Roles of bioavailable iron and calcium in coal dust-induced oxidative stress: Possible implications in coal workers' lung disease

Marked regional differences in prevalence of pneumoconiosis are apparent in the US despite comparable dust exposure. In the present study, we examined the ability of 28 coal samples to release bioavailable iron (BAI) and calcium, as well as other metals such as Cr, Ni, Cu, and Co, from three coalmine regions in Utah (UT), West Virginia (WV), and Pennsylvania (PA), respectively. BAI is defined as iron (both Fe2+ and Fe3+) released by the coals in 10 mM phosphate solution, pH 4.5, which mimics conditions of the phagolysosomes in cells. We found that coals from the UT, WV, and PA regions released average levels of BAI of 9.6, 4658.8, and 12149 parts per million (ppm, w/w), respectively, which correlated well with the prevalence of pneumoconiosis from that region (correlation coefficient r = 0.92). The low levels of BAI in the UT coals were due to the presence of calcite (CaCO3), which was shown to be preferentially acid solubilized before iron compounds. Release of iron by two coal samples from the PA and UT regions was further examined in vitro in human lung epithelial A549 cells. We found that the coal from PA, with a high prevalence of pneumoconiosis, released BAI in a dose-dependent manner, both in tissue culture media and in A549 cells. At 2 mug/cm(2), levels of lipid peroxidation induced by the PA coal were increased 112% over control cells at 24 h treatment, and were sustained at this level for 3 days. The coal from UT with a low prevalence of pneumoconiosis, induced a marginal increase in cellular iron at 5 and 10 mug/cm(2) treatments and had no effect on lipid peroxidation. Calcium levels in the cells treated with the PA and UT coals were 8.6 and 11.5 mumoles/10(6) cells, respectively, and were significantly higher than that in the controls (53 mumoles/10(6) cells). Our results suggest that the differences in the BAI content in the coals may be responsible for the observed regional differences in the prevalence of pneumoconiosis. Therefore, BAI may be a useful characteristic of coal for predicting coal's toxicity.