susceptibility to copper or endothelial cell-mediated oxidative modification was correlated with the content of probucol in LDL (r = 0.73, r = 0.65, p less than 0.005). Additionally, the decrease in HDL cholesterol level was correlated with the increase in protection to LDL from oxidative modification (r = 0.67 for copper, r = 0.58 for endothelial cells, p less than 0.05 for both) and also with the content of probucol in LDL (r = 0.6, p = 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Cholesterol, HDL↗
PubMed1992

Evidence for an additional intracellular site of action of probucol in the prevention of oxidative modification of low density lipoprotein. Use of a new water-soluble probucol derivative.

Oxidative modification of low density lipoprotein (LDL) renders it more atherogenic. Probucol, a highly nonpolar antioxidant, is transported in lipoproteins, including LDL, and inhibits oxidative modification of LDL in vitro. The ability of probucol to inhibit atherogenesis in the LDL receptor-deficient rabbit has been attributed to its antioxidant effect. We report synthesis of a new water-soluble analogue of probucol that is very effective in preventing cell-induced LDL oxidation. The polar probucol derivative, diglutaryl probucol, is efficiently taken up by endothelial cells and macrophages in culture and is hydrolyzed to release the active antioxidant, probucol. The treated cells, after thorough washing, show a marked decrease in their capacity to oxidize LDL during a subsequent incubation. At high concentrations of the derivative, the cells also released free probucol into the medium. Thus, the effectiveness of probucol in vivo may be related both to its presence in LDL, acting as a nonspecific antioxidant, and to an additional ability to inhibit cell-mediated oxidation of LDL by virtue of its uptake into cells.

Animals↗
PubMed1991

Analysis of the monocyte chemotactic response to lysophosphatidylcholine: role of lysophospholipase C.

Previously, we reported that lysophosphatidylcholine (lyso-PtdCho), a component of oxidized low-density lipoprotein, was a monocyte chemoattractant (M.T. Quinn et al. (1988) Proc. Natl. Acad. Sci. USA 85, 2805-2809). Monocyte chemotaxis was also stimulated by lyso-platelet activating factor but not by platelet activating factor itself. In the present studies, we used other analogs of lyso-PtdCho to determine structural and metabolic features required for chemotactic activity. Although both D- and L-lyso-PtdCho stimulated chemotaxis, suggesting a lack of stereospecificity, studies using propanediol and ethanediol analogs of lyso-PtdCho suggested that a free hydroxyl moiety or an ester-linked fatty acid vicinal to the phosphocholine group of the lysophospholipid was required for the expression of activity. Incubation of [3H]choline-labeled lyso-PtdCho with monocytes resulted in the formation of labeled PtdCho, glycerophosphocholine (GPC), phosphocholine, and free choline, while resident peritoneal macrophages, cells which we show do not respond chemotactically to lyso-PtdCho, metabolized the labeled substrate to generate only labeled PtdCho and GPC; no labeled phosphocholine was found, suggesting a possible role for lysophospholipase C activity in the monocyte chemotactic response. Although monoacylglycerol, the product of lysophospholipase C hydrolysis of lyso-PtdCho, was not chemotactic for monocytes, diacylglycerol demonstrated chemotactic activity, suggesting that the subsequent acylation to diacylglycerol may be involved in the monocyte chemotactic response to lyso-PtdCho. Indeed, monocytes incorporated [3H]glycerol from [3H]glycerol-labeled lyso-PtdCho into di- and triacylglycerol. Based on these results, a model is proposed whereby the monocyte chemotactic response to lyso-PtdCho involves a sequence of metabolic steps which includes hydrolysis of lyso-PtdCho to monoacylglycerol and phosphocholine by lysophospholipase C followed by acylation of monoacylglycerol to diacylglycerol. Diacylglycerol would then act as an intracellular second messenger that could activate or facilitate the chemotactic response.

Animals↗
PubMed1991

The spin trap, alpha-phenyl N-tert-butylnitrone, inhibits the oxidative modification of low density lipoprotein.

The lipophilic spin trap, N-tert-butylnitrone (PBN) inhibits the formation of the oxidatively-modified low density lipoprotein (LDL) by endothelial cells and by cupric ions. The LDL incubated in the presence of PBN with cells or cupric ions was less readily degraded by macrophages than the LDL incubated in the absence of PBN. A lipid-derived radical formed during oxidation of LDL was detected by spin trapping with PBN. It is likely that PBN inhibits the oxidative and biological modification of LDL by scavenging the LDL-lipid-derived radical.

Animals↗
PubMed1991

Feasibility of using an oleate-rich diet to reduce the susceptibility of low-density lipoprotein to oxidative modification in humans.

Oxidized low-density lipoprotein (LDL) is more atherogenic than native LDL. The initial step in the oxidation is the peroxidation of polyunsaturated fatty acids. Thus, decreasing the concentration of polyunsaturated fatty acids should reduce the susceptibility of LDL to oxidation. Therefore, we tested the possibility that diets enriched in oleate might result in LDL that is less susceptible to oxidative modification. LDL isolated from subjects consuming an oleate-enriched diet, compared with LDL from subjects on a linoleate-enriched diet, contained significantly more oleate (28.7% vs 11.5%) and less linoleate (31.9% vs 50.9%). Generation of conjugated dienes was significantly lower in the LDL from the oleate group. Most important, after incubation with endothelial cells, LDL from the oleate group underwent less degradation by macrophages. These studies demonstrate the feasibility of altering the diet in a way that will not raise LDL cholesterol concentrations and yet will decrease the susceptibility of LDL to oxidative modification.

Adult↗
PubMed1991

Macrophage-derived foam cells freshly isolated from rabbit atherosclerotic lesions degrade modified lipoproteins, promote oxidation of low-density lipoproteins, and contain oxidation-specific lipid-protein adducts.

Pure macrophage-derived foam cells (MFC) were isolated from the aortas of rabbits made atherosclerotic by balloon deendothelialization followed by diet-induced hypercholesterolemia. The MFC were isolated under sterile conditions using an enzymatic digestion procedure and discontinuous density gradient centrifugation. The purity of the MFC preparations was verified immunocytochemically with the macrophage specific monoclonal antibody RAM-11. MFC plated in medium containing 0.5% FCS for 24 h contained approximately 600 micrograms cholesterol per mg cell protein, 80% of which was esterified cholesterol. The MFC specifically degraded low density lipoprotein (LDL), acetyl-LDL, copper oxidized LDL, and beta-very low density lipoprotein (beta-VLDL) at rates comparable to mouse peritoneal macrophages (MPM) in 5-h assays. MFC within sections of the atherosclerotic lesions from the ballooned rabbits as well as the MFC isolated from the same lesions in the presence of antioxidants, exhibited positive immunoreactivity with polyclonal guinea pig antisera and mouse monoclonal antibodies directed against malondialdehyde-LDL, and 4-hydroxynonal-LDL. The MFC also exhibited the capacity to induce the oxidation of LDL at rates comparable to those exhibited by MPM and rabbit aortic endothelial cells. These data provide direct evidence that arterial wall macrophages express modified LDL receptors in vivo, contain epitopes found in oxidized-LDL and are capable of oxidizing LDL even when maximally loaded with cholesterol.

Animals↗
PubMed1991

Gene expression in macrophage-rich human atherosclerotic lesions. 15-lipoxygenase and acetyl low density lipoprotein receptor messenger RNA colocalize with oxidation specific lipid-protein adducts.

Oxidatively modified low density lipoprotein (LDL) exhibits several potentially atherogenic properties, and inhibition of LDL oxidation in rabbits decreases the rate of the development of atherosclerotic lesions. In vitro studies have suggested that cellular lipoxygenases may be involved in LDL oxidation, and we have shown previously that 15-lipoxygenase and oxidized LDL are present in rabbit atherosclerotic lesions. We now report that epitopes of oxidized LDL are also found in macrophage-rich areas of human fatty streaks as well as in more advanced human atherosclerotic lesions. Using in situ hybridization and immunostaining techniques, we also report that 15-lipoxygenase mRNA and protein colocalize to the same macrophage-rich areas. Moreover, these same lesions express abundant mRNA for the acetyl LDL receptor but no detectable mRNA for the LDL receptor. We suggest that atherogenesis in human arteries may be linked to macrophage-induced oxidative modification of LDL mediated by 15-lipoxygenase, leading to subsequent enhanced macrophage uptake, partly by way of the acetyl LDL receptor.

Arachidonate 15-Lipoxygenase↗
PubMed1991

Evidence for a dominant role of lipoxygenase(s) in the oxidation of LDL by mouse peritoneal macrophages.

It has been suggested that the oxidative modification of low density lipoprotein (LDL) is a key event in atherogenesis. Several mechanisms have been proposed to explain how different types of cells modify LDL. In this study we examine the relative contributions of superoxide anions and cellular lipoxygenase (LO) in the modification of LDL by macrophages. Superoxide dismutase (SOD) inhibited LDL oxidation by macrophages but only by 25%. Under the same conditions, several LO inhibitors (eicosatetraynoic acid (ETYA), piriprost, and A-64077) almost completely inhibited the modification of LDL by macrophages. SOD had a greater inhibitory effect on the modification of LDL by U937 cells and fibroblasts (32% and 64%, respectively) but again LO inhibitors had a much greater effect (79 to 100% inhibition). Incubation of [1-14C]linoleic acid with mouse peritoneal macrophages resulted in its conversion to a single more polar product coeluting with 13- and 9-HODE by reverse phase HPLC. When the cells were preincubated with LO inhibitors, formation of this product was significantly inhibited. It is concluded that the modification of LDL by macrophages is mediated in large part by lipoxygenase-type activity.

Animals↗
PubMed1990

Oxidation of low-density lipoprotein by Cu2+ and lipoxygenase: an electron spin resonance study.

The aim of this work was to obtain spectroscopic evidence for free radicals formed during copper ion- and lipoxygenase-catalyzed oxidation of the low-density lipoprotein. During the initial oxidation phase, a free-radical metabolite derived from the endogenous alpha-tocopherol present in the low-density lipoprotein was detected by the electron spin resonance technique. The divalent copper ions were bound to the residual EDTA present in the low-density lipoprotein and to the protein. Production of the alpha-tocopherol radical was suppressed in the presence of spin traps. Evidence for the low-density lipoprotein-lipid derived radicals was obtained by ESR-spin trapping methods. Implications of these findings in the oxidative modification of the low-density lipoprotein are discussed.

Copper↗