There is considerable evidence that inhaled toxicants such as cigarette smoke can cause both irreversible changes to the genetic material (DNA mutations) and putatively reversible changes to the epigenetic landscape (changes in the DNA methylation and chromatin modification state). The diseases that are believed to involve genetic and epigenetic perturbations include lung cancer, chronic obstructive pulmonary disease (COPD), and cardiovascular disease (CVD), all of which are strongly linked epidemiologically to cigarette smoking. In this review, we highlight the significance of genomics and epigenomics in these major smoking-related diseases. We also summarize the in vitro and in vivo findings on the specific perturbations that smoke and its constituent compounds can inflict upon the genome, particularly on the pulmonary system. Finally, we review state-of-the-art genomics and new techniques such as high-throughput sequencing and genome-wide chromatin assays, rapidly evolving techniques which have allowed epigenetic changes to be characterized at the genome level. These techniques have the potential to significantly improve our understanding of the specific mechanisms by which exposure to environmental chemicals causes disease. Such mechanistic knowledge provides a variety of opportunities for enhanced product safety assessment and the discovery of novel therapeutic interventions.
In this paper, I review the results of a representative selection of chronic inhalation studies with rats and mice exposed to mainstream cigarette smoke and describe the inhalation exposures and the histopathological changes reported by various authors. Many of the studies used nose-only exposure systems, whereas others simply used large whole-body chambcrs. Smoke-induced epithelial hypertrophy, hyperplasia, and squarnous metaplasia were reported in the conducting airways in most of the studies, along with increased numbers of intra-alveolar macrophages that were occasionally associated with alveolar metaplasia. Lung adenomas and adenocarcinomas were reported in only a few of the studies. No statistically significant increase in the incidence of malignant lung tumors was secn in either species as a result of smoke exposure, a finding that does not agree with the results of epidemiological studies in humans. Possible reasons for this lack of correlation are given.
Keywords. Cigarette smoke; inhalation toxicology; rodent respiratory tract
INTRODUCTIONRats and mice are used routinely in studies of experimental carcinogenesis (12, 30, 33), leading others (1 1) to consider that "these laboratory results are the main source of information that is used to set regulatory standards for potential human exposures." The (unstated) presumption is that the rodent carcinogenic responses accurately predict human carcinogenic responses (27). The present review was performed to verify whether the measured response to cigarette smoke in rodents reflects the strong epidemiological findings in human smokers (1 9, 3 1).This paper is a critical review of the scientific literature relating to the' pulmonary pathology of rats and mice exposed by inhalation to cigarette smoke. There is a large amount of published literature on this subject, so for space reasons only a selection of the major papers is presented: the reviewed. findings are similar to those in papers not so selected. Although there are numerous reports on smoke inhalation studies with other small animals (e.g., hamsters and rabbits) and larger animals (e.g., dogs and primates), these species will be considered in future reviews. Rigorous criteria by which to evaluate the results were selected in accord with accepted standards of toxicology, pathology, and carcinogenesis.Reviews of the earlier literature are available (32, 36), so the present review includes o d y the more recent work on animal inhalation studies with cigarette smoke. Cri-
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