Inhibition by serum components of oxidation and collagen-binding of low-density lipoprotein

Effects of chromium histidinate on renal function, oxidative stress, and heat-shock proteins in fat-fed and streptozotocin-treated rats

OBJECTIVE

Chromium is an essential element for carbohydrate, fat, and protein metabolism. The therapeutic potential of chromium histidinate (CrHis) in the treatment of diabetes has been elucidated. The present study investigated the effects of CrHis on serum parameters of renal function, on oxidative stress markers (malondialdehyde [MDA] and 8-isoprostane), and on the expression of heat-shock proteins (HSPs) in rats.

METHODS

Male Wistar rats (n=60, 8 weeks old) were divided into four groups. Group 1 received a standard diet (12% of calories as fat). Group 2 received a standard diet, plus CrHis. Group 3 received a high-fat diet (40% of calories as fat) for 2 weeks, and was then injected with streptozotocin (STZ) on day 14 (STZ, 40 mg/kg intraperitoneally). Group 4 was treated in the same way as group 3 (HFD/STZ), but was supplemented with 110 microg CrHis/kg/body weight/day. Oxidative stress in the kidneys of diabetic rats was evidenced by an elevation in levels of MDA and 8-isoprostane. Protein concentrations of HSP60 and HSP70 in renal tissue were determined by Western blot analyses.

RESULTS

Chromium histidinate supplementation lowered kidney concentrations of MDA, 8-isoprostane levels, serum urea-N, and creatinine, and reduced the severity of renal damage in the STZ-treated group (i.e., the diabetes-induced group). The expression of HSP60 and HSP70 was lower in the STZ group that received CrHis than in the group that did not. No significant effect of CrHis supplementation was detected in regard to the overall measured parameters in the control group.

CONCLUSIONS

Chromium histidinate significantly decreased lipid peroxidation levels and HSP expression in the kidneys of experimentally induced diabetic rats. This study supported the efficacy of CrHis in reducing renal risk factors and impairment because of diabetes.

Consequences of low plasma histidine in chronic kidney disease patients: associations with inflammation, oxidative stress, and mortality

BACKGROUND

Histidine is considered as an antiinflammatory and antioxidant factor. Histidine deficiency may contribute to an impaired nutritional state in patients with chronic kidney disease (CKD).

OBJECTIVE

We aimed to investigate the consequences of plasma histidine deficiency in CKD patients.

DESIGN

CKD patients (n = 325; 203 M) with a median age of 54 y (range: 19-70 y) were evaluated shortly before the beginning of renal replacement therapy. The median glomerular filtration rate was 6.4 mL/min (range: 0.8-14.5 mL/min). Nutritional status was assessed by subjective global assessment. Survival was followed for up to 60 mo; 101 patients died.

RESULTS

Plasma histidine concentrations were significantly lower in CKD patients with history of cardiovascular disease, presence of plaques, protein-energy wasting, and inflammation. Plasma histidine was negatively associated with age, C-reactive protein, interleukin-6, leukocytes, thrombocytes, fibrinogen, hepatocyte growth factor, adhesion molecules, insulin-like growth factor-1, and 8-hydroxy-2'-deoxyguanosine and was positively associated with handgrip strength, hemoglobin, S-albumin and fetuin-A. A multivariate regression analysis showed that histidine concentrations were independently associated with hepatocyte growth factor, hemoglobin, and fetuin-A. In unadjusted analysis, a low histidine concentration was associated with all-cause mortality (log rank chi-square test = 8.9; P = 0.002). After adjustment for age, sex, cardiovascular disease, inflammation, diabetes mellitus, serum S-albumin, and amino acid supplementation, the association between low histidine and mortality remained significant (hazard ratio: 1.55; 95% CI: 1.02, 2.40; P < 0.05).

CONCLUSION

Low plasma concentrations of histidine are associated with protein-energy wasting, inflammation, oxidative stress, and greater mortality in CKD patients.

Prooxidant and antioxidant properties of human serum ultrafiltrates toward LDL: important role of uric acid

Oxidized LDL is present within atherosclerotic lesions, demonstrating a failure of antioxidant protection. A normal human serum ultrafiltrate of Mr below 500 was prepared as a model for the low Mr components of interstitial fluid, and its effects on LDL oxidation were investigated. The ultrafiltrate (0.3%, v/v) was a potent antioxidant for native LDL, but was a strong prooxidant for mildly oxidized LDL when copper, but not a water-soluble azo initiator, was used to oxidize LDL. Adding a lipid hydroperoxide to native LDL induced the antioxidant to prooxidant switch of the ultrafiltrate. Uric acid was identified, using uricase and add-back experiments, as both the major antioxidant and prooxidant within the ultrafiltrate for LDL. The ultrafiltrate or uric acid rapidly reduced Cu2+ to Cu+. The reduction of Cu2+ to Cu+ may help to explain both the antioxidant and prooxidant effects observed. The decreased concentration of Cu2+ would inhibit tocopherol-mediated peroxidation in native LDL, and the generation of Cu+ would promote the rapid breakdown of lipid hydroperoxides in mildly oxidized LDL into lipid radicals. The net effect of the low Mr serum components would therefore depend on the preexisting levels of lipid hydroperoxides in LDL. These findings may help to explain why LDL oxidation occurs in atherosclerotic lesions in the presence of compounds that are usually considered to be antioxidants.

Human serum, cysteine and histidine inhibit the oxidation of low density lipoprotein less at acidic pH

Low concentrations of serum or interstitial fluid have been shown to inhibit the oxidation of low density lipoprotein (LDL) catalysed by copper or iron, and may therefore protect against the development of atherosclerosis. As atherosclerotic lesions may have an acidic extracellular pH, we have investigated the effect of pH on the inhibition of LDL oxidation by serum and certain components of serum. Human serum (0.5%, v/v), lipoprotein-deficient human serum at an equivalent concentration and the amino acids L-cysteine (25 microM) and L-histidine (25 microM), but not L-alanine (25 microM), inhibited effectively the oxidation of LDL by copper at pH 7.4, as measured by the formation of conjugated dienes. The antioxidant protection was reduced considerably at pH 6.5, and was decreased further at pH 6.0. These observations may help to explain why LDL becomes oxidised locally in atherosclerotic lesions in the presence of the strong antioxidant protection offered by extracellular fluid.

Oxidation of lipoprotein(a) and low density lipoprotein containing density gradient ultracentrifugation fractions

Increased plasma concentrations of lipoprotein(a) (Lp(a)) are associated with an increased risk for atherosclerotic cardiovascular disease. It is thought that the atherogenicity of Lp(a) is mediated both through its LDL-like properties and its plasminogen-like properties. In this study we have investigated the LDL-like atherogenic properties of Lp(a) by comparing the susceptibility to in vitro oxidation of Lp(a) and LDL isolated from the same subject. The subjects studied varied widely in plasma Lp(a) concentration (331-1829 mg/l) and Lp(a) phenotype (from B to S4). Lipoproteins are notoriously unstable in vitro, consequently differences in in vitro handling could influence oxidizability. Therefore, the isolation and handling of Lp(a) and LDL were performed in an identical fashion. Lp(a) and LDL containing fractions were obtained by density gradient ultracentrifugation. Separate fractions containing various amounts of Lp(a) and LDL, quantitated by measuring both Lp(a) and apo B-100, were subsequently oxidized on equimolar apo B-100 basis. Despite large differences in the Lp(a)/apo B-100 ratio of the various fractions (ranging from 5.3 +/- 1.7 to 0.2 +/- 0.1) they showed quite similar oxidation characteristics. The most dense Lp(a) containing fractions showed an aberrant susceptibility to oxidation. Subsequent gel filtration and reconstitution experiments showed that this was due to protein (i.e., albumin) contamination. Removal of excess protein revealed an oxidation pattern similar to that of LDL. It is concluded that the susceptibility of Lp(a) to lipid-peroxidation is similar to that of LDL when isolated simultaneously and in the same way from the same subject. Thus, lipid-peroxidation of Lp(a) is not influenced by the presence of its distinguishing apolipoprotein(a).

Ceruloplasmin enhances smooth muscle cell- and endothelial cell-mediated low density lipoprotein oxidation by a superoxide-dependent mechanism

Cultured vascular smooth muscle cells (SMC) and endothelial cells (EC) stimulate low density lipoprotein (LDL) oxidation by free radical-mediated, transition metal-dependent mechanisms. The physiological source(s) of metal ions is not known; however, purified ceruloplasmin, a plasma protein containing 7 coppers, oxidizes LDL in vitro. We now show that ceruloplasmin also increases LDL oxidation by vascular cells. In metal ion-free medium, human ceruloplasmin increased bovine aortic SMC- and EC-mediated LDL oxidation by up to 30- and 15-fold, respectively. The maximal response was at 100-300 microg ceruloplasmin/ml, a level at or below the unevoked physiological plasma concentration. Oxidant activity was dependent on protein structure as a specific proteolytic cleavage or removal of one of the seven ceruloplasmin copper atoms inhibited activity. Three lines of evidence indicated a critical role for cellular superoxide (O2.) in ceruloplasmin-stimulated oxidation. First, the rate of production of O2. by cells correlated with their rates of LDL oxidation. Second, superoxide dismutase effectively blocked ceruloplasmin-stimulated oxidation by both cell types. Finally, O2. production by SMC quantitatively accounted for the observed rate of LDL oxidation. To show this, the course of O2. production by SMC was simulated by repeated addition of xanthine and xanthine oxidase to culture medium under cell-free conditions. Neither ceruloplasmin nor O2. alone increased LDL oxidation, but together they completely reconstituted the oxidation rate of ceruloplasmin-stimulated SMC. These results are the first to show that ceruloplasmin stimulates EC- and SMC-mediated oxidation of LDL and that cell-derived O2. accounts quantitatively for metal-dependent, free radical-initiated oxidation of LDL by these cells.

Human suction blister interstitial fluid prevents metal ion-dependent oxidation of low density lipoprotein by macrophages and in cell-free systems

LDL in the circulation is well protected against oxidation by the highly efficient antioxidant defense mechanisms of human plasma. LDL oxidation contributing to atherosclerosis, therefore, has been hypothesized to take place in the interstitial fluid of the arterial wall. We investigated the antioxidant composition and the capacity to inhibit LDL oxidation of human suction blister interstitial fluid (SBIF), a suitable representative of interstitial fluid. We found that the concentrations in SBIF of the aqueous small-molecule antioxidants ascorbate and urate were, respectively, significantly higher (P < 0.05) and identical to plasma concentrations. In contrast, lipoprotein-associated lipids and lipid-soluble antioxidants (alpha-tocopherol, ubiquinol-10, lycopene, and beta-carotene) were present at only 8-23% of the concentrations in plasma. No lipid hydroperoxides could be detected ( < 5 nM) in either fluid. The capacity of serum and SBIF to protect LDL from oxidation was investigated in three metal ion-dependent systems: copper, iron, and murine macrophages in Ham's F-10 medium. In all three systems, addition of > or = 6% (vol/vol) of either serum or SBIF inhibited LDL oxidation by > 90%. The concentration that inhibited macrophage-mediated LDL oxidation by 50% was as low as 0.3% serum and 0.7% SBIF. The enzymatic or physical removal of ascorbate or urate and other low molecular weight components did not affect the ability of either fluid to prevent LDL oxidation, and the high molecular weight fraction was as protective as whole serum or SBIF. These data demonstrate that both serum and SBIF very effectively protect LDL from metal ion-dependent oxidation, most probably because of a cumulative metal-binding effect of several proteins. Our data suggest that LDL in the interstitial fluid of the arterial wall is very unlikely to get modified by metal ion-mediated oxidation.

Adhesion of washed blood platelets in vitro is advanced, accelerated, and enlarged by oxidized low-density lipoprotein

In order to study the influence of oxidized low-density lipoprotein (Ox-LDL) on platelet functional morphology at an early activation stage, washed human blood platelets were stimulated by 100 micrograms/ml Ox-LDL at 37 degrees C. The settling and spreading process of stimulated and unstimulated platelets on Formvar-coated glass was observed for approximately 20 min by reflection contrast microscopy (RCM) and quantified by image analysis. Each group consisted of at least 250 platelets. The results show that incubation with Ox-LDL causes platelet shape change and pseudopodia formation. The sedimentation of stimulated platelets precedes that of unstimulated platelets by approximately 3 min. The increase of the total adhesion area of all Ox-LDL treated platelets is significantly accelerated in comparison to normal platelets (20.45 microns2/min vs. 15.45 microns2/min; P < 0.01). The mean total adhesion area of Ox-LDL-treated platelets was generally larger than that of untreated platelets (189.7 microns2 vs. 144.7 microns2; P < 0.01). The disappearance of intracellular granules after platelet activation, observed by RCM, is supported by transmission electron microscopy. Our results suggest that Ox-LDL activates platelets and advances and accelerates their adhesion and thereby may contribute to pathological thrombosis and arteriosclerosis.

Functional morphological alterations of human blood platelets induced by oxidized low density lipoprotein

The effect of oxidized low density lipoprotein (LDL) on the functional morphology of human platelets in vitro was studied by means of transmission electron microscopy. The washed platelets, stimulated by oxidized LDL (50-300 micrograms protein/ml), showed disc-sphere transformation, centralization of granules and complete degranulation in a dose- and time-dependent manner. A cytodamage in platelet membrane was induced by oxidized LDL leading to a lower electron density of cytoplasm compared to control. The morphological observations were supported by an analysis of the platelet shape-change parameter. Since the shape change, induced by oxidized LDL (50 micrograms/ml), was inhibited by a preincubation of platelets with staurosporine (10 nM), the protein kinase C was probably involved in the platelet activation initiated by oxidized LDL. The present results suggest that oxidized LDL could contribute to pathological thrombosis and atherogenesis by activating platelets.

Intact human ceruloplasmin oxidatively modifies low density lipoprotein

Ceruloplasmin is a plasma protein that carries most of the copper found in the blood. Although its elevation after inflammation and trauma has led to its classification as an acute phase protein, its physiological role is uncertain. A frequently reported activity of ceruloplasmin is its ability to suppress oxidation of lipids. In light of the intense recent interest in the oxidation of plasma LDL, we investigated the effects of ceruloplasmin on the oxidation of this lipoprotein. In contrast to our expectations, highly purified, undegraded human ceruloplasmin enhanced rather than suppressed copper ion-mediated oxidation of LDL. Ceruloplasmin increased the oxidative modification of LDL as measured by thiobarbituric acid-reacting substances by at least 25-fold in 20 h, and increased electrophoretic mobility, conjugated dienes, and total lipid peroxides. In contrast, ceruloplasmin that was degraded to a complex containing 115- and 19-kD fragments inhibited cupric ion oxidation of LDL, as did commercial preparations, which were also degraded. However, the antioxidant capability of degraded ceruloplasmin in this system was similar to that of other proteins, including albumin. The copper in ceruloplasmin responsible for oxidant activity was not removed by ultrafiltration, indicating a tight association. Treatment of ceruloplasmin with Chelex-100 removed one of seven copper atoms per molecule and completely blocked oxidant activity. Restoration of the copper to ceruloplasmin also restored oxidant activity. These data indicate that ceruloplasmin, depending on the integrity of its structure and its bound copper, can exert a potent oxidant rather than antioxidant action on LDL. Our results invite speculation that ceruloplasmin may be in part responsible for oxidation of LDL in blood or in the arterial wall and may thus have a physiological role that is quite distinct from what is commonly believed.