Influence of nutrients and cytokines on endothelial cell metabolism

Zinc Modulates PPARγ Signaling and Activation of Porcine Endothelial Cells

Dietary zinc has potent antioxidant and anti-inflammatory properties and is a critical component of peroxisome proliferator-activated receptor (PPAR) gene expression and regulation. To assess the protective mechanisms of PPARgamma in endothelial cell dysfunction and the role of zinc in the modulation of PPARgamma signaling, cultured porcine pulmonary artery endothelial cells were exposed to the membrane-permeable zinc chelator N,N,N'N'-tetrakis (2-pyridylmethyl)-ethylene diamine (TPEN), thiazolidinedione (TZD; PPARgamma agonist) or bisphenol A diglycidyl ether (BADGE; PPARgamma antagonist). Subsequently, endothelial cells were activated by treatment with linoleic acid (90 micro mol/L) for 6 h. Zinc chelation by TPEN increased the DNA binding activity of nuclear factor (NF)-kappaB and activator protein (AP)-1, decreased PPARgamma expression and activation as well as up-regulated interleukin (IL)-6 expression and production. These effects were fully reversed by zinc supplementation. In addition, exposure to TZD down-regulated linoleic acid-induced DNA binding activity of NF-kappaB and AP-1, whereas BADGE further induced activation of these oxidative stress-sensitive transcription factors. Most importantly, the TZD-mediated down-regulation of NF-kappaB and AP-1 and reduced inflammatory response were impaired during zinc chelation. These data suggest that zinc plays a critical role in PPARgamma signaling in linoleic acid-induced endothelial cell activation and indicate that PPARgamma signaling is impaired during zinc deficiency.

Nutrient involvement in preeclampsia

Preeclampsia is a pregnancy-specific condition that increases maternal and infant mortality and morbidity. It is diagnosed by new-onset increased blood pressure and proteinuria during gestation; for many years these markers were the sole targets for study. More recently, increased attention to the multisystemic nature of the syndrome with involvement of almost all organs, activation of coagulation and increased sensitivity to pressor agents has expanded understanding of the disorder. The epidemiology of preeclampsia, being more common in poor women, long ago suggested that nutrients might be involved in the disorder. Numerous conflicting hypotheses were advanced but the testing of these hypotheses has either been done poorly or not at all. Review of the available data indicates very few studies that provide useful insights. In many studies the syndrome is poorly defined and in most studies nutritional data (questionnaires or biomarkers) are obtained on women with the clinical syndrome. In overtly preeclamptic women it is impossible to decipher cause from effect. Nonetheless, current concepts of the genesis of preeclampsia that include endothelial dysfunction, inflammatory activation, oxidative stress and predisposing maternal factors provide targets for well-designed nutritional investigation. In this review the current concepts of the pathogenesis of preeclampsia are reviewed and available data are assessed in light of these concepts. Targets for nutritional investigation based on the current knowledge of pathophysiology are suggested.

High-Energy Diets, Fatty Acids and Endothelial Cell Function: Implications for Atherosclerosis

Diets high in fat and/or calories can lead to hypertriglyceridemia and postprandial lipemia and thus are considered a risk factor for the development of atherosclerosis. Plasma chylomicron levels are elevated in humans after consuming a high-fat meal, and hepatic synthesis of VLDL is increased when caloric intake is in excess of body needs. High lipoprotein lipase activity and subsequent hydrolysis of triglyceride-rich lipoproteins may be an important source of elevated concentrations of fatty acid anions in the proximity to the endothelium and hence a major risk factor for atherosclerosis. We have shown that selected fatty acids, as well as lipoprotein lipase-derived remnants of lipoproteins isolated from hypertriglyceridemic subjects, can activate vascular endothelial cells and disrupt endothelial integrity. Our studies suggest that omega-6 fatty acids, and especially linoleic acid, cause endothelial cell dysfunction most markedly as well as can potentiate TNF-mediated endothelial cell injury. We propose that high-energy diets, and especially diets rich in linoleic acid, are atherogenic by contributing to an imbalance in cellular oxidative stress/antioxidant status of the endothelium, which can lead to activation of oxidative stress-responsive transcription factors, inflammatory cytokine production and the expression of adhesion molecules. Our data also suggest that nutrients, which have antioxidant and/or membrane stabilizing properties, can protect endothelial cells. These findings contribute to the understanding of the interactive role of high fat/calorie diets and subsequent hypertriglyceridemia with inflammatory components and nutrients that exhibit antiatherogenic properties in the development of atherosclerosis. Moreover, results from our research further support the concept that high-fat/calorie diets and associated postprandial hypertriglyceridemia are significant risk factors for atherosclerosis.

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 microM 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-kappaB (NF-kappaB), 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.

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.

Antioxidant-like properties of zinc in activated endothelial cells

OBJECTIVE

The objective of this study was to test the hypothesis that zinc deficiency in endothelial cells may potentiate the inflammatory response mediated by certain lipids and cytokines, possibly via mechanisms associated with increased cellular oxidative stress. Our experimental approach was to compare conditions of cellular zinc deficiency and zinc supplementation with oxidative stress-mediated molecular and biochemical changes in vascular endothelial cells.

METHODS

To investigate our hypothesis, porcine pulmonary artery-derived endothelial cells were depleted of zinc by culture in media containing 1% fetal bovine serum for eight days. Subsequently, endothelial cells were exposed to media enriched with or without zinc (10 microM) for two days, followed by exposure to either tumor necrosis factor-alpha (TNF, 500 U/mL) or linoleic acid (90 microM), before measurement of oxidative stress (DCF fluorescence), activation of nuclear factor kappaB (NF-kappaB) or activator protein-1 (AP-1) and production of the inflammatory cytokine interleukin 6 (IL-6).

RESULTS

Oxidative stress was increased markedly in zinc-deficient endothelial cells following treatment with fatty acid or TNF. This increase in oxidative stress was partially blocked by prior zinc supplementation. The oxidative stress-sensitive transcription factor NF-kappaB was up-regulated by zinc deficiency and fatty acid treatment. The up-regulation mediated by fatty acids was markedly reduced by zinc supplementation. Similar results were obtained with AP-1. Furthermore, endothelial cell production of IL-6 was increased in zinc-deficient endothelial cells following treatment with fatty acids or TNF. This increase in production of inflammatory cytokines was partially blocked by zinc supplementation.

DISCUSSION

Our previous data clearly show that zinc is a protective and critical nutrient for maintenance of endothelial integrity. The present data suggest that zinc may in part be antiatherogenic by inhibiting oxidative stress-responsive events in endothelial cell dysfunction. This may have implications in understanding mechanisms of atherosclerosis.

Effect of linoleic acid on endothelial cell inflammatory mediators

Selected lipids may influence the inflammatory cascade within the vascular endothelium. To test this hypothesis, endothelial cells were treated with linoleic acid (18:2, n - 6) for 12 hours and/or tumor necrosis factor-alpha (TNF) for 4 hours. For a combined exposure to 18:2 and TNF (18:2 + TNF), cells were first preenriched with 18:2 for 8 hours before exposure to TNF for an additional 4 hours. Exposure to 18:2 increased cellular oxidative stress, activated nuclear factor-kappaB (NF-kappaB), increased interleukin-8 (IL-8) production, and elevated intercellular adhesion molecule-1 (ICAM-1) levels. A combined exposure to 18:2+ TNF resulted in decreased NF-kappaB activation compared with TNF treatment alone. In addition, preexposure to 18:2 altered TNF-mediated IkappaB-alpha signaling. Within the first 15 minutes of a 90-minute period, cytoplasmic levels of IkappaB-alpha decreased more rapidly in cells treated with 18:2 + TNF compared with TNF, suggesting translocation and activation of NF-kappaB in cultures that were pretreated with 18:2 before TNF exposure. A combined exposure to 18:2+TNF had various effects on IL-8 production and ICAM-1 levels depending on the time of exposure. For example, 18:2 + TNF treatment increased ICAM-1 levels at 12 hours but decreased ICAM-1 levels at 24 hours compared with treatment with TNF alone. These data suggest that selected fatty acids such as 18:2 can exert proinflammatory effects and, in addition, may markedly alter TNF-mediated inflammatory events.

Dietary fat, genes, and human health

These studies show that a macronutrient like dietary fat plays an important role in gene expression. In the cases presented here, dietary fat regulates gene expression leading to changes in carbohydrate and lipid metabolism. The interesting outcome of these studies is the finding that the molecular targets for dietary fat action did not converge with the principal targets for hormonal regulation of gene transcription, like hormone receptors. Instead, PUFA-RF targets elements that play key ancillary roles in gene transcription. This is important because it shows how PUFA can interfere with hormone regulation of a specific gene without having generalized effect on overall hormonal control, i.e. PUFA effects are promoter-specific. How PUFA-RF interferes with gene transcription will require the isolation and characterization of PUFA-RF along with the tissue-specific factors targeted by PUFA-RF. A different story emerges when fatty acids activate PPAR. Based on the studies presented here and elsewhere, long chain-highly unsaturated fatty acids (like 20:5,n-3 and 22:6, n-3) or high levels of fat activate PPAR. PPAR directly activates genes like AOX, but also inhibits transcription of genes like S14, FAS, apolipoprotein CIII, transferrin. For S14, the mechanism of inhibition involves sequestration of RXR, a critical factor for T3 receptor binding to DNA. Thus, PPAR can have generalized effects on T3 action or on other nuclear receptors, like vit. D (VDR) and retinoic acid (RAR) receptors, that require RXR for action. For apolipoprotein CIII and transferrin, PPAR/RXR heterodimers compete for HNF-4 binding sites (DR + 1). In addition to HNF-4, COUP-TF, ARP-1 and RXR all bind the DR + 1 type motif. These factors are important for tissue-specific regulation of gene transcription. PPAR can potentially interfere with the transcription of multiple genes through disruption of nuclear receptor signaling leading to changes in phenotype. Clearly, more studies are required to assess the role PPAR plays in the fatty acid regulation of gene transcription and its contribution to chronic disease. Finally, it is clear that dietary fat has the potential to affect gene expression through multiple pathways. Depending on the gene examined, PUFA might augment or abrogate gene transcription which leads to specific phenotypic changes altering metabolism, differentiation or cell growth. These effects can be beneficial to the organism, such as the n-3 PUFA-mediated suppression of serum triglycerides or detrimental, like the saturated and n-6 PUFA-mediated promotion of insulin resistance. How such effects contribute to the onset or progression of specific neoplasia is unclear. However, studies in metabolism might provide important clues for this connection.

Zinc attenuates tumor necrosis factor-mediated activation of transcription factors in endothelial cells

OBJECTIVE

The objective of the study was to test the hypothesis that zinc can protect against endothelial dysfunction by interfering with oxidative stress-mediated cellular signaling and subsequent inhibition of an endothelial cell inflammatory response. Our approach was to compare alterations on molecular and biochemical levels with changes in endothelial barrier function that occur in zinc deficient conditions.

METHODS

To investigate our hypothesis, endothelial cells were exposed to zinc deficient media for 2 to 10 days to deplete cellular zinc stores. Following this, half of the groups received zinc supplementation (9.2 microM) for 48 hours. The other half served as zinc deficient controls. These cells were then challenged with tumor necrosis factor-alpha (TNF) for varying time periods. Nuclear extracts were prepared from cells and analyzed for nuclear factor kappa B (NF-kappa B) and activator protein-1 (AP-1) binding. Media from cells were analyzed for interleukin 8 (IL-8) production, and cellular proteins were determined.

RESULTS

Zinc supplementation resulted in a 74% increase in cellular zinc content. It was also shown that a 1.5 hour exposure to TNF (100 U/mL medium) significantly increased NF-kappa B and AP-1 binding, which was lowered considerably when cells were supplemented with physiological levels of zinc. Zinc supplementation also caused a marked attenuation in IL-8 expression by endothelial cells in response to TNF-mediated cell activation.

DISCUSSION

Our previous data clearly show that zinc is a protective and critical nutrient for maintenance of endothelial integrity. The present data suggest that zinc may protect against cytokine-mediated activation of oxidative stress sensitive transcription factors, upregulation of inflammatory cytokines and endothelial cell dysfunction. This may have implications in understanding mechanisms of atherosclerosis.

Cyclic changes in the organization of cell adhesions and the associated cytoskeleton, induced by stimulation of tyrosine phosphorylation in bovine aortic endothelial cells

In this study we have investigated the relationships between the stimulation of tyrosine-specific protein phosphorylation and the state of assembly of cell-cell and cell-matrix adherens-type junctions. Bovine aortic endothelial (BAE) cells were treated with either the phosphotyrosine phosphatase inhibitor pervanadate or with epidermal growth factor (EGF), and the effect of the treatment on the organization of cell contacts and the actin cytoskeleton was evaluated by digital immunomicroscopy. We show here that pervanadate induced a dramatic (about 40-fold) increase in the level of phosphotyrosine labeling of cell-cell junctions, which reached maximal values following 20 minutes of incubation. Concomitantly, the junctional levels of vinculin, actin and plakoglobin increased, followed by a slower recruitment of cadherins to these sites. Upon longer incubation cell-cell junctions deteriorated and stress fibers and focal adhesions were formed. EGF stimulation of serum-starved BAE cells induced a rapid ‘wave’ of junctional tyrosine phosphorylation, followed by cyclic changes in the local levels of phosphotyrosine labeling. Periodic changes were also found in the intensity of labeling of junctional actin, vinculin and cadherins. These results suggest that tyrosine phosphorylation and the assembly of cell-cell adherens junctions are interdependent processes, and raise the possibility that the cross-talk between the two is responsible both for the regulation of junction formation and for adhesion-mediated signaling.

Lipid composition of cultured endothelial cells in relation to their growth

Human endothelial cells in culture were examined in different growth conditions. The human endothelial cell line, EA.hy 926 cell line, was used and cells were studied either in exponential growth phase, at confluence, or growth-arrested by serum deprivation. Phospholipids were separated and analyzed by high-performance thin-layer chromatography, and their fatty acids were quantified by gas-liquid chromatography. No significant differences in the phospholipid distributions were found between exponentially growing and confluent endothelial cells in which phosphatidylcholine (PC) represented the major phospholipid. In comparison, serum-deprived cells exhibited higher proportions of sphingomyelin and lower content of PC. We also found that among the total lipids, cholesterol level for dividing endothelial cells was lower than for cells growth-arrested either by serum deprivation or by contact inhibition at confluence. The global fatty acid distribution was not affected by the growth conditions. Thus, oleate (18:1 n-9 and 18:1 n-7), palmitate (C16:0), and stearate (C18:0) were the main components of endothelial cell membranes. However, the fatty acid distributions obtained from each phospholipid species differed with the growth status. Altogether, the data indicated that subtle modulations of endothelial cell metabolism appear upon cell growth. The resulting membrane-dependent cellular functions such as cholesterol transport and receptor activities can be expected to be relevant for lipid trafficking within the vessel wall in vitro and in vivo.

Antiatherogenic properties of zinc: implications in endothelial cell metabolism

Zinc is an essential component of biomembranes and is necessary for maintenance of membrane structure and function. There is evidence that zinc can provide antiatherogenic properties by preventing metabolic physiologic derangements of the vascular endothelium. Because of its antioxidant and membrane-stabilizing properties, zinc appears to be crucial for the protection against cell-destabilizing agents such as polyunsaturated lipids and inflammatory cytokines. Zinc also may be antiatherogenic by interfering with signaling pathways involved in apoptosis. Most importantly, we have evidence that zinc can protect against inflammatory cytokine-mediated activation of oxidative stress-responsive transcription factors, such as nuclear factor kappa B and AP-1. It is very likely that certain lipids and zinc deficiency may potentiate the cytokine-mediated inflammatory response and endothelial cell dysfunction in atherosclerosis. Thus, the antiatherogenic role of zinc appears to be in its ability to inhibit oxidative stress-responsive factors involved in disruption of endothelial integrity and atherosclerosis. We discuss antiatherogenic properties of zinc with a focus on endothelial cell metabolism.

Growth requirements of endothelial cells in culture: variations in serum and amino acid concentrations

Endothelial cell growth in vitro is limited to the availability of nutrients from commercially available media and added serum. Nutrients, such as amino acids, are chiefly derived from the cell culture medium, rather than from added serum, and optimal endothelial cell growth may be dependent on amino acid levels in the culture media. To test this hypothesis, porcine pulmonary artery-derived endothelial cells were exposed to culture medium 199 (M199), amino acid-deficient M199 (dM199), as well as dM199 supplemented with amino acids. Cell protein was similar in cells cultured for 3 d in M199 supplemented with 1, 3, 5 or 10% bovine calf serum, respectively. Addition of amino acid solutions (L-amino acids [Laa], DL-amino acids [DLaa], 2Laa, or Laa+glutamine) to dM199 demonstrated a cell dependence for optimal growth on the type of amino acids as well as on the total available nitrogen in the media. Compared with M199, dM199 supplemented with Laa only partially supported long-term growth of endothelial cells in culture. On the other hand, dM199 supplemented with either 2Laa, DLaa, or Laa+ glutamine was superior over M199 with regard to endothelial cell growth. The addition of Laa+glutamine to dM199 was most growth-supporting, with an increase of over 2.6-fold in total cell protein compared with cells cultured with M199. These results suggest that, in addition to the presence of essential amino acids, total available nitrogen in culture media may be a critical factor for optimal endothelial cell growth.

What turns on the endothelins?

Production of the potent vasoconstrictor peptide endothelin-1 (ET-1) within the circulation is increased markedly in a number of pathologies, such as the damage following from ischaemia and reperfusion, vasculitis, congestive heart failure, and systemic inflammatory response (septic shock syndrome) and related pathological states. All these conditions are associated with marked increases in the production of cytokines such as tumour necrosis factor-alpha and interleukin-2. Our experiments indicate that in rats administration of either of these cytokines results in a rapid increase in the circulating levels of ET-1 and a very pronounced ET-1-dependent coronary vasoconstriction. Furthermore, in rats suffering from adjuvant polyarthritis, in which there is marked joint inflammation and associated cytokine production, there are dramatic increases in coronary perfusion pressure which are absent when rats are treated with an endothelin receptor antagonist. This does not imply that inflammation must be associated with coronary vasoconstriction and myocardial dysfunction, but rather that in the rat, where ET-1 is a strong and almost irreversible constrictor of coronary vessels, coronary perfusion pressure serves as a very good bioassay for ET-1 activity. What our results do imply is that the ET-1 system is latent, but that in numerous disease states it can become rapidly upregulated following endothelial activation by cytokines.

Role of fatty acids and eicosanoids in modulating proteoglycan metabolism in endothelial cells

Endothelial cell dysfunction is considered to be a critical event in the etiology of atherosclerosis. Thus, the preservation of endothelial structure and function are a prerequisite for normal control of vascular permeability properties, mediation of both inflammatory and immunologic responses and the general 'communication' between blood-borne cells and abluminal tissues. Many of these properties can be influenced by proteoglycans present in vascular tissues. There is evidence that selected lipids can be atherogenic by altering endothelial proteoglycan metabolism. Little is known about the role of fatty acids in modulating proteoglycan composition in endothelial cells. Data suggest, however, that linoleic acid in particular can adversely alter proteoglycan metabolism, which may be related to an imbalance in eicosanoid synthesis patterns. These events could be sufficient to disrupt normal endothelial barrier function, initiate smooth muscle migration and proliferation, and result in other metabolic dysfunctions associated with the etiology of vascular diseases such as atherosclerosis. Thus, the focus of this review is on fatty acids and eicosanoids as they may alter proteoglycan metabolism of vascular tissues and in particular of the endothelium.