Endogenous mutagens derived from amino acids

L-Cysteine, glutathione and the therapeutically used L-cysteine precursor, N-acetyl-L-cysteine, induced strong mutagenic effects in Salmonella typhimurium (reversion of the his- strains TA97, TA92 and TA104), when tested in the presence of subcellular kidney preparations. The tyrosine metabolites, levodopa (an ortho-hydroquinone) and homogentisic acid (a para-hydroquinone) reverted various his- strains as well. This mutagenicity did not require the presence of mammalian enzymes, and was relatively weak. The induction of gene mutations was also studied in mammalian cells (V79 Chinese hamster cells), using acquisition of resistance toward 6-thioguanine as the marker. L-Cysteine and N-acetyl-L-cysteine were found to be inactive, levodopa was weakly mutagenic, and homogentisic acid was strongly mutagenic (enhancing the mutation frequency 135-fold above background at an exposure concentration of 50 microM). This finding is striking as the urinary concentration of homogentisic acid is about 1000 times higher in patients with a genetic defect in homogentisic acid 1,2-dioxygenase (alkaptonuria). Genotoxic and carcinogenic effects of other amino acids and metabolites, reported in the literature, are discussed as well.     

Some substrates and inhibitors of cytosolic epoxide hydrolase induce sister-chromatid exchanges in mammalian cells, but do not induce gene mutations in Salmonella typhimurium and V79 cells

Trans-stilbene oxide, trans-β-methylstyrene, 7,8-oxide, trans-β-ethylstyrene, 7,8-oxide, trans-β-propylstyrene 7,8-oxide and 4-fluorochalcone oxide were investigated for genotoxic activity in bacterial and mammalian cells, in the absence of external xenobiotic-metabolising systems. All compounds strongly enhanced the frequency of sister-chromatid exchanges (SCE) in cultured human lymphocytes. None of them was mutagenic in Salmonella typhimurium (reversion of the his⁻ strains TA98, TA100 and TA104). The limit of detection was 1/20,000 to 1/10⁶ of the activity of the positive control, benzo[a]pyrene 4,5-oxide, depending on the compound and the bacterial strain. Trans-β-methylstyrene 7,8-oxide and 4-fluorochalcone oxide were additionally tested for induction of SCE and gene mutations in the same target cells, namely Chinese hamster V79 cells. Their influence on the level of SCE was similar to that observed in human lymphocytes, whilst gene mutations (at the hprt locus) were not induced. The four investigated styrene oxide derivatives are known to be excellent substrates for a mammalian enzyme, cytosolic epoxide hydrolase (cEH). 4-Fluorochalcone oxide is a potent selective inhibitor of this enzyme and is structurally similar to the investigated styrene oxide derivatives. These properties of the test compounds however cannot explain the observed discrepancies in the results, since the genetic end point (SCE versus gene mutations) was decisive, and SCE were induced in cEH-proficient human lymphocytes as well as in cEH-deficient V79 cells.     

Relationship between mutagenicity and DNA adduct formation in mammalian cells for fjord- and bay-region diol-epoxides of polycyclic aromatic hydrocarbons

Chinese hamster V79 cells were treated with the anti- and syn-diastereomers of the bay- or fjord-region diol-epoxides of four polycyclic aromatic hydrocarbons, namely benzo[a]pyrene (BP), benzo[c]chrysene (BcC), benzo[g]chrysene (BgC) and benzo[c]phenanthrene (BcPh). The frequency of induction of 6-thioguanine-resistant mutations was determined, and the extent of formation of DNA adducts was measured by 32P-postlabelling. When expressed as mutation frequency per nanomoles compound per millilitre incubation medium, this group of chemicals expressed a 160-fold range in potency. In agreement with previous experimental studies, the anti-diol-epoxide of BcC was highly mutagenic, inducing in excess of 3 x 10(4) mutations/10(6) cells per nmol compound/ml. The mutagenic activities of the anti- and syn-diol-epoxides of BP were 10- and 100-fold lower, respectively. Both diol-epoxides of BgC, the syn-BcC and the anti-BcPh derivatives were also highly mutagenic, and only the syn-BcPh diol-epoxide was less mutagenic than the anti-diol-epoxide of BP. Determination of the levels of DNA adducts formed by the diol-epoxides indicated that the most mutagenic compounds were the most DNA reactive, although the fjord-region diol-epoxides gave rise to more complex patterns of adducts than those of the BP diol-epoxides. When the mutagenicity results were expressed as mutations per femtomoles total adducts formed, all compounds showed similar activities. Thus the potent mutagenicity of the fjord region diol-epoxides appears to be due to the high frequency with which they form DNA adducts in V79 cells, rather than to formation of adducts with greater mutagenic potential.     

Biphenyl and fluorinated derivatives: liver enzyme-mediated mutagenicity detected in Salmonella typhimurium and Chinese hamster V79 cells.

Hepatocarcinogenic polychlorinated and polybrominated biphenyls usually show negative results in in vitro mutagenicity assays. Problems in their testing result from their low water solubility and their slow rate of metabolism. We therefore investigated better soluble model compounds, namely biphenyl and its 3 possible monofluorinated derivatives. In the direct test, these compounds proved to be nonmutagenic in Salmonella typhimurium TA98 and TA100 (reversion to histidine prototrophy) and in Chinese hamster V79 cells (acquisition of resistance to 6-thioguanine). However, when the exposure was carried out in the presence of NADPH-fortified postmitochondrial fraction of liver homogenate from Aroclor 1254-treated rats, all 4 compounds showed mutagenic activity in V79 cells. 3-Fluorobiphenyl produced strong mutagenic effects in S. typhimurium TA100 as well, whereas the other biphenyls were inactive. In strain TA98, 3- and 4-fluorobiphenyl showed mutagenic activity. This mutagenicity was enhanced in the presence of 1,1,1-trichloropropene 2,3-oxide, an inhibitor of microsomal epoxide hydrolase, thus suggesting that epoxides may be active metabolites.     

Glutaraldehyde-fixed transformed and non-transformed cells induce contact-dependent inhibition of growth in non-transformed C3H/10T1/2 mouse fibroblasts, but not in 3-methylcholanthrene-transformed cells

C3H/10T1/2 mouse fibroblasts showed a pronounced inhibition of growth when reaching a critical cell density. The situation of high cell density could be mimicked by the addition of glutaraldehyde-fixed cells to sparsely seeded proliferating cells. Treatment of the C3H/10T1/2 cells with 3-methylcholanthrene led to a high frequency of piled up foci (118 type II and type III foci in 78 cultures). Cells of a type III focus of a treated culture were cloned. These cells grew in soft-agar and reached 10 times higher cell densities when grown in culture dishes, than did their non-transformed counterparts. Glutaraldehyde-fixed transformed cells did not differ from fixed non-transformed cells in the ability to inhibit the growth of sparsely seeded non-transformed cells. On the other hand, both the addition of fixed normal or transformed C3H/10T1/2 cells did not affect the growth rate of transformed cells. In a concept explaining the density-dependent inhibition of growth of non-transformed cells by a specific interaction of plasma membrane-localized effectors with plasma membrane-localized receptors, the present findings would indicate that the transformed cells used express active effectors but are functionally defective in the receptors or in the signal transmission.     

Characterization of a surrogate murine antibody to model anti-human CD3 therapies

Fc-modified anti-human CD3ε monoclonal antibodies (mAbs) are in clinical development for the treatment of autoimmune diseases. These next generation mAbs have completed clinical trials in patients with type-1 diabetes and inflammatory bowel disease demonstrating a narrow therapeutic window. Lowered doses are ineffective, yet higher pharmacologically-active doses cause an undesirable level of adverse events. Thus, there is a critical need for a return to bench research to explore ways of improving clinical outcomes. Indeed, we recently reported that a short course of treatment affords synergy, providing long-term disease amelioration when combining anti-mouse CD3 and anti-mouse tumor necrosis factor mAbs in experimental arthritis. Such strategies may widen the window between risk and benefit; however, to more accurately assess experimentally the biology and pharmacology, reagents that mimic the current development candidates were required. Consequently, we engineered an Fc-modified anti-mouse CD3ε mAb, 2C11-Novi. Here, we report the functional characterization of 2C11-Novi demonstrating that it does not bind FcγR in vitro and elicits little cytokine release in vivo, while maintaining classical pharmacodynamic effects (CD3-TCR downregulation and T cell killing). Furthermore, we observed that oral administration of 2C11-Novi ameliorated progression of remitting-relapsing experimental autoimmune encephalitis in mice, significantly reducing the primary acute and subsequent relapse phase of the disease. With innovative approaches validated in two experimental models of human disease, 2C11-Novi represents a meaningful tool to conduct further mechanistic studies aiming at exploiting the immunoregulatory properties of Fc-modified anti-CD3 therapies via combination therapy using parenteral or oral routes of administration.     

Human cytosolic sulphotransferases: genetics, characteristics, toxicological aspects

Cytosolic sulphotransferases transfer the sulpho moiety from the cofactor 5'-phosphoadenosine-3'-phosphosulphate (PAPS) to nucleophilic groups of xenobiotics and small endogenous compounds (such as hormones and neurotransmitters). This reaction often leads to products that can be excreted readily. However, other sulpho conjugates are strong electrophiles and may covalently bind with DNA and proteins. All known cytosolic sulphotransferases are members of an enzyme/gene superfamily termed SULT. In humans, 10 SULT genes are known. One of these genes encodes two different enzyme forms due to the use of alternative first exons. Different SULT forms substantially differ in their substrate specificity and tissue distribution. Genetic polymorphisms have been described for three human SULTs. Several allelic variants differ in functional properties, including the activation of promutagens. Only initial results are available from the analysis of SULT allele frequencies in different population groups, e.g. subjects suffering from specific diseases and corresponding controls.