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Biomedical subjects

K W Bock

Publications and source records attributed to K W Bock.

At least 55 records · Page 3Linked to original sources
PubMed1994

2,3,7,8-Tetrachlorodibenzo-p-dioxin as growth modulator in mouse hepatocytes with high and low affinity Ah receptor.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a highly potent tumor promoter in rodent liver, has been shown to increase epidermal growth factor (EGF)-stimulated DNA synthesis in rat hepatocytes in primary culture, a comitogenic effect possibly linked to the mechanism of liver tumor promotion. In hepatocytes isolated from male congenic mice (C57BL/6J) with high-affinity (AhbAhb) or low-affinity (AhdAhd) Ah receptor the association of the comitogenic action of TCDD with the Ah receptor was investigated. The potency of TCDD as inducer of CYP1A-catalyzed 7-ethoxyresorufin O-deethylase (EROD) activity was approximately 10-fold lower in AhdAhd compared to AhbAhb cells. In both cell types, TCDD did not stimulate DNA synthesis in the absence of EGF. When added together with EGF, TCDD exhibited two opposing effects on DNA synthesis, measured as incorporation of [3H]thymidine into DNA: (i) At 3 x 10(-14) M, TCDD increased EGF-stimulated DNA synthesis approximately 1.4-fold in AhbAhb but not in AhdAhd cells at a plating density of 35,000 cells/cm2. In hepatocytes from AhdAhd mice, 3 x 10(-12) M TCDD was required to elicit a similar comitogenic response. (ii) At a density of 10,000 cells/cm2, 3 x 10(-12) M TCDD had a pronounced inhibitory effect on EGF-stimulated DNA synthesis in AhbAhb but not in AhdAhd cells. Essentially similar results were obtained by counting of 5-bromo-2'-deoxyuridine-labeled nuclei. These findings demonstrate that TCDD can enhance or antagonize EGF-stimulated DNA synthesis in mouse hepatocytes modified by the cell density. The different concentration-response relationships in hepatocytes from both strains suggest that the Ah receptor regulates these responses.

Animals↗
PubMed1994

Consequences of 3-methylcholanthrene-type induction for the metabolism of 4-aminobiphenyl in isolated rat hepatocytes.

Carcinogenic aromatic amines such as 4-aminobiphenyl (4-ABP) are extensively metabolized by both oxidative and conjugation reactions. Thus the burden of genotoxic metabolites of 4-ABP in a target organ is probably influenced by the balance of N-hydroxylation and alternative metabolic pathways in the hepatocyte. In freshly isolated rat hepatocytes, 4-ABP (at a substrate concentration of 10 microM) was mainly N-acetylated (54% of total metabolites), while 2% N-hydroxy-4-ABP-N-glucuronide and 21% of unconjugated N-hydroxylated metabolites were detectable. Ring-hydroxylated metabolites and the primary N-glucuronide of 4-ABP accounted for 8% and 4%, respectively. Pretreatment of rats with 3-methylcholanthrene (MC), a dioxin-type inducer of CYP1A isozymes and phenol UDP-glucuronosyltransferase (UGT1A1), led to a dramatic decrease of N-acetylated (2% of total metabolites) and an increase of N-hydroxylated (54% as free and glucuronidated compound) and ring-hydroxylated (35%) metabolites. Essentially similar effects were seen at a substrate concentration of 50 microM. Consistently, MC-type induction with beta-naphthoflavone resulted in a significant increase in the formation of DNA adducts of 4-ABP, detected by 32P-postlabeling of hepatocellular DNA. The results suggest that, similar to a previous study with 2-naphthylamine (2-NA), MC treatment leads to a marked shift from conjugation to N-oxidation. However, N-hydroxy-4-ABP (in contrast to N-hydroxy-2-NA) is mostly released from hepatocytes in the unconjugated form.

Aminobiphenyl Compounds↗
PubMed1994

Promotion of preneoplastic foci in rat liver with 2,3,7,8-tetrachlorodibenzo-p-dioxin, 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin and a defined mixture of 49 polychlorinated dibenzo-p-dioxins.

In a two-stage initiation-promotion experiment the hypothesis was investigated that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) equivalents (TE), calculated from data of CYP1A induction in hepatocytes in primary culture, or international TCDD equivalents (ITE) are useful for evaluating the tumor-promoting potency of 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HpCDD) and of a defined mixture (M2) of 49 polychlorinated dibenzo-p-dioxins (PCDDs) in comparison with TCDD. Therefore, female Wistar rats were treated with an initiating dose of N-nitrosomorpholine, and subsequently received daily doses of 2, 20 and 200 ng TCDD/kg or equivalent doses of HpCDD or M2, based on TE values. After a promotion phase of 13 weeks, hepatic PCDD levels, CYP1A activity and the relative hepatic volume of adenosinetriphosphatase-negative or glutathione S-transferase P-positive preneoplastic foci were determined. After logarithmic transformation, linear PCDD level-response relationships were obtained for induction of CYP1A activity with TCDD, HpCDD or M2. Based on TE values, inducing potencies of both HpCDD and M2 were over-estimated at higher doses, whereas induction was approximately equivalent at the lowest dose. The best fit of PCDD level-response relationships of relative hepatic volumes of preneoplastic lesions was achieved using a four-parameter logistic model. Significantly different functions were calculated for promotion with TCDD or HpCDD. It is concluded that (i) different PCDD level-response relationships exist for the induction of hepatic CYP1A activity and the promotion of preneoplastic liver foci, and (ii) that TE or ITE factors provide only a rough estimate of the tumor-promoting potency of a PCDD mixture but may overestimate the risk from exposure to higher-chlorinated 2,3,7,8-substituted congeners such as HpCDD.

Animals↗
PubMed1994

Effects of 2,3,7,8-tetrachloro- and 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin on the proliferation of preneoplastic liver cells in the rat.

Using an initiation-promotion system, enzyme-altered putative preneoplastic liver foci were induced in female Wistar rats by application of diethylnitrosamine (10 mg/kg/day) for 5 days, followed by bi-weekly treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; corresponding to 100 ng/kg/day) or 1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin (HCDD; corresponding to 5 micrograms/kg/day) for up to 17 weeks. Groups of animals were killed at various time intervals after start of promoter treatment. For evaluation of DNA synthesis, 5-bromo-2'-deoxyuridine was administered 24 h prior to killing the animals. Quantitative analysis of the number and volume fraction of adenosine-triphosphatase-deficient liver foci revealed that the promoting activity of both dioxins was roughly comparable under the experimental conditions employed. Nuclear labelling indices (LIs) of normal hepatocytes were not altered by TCDD or HCDD treatment, while a slight increase in LIs of non-parenchymal liver cells was observed. Using an immunohistochemical double-staining technique, hepatocytes within glutathione-transferase P-positive liver foci were found to show an approximately 5-to 10-fold higher LI than normal hepatocytes throughout all periods of investigation. During the time course of the experiment, LIs of foci from all treatment groups decreased with time. However, in TCDD-treated rats, and less pronounced in HCDD-treated rats, the initially high rate of proliferation persisted for a greater length of time than in non-dioxin-treated control animals. Assignment of liver foci into four transection size classes revealed that LIs in larger size classes varied considerably, indicating heterogeneity in the growth behaviour of individual liver lesions. Overall, both dioxins had no effects on the proliferation of normal hepatocytes, while LIs of enzyme-altered liver lesions were slightly enhanced by treatment with TCDD or HCDD. Whether the selective, albeit moderate increase in the proliferation of enzyme-altered liver cells is sufficient to explain the promoting activity of dioxins, or if additional factors (e.g. decrease in death rates of foci cells) are equally important, remains to be determined in further experiments.

Animals↗
PubMed1994

Glucuronidation of carcinogenic arylamines and their N-hydroxy derivatives by rat and human phenol UDP-glucuronosyltransferase of the UGT1 gene complex.

Since carcinogenic arylamines are sequentially oxidized and conjugated with glucuronic acid, differences in glucuronidation may critically determine the toxic potential of these compounds. Therefore, N-glucuronidation of 1- and 2-naphthylamine (1-NA and 2-NA),4-aminobiphenyl(4-ABP) and their N-hydroxy derivatives was investigated using rat and human liver microsomes and V79 cell-expressed phenol UDP-glucuronosyltransferases (UGT) of the UGT1 gene complex. Cell-expressed UGTs included rat and human UGT1.6, which are known to conjugate planar phenols, and human UGT1.7, conjugating both planar and bulky phenol. (i) N-Glucuronidation of 1- and 2-NA and of N-hydroxy-2-NA was inducible by 3-methylcholanthrene in rat liver microsomes whereas N-glucuronidation of the bulky arylamines 4-ABP and N-hydroxy-4-ABP was not. In support of these findings mutagenicity of N-hydroxy-2-NA in the Ames test was markedly reduced upon addition of UDP-glucuronic acid using liver homogenates from 3-methylcholanthrene-treated rats. (ii) With cell-expressed rat UGT1.6, non-carcinogenic 1-NA was conjugated with the highest rate and with higher affinity than 2-NA. UGT1.6 showed poor activity towards N-hydroxy-4-ABP and 4-ABP. (iii) Substrate specificity of human UGT1.6 also appeared to be limited to planar 1-NA, 2-NA and its N-hydroxy derivative, whereas UGT1.7 showed broader substrate specificity, including the bulky arylamine 4-ABP and its N-hydroxy derivative. The results suggest marked differences in substrate specificity of different UGT isozymes for arylamines and their N-hyroxy derivatives.

1-Naphthylamine↗
PubMed1994

The influence of environmental and genetic factors on CYP2D6, CYP1A2 and UDP-glucuronosyltransferases in man using sparteine, caffeine, and paracetamol as probes.

The impact of gender, use of oral contraceptive steroids (OCS), coffee consumption and of smoking on the metabolism of sparteine, caffeine, and paracetamol was studied in 194 randomly selected subjects (98 male and 95 female). Thirty-eight of the male volunteers were cigarette smokers, 40 of the female subjects were smokers and/or users of OCS. The metabolic ratio of sparteine oxidation (MRs) showed a trimodal distribution. 7.7% of the subjects had a MRs > 20 and thus were poor metabolizers (PMs). Within the extensive metabolizer (EM) subjects, a distinct subgroup accounting for 11% was observed with 20 > MRs > 1.2. Six of the 15 phenotypical PMs were heterozygous EMs by genotyping. This indicates the existence of one or several CYP2D6 mutations which cannot be identified by the currently employed genotyping methods. In each subgroup, i.e. smokers/OCS and non-smokers/non-OCS, the cumulative frequency distribution of the heterozygous (wt/B) phenotype caused a shift to higher MRs compared with the wild-type homozygotes (wt/wt). Thus, for the in vivo activity of CYP2D6, genetic determinants prevail over environmental factors. Smoking, use of oral contraceptive steroids, caffeine consumption, or gender had no influence on sparteine metabolism. The distribution of the paracetamol glucuronide/paracetamol metabolic ratio appeared to be unimodal although skewed. Glucuronidation capacity was clearly affected by gender, OCS use and smoking. It was higher in male than in female subjects. Male smokers had the highest, and female non-smokers/non-OCS users the lowest metabolic ratio. CYP1A2 activity, as determined by a caffeine metabolic ratio ((AFMU + 1X + 1U)/1, 7U), was multimodally distributed and was clearly increased in smokers. It was significantly correlated to paracetamol glucoronidation in male heavy smokers (r=0.85), suggesting an element of co-regulation of CYP1A2 and of paracetamol conjugating UDP-glucuronosyltransferase isozymes, including UGTI.6.

Acetaminophen↗
PubMed1993

Stereoselective glucuronidation of (R)- and (S)-naproxen by recombinant rat phenol UDP-glucuronosyltransferase (UGT1A1) and its human orthologue.

Recombinant rat phenol UDP-glucuronosyltransferase (UGT1A1) conjugates (R)-naproxen at a much higher rate (> 17-fold) than its (S)-enantiomer, substantiating previous findings on stereoselective glucuronidation of racemic naproxen. In contrast, the recombinant human orthologue conjugated both enantiomers at equal rates. In line with high constitutive expression of UGT1A1 in extrahepatic tissues, a high R/S ratio of naproxen glucuronidation was found in rat testes, intestine, lung and kidney. The results demonstrate that (R)-naproxen represents a stereoselective substrate of rat UGT1A1, but not of the human orthologous UGT1A1.

Animals↗
PubMed1993

Paracetamol glucuronidation by recombinant rat and human phenol UDP-glucuronosyltransferases.

Stably expressed human and rat phenol UDP-glucuronosyltransferases (UGTs) of the UGT1 complex (HlugP1, HlugP4 and 3-methylcholanthrene-inducible rat UGT1A1, the latter considered to be an orthologous enzyme to HlugP1) have been used to investigate the role of UGTs in paracetamol glucuronidation. Kinetic analysis of recombinant UGTs was compared to that of total UGT activities in liver microsomes. Paracetamol was found to be an overlapping substrate of several UGTs. It shows higher affinity for HlugP1 and rat UGT1A1 (apparent Km values of 2 and 3 mM, respectively) than for HlugP4 (Km = 50 mM) and other UGTs present in liver microsomes (Km values of > 12 mM). Glucuronidation of paracetamol with HlugP1 contrasts with that of 6-hydroxychrysene and of 4-methylumbelliferone, which are conjugated with higher affinity by HlugP4 than by HlugP1. Due to the wide tissue distribution of rat UGT1A1, paracetamol glucuronidation was also investigated in extrahepatic rat and human tissues. Paracetamol UGT activity was present and inducible by 2,3,7,8-tetrachlorodibenzo-p-dioxin in rat kidney, lung and spleen. It was also detected in human kidney. A selective cDNA probe for exon 1 of HlugP1 cross-reacted with mRNA from both human liver and kidney. The results demonstrate that paracetamol is conjugated by HlugP1 and its rat orthologue UGT1A1 with higher affinity than by HlugP4 and other UGTs.

Acetaminophen↗
PubMed1993

Immunohistochemical localization of the cytochrome P450 isozymes LMC2 and LM4B (P4501A1) in 2,3,7,8-tetrachlorodibenzo-p-dioxin-treated zebrafish (Brachydanio rerio).

Dibenzo-p-dioxins are persistent pollutants in our environment that exert a variety of biological and toxic effects in various species, in particular developmental toxicity in fish. Using zebrafish (Brachydanio rerio) as a model system, we analyzed the expression and inducibility of a constitutive isozyme (LMC2) and a dioxin-inducible isozyme (LM4B or P4501A1) of cytochrome (cyt.) P450 in different organs by immunohistochemical techniques. In untreated zebrafish, LMC2 was constitutively expressed at high levels in the liver, kidney, skin, and oral mucosa, while moderate expression was detected in gills, pseudobranch, intestine, and ovaries. LM4B was either not expressed in these organs or was found at comparatively low levels. After treatment of zebrafish with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; total dose of 15 ng per fish), LM4B expression was strongly induced in livers and kidneys and less pronounced in gills and pseudobranches, whereas no or only marginal induction was seen in skin, intestine, oral mucosa, and ovaries. In addition, cyt. P450 catalyzed 7-ethoxyresorufin-O-deethylation and phenol UDP-glucuronosyltransferase activity were also found to be inducible in liver and kidney. Our results demonstrate that zebrafish respond to inducing stimuli of TCDD in a fashion similar to that observed with related compounds in other fish species, suggesting that zebrafish may be a useful experimental model for studying biological and toxic effects of TCDD and other environmental pollutants in fish.

Animals↗
PubMed1992

Assessment of biological activities of mixtures of polychlorinated dibenzo-p-dioxins (PCDDs) and their constituents in human HepG2 cells.

Dose-response curves of the induction of P4501A1-dependent 7-ethoxyresorufin O-deethylase (EROD) were analyzed in human hepatoma HepG2 cells treated with defined mixtures of polychlorinated dibenzo-p-dioxins (PCDDs) and their 2,3,7,8-substituted constituents, similar to previous studies with rat hepatocytes and H4IIE cells (Schrenk et al. 1991). PCDDs appear to act less potent in human HepG2 cells in comparison with rat cells. For example, EC50 values of 2,3,7,8-Cl4DD were 8-fold and 19-fold higher than in rat H4IIE cells and hepatocytes, respectively. EC50 values of PCDDs were compared with that of 2,3,7,8-Cl4DD and expressed as 2,3,7,8-Cl4DD equivalents (TEs). Although the rank order of PCDD potencies was similar, TEs for some PCDDs (1,2,3,7,8-Cl5DD; TE = 0.75 and 1,2,3,7,8-Cl6DD; TE = 0.61) were found to be higher than in the rat system. In contrast to rat cells no significant induction of EROD could be detected with Cl8DD in HepG2 cells up to its limit of solubility. Experimentally determined TEs of PCDD mixtures containing 49 constituents were found to be largely due to additive effects of their 2,3,7,8-substituted constituents.

Animals↗
PubMed1992

Metabolism of 1- and 2-naphthylamine in isolated rat hepatocytes.

The liver probably plays a major role in the metabolic activation of the bladder carcinogen 2-naphthylamine (2-NA) and in the inactivation of the non-carcinogenic isomer 1-naphthylamine (1-NA). However, metabolic profiles of these compounds (including primary metabolites and directly determined conjugates) in hepatocytes are not available. Therefore metabolism of 1- and 2-NA was compared in freshly isolated hepatocytes from 3-methylcholanthrene (MC)-treated and untreated rats. At 10 microM, 2-NA was found to be mainly N-acetylated (66% of total metabolites after 1 h incubation) and N-glucuronidated (19%). Minor pathways led to C-oxidation (7%) and N-oxidation (3%; 2% present as the N-glucuronide). In hepatocytes from MC-treated rats total metabolism was slightly affected (1.5-fold increase). However, C- and N-oxidation were markedly increased (63 and 18% respectively), while N-acetylation and N-glucuronidation were diminished (5 and 2% respectively). Similar experiments were carried out with 1-NA. Its N-glucuronide was the predominant metabolite (68%) followed by the N-acetylated compound (15%) while C-oxidation was low and N-oxidized metabolites could not be detected, even after induction. The results demonstrate that MC treatment markedly shifted 2-NA metabolism from N-acetylation and N-glucuronidation to N- and C-oxidation. In the case of 1-NA metabolism extensive N-glucuronidation together with the lack of N-oxidation may prevent carcinogenesis.

1-Naphthylamine↗
PubMed1992

2,3,7,8-Tetrachlorodibenzo-p-dioxin and ethinylestradiol as co-mitogens in cultured rat hepatocytes.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) acts as a potent liver tumor promoter in female but not in male rats. As a basis for studying mechanisms of growth control by liver tumor promoters, the effects of TCDD, of two congeners and ethinylestradiol have been examined in primary cultures of hepatocytes. The agents alone were relatively ineffective but acted as co-mitogens when DNA synthesis was stimulated by epidermal growth factor (EGF). The co-mitogenic effect of TCDD was only observed in adult animals, which are less sensitive to EGF than juvenile animals. Similar effects were seen with two TCDD congeners (1,2,3,4,6,7,8-heptachlorodibenzo-p-dioxin and octachlorodibenzo-p-dioxin) in the rank order of their affinity to the Ah receptor. The concentration maximum required for their co-mitogenic action (3 x 10(-12) M for TCDD) was lower than that required for enzyme induction. TCDD was not able to overcome the inhibitory action of TGF-beta (1 ng/ml). Ethinylestradiol additively or even synergistically increased the effect of TCDD. The results suggest: (i) co-mitogenic actions of TCDD and congeners are mediated by the Ah receptor. They are elicited at lower concentrations than those required for the induction of drug-metabolizing enzymes. (ii) Estrogens enhance the co-mitogenic actions of dioxins.

Age Factors↗
PubMed1992

Enzyme and immunohistochemical phenotyping of diethylnitrosamine-induced liver lesions of male C3H/He, B6C3F1 and C57BL/6J mice.

Male C3H/He, B6C3F1 and C57BL/6J mice were given a single injection of diethylnitrosamine (20 micrograms/g body wt) on day 15 after birth and animals were killed 17-29 (C3H/He and B6C3F1) and 29-46 weeks (C57BL/6J) after treatment. Carcinogen-induced liver lesions were identified by a deficiency in the marker enzyme glucose-6-phosphatase and the enzymatic phenotypes of these lesions were studied by enzyme and immunohistochemical methods using serial liver sections stained for seven additional histochemical markers. In all three mouse strains, liver lesions were characterized by an increased basophilia and a decreased expression of UDP-glucuronosyl-transferase, microsomal epoxide hydrolase and NADPH-cytochrome P450 reductase, while the cytochrome P450 isoenzymes 1A2, 2C6 and 2E1 were virtually unexpressed. Quantitative analyses revealed that throughout all periods of investigation, on average greater than 70% of the glucose-6-phosphatase-deficient lesions occupying up to 99% of the total volumetric fraction expressed concomitant alterations in at least one of these additional marker stainings. Upon determination of the phenotypic complexity levels, between 70 and 90% of lesions were found to contain alterations in at least six of the markers analysed, while lesions with alterations in less than three markers were comparatively infrequent. In the light of previous observations in the rat liver system, the relative homogeneity of enzyme phenotypes and the apparent lack of time-dependent changes in enzyme expression suggest that the majority of lesions of all three mouse strains possess an increased neoplastic character already from the very early beginning of the carcinogenic process in liver.

Animals↗
PubMed1992

Mono- and diglucuronide formation from chrysene and benzo(a)pyrene phenols by 3-methylcholanthrene-inducible phenol UDP-glucuronosyltransferase (UGT1A1).

Mono- and diphenols of chrysene and benzo(a)pyrene are suspected substrates of a 3-methylcholanthrene (MC)-inducible phenol UDP-glucuronosyltransferase (UGT1A1). Mono- and diglucuronide formation from these compounds was studied in two systems, (a) livers of MC-treated rats (homologous expression) and (b) a Chinese hamster lung fibroblast cell line (V79) containing rat UGT1A1 cDNA and stably expressing this isozyme (heterologous expression). In liver microsomes of MC-treated rats, glucuronidation of 6-hydroxychrysene was stimulated 11-fold by MC treatment. With 3,6-dihydroxychrysene, formation of its 3-hydroxy-6-monoglucuronide and of the diglucuronide was increased 24- and 310-fold, respectively. Induction factors obtained for monoglucuronide formation (but not for diglucuronide formation) were in line with published data on the increase of immunodetectable UGT1A1 protein and of its mRNA. It is suggested that the high induction factors for diglucuronide formation are the result of a combination of the induction of UGT1A1 and facilitated interaction of neighboring UGT1A1 molecules in endoplasmic reticulum membranes. Glucuronidation of 6-hydroxychrysene, 3-hydroxybenzo(a)pyrene, and 3,6-dihydroxybenzo(a)pyrene to their mono- and diglucuronides was clearly detectable in V79 cells expressing UGT1A1. However, conjugation of 3,6-dihydroxychrysene to its monoglucuronides was low and diglucuronide formation was not detectable, suggesting that UGT isozymes other than UGT1A1 are responsible for these reactions.

Animals↗
PubMed1991

Site-specific hypomethylation of c-myc protooncogene in liver nodules and inhibition of DNA methylation by N-nitrosomorpholine.

The protooncogene c-myc was investigated in N-nitrosomorpholine-induced rat liver nodules to elucidate the role of altered DNA methylation in chemical carcinogenesis. Furthermore, Micrococcus luteus DNA and chicken erythrocyte DNA were modified in vitro by reactive metabolites of N-nitrosomorpholine, generated by P450-dependent monooxygenases. The modified DNAs were less methylated in vitro than control DNAs by DNA-(cytosine-5)-methyltransferase (DNA methylase). The DNA methylase assay and 32P-postlabeling analysis revealed lowered levels of DNA methylation in nodular DNA. In nodular tissue, c-myc messenger RNA levels were found to be increased compared to normal liver. DNA methylation analysis using the restriction endonucleases HpaII/MspI indicated hypomethylation in the first intron of c-myc DNA in liver nodules. The results suggest that genotoxic lesions may cause stably inherited, aberrant DNA methylation patterns which may be responsible for site-specific hypomethylation of the c-myc protooncogene in liver nodules.

Animals↗
PubMed1991

Drug metabolizing enzyme activities in rat liver epithelial cell lines, hepatocytes and bile duct cells.

P450-dependent mono-oxygenase and conjugating enzyme activities were studied in rat liver epithelial cells (RLEs) and compared to those in hepatocytes and bile duct cells. Various RLE cell lines were investigated since (a) they are suspected to be derived from cells in the lineage from putative pluripotent stem cells to either hepatocytes or bile duct cells, and (b) they may represent targets of chemical carcinogens. Despite considerable variation between lines, common features were recognized. P450-dependent monooxygenase activities (7-ethoxyresorufin O-deethylase and 7-ethoxycoumarin O-deethylase) were undetectable in all RLEs and bile duct cells, and were uninducible by benz(a)anthracene. In contrast, glucuronosyltransferase (GT), sulfotransferase and GSH transferase activities were clearly detectable. Conjugating enzyme activities increased until confluency of the cell cultures was reached. Under the latter conditions, GT activities towards 4-methylumbelliferone or benzo(a)pyrene-3,6-quinol (substrates of a 3-methylcholanthrene-inducible phenol GT) were similar to those found in hepatocytes or bile duct cells. Using a selective cDNA probe, phenol GT mRNA was clearly detectable in RLE1. In contrast, GT activity towards 4-hydroxybiphenyl was much lower than in hepatocytes or bile duct cells (0.04- and 0.03-fold). Sulfotransferase and GSH transferase activities were also roughly comparable to those found in hepatocytes and in bile duct cells. The results suggest that RLEs and bile duct cells exhibit both high conjugating enzyme activities and a lack of P450-dependent mono-oxygenase activities, a pattern resembling the 'toxin-resistance phenotype' found in putative preneoplastic hepatocyte foci and nodules.

Aging↗