Silicosis and lung cancer: current perspectives

Abstract

"Silica" refers to crystalline particles formed by the combination of silicon with oxygen. Inhalation of silica particles promotes the development of pulmonary fibrosis that over prolonged periods increases the risk of lung cancer. The International Agency for Research on Cancer (IARC) classified crystalline silica as a human carcinogen in 1997. This categorization was questioned due to 1) the absence of dose-response findings, 2) the presence of confounding variables that complicated interpretation of the data and 3) potential selection bias for compensated silicosis. Yet, recent epidemiologic studies strongly support the conclusion that silica exposure increases the risk of lung cancer in humans independent of confounding factors including cigarette smoke. Based on this evidence, the US Occupational Safety and Health Administration (OSHA) lowered the occupational exposure limit for crystalline silica from 0.1 to 0.05 mg/m³ in 2013. Further supporting the human epidemiologic data, murine models show that chronic silicosis is associated with an increased risk of lung cancer. In animals, the initial inflammation induced by silica exposure is followed by the development of an immunosuppressive microenvironment that supports the growth of lung tumors. This work will review our current knowledge of silica-associated lung cancers, highlighting how recent mechanistic insights support the use of cutting-edge approaches to diagnose and treat silica-related lung cancer.

Keywords: fibrosis; inflammation; lung cancer; occupational lung disease; silicosis.

Figure 1

Proposed mechanism of silica-induced pulmonary toxicity. Notes: Studies in the rat model suggest that silica can influence the process of epithelial cell damage and subsequent carcinogenesis via several pathways. 1) Impairment of particle clearance by alveolar macrophages due to silica-dependent toxicity and/or inflammation, 2) persistence of silica particles in the lungs activating macrophages/neutrophils to produce cytokines/chemokines/oxidants known to facilitate cancer development and 3) oxidant-induced epithelial cell genotoxicity/injury/proliferation. The impact of these pathways on tumorigenesis is described in the text.

Figure 2

Proposed mechanism by which the immunosuppressive microenvironment induced by chronic silicosis supports tumor growth. Notes: In response to the chronic inflammation induced by prolonged silica exposure (upper portion of figure), physiological host processes designed to minimize ongoing immune stimulation are triggered. This results in the creation of an immunosuppressive microenvironment characterized by the presence of regulatory T cells and MDSC. Within this microenvironment, PD-1:PD-L1 interactions inhibit the activity of tumorigenic T cells, allowing for the growth of transformed cells. Abbreviations: MDSC, myeloid-derived suppressor cells; PD-1, programmed cell death protein 1; TNF-α, tumor necrosis factor-α; Treg, regulatory T cells.