In vitro comet and micronucleus assays do not predict morphological transforming effects of silica particles in Syrian Hamster Embryo cells

Lung injury and expression of p53 and p16 in Wistar rats induced by respirable chrysotile fiber dust from four primary areas of China

Chrysotile products were widely used in daily life, and a large amount of respirable dust was produced in the process of production and application. At present, there was seldom research on the safety of chrysotile fiber dust, and whether its long-term inhalation can lead to lung cancer was unknown. In order to determine whether respirable chrysotile fiber dust of China caused lung cancer, four major chrysotile-producing mine areas in China were selected for this study. Chrysotile fibers were prepared into respirable dust. Particle size was measured by laser particle analysis, morphology was observed by scanning electron microscope, chrysotile fiber phase was analyzed by X-ray diffraction, trace chemical elements were identified by X-ray fluorescence, and the structure and the active groups of the dust were determined after grinding by Fourier transform infrared spectroscopy. Male Wistar rats were exposed to non-exposed intratracheal instillation with different concentrations of chrysotile fiber dust. The rats were weighed after 1, 3, and 6 months, then the lung tissues were separated, the lung morphology was observed, and the pulmonary index was calculated. Pathological changes in lung tissues were observed by optical microscope after the HE staining of tissues, and the gene expression of p53 and p16 was determined by reverse transcription polymerase chain reaction. First, the results showed that the particle sizes of the four fibers were less than 10 μm. Four primary areas of chrysotile had similar fibrous structure, arranged in fascicles, or mixed with thin chunks of material. Second, the elementary composition of the four fibers was mainly chrysotile, and the structure and the active groups of the grinding dust were not damaged. Third, the weights of the treated rats were obviously lower, and the lung weights and the pulmonary index increased significantly (P < 0.05). Fourth, the treated Wistar rat lung tissues revealed different degrees of congestion, edema, inflammatory cell infiltration, and mild fibrosis. Fifth, the p53 and p16 genes decreased in the Mangnai group after 1 month of exposure, and the other groups increased. The expression of p53 and p16 in each group decreased significantly after 6 months (P < 0.05). In conclusion, the respirable chrysotile fiber dust from the four primary areas of China had the risk of causing lung injury, and these changes may be related to the physical and chemical characteristics of chrysotile from different production areas.

An updated review of the genotoxicity of respirable crystalline silica

Human exposure to (certain forms of) crystalline silica (CS) potentially results in adverse effects on human health. Since 1997 IARC has classified CS as a Group 1 carcinogen [1], which was confirmed in a later review in 2012 [2]. The genotoxic potential and mode of genotoxic action of CS was not conclusive in either of the IARC reviews, although a proposal for mode of actions was made in an extensive review of the genotoxicity of CS by Borm, Tran and Donaldson in 2011 [3]. The present study identified 141 new papers from search strings related to genotoxicity of respirable CS (RCS) since 2011 and, of these, 17 relevant publications with genotoxicity data were included in this detailed review.

Studies on in vitro genotoxic endpoints primarily included micronucleus (MN) frequency and % fragmented DNA as measured in the comet assay, and were mostly negative, apart from two studies using primary or cultured macrophages. In vivo studies confirmed the role of persistent inflammation due to quartz surface toxicity leading to anti-oxidant responses in mice and rats, but DNA damage was only seen in rats. The role of surface characteristics was strengthened by in vitro and in vivo studies using aluminium or hydrophobic treatment to quench the silanol groups on the CS surface.

In conclusion, the different modes of action of RCS-induced genotoxicity have been evaluated in a series of independent, adequate studies since 2011. Earlier conclusions on the role of inflammation driven by quartz surface in genotoxic and carcinogenic effects after inhalation are confirmed and findings support a practical threshold. Whereas classic in vitro genotoxicity studies confirm an earlier no-observed effect level (NOEL) in cell cultures of 60-70 μg/cm², transformation frequency in SHE cells suggests a lower threshold around 5 μg/cm². Both levels are only achieved in vivo at doses (2–4 mg) beyond in vivo doses (> 200 μg) that cause persistent inflammation and tissue remodelling in the rat lung.

Epigenetic changes in the early stage of silica-induced cell transformation

The increasing use of nanomaterials in numerous domains has led to growing concern about their potential toxicological properties, and the potential risk to human health posed by silica nanoparticles remains under debate. Recent studies proposed that these particles could alter gene expression through the modulation of epigenetic marks, and the possible relationship between particle exposure and these mechanisms could represent a critical factor in carcinogenicity. In this study, using the Bhas 42 cell model, we compare the effects of exposure to two transforming particles, a pyrogenic amorphous silica nanoparticle NM-203 to those of the crystalline silica particle Min-U-Sil® 5. Short-term treatment by Min-U-Sil® 5 decreased global DNA methylation and increased the expression of the two de novo DNMTs, DNMT3a and DNMT3b. NM-203 treatment affected neither the expression of these enzymes nor DNA methylation. Moreover, modified global histone H4 acetylation status and HDAC protein levels were observed only in the Min-U-Sil® 5-treated cells. Finally, both types of particle treatment induced strong c-Myc expression in the early stage of cell transformation and this correlated with enrichment in RNA polymerase II as well as histone active marks on its promoter. Lastly, almost all parameters that were modulated in the early stage were restored in transformed cells suggesting their involvement mainly in the first steps of cell transformation.