Dissociation of intracellular lysosomal rupture from the cell death caused by silica

Abstract

The relationship between intracellular lysosomal rupture and cell death caused by silica was studied in P388D1 macrophages. After 3 h of exposure to 150 µg silica in medium containing 1.8 mM Ca2+, 60% of the cells were unable to exclude trypan blue. In the absence of extracellular Ca2+, however, all of the cells remained viable. Phagocytosis of silica particles occurred to the same extent in the presence or absence of Ca2+. The percentage of P388D, cells killed by silica depended on the dose and the concentration of Ca2+ in the medium. Intracellular lysosomal rupture after exposure to silica was measured by acridine orange fluorescence or histochemical assay of horseradish peroxidase. With either assay, 60% of the cells exposed to 150 µg silica for 3 h in the presence or absence of Ca2+ showed intracellular lysosomal rupture, whereas cell death occurred only in the presence of Ca2+. Intracellular lysosomal rupture was not associated with measurable degradation of total DNA, RNA, protein, or phospholipid or accelerated turnover of exogenous horseradish peroxidase. Pretreatment with promethazine (20 µg/ml) protected 80% of P388D, macrophages against silica toxicity although lysosomal rupture occurred in 60-70% of the cells. Intracellular lysosomal rupture was prevented in 80% of the cells by pretreatment with indomethacin (5 × 10-5 M), yet 40-50% of the cells died after 3 h of exposure to 150 µg silica in 1.8 mM extracellular Ca2+. The calcium ionophore A23187 also caused intracellular lysosomal rupture in 90-98% of the cells treated for 1 h in either the presence or absence of extracellular Ca2+. With the addition of 1.8 mM Ca2+, 80% of the cells was killed after 3 h, whereas all of the cells remained viable in the absence of-Ca2+. These experiments suggest that intracellular lysosomal rupture is not causally related to the cell death caused by silica or A23187. Cell death is dependent on extracellular Ca2+ and may be mediated by an influx of these ions across the plasma membrane permeability barrier damaged directly by exposure to these toxins. © 1980, Rockefeller University Press., All rights reserved.