Reductive Activation of Hexavalent Chromium by Human Lung Epithelial Cells: Generation of Cr(V) and Cr(V)-Thiol Species

摘要

Chromium(VI) compounds (e.g. chromates) are cytotoxic, mutagenic, and potentially carcinogenic. The reduction of Cr(VI) can yield reactive intermediates such as Cr(V) and reactive oxygen species. Bronchial epithelial cells are the primary site of pulmonary exposure to inhaled Cr(VI) and are the primary cells from which Cr(VI)-associated human cancers arise. BEAS-2B cells were used here as a model of normal human bronchial epithelium for studies on the reductive activation of Cr(VI). Cells incubated with Na2CrO4 exhibited two Cr(V) ESR signals, g = 1.979 and 1.985, which persisted for at least one hour. The g = 1.979 signal is similar to that generated in vitro by human microsomes and by proteoliposomes containing P450 reductase and cytochrome b5. Unlike many cells in culture, these cells continued to express P450 reductase and cytochrome b5. Studies with the non-selective thiol oxidant diamide indicated that the g = 1.985 signal was thiol-dependent whereas the g = 1.979 signal was not. Pretreatment with phenazine methosulfate eliminated both Cr(V) signals suggesting that Cr(V) generation is largely NAD(P)H-dependent. ESR spectra indicated that a portion of the Cr(VI) was rapidly reduced to Cr(III). Cells incubated with an insoluble chromate, ZnCrO4, also generated both Cr(V) signals, whereas Cr(V) was not detected with insoluble PbCrO4. In clonogenic assays, the cells were very sensitive to Na2CrO4 and ZnCrO4, but considerably less sensitive to PbCrO4.