Metabolic degradation of 3,4-benzopyrene in the cultures of normal and neoplastic fibroblasts

The carcinogen benzo(e)pyrene is metabolized by DM15 cells without an uncoupling effect on their gap junctions

Benzo(e)pyrene (B(e)P) promotes carcinogenesis in the skin. Unlike some other promoters however, B(e)P does not produce an uncoupling effect on gap junction permeability in DM15 transformed fibroblasts. This study demonstrates that DM15 cells exhibit a relatively high level of B(e)P metabolism. Moreover, although pretreatment of DM15 cells with benz(a)anthracene results in an 8-fold increase of arylhydrocarbon hydroxylase activity and a 2-fold increase in the rate of B(e)P metabolism, it did not enable B(e)P to affect Lucifer Yellow transfer between DM15 cells. We conclude that neither B(e)P nor its metabolites are capable of uncoupling gap junction permeability in DM15 cells.

The effect of complete carcinogens on intercellular transfer of lucifer yellow in fibroblast culture

The effect on permeability of gap junctions of complete powerful carcinogens, 3-methylcholanthrene (MC), 7,12-dimethylbenz(a)anthracene (DMBA), ethyl methanesulfonate (EMS), and weak carcinogens, benz(a)anthracene (BA), benzo(e)pyrene (B(e)P) as well as the aryl-hydroxylase inhibitor 7,8-benzoflavone (7,8-BF) has been studied with the use of a dye-coupling technique and transformed Djungarian hamster DM15 fibroblasts. MC, EMS and 7,8-BF were found to exert a strong inhibitory effect on cell-to-cell dye transfer. BA and DMBA had the uncoupling activity only in 2 out of 4 experiments. B(e)P was not shown to affect LY transfer between DM15 cells. The uncoupling effect of MC, 7,8-BF and EMS (only when EMS used at the concentration of 600 micrograms/ml but not 1000 micrograms/ml) appeared reversible. The causes of failure to detect DMBA and B(e)P effects on gap junctions 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.

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.

Microsomal aspects of carcinogenesis and neoplasia

Neither immunologic nor genetic concepts of carcinogenesis have yet been decisively confirmed, and epigenetic theories, as formulated so far, are either non-predictive or insufficiently consistent with morphologic and experimental evidence. Computing data, concerned with carcinogenic mechanisms and neoplastic changes at the level of the endoplasmic reticulum, may lead to a new coherent understanding of tumor pathogenesis. Carcinogenic agents initiate biophysical perturbations, chemical alterations and conformational transitions in the membrane lattice of the endoplasmic reticulum. Foremost among the resulting neoplastic changes is an increased, irreversible separation of polyribosomes from membranes of the ergastoplasm. The carcinogenic process, apparently, deletes a protein required for polysome attachment. Since microsomal cytochromes can be synthesized by membrane-bound polysomes only, the translation of genetic information for their biosynthesis is irreversibly restricted. A similar, self-perpetuating deficiency may be postulated for the polysome attachment protein. Activities, depending on cytochromes P-450 and b5, are hampered, e.g. those of the monoxygenase system. Cholesterogenesis is derepressed. Ratios of phospholipids/cholesterol are decreased, and lipid-protein complexes, altered both in structure and function. Another distinct effect of the membrane-polysome separation is the unmasking of thiol-disulfide exchange enzymes which, in turn, stimulate the biosyntehsis of proteins and of deoxyribonucleotides involved in cell replication.

Sulfate conjugation of benzo[alpha]pyrene metabolites and derivates

Sulfate conjugation of benzo[alpha]pyrene(BP) metabolites and derivatives was studied. The reaction sequence consisted of two steps; activation of sulfate ion to 3'-phosphoadenosine-5'-phosphosulfate and transfer of the activated sulfate to the BP-derivatives. Both reactions were carried out by enzymes located in the rat liver 105 000 g supernatant. The reactions required MgCl2. Phenol and quinone derivatives were generally good substrates for sulfate conjugation and different reactivities were observed with the dihydrodiol derivatives. Sulfate conjugates were more polar than their parent BP-derivatives and except for quinone conjugates were easily extracted with ethyl acetate. The role of sulfate conjugation in BP carcinogenesis is discussed.

Glucuronidation of benzo[a]pyrene in hamster embryo cells

The formation of water-soluble metabolites of tritium-labeled benzo[a]pyrene (BP) by cultured hamster embryo cells was studied. The ratio of the radioactivity in the aqueous phase to that in the organic phase increased with the incubation period. After incubation for 48 h with 3.75 nmol/ml of [3H] BP in the medium more than 90% of the 3H-radioactivity was found in the aqueous phase, whereas with 10-fold more BP about half the radioactivity remained in the organic phase. The main metabolites extracted from the medium at 37.5 nmol/ml BP with ethyl acetate by high pressure liquid chromatography (HPLC) were 9,10-diol and 7,8-diol; but after treatment of the medium with beta-glucuronidase the main oxygenated metabolites were phenols, the amount of 9-OH BP being more than that of 3-OH BP. beta-Glucuronidase also released 9,10-diol and 7,8-diol, but most of these diols were in the free form in the medium. The medium from cells treated with 3.75 nmol/ml BP has a quantitatively different profile, and most of the radioactivity obtained by extraction with organic solvent and digestion with beta-glucuronidase was eluted in the regions of phenols. These results show that in hamster embryo cells BP is mainly metabolised to conjugates of phenols with glucuronic acid.

beta-Glucuronidase catalyzed hydrolysis of benzo(a)pyrene-3-glucuronide and binding to DNA

beta-Glucuronidase catalyzes the hydrolysis of benzo[a]pyrene-3-glucuronide to 3-hydroxybenzo[a]pyrene. During the enzymatic hydrolysis, a benzo[a]pyrene derivative is formed which binds to DNA to a far greater extent than either the 3-hydroxybenzo[a]pyrene or its glucuronide. These results suggest that conjugates of benzo(a)pyrene may be converted by beta-glucuronidase at intracellular and organ sites distal to the initial sites of oxygenation and conjugation of benzo(a)pyrene to activated intermediates that are possibly carcinogenic.

Serum dependence of expression of the transformed phenotype: experiments with subline of mouse L fibroblasts adapted to growth in serum-free medium

Characteristics of LSF cells grown in serum-containing and serum-free medium were compared. LSF is a subline of the L-strain of mouse transformed fibroblasts adapted to continuous growth is serum-free medium. Proliferation of LSF cells in monolayer on solid substratum was almost similar in serum-containing and in serum-free media. However, several other characters were found to be altered by the addition of serum to the serum-free medium: ability of cells to form colonies in semi-solid medium increased considerably; agglutinability of cells by Concanavalin A increased; uptake of 2-deoxy-D-glucose by the cells increased considerably; ability of cells to metabolize benzo (a) pyrene was inhibited; cell morphology was altered and, in particular, the cells became less spread on the substratum and density of microvilli on the cell surface increased. All these changes induced by serum were reversed by transfer of the cells back into serum-free medium. Thus, addition of serum increased the expression of a number of cellular traits characteristic of transformed phenotype, while in serum-free medium partial phenotypic reversion of transformation was observed. A possible role of serum in the expression of the transformed phenotype is discussed. It is pointed out that cell lines adapted to growth in serum-free medium provide an experimental system convenient for analysis of the effects of different serum components on the cell phenotype.

Aryl hydrocarbon hydroxylase and polycyclic hydrocarbon tumorigenesis: effect of the enzyme inhibitor 7,8-benzoflavone on tumorigenesis and macromolecule binding

Aryl hydrocarbon hydroxylase is present and is inducible in mouse skin. 7,8-Benzoflavone, an inhibitor of the enzyme, markedly inhibits tumorigenesis by 7,12-dimethylbenz(a)anthracene, but has either no effect on or stimulates benzo(a)pyrene tumorigenesis. Thus, the role of aryl hydrocarbon hydroxylase appears highly specific for each polycyclic hydrocarbon, in respect to detoxification and/or activation of the hydrocarbon to a carcinogenic form. In parallel studies, we found that 7,8-benzoflavone significantly reduces the amount of 7,12-dimethylbenz(a)anthracene binding to mouse skin DNA, RNA, and protein, and the binding of benzo(a)pyrene to RNA and protein of mouse skin. 7,8-Benzoflavone exhibited a markedly lesser effect on the binding of benzo(a)pyrene to DNA.

Enzymatic hydroxylation of benzopyrene and its relationship to cytotoxicity

Aryl hydrocarbon hydroxylase, an inducible microsomal enzyme system, has been measured in cultures of normal and transformed hamster, mouse, and human cells. In order to determine the highest level of enzyme, the cells were induced by pretreatment with benz(a)anthracene. A correlation was found between the level of enzyme activity and the susceptibility of the cells to the cytotoxicity produced after treatment with benzopyrene. The results indicate that aryl hydrocarbon hydroxylase is the enzyme system responsible for cell susceptibility to the cytotoxic effect of benzopyrene and the toxic effect of benzopyrene is due to its enzymatic conversion to a cytotoxic metabolite. 3-Hydroxybenzopyrene, one of the products of the enzymatic hydroxylation of benzopyrene, was found to be cytotoxic to cells that were either susceptible or resistant to the cytotoxic effect of benzopyrene.