Electronegative very-low-density lipoprotein induces brain inflammation and cognitive dysfunction in mice.

Epidemiologic studies have indicated that dyslipidemia may facilitate the progression of cognitive dysfunction. We previously showed that patients with metabolic syndrome (MetS) had significantly higher plasma levels of electronegative very-low-density lipoprotein (VLDL) than did healthy controls. However, the effects of electronegative-VLDL on the brain and cognitive function remain unclear. In this study, VLDL isolated from healthy volunteers (nVLDL) or patients with MetS (metVLDL) was administered to mice by means of tail vein injection. Cognitive function was assessed by using the Y maze test, and plasma and brain tissues were analyzed. We found that mice injected with metVLDL but not nVLDL exhibited significant hippocampus CA3 neuronal cell loss and cognitive dysfunction. In mice injected with nVLDL, we observed mild glial cell activation in the medial prefrontal cortex (mPFC) and hippocampus CA3. However, in mice injected with metVLDL, plasma and brain TNF-α and Aß-42 levels and glial cell activation in the mPFC and whole hippocampus were higher than those in control mice. In conclusion, long-term exposure to metVLDL induced levels of TNF-α, Aß-42, and glial cells in the brain, contributing to the progression of cognitive dysfunction. Our findings suggest that electronegative-VLDL levels may represent a new therapeutic target for cognitive dysfunction.

VLDL from Metabolic Syndrome Individuals Enhanced Lipid Accumulation in Atria with Association of Susceptibility to Atrial Fibrillation.

Metabolic syndrome (MetS) represents a cluster of metabolic derangements. Dyslipidemia is an important factor in MetS and is related to atrial fibrillation (AF). We hypothesized that very low density lipoproteins (VLDL) in MetS (MetS-VLDL) may induce atrial dilatation and vulnerability to AF. VLDL was therefore separated from normal (normal-VLDL) and MetS individuals. Wild type C57BL/6 male mice were divided into control, normal-VLDL (nVLDL), and MetS-VLDL (msVLDL) groups. VLDL (15 µg/g) and equivalent volumes of saline were injected via tail vein three times a week for six consecutive weeks. Cardiac chamber size and function were measured by echocardiography. MetS-VLDL significantly caused left atrial dilation (control, n = 10, 1.64 ± 0.23 mm; nVLDL, n = 7, 1.84 ± 0.13 mm; msVLDL, n = 10, 2.18 ± 0.24 mm; p < 0.0001) at week 6, associated with decreased ejection fraction (control, n = 10, 62.5% ± 7.7%, vs. msVLDL, n = 10, 52.9% ± 9.6%; p < 0.05). Isoproterenol-challenge experiment resulted in AF in young msVLDL mice. Unprovoked AF occurred only in elderly msVLDL mice. Immunohistochemistry showed excess lipid accumulation and apoptosis in msVLDL mice atria. These findings suggest a pivotal role of VLDL in AF pathogenesis for MetS individuals.

Stimulation of stellate cells by injured acinar cells: a model of acute pancreatitis induced by alcohol and fat (VLDL).

Mechanisms leading to acute pancreatitis after a fat-enriched meal combined with excess alcohol are incompletely understood. We have studied the effects of alcohol and fat (VLDL) on pancreatic acinar cell (PAC) function, oxidative stress, and repair mechanisms by pancreatic stellate cells (PSC) leading to fibrogenesis. To do so, PAC (rat) were isolated and cultured up to 24 h. Ethanol and/or VLDL were added to PAC. We measured PAC function (amylase, lipase), injury (lactic dehydrogenase), apoptosis (TUNEL, Apo2.7, annexin V binding), oxidative stress, and lipid peroxidation (conjugated dienes, malondialdehyde, chemoluminescence); we also measured PSC proliferation (bromodeoxyuridine incorporation), matrix synthesis (immunofluorescence of collagens and fibronectin, fibronectin immunoassay), and fatty acids in PAC supernatants (gas chromatography). Within 6 h, cultured PAC degraded and hydrolyzed VLDL completely. VLDL alone (50 microg/ml) and in combination with alcohol (0.2, 0.5, and 1% vol/vol) induced PAC injury (LDL, amylase, and lipase release) within 2 h through generation of oxidative stress. Depending on the dose of VLDL and alcohol, apoptosis and/or necrosis were induced. Antioxidants (Trolox, Probucol) reduced the cytotoxic effect of alcohol and VLDL. Supernatants of alcohol/VLDL-treated PAC stimulated stellate cell proliferation and extracellular matrix synthesis. We concluded that, in the presence of lipoproteins, alcohol induces acinar cell injury. Our results provide a biochemical pathway for the clinical observation that a fat-enriched meal combined with excess alcohol consumption can induce acinar cell injury (acute pancreatitis) followed by repair mechanisms (proliferation and increased matrix synthesis in PSC).

[Effects of very low-density lipoprotein on cellular lipid accumulation and the expression of monocyte chemoattractant protein-1 in human mesangial cells].

OBJECTIVE: To investigate the effects of very low-density lipoprotein (VLDL) on cellular lipid accumulation and the expression of monocyte chemoattractant protein-1 (MCP-1) in human mesangial cells. METHODS: An established stable human mesangial cell line (HMCL) was used in all experiments. VLDL-induced cellular lipid deposition was visualized by Oil Red O staining and analyzed quantitatively by standard enzymatic procedures. MCP-1 mRNA and protein expression levels in treated HMCLs were determined by real-time quantitative RT-PCR and enzyme-linked immunosorbent assay, respectively. For adhesion study, HMCLs were treated with VLDL for 12 hours, followed by a one-hour incubation with THP-1 cells. RESULTS: VLDL induced cellular lipid accumulation in HMCLs in a time- (0-24 h) and dose- (0-200 microg/ml) dependent manner, and the principal component of accumulated lipid is triglyceride. In HMCLs, MCP-1 mRNA expression was promoted by VLDL in a time- (0-6 h) and dose- (0-100 microg/ml) dependent manner, and VLDL also enhanced MCP-1 secretion in a dose-dependent manner. Such an effect was accompanied by increased adhesion of monocytes to HMCLs. CONCLUSIONS: VLDL can induce cellular triglyceride accumulation and upregulate the expression of MCP-1 in human mesangial cells. Hence, VLDL may be involved in the pathogenesis of lipid-mediated renal injury.

Triglyceride-rich lipoproteins prime aortic endothelium for an enhanced inflammatory response to tumor necrosis factor-alpha.

High levels of triglyceride-rich lipoproteins (TGRLs) in blood are linked to development of atherosclerosis, yet the mechanisms by which these particles initiate inflammation of endothelium are unknown. TGRL isolated from human plasma during the postprandial state was examined for its capacity to bind to cultured human aortic endothelial cells (HAECs) and alter the acute inflammatory response to tumor necrosis factor-alpha. HAECs were repetitively incubated with dietary levels of freshly isolated TGRL for 2 hours per day for 1 to 3 days to mimic postprandial lipidemia. TGRL induced membrane upregulation of the low-density lipoprotein family receptors LRP and LR11, which was inhibited by the low-density lipoprotein receptor-associated protein-1. TGRLs alone did not elicit inflammation in HAECs but enhanced the inflammatory response via a 10-fold increase in sensitivity to cytokine stimulation. This was reflected by increased mitogen-activated protein kinase activation, nuclear translocation of NF-kappaB, amplified expression of endothelial selectin and VCAM-1, and a subsequent increase in monocyte-specific recruitment under shear flow as quantified in a microfabricated vascular mimetic device.

Oxidized lipoproteins may play a role in neuronal cell death in Alzheimer disease.

Oxidative stress in the central nervous system (CNS) may cause oxidation of lipoprotein particles. The oxidized lipoproteins may damage cellular and subcellular membranes, leading to tissue injury and cell death. Human low-density lipoprotein (LDL) and very low-density lipoprotein (VLDL) are oxidized by transition metal ions, such as Cu2+. Using PC 12 cells, we tested the cytotoxicity of oxidized LDL and VLDL. Cell death was increased in a dose-dependent manner. Antioxidants added to the incubation medium, such as vitamins E or C, or resveratrol showed some protection. Results indicated that oxidized lipoproteins may serve as an oxidative stressor, which may initiate the neuronal cell death leading to the manifestation of Alzheimer disease (AD).

Aortic endothelial cell damage induced by beta-VLDL and macrophages in vitro.

We performed an in vitro study to assess damage to swine aortic endothelial cells by rabbit beta-VLDL and/or rabbit peritoneal macrophages. Incubation of cultured aortic endothelial cells with beta-VLDL, macrophages, or macrophage lysate induced endothelial cell damage time- and dose-dependently as estimated by [3H]adenine release. Incubation of endothelial cells with both beta-VLDL and macrophages produced a synergistic effect on the increase of [3H]adenine release. Pretreatment of the endothelial cells with some kinds of antioxidants (probucol 50 micrograms/ml, vitamin E 50 microM, superoxide dismutase-polyethylene glycol 0.5 mg/ml, or catalase-polyethylene glycol 0.5-1.0 mg/ml) significantly prevented the endothelial damage by beta-VLDL or macrophage lysate. We conclude that beta-VLDL and/or macrophages could induce endothelial cell damage and that some kinds of antioxidants could prevent it.

Lipolytic surface remnants of triglyceride-rich lipoproteins are cytotoxic to macrophages but not in the presence of high density lipoprotein. A possible mechanism of atherogenesis?

Hypertriglyceridemic (HTG) serum, lipolyzed in vitro by purified bovine milk lipoprotein lipase, was found to be cytotoxic to cultured macrophages. Surviving macrophages contained numerous lipid inclusions similar to those found in foam cells. Individual lipoprotein fractions isolated from the lipolyzed HTG serum, including HDL, were also cytotoxic. Lipolysis of isolated lipoprotein fractions (either HTG or normal) allowed localization of cytotoxicity to postlipolysis remnant VLDL and chylomicron particles. The presence of a critical concentration of HDL in either the lipolysis mixture or the culture dishes inhibited the cytotoxicity. Below this critical concentration HDL itself became cytotoxic, producing lipid inclusions in surviving macrophages. The lipid fraction of the cytotoxic remnants contained the cytotoxic factor(s); neither FFA nor lysolecithin alone could account for this cytotoxicity. Postprandial lipemic sera from subjects with a brisk chylomicron response, when lipolyzed in vitro, were cytotoxic to cultured macrophages; neither fasted sera from these subjects, nor postprandial sera from normolipidemic subjects with a normal chylomicron response, were cytotoxic. Postheparin (in vivo lipolyzed) serum and its isolated lipoprotein fractions obtained 30 min after heparin injection in subjects with HTG were shown to be cytotoxic to macrophages; by 60 min most of the cytotoxicity had disappeared. The postprandial and postheparin observations support an in vivo significance for remnant-associated cytotoxicity. We hypothesize that cytotoxic remnants of lipolyzed VLDL and chylomicrons may be one of the major atherogenic lipoproteins. Further, we suggest that inhibition of the cytotoxicity of these remnants may be one important way that HDL prevents atherosclerosis.

Injury of arterial endothelial cells in diabetic, sucrose-fed and aged rats.

The toxicity of elevated levels of very low density lipoproteins (VLDL, d less than 1.006 g/ml) was investigated using porcine aortic endothelial cells in vitro. VLDL isolated from normal rat serum and added at elevated levels was as toxic as VLDL isolated from streptozotocin-induced diabetic rat serum. Injury was detected by scanning electron microscopy in 4-day-old primary cultures of endothelial cells after a 1/2-h exposure to diabetic rat serum. Bleb formation and contraction was seen first in isolated cells (1/2 h), followed by cells at the periphery of the monolayer (1 h) and finally in cells throughout the monolayer (4 h). By 10 h few cells remained attached to the dish. A similar sequence of events occurred in 1-day-old cultures after a 3-h lag period. Serum from sucrose-fed as well as aged rats was also found to be toxic to endothelial cells in vitro. Elevated levels of VLDL were responsible for the toxicities of these sera. Scanning electron microscopy of the aortas from diabetic and sucrose-fed rats revealed endothelial desquamation, platelet and leukocyte attachment, fibrin deposition and the presence of microthrombi. The common occurrence of both micro- and macrovascular disease in diabetic, sucrose-fed, and aged rats and the toxicity of their serum in vitro suggest that elevated levels of VLDL may initiate vascular disease in these models.

Lipoprotein oxidation and lipoprotein-induced cytotoxicity.

The results of this study indicate that when human VLDL or LDL is prepared under conditions allowing oxidation, such oxidation renders the molecular complexes highly toxic to human skin fibroblasts growing in culture. The cytotoxicity can be predicted by assaying for the presence of thiobarbituric acid-reacting substances on the lipoprotein. However, malondialdehyde, which reacts with thiobarbituric acid and is known to be injurious to cells, was not cytotoxic in the same experimental system when dissolved in culture medium or covalently bound to non-toxic LDL. The toxic agent(s) on oxidized LDL is(are) located in a lipid-extractable moiety. Since lipid peroxides and oxidized sterols can occur in vivo under various pathological conditions, the cytotoxicity of these lipoprotein-associated substances observed in vitro may be related to certain manifestations of these conditions.