Metallothionein, glutathione, and cystine transport in pulmonary artery endothelial cells and NIH/3T3 cells

A Systematic Review of the Various Effect of Arsenic on Glutathione Synthesis In Vitro and In Vivo

Background

Arsenic is a toxic metalloid widely present in nature, and arsenic poisoning in drinking water is a serious global public problem. Glutathione is an important reducing agent that inhibits arsenic-induced oxidative stress and participates in arsenic methylation metabolism. Therefore, glutathione plays an important role in regulating arsenic toxicity. In recent years, a large number of studies have shown that arsenic can regulate glutathione synthesis in many ways, but there are many contradictions in the research results. At present, the mechanism of the effect of arsenic on glutathione synthesis has not been elucidated.

Objective

We will conduct a meta-analysis to illustrate the effects of arsenic on GSH synthesis precursors Glu, Cys, Gly, and rate-limiting enzyme γ-GCS in mammalian models, as well as the regulation of p38/Nrf2 of γ-GCS subunit GCLC, and further explore the molecular mechanism of arsenic affecting glutathione synthesis.

Results

This meta-analysis included 30 studies in vivo and 58 studies in vitro, among which in vivo studies showed that arsenic exposure could reduce the contents of GSH (SMD = -2.86, 95% CI (-4.45, -1.27)), Glu (SMD = -1.11, 95% CI (-2.20,-0.02)), and Cys (SMD = -1.48, 95% CI (-2.63, -0.33)), with no statistically significant difference in p38/Nrf2, GCLC, and GCLM. In vitro studies showed that arsenic exposure increased intracellular GSH content (SMD = 1.87, 95% CI (0.18, 3.56)) and promoted the expression of p-p38 (SMD = 4.19, 95% CI (2.34, 6.05)), Nrf2 (SMD = 4.60, 95% CI (2.34, 6.86)), and GCLC (SMD = 1.32, 95% CI (0.23, 2.41)); the p38 inhibitor inhibited the expression of Nrf2 (SMD = -1.27, 95% CI (-2.46, -0.09)) and GCLC (SMD = -5.37, 95% CI (-5.37, -2.20)); siNrf2 inhibited the expression of GCLC, and BSO inhibited the synthesis of GSH. There is a dose-dependent relationship between the effects of exposure on GSH in vitro. Conclusions. These indicate the difference between in vivo and in vitro studies of the effect of arsenic on glutathione synthesis. In vivo studies have shown that arsenic exposure can reduce glutamate and cysteine levels and inhibit glutathione synthesis, while in vitro studies have shown that chronic low-dose arsenic exposure can activate the p38/Nrf2 pathway, upregulate GCLC expression, and promote glutathione synthesis.

Gene expression, glutathione status, and indicators of hepatic oxidative stress in laughing gull (Larus atricilla) hatchlings exposed to methylmercury

Despite extensive studies of methylmercury (MeHg) toxicity in birds, molecular effects on birds are poorly characterized. To improve our understanding of toxicity pathways and identify novel indicators of avian exposure to Hg, the authors investigated genomic changes, glutathione status, and oxidative status indicators in liver from laughing gull (Larus atricilla) hatchlings that were exposed in ovo to MeHg (0.05-1.6 µg/g). Genes involved in the transsulfuration pathway, iron transport and storage, thyroid-hormone related processes, and cellular respiration were identified by suppression subtractive hybridization as differentially expressed. Quantitative polymerase chain reaction (qPCR) identified statistically significant effects of Hg on cytochrome C oxidase subunits I and II, transferrin, and methionine adenosyltransferase RNA expression. Glutathione-S-transferase activity and protein-bound sulfhydryl levels decreased, whereas glucose-6-phosphate dehydrogenase activity increased dose-dependently. Total sulfhydryl concentrations were significantly lower at 0.4 µg/g Hg than in controls. Together, these endpoints provided some evidence of compensatory effects, but little indication of oxidative damage at the tested doses, and suggest that sequestration of Hg through various pathways may be important for minimizing toxicity in laughing gulls. This is the first study to describe the genomic response of an avian species to Hg. Laughing gulls are among the less sensitive avian species with regard to Hg toxicity, and their ability to prevent hepatic oxidative stress may be important for surviving levels of MeHg exposures at which other species succumb.

DIESEL particulate exposed macrophages alter endothelial cell expression of eNOS, iNOS, MCP1, and glutathione synthesis genes

There is considerable debate regarding inhaled diesel exhaust particulate (DEP) causing impairments in vascular reactivity. Although there is evidence that inhaled particles can translocate from the lung into the systemic circulation, it has been suggested that inflammatory factors produced in the lung following macrophage particle engulfment also pass into the circulation. To investigate these differing hypotheses, we used in vitro systems to model each exposure. By using a direct exposure system and a macrophage-endothelial cell co-culture model, we compared the effects of direct DEP exposure and exposure to inflammatory factors produced by DEP-treated macrophages, on endothelial cell mRNA levels for eNOS, iNOS, endothelin-1, and endothelin-converting-enzyme-1. As markers of oxidative stress, we measured the effects of DEP treatment on glutathione (GSH) synthesis genes and on total GSH. In addition, we analyzed the effect of DEP treatment on monocyte chemo-attractant protein-1. Direct DEP exposure increased endothelial GCLC and GCLM as well as total GSH in addition to increased eNOS, iNOS, and Mcp1 mRNA. Alternatively, inflammatory factors released from DEP-exposed macrophages markedly up-regulated endothelial iNOS and Mcp1 while modestly down-regulating eNOS. These data support both direct exposure to DEP and the release of inflammatory cytokines as explanations for DEP-induced impairments in vascular reactivity.

Effects of exogenous metallothionein against thallium-induced oxidative stress in rat liver

Metallothionein (MT) is a low-molecular weight sulfur-rich protein that plays role in metal homeostasis/detoxification and radical scavenging. The following study investigated the ability of exogenous MT to protect against oxidative damage induced by thallium (TI) in rat liver. Male Wistar rats were divided into four groups; a control and three experimental groups. The control group received physiological saline. Group 1 animals were injected with thallium acetate intraperitoneally (i.p.) at a single dose of LD(50) (32 mg/kg). In group 2 and group 3, metallothionein I was administrated once at two different doses (1 or 2.5mg/kg i.p., respectively) 1h before TI intoxication. Levels of endogenous antioxidants, oxidative stress markers were measured and histopathological examinations were performed 4 days after TI administration. TI accumulation in liver decreased related to the dose of MT. Mostly all of the alterations in the levels antioxidants restored to normal levels in MT administrated animals. H(2)O(2) levels and lipid peroxidation decreased, integrity of hepatocytes and membranous structures inside the cells were preserved. The toxic effects of TI were modulated in MT administrated animals particularly at the dose of 2.5mg/kg. These findings suggest an active role of exogenous MT against TI-induced oxidative stress in rat liver.

Identification and characterization of eleven glutathione S-transferase genes from the aquatic midge Chironomus tentans (Diptera: Chironomidae)

Eleven cDNAs encoding glutathione S-transferases (GSTs) were sequenced and characterized in Chironomus tentans, an ecologically important aquatic midge. Phylogenetic analysis revealed seven GSTs in three different cytosolic classes including 4 in sigma (CtGSTs1, CtGSTs2, CtGSTs3, CtGSTs4), 2 in delta (CtGSTd1, CtGSTd2), and 1 in omega (CtGSTo1). The remaining four GSTs (CtGSTu1, CtGSTu2, CtGSTu3, CtGSTu4) were unclassified due to their low relatedness to currently known classes of insect GSTs. Reverse-transcription (RT)-PCR analysis of the 11 GST genes showed that CtGSTd1, CtGSTu2, CtGSTu4, CtGSTs1, CtGSTs2, CtGSTs3, CtGSTs4 and CtGSTo1 were expressed in all tissues examined, including salivary glands, hemolymph, midgut, Malpighian tubules, fatbodies and carcass, whereas CtGSTd2 and CtGSTu1 were expressed in a limited number of tissues. CtGSTs1 and CtGSTs4 appeared to be the only two genes, of which expressions can be detected in eggs, whereas all the 11 GST genes showed various expression patterns in the four larval instars. However, expressions of CtGSTd2, CtGSTu1 and CtGSTu2 were not detectable in pupal and adult stages. Real-time quantitative PCR confirmed that the herbicide alachlor increased CtGSTd1, CtGSTs2 and CtGSTs3 gene expression by 2.1-, 2.8- and 4.3-fold, respectively, when fourth-instar midges were exposed to alachlor at 1000 microg/L for 72 h. Such increased gene expressions were associated with 2.2- and 1.8-fold decreases of total GST activities in vivo when CDNB and DCNB were used as substrates, respectively. Further studies showed that 65.5 and 73.5% of GST activities were inhibited in vitro by alachlor at 100 and 1000 microg/L, respectively. Because alachlor has been known as an electrophilic substrate that can be conjugated by glutathione (GSH), rapid in vitro inhibition of GST activities by alachlor suggested that decreased GST activities were likely caused by the depletion of GSH. However, alachlor may regulate different GST genes, as found in other organisms, leading to significantly increased transcriptional levels of CtGSTd1, CtGSTs2 and CtGSTs3 in out of 11 GST genes examined in this study.

xCT expression reduces the early cell cycle requirement for calcium signaling

Calcium has long been recognized as an important regulator of cell cycle transitions although the mechanisms are largely unknown. A functional genomic screen has identified genes involved in the regulation of early cell cycle progression by calcium. These genes when overexpressed confer the ability to bypass the G1/S arrest induced by Ca(2+)-channel antagonists in mouse fibroblasts. Overexpression of the cystine-glutamate exchanger, xCT, had the greatest ability to evade calcium antagonist-induced cell cycle arrest. xCT carries out the rate limiting step of glutathione synthesis in many cell types and is responsible for the uptake of cystine in most human cancer cell lines. Functional analysis indicates that the cystine uptake activity of xCT overcomes the G1/S arrest induced by Ca(2+)-channel antagonists by bypassing the requirement for calcium signaling. Since cells overexpressing xCT were found to have increased levels and activity of the AP-1 transcription factor in G1, redox stimulation of AP-1 activity accounts for the observed growth of these cells in the presence of calcium channel antagonists. These results suggest that reduced calcium signaling impairs AP-1 activation and that xCT expression may directly affect cell proliferation.

BspA (CyuC) in Lactobacillus fermentum BR11 is a highly expressed high-affinity L-cystine-binding protein

The BspA protein of Lactobacillus fermentum BR11 (BR11) is a cell envelope constituent that is similar to known solute-binding proteins and putative adhesins. BspA is required for L-cystine uptake and oxidative defense and is likely to be an L-cystine-binding protein. The aim of this study was to directly measure L-cystine-BspA binding and BspA expression. De-energized BR11 cells bound radiolabelled L-cystine with a Kd of 0.2 microM. A bspA mutant could not bind L-cystine. L-cystine-BR11 binding was unaffected by large excesses of L-glutamine, L-methionine, or collagen, indicating L-cystine specificity. BR11 and the bspA mutant were identical in their abilities to bind L-cysteine, indicating that L-cysteine is not a BspA ligand. BspA expression levels were deduced from radiolabelled L-cystine binding and it was found that there are 1-2 x 10(5) BspA molecules per cell, and that expression is slightly higher under oxidizing conditions. It is proposed that BspA be renamed CyuC.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

High-glucose-induced metallothionein expression in endothelial cells: an endothelin-mediated mechanism

Vascular endothelial cells are constantly exposed to oxidative stress and must be protected by physiological responses. In diabetes mellitus, endothelial cell permeability is impaired and may be increased by high extracellular glucose concentrations. It has been postulated that metallothionein (MT) can protect endothelial cells from oxidative stress with its increased expression by cytokines, thrombin, and endothelin (ET)-1. In this study, we demonstrate that high glucose concentration can induce MT expression in endothelial cells through a distinct ET-dependent pathway. Exposure of human umbilical vein endothelial cells (HUVEC) to increasing concentrations of glucose resulted in a rapid dose-dependent increase in MT-2 and ET-1 mRNA expression. MT expression may be further augmented with addition of ET-1. Preincubation of the cells with the specific ET(B) antagonist BQ-788 blocked MT-2 mRNA expression more effectively than the ET(A) inhibitor TBC-11251. High glucose also increased immunoreactive MT protein expression and induced translocation of MT into the perinuclear area. Perinuclear localization of MT was related to high-glucose-induced reorganization of F-actin filaments. These results demonstrate that an increase in extracellular glucose in HUVEC can lead to a rapid dose-dependent increase in MT-2 mRNA expression and to perinuclear localization of MT protein with changes to the cytoskeleton. These effects are mediated via the ET receptor-dependent pathway.

Arsenite stimulates plasma membrane NADPH oxidase in vascular endothelial cells

Low-level arsenite treatment of porcine aortic endothelial cells (PAEC) stimulated superoxide accumulation that was attenuated by inhibitors of NAD(P)H oxidase. To demonstrate whether arsenite stimulated NADPH oxidase, intact PAEC were treated with arsenite for up to 2 h and membrane fractions were prepared to measure NADPH oxidase activity. Arsenite (5 microM) stimulated a twofold increase in activity by 1 h, which was inhibited by the oxidase inhibitor diphenyleneiodonium chloride. Direct treatment of isolated membranes with arsenite had no effect. Analysis of NADPH oxidase components revealed that p67(phox) localized exclusively to membranes of both control and treated cells. In contrast, cytosolic Rac1 translocated to the membrane fractions of cells treated with arsenite or angiotensin II but not with tumor necrosis factor. Immunodepletion of p67(phox) blocked oxidase activity stimulated by all three compounds. However, depleting Rac1 inhibited responses only to arsenite and angiotensin II. These data demonstrate that stimulus-specific activation of NADPH oxidase in endothelial cells was the source of reactive oxygen in endothelial cells after noncytotoxic arsenite exposure.

Adaptive responses to peroxynitrite: increased glutathione levels and cystine uptake in vascular cells

We and others recently demonstrated increased glutathione levels, stimulated cystine uptake, and induced gamma-glutamylcysteinyl synthase (gamma-GCS) in vascular cells exposed to nitric oxide donors. Here we report the effects of peroxynitrite on glutathione levels and cystine uptake. Treatment of bovine aortic endothelial and smooth muscle cells with 3-morpholinosydnonimine (SIN-1), a peroxynitrite donor, resulted in transient depletion of glutathione followed by a prolonged increase beginning at 8-9 h. Concentration-dependent increases in glutathione of up to sixfold occurred 16-18 h after 0.05-2.5 mM SIN-1. Responses to SIN-1 were inhibited by copper-zinc superoxide dismutases and manganese(III)tetrakis(1-methyl-4-pyridyl)porphyrin pentachloride, providing evidence for peroxynitrite involvement. Because glutathione synthesis is regulated by amino acid availability, we also studied cystine uptake. SIN-1 treatment resulted in a prolonged increase in cystine uptake beginning at 6-9 h. Increases in cystine uptake after SIN-1 were blocked by inhibitors of protein and RNA synthesis, by extracellular glutamate but not by extracellular sodium. These studies suggest induction of the x(c)(-) pathway of amino acid uptake. A close correlation over time was observed for increases in cystine uptake and glutathione levels. In summary, vascular cells respond to chronic peroxynitrite exposure with adaptive increases in cellular glutathione and cystine transport.