Hexavalent chromium (Cr(VI)) is a well-designated human lung carcinogen, with solubility playing an important role in its carcinogenic potential. Although it is known that particulate or water-insoluble Cr(VI) compounds are more potent than the soluble species of this metal, the mechanisms of action are not fully elucidated. In this study, we investigated the hypothesis that the difference in potency between particulate and soluble Cr(VI) is due to more chronic exposures with particulate chromate because it can deposit and persist in the lungs while soluble chromate is rapidly cleared. Chronic exposure to both insoluble lead chromate and soluble sodium chromate induced a concentration and time-dependent increase in intracellular Cr ion concentrations in cultured human lung fibroblasts. Intracellular Pb levels after chronic exposure to lead chromate increased in a concentration-dependent manner but did not increase with longer exposure times up to 72 h. We also investigated the effects of chronic exposure to Cr(VI) on clastogenicity and found that chronic exposure to lead chromate induces persistent or increasing chromosome damage. Specifically, exposure to 0.5 microg/cm(2) lead chromate for 24, 48 and 72 h induced 23, 23 and 27% damaged metaphases, respectively. Contrary to lead chromate, the amount of chromosome damage after chronic exposure to sodium chromate decreased with time. For example, cells exposed to 1 microM sodium chromate for 24, 48 and 72 h induced 23, 13 and 17% damaged metaphases, respectively. Our data suggest a possible mechanism for the observed potency difference between soluble and insoluble Cr(VI) compounds is that chronic exposure to particulate Cr(VI) induces persistent chromosome damage and chromosome instability while chromosome damage is repaired with chronic exposure to soluble Cr(VI).
