Oncogenic response of strain A/J mice to inhaled chemicals

Health Effects of Naphthalene Exposure: A Systematic Evidence Map and Analysis of Potential Considerations for Dose-Response Evaluation

BACKGROUND

Naphthalene is a polycyclic aromatic hydrocarbon that has been associated with health effects, including cancer. As the state of the science on naphthalene toxicity continues to evolve, updated toxicity reference value(s) may be required to support human health risk assessment.

OBJECTIVES

We present a systematic evidence map of studies that could be used to derive toxicity reference value(s) for naphthalene.

METHODS

Human and animal health effect studies and physiologically based pharmacokinetic (PBPK) models were identified from a literature search based on populations, exposures, comparators, and outcomes (PECO) criteria. Human and animal studies meeting PECO criteria were refined to a smaller subset considered most informative for deriving chronic reference value(s), which are preferred for assessing risk to the general public. This subset was evaluated for risk of bias and sensitivity, and the suitability of each study for dose-response analysis was qualitatively assessed. Lowest observed adverse effect levels (LOAELs) were extracted and summarized. Other potentially relevant studies (e.g., mechanistic and toxicokinetic studies) were tracked as supplemental information but not evaluated further. Existing reference values for naphthalene are also summarized.

RESULTS

We identified 26 epidemiology studies and 16 animal studies that were considered most informative for further analysis. Eleven PBPK models were identified. The available epidemiology studies generally had significant risk of bias and/or sensitivity concerns and were mostly found to have low suitability for dose-response analysis due to the nature of the exposure measurements. The animal studies had fewer risk of bias and sensitivity concerns and were mostly found to be suitable for dose-response analysis.

CONCLUSION

Although both epidemiological and animal studies of naphthalene provide weight of evidence for hazard identification, the available animal studies appear more suitable for reference value derivation. PBPK models and mechanistic and toxicokinetic data can be applied to extrapolate these animal data to humans, considering mode of action and interspecies metabolic differences. https://doi.org/10.1289/EHP7381.

Coexposure to Inhaled Aldehydes or Carbon Dioxide Enhances the Carcinogenic Properties of the Tobacco-Specific Nitrosamine 4-Methylnitrosamino-1-(3-pyridyl)-1-butanone in the A/J Mouse Lung

Tobacco smoke is a complex mixture of chemicals, many of which are toxic and carcinogenic. Hazard assessments of tobacco smoke exposure have predominantly focused on either single chemical exposures or the more complex mixtures of tobacco smoke or its fractions. There are fewer studies exploring interactions between specific tobacco smoke chemicals. Aldehydes such as formaldehyde and acetaldehyde were hypothesized to enhance the carcinogenic properties of the human carcinogen, 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK) through a variety of mechanisms. This hypothesis was tested in the established NNK-induced A/J mouse lung tumor model. A/J mice were exposed to NNK (intraperitoneal injection, 0, 2.5, or 7.5 μmol in saline) in the presence or absence of acetaldehyde (0 or 360 ppmv) or formaldehyde (0 or 17 ppmv) for 3 h in a nose-only inhalation chamber, and lung tumors were counted 16 weeks later. Neither aldehyde by itself induced lung tumors. However, mice receiving both NNK and acetaldehyde or formaldehyde had more adenomas with dysplasia or progression than those receiving only NNK, suggesting that aldehydes may increase the severity of NNK-induced lung adenomas. The aldehyde coexposure did not affect the levels of NNK-derived DNA adduct levels. Similar studies tested the ability of a 3 h nose-only carbon dioxide (0, 5, 10, or 15%) coexposure to influence lung adenoma formation by NNK. While carbon dioxide alone was not carcinogenic, it significantly increased the number of NNK-derived lung adenomas without affecting NNK-derived DNA damage. These studies indicate that the chemicals in tobacco smoke work together to form a potent lung carcinogenic mixture.

Inhaled Furan Selectively Damages Club Cells in Lungs of A/J Mice

Furan, a possible human carcinogen, is a product of incomplete combustion and is present in cigarette smoke, engine exhaust, and processed food. Oral administration induces liver toxicity and carcinogenesis in F344 rats and B6C3F1 mice. To assess possible adverse effects from inhalation, A/J mice were nose-only exposed for 3 hours to furan (0, 30, 75, 150, 300, or 600 ppmv) and euthanized after 24 hours, 48 hours, or 1 week. Histopathology evaluation revealed bronchiolar club cell necrosis (diffuse, marked) with airway denudation following exposure to 300 and 600 ppmv furan with evidence of club cell regeneration and partial repair after 1 week. Initial signs of hepatotoxicity were observed in the 150 ppmv furan-exposed group. Acute necrosis and mineralization were observed in livers at 24 and 48 hours with hepatocyte regeneration by 1-week postexposure in mice exposed to 300 and 600 ppmv furan; the 300 ppmv exposed group had multifocal mineralization that evoked a mild granulomatous response. Measurement of urinary furan metabolites confirmed that the mice metabolized furan to the toxic intermediate, cis-2-butene-1,4-dial. These observations indicate that inhaled furan is toxic to lungs with club cells as the target as well as liver.

Tobacco Smoke–Induced Lung Cancer in Animals—A Challenge to Toxicology (?)

Tobacco smoke is a known human carcinogen that primarily produces malignant lesions in the respiratory tract, although it also affects multiple other sites. A reliable and practical animal model of tobacco smoke–induced lung cancer would be helpful for in studies of product modification and chemoprevention. Over the years, many attempts to reproduce lung cancer in experimental animals exposed to tobacco smoke have been made, most often with negative or only marginally positive results. In hamsters, malignant lesions have been produced in the larynx, but not in the deeper lung. Female rats and female B6C3F1 mice, when exposed over lifetime to tobacco smoke, develop tumors in the nasal passages and also in the lung. Contrary to what is seen in human lung cancers, most rodent tumors are located peripherally and only about half of them show frank malignant features. Distant metastases are extremely rare. Male and female strain A mice exposed to 5 months to tobacco smoke and then kept for another 4 months in air respond to tobacco smoke with increased lung tumor multiplicities. However, the increase over background levels is comparatively small, making it difficult to detect significant differences when the effects of chemopreventive agents are evaluated. On the other hand, biomarkers of exposure and of effect as well as evaluation of putative carcinogenic mechanisms in rats and mice exposed to tobacco smoke allow detection of early events and their modification by different smoke types or chemopreventive agents. The challenge will be to make such data broadly acceptable and accepted in lieu of having to do more and more long term studies involving larger and larger number of animals.

Hypothesis-based weight-of-evidence evaluation and risk assessment for naphthalene carcinogenesis

Inhalation of naphthalene causes olfactory epithelial nasal tumors in rats (but not in mice) and benign lung adenomas in mice (but not in rats). The limited available human data have not identified an association between naphthalene exposure and increased respiratory cancer risk. Assessing naphthalene's carcinogenicity in humans, therefore, depends entirely on experimental evidence from rodents. We evaluated the respiratory carcinogenicity of naphthalene in rodents, and its potential relevance to humans, using our Hypothesis-Based Weight-of-Evidence (HBWoE) approach. We systematically and comparatively reviewed data relevant to key elements in the hypothesized modes of action (MoA) to determine which is best supported by the available data, allowing all of the data from each realm of investigation to inform interpretation of one another. Our analysis supports a mechanism that involves initial metabolism of naphthalene to the epoxide, followed by GSH depletion, cytotoxicity, chronic inflammation, regenerative hyperplasia, and tumor formation, with possible weak genotoxicity from downstream metabolites occurring only at high cytotoxic doses, strongly supporting a non-mutagenic threshold MoA in the rat nose. We also conducted a dose-response analysis, based on the likely MoA, which suggests that the rat nasal MoA is not relevant in human respiratory tissues at typical environmental exposures. Our analysis illustrates how a thorough WoE evaluation can be used to support a MoA, even when a mechanism of action cannot be fully elucidated. A non-mutagenic threshold MoA for naphthalene-induced rat nasal tumors should be considered as a basis to determine human relevance and to guide regulatory and risk-management decisions.

A novel application of the Margin of Exposure approach: segregation of tobacco smoke toxicants

This paper presents a rationale for utilising a Margin of Exposure (MOE) approach to the segregation of tobacco smoke toxicants for risk assessment and management purposes. Future regulatory frameworks and product modifications aimed at tobacco harm reduction could utilise data that segregate toxicants using associations with specific diseases caused by cigarette smoking together with an indication of their relative contribution to that disease. Compounds with MOEs >10,000 accompanied by appropriate narrative are considered "low priority for risk management actions". This paper applies the MOE model to representative examples of tobacco smoke toxicants associated with respiratory tract carcinogenesis and other respiratory diseases. A multiplicity of published dose response data on individual toxicants has been used to determine the range of possible MOE values, thus demonstrating the consistency of the relationships. Acetaldehyde, acrolein, acrylonitrile, cadmium, ethylene oxide, formaldehyde and isoprene all segregate with MOEs <10,000 and should be considered as high priority for exposure reduction research whereas benzo(a)pyrene and vinyl chloride segregate with an MOE >10,000 and therefore may be considered as a low priority. 1,3-Butadiene, m-/p-cresols, NNK and NNN are assumed to segregate with high priority although additional data would be required to complete a full MOE assessment.

Mechanisms Involved in A/J Mouse Lung Tumorigenesis Induced by Inhalation of an Environmental Tobacco Smoke Surrogate

Lung tumors have been reproducibly induced in A/J mice exposed to a surrogate for experimental environmental tobacco smoke (ETSS) in a 5-mo inhalation period followed by 4 mo without further exposure. In order to increase our mechanistic understanding of this model, male mice were whole-body exposed for 6 h/d, 5 d/wk to ETSS with a particulate matter concentration of 100 mg/m(3). Food restriction regimens were included to model or exceed the ETSS-related impairment of body weight development. Half of the mice were pretreated with a single ip injection of urethane to study the effect of the above treatments on lung tumor development induced by this substance. At 5 mo, the tumor response was statistically the same for all groups of non-pretreated mice; however, the expected urethane-induced lung tumorigenesis was significantly inhibited by approximately 25% by ETSS and food restriction. This inhibition was accompanied by a threefold increase in blood corticosterone as a common stress marker for both ETSS and food restriction. At 9 mo, in mice not pretreated, the lung tumor incidence and multiplicity were significantly increased by twofold in the ETSS group; in the urethane-treated groups, the same high tumor multiplicity was reached regardless of previous treatment. The predominant tumor type in all groups was bronchiolo-alveolar adenoma. There was no induction of a specific K-ras mutation pattern by ETSS exposure. These data suggest a stress-induced inhibition of lung tumorigenesis in this model, explaining the need for the posttreatment period.

An adjustment factor for mode-of-action uncertainty with dual-mode carcinogens: the case of naphthalene-induced nasal tumors in rats

The U.S. Environmental Protection Agency (USEPA) guidelines for cancer risk assessment recognize that some chemical carcinogens may have a site-specific mode of action (MOA) involving mutation and cell-killing-induced hyperplasia. The guidelines recommend that for such dual MOA (DMOA) carcinogens, judgment should be used to compare and assess results using separate "linear" (genotoxic) versus "nonlinear" (nongenotoxic) approaches to low-level risk extrapolation. Because the guidelines allow this only when evidence supports reliable risk extrapolation using a validated mechanistic model, they effectively prevent addressing MOA uncertainty when data do not fully validate such a model but otherwise clearly support a DMOA. An adjustment-factor approach is proposed to address this gap, analogous to reference-dose procedures used for classic toxicity endpoints. By this method, even when a "nonlinear" toxicokinetic model cannot be fully validated, the effect of DMOA uncertainty on low-dose risk can be addressed. Application of the proposed approach was illustrated for the case of risk extrapolation from bioassay data on rat nasal tumors induced by chronic lifetime exposure to naphthalene. Bioassay data, toxicokinetic data, and pharmacokinetic analyses were determined to indicate that naphthalene is almost certainly a DMOA carcinogen. Plausibility bounds on rat-tumor-type-specific DMOA-related uncertainty were obtained using a mechanistic two-stage cancer risk model adapted to reflect the empirical link between genotoxic and cytotoxic effects of the most potent identified genotoxic naphthalene metabolites, 1,2- and 1,4-naphthoquinone. Bound-specific adjustment factors were then used to reduce naphthalene risk estimated by linear extrapolation (under the default genotoxic MOA assumption), to account for the DMOA exhibited by this compound.

Naphthalene metabolism in relation to target tissue anatomy, physiology, cytotoxicity and tumorigenic mechanism of action

This report provides a summary of deliberations conducted under the charge for members of Module C Panel participating in the Naphthalene State-of-the-Science Symposium (NS(3)), Monterey, CA, October 9-12, 2006. The panel was charged with reviewing the current state of knowledge and uncertainty about naphthalene metabolism in relation to anatomy, physiology and cytotoxicity in tissues observed to have elevated tumor incidence in these rodent bioassays. Major conclusions reached concerning scientific claims of high confidence were that: (1) rat nasal tumor occurrence was greatly enhanced, if not enabled, by adjacent, histologically related focal cellular proliferation; (2) elevated incidence of mouse lung tumors occurred at a concentration (30 ppm) cytotoxic to the same lung region at which tumors occurred, but not at a lower and less cytotoxic concentration (tumorigenesis NOAEL=10 ppm); (3) naphthalene cytotoxicity requires metabolic activation (unmetabolized naphthalene is not a proximate cause of observed toxicity or tumors); (4) there are clear regional and species differences in naphthalene bioactivation; and (5) target tissue anatomy and physiology is sufficiently well understood for rodents, non-human primates and humans to parameterize species-specific physiologically based pharmacokinetic (PBPK) models for nasal and lung effects. Critical areas of uncertainty requiring resolution to enable improved human cancer risk assessment were considered to be that: (1) cytotoxic naphthalene metabolites, their modes of cytotoxic action, and detailed low-dose dose-response need to be clarified, including in primate and human tissues, and neonatal tissues; (2) mouse, rat, and monkey inhalation studies are needed to better define in vivo naphthalene uptake and metabolism in the upper respiratory tract; (3) in vivo validation studies are needed for a PBPK model for monkeys exposed to naphthalene by inhalation, coupled to cytotoxicity studies referred to above; and (4) in vivo studies are needed to validate a human PBPK model for naphthalene. To address these uncertainties, the Panel proposed specific research studies that should be feasible to complete relatively promptly. Concerning residual uncertainty far less easy to resolve, the Panel concluded that environmental, non-cytotoxic exposure levels of naphthalene do not induce tumors at rates that can be predicted meaningfully by simple linear extrapolation from those observed in rodents chronically exposed to far greater, cytotoxic naphthalene concentrations.

A review of whole animal bioassays of the carcinogenic potential of naphthalene

This report provides a summary of deliberations conducted under the charge for members of Module A participating in the Naphthalene State-of-the-Science Symposium (NS3), Monterey, CA, October 9-12, 2006. Whole animal bioassays have been performed by the National Toxicology Program in mice and rats to ascertain the carcinogenic potential of naphthalene by inhalation exposure. A statistically significant increased incidence of pulmonary alveolar/bronchiolar adenoma (a benign lesion), was observed among female mice; an observed increase among the males did not reach statistical significance. No nasal tumors were observed in either sex. A tumorigenic response was observed in both sexes of rats, in males an increased incidence of nasal respiratory epithelium adenoma (a benign rather than malignant lesion) and in females, olfactory epithelial neuroblastoma. Interpretations of these studies vary. On the one hand, evidence of extensive non-neoplastic response in both sexes of both species indicates cytotoxicity occurred at all doses, and strongly suggests that cytotoxicity played a significant role in the tumor responses observed in the target tissues. On the other hand, olfactory epithelial neuroblastoma has rarely been observed in NTP bioassays. This review seeks to develop a consensus understanding of the scientific evidence provided by these studies, taking into account that they have been used as the basis for quantitative human cancer risk assessment, and suggests scientific studies that, if performed, could resolve scientific uncertainties.

Screening and characterization of variant Theta-class glutathione transferases catalyzing the activation of ethylene dibromide to a mutagen

Ethylene dibromide (EDB) is a widespread environmental pollutant and mutagen/carcinogen. Certain Theta-class glutathione transferases (GSTs), enzymes that catalyze the reaction of reduced glutathione (GSH) with electrophiles, activate EDB to a mutagen. Previous studies have shown that human GST T1-1, but not rat GST T2-2, activates EDB. We have constructed an E. coli lacZ reversion mutagenicity assay system in which expression of recombinant GST supports activation of EDB to a mutagen. Hexa-histidine N-terminal tagging of GST T1-1 results in greatly enhanced expression of the recombinant enzyme and gives a lacZ strain that shows a mutagenic response to EDB at extremely low levels (approximately 1 ng EDB per plate). The hexa-histidine-tagged enzyme was purified in one step by Ni(2+)-affinity chromatography. We applied the lacZ mutagenicity assay to the rapid screening of a library of variant GST Theta enzymes. Sequence variants with altered catalytic activities were identified, purified, and characterized.

Lung carcinogenesis: pivotal role of metals in tobacco smoke

Although significant progress has been made in unraveling the molecular mechanisms responsible for tobacco smoke toxicity and carcinogenicity, only limited information is available concerning the mechanisms by which tar particles and the gaseous phase constituents of tobacco smoke participate and contribute to carcinogenic processes in lung cancer. The present review critically evaluates how metals contained in the tar particles and the gaseous phase of tobacco smoke play a leading role in the carcinogenic process, taking into consideration the physiology and pathophysiology of the bronchial epithelium. Overwhelmingly, the published data indicate that the bronchopulmonary epithelial cells may represent the first and most critical line of defense against cigarette smoke.

Naphthalene and beta-naphthoflavone effects on Anguilla anguilla L. hepatic metabolism and erythrocytic nuclear abnormalities

The effects of a polycyclic aromatic hydrocarbon (PAH) such as naphthalene (NAP)--an environmental contaminant--and beta-naphthoflavone (BNF)--a model substance (PAH-like compound)--were investigated in European eel (Anguilla anguilla L.) over 3-, 6-, and 9-day exposure (0.1-2.7 microM). Both xenobiotics revealed to be strong biotransformation (phase I) inducers. After 3-day exposure, liver ethoxyresorufin O-deethylase (EROD) activity was significantly increased by all NAP and BNF tested concentrations. At 6 and 9 days, liver EROD activity was significantly induced mainly by the highest NAP and BNF concentrations. Liver cytochrome P450 content was significantly induced after 3-day exposure to 0.9 and 2.7 microM BNF and 9-day exposure to 0.1, 0.3 and 0.9 microM NAP. Liver alanine transaminase (ALT) activity was measured as an indicator of hepatic health condition, revealing a significant decrease after 6-day exposure to 0.9 microM BNF. Genotoxicity measured as erythrocytic nuclear abnormalities (ENA) was detected in all BNF treated fish on day 6, whereas on day 9, ENA frequencies returned to control levels, significantly decreasing at 0.9 microM BNF exposure. Immature erythrocytes (IE) frequency demonstrated a decreasing tendency along the BNF experiment and concomitantly with the above ENA response. The present experimental results elect EROD activity in A. anguilla as a useful short- to medium-term biomarker of exposure to both PAH and PAH-like compounds. However, some problems can emerge in the presence of high xenobiotic concentrations. Concerning genotoxicity, it is hypothesized that ENA response depends on different factors such as the exhaustion of the detoxification process, the balance erythropoiesis/erythrocytic catabolism and the DNA repairing capacity.

Review of the genotoxicity of nitrogen oxides

Nitrogen oxides (NOx) are formed in combustion processes and are major pollutants in urban air. Relatively few studies on the genotoxicity of NO2 and NO have been performed. These studies indicate that NO2 is genotoxic in vitro, but the effect of NO seems to be very slight. One in vivo study showed chromosome aberrations and mutations in lung cells after inhalation of NO2 (and NO), but tests for chromosome aberrations in lymphocytes and spermatocytes or micronuclei in bone marrow were negative after inhalation of NO2. Based on present studies, there is no clear evidence of a carcinogenic potential of NO2, although lung adenomas were induced in the susceptible strain A/J mouse. The primary metabolites of NOx are nitrite and nitrate. Nitrate seems to be devoid of genotoxic properties, but nitrite is genotoxic in vitro, and there are also positive in vivo results. Cancer studies have been mainly negative. However, carcinogenic nitrosamines have been shown to be formed in vivo after inhalation of NO2. Nitrogen oxides are key components in atmospheric smog formation, which may lead to secondary effects. Strongly mutagenic nitro-PAH compounds are easily formed, and mutagenic reaction products may be formed photochemically from alkenes.

NO2-induced DNA single strand breaks are inhibited by antioxidative vitamins in V79 cells

Recently, we were able to show that nitrogen dioxide (NO2), a strong oxidant, induced DNA single strand breaks (SSBs) in V79 cells. Possibly, special scavengers, e.g. antioxidative vitamins, may protect cells from NO2-induced damage. Therefore, the effect of various tocopherols, beta-carotene, retinol, and ascorbic acid on NO2-induced SSBs in V79 cells was investigated. Cells were preincubated with vitamins and treated for 10 min with 200 ppm NO2. The rate of SSBs was measured by the alkaline elution assay, the amount of DNA by a fluorimetric assay. Micromolar concentrations of d-gamma-tocopherol inhibited the rate of NO2-induced SSBs by 40%, beta-carotene and ascorbic acid by 25%. None of these vitamins had any effects on DNA or the viability of cells. When incubating the cells with retinol in a medium with pH 8.5, this vitamin inhibited NO2-induced SSBs, reducing them by 35%. However, in high concentrations, retinol itself induced SSBs and influenced cell viability. The results are discussed with regard to many toxic effects of NO2.

Experimental studies on tumor promotion by nitrogen dioxide

The effects of nitrogen dioxide (NO2) on promotion of lung tumorigenesis induced by N-bis(2-hydroxypropyl) nitrosamine (BHPN) were investigated in male Wistar rats. In a preliminary study, the highest non-effective dose of BHPN was found to be 0.5 g per kg body weight. Rats were given a single intraperitoneal injection of BHPN at a dose of 0.5 g per kg body weight or saline at 6 weeks of age, and then exposed to clean air, 0.04 ppm, 0.4 ppm or 4 ppm of NO2 for 17 months, respectively. The incidence of pulmonary tumors in rats exposed to BHPN plus 4 ppm of NO2 was 12.5%; the tumors were adenomas and adenocarcinomas. Adenomas were found in 4 out of 40 rats (10%) and adenocarcinomas were found in 1 out of 40 rats (2.5%). The tumor incidence in the lungs of rats kept in BHPN plus clean air and BHPN plus 0.04 ppm of NO2 was 2.5% (1/40). In both groups adenomas were found. There was no significant difference in tumor incidence between animals exposed to BHPN plus clean air and to BHPN plus 4 ppm of NO2. No lung tumors were found in the group of BHPN plus 0.4 ppm NO2 and in animals exposed to NO2 without BHPN treatment. A high incidence of alveolar cell hyperplasia was observed in the lungs of rats injected with BHPN, and the effect of NO2 on development of alveolar cell hyperplasia was slight. On the other hand, marked bronchiolar mucosal hyperplasia was found in 17 out of 40 rats (42.5%) in the group of BHPN plus 4 ppm of NO2, and in 1 out of 40 rats (2.5%) in each of the group exposed to clean air, 0.04 ppm or 0.4 ppm of NO2 with BHPN treatment, respectively. The hyperplasia in lungs of rats exposed to 4 ppm of NO2 without BHPN treatment was slighter than that in lung of rat exposed to 4 ppm of NO2 with BHPN treatment. On the other hand, tumor incidence in the nasal cavity of rats in each of group exposed to clean air and NO2 with BHPN treatment was 97-100%. Incidence of tumors in other organs in the groups exposed to clean air and NO2 with and without BHPN treatment was very low, and NO2 had no effect on tumor development in the nasal cavity and other organs whether animals were treated with BHPN or not.(ABSTRACT TRUNCATED AT 400 WORDS)

Ozone, nitrogen dioxide and lung cancer: a review of some recent issues and problems

Lung cancer in man is a common disease. There is some recent concern that oxidant air pollutants might be a contributing risk factor. Experimental data show that ozone and NO2 increase incidence and multiplicity of lung tumors in strain A mice; however, the data are not always statistically significant. Also it depends on experimental design whether ozone enhances or inhibits the development of lung tumors in mice. Similarly, ozone and nitrogen dioxide enhance lung colonization by cancer cells injected intravenously following exposure to the air pollutants, whereas NO2 kills lung metastases if cells are injected prior to exposure. Both ozone and NO2 modulate the proliferation of pulmonary neuroendocrine cells, the precursor cells for small cell lung cancer. It is concluded that there is little evidence to implicate ozone or NO2 directly as pulmonary carcinogens, but that they might modify and influence the carcinogenic process in the lung.

Failure of carbon disulfide and levothyroxine to modify the cardiovascular response of rabbits to a high-cholesterol diet

Exposure of rabbits for 12 weeks to 300 ppm carbon disulfide (CS2) for 6 h/day, 5 days/week, or to 25 mg/day of thiourea or 2% cholesterol in the diet, or to any combination thereof caused a significant reduction in the concentration of serum thyroxine (T4). The reduction of the concentration of serum T4 in rabbits by the treatments was completely offset by the inclusion of 0.1 mg/day of sodium levothyroxine in the diet. Ingestion of feed containing 2% cholesterol significantly increased the degree of atherosclerosis present in the aortic arch and significantly increased the oil red O positive lipid present in the heart and the aorta, with the aortic arch being the most severely affected. The response of the aorta and the heart to the 2% cholesterol diet was not significantly modified by concurrent exposure to CS2 by inhalation or by treatment with thiourea, a metabolite of CS2. We found no evidence that the development of cardiovascular lesions induced by a 2% cholesterol diet in rabbits was mediated by a mechanism involving a component of hypothyroidism.