Pure-Silica Zeolites (Porosils) as Model Solids for the Evaluation of the Physicochemical Features Determining Silica Toxicity to Macrophages

Abstract

The interaction between inhaled particles and alveolar macrophages plays a key role in silica-related diseases. It has been previously shown [Fubini, B., et al. (1999) Chem. Res. Toxicol. 12, 737-745] that a monocyte-macrophage cell line (J774) may be employed in the evaluation of the degree of cytotoxicity to alveolar macrophages of various silica dusts. In this paper, pure-silica zeolites (porosils) in microcrystalline form have been employed as "model solids" in an effort to show which physicochemical properties of the silica particle are playing a major role in the toxicity to macrophages. The samples employed covered four different porosil crystal structures (MFI, FAU, TON, and MTT) and also include a synthetic rodlike cristobalite (CRIS-rd). When compared at equal weight, the samples cover a wide range of cytotoxicity from inert to toxic as unheated mineral cristobalite [Fubini, B., et al. (1999) Chem. Res. Toxicol. 12, 737-745]. Mild grinding did not affect cytotoxicity. Calcined (open pores) and uncalcined (pore filled with template) TON exhibited the same cytotoxicity, indicating that only the outer surface is implied. The hydrophobic and/or hydrophilic character of TON, evaluated by adsorption calorimetry, is close to what has been previously found for silicalite and is consistent with a hydrophilic outer surface and hydrophobic pore walls. The potential for generating hydroxyl radicals from hydrogen peroxide varies among the various porosils that have been studied. A model is proposed for the correlation between inhibition of growth on proliferating cells and physicochemical properties varying from one to the other sample. The extent of external surface and the aspect ratio were related to the intensity of the cytotoxic effect, while the level of radical release was not. This suggests, on one hand, that comparison of toxicity among various dusts should be made at equal particle surface and, on the other, that in the model studied, free radical release does not play a crucial role in the primary event of toxicity to alveolar macrophages.