Reductive activation of hexavalent chromium by human lung epithelial cells: Generation of Cr(V) and Cr(V)-thiol species

Abstract

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 Na 2 CrO 4 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 b 5 . Unlike many cells in culture, these cells continued to express P450 reductase and cytochrome b 5 . 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, ZnCrO 4 , also generated both Cr(V) signals, whereas Cr(V) was not detected with insoluble PbCrO 4 . In clonogenic assays, the cells were very sensitive to Na 2 CrO 4 and ZnCrO 4 , but considerably less sensitive to PbCrO 4 .

The effects of chromium(VI) on the thioredoxin system: Implications for redox regulation

“…This is consistent with the expected slower penetration of particulate forms and with the smaller Cr(V) signals seen at early times with ZnCrO 4 [26]. Trx2 is more sensitive to Cr(VI) treatment than Trx1, suggesting greater redox stress in the mitochondria.…”

“…Overall, several reactive and pro-oxidant species can be generated by intracellular Cr(VI) reduction, and pro-oxidant effects can contribute to Cr(VI) toxicity [26,33,54–56,64,69–80] and to its ability to promote mitochondrial-dependent apoptosis [81–83]. The redox cycling of Cr could increase the generation of ROS and thereby enhance oxidative stress [41,55,70,71,84].…”

Oxidative damage induced by chromium (VI) in rat erythrocytes: protective effect of selenium

“…In fact, the US Environmental Protection Agency and the International Agency for Research on Cancer classify Cr as a human carcinogen, currently making Cr(VI) one of 33 compounds listed to pose the greatest potential health threat in urban areas [3,81]. Indeed, once inside cells, Cr(VI) can be reduced by several chemical and enzymatic reductants including ascorbate, cysteine, glutathione, glutathione reductase, and multiple microsomal enzymes like cytochrome b5 [12,13,75]. So, reactive Cr intermediates, Cr(V) and Cr(IV), can be generated and exert their cytotoxic effects [12,74].…”

“…So, reactive Cr intermediates, Cr(V) and Cr(IV), can be generated and exert their cytotoxic effects [12,74]. Cr(VI) reduction results in several oxidants: (1) Cr(V) can directly oxidize cell components [78], (2) Cr(V) and Cr(IV) catalyze robust hydroxyl radical (HO • ) 1 generation in Fentonlike reactions with H 2 O 2 [12,13,52,66,67], and (3) some enzymes simultaneously reduce Cr(VI) to Cr(V) and generate superoxide (O2 •− ) [12,49]. Potassium dichromate that enters the bloodstream is transported into red blood cells (RBCs) via sulfate anion channels [83].…”

The pro-oxidant chromium(VI) inhibits mitochondrial complex I, complex II, and aconitase in the bronchial epithelium: EPR markers for Fe–S proteins

“…Once inside cells, there are several chemical and enzymatic reductants that can reduce Cr(VI), including ascorbate, cysteine, glutathione, glutathione reductase, and multiple microsomal enzymes including cytochrome b 5 [18-26]. These 1- and 2-electron reductants generate reactive Cr intermediates, C(V) and Cr(IV), which are important for the cytotoxic effects [8, 27-33]. Oxidative damage is one likely outcome given that Cr(VI) reduction results in several oxidants: (a) Cr(V) can directly oxidize cell components [34, 35]; (b) Cr(V) and Cr(IV) catalyze robust hydroxyl radical (HO • ) generation in Fenton-like reactions with H 2 O 2 [26, 29, 36-39]; and (c) some enzymes simultaneously reduce Cr(VI) to Cr(V) and generate superoxide (O 2 •− ) [26, 40].…”