Asbestos upregulates expression of the urokinase-type plasminogen activator receptor on mesothelial cells

Our plain-English summary of Asbestos upregulates expression of the urokinase-type plasminogen activator receptor on mesothelial cells, by R.C. Perkins, V.C. Broaddus, S. Shetty, et al (American Journal of Respiratory Cell and Molecular Biology, 1999).

The study

• Authors: R.C. Perkins, V.C. Broaddus, S. Shetty, S. Hamilton, S. Idell
• Journal: American Journal of Respiratory Cell and Molecular Biology
• Published: 1999;21(5):637-46
• Cited by: 17 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Inhalation of asbestos is associated with pathologic changes in the pleural space, including pleural thickening, pleural plaques, and mesothelioma. These processes are characterized by altered local proteolysis, cellular proliferation, and cell migration, suggesting that the urokinase-type plasminogen activator receptor (uPAR) could be involved in the pathogenesis of asbestos-induced pleural disease. We hypothesized that mesothelial cell uPAR expression is induced by exposure to asbestos. To test this hypothesis, we used complementary techniques in rabbit and human mesothelial cells to determine whether uPAR expression is altered by exposure to asbestos. uPAR expression was induced by chrysotile and crocidolite asbestos, but not by wollastonite, as indicated by binding of radiolabeled urokinase-type plasminogen activator (uPA) to rabbit or human mesothelial cells. uPA was not induced by fiber exposure. Exposure to exogenous uPA increased uPA activity of cells exposed to wollastonite but not asbestos-treated MeT5A cells. uPAR expression increased further when asbestos was preincubated with vitronectin (VN) or serum. Increases in uPAR expression were confirmed by binding of uPA to uPAR in cell membrane preparations and immunofluorescent staining of uPAR at the cell surface, and were associated with increases in steady-state uPAR messenger RNA. Mesothelial cell uPAR expression was also induced by media from monocytes cultured with asbestos incubated with VN and serum. By antibody neutralization, the latter effect appeared to be in part mediated by transforming growth factor-beta. We found that asbestos increases uPAR at the surface of rabbit and human mesothelial cells, suggesting that altered expression of this receptor could be involved in asbestos-induced remodeling of the pleural mesothelium.

In plain English

This was a laboratory study of cultured cells, not of people. The authors exposed rabbit pleural mesothelial cells and a human pleural cell line (MeT5A) to chrysotile or crocidolite asbestos, or to wollastonite, which the article calls a nonfibrogenic fiber. They measured uPAR, a cell-surface receptor for the enzyme urokinase (uPA). After 18 hours, both asbestos types significantly increased uPAR on rabbit and human cells, and wollastonite did not. Asbestos preincubated with vitronectin, a blood protein, or with serum raised uPAR further. Fluid from human blood monocytes exposed to asbestos also raised uPAR on the human cells. When the authors tested fluid from monocytes exposed to serum-coated chrysotile, an antibody against the signaling protein TGF-beta reduced this effect, suggesting TGF-beta contributed in part. Asbestos raised uPAR messenger RNA but did not increase uPA itself. Asbestos-treated cells released no more of the cell-damage marker LDH than controls. The authors noted that these increases were smaller than those caused by cytokines in earlier work. They concluded that asbestos increases uPAR on mesothelial cells in vitro, suggesting a possible mechanism in pleural injury and repair.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory (in vitro) cell culture experiment
• Who was studied: Mesothelial cells freshly isolated from the pleura of New Zealand white rabbits, and MeT5A, an immortalized human pleural mesothelial cell line. Some experiments also used isolated human blood monocytes. No human subjects were studied.
• Exposure: Chrysotile and crocidolite asbestos, compared with wollastonite (described as a nonfibrogenic fiber). Fibers were used uncoated or after preincubation with vitronectin or serum. Cells were exposed for 18 h in most experiments. In other experiments, cells received culture fluid from monocytes that had been exposed to chrysotile.
• Main results:
  • In rabbit mesothelial cells exposed to asbestos for 18 h, uPAR expression (measured by binding of radiolabeled uPA) increased significantly. The figure is described as representative of 8 separate experiments.
  • In human MeT5A mesothelial cells exposed to asbestos, uPAR expression also increased significantly over cells in media alone. The figure is described as representative of 12 separate experiments.
  • uPAR expression was induced by chrysotile and crocidolite asbestos but not by wollastonite.
  • In mesothelial cells, asbestos coated with vitronectin or serum produced greater uPAR expression than uncoated asbestos or no particulate.
  • Culture fluid from monocytes exposed to asbestos increased uPAR on mesothelial cells, especially when the asbestos was coated with vitronectin or serum.
  • Triplicate experiments used fluid from monocytes exposed to serum-coated chrysotile. In these, only the TGF-beta antibody reduced radiolabeled uPA binding to MeT5A cells, to an average of 77.9 pmol per 10^5 cells (range 60.4 to 94.2). By comparison, cells treated with media alone bound 148.3 (range 135.4 to 165.9), cells with media plus control IgG bound 118.5 (range 110.1 to 127.1), and cells with the TNF-alpha antibody bound 141.7 (range 94.6 to 173.9).
  • Fluorescent staining of uPAR in mesothelial cells treated with crocidolite or chrysotile was increased at 6 and 18 h compared with control cells and wollastonite-treated cells.
  • uPAR messenger RNA increased in MeT5A cells exposed to crocidolite for 6 h, and increased further with vitronectin-coated crocidolite. Chrysotile-treated cells showed a similar increase.
  • Fiber treatment did not increase uPA activity in MeT5A cell lysates or culture media. Western blotting likewise showed no induction of uPA antigen.
  • After exogenous uPA was added, chrysotile- or crocidolite-pretreated MeT5A cells showed no increase in uPA activity, but wollastonite-treated cells consistently did.
  • After 18 h, LDH in the media of asbestos-treated rabbit or MeT5A cells did not differ from controls.
  • At 18 h there were, on average, about 10% fewer cells with chrysotile or crocidolite treatment than with media or wollastonite.
• The authors' conclusion: The authors concluded that asbestos increases uPAR expression on mesothelial cells in vitro, either directly or through monocyte-derived products. They suggested this is a novel mechanism by which asbestos could influence uPAR-related cell functions in pleural injury and repair.
• Limitations the authors state:
  • The asbestos-induced increases in uPAR were not as pronounced as increases previously described from cytokine exposure.
  • Small amounts of receptor-bound active uPA might have escaped detection by the functional assay.
  • Ligand blotting required large numbers of cells and was performed only with MeT5A cells.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Chrysotile, Crocidolite.

Medical terms in this study, explained in our Lexicon: Mesothelial Cells, Pleural Thickening, Pleural Plaques, Mesothelioma, Chrysotile, Crocidolite, Wollastonite, Vitronectin.

Related studies

• Expression and activity of urokinase and its receptor in endothelial and pulmonary epithelial cells exposed to asbestos — A. Barchowsky, R.R. Roussel, et al. (1998)
• Urokinase receptor in human malignant mesothelioma cells: role in tumor cell mitogenesis and proteolysis — S. Shetty, A. Kumar, et al. (1995)
• Post-transcriptional regulation of plasminogen activator inhibitor type-1 expression in human pleural mesothelial cells — S. Shetty, T. Velusamy, et al. (2010)
• Increased expression of epidermal growth factor receptor in rat pleural mesothelial cells correlates with carcinogenicity of mineral fibres — S.P. Faux, C.E. Houghton, et al. (2000)
• Urokinase-type plasminogen activator receptor promotes proliferation and invasion with reduced cisplatin sensitivity in malignant mesothelioma — S. Wang, L. Jiang, et al. (2016)
• Expression of growth factor and growth factor receptor RNA in rat pleural mesothelial cells in culture — E. Bermudez, J. Everitt, et al. (1990)

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