Fourteen-day inhalation study in rats, using aged and diluted sidestream smoke from a reference cigarette. I. Inhalation toxicology and histopathology

[An inhalation chamber model for controlled studies of tobacco smoke toxicity in rodents]

INTRODUCTION

Smoking is a serious worldwide public health problem. Animal models act as a bridge between laboratory and human studies. The models applied are difficult to reproduce because of the use of different types of inhalation chambers and mainly because of the lack of continuous monitoring of smoke concentration.

OBJECTIVE

To develop an inhalation chamber for rats (with only the nose exposed) in which the amount of carbon monoxide (CO) can be maintained and monitored constantly.

MATERIAL AND METHODS

Male Wistar rats weighing 250g were exposed to 50ppm CO produced by the smoke from a filter-free cigarette. The animals were submitted to a single 2-h exposure and then sacrificed at 0, 4, 24 and 48h. The control group was left restrained inside the small perpendicular chambers, receiving only 5L/min of compressed air.

RESULTS

The model was able to increase HbCO levels immediately after the end of exposure (p<0.001), with a decrease being observed from 2h onwards when compared to the levels of the control group. Plasma cotinine increased immediately after exposure, and showed still detectable levels at 2 and 4h (p<0.05).

CONCLUSION

We conclude that the presented inhalation chamber system is able to maintain a controlled CO concentration in a model in which small animals are exposed to the inhalation of cigarette smoke, permitting well-controlled studies, as well as investigations involving other toxic gases and air pollutants.

Chronic nose-only inhalation study in rats, comparing room-aged sidestream cigarette smoke and diesel engine exhaust

Nose-only exposure of male and female Wistar rats to a surrogate for environmental tobacco smoke, termed room-aged sidestream smoke (RASS), to diesel engine exhaust (DEE), or to filtered, fresh air (sham) was performed 6 hours/day, 7 days/week for 2 years, followed by a 6-month post-exposure period. The particulate concentrations were 3 and 10 mg/m3. Markers of inflammation in bronchoalveolar lavage showed that DEE (but not RASS) produced a dose-related and persistent inflammatory response. Lung weights were increased markedly in the DEE (but not RASS) groups and did not decrease during the 6-month post-exposure period. Bulky lung DNA adducts increased in the RASS groups, but not in the DEE groups. Cell proliferation in the lungs was unaffected by either experimental treatment. Histopathological responses in the RASS groups were minimal and almost completely reversible; lung tumors were similar in number to those seen in the sham-exposed groups. Rats exposed to DEE showed a panoply of dose-related histopathological responses: largely irreversible and in some cases progressive. Malignant and multiple tumors were seen only in the DEE groups; after 30 months, the tumor incidence (predominantly bronchiolo-alveolar adenomas) was 2% in the sham-exposed groups, 5%in the high RASS groups, and 46% in the high DEE groups (sexes combined). Our results suggest that in rats exposed to DEE, but not to RASS, the following series of events occurs: particle deposition in lungs --> lung "overload" --> pulmonary inflammation --> tumorigenesis, without a significant modifying role of cell proliferation or DNA adduct formation.

A responsive, sensitive, and reproducible dermal tumor promotion assay for the comparative evaluation of cigarette smoke condensates

The mouse dermal initiation/promotion bioassay has been used for several decades to study cigarette smoke condensates (CSCs). However, these studies have used highly variable methodologies that differ in the manner of CSC collection, duration of treatment, mouse strain, number of mice and endpoints measured. In this report, a protocol that uses female SENCAR mice and standardizes many of the procedures is presented. A reference cigarette (University of Kentucky 1R4F), readily available to researchers, was used. This report presents the combined data from four independent studies. Female, SENCAR mice (40/group) were treated with a single dose (75microg) of dimethylbenz[a]anthracene (DMBA) as an initiator, followed 1 week later by treatment (three times/week) with 10, 20 or 40mg "tar"/application of 1R4F CSC for 29 weeks. There were no treatment-related effects on body weights. Histological diagnosis of all masses at study termination indicated a dose-dependent increase in the number of tumor-bearing mice and total tumor number. These studies support the conclusion that the 1R4F cigarette is suitable for use as a reference standard and the protocol presented is an appropriate and standardized model suitable for the comparative evaluation of CSC.

Health risks associated with inhaled nasal toxicants

Health risks of inhaled nasal toxicants were reviewed with emphasis on chemically induced nasal lesions in humans, sensory irritation, olfactory and trigeminal nerve toxicity, nasal immunopathology and carcinogenesis, nasal responses to chemical mixtures, in vitro models, and nasal dosimetry- and metabolism-based extrapolation of nasal data in animals to humans. Conspicuous findings in humans are the effects of outdoor air pollution on the nasal mucosa, and tobacco smoking as a risk factor for sinonasal squamous cell carcinoma. Objective methods in humans to discriminate between sensory irritation and olfactory stimulation and between adaptation and habituation have been introduced successfully, providing more relevant information than sensory irritation studies in animals. Against the background of chemoperception as a dominant window of the brain on the outside world, nasal neurotoxicology is rapidly developing, focusing on olfactory and trigeminal nerve toxicity. Better insight in the processes underlying neurogenic inflammation may increase our knowledge of the causes of the various chemical sensitivity syndromes. Nasal immunotoxicology is extremely complex, which is mainly due to the pivotal role of nasal lymphoid tissue in the defense of the middle ear, eye, and oral cavity against antigenic substances, and the important function of the nasal passages in brain drainage in rats. The crucial role of tissue damage and reactive epithelial hyperproliferation in nasal carcinogenesis has become overwhelmingly clear as demonstrated by the recently developed biologically based model for predicting formaldehyde nasal cancer risk in humans. The evidence of carcinogenicity of inhaled complex mixtures in experimental animals is very limited, while there is ample evidence that occupational exposure to mixtures such as wood, leather, or textile dust or chromium- and nickel-containing materials is associated with increased risk of nasal cancer. It is remarkable that these mixtures are aerosols, suggesting that their "particulate nature" may be a major factor in their potential to induce nasal cancer. Studies in rats have been conducted with defined mixtures of nasal irritants such as aldehydes, using a model for competitive agonism to predict the outcome of such mixed exposures. When exposure levels in a mixture of nasal cytotoxicants were equal to or below the "No-Observed-Adverse-Effect-Levels" (NOAELs) of the individual chemicals, neither additivity nor potentiation was found, indicating that the NOAEL of the "most risky chemical" in the mixture would also be the NOAEL of the mixture. In vitro models are increasingly being used to study mechanisms of nasal toxicity. However, considering the complexity of the nasal cavity and the many factors that contribute to nasal toxicity, it is unlikely that in vitro experiments ever will be substitutes for in vivo inhalation studies. It is widely recognized that a strategic approach should be available for the interpretation of nasal effects in experimental animals with regard to potential human health risk. Mapping of nasal lesions combined with airflow-driven dosimetry and knowledge about local metabolism is a solid basis for extrapolation of animal data to humans. However, more research is needed to better understand factors that determine the susceptibility of human and animal tissues to nasal toxicants, in particular nasal carcinogens.

The composition of cigarette smoke: a retrospective, with emphasis on polycyclic components

The difficulties encountered in extrapolating biological activity from cigarette smoke composition provide generally applicable lessons as they are representative of the problems encountered with other complex mixtures. Researchers attempting to assess risk are faced with attempting to interpret data from a number of areas including: tobacco science; smoke/aerosol chemistry specific to tobacco; sophisticated analytical chemistry applications and techniques for trapping, collecting, separating, and quantifying very specific compounds at nanogram to picogram levels; numerous biological testing methodologies; and animal models of tumors and carcinogenesis. Numerous hypotheses have been developed over the past five decades and tested with the technology of the day in attempts to interpret the biological activity of cigarette smoke in relation to the chemistry of this complex mixture. These hypotheses fall into several categories discussed in this review: mechanisms of pyrogenesis of polycyclic aromatic hydrocarbons (PAHs) in tobacco smoke; levels of PAHs in cigarette mainstream smoke (MS) and its tumorigenicity in mouse skin-painting experiments; control of PAH levels in MS; chemical indicators of cigarette smoke condensate (CSC) tumorigenicity; control of levels of MS components partitioned between the vapor phase and particulate phase of MS; tumorigenic threshold limits of CSC and many of its components; tumorigenic aza-arenes in tobacco smoke; MS components reported to be ciliastatic to smokers' respiratory tract cilia; anticarcinogenic tobacco-smoke components. Of 52 hypotheses reviewed in this paper, 15 have excellent data supporting the hypothesis based on today's technology. The remaining 37 hypotheses, although originally plausible, have since become insupportable in light of new and contradictory data generated over the years. Such data were generated sometimes by the original authors of the hypotheses and sometimes by other investigators. The hypotheses presented today are less likely to be supplanted because they are well conceived and have a strong mechanistic basis. The challenge for the future is the generation and interpretation of data relating the chemistry and biological activity associated with the dynamic and complex mixture of tobacco smoke.

Cytokeratin expression patterns in the rat respiratory tract as markers of epithelial differentiation in inhalation toxicology. II. Changes in cytokeratin expression patterns following 8-day exposure to room-aged cigarette sidestream smoke

The expression of specific cytokeratin (CK) polypeptide patterns is a sensitive marker of the cytoskeletal differentiation of epithelial cells. We developed an immunohistochemical method to assess CK expression patterns in the rat respiratory tract using serial paraffin-embedded sections from the nasal cavity, trachea, and lung. In the present study, this method was used to detect exposure-related differences in CK expression patterns in adult Wistar rats following inhalation of room-aged sidestream smoke (11 mg total particulate matter/m3 air, 8 days, 12 hr/day, whole body). In the anterior nasal cavity level 1 (NL1), changes in CK expression patterns were observed in the respiratory epithelium of the lateral wall and the maxilloturbinate (CK14, CK15, and CK18) and in the squamous epithelium of the ventral meatus (CK13). At nasal cavity level 2 (NL2), immediately behind NL1, changes were observed in the olfactory epithelium (CK13, CK14, and CK18) and in the respiratory epithelium of the septum (CK7 and CK19), the lateral wall (CK7 and CK13), and the lateral aspect of the maxilloturbinate (CK14). Changes were also observed in the submucosal glands, nasolacrimal duct, and vomeronasal organ. In the trachea only CK7 expression changed, and in the lung expression of CK7 (bronchioli) and CK8 (bronchus) changed; the expression of other CK polypeptides did not change. The observed changes in CK expression at NL1 correlated with the histomorphological changes, whereas CK expression changes were also seen in the olfactory and respiratory epithelia at NL2 and in the trachea and lung, where no histomorphological changes were seen. These findings indicate that changes in CK expression in respiratory tract epithelial cells are a sensitive marker for cellular stress response.

The toxicology of environmental tobacco smoke

It has by now become obvious that environmental tobacco smoke (ETS) may pose a health risk to nonsmokers. Epidemiological data suggest that exposure to ETS may increase the risk of developing lung cancer, cardiovascular disease, intrauterine growth retardation, predisposition to chronic lung disease, and sudden infant death syndrome. The human populations most at risk from ETS exposure appear to be neonates, young children, and possibly the fetus while in utero. Experimental studies with cigarette sidestream smoke (SS) have successfully duplicated several of these disease conditions in laboratory animals, particularly the effects of SS on fetal growth, lung maturation, and altered airway reactivity. The availability of animal models may open the way to fruitful experimental studies on mechanisms that help us to better understand disease.

Molecular toxicology endpoints in rodent inhalation studies

Although histopathology will continue to be essential for assessing the results of rodent inhalation studies, molecular toxicology endpoints are of increasing importance, as these techniques often complement and extend histopathological examinations. One of the primary uses of molecular toxicology is determining the delivered dose of the inhaled material to macromolecules in target tissues. During inhalation studies this is most often done by measuring DNA adducts in the respiratory tract. DNA adducts may be measured specifically (e.g. using monoclonal antibodies or mass spectrometry) or non-specifically (e.g. by using the 32P-post-labeling assay). Another major use of molecular toxicology techniques is the assessment of cellular and molecular changes in target tissues which may precede or be more sensitive than histopathologic alterations. For example, rates of cellular DNA synthesis occurring in target tissues may be quantified at any time during the study by administering the animals either radiolabelled thymidine or the non-radiolabelled thymidine analog bromodeoxyuridine (BrdU). Pulmonary changes may be assessed in bronchoalveolar lavage fluid using either cellular (e.g. macrophage number, granulocyte number) or biochemical (e.g. alkaline phosphatase, lactate dehydrogenase) techniques. The potential of the inhaled material to produce genetic alterations may be evaluated by examining the chromosomes of pulmonary alveolar macrophages for cytogenetic changes. To illustrate the use of these endpoints, an experiment was conducted to determine the molecular toxicology of aged and diluted sidestream smoke (a surrogate for environmental tobacco smoke) in rodent inhalation studies. The endpoints measured were DNA adducts in target and non-target tissue, chromosome aberrations in pulmonary alveolar macrophages, and DNA synthesis in the epithelial lining of the nasal turbinates.

Environmental tobacco smoke: current assessment and future directions

Scientific information on environmental tobacco smoke (ETS) is critically reviewed. Key areas addressed are: differences in chemical composition between mainstream smoke, sidestream smoke, and ETS; techniques for measurement of ETS; epidemiology; in vitro and in vivo toxicology; and chamber and field studies of perceptual or physiological effects. Questions concerning estimation of ETS exposure, suitability of various biomarkers, calculation of lifetime dose, control of confounding variables, use of meta-analysis, and the relationship between ETS concentrations and human responses all emphasize the need for additional research in order to assess potential effects of ETS on health or comfort.