Elimination of MNU-induced mutational lesions in V79 Chinese hamster cells

Quinoid metabolites of 4-monochlorobiphenyl induce gene mutations in cultured Chinese hamster v79 cells

4-Monochlorobiphenyl (PCB3) is a component of commercial polychlorinated biphenyl (PCB) products and is an airborne environmental pollutant. Our recent study with transgenic Fischer 344 rats revealed the mutagenic potential of PCB3 in the livers of male rats. PCB3 is converted in vitro to hydroxylated metabolites, to hydroquinones (HQs, e.g., 2',5'-HQ and 3',4'-HQ), and can be further oxidized to quinones (Qs, e.g., 2',5'-Q and 3',4'-Q). This raises the question whether the mutagenic potential of PCB3 is due to the mutagenicity of PCB3 itself or of one of the metabolites. In this study, we investigated the mutagenicity of PCB3, of the monohydroxylated metabolites 2'-hydroxy (HO)-, 3'-HO-, and 4'-HO, of the HQs 3',4'-HQ and 2',5'-HQ and of the Qs 3',4'-Q and 2',5'-Q in cultured Chinese hamster V79 cells. The induction of gene mutations was determined at the hypoxanthine-guanine phosphoribosyltransferase (hprt) gene locus by selection with 6-thioguanine. The induction of chromosome and genome mutations was assessed using the micronucleus assay and immunochemical differentiation of micronuclei containing whole chromosomes (kinetochore positive) and DNA fragments (kinetochore negative). The induction of chromosome and genome mutations, detected as micronuclei, was only observed at higher, cytotoxic concentrations of monohydroxylated, catecholic, and quinoid metabolites of PCB3. However, both PCB3-Qs induced a significant increase in the mutant frequency of the hprt gene and did so at submicromolar concentrations. Thus, the present study demonstrates for the first time the mutagenicity of PCB3 metabolites in mammalian cells and identifies quinoid metabolites of PCB3 as potential ultimate mutagens.

Effect of total plasma homocysteine on cervical dysplasia risk

We investigated whether total plasma homocysteine (tHcy) is associated with risk for cervical intraepithelial neoplasia (CIN). tHcy was evaluated, along with numerous risk factors for CIN and biochemical indexes of nutrients, in a previously reported study population of 294 subjects with CIN and 170 female controls without CIN. tHcy was significantly higher in cases than in controls (9.1 vs. 8.3 mumol/l, p = 0.002). Human papillomavirus type 16 infection [odds ratio (OR) = 6.7], oral contraceptive use (OR = 6.0), parity (OR = 2.2), and cigarette smoking (OR = 1.9) were significantly associated with CIN after adjustment for each other and for age, number of sexual partners, and plasma tHcy, folate, iron, and zinc. Human papillomavirus type 16 positivity increased risk for CIN more when tHcy was > 9.12 mumol/l (OR = 4.7) than when it was < or = 9.12 mumol/l (OR = 3.0). Cigarette use increased risk for CIN when tHcy was > 9.12 mumol/l (OR = 3.9), but not when tHcy was < or = 9.12 mumol/l (OR = 1.5). Parity increased risk for CIN more when tHcy was > 9.12 mumol/l (OR = 4.0) than when tHcy was < or = 9.12 mumol/l (OR = 2.0). These results suggest that elevated plasma tHcy is a risk factor for cervical dysplasia and that it enhances the effects of other risk factors. It is unknown whether tHcy is serving as a marker of folate deficiency or is acting through other mechanisms.

Streptozotocin-induced toxicity in CHO-9 and V79 cells

The cytotoxicity of streptozotocin (STZ) was investigated in Chinese hamster fibroblast lines (CHO-9 and V79) in comparison to two other alkylating agents, methylnitrosourea (MNU) and ethylnitrosourea (ENU), using cell survival as the endpoint. It was found that V79 cells were far more resistant to methylation induced by STZ and MNU than CHO-9 cells (20 and four times, respectively) but equally sensitive to the ethylating agent ENU. The extent of STZ-induced DNA methylation was estimated by analyzing the extent of O6-metG and N7-metG adducts in the DNA of treated cells through high-performance liquid chromatography (HPLC) with electrochemical detection. The number of adducts as well the efficiencies of their removal from the DNA were similar in both cell lines. The response of these cells to the presence of DNA damage was evaluated by analysis of STZ effects on DNA replication and cell cycle progression. Measurement of [3H]-thymidine incorporation showed a similar pattern of response at the level of inhibition of DNA synthesis in both cell lines. However, analysis of metaphase cells 36 hr after STZ exposure showed an accumulation of cells in the second cycle in the CHO-9 line, indicating induction of a cell cycle arrest. Only a slight effect was observed on cell cycle progression in V79 cells, indicating that the methylation resistance of these cells could be related to their ability to progress through the cell cycle despite the presence of DNA lesions.

Mechanisms of inhibitors of mutagenesis and carcinogenesis. Classification and overview

The mechanisms of action of inhibitors of mutagenesis and carcinogenesis are reviewed in the light of our present knowledge. The identified mechanisms are classified into several categories and sub-categories, depending on the stage of intervention in the mutagenesis and carcinogenesis processes, and on the patterns of modulation of the host defense devices. Although a number of the known mechanisms fit into the proposed scheme, the available information on these problems is still fragmentary, and often inhibitors act through multiple mechanisms or can interact with other inhibitors. Moreover, due to the double-edged nature of many protective factors of the organism, and to the wide array of biological properties displayed by several inhibitors, the beneficial effects are in many instances counter-balanced by adverse reactions. Nevertheless, the present data-base on mechanisms of inhibitors, which is expected to grow rapidly in the near future, provides an extremely useful scientific premise for the primary prevention of mutation-related diseases. In this prospect, the elucidation of the underlying mechanisms complements the results emerging from the monitoring of protective end-points in mutagenicity and carcinogenicity test systems.

The elimination of mutagenic lesions induced by alkylating agents and UV in V79 Chinese hamster cells

The mutagenicity of MNU, EMS, BMS and UV light was compared by analyzing the dose-response curve just before and after the replicative process of the HGPRT gene in synchronized V79 Chinese hamster cells. This system makes it possible to compare a 10-h period for repair of different mutagenic lesions with no time for repair. Additional time for repair in synchronized V79 cells resulted in a reduced response for MNU and UV, but not for EMS and BMS. This result suggests that an error-free repair process operates on mutagenic lesions in methylated DNA and on thymine dimers, but not on ethylated and butylated DNA. Based on these results, it is concluded that the repair capacity of V79 cells to remove mutagenic lesions is characterized as low for UV, moderate for MNU and not detectable for the mutagenic lesions induced by EMS and BMS.

Role of vitamin B12 and folate deficiencies in carcinogenesis

A significant body of experimental evidence supports the notion that a deficiency of either vitamin B12 or folic acid enhances the activity of various carcinogens. Unifying mechanisms are proposed to explain this cocarcinogenic role.