Studies of the in vitro genotoxic effects of nanostructured silica particles produced by different manufacturing processes

Outline of reasons and objectives
Nanoparticles of synthetic amorphous silica (SAS) are produced in large quantities worldwide (1 million metric tons/year) and they have numerous industrial uses. SASs can be broken down into two main types depending on the process of synthesising them: pyrogenic SASs (powders) and precipitated SASs (powders or colloids). The toxicity of such amorphous silicas as manufactured nanomaterials is currently being debated, and the genotoxicity data (genotoxicity referring to damage to the structural integrity of the genome) concerning industrial SASs remains limited. The objective of this study was, in two different cellular models, namely the V79 hamster lung cell line, and the A549 human bronchial epithelial cell line, to assess the cytotoxicity and the genotoxicity of representative samples of pyrogenic and precipitated SASs coming from industry.
Approach
Pyrogenic and precipitated SAS samples supplied by industry were physiochemically characterised at INRS. The V79 and A549 cells were exposed to the SASs at different concentrations and with different treatment periods. The genotoxic effects were assessed by measuring the damage to the DNA, the formation of micronuclei and the formation of mutations. Other cytotoxic effects (cell viability, apoptosis) and production of reactive species of oxygen were also studied in order to understand the observed effects better.
Main results
The pyrogenic and precipitated SASs with a size of about 20 nanometres produced significant cytotoxic effects on the V79 cells, exposed for 24 hours. At this size, the colloidal precipitated and pyrogenic SASs also induced damage to the DNA on the V79 cells. Conversely, the SASs of slightly larger size, on average around 50 nm, did not produce any significant toxic effects regardless of their chemical nature or of the physical state (powder or colloid). In addition, no SAS induced any micronuclei or mutation of the hprt gene on the V79 cells. Finally, all of the toxicity tests conducted on the A549 cells proved to be negative, indicating that that line was significantly less sensitive than then V79 line for showing up the harmful effects of the tested SAS samples.
Discussion
This study confirms the data from the literature, such data being sparse, on the possibility of a cytotoxic and genotoxic activity of certain manufactured silica nanoparticles. The useful information that this study can bring as regards prevention is that it would appear that the toxicity of the SASs is related more to their size than to their chemical nature (pyrogenic or precipitated) or to their physical state (powder or colloidal). Our results thus highlight that SAS nanoparticles having a small difference in size, e.g. 20 and 25/70 nanometres, have genotoxic properties that are very different. Additional mechanistic studies would be necessary to explain these differences in effect, but these results already make it possible to alert nano-silica producers that a small variation in the primary size of the particles can influence their toxicity considerably.