Glucuronidation of benzo[a]pyrene in hamster embryo cells

Glucuronidation: an important mechanism for detoxification of benzo[a]pyrene metabolites in aerodigestive tract tissues

UDP-glucuronosyltransferases (UGTs) have been implicated as important detoxifying enzymes for several major tobacco carcinogens. Because the aerodigestive tract is a primary target for exposure to tobacco smoke carcinogens, the major goal of the present study was to determine whether aerodigestive tract tissues exhibit glucuronidating activity against metabolites of benzo[a]pyrene (BaP) and to explore the pattern of expression of UGT genes in a series of aerodigestive tract tissue specimens. Glucuronidation of the phenolic BaP metabolites 3-, 7-, and 9-hydroxy-BaP was observed in all upper aerodigestive tract tissue microsome specimens tested, as determined by high-pressure liquid chromatography analysis. Glucuronidating activity toward the procarcinogenic BaP metabolite trans-BaP-7,8-dihydrodiol(+/-) was also detected in aerodigestive tract tissues. By semiquantitative duplex reverse transcription-polymerase chain reaction analysis, UGT1A7 and UGT1A10 were shown to be well expressed in all aerodigestive tract tissues examined, including tongue, tonsil, floor of mouth, larynx, and esophagus. UGT1A8 and UGT1A6 were expressed primarily in larynx; no expression was observed for UGTs 1A1, 1A3, 1A4, 1A5, 1A9. Of the family 2B UGTs, only UGT2B4 and UGT2B17 exhibited significant levels of expression in aerodigestive tract tissues. Of the aerodigestive tract-expressing UGTs, only UGTs 1A7, 1A8, and 1A10 exhibited glucuronidating activity against 7-hydroxy-BaP, with UGT1A10 exhibiting the highest affinity as determined by kinetic analysis (K(m) = 49 microM). No UGT expression or glucuronidating activity was observed for any of the lung specimens analyzed in this study. These results suggest that several family 1 UGTs may potentially play an important role in BaP detoxification in the aerodigestive tract.

3-Hydroxybenzo[a]pyrene glucuronidation by cells: a direct fluorometric assay in culture medium

A direct fluorometric assay for measuring the glucuronidation by cultured cells using 3-hydroxybenzo[a]pyrene (less than 1 microM) as substrate is described. The method is based on the different characteristics in the fluorescence spectra of 3-hydroxybenzo[a]pyrene and benzo[a]pyrene-3-glucuronide. The analytical procedure operates directly on the culture medium without extraction steps. The sensitivity of detection is 1 nmol glucuronide/1 mumol substrate.

Effect of 2(3)-tert-butyl-4-hydroxyanisole on benzo[a]pyrene metabolism and DNA-binding of benzo[a]pyrene metabolites in isolated mouse hepatocytes

Benzo[a]pyrene (BP) metabolism and the conjugation and DNA-binding of BP metabolites, was studied using isolated hepatocytes from mice maintained on a diet containing 2(3)-tert-butyl-4-hydroxyanisole (BHA) (7.5 g/kg food) to discover the mechanisms involved in the anticarcinogenic effects of this antioxidant. The antioxidant feeding produced: (a) profound differences in the BP metabolite pattern, (b) no increase in the levels of either the glucuronic acid, the sulfate or the glutathione conjugates and(c) a marked decrease in the level of BP metabolites bound to intracellular DNA. Therefore, the inhibition of DNA-binding observed after administration of BHA, may be due to the change in BP metabolism rather than to an increase in the conjugation of reactive metabolites.

Metabolic activation of benzo[a]pyrene, aflatoxin B1, and dimethylnitrosamine by a human hepatoma cell line

The metabolism of chemical carcinogens by a human hepatoma cell line, huH-1, was studied. The huH-1 line has been derived from a hepatoma of a 57-year-old HBs-antigen carrier and cultivated for several years. The hepatoma cells metabolized about 90% of 5 microM benzo[a]pyrene into water-soluble products within 24 h. Aryl hydrocarbon hydroxylase activity in huH-1 cells was induced to 24 times higher than the basal level by treatment with 13 microM benz[a]anthracene for 24 h. Metabolic activation of benzo[a]pyrene, dimethylnitrosamine and aflatoxin B1 by huH-1 cells was observed by cell-mediated sister-chromatid exchange assay. Sister-chromatid exchanges in human diploid fibroblasts were observed in the cultures mixed with or without huH-1 cells. All 3 chemicals induced sister-chromatid exchanges in human fibroblasts far more efficiently in the cultures mixed with huH-1 cells than in those without huH-1 cells. Some characteristics of huH-1 cells as a human cell-mediated metabolic activation system for carcinogens are discussed.

Genetic differences between C57BL/6 and DBA/2 mice in the inductions of UDP-glucuronyl transferases for 3-hydroxybenzo(a)pyrene, p-nitrophenol, and bilirubin by 3-methylcholanthrene

In C57BL/6 mice, aryl hydrocarbon hydroxylase (AHH) increased 1 day after treatment with 3-methylcholanthrene, and induction of UDP-glucuronyl transferases for 3-hydroxybenzo(a)pyrene, p-nitrophenol and bilirubin in the liver microsomes was observed 2 to 5 days later. In DBA/2 mice, neither AHH nor transferase activities were influenced by 3-methylcholanthrene. These results suggest that induction of activating and detoxicating enzymes is genetically linked.

Effects of asbetos, iron oxide, silica, and carbon black on the microsomal availability of benzo[a]pyrene

Particulate matter and polynuclear aromatic hydrocarbons are known to be cocarcinogenic. Using fluorescence spectroscopy, we determined that adsorption of benzoic] pyrene (BP) to iron oxide, silica, and asbestos (anthophyllite and Canadian chrysotile) results in a greatly enhanced rate of BP uptake into rat liver microsomes when compared to uptake from aqueous dispersions of BP microcrystals. Simple mixtures of BP microcrystals and particulates do not display enhanced microsomal uptake rates, an observation which indicates that adsorption of BP to the surface of the particle is necessary for enhanced microsomal uptake. BP was not released into microsomes from carbon black. Most importantly, the data indicate that asbestos particles are more effective than silica and iron oxide in enhancing the microsomal availability of BP. These observations suggest that particles, and especially the fibrous mineral particulates, could be cocarcinogenic as a result of their ability to adsorb polynuclear aromatic hydrocarbons and to transport these carcinogens into cells. Except for chrysotile, the particles did not disrupt microsomal integrity as determined by NADPH-dependent lipid peroxidation activity. Binding of the microsomes to the particles did not affect the BP uptake rates. In addition, these BP uptake rates were independent of both the concentrations of microsomes and of particles. These observations are consistent with the mechanism of particle-enhanced transport being an increased rate of BP solubilization from the adsorbed state into the aqueous phase, followed by rapid partitioning of BP into the microsomal membranes.

Metabolism of benzo[A]pyrene and the related enzyme activities in hamster embryo cells

The rate of metabolism of benzo[a]pyrene (BP) and changes in related enzyme activities in cultured hamster embryo cells during successive subculture were studied. The activity of aryl hydrocarbon hydroxylase (AHH) was the highest when embryo cells were first dispersed in tissue culture flasks and decreased during subsequent passages. On the other hand, UDP-glucuronyl transferase activity increased gradually during successive subculture. Treatment of the cells with 13 nmol/ml of benz[a]anthracene (BA) for 24 h increased the activity of AHH but not that of UDP-glucuronyl transferase. The metabolism of BP was measured in cells of the passages 1, 3 and 7; metabolism of BP was most efficient in cells in passage 3 and their formation of glucuronic acid conjugates of BP, one of the major metabolites found in the medium, was 3- and 10-fold more than those of cells in passages 1 and 7, respectively. Analysis of BP-metabolites extracted from the medium with ethylacetate showed that the main metabolites were 9,10-diol and 7,8-diol. Phenols and quinones were released by treatment of the medium with beta- glucuronidase and their amounts were larger than those of diols at all passages. These results show that in hamster embryo cells in early passage, BP is metabolized to conjugates of phenols with glucuronic acid.

Metabolism of benzo[A]pyrene in hamster embryo cells. Effect of the concentration of benzo[A]pyrene on its metabolism

The metabolism of benzo[a]pyrene (BP) by hamster embryo cells was studied. The production of water-soluble metabolites, shown to be conjugates with glucuronic acid, depended on BP concentration. With increased BP concentration the amount of glucuronic acid conjugates increased, but the proportion of conjugates in BP or its metabolites present in the medium decreased. The metabolites extracted with ethylacetate were trans-7,8-dihydrodiol-BP (7,8-dihydrodiol) and trans-9,10-dihydrodiol-BP (9,10-dihydrodiol), but large peaks of phenolic metabolites were found by high pressure liquid chromatography (HPLC) after digesting the medium with beta-glucuronidase. Therefore, BP is metabolized to oxygenated forms, and of these, most of the phenolic metabolites and parts of the dihydrodiols are conjugated with glucuronic acid. The proportions of dihydrodiols to phenols, estimated by HPLC after beta-glucuronidase digestion, decreased when the BP concentration was decreased. The results suggest that dihydrodiols are less readily glucuronidated than phenols and so may be metabolized further to metabolites other than glucuronic acid conjugates.