Expression of rat liver reduced glutathione transport in Xenopus laevis oocytes

Glutathione preservation during storage of rat lenses in optisol-GS and castor oil

Background

Glutathione concentration in the lens decreases in aging and cataractous lenses, providing a marker for tissue condition. Experimental procedures requiring unfrozen lenses from donor banks rely on transportation in storage medium, affecting lens homeostasis and alterations in glutathione levels. The aim of the study was to examine the effects of Optisol-GS and castor oil on lens condition, determined from their ability to maintain glutathione concentrations.

Methodology/Principal Findings

Rat lenses were stored in the two types of storage media at varying time intervals up to 3 days. Glutathione concentration was afterwards determined in an enzymatic detection assay, specific for both reduced and oxidized forms. Lenses removed immediately after death exhibited a glutathione concentration of 4.70±0.29 mM. In vitro stored lenses in Optisol-GS lost glutathione quickly, ending with a concentration of 0.60±0.34 mM after 3 days while castor oil stored lenses exhibited a slower decline and ended at 3 times the concentration. A group of lenses were additionally stored under post mortem conditions within the host for 6 hours before its removal. Total glutathione after 6 hours was similar to that of lenses removed immediately after death, but with altered GSH and GSSG concentrations. Subsequent storage of these lenses in media showed changes similar to those in the first series of experiments, albeit to a lesser degree.

Conclusions/Significance

It was determined that storage in Optisol-GS resulted in a higher loss of glutathione than lenses stored in castor oil. Storage for more than 12 hours reduced glutathione to half its original concentration, and was considered unusable after 24 hours.

Glutathione participates in the modulation of starvation-induced autophagy in carcinoma cells

Glutathione (γ-L-glutamyl-L-cysteinyl-glycine, GSH) is the most abundant low molecular weight, thiol-containing compound within the cells and has a primary role in the antioxidant defense and intracellular signaling. Here we demonstrated that nutrient deprivation led to a significant decrease of intracellular GSH levels in three different carcinoma cell lines. This phenomenon was dependent on ABCC1-mediated GSH extrusion, along with GCL inhibition and, to a minor extent, the formation of GSH-protein mixed disulfides that synergistically contributed to the modulation of autophagy by shifting the intracellular redox state toward more oxidizing conditions. Modulation of intracellular GSH by inhibiting its de novo synthesis through incubation with buthionine sulfoximine, or by maintaining its levels through GSH ethyl ester, affected the oxidation of protein thiols, such as PRDXs and consequently the kinetics of autophagy activation. We also demonstrated that thiol-oxidizing or -alkylating agents, such as diamide and diethyl maleate activated autophagy, corroborating the evidence that changes in thiol redox state contributed to the occurrence of autophagy.

Neutral sphingomyelinase-induced ceramide triggers germinal vesicle breakdown and oxidant-dependent apoptosis in Xenopus laevis oocytes

Ceramide regulates many cellular processes, including cell growth, differentiation, and apoptosis. Although the effects of exogenous bacterial neutral sphingomyelinase (SMase) in Xenopus laevis oocytes have been investigated, its microinjection into oocytes has not been reported previously. Thus, we compared the incubation versus microinjection of the neutral Bacillus cereus sphingomyelinase (bSMase) to examine whether the topology of ceramide generation determines its effects on the fate of oocytes. In agreement with previous findings, incubation of mature stage VI oocytes with bSMase increased ceramide levels in oocyte extracts over time, causing the germinal vesicle breakdown indicative of maturation, without evidence of cytotoxicity. In contrast, bSMase microinjection, which increased ceramide levels in a time- and dose-dependent manner, resulted in oocyte apoptosis characterized by reactive oxygen species (ROS) generation, reduced glutathione (GSH) depletion in cytosol and mitochondria, release of cytochrome c and Smac/Diablo from mitochondria, and caspase-3 activation. Microinjection of acidic SMase from human placenta recapitulated the apoptotic effects of bSMase microinjection. Preincubation of oocytes with GSH-ethyl ester before bSMase microinjection prevented ROS generation and mitochondrial downstream events, thus protecting oocytes from bSMase-induced death. These findings show a divergent action of bSMase-induced ceramide on oocyte maturation or apoptosis depending on the intracellular site where ceramide is generated.

Enhanced γ-glutamylcysteine synthetase activity decreases drug-induced oxidative stress levels and cytotoxicity

Multidrug resistance of cancer cells can be intrinsic or acquired and occurs due to various reasons, including increased repair of genotoxic damage, an enhanced ability to remove/detoxify chemical agents, or reactive oxygen species (ROS), and repression of apoptosis. Human A2780/100 ovarian carcinoma cells exhibit resistance to DNA cross-linking agents, chlorambucil (Cbl), cisplatin (Cpl), melphalan (Mel), and ionizing radiation (IR) compared to the parental cell line, A2780. In the present study, we show that when A2780/100 and A2780 cells were treated with Cbl, GSH was extruded via methionine or cystathionine-inhibitable transporters of intact plasma membrane. GSH loss was followed by a rapid increase in ROS levels. The resistant, but not drug-sensitive cells normalized the intracellular GSH concentration along with ROS levels within 4-6 h after Cbl addition, and survived drug treatment. Normalization of GSH and ROS levels in A2780/100 cells correlated well with elevated gamma-glutamylcysteine synthetase (gamma-GCS) activity (10 +/- 1.8-fold over A2780 cells). Ectopic overexpression of the gamma-GCS heavy subunit in drug-sensitive cells nearly restored GSH and ROS to pre-treatment levels consequently increased cellular resistance to genotoxic agents (Cbl, Cpl, and IR), while overexpression of gamma-GCS light subunit had no such effects. Thus, in our model system, drug-resistant cells have the inherent ability to maintain increased gamma-GCS activity, reestablish physiological GSH, and cellular redox state and maintain increased cellular resistance to DNA cross-linking agents and IR.

GSH transport in immortalized mouse brain endothelial cells: evidence for apical localization of a sodium-dependent GSH transporter

We have previously shown GSH transport across the blood-brain barrier in vivo and expression of transport in Xenopus laevis oocytes injected with bovine brain capillary mRNA. In the present study, we have used MBEC-4, an immortalized mouse brain endothelial cell line, to establish the presence of Na+-dependent and Na+-independent GSH transport and have localized the Na+-dependent transporter using domain-enriched plasma membrane vesicles. In cells depleted of GSH with buthionine sulfoximine, a significant increase of intracellular GSH could be demonstrated only in the presence of Na+. Partial but significant Na+ dependency of [35S]GSH uptake was observed for two GSH concentrations in MBEC-4 cells in which gamma-glutamyltranspeptidase and gamma-glutamylcysteine synthetase were inhibited to ensure absence of breakdown and resynthesis of GSH. Uniqueness of Na+-dependent uptake in MBEC-4 cells was confirmed with parallel uptake studies with Cos-7 cells that did not show this activity. Molecular form of uptake was verified as predominantly GSH, and very little conversion of [35S]cysteine to GSH occurred under the same incubation conditions. Poly(A)+ RNA from MBEC expressed GSH uptake with significant (approximately 40-70%) Na+ dependency, whereas uptake expressed by poly(A)+ RNA from HepG2 and Cos-1 cells was Na+ independent. Plasma membrane vesicles from MBEC were separated into three fractions (30, 34, and 38% sucrose, by wt) by density gradient centrifugation. Na+-dependent glucose transport, reported to be localized to the abluminal membrane, was found to be associated with the 38% fraction (abluminal). Na+-dependent GSH transport was present in the 30% fraction, which was identified as the apical (luminal) membrane by localization of P-glycoprotein 170 by western blot analysis. Localization of Na+-dependent GSH transport to the luminal membrane and its ability to drive up intracellular GSH may find application in the delivery of supplemented GSH to the brain in vivo.

Glutathione protects the rat liver against reperfusion injury after hypothermic preservation

BACKGROUND & AIMS

The extracellular generation of reactive oxygen species (ROS) by Kupffer cells contributes to reperfusion injury of the liver allograft. The endogenous antioxidant glutathione (GSH) can detoxify these ROS; however, this effect might be limited by the low extracellular concentration of GSH. We therefore investigated whether an increase of extracellular GSH protects the liver against reperfusion injury after cold preservation.

METHODS

Livers of male Sprague-Dawley rats subjected to 24 hours of cold ischemia in University of Wisconsin solution (4 degrees C) were reperfused for 2 hours in the absence (controls) or presence of 0.5, 1, 2, or 4 mmol/L GSH (n = 4-6 each).

RESULTS

Two hours after starting reperfusion of control livers, the sinusoidal release of lactate dehydrogenase and purine nucleoside phosphorylase increased to 247 +/- 96 and 27 +/- 13 mU. min(-1). g liver(-1), respectively, but only to 76 +/- 43 and 10 +/- 4 mU. min(-1). g liver(-1) in the presence of 4 mmol/L GSH. This cytoprotective effect was confirmed histologically by a marked reduction of trypan blue staining of hepatocytes. Compared with control livers, postischemic bile flow was significantly enhanced by GSH (0.15 +/- 0.02 vs. 0.41 +/- 0.11 microL. min(-1). g liver(-1)), indicating improved liver function. During reperfusion of control livers, intracellular GSH content declined from 4.5 +/- 0.3 to 2.3 +/- 0.1 micromol/g liver, but only to 3.8 +/- 0.4 micromol/g liver in the presence of 4 mmol/L GSH. Reperfusion of untreated livers was accompanied by a prolonged increase of portal pressure to maximally 12.5 +/- 1.9 cm H2O, which was significantly attenuated by 4 mmol/L GSH (7.2 +/- 1.4 cm H2O). Similar cytoprotective and hemodynamic effects were observed with 2 mmol/L GSH, but not with 0.5 and 1 mmol/L GSH.

CONCLUSIONS

Treatment of cold-preserved livers with GSH upon reperfusion prevents damage of hepatocytes, deterioration of the hepatic circulation, and loss of intracellular GSH. In view of these protective effects and its low toxicity in humans, GSH should be considered a candidate drug for prevention of ROS-related reperfusion injury of the liver allograft.

ATP modulates sulfobromophthalein uptake in rat liver plasma membrane vesicles

The hepatic uptake of the bilirubin-bilirubin-sulfobromophthalein (BSP) group of organic anions is a carrier-mediated process and is accounted for by at least four distinct plasma membrane proteins (bilitranslocase, BSP/bilirubin-binding protein, organic anion-binding protein and the organic anion transport protein). In order to investigate the regulation of basolateral organic anion uptake, BSP transport was measured in rat basolateral liver plasma membrane vesicles in the presence of ATP. ATP significantly stimulated the electroneutral uptake of BSP with an increment in Vmax compared with control (1.57 +/- 0.14 vs 0.73 +/- 0.06 nmol BSP/mg protein per 15 s, respectively; P < 0.001) while the apparent K(m) was not changed significantly (12 +/- 1 vs 12 +/- 2 mumol/L). The stimulatory effect was dose-dependent for ATP (K(m) 1.01 +/- 0.37 mmol/L). ATP had no detectable effect on the electrogenic component of BSP transport. Other nucleotides (ADP, AMP, GTP) and non-hydrolysable ATP did not enhance BSP uptake, suggesting that ATP hydrolysis was necessary for the effect. This was supported by the lack of effect on BSP uptake when ATP was added in the presence of vanadate. The addition of phorbol-12-myristate 13-acetate, an activator of protein kinase C (PKC), increased BSP uptake in a dose-dependent manner in the presence, but not in the absence, of ATP. Incubation of vesicles with staurosporine, an inhibitor of PKC activity, resulted in a dose-dependent inhibition of ATP-sensitive BSP transport. These data indicate that electroneutral BSP hepatic uptake is modulated by ATP. The effect is related to ATP hydrolysis and involves the activity of PKC.

Bile secretion of cadmium, silver, zinc and copper in the rat. Involvement of various transport systems

In the present study we compared, in vivo in rats, the hepatobiliary transport of monovalent (silver:Ag) and divalent metals (zinc:Zn; cadmium:Cd) with that of copper (Cu). Cu can have two oxidation states in vivo, i.e. Cu(I) and Cu(II). Studies were performed in normal Wistar (NW) rats and mutant GY Wistar rats. The latter express defective canalicular ATP-dependent glutathione-conjugate transport (cMOAT); reduced glutathione (GSH) is virtually absent in bile of these mutants. Cd (400 nmol/100g body wt, i.v.) was rapidly secreted into bile in NW rats concommitant with a 4-fold increase in biliary GSH secretion. In contrast, biliary Cd concentrations remained below detection limits in GY rats. Injection of Zn (1500 nmol/100g body wt) did not affect Zn secretion in GY rats and resulted only in a very small increase in NW rats (recovery < 2%). The biliary secretion pattern of Ag (800 nmol/100g body wt, i.v.) was highly similar to that of Cu (260 nmol/100g body wt). A biphasic pattern composed of a rapid and slow phase was observed in NW rats for both metals with a recovery of 48.5 +/- 10.6% and 44.9 +/- 8.4% of the dose for Ag and Cu, respectively. In GY rats, the rapid phase of both Ag and Cu secretion was absent and recoveries were 23.2 +/- 3.6% and 19.7 +/- 3.2%, respectively. When Ag and Cu were administered simultaneously, the recoveries of Ag and Cu were decreased in NW and GY rats when compared to single administration. Our data indicate that divalent and monovalent metals are secreted into bile via different transport systems in the rat. The absence of Cd and Zn secretion into bile of GY rats after their i.v. administration suggest a role of cMOAT in their biliary elimination. Cu and Ag probably share common transport systems for hepatobiliary removal, being in part dependent on the presence of either GSH in bile or cMOAT activity or on both. The GSH-independent portion of transport, i.e. the slow phase, may be mediated by the newly identified Cu transporting P-type ATPase (cCOP).

Rapid and specific efflux of reduced glutathione during apoptosis induced by anti-Fas/APO-1 antibody

Although human JURKAT T lymphocytes induced to undergo apoptosis with anti-Fas/APO-1 antibody were observed to rapidly lose reduced glutathione (GSH), increased concentrations of oxidized products were not detectable. Unexpectedly, the reduced tripeptide was instead quantitatively recovered in the incubation medium of the cells. As GSH loss was blocked by bromosulfophthalein and dibromosulfophthalein, known inhibitors of hepatocyte GSH transport, a specific export rather than nonspecific leakiness through plasma membranes is proposed to be responsible. Apoptosis was delayed when GSH-diethylesters were used to elevate intracellular GSH, although the high capacity of the activated efflux system quickly negated the benefit of this treatment. Stimulation of GSH efflux provides a novel mechanism whereby Fas/APO-1 ligation can deplete GSH. We speculate that it enhances the oxidative tonus of a responding cell without requiring an increase in the production of reactive oxygen species.

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.

Evidence for the existence of a sodium-dependent glutathione (GSH) transporter. Expression of bovine brain capillary mRNA and size fractions in Xenopus laevis oocytes and dissociation from gamma-glutamyltranspeptidase and facilitative GSH transporters

Our laboratory previously has shown apparent carrier-mediated glutathione (GSH) uptake across the blood-brain barrier (BBB) in two animal models. In the present study, when Xenopus oocytes were injected with bovine brain capillary mRNA expression of intact GSH, uptake was observed after 3 days. When total mRNA was converted to cDNA and subfractionated with subsequent cRNA injection into oocytes, three distinct fractions (5, 7-8, and 11-12) expressed carrier-mediated intact GSH transport. Northern blot analysis established the presence of RcGshT, the previously cloned sodium-independent hepatic canalicular transporter, only in fraction 5. GSH transport activity in fraction 7 was significantly inhibited by replacement of NaCl with choline chloride and by sulfobromophthalein-GSH, neither of which affects RcGshT. The Na(+)-dependent GSH uptake kinetics exhibited high affinity (approximately 400 micron) and low affinity (approximately 10 mM) components. Fraction 11 expressed Na(+)-independent transport of intact GSH and also contained the GGT transcript. In conclusion, we have identified three distinct sized transcripts from bovine brain capillary mRNA which express GSH transport: one fraction expresses a novel Na(+)-dependent GSH uptake which can be dissociated unequivocally from both GGT and RcGshT for the first time and which may account for uptake of GSH against its electrochemical gradient at the BBB.

Evidence that the rat hepatic mitochondrial carrier is distinct from the sinusoidal and canalicular transporters for reduced glutathione. Expression studies in Xenopus laevis oocytes

Mitochondrial GSH derives from a mitochondrial transport system (RmGshT), which translocates cytosol GSH into the mitochondrial matrix. Mitochondria of oocytes, isolated 3-4 days after microinjection of total liver mRNA, expressed a RmGshT compared with water-injected oocytes. The expressed RmGshT exhibited similar functional features as reported in isolated mitochondria of rat liver such as ATP stimulation, inhibition by glutamate, and insensitivity to inhibition by sulfobromophthalein-glutathione (BSP-GSH) and S-(2,4-dinitrophenyl)glutathione (DNP-GSH). The expressed RmGshT is localized in the inner mitochondrial membrane since expression is still observed in mitoplasts prepared from total liver mRNA-injected oocytes. Fractionation of poly(A)+ RNA identified a single mRNA species of approximately 3-3.5 kilobases encoding for the RmGshT, which was stimulated by ATP and inhibited by glutamate but not by BSP-GSH or DNP-GSH. Microinjection of this fraction did not lead to expression of plasma membrane GSH transport in intact oocytes, and conversely, oocytes microinjected with cRNA for rat liver sinusoidal GSH transporter (RsGshT) or rat liver canalicular GSH transporter (RcGshT) did not express mitochondrial GSH transport activity. Thus, our results show the successful expression of the rat hepatic mitochondrial GSH carrier, which is different from RsGshT and RcGshT, and provide the strategic basis for the cloning of this important carrier.

Evidence for a hepatocyte membrane fatty acid transport protein using rat liver mRNA expression in Xenopus laevis oocytes

The aim of the present study was to directly demonstrate that hepatocellular uptake of long-chain fatty acids represents a non-diffusional uptake mechanism. Xenopus laevis oocytes were used for expression of rat liver mRNA to identify the liver fatty acid uptake system. Injection of total rat liver poly(A)+ RNA into oocytes resulted in a dose-dependent increase in fatty acid uptake. The most active mRNA was found in the 1.1-2.1 kb subfraction. In contrast, expression of the liver cytosolic fatty acid binding protein (L-FABP) or the previously suggested candidate carrier protein, mitochondrial aspartate aminotransferase (mGOT), did not induce fatty acid uptake. It is concluded that in rat liver, fatty acid transport represents a protein-mediated transport system.