Cellular and molecular basis of the asbestos-related diseases
Asbestosis is an inflammatory and fibrotic process of the alveolar structures mediated, at least in part, by cytokines released by "activated" alveolar macrophages. The process of phagocytosis and "activation" of alveolar macrophages is poorly understood. Are all macrophages activated or only subpopulations? Which cytokines are up-regulated? How does the local milieu modulate profibrotic and antifibrotic mediators? Is protein release accompanied by up-regulation of gene transcription? Is there an ordered sequence of cytokine activity? What roles do neutrophils and lymphocytes play? How can disease progression best be quantified absent further exposure? Answers to these questions are important to direct rational strategies at interdicting the fibrotic process. The question of cancer and asbestos is more vexing. The processes of inflammation, fibrosis, and carcinogenesis appear to be closely intertwined. For example, proto-oncogenes such as c-sis (PDGF B-chain) are up-regulated in activated alveolar macrophages from fibrotic lungs; these and possibly others may play an important role in asbestos carcinogenesis. Second, asbestos can transfect DNA into cells. Furthermore, DNA can adhere to asbestos fibers, and these fibers are capable of direct transmigration into cells. The questions of the mechanisms of cigarette smoke cocarcinogenicity and latency remain. Lastly, if the bronchial epithelium is highly metaplastic throughout from cigarette smoking, what triggers a single (or several) nidus of cells to transform into carcinoma? Malignant mesothelioma poses the most challenging questions because of association with brief asbestos exposure by history. Mesothelial cells are susceptible to minute environmental manipulations, and changes occur after exposure to all fiber types. Yet epidemiologic studies point toward long amphiboles as having greater mesothelioma risk. To test this hypothesis, experimental data must be generated differentiating tumorigenesis risk from short, chrysotile fibers that can migrate to the parietal pleura from the associations of long amphiboles persisting in lung tissue. Despite the future decreasing numbers of clinical cases of asbestos-related disease, solving the important mechanistic questions remaining will contribute significantly to our understanding of fibrosis and cancer.