Mechanisms associated with human alveolar macrophage stimulation by particulates

Our plain-English summary of Mechanisms associated with human alveolar macrophage stimulation by particulates, by A. Holian, K. Kelley, R.F. Hamilton (Environmental Health Perspectives, 1994).

The study

• Authors: A. Holian, K. Kelley, R.F. Hamilton
• Journal: Environmental Health Perspectives
• Published: 1994;102 Suppl 10(Suppl 10):69-74
• Cited by: 27 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Asbestos and silica are well-known fibrogenic dusts. However, there is no comprehensive understanding of the molecular and cellular events that lead to fibrosis as a consequence of asbestos or silica inhalation. Previous studies have shown that asbestos stimulates superoxide anion production in alveolar macrophages through the phospholipase C/protein kinase C pathway. In contrast, silica does not appear to activate this pathway nor stimulate superoxide anion production, but silica does stimulate cytokine release by some undetermined pathway. Therefore, using human alveolar macrophages isolated from normal healthy volunteers, we evaluated the potential involvement of intracellular calcium and tyrosine kinases as potential signal transduction pathways. In the absence of serum, crystalline silica, and to a lesser extent amorphous silica, caused a rapid and dose-dependent elevation of intracellular calcium coming from the extracellular space. However, in the presence of serum, which is required for silica-stimulated cytokine release, neither form of silica caused noticeable elevation of intracellular calcium. Silica, however, did increase the extent of tyrosine phosphorylation, most notably of proteins at approximately 46 and 50 kDa, suggesting activation of a tyrosine kinase pathway. Preincubation of alveolar macrophages for 24 hr with silica-primed human alveolar macrophages for enhanced interleukin-1 beta (IL-1 beta) release stimulated by endotoxin (LPS) that was dose dependent. The enhanced LPS-stimulated release of IL-1 beta correlated with enhanced mitogen-activated protein kinase activity. Taken together, these results indicate that a tyrosine kinase pathway is activated during silica stimulation of human alveolar macrophages.

In plain English

This laboratory study exposed alveolar macrophages, immune cells collected by lung washing from healthy nonsmoking volunteers, to particles in the dish. The particles were chrysotile asbestos, crystalline silica, amorphous silica and titanium dioxide, but most experiments tested silica. Without serum, crystalline silica raised calcium inside the cells from about 150 nM to about 300 nM or almost 500 nM, depending on dose. Amorphous silica had a smaller effect. Removing calcium from outside the cells prevented the rise. With serum present, neither form of silica noticeably raised calcium. Silica quickly increased tyrosine phosphorylation of proteins at about 46 and 50 kDa. Chrysotile and the other particles were also tested, but only crystalline silica primed the cells to release more of the inflammatory signal IL-1β when later exposed to bacterial endotoxin. The chrysotile data were not shown. This increase was slight but significant at the lower silica dose and almost reached significance (p=0.05) at the higher dose. After endotoxin stimulation, silica-primed cells showed greater MAP kinase activity than unprimed cells. The authors concluded that silica activates and modifies a tyrosine kinase pathway that may be important in the development of fibrosis.

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 study of human alveolar macrophages exposed to particulates
• Who was studied: Alveolar macrophages obtained by bronchoalveolar lavage of normal nonsmoking volunteers. The total number of volunteers is not stated. Figure legends give n=7 and n=6 for the IL-1β experiments, and the calcium traces are representative of three experiments.
• Exposure: Doses were chosen to give equivalent surface areas:
• Chrysotile asbestos (Calidria RG-144), 25 µg/ml
• Crystalline silica (min-U-sil 5), 133 µg/ml, also tested at 33 and 67 µg/ml
• Amorphous silica, 80 µg/ml, also tested at 40 µg/ml
• Titanium dioxide, a nonfibrogenic particulate, 60 µg/ml

Bacterial endotoxin (LPS) was used as a second stimulus.

• Main results:
  • Without serum, crystalline silica (67 or 133 µg/ml) raised intracellular calcium in human alveolar macrophages from about 150 nM to about 300 nM, or almost 500 nM, in about 9 minutes. Calcium stayed constant in untreated cells.
  • When EDTA was added just before silica, there was no calcium rise. The authors took this to mean that the rise depended on calcium from outside the cell.
  • Amorphous silica at 40 and 80 µg/ml raised intracellular calcium less than crystalline silica did, to about 200 and 300 nM respectively.
  • In the presence of 10% fetal calf serum, neither crystalline nor amorphous silica noticeably raised intracellular calcium.
  • Both LPS and silica rapidly (under 7.5 minutes) increased tyrosine phosphorylation: of proteins at about 46 and 80 kDa with LPS, and at about 46 and 50 kDa with silica.
  • Chrysotile, titanium dioxide, amorphous silica and crystalline silica were all tested for priming; the data for the first three were not shown. Only crystalline silica primed human alveolar macrophages for enhanced IL-1β release in response to LPS.
  • The enhanced IL-1β release was significant at the lower dose: p less than 0.05 for 33 µg/ml silica plus 100 ng/ml LPS versus control (n=6). At 133 µg/ml silica plus 10 µg/ml LPS (n=7), p was 0.05, which the authors described as almost reaching significance.
  • LPS stimulated MAP kinase activity, which peaked around 15 minutes. When cells pre-treated with 33 µg/ml silica were then stimulated with 100 ng/ml LPS, MAP kinase activity was greater than in cells given LPS without silica pre-treatment. This correlated with the enhanced IL-1β release.
• The authors' conclusion: The authors concluded that silica activates and modifies a tyrosine kinase pathway in human alveolar macrophages that may be important in the development of fibrosis.
• Limitations the authors state:
  • The authors stated that neither the serum-free nor the serum-containing test model is ideal as a reflection of conditions in the lung.
  • The proteins showing increased tyrosine phosphorylation were not identified.
  • The mechanism by which calcium enters the macrophages is uncertain.
  • In the presence of serum, silica might raise intracellular calcium at a later time point than was measured.

Mines named here

Asbestos mines this study is about. Each mine's page says who ran it, when it worked, and what it produced.

• Santa Rita mine, Joaquin Ridge near Coalinga, California

All asbestos mines

Topics

Medical terms in this study, explained in our Lexicon: Alveolar Macrophage, Fibrogenic, Fibrosis, Superoxide, Cytokine, Crystalline.

Related studies

• Asbestos stimulation triggers differential cytokine release from human monocytes and alveolar macrophages — L.J. Geist, L.S. Powers, et al. (2000)
• Asbestos and silica-induced changes in human alveolar macrophage phenotype — A. Holian, M.O. Uthman, et al. (1997)
• A comparison of asbestos and urban particulate matter in the in vitro modification of human alveolar macrophage antigen-presenting cell function — R.F. Hamilton, A. Holian, et al. (2004)
• Involvement of the ICE family of proteases in silica-induced apoptosis in human alveolar macrophages — R. Iyer, A. Holian (1997)
• Rapid screening of possible cytotoxic effects of particulate air pollutants by measurement of changes in cytoplasmic free calcium, cytosolic pH, and plasma membrane potential in alveolar macrophages by flow cytometry — A. Tárnok, M. Dörger, et al. (2001)
• Asbestos induces apoptosis in human alveolar macrophages — R.F. Hamilton, L.L. Iyer, et al. (1996)

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