Recurrent drug-induced liver injury (DILI) with different drugs in the Spanish Registry: the dilemma of the relationship to autoimmune hepatitis.

BACKGROUND & AIMS: Multiple instances of DILI in the same patient with drugs of similar structure or function as well as completely unrelated drugs are not well understood and poorly documented. We have sought evidence of the frequency and characteristics of patients who have experienced two DILI episodes due to different drugs. METHODS: All cases of DILI systematically collected in the Spanish DILI Registry between 1994 and 2009 were retrieved. Data on demographics, clinical, laboratory and pathological findings, and outcome were analyzed. RESULTS: Nine patients (mean age 67 years, four women) out of 742, 1.21%, had evidence of two DILI episodes caused by different drugs. In four cases DILI was associated with structurally related drugs and in an additional two cases the drugs had a common target. In another case, unrelated antibiotics were implicated. In only two cases, the two drugs/herbals were not related in structure or function. All but one patient exhibited hepatocellular damage. The type of damage was consistent in both DILI episodes. Four cases presented as autoimmune hepatitis (AIH) in the second episode. CONCLUSIONS: Multiple episodes of DILI in association with different drugs occur infrequently. In each individual, the type of injury was similar during the two DILI episodes, regardless of the causative drug. Second episodes of DILI are more likely to be associated with features of AIH. It remains uncertain if this is drug-induced unmasking of true AIH or DILI with autoimmune features. These cases illustrate the dilemma faced by clinicians in distinguishing these possibilities.

Role of hepatic anion-binding protein in bromsulphthalein conjugation.

Using gel filtration, the binding of both glutathione and Bromsulphthalein (BSP) to a liver-soluble protein was found to be identical. BSP-conjugating activity (glutathione S-aryltransferase) was present only in the fractions corresponding to the two protein-bound markers. Using a highly sensitive assay, with 3,4-dichloronitrobenzene, the pattern of glutathione S-aryltransferase activity was found to coincide with Y protein. This evidence suggests that Y protein, or ligandin, has a dual role in hepatic transport: a specific enzymic function in the conjugation of certain anions with glutathione in addition to a transport function in the intracellular binding of organic anions.

Diagnosis, management and prevention of drug-induced liver injury.

Drug-induced liver injury (DILI) is increasingly being recognised as a significant cause of both acute and chronic liver disease. The most commonly implicated agents are paracetamol, antimicrobials, non-steroidal anti-inflammatory drugs, statins, isoniazid and herbal remedies. Drug-induced hepatotoxicity is generally idiosyncratic in nature. The pathogenesis of DILI remains enigmatic, but involves exposure to the toxic agent, mitochondrial injury, failure of adaptation, and innate and adaptive immune responses. Diagnosis of drug-induced liver diseases can be difficult, but the key to causality is to diligently exclude other causes of liver injury, and to identify a characteristic clinical drug-related signature. Management of drug-induced liver injury is symptomatic, with early referral to a liver transplant unit at the first hint of liver failure, especially in those with non-paracetamol-induced liver injury. Prevention of drug hepatotoxicity includes increased vigilance during pre-clinical drug development and clinical trials, alanine aminotransferase monitoring with certain drugs, better marketing strategies, and the future identification of both diagnostic and prognostic biomarkers.

Colchicine protects mice from the lethal effect of an agonistic anti-Fas antibody.

The aim of this study was to determine whether colchicine, which has been reported to protect against various hepatotoxic insults, influences the susceptibility of mice to the agonistic anti-Fas antibody, Jo2. All mice that were pretreated with colchicine (2 mg/kg) survived the lethal challenge of intraperitoneal administration of 10 microg of Jo2, whereas all control mice pretreated with gamma-lumicolchicine succumbed to the challenge. Twelve micrograms of Jo2 killed less than half of colchicine-pretreated mice and its lethal effects were delayed relative to control mice, which all died within 8 hours. Other microtubule-disrupting agents such as Taxol, vinblastine, and nocodazole also improved the survival of mice treated with the lethal dose of Jo2. Histologic examination showed that colchicine protected against Jo2-induced fulminant liver injury, and TUNEL assay demonstrated that colchicine protected against massive apoptosis of hepatocytes. Hepatocytes isolated from colchicine-pretreated mice exhibited decreased susceptibility to Jo2-induced apoptosis. In addition, colchicine pretreatment reduced surface expression of Fas and decreased Jo2- and TNF-alpha-induced apoptosis of cultured hepatocytes in the presence of actinomycin D, but did not affect the susceptibility of cultured sinusoidal endothelial cells to Jo2-induced apoptosis. Remarkably, Fas and TNF receptor-1 mRNA and intracellular protein levels increased after colchicine treatment, indicating that colchicine protects against death ligand-induced apoptosis in the liver by decreasing death-receptor targeting to the cell surface.

Cyclical oxidation-reduction of the C3 position on bile acids catalyzed by rat hepatic 3 alpha-hydroxysteroid dehydrogenase. I. Studies with the purified enzyme, isolated rat hepatocytes, and inhibition by indomethacin.

We recently identified that the Y' bile acid binders are 3 alpha-hydroxysteroid dehydrogenases (3 alpha-HSD). In the present studies, purified 3 alpha-HSD catalyzed rapid 3H loss from [3 beta-3H, C24-14C]lithocholic and chenodeoxycholic acids without net conversion to 3-oxo bile acids under physiologic pH and redox conditions. [3 beta-3H]Cholic acid was a poor substrate. The Y' fraction of hepatic cytosol was exclusively responsible for this activity and 3H was transferred selectively to NADP+. Time-dependent 3H loss was also seen in isolated hepatocytes. Further hydroxylation products of lithocholic and chenodeoxycholic acids lost 3H at the same rate, whereas 3H loss from lithocholic acid rapidly ceased, which suggests compartmentation of this bile acid in hepatocytes. Indomethacin inhibited 3H loss from bile acids either in incubations with the pure enzyme or in isolated hepatocytes. Indomethacin did not alter the initial uptake rate of bile acids by hepatocytes, but caused a redistribution of unconjugated bile acids into the medium at early time points (2.5 and 5.0 min) and that of conjugated bile acids at later time intervals (30 min). 3H loss from the 3 beta position therefore can be used to probe the interaction between bile acids and cytosolic 3 alpha-HSD in intact cells, and indomethacin is capable of inhibiting this interaction.

Coproporphyrin I and 3 excretion in bile and urine.

The excretion of coproporphyrin isomers I and III was studied in the rat. Both isomers were found to bind equally to rat plasma and liver cytosol in vitro and to disappear from plasma at equal rates after single injections in vivo. During equimolar infusions of isomers into bile fistula animals, both the I and III isomers were excreted in bile in a concentration ratio of 2:1, respectively. Pretreatment of animals with ethinylestradiol or simultaneous infusions of phenoldibromophthalein disulfonate caused a reduction in total hepatic excretion with no change in the 2:1 ratio in bile. As hepatic excretion fell, excretion of both isomers in urine rose, with an increase in the proportion of the I isomer. The findings mimic those reported to occur in man and can be explained by inhibition of a common carrier which requires a stereospecific configuration that statistically favors the hepatic transport of the symmetrical coproporphyrin I isomer.

Inhibition of glutathione efflux in the perfused rat liver and isolated hepatocytes by organic anions and bilirubin. Kinetics, sidedness, and molecular forms.

Using isolated, in situ, single-pass perfused rat livers, incubations of freshly isolated hepatocytes, and sinusoidal membrane-enriched vesicles, we and others have shown the saturability of transport (efflux) of hepatic glutathione (GSH). These observations have implicated a carrier mechanism. Our present studies were designed to provide further evidence in support of a carrier mechanism for hepatic GSH efflux by demonstrating competition by liver-specific ligands which are taken up by hepatocytes. Perfusing livers with different substances, we found that: (a) sulfobromophthalein-GSH (BSP-GSH) had a dose-dependent and fully reversible inhibitory effect on GSH efflux, while GSH alone did not have any effect; (b) taurocholate had no inhibitory effect; (c) all of the organic anions studied, i.e., BSP, rose bengal, indocyanine green, and unconjugated bilirubin (UCB), manifested potent, dose-dependent inhibitory effects, with absence of toxic effects and complete reversibility of inhibition in the case of UCB. The inhibitory effects of UCB could be overcome partially by raising (CoCl2-induced) hepatic GSH concentration. Because of the physiological importance of UCB, we conducted a detailed study of its inhibitory kinetics in the isolated hepatocyte model in the range of circulating concentrations of UCB. Studies with Cl- -free media, to inhibit the uptake of UCB by hepatocytes, showed that the inhibition of GSH efflux by UCB is apparently from inside the cell. This point was confirmed by showing that the inhibition is overcome only when bilirubin-loaded cells are cleared of bilirubin (incubation with 5% bovine serum albumin). Using Gunn rat hepatocytes and purified bilirubin mono- and diglucuronides, we found that both UCB and glucuronide forms of bilirubin inhibit GSH efflux in a dose-dependent manner. We conclude that the organic anions, although taken up by a mechanism independent of GSH, may competitively inhibit the carrier for GSH efflux from inside the hepatocyte.

Effect of chronic ethanol feeding on rat hepatocytic glutathione. Compartmentation, efflux, and response to incubation with ethanol.

Hepatocytes from rats that were fed ethanol chronically for 6-8 wk were found to have a modest decrease in cytosolic GSH (24%) and a marked decrease in mitochondrial GSH (65%) as compared with pair-fed controls. Incubation of hepatocytes from ethanol-fed rats for 4 h in modified Fisher's medium revealed a greater absolute and fractional GSH efflux rate than controls with maintenance of constant cellular GSH, indicating increased net GSH synthesis. Inhibition of gamma-glutamyltransferase had no effect on these results, which indicates that no degradation of GSH had occurred during these studies. Enhanced fractional efflux was also noted in the perfused livers from ethanol-fed rats. Incubation of hepatocytes in medium containing up to 50 mM ethanol had no effect on cellular GSH, accumulation of GSH in the medium, or cell viability. Thus, chronic ethanol feeding causes a modest fall in cytosolic and a marked fall in mitochondrial GSH. Fractional GSH efflux and therefore synthesis are increased under basal conditions by chronic ethanol feeding, whereas the cellular concentration of GSH drops to a lower steady state level. Incubation of hepatocytes with ethanol indicates that it has no direct, acute effect on hepatic GSH homeostasis.

Cyclical oxidation-reduction of the C3 position on bile acids catalyzed by 3 alpha-hydroxysteroid dehydrogenase. II. Studies in the prograde and retrograde single-pass, perfused rat liver and inhibition by indomethacin.

[3 beta-3H, 24-14C]Lithocholic, chenodeoxycholic, and cholic acids were administered in tracer bolus doses either prograde or retrograde in the isolated perfused rat liver. Little 3H loss from cholic acid was observed, whereas with the other bile acids, 20-40% of the administered 3H was lost in a single pass from perfusate to bile. Most of the 3H loss occurred rapidly (5 min) and was recovered as [3H]water in perfusate. Excretion of bile acids was delayed with retrograde administration, and 3H loss was more extensive. In both prograde and retrograde studies, indomethacin markedly inhibited the excretion of the bolus of bile acid into bile. Indomethacin inhibited the extraction of glycocholate (50 microM) during steady state perfusion without affecting transport maximum for excretion. At lower glycocholate concentration (5 microM), indomethacin inhibited both extraction and excretion. A greater effect was seen on excretion in the latter case, which suggests that displacement of bile acid from the cytosolic protein lead to redistribution in the hepatocyte as well as reflux into the sinusoid. These data suggest that binding of bile acids to cytosolic 3 alpha-hydroxysteroid dehydrogenases occurs extensively during hepatic transit and is important in mediating the translocation of bile acids from the sinusoidal to canalicular pole of the cell.

Effects of chronic ethanol feeding on rat hepatocytic glutathione. Relationship of cytosolic glutathione to efflux and mitochondrial sequestration.

Chronic ethanol feeding to rats increases the sinusoidal component of hepatic glutathione (GSH) efflux, despite a lower steady-state GSH pool size. In the present studies, no increase of biliary GSH efflux in vivo was found in chronic ethanol-fed cells. Studies were performed on ethanol-fed and pair-fed cells to identify the kinetic parameters of cellular GSH concentration-dependent efflux. The relationship between cytosolic GSH and the rate of efflux was modeled by the Hill equation, revealing a similar Vmax, 0.22 +/- 0.013 vs. 0.20 +/- 0.014 nmol/min per 10(6) cells for ethanol-fed and pair-fed cells, respectively, whereas the Km was significantly decreased (25.3 +/- 2.3 vs. 33.5 +/- 1.4 nmol/10(6) cells) in ethanol-fed cells. The difference in Km was larger when the data were corrected for the increased water content in ethanol-fed cells. We found a direct correlation between mitochondria and cytosolic GSH, revealing that mitochondria from ethanol-fed cells have less GSH at all cytosolic GSH values. The rate of resynthesis in depleted ethanol-fed cells in the presence of methionine and serine was similar to control cells and gamma-glutamylcysteine synthetase remained unaffected by chronic ethanol. However, the reaccumulation of mitochondrial GSH as the cytosolic pool increased was impaired in the ethanol cells. The earliest time change in GSH regulation was a 50% decrease in the mitochondrial GSH at 2 wk.

Direct protection against acetaminophen hepatotoxicity by propylthiouracil. In vivo and in vitro studies in rats and mice.

Hepatotoxicity caused by acetaminophen can be prevented by enzyme-catalyzed conjugation of its reactive metabolite with glutathione (GSH). Since we have shown in previous studies that 6-N-propyl-2-thiouracil (PTU) can substitute for GSH as a substrate for the GSH S-transferases, we examined the possibility that PTU might also protect against acetaminophen hepatotoxicity by direct chemical interaction with the reactive metabolite of acetaminophen. In an in vitro system consisting of [(3)H]acetaminophen, liver microsomes from phenobarbital-pretreated rats, and an NADPH-generating system, we found that PTU had a dose-dependent additive effect with GSH on inhibition of acetaminophen covalent binding. PTU administration also resulted in a dose-dependent decrease in both GSH depletion and covalent binding in vivo in acetaminophen-treated mice. To examine the possible mechanisms by which PTU exerts its protective effect, we studied the action of PTU on both acetaminophen conjugation and metabolic activation. PTU had no effect upon acetaminophen pharmacokinetics in phenobarbital-pretreated rats, as examined by measuring acetaminophen concentration in bile, urine, and blood after an intraperitoneal dose, nor did it alter the total amount of polar conjugates formed. Microsomes from PTU-treated rats were unaltered in cytochrome P-450 concentrations and p-nitroanisole-O-demethylase, benzo-alpha-pyrene hydroxylase, and cytochrome c-reductase activities. Furthermore PTU did not decrease acetaminophen-GSH adduct formation in vitro, suggesting that there was no reduction in drug activation. However, in bile from [(35)S]PTU and [(3)H]acetaminophen treated rats, as well as in incubates of the two drugs with liver microsomes, a new (35)S- and (3)H-containing product could be identified. By both thin layer chromatography and high pressure liquid chromatography this new product, which co-eluted with [(3)H]acetaminophen, was separated from unreacted [(35)S]PTU. The formation of this product in vitro was a function of PTU concentration and reached a maximum of 0.06 mumol/min per mg protein at 0.5 mM PTU. In vivo, the total biliary excretion of this product over 4 h (116 nmol) equaled the net reduction in acetaminophen metabolite covalent binding in the liver of phenobarbital-pretreated rats (108 nmol). We conclude that PTU, independent of its antithyroid effect, diminishes hepatic macromolecular covalent binding of acetaminophen reactive metabolite both in vivo and in vitro, and it does so by detoxifying the reactive metabolite through direct chemical interaction in a manner similar to GSH. These observations may define the mechanism by which PTU is protective against liver injury caused by acetaminophen.

Impaired uptake of glutathione by hepatic mitochondria from chronic ethanol-fed rats. Tracer kinetic studies in vitro and in vivo and susceptibility to oxidant stress.

Isolated hepatocytes incubated with [35S]-methionine were examined for the time-dependent accumulation of [35S]-glutathione (GSH) in cytosol and mitochondria, the latter confirmed by density gradient purification. In GSH-depleted and -repleted hepatocytes, the increase of specific activity of mitochondrial GSH lagged behind cytosol, reaching nearly the same specific activity by 1-2 h. However, in hepatocytes from ethanol-fed rats, the rate of increase of total GSH specific radioactivity in mitochondria was markedly suppressed. In in vivo steady-state experiments, the mass transport of GSH from cytosol to mitochondria and vice versa was 18 nmol/min per g liver, indicating that the half-life of mitochondrial GSH was approximately 18 min in controls. The fractional transport rate of GSH from cytosol to mitochondria, but not mitochondria to cytosol, was significantly reduced in the livers of ethanol-fed rats. Thus, ethanol-fed rats exhibit a decreased mitochondrial GSH pool size due to an impaired entry of cytosol GSH into mitochondria. Hepatocytes from ethanol-fed rats exhibited a greater susceptibility to the oxidant stress-induced cell death from tert-butylhydroperoxide. Incubation with glutathione monoethyl ester normalized the mitochondrial GSH and protected against the increased susceptibility to t-butylhydroperoxide, which was directly related to the lowered mitochondrial GSH pool size in ethanol-fed cells.

Insulin and glucocorticoid dependence of hepatic gamma-glutamylcysteine synthetase and glutathione synthesis in the rat. Studies in cultured hepatocytes and in vivo.

We reported that glucagon and phenylephrine decrease hepatocyte GSH by inhibiting gamma-glutamylcysteine synthetase (GCS), the rate-limiting enzyme in GSH synthesis (Lu, S.C., J. Kuhlenkamp, C. Garcia-Ruiz, and N. Kaplowitz. 1991. J. Clin. Invest. 88:260-269). In contrast, we have found that insulin (In, 1 microgram/ml) and hydrocortisone (HC, 50 nM) increased GSH of cultured hepatocytes up to 50-70% (earliest significant change at 6 h) with either methionine or cystine alone as the sole sulfur amino acid in the medium. The effect of In occurred independent of glucose concentration in the medium. Changes in steady-state cellular cysteine levels, cell volume, GSH efflux, or expression of gamma-glutamyl transpeptidase were excluded as possible mechanisms. Both hormones are known to induce cystine/glutamate transport, but this was excluded as the predominant mechanism since the induction in cystine uptake required a lag period of greater than 6 h, and the increase in cell GSH still occurred when cystine uptake was blocked. Assay of GSH synthesis in extracts of detergent-treated cells revealed that In and HC increased the activity of GCS by 45-65% (earliest significant change at 4 h) but not GSH synthetase. In and HC treatment increased the Vmax of GCS by 31-43% with no change in Km. Both the hormone-mediated increase in cell GSH and GCS activity were blocked with either cycloheximide or actinomycin D. Finally, when studied in vivo, streptozotocin-treated diabetic and adrenalectomized rats exhibited lower hepatic GSH levels and GCS activities than respective controls. Both of these abnormalities were prevented with hormone replacement. Thus, both in vitro and in vivo, In and glucocorticoids are required for normal expression of GCS.

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

In cultured rat hepatocytes, cystine led to an inhibition of GSH efflux by lowering the Vmax by approximately 35% without affecting the Km. The cystine-mediated inhibition of GSH efflux was rapid in onset (< 1 h), with near maximum effect at 0.1 mM. Inhibition was still observed when cystine uptake was prevented. Cystine and sulfobromophthalein-GSH, a selective inhibitor of sinusoidal transport of GSH, did not exhibit additive inhibitory effects on GSH efflux. Depletion of ATP or membrane depolarization after cystine treatment were excluded as potential mechanisms. DTT not only reversed the cystine-mediated inhibition of GSH efflux, it stimulated GSH efflux up to 400-500%. The DTT effect was immediate in onset, reaching maximum after 30 min, and was partially reversed by cystine, suggesting that the two share a common site(s) of action. DTT treatment did not alter cellular ATP levels or change the membrane potential. In cultured hepatocytes, DTT treatment increased the Vmax of GSH efflux by approximately 500% without affecting the Km. Inhibition of microtubular function and vesicular acidification did not affect basal or DTT stimulated efflux. Both cystine and DTT effects on sinusoidal GSH efflux were confirmed in perfused livers. In summary, the capacity of the sinusoidal GSH transporter is markedly influenced by thiol-disulfide status.

3 alpha-hydroxysteroid dehydrogenase activity of the Y' bile acid binders in rat liver cytosol. Identification, kinetics, and physiologic significance.

Rat Y' bile acid binders (33 kD) have been previously recognized as cytosolic bile acid binding proteins (Sugiyama, Y., T. Yamada, and N. Kaplowitz, 1983, J. Biol. Chem., 258:3602-3607). We have now determined that these Y' binders are 3 alpha-hydroxysteroid dehydrogenases (3 alpha-HSD), bile acid-metabolizing enzymes. 3 alpha-HSD activity copurified with lithocholic acid-binding activity after sequential gel filtration, chromatofocusing, and affinity chromatography. Three peaks of 3 alpha-HSD activity (I, II, III) were observed in chromatofocusing and all were identified on Western blot by a specific Y' binder antiserum. 3 alpha-HSD-I, the predominant form, was purified and functioned best as a reductase at pH 7.0 with a marked preference for NADPH. Michaelis constant values for mono- and dihydroxy bile acids were 1-2 microM, and cholic acid competitively inhibited the reduction of 3-oxo-cholic acid. Under normal redox conditions, partially purified 3 alpha-HSD-I and freshly isolated hepatocytes catalyzed the rapid reduction of 3-oxo-cholic to cholic acid without formation of isocholic acid, whereas the reverse reaction was negligible. The Y' bile acid binders are therefore 3 alpha-HSD, which preferentially and stereospecifically catalyze the reduction of 3-oxo-bile acids to 3 alpha-hydroxy bile acids.

Hormone-mediated down-regulation of hepatic glutathione synthesis in the rat.

Our present work characterized the role of hormone-mediated signal transduction pathways in regulating hepatic reduced glutathione (GSH) synthesis. Cholera toxin, dibutyryl cAMP (DBcAMP), and glucagon inhibited GSH synthesis in cultured hepatocytes by 25-43%. Cellular cAMP levels exhibited a lower threshold for stimulation of the GSH efflux than inhibition of its synthesis. The effect of DBcAMP was independent of the type of sulfur amino acid precursor and cellular ATP levels and unassociated with increased GSH mixed disulfide formation or altered GSH/oxidized glutathione ratio. In liver cytosols, addition of DBcAMP and cAMP-dependent protein kinase (A-kinase) inhibited GSH synthesis from substrates (cysteine, ATP, glutamate, and glycine) by approximately 20% which was prevented by the A-kinase inhibitor. However, if only substrates of the second step in GSH synthesis were used (gamma-glutamylcysteine, glycine, and ATP), DBcAMP and A-kinase exerted no inhibitory effect. Phenylephrine, vasopressin, and phorbol ester also inhibited GSH synthesis in cultured cells by approximately 20%, and depleted cell GSH independent of the type of sulfur amino acid precursor. Cellular cysteine level was unchanged despite the significant fall in GSH after glucagon or phenylephrine treatment. Pretreatment with either staurosporine, C-kinase inhibitor, or calmidazolium, a calmodulin inhibitor, partially prevented but, together, completely prevented the inhibitory effect of phenylephrine. The same combination had no effect on the inhibitory effect of glucagon. The effects of hormones were confirmed in both the intact perfused liver and after in vivo administration. Thus, two classes of hormones acting through distinct signal transduction pathways may down-regulate hepatic GSH synthesis by phosphorylation of gamma-glutamylcysteine synthetase.

Evidence for carrier-mediated transport of glutathione across the blood-brain barrier in the rat.

Information on the origin of brain glutathione and the possibility of its transport from blood to brain is limited. We found a substantial uptake of 35S-labeled glutathione by the rat brain using the carotid artery injection technique. The brain uptake index of glutathione with and without an irreversible gamma-glutamyl transpeptidase inhibitor, acivicin, was similar. No significant differences in the regional uptake of labeled glutathione were found in rats pretreated with acivicin. The brain uptake index of tracer glutathione was similar to that of cysteine tracer and was lower than that of phenylalanine. The transport of oxidized glutathione (glutathione disfulfide) across the blood-brain barrier was not significantly different from that of sucrose, an impermeable marker. Brain radioactivity 15 s after carotid artery injection of labeled glutathione to rats pretreated with acivicin was predominantly in the form of glutathione. The in vivo glutathione uptake was saturable with an apparent Km of 5.84 mM. Amino acids, amino acid analogues, and other compounds [cysteine, phenylalanine, glutathione disulfide, gamma-glutamylglutamate, gamma-glutamyl p-nitroanilide, 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH)] did not affect glutathione transport. Our data suggest that glutathione is transported across the blood-brain barrier by a saturable and specific mechanism.

Expression cloning of a rat hepatic reduced glutathione transporter with canalicular characteristics.

Using the Xenopus oocyte expression system, we have previously identified an approximately 4-kb fraction of mRNA from rat liver that expresses sulfobromophthalein-glutathione (BSP-GSH)-insensitive reduced glutathione (GSH) transport (Fernandez-Checa, J., J. R. Yi, C. Garcia-Ruiz, Z. Knezic, S. Tahara, and N. Kaplowitz. 1993. J. Biol. Chem. 268:2324-2328). Starting with a cDNA library constructed from this fraction, we have now isolated a single clone that expresses GSH transporter activity. The cDNA for the rat canalicular GSH transporter (RcGshT) is 4.05 kb with an open reading frame of 2,505 nucleotides encoding for a polypeptide of 835 amino acids (95,785 daltons). No identifiable homologies were found in searching various databases. An approximately 96-kD protein is generated in in vitro translation of cRNA for RcGshT. Northern blot analysis reveals a single 4-kb transcript in liver, kidney, intestine, lung, and brain. The abundance of mRNA for RcGshT in rat liver increased 3, 6, and 12 h after a single dose of phenobarbital. Insensitivity to BSP-GSH and induction by phenobarbital, unique characteristics of canalicular GSH secretion, suggest that RcGshT encodes for the canalicular GSH transporter.

Sinusoidal efflux of glutathione in the perfused rat liver. Evidence for a carrier-mediated process.

Turnover of hepatic glutathione in vivo in the rat is almost entirely accounted for by cellular efflux, of which 80-90% is sinusoidal. Thus, sinusoidal efflux play a major quantitative role in homeostasis of hepatic glutathione. Som preliminary observations from our laboratory (1983. J. Pharmacol. Exp. Ther. 224:141-147.) and circumstantial evidence in the literature seemed to imply that the raising of the hepatic glutathione concentration above normal was not accompanied by a rise in the rate of sinusoidal efflux. Based on these observations, we hypothesized that the sinusoidal efflux was probably a saturable process and that at normal levels of hepatic glutathione the efflux behaved as a zero-order process (near-saturation). We tested our hypothesis by the use of isolated rat livers perfused in situ, single pass, with hemoglobin-free, oxygenated buffer medium at pH 7.4 and 37 degrees C. Preliminary experiments established a range of perfusion rates (3-4 ml/min per g) for adequacy of oxygenation, lack of cell injury, and minimization of variability contributed by perfusion rates. Hepatic glutathione was lowered to below normal by a 48-h fast, diethylmaleate (0.1-1.0 ml/kg i.p.), and buthionine sulfoximine (8 mmol/kg i.p.), and raised to above normal by 3-methylcholanthrene (20 mg/kg x 3 d i.p.) and cobalt chloride (0.05-0.27 g/kg-1 subcutaneously). Steady state sinusoidal efflux from each liver was measured over a 1-h perfusion, during which the coefficient of variation of glutathione in perfusates stayed within 10%. Hepatic glutathione efflux as a function of hepatic concentration was characterized by saturable kinetics with sigmoidal (non-hyperbolic) features. The data were fitted best with the Hill model and the following parameter values were estimated: Vmax = 20 nmol/min per g, Km = 3.2 mumol/g, and n = 3 binding/transport sites. The efflux could be inhibited reversibly by sulfobromophthalein-glutathione conjugate but was not affected by the addition of glutathione to the perfusion medium. The results support our hypothesis that sinusoidal efflux of glutathione is near saturation (approximately equal to 80% of Vmax) at normal (fed and fasted) liver glutathione concentrations. The phenomenon of saturability coupled with the ability to inhibit the efflux leads us to propose that sinusoidal efflux from hepatocytes appears to be a carrier-mediated process. Some recent studies by others, using sinusoidal membrane-enriched vesicles, also support these conclusions.