Crystalline silica and lung cancer: a review of recent experimental evidence.

The International Agency for Research in Cancer (IARC) has recently stated that crystalline silica should be regarded as a potential carcinogen. The IARC bases this statement on a finding that there is limited evidence of carcinogenicity in humans and sufficient evidence for carcinogenicity in animals. Recent laboratory animal experiments demonstrating a carcinogenic response to silica exposure have intensified scientific and regulatory concern for crystalline silica as a respiratory carcinogen. Studies of human populations have been contradictory in demonstrating a causal relationship between crystalline silica exposure and lung cancer. This paper reviews recent experimental evidence and attempts to identify the gaps and inconsistencies in our understanding of the relationship between exposure to crystalline silica and the two diseases of concern: silicosis and pulmonary neoplasia. Given our current level of understanding and the need for more scientific data it seems premature to initiate changes in exposure regulations at this time.

Periodic response difference in mouse epidermis chronically exposed to crude-oils or BaP: males vs. females.

Skin cancer has long been associated with industrial and environmental exposure to individual chemical compounds, complex chemical mixtures, X-irradiation and UV-irradiation. Skin carcinogenesis in animal models has been used as a toxicity assay and to study mechanisms of neoplastic progression. Using a mouse-skin carcinogenesis model, we conducted assays of tumorigenic activity using several crude oils and benzo[a]pyrene. During analysis of the data for assessment of tumor production by these compounds, differences were observed in male and female time-to-tumor values. There were 4 possible observations for males and females in each of the treatment groups: (1) no significantly different time-to-tumor values between the sexes; (2) accelerated or delayed response by one or the other sex; (3) production of a greater number of tumors in one or the other sex; and (4) production of different tumor types in one of the sexes. Our data show that 3 of these 4 do occur in mice epidermally-exposed to these compounds. There were groups with no differences between male/female response to treatment. Of specific interest were periodic male/female differences in time-to-time values. Only 1 compound, a shale-derived crude oil, produced tumors more extensively in one sex than the other. While benzo[a]pyrene produced predominantly carcinomas, the crude oils produced papillomas and carcinomas to varying degrees. However, the types of tumors produced showed no preference for one or the other sex. In the groups with different time-to-tumor values for male and female animals, the differences occurred either throughout the course of the experiment or in specific time domains. In 1 treatment group, the females had significantly earlier time-to-tumor values than males. In several of the other treatment groups the females had significantly longer time-to-tumor values. Of these treatment groups, the female mean time-to-tumor values were either cumulatively significantly longer or periodically delayed compared to the male values. The presence of significantly accelerated or delayed tumor production by one sex in these animal systems implies sex-related modulation of neoplastic progression.

Silica-induced alveolar cell tumors in rats.

Min-U-Sil5, a form of alpha quartz, has been shown to induce peripheral lung tumors in rats exposed to the dust by inhalation. The animals were exposed to a nominal particle concentration of 12.4 mg/m3 for 8 hr/day, 4 days/week, for 2 years. The induced tumors were large and peripheral, and, when examined by electron microscopy, were found to be composed predominantly of alveolar type II cells. These cells were found in papillary, acinar, and solid forms of the tumors and were characterized by lamellar inclusion bodies. This is in contrast to the mouse, in which the papillary form was associated with Clara cells and the acinar form was linked with the type II cell. In this study, the Clara cell was a minor component of the tumor mass. No clear risk is established in man linking silica exposure to increased lung tumor rates.

Nitrogen dioxide exposure and development of pulmonary emphysema.

This study assessed the relationship between nitrogen dioxide inhalation and the development of pulmonary emphysema and investigated how the severity of preexisting emphysema brought about by protease (elastase) instillation into the lung may be augmented by a subchronic exposure to a relatively high concentration of nitrogen dioxide. Lungs of adult Fischer-344 rats were evaluated for emphysematous changes after (1) a single intratracheal instillation of elastase (E), (2) a 25-d exposure to 35 ppm nitrogen dioxide (NO2), and (3) elastase instillation followed by 25-d exposure to 35 ppm NO2 (E + NO2). Rats instilled with sterile normal saline and subsequently exposed to filtered air served as a control group (NS). Residual volumes (RV) of the NO2 and NS groups were virtually identical, whereas the RV of the E and E + NO2 lungs (2.3 and 2.3 ml, respectively) were significantly greater than those of the NS and NO2 lungs (1.3 and 1.4 ml, respectively). Directionally similar changes in the excised lung volumes and total lung capacities were obtained with the E and E + NO2 groups; NO2 alone, however, did not alter these volumetric parameters. No differences in arterial blood gases and pH values, minute ventilation, or breathing frequencies were found among the experimental groups. The mean linear intercept values (MLI) obtained with the NS and NO2 exposed lungs were essentially identical with average values of approximately 62 micron. This morphometric parameter was substantially increased in the E- and E + NO2-exposed lungs; no significant differences, however, were found between the MLI values obtained with the E and E + NO2 lungs (approximately 95 and approximately 97 micron, respectively). From these data, as well as histologic examinations of lung sections for evidence of emphysema, we conclude that (1) a subchronic, moderately high level of NO2 exposure does not produce an irreversible emphysematous lesion in the rat model and (2) exposure of rats to 35 ppm for 25 d after elastase instillation into the lungs does not potentiate protease-induced emphysema or bring about a progression in preexisting emphysema.

Pulmonary macrophages: alveolar and interstitial populations.

The sizes of the alveolar macrophage (AM) and interstitial macrophage (IM) populations in the lungs of adult Fischer-344 rats were determined during steady state. AM labeled with opsonized erythrocytes during an in situ phagocytic assay were lavaged from excised lungs. The lungs were then dispersed into single-cell suspensions with collagenase and mechanical agitation, and the remaining mononuclear phagocytes were identified following a second labeling step. The size of the AM population was 1.3 X 10(7) cells, or approximately equal to 3% of the total lung cell population. The AM were negative for cytoplasmic myeloperoxidase granules. The size of the IM population was 8 X 10(6) cells, or approximately equal to 2% of the total lung cell population. IM were also negative for myeloperoxidase and, like AM, demonstrated marked Fc gamma R-mediated phagocytic activity. The high cell yields (approximately equal to 4 X 10(8) cells/lung; viability, greater than 85%) and the percentages of type II cells (11%) and ciliated epithelial cells (less than 0.5%) indicated the enzymatic dispersion method resulted in a highly efficient and representative sampling of the lung parenchyma. The collagenase method used in this study to disperse the lung cells into single-cell suspensions, in conjunction with additional cell separation techniques, may be of potential use for isolating enriched populations of IM, as well as other lung cell types, for in vitro study.

Role of anchorage in the expression of tumorigenicity of untransformed mouse cell lines.

Cultured mouse cells were tested for tumorigenicity in nude mice with both a conventional assay (injection of cell suspensions) and a new test involving implantation of cells grown on gelatin sponges. Sublines of Balb/3T3 cells, obtained from different sources, varied in their tumorigenic potential with either assay. One subline (A) formed distinctive precancerous nodules only in the sponge assay; these nodules often became progressive after a latent period of 3-4 months. However, suspensions of cells of this subline also caused tumors after a similar latent period, but no nodular phase preceded tumor formation. Another subline of Balb/3T3 (M) has failed to form tumors in either assay. The Balb/3T3 sublines did not differ in vitro properties, such as low saturation density, failure to grow in methylcellulose, and monolayer morphology. A second experimental approach involved tests on nude BALB/c mouse-embryo fibroblasts at various passage levels. The cells were passaged from primary culture, through crisis, to heteroploid, established cell lines. Tumorigenicity was demonstrable earlier in the sponge assay, at which time in vitro parameters putatively associated with malignant behavior were unchanged. Possible relationships with the in vivo phenomenon of solid-surface sarcomagenesis are discussed.

The effect of application frequency on epidermal carcinogenesis assays.

Classic mouse skin-painting assays of such highly-complex mixtures as petroleums and synthetic fuels may lead to severe local cytotoxic effects that can alter tumorigenesis. This effect can be particularly disruptive in dose-response studies where a wide range of doses is employed. The experiments described here illustrate that frequency of exposure may be more important than the concentration of individual doses. C3Hf/Bd mice were exposed to 2 shale oils in experiments in which a constant weekly dose was either applied once or divided into 2 or 4 equal parts and applied either twice or 4 times each week. In experiments with either oil, a single weekly application was more effective in total tumor production and less cytotoxic than more frequent application of less-concentrated doses.

Biological effects of raw and processed oil shale particles in the lungs of laboratory animals.

Environmental and occupational health concerns will have an effect on the developing oil shale technologies. The mining and crushing of large volumes of rock will be a characteristic of at least some of these technologies, and above ground disposal of proceessed shale will require adequate control measures. Exposure by inhalation to the dusts that may arise from shale oil technologies may present a hazard both in the work force and in the local population. Animal studies dealing with the effects of oil shale-related materials in the lung are in progress. Experiments involving Syrian hamsters exposed by inhalation and by intratracheal instillation are described.

Rapid radioiodination of rose bengal at room temperature.

Published methods for radioiodination of rose bengal require reaction times of 1 hr or more at temperature from 50 to 120 degrees C. Through the use of an acidified ethanol solvent and potassium iodate oxidant, purified rose bengal is radioiodinated at room temperature within 15 min with chemical yields ranging between 93 and 97%. Radiochemical impurities are sufficiently minimized to permit preparation in a single 10-ml serum vial, requiring no additional purification steps. The method reported here is readily adaptable to cold-kit preparation.

Lung response to localized irradiation from plutonium microspheres.

Uniform spherical 10-micrometer diameter particles of ZrO2 ceramic, containing various concentrations of PuO2, have been injected into the jugular vein of Syrian hamsters with subsequent permanent lodgement in the lung capillaries. The number of particles injected has varied from 2000 to 2 000 000 and the specific activity has been from 0.16 to 59 pCi/sphere so that lung burdens range from 0.2 to 700 nCi. To date, approximately 3000 hamsters have been committed to the experiment and two-thirds have died--the expected rate for normal animals. Little biological damage has been observed, and only five primary lung tumours have been found that may be due to radiation delivered to the lung. To provide a comparison to more uniform radiation, soluble polonium has also been instilled intratracheally. Results from the microspheres suggest that localized lung irradiation alone is not sufficient cause for tumour induction and is much less hazardous than diffuse exposure.

Ocular hazard from picosecond pulses of Nd: YAG laser radiation.

Seven rhesus monkeys (14 eyes) were exposed to 1064-nanometer radiation in single pulses of 25 to 35 picoseconds fromn a mode-locked Nd: YA G laser. Threshold injury resulted from single pulses with a mnean energy of 13 +/- 3 mnicrojoules. Electron microscopy of the retina revealed that damnage was highly localized in the photoreceptor and pigmented epithelial cells at the oluter retina. Membrane disruption, distorted outer segmtzents, and abnormnal melanin granules resembling fetal premelanosomnies were observed.

The effects of dose protraction on hematopoiesis in the primate and dog.

The modifying effects of dose protraction by fractionation or continuous low dose-rate exposure are not well known in the primate. Comparative studies between the two mammalian species dog and monkey with widely differing acute LD50 values have not been made. A program to study dose-rate effects on injury and recovery of the bone marrow in dogs and monkeys under simultaneous and similar exposure conditions is in progress. The equivalent residual dose (ERD) assumptions of 10% irreparable injury and a 28-day recovery half-time for the reparable portion (90%) of radiation were studied in dogs (beagles) and monkeys (Macaca arctoides) using variable recovery times and appropriate fractionated exposures to attain but not exceed theoretical effective residual body burdens of 200 rads over a 1-year period. The ERD assumptions were tolerated by the dogs but resulted in death to 7 of 8 monkeys tested. The same ERD assumptions were tested with monkeys (Macaca mulatta) using a fixed recovery time span and an upper ERD limit of 100 rads. Monkeys in this study have tolerated 1900 rads of gamma-ray exposure (approximately 3 times the acute lethal dose) with minimal suppression of the hematopoietic system.