Differential expression and oxidation of MKP-1 modulates TNF-alpha gene expression

Our plain-English summary of Differential expression and oxidation of MKP-1 modulates TNF-alpha gene expression, by L.A. Tephly, A.B. Carter (American Journal of Respiratory Cell and Molecular Biology, 2007).

The study

• Authors: L.A. Tephly, A.B. Carter
• Journal: American Journal of Respiratory Cell and Molecular Biology
• Published: 2007;37(3):366-74
• Cited by: 14 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Monocytic cells are integral in the pathogenesis of inflammatory disorders. We have shown previously that asbestos-induced p38 mitogen-activated protein (MAP) kinase activation and TNF-alpha expression are mediated by H(2)O(2) in blood monocytes. Due to the high expression and activity of catalase and glutathione peroxidase, normal alveolar macrophages do not respond in a manner similar to that of blood monocytes. Since kinase activity is tightly regulated by phosphatases, we hypothesized that the dual specificity phosphatase MAP kinase phosphatase (MKP)-1 regulates p38 activity and TNF-alpha production in alveolar macrophages due to insufficient H(2)O(2) generation in response to asbestos. We found that MKP-1 was highly expressed in alveolar macrophages, while blood monocytes had minimal expression. Inhibition of expression and activity of MKP-1 or overexpression of a catalytic mutant MKP-1 recovered p38 activity in alveolar macrophages. We questioned whether MKP-1 oxidation played a role dictating the contrasting responses of these cells to asbestos exposure, and found that overexpressed wild-type MKP-1 in monocytes was oxidized, while the mutant MKP-1 remained in the reduced form. Monocytes overexpressing either catalase or wild-type MKP-1 had decreased p38 activation and TNF-alpha production, respectively. In addition, TNF-alpha gene expression was regained in alveolar macrophages overexpressing the catalytic mutant MKP-1. These data suggest that MKP-1, through increased expression and lack of oxidation, modulates the inflammatory response in alveolar macrophages exposed to asbestos.

In plain English

This was a laboratory study of cells, not of people. The authors compared blood monocytes and alveolar macrophages (lung cells collected by washing the airways) from healthy, nonsmoking volunteers aged 18 to 45. They also used a human monocyte cell line and a mouse macrophage cell line. The cells were exposed to crocidolite asbestos. In blood monocytes, asbestos activated a signaling protein called p38. In alveolar macrophages, p38 activity was essentially absent. The macrophages had much more MKP-1, an enzyme that switches p38 off, and blocking or removing MKP-1 let p38 activate. In the monocyte cell line, most added MKP-1 was oxidized, which inactivates it, and asbestos increased this. After asbestos, blood monocytes, but not macrophages, increased TNF-alpha mRNA (TNF-alpha is an inflammatory signal). Extra MKP-1 partly reduced asbestos-triggered activity of an added test gene controlled by the TNF-alpha promoter in monocyte-line cells. A mutant MKP-1 raised a similar test gene's activity almost 20-fold after asbestos in mouse macrophage-line cells. The authors concluded that MKP-1, through high levels and lack of oxidation, modulates the inflammatory response of alveolar macrophages exposed to asbestos.

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 experiments comparing human alveolar macrophages and blood monocytes, plus cell lines, exposed to asbestos in culture
• Who was studied: Alveolar macrophages (by bronchoalveolar lavage) and blood monocytes (by blood draw) from normal volunteers aged 18 to 45 with no cardiopulmonary or other chronic disease, no medication except oral contraceptives, no infection, and lifetime nonsmokers, at the University of Iowa. The number of volunteers is not stated. Also used: the human monocyte cell line THP-1 and the macrophage cell line RAW 264.7, which the authors describe as murine.
• Exposure: Crocidolite asbestos from the NAIMA Fiber Repository, applied to cells in culture. The dose is printed as "10 mg/cm2" (the unit may have been garbled in conversion; see text problems). Exposure times ranged from minutes to 24 hours depending on the experiment.
• Main results:
  • In alveolar macrophages, activated (phosphorylated) p38 was essentially absent even after 2 hours of asbestos exposure. Blood monocytes had more activated p38 at baseline and a time-dependent increase after asbestos.
  • Alveolar macrophages had higher MKP-1 mRNA levels than blood monocytes. The figure reports P less than 0.0004 for untreated cells and P less than 0.0003 for asbestos-exposed cells. Blood monocytes had minimal MKP-1 protein, while macrophage MKP-1 protein increased with asbestos.
  • In alveolar macrophages, blocking MKP-1 production (with cycloheximide or triptolide) or its activity (with sodium vanadate) resulted in p38 activation after asbestos exposure.
  • In THP-1 monocytes given normal (wild-type) MKP-1, the MKP-1 appeared in an oxidized form, which increased with crocidolite exposure. The mutant MKP-1 (C258S) did not show this shift.
  • In THP-1 monocytes engineered to make extra catalase (an enzyme that breaks down hydrogen peroxide), asbestos-induced p38 activation was significantly decreased.
  • In RAW 264.7 macrophage-line cells, the mutant MKP-1 gave a greater than 5-fold increase in a p38 activity reporter (luciferase) after asbestos exposure.
  • Blood monocytes significantly increased TNF-alpha mRNA after asbestos exposure. In alveolar macrophages, TNF-alpha mRNA was no different from unexposed controls.
  • In THP-1 monocytes given an added reporter gene (CAT) driven by the TNF-alpha promoter, extra wild-type MKP-1 partially but significantly reduced the reporter's activity after crocidolite (three experiments; the figure reports P less than 0.003).
  • In RAW 264.7 macrophage-line cells given an added TNF-alpha promoter reporter gene (luciferase), the mutant MKP-1 (C258S) gave almost a 20-fold increase in reporter activity after asbestos exposure. Cells with an empty vector were no different from controls.
• The authors' conclusion: The authors concluded that MKP-1, through increased expression and lack of oxidation, modulates the inflammatory response in alveolar macrophages exposed to asbestos.
• Limitations the authors state:
  • The authors did not determine the chemical products of MKP-1 oxidation.
  • The authors state that their data do not rule out a role for other phosphatases.

Topics

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

Medical terms in this study, explained in our Lexicon: Glutathione (GSH), Alveolar Macrophage, Asbestos Exposure.

Related studies

• Iron and reactive oxygen species in the asbestos-induced tumor necrosis factor-alpha response from alveolar macrophages — P.P. Simeonova, M.I. Luster (1995)
• High levels of catalase and glutathione peroxidase activity dampen H2O2 signaling in human alveolar macrophages — A.B. Carter, L.A. Tephly, et al. (2004)
• Asbestos fibres and man made mineral fibres: induction and release of tumour necrosis factor-alpha from rat alveolar macrophages — A.G. Ljungman, M. Lindahl, et al. (1994)
• Chrysotile asbestos upregulates gene expression and production of alpha-receptors for platelet-derived growth factor (PDGF-AA) on rat lung fibroblasts — J.C. Bonner, A.L. Goodell, et al. (1993)
• Tumour necrosis factor-alpha gene polymorphisms in asbestos-induced diseases — S. Helmig, N. Aliahmadi, et al. (2010)
• Reduced tumor necrosis factor-alpha and transforming growth factor-beta1 expression in the lungs of inbred mice that fail to develop fibroproliferative lesions consequent to asbestos exposure — D.M. Brass, G.W. Hoyle, et al. (1999)

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