In vitro cleavage by asbestos fibers of the fifth component of human complement through free-radical generation and kallikrein activation

Our plain-English summary of In vitro cleavage by asbestos fibers of the fifth component of human complement through free-radical generation and kallikrein activation, by M. Governa, M. Amati, M. Valentino, et al (Journal of Toxicology and Environmental Health. Part a, 2000).

The study

• Authors: M. Governa, M. Amati, M. Valentino, I. Visonà, B. Fubini, G.C. Botta, A.R. Volpe, M. Carmignani
• Journal: Journal of Toxicology and Environmental Health. Part a
• Published: 2000;59(7):539-52
• Cited by: 13 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Chrysotile and crocidolite fibers incubated in normal human plasma (NHP) generated from the C5 component of complement C5a-type fragments that stimulated polymorphonuclear leukocyte (PMN) chemotaxis. Absorption of NHP with antiserum against C5a totally abolished neutrophil chemotactic activity. Asbestos fibers also produced C5a small peptides in the presence of ethylene glycol bis(beta-aminoethyl ether) N,N,N'N'-tetraacetic acid (EGTA) but not ethylene diamine tetraacetic acid (EDTA). Activation of C5 was significantly inhibited when asbestos fibers were pretreated with iron chelators such as sodium dithionite (DTN), deferoxamine (DFX), or ascorbate (AA). Concentration-related inhibition of C5 activation was also observed when asbestos fibers were added concurrently to plasma in the presence of DFX, 1,3-dimethyl-2-thiourea (DMTU), a strong hydroxyl scavenger, or aprotinin (APR), a specific protease inhibitor. Further, chrysotile and crocidolite significantly increased plasma kallikrein activity. Data demonstrate that asbestos-induced C5 activation plays a role in inflammatory reactions characteristic of asbestosis through mechanisms involving iron ions, hydroxyl radicals, and oxidized C5-ike fragments. The ferrous ions present at the asbestos fiber surface trigger this activation and catalyze, via Fenton reaction, the production of hydroxyl radicals, which in turn convert native C5 to an oxidized C5-like form. This product is then cleaved by kallikrein, activated by the same asbestos fibers, yielding an oxidized C5a with the same functional properties as C5a.

In plain English

This was a laboratory (in vitro) study by researchers in Italy. It asked whether asbestos fibers can activate C5, a blood protein of the complement system that helps drive inflammation. The authors added UICC Canadian chrysotile and crocidolite fibers, in equal surface areas, to pooled plasma from 12 healthy volunteers. They then measured how strongly the treated plasma attracted white blood cells called neutrophils. Both fiber types produced C5a-type fragments that attracted these cells. The effect rose with fiber dose, except at the highest doses. Blocking C5a with an antiserum abolished it. The effect was reduced in three situations: when iron was removed from the fiber surfaces, when an iron chelator or a hydroxyl radical scavenger was added, or when aprotinin, a protease inhibitor, was added. Fibers treated with sodium dithionite produced no activity. Both fibers raised plasma kallikrein activity, crocidolite more strongly. The authors concluded that iron on the fiber surface produces hydroxyl radicals that alter C5, which fiber-activated kallikrein then cleaves. They proposed that this may occur in the lung's air sacs and contribute to inflammation in early asbestosis.

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 (asbestos fibers incubated in human plasma; no people or animals were exposed)
• Who was studied: Pooled plasma from 12 healthy volunteers (7 men and 5 women, aged 28 to 46) who worked in administrative university offices; laboratory work at Italian universities (Ancona, Turin, Rome, L'Aquila); dates not given
• Exposure: UICC Canadian chrysotile type B and UICC crocidolite, dosed by equal fiber surface area (50 to 800 square centimetres of fiber surface per ml of plasma), some surface-treated with ascorbate, deferoxamine or sodium dithionite to remove iron
• Main results:
  • Untreated fibers in normal plasma generated neutrophil-attracting activity that rose with dose except at the highest doses. As a percentage of a zymosan-activated plasma control, chrysotile went from 24.1 ± 6.3 at 50 cm2/ml to 86.6 ± 7.9 at 200 cm2/ml, and crocidolite from 25.1 ± 3.4 at 50 cm2/ml to 70.3 ± 6.9 at 400 cm2/ml (61.1 ± 5.8 at 800 cm2/ml).
  • Heat-inactivating the plasma, or absorbing it with antiserum against C5a, totally abolished the neutrophil-attracting activity.
  • In plasma treated with EGTA, activity was 67.9 ± 6.8 for chrysotile and 87.0 ± 7.2 for crocidolite (percent of zymosan-activated plasma control). No activity was detected in plasma treated with EDTA. The authors took this to indicate the alternate complement pathway.
  • Iron-depleted fibers produced less activity than untreated fibers (percent of untreated-fiber control). Chrysotile: 32.3 ± 8.9 after ascorbate and 38.3 ± 7.9 after deferoxamine. Crocidolite: 42.4 ± 7.3 after ascorbate and 43.0 ± 9.0 after deferoxamine. Neither fiber produced any chemotactic activity after sodium dithionite treatment.
  • Share of total iron removed: chrysotile 49%, 23% and 15%, and crocidolite 7.4%, 4.4% and 0.2%, with ascorbate, deferoxamine and sodium dithionite respectively.
  • Adding DMTU (a hydroxyl radical scavenger) or deferoxamine to plasma with fibers lowered activity as their concentration increased. The chemicals added to plasma without fibers did not significantly reduce chemotaxis.
  • Aprotinin, a protease inhibitor, reduced C5 cleavage by the fibers in a concentration-related manner.
  • Plasma kallikrein activity rose significantly with fiber concentration, and crocidolite had a stronger effect. Table 4 values: chrysotile from 138 ± 3.45 at 50 cm2/ml to 705 ± 7.81 at 400 cm2/ml; crocidolite from 3254 ± 25.54 at 50 cm2/ml to 6213 ± 27.26 at 800 cm2/ml. The table note describes these as percentages of the normal plasma control.
• The authors' conclusion: The authors concluded that ferrous iron at the surface of chrysotile and crocidolite fibers catalyzes production of hydroxyl radicals, which convert C5 to an oxidized form. Kallikrein, activated by the same fibers, then cleaves this form into a product that acts like C5a. They proposed that this may occur in the lung's alveolar spaces and contribute to inflammatory reactions characteristic of the early phases of asbestosis.

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: Chrysotile, Crocidolite, Hydroxyl Radical, Fenton Reaction.

Related studies

• Modulation of genotoxic effects in asbestos-exposed primary human mesothelial cells by radical scavengers, metal chelators and a glutathione precursor — I. Poser, Q. Rahman, et al. (2004)
• Free radical activity of synthetic vitreous fibers: iron chelation inhibits hydroxyl radical generation by refractory ceramic fiber — D.M. Brown, C. Fisher, et al. (1998)
• Free radical generation at the solid/liquid interface in iron containing minerals — B. Fubini, L. Mollo, et al. (1995)
• Enhanced generation of free radicals from phagocytes induced by mineral dusts — V. Vallyathan, J.F. Mega, et al. (1992)
• Wollastonite fibers in vitro generate reactive oxygen species able to lyse erythrocytes and activate the complement alternate pathway — M. Governa, L. Camilucci, et al. (1998)
• Time course of chemotactic factor generation and the corresponding macrophage response to asbestos inhalation — D.B. Warheit, L.H. Hill, et al. (1986)

Browse more research in Journal of Toxicology and Environmental Health. Part a, or return to medical research.