The distribution of UDP-glucuronosyltransferases in rat liver parenchymal and nonparenchymal cells

Isoform-specific expression and induction of udp-glucuronosyltransferase in immunoactivated peritoneal macrophages of the rat

Phase I drug-metabolizing enzymes such as cytochrome P450 in immunocytes are known to play a role in metabolic activation of toxic and immunosuppressive compounds such as polycyclic aromatic hydrocarbon (PAH). UDP-glucuronosyltransferase (UGT), a drug-metabolizing phase II enzyme, accelerates elimination of these compounds; however, there is little information on the expression and function of UGT in immunocytes. In this study, we investigated the expressions of UGT isoforms in rat peritoneal macrophages and the role of UGT in macrophage functions. Expressions of UGT1A1, 1A6, and 1A7 were observed in macrophages by immunohistochemical staining and reverse transcriptase-polymerase chain reaction. When macrophage cells cultured in plates were exposed to 1-naphthol and 3-hydroxybenzo-[a]pyrene (3-OH-B[a]P), these glucuronides increased in the medium, indicating that macrophages glucuronidated the chemicals. The production of the glucuronides of 1-naphthol and 3-OH-B[a]P was induced by lipopolysaccharide (LPS) treatment of the cultured macrophage cells. Northern blot analysis revealed that UGT1A7 mRNA was induced by LPS treatment. This result is the first evidence that a drug-metabolizing enzyme is induced by immunoactivation. The results indicated that macrophages can detoxify various toxic and immunosuppressive compounds with UGT, and that ability is enhanced by immunoactivation. We propose that macrophages contribute to protection against not only macromolecules as immunocytes but also small molecules such as the immunosuppressive agents PAHs in peripheral blood and interstitial tissues.

ASSESSMENT OF CATECHOL INDUCTION AND GLUCURONIDATION IN RAT LIVER MICROSOMES

Catechols are substances with a 1,2-dihydroxybenzene group from natural or synthetic origin. The aim of this study was to determine whether catechols (4-methylcatechol, 4-nitrocatechol, 2,3-dihydroxynaphthalene) and the antiparkinsonian drugs, entacapone and tolcapone, at doses 150 to 300 mg/kg/day, for 3 days, are able to enhance their own glucuronidation. The induction potency of catechols on rat liver UDP-glucuronosyltransferases (UGTs) was compared with that of a standard polychlorinated biphenyl (PCB) inducer, Aroclor 1254. The glucuronidation rate of these catechols was enhanced up to 15-fold in the liver microsomes of PCB-treated rats, whereas treatment with catechols had little effect. Entacapone, tolcapone, 4-methylcatechol, catechol, 2,3-dihydroxynaphthalene, and 4-nitrocatechol were glucuronidated in control microsomes at rates ranging from 0.12 for entacapone to 22.0 nmol/min/mg for 4-nitrocatechol. Using 1-naphthol, entacapone, and 1-hydroxypyrene as substrates, a 5-, 8-, and 16-fold induction was detected in the PCB rats, respectively, whereas the catechol-induced activities were 1.1- to 1.5-fold only. Entacapone was glucuronidated more efficiently by PCB microsomes than by control microsomes (Vmax/Km, 0.0125 and 0.0016 ml/min/mg protein, respectively). Similar kinetic results were obtained for 1-hydroxypyrene. The Eadie-Hofstee plots suggested the contribution of multiple UGTs for the glucuronidation of 1-hydroxypyrene (Km1, Km2, Km3 = 0.8, 9.7, and 63 microM, and Vmax1, Vmax2, Vmax3 = 11, 24, and 55 nmol/min/mg, respectively), whereas only one UGT could be implicated in the glucuronidation of entacapone (Km = 130 microM, Vmax = 1.6 nmol/min/mg). In conclusion, catechols are poor inducers of their own glucuronidation supported by several UGT isoforms. Their administration is unlikely to affect the glucuronidation of other drugs administered concomitantly.

Presence of phenol UDP-glucuronosyltransferase in bovine alveolar macrophages and bronchial epithelial cells

Pulmonary organ and cells are the primary target of atmospheric pollutants. Phenol UDP-glucuronosyltransferase (UGT) was found in bovine alveolar macrophage cells by immunohistochemical staining and also was observed in bronchial epithelial cells of the lung. A high level of activity of UGT, which is one of the phase II drug-metabolizing enzymes, toward 1-naphthol was observed in the microsomes of both cell types. By Western blotting analysis, a 54-kDa band was detected in alveolar macrophage cells and in bovine lung using polyclonal antibodies against a purified rat UGT, which catalyze the glucuronidation of various phenolic xenobiotics such as 1-naphthol and have the same molecular mass (54 kDa). Reverse transcriptase-polymerase chain reaction (RT-PCR) amplified the common cDNA region in UGT1A subfamily isoforms, indicating that UGT1A subfamily isoform was expressed in alveolar macrophages and in bronchial epithelial cells of the lung. These results suggest that phenol UGT act as a primary barrier against various phenolic chemicals in the lung.

Nonresponsiveness to 2,3,7,8-tetrachlorodibenzo-p-dioxin of transforming growth factor beta1 and CYP 1A1 gene expression in rat liver fat-storing cells

Exposure to the potent tumor promoter 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) or related agonists of the aryl hydrocarbon receptor (AhR) can result in hepatocarcinogenesis in rodents. Changes in the expression and/or the level of growth factors may be a critical event in TCDD-mediated hepatocarcinogenesis. In this study, the influence of TCDD, the most potent AhR agonist, on the expression of transforming growth factor beta1 (TGF beta1), an inhibitor of hepatocellular proliferation synthesized in fat-storing cells (FSC) of the liver, was investigated in Wistar rats of both sexes in vitro and ex vivo. FSC were isolated from rat liver, cultured, and treated with 10(-10) and 10(-8) M TCDD, respectively, and TGF beta1 gene expression was determined at the levels of mRNA and protein. Furthermore, adult rats were treated with TCDD (10 micrograms/kg body wt, given by a single ip injection), FSC were isolated, and TGF beta1 gene expression was analyzed at different time points. Exposure to TCDD had no effect on the expression of TGF beta1 either at the RNA or at the protein level. Surprisingly, expression of CYP1A1, an AhR-regulated gene, was also not detectable either in untreated FSC or after TCDD treatment in vitro or ex vivo. Western blot analysis revealed that the lack of TCDD responsiveness of CYP1A1 is due to the absence of detectable amounts of the AhR in FSC. Based on these results we conclude that FCS may be the only liver cell type that lacks AhR-dependent inducibility of drug metabolism.

Distribution and induction of aflatoxin B1-9a-hydroxylase activity in rat liver parenchymal and non-parenchymal cells

Chronic administration of aflatoxin B1 (AFB1) to rats gives rise to hepatocellular and cholangiocellular carcinomas without affecting Kupffer and endothelial cells. The enzymatic conversion of AFB1 to AFB1-8,9-epoxide is the critical step in the activation of the myocotoxin, while the conversion of AFB1 to aflatoxin M1 (AFM1), catalyzed by the AFB1-9a-hydroxylase, is considered to be a detoxication route for the toxin. In the present study the distribution and inducibility of AFB1-9a-hydroxylase were analyzed in microsomes derived from freshly isolated liver parenchymal (PC) and nonparenchymal cells (i.e. Kupffer + endothelial cells, NPC). AFB1-9a-hydroxylase activity was clearly measurable in NPC and similar to that of PC. In NPC the rate of formation of AFM1 was higher (when incubating with 16 microM AFB1) than or similar (with 128 microM AFB1) to that of AFB1-8,9-epoxide, while in PC it was significantly lower. Taken together, these results suggest that the AFB1-9a-hydroxylase activity might be particularly important in NPC to protect these cells from AFB1 by converting it to a significantly less mutagenic metabolite and by reducing the amount of AFB1 available for epoxidation. Furthermore, it is shown that AFB1-9a-hydroxylase activity is inducible by phenobarbital (only in PC), 3-methylcholanthrene, isosafrole and Aroclor 1254, thus indicating that in rat liver the conversion of AFB1 to AFM1 is catalyzed by members of the cytochrome 1A and 2B families.

Different biotransformation of morphine in isolated liver cells from guinea pig and rat

The biotransformation of morphine was characterized in freshly isolated parenchymal and non-parenchymal liver cells from rats and guinea pigs in suspension culture to establish an in vitro model for morphine metabolism. Liver cells were prepared by a collagenase perfusion technique, and separated by differential centrifugation. Morphine metabolism was investigated at different concentrations (1, 5, 100 and 200 microM). Samples were taken repeatedly during 2-4 hr of incubation, and subsequently analysed on a HPLC system employing both UV and electrochemical detection. In suspensions of hepatocytes from both animal species morphine-3-glucuronide (M3G) was the major metabolite of morphine, and was formed at comparable rates at all concentrations examined. Guinea pig hepatocytes in addition produced considerable quantities of morphine-6-glucuronide (M6G), whereas this metabolite was detected only intracellularly in minor quantities in rat hepatocytes. The ratio between the two morphine glucuronides (M3G/M6G) in suspensions of guinea pig hepatocytes was approximately 4:1. N-Demethylation of morphine was more pronounced per mg cell protein in rat hepatocytes compared to guinea pig cells. Metabolic activity towards morphine was not detected in non-parenchymal cells of the two species. The morphine glucuronidation pattern found in guinea pig hepatocytes resembles to a greater extent than that found in rat hepatocytes the pattern found in in vivo studies of humans. It was concluded that isolated guinea pig parenchymal cells appeared to be a promising in vitro system for studies of morphine glucuronidation, and to observe metabolism in general.

Zonation of acetaminophen metabolism and cytochrome P450 2E1-mediated toxicity studied in isolated periportal and perivenous hepatocytes

To study the mechanism of centrilobular damage developing in the centrilobular region after high doses of acetaminophen (APAP), its metabolism and toxicity were compared in periportal and perivenous hepatocytes isolated by digitonin/collagenase perfusion. Contrary to earlier reports, based on perfusions, no evidence for a periportal dominance of APAP sulfation could be observed. Glucuronidation, the dominant pathway of conjugation at high (5 mM) APAP concentration, was faster in perivenous cells. During primary culture, prolonged exposure (> or = 24 hr) to 5 mM APAP damaged perivenous cells, with a higher P450 2E1 level than periportal cells. When cells were isolated from ethanol-pretreated rats, to induce P450 2E1 levels specifically in the perivenous region, perivenous hepatocytes exhibited enhanced APAP vulnerability and extensive glutathione depletion. In contrast, corresponding periportal cells retained good viability. Isoniazid, an inhibitor of cytochrome P450 2E1, protected cells against APAP toxicity and prevented glutathione depletion. Induction of P450 2E1 also caused a 3-fold increase in the covalent binding of reactive intermediates from [14C]APAP, and this increase was mainly confined to perivenous cells. These results indicate that in rat liver there is only slight perivenous zonation of APAP conjugation and suggest that zone-specific APAP activation, mediated by the regional expression of ethanol-inducible cytochrome P450 2E1, is responsible for the characteristic centrilobular liver damage elicited by APAP.