Oxygen tension within the arterial wall: relationship to altered bioenergetic metabolism and lipid accumulation

Defining Physiological Normoxia for Improved Translation of Cell Physiology to Animal Models and Humans

The extensive oxygen gradient between the air we breathe (Po₂ ~21 kPa) and its ultimate distribution within mitochondria (as low as ~0.5-1 kPa) is testament to the efforts expended in limiting its inherent toxicity. It has long been recognized that cell culture undertaken under room air conditions falls short of replicating this protection in vitro. Despite this, difficulty in accurately determining the appropriate O₂ levels in which to culture cells, coupled with a lack of the technology to replicate and maintain a physiological O₂ environment in vitro, has hindered addressing this issue thus far. In this review, we aim to address the current understanding of tissue Po₂ distribution in vivo and summarize the attempts made to replicate these conditions in vitro. The state-of-the-art techniques employed to accurately determine O₂ levels, as well as the issues associated with reproducing physiological O₂ levels in vitro, are also critically reviewed. We aim to provide the framework for researchers to undertake cell culture under O₂ levels relevant to specific tissues and organs. We envisage that this review will facilitate a paradigm shift, enabling translation of findings under physiological conditions in vitro to disease pathology and the design of novel therapeutics.

The pigeon (Columba livia) model of spontaneous atherosclerosis

Multiple animal models have been employed to study human atherosclerosis, the principal cause of mortality in the United States. Each model has individual advantages related to specific pathologies. Initiation, the earliest disease phase, is best modeled by the White Carneau (WC-As) pigeon. Atherosclerosis develops spontaneously in the WC-As without either external manipulation or known risk factors. Furthermore, susceptibility is caused by a single gene defect inherited in an autosomal recessive manner. The Show Racer (SR-Ar) pigeon is resistant to atherosclerosis. Breed differences in the biochemistry and metabolism of celiac foci cells have been described. For example, WC-As have lower oxidative metabolism but higher amounts of chondroitin-6-sulfate and nonesterified fatty acids compared with SR-Ar. Gene expression in aortic smooth muscle cells was compared between breeds using representational difference analysis and microarray analysis. Energy metabolism and cellular phenotype were the chief gene expression differences. Glycolysis and synthetic cell types were related to the WC-As but oxidative metabolism and contractile cell types were related to the SR-Ar. Rosiglitazone, a PPARγ agonist, blocked RNA binding motif (RBMS1) expression in WC-As cells. The drug may act through the c-myc oncogene as RBMS1 is a c-myc target. Proteomic tests of aortic smooth muscle cells supported greater glycosylation in the WC-As and a transforming growth factor β effect in SR-Ar. Unoxidized fatty acids build up in WC-As cells because of their metabolic deficiency, ultimately preventing the contractile phenotype in these cells. The single gene responsible for the disease is likely regulatory in nature.

Differentially expressed genes in aortic smooth muscle cells from atherosclerosis-susceptible and atherosclerosis-resistant pigeons

Susceptibility to spontaneous atherosclerosis in the White Carneau (WC-As) pigeon shows autosomal recessive inheritance. Aortic smooth muscle cells (SMC) cultured from susceptible WC-As and resistant Show Racer (SR-Ar) pigeons exhibit developmental and degenerative features corresponding to the respective SMC at atherosclerosis-prone sites in vivo. We used representational difference analysis to identify differentially expressed genes between WC-As and SR-Ar aortic SMC. Total RNA was extracted from cultured primary SMC of each breed, converted to double-stranded cDNA, followed by direct comparison in reciprocal representational difference analysis experiments. Difference products were cloned, sequenced, and identified by BLAST against the chicken genome. Six putative biochemical pathways were distinctly different between breeds with genes involved in energy metabolism and contractility exhibiting the most striking disparity. Genes associated with glycolysis and a synthetic SMC phenotype were expressed in WC-As cells. In contrast, SR-Ar cells expressed genes indicative of oxidative phosphorylation and a contractile SMC phenotype. In WC-As cells, the alternatives of insufficient ATP production limiting contractile function or the lack of functional contractile elements downregulating ATP synthesis cannot be distinguished due to the compressed in vitro versus in vivo developmental time frame. However, the genetic potential for effectively coupling energy production to muscle contraction present in the resistant SR-Ar was lacking in the susceptible WC-As.

Mechanisms of LDL oxidation

BACKGROUNDS

Many lines of evidence suggest that oxidized low-density lipoprotein (LDL) is implicated in the pathogenesis of atherosclerotic vascular diseases. This review summarizes a diversity of mechanisms proposed for LDL oxidation serving for the so-called "LDL oxidation hypothesis of atherogenesis".

METHODS AND RESULTS

We investigated the literature and our research results related to mechanisms of LDL oxidation and its atherogenesis. LDL oxidation is catalyzed by transition metal ions and several free radicals, and LDL is also oxidized by some oxidizing enzymes. In this way, LDL can be converted to a form that is recognized specifically by and with high affinity to macrophage scavenger receptors, leading to foam cell formation, the defining characteristic of fatty streak lesions.

CONCLUSIONS

Several pathways are involved in the promotion of LDL oxidation in vitro and in vivo, but it would appear that the physiologically relevant mechanisms of LDL oxidation are still imperfectly understood. The underlying mechanisms of LDL oxidation must be further explored to reveal appropriate ways for the diagnosis and treatment of atherosclerosis and its relevant diseases.

Lysosomal enzymes are released from cultured human macrophages, hydrolyze LDL in vitro, and are present extracellularly in human atherosclerotic lesions

OBJECTIVE

Human atherosclerotic lesions have been shown to contain lipid droplets and vesicles resembling those of in vitro enzymatically modified LDL. However, little is known about the hydrolytic enzymes in the arterial intima that induce fusion of LDL particles and so produce lipid droplets or that induce foam cell formation.

METHODS AND RESULTS

Human coronary atherosclerotic lesions obtained at surgery and at autopsy were stained for lysosomal acid lipase and cathepsin D. The extracellular areas of macrophage-rich intimal regions of the atherosclerotic lesions stained positively for both cathepsin D and lysosomal acid lipase, whereas normal arteries were negative. When monocyte-derived macrophages were incubated with opsonized zymosan to stimulate the release of lysosomal enzymes from the cells and LDL was incubated with the macrophage-conditioned media, the apolipoprotein B-100, cholesteryl esters, and triacylglycerols of LDL were hydrolyzed. These hydrolytic modifications rendered the LDL particles unstable and induced their fusion. Cultured macrophages and smooth muscle cells took up the hydrolase-modified LDL particles avidly and were transformed into foam cells.

CONCLUSIONS

Our in vivo and in vitro results suggest that lysosomal enzymes released from macrophages may induce hydrolytic modification of LDL and foam cell formation in the human arterial intima.

Cu2+ -induced low density lipoprotein peroxidation is dependent on the initial O2 concentration: an O2 consumption study

Atherosclerotic plaques form in the arterial intima, where low density lipoprotein (LDL) is thought to be oxidatively modified at sites which may contain catalytic amounts of copper in the presence of low O2 tension. We have investigated O2 consumption during LDL peroxidation induced by Cu2+ ions in vitro and found two phases: a lag phase followed by a phase of rapid O2 consumption. The length of the lag phase was dependent on Cu2+ and on initial O2 concentrations; increasing either decreased the lag time; however, LDL. concentration had no effect. LDL-induced Cu2+ reduction, however, was not affected by low initial O2 concentrations, suggesting that O2 is not required for LDL-mediated reduction of Cu2+. Following the lag phase, O2 consumption was dependent upon LDL or initial O2 concentrations; Cu2+ concentrations had little effect, suggesting that the propagation phase is more dependent on the presence of LDL lipids and O2 as substrates for the reaction. In summary, LDL peroxidation takes place in the presence of Cu2+ at low O2 tension; however, the reaction is dependent upon initial O2 concentrations; increases shorten the lag phase and accelerate O2 consumption.

Human serum, cysteine and histidine inhibit the oxidation of low density lipoprotein less at acidic pH

Low concentrations of serum or interstitial fluid have been shown to inhibit the oxidation of low density lipoprotein (LDL) catalysed by copper or iron, and may therefore protect against the development of atherosclerosis. As atherosclerotic lesions may have an acidic extracellular pH, we have investigated the effect of pH on the inhibition of LDL oxidation by serum and certain components of serum. Human serum (0.5%, v/v), lipoprotein-deficient human serum at an equivalent concentration and the amino acids L-cysteine (25 microM) and L-histidine (25 microM), but not L-alanine (25 microM), inhibited effectively the oxidation of LDL by copper at pH 7.4, as measured by the formation of conjugated dienes. The antioxidant protection was reduced considerably at pH 6.5, and was decreased further at pH 6.0. These observations may help to explain why LDL becomes oxidised locally in atherosclerotic lesions in the presence of the strong antioxidant protection offered by extracellular fluid.

Oxygen mass transfer calculations in large arteries

The purpose of this study was to model the transport of oxygen in large arteries, including the physiologically important effects of oxygen transport by hemoglobin, coupling of transport between oxygen in the blood and in wall tissue, and metabolic consumption of oxygen by the wall. Numerical calculations were carried out in an 89 percent area reduction axisymmetric stenosis model for several wall thicknesses. The effects of different boundary conditions, different schemes for linearizing the oxyhemoglobin saturation curve, and different Schmidt numbers were all examined by comparing results against a reference solution obtained from solving the full nonlinear governing equations with physiologic values of Schmidt number. Our results showed that for parameters typical of oxygen mass transfer in the large arteries, oxygen transport was primarily determined by wall-side effects, specifically oxygen consumption by wall tissue and wall-side mass transfer resistance. Hemodynamic factors played a secondary role, producing maximum local variations in intimal oxygen tension on the order of only 5-6 mmHg. For purposes of modeling blood-side oxygen transport only, accurate results were obtained through use of a computationally efficient linearized form of the convection-diffusion equation, so long as blood-side oxygen tensions remained in the physiologic range for large arteries. Neglect of oxygen binding by hemoglobin led to large errors, while arbitrary reduction of the Schmidt number led to more modest errors. We conclude that further studies of oxygen transport in large arteries must couple blood-side oxygen mass transport to transport in the wall, and accurately model local oxygen consumption within the wall.

Molecular chlorine generated by the myeloperoxidase-hydrogen peroxide-chloride system of phagocytes converts low density lipoprotein cholesterol into a family of chlorinated sterols

Oxidation of low density lipoprotein (LDL) may be of critical importance in triggering the pathological events of atherosclerosis. Myeloperoxidase, a heme protein secreted by phagocytes, is a potent catalyst for LDL oxidation in vitro, and active enzyme is present in human atherosclerotic lesions. We have explored the possibility that reactive intermediates generated by myeloperoxidase target LDL cholesterol for oxidation. LDL exposed to the myeloperoxidase-H2O2-Cl- system at acidic pH yielded a family of chlorinated sterols. The products were identified by mass spectrometry as a novel dichlorinated sterol, cholesterol alpha-chlorohydrin (6beta-chlorocholestane-(3beta,5alpha)-diol), cholesterol beta-chlorohydrin (5alpha-chlorocholestane-(3beta, 6beta)-diol), and a structurally related cholesterol chlorohydrin. Oxidation of LDL cholesterol by myeloperoxidase required H2O2 and Cl-, suggesting that hypochlorous acid (HOCl) was an intermediate in the reaction. However, HOCl failed to generate chlorinated sterols under chloride-free conditions. Since HOCl is in equilibrium with molecular chlorine (Cl2) through a reaction which requires Cl- and H+, this raised the possibility that Cl2 was the actual chlorinating intermediate. Consonant with this hypothesis, HOCl oxidized LDL cholesterol in the presence of Cl- and at acidic pH. Moreover, in the absence of Cl- and at neutral pH, Cl2 generated the same family of chlorinated sterols as the myeloperoxidase-H2O2-Cl- system. Finally, direct addition of Cl2 to the double bond of cholesterol accounts for dichlorinated sterol formation by myeloperoxidase. Collectively, these results indicate that Cl2 derived from HOCl is the chlorinating intermediate in the oxidation of cholesterol by myeloperoxidase. Our observations suggest that Cl2 generation in acidic compartments may constitute one pathway for oxidation of LDL cholesterol in the artery wall.

Human neutrophils employ chlorine gas as an oxidant during phagocytosis

Reactive oxidants generated by phagocytes are of central importance in host defenses, tumor surveillance, and inflammation. One important pathway involves the generation of potent halogenating agents by the myeloperoxidase-hydrogen peroxide-chloride system. The chlorinating intermediate in these reactions is generally believed to be HOCl or its conjugate base, ClO-. However, HOCl is also in equilibrium with Cl2, raising the possibility that Cl2 executes oxidation/ halogenation reactions that have previously been attributed to HOCl/ClO-. In this study gas chromatography-mass spectrometric analysis of head space gas revealed that the complete myeloperoxidase-hydrogen peroxide-chloride system generated Cl2. In vitro studies demonstrated that chlorination of the aromatic ring of free L-tyrosine was mediated by Cl2 and not by HOCl/ClO-. Thus, 3-chlorotyrosine serves as a specific marker for Cl2-dependent oxidation of free L-tyrosine. Phagocytosis of L-tyrosine encapsulated in immunoglobulin- and complement-coated sheep red blood cells resulted in the generation of 3-chlorotyrosine. Moreover, activation of human neutrophils adherent to a L-tyrosine coated glass surface also stimulated 3-chlorotyrosine formation. Thus, in two independent models of phagocytosis human neutrophils convert L-tyrosine to 3-chlorotyrosine, indicating that a Cl2-like oxidant is generated in the phagolysosome. In both models, synthesis of 3-chlorotyrosine was inhibited by heme poisons and the peroxide scavenger catalase, implicating the myeloperoxidase-hydrogen peroxide system in the reaction. Collectively, these results demonstrate that myeloperoxidase generates Cl2 and that human neutrophils use an oxidant with characteristics identical to those of Cl2 during phagocytosis. Moreover, our observations suggest that phagocytes exploit the chlorinating properties of Cl2 to execute oxidative and cytotoxic reactions at sites of inflammation and vascular disease.

Pyrene lipids as markers of peroxidative processes in different regions of low and high density lipoproteins

Three different pyrene derivatives, pyrene decanoyl phosphatidylcholine (P10PC), pyrene dodecanoyl sulfatide (P12CS) and cholesteryl pyrenyl hexanoate (P6Chol), were used to follow lipid peroxidation in low and high density lipoproteins. Probe-labelled lipoproteins were subjected to Cu2+ catalyzed peroxidation. In all cases the fluorescence of the probes progressively decreased due to the involvement of pyrene in the peroxidative reaction. Thus, we used the fluorescence decrease of P6Chol to monitor the lipid peroxidation in the hydrophobic core of LDL and HDL, and that of the amphipatic probes, P10PC and P12CS, to follow lipid peroxidation in the envelope of both lipoproteins. The possibility of following lipid peroxidation in individual lipoprotein regions could lead to more detailed information on the oxidative modifications that play an important role in the altered cholesterol homeostasis involved in the formation of atherosclerotic lesions. No differences were observed in the peroxidation kinetics of the hydrophobic core of HDL and LDL monitored with P6Chol. On the contrary kinetics obtained with P10PC and P12 CS demonstrated the HDL envelope to be more susceptible to Cu2+ -dependent lipid peroxidation than that of the LDL. This could be due to a greater radical generating capacity of the HDL envelope and can be explained on the basis of low vitamin E levels and large amounts of polyunsaturated fatty acids esterified on phospholipids determined in HDL, and on literature evidence that indicates HDL as the principal vehicle of circulating plasma lipids peroxides.

Iron released from transferrin at acidic pH can catalyse the oxidation of low density lipoprotein

Low density lipoprotein (LDL) oxidation within the arterial wall may contribute to the disease of atherosclerosis. We have investigated the conditions under which transferrin (the major iron-carrying protein in plasma) may release iron ions to catalyse the oxidation of LDL. Transferrin that had been incubated at pH 5.5 released approximately 10% of its bound iron in 24 h, as measured by ultrafiltration and atomic absorption spectroscopy. Furthermore, transferrin co-incubated with LDL and L-cysteine at pH 5.5 resulted in the oxidation of the LDL as measured by thiobarbituric acid-reactive substances and electrophoretic mobility. This effect was observed at transferrin concentrations as low as 40% of its average plasma concentration. The release of iron from transferrin in atherosclerotic lesions due to a localised acidic pH may help to explain why LDL oxidation occurs in these lesions.

Acidic pH enables caeruloplasmin to catalyse the modification of low-density lipoprotein

LDL oxidation within the arterial wall may contribute to the disease of atherosclerosis. There is some evidence that elevated plasma levels of copper are associated with an increased risk of coronary artery disease. We have investigated the conditions under which caeruloplasmin (the plasma copper carrier protein) can catalyse the macrophage-mediated modification of LDL. Low concentrations of CuSO4 (< 1 microM) could catalyse the macrophage-mediated modification of LDL. Native caeruloplasmin was unable to catalyse the modification of LDL at pH 7.4, but could do so after preincubation at acidic pH. After preincubation at acidic pH, concentrations of caeruloplasmin as low as 30 micrograms/ml (about one-tenth of the human plasma level) could catalyse significant LDL oxidation when added to macrophages. The activation of copper in caeruloplasmin in atherosclerotic lesions due to a localised acidic pH may help to explain why LDL oxidation occurs in these areas of the body.

Acidic pH increases the oxidation of LDL by macrophages

We have investigated the effect of pH on LDL oxidation by macrophages (in the presence of iron ions), using a modification of Hanks' balanced salt solution. Increasing the acidity of the medium greatly increased the oxidation of the LDL by the macrophages as measured by thiobarbituric acid-reactive substances or increased uptake and degradation by a second set of macrophages. The rate of oxidation of LDL by iron ions alone, measured in terms of conjugated dienes, was also increased greatly even at mildly acidic pH. It is quite possible that atherosclerotic lesions have an acidic extracellular pH, particularly in the vicinity of macrophages, and the observation that LDL oxidation by macrophages is increased at acidic pH may therefore help to explain why atherosclerotic lesions are apparently one of the very few sites in the body where LDL oxidation occurs.

Macrophage proteases can modify low density lipoproteins to increase their uptake by macrophages

When low density lipoprotein (LDL) was incubated with sonicated macrophages at acidic pH, its protein moiety was partially degraded by cathepsins B and D. The reisolated LDL was taken up by intact macrophages up to about 20 times as fast as control LDL. LDL proteolysis and its enhanced uptake could be inhibited almost entirely by the selective protease inhibitors leupeptin and pepstatin. If macrophages in atherosclerotic lesions were to release acidic proteases (either by exocytosis or following cell death) and these were to modify LDL, this may help to explain why so much cholesteryl ester accumulates in these cells.