Involvement of the ICE family of proteases in silica-induced apoptosis in human alveolar macrophages

Our plain-English summary of Involvement of the ICE family of proteases in silica-induced apoptosis in human alveolar macrophages, by R. Iyer, A. Holian (The American Journal of Physiology, 1997).

The study

• Authors: R. Iyer, A. Holian
• Journal: The American Journal of Physiology
• Published: 1997;273(4):L760-7
• Cited by: 39 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Exposure to silica dust can result in lung inflammation that may progress to fibrosis for which there is no effective clinical treatment. The mechanisms involved in the development of pulmonary silicosis have not been well defined; however, most current evidence implicates a central role for alveolar macrophages in this process. We have previously demonstrated that fibrotic agents, such as asbestos and silica, induce apoptosis in human alveolar macrophages. The goal of this study was to identify molecular events in the silica-induced apoptotic process to better understand the mechanism by which fibrotic agents may be inducing apoptosis in human alveolar macrophages. To elucidate the possible mechanism by which silica causes apoptosis, we investigated the involvement of the interleukin-converting enzyme (ICE) family of proteases. Human alveolar macrophages were treated with silica in vitro and were examined for the involvement of ICE, Ich-1L, and cpp32beta in silica-induced apoptosis. Pretreatment of cells with 10 microM of the ICE inhibitor z-Val-Ala-Asp-fluoromethyl ketone and the cpp32beta inhibitor Asp-Glu-Val-Asp-fluoromethyl ketone completely blocked silica-induced apoptosis. Additionally, an increased formation of the active p20 fragments of ICE and Ich-1L as well as degradation of the inactive zymogen form of cpp32beta protein were observed in silica-treated human alveolar macrophages, indicating activation of these proteases. Furthermore, degradation of the nuclear protein poly(ADP-ribose) polymerase was observed within 2 h of silica treatment. These results suggest that silica-induced apoptosis involves activation of the ICE family of proteases and is the first step in elucidating the intracellular mechanism of particulate-induced apoptosis in human alveolar macrophages.

In plain English

This laboratory study looked at how crystalline silica (not asbestos) causes human lung immune cells called alveolar macrophages to die by apoptosis, a form of programmed cell death. The cells came from lung washings of healthy nonsmoking adult volunteers at the University of Texas in Houston. In test tubes they were exposed to 133 µg/ml of crystalline silica (Min-U-Sil-5). The authors tested whether a family of enzymes, the ICE family of proteases (ICE, Ich-1L and cpp32b), was involved. Silica significantly increased a laboratory measure of apoptosis compared with untreated cells. Treated cells also showed DNA breakdown and shrunken nuclei. Pretreating the cells with 10 µM of either of two enzyme inhibitors, zVAD-FMK or DEVD-FMK, significantly reduced this cell death. Active forms of ICE and Ich-1L rose within 1 hour of silica treatment, the inactive form of cpp32b decreased, and a breakdown product of the protein PARP appeared within 2 hours. The authors concluded that silica induces apoptosis in these cells by stimulating these enzymes. They suggested that asbestos, which this study did not test, may use the same pathway. They noted that zVAD-FMK may not act only on ICE.

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: In vitro laboratory experiment on human cells
• Who was studied: Alveolar macrophages obtained by bronchoalveolar lavage of normal, nonsmoking adult volunteers of either gender, University of Texas Medical School, Houston. The total number of volunteers is not stated; immunoblots are described as representative of 3 experiments (2 for the cpp32 p20 blot).
• Exposure: Crystalline silica (acid-washed Min-U-Sil-5, average size 5 µm) at 133 µg/ml, which the authors describe as a bioactive but nonnecrotic dose, for up to 6 h. Some cells were pretreated with the inhibitors zVAD-FMK (10 µM) or DEVD-FMK (10 µM). Asbestos was not tested.
• Main results:
  • In silica-treated macrophages, a measure of apoptosis (cytosolic histone-bound DNA fragments) increased significantly compared with control. In cells also given 10 µM zVAD-FMK, silica-induced apoptosis was significantly inhibited.
  • Silica-treated cells showed dark, shrunken nuclei. Cells treated with silica plus zVAD-FMK looked like control cells.
  • Silica caused DNA ladder formation at 6 h. Pretreatment with zVAD-FMK blocked it.
  • The active p20 form of ICE increased within 1 h of silica treatment. With zVAD-FMK it was only marginally higher than control.
  • Ich-1L p20 increased as early as 1 h in silica-treated cells. zVAD-FMK pretreatment blocked this increase.
  • The cpp32b inhibitor DEVD-FMK significantly inhibited apoptosis in silica-treated macrophages.
  • The inactive p32 form of cpp32b decreased at 1 and 3 h in silica-treated cells, and the p20 form increased at 1 h.
  • The 85-kDa breakdown product of the nuclear protein PARP increased within 2 h of silica treatment.
• The authors' conclusion: The authors concluded that silica induces apoptosis in human alveolar macrophages by stimulating ICE, Ich-1L and cpp32b. They proposed that this pathway may be used by other fibrogenic agents such as asbestos.
• Limitations the authors state:
  • The inhibitor zVAD-FMK may not be specific for ICE and may inhibit Ich-1L itself.
  • The individual roles of these proteases are not clear, including whether one or all are required.

Topics

Medical terms in this study, explained in our Lexicon: Apoptosis, Alveolar Macrophage, Inflammation, Fibrosis, Silicosis.

Related studies

• Asbestos and silica-induced changes in human alveolar macrophage phenotype — A. Holian, M.O. Uthman, et al. (1997)
• Asbestos induces apoptosis in human alveolar macrophages — R.F. Hamilton, L.L. Iyer, et al. (1996)
• Asbestos causes apoptosis in alveolar epithelial cells: role of iron-induced free radicals — A. Aljandali, H. Pollack, et al. (2001)
• Expression of the T cell receptor Vbeta repertoire in a human T cell resistant to asbestos-induced apoptosis and peripheral blood T cells from patients with silica and asbestos-related diseases — Y. Nishimura, Y. Miura, et al. (2006)
• Mechanisms associated with human alveolar macrophage stimulation by particulates — A. Holian, K. Kelley, et al. (1994)
• Long-lasting production of TGF-beta1 by alveolar macrophages exposed to low doses of asbestos without apoptosis — Y. Nishimura, T. Nishiike-Wada, et al. (2007)

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