Search

Differences among species in the anatomic sites of particle retention could influence responses to inhaled particles. In this study, we used morphometric techniques to examine the influence of exposure concentration on particle retention in histologic sections from rats and humans. The rats had been exposed for 24 months to diesel exhaust at 0.35, 3.5, or 7.0 mg soot/m(3). The human subjects were nonsmokers who did not work as miners, nonsmoking coal miners who worked under the current standard of 2 mg dust/m(3) for 10-20 years (mean = 14 years), and nonsmoking coal miners who worked under the former standard of < 10 mg dust/m(3) for 33-50 years (mean = 40 years). The distribution of retained particles within the lung compartments was markedly different between species. In all three groups of rats, 82-85% of the retained particulate material was located in the alveolar and alveolar duct lumens, primarily in macrophages. In humans, 57, 68, and 91% of the retained particulate material was located in the interstitium of the lung in the non-miners, coal miners under the current standard, and coal miners under the former standard, respectively. These results show that chronically inhaled diesel soot is retained predominantly in the airspaces of rats over a wide range of exposures, whereas in humans, chronically inhaled particulate material is retained primarily in the interstitium. In humans, the percentage of particles in the interstitium is increased with increased dose (exposure concentration, years of exposure, and/or lung burden). This difference in distribution may bring different lung cells into contact with the retained particles or particle-containing macrophages in rats and humans and may account for differences in species response to inhaled particles.

Activation of the K-ras protooncogene and inactivation of the p53 tumor suppressor gene are events common to many types of human cancers. Molecular epidemiology studies have associated mutational profiles in these genes with specific exposures. The purpose of this paper is to review investigations that have examined the role of the K-ras and p53 genes in lung tumors induced in the F344 rat by mutagenic and nonmutagenic exposures. Mutation profiles within the K-ras and p53 genes, if present in rat lung tumors, would help to define some of the molecular mechanisms underlying cancer induction by various environmental agents. Pulmonary adenocarcinomas or squamous cell carcinomas were induced by tetranitromethane (TNM), 4-methylnitrosamino-1-(3-pyridyl)-1-butanone (NNK), beryllium metal, plutonium-239, X-ray, diesel exhaust, or carbon black. These agents were chosen because the tumors they produced could arise via different types of DNA damage. Mutation of the K-ras gene was determined by approaches that included DNA transfection, direct sequencing, mismatch hybridization, and restriction fragment length polymorphism analysis. The frequency for mutation of the K-ras gene was exposure dependent. Only two agents, TNM and plutonium, led to mutation frequencies of > 10%. In both cases, the transition mutations formed could have been derived from deamination of cytosine. The identification of non-ras transforming genes in rat lung tumors induced by mutagenic and nonmutagenic exposures such as NNK and beryllium would help define some of the mechanisms underlying cancer induction by different types of DNA damage. Alteration in the p53 gene was assessed by immunohistochemical analysis for p53 protein and single-strand conformation polymorphism (SSCP) analysis of exons 4 to 9. None of the 93 adenocarcinomas examined was immunoreactive toward the anti-p53 antibody CM1. In contrast, 14 to 71 squamous cell carcinomas exhibited nuclear p53 immunoreactivity with no correlation to type of exposure. However, SSCP analysis only detected mutations in 2 of 14 squamous cell tumors that were immunoreactive, suggesting that protein stabilization did not stem from mutations within the p53 gene. Thus, the p53 gene does not appear to be involved in the genesis of most rat lung tumors. Images Figure 1. Figure 2.

The usefulness of pulmonary carcinogenicity data from rats exposed to high concentrations of particles for quantitatively predicting lung cancer risk in humans exposed to much lower environmental or occupational concentrations has been questioned. The results of several chronic inhalation bioassays of poorly soluble, nonfibrous particles have suggested that rats may be more prone than other rodent species to develop persistent pulmonary epithelial hyperplasia, metaplasia, and tumors in response to the accumulation of inhaled particles. In addition, rats and primates differ in their pulmonary anatomy and rate of particle clearance from the lung. This paper reviews results of recent Lovelace Respiratory Research Institute (Albuquerque, NM) investigations that directly compared the anatomical patterns of particle retention and the lung tissue responses of rats and monkeys exposed chronically to high occupational concentrations of poorly soluble particles. Lung sections from male cynomolgus monkeys and F344 rats exposed 7 hr/day, 5 days/week for 24 months to filtered ambient air, diesel exhaust (2 mg soot/m3), coal dust (2 mg respirable particulate material/m3), or diesel exhaust and coal dust combined (1 mg soot and 1 mg respirable coal dust/m3) were obtained from a study conducted at the U.S. National Institute for Occupational Safety and Health and examined histopathologically and morphometrically. Within each species, the sites of particle retention and lung tissue responses were the same for diesel soot, coal dust, and combined material. Rats retained a significantly greater portion of the particulate material in the lumens of alveolar ducts and alveoli than monkeys. Conversely, monkeys retained a significantly greater portion of the particulate material in the interstitium than rats. Rats, but not monkeys, had significant alveolar epithelial hyperplastic, inflammatory, and septal fibrotic responses to the retained particles. These results suggest that anatomic patterns of particle retention and lung tissue reactions in rats may not be predictive of retention patterns and tissue responses in primates that inhale poorly soluble particles at concentrations representing high occupational exposures.

The inhalation Toxicology Research Institute (ITRI) is conducting research to improve the understanding of chronic beryllium disease (CBD) and beryllium-induced lung cancer. Initial animal studies examined beagle dogs that inhaled BeO calcined at either 500 or 1000 degrees C. At similar lung burdens, the 500 degrees C BeO induced more severe and extensive granulomatous pneumonia, lymphocytic infiltration into the lung, and positive Be-specific lymphocyte proliferative responses in vitro than the 1000 degrees C BeO. However, the progressive nature of human CBD was not duplicated. More recently, Strains A/J and C3H/Hej mice were exposed to Be metal by inhalation. This produced a marked granulomatous pneumonia, diffuse infiltrates, and multifocal aggregates of interstitial lymphocytes with a pronounced T helper component and pulmonary in situ lymphocyte proliferation. With respect to lung cancer, at a mean lung burden as low as 17 micrograms Be/g lung, inhaled Be metal induced benign and/or malignant lung tumors in over 50% of male and female F344 rats surviving > or = 1 year on study. Substantial tumor multiplicity was found, but K-ras and p53 gene mutations were virtually absent. In mice, however, a lung burden of approximately 60 micrograms (-300 micrograms Be/g lung) caused only a slight increase in crude lung tumor incidence and multiplicity over controls in strain A/J mice and no elevated incidence in strain C3H mice. Taken together, this research program constitutes a coordinated effort to understand beryllium-induced lung disease in experimental animal models.