Hyperhomocysteinemic Subjects Have Enhanced Expression of Lectin-Like Oxidized LDL Receptor-1 in Mononuclear Cells

An elevated plasma concentration of homocysteine is an independent risk factor for cardiovascular disease. However, the mechanisms are still unclear. Lectin-like oxidized LDL receptor-1 (LOX-1) has ligand specificity for oxidized LDL (oxLDL). We hypothesized that homocysteine's atherogenic effects may involve LOX-1-mediated mechanisms. We examined the effect of folic acid supplementation for 6 wk and 12 mo (5 mg/d for 1 wk, 1 mg/d for 37 wk and 0.4 mg/d for the remaining 14 wk) on LOX-1 mRNA levels and on oxLDL-induced release of tumor necrosis factor alpha from peripheral blood mononuclear cells in hyperhomocysteinemic individuals. Compared with healthy controls, hyperhomocysteinemic subjects had elevated mRNA levels of LOX-1 in mononuclear cells (P < 0.001), and their mononuclear cells released more tumor necrosis factor alpha (TNFalpha) upon oxLDL stimulation (P = 0.01). This oxLDL-stimulated release of TNFalpha correlated with LOX-1 expression (r = 0.57, P = 0.026). Folic acid treatment led to a normalization of homocysteine levels accompanied by a reduction in LOX-1 gene expression (P < 0.02) and in oxLDL-stimulated release of TNFalpha (P < 0.05). These novel findings suggest both that homocysteine exerts its atherogenic effect in part by elevating levels of LOX-1, thereby enhancing oxLDL-induced inflammatory responses, and most important, that folic acid supplementation may downregulate these responses.

Patients with familial hypercholesterolaemia show enhanced spontaneous chemokine release from peripheral blood mononuclear cells ex vivo Dependency of xanthomas/xanthelasms, smoking and gender

AIMS

Familial hypercholesterolaemia (FH) is associated with increased risk of premature atherosclerosis and coronary artery disease (CAD). However, onset of clinically manifested CAD varies widely among patients with heterozygous FH, and we hypothesized that inflammatory mediators such as chemokines could contribute to atherogenesis in these patients.

METHODS AND RESULTS

We compared peripheral blood mononuclear cells (PBMCs) from FH patients with an identical mutation with PBMCs from sex- and age-matched healthy controls with respect to spontaneous and oxidized low density lipoprotein (oxLDL)-stimulated release of chemokines. Our main findings were: (1) PBMCs from FH patients spontaneously released significantly higher levels of macrophage inflammatory protein (MIP)-1alpha, MIP-1beta and interleukin (IL)-8, and had a significantly lower oxLDL-stimulatory ratio for MIP-1alpha and MIP-1beta than cells from healthy controls. (2) Spontaneous release of these chemokines correlated positively and stimulatory ratio correlated negatively with plasma concentrations of total and LDL cholesterol. (3) Among FH patients, release of MIP-1alpha, MIP-1beta and IL-8 from PBMCs varied with the presence of xanthomas/xanthelasms, smoking and gender. (4) In vitro studies showed that FH serum but not control serum was able to induce enhanced spontaneous release of chemokines in PBMCs from both FH patients and control subjects.

CONCLUSIONS

Our data may suggest that a pathophysiological consequence of FH is enhanced chemokine responses, which in turn may promote recruitment and activation of leukocytes within the vessel wall, contributing to atherosclerosis as well as to the different phenotypes in these patients with an identical FH mutation.

Expression of matrix metalloproteinase-9 in mononuclear cells of hyperhomocysteinaemic subjects

Atherosclerotic plaque instability and rupture requires extracellular matrix modification, a complex process regulated by matrix metalloproteinases (MMPs). We hypothesized that homocysteine's atherogenic effects may involve MMP-mediated mechanisms. Our results showed the following: (i) Compared with healthy control subjects (n = 9), patients with hyperhomocysteinaemia (n = 9) had elevated mRNA levels of MMP-9 and tissue inhibitors of metalloproteinases-1 (TIMP-1) in freshly isolated peripheral blood mononuclear cells (PBMCs), which were positively correlated with homocysteine and negatively correlated with folate and vitamin B12 levels. (ii) Peripheral blood mononuclear cells obtained from these patients released markedly enhanced the amount of MMP-9 upon oxidized LDL (oxLDL) stimulation, whereas no such enhancing effect was seen in cells from healthy controls. (iii) During folic acid 6 weeks' treatment, normalization of homocysteine levels was accompanied by a significant reduction in mRNA levels of MMP-9 and TIMP-1 in PBMCs, as well as a marked reduction in oxLDL-stimulated release of MMP enzyme activity. These novel findings may suggest that homocysteine exerts its atherogenic effect in part by elevating levels and activity of MMPs, which in turn may enhance matrix degradation, potentially promoting atherogenesis and plaque instability. Moreover, our findings suggest that folic acid supplementation may down-regulate these inappropriate responses in these patients.

Folic acid treatment reduces elevated plasma levels of asymmetric dimethylarginine in hyperhomocysteinaemic subjects

Asymmetric dimethylarginine (ADMA), an endogenous inhibitor of NO synthase, has been suggested to be a novel risk factor for endothelial dysfunction. It has previously been reported that hyperhomocysteinaemia may be associated with impaired endothelium-dependent vasodilation and reduced plasma level of NO-derived endproducts (NOx). In the present study, plasma levels of arginine and ADMA were measured in twenty-one healthy control subjects, and in twenty-one hyperhomocysteinaemic subjects before and after 6 weeks and 12 months of folic acid supplementation, and compared with previously measured plasma NOx values in the hyperhomocysteinaemic subjects. Compared with control subjects, hyperhomocysteinaemic subjects had higher plasma levels of arginine and ADMA. More importantly, folic acid therapy significantly reduced plasma levels of arginine and ADMA. Furthermore, plasma levels of arginine and ADMA were positively correlated with plasma homocysteine levels and negatively correlated with plasma folate, as well as negatively correlated with plasma NOx. Our results suggest that ADMA may be a mediator of the atherogenic effects of homocysteine.

Folic acid treatment reduces chemokine release from peripheral blood mononuclear cells in hyperhomocysteinemic subjects

Elevated plasma homocysteine concentration is an independent risk factor for cardiovascular disease. However, the mechanisms by which hyperhomocysteinemia induces vascular disease are uncertain. An early step in atherogenesis involves leukocyte migration into the arterial wall, a process regulated in part by chemokines. We hypothesized that homocysteine may exert its atherogenic effect in part through chemokine-mediated mechanisms, and in the present study, we examined the effects of folic acid supplementation for 6 weeks on chemokine levels in hyperhomocysteinemic individuals. Data showed the following: (1) Compared with control subjects, hyperhomocysteinemic subjects had elevated plasma levels of the CXC chemokines, epithelial neutrophil-activating peptide (ENA)-78 (P<0.05), and growth-regulated oncogene (GRO)alpha (P=0.088), and homocysteine was significantly correlated with ENA-78 and GROalpha. (2) During folic acid treatment, normalization of homocysteine levels was accompanied by a marked reduction in oxidized low density lipoprotein-stimulated release of CXC chemokines (ie, GROalpha, ENA-78, and interleukin-8) and CC chemokines (ie, monocyte chemoattractant peptide-1 and RANTES) in peripheral blood mononuclear cells from these individuals. (3) The oxidized low density lipoprotein-induced release of ENA-78 from peripheral blood mononuclear cells from control subjects was significantly reduced when cells were incubated in the presence of folic acid. These data may suggest that homocysteine exerts atherogenic effects in part by enhancing chemokine responses in cells involved in atherogenesis and that folic acid supplementation may downregulate these inflammatory responses.

Effect of folic acid treatment on endothelium-dependent vasodilation and nitric oxide-derived end products in hyperhomocysteinemic subjects

PURPOSE

An elevated plasma homocysteine concentration is an independent risk factor for cardiovascular diseases. In this study, we tested the hypothesis that hyperhomocysteinemia induces endothelial dysfunction mediated, at least in part, through nitric oxide-dependent mechanisms and that folic acid supplementation improves endothelial function in hyperhomocysteinemic subjects.

SUBJECTS AND METHODS

Endothelial function was evaluated in healthy controls and hyperhomocysteinemic subjects by measuring plasma levels of the nitric oxide-derived end products nitrite and nitrate and by assessing vasodilatory responses in the skin microcirculation and forearm vasculature. In the subjects with hyperhomocysteinemia, these measurements were repeated after 6 weeks and 12 months of folic acid supplementation.

RESULTS

Compared with healthy controls, hyperhomocysteinemic subjects had significantly lower median plasma levels of nitric oxide-derived end products (12.1 microM [range 4.4 to 41.8] versus 24.6 microM [13.6 to 53.2]; P <0.001), a significantly lower endothelium-dependent vasodilatory response to acetylcholine (P <0.01), hyperemic response in the microcirculation (P <0.01), and total forearm blood flow during reactive hyperemia (P = 0.01). There was no significant difference in the endothelium-independent response. Folic acid treatment for 12 months increased the plasma level of nitric oxide-derived end products by 121% (95% confidence interval [CI], 72% to 170%), the vasodilatory response to acetylcholine by 124% (95% CI, 36% to 212%), and the ischemia-mediated hyperemic responses in the microcirculation by 60% (95% CI, 25% to 96%) and in the forearm vasculature by 47% (95% CI, 21% to 73%).

CONCLUSIONS

Homocysteine appears to induce its atherogenic effect, at least in part, by depressing endothelial function, possibly through nitric oxide-dependent mechanisms. This effect can be reversed by folic acid supplementation.

Effect of Norwegian fish powder on risk factors for coronary heart disease among hypercholesterolemic individuals

BACKGROUND AND AIM

Numerous studies suggest an association between high intake of fatty fish and reduced risk of coronary heart disease. Very long-chain omega-3 fatty acids are thought to be responsible for the benefits observed, though other fatty fish components may act in concert with them. Norwegian fish powder is a dry herring product that contains essential amino acids, marine omega-3 fatty acids, vitamins and minerals. The aim of the present study was to determine whether it has beneficial effects on risk factors for coronary heart disease in man.

METHODS AND RESULTS

A single center, randomized, double-blind, parallel-treatment study was carried out for 12 weeks. Subjects with primary hypercholesterolemia were randomly allocated to 10 g fish powder or placebo (20 tablets/day). Participants were instructed to follow National Cholesterol Education Program (NCEP) Step I Diet during a 4-week diet run-in phase and during the study. Concentrations of lipids, lipoproteins, hemostatic variables and endothelial cell markers were determined before and after supplementation. Our data showed that the fish powder supplement was well tolerated. A significant decrease and increase respectively were observed in plasma alpha-linolenic acid (p = 0.03) and docosahexaenoic acid (DHA) (p = 0.03). Concentrations of lipids, lipoproteins, homocysteine, factor VII, fibrinogen, tissue plasminogen activator (tPA), plasminogen activator inhibitor (PAI)-1, soluble intercellular adhesion molecule (ICAM)-1, P-selectin and interleukin (IL)-8 were not beneficially affected.

CONCLUSIONS

Fish powder supplementation does not seem an effective approach to improve risk factors for coronary heart disease in hypercholesterolemic subjects following the NCEP Step I Diet.

Retinoylation of proteins in rat hepatocytes following uptake of chylomicron remnant retinyl ester

Several proteins may covalently bind retinoic acid, a process called retinoylation. Recently, we have demonstrated that proteins were retinoylated in vivo in liver, kidney and lung. In order to gain further knowledge about the mechanism of this process, we studied retinoylation in rat hepatocytes administered vitamin A as [3H]retinyl esters in chylomicron remnants. This resembles the normal physiological uptake of vitamin A. After 24 h incubation, about 0.0017 mol [3H]retinoid was covalently bound per mol protein. Citral, an inhibitor of the oxidation of retinol to retinoic acid, reduced retinoylation about 40%, indicating that oxidation of retinol to retinoic acid is necessary for a large fraction of the observed covalent modification of proteins. When cells were incubated with physiological concentrations of [3H]retinol or [3H]retinoic acid dissolved in ethanol, much less retinoid was covalently bound per mol protein compared with cells incubated with chylomicron remnant. Saturation of the retinoylation was apparent with retinoic acid around the physiological concentration. Retinoylated proteins were also analysed by SDS-PAGE. In general, the same protein bands were labelled with both [3H]retinol and [3H]retinoic acid, although the intensity of the bands varied. Major bands had an apparent molecular weight of about 16, 35, 50 and 120 kDa. In a parallel experiment in which liver stellate cells were incubated with [3H]retinol, major retinoylated protein bands were about 35, 60 and 65 kDa. Thus, different proteins appear to be retinoylated in hepatocytes and liver stellate cells, suggesting that protein retinoylation is a cell specific phenomenon. These results demonstrate that retinoids presented to hepatocytes as chylomicron remnant retinyl esters are covalently linked to proteins. We therefore suggest that retinoylation of proteins represents a minor but significant pathway whereby cells metabolize vitamin A.

Expression of retinoic acid receptor and retinoid X receptor subtypes in rat liver cells: implications for retinoid signalling in parenchymal, endothelial, Kupffer and stellate cells

In the present study, a systematic examination of the relative expression pattern of the nuclear retinoid receptors (RAR and RXR) in various liver cells was performed. Our data demonstrate that RXRalpha is the dominant receptor in all liver cells, and that RARbeta is also expressed at a high level in most cells. More specifically, RARbeta and RARalpha were the most predominant of the RAR subtypes in parenchymal cells, while all three RAR subtypes were equally expressed in endothelial and Kupffer cells. The total level of expression of all the RXR subtypes were larger than the total level of expression of all the RAR subtypes in all liver cells. This is in agreement with the observation that RXR is a heterodimer partner not only for RAR, but also for other members in the steroid/thyroid receptor superfamily of ligand-dependent transcription factors.

Secretion of N-(4-hydroxyphenyl) retinamide-retinol-binding protein from liver parenchymal cells: evidence for reduced affinity of the complex for transthyretin

The synthetic retinoid 4-HPR has been shown to markedly lower the plasma concentration of both retinol and RBP in rats and humans. We have studied the effect of 4-HPR on the secretion of retinol-RBP from liver cells in vivo and in vitro. In rats maintained with a normal diet, a vitamin A-deficient diet or a normal diet supplemented with 4-HPR, chylomicrons [3H]retinyl esters were rapidly cleared from the plasma. The secretion of chylomicron-derived [3H]retinol from tissues to the circulation, however, was different. In control rats, the lymph-derived [3H]retinol peaked after about 2 hr, whereas 4-HPR treatment effectively reduced this peak of [3H]retinol. Our results suggest that 4-HPR inhibits secretion of retinol-RBP from the liver. Therefore, we decided to study the effect of 4-HPR on the secretion of RBP using the human hepatoma cell line HepG2. Retinol and 4-HPR were found to induce the secretion of RBP. The medium from cells treated with 4-HPR was immunoprecipitated with antibodies against human RBP. HPLC analysis of the precipitated RBP revealed the presence of 4-HPR. When the medium from cells incubated with either 4-HPR or retinol was applied to a TTR affinity column, we found that RBP from cells incubated with 4-HPR had a considerably reduced affinity for TTR. We conclude that 4-HPR binds RBP and thereby induces secretion of RBP in HepG2 cells, and that the secreted 4-HPR-RBP complex has a reduced affinity for TTR. This observation may explain the 4-HPR-induced reduction of plasma retinol and RBP observed in in vivo studies.

Secretion of N-(4-hydroxyphenyl) retinamide-retinol-binding protein from liver parenchymal cells: evidence for reduced affinity of the complex for transthyretin

The synthetic retinoid 4-HPR has been shown to markedly lower the plasma concentration of both retinol and RBP in rats and humans. We have studied the effect of 4-HPR on the secretion of retinol-RBP from liver cells in vivo and in vitro. In rats maintained with a normal diet, a vitamin A-deficient diet or a normal diet supplemented with 4-HPR, chylomicrons [3H]retinyl esters were rapidly cleared from the plasma. The secretion of chylomicron-derived [3H]retinol from tissues to the circulation, however, was different. In control rats, the lymph-derived [3H]retinol peaked after about 2 hr, whereas 4-HPR treatment effectively reduced this peak of [3H]retinol. Our results suggest that 4-HPR inhibits secretion of retinol-RBP from the liver. Therefore, we decided to study the effect of 4-HPR on the secretion of RBP using the human hepatoma cell line HepG2. Retinol and 4-HPR were found to induce the secretion of RBP. The medium from cells treated with 4-HPR was immunoprecipitated with antibodies against human RBP. HPLC analysis of the precipitated RBP revealed the presence of 4-HPR. When the medium from cells incubated with either 4-HPR or retinol was applied to a TTR affinity column, we found that RBP from cells incubated with 4-HPR had a considerably reduced affinity for TTR. We conclude that 4-HPR binds RBP and thereby induces secretion of RBP in HepG2 cells, and that the secreted 4-HPR-RBP complex has a reduced affinity for TTR. This observation may explain the 4-HPR-induced reduction of plasma retinol and RBP observed in in vivo studies.

Storage of vitamin A in extrahepatic stellate cells in normal rats

In mammals, vitamin A is primarily stored as retinyl esters in hepatic stellate cells under normal dietary intake of the vitamin. Previously, extrahepatic vitamin A-storing stellate cells have only been identified in animals maintained on a vitamin A-rich diet, and it has not been known whether these cells play a role in normal vitamin A metabolism. The purpose of this study was, to quantify the stellate cell lipid droplet area in hepatic and extrahepatic stellate cells in control rats and in rats fed excess vitamin A. The stellate cells were identified by the gold chloride staining technique, specific autofluorescence of retinyl ester, and by electron microscopy. The stellate cell lipid droplet area was then quantitated by the use of morphometric quantitation. We demonstrated that lipid droplet-containing stellate cells were identified in liver, lung, kidney, and intestine, in normal as well as vitamin A-fed rats. The area of lipid droplets in liver, lung, and intestine stellate cells of normal rats was 0.2, 0.3, and 0.04 mm2 per cm2 tissue, respectively. When the rats were administered excess vitamin A, the hepatic, lung, and intestinal stellate cell lipid droplet area increased about 10-fold, 2-fold, and 40-fold, respectively. Thus the present study shows that extrahepatic stellate cells in lung and intestine of normal rats contain lipid droplets, and that these lipid droplets increase in area when high doses of vitamin A are fed to the animals. These data suggest that not only liver stellate cells but also extrahepatic stellate cells play an important role in vitamin A storage in normal as well as vitamin A-fed animals.

The C-terminal RNLL sequence of the plasma retinol-binding protein is not responsible for its intracellular retention

An in vitro model system using COS cells that transiently express human plasma retinol binding protein has been set up in which we are able to mimic the retinol dependent secretion of this protein observed in hepatocytes. In the absence of its ligand, plasma retinol binding protein is retained in the endoplasmic reticulum. It contains a C-terminal sequence, RNLL, that could function as a cryptic KDEL motif and thus be responsible for its retention in the endoplasmic reticulum. The model system has been used to test a mutant lacking these four last amino acids for retention and retinol induced secretion. The results obtained show that although plasma retinol binding protein is retained in the endoplasmic reticulum, the RNLL sequence does not seem to be responsible for its retention.