Silicosis and lung cancer: a fifty-year perspective.

The development of our studies on silica carcinogenesis and its mechanisms is reviewed. Starting from an analysis of the cellular reactions to silica in the pathogenesis of silicosis in the rat, followed by an analysis of the carcinogenic response to silica in the lungs of rats (but not in mice and hamsters), we went on to develop cellular models for culture and neoplastic transformation of rat alveolar epithelial cells. We studied the binding of silica to DNA, the generation of reactive oxygen species and the DNA damage mediated by hydroxyl radicals, showing marked differences among silica samples of varying purity. Then we investigated the role of peptides induced by silica in various cells, including cytokines and growth factors. Tumor necrosis factor (TNF)-alpha, which can cause activation of DNA transcription and is required for silica-induced fibrosis, was found to inhibit neoplastic transformation by quartz in cell cultures. Transforming growth factor (TGF)-beta was found to be produced in hyperplastic alveolar type II cells and to reach fibroblasts, macrophages and the connective tissue matrix adjacent to silicotic granulomas. Neuroendocrine cells and their peptides were found to be increased in alveolar and bronchiolar epithelia of silica lesions in rats, in contrast with mice and hamsters. Expression of adhesion molecules was found to be altered in silica-induced carcinogenesis and epithelial-mesenchymal transition was revealed by mesenchymal markers in the induced carcinomas. Promoter hypermethylation of adhesion genes in the induced carcinomas indicated a role for epigenetic mechanisms.

Dietary cooked navy beans and their fractions attenuate colon carcinogenesis in azoxymethane-induced ob/ob mice.

Based on the protective effects of cooked dry bean consumption in a human intervention study, we evaluated which fraction of cooked dry beans is responsible for its cancer-preventive effects. Cooked navy beans (whole beans), the insoluble fraction (bean residue) or soluble fraction of the 60% (vol:vol) ethanol extract of cooked navy beans (bean extract), or a modified AIN-93G diet (16.6% fat including 12.9% lard) as control diet were fed to 160 male obese ob/ob mice after 2 azoxymethane injections. In comparison to control-fed mice, dysplasia, adenomas, or adenocarcinomas were detected in fewer mice on either bean fraction diet (percent reduction from control: whole beans 54%, P=0.10; bean residue 81%, P=0.003; bean extract 91%, P=0.007), and any type of colon lesions, including focal hyperplasia, were found in fewer mice on each of the 3 bean diets percent reduction from control: whole bean 56%, P=0.04; bean residue 67%, P=0.01; bean extract 87%, P=0.0003. These results suggest that both the soluble and the insoluble fraction of the extract contribute to the cancer-protective effect of cooked navy beans.

Altered expression of adhesion molecules and epithelial-mesenchymal transition in silica-induced rat lung carcinogenesis.

Loss of the epithelial phenotype and disruption of adhesion molecules is a hallmark in the epithelial-mesenchymal transition (EMT) reported in several types of cancer. Most of the studies about the relevance of adhesion and junction molecules in lung cancer have been performed using established tumors or in vitro models. The sequential molecular events leading to EMT during lung cancer progression are still not well understood. We have used a rat model for multistep lung carcinogenesis to study the status of adherens and tight junction proteins and mesenchymal markers during EMT. After silica-induced chronic inflammation, rats sequentially develop epithelial hyperplasia, preneoplastic lesions, and tumors such as adenocarcinomas and squamous cell carcinomas. In comparison with normal and hyperplastic bronchiolar epithelium and with hyperplastic alveolar type II cells, the expression levels of E-cadherin, alpha-catenin and beta-catenin were significantly reduced in adenomatoid preneoplastic lesions and in late tumors. The loss of E-cadherin in tumors was associated with its promoter hypermethylation. alpha- and beta-catenin dysregulation lead to cytoplasmic accumulation in some carcinomas. No nuclear beta-catenin localization was found at any stage of any preneoplastic or neoplastic lesion. Zonula occludens protein-1 was markedly decreased in 66% of adenocarcinomas and in 100% squamous cell carcinomas. The mesenchymal-associated proteins N-cadherin and vimentin were analyzed as markers for EMT. N-cadherin was de novo expressed in 32% of adenocarcinomas and 33% of squamous cell carcinomas. Vimentin-positive tumor cells were found in 35% of adenocarcinomas and 88% of squamous cell carcinomas. Mesenchymal markers were absent in precursor lesions, both hyperplastic and adenomatoid. The present results show that silica-induced rat lung carcinogenesis is a good model to study EMT in vivo, and also provide in vivo evidence suggesting that the changes in cell-cell adhesion molecules are an early event in lung carcinogenesis, while EMT occurs at a later stage.