Synthetic silica fibers of different length, diameter and shape: synthesis and interaction with rat (NR8383) and human (THP-1) macrophages in vitro, including chemotaxis and gene expression profile

Our plain-English summary of Synthetic silica fibers of different length, diameter and shape: synthesis and interaction with rat (NR8383) and human (THP-1) macrophages in vitro, including chemotaxis and gene expression profile, by N. Białas, N. Rosenkranz, D.G. Weber, et al (Particle and Fibre Toxicology, 2024).

The study

• Authors: N. Białas, N. Rosenkranz, D.G. Weber, K. Kostka, G. Johnen, A. Winter, A. Brik, K. Loza, K. Szafranski, T. Brüning, J. Bünger, G. Westphal, M. Epple
• Journal: Particle and Fibre Toxicology
• Published: 2024;21(1):23
• Cited by: 2 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Background: Inhalation of biopersistent fibers like asbestos can cause strong chronic inflammatory effects, often resulting in fibrosis or even cancer. The interplay between fiber shape, fiber size and the resulting biological effects is still poorly understood due to the lack of reference materials.

Results: We investigated how length, diameter, aspect ratio, and shape of synthetic silica fibers influence inflammatory effects at doses up to 250 µg cm-2. Silica nanofibers were prepared with different diameter and shape. Straight (length ca. 6 to 8 µm, thickness ca. 0.25 to 0.35 µm, aspect ratio ca. 17:1 to 32:1) and curly fibers (length ca. 9 µm, thickness ca. 0.13 µm, radius of curvature ca. 0.5 µm, aspect ratio ca. 70:1) were dispersed in water with no apparent change in the fiber shape during up to 28 days. Upon immersion in aqueous saline (DPBS), the fibers released about 5 wt% silica after 7 days irrespectively of their shape. The uptake of the fibers by macrophages (human THP-1 and rat NR8383) was studied by scanning electron microscopy and confocal laser scanning microscopy. Some fibers were completely taken up whereas others were only partially internalized, leading to visual damage of the cell wall. The biological effects were assessed by determining cell toxicity, particle-induced chemotaxis, and the induction of gene expression of inflammatory mediators.

Conclusions: Straight fibers were only slightly cytotoxic and caused weak cell migration, regardless of their thickness, while the curly fibers were more toxic and caused significantly stronger chemotaxis. Curly fibers also had the strongest effect on the expression of cytokines and chemokines. This may be due to the different aspect ratio or its twisted shape.

© 2024. The Author(s).

In plain English

This laboratory study made synthetic, non-crystalline (amorphous) silica fibers of three kinds and tested them on macrophages: a rat cell line (NR8383) and a human cell line (THP-1). No asbestos was tested. The fibers were straight thick (about 6 µm long by 0.35 µm wide, aspect ratio 17:1), straight thin (about 8 µm by 0.25 µm, 32:1) and curly (usually around 9 µm or more, aspect ratio about 70:1). The fibers kept their shape in water for 28 days and released 4.7 to 5.8 percent of their silica over 7 days in saline. Both cell types took up the fibers, and some fibers damaged cell membranes. In rat cells, curly fibers were the most toxic, although significant toxicity appeared only at very high doses. Curly fibers also caused the strongest migration of test immune cells in a chemotaxis assay. In rat cells exposed to straight thick or curly fibers, 27 of 84 inflammation-related genes changed significantly, and the changes were generally larger after curly fibers. The authors concluded that fiber shape may influence toxicity and inflammation. They said it remains unclear whether the curling or the length-to-width ratio was responsible.

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 using specially synthesized silica fibers
• Who was studied: Cultured macrophage cell lines: NR8383 rat alveolar macrophages and human THP-1 macrophages. Differentiated human HL-60 (dHL-60) cells were the migrating cells in the chemotaxis assay. Gene expression was measured only in the rat NR8383 cells.
• Exposure: Synthetic amorphous silica fibers of three kinds: straight thick (about 6 µm by 0.35 µm, aspect ratio 17:1), straight thin (about 8 µm by 0.25 µm, aspect ratio 32:1) and curly. Viability results are shown for doses up to 250 µg per square centimeter. Gene expression was measured at 16 and 32 µg per square centimeter, for straight thick and curly fibers only. Silica nanoparticles served as a granular particle control.
• Main results:
  • The curly fibers were usually around 9 µm or more long, giving an aspect ratio of about 70:1.
  • All the fibers were non-crystalline (X-ray amorphous) silica.
  • In saline (DPBS), the fibers released between 4.7 and 5.8 percent by weight of their silica after 7 days.
  • The fibers did not change shape significantly during 28 days in water.
  • In rat NR8383 cells, straight thin fibers were more toxic than straight thick fibers, and curly fibers were the most toxic. Significant toxicity was seen only at very high doses.
  • In human THP-1 cells, none of the fibers had a strong toxic effect at doses up to 26 µg per square centimeter.
  • Both types of macrophages took up the fibers, and the fibers visibly damaged the cell membrane.
  • Curly fibers caused the strongest chemotaxis (migration of dHL-60 cells) of all the fibers. Straight fibers caused less, and the difference between curly and straight thin fibers was statistically significant.
  • Of 84 chemokine and cytokine genes measured in rat NR8383 cells, 27 showed statistically significant changes in expression after exposure to straight thick or curly fibers.
  • In rat NR8383 cells, the fold changes in gene expression were generally higher after curly fibers than after straight thick fibers.
  • Example gene, Cxcl1: expression rose 33.61-fold with straight thick fibers at 16 µg per square centimeter and 125.90-fold at 32 µg per square centimeter. With curly fibers it rose 385.25-fold at 16 µg and 820.07-fold at 32 µg.
• The authors' conclusion: The authors concluded that fiber shape may influence the toxic and inflammatory properties of fibers, since curly fibers were significantly more active than straight fibers.
• Limitations the authors state:
  • Curly fibers were also the thinnest and longest, so the authors could not tell whether the curly shape or the aspect ratio caused their stronger effects.
  • The fibers did not reach a length, diameter or aspect ratio that strongly enhanced toxicity, at least within the 24-hour time scale studied.
  • The actual number of fibers taken up by each cell could not be determined.
  • The synthesis could not be scaled up, so only small batches were available.
  • The radius of curvature of the curly fibers is only an approximate value, because the curly fibers varied.

Topics

Medical terms in this study, explained in our Lexicon: Macrophage, Fibrosis, Aspect Ratio (Length-to-diameter ratio), Scanning Electron Microscopy (SEM), Cytokine.

Related studies

• Effect of SiO2-liberated macrophage factor on protein synthesis in connective tissue in vitro — M. Aalto, H. Turakainen, et al. (1979)
• Functional expression of system x(c)- is upregulated by asbestos but not crystalline silica in murine macrophages — J.C. Pfau, T. Seib, et al. (2012)
• Asbestos fibers and silica particles stimulate rat alveolar macrophages to release tumor necrosis factor. Autoregulatory role of leukotriene B4 — C.M. Dubois, E. Bissonnette, et al. (1989)
• Standardized methods for preparation and bi-variate length & diameter counting/sizing of aerosol and tissue digestion fiber samples — D.M. Bernstein, P. Kunzendorf (2018)
• Fibrogenesis by mineral fibres: an in-vitro study of the roles of the macrophage and fibre length — M. Aalto, A.G. Heppleston (1984)
• Rat alveolar macrophage-derived platelet-derived growth factor is chemotactic for rat lung fibroblasts — A.R. Osornio-Vargas, J.C. Bonner, et al. (1990)

Browse more research in Particle and Fibre Toxicology, or return to medical research.