Correlations between common tests for assessment of liver damage: indices of the hepatoprotective activity of promethazine in carbon tetrachloride hepatotoxicity

The effects of promethazine (PM) on different aspects of the hepatotoxic action of CCl4 in the rat were investigated with the objective of finding rapid and reliable indicators of hepatoprotective effects. The study was based on definitive histological assessment of liver damage caused by CCl4 in the presence and absence of PM: PM (78 mumol kg-1, i.p.) protected against CCl4-induced hepatic necrosis 24 h after a low dose of CCl4 (1.3 mmol kg-1) but not against a higher dose (13.0 mmol kg-1). The large increases in plasma activities of GOT, GPT and LDH produced by dosing with CCl4 were partially inhibited by the administration of PM. PM and CCl4 caused a synergistic and long-lasting decrease in body temperature (2-3 degrees C for 8-10 h). Modifying the toxicity with PM, together with a low dose of CCl4, helped to minimize secondary effects of CCl4, to clarify the sequence of toxic events, and to assess the sensitivity of some standard tests of hepatotoxicity. Simultaneous measurement of over 20 commonly used biochemical screening tests in individual animals 3 or 6 h after treatment permitted direct correlation of a wide variety of concentrations, activities and effects. For example, liver CHCl3 concentrations (as a measure of CCl4 metabolism) correlate strongly with increases in diene conjugation of microsomal lipids (as a measure of CCl4-induced lipid peroxidation); malonaldehyde production appears to be less sensitive as a measure of lipid peroxidation in vivo than diene conjugation. The changes induced in each parameter and the correlations between them are discussed with reference to the overall nature of the hepatotoxic reaction and its modification by PM.     

Dose- and Time-Dependent Effects of Luteolin on Liver Metallothioneins and Metals in Carbon Tetrachloride-Induced Hepatotoxicity in Mice

The aim of this study was to investigate the protective effect of luteolin on liver Ca, Mg, Zn, Cu, Fe, and Mn content in mice with carbon tetrachloride (CCl4)-induced hepatotoxicity. Additionally, liver metallothionein (MT) expression was studied. Luteolin was administered intraperitoneally (i.p.) as a single 5- or 50-mg/kg dose or once daily for two consecutive days, respectively. Two hours after the last injection, the mice were treated with CCl4 (20 mg/kg, i.p.). CCl4 injection reduced hepatic level of all metals except Ca, with an intense cytoplasmic staining pattern in hepatocytes located in periportal areas, indicating induction of MTs. Pretreatment with 50 mg/kg of luteolin for 2 days remarkably elevated metal content to control values (Mg and Cu) or even above them (Zn and Fe). Luteolin pretreatment increased pericentral MTs immunopositivity and histological architecture improvement in a time- and dose-dependent manner, being the most prominent in mice pretreated with 50 mg/kg for 2 days. The liver in this group showed pronounced MT expression in almost all hepatocytes throughout the liver parenchyma. In conclusion, these results suggest the protective effect of luteolin on CCl4-induced hepatotoxicity and an enhancement of hepatocyte proliferative capabilities.     

Involvement of endogenous CRF in carbon tetrachloride-induced acute liver injury in rats

Central neuropeptides play important roles in many physiological and pathophysiological regulation mediated through the autonomic nervous system. In regard to the hepatobiliary system, several neuropeptides act in the brain to regulate bile secretion, hepatic blood flow, and hepatic proliferation. Central injection of corticotropin-releasing factor (CRF) aggravates carbon tetrachloride (CCl4)-induced acute liver injury through the sympathetic nervous pathway in rats. However, still nothing is known about a role of endogenous neuropeptides in the brain in hepatic pathophysiological regulations. Involvement of endogenous CRF in the brain in CCl4-induced acute liver injury was investigated by centrally injecting a CRF receptor antagonist in rats. Male fasted Wistar rats were injected with CRF receptor antagonist alpha-helical CRF-(9-41) (0.125-5 microg) intracisternally just before and 6 h after CCl4 (2 ml/kg) administration, and blood samples were obtained before and 24 h after CCl4 injection for measurement of hepatic enzymes. The liver sample was removed 24 h after CCl4 injection, and histological changes were examined. Intracisternal alpha-helical CRF-(9-41) dose dependently (0.25-2 microg) reduced the elevation of alanine aminotransferase and aspartate aminotransferase levels induced by CCl4. Intracisternal alpha-helical CRF-(9-41) reduced CCl4-induced liver histological changes, such as centrilobular necrosis. The effect of central CRF receptor antagonist on CCl4-induced liver injury was abolished by sympathectomy and 6-hydroxydopamine pretreatment but not by hepatic branch vagotomy or atropine pretreatment. These findings suggest the regulatory role of endogenous CRF in the brain in experimental liver injury in rats.     

Enhanced lipid peroxidation is not necessary for induction of metallothionein-I by oxidative stress

A study has been made of factors which may influence the induction of metallothionein-I (MT-I) synthesis by the superoxide radical generating agent, paraquat (PQ). Hepatic concentrations of zinc (Zn) and MT-I increased in rats injected with PQ (40 mg/kg, s.c.) or fasting, but were greater in the former. Renal concentration of MT-I increased in fasted rats but not in PQ-treated rats. The data suggest that the increase in MT-I concentrations in PQ-treated rats is not caused by reduction in food intake. Administration of PQ increased hepatic concentrations of Zn, MT-I and thiobarbituric acid-reactive substances (TBA-RS), indicating the occurrence of lipid peroxidation. Treatment of rats with vitamin E (400 mg/kg, s.c.) on 4 successive days before injection of PQ prevented only the enhancement of lipid peroxidation. The data indicate that the induction of MT synthesis by PQ is not correlated with enhancement of lipid peroxidation. Similar results were obtained in the liver of rats subjected to the radical-generating conditions, such as fasting and exposure to carbon tetrachloride. Free radicals may induce MT synthesis by direct or indirect mechanisms.     

Changes in plasma zinc,copper, iron,and hepatic metallothionein in adjuvant-induced arthritis treated with cyclosporin

The early changes in hepatic metallothionein (MT) and plasma zinc (Zn), copper (Cu), and iron (Fe) were investigated during the induction of adjuvant (AJ) arthritis in rats in conjunction with cyclosporin (CSA) treatment. Plasma Zn decreased after AJ injection (60% of control values at 8 h), and this was associated with a 4.5-fold increase in hepatic MT at 8 h. Plasma Zn was lowest at 16 h (40% of control), whereas hepatic MT concentrations increased to a maximum of 20-fold at 16 h. Changes in plasma Fe paralleled those of Zn, whereas plasma Cu levels were increased. Plasma metal and hepatic MT concentrations returned toward normal from d 1–7. At d 14, when marked paw swelling was apparent, hepatic MT and plasma Cu were again increased and plasma Zn decreased. Administration of CsA decreased MT induction in rats injected with AJ and also caused a marked recovery in plasma Zn and Fe levels. These changes were small but significant even in the early stages (up to 24 h) after AJ injection and were followed by a sustained improvement in all parameters, corresponding to the nonappearance of clinical arthropathy in CsA-treated rats. TNF-α and IL-6 production by peritoneal macrophages isolated from AJ-injected rats was significantly decreased by CsA treatment at d 7 and 14. The inhibition of hepatic MT induction during acute and chronic inflammation by cyclosporin emphasizes the role of the immune system in altered metal homeostasis in inflammation.     

C-terminus modification of Streptomyces clavuligerus deacetoxycephalosporin C synthase improves catalysis with an expanded substrate specificity

Here we investigated the effect of pioglitazone, a peroxisome proliferator-activated receptor (PPAR)-gamma ligand, on early-phase hepatic fibrogenesis in vivo caused by acute carbon tetrachloride (CCl(4)) administration in the rat. Pioglitazone (1 mg/kg BW) prevented pericentral fibrosis and induction of alpha-smooth muscle actin (SMA) 72 h after CCl(4) administration (1 ml/kg BW). CCl(4) induction of alpha1(I)procollagen mRNA in the liver was blunted by pioglitazone to the levels almost 2/3 of CCl(4) alone. Pioglitazone also prevented CCl(4)-induced hepatic inflammation and necrosis, as well as increases in serum tumor necrosis factor-alpha levels. Further, pioglitazone inhibited the induction of alphaSMA and type I collagen in primary cultured hepatic stellate cells in a dose-dependent manner. In conclusion, pioglitazone inhibits both hepatic inflammation and activation of hepatic stellate cells, thereby ameliorating early-phase fibrogenesis in the liver following acute CCl(4).     

Effect of surgically induced cholestasis on the levels of hepatic zinc and metallothionein in rat liver

Early effects of experimental cholestasis on the homeostasis of zinc (Zn) and metallothionein (MT) were studied in rats which had undergone bile duct ligation for 0, 3, 6, 9, 12, 16, 20, and 24 h. Transient increases in hepatic Zn levels were observed at 9 h but returned to control values at 12 h. Serum Zn levels increased at 24 h. Cholestasis was confirmed by increased serum alkaline phosphatase (AP) activity. MT increased at 3 h and reached a maximum level at 12 h and remained elevated even at 24 h after the onset of experimental cholestasis. No hepatocellular damage was detected according to the results of alanine aminotransferase (ALT) activities in serum. These results shown that the increases in Zn observed in liver are related to bile stagnation rather than to a hepatocellular damage and that increased MT occurs concurrently with increased hepatic Zn. These observations suggest that the cellular levels of Zn in cholestasis is regulated by homeostatic mechanisms, of which one could be mediated by MT.     

Effect of Emblica officinalis (Gaertn) on CCl4 induced hepatic toxicity and DNA synthesis in Wistar rats

A single dose of CCl4 (1 ml/kg body weight, po in corn oil) increased the levels of SGOT (serum glutamate oxaloacetate transaminase), SGPT (serum glutamate pyruvate transaminase), LDH (lactate dehydrogenase), glutathione-S-transferase and depletion in reduced glutathione, glutathione peroxidase and glutathione reductase. It also caused enhancement in the levels of lipid peroxidation (LPO) and DNA synthesis. There was also pathological deterioration of hepatic tissue as evident from multivacuolated hepatocytes containing fat globules around central vein. The pretreatment of E. officinalis for 7 consecutive days showed a profound pathological protection to liver cell as depicted by univacuolated hepatocytes. Pretreatment with E. officinalis at doses of 100 and 200 mg/kg body weight, prior to CCl4 intoxication showed significant reduction in the levels of SGOT, SGPT, LDH, glutathione-S-transferase, LPO and DNA synthesis. There was also increase in reduced glutathione, glutathione peroxidase and glutathione reductase. The results suggest that E. officinalis inhibits hepatic toxicity in Wistar rats.     

Protective effect of aminoguanidine, a nitric oxide synthase inhibitor, against carbon tetrachloride induced hepatotoxicity in mice

The present study was undertaken to evaluate the effect of aminoguanidine (AG) on carbon tetrachloride (CCl4)-induced hepatotoxicity. Treatment of mice with CCl4 (20 microl/kg, i.p.) resulted in damage to centrilobular regions of the liver, increase in serum aminotransferase and rise in lipid peroxides level 24 hours after CCl4 administration. Pretreatment of mice with AG (50 mg/kg, i.p.) 30 minutes before CCl4 was found to protect mice from the CCl4-induced hepatic toxicity. This protection was evident from the significant reduction in serum aminotransferase, inhibition of lipid peroxidation and prevention of CCl4-induced hepatic necrosis revealed by histopathology. Aminoguanidine, a relatively specific inhibitor of inducible nitric oxide synthase, did not inhibit the in vitro lipid peroxidation. Taken together, these data suggest a potential role of nitric oxide as an important mediator of CCl4-induced hepatotoxicity.     

Protective role of fructose 1,6-bisphosphate during CCl4 hepatotoxicity in rats.

Rats were injected intraperitoneally with CCl4 (2.5 ml/kg body wt.) and the hepatotoxicity was compared with that of rats receiving the same dose of CCl4 and an intraperitoneal injection of fructose 1,6-bisphosphate (2 g/kg body wt.). A 50-70% decrease in plasma aspartate aminotransferase and alanine aminotransferase activities was observed in the latter treatment, indicating a protective role of the sugar bisphosphate in CCl4 hepatotoxicity. The protection was accompanied by elevated hepatic activities of ornithine decarboxylase at 2, 6 and 24 h, S-adenosylmethionine decarboxylase at 6 h, and spermidine N1-acetyltransferase at 2 h. The increase in the enzymes involved in polyamine metabolism was shown in our previous work [Rao, Young & Mehendale (1989) J. Biochem. Toxicol. 4, 55-63] to correlate with increased polyamine synthesis or interconversion, which was related to the extent of hepatocellular regeneration. The hepatic contents of fructose 1,6-bisphosphate and ATP significantly decreased after CCl4 treatment, and administration of the sugar bisphosphate increased hepatic ATP. Fructose 1,6-bisphosphate, an intermediary metabolite of the glycolytic pathway, may decrease CCl4 toxicity by increasing the ATP in the hepatocytes. The ATP generated is useful for hepatocellular regeneration and tissue repair, events which enable the liver to overcome CCl4 injury.     

Effect of gamma irradiation on liver metallothionein synthesis and lipid peroxidation in rats.

Several studies have recently shown that metallothionein (MT), a protein characterized by a high thiol content and that binds Zn2+ and Cu+, might be involved in the protection against oxidative stress and can act as a free radical scavenger. Oxidative stresses, such as irradiation, increase lipid peroxidation (LP) and subsequent tissue damage through free radical production. The induction of hepatic MT synthesis by gamma-irradiation (20 Gy) at 8, 24, 30 and 48 hrs. post-irradiation in two different age groups of Sprague-Dawley rats (39-40 and 48-49 days old) was studied. LP measured by the thiobarbituric acid reactive substances (TBARS) assay and Cu and Zn levels in liver have also been determined. In the younger group, the gamma-irradiation induced hepatic MT synthesis and increased LP that peaked 24 hrs. after irradiation. During the first 30 hrs. post-irradiation, a positive and statistically significant correlation between hepatic MT content and LP level in liver was found. In the older group, liver MT synthesis was only increased 1.7-fold and LP levels were not altered at 24 hrs. post-irradiation compared with sham-irradiated rats.Therefore it appears that LP is not necessary for induction of MT synthesis by gamma-irradiation.     

Increased synthetic capacity for metallothionein in cultured liver cells following dimethyl sulfoxide pretreatment

Cultured TRL 1215 cells in log phase of growth were exposed to dimethyl sulfoxide (DMSO; 14-280 mM) followed 48 h later by cadmium (10 micron). Intracellular concentrations of metallothionein (MT) were measured 24 h after cadmium addition. Cadmium alone caused a 10-fold increase in the levels of MT, while DMSO alone had no effect on cellular MT levels. DMSO pretreatment followed by cadmium exposure, however, resulted in MT levels that were elevated by a factor of as much as 25-fold those observed in control cells. Concurrent treatment with the DNA synthesis inhibitor hydroxyurea (HU) eliminated the enhancing effect of DMSO pretreatment on cadmium induction of MT, indicating the requirement of DNA synthesis. An enhancement of the cellular accumulation of the metal ion did not account for the increased cadmium-induced MT synthesis in DMSO-pretreated cells as these cells did not show significantly increased uptake of cadmium during the initial period of exposure. DMSO pretreatment enhances cadmium induction of MT synthesis through a mechanism that appears to be dependent on the synthesis of DNA.     

Hepatoprotective mechanism of schisandrin B: role of mitochondrial glutathione antioxidant status and heat shock proteins

In this study, the time course of schisandrin B- (Sch B-) induced changes in hepatic mitochondrial glutathione antioxidant status (mtGAS) and heat shock protein (HSP) 25/70 induction was examined to study their differential roles in the hepatoprotection afforded by Sch B pretreatment against carbon tetrachloride (CCl(4)) toxicity in mice. Dimethyl diphenyl bicarboxylate (DDB), a nonhepatoprotective analog of Sch B, was also included for comparison. The results indicate that Sch B treatment (2 mmol/kg) produced maximum enhancement in hepatic mtGAS and increases in both hepatic HSP 25 and HSP 70 levels at 24 h after dosing. While the extent of hepatoprotection afforded by Sch B pretreatment against CCl(4) was found to correlate inversely with the elapsed time postdosing, the protective effect was associated with the ability to sustain mtGAS and/or HSP 70 levels in a CCl(4)-intoxicated condition. On the other hand, DDB (2 mmol/kg) treatment, which did not sustain mtGAS and HSP 70 level, could not protect against CCl(4) toxicity. Abolition of the Sch B-mediated enhancement of mtGAS by buthionine sulfoximine/phorone did not completely abrogate the hepatoprotective action of Sch B. The results indicate that Sch B pretreatment independently enhances mtGAS and induces HSP 25/70 production, particularly under conditions of oxidative stress, thereby protecting against CCl(4) hepatotoxicity.     

Thymoquinone protects against carbon tetrachloride hepatotoxicity in mice via an antioxidant mechanism

Thymoquinone (TQ) is the major active component of the volatile oil of Nigella sativa seeds. The effects of TQ on carbon tetrachloride (CCl4)-induced hepatotoxicity was investigated in male Swiss albino mice. Carbon tetrachloride (20 microliters/Kg, i.p.) injected into mice, induced damage to liver cells and was followed by the increase in serum alanine aminotransferase (ALT) activity after 24 h. Oral administration of TQ in a single dose (100 mg/Kg) resulted in significant (p < 0.001) protection against the hepatotoxic effects of CCl4. TQ was tested as a substrate for mice hepatic DT-diaphorase in the presence of NADH. TQ appears to undergo reduction to dihydrothymoquinone (DHTQ). Reduction rates as a function of protein (liver homogenate) and substrate (TQ) concentrations are reported. An apparent K(m) of 0.1 mM and an apparent Vmax of 74 mumol/min/g liver were measured. TQ and DHTQ inhibited the in vitro non-enzymatic lipid peroxidation in liver homogenate (induced by Fe(3+)-ascorbate) in a dose dependent manner. In this in vitro model DHTQ was more potent in comparison with TQ and butylated hydroxytoluene (BHT). The IC50 for DHTQ, TQ and BHT were found to be 0.34, 0.87 and 0.58 microM respectively. The data suggest that the in vivo protective action of TQ against CCl4-induced hepatotoxicity may be mediated through the combined antioxidant properties of TQ and its metabolite DHTQ.     

Cytokine-responsive gene-2/IFN-inducible protein-10 expression in multiple models of liver and bile duct injury suggests a role in tissue regeneration.

IFN-inducible protein-10 (IP-10/CXCL10) is a CXC chemokine that targets both T cells and NK cells. Elevation of IP-10 expression has been demonstrated in a number of human diseases, including chronic cirrhosis and biliary atresia. Cytokine-responsive gene-2 (Crg-2), the murine ortholog of IP-10, was induced following CCl(4) treatment of the hepatocyte-like cell line AML-12. Crg-2 expression was noted in vivo in multiple models of hepatic and bile duct injury, including bile duct ligation and CCl(4), D-galactosamine, and methylene dianiline toxic liver injuries. Induction of Crg-2 was also examined following two-thirds hepatectomy, a model that minimally injures the remaining liver, but that requires a large hepatic regenerative response. Crg-2 was induced in a biphasic fashion after two-thirds hepatectomy, preceding each known peak of hepatocyte DNA synthesis. Induction of Crg-2 was also observed in the kidney, gut, thymus, and spleen within 1 h of two-thirds hepatectomy. Characteristic of an immediate early gene, pretreatment of mice with the protein synthesis inhibitor cycloheximide before either two-thirds hepatectomy or CCl(4) injection led to Crg-2 superinduction. rIP-10 was demonstrated to have hepatocyte growth factor-inducing activity in vitro, but alone had no direct mitogenic effect on hepatocytes. Our data demonstrate that induction of Crg-2 occurs in several distinct models of liver injury and regeneration, and suggest a role for CRG-2/IP-10 in these processes.     

CCl4-induced alterations in Ca++ homeostasis in chlordecone and phenobarbital pretreated animals

Male S-D rats were maintained on normal powdered diet or on the same diet containing 10 ppm chlordecone or 225 ppm phenobarbital for 15 days. On day 15, all the animals received a single ip injection of either corn oil or a subtoxic dose of CCl4 (25-200 microliter/kg) in corn oil vehicle (1 ml/kg). The animals were sacrificed 12 hrs later. Liver microsomal cytochrome P-450 and Ca++ levels in whole liver, mitochondria, microsomes and cytosol were determined. Cytochrome P-450 induction was greater with phenobarbital pretreatment than with chlordecone but the CCl4 induced destruction of cytochrome P-450 was almost similar in both groups and progressive with the dose of CCl4. CCl4 given to animals on normal diet in a dose range of 25-200 microliter/kg did not significantly alter the cytochrome P-450 levels. These findings are consistent with greater bioactivation of CCl4 after the above two pretreatments. There was a massive accumulation of Ca++ in chlordecone and phenobarbital pretreated animals after CCl4 administration. Cytosolic Ca++ levels remained high despite the mitochondrial and microsomal sequestration. This perturbation of hepatocellular Ca++ homeostasis might lead to hepatic lesion and hepatic failure. Chlordecone or phenobarbital alone do not alter hepatic Ca++ levels. These findings suggest that excessive accumulation of Ca++ may be causally related to the progression of hepatotoxic response due to CCl4 in chlordecone treated animals.     

Induction of metallothionein in a human trophoblast cell line by cadmium and zinc

The induction of metallothionein (MT) in a cell line derived from a malignant trophoblastic tumor (JAr cells) was demonstrated using the Cd/heme radioassay following exposure to Cd or Zn. Cd at an optimal concentration of 1 microM produced a 30-fold increase in MT following a 24 hr incubation. Induction by Cd was both time and dose dependent, with a significant increase in MT noted as early as 3 hr, with levels continuing to increase up to 24 hr. Zn was also quite effective in inducing MT synthesis in this cell line. Exposure to 80 microM Zn for 24 hr produced a 70-fold increase in MT. Although Cd was a more potent inducer of MT, exposure to Zn resulted in a greater magnitude of induction. The magnitude of MT induction in JAr cells was much greater than that seen in cultured trophoblasts from term chorion laeve. The degree of induction seen in this cell line makes it an interesting model for the study of MT's role in trophoblast function. MT induction in trophoblasts may reflect a protective mechanism against heavy metal toxicity and/or an integral aspect of normal zinc homeostasis.     

Metallothionein induction in freshly isolated rat hepatocytes

The control of metallothionein (MT) synthesis was investigated in freshly prepared rat hepatocytes in experiments of short-term duration. Viability and metabolic function were maintained in incubations of 6-h duration. MT synthesis was measurable in hepatocytes from fed rats at Zn concentrations down to 1 μM. Zn and dexamethasone induced concentration-dependent increases in the synthesis of MT with maximal increases above the 5-h control of 3.2- and 2.5-fold, respectively. Zn induction of MT was first measurable at 2 h and was inhibited by actinomycin C. Although initial (0 h) MT concentrations in hepatocytes from fasted rats were double those from fed rats, after 6-h incubation in the presence of 50 μM Zn, the fasted rat hepatocytes showed only half the MT concentrations of the fed rat hepatocytes. Glucagon and interleukin-6 (IL-6) were less effective inducers and increased MT synthesis by 28 and 17%, respectively. IL-6 (100 U/mL) was found to have an additive effect on MT synthesis above that of Zn alone (1–50 μM) or Zn plus dexamethasone (1 μM). A supernatant from LPS-stimulated macrophages increased MT synthesis by 40%. The basal MT synthesis was not increased by either tumor necrosis factor-α (TNF-α) or interleukin-1 (IL-1). All incubations were carried out in the presence of RPMI 1640 medium with Hepes (20 mM), bicarbonate (24 mM), and fatty acid-free albumin (FAFA; 0.5% w/v). MT synthesis was also seen using Krebs bicarbonate buffer with glucose (10 mM), Hepes (20 mM), and FAFA (0.5% w/v), and although the level of MT synthesis was less than in RPMI, the increases in concentrations of MT at 5 h were 225, 139, 36 and 20% for Zn, dexamethasone, glucagon, and control, respectively. It is concluded that MT synthesis occurs in freshly prepared hepatocytes and that these cells are responsive to some of the established inducers of MT. This system enables the study of MT synthesis in individual rats in various metabolic and pathological states.     

Increased metallothionein gene expression in 5-aza-2'-deoxycytidine-induced resistance to cadmium cytotoxicity

The pyrimidine analog, 5-azacytidine (AZA-CR), has been shown to increase the expression of the metallothionein (MT) gene and to induce tolerance to cadmium toxicity. Since incorporation into DNA of AZA-CR appears to be required for this effect, the deoxynucleoside of AZA-CR should also be effective. Therefore, this study was undertaken to assess the effect of 5-aza-2'-deoxycytidine (AZA-CdR) pretreatment on cadmium-induced cytotoxicity and MT expression in cultured cells. TRL 1215 cells in log phase of growth were exposed to AZA-CdR (0.4, 0.8, 4.0, 8.0 microM) followed 48 h later by the addition of cadmium (10 microM). MT concentrations were measured 24 h after the addition of cadmium. AZA-CdR alone caused modest, dose-related increases in MT levels (2.3-fold maximum), while cadmium alone resulted in a 9.5-fold increase. Pretreatment with AZA-CdR in combination with cadmium caused a 19--24-fold increase in cellular MT at all doses of AZA-CdR. Addition of the DNA synthesis inhibitor, hydroxyurea (HU), to the incubation medium during AZA-CdR exposure prevented the enhancing effect of the analog on cadmium induction of MT accumulation. Time course studies revealed that AZA-CdR pretreatment reduced the time required for cadmium to induce MT levels from 4--8 h to 0--2 h. AZA-CdR pretreated cells placed in suspension with cadmium (125 microM) showed a marked reduction in cadmium-induced cytotoxicity as reflected by reduced glutamic-oxaloacetic transaminase (GOT) loss. Uptake studies showed that AZA-CdR pretreatment had no effect on cadmium transport during the initial phases of exposure, indicating that an alteration in the toxicokinetics of the metal did not account for the reduction in toxicity. AZA-CdR did, however, cause hypomethylation of the MT-I gene. These results suggest that AZA-CdR pretreatment induces tolerance to cadmium toxicity by increasing the genetic expression of MT possibly through hypomethylation of the MT gene.     

Potentiation by chlordecone of the defect in hepatic microsomal calcium sequestration induced by carbon tetrachloride

The effect of carbon tetrachloride (CCl4) on the capacity of hepatic microsomes to sequester calcium was studied following pretreatment of rats with chlordecone. Chlordecone pretreatment alone had no effect on the kinetics of calcium uptake by hepatic microsomes. It was found, however, that chlordecone pretreatment of rats potentiated by sixfold the potency of CCl4 to suppress microsomal calcium sequestration capacity when measured one hour after CCl4 administration.