Molecular toxicology studies on the quartz hazard
Silicon makes up almost 28% of the Earth's crust and within that crust, quartz (crystalline silica) is one of the most abundant minerals. Exposure to quartz can occur in a number of occupations, including the mining and construction industries in which respirable quartz particles are generated and become airborne. Inhalation of quartz can lead to the fibrosing lung disease silicosis and cancer. Silicosis has been recognised for many decades as one of the most prevalent occupational lung diseases. In 1997, an IARC working Group classified quartz as a class 1 lung carcinogen, but only in some industries, suggesting that the quartz hazard is a variable entity. The reactivity of the quartz surface may underlie its ability to cause inflammation and treatments that ameliorate this reactivity would then reduce the quartz hazard. In the present study the effect of treating quartz with aluminium lactate, a procedure reported to decrease the quartz hazard, on the highly reactive quartz surface and on proinflammatory events in the rat lung were explored. Aluminium lactate-treated quartz showed a reduced surface reactivity as measured by electron spin resonance. Eighteen hours post-instillation of quartz into the rat lung, there was massive inflammation as indicated by the number of neutrophils in the bronchoalveolar lavage (BAL) and an increase in BAL macrophage inflammatory protein-2 (MIP-2). However, aluminium lactate-treated quartz had no significant effect when compared to control. Epithelial damage as indicated by BAL protein and gamma glutamyl transpeptidasea lso increased with quartz instillation but not with aluminium lactate-treated quartz and furthermore, quartz induced an increase in MIP-2 mRNA content of BAL cells while aluminium lactate-treated quartz had no effect compared to controls. There was an increase in nuclear binding of the transcription factor nuclear factor-kappa B (NF-xB) in the quartz exposed BAL cells and again, no effect on nuclear NF-xB binding in BAL cells from aluminium lactate-treated quartz instilled rats. In addition, the effect of aluminium lactate and PVNO quartz treatment on DNA damage, cell cytotoxicity and particle uptake by A549 cells was assessed. DNA strand breakage, as produced by quartz at non-toxic concentrations, could be completely prevented by both coating materials. Particle uptake by A549 cells appeared to be significantly inhibited by the PVNO coating, and to a lesser extent by the aluminium lactate coating, demonstrating that respirable quartz particles induce oxidative DNA damage in human lung epithelial cells and indicating that the surface properties of the quartz as well as particle uptake by these target cells are important in the cytotoxic and genotoxic effects of quartz in vitro. Finally, the role played by surface area and specific reactivity in the acute inflammatory response to particles was investigated. Acute inflammatory response following instillation of particles has been used to evaluate hazard but has been criticised because of the non-physiological delivery and the problems of local overload. Here, a number of low toxicity dusts of various particle sizes were instilled and the neutrophil influx into the lung 18-24 hours post-instillation assessed. The extent of inflammation was shown to be a function of the surface area instilled and ultrafine particles, which present a case of high surface area per unit mass, were inflammogenic pro rata with their surface area. There is no evidence that ultrafine particles of carbon black, titanium dioxide or polystyrene have any special reactivity in addition to their large surface area. We further tested whether this approach could be used to model the reactivity of highly toxic dusts. Rats were instilled with either quartz or aluminium lactate-treated quartz and, as anticipated, the high specific surface reactivity of quartz meant that it was much more inflammogenic than was predicted using the relationship described for `low toxicity' dusts. This approach represents the possibility of modelling potential toxicity for nuisance dusts based on the inflammatory response of a given instilled surface area dose.