Glutathione analogues as novel inhibitors of rat and human glutathione S-transferase isoenzymes, as well as of glutathione conjugation in isolated rat hepatocytes and in the rat in vivo

Glutathione S-Transferases Mediate In Vitro and In Vivo Inactivation of Genipin: Implications for an Underlying Detoxification Mechanism

Genipin (GP), the reactive metabolite of geniposide (GE), is responsible for GE-induced hepatotoxicity. As a potential detoxification pathway, the inactivation of GP by glutathione S-transferases (GSTs) has not yet been characterized. In this study, the thiol-GSH conjugates of GP, M532-1 and M532-2 were first identified and the catalytic activities of GSTs were investigated both in vitro and in vivo. GSTA1-1 and GSTA4-4 showed high activity in the formation of both thiol-GSH conjugates, whereas GSTA4-4 specifically catalyzed M532-2 formation in vitro. The active GST isoforms protect against alkylation of N-acetylcysteine (NAC), a classic model nucleophile. GST inhibition attenuated M532-1 formation in rat bile, confirming the in vivo catalytic role of GSTs. In conclusion, this study demonstrated the inactivation of GP by GSTs and implied that interindividual variability of GSTs may be a risk factor for susceptibility to GE-induced hepatotoxicity.

The mercapturic acid pathway

The mercapturic acid pathway is a major route for the biotransformation of xenobiotic and endobiotic electrophilic compounds and their metabolites. Mercapturic acids (N-acetyl-l-cysteine S-conjugates) are formed by the sequential action of the glutathione transferases, γ-glutamyltransferases, dipeptidases, and cysteine S-conjugate N-acetyltransferase to yield glutathione S-conjugates, l-cysteinylglycine S-conjugates, l-cysteine S-conjugates, and mercapturic acids; these metabolites constitute a "mercapturomic" profile. Aminoacylases catalyze the hydrolysis of mercapturic acids to form cysteine S-conjugates. Several renal transport systems facilitate the urinary elimination of mercapturic acids; urinary mercapturic acids may serve as biomarkers for exposure to chemicals. Although mercapturic acid formation and elimination is a detoxication reaction, l-cysteine S-conjugates may undergo bioactivation by cysteine S-conjugate β-lyase. Moreover, some l-cysteine S-conjugates, particularly l-cysteinyl-leukotrienes, exert significant pathophysiological effects. Finally, some enzymes of the mercapturic acid pathway are described as the so-called "moonlighting proteins," catalytic proteins that exert multiple biochemical or biophysical functions apart from catalysis.

Influence of glutathione fructosylation on its properties

Incubation of fructose and glutathione leads to the formation of N-2-deoxy-glucos-2-yl glutathione as the major glycation product, with characteristic positive ion at 470 Th in LC-MS spectra. Glutathione disulfide and fructose generate two compounds: N-2-deoxy-glucos-2-yl glutathione disulfide (m/z=775 Th) and bis di-N,N'-2-deoxy-glucos-2-yl glutathione disulfide (m/z=937 Th). N-2-deoxy-glucos-2-yl glutathione is 2.5-fold less effective than glutathione in reducing dehydroascorbic acid. Glutathione peroxidase and glutahione-S-transferase exhibit marginal activity toward N-2-deoxy-glucos-2-yl glutathione, while glyoxalase I shows 44.9% of the enzyme's specific activity. Glutathione reductase demonstrates 6.9% of the enzyme's specific activity with bis di-N,N'-2-deoxy-glucos-2-yl glutathione, while with mono-N-glucosyl glutathione disulfide retained 5 6.1% of the original activity. Glutathione reductase could not reduce N-2-deoxy-glucos-2-yl glutathione in mixed disulfide with gammaS-crystallin, but reduced glutathione in mixed disulfide with gammaS-crystallin by 90%. The presence of N-2-deoxy-glucos-2-yl glutathione in mixed disulfide with gammaS-crystallin makes this molecule more susceptible to unfolding than native gammaS-crystallin.

Glucose-derived Amadori compounds of glutathione

Under the chromatographic conditions used in these studies we observed time- and concentration-dependent formation of N-1-Deoxy-fructos-1-yl glutathione as the major glycation product formed in the mixtures of GSH with glucose. N-1-Deoxy-fructos-1-yl glutathione had a characteristic positively charged ion with m/z=470 Th in its LC-MS spectra. Mixtures of glutathione disulfide and glucose generated two compounds: N-1-Deoxy-fructos-1-yl GSSG (m/z=775 Th) as major adduct and bis di-N, N'-1-Deoxy-fructos-1-yl GSSG (m/z=937 Th) as the minor one. All three compounds showed a resonance signal at 55.2 ppm in the 13C-NMR spectra as C1 methylene group of deoxyfructosyl, which represents direct evidence that they are Amadori compounds. All three compounds purified from GSSG/Glc or GSH/Glc mixtures also showed LC-MS/MS fragmentation patterns identical to those of the synthetically synthesized N-1-Deoxy-fructos-1-yl glutathione, N-1-Deoxy-fructos-1-yl GSSG and bis di-N, N'-1-Deoxy-fructos-1-yl GSSG. N-1-Deoxy-fructos-1-yl glutathione was shown to be a poor substrate for glutathione peroxidase (6.7% of the enzyme's original specific activity) and glutathione-S-transferase (25.7% of the original enzyme's specific activity). Glutathione reductase failed to recycle the disulfide bond within the structure of di-substituted bis di-N, N'-1-Deoxy-fructos-1-yl GSSG. It showed only 1% of the original enzyme's specific activity, but retained its ability to reduce the disulfide bond within the structure of N-1-Deoxy-fructos-1-yl GSSG by 57% of its original specific activity. Since the GSH concentration in diabetic lens is significantly decreased and the glucose concentration can increase 10-fold and higher, the formation of Amadori products of the different forms of glutathione with this monosaccharide may be favored under these conditions and could contribute to a lowering of glutathione levels and an increase of oxidative stress observed in diabetic lens.

Glutathione conjugates and their synthetic derivatives as inhibitors of glutathione-dependent enzymes involved in cancer and drug resistance

Alterations in levels of glutathione (GSH) and glutathione-dependent enzymes have been implicated in cancer and multidrug resistance of tumor cells. The activity of a number of these, the multidrug resistance-associated protein 1, glutathione S-transferase, DNA-dependent protein kinase, glyoxalase I, and gamma-glutamyl transpeptidase, can be inhibited by GSH-conjugates and synthetic analogs thereof. In this review we focus on the function of these enzymes and carriers in cancer and anti-cancer drug resistance, in relation to their inhibition by GSH-conjugate analogs.

Inhibition of glutathione S-transferase in rat hepatocytes by a glycine-tetrazole modified S-alkyl-GSH analogue

Glutathione (GSH) conjugates inhibit enzymes that are involved in drug metabolism and drug resistance, but their cellular uptake is very low. To improve membrane-permeability, we synthesized a novel GSH-conjugate analogue with a tetrazole carboxylate isostere at the glycine position. Introduction of the tetrazole decreases inhibitory potency towards CDNB conjugation by glutathione S-transferase. However, the tetrazole derivative inhibited 2-bromoisovalerylurea conjugation in rat liver cytosol, as well as in hepatocytes.

Peptidomimetic glutathione analogues as novel gammaGT stable GST inhibitors

Elevated levels of glutathione-S-transferase (GST) isoenzymes are found in many tumor cells and are thought to play a role in the onset of multidrug resistance (MDR). To evaluate the contribution of GST to this process, inhibitors are needed. Glutathione (GSH) conjugates, although good GST inhibitors, cannot be used in vivo, because they are eliminated rapidly. In this paper, we describe the synthesis of a series of novel peptidomimetic glutathione analogues that are stabilized against peptidase mediated breakdown. The peptide bonds in GSH were replaced by isosteres, such as the 'reduced' amide (which was prepared using a novel method), N-methylamide, urethane, and methylene linkages. The in vitro evaluation of the compounds focuses on GST inhibition and stability towards gamma-glutamyl-transpeptidase (gammaGT), the main enzyme involved in GSH breakdown. The compounds were conjugated to the model electrophile ethacrynic acid (EA) to resemble GS-EA, an efficient GST inhibitor. All novel GSH-analogues were shown to inhibit rat liver cytosolic GSTs. Furthermore, peptidomimetic changes of the gamma-glutamyl-cysteine amide bond greatly improved stability towards gammaGT. These compounds may therefore be useful in the design of novel in vivo applicable GST inhibitors.

Carbon tetrachloride-induced changes in the activity of phase II drug-metabolizing enzyme in the liver of male rats: role of antioxidants

Glutathione S-transferases and glutathione play an important role in the detoxification of most toxic agents. In the present study, the protective effects of some antioxidants (L-ascorbic acid (AA), vitamin E (VE) or garlic) on carbon tetrachloride-induced changes in the activity of alanine amino transferase (ALT), aspartate amino transferase (AST), glutathione S-transferase (GST), and the level of glutathione (GSH) and thiobarbituric acid reactive substances (TBARS) were studied. The activities of ALT, and AST were assayed in plasma, whereas the activity of GST and the levels of GSH and TBARS were determined in the livers of rats. The current study included two experiments. In the first experiment, animals received single oral dose of CCl4 (400 mg/kg body weight) after administration of AA (100 mg/kg b.w.), VE (100 mg/kg b.w.) or garlic (800 mg/kg b.w.) as single oral doses. In the second experiment, rats received repeated oral doses of antioxidants for 12 consecutive days followed by a single oral dose of CCl4 on the 13th day and killed after that by 24 h. Treatment of male rats with CCl4 significantly increased the activity of ALT and AST in plasma, and the levels of both GSH and TBARS in the liver. On the other hand, CCl4 inhibited the activity of GST after single dose treatment. Single-dose treatments of rats with AA, VE or garlic prior to the administration of CCl4 could not restore the alterations in the activity of ALT and AST caused by CCl4 to the normal control level. However, repeated dose treatments with these agents restored such alterations to the normal level. We observed that VE is more effective than AA and garlic in restoring the inhibition of GST activity caused by CCl4 to the normal level after single dose treatments. On the other hand, AA and VE are more effective than garlic in restoring the induced TBARS level caused by CCl4 to the normal control level after repeated dose treatments. It has been observed that the tested antioxidants were able to antagonize the toxic effects of CCl4, and such counteracting effects were more pronounced when they were administered as repeated doses prior to administration of CCl4.

Glutathione S-transferases as emerging therapeutic targets

Glutathione S-transferases (GST) represent a large family of Phase II detoxification enzymes widely expressed in animals and plants. These enzymes catalyse the conjugation of glutathione with some endogenous molecules and a broad range of exogenous substrates including various anticancer drugs. Due to high expression of GSTs in tumours when compared to normal tissues and their high level in plasma from cancer patients, these enzymes are considered to be cancer markers. Their involvement in resistance to anticancer drugs and an inverse correlation between expression and prognosis in many tumours provided a rationale for the design of inhibitors and prodrugs to enhance therapeutic index. The first generation of GST inhibitors included ethacrynic acid and showed promising potentiating activity in vitro but lack of isoenzyme specificity and diuretic side effects restricted clinical use. Novel GST inhibitors include glutathione analogues and demonstrate better specificities with fewer limiting toxicities. One lead compound is a potent inhibitor of the GSTP1-1 isoform in both cell lines and animal models. A GSTP1-1 activated prodrug has also been developed. Testing of the preclinical and clinical efficacy of these agents is presently in progress. Their rational design provides a promising new approach to targeting tumour-specific characteristics in a manner consistent with improving therapeutic index.

Novel therapy for COPD

The incidence of chronic obstructive pulmonary disease (COPD) is increasing throughout the world. Much less is known about the pathogenesis of COPD than that of asthma and there is little response to current therapy. Most patients with COPD have acquired their lung disease through smoking cigarettes, and the major step in management is to minimise further damage by stopping this habit. A number of therapies are being developed for the treatment of COPD; including new bronchodilators such as tiotropium bromide, agents to block inflammation induced by neutrophils and macrophages, as well as strategies to combat proteases and oxidants. The long-term goal is to provide therapy that retards the accelerated loss of lung function occurring in COPD. Development of novel therapies for COPD requires reliable Phase II decision making before entering large scale Phase III studies. The patient with COPD is often overlooked compared to their asthmatic counterpart, who benefit from an urgent need to identify novel targets and better therapy.

Synthesis and glutathione S-transferase structure-affinity relationships of nonpeptide and peptidase-stable glutathione analogues

A series of nonpeptidic glutathione analogues where the peptide bonds were replaced by simple carbon-carbon bonds or isosteric E double bonds were prepared. The optimal length for the two alkyl chains on either side of the mercaptomethyl group was evaluated using structure-affinity relationships. Affinities of the analogues 14a-f, 23, and 25 were evaluated for a recombinant GST enzyme using a new affinity chromatography method previously developed in our laboratory. Analysis of these analogues gives an additional understanding for GST affinity requirements: (a) the carbon skeleton must conserve that of glutathione since analogue 14a showed the best affinity (IC50 = 5.2 microM); (b) the GST G site is not able to accommodate a chain length elongation of one methylene group (no affinity for analogues 14c,f); (c) a one-methylene group chain length reduction is tolerated, much more for the "Glu side" (14d, IC50 = 10.1 microM) than for the "Gly side" (14b, IC50 = 1800 microM); (d) the mercaptomethyl group must remain at position 5 as shown from the null affinity of the 6-mercaptomethyl analogue 14e; (e) the additional peptide isosteric E double bond (25) or hydroxyl derivative (23) in 14e did not help to retrieve affinity. This work reveals useful information for the design of new selective nonpeptidic and peptidase-stable glutathione analogues.

Inhibition of glutathione conjugation in the rat in vivo by analogues of glutathione conjugates

Glutathione (GSH) conjugation plays an important role in (de-)toxification of its substrates in vivo. We have developed inhibitors of GSH conjugation that are active in the rat in vivo which are derived from the structure of GSH conjugates: they contain a backbone of gamma-L-Glu-D-2-aminoadipic acid that is virtually isosteric with the gamma-L-Glu-L-Cys-Gly structure of GSH. In addition, a hydrophobic alkyl group is attached such that it may interact with the H-site of the enzyme. Finally, the carboxyl groups were esterified with alcohols of varying chain length. The results show that all these compounds preferentially inhibit alpha-GST's 1-1 and 2-2, have less effect on mu isoenzymes 3-3 and 4-4, and finally, have little effect on rat theta (G.J. Mulder, S. Ouwerkerk-Mahadevan, Modulation of glutathione conjugation in vivo: How to decrease glutathione conjugation in vivo or in intact cellular systems in vitro, Chem. Biol. Interact. 105 (1997) 17-34) and pi (S. Ouwerkerk-Mahadevan, J.H. van Boom, M.C. Dreef-Tromp, J.H.T.M. Ploemen, D.J. Meyer, G.J. Mulder, Glutathione analogues as novel inhibitors of rat and human glutathione S-transferase isoenzymes, as well as of glutathione conjugation in isolated rat hepatocytes and the rat in vivo, Bioche. J., 308 (1995) 283-290). Several of the compounds inhibit the GSH conjugation of bromsulfophthalein and (S)-2-bromisovalerylurea in hepatocytes, in the situ recirculating rat liver perfusion and in the rat in vivo (after i.v. administration). The most effective compound contains a 2-heptylamine group linked as an amide to the 1-carboxyl group of the aminoadipic acid moiety at the H-site, and an ethyl ester at the 5-carboxylic acid group of aminoadipic acid.

Modulation of glutathione conjugation in vivo: how to decrease glutathione conjugation in vivo or in intact cellular systems in vitro

Glutathione conjugation is involved in detoxification and toxification of a variety of electrophilic substrates. Thus it plays a major role in protection against reactive intermediates. At the same time this conjugation may cause resistance of tumor cells against certain cytostatics. In this review the methods available to decrease glutathione conjugation in vivo are discussed. So far the only in vivo active inhibitors of glutathione S-transferases are ethacrynic acid and a number of glutathione-derived structures; the latter seem very promising for further development. For (chronic) glutathione-depletion, buthionine sulfoximine is most effective, and surprisingly safe in clinical studies. Diethylmaleate can be used for acute depletion. Inhibition of glutathione transferases offers advantages over glutathione depletion as a method of decreasing glutathione conjugation since inhibition may be accomplished without changing the activities of other glutathione-dependent reactions in the cell. However, clinically safe, in vivo effective and isoenzyme-selective glutathione S-transferase inhibitors have not yet been developed.

Inhibition of glutathione S-transferase activity in human melanoma cells by alpha,beta-unsaturated carbonyl derivatives. Effects of acrolein, cinnamaldehyde, citral, crotonaldehyde, curcumin, ethacrynic acid, and trans-2-hexenal

The glutathione S-transferase (GST) activity towards 1-chloro-2,4-dinitrobenzene in intact human IGR-39 melanoma cells was determined by the quantification by HPLC-analysis of the excreted glutathione (GSH) conjugate (S-(2,4-dinitrophenyl)glutathione; DNPSG). The major GST subunit expressed in these melanoma cells is the pi-class GST subunit P1. Using this system, the effect of exposure for 1 h to a series of alpha, beta-unsaturated carbonyl compounds at non-toxic concentrations was studied. Curcumin was the most potent inhibitor (96% inhibition at 25 microM), while 67 and 61% inhibition at 25 microM was observed for ethacrynic acid and trans-2-hexenal, respectively. Moderate inhibition was observed for cinnamaldehyde and crotonaldehyde, while no inhibition was found for citral. The reactive acrolein did not inhibit the DNPSG-excretion at 2.5 microM, the highest non-toxic concentration. Up to about 50% GSH-depletion was found after treatment with crotonaldehyde, curcumin and ethacrynic acid, however the consequences for GST conjugation are presumably small. Reversible inhibition of GST was the major mechanism of inhibition of DNPSG-excretion in melanoma cells, except in the cases of curcumin and ethacrynic acid, which compounds also inactivated GSTP1-1 by covalent modification. This was clear from the fact that depending on the dose between 30 and 80% inhibition was still observed after lysis of the cells, under which conditions reversible inhibition was is absent. Intracellular levels of DNPSG remained relatively high in the case of ethacrynic acid. It is possible that ethacrynic acid also inhibits the transport of DNPSG by inhibition of the multidrug resistance-associated protein gene encoding glutathione conjugate export pump (MRP/GS-X pump) in some way.

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)