Oxidized lipoprotein(a) inhibits endothelium-dependent dilation: prevention by high density lipoprotein

Thiamine (Vitamin B1) Improves Endothelium-Dependent Vasodilatation in the Presence of Hyperglycemia

Brachial artery vasoactivity (BAVA) is a reliable, noninvasive method of assessing endothelium-dependent vasodilatation (EDV) in vivo. Acute hyperglycemia, impaired glucose tolerance (IGT), and diabetes mellitus impair EDV, a precursor to atherosclerosis. Thiamine is a coenzyme important in intracellular glucose metabolism. The purpose of this study was to evaluate the effect of thiamine on BAVA in the presence of hyperglycemia. Ten healthy subjects (group H, mean age 27 years), 10 patients with impaired glucose tolerance by World Health Organization criteria (group IGT, mean age 65 years), and 10 patients with non-insulin-dependent diabetes mellitus (group NIDDM, mean age 50 years) were studied. Duplex ultrasound was used to measure brachial artery flow changes in response to reactive hyperemia following brachial artery tourniquet occlusion for 5 min. This test was performed after a 10 hr fast and at 30, 60, and 120 min after a 75 g oral glucose challenge along with measurements of blood glucose level (BGL). A week later, BAVA evaluation was repeated after administration of 100 mg of intravenous thiamine. BAVA (% increased blood flow) at peak and trough BGL was compared with and without thiamine. BAVA at peak glucose improved from 69.0 +/- 6.4% to 152.8 +/- 22.9% in group H (p < 0.005), from 57.6 +/- 12.6% to 139.7 +/- 12.4% in group IGT (p < 0.005), and from 57.8 +/- 8.3% to 167.8 +/- 11.6% in group NIDDM (p < 0.005) following administration of thiamine. On the other hand, at trough glucose levels, BAVA remained essentially unchanged in group H (prethiamine 83.8 +/- 6.5% vs. post-thiamine 83.8 +/- 17.0%, p > 0.05) as well as group IGT (prethiamine 96.7 +/- 8.5% vs. post-thiamine 104.0 +/- 17.4%, p > 0.05). BAVA at trough glucose was not measured in group NIDDM secondary to trough BGL > 140 mg/dL. EDV was improved by thiamine in the presence of hyperglycemia in healthy subjects and in patients with IGT and NIDDM. The mechanism by which thiamine improves EDV is not due to a glucose-lowering effect as thiamine had no effect on EDV under normoglycemic conditions. Routine administration of thiamine might improve endothelial function and therefore slow the development and progression of atherosclerosis, especially in patients with IGT and NIDDM who are prone to develop accelerated atherosclerosis.

Lipids and endothelium-dependent vasodilation--a review

Studies using both in vitro and in vivo techniques have repeatedly shown that endothelium-dependent vasodilation (EDV) is impaired in different forms of experimental as well as human hypercholesterolemia. Clearly this impaired EDV can be reversed by lowering cholesterol levels by diet or medical therapy. Competitive blocking of L-arginine, changes in nitric oxide synthase activity, increased release of endothelin-1, and inactivation of nitric oxide due to superoxide ions all contribute to the impairment in EDV during dyslipidemia. The oxidation of low density lipoprotein, with its compound lysophosphatidylcholine, plays a critical role in these events. However, data on the role of triglycerides and fat-rich meals regarding EDV are not so consistent as data for cholesterol, although a view that the compositions of individual fatty acids and antioxidants are of major importance is emerging. Thus, this review shows that while impaired EDV is a general feature of hypercholesterolemia, the mechanisms involved and the therapeutic opportunities available still have to be investigated. Furthermore, discrepancies regarding the role of triglycerides and fat content in food may be explained by divergent effects of different fatty acids on the endothelium.

Endothelium-dependent vasorelaxation in the aorta of transgenic mice expressing human apolipoprotein(a) or lipoprotein(a)

Elevated plasma level of lipoprotein(a) (Lp(a)) is a well established risk factor for premature atherosclerosis and coronary artery disease. Recent studies showed impaired endothelium-dependent vasodilatation in humans with elevated plasma Lp(a). However, these human studies could not determine whether (1) elevated Lp(a) levels alone are the cause of endothelial dysfunction (these patients had multiple risk factors), and (2) native or oxidatively modified Lp(a) contributes to endothelial dysfunction (no measurements of native/oxidized Lp(a) ratio was reported in humans). In order to test whether apo(a) (an essential component of Lp(a) which is required for binding to endothelial cells) and native Lp(a) cause endothelial dysfunction, in the present study we tested endothelium-dependent vasorelaxation in aortic rings isolated from control and transgenic male mice either expressing the human apo(a) gene (TgA) or both the human apo(a) and human apo B100 genes (TgL). The TgA mice had plasma apo(a) levels of 8.8 +/- 1.2 mg/dl (n=6) and the double transgenic TgL mice had plasma Lp(a) levels of 15.3 +/- 1.4 mg/dl (n=8). Isolated aortic rings with and without endothelium were mounted in organ chambers and contracted with U46619 (10(-8) M) in the presence of ibuprofen (10(-5) M). Acetylcholine caused concentration-dependent (10(-9)-10(-5) M) relaxation, which could be prevented by endothelium removal and by NG-L-nitro-arginine (10(-4) M). Basal and acetylcholine-stimulated endothelium-dependent relaxation and endothelium-independent relaxation to nitroglycerin (10(-6) M) were not significantly different in aortic rings isolated from control and TgA or TgL mice. Twenty-four hour incubation of aortic rings isolated from control mice with recombinant human apo(a) or native Lp(a) (up to 300 microg/ml) caused no impairment of endothelium-dependent relaxations. In contrast, incubation with oxidized Lp(a) (50 microg/ml) or oxidized LDL (250 microg/ml) caused significant suppression of acetylcholine-induced endothelium-dependent vasorelaxation. These results show for the first time that elevated plasma levels of apo(a) and Lp(a) do not cause endothelial dysfunction in transgenic mice.

Co-incubation of native and oxidized low-density lipoproteins: potentiation of relaxation impairment

The influence of native low-density lipoprotein (LDL) on the inhibition of endothelium-dependent relaxation previously induced by oxidized LDL was investigated with intact rabbit aortic rings. We also tried to assess oxysterol involvement in the native lipoprotein effects. Lipoprotein fractions (1 mg protein/ml) were tested for their ability to inhibit the vasorelaxation induced by acetylcholine in aorta rings previously precontracted by noradrenaline vs. that in control strips in Krebs buffer. Co-incubation of oxidized and native LDL reinforced the oxidized LDL-induced inhibition, compared to the impairment evoked by oxidized LDL alone (E(max)=43.3+/-6.7% and 61. 4+/-5.4%, respectively; P<0.05). Finally, smaller amounts of 7-oxy-cholesterols were recovered in organ baths after co-incubation of native and oxidized LDL than after incubation of oxidized LDL alone. Conversely, more oxy-cholesterols were found in the strip vessels under the same conditions (% of oxysterol incorporation: 0. 05158 vs. 0.10199, r=0.703). Together these results suggest that the strengthening of oxidized LDL-induced inhibition by native LDL is dependent on an oxysterol effect on arterial wall cells. Mechanisms involved in this phenomenon remain to be investigated.

Native and gamma radiolysis-oxidized lipoprotein(a) increase the adhesiveness of rabbit aortic endothelium

Accumulation of monocyte-derived foam cells in the arterial intima is a major event in the development of atherogenesis. We have examined whether native and oxidized lipoprotein(a) (Lp(a)) can induce adhesion of monocytic cells to aortic endothelium. The extensive oxidation of paired samples of Lp(a) and low-density lipoprotein (LDL) was achieved by O2.-/OH. free radicals produced by gamma radiolysis of water, leading to similar values for the formation of peroxidation markers (conjugated dienes, TBARS, 8-epi-PGF2alpha) for both Lp(a) and LDL. Rabbit aortic segments were incubated for 5 h in the presence of equimolar concentrations of native and oxidized preparations of Lp(a) and LDL (125 micromol cholesterol/l, corresponding to 40 and 30 mg protein/l for Lp(a) and LDL, respectively). The aortic segments were incubated with rhodamin-isothiocyanate labeled U937 monocytic cells for 30 min and cell adhesion was quantified by fluorescent microscopy. Native Lp(a), and to a larger extent oxidized Lp(a), significantly increased U937 cell adhesion by 2.3 and 2.7 fold compared to controls (P < 0.005 and P < 0.001, respectively). Monocytic cell adhesion was also increased by native LDL (1.6 fold, P < 0.005), and to a greater extent by oxidized LDL (2.3 fold, P < 0.001). Thus native Lp(a) enhances the adhesive properties of the arterial endothelium which may account for its proatherogenic action. Furthermore, our results show that oxidized Lp(a), as well as oxidized LDL, are potent stimuli of monocyte adhesion to endothelial cells.

Effect of HDL and atherogenic lipoproteins on formation of O2- and renin release in juxtaglomerular cells

Atherogenic lipoproteins and reactive oxygen species stimulate renin release from isolated juxtaglomerular (JG) cells. Here we assessed whether stimulation of renin release is mediated by formation of superoxide anion (O2-), and whether the effects of oxidized lipoproteins, like in many other biological systems, can be prevented by the antiatherogenic high density lipoprotein (HDL). Lipoproteins were prepared from human plasma, and JG cells from mouse and rat kidneys. Basal renal activity of JG cells was measured in culture supernatants and cells, and was dose-dependently and significantly stimulated by oxidized LDL (50 and 300 micrograms/ml) and by oxidized Lp(a) (1, 10 and 30 micrograms/ml). Administration of HDL alone had no effect on renin release. However, coincubation with 100 micrograms/ml HDL significantly suppressed oxidized LDL- and oxidized Lp(a)-stimulated renin release. O2- production of JG cells was directly measured using a chemiluminescence assay. Stimulation with 10 micrograms/ml oxidized LDL and oxidized Lp(a) significantly increased the O2- generation of JG cells. In the presence of 5 micrograms/mL HDL, O2- production was reduced to control levels. These data indicate that stimulation of JG cells with oxidized LDL and Lp(a) induces formation of O2-, which may stimulate renin release in an autocrine fashion. Renin release can be prevented by HDL, presumably by preventing the formation of O2-.

Effect of antioxidants alone and in combination with monounsaturated fatty acid-enriched diets on lipoprotein oxidation

Previous studies have demonstrated that compared with more buoyant LDL, dense LDL (D-LDL) is more susceptible to oxidation and less readily protected from oxidation by antioxidant enrichment. However, diets enriched in monounsaturated fatty acids (MUFAs) appear particularly effective in protecting D-LDL from oxidation. We therefore evaluated in 12 non-insulin-dependent diabetes mellitus subjects the effects of supplementation with alpha-tocopherol (1600 IU/d) and probucol (1 g/d) alone and in combination with an MUFA-enriched diet on LDL and LDL subfraction susceptibility to oxidation and monocyte release of superoxide anion. Subjects received either alpha-tocopherol or probucol for 4 months, and during the fourth month both groups also received an MUFA-enriched diet. alpha-Tocopherol levels were significantly increased in LDL and LDL subfractions (P < .05) after 3 months of supplementation. MUFA-enriched diets led to further increases in alpha-tocopherol in LDL fractions in the alpha-tocopherol group as well as in those receiving probucol. In the alpha-tocopherol-supplemented group, lag times were increased significantly (1.6- to 2.0-fold) for all LDL fractions, although the absolute increase was least for D-LDL. Although probucol supplementation increased lag times of LDL and LDL subfractions three- to fourfold, D-LDL was still more readily oxidized. In both the alpha-tocopherol- and probucol-supplemented groups the benefit of adding MUFA-enriched diets was greatest for D-LDL, with further increases in lag time of 26% and 18%, respectively. Neither antioxidant supplementation nor the addition of an MUFA-enriched diet reduced unstimulated or phorbol ester-stimulated monocyte superoxide anion production. These data demonstrate the markedly different effects that antioxidants and diet may have on different LDL subfractions, which may be particularly important in individuals with non-insulin-dependent diabetes mellitus, who frequently have increased amounts of D-LDL.