In vitro cell transformation induced by amorphous silica nanoparticles

Worldwide production of silica nanoparticles is estimated to be one million tons per year. Unlike crystalline silica, silica nanoparticles which have an amorphous structure are so far not considered as carcinogen to humans by IARC. However, numerous in vitro and in vivo studies have pointed out toxicological properties of manufactured silica nanomaterials involving inflammation, oxidative stress and genotoxicity. On the other hand, little is known about their carcinogenic potential.
The aim of this study is to investigate the capacity of four different silica nanomaterials provided by the EC-JRC Nanomaterials Repository (NM200, NM201, NM202 and NM203) to induce carcinogenesis. We therefore used the in vitro Bhas 42 cell transformation assay (CTA) that has been recently the subject of an OECD draft guidance document. The genome of the transgenic Bhas 42 cells contains several copies of the v-Ha-ras gene providing to them the characteristic of “initiated” cells, particularly sensitive to promoting agents. The originality of the Bhas 42 CTA is that it is based on two assays performed in parallel:
i) the initiation assay, where cells are seeded at low density and then treated at the beginning of growth phase. This condition allows DNA damage and mutation favouring the initiating activity of chemicals;
ii) the promotion assay, where cells are seeded at higher density and then repeatedly treated at stationary phase. This condition allows cell-to-cell communication and particularly favours the promoting activity of chemicals.
Results show that all four silica nanomaterials are able to induce significantly transformed cell foci in the promotion condition of the assay, but not in the initiation condition. Interestingly, the crystalline silica micro-size particle Min-U-Sil also responds positively only in the promotion condition, while a sample of amorphous silica microparticles (diatomaceous earth) is negative in both condition. This study brings to light for the first time the transforming potential of manufactured silica nanoparticles. Molecular mechanisms by which silica nanomaterials trigger cell transformation are under investigation.