Thiol-disulfide effects on hepatic glutathione transport. Studies in cultured rat hepatocytes and perfused livers

Glutathione Participation in the Prevention of Cardiovascular Diseases

Cardiovascular diseases (CVD) (such as occlusion of the coronary arteries, hypertensive heart diseases and strokes) are diseases that generate thousands of patients with a high mortality rate worldwide. Many of these cardiovascular pathologies, during their development, generate a state of oxidative stress that leads to a deterioration in the patient’s conditions associated with the generation of reactive oxygen species (ROS) and reactive nitrogen species (RNS). Within these reactive species we find superoxide anion (O₂^•–), hydroxyl radical (^•OH), nitric oxide (NO^•), as well as other species of non-free radicals such as hydrogen peroxide (H₂O₂), hypochlorous acid (HClO) and peroxynitrite (ONOO^–). A molecule that actively participates in counteracting the oxidizing effect of reactive species is reduced glutathione (GSH), a tripeptide that is present in all tissues and that its synthesis and/or regeneration is very important to be able to respond to the increase in oxidizing agents. In this review, we will address the role of glutathione, its synthesis in both the heart and the liver, and its importance in preventing or reducing deleterious ROS effects in cardiovascular diseases.

GSH protects against oxidative stress and toxicity in VL-17A cells exposed to high glucose

PURPOSE

The deficiency of glutathione (GSH) has been linked to several diseases. The study investigated the role of GSH as a protective factor against hyperglycemia-mediated injury in VL-17A cells treated with 50 mM glucose.

METHODS

The cell viability and different oxidative stress parameters including glyoxalase I activity were measured.

RESULTS

GSH supplementation with 2 mM N-acetyl cysteine (NAC) or 0.1 mM ursodeoxycholic acid (UDCA) increased the viability, GSH level and the GSH-dependent glyoxalase I activity in 50 mM glucose-treated VL-17A cells. Further, pretreatment of 50 mM glucose-treated VL-17A cells with NAC or UDCA decreased oxidative stress (levels of reactive oxygen species and protein carbonylation), apoptosis (caspase 3 activity and annexin V-propidium iodide positive cells) and glutathionylated protein formation, a measure of oxidative stress. GSH depletion with 0.4 mM buthionine sulfoximine (BSO) or 1 mM diethyl maleate (DEM) potentiated the decrease in viability, glyoxalase I activity and increase in oxidative stress and apoptosis, with decreased GSH levels in 50 mM glucose-treated VL-17A cells.

CONCLUSION

Thus, changes in GSH levels with exogenous agents such as NAC, UDCA, BSO or DEM modulate hyperglycemia-mediated injury in a cell model of VL-17A liver cells.

Liv.52 attenuate copper induced toxicity by inhibiting glutathione depletion and increased antioxidant enzyme activity in HepG2 cells

Altered copper metabolism plays a pivotal role in the onset of several hepatic disorders and glutathione (GSH) plays an important role in its homeostasis. Hepatic diseases are often implicated with decreased content of intracellular GSH. GSH depleted cells are prone to increased oxidative damage eventually leading to its death. Liv.52 is used to treat hepatic ailments since long time. Hence, in the present study the potential cytoprotective effect of Liv.52 against toxicity induced by copper (Cu2+) was evaluated in HepG2 cells. Cu2+ at 750 microM induced cytotoxicity to HepG2 cells as determined by MTT assay. The toxicity was brought about by increased lipid peroxidation, DNA fragmentation and decreased GSH content. But, upon treatment with Liv.52 cell death induced by Cu2+ was significantly abrogated by inhibition of lipid peroxidation by 58% and DNA fragmentation by 37%. Liv.52 increased the GSH content by 74%. Activities of the antioxidant enzymes catalase, glutathione peroxidase and superoxide dismutase were increased by 46%, 22% and 81% respectively in Liv.52 treated cells. Thus, it is apparent from these results that Liv.52 abrogates Cu2+ induced cytotoxicity in HepG2 cells by inhibiting lipid peroxidation and increased GSH content and antioxidant enzyme activity.

Glutathione in liver diseases and hepatotoxicity

Glutathione (GSH) is a major antioxidant as well as redox and cell signaling regulator. GSH guards cells against oxidative injury by reducing H(2)O(2) and scavenging reactive oxygen and nitrogen radicals. In addition, GSH-induced redox shift with or without ROS subjects some cellular proteins to varied forms of oxidation, altering the function of signal transduction and transcription factor molecules. Increasing evidence supports the important role of ROS and GSH in modulating multiple signaling pathways. TNF-alpha and Fas signaling, NF-kappaB, JNK and mitochondrial apoptotic pathways are the focus of this review. The redox regulation either can switch on/off or regulate the threshold for some crucial events in these pathways. Notably, mitochondrial GSH depletion induces increased mitochondrial ROS exposure which impairs bioenergetics and promotes mitochondrial permeability transition pore opening which is critical for cell death. Depending on the extent of mitochondrial damage, NF-kappaB inhibition and JNK activation, hepatocytes may either undergo different modes of cell death (apoptosis or necrosis) or be sensitized to cell-death stimuli (i.e. TNF-alpha). These processes have been implicated in the pathogenesis of many liver diseases.

Human gastric signet ring carcinoma (KATO-III) cell apoptosis induced by Vitex agnus-castus fruit extract through intracellular oxidative stress

We have previously reported that an ethanol extract of the dried ripe fruit of Vitex agnus-castus (Vitex) displays cytotoxic activity against certain kinds of human cancer cell line resulting in the induction of apoptosis. In this paper, we investigate the molecular mechanism of apoptosis induced by Vitex using a human gastric signet ring carcinoma cell line, KATO-III. DNA fragmentation was observed in Vitex-treated KATO-III cells in a time- and dose-dependent manner. DNA fragmentation was accompanied by the following phenomena: elevation in the level of hemeoxygenase-1 protein and thioredoxin reductase mRNA; repression of Mn-superoxide dismutase and catalase mRNAs; release of cytochrome c from mitochondria into the cytosol; activation of caspases-8, -9 and -3; decrease in the level of Bcl-2, Bcl-XL and Bid protein; increase in the level of Bad protein. The intracellular oxidized state, measured using 2',7'-dichlorofluorescin diacetate, increased after Vitex treatment. While the amount of intracellular GSH decreased significantly after treatment with Vitex, the level of GSSG was unaffected. Furthermore, no significant perturbation in the amount of proteins/mRNAs related to glutathione metabolism could be detected. These apoptotic alterations induced by exposure to Vitex were blocked by the presence of an anti-oxidative reagent, N-acetyl-l-cysteine, or the addition of exogenous GSH. Our results demonstrate that intracellular oxidative stress and mitochondrial membrane damage is responsible for Vitex-induced apoptosis, which may be mediated by a diminution of reduced type glutathione within the cell.

Signaling role of intracellular iron in NF-kappaB activation

Iron chelators inhibit endotoxin-induced NF-kappaB activation in hepatic macrophages (HMs), suggesting a role for the intracellular chelatable pool of iron in NF-kappaB activation. The present study tested this hypothesis. Analysis of Fe(59)-loaded HMs stimulated with lipopolysaccharide (LPS), revealed a previously unreported, transient rise in intracellular low molecular weight (LMW).Fe(59) complex ([LMW.Fe](i)) at </=2 min returning to the basal level within 15 min. The [LMW.Fe](i) response preceded IkappaB kinase (IKK) (>/=15 min) and NF-kappaB (>/=30 min) activation. Iron chelators (1,2-dimethyl-3-hydroxypyridin-4-one and N,N'-bis-2-hydroxybenzylethylenediamine-N,N'-diacetic acid) abrogated the [LMW.Fe](i) response and IKK and NF-kappaB activation. The [LMW.Fe](i) response was also observed in tumor necrosis factor alpha (TNFalpha)-stimulated HMs and RAW264.7 cells treated with LPS and interferon-gamma but not in primary rat hepatocytes or myofibroblastic cells exposed to LPS or TNFalpha. Both [LMW.Fe](i) response and IKK activation in LPS-stimulated HMs were inhibited by diphenylene iodonium (nonspecific inhibitor for flavin-containing oxidases), l-N(6)-(1-iminoethyl)lysine (selective iNOS inhibitor), and adenoviral-mediated expression of a dominant negative mutant of Rac1 or Cu,Zn-superoxide dismutase, suggesting the role of (.)NO and O(2)() in mediating the iron signaling. In fact, this inhibition was recapitulated by a cell-permeable scavenger of ONOO(-), 5,10,15,20-tetrakis (4-sulfonatophenyl)porphyrinato iron (III) chloride. Conversely, ONOO(-) alone induced both [LMW.Fe](i) response and IKK activation. Finally, direct addition of ferrous iron to cultured HMs activated IKK and NF-kappaB. These results support a novel signaling role for [LMW.Fe](i) in IKK activation, which appears to be induced by ONOO(-) and selectively operative in macrophages.

Chronic exposure of HepG2 cells to excess copper results in depletion of glutathione and induction of metallothionein

Metallothionein (MT) and reduced glutathione (GSH) play an important role in the intracellular handling of copper by preventing the generation and favouring the removal of copper-derived free radicals. The present study addressed the changes in MT and GSH that follow chronic (2 or 5 weeks) exposure of human hepatoblastoma cells (HepG2) to excess copper. Copper treatment (100 microM, 2 weeks) led to a 28-fold elevation in intracellular copper. Concomitantly, cells exhibited a seven-fold increase in total MT and an increment in its saturation with copper from 45 to 86%. Around 38% of copper in the cytosolic fraction could be accounted for by MT. GSH equivalents were substantially lowered (to 37% of basal levels) in treated cells, with only part of it being accounted for by an increase in GSSG. Copper-treatment induced no changes in catalase or GSH-peroxidase activities but it was associated with a small reduction in SOD (20%) and GSH-reductase (26%) activities. Copper-loaded cells did not differ from controls in their basal oxidative tone; however, when exposed to tert-butylhydroperoxide they exhibited a markedly greater susceptibility to undergo both oxidative stress and cell lysis. It is proposed that chronic exposure of HepG2 cells to excess copper is accompanied by "adaptive changes" in GSH and MT metabolism that would render cells substantially more susceptibility to undergo oxidative stress-related cytotoxicity.

The thiol sensitivity of glutathione transport in sidedness-sorted basolateral liver plasma membrane and in Oatp1-expressing HeLa cell membrane

Sinusoidal efflux of hepatic reduced glutathione (GSH) is a key step in interorgan GSH/cysteine homeostasis and extracellular detoxification. Rat organic anion transporter polypeptide1 (Oatp1) is known to transport GSH but several features of sinusoidal GSH uptake, such as electrogenic property and asymmetric effects of uncharged thiols (increased efflux, decreased uptake), either cannot be accounted for by Oatp1 or have not been studied. The asymmetric effect of thiols has only been studied in intact cells and not directly in membrane vesicles. To accomplish the latter, we studied GSH uptake in inside-out-(IO) and rightside-out-(RO) oriented basolateral plasma membrane vesicles (bLPM). We also studied the kinetics and effect of thiols on GSH transport by Oatp1 stably expressed in HeLa cells. GSH uptake was approximately 2- to 3-fold higher in IO than RO bLPM. Dithiothreitol-stimulated GSH uptake in IO but inhibited uptake in RO bLPM, demonstrating that thiols exert direct asymmetric side-specific effects on GSH transport. Uptake in IO and RO bLPM was sigmoid (K(m) approximately 13 mM) with a 2-fold higher capacity in IO compared with RO bLPM. In both IO and RO bLPM, a component with a high affinity but low capacity for GSH (K(m) approximately 100 microM) was also present. Endogenous GSH transporter in HeLa cells was thiol-sensitive, electrogenic, and described by a single Michaelis-Menten component (K(m) approximately 15 mM). In contrast, GSH transport mediated by Oatp1 was insensitive to thiols and membrane potential, inhibited by cystine, and stimulated by an inward H(+) gradient. These findings identify novel functional asymmetries in sinusoidal efflux and uptake of GSH and further clarify the role of Oatp1 in GSH transport.

Radiation inactivation studies of hepatic sinusoidal reduced glutathione transport system

Sinusoidal transport of reduced glutathione (GSH) is a carrier-mediated process. Perfused liver and isolated hepatocyte models revealed a low-affinity transporter with sigmoidal kinetics (K(m) approximately 3.2-12 mM), while studies with sinusoidal membrane vesicles (SMV) revealed a high-affinity unit (K(m) approximately 0.34 mM) besides a low-affinity one (K(m) approximately 3.5-7 mM). However, in SMV, both the high- and low-affinity units manifested Michaelis-Menten kinetics of GSH transport. We have now established the sigmoidicity of the low-affinity unit (K(m) approximately 9) in SMV, consistent with other models, while the high-affinity unit has been retained intact with Michaelis-Menten kinetics (K(m) approximately 0.13 mM). We capitalized on the negligible cross-contributions of the two units to total transport at the low and high ends of GSH concentrations and investigated their characteristics separately, using radiation inactivation, as we did in canalicular GSH transport (Am. J. Physiol. 274 (1998) G923-G930). We studied the functional sizes of the proteins that mediate high- and low-affinity GSH transport in SMV by inactivation of transport at low (trace and 0.02 mM) and high (25 and 50 mM) concentrations of GSH. The low-affinity unit in SMV was much less affected by radiation than in canalicular membrane vesicles (CMV). The target size of the low-affinity sinusoidal GSH transporter appeared to be considerably smaller than both the canalicular low- and high-affinity transporters. The high-affinity unit in SMV was markedly inactivated upon irradiation, revealing a single protein structure with a functional size of approximately 70 kDa. This size is indistinguishable from that of the high-affinity GSH transporter in CMV reported earlier.

Lung glutathione and oxidative stress: implications in cigarette smoke-induced airway disease

Glutathione (GSH), a ubiquitous tripeptide thiol, is a vital intra- and extracellular protective antioxidant in the lungs. The rate-limiting enzyme in GSH synthesis is gamma-glutamylcysteine synthetase (gamma-GCS). The promoter (5'-flanking) region of the human gamma-GCS heavy and light subunits are regulated by activator protein-1 and antioxidant response elements. Both GSH and gamma-GCS expression are modulated by oxidants, phenolic antioxidants, and inflammatory and anti-inflammatory agents in lung cells. gamma-GCS is regulated at both the transcriptional and posttranscriptional levels. GSH plays a key role in maintaining oxidant-induced lung epithelial cell function and also in the control of proinflammatory processes. Alterations in alveolar and lung GSH metabolism are widely recognized as a central feature of many inflammatory lung diseases including chronic obstructive pulmonary disease (COPD). Cigarette smoking, the major factor in the pathogenesis of COPD, increases GSH in the lung epithelial lining fluid of chronic smokers, whereas in acute smoking, the levels are depleted. These changes in GSH may result from altered gene expression of gamma-GCS in the lungs. The mechanism of regulation of GSH in the epithelial lining fluid in the lungs of smokers and patients with COPD is not known. Knowledge of the mechanisms of GSH regulation in the lungs could lead to the development of novel therapies based on the pharmacological or genetic manipulation of the production of this important antioxidant in lung inflammation and injury. This review outlines 1) the regulation of cellular GSH levels and gamma-GCS expression under oxidative stress and 2) the evidence for lung oxidant stress and the potential role of GSH in the pathogenesis of COPD.

Investigation of the transport of intact glutathione in human and rat type II pneumocytes

The aim of the study was to investigate whether there is transmembrane transport of intact glutathione ([3H]-GSH, 0.1 microCi) in rat and human type II pneumocytes (T2P), and if this transport might be dependent on the redox state of the extracellular fluid. The T2P were pretreated with acivicin (250 microM) to inhibit gamma-glutamyltransferase activity and with L-buthionine-[SR]-sulfoximine (1 mM) to inhibit intracellular GSH synthesis. After 48 h in culture, initial GSH influx rate was 0.70 +/- 0.20 nmol/min/mg protein (37 degrees C) and 0.35 +/- 0.04 nmol/min/mg protein (4 degrees C) during the first 5 min in rat T2P. In human T2P, the initial GSH influx rate was 0.36 +/- 0.30 nmol/min/mg protein (37 degrees C) and 0.32 +/- 0.06 nmol/min/mg protein (4 degrees C) during the first 10 min. Thereafter no further influx was found. The influx of 1 mM GSH in freshly isolated rat and human T2P in suspension was 2.3 +/- 0.3 and 1.2 +/- 0.3 nmol/mg protein after 15 min at 37 degrees C, and 2.8 +/- 0.2 and 1.0 +/- 0.3 nmol/mg protein at 4 degrees C, respectively. When GSH influx was studied at different concentrations between 0 and 40 mM, a linear increase without saturation or difference between 37 degrees C and 4 degrees C was found. Pre-exposure to ouabain had no effect on GSH influx. Efflux of GSH was stimulated and influx inhibited by pre-exposure of the cells to reduced thiols, while disulphides inhibited efflux and favoured inward uptake. Thus, in human and rat T2P a GSH-carrier exists which operates as an effluxer. At GSH concentrations in the physiological range no uptake is seen, but some uptake can be observed at GSH concentrations above normal physiological levels. The uptake appears to be energy-independent and non-saturable. Efflux of GSH is stimulated and influx inhibited by reduced thiols, while disulphides inhibit the efflux and favour inward uptake. GSH uptake in T2P thus may depend on concentration gradients and driving forces, such as the redox state of the extracellular fluid.

Involvement of N-acetylcysteine-sensitive pathways in ricin-induced apoptotic cell death in U937 cells

We have found that the antioxidant N-acetylcysteine (NAC) strongly inhibited ricin-induced apoptotic cell death in U937 cells (human myeloid leukemia), as judged by cytotoxicity, nuclear morphological change, and DNA fragmentation. Consistent with these observations, a significant depletion of cellular glutathione was observed in ricin-treated cells, and NAC prevented the decrease in cellular glutathione. On the other hand, among the caspase inhibitors tested, Z-Asp-CH2-DCB, which inhibited ricin cytotoxicity, also suppressed ricin-mediated glutathione depletion, while NAC did not affect the generation of caspase-3 like activity in ricin-treated cells. These results suggest that glutathione loss takes place downstream from caspase activation during the ricin-induced apoptotic process. Treatment with a specific inhibitor of glutathione biosynthesis, buthionine sulfoximine (BSO) failed to induce apoptosis, and had no effect on the overall extent of ricin-induced apoptosis, even though the glutathione level was decreased to less than 5% of the control level. However, NAC still protected against ricin-induced apoptosis in the BSO-treated cells. We conclude that glutathione loss is one of several apoptotic changes caused by ricin, but is not a sufficient factor for the progress of apoptosis. NAC may prevent ricin-induced apoptosis through maintaining an intracellular reducing condition by acting as a thiol supplier.

Identification of glutathione as a driving force and leukotriene C4 as a substrate for oatp1, the hepatic sinusoidal organic solute transporter

oatp1 is an hepatic sinusoidal organic anion transporter that mediates uptake of various structurally unrelated organic compounds from blood. The driving force for uptake on oatp1 has not been identified, although a role for bicarbonate has recently been proposed. The present study examined whether oatp1-mediated uptake is energized by efflux (countertransport) of intracellular reduced glutathione (GSH), and whether hydrophobic glutathione S-conjugates such as leukotriene C4 (LTC4) and S-dinitrophenyl glutathione (DNP-SG) form a novel class of substrates for oatp1. Xenopus laevis oocytes injected with the complementary RNA for oapt1 demonstrated higher uptake of 10 nM [3H]LTC4 and 50 microM [3H]DNP-SG, and higher efflux of [3H]GSH (2.5 mM endogenous intracellular GSH concentration). The oatp1-stimulated LTC4 and DNP-SG uptake was independent of the Na+ gradient, cis-inhibited by known substrates of this transport protein and by 1 mM GSH, and was saturable, with apparent Km values of 0.27 +/- 0.06 and 408 +/- 95 microM, respectively. Uptake of [3H]taurocholate, an endogenous substrate of oatp1, was competitively inhibited by DNP-SG. Of significance, oatp1-mediated taurocholate and LTC4 uptake was cis-inhibited and trans-stimulated by GSH, and [3H]GSH efflux was enhanced in the presence of extracellular taurocholate or sulfobromophthalein, indicating that GSH efflux down its large electrochemical gradient provides the driving force for uptake via oatp1. The stoichiometry of GSH/taurocholate exchange was 1:1. These findings identify a new class of substrates for oatp1 and provide evidence for GSH-dependent oatp1-mediated substrate transport.

Glutathione transport system in human small intestine epithelial cells

The present study characterizes for the first time a GSH specific transporter in a human intestinal epithelial cell line (I407). GSH metabolism is very important for the antioxidant and detoxifying action of intestine and for the maintenance of the luminal thiol-disulfide ratio involved in regulation mechanisms of the protein activity of epithelial cells. GSH level decreases have been related to physio-pathological alterations either of intestine or other organs. GSH specific transport systems have been identified in membranes of various cell types of rat, mice and rabbit. The presence of a Na+-independent transport system of GSH is confirmed by the similar behaviour of GSH uptake time-courses when Na+ in extracellular uptake medium was replaced with choline+ or K+ as well as by kinetic saturation and by the trans-stimulation effect on GSH uptake in GSH preloaded cells. Moreover, this transporter is activated when cations are present in extracellular medium and it is affected by membrane potential changes with an increase in GSH uptake values when membrane depolarization occurs. The present results also show a remarkable affinity and specificity of this transporter for GSH; in fact, Km value is very low (90 +/- 20 microM) and only compounds strictly related to GSH structure, such as GSH S-conjugates and GSH-ethyl ester, inhibit GSH uptake in 1407 cells. Finally, a possible hormonal control and modulation by the thiol-disulfide status of GSH transporter activity is suggested.

Rapid and specific efflux of glutathione before hepatocyte injury induced by hypoxia

Hypoxia caused the efflux of glutathione (GSH) from hepatocytes before membrane lysis occurred. Dithiothreitol (DTT), a thiol reductant, greatly increased the hypoxia induced GSH efflux as well as the subsequent membrane lysis. The NADH generating nutrients sorbitol and beta-hydroxybutyrate as well as ethanol also enhanced hepatocyte GSH efflux and cell injury, whereas on the other hand NADH oxidising metabolic intermediates, e.g., acetoacetate or the artificial electron acceptor methylene blue, partly prevented GSH efflux and membrane lysis. Hypoxia induced GSH efflux and cytotoxicity were also prevented by oxypurinol, a xanthine oxidase inhibitor, as well as by the polyphenolic antioxidant quercetin, suggesting that reactive oxygen species contributed to the GSH efflux and cell lysis. The above results suggest that reductive stress caused by hypoxia activates the redox sensitive sinusoidal GSH transporter that is likely responsible for the GSH efflux before membrane lysis ensues.

Role of cellular thiol status in tocopheryl hemisuccinate cytoprotection against ethyl methanesulfonate-induced toxicity

Suspensions of rat hepatocytes treated with the alkylating agent ethyl methanesulfonate (EMS) exhibited extensive lipid peroxidation as well as rapid and near complete depletion of cellular reduced glutathione (GSH) levels prior to cell death. Pretreatment of hepatocytes with medium deficient in sulfur amino acids accelerated cell death induced by EMS, confirming the previously reported cytoprotective role for GSH in this toxic event. Nearly all of the cellular GSH lost following 50 mM EMS treatment was accounted for as S-ethyl glutathione (GS-Et). No significant formation of glutathione disulfide was observed. The GS-Et formed was not exported from the cell but remained at high intracellular concentrations throughout the course of the experiment. In addition, EMS treatment inhibited the efflux of intracellular GSH and inhibited the cellular accumulation of glutamate (Glu). Supplementation of hepatocytes with 25 microM d-alpha-tocopheryl hemisuccinate (TS) protected these cells against EMS-induced lipid peroxidation and cell death. Cytoprotection with TS had no effect on EMS-induced depletion of intracellular GSH or intracellular levels of GS-Et or Glu. However, TS supplementation did prevent EMS-induced depletion of cellular protein thiols. Interestingly, the pretreatment of hepatocytes with 1 mM dithiothreitol promoted EMS toxicity. The results of this study suggest that the cytoprotective abilities of TS are related to the prevention of both EMS-induced lipid peroxidation and protein thiol depletion. Thus, the onset of lipid peroxidation and the loss of protein thiols in hepatocytes appear to be critical cellular events leading to EMS-induced cell death.

GSH transporters: molecular characterization and role in GSH homeostasis

Considerable progress has been made in the last few years in the molecular identification and characterization of hepatic GSH transporter-associated polypeptides. We are now poised to determine their precise mechanisms of action and regulation at the transcriptional and post-translational level. It is also anticipated that molecular characterization of the mitochondrial GSH transporter and sodium GSH co-transporters will be accomplished in the near future. With this information, a more complete understanding of GSH/cysteine homeostasis can be achieved which can be applied to furthering the prevention and treatment of the diseases of oxidative stress, such as aging, HIV, cataract, atherosclerosis, cancer and alcoholic liver disease.

Role of two recently cloned rat liver GSH transporters in the ubiquitous transport of GSH in mammalian cells

Recently our laboratory has cloned both the rat canalicular and sinusoidal GSH transporters (RcGshT and RsGshT, respectively; Yi, J., S. Lu, J. Fernandez-Checa, and N. Kaplowitz. 1994. J. Clin. Invest. 93:1841-1845; and 1995. Proc. Natl. Acad. Sci. USA. 92:1495-1499). The current work characterized GSH transport and the expression of these two GSH transporters in various mammalian cell lines. The average cell GSH levels (nmol/10(6) cells) were 25, 22, 32, 13, and 13 in HepG2, HeLa, CaCo-2, MDCK, and Cos-1 cells, respectively. GSH efflux was temperature dependent and averaged 0.018, 0.018, 0.012, 0.007, and 0.019 nmol/10(6) cells/min from HepG2, HeLa, CaCo-2, MDCK, and Cos-1 cells, respectively. Dithiothreitol (DTT), which stimulates rat sinusoidal GSH efflux, stimulated GSH efflux only in HepG2 and HeLa cells which was partially reversed by subsequent cystine treatment. GSH uptake (1 mM plus 35S-GSH) was temperature dependent, linear up to 45 min, and Na+-independent with average rates of 1.12, 0.91, 0.45, and 0.45 nmol/10(6) cells/30 min for HepG2, HeLa, CaCo-2, MDCK, and Cos-1 cells, respectively. BSP-GSH (2mM), which cis-inhibits sinusoidal GSH uptake in rat liver and HepG2 cells, inhibited GSH uptake only in HeLa cells. mRNA and polypeptide of RcGshT are expressed in all cells whereas those of RsGshT are expressed only in HepG2 and HeLa cells. In conclusion, bidirectional GSH transport, mediated by the "canalicular" GSH transporter, is ubiquitous in mammalian cells. Sinusoidal GSH transporter expression is more restricted, being present in HepG2 and HeLa cells. DTT and BSP-GSH affect GSH transport only in cells expressing the sinusoidal transporter confirming their selective action on this transporter.

Platelet glutathione transport: characteristics and evidence for regulation by intraplatelet thiol status

The present study demonstrates the carrier-mediated uptake of intact glutathione (GSH) by human platelets. Platelet GSH uptake was characterized as being Na+ independent and saturable. The KM, apparent and Vmax, apparent for GSH uptake in platelet plasma membrane vesicles were 28.0 +/- 8.4 microM and 263.5 +/- 28.5 pmol/min per mg protein, respectively. The transport was inhibited by GSH analogs and enhanced by KCl-induced membrane depolarization. GSH transport may be regulated by the intracellular thiol status, since the depletion of intraplatelet GSH with 100 microM 1-chloro-2,4-dinitrobenzene (CDNB) increased GSH uptake by approximately 40%. The KM, apparent and Vmax, apparent for GSH uptake in intact platelets changed from 99.5 +/- 15 microM and 42 +/- 7.5 pmol/min per 10(9) platelets, respectively, to 33.7 +/- 6.7 microM and 21.5 +/- 6.9 pmol/min per 10(9) platelets, respectively, on reducing intraplatelet GSH with 100 microM CDNB.

Cocaine-induced liver injury in mice elicits specific changes in DNA ploidy and induces programmed death of hepatocytes

Liver injury was induced by a single dose (60 mg/kg) of cocaine in male albino Swiss mice untreated or pretreated with phenobarbital (in drinking water 1 gm/L), for 5 days before cocaine administration. One parameter of liver injury, serum isocitrate dehydrogenase activity, showed sharp increases at 24 hr of cocaine treatment; we also noted decrease hepatic levels of ATP, GSH, cytochrome P-450 and NADPH/NADP+ ratio and increases in malondialdehyde concentration. Histopathological study of liver slices showed perivenous and periportal necrosis induced by cocaine in untreated mice and mice pretreated with phenobarbital, respectively. A regenerative postnecrotic response, which peaked at 48 hr, was demonstrated by the appearance of mitotic cells. Mitotic index analysis showed that proliferative cells appear to be unevenly distributed in the hepatic acinus and were mainly located in the vicinity of the damaged acinar region. Genomic DNA ploidy and the distribution of DNA in the phases of the cell cycle were studied in nuclei of isolated hepatocytes. At 12 hr of cocaine administration, both in untreated and phenobarbital-pretreated mice, the following changes were observed: a sharp decrease in tetraploid (4N) cells (40% to 17% and 25% to 6%, respectively) and octoploid (8N) cells (5% to 2% and 2% to 1%, respectively), together with the appearance of a hypodiploid population (13% and 31%, respectively). Hypodiploid population was characterized as apoptotic cells by detection of DNA fragmentation in agarose gel. These results suggest that a significant percentage of cell death induced by cocaine occurs by means of the apoptosis death program. Comparison of the initial values of DNA ploidy with those obtained at 7 days of cocaine administration showed remarkable increases in polyploid populations (4N and 8N) and a decrease in diploid cells (2N), indicating that the process of differentiation occurs when liver restores its functionality.