Surface Reactivity, Cytotoxic, and Morphological Transforming Effects of Diatomaceous Earth Products in Syrian Hamster Embryo Cells

Abstract

In order to evaluate the effect of thermal treatments on the surface reactivity and carcinogenic potential of diatomaceous earth (DE) products, the physicochemical features of some specimens--derived by heating the same original material--were compared with their cytotoxic and transforming potency. The samples were an untreated DE (amorphous) progressively heated in the laboratory at 900 degrees C (DE 900) and 1200 degrees C (DE 1200) and a commercial product manufactured from the same DE (Chd) from which the finer fraction (< 10-microm diameter) was separated (Chd-F). Quartz (Min-U-Sil 5) and a vitreous silica (amorphous) smoothed up with hydrofluoric acid and were used as positive and negative controls, respectively. All samples were analyzed for their degree of crystallization, for their ability to release free radicals and reactive oxygen species, and for their cytotoxic and transforming potencies in Syrian hamster embryo (SHE) cells. X-ray diffractometry showed that DE 900, like DE, was still amorphous, whereas DE 1200 as well as the commercial product (Chd) were partially crystallized into cristobalite. The ability of the dust to release hydroxyl (*OH) radicals in the presence of hydrogen peroxide, as revealed by the spin-trapping technique, was as follows: Chd-F, DE 1200 > Chd > DE 900 > DE, suggesting that on heating, the surface acquires a higher potential for free radical release. Most of the silica samples generated COO* radicals from the formate ion, following homolytic rupture of the carbon-hydrogen bond, in the presence of ascorbic acid. A concentration-dependent decrease in cell proliferation and colony-forming efficiency was observed in SHE cultures treated with Chd-F, Chd, and DE. Heating abolished DE cytotoxicity but conferred a transforming ability to thermal treated particles. DE was the only sample that did not induce morphological transformation of cells. According to their transformation capacity, the samples were classified as follows: Chd-F > Chd, DE 1200 > DE 900 >> DE. Taken together, the reported results suggest that (1) the transforming potential of a biogenic amorphous silica is related to the thermal treatment that transforms the original structure in cristobalite and generates surface active sites; (2) the reactivity of samples in releasing *OH radicals correlates to their transforming ability; (3) the finer fraction of the commercial product is significantly more toxic and transforming than the coarse dust; and (4) opposite to silica dusts of mineral origin, which loose both cytotoxicity and transforming ability upon heating, heated diatomite acquires a cell-transforming potency. DE products should be thus considered a set apart of silica-based potentially toxic materials.

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

“…In our study, concentrations of 3.8 to 15.2 µg/cm 2 induced similar TF (0.24-0.71%) to that shown by Hesterberg and Barrett [21], and the crystallized silica was again found to be less potent than the asbestos (0.28 % TF with 1.91 µg/cm 2 of chrysotile vs. 0.4 % TF with 30.4 µg/ cm 2 of Chd or 0.24% TF with 3.81 µg/ cm 2 of Min-U-Sil). All of these data are in agreement with the previously reported data [18].…”

“…In the present work, we have shown that crystallized silica (Chd and Min-U-Sil) was able to induce transformed SHE colonies in a dose-dependent manner after seven days of treatment. Our results are in agreement with previous transformation studies on quartz [18], [21], [22], [23] and [24]. Hesterberg and Barrett [21] showed that two quartz samples induced a concentration-dependent increase in TF, and both of these samples were less potent than asbestos fibres.…”

“…With regards to natural amorphous silica materials, these have been evaluated as nonfibrogenic and unlikely to be carcinogenic [1]. In the present work, and as expected [18], the DE was unable to induce transformed colonies (except for three colonies in one experiment).…”

“…We have previously shown that both crystallized silica (Chd and Min-U-Sil 5) and chrysotile asbestos are able to generate ROS. The reactivity of these samples in releasing  OH radicals correlated to their transforming ability, whereas amorphous silica DE was unable to generate ROS and did not show any transforming capacity [13] and [18]. Some other works have shown genotoxic effects, but no link to the carcinogenicity of these products has been made [5], [6] and [19].…”

In vitro cell transformation induced by synthetic amorphous silica nanoparticles

“…In the present study, the suitability of the Bhas 42 CTA for particle-type substances was assessed using crystalline (quartz Min-U-Sil 5) and amorphous (diatomaceous earth) silica microparticles that have been previously evaluated for their transforming activity in the Syrian hamster embryonic (SHE) CTA [17,18]. [19] (Table 1).…”