The human liver glutathione S-transferase gene superfamily: expression and chromosome mapping of an Hb subunit cDNA

Glutathione transferases, regulators of cellular metabolism and physiology

BACKGROUND

The cytosolic glutathione transferases (GSTs) comprise a super family of proteins that can be categorized into multiple classes with a mixture of highly specific and overlapping functions.

SCOPE OF REVIEW

The review covers the genetics, structure and function of the human cytosolic GSTs with particular attention to their emerging roles in cellular metabolism.

MAJOR CONCLUSIONS

All the catalytically active GSTs contribute to the glutathione conjugation or glutathione dependant-biotransformation of xenobiotics and many catalyze glutathione peroxidase or thiol transferase reactions. GSTs also catalyze glutathione dependent isomerization reactions required for the synthesis of several prostaglandins and steroid hormones and the catabolism of tyrosine. An increasing body of work has implicated several GSTs in the regulation of cell signaling pathways mediated by stress-activated kinases like Jun N-terminal kinase. In addition, some members of the cytosolic GST family have been shown to form ion channels in intracellular membranes and to modulate ryanodine receptor Ca(2+) channels in skeletal and cardiac muscle.

GENERAL SIGNIFICANCE

In addition to their well established roles in the conjugation and biotransformation of xenobiotics, GSTs have emerged as significant regulators of pathways determining cell proliferation and survival and as regulators of ryanodine receptors that are essential for muscle function. This article is part of a Special Issue entitled Cellular functions of glutathione.

Pharmacogenomics of cisplatin-induced ototoxicity

Cisplatin ototoxicity affects different individuals in a widely variable manner. These variations are likely to be explained by genetic differences among those affected. It would be highly advantageous to identify genetic variants that predispose to cisplatin ototoxicity in order to minimize the risk to susceptible subgroups. Although this area of research is very important, only a few studies have rigorously examined the genetic basis for cisplatin-induced susceptibility to hearing loss. This article addresses recent progress in clarifying the incidence of cisplatin ototoxicity and the risk factors and controversies regarding the identification of genetic variants associated with cisplatin-induced hearing loss.

Age-related macular degeneration and genetic polymorphisms of glutathione S-transferases M1 (GSTM1) and T1 (GSTT1)

The aim of this study is to understand the multifactorial causes of age-related macular degeneration (ARMD), and, therefore, it is reasonable to investigate whether genetic polymorphisms of antioxidant enzymes (GSTM1 and GSTT1) contribute to the development of ARMD. This study consisted of 112 subjects (44 females, 68 males) with exudative ARMD, who were recruited from Khalili Hospital ophthalmic clinic in Shiraz (southern Iran), referred by vitreoretinal surgeon. Also 112 sex-matched controls (44 females, 68 males) were randomly selected from unrelated volunteers in the same clinic. We excluded patients and controls with cataract or past history of cataract surgery, asthma, past history of malignancy, cardiovascular disease that on medication and known cases of glaucoma, because these traits were associated with GSTM1 and/or GSTT1 polymorphisms. There was no association between polymorphisms of neither GSTM1 nor GSTT1 and risk of ARMD. The combination genotypes of GSTM1 and GSTT1 were not associated with the risk of ARMD. We considered the time of deterioration of vision as the time of onset of exudative ARMD. The Kaplan-Meier analysis revealed that there was significant difference between genotypes of GSTM1 (log rank statistic = 7.03, df = 1, P = 0.008). The age at onset among GSTM1 null genotype was lower than the active genotype of GSTM1. Our results support the hypothesis that the protein encoded by the GSTM1 gene might have a protective function against oxidative stress in retina. Since the age at onset is influenced by the GSTM1 polymorphism, this implies that GSTM1 is a modifier gene.

An Interethnic Comparison of Polymorphisms of the Genes Encoding Drug-Metabolizing Enzymes and Drug Transporters: Experience in Singapore

Much of the interindividual variability in drug response is attributable to the presence of single nucleotide polymorphisms (SNPs) in genes encoding drug-metabolizing enzymes and drug transporters. In recent years, we have investigated the polymorphisms in a number of genes encoding phase I and II drug-metabolizing enzymes including CYPIA1, CYP3A4, CYP3A5, GSTM1, NAT2, UGT1A1, and TPMT and drug transporter (MDR1) in three distinct Asian populations in Singapore, namely the Chinese, Malays, and Indians. Significant differences in the frequencies of common alleles encoding these proteins have been observed among these three ethnic groups. For example, the frequency of the variant A2455G polymorphism of CYP1A1 was 28% in Chinese and 31% in Malays, but only 18% in Indians. CYP3A4*4 was detected in two of 110 Chinese subjects, but absent in Indians and Malays. Many Chinese and Malays (61-63%) were homozygous for the GSTM1*0 null genotype compared with 33% of Indians. The frequency of the UGTIA1*28 allele was highest in the Indian population (35%) compared to similar frequencies that were found in the Chinese (16%) and Malay (19%) populations. More importantly, our experience over the years has shown that the pharmacogenetics of these drug-metabolizing enzymes and MDR1 in the Asian populations are different from these in the Caucasian and African populations. For example, the CYP3A4*1B allele, which contains an A-290G substitution in the promoter region of CYP3A4, is absent in all three Asian populations of Singapore studied, but occurs in more than 54% of Africans and 5% of Caucasians. There were no difference in genotype and allelic variant frequencies in exon 12 of MDR1 between the Chinese, Malay, and Indian populations. When compared with other ethnic groups, the distribution of the wild-type C allele in exon 12 in the Malays (34.2%) and Indians (32.8%) was relatively high and similar to the Japanese (38.55%) and Caucasians (41%) but different from African-Americans (15%). The frequency of wild-type TT genotype in Asians (43.5% to 52.1%) and Japanese (61.5%) was much higher than those found in Caucasians (13.3%). All the proteins we studied represent the primary hepatic or extrahepatic enzymes, and their polymorphic expression may be implicated in disease risk and the disposition of drugs or endogenous substances. As such, dose requirements of certain drugs may not be optimal for Asian populations, and a second look at the factors responsible for this difference is necessary.

Different susceptibility to peroxisome proliferator-induced hepatocarcinogenesis in rats with polymorphic glutathione transferase genes

Although peroxisomal bifunctional enzyme (enoyl-CoA hydratase/L-3-hydroxyacyl-CoA dehydrogenase; BE) is a positive marker for peroxisome proliferation, it is completely absent or expressed very weakly in rat hepatic preneoplastic and neoplastic lesions induced by peroxisome proliferators (PP). After administration of PP for 8-15 weeks, some rats exhibit BE-negative preneoplastic foci but other rats do not. In the present study, to investigate the involvement of glutathione S-transferase (GST) M1 gene polymorphism in interindividual differences in susceptibility to PP, we developed a method to determine the genotypes of rats. We then examined whether rats with one type encoding 198Asn-199Cys (NC-type) or another encoding 198Lys-199Ser (KS-type) exhibit differences in clofibrate (CF) susceptibility. After administration of 0.3% CF for 6 weeks or more, BE-negative foci were found immunohistochemically in KS/KS-type rats, but not in NC/NC-type rats. The number of BE-negative foci in KS/KS rats was 15.3 +/- 9.0 foci/cm2 of liver section after 6 weeks of CF administration, and the values did not alter thereafter. The mean areas of BE-negative foci in KS/KS rat livers increased during the period from 6 to 60 weeks. At weeks 30 and 60, almost all BE-negative foci exhibited a clear cell phenotype, a type of preneoplastic hepatic lesion. BE-negative foci were devoid of peroxisome proliferator-activated receptor alpha, whereas surrounding tissues were positive for the receptor. These results indicate that rats that are polymorphic for the GST M1 gene exhibit different susceptibilities to CF in vivo.

Polymorphism of human mu class glutathione transferases

OBJECTIVES AND METHODS

A combined database mining approach was used to detect polymorphisms in the mu class glutathione-S-transferase (GST) genes. Although a large number of potential polymorphisms were detected in the five genes that comprise the Mu class GSTs using sequence alignment programs and by searching single nucleotide polymorphism databases, the majority were not validated or detected in three major ethnic populations (African, Southern Chinese and Australian European).

RESULTS

Two new polymorphisms were detected and characterized in the GSTM3 gene. A rare pG147W substitution was detected only in the Southern Chinese subjects. A more common pV224I substitution was found in each of the ethnic groups studied, and significant differences in allele frequencies were observed between each group. These two polymorphisms can combine to form four distinct haplotypes (GSTM3A [p.G147;V224], GSTM3C [p.G147;I224], GSTM3D [p.W147;V224], GSTM3E [p.W147;I224]). The four isoforms were expressed in Escherichia coli and characterized enzymatically with several substrates including 1-chloro-2,4-dinitrobenzene (CDNB), cumene hydroperoxide and t-nonenal. GSTM3-3 containing the variant p.W147 residue tended to show diminished specific activity and catalytic efficiency with CDNB. In contrast, GSTM3-3 containing the variant p.I224 residue tended to show increased specific activity and catalytic efficiency with CDNB. Interactions between the different p.147 and p.224 residues were also observed, with the GSTM3C isoform exhibiting the greatest activity with each substrate, and GSTM3E the lowest.

CONCLUSION

These functional polymorphisms may play a significant role in modulating the ability of GSTM3-3 to metabolize substrates such as the chemotherapeutic agent 1,3-bis(2-chloroethyl)-1-nitrosourea.

Oxygen free radical scavenger enzyme polymorphisms in systemic sclerosis

We performed a case-control study of polymorphisms of glutathione S-transferase (GST) isoenzymes and manganese superoxide dismutase (MnSOD) in black South Africans with systemic sclerosis (SSc). The frequency of the GSTM1*B phenotype was significantly decreased in the overall SSc group compared with controls (OR=0.19, p(corr)<.05), implying a possible protective effect against development of the disease. There was also a trend toward increased MnSODAla allele and phenotype frequencies in the diffuse cutaneous SSc subset compared with controls (OR=2.11 and 3.15, respectively, p(corr)<.1). Our findings provide new data on the distribution of GST and MnSOD polymorphisms in healthy Africans and further evidence that genetic factors may have a contributory role to play in predisposing to oxidative stress in SSc.

Mapping of glutathione transferase (GST) genes in the rat

Glutathione transferases (GST) make up a large group of related enzymes in mammalian tissues. The enzyme molecules are dimeric and at least 13 different subunits occur in the rat. Each subunit appears to be coded for by a distinct gene, and thus there is a large GST gene family in the rat. Recently, there have been several reports of the mapping of rat GST genes. In the present communication we confirm the previous assignments and extend the data with the mapping to rat chromosome 2 of a previously unmapped GST gene (Gstm1), and with the regional mapping of seven Gstp genes. These mappings provide further evidence for conservation of syntenic gene relationships among mammals. The human homologs of Gstm1 map to chromosome 1, and belong to a group of 9 genes that show conserved synteny on rat chromosome 2. The corresponding murine genes in most cases map to mouse chromosome 3. Similarly, the human homolog of Gstp maps to chromosome 11, and is one of 10 genes that exhibit conserved synteny on rat chromosome 1. The corresponding mouse genes map to mouse chromosome 7. Previously only one gene on rat chromosome 8 had a human homolog on chromosome 6, and rat Gsta1 is the second instance. Based on these mappings it appears that a new group of genes will exhibit conserved synteny on rat chromosome 8, human chromosome 6 and mouse chromosome 9. Interestingly, each of the three groups of conserved synteny seems to span the region across the centromeres of the human chromosomes.

Glutathione S-Transferases (GSTs) Inhibit Transcriptional Activation by the Peroxisomal Proliferator-Activated Receptor γ (PPARγ) Ligand, 15-Deoxy-Δ12,14Prostaglandin J2 (15-d-PGJ2)

15-Deoxy-Delta(12,14)prostaglandin J(2) (15-d-PGJ(2)), a terminal metabolite of the J-series cyclopentenone prostaglandins, influences a variety of cellular processes including gene expression, differentiation, growth, and apoptosis. As a ligand of peroxisomal proliferator-activated receptor gamma (PPAR gamma), 15-d-PGJ(2) can transactivate PPAR gamma-responsive promoters. Previously, we showed that multidrug resistance proteins MRP1 and MRP3 attenuate cytotoxic and transactivating activities of 15-d-PGJ(2) in MCF7 breast cancer cells. Attenuation was glutathione-dependent and was associated with formation of the glutathione conjugate of 15-d-PGJ(2), 15-d-PGJ(2)-SG, and its active efflux by MRP. Here we have investigated whether the glutathione S-transferases (GST) can influence biological activities of 15-d-PGJ(2). MCF7 cells were stably transduced with human cytosolic GST isozymes M1a, A1, or P1a. These GSTs had no effect on 15-d-PGJ(2) cytotoxicity when expressed either alone or in combination with MRP1. However, expression of any of the three GSTs significantly inhibited 15-d-PGJ(2)-dependent transactivation of a PPAR gamma-responsive reporter gene. The degree of inhibition correlated with the level of GST expressed. Under physiologic conditions, the nonenzymatic rate of 15-d-PGJ(2) conjugation with glutathione was significant. Of the three GST isozymes, only GSTM1a-1a further stimulated the rate of 15-d-PGJ(2)-SG formation. Moreover, GSTM1a-1a rate enhancement was only a transient burst that was complete within 15 s. Hence, catalysis plays little, if any, role in GST inhibition of 15-d-PGJ(2)-dependent transactivation. In contrast, inhibition of transactivation was associated with strong GST/15-d-PGJ(2) interactions. Potent inhibition by 15-d-PGJ(2) and 15-d-PGJ(2)-SG of GST activity was observed with K(i) in the 0.15-2.0 microM range for the three GST isozymes, results suggesting avid associations between GST and 15-d-PGJ(2) or 15-d-PGJ(2)-SG. Electrospray ionization mass spectrometry (ESI/MS) studies revealed no stable adducts of GST and 15-d-PGJ(2) indicating that GST/15-d-PGJ(2) interactions are primarily noncovalent. These results are consistent with a mechanism of GST-mediated inhibition of transactivation in which GST binds 15-d-PGJ(2) and 15-d-PGJ(2)-SG thereby sequestering the ligands in the cytosol away from their nuclear target, PPAR gamma.

Role of Multidrug Resistance Protein 2 (MRP2, ABCC2) in Alkylating Agent Detoxification: MRP2 Potentiates GlutathioneS-Transferase A1-1-Mediated Resistance to Chlorambucil Cytotoxicity

Our previous studies have shown that the glutathione S-transferases (GSTs) can operate in synergy with the efflux transporter multidrug resistance protein 1 (MRP1, ABCC1) to confer resistance to the cyto- and genotoxicities of some anticancer drugs and carcinogens. The current study was designed to determine whether the alternative efflux transporter, MRP2 (ABCC2), can also potentiate GST-mediated detoxifications in HepG2 cells. HepG2 cells, which express high-level MRP2 but not MRP1, were stably transduced with GST expression vectors under tetracycline-repressible transcriptional control. MRP2 was able to support GSTA1-1-mediated resistance to chlorambucil (CHB) cytotoxicity in HepG2 cells. Resistance was GST isozyme-specific in that GSTP1a-1a and GSTM1a-1a failed to confer protection from CHB toxicity. Moreover, inhibition of MRP2 with sulfinpyrazone completely reversed GSTA1-1-associated resistance, indicating that MRP2-efflux function is required to potentiate GSTA1-1-mediated resistance. Relative transport by MRP1 versus MRP2 of monoglutathionyl-CHB (CHB-SG) was examined using inside-out plasma membrane vesicles derived from MCF7 cells transduced with MRP1 or MRP2 expression vectors. Both MRP1 and MRP2 transported CHB-SG efficiently, at the levels of protein expressed, with similar Vmax and with Km of 0.39 and 10 microM, respectively. We conclude that detoxification of CHB by GSTA1-1 requires the removal of the glutathione conjugate formed and that either MRP1 or MRP2 can serve this efflux function. These findings have implications for the role of MRP2 in detoxification of alkylating agents in the apical epithelium of liver and kidney where it is highly expressed as well as the role of MRP2 in the emergence of alkylating drug resistance in cancer cells.

Genetic determinants of delayed graft function after kidney transplantation

BACKGROUND

Intracellular concentration of reactive oxygen species is held within tight physiological limits by enzymes with scavenging and repair functions. Under extreme conditions such as prolonged cold ischemia, these enzymes may be unable to adequately protect the organ, resulting in reperfusion injury that renders the graft dysfunctional after transplantation. In this study, we investigated normal human variation of some of these inducible enzymes to determine if certain phenotypes could be identified that are associated with a reduced risk of delayed graft function (DGF).

METHODS

Polymerase chain reaction was performed to differentiate polymorphisms for manganese superoxide dismutase and three classes of glutathione-S-transferase in donors and recipients of transplants with over 24 hr of cold ischemia. The data attained was analyzed compared with the presence or absence of DGF, defined as the requirement of hemodialysis in the first week after transplantation.

RESULTS

Enzyme polymorphisms were defined for 229 recipients and 104 of their respective donors. Patients receiving a kidney from a donor who expressed GSTM1*B either alone or in combination with GSTM1*A experienced significantly lower rates of DGF (P <0.05). No association was found between any enzyme polymorphism in the recipients and the development of DGF.

CONCLUSIONS

The identification of a genetic allele, which is protective against reperfusion injury, generates the possibility for defining polymorphisms at the time of tissue typing to give insight to the inherent biological risk of DGF that an organ possesses.

Differential effects of iron overload on GST isoform expression in mouse liver and kidney and correlation between GSTA4 induction and overproduction of free radicles

We have investigated the effect of iron overload on the expression of mouse GSTA1, A4, M1, and P1 in liver, the main iron storage site during iron overload, and in kidney. In iron-overloaded animals, mRNA and protein levels of GSTA1, A4, and M1 were increased in liver. In kidney, GSTA4 protein level was also increased while, unexpectedly, GSTA1 and M1 expression was strongly decreased. We showed, by immunohistochemistry, that GSTA4 was more abundant in hepatocytes of periportal areas and in convoluted proximal tubular cells in normal liver and kidney, respectively. In iron-overloaded mice, GSTA4 staining was more intense in cells that preferentially accumulated iron, and conjugation of 4-hydroxynonenal, a specific substrate of GSTA4, was enhanced in both organs. Moreover an acute exposure of primary cultures of mouse hepatocytes to iron-citrate strongly induced oxidative stress and cellular injury and resulted in an increase in GSTA4 expression, while cotreatment with iron-citrate and either desferrioxamine or vitamin E prevented both toxicity and GSTA4 induction. These data demonstrate that GSTA1 and M1 are differentially regulated in liver and kidney while GSTA4 is induced in both organs during iron overload. Moreover, they support the view that iron-induction of GSTA4 is related to an overproduction of free radicals.

Immunohistological analysis of glutathione transferase A4 distribution in several human tissues using a specific polyclonal antibody

We examined the cellular distribution of glutathione transferase A4 (GSTA4) in various human tissues by indirect immunoperoxidase using a specific polyclonal antibody raised in rabbit. This enzyme was localized in hepatocytes, bile duct cells, and vascular endothelial cells in liver, upper layers of keratinocytes and sebaceous and sweat glands in skin, proximal convoluted tubules in kidney, epithelial cells of mucosa and muscle cells in colon, muscle cells in heart, and neurons in brain. Staining was increased in pathological situations such as cirrhosis, UV-irradiated skin, and myocardial infarction and was strongly decreased in hepatocellular carcinoma. These results strongly support the view of a close correlation between cellular GSTA4 localization and the formation of reactive oxygen species in the tissues investigated.

Glutathione S-transferase M1 (GSTM1) polymorphism in two Brazilian populations

The distribution of GSTM1 phenotype frequencies was studied in two Brazilian samples composed of healthy and unrelated blood donors of both sexes ranging in age from 18 to 61 years. The first sample consisted of 658 individuals from Rio de Janeiro, and the second included 179 individuals from Brasília. The GSTM1 phenotypes were detected using PCR reactions and subsequent digestion by the restriction enzyme HaeII. The GSTM1 null phenotype frequency was 46% and 49% for Rio de Janeiro and Brasília samples, respectively. The GSTM1 phenotype distributions were not in agreement with Hardy-Weinberg equilibrium in either sample, chi²1 = 11.49 (P < 0.001) for Rio de Janeiro and chi²1 = 6.77 (P < 0.01) for Brasília. This deviation from Hardy-Weinberg equilibrium may be due to factors such as selection, errors in the phenotype determination or incomplete panmixia of the Brazilian population, whose main racial components are Caucasians, Africans and Indians.

Glutathione S-transferase polymorphisms and skin cancer after renal transplantation

BACKGROUND

Susceptibility to skin cancer after transplantation is multifactorial, and risk factors include skin type, sun exposure, and level of immunosuppression. A major mechanism of carcinogenesis is ultraviolet radiation-induced free radical damage, and genetically determined ability to metabolize free radicals may also predispose to skin cancer. The glutathione S-transferase enzymes play a major role in limiting the toxic effects of reactive oxygen species, and this study was designed to determine whether polymorphisms in these enzymes are associated with skin cancers in renal transplant recipients.

METHODS

Two hundred twenty-two long-term survivors of renal transplantation were examined for polymorphisms in the GSTM1, GSTT1, and GSTP1 genes, using a unified polymerase chain reaction with sequence specific primers (PCR-SSP) genotyping method.

RESULTS

The GSTP1*C allele was associated with the development of squamous cell carcinomas (SCCs; P = 0.01). No associations of the GSTM1 null genotype or the GSTT1 null genotype were identified, and the development of basal cell carcinomas was not associated with any GST polymorphism studied.

CONCLUSIONS

These results indicate that genetic variation in enzymes involved in free radical metabolism in the skin are associated with the development of skin cancer. While all renal transplant recipients should be advised to protect themselves from the sun, the identification of transplant patients with a genetic predisposition to skin tumors may permit the targeting of preventative and early intervention strategies to high-risk individuals.

Tyr115, gln165 and trp209 contribute to the 1, 2-epoxy-3-(p-nitrophenoxy)propane-conjugating activity of glutathione S-transferase cGSTM1-1

We investigated the epoxidase activity of a class mu glutathione S-transferase (cGSTM1-1), using 1,2-epoxy-3-(p-nitrophenoxy)propane (EPNP) as substrate. Trp209 on the C-terminal tail, Arg107 on the alpha4 helix, Asp161 and Gln165 on the alpha6 helix of cGSTM1-1 were selected for mutagenesis and kinetic studies. A hydrophobic side-chain at residue 209 is needed for the epoxidase activity of cGSTM1-1. Replacing Trp209 with histidine, isoleucine or proline resulted in a fivefold to 28-fold decrease in the k(cat)(app) of the enzyme, while a modest 25 % decrease in the k(cat)(app) was observed for the W209F mutant. The rGSTM1-1 enzyme has serine at the correponding position. The k(cat)(app) of the S209W mutant is 2. 5-fold higher than that of the wild-type rGSTM1-1. A charged residue is needed at position 107 of cGSTM1-1. The K(m)(app)(GSH) of the R107L mutant is 38-fold lower than that of the wild-type enzyme. On the contrary, the R107E mutant has a K(m)(app)(GSH) and a k(cat)(app) that are 11-fold and 35 % lower than those of the wild-type cGSTM1-1. The substitutions of Gln165 with Glu or Leu have minimal effect on the affinity of the mutants towards GSH or EPNP. However, a discernible reduction in k(cat)(app) was observed. Asp161 is involved in maintaining the structural integrity of the enzyme. The K(m)(app)(GSH) of the D161L mutant is 616-fold higher than that of the wild-type enzyme. In the hydrogen/deuterium exchange experiments, this mutant has the highest level of deuteration among all the proteins tested. We also elucidated the structure of cGSTM1-1 co-crystallized with the glutathionyl-conjugated 1, 2-epoxy-3-(p-nitrophenoxy)propane (EPNP) at 2.8 A resolution. The product found in the active site was 1-hydroxy-2-(S-glutathionyl)-3-(p-nitrophenoxy)propane, instead of the conventional 2-hydroxy isomer. The EPNP moiety orients towards Arg107 and Gln165 in dimer AB, and protrudes into a hydrophobic region formed by the loop connecting beta1 and alpha1 and part of the C-terminal tail in dimer CD. The phenoxyl ring forms strong ring stacking with the Trp209 side-chain in dimer CD. We hypothesize that these two conformations represent the EPNP moiety close to the initial and final stages of the reaction mechanism, respectively.

The house musk shrew (Suncus murinus): a unique animal with extremely low level of expression of mRNAs for CYP3A and flavin-containing monooxygenase

Expression of drug-metabolizing enzymes including cytochrome P450 (CYP) and flavin-containing monooxygenase (FMO) in various tissues of Suncus murinus (Suncus) were examined. Northern blot analysis showed that mRNAs hybridizable with cDNAs for rat CYP1A2, human CYP2A6, rat CYP2B1, human CYP2C8, human CYP2D6, rat CYP2E1, human CYP3A4 and rat CYP4A1 were expressed in various tissues from Suncus. The mRNA level of CYP2A in the Suncus lung was very high. Furthermore, it was found that the level of CYP2A mRNA in the Suncus lung was higher compared to the Suncus liver. The expression level of mRNA hybridizable with cDNA for human CYP3A4 was very low. The presence of CYP3A gene in Suncus was proven by the induction of the CYP with dexamethasone. Very low expression levels of mRNAs hybridizable with cDNAs for rat FMO1, rat FMO2, rat FMO3 and rat FMO5 were also seen in Suncus liver. No apparent hybridization band appeared when human FMO4 cDNA was used as a probe. The hepatic expression of mRNAs hybridizable with cDNAs for UDP-glucuronosyltransferase 1*6, aryl sulfotransferase, glutathione S-transferase 1, carboxyesterase and microsomal epoxide hydrolase in the Suncus were observed. These results indicate that the Suncus is a unique animal species in that mRNAs for CYP3A and FMO are expressed at very low levels.

Phase I and phase II drug-metabolizing enzymes are expressed and heterogeneously distributed in the biliary epithelium

Tissue expression of drug-metabolizing enzymes influences susceptibility to drugs and carcinogens. Because the biliary epithelium, exposed to bile-borne chemicals, may give rise to drug-induced cholangiopathies and to cholangiocarcinomas, we determined the pattern of expression of drug-metabolizing enzymes in this epithelium. We first demonstrated by blot analyses that biliary epithelial cells (BEC) isolated from human gallbladders display cytochrome P450 (CYP) 1A, 2E1, and 3A, microsomal epoxide hydrolase (mEH), alpha, mu, and pi glutathione S-transferase (GST), transcripts and proteins. We also identified CYP-associated steroid 6beta-hydroxylase activity in BEC. CYP and mEH expression was 5- to 20-fold lower in BEC than in autologous hepatocytes, and further differed by a higher ratio of CYP3A5/CYP3A4, and by CYP1A1 predominance over CYP1A2. alphaGST was highly expressed in both hepatocytes and BEC, while piGST was restricted to BEC. In approximately 50% of individuals, muGST was expressed in hepatocytes and at lower levels in BEC. By using the same antibodies as those used in immunoblots, we could show by immunohistochemistry that CYP2E1, CYP3A, mEH, alpha, mu, and piGST immunoreactivities are expressed and display a heterogeneous distribution in the epithelium lining the entire biliary tract except for small intrahepatic bile ducts that were devoid of CYP3A and alphaGST immunoreactivities. In conclusion, BEC contribute to phase II, and although to a lesser extent than hepatocytes, to phase I biotransformation. The distribution of drug-metabolizing enzymes in BEC suggest that they are heterogeneous in their ability to generate and detoxicate reactive metabolites, which may contribute to specific distributions of cholangiopathies.

Polymorphism of GSTM1 gene in patients with colorectal cancer and colonic polyps

The frequency of the GSTM1 gene in patients with nonpolyposis colorectal cancer (CRC) (n = 70) and in subjects with colonic polyps (n = 27) was evaluated and compared with healthy individuals (n = 145). Patients with CRC were divided into the three groups: patients coming from the families with hereditary nonpolyposis colorectal cancer (HNPCC) (n = 17); patients with a high risk of HNPCC who were referred to as suspected of HNPCC (n = 25); patients with sporadic colorectal cancer without clinical features of hereditary tumours (n = 28). A simple polymerase chain reaction (PCR) - based assay to identify GSTM1 nulled and positive (non-nulled) genotype was used. No significant differences in frequency of nulled individuals were observed in both patients with HNPCC and patients suspected of HNPCC as well as in subjects with colonic polyps. The most interesting observation was made in the group of patients with sporadic CRC. Twenty individuals (71.4 %) of the group were GSTM 1 deficient which was significantly different from the control population (p < 0.04). The above data indicate that the absence of the GSTM1 gene is associated with a greater risk of sporadic colorectal cancer. There is an increase in the overall risk of approximately 2.5 as compared with the control population.

Glutathione S-transferase T1 and M1 genotypes in normal mucosa, transitional mucosa and colorectal adenocarcinoma

Gene codings for glutathione S-transferase T1 (GSTT1) and M1 (GSTM1) are polymorphic in humans with null genotypes present in approximately 20 and 50%, respectively. A significant excess of homozygous null GSTT1 and GSTM1 genotypes has been demonstrated among individuals with certain types of cancers. This finding suggests that GSTT1 and GSTM1 may play a role in tumour susceptibility. However, reports concerning colorectal cancer susceptibility are controversial. In the present study, we used a multiplex polymerase chain reaction (PCR) approach to identify and analyze simultaneously the genotypes of both the genes in 99 patients with colorectal cancer and 109 healthy controls. Compared with the control group, a significant excess of homozygous null genotypes for GSTT1 was found in normal mucosa among the cancer patients, but not for GSTM1. Both genes were more frequently deleted in tumours than in corresponding normal mucosa. Furthermore, GSTT1 null genotype in tumour tissue, was significantly related to old age and to poor differentiation of tumours. GSTM1 null genotype in tumour was more frequent in the rectal tumours compared with tumours of left colon and right colon. Our results suggest that individuals with GSTT1 null genotype may be genetically predisposed for an increased risk of developing colorectal cancer. Allele loss in tumour tissue, which reflects genetic instability, may be considered as a marker for evaluating clinico-pathological characteristics of the cancer patients.

Evolution of differential substrate specificities in Mu class glutathione transferases probed by DNA shuffling

A library of variant enzymes was created by combined shuffling of the DNA encoding the human Mu class glutathione transferases GST M1-1 and GST M2-2. The parental GSTs are 84 % sequence identical at the protein level, but their specific activities with the substrates aminochrome and 2-cyano-1,3-dimethyl-1-nitrosoguanidine (cyanoDMNG) differ by more than 100-fold. Aminochrome is of particular interest as an oxidation product of dopamine and of possible significance in the etiology of Parkinson's disease, and cyanoDMNG is a model for genotoxic and potentially carcinogenic nitroso compounds. GST M2-2 has at least two orders of magnitude higher catalytic activity with both of the substrates than any of the other known GSTs, including GST M1-1. The DNA library of variant Mu class GST sequences contained "mosaic" structures composed of alternating segments of both parental sequences. All clones contained the 5'-end of a GST M1-1 clone optimized for high-level expression in Escherichia coli. The remainder of the sequences derived from segments of GST M2-2 and GST M1-1 DNA. All of the clones analyzed contained between two and seven distinct DNA segments. In addition, each clone contained an average of approximately one point mutation. None of the library clones analyzed was identical with either of the two parental structures. Variant GST sequences were expressed in E. coli, and their enzymatic activities with aminochrome, cyanoDMNG, and 1-chloro-2,4-dinitrobenzene (CDNB) were determined in bacterial lysates. Such screening of more than 70 clones demonstrated a continuous range of activities covering at least two orders of magnitude for each of the substrates. For a given clone, the activities with aminochrome and cyanoDMNG, in spite of their different chemistries, were clearly correlated, whereas no strong correlation was found with CDNB. This functional correlation suggests a common structural basis for the enzymatic mechanisms for conjugation of aminochrome and denitrosation of cyanoDMNG. From an evolutionary perspective, the results show that recombination of segments from homologous proteins gives rise to a large proportion of functionally competent proteins with a range of activities. The data support the proposal that natural evolution of protein functions may involve recombination of DNA segments followed by selection for advantageous functional properties of the resulting proteins. Clearly, the same approach can be utilized in the engineering of proteins displaying novel functions by in vitro evolution.

Genetic polymorphisms of human N-acetyltransferase, cytochrome P450, glutathione-S-transferase, and epoxide hydrolase enzymes: relevance to xenobiotic metabolism and toxicity

In this review, an overview is presented of the current knowledge of genetic polymorphisms of four of the most important enzyme families involved in the metabolism of xenobiotics, that is, the N-acetyltransferase (NAT), cytochrome P450 (P450), glutathione-S-transferase (GST), and microsomal epoxide hydrolase (mEH) enzymes. The emphasis is on two main topics, the molecular genetics of the polymorphisms and the consequences for xenobiotic metabolism and toxicity. Studies are described in which wild-type and mutant alleles of biotransformation enzymes have been expressed in heterologous systems to study the molecular genetics and the metabolism and pharmacological or toxicological effects of xenobiotics. Furthermore, studies are described that have investigated the effects of genetic polymorphisms of biotransformation enzymes on the metabolism of drugs in humans and on the metabolism of genotoxic compounds in vivo as well. The effects of the polymorphisms are highly dependent on the enzyme systems involved and the compounds being metabolized. Several polymorphisms are described that also clearly influence the metabolism and effects of drugs and toxic compounds, in vivo in humans. Future perspectives in studies on genetic polymorphisms of biotransformation enzymes are also discussed. It is concluded that genetic polymorphisms of biotransformation enzymes are in a number of cases a major factor involved in the interindividual variability in xenobiotic metabolism and toxicity. This may lead to interindividual variability in efficacy of drugs and disease susceptibility.

Glutathione S-transferase M1 null genotype: lack of association with tumour characteristics and survival in advanced breast cancer

BACKGROUND

Glutathione S-transferase (GST)M1, a member of the mu class GST gene family, has been shown to be polymorphic because of a partial gene deletion. This results in a failure to express the GSTM1 gene in 50-60% of individuals. Several studies have demonstrated a possible link with the GSTM1-null genotype and susceptibility to cancer. Furthermore, a GSTM1 isoenzyme has been positively associated with protective effect against mutagenic drugs, such as alkylating agents and anthracyclines.

OBJECTIVES

To determine whether GSTM1 polymorphisms are associated with tumour characteristics and survival in advanced breast cancer patients, and whether it may constitute a prognostic factor.

METHODS

We genotyped 92 patients receiving primary chemotherapy, which included cyclophosphamide, doxorubicine and 5-fluorouracil. The relationships between allelism at GSTM1 and clinicopathological parameters including age, menopausal status, tumour size, grade hormone receptors, involved nodes and p53 gene mutations were analysed. Of the patients with GSTM1-positive genotype, tissue samples obtained before and after treatment were available from 28 cases, allowing RNA extraction and GSTM1 expression by reverse transcription polymerase chain reaction. Relationships with clinical response to chemotherapy, and disease-free and overall survival were also evaluated. The data obtained was analysed using logistic regression to estimate the odds ratio and 95% confidence interval.

RESULTS

Of 92 patients, 57.6% (n = 53) were classified as heritably GSTM1-deficient, and 42.4% (n = 39) were of the GSTM1-positive genotype. There were no statistically significant relationships between GSTM1-null genotype and the clinicopathological parameters analysed. No relationship was observed between GSTM1 RNA expression and objective clinical response to chemotherapy. Objective clinical response to chemotherapy was related only to clinical tumour size (P = 0.0177) and to the absence of intraductal carcinoma (P = 0.0013). GSTM1-null genotype had no effect on disease-free or overall survival. The absence of hormone receptors (P = 0.002), the presence of a mutated p53 gene (P = 0.0098) and lack of response to primary chemotherapy (P = 0.0086) were the only factors associated with reduced disease-free or overall survival.

CONCLUSIONS

GSTM1-null genotype alone had no effect on tumour characteristics and outcome of patients with advanced breast cancers. The lack of correlation of GSTM1 genotype with clinical tumour features, clinical response to chemotherapy and survival exclude a role for GSTM1 polymorphism as a prognostic factor in advanced breast cancer.

Thioltransferase activity of bovine lens glutathione S-transferase

A Mu-class glutathione S-transferase purified to electrophoretic homogeneity from bovine lens displayed thioltransferase activity, catalysing the transthiolation reaction between GSH and hydroxyethyldisulphide. The thiol-transfer reaction is composed of two steps, the formation of GSSG occurring through the generation of an intermediate mixed disulphide between GSH and the target disulphide. Unlike glutaredoxin, which is only able to catalyse the second step of the transthiolation process, glutathioneS-transferase catalyses both steps of the reaction. Data are presented showing that bovine lens glutathione S-transferase and rat liver glutaredoxin, which was used as a thioltransferase enzyme model, can operate in synergy to catalyse the GSH-dependent reduction of hydroxyethyldisulphide.

Coordinated action of glutathione S-transferases (GSTs) and multidrug resistance protein 1 (MRP1) in antineoplastic drug detoxification. Mechanism of GST A1-1- and MRP1-associated resistance to chlorambucil in MCF7 breast carcinoma cells

To examine the role of multidrug resistance protein 1 (MRP1) and glutathione S-transferases (GSTs) in cellular resistance to antineoplastic drugs, derivatives of MCF7 breast carcinoma cells were developed that express MRP1 in combination with one of three human cytosolic isozymes of GST. Expression of MRP1 alone confers resistance to several drugs representing the multidrug resistance phenotype, drugs including doxorubicin, vincristine, etoposide, and mitoxantrone. However, co-expression with MRP1 of any of the human GST isozymes A1-1, M1-1, or P1-1 failed to augment MRP1-associated resistance to these drugs. In contrast, combined expression of MRP1 and GST A1-1 conferred approximately 4-fold resistance to the anticancer drug chlorambucil. Expression of MRP1 alone failed to confer resistance to chlorambucil, showing that the observed protection from chlorambucil cytotoxicity was absolutely dependent upon GST A1-1 protein. Moreover, using inhibitors of GST (dicumarol) or MRP1 (sulfinpyrazone), it was shown that in MCF7 cells resistance to chlorambucil requires both intact MRP1-dependent efflux pump activity and, for full protection, GST A1-1 catalytic activity. These results are the first demonstration that GST A1-1 and MRP1 can act in synergy to protect cells from the cytotoxicity of a nitrogen mustard, chlorambucil.

Guinea pig liver Mu-class glutathione S-transferase M1-2 cross-reacts with antibodies to both rat Mu- and theta-class glutathione S-transferases

Two novel major heterodimeric Mu-class glutathione (GSH) S-transferases (GSTs), designated M1-2 and M1-3*, were isolated from guinea pig (gp) liver cytosol and purified to homogeneity together with a known major homodimeric Mu-class gpGSTM1-1 (reported as GST b by R. Oshino, K. Kamei, M. Nishioka, and M. Shin, 1990, J. Biochem. 107, 105-110). These three gpGSTs were quantitatively retained on an S-hexyl-GSH affinity column and separated as homogeneous proteins by chromatofocusing. Subunits of the heterodimers were inseparable on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but could be completely separated by reverse-phase partition high-performance liquid chromatography. A molecular cloning study demonstrated that the gpGST subunit M2 consisted of 217 amino acid residues with a calculated molecular mass of 25,562 and shared 84% identity in overall amino acid sequence with gpGSTM1-1. N-terminal amino acid sequences of peptides from the gpGST subunit M3* with a blocked N-terminus strongly suggested that it should belong to the Mu class. Western blot analysis using antisera raised against purified rat (r) GSTsA1-2 (Alpha), M1-1, P1-1 (Pi), and T2-2 (Theta) indicated that gpGSTsM1-1 and M1-3* cross-reacted only with anti-rGSTM1 antibody. However, gpGSTM1-2 cross-reacted intensely to almost the same extent with antibodies to both rGSTsM1-1 and T2-2. A homodimeric gpGSTM2-2, artificially constructed from native gpGSTM1-2 by treatment with guanidine hydrochloride followed by dialysis, intensely cross-reacted with antibodies to both the rat Mu- and Theta-class GSTs. Thus, the gpGST subunit M2 provided the first evidence for the double immuno-cross-reaction of a GST with polyclonal antibodies to two different classes of GSTs.

Chemoprotective functions of glutathione S-transferases in cell lines induced to express specific isozymes by stable transfection

The authors have shown that expression of mGSTM1-1 or hGSTP1-1 in MCF-7 cells protects against DNA alkylation by 4-nitroquinoline-1-oxide (NQO) in an isozyme-specific manner and is commensurate with relative specific activity. Expression of GSTs also conferred protection against both DNA strand breaks and sister-chromatid exchange induced by NQO. Interestingly, GST expression did not protect against NQO cytotoxicity in transfected MCF-7 cell lines, although resistance to NQO cytotoxicity was observed in a T47D pi transfectant line, expressing much higher specific activity of the transfected hGSTP1-1. However, high level expression of hGSTP1-1 or mGSTM1-1 in V79 transfectants did not confer resistance to cytotoxicity, indicating that expression of GST alone is not sufficient. The authors have also shown protection against AFB1 in cell lines expressing transfected rat CYP2B1 (V79MZr2B1) and transfected mGST-Yc (mGSTA3-3). Protection was observed against both alkylation of DNA (3-fold) by [3H]AFB1 and against AFB1 cytotoxicity (7-fold). Similarly, V79MZr1A1 cells that express CYP1A1 and either transfected human or murine GSTP1-1 (< 5000 mIU/mg, CDNB) exhibited > 70% decrease in covalent labeling of total nucleic acids by [3H]BPDE. However, no protection against the cytotoxicity of BPDE was conferred by expression of hGSTP1-1. Overall, these results indicate that in some (NQO or BPDE), but not all (AFB1) cases, protection by GST expression against DNA damage is more effective than protection against cytotoxicity. In addition, there is evidence to indicate that additional factor(s) other than high GST isozyme expression level and good substrate efficacy affect the degree of protection against cytotoxicity of reactive electrophiles. This includes the differential protection against NQO cytotoxicity in T47D pi, but not V79 Xh pi-33 cells and also the recent studies which showed that expression of the MRP GS-X conjugate efflux transporter confers synergistic protection against NQO cytotoxicity when co-expressed with transfected human GSTP1-1 in MCF-7 cells. Thus, protective efficacy conferred by GST expression can vary with different cellular targets and/or experimental end-points, as well as with variations in relative specific activity or in different cellular phenotypic contexts.

Characterization of a human glutathione S-transferase mu cluster containing a duplicated GSTM1 gene that causes ultrarapid enzyme activity

The mu class glutathione S-transferase gene GSTM1 is polymorphic in humans, with approximately half of the Caucasian population being homozygous deleted for this gene. GSTM1 enzyme deficiency has been suggested to predispose people to lung and bladder cancer. Some people in a Saudi Arabian population, however, have been described previously with ultrarapid GSTM1 enzyme activity. Here we have evaluated the molecular genetic basis for this observation. Genomic DNA from two Saudi Arabian subjects exhibiting ultrarapid enzyme activity and from 13 Swedish subjects having null, one, or two GSTM1 genes were subjected to restriction fragment length polymorphism analysis using the restriction enzymes EcoRI, EcoRV, and HindIII and combinations thereof. Hybridization was carried out using a full-length GSTM1 cDNA or the 5' and 3' parts of the cDNA. The restriction mapping data revealed the presence of a GST mu cluster with two GSTM1 genes in tandem situated between the GSTM2 and GSTM5 genes. A quantitative multiplex polymerase chain reaction method, which simultaneously amplified a fragment of the GSTM1 gene and the beta-globin gene, was developed, and the genomic GSTM1 copy number was determined from the GSTM1/beta-globin ratio. This method clearly separated GSTM1 +/- subjects (ratios between 0.4 and 0.7) from GSTM1 +/+ subjects (ratios between 0.8 and 1.2). The two Saudi Arabians with ultrarapid GSTM1 activities had ratios of approximately 1.5, indicating that they carried three GSTM1 genes. These results demonstrate the existence of a novel mu class GST cluster containing a duplicated active GSTM1 gene causing ultrarapid enzyme activity.

Subunit diversity and tissue distribution of human glutathione S-transferases: interpretations based on electrospray ionization-MS and peptide sequence-specific antisera

Uncertainties about the composition and identities of glutathione S-transferases (GSTs) in human tissue have impeded studies on their biological functions. A rigorous protocol has therefore been developed to characterize the human proteins. Cytosolic GST subunits were resolved by reverse-phase HPLC methods, individual components were assigned to Alpha, Mu and Pi classes on the basis of their immunoreactivities, and peptide-sequence-specific antisera were used to distinguish among five different Mu-class subunits (GSTM1-GSTM5). Each subunit type was characterized and identified unambiguously by electrospray ionization-MS. Acetylation of N-terminal residues in the GSTA1, GSTA2, GSTM3 and GSTM4 subunits were the only natural post-translational modifications detected. The unique structure of GSTM3, with N- and C-terminal peptide extensions predicted from cDNA sequences, was confirmed. Only testis and brain were rich sources of GSTM3 subunits. Subunit profiles were distinct and characteristic of the particular tissue type, and this tissue specificity in GST expression was evident even in organs from different individuals. For instance, livers had relatively simple GST compositions, consisting of a preponderance of Alpha-class subunits and GSTM1 (when present). By contrast, representation of most subunit types was a characteristic feature of testis, which had the highest levels of GSTs. GSTM4 and GSTM5 subunits, here identified for the first time in human tissue extracts, were minor components, with GSTM5 found only in brain, lung and testis. Specimens devoid of GSTM1 subunits, particularly those from null-genotype individuals, were readily discerned at the protein level. Liver was the only rich source of the GSTM1 subunit (although it also constituted a major fraction of adrenal GSTs), and so the functional consequences of the GSTM1 gene deletion are likely to vary in extrahepatic tissues.

The role of human glutathione transferases and epoxide hydrolases in the metabolism of xenobiotics

Human glutathione transferases (GSTs) are a multigene family of enzymes that are involved in the metabolism of a wide range of electrophilic compounds of both exogenous and endogenous origin. GSTs are generally recognized as detoxifying enzymes by catalyzing the conjugation of these compounds with glutathione, but they may also be involved in activation of some carcinogens. The memmalian GSTs can be differentiated into four classes of cytosolic enzymes and two membrane bound enzymes. Human epoxide hydrolases (EHs) catalyze the addition of water to epoxides to form the corresponding dihydrodiol. The enzymatic hydration is essentially irreversible and produces mainly metabolites of lower reactivity that can be conjugated and excreted. The reaction of EHs is therefore generally regarded as detoxifying. The mammalian EHs can be distinguished by their physical and enzymatic properties. Microsomal EH (mEH) exhibits a broad substrate specificity, while the soluble EH (sEH) is an enzyme with a "complementary" substrate specificity to mEH. Cholesterol EH and leukotriene A4 hydrolase are two EHs with very limited substrate specificity. The activities of either GSTs or EHs expressed in vivo exhibit a relatively large interindividual variation, which might be explained by induction, inhibition, or genetic factors. These variations in levels or activities of individual isoenzymes are of importance with respect to an individual's susceptibility to genotoxic effects. This article gives a general overview of GSTs and EHs, discussing the modulation of activities, determination of these enzymes ex vivo, and the polymorphic expression of some isoenzymes.

De novo expression of transfected human class 1 aldehyde dehydrogenase (ALDH) causes resistance to oxazaphosphorine anti-cancer alkylating agents in hamster V79 cell lines. Elevated class 1 ALDH activity is closely correlated with reduction in DNA interstrand cross-linking and lethality

Human class 1 aldehyde dehydrogenase (hALDH-1) can oxidize aldophosphamide, a key aldehyde intermediate in the activation pathway of cyclophosphamide and other oxazaphosphorine (OAP) anti-cancer alkylating agents. Overexpression of class 1 ALDH (ALDH-1) has been observed in cells selected for survival in the presence of OAPs. We used transfection to induce de novo expression of human ALDH-1 in V79/SD1 Chinese hamster cells to clearly quantitate the role of hALDH-1 expression in OAP resistance. Messenger RNA levels correlated well with hALDH-1 protein levels and enzyme activities (1.5-13.6 milliunits/mg with propionaldehyde/NAD+ substrate, compared to < 1 milliunit/mg in controls) in individual clonal transfectant lines, and slot blot analysis confirmed the presence of the transfected cDNA. Expressed ALDH activity was closely correlated (r = 0.99) with resistance to mafosfamide, up to 21-fold relative to controls. Transfectants were cross-resistant to other OAPs but not to phosphoramide mustard, ifosfamide mustard, melphalan, or acrolein. Resistance was completely reversed by pretreatment with 25 microM diethylaminobenzaldehyde, a potent ALDH inhibitor. Alkaline elution studies showed that expression of ALDH-1 reduced the number of DNA cross-links commensurate with mafosfamide resistance, and this reduction in cross-links was fully reversed by the inhibitor. Thus, overexpression of human class 1 ALDH alone is sufficient to confer OAP-specific drug resistance.

Turnover of glutathione S-transferase alpha mRNAs is accelerated by 12-O-tetradecanoyl phorbol-13-acetate in human hepatoma and colon carcinoma cell lines

The phorbol ester, 12-O-tetradecanoyl phorbol-13-acetate (TPA), known to induce murine glutathione S-transferase (GST) Ya, was examined for its effect on the expression of human GST alpha. Unexpectedly, 24-h treatment of the human hepatoma cell line HepG2 with 100 nmol/l TPA caused a decrease of the GST alpha mRNA level to below 5% of controls, i.e. opposite to the known response in the mouse. The level of mRNA for GST Mu was also decreased, but the mRNAs of c-jun and jun-B were elevated after 2 h. The decrease of GST alpha mRNAs was inhibited by staurosporine, suggesting an involvement of protein kinase C. Inhibition of transcription and translation by actinomycin D and cycloheximide also partially inhibited the effect of TPA on the expression of GST alpha. In the presence of actinomycin D, GST alpha mRNA halflife was 14.5 h, compared to 3.5 h in the presence of TPA. The calcium ionophore A23187 caused a loss of GST alpha mRNAs to levels almost as low as those obtained with TPA. The effects of TPA and the calcium ionophore were also observed in CaCo2 colon carcinoma cells. As a consequence of the decrease of mRNA levels, GST alpha protein levels and total GST enzyme activity were also diminished. Also, the morphology of the cells was changed after 3 h exposure to TPA. These data suggest that human GST alpha expression can be regulated at the level of mRNA stability by a pathway involving protein kinase C.

Potential contribution of the glutathione S-transferase supergene family to resistance to oxidative stress

The glutathione S-transferase (GST) supergene family comprises gene families that encode isoenzymes that are widely expressed in mammalian tissue cytosols and membranes. Both cytosolic (particularly the isoenzymes encoded by the alpha, mu and theta gene families) and microsomal GST catalyse the conjugation of reduced glutathione (GSH) with a wide variety of electrophiles which include known carcinogens as well as various compounds that are products of oxidative stress including oxidised DNA and lipid. Indeed, several lines of evidence suggest certain of these isoenzymes play a pivotal role in protecting cells from the consequences of such stress. An assessment of the importance of these GST in humans is presently difficult however, because the number of alpha and theta class genes is not known and, the catalytic preferences of even identified isoforms is not always clear.

The glutathione S-transferase supergene family: regulation of GST and the contribution of the isoenzymes to cancer chemoprotection and drug resistance

The glutathione S-transferases (GST) represent a major group of detoxification enzymes. All eukaryotic species possess multiple cytosolic and membrane-bound GST isoenzymes, each of which displays distinct catalytic as well as noncatalytic binding properties: the cytosolic enzymes are encoded by at least five distantly related gene families (designated class alpha, mu, pi, sigma, and theta GST), whereas the membrane-bound enzymes, microsomal GST and leukotriene C4 synthetase, are encoded by single genes and both have arisen separately from the soluble GST. Evidence suggests that the level of expression of GST is a crucial factor in determining the sensitivity of cells to a broad spectrum of toxic chemicals. In this article the biochemical functions of GST are described to show how individual isoenzymes contribute to resistance to carcinogens, antitumor drugs, environmental pollutants, and products of oxidative stress. A description of the mechanisms of transcriptional and posttranscriptional regulation of GST isoenzymes is provided to allow identification of factors that may modulate resistance to specific noxious chemicals. The most abundant mammalian GST are the class alpha, mu, and pi enzymes and their regulation has been studied in detail. The biological control of these families is complex as they exhibit sex-, age-, tissue-, species-, and tumor-specific patterns of expression. In addition, GST are regulated by a structurally diverse range of xenobiotics and, to date, at least 100 chemicals have been identified that induce GST; a significant number of these chemical inducers occur naturally and, as they are found as nonnutrient components in vegetables and citrus fruits, it is apparent that humans are likely to be exposed regularly to such compounds. Many inducers, but not all, effect transcriptional activation of GST genes through either the antioxidant-responsive element (ARE), the xenobiotic-responsive element (XRE), the GST P enhancer 1(GPE), or the glucocorticoid-responsive element (GRE). Barbiturates may transcriptionally activate GST through a Barbie box element. The involvement of the Ah-receptor, Maf, Nrl, Jun, Fos, and NF-kappa B in GST induction is discussed. Many of the compounds that induce GST are themselves substrates for these enzymes, or are metabolized (by cytochrome P-450 monooxygenases) to compounds that can serve as GST substrates, suggesting that GST induction represents part of an adaptive response mechanism to chemical stress caused by electrophiles. It also appears probable that GST are regulated in vivo by reactive oxygen species (ROS), because not only are some of the most potent inducers capable of generating free radicals by redox-cycling, but H2O2 has been shown to induce GST in plant and mammalian cells: induction of GST by ROS would appear to represent an adaptive response as these enzymes detoxify some of the toxic carbonyl-, peroxide-, and epoxide-containing metabolites produced within the cell by oxidative stress. Class alpha, mu, and pi GST isoenzymes are overexpressed in rat hepatic preneoplastic nodules and the increased levels of these enzymes are believed to contribute to the multidrug-resistant phenotype observed in these lesions. The majority of human tumors and human tumor cell lines express significant amounts of class pi GST. Cell lines selected in vitro for resistance to anticancer drugs frequently overexpress class pi GST, although overexpression of class alpha and mu isoenzymes is also often observed. The mechanisms responsible for overexpression of GST include transcriptional activation, stabilization of either mRNA or protein, and gene amplification. In humans, marked interindividual differences exist in the expression of class alpha, mu, and theta GST. The molecular basis for the variation in class alpha GST is not known. (ABSTRACT TRUNCATED)

Genetic differences in drug disposition

Genetic polymorphisms of drug metabolizing enzymes are well recognized. This review presents molecular mechanisms, ontogeny and clinical implications of genetically determined intersubject variation in some of these enzymes. Included are the polymorphic enzymes N-acetyl transferase, cytochromes P4502D6 and 2C, which have been well described in humans. Information regarding other Phase I and Phase II polymorphic pathways, such as glutathione and methyl conjugation and alcohol and acetaldehyde oxidation continues to increase and are also discussed. Genetic factors effecting enzyme activity are frequently important determinants of the disposition of drugs and their efficacy and toxicity. In addition, associations between genetic differences in these enzymes and susceptibility to carcinogens and teratogens have been reported. Ultimately, the application of knowledge regarding these genetic factors of enzyme activity may guide medical therapy and minimize xenobiotic-induced disease.

Human glutathione S-transferases

1. Multiple forms of glutathione S-transferase (GST) isoenzymes present in human tissues are dimers of subunits belonging to three distinct gene families namely alpha, mu and pi. Only the subunits within each class hybridize to give active dimers. 2. These subunits are differentially expressed in a tissue-specific manner and the composition of glutathione S-transferases in various tissues differs significantly. 3. Minor GST subunits not belonging to these three classes are also present in some tissues. 4. An ortholog of rat GST 8-8 and mouse mGSTA4-4 is selectively expressed in some human tissues including bladder, brain, heart, liver, and pancreas. This isoenzyme designated as GST 5.8 expresses several fold higher activity towards 4-hydroxy-2,3-trans-nonenal as compared to the routinely used substrate 1-chloro-2,4-dinitrobenzene.

Isoenzyme-specific quantitative immunoassays for cytosolic glutathione transferases and measurement of the enzymes in blood plasma from cancer patients and in tumor cell lines

Enzyme-linked immunoassays (ELISAs) based on the double-antibody sandwich technique have been developed for the quantitative analysis of the major human cytosolic class Pi, Mu and Alpha glutathione transferases (GSTs). The procedures were optimized with respect to antibody concentration for coating of plates as well as other parameters in order to achieve high sensitivity and accuracy. No cross-reactivity was detected between members of the three different classes of GSTs or among the Mu class GSTs M2-2, M3-3 and M4-4 with the ELISA for GST M1-1. The ELISAs have been applied to establish the cytosolic GST profiles of 10 cell lines and to monitor the plasma GST levels in cancer patients. The results revealed that the class Pi GST was the dominant isoenzyme in six (LS 174T, HCT-8, Hu 549 Pat, K-562, U-937 and Hu 549) out of nine tumor cell lines and immortalized hepatocytes (Chang Liver). The isoenzymes A1-1 and M1-1 were determined to be the major GST components in Hep G2 and HeLa cells, respectively. In a clinical study, the majority of the patients with urinary bladder cancer were found to have increased plasma levels of both GST A1-1 and GST P1-1 (10/15), while patients with renal cancer frequently showed increases only in GST P1-1 (5/8). The results demonstrate that the ELISAs are suitable for analyzing GST phenotypes in both normal and tumor cells and in monitoring plasma levels of GSTs in cancer patients.

Co-variation of glutathione transferase expression and cytostatic drug resistance in HeLa cells: establishment of class Mu glutathione transferase M3-3 as the dominating isoenzyme

Qualitative and quantitative analyses of glutathione, glutathione transferases (GSTs) and other glutathione-linked enzymes in HeLa cells have been made in order to study their significance in cellular resistance to electrophilic cytotoxic agents. The cytosolic concentrations of three GSTs, GST M1-1 (53 +/- 9 ng/mg of cytosolic protein), GST P1-1 (11 +/- 3 ng/mg) and GST A1-1 (1.1 +/- 0.4 ng/mg) were quantified by isoenzyme-specific enzyme-linked immunoassays. Electrophoretic analysis and immunoblotting demonstrated another component, GST M3-3, which was identified by amino acid sequence analysis. GST M3-3 was quantified (1550 +/- 250 ng/mg) by slot-blot immunoanalysis and was the most abundant GST in HeLa cells. An additional cytosolic 13 kDa protein with high affinity for immobilized glutathione or S-hexyglutathione was found to be identical with a macrophage migration-inhibitory factor, previously identified as a lymphokine. Cells grown in roller bottles (HR) rather than in ordinary culture flasks contain a significantly lower concentration of all the GSTs and were found to be more sensitive to the cytostatic agents doxorubicin (2.3-fold), cisplatin (1.7-fold) and melphalan (1.4-fold). The cytosolic concentrations of glutathione reductase and glyoxalase I were also lower in HR cells, whereas the total glutathione concentration was unchanged and the glutathione peroxidase activity was increased. The results indicate that GSTs contribute to the cellular resistance phenotype.

Human Mu-class glutathione S-transferases present in liver, skeletal muscle and testicular tissue

The major human Mu-class glutathione S-transferases (GST) have been purified to allow comparisons of their catalytic, physicochemical and immunochemical properties. GST isoenzymes, purified from hepatic, testicular and skeletal muscle tissue were found to comprise three distinct subunits (M1, M2 and M3) which may combine to form both homodimeric and heterodimeric proteins. Two distinct subunits, M1a and M1b, which represent allelic charge variants have been isolated but no polymorphic forms encoded at the GST M2 and M3 loci have been observed. Three GST isoenzymes (M1a-1a, M1a-1b and M1b-1b) have been purified from a single liver specimen. In addition, GST M1a-2, M1b-2, M2-2 and M2-3 have been isolated from muscle, whilst the M3-3 homodimer has been purified from human testis. The homodimeric enzymes GST M1a-1a, M1b-1b, M2-2 and M3-3 have pI values of 6.1, 5.5, 5.3 and 5.0, whilst SDS-PAGE indicated that M1a, M1b, M2 and M3 have molecular masses of 26.7, 26.6, 26.0 and 26.3 kDa, respectively. The M1, M2 and M3 subunits isolated from either liver, skeletal muscle or testis, are catalytically distinct. Both M1-type subunits (M1a and M1b) possess a high activity for trans-4-phenyl-3-buten-2-one, whereas, the skeletal muscle subunit M2 has a high activity towards 1,2-dichloro-4-nitrobenzene. By contrast, the testicular GST subunit M3 has no detectable activity towards either of these substrates. However, all three Mu-class subunits are active towards the compounds 4-hydroxynonenal and 4-hydroxydecinal, possible endogenous substrates which are produced by lipid peroxidation. The human Mu-class subunits can be distinguished immunochemically; antisera raised against the testicular GST M3-3 showed no reactivity towards either the M1 or M2 subunits. The M3 subunit has a blocked N-terminus but automated amino-acid sequencing of a CNBr-derived peptide allowed 14 residues of the M3 subunit to be identified. These data indicated that testicular GST M3-3 is likely to correspond to the brain/testis Mu-class GST cDNA described by Campbell et al. (Campbell E., Takahashi Y., Abramovitz M., Peretz M., & Listowsky I. (1990) J. Biol. Chem. 265, 9188-9193).

Molecular cloning and heterologous expression of an alternatively spliced human Mu class glutathione S-transferase transcript

Two cDNA clones encoding a new Mu class glutathione S-transferase (GST) have been isolated from a human testis cDNA library. Both clones are incomplete and appear to result from alternative splicing. One clone is missing the sequence encoding exon 4 and the other is missing exon 8. The complete sequence of the previously undescribed isoenzyme can be deduced from the two cDNA clones. This is the first report of alternative splicing in a GST transcript and may represent either a novel form of regulation in this multigene family or illegitimate transcription and experimental alternative splicing as part of the evolutionary process. By combining components from each clone a complete cDNA has been constructed and the encoded protein expressed in Escherichia coli. In general, the recombinant enzyme has relatively low activity when compared with all the previously described human Mu class GST isoenzymes.

Identification of class-mu glutathione transferase genes GSTM1-GSTM5 on human chromosome 1p13

The GSTM1, GSTM2, GSTM3, GSTM4, and GSTM5 glutathione transferase genes have been mapped to human chromosome 1 by using locus-specific PCR primer pairs spanning exon 6, intron 6, and exon 7, as probes on DNA from human/hamster somatic cell hybrids. For GSTM1, the assignment was confirmed by Southern blot hybridization to a pair of 12.5/2.4-kb HindIII fragments. The GSTM1-specific primer pairs can be used to identify individuals carrying non-null GSTM1 alleles. The organization of these five genes was confirmed by the isolation of a yeast artificial chromosome clone (GSTM-YAC2) that contains all five genes. With this clone, the location of the GSTM1-GSTM5 gene cluster on chromosome 1 was confirmed by fluorescence in situ hybridization. Both regional assignment using the fractional length method and examination of probe signal with reference to R-banded chromosomes induced by BrdU places the gene cluster in or near the 1p13.3 region. The close physical proximity of the GSTM1 and GSTM2 loci, which share 99% nucleotide sequence identity over 460 nucleotides of 3'-untranslated mRNA, suggests that the GSTM1-null allele may result from unequal crossing-over.

Anionic glutathione S-transferases in shrimp eyes

1. Two anionic isoenzymes of glutathione transferases (EC 2.5.1.18), QI and QII, have been purified from the eyes of the shrimp Penaeus japonicus by using a combination of S-hexylglutathione affinity column chromatography and Mono-Q fast protein liquid chromatography (f.p.l.c.). 2. Both QI and II glutathione S-transferases are homodimers. They show similarity in substrate specificities and pH optima, but not in isoelectric points. 3. QI is distinct from QII by anion-exchange f.p.l.c., reverse-phase h.p.l.c. chromatography and amino acid sequencing analysis. QI has N-terminal amino acid sequences homologous to mu glutathione S-transferase, whereas QII is homologous to theta glutathione S-transferases.