Long-term mutagenicity studies with chloroform and dimethylnitrosamine in femalelacI transgenic B6C3F1 mice

Abstract

The weight of evidence indicates that chloroform induces cancer in the female B6C3F1 mouse liver via a nongenotoxic‐cytotoxic mode of action. However, it is probable that DNA damage occurs secondary to events associated with cytolethality and regenerative cell proliferation. The purpose of the present study was to evaluate the potential mutagenic activity of chloroform in the B6C3F1 lacI transgenic mouse liver mutagenesis assay including mutagenic events that might occur secondary to cytolethality. The positive control, dimethylnitrosamine (DMN) is a DNA‐reactive mutagen and carcinogen. DMN‐induced mutations were anticipated to require only a brief exposure and without further treatment were predicted to remain unchanged over time at those frequencies. Chloroform‐induced mutations secondary to toxicity were anticipated to require longer exposure periods and to occur only under conditions that produced sustained cytolethality and regenerative cell proliferation. Female B6C3F1 lacI transgenic mice were treated with daily doses of 2, 4, or 8 mg/kg of DMN by gavage for 4 days and then held until analysis 10, 30, 90, and 180 days postexposure. Livers from DMN‐treated mice exhibited a dose‐related 2‐ to 5‐fold increase over control mutant frequencies and remained at those levels for 10 through 180 days postexposure. Thus, following the initial induction by DMN no selective mutation amplification or loss was seen for this extended period of time. Female B6C3F1 lacI mice were exposed daily for 6 hr/day 7 days/week to 0, 10, 30, or 90 ppm chloroform by inhalation, representing nonhepatotoxic, borderline, or overtly hepatotoxic chloroform exposures. Timepoints for determination of lacI mutant frequency were 10, 30, 90, and 180 days of exposure. No increase in lacI mutant frequency in the liver was observed at any dose or timepoint with chloroform, indicating a lack of DNA reactivity. DNA alterations secondaryto toxicity either did not occur or were of a typenot detectable by lacI mutant frequency analysis,such as large deletions. Environ. Mol. Mutagen. 31:248–256, 1998 © 1998 Wiley‐Liss, Inc.

Chemical carcinogenesis

“…The conclusions reached from several experiments enabled the conclusion to be drawn that initiation is caused by irreversible genetic changes which predispose susceptible normal cells to malign evolution and immortality (Beremblum and Shubik 1947, Stenbäck et al 1981, Butterworth et al 1992, Mehta 1995, Dybing and Sanner 1999, Trosko 2001, Shacter and Weitzman 2002. The initiated cell is not a neoplasic cell but has taken its first step towards this state, after successive genotypical and phenotypical changes have occurred (Trosko 2003).…”

“…Promoter compounds do not interact directly with DNA and unchain biological effects without being metabolically activated (Yuspa et al 1983, Butterworth et al 1992, Weisburger 1998, Williams 2001. These agents increase cell proliferation in susceptible tissues, contribute towards fixing mutations, enhance alterations in genetic expression and cause changes in cellular growth control (Mehta 1995, Gomes-Carneiro et al 1997).…”

“…The promoters' most important activity is mitogenesis -genotoxical and mutational actions are not necessary at this stage (Pitot and Dragan 1991). The promoter must be present for weeks, months and years in order to be effective and its effectiveness depends on its concentration in the target tissue (Butterworth et al 1992). Promotion is a reversible stage, after a promoter's disappearance a regression in cell proliferation can occur, probably by apoptosis.…”

“…Progression is characterised by irreversibility, genetic instability, faster growth, invasion, metastization, and changes in the biochemical, metabolical and morphological characteristics of cells (Pitot and Dragan 1991, Butterworth et al 1998, Loeb 1998, Klaunig et al 2000, Gutiérrez and Salsamendi 2001, Dixon and Kopras 2004.…”

“…After cardiovascular diseases, it is the second cause of death amongst the global population (Huff 1994, Weisburger Carneiro et al 1997, Huff 1999. With the discovery of different mechanisms involved in carcinogenesis, this definition is now incomplete (Butterworth and Bogdanffy 1999). From an experimental point of view, a compound is considered carcinogenic when its administration to laboratory animals induces a statistically significant rise in the incidence of one or more histological types of neoplasia, compared with the animals in the control group which are not exposed to the substance (Gutiérrez and Salsamendi 2001).…”