Nitrapyrin: a scientific advisory group review of the mode of action and carcinogenicity in B6C3F1 mice.

Nitrapyrin has been registered as a nitrogen stabilizer in the United States for many years based on a robust set of regulatory data. These data demonstrated that nitrapyrin was not genotoxic and that there were no tumors elicited in rats or mice that were relevant for human risk assessment. A repeat carcinogenicity study in B6C3F1 mice, conducted at two substantially higher-dose levels (0, 125 or 250 mg/kg/day) than the original study (0, 5, 25 or 75 mg/kg/day) identified liver, stomach, epididymal and Harderian gland tumors. In order to assess the relevance of these findings for human risk assessment, a Scientific Advisory Group (SAG) examined relevant microscopic changes in these tissues and also evaluated genotoxicity and mechanistic data. The SAG determined that the maximum tolerated dose had been exceeded in mice given 125 or 250 mg/kg/day, based on 26-33% decreased body weight gains (males-250 mg/kg/day), hepatocellular necrosis and compensatory hepatocellular proliferation (males and females-125 and 250 mg/kg/day). The SAG believed that the increased incidences of hepatocellular foci of alteration and hepatocellular neoplasms represented an epigenetic response to hepatocellular necrosis and increased mitogenesis. Increased incidences of proliferative lesions in the forestomach mucosa were likely secondary to the irritant effects of nitrapyrin. Neither the liver nor forestomach effects were interpreted to be a direct carcinogenic effect. Higher incidences of Harderian gland adenomas (females) and undifferentiated sarcomas in the epididymis represented normal biological variations in incidence and were unrelated to nitrapyrin. Therefore, it was the SAG's opinion that nitrapyrin exposure that does not produce target organ toxicity in exposed individuals would not be expected to increase the risk of cancer.

Nephrotoxicity and hepatotoxicity induced by inhaled bromodichloromethane in wild-type and p53-heterozygous mice.

Bromodichloromethane (BDCM) is a common municipal drinking water disinfection by-product, resulting in widespread trace human exposure via ingestion and inhalation. The present studies were designed to define organ-specific, BDCM-induced toxicity in wild type (p53(+/+)) and heterozygous (p53(+/-)) mice on both the FVB/N and C57BL/6 genetic backgrounds. Mice were exposed to BDCM vapor daily for 6 h/day and 7 days/week at concentrations of 0, 1, 10, 30, 100, or 150 ppm for 1 week and at 0, 0.3, 1, 3, 10, or 30 ppm for 3 weeks. In the 1-week exposure study, dose-dependent mortality and morbidity were observed at concentrations of 30 ppm and above and were as high as 100% at 150 ppm. In the 3-week exposure study, mortality and morbidity were found only in the 30-ppm exposure groups and were 0, 17, 67, and 33% for the wild-type C57BL/6, p53(+/-) C57BL/6, wild-type FVB/N, and p53(+/-) FVB/N mice, respectively. BDCM was a particularly potent kidney cytotoxicant. Dose-dependent tubular degeneration, necrosis, and associated regenerative cell proliferation greater than 10-fold over controls were seen at concentrations as low as 10 ppm in the kidneys of all strains at 1 week. Similar dose-dependent increases in hepatic necrosis, degeneration, and regenerative cell proliferation were observed but were induced only at concentrations of 30 ppm and higher. Pathological changes were more severe in the FVB/N compared to the C57BL/6 mice and were more severe in the heterozygotes compared to the wild-type mice. However, recovery and return of the percentage of kidney cells in S-phase to control levels was seen at 3 weeks. The estimated maximum tolerated dose for longer-term exposures was 15 ppm, based on mortality, induced kidney pathology, and regenerative cell proliferation. A one-year cancer bioassay was initiated with doses of 0, 0.5, 3, 10, and 15 ppm, based on this information. No pathological changes in the livers were found at the 13-week time point of that study. At 13 weeks, the kidney lesions and regenerative cell proliferation seen at the 1-week time point at doses of 10 ppm and above had resolved, and the cell proliferation rates had returned to baseline. Differences in toxicity indicate that caution be used in substituting wild-type mice for transgenic mice for range-finding studies to select doses for p53(+/-) cancer studies. Resolution of the kidney lesions indicates that periods of very high regenerative cell proliferation, potentially important in the carcinogenic process, may not be observed if measurements are taken only at 3 weeks of exposure or later.

The preparation of anthraquinone used in the National Toxicology Program cancer bioassay was contaminated with the mutagen 9-nitroanthracene.

Commercial anthraquinone (AQ) (9,10-anthracenedione) is produced by at least three different production methods worldwide: oxidation of anthracene (AQ-OX), Friedel-Crafts technology (AQ-FC) and by Diels-Alder chemistry (AQ-DA), with the final product varying in color and purity. AQ-OX begins with anthracene produced from coal tar and different lots can contain various contaminants, particularly the mutagenic isomers of nitroanthracene. AQ has been reported to be negative in a variety of genotoxicity tests including numerous Ames Salmonella mutagenicity assays. In addition, we report that AQ-DA is negative in the Salmonella-Escherichia coli reverse mutation assays, the L5178Y mouse lymphoma forward mutation assay, for inducing chromosomal aberrations, polyploidy or endoreduplication in Chinese hamster ovary cells, and in the in vivo mouse micronucleus assay. Further, a previous 18 month bioassay conducted with AQ administered to male and female B6C3F(1) and (C57BL/6xAKR)F(1) mice reported no induction of cancer. Thus, it was somewhat unexpected that in a long-term study conducted by the National Toxicology Program (NTP) AQ-OX induced a weak to modest increase in tumors in the kidney and bladder of male and female F344/N rats and a strong increase in the livers of male and female B6C3F(1) mice. In the studies reported here, a sample of the AQ-OX used in the NTP bioassay was shown to be mutagenic in the Ames tester strains TA98, TA100 and TA1537. Addition of an S9 metabolic activation system decreased or eliminated the mutagenic activity. In contrast, the purified NTP AQ-OX as well as the technical grade samples AQ-FC and AQ-DA were not mutagenic in the Ames test. The chemical structure of AQ does not suggest that the parent compound would be DNA reactive. Therefore, a mutagenic contaminant was present in the NTP bioassay sample that is either directly mutagenic or can be activated by bacterial metabolism. Analytical studies showed that the primary contaminant 9-nitroanthracene (9-NA) was present in the NTP AQ-OX at a concentration of 1200 p.p.m., but not in the purified material. The 9-NA and any other contaminants that might have been present in the NTP AQ-OX induced measurable mutagenicity at 9-NA concentrations as low as 0.15 microg/plate in tester strain TA98, indicating potent mutagenic activity. On the basis of revertants per microgram, 9-NA was more potent than benzo[a]pyrene (B[a]P) and was about equally as potent as the 2-nitrofluorene run concurrently as positive controls. TD(50) quantitative carcinogenicity potency estimates indicate that a carcinogen of a potency in the range between B[a]P and dimethylnitrosamine would be required to produce the observed carcinogenic response at the levels of the contaminants found in the test sample. While recognizing that there are limitations in extrapolating mutagenic potency to potential carcinogenic potency, these estimates do indicate that it is plausible that the 9-NA contaminant might have been responsible for all of the tumor induction observed in the NTP study. In fact, in the absence of reliable cancer data, the genetic toxicology profile indicates that AQ would not be a genotoxic carcinogen. Thus, no conclusion as to the carcinogenic activity of AQ can be made at this time.

Expression of base excision repair enzymes in rat and mouse liver is induced by peroxisome proliferators and is dependent upon carcinogenic potency.

Elevated and sustained cell replication, together with a decrease in apoptosis, is considered to be the main mechanism of hepatic tumor promotion due to peroxisome proliferators. In contrast, the role of oxidative stress and DNA damage in the carcinogenic mechanism is less well understood. In view of possible induction of DNA damage by peroxisome proliferators, DNA repair mechanisms may be an important factor to consider in the mechanism of action of these compounds. Here, the ability of peroxisome proliferators to induce expression of base excision repair enzymes was examined. WY-14,643, a potent carcinogen, increased expression of several base excision DNA repair enzymes in a dose- and time-dependent manner. Importantly, expression of enzymes that do not repair oxidative DNA damage was not changed. Moreover, less potent members of the peroxisome proliferator group had much weaker or no effects on expression of DNA repair enzymes when compared with WY-14,643. Collectively, these data suggest that DNA base excision repair may be an important factor in peroxisome proliferator-induced carcinogenesis and that induction of DNA repair might provide further evidence supporting a role of oxidative DNA damage by peroxisome proliferators.

Metabolism of chloroform by cytochrome P450 2E1 is required for induction of toxicity in the liver, kidney, and nose of male mice.

Chloroform is a nongenotoxic-cytotoxic liver and kidney carcinogen and nasal toxicant in some strains and sexes of rodents. Substantial evidence indicates that tumor induction is secondary to events associated with cytolethality and regenerative cell proliferation. Therefore, pathways leading to toxicity, such as metabolic activation, become critical information in mechanism-based risk assessments. The purpose of this study was to determine the degree to which chloroform-induced cytotoxicity is dependent on the cytochromes P450 in general and P450 2E1 in particular. Male B6C3F(1), Sv/129 wild-type (Cyp2e1+/+), and Sv/129 CYP2E1 knockout (Cyp2e1-/- or Cyp2e1-null) mice were exposed 6 h/day for 4 consecutive days to 90 ppm chloroform by inhalation. Parallel control and treated groups, excluding Cyp2e1-null mice, also received an i.p. injection (150 mg/kg) of the irreversible cytochrome P450 inhibitor 1-aminobenzotriazole (ABT) twice on the day before exposures began and 1 h before every exposure. Cells in S-phase were labeled by infusion of BrdU via an implanted osmotic pump for 3.5 days prior to necropsy, and the labeling index was quantified immunohistochemically. B6C3F(1) and Sv/129 wild-type mice exposed to chloroform alone had extensive hepatic and renal necrosis with significant regenerative cell proliferation. These animals had minimal toxicity in the nasal turbinates with focal periosteal cell proliferation. Administration of ABT completely protected against the hepatic, renal, and nasal toxic effects of chloroform. Induced pathological changes and regenerative cell proliferation were absent in these target sites in Cyp2e1-/- mice exposed to 90 ppm chloroform. These findings indicate that metabolism is obligatory for the development of chloroform-induced hepatic, renal, and nasal toxicity and that cytochrome P450 2E1 appears to be the only enzyme responsible for this cytotoxic-related metabolic conversion under these exposure conditions.

A comprehensive approach for integration of toxicity and cancer risk assessments.

Experimental observations and theoretical considerations indicate a dose threshold for most chemically induced noncancer toxic effects below which the increased risk of toxicity is zero. Thus, the historical approach for minimizing risk from toxic chemicals has been to experimentally determine a no-observed-adverse-effect-level (NOAEL) and then to apply safety or uncertainty factors to estimate a dose not expected to produce that toxic effect in humans. In contrast, for radiation and chemically induced cancer, it has been assumed that all agents operate by a genotoxic mode of action and that some risk can be assigned to even vanishingly small doses. Accordingly, risk assessments for carcinogens have commonly been based on the assumption that the tumor dose-response curve at low doses is linear and passes through the origin. Mode of action is defined as a fundamental obligatory step in the induction of toxicity or cancer. It is now clear that tumor induction can arise in a variety of ways including not only a DNA-reactive genotoxic mode of action, but also non-DNA-reactive nongenotoxic-cytotoxic and nongenotoxic-mitogenic modes of action. Initial risk assessment approaches that recognized this distinction identified a chemical carcinogen as either genotoxic or nongenotoxic, with no middle ground. The realization that there is a continuum whereby different chemicals can act by a combination of modes of action and the recent explosion of research into molecular mechanisms of carcinogenesis indicate that all relevant information should be integrated into the risk assessment process on a case by case basis. A comprehensive approach to risk assessment demands that default assumptions be replaced with an integrated understanding of the rate-limiting steps in the induction of toxicity or cancer along with quantitative measures of the shapes of those dose-response curves. The examples of more contemporary risk assessments are presented for chloroform and vinyl acetate.

Long-term mutagenicity studies with chloroform and dimethylnitrosamine in female lacI transgenic B6C3F1 mice.

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 secondary to toxicity either did not occur or were of a type not detectable by lacI mutant frequency analysis, such as large deletions.

Patterns of chloroform-induced regenerative cell proliferation in BDF1 mice correlate with organ specificity and dose-response of tumor formation.

It has been reported that chloroform administered to BDF1 mice by inhalation for 2 years at concentrations of 5, 30 or 90 p.p.m. for 6 h/day, 5 days/week induced an increase in renal cell tumors in male but not female mice exposed to the doses of 30 and 90 p.p.m. A small increase in liver tumors was statistically significant in the female mice at 90 p.p.m. if the incidences of carcinomas and adenomas were combined. Because chloroform is not a DNA reactive mutagen, a 13-week time-course and dose-response study was conducted under conditions of the original bioassay to examine whether regenerative cell proliferation was an underlying mechanism of carcinogenesis. Mice were given bromodeoxyuridine via infusion during the last 3.5 days prior to necropsy to label cells in S-phase. Chloroform induced pathology and regenerative cell proliferation, measured as the labeling index (LI, percentage of cells in S-phase), were assessed microscopically and immunohistochemically. Male mice exposed to 30 and 90 p.p.m. exhibited a dose-dependent increase in regenerating tubules within the renal cortex and up to a 31-fold increase in LI. No renal lesions or increased LI were observed in females. Increased centrilobular to midzonal hepatocyte degeneration and vacuolation and a 7-fold increase over controls in the hepatocyte LI were observed in the female mice at 90 p.p.m. at 13 weeks. Males exhibited similar pathology, but the increase in LI was not sustained. The observed correlations between cytolethality and regenerative cell proliferation with tumor formation supports extensive evidence that chloroform induces cancer via a non-genotoxic-cytotoxic mode of action. A concentration of 5 p.p.m. is the no-observed-adverse-effect level for nephrotoxicity, cell proliferation and cancer. An appropriate safety factor applied to this value is a straightforward approach to cancer risk assessment that is consistent with the mode of action of chloroform.

Differential display identified changes in mRNA levels in regenerating livers from chloroform-treated mice.

The nongenotoxic-cytotoxic carcinogen chloroform induces liver necrosis, regenerative cell proliferation, and, eventually, liver tumors in female B6C3F1 mice when administered by gavage at doses of 238 or 477 mg/kg/d. Administration of 1800 ppm of chloroform in the drinking water results in similar daily doses but does not produce liver toxicity or cancer. The differential-display technique was used to compare the expression of a subset of mRNAs in normal (control) and regenerating liver after chloroform-induced toxicity to define the proportion of genes whose expression changes under hepatotoxic conditions and to identify the genes that might play a role in regeneration and perhaps cancer. RNA was purified from the livers of female B6C3F1 mice after 4 d or 3 wk of gavage treatment with 3, 238, or 477 mg/kg/d of chloroform or treatment with 1800 ppm chloroform in drinking water. There was a remarkably high degree of consistency of gene expression among the animals and across dose and treatment groups as visualized by the differential-display technique. Of the 387 bands observed, only four (about 1%) changed expression in regenerating liver. The genes were assigned locus names by GenBank after sequence submission. The genes with increased mRNA levels as confirmed by northern blot analysis were MUSTIS21, a mouse primary response gene induced by growth factors and tumor promoters; MUSMRNAH, a gene highly homologous to a human gene isolated from a prostate carcinoma cell line; and MUSFRA, a novel gene. The novel gene MUSFRB exhibited decreased mRNA levels. No change in expression was seen among control mice given the nontoxic regimens of 3 mg/kg/d chloroform or 1800 ppm chloroform in drinking water, indicating that changes in expression were associated with toxicity and regeneration rather than chloroform per se. These genes and others that may be identified by expanding this approach may play a role in regeneration and perhaps in the process of chloroform-induced carcinogenesis in rodent liver.

Risk assessment of inhaled chloroform based on its mode of action.

The development of scientifically sound risk assessments based on mechanistic data will enable society to better allocate scarce resources. Inadequate risk assessments may result in potentially dangerous levels of hazardous chemicals, whereas overly conservative estimates can result in unnecessary loss of products or industries and waste limited resources. Risk models are used to extrapolate from high-dose rodent studies to estimate potential effects in humans at low environmental exposures and determine a virtually safe dose (VSD). When information to the contrary is not available, the linearized multistage (LMS) model, a conservative model that assumes some risk of cancer at any dose, is traditionally employed. In the case of airborne chloroform, the dose at which an increased lifetime cancer risk of 10(-6) could be calculated was chosen as the target VSD. Applying the LMS model to the mouse liver tumor data from a corn-oil gavage bioassay yields a VSD of 0.000008 ppm chloroform in the air. The weight of evidence indicates that chloroform is not directly mutagenic but, rather, acts through a nongenotoxic-cytotoxic mode of action. In this case, tumor formation results from events secondary to induced cytolethality and regenerative cell proliferation. Toxicity is not observed in rodents when chloroform is not converted to toxic metabolites at a rate sufficient to kill cells. Thus, tumors would not be anticipated at doses that do not induce cytolethality, contrary to the predictions of the LMS model. Inhalation studies in rodents show no cytolethality or regenerative cell proliferation in mouse liver at a chloroform concentration of 10 ppm as the no observed effect level (NOEL) or below. Using that NOEL and a safety factor approach, one can develop a VSD of 0.01 ppm. Integrating these data into the risk assessment process will yield risk estimates that are appropriate to the route of administration and consistent with the mode of action.

Chloroform-induced cytotoxicity and regenerative cell proliferation in the kidneys and liver of BDF1 mice.

In a 2-year chloroform inhalation bioassay, an increased incidence of tumors was observed in the kidneys of male BDF1 mice and the liver of female BDF1 mice exposed to the highest exposure concentration of 90 ppm. To investigate the role of cytotoxicity and regenerative cell proliferation in tumor formation, male and female BDF1 mice were exposed to chloroform vapor concentrations of 0, 0.3, 5, 30, or 90 ppm 6 h/day for 4 days. Bromodeoxyuridine (BrdU) was administered via osmotic pumps implanted 3.5 days prior to necropsy, and the labeling index (LI), or percentage of cells in S-phase, was quantified using BrdU immunohistochemistry. To assess longer-term responses, additional male mice were exposed 5 days/week for 2 weeks to 0, 30, or 90 ppm. Degenerative lesions and an increase in the LI of seven- to ten-fold over controls were observed in the kidneys of male but not female mice exposed to 30 or 90 ppm. Liver lesions and increased hepatocyte LI were observed in male mice exposed to 30 or 90 ppm and in female mice exposed to 90 ppm. In the 2-week exposure groups 40% of the 30 ppm group and 80% of the 90 ppm group died with severe kidney damage, indicating that both 30 and 90 ppm exceed a maximum tolerated dose. Thus, in the 2-year bioassay chloroform concentrations had to be stepped-up over a period of weeks in order for the male mice exposed to 30 or 90 ppm to survive. The extrapolation of tumor data from such an unusual procedure is questionable. These observations are consistent with a substantial database that indicates that tumor induction by chloroform occurs via a non-genotoxic-cytotoxic mode of action and is secondary to organ-specific toxicity. These data further support the premise that doses that do not induce regenerative cell proliferation do not present an increased risk of cancer.

A 90-day chloroform inhalation study in F-344 rats: profile of toxicity and relevance to cancer studies.

Chloroform acts via a nongenotoxic-cytotoxic mode of action to produce cancer if given in doses and at dose rates sufficiently high to produce organ-specific toxicity. In a recent study, chloroform failed to induce cancer in male or female F-344 rats when administered by inhalation for 2 years at 90 ppm, 5 days/week. The present study was undertaken to define the concentration-response curves for chloroform-induced lesions and regenerative cell proliferation in the F-344 rat when exposed by inhalation and to correlate those patterns of toxicity with the results from the inhalation cancer bioassay. Male and female F-344 rats were exposed to airborne concentrations of 0, 2, 10, 30, 90, or 300 ppm chloroform 6 hr/day, 7 days/week for 4 days or 3, 6, 13 weeks. Additional treatment groups were exposed 5 days/week for 13 weeks or were exposed for 6 weeks and held until Week 13. Bromodeoxyuridine was administered via osmotic pumps implanted 3.5 days prior to necropsy and the labeling index (LI, percentage of nuclei in S-phase) was evaluated immunohistochemically. A full-screen necropsy identified the kidney, liver, and nasal passages as the only target organs. This study confirmed that 300 ppm is extremely toxic and would be inappropriate for longer-term cancer studies. The primary target in the kidney was the epithelial cells of the proximal tubules of the cortex, with significantly elevated increases in the LI at concentrations of 30 ppm and above. However, only a marginal increase in the renal LI in the males was seen after exposures of 90 ppm, 5 days/week. Chloroform induced hepatic lesions in the midzonal and centrilobular regions with increases in the LI throughout the liver, but only at 300 ppm exposures. An additional liver lesion seen only at the highly hepatotoxic concentration of 300 ppm was numerous intestinal crypt-like ducts surrounded by dense connective tissue. Enhanced bone growth and hypercellularity in the lamina propria of the ethmoid turbinates of the nose occurred at the early time points at concentrations of 10 ppm and above. At 90 days there was a generalized atrophy of the ethmoid turbinates at concentrations of 2 ppm and above. Cytolethality and regenerative cell proliferation are necessary but not always sufficient to induce cancer because of tissue, sex, and species differences in susceptibility. A combination of a lack of direct genotoxic activity by chloroform, only a marginal induction of cell proliferation in the male rat kidney, and lower tissue-specific susceptibility in the female rat is apparently responsible for the reported lack of chloroform-induced cancer in a long-term inhalation bioassay with F-344 rats.

Comparison of chloroform-induced toxicity in the kidneys, liver, and nasal passages of male Osborne-Mendel and F-344 rats.

Chloroform given by gavage in corn oil at 180 mg/kg per day induced kidney tumors in male Osborne-Mendel rats. Chloroform-induced cytotoxicity and regenerative cell proliferation have been observed in the kidneys of male F-344 rats. In order to compare the acute sensitivity of male Osborne-Mendel with F-344 rats, animals from both strains were administered a single gavage dose of 0, 10, 34, 90, 180, or 477 mg/kg chloroform and necropsied 48 h later. Known target tissues were examined for histological changes. Regenerative cell proliferation was assessed as a labeling index (LI, percent of cells in S-phase) as determined by nuclear incorporation of bromodeoxyuridine. The epithelial cells of the proximal tubules of the kidney cortex were the primary target cells for cytotoxicity and regenerative cell proliferation. A dose-dependent increase in the LI was present in the kidney of Osborne-Mendel rats given doses of 10 mg/kg chloroform and above and in F-344 rats given 90 mg/kg and above. The maximal increase in the LI was 4.5- or 3.7-fold over control in Osborne-Mendel or F-344 given 477 mg/kg, respectively. The only increase in the hepatocyte LI was in the F-344 rats given 477 mg/kg. Edema and periosteal hypercellularity were observed in the nasal passages of both strains at doses of 90 mg/kg and above. These data indicate that Osborne-Mendel and F-344 rats are about equally susceptible to chloroform-induced nephrotoxicity. These results provide a basis for linking the extensive data base on mechanisms of action of chloroform toxicity in F-344 rats to the Osborne-Mendel rat and support the hypothesis that events secondary to chloroform-induced cytolethality and regenerative cell proliferation played a role in the induction of renal tumors in the Osborne-Mendel rat.

Measures of cell replication in risk/safety assessment of xenobiotic-induced, nongenotoxic carcinogenesis.

The phenomena associated with nongenotoxic carcinogenesis are multifaceted and complex. Nongenotoxic carcinogens stimulate cell replication in the presence or the absence of cytotoxicity. Cell proliferation is pivotal in the neoplastic process, but the extent of its contribution to the development of xenobiotic-induced cancer remains an open question. The search for a better understanding of this process has generated considerable interest and effort, often with the objective of obtaining useful predictors of the tumourigenic potential of xenobiotics. Alterations in the natural balance of endogenous humoural agents that maintain replicative homeostasis results in proliferative stimulation (or inhibition) which may be transient or sustained. The bases for the molecular interaction of these mediators with cellular receptors, trans-cytoplasmic message conveyance, and subsequent nuclear responses leading to xenobiotic-induced mitosis are becoming better understood. Assessment of tissue replicative status has now become established and utilizes biochemical and histological methodology in a routine manner. The increasingly challenging international regulatory environment is demanding greater understanding of the mechanisms that underlie fundamental phenomena and the influences exerted by xenobiotics prior to their registration. While the precise mode of action of an individual xenobiotic may not be known, sound interpretation of toxicological data, including the contribution made by cell replication, creates greater confidence of its safety in the scientific, regulatory, and commercial communities. This article offers a view of cell proliferation from molecular interactions at the cellular level, through practical assessment of cell and tissue replicative status to its utility in contributing to the registration of new drugs and chemicals.

A non-bile duct origin for intestinal crypt-like ducts with periductular fibrosis induced in livers of F344 rats by chloroform inhalation.

To evaluate the toxic effects of prolonged exposure to chloroform vapors, female and male F344 rats were exposed to 0, 2, 10, 30, 90 and 300 p.p.m. chloroform by inhalation for 7 or 5 days/week for up to 13 weeks. The purpose of this study was to characterize a lesion that occurred in the livers of rats in the 300 p.p.m. exposure groups. Atypical glandular structures lined by intestinal-like epithelium and surrounded by dense connective tissue occurred in the livers of rats exposed to strongly hepatotoxic atmospheric concentrations of chloroform. Bile duct bromodeoxyuridine labeling indices as well as observations of the locations of the early lesions at the 3 and 6 week time points indicate that these lesions arose from a population of cells remote from the bile ducts. We refer to these lesions as intestinal crypt-like ducts with periductular fibrosis to distinguish them from true cholangiofibrosis. Here, intestinal crypt-like ducts with periductular fibrosis were seen only in rats exposed to 300 p.p.m. chloroform, and the multiplicity and severity of the lesions were greater in the right liver lobe. The lesion only occurred in association with liver necrosis and dramatic increases in hepatocyte labeling indices, while labeling indices in bile ducts in the same animals were not significantly different from controls. There was a treatment-related increase of transforming growth factor-alpha immunoreactivity in hepatocytes, bile duct epithelium, bile canaliculi and oval cells, and an increase in transforming growth factor-beta immunoreactivity in hepatocytes, bile duct epithelium and intestinal crypt-like ducts. Thus, intestinal crypt-like ducts with periductular fibrosis appeared to develop from a population of cells unrelated to bile ducts. Also, they occurred only in animals exposed to chloroform concentrations that induced significant hepatocyte necrosis and regenerative cell proliferation and were associated with increased growth factor expression or uptake.

A 90-day chloroform inhalation study in female and male B6C3F1 mice: implications for cancer risk assessment.

High doses of chloroform induced liver cancer in male and female B6C3F1 mice when administered by gavage, kidney cancer in male Osborne-Mendel rats when given by gavage or in the drinking water, and kidney cancer in male BDF1 mice when administered by inhalation. The weight of evidence indicates that chloroform is acting through a nongenotoxic-cytotoxic mode of action. The present study was designed to investigate the dose-response relationships for chloroform-induced lesions and regenerative cell proliferation in B6C3F1 mice as the basis for formulation of a biologically based risk assessment for inhaled chloroform. Different groups of female and male B6C3F1 mice were exposed to atmospheric concentrations of 0, 0.3, 2, 10, 30, and 90 ppm chloroform 6 hr/day, 7 days/week for exposure periods of 4 days or 3, 6, or 13 consecutive weeks. Some additional exposure groups were exposed for 5 days/week for 13 weeks or were exposed for 6 weeks and then examined at 13 weeks. Bromodeoxyuridine was administered via osmotic pumps implanted 3.5 days prior to necropsy, and the labeling index (LI, percentage of nuclei in S-phase) was evaluated immunohistochemically from histological sections. Complete necropsy and microscopic evaluation revealed treatment-induced dose- and time-dependent lesions only in the livers and nasal passage of the female and male mice and in the kidneys of the male mice. Large, sustained increases in the liver LI were seen in the 90-ppm groups at all time points. The female mice were most sensitive, with a no-observed-adverse-effect level (NOAEL) for induced hepatic cell proliferation of 10 ppm. The hepatic LI in the 5 days/week groups were about half of those seen in the 7 days/week groups and had returned to the normal baseline in the 6-week recovery groups. Induced renal histologic changes and regenerative cell proliferation were seen in the male mice at 30 and 90 ppm with 7 days/week exposures and also at 10 ppm with the 5 days/week regimen. Nasal lesions were transient and confined to mice exposed to 10, 30, or 90 ppm for 4 days. In a previous cancer bioassay, a gavage dose of 477 mg/kg/day produced a 95% liver tumor incidence in female B6C3F1 mice. This gavage dose is equivalent to a daily 6 hr/day inhalation exposure of approximately 80 ppm, based on the observed induced increases in the LI as an internal dosimeter. The United States Environmental Protection Agency currently uses the linearized multistage model applied to the mouse liver tumor data from the chloroform gavage study to estimate a virtually safe dose (VSD) as a one in a million increased lifetime risk of cancer. The resulting value is an airborne exposure concentration of 0.000008 ppm. Assuming that chloroform-induced female mouse liver cancer is secondary to events associated with necrosis and regenerative cell proliferation, then no increases in liver cancer in female mice would be predicted at the NOAEL of 10 ppm or below based on the results reported here. Applying an uncertainty factor of 1000 yields an estimate of a VSD at 0.01 ppm. This estimate relies on inhalation data and is more consistent with the mode of action of chloroform.

Levels of myc, fos, Ha-ras, met and hepatocyte growth factor mRNA during regenerative cell proliferation in female mouse liver and male rat kidney after a cytotoxic dose of chloroform.

Chloroform is a liver carcinogen in mice and a kidney carcinogen in rats. It is thought to act through a non-genotoxic-cytotoxic mode of action. Changes in expression of growth control genes accompanying chloroform-induced cytolethality and regeneration may play a part in the development of chloroform-induced tumors. In this experiment, we examined the levels of the myc, fos, Ha-ras, met and hepatocyte growth factor mRNA in livers of female B6C3F(1) mice and kidneys of male F-344 rats to detect changes in gene expression following a single, cytotoxic gavage dose of chloroform in corn oil. Poly A+ RNA was purified from homogenates of livers of mice treated with 350mg/kg chloroform and kidneys of rats treated with 180 mg/kg chloroform and used for Northern blot analysis. Livers of female mice showed large transient increases in levels of myc and fos mRNA while levels of Ha-ras, met and the hepatocyte growth factor gene mRNA remained near control levels. In the male rat kidney, levels of myc mRNA increased after treatement, while levels of mRNA of all other genes examined remained near control levels. This pattern of gene expression is consistent with that induced by other cytotoxic carcinogens and suggest that alteration of the myc and fos genes could be involved in the regenerative cell proliferation that ultimately could play a role in chloroform-induced tumors.