Malignant mesothelioma (MM) is a relatively rare but devastating tumor that is increasing worldwide. Yet, because of difficulties in early diagnosis and resistance to conventional therapies, MM remains a challenge for pathologists and clinicians to treat. In recent years, much has been revealed regarding the mechanisms of interactions of pathogenic fibers with mesothelial cells, crucial signaling pathways, and genetic and epigenetic events that may occur during the pathogenesis of these unusual, pleiomorphic tumors. These observations support a scenario whereby mesothelial cells undergo a series of chronic injury, inflammation, and proliferation in the long latency period of MM development that may be perpetuated by durable fibers, the tumor microenvironment, and inflammatory stimuli. One culprit in sustained inflammation is the activated inflammasome, a component of macrophages or mesothelial cells that leads to production of chemotactic, growth-promoting, and angiogenic cytokines. This information has been vital to designing novel therapeutic approaches for patients with MM that focus on immunotherapy, targeting growth factor receptors and pathways, overcoming resistance to apoptosis, and modifying epigenetic changes.CME Accreditation Statement: This activity (“ASIP 2013 AJP CME Program in Pathogenesis”) has been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Society for Clinical Pathology (ASCP) and the American Society for Investigative Pathology (ASIP). ASCP is accredited by the ACCME to provide continuing medical education for physicians.The ASCP designates this journal-based CME activity (“ASIP 2013 AJP CME Program in Pathogenesis”) for a maximum of 48 AMA PRA Category 1 Credit(s)™. Physicians should only claim credit commensurate with the extent of their participation in the activity.CME Disclosures: The authors of this article and the planning committee members and staff have no relevant financial relationships with commercial interests to disclose.Malignant mesotheliomas (MMs), among the most aggressive tumors, arise most often from the mesothelial cells that line the pleura, peritoneum, and, occasionally, the pericardium. Because of the multifaceted properties of mesothelium that maintain a protective barrier but also produce components of the extracellular matrix, hyaluronan and other lubricants, chemokines and cytokines, and fibrinolytic and procoagulant factors, understanding its complex biology is a challenge. The intermediate filament pattern of mesothelial cells, suggesting an epithelial–mesodermal hybrid morphology, and their several patterns of differentiation during the neoplastic process suggest their transformation to malignancy is complicated and raises the question of whether one is studying a single tumor type or multiple subgroups of tumors.MMs are most commonly attributed to occupational exposures to asbestos, a regulatory term for a group of fibrous silicates that occur as needle-like amphiboles (crocidolite, amosite tremolite, anthophyllite, and antigorite) or curly serpentine (chrysotile) fibers. Although each of these fibers has its own distinctive properties, the fibrous nature and biopersistance of these inhaled fibers may be key to carcinogenic events that occur during the long latency periods (mean, 30 to 45 years) of most MMs. Most intensely investigated are chrysotile, the most commonly used type of asbestos historically (>90% use worldwide), and crocidolite, the asbestos type associated most often with MMs in humans
1,2 ( and ). The morphology of crocidolite asbestos is similar to nonasbestos fibers of erionite or Libby amphibole, other naturally occurring minerals associated with the development of MMs.
5,6 However, 20% to 25% of individuals with MM have no documented exposure to asbestos or other fibers, suggesting familial susceptibility (sporadic or idiopathic MM), unknown exposure to in-place or naturally occurring asbestos, or other causative agents, such as chemicals, radiation, and viruses.
7Open in a separate windowProperties of chrysotile (white) asbestos. A: Image of bundle of curly chrysotile fibers before processing. B: Scanning electron micrograph of chrysotile fibers (arrows) causing deformation of red blood cells. Chrysotile is positively charged, hemolytic, and cytolytic, primarily due to its magnesium content. Leaching of magnesium renders chrysotile less toxic and also results in chrysotile fiber dissolution over time. C: Scanning electron micrograph of interaction of long chrysotile fiber with the respiratory epithelium of the alveolar duct junction after inhalation by rats. Arrowheads show points of contact with and between epithelial cells. Subsequent penetration into and between cells leads to fiber deposition in the lung interstitum and access to the visceral pleura and pleural space. D: Polarized microscopy showing chrysotile fibers and fibrils.Photomicrograph is a courtesy of Lee Poye (J3 Resources, Inc., Houston, TX) Original magnification, ×100.
Open in a separate windowProperties of crocidolite, or blue, asbestos. A: Riebeckito ore showing veins of crocidolite asbestos fibers (arrow) before processing. B: Scanning electron micrograph showing morphology of needle-like fibers. C: Early penetration of a crocidolite fiber into the differentiated tracheobronchial epithelium in tracheal organ culture. D: Growth of metaplastic cells over long fibers of crocidolite observed at 1 month in this model.
3 These events have not been captured in the pleura in animal inhalation models or in clinical specimens in humans, but mesothelial cells undergo proliferation, as measured by cell counts, or immunochemical markers have been observed in response to crocidolite asbestos
in vitro and after inhalation by rats.
4Because asbestos fibers neither appear to be metabolized nor directly interact with DNA, they are unlike most chemical carcinogens. The sensitivity of human mesothelial cells to fibers of high aspect (length to diameter) ratio is also perplexing, as are the phenomena governing fiber transport to the parietal pleura where most MMs are thought to develop. Although much insight exists on understanding how fibers (particularly high iron-containing amphibole asbestos types) generate reactive oxygen and nitrogen species to induce inflammation and cell signaling pathways important in proliferation and transformation, how these cellular events converge in the pathogenesis of MM remains enigmatic. This review amalgamates current observations in the field and their implications in strategies to prevent and manage MMs in patients.
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