Role of glutathione in the biliary excretion of the arsenical drugs trimelarsan and melarsoprol

Ex vivo and in vivo effects of arsenite on GST and ABCC2 activity and expression in the middle intestine of the rainbow trout Oncorhynchus mykiss

In fish of freshwaters environments, the accumulation and toxic effects of arsenite (AsIII) can be attenuated by detoxification proteins such as GST and ABCC transporters. We studied the effects of AsIII on the middle intestine of O. mykiss in ex-vivo and in vivo/ex vivo assays. For the ex vivo assays, we measured the transport rate of the ABCC substrate DNP-SG and GST activity in intestinal strips and everted sacs. AsIII inhibited DNP-SG transport in a concentration-dependent manner, specifically when we applied it on the basolateral side. GST activity increased when we applied a maximum concentration of AsIII. For the in vivo/ex vivo assays, we kept fish in water with or without 7.7 μmol L^-1 of AsIII for 48 h. Then, we measured DNP-SG transport rate, GST activity, and PP1 activity in intestine strips during one hour. For PP1 activity, we incubated the strips with or without microcystin-LR (MCLR), a toxin excreted through ABCC2 proteins. We also analyzed Abcc2 and Gst-π mRNA expression in intestine and liver tissue. In the group exposed in vivo to AsIII, DNP-SG transport rate and GST activity were higher and the effect of MCLR over PP1 activity was attenuated. AsIII significantly induced only Abcc2 mRNA expression in both middle intestine and liver. Our results suggest that, in the middle intestine of O. mykiss, AsIII is absorbed mainly at the basolateral side of the enterocytes, excreted to the lumen by ABCC2 transporters, and is capable of modulating Abcc2 mRNA expression by a transcriptional mechanism.

NRF2 Is a Potential Modulator of Hyperresistance to Arsenic Toxicity in Stem-Like Keratinocytes

Arsenic is a well-known human carcinogen. Stem cells are indicated to be involved in arsenic carcinogenesis and have a survival selection advantage during arsenic exposure with underlying mechanisms undefined. In the present study, we demonstrated that CD34high-enriched cells derived from HaCaT human keratinocytes showed stem-like phenotypes. These cells were more resistant to arsenic toxicity and had higher arsenic efflux ability than their mature compartments. The master transcription factor in antioxidant defense, nuclear factor erythroid 2-related factor 2 (NRF2) with its downstream genes, was highly expressed in CD34high-enriched cells. Stable knockdown of NRF2 abolished the hyperresistance to arsenic toxicity and holoclone-forming ability of CD34high-enriched cells. Our results suggest that skin epithelial stem/progenitor cells are more resistant to arsenic toxicity than mature cells, which is associated with the high innate expression of NRF2 in skin epithelial stem/progenitor cells.

Up-regulation of hepatic ABCC2, ABCG2, CYP1A1 and GST in multixenobiotic-resistant killifish (Fundulus heteroclitus) from the Sydney Tar Ponds, Nova Scotia, Canada

Cellular defence against accumulation of toxic xenobiotics includes metabolism by phase I and II enzymes and export of toxicants and their metabolites via ATP-binding cassette (ABC) transporters. Liver gene expression of representatives of these three protein groups was examined in a population of multixenobiotic-resistant killifish (Fundulus heteroclitus) from the Sydney Tar Ponds, Nova Scotia, Canada. The Tar Ponds are heavily polluted with polycyclic aromatic hydrocarbons, polychlorinated biphenyls and heavy metals. The relationship among ABC transporters ABCB1, ABCB11, ABCC2, ABCG2, phase I enzyme cytochrome P4501A1 (CYP1A1) and phase II enzyme glutathione-S-transferase (GST-mu) was investigated by quantifying hepatic transcript abundance. In Tar Pond killifish, hepatic mRNA expression levels of ABCC2, ABCG2, CYP1A1 and GST-mu were elevated compared to reference sites, suggesting that hydrophobic contaminants undergo phase I and II metabolism and are then excreted into the bile of these fish. Hepatic ABCB1 and ABCB11 mRNA were not up-regulated in Tar Pond fish compared to two reference sites, indicating that these two proteins are not involved in conferring multixenobiotic resistance to Tar Pond killifish. The results suggest instead that liver up-regulation of phase I and II enzymes and complementary ABC transporters ABCC2 and ABCG2 may confer contaminant resistance to Tar Pond fish.

A case-control study of GST polymorphisms and arsenic related skin lesions

BACKGROUND

Polymorphisms in GSTT1, GSTM1 and GSTP1 impact detoxification of carcinogens by GSTs and have been reported to increase susceptibility to environmentally related health outcomes. Individual factors in arsenic biotransformation may influence disease susceptibility. GST activity is involved in the metabolism of endogenous and exogenous compounds, including catalyzing the formation of arsenic-GSH conjugates.

METHODS

We investigated whether polymorphisms in GSTT1, GSTP1 and GSTM1 were associated with risk of skin lesions and whether these polymorphisms modify the relationship between drinking water arsenic exposure and skin lesions in a case control study of 1200 subjects frequency matched on age and gender in community clinics in Pabna, Bangladesh in 2001-2002.

RESULTS AND DISCUSSION

GSTT1 homozygous wildtype status was associated with increased odds of skin lesions compared to the null status (OR1.56 95% CI 1.10-2.19). The GSTP1 GG polymorphism was associated with greater odds of skin lesions compared to GSTP1 AA, (OR 1.86 (95%CI 1.15-3.00). No evidence of effect modification by GSTT1, GSTM1 or GSTP1 polymorphisms on the association between arsenic exposure and skin lesions was detected.

CONCLUSION

GSTT1 wildtype and GSTP1 GG are associated with increased risk of skin lesions.

Role of oxidative stress in the apoptosis of hepatocellular carcinoma induced by combination of arsenic trioxide and ascorbic acid

AIM

The present study was designed to determine the possible pathway underlying the enhancement of apoptosis induced by the combined use of arsenic trioxide (As(2)O(3)) and ascorbic acid (AA).

METHODS

The level of intracellular reactive oxygen species (ROS) was detected by means of flow cytometry analysis with an oxidation-sensitive fluorescent probe (6-carboxy-2',7' dichlorodihydrofluorescein diacetate) uploading. The activity of glutathione (GSH), glutathione peroxidase (GPx), and superoxide dismutase (SOD) were detected by biochemical methods. The mitochondrial membrane potential was measured by flow cytometry analysis with rhodamine 123 staining. Bcl-2, Bax, and p17 subunit of caspase-3 were analyzed using the Western blot method. The apoptosis rate was determined by flow cytometry with annexin-V/propidium iodide staining.

RESULTS

Compared with As(2)O(3) (2.0 micromol/L) treated alone, As(2)O(3) (2.0 micromol/L) in combination with AA (100 micromol/L) decreased intracellular GSH content from 101.30+/-5.76 to 81.91+/-3.12 mg/g protein, and increased ROS level from 127.61+/-5.12 to 152.60+/-5.88, which was represented by the 2, 7-dichlorofluorescein intensity. The loss of mitochondria membrane potential was increased from 1269.97+/-36.11 to 1540.52+/-52.63, which was presented by fluorescence intensity. The p17 subunit of caspase-3 expression was increased approximately 2-fold. However, SOD and GPx depletion and the ratio of Bcl-2 to Bax were equal to that of As2O3 treated alone (P>0.05). When the ROS scavenger, N-acetyl-L-cysteine, was added to As(2)O(3) and AA combined treatment group, the apoptosis rate decreased from 15.60 %+/-1.14% to 9.48%+/-0.67%, and the ROS level decreased from 152.60+/-5.88 to 102.77+/-10.25.

CONCLUSION

AA potentiated As(2)O(3)-induced apoptosis through the oxidative pathway by increasing the ROS level. This may be the result of depleting intracellular GSH. It may influence the downstream cascade following ROS, including mitochondria depolarization and caspase-3 activation. However, SOD and GPx depletion and the ratio of Bcl-2 to Bax influenced by As(2)O(3) was not found to be potentiated by AA.

Role of glutathione in reduction of arsenate and of γ-glutamyltranspeptidase in disposition of arsenite in rats

Arsenate (AsV), the environmentally prevalent form of arsenic, is converted sequentially in the body to arsenite (AsIII), monomethylarsonic acid (MMAsV), monomethylarsonous acid (MMAsIII), and dimethylarsinic acid (DMAsV) and some trimethylated metabolites. Although the biliary excretion of arsenic in rats is known to be glutathione (GSH)-dependent, involving transport of arsenic-GSH conjugates, the role of GSH in the reduction of AsV to the more toxic AsIII in vivo has not been defined. Therefore, we studied how the fate of AsV is influenced by buthionine sulfoximine (BSO), which depletes GSH in tissues. Control and BSO-treated rats were given AsV (50 micromol/kg, i.v.) and arsenic metabolites in bile, urine, blood and tissues were analysed by HPLC-HG-AFS. BSO increased retention of AsV in blood and tissues and decreased appearance of AsIII in blood, bile (by 96%) and urine (by 63%). The biliary excretion of MMAsIII was also nearly abolished, the appearance of MMAsIII and MMAsV in the blood was delayed and the renal concentrations of these monomethylated arsenicals were decreased by BSO. Interestingly, appearance of DMAsV in blood and urine remained unchanged and the concentrations of this metabolite in the kidneys and muscle were even increased in response to BSO. To test the role of gamma-glutamyltranspeptidase (GGT) in arsenic disposition, the effect of the of the GGT inhibitor acivicin was investigated in rats injected with AsIII (50 micromol/kg, i.v.). Acivicin lowered the hepatic and renal GGT activities and increased the biliary as well as urinary excretion of GSH, but failed to alter the disposition (i.e. blood and tissue concentrations, biliary and urinary excretion) of AsIII and its metabolites. In conclusion, shortage of GSH decreases not only the hepatobiliary transport of arsenic, but also reduction of AsV and the formation of monomethylated arsenic, while not hindering the production of dimethylated arsenic. While GSH plays an important role in the disposition and toxicity of arsenic, GGT, which hydrolyses GSH and GSH conjugates, apparently does not influence the fate of the GSH-reactive trivalent arsenicals in rats.

Nitric oxide prodrugs and metallochemotherapeutics: JS-K and CB-3-100 enhance arsenic and cisplatin cytolethality by increasing cellular accumulation

Development of chemotherapeutic resistance is a major cause of pharmacologic failure in cancer treatment. One mechanism of resistance in tumor cells is the overexpression of glutathione S-transferases (GSTs) that serve two distinct roles in the development of drug resistance via the formation of glutathione conjugates with drugs for their cellular efflux, and the inhibition of the mitogen-activated protein kinase pathway. To target GST-based resistance to chemotherapeutics, a series of nitric oxide (NO)-releasing diazeniumdiolates was synthesized and shown to release NO on reaction with GST and/or glutathione. Two diazeniumdiolates, JS-K [O2-(2,4-dinitrophenyl) 1-[(4-ethoxycarbonyl)piperazin-1-yl]diazen-1-ium-1,2-diolate] and CB-3-100 [O2-(2,4-dinitrophenyl) 1-[4-(N,N-diethylcarboxamido)piperazin-1-yl]diazen-1-ium-1,2-diolate], were studied on their ability in reversing arsenic and cisplatin resistance in a rat liver cell line that is tumorigenic and shows acquired tolerance to arsenic and cisplatin, with overexpression of GSTs. The enhanced cytolethality produced by the NO donors was accompanied by increased accumulation of arsenic and platinum within cells and by enhanced activation of mitogen-activated protein kinase members c-jun-NH-kinase and extracellular signal-regulated kinase. Our data indicate that JS-K and CB-3-100 are promising lead compounds for the possible development of a novel class of adjuvant chemotherapeutic agents potentially capable of reversing arsenic and cisplatin resistance in certain tumor cells.

Toxicokinetic and genomic analysis of chronic arsenic exposure in multidrug-resistance mdr1a/1b(-/-) double knockout mice

Multidrug-resistance gene knockout mdr1a/1b(-/-) mice, which are deficient in P-glycoproteins, are more sensitive than wild-type (WT) mice to acute arsenic toxicity. This study assessed toxic manifestations of chronic oral arsenic in mdr1a/1b(-/-) mice, including oxidative stress and altered gene expression, and investigated altered toxicokinetics as a potential basis of enhanced arsenic toxicity. Thus, mdr1a/1b(-/-) and WT mice were exposed to sodium arsenite (0-80 ppm as arsenic) in the drinking water for 10 weeks at which time hepatic arsenic accumulation, lipid peroxidation (LPO), redox status and change in gene expression level were assessed. All mice survived the arsenic exposure, but body weight gain in the highest dose group was reduced in both mdr1a/1b(-/-) and WT mice. Arsenic induced pathological changes, elevated LPO levels and enhanced glutathione S-transferase (GST) activity, in the liver to a greater extent in mdr1a/1b(-/-) than in WT mice. Arsenic also decreased Cu/Zn superoxide dismutase activity in both mdr1a/1b(-/-) and WT mice. The expressions of certain genes, such as those encoding cell proliferation, GST, acute-phase proteins and metabolic enzymes, were modestly altered in arsenic-exposed mice. The expression of cyclin D1, a potential hepatic oncogene, was enhanced in arsenic-exposed mdr1a/1b(-/-) mice only. At the highest level of exposure, hepatic arsenic content was higher in mdr1a/1b(-/-) than in WT mice, suggesting that enhanced accumulation due to transport deficiency may, in part, account for the enhanced toxicity in these mice. In summary, this study shows that chronic arsenic toxicity, including liver pathology and oxidative stress, is enhanced in mdr1a/1b(-/-) mice, possibly due to enhanced accumulation of arsenic as a result of transport system deficiency.

The MRP1-mediated effluxes of arsenic and antimony do not require arsenic-glutathione and antimony-glutathione complex formation

Arsenic trioxide is an effective treatment for acute promyelocytic leukemia, but resistance to metalloid salts is found in humans. Using atomic absorption spectroscopy, we have measured the rate of uptake of arsenic trioxide and of antimony tartrate in GLC4 and GLC4/ADR cells overexpressing MRP1 and the rate of their MRP1-mediated effluxes as a function of the intracellular GSH concentration. In sensitive cells, after 1 h, a pseudosteady state is reached where intra- and extracellular concentrations of metalloid are the same. This precludes the formation, at short term, of complexes between arsenic or antimony with GSH. In resistant cells reduced intracellular accumulation of arsenic (or antimony), reflecting an increased rate of arsenic (or antimony) efflux from the cells, is observed. No efflux of the metalloid is observed in GSH depleted cells. The two metalloids and GSH are pumped out by MRP1 with the same efficiency. Moreover for the three compounds 50% of the efflux is inhibited by 2 microM MK571. This led us to suggest that As- and Sb-containing species could be cotransported with GSH.

Species variations in the biliary and urinary excretion of arsenate, arsenite and their metabolites

In most mammalian species, inorganic arsenicals are extensively biotransformed and excreted both in unchanged form and as metabolites. In the bile of rats receiving arsenate (AsV) or arsenite (AsIII) we have identified monomethylarsonous acid (MMAsIII), purportedly the most toxic metabolite of inorganic arsenic. As rats are not commonly accepted for studying arsenic metabolism, we carried out a comparative investigation on the excretion of AsV, AsIII and their metabolites in five animal species in order to determine whether they also form MMAsIII from AsV and AsIII. Anaesthetised bile duct-cannulated rats, mice, hamsters, rabbits, and guinea pigs were injected with AsV or AsIII (50 micromol/kg, i.v.) and their bile and urine was collected for 2 h. Arsenic in bile and urine was speciated by HPLC-hydride generation-atomic fluorescence spectrometry and the excretion rates of AsV, AsIII, monomethylarsonic acid (MMAsV), MMAsIII and dimethylarsinic acid (DMAsV) were quantified. All species injected with AsV excreted arsenic preferentially into urine, whereas all animals receiving AsIII, except rabbits, delivered more arsenic into bile than urine. Bile contained almost exclusively trivalent arsenic (i.e. AsIII and/or MMAsIII), whereas AsV, AsIII and DMAsV appeared in urine. Except for guinea pigs, which do not methylate arsenic, the other species formed MMAsIII and excreted it into bile. Having excreted as much as 8% of the dose of AsIII or AsV in 2 h as MMAsIII, rats were by far the most efficient producers of this supertoxic metabolite. Thus, although the rat is not a good model for studying long-term arsenic disposition, this species appears especially valuable in studies on AsIII methyltransferase and in vivo formation of MMAsIII.

Multidrug-resistance mdr1a/1b double knockout mice are more sensitive than wild type mice to acute arsenic toxicity, with higher arsenic accumulation in tissues

Arsenic is an environmental toxicant. Active extrusion via the ArsAB pump is a mechanism for arsenic detoxication in bacteria. However, how arsenic is effluxed from mammalian cells is not completely known. Our recent work shows that acquired arsenic resistance is associated with overexpression of P-glycoprotein and can be reversed by PSC833, an inhibitor for P-glycoprotein. This study utilized the mdr1a/1b(-/-) mice, which lack mdr1-type P-glycoproteins, to examine whether these mice are sensitive to arsenic toxicity and have higher arsenic accumulation in their tissues. The mdr1a/1b(-/-) and wild-type FVB mice were given arsenic as sodium arsenite (12-19 mg/kg, sc) and toxicity was examined 24 h later. The mdr1a/1b(-/-) mice were more sensitive than wild-type mice to arsenite-induced lethality, with LD(50) of 14.5 and 17 mg/kg, respectively. Histologically, arsenite produced more frequent and more severe lesions in the liver and kidney of the mdr1a/1b(-/-) mice than in wild-type mice. Serum alanine aminotransferase activity and blood urea nitrogen levels, indicative of hepatic and renal damage respectively, were increased 4 to 6-fold in the mdr1a/1b(-/-) mice as compared with 1-2-fold increases in wild-type mice. The mdr1a/1b(-/-) mice accumulated more arsenic in the liver (15.3 vs. 5.2 microg/g), kidney (7.23 vs. 3.22 microg/g), small intestine (3.98 vs. 1.57 microg/g) and brain (0.45 vs. 0.17 microg/g), as compared with wild-type mice 24 h after sodium arsenite (14 mg/kg, s.c.) administration. In summary, this study demonstrated that the mdr1a/1b(-/-) mice were more sensitive to acute arsenic toxicity and accumulated more arsenic than wild-type mice, suggesting that P-glycoproteins are involved, at least in part, in arsenic efflux in mammalians.

Overexpression of glutathione S-transferase II and multidrug resistance transport proteins is associated with acquired tolerance to inorganic arsenic

Recent work shows that long-term exposure to low levels of arsenite induces malignant transformation in a rat liver epithelial cell line. Importantly, these chronic arsenic-exposed (CAsE) cells also develop self-tolerance to acute arsenic exposure. Tolerance is accompanied by reduced cellular arsenic accumulation, suggesting a mechanistic basis for reduced arsenic sensitivity. The present study examined the role of xenobiotic export pumps in acquired arsenic tolerance. Microarray analysis of CAsE cells showed increased expression of the genes encoding for glutathione S-transferase Pi (GST-Pi), multidrug resistance-associated protein genes (MRP1/MRP2, which encode for the efflux transporter Mrp1/Mrp2) and the multidrug resistance gene (MDR1, which encodes for the efflux transporter P-glycoprotein). These findings were confirmed at the transcription level by reverse transcription-polymerase chain reaction and at the translation level by Western-blot analysis. Acquired arsenic tolerance was abolished when cells were exposed to ethacrynic acid (an inhibitor of GST-Pi), buthionine sulfoximine (a glutathione synthesis inhibitor), MK571 (a specific inhibitor for Mrps), and PSC833 (a specific inhibitor for P-glycoprotein) in dose-dependent fashions. MK571, PSC833, and buthionine sulfoximine markedly increased cellular arsenic accumulation. Consistent with a role for multidrug resistance efflux pumps in arsenic resistance, CAsE cells were found to be cross-resistant to cytotoxicity of several anticancer drugs, such as vinblastine, doxorubicin, actinomycin-D, and cisplatin, that are also substrates for Mrps and P-glycoprotein. Thus, acquired tolerance to arsenic is associated with increased expression GST-Pi, Mrp1/Mrp2 and P-glycoprotein, which function together to reduce cellular arsenic accumulation.

Effects of methylmercury and organic acid mercurials on the disposition of exogenous selenium in rats

Interaction of methylmercury (MM), an environmental and industrial toxicant, with selenium is well known but incompletely understood. Therefore, the effects of MM (10 micromol/kg i.v.) on the disposition of exogenous selenium were compared with those of other organic mercurials (merbromine, mercuribenzene sulfonic acid, and mercuribenzoic acid) in anesthetized bile duct-cannulated rats injected with sodium [(75)Se]selenite (10 micromol/kg i.v.). The mercurial organic acids (10 micromol/kg i.v.) differed strikingly from MM in their influence on selenium disposition. They promoted renal and hepatic accumulation as well as biliary excretion of selenium but decreased distribution to the muscle, testis, and brain as well as the pulmonary excretion of selenium. In contrast, MM altered selenium distribution in an opposite fashion: it diminished the biliary output of selenium and enhanced selenium exhalation. GC-MS analysis verified that this latter paradoxical effect resulted from increased exhalation of dimethyl selenide. Further studies indicated that the MM-induced increase in pulmonary excretion of dimethyl selenide cannot be due to a diminished conversion of this volatile selenium compound to trimethylselenonium ion (TMSe(+)), because MM influenced neither the urinary excretion nor the hepatic and renal concentration of TMSe(+) in selenite-injected rats. Compared to the selenite-exposed rats, the selenite plus MM-injected animals exhibited a significant rise in the hepatic level of S-adenosylmethionine (SAME), the endogenous methyl donor in selenium methylation, and the ratio of SAME to S-adenosylhomocysteine. Based on these and others' observations, it is hypothesized that MM may increase hepatic availability of SAME in selenite-dosed rats by counteracting selenite-induced inactivation of SAME synthetase, thereby facilitating SAME synthesis, and/or by acting as a methyl donor in formation of dimethyl selenide, thereby sparing SAME. In summary, the toxicologically and ecologically relevant interaction of MM and selenite is not mimicked by organic acid mercurials, possibly because it results in formation of lipophilic Hg- and Se-containing common compound(s) and because it also appears to involve methyl transfer from MM to selenium.