Kinetics of metabolism of deltamethrin and cis- and trans-permethrin in vitro. Studies using rat and human liver microsomes, isolated rat hepatocytes and rat liver cytosol.

The kinetics of metabolism of deltamethrin (DLM) and cis- and trans-permethrin (CPM and TPM) was studied in male Sprague-Dawley rat and human liver microsomes. DLM metabolism kinetics was also studied in isolated rat hepatocytes, liver microsomes and cytosol. Apparent intrinsic clearance (CLint) values for the metabolism of DLM, CPM and TPM by cytochrome P450 (CYP) and carboxylesterase (CES) enzymes in rat and human liver microsomes decreased with increasing microsomal protein concentration. However, when apparent CLint values were corrected for nonspecific binding to allow calculation of unbound (i.e., corrected) CLint values, the unbound values did not vary greatly with microsomal protein concentration. Unbound CLint values for metabolism of 0.05-1 µM DLM in rat liver microsomes (CYP and CES enzymes) and cytosol (CES enzymes) were not significantly different from rates of DLM metabolism in isolated rat hepatocytes. This study demonstrates that the nonspecific binding of these highly lipophilic compounds needs to be taken into account in order to obtain accurate estimates of rates of in vitro metabolism of these pyrethroids. While DLM is rapidly metabolised in vitro, the hepatocyte membrane does not appear to represent a barrier to the absorption and hence subsequent hepatic metabolism of this pyrethroid.

Piperonyl butoxide: Mode of action analysis for mouse liver tumour formation and human relevance.

In a 79 week bioassay the pesticide synergist piperonyl butoxide (PBO) was shown to significantly increase the incidence of hepatocellular adenoma (but not hepatocellular carcinoma) in male CD-1 mice at dietary levels of 100 and 300 mg/kg/day PBO and in female mice at a dietary level of 300 mg/kg/day. As PBO is not a genotoxic agent, a series of investigative studies were undertaken to elucidate the mode of action (MOA) for PBO-induced mouse liver tumour formation. Male CD-1 mice were fed diets to provide intakes of 0 (control), 30, 100 and 300 mg/kg/day PBO and for purposes of comparison 500 ppm sodium phenobarbital (NaPB), a known constitutive androstane receptor (CAR) activator, for 7 and 14 days. Treatment with 100 and 300 mg/kg/day PBO and 500 ppm NaPB increased relative liver weight which was associated with hepatocyte hypertrophy, with hepatocyte replicative DNA synthesis (RDS) being increased after 7 days treatment. The treatment of CD-1 mice with 30-300 mg/kg/day PBO for 14 days resulted in significant dose-dependent increases in hepatic microsomal cytochrome P450 (CYP) content and 7-pentoxyresorufin O-depentylase (PROD) activity and in hepatic Cyp2b10 mRNA levels. In contrast, PBO produced a biphasic effect on markers of activation of the peroxisome proliferator-activated receptor alpha (PPARα), with small increases in microsomal lauric acid 12-hydroxylase activity and hepatic Cyp4a10 mRNA levels being observed in mice given 100 mg/kg/day with PBO, with either no increase or a significant inhibition being observed in mice given 300 mg/kg/day PBO. The hepatic effects of PBO in male CD-1 mice were generally similar to those produced by NaPB and were reversible after the cessation of treatment for 28 days. Studies were also performed in male C57BL/6J (wild type) mice and in hepatic CAR and pregnane X receptor (PXR) knockout mice (CAR KO/PXR KO mice), where in the CAR KO/PXR KO mice PBO had little effect on markers of CAR activation, but produced some increases in markers of PPARα activation. The treatment of male CD-1 mouse hepatocytes for 4 days with 5-50 µM PBO, 10-1000 µM NaPB and 25 ng/mL epidermal growth factor (EGF) resulted in significant increases in hepatocyte RDS. While treatment of hepatocytes from one male and one female human donor with 5-500 µM PBO and 10-1000 µM NaPB for 4 days had no effect on hepatocyte RDS, treatment with EGF resulted in significant increases in RDS in both human hepatocyte preparations. In summary, PBO is predominantly a hepatic CAR activator at carcinogenic dose levels in CD-1 mice, with activation of hepatic CAR resulting in a suppression of the effect of PBO on hepatic PPARα. A robust MOA for PBO-induced mouse liver tumour formation has been established, this MOA being similar to that previously identified for NaPB and some other rodent liver CAR activators. Based on the lack of effect of PBO on RDS in human hepatocytes, it is considered that the MOA for PBO-induced mouse liver tumour formation is qualitatively not plausible for humans.

Transcriptome analysis provides new insights into liver changes induced in the rat upon dietary administration of the food additives butylated hydroxytoluene, curcumin, propyl gallate and thiabendazole.

Transcriptomics was performed to gain insight into mechanisms of food additives butylated hydroxytoluene (BHT), curcumin (CC), propyl gallate (PG), and thiabendazole (TB), additives for which interactions in the liver can not be excluded. Additives were administered in diets for 28 days to Sprague-Dawley rats and cDNA microarray experiments were performed on hepatic RNA. BHT induced changes in the expression of 10 genes, including phase I (CYP2B1/2; CYP3A9; CYP2C6) and phase II metabolism (GST mu2). The CYP2B1/2 and GST expression findings were confirmed by real time RT-PCR, western blotting, and increased GST activity towards DCNB. CC altered the expression of 12 genes. Three out of these were related to peroxisomes (phytanoyl-CoA dioxygenase, enoyl-CoA hydratase; CYP4A3). Increased cyanide insensitive palmitoyl-CoA oxidation was observed, suggesting that CC is a weak peroxisome proliferator. TB changed the expression of 12 genes, including CYP1A2. In line, CYP1A2 protein expression was increased. The expression level of five genes, associated with p53 was found to change upon TB treatment, including p53 itself, GADD45alpha, DN-7, protein kinase C beta and serum albumin. These array experiments led to the novel finding that TB is capable of inducing p53 at the protein level, at least at the highest dose levels employed above the current NOAEL. The expression of eight genes changed upon PG administration. This study shows the value of gene expression profiling in food toxicology in terms of generating novel hypotheses on the mechanisms of action of food additives in relation to pathology.

Effect of Pyrethrins on cytochrome P450 forms in cultured rat and human hepatocytes.

High doses of Pyrethrins produce liver and thyroid gland tumours in rats by modes of action involving the induction of hepatic xenobiotic metabolising enzymes. The aim of this study was to compare the effects of Pyrethrins with those of the rat liver and thyroid tumour promoter sodium Phenobarbital on some cytochrome P450 (CYP) forms in cultured rat and human hepatocytes. The treatment of female Sprague-Dawley rat and human (both male and female) hepatocytes for 72 h with 0-1000 microM Pyrethrins and 0-1000 microM Phenobarbital did not result in any marked cytotoxicity. In rat hepatocytes both Pyrethrins and Phenobarbital produced an induction of 7-benzyloxy-4-trifluoromethylcoumarin O-debenzylase activity (a CYP1A/2B form marker) and CYP2B1 and CYP2B1/2 mRNA levels. Pyrethrins and Phenobarbital also induced CYP3A-dependent testosterone 6beta-hydroxylase activity in rat hepatocytes. In human hepatocytes Pyrethrins and Phenobarbital induced both testosterone 6beta-hydroxylase activity and CYP3A4 mRNA levels and also increased CYP2B6 mRNA levels. The effects of Pyrethrins and Phenobarbital were concentration-dependent and exhibited a threshold. These results demonstrate that the effects of Pyrethrins on CYP forms in cultured rat and human hepatocytes are qualitatively similar to those of Phenobarbital. Pyrethrins induce CYP2B and CYP3A forms in cultured rat hepatocytes and can induce CYP3A and CYP2B forms in human hepatocytes. While CYP form induction by Pyrethrins, Phenobarbital and related compounds can be associated with liver and thyroid gland tumour formation in rodents, epidemiological data for Phenobarbital suggests that such effects do not occur in humans.

Effect of thiabendazole on some rat hepatic xenobiotic metabolising enzymes.

The effect of thiabendazole (TB) on some rat hepatic xenobiotic metabolising enzymes has been investigated. Male Sprague-Dawley rats were fed control diet or diets containing 102-5188 ppm TB for 28 days. As a positive control for induction of hepatic xenobiotic metabolism, rats were also fed diets containing 1457 and 10,155 ppm butylated hydroxytoluene (BHT). Treatment with TB and BHT resulted in dose-dependent increases in relative liver weight. TB was found to be a mixed inducer of cytochrome P450 (CYP) forms in the CYP1A and CYP2B subfamilies. The administration of high doses of TB resulted in the induction of 7-ethoxyresorufin O-deethylase and 7-pentoxyresorufin O-depentylase activities, CYP1A1, CYP1A2, CYP2B1 and CYP2B1/2 mRNA levels and CYP1A2 and CYP2B1/2 apoprotein levels. In contrast, BHT was a CYP2B form inducer, increasing 7-pentoxyresorufin O-depentylase activity, CYP2B1 and CYP2B1/2 mRNA levels and CYP2B1/2 apoprotein levels. Both TB and BHT induced GSH S-transferase activities towards a range of substrates. In addition, TB and BHT markedly induced GSTP1 mRNA levels, but had only a small effect on GSTT1 mRNA levels. In summary, these results demonstrate that TB induces both phase I and II xenobiotic metabolising enzymes in rat liver.