The hepatic metabolism and biliary excretion of benzo[a]pyrene in guinea-pigs fed normal, high-fat or high-cholesterol diets

H1-antihistamines exacerbate high-fat diet-induced hepatic steatosis in wild-type but not in apolipoprotein E knockout mice

We examined the effects of two over-the-counter H1-antihistamines on the progression of fatty liver disease in male C57Bl/6 wild-type and apolipoprotein E (ApoE)-/- mice. Mice were fed a high-fat diet (HFD) for 3 mo, together with administration of either cetirizine (4 mg/kg body wt) or fexofenadine (40 mg/kg body wt) in drinking water. Antihistamine treatments increased body weight gain, gonadal fat deposition, liver weight, and hepatic steatosis in wild-type mice but not in ApoE-/- mice. Lobular inflammation, acute inflammation, and necrosis were not affected by H1-antihistamines in either genotype. Serum biomarkers of liver injury tended to increase in antihistamine-treated wild-type mice. Serum level of glucose was increased by fexofenadine, whereas lipase was increased by cetirizine. H1-antihistamines reduced the mRNA expression of ApoE and carbohydrate response element-binding protein in wild-type mice, without altering the mRNA expression of sterol regulatory element-binding protein 1c, fatty acid synthase, or ApoB100, in either genotype. Fexofenadine increased both triglycerides and cholesterol ester, whereas cetirizine increased only cholesterol ester in liver, with a concomitant decrease in serum triglycerides by both antihistamines in wild-type mice. Antihistamines increased hepatic levels of conjugated bile acids in wild-type mice, with the effect being significant in fexofenadine-treated animals. The increase was associated with changes in the expression of organic anion transport polypeptide 1b2 and bile salt export pump. These results suggest that H1-antihistamines increase the progression of fatty liver disease in wild-type mice, and there seems to be an association between the severity of disease, presence of ApoE, and increase in hepatic bile acid levels.

Gender-specific reduction of hepatic Mrp2 expression by high-fat diet protects female mice from ANIT toxicity

Emerging evidence suggests that feeding a high-fat diet (HFD) to rodents affects the expression of genes involved in drug transport. However, gender-specific effects of HFD on drug transport are not known. The multidrug resistance-associated protein 2 (Mrp2, Abcc2) is a transporter highly expressed in the hepatocyte canalicular membrane and is important for biliary excretion of glutathione-conjugated chemicals. The current study showed that hepatic Mrp2 expression was reduced by HFD feeding only in female, but not male, C57BL/6J mice. In order to determine whether down-regulation of Mrp2 in female mice altered chemical disposition and toxicity, the biliary excretion and hepatotoxicity of the Mrp2 substrate, α-naphthylisothiocyanate (ANIT), were assessed in male and female mice fed control diet or HFD for 4weeks. ANIT-induced biliary injury is a commonly used model of experimental cholestasis and has been shown to be dependent upon Mrp2-mediated efflux of an ANIT glutathione conjugate that selectively injures biliary epithelial cells. Interestingly, HFD feeding significantly reduced early-phase biliary ANIT excretion in female mice and largely protected against ANIT-induced liver injury. In summary, the current study showed that, at least in mice, HFD feeding can differentially regulate Mrp2 expression and function and depending upon the chemical exposure may enhance or reduce susceptibility to toxicity. Taken together, these data provide a novel interaction between diet and gender in regulating hepatobiliary excretion and susceptibility to injury.

Bioavailability and risk assessment of orally ingested polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs) are a family of toxicants that are ubiquitous in the environment. These contaminants generate considerable interest, because some of them are highly carcinogenic in laboratory animals and have been implicated in breast, lung, and colon cancers in humans. These chemicals commonly enter the human body through inhalation of cigarette smoke or consumption of contaminated food. Of these two pathways, dietary intake of PAHs constitutes a major source of exposure in humans. Although many reviews and books on PAHs have been published, factors affecting the accumulation of PAHs in the diet, their absorption following ingestion, and strategies to assess risk from exposure to these hydrocarbons following ingestion have received much less attention. This review, therefore, focuses on concentrations of PAHs in widely consumed dietary ingredients along with gastrointestinal absorption rates in humans. Metabolism and bioavailability of PAHs in animal models and the processes, which influence the disposition of these chemicals, are discussed. The utilitarian value of structure and metabolism in predicting PAH toxicity and carcinogenesis is also emphasized. Finally, based on intake, disposition, and tumorigenesis data, the exposure risk to PAHs from diet, and contaminated soil is presented. This information is expected to provide a framework for refinements in risk assessment of PAHs from a multimedia exposure perspective.

Benzo(a)pyrene toxicokinetics in the cricket following injection into the haemolymph

The metabolic disposition of (14)C-labelled benzo(a)pyrene (BP) in the cricket (Acheta domesticus) was investigated after injection into the haemolymph. (14)C-BP was taken up rapidly by the nerve cord, malpighian tubules, reproductive organs, gut, and muscle:cuticle of the cricket. The elimination half-lives of (14)C-BP in these tissues ranged from 8.9 to 17.8 h. The haemolymph (14)C-BP concentration-time curve could be described by a one-compartment open pharmacokinetic model. (14)C-BP was metabolized by the cricket mainly to unconjugated and conjugated BP metabolites since very little unchanged (14)C-BP was found in the excreta at 48 h post-dosing. GLPC-MSD and HPLC/ES-MS analyses showed the presence of at least two BP metabolites in the excreta. The BP metabolites were identified tenatively as the diol derivatives of benzo(a)pyrene and benzo(a)pyrene quinone.

Effect of dietary lipids on hepatic and intestinal monooxygenases in mice

The effect of dietary lipids on hepatic and intestinal monooxygenases was studied by feeding C57BL/6N mice (for 2 wks) diets containing 5% and 23.5% (wt/wt) olive oil or corn oil. At the end of the feeding period, we measured arylhydrocarbon hydroxylase (AHH) activity in S9 preparations from liver, small intestine, and colon; and, using the same S9 preparations from the liver, we observed the activation of the following three dietary promutagens: 2-amino-3-methylimidazo(4,5-f)quinoline, 2-amino-3,8-dimethylimidazo(4,5-f) quinoxaline, and 2-amino-6-methyldipyrido(1,2-a:3',2'-d)imidazole. The results showed that high-fat diets increased hepatic AHH activity both in corn oil and olive oil diets compared with the low-fat diets; also, a 5% corn oil diet had significantly higher AHH activity compared with the 5% olive oil diet. AHH activity was, respectively, 48.6 +/- 5.1 and 79.5 +/- 11.4 pmol 3OH-benzo[a]pyrene formed/mg/min in the 5% and 23.5% olive oil diets and 66.1 +/- 5.1 and 83.9 +/- 12.2 in the 5% and 23.5% corn oil diets; values are means +/- SE, n = 16. The results also showed a significant increase in the ability of hepatic S9 fractions from animals on high-fat diets to activate promutagens in the Salmonella/plate test. On the contrary, AHH activity in the small intestine and colon was not affected by the fat content of the diet.

Pharmacokinetics of low doses of benzo[a]pyrene in the rat

Intestinal absorption, bioavailability, hepatic and pulmonary extraction and elimination of low doses of benzo[a]pyrene (BP; 0.7-4.4 nmol) were studied in the rat using [G-3H]BP. The hepatic extraction ratio was 0.4 both in a liver perfusion model and in vivo as determined by comparison of intravenous and intraportal infusion experiments in anaesthetized rats. The pulmonary extraction ratio in vivo was 0.11 in control rats and 0.16 in rats pretreated with an inducer of cytochrome P-448. Analysis of BP concentrations in atrial blood and in the bile after continuous BP infusion into the duodenum of anaesthetized rats indicated that at least 30% of the dose must have been absorbed from the gut. Studies have also been performed in conscious rats given BP either as an intravenous bolus or by gavage. The bioavailability was determined to be about 10% in these experiments. Elimination proceeded in a triphasic manner with a half-life of 16.6 hr for the terminal phase.

The intestinal metabolism and DNA binding of benzo[a]pyrene in guinea-pigs fed normal, high-fat and high-cholesterol diets

Strains of intestinal bacteria were capable of deconjugating benzo[a]pyrene metabolites in vitro. The hydrolysis products, and other primary oxidative metabolites of benzo[a]pyrene, were stable to further degradation by the strains tested. Cytochromes P-450 and b5 were detectable in the mucosa of the guinea-pig small intestine, but not in the mucosae of the colon or rectum. The concentrations were unaltered by administration of benzo[a]pyrene and/or the feeding of high-fat or high-cholesterol diets. Benzo[a]pyrene hydroxylase was measurable in the mucosa of the upper intestine, but was present in the lower gut only at very low levels in some animals. The activity was inducible, by oral administration of benzo[a]pyrene, in the small intestinal mucosa of guinea-pigs fed normal diet but not in those fed high-fat and high-cholesterol diets. Low levels of covalent binding of 3H to DNA of liver and gut mucosa were obtained in guinea-pigs dosed orally with 3H-benzo[a]pyrene. Comparison with data for animals given 3H2O suggested that approx. one quarter of the binding was probably due to 3H exchange during metabolism. The feeding of high-fat and high-cholesterol diets did not increase this binding. Guinea-pigs fed high-fat and high-cholesterol diets excreted a greater proportion of an oral dose of 3H-benzo[a]pyrene in urine, and less in faeces than animals fed a normal diet. Due to the low, and apparently non-inducible, levels of benzo[a]pyrene hydroxylase activity and of covalent binding in the colonic mucosa, the administration of benzo[a]pyrene to guinea-pigs fed high-fat or high-cholesterol diets appears unlikely to provide a novel animal model for studies on mechanisms of colon carcinogenesis.