Effects of diethyl maleate on aryl hydrocarbon hydroxylase and on 3-methyl-cholanthrene-induced skin tumorigenesis in rats and mice

Mouse Skin Bioassay for Chemical Carcinogens

The mouse skin initiation/promotion bioassay is one of the proposed bioassays of the Carcinogenesis Testing Matrix for tier II (Bull and Pereira, 1980). A review of the literature indicated that 544 chemicals and substances have been examined by application to mouse skin for carcinogenic activity. Poly-cyclic aromatic hydrocarbons, direct acting alkylating agents, and environmental samples of complex mixtures and subtractions of them that include condensates of automobile exhaust and cigarette smoke have been demonstrated to be carcinogenic by the mouse skin bioassay. Chemical classes of carcinogens that have not been demonstrated to contain initiation and carcinogens in mouse skin include azoxy, diazo, halogenated methanes, hydrazine, inorganics, steroids, and sulfonates. The mouse skin assay can be modified so mat the test substance is administered systemically i.e., oral and intraperitoneal and the promoter applied topically. This modification has the potential of increasing the number of chemical classes detected in the mouse skin initiation/promotion bioassay.

The effect of N-acetylcysteine on ifosfamide-induced nephrotoxicity: in vitro studies in renal tubular cells

Ifosfamide (IF) nephrotoxicity is a serious adverse effect in children undergoing chemotherapy. Previous studies have shown that, in addition to the renal production of chloroacetaldehyde, a toxic metabolite of IF, lower levels of glutathione (GSH) may predispose the kidney to damage. The antioxidant N-acetylcysteine (NAC) is used extensively as an antidote for acetaminophen poisoning in children by replenishing GSH levels. As it has been safely and effectively used clinically, the objective of this study was to test whether the reversal of ifosfamide-induced nephrotoxicity can be achieved by administering NAC. Supplementation with NAC may reduce or prevent the degree of cellular cytotoxicity induced by IF. Porcine renal proximal tubular (LLCPK-1) cells were treated with NAC (0.4 mM or 2.5 mM) concurrently with 1 mM IF and 50 microM L-buthionine sulfoximine (BSO). Cellular viability was assessed by alamarBlue assay at 96 h. Intracellular GSH and oxidized GSH (GSSG) levels were determined using a GSH/GSSG colorimetric detection kit. A significant 60% decrease in cellular viability occurred when cells were treated daily with BSO and IF for 96 h. This decrease was significantly reduced when cells were concurrently treated with NAC in a concentration-dependent manner. Intracellular and total GSH levels in cells receiving concurrent treatment of NAC were significantly higher than those without NAC treatment. NAC protects renal tubular cells from IF-induced cytotoxicity. It is likely that NAC is protecting the cells by partially acting as a precursor for GSH synthesis. This mode of therapy may allow for protecting children from life-threatening nephrotoxicity induced by IF.

Changes in the levels of glutathione after cellular and cutaneous damage induced by squalene monohydroperoxide

Squalene monohydroperoxide (Sq-OOH), the initial product of ultraviolet-peroxidated squalene, was used to investigate the effect of peroxidative challenge upon the glutathione contents in rabbit ear skin and primary-cultured fibroblasts derived from rabbit ear skin. The cellular reduced glutathione (GSH) contents decreased during 30-minute incubations in vitro with Sq-OOH, and oxidized glutathione (GSSG) was formed concomitantly, indicating that Sq-OOH had a potential for GSH-depleting activity in vitro. When Sq-OOH was applied topically to the skin in vivo, only GSSG contents increased significantly within 30 minutes. Moreover, pretreatment with the GSH depletors, DL-buthionine sulfoximine (BSO) and diethyl maleate (DEM), could potentiate the cytotoxicity and comedogenicity induced by Sq-OOH. These findings suggest that the endogenous antioxidant, glutathione, is quite sensitive to Sq-OOH and may be an important material for protecting cells and/or tissues against the oxidative stress induced by Sq-OOH treatment.

Effect of glutathione manipulation on prostaglandin synthesis in renal medullary homogenates

1. The effect of glutathione (GSH) manipulation on arachidonic acid (AA) metabolism in renal medullary (RM) homogenates was investigated. 2. Diethyl maleate (DEM) depleted GSH initially by 50% (P less than 0.05) and produced a general suppression (P less than 0.05) of all PGs with the exception of TXB2. GSH was further depleted during homogenization and a 30-min incubation period (P less than 0.01). 3. Adding glutathione monoethyl ester (GSH-MEE) (0, 0.8, 1.6 or 3.2 mmol/ml) to RM homogenates increased GSH (P less than 0.01) and decreased RM homogenates' PGs-synthesizing capability (P less than 0.05), with the exception of PGE2 and TXB2 at the highest concentration. 4. The results indicate that homogenization has a significant impact (P less than 0.05) on GSH concentration of the media and alterations in GSH concentration affect the profile and quantity of AA metabolites in renal medullary homogenates.

Impairment of bromosulfophthalein hepatic transport and cholestasis induced by diethyl maleate in the rabbit

The present study was designed to investigate the effect of hepatic glutathione depletion induced by intraperitoneal administration of diethyl maleate (DEM) on the maximum biliary transport (Tm) and on the biliary excretion of bromosulfophthalein (BSP) in anaesthetized rabbits when the dye was perfused endovenously at doses exceeding Tm. The Tm of total BSP (BSPt) and that of conjugated BSP (BSPc) were significantly reduced after DEM administration whereas that of unconjugated BSP (BSPu) was markedly increased. A reduction in the biliary excretion of BSPt and BSPc, in the percentage of BSPc, in the cumulative excretion of BSPt and in the percent-dose recovery were also observed. However, no change in hepatic glutathione S-transferase activity was noted after DEM. The cholestasis observed following DEM administration coursed with falls in the biliary secretion of sodium, chloride and bicarbonate.

The detection of cytotoxicity produced by short-lived reactive intermediates: a study with bromobenzene

1. A V79 cell incubation incorporating rat liver 9000 g supernatant (S9) fractions, used previously to detect the toxicity due to long-lived, stable metabolites of cyclophosphamide, has been used to study the toxicity of short-lived, reactive metabolites generated from bromobenzene. 2. Cytotoxicity was observed in the presence of S9 fractions from rats treated with phenobarbitone but not in the presence of S9 fractions from untreated or beta-naphthoflavone-treated animals. This toxicity was enhanced by depletion of the glutathione in the S9 fraction by prior treatment of the animals with diethyl maleate and was reduced by SKF 525 A, in agreement with results in vivo on the mechanism of bromobenzene-induced hepatotoxicity. 3. This study demonstrates that cytotoxicity due to the generation of short-lived, reactive metabolites can be detected in this system in vitro provided that procedures are used to modify the activating and detoxifying enzyme systems within the S9 fraction.

Effects of phorone (diisopropylidene acetone), a glutathione (GSH) depletor, on hepatic enzymes involved in drug and heme metabolism in rats: evidence that phorone is a potent inducer of heme oxygenase

Concomitant with the depletion of glutathione content, phorone (250 mg/kg, ip.) produced a marked increase in heme oxygenase activity, biphasic effect on delta-aminolevulinic acid synthetase activity, and slight decreases in cytochrome P-450 content and aminopyrine demethylase activity in the liver of rats. The increase in heme oxygenase activity evoked by phorone was almost completely blocked by pretreatment of rats with actinomycin D and cycloheximide. Phorone was able to produce the changes in these parameters in a dose-dependent manner. Buthionine sulfoximine, a GSH depletor by inhibition of biosynthesis, failed to affect these hepatic parameters.

Changes in biliary secretion and lactate metabolism induced by diethyl maleate in rabbits

Diethyl maleate is a compound which binds with glutathione by means of a glutathione S-transferase and is excreted into bile leading to a rapid depletion of hepatic glutathione. In the rabbit, the activity of the enzyme is fairly low and we were thus prompted to study the possible effects of diethyl maleate on biliary secretion and metabolic status in this species. The administration of diethyl maleate induced a transient choleresis followed by cholestasis. The choleresis coursed with increases in the biliary output of sodium and unaccounted anions, whereas those of chloride, bicarbonate and bile acids were unaffected. Our data seem to confirm that choleresis is due to the osmotic activity of diethyl maleate compounds excreted into bile, as has been reported in rats and dogs. The cholestasis observed coursed with falls in the outputs of sodium, chloride and bicarbonate though that of bile acids remained constant. Following diethyl maleate administration, a metabolic acidosis appeared with progressive increases of blood lactate concentration. In bile the concentration of this anion closely followed that of plasma. The cholestasis is attributed to a lowered biliary secretion of bicarbonate probably secondary to the metabolic alteration. The hepatic values of cytoplasmatic and mitochondrial NADH/NAD ratios and of adenine nucleotide concentrations suggest that the increase in blood lactate results rather from a fall in its hepatic utilization that from an increase in its production.

Effect of diethylmaleate and other glutathione depletors on protein synthesis

The alpha, beta-unsaturated carbonyl compound diethylmaleate (DEM) depletes glutathione (GSH) from liver and other tissues, and for this reason it is often used in toxicological research to study the GSH-mediated metabolism of xenobiotics. In addition to GSH depletion, however, DEM has been shown to have other nonspecific effects, such as alteration of monooxygenase activities or glycogen metabolism. In this study we found that DEM (1 ml/kg) inhibited protein synthesis in brain and liver, following in vivo administration to mice. Protein synthesis was measured as the incorporation of [3H]valine into trichloroacetic acid-precipitable material. Administration of DEM also decreased body temperature by 2-3 degrees. By increasing the environmental temperature from 22 degrees to 35 degrees the hypothermic effect of DEM was prevented, without affecting its ability to deplete GSH from brain and liver. Furthermore, when mice were maintained at 35 degrees, DEM still caused a significant decrease in protein synthesis, suggesting that this effect was only partially due to hypothermia. To test whether inhibition of protein synthesis was related to GSH depletion, groups of animals were dosed with the alpha, beta-unsaturated carbonyl phorone (diisopropylidenacetone) or the specific inhibitor of GSH synthesis, buthionine sulfoximine (BSO). Phorone decreased GSH in liver and brain; however, it had no effect on protein synthesis. BSO decreased GSH levels in liver and kidney, but not in brain, and did not have any effect on protein synthesis in any of these tissues, nor did it cause any hypothermia. Furthermore, when hepatic GSH content was decreased by in vivo administration of DEM or BSO, there was no inhibition of protein synthesis measured in vitro. These results indicate that, at the dose normally used to deplete GSH from various tissues. DEM also exerts an inhibitory effect on protein synthesis, which appears to be only partially due to its hypothermic effect, and is independent from GSH depletion. BSO, which, in our experimental conditions, lacks this and other nonspecific effects, might be a good alternative for studies aimed at characterizing the role of GSH in the metabolism and toxicity of chemicals.

Skin-tumour promoting activity of methyl ethyl ketone peroxide—a potent lipid-peroxidizing agent

The tumour-promoting activity of methyl ethyl ketone peroxide (MEKP) was tested on the skin of hairless mice using a two-stage initiation-promotion protocol. When ultraviolet radiation in the UVB region (280-320 nm) was used as tumour initiator, MEKP showed weak promoting activity. The promotional activity of MEKP was potentiated by diethyl maleate, which is known to deplete intracellular glutathione, suggesting that lipid peroxidation may be important in the tumour promotion.

The effects of buthionine sulphoximine (BSO) on glutathione depletion and xenobiotic biotransformation

Buthionine sulphoximine (BSO) is an inhibitor of gamma-glutamylcysteine synthetase (gamma-GCS) and, consequently lowers tissue glutathione (GSH) concentrations. In fed male C3H mice, liver and kidney GSH levels were depleted by BSO in a dose dependent manner with maximum effect (35% of initial levels) occurring with doses between 0.8 and 1.6 g/kg, i.p. At these doses maximum effects on gamma-GCS and GSH were observed 2-4 hr after BSO administration; initial gamma-GCS activity and GSH content were restored approximately 16 hr post BSO. BSO, either in vivo or in vitro, had no effect on hepatic microsomal cytochrome P-450 levels, a range of cytochrome P-450 dependent enzyme activities or p-nitrophenol glucuronyl transferase activity. Similarly, BSO had no effect on phenol sulphotransferase and two GSH-transferase activities in the 105,000 g supernatant fraction. BSO had no effect on the duration of hexobarbitone induced narcosis in mice. Consistent with specific inhibition of GSH synthesis, BSO pretreatment of mice decreased the proportion of a 50 mg/kg dose of paracetamol excreted in the urine as GSH-derived conjugates but did not affect paracetamol clearance through the glucuronidation or sulphation pathways. Since BSO does not affect cytochrome P-450 or conjugating enzyme activity, its use as a specific depletor of tissue GSH in the investigation of mechanisms of xenobiotic-induced toxicities is preferable to the standard GSH-depleting agents as these have other enzymic effects.

Protection against 3-methylcholanthrene-induced skin tumorigenesis in Balb/C mice by ellagic acid

Topical application of ellagic acid, a naturally occurring dietary plant phenol, to Balb/C mice resulted in significant protection against 3-methylcholanthrene (MCA)-induced skin tumorigenesis. Ellagic acid was found to be an effective inhibitor of tumor formation whether the tumor data are considered as percent mice with tumors, cumulative number of tumors, tumors per mouse or tumors per tumor bearing animal as a function of the number of weeks on test. By 8, 10, 12, 14, and 16 weeks of testing, the number of tumors per mouse in the group receiving MCA alone was 2.0, 3.4, 4.0, 4.9 and 5.3, respectively, whereas the corresponding numbers in the group receiving MCA plus 2 mumol ellagic acid were 0, 0.3, 0.4, 0.6 and 1.2, respectively. At the termination of the experiment (16 weeks) aryl hydrocarbon hydroxylase (AHH) activity in skin and liver and the extent of 3H-BP-binding to skin, liver and lung DNA were determined and both of these parameters were found to be significantly inhibited in the animals treated with ellagic acid. These results indicate that ellagic acid can inhibit the metabolism of polyaromatic hydrocarbons and modulate skin carcinogenesis induced by these chemicals.

Effects of diethyl maleate on protein synthesis in isolated hepatocytes

Diethyl maleate is commonly used in toxicological and drug metabolism research using isolated adult rat hepatocytes. At the highest concentrations used the effect of diethyl maleate is, however, not limited to glutathione depletion. In these conditions it inhibits protein synthesis and it impairs the "L" system for amino acid transport. It has, however, no effect on the cytochrome P-450 content or its dependent aldrin monooxygenase. The present report shows that a concentration of diethyl maleate as low as 0.2 mM is sufficient to deplete glutathione without affecting glycogen and protein synthesis, transport of amino acid or monooxygenase activity in isolated adult rat hepatocytes.

Glutathione and hepatic mixed-function oxidase activity

It is apparent that hepatic GSH may function in drug metabolism not only as a substrate for conjugation but also in regulation of cytochrome P-450 activity. The remarkable aspect of the latter activity is its specificity. Loss of hepatic GSH depresses N-demethylation of DAB while ring hydroxylation is unaffected. On the other hand, the effect is to some degree nonspecific in that control as well as PB- or MC-induced N-demethylation is inhibited. Thus the response may not simply be specific to one isozyme of cytochrome P-450 but may be associated with one aspect of the enzymic activity of several cytochrome P-450 isozymes (i.e., N-demethylation). We have postulated that sensitivity of this activity to lipid peroxidation underlies the relationship to GSH since the tripeptide serves as a major protection against hepatic lipid peroxidation and its consequences. It is as yet not clear as to how or why this particular aspect of P-450 activity is more sensitive to lipid peroxidation than are other activities such as ring hydroxylation. Ongoing investigations include attempts to identify the cytochrome P-450 isozyme(s) which inhibit this response to GSH depletion. GSH-lipid peroxidation relationships have already been reported with isolated hepatocytes, and there may be a possible connection between this and the relative instability of cytochrome P-450 in cultured hepatocytes. Another factor which may be involved is heme oxygenase activity, which is markedly induced in the liver after GSH depletion, after cobalt administration (which also depresses cytochrome P-450 activity), and during incubation of isolated hepatocytes. This enzyme catalyzes the rate-limiting step in heme breakdown and may contribute to the loss of cytochrome P-450 activity associated with GSH depletion.

Chemically-induced glutathione depletion and lipid peroxidation

Malondialdehyde (MDA) formation in mouse liver homogenates was measured in the presence of various glutathione depletors (5 mmol/l). After a lag phase of 90 min, the MDA formation increased from 1.25 nmol/mg protein to 14.5 nmol/mg in the presence of diethyl maleate (DEM), to 10.5 with diethyl fumarate (DEF) and to 4 with cyclohexenon by 150 min. It remained at 1.25 nmol/mg with phorone and in the control. On the other hand, glutathione (GSH) dropped from 55 nmol/mg to 50 nmol/mg in the control to, less than 1 with DEM, to 46 with DEF, to 3 with cyclohexenon and to 7 with phorone. The data show that the potency to deplete GSH is not related to MDA production in this system. DEM stimulated in vitro ethane evolution in a concentration-dependent manner and was strongly inhibited by SKF 525A. From type I binding spectra to microsomal pigments the following spectroscopic binding constants were determined: 2.5 mmol/l for phorone, 1.2 mmol/l for cyclohexenon, 0.5 mmol/l for DEM and 0.3 mmol/l for DEF. In isolated mouse liver microsomes NADPH-cytochrome P-450 reductase and NADH-cytochrome b5 reductase activity were unaffected by the presence of DEM, whereas ethoxycoumarin dealkylation was inhibited. Following in vivo pretreatment, hepatic microsomal electron flow as determined in vitro was augmented in the presence of depleting as well as non-depleting agents, accompanied by a shift from O2- to H2O2 production. It is concluded that it is not the absence of GSH which causes lipid peroxidation after chemically-induced GSH depletion but rather the interaction of the chemicals with the microsomal monoxygenase system.