Rabih Slim

PCB-induced oxidative stress in endothelial cells: modulation by nutrients

There is an increasing body of evidence suggesting that exposure to Superfund chemicals may have adverse consequences on many organ systems, as well as carcinogenic and atherogenic effects. This is particularly true for polyhalogenated aromatic hydrocarbons such as the polychlorinated biphenyls (PCBs). The vascular endothelium, which is constantly exposed to blood components including environmental contaminants, is extremely vulnerable to chemical insult as well as necrotic and apoptotic injury. Our recent studies suggest that certain PCBs, especially coplanar PCBs, can compromise normal functions of vascular endothelial cells by activating oxidative stress-sensitive signaling pathways and subsequent proinflammatory events critical in the pathology of atherosclerosis and cardiovascular disease. Our findings suggest that an increase in the level of cellular oxidative stress is a significant event in PCB-mediated endothelial cell dysfunction and that nutrients can modulate PCB-induced oxidative stress and endothelial toxicity. We have demonstrated that the dietary fat linoleic acid, the parent unsaturated fatty acid of the omega-6 family, can increase endothelial dysfunction induced by selected PCBs, probably by contributing to oxidative stress and as the result of the production of toxic metabolites called leukotoxins. The subsequent imbalance in the overall cellular oxidant/antioxidant status can activate oxidative stress- or redoxsensitive transcription factors, which in turn promote gene expression for inflammatory cytokines and adhesion molecules, intensifying the inflammatory response and endothelial cell dysfunction. Our data also suggest that antioxidant nutrients such as vitamin E can protect against endothelial cell damage mediated by PCBs or polyunsaturated dietary fats by interfering with oxidative stress-sensitive and proinflammatory signaling pathways. The concept that nutrition can modify or ameliorate the toxicity of Superfund chemicals is provocative and warrants further study as the implications for human health are significant. The information from such studies could be used to develop dietary recommendations and nutritional interventions for populations at high risk for exposure to PCBs, including communities living near Superfund sites and those exposed via occupation or diet.

Susceptibility to hepatic oxidative stress in rabbits fed different animal and plant fats.

OBJECTIVE This study was designed to determine the effect of diets enriched with plant and animal fats on oxidative stress and glutathione metabolism in rabbit liver tissues. This study was conducted to investigate whether the type of dietary fat will impact fatty acid composition and oxidant/antioxidant status in tissues. METHODS Rabbits were fed diets containing 2 g corn oil/100 g diet (low fat diet, LF) and LF supplemented with 16 g/100 g diet of either corn oil (CO), CO with added cholesterol (CO + C), milk fat (MF), chicken fat (CF), beef tallow (BT), or lard (L) for 30 days. After the feeding period, livers were analyzed for total fatty acid composition, thiobarbituric acid reactive substances (TBARS), conjugated dienes, and reduced glutathione (GSH), as well as for activities of glutathione peroxidase (GP) and glutathione reductase (GR). Moreover, to fully determine the oxidative stability and free radical trapping capacity, TBARS levels were measured after additional exposure of liver homogenates to 10 mM 2,2(1)-azo-bis-amidinopropane- hydrochloride (AAPH) for up to 21 hours. RESULTS CO and CF, but not saturated fats such as MF, increased liver conjugated diene and TBARS levels and decreased liver GSH levels and GP activity. In tissues additionally exposed to AAPH, the maximum oxidation, measured as TBARS, was reached between 6 and 7 hours of treatment, independent of dietary fat. In addition, there was a marked effect of AAPH on the maximum rate of TBARS formation with the following descending order: CO > CF > CO + C > L > MF > BT > LF. This high susceptibility to oxidative stress in liver tissues of rabbits fed the CO diet may be explained in part by the significant elevation in linoleic acid (18:2n-6). DISCUSSION There appears to be an inverse correlation between dietary fat-mediated oxidative stress and antioxidant enzyme activities. The present data suggest that high levels of dietary unsaturated fat should be avoided if oxidative stress is a critical issue in nutrition-related diseases. In addition, these data support our hypothesis that diets rich in MF provide a lipid environment with low susceptibility to oxidative stress.

Zinc nutrition and apoptosis of vascular endothelial cells: implications in atherosclerosis

Little is known about the requirements and function of zinc in maintaining endothelial cell integrity, especially during stressful conditions, such as the inflammatory response in cardiovascular disease. There is evidence that zinc requirements of the vascular endothelium are increased during inflammatory conditions such as atherosclerosis, where apoptotic cell death is also prevalent. Apoptosis is a morphologically distinct mechanism of programmed cell death which involves the activation of a cell-intrinsic suicide program, and there is evidence that factors such as inflammatory cytokines (e.g., tumor necrosis factor [TNF]) and pure or oxidized lipids are necessary to induce the cell death pathway. Because of its constant exposure to blood components, including prooxidants, diet-derived fats, and their derivatives, the endothelium is very susceptible to oxidative stress and to apoptotic injury mediated by blood lipid components, prooxidants, and cytokines. Thus, it is likely that the cellular lipid environment, primarily polyunsaturated fatty acids, can potentiate the overall endothelial cell injury by increasing cellular oxidative stress and cytokine release in proximity to the endothelium, which then could further induce apoptosis and disrupt endothelial barrier function. Our data suggest that zinc deficiency exacerbates the detrimental effects of specific fatty acids (e.g., linoleic acid) and inflammatory cytokines, such as TNF, on vascular endothelial functions. We propose that a major mechanism of zinc protection against disruption of endothelial cell integrity during inflammatory conditions, is by the ability of zinc to inhibit the pathways of signal transduction leading to apoptosis and especially mechanisms that lead to upregulation of caspase genes.

Linoleic acid amplifies polychlorinated biphenyl-mediated dysfunction of endothelial cells

Selected dietary lipids may increase the atherogenicity of environmental chemicals, such as polychlorinated biphenyls (PCBs), by cross‐amplifying mechanisms leading to dysfunction of the vascular endothelium. To investigate this hypothesis, cultured endothelial cells were treated with 90 μM linoleic acid (18:2n‐6), followed by either one of two PCBs, 3,3′,4,4′‐tetrachlorobiphenyl (PCB 77) or 2,2′4,4′,5,5′‐hexachlorobiphenyl (PCB 153). These PCBs were selected for their varying binding activities with the aryl hydrocarbon (Ah) receptor and differences in their induction of cytochrome P450. PCB 77 disrupted endothelial barrier function by allowing an increase in albumin transfer across endothelial monolayers. Prior cellular enrichment with 18:2 before PCB treatment further diminished endothelial barrier function, as compared to cells treated only with the PCB. This phenomenon appears to be mediated by increased oxidative stress, which is supported by enhanced 2,7‐dichlorofluorescein fluorescence, activation data of the oxidative stress‐sensitive nuclear transcription factor‐κB (NF‐κB), as well as an observed decrease in vitamin E content in the culture media. Similar to the endothelial permeability data, pre‐enrichment of cells with 18:2 further increased the PCB‐mediated induction of cytochrome P450 1A. In contrast to PCB 77, PCB 153 (or 18:2 plus PCB 153) had little or no effect on endothelial barrier function. Our results suggest that certain unsaturated fatty acids can potentiate PCB‐mediated endothelial cell dysfunction and that oxidative stress and activation of the cytochrome P450 1A subfamily may be, in part, responsible for these metabolic events. These findings have implications for understanding the involvement of certain environmental contaminants in diseases that involve dysfunction of the vascular endothelium.

Dietary cholesterol supplementation protects against endothelial cell dysfunction mediated by native and lipolyzed lipoproteins derived from rabbits fed high-corn oil diets

Abstract Epidemiologic studies indicate that dietary plant fats are negatively and dietary cholesterol positively correlated with the incidence of cardiovascular disease. However, recent research suggests that unsaturated fats may be atherogenic because of their contribution to cellular oxidative stress and that cholesterol may exhibit antioxidant properties. To test this hypothesis, rabbits were fed diets containing 2 g corn oil/100 g diet, supplemented either with 16 g/100 g diet of corn oil (CO) or CO plus added cholesterol (CO+C) for 10 weeks. The cholesterol concentration in the CO+C diet was less than 30 mg cholesterol/100 g diet, which was sufficient to cause a significant rise in plasma and LDL cholesterol, but that did not lead to any detectable fatty steak or lesion formation. Compared with the native CO group, lipid hydroperoxide levels were significantly lower in both native lipoproteins ( VLDL LDL ) and lipolyzed remnants of this lipoprotein mixture derived from rabbits fed CO+C. This may be attributable in part to the observed decrease in unsaturated fatty acids as a result of dietary cholesterol supplementation. Relative to native lipoproteins, endothelial cells exposed to lipolyzed lipoprotein remnants experienced a significant increase in oxidative stress, as evidenced by increased DCF fluorescence, NF-ϰB, an oxidative stress sensitive transcription factor, was markedly induced in cells treated with both native and lipolyzed lipoproteins derived from the CO group, compared to the CO+C group. Independent of lipolysis, oxidative stress was markedly decreased in cells exposed to lipoproteins derived from animals fed CO+C, Furthermore, dietary cholesterol supplementation protected endothelial cells against lipolytic remnant-mediated barrier dysfunction. These data continue to support the hypothesis that lipolytic lipoprotein remnants are atherogenic and that small amounts of supplemental cholesterol may provide antioxidant protection.