The ubiquitous toxic metalloid arsenic elicits pleiotropic adverse and adaptive responses in mammalian species. The biological targets of arsenic are largely unknown at present. We analyzed the signaling pathway for induction of detoxification gene NAD(P)H-quinone oxidoreductase (Nqo1) by arsenic. Genetic and biochemical evidence revealed that induction required cap 'n' collar basic leucine zipper transcription factor Nrf2 and the antioxidant response element (ARE) of Nqo1. Arsenic stabilized Nrf2 protein, extending the t1 ⁄ 2 of Nrf2 from 21 to 200 min by inhibiting the Keap1⅐Cul3-dependent ubiquitination and proteasomal turnover of Nrf2. Arsenic markedly inhibited the ubiquitination of Nrf2 but did not disrupt the Nrf2⅐Keap1⅐Cul3 association in the cytoplasm. In the nucleus, arsenic, but not phenolic antioxidant tert-butylhydroquinone, dissociated Nrf2 from Keap1 and Cul3 followed by dimerization of Nrf2 with a Maf protein (Maf G/Maf K). Chromatin immunoprecipitation demonstrated that Nrf2 and Maf associated with the endogenous Nqo1 ARE enhancer constitutively. Arsenic substantially increased the ARE occupancy by Nrf2 and Maf. In addition, Keap1 was shown to be ubiquitinated in the cytoplasm and deubiquitinated in the nucleus in the presence of arsenic without changing the protein level, implicating nuclear-cytoplasmic recycling of Keap1. Our data reveal that arsenic activates the Nrf2/Keap1 signaling pathway through a distinct mechanism from that by antioxidants and suggest an "onswitch" model of Nqo1 transcription in which the binding of Nrf2⅐Maf to ARE controls both the basal and inducible expression of Nqo1.Environmental toxic metal arsenic, which originates from both geochemical and anthropogenic activities, is a ubiquitous contaminant and an established human carcinogen (1, 2). Arsenic has become a major public health concern worldwide because millions of people are at risk of drinking water contaminated with arsenic that has been associated with multiple human diseases or lesions (3-5). The anthropogenic sources of arsenic contaminating soil and water include mining, metallurgical activities, and manufacture and agricultural use of pesticides and herbicides. Arsenic causes a spectrum of adverse responses in many species. Whereas acute exposure to inorganic arsenic in humans results in cardiac failure, peripheral neuropathy, anemia, leucopenia, and death, chronic arsenic exposure can cause a range of cancers (particularly of the skin, lung, bladder, and liver) as well as liver injury, neuropathy, cardiovascular lesions, ovarian dysfunction, aberrant embryo development, and postnatal growth retardation (4, 6 -10). Interestingly, arsenic-containing compounds have proven to be effective as therapeutic agents in treating cancer such as leukemia (11), chronic inflammatory disease such as psoriasis (12), and parasitic infection such as sleeping sickness (13, 14). Although chemical interaction with protein thiol groups and generation of reactive oxygen species have been implicated in the actions of arsenic, the...

The potential for development of Parkinson's disease (PD)-like neurological dysfunction following occupational exposure to aerosolized welding fumes (WF) is an area of emerging concern. Welding consumables contain a complex mixture of metals, including iron (Fe) and manganese (Mn), which are known to be neurotoxic. To determine whether WF exposure poses a neurological risk particularly to the dopaminergic system, we treated Sprague-Dawley rats with WF particulates generated from two different welding processes, gas metal arc-mild steel (GMA-MS; low Mn, less water-soluble) and manual metal arc-hard surfacing (MMA-HS; high Mn, more water-soluble) welding. Following repeated intratracheal instillations (0.5 mg/rat, 1/week x 7 weeks) of GMA-MS or MMA-HS, elemental analysis and various molecular indices of neurotoxicity were measured at 1, 4, 35 or 105 days after last exposure. MMA-HS exposure, in particular, led to increased deposition of Mn in striatum and midbrain. Both fumes also caused loss of tyrosine hydroxylase (TH) protein in the striatum (~20%) and midbrain (~30%) by 1 day post-exposure. While the loss of TH following GMA-MS was transient, a sustained loss (34%) was observed in the midbrain 105 days after cessation of MMA-HS exposure. In addition, both fumes caused persistent down-regulation of dopamine D2 receptor (Drd2; 30-40%) and vesicular monoamine transporter 2 (Vmat2; 30-55%) mRNAs in the midbrain. WF exposure also modulated factors associated with synaptic transmission, oxidative stress, neuroinflammation and gliosis. Collectively, our findings demonstrate that repeated exposure to Mn-containing WF can cause persistent molecular alterations in dopaminergic targets. Whether such perturbations will lead to PD-like neuropathological manifestations remains to be elucidated.

Chromium (Cr) (VI) is a major environmental toxic metal and a human carcinogen. The molecular events mediating cellular responses to Cr(VI) are not clear at present. We show that Cr(VI) potently induced apoptosis and production of reactive oxygen species (ROS) in mouse hepa1c1c7 cells in a concentration-dependent manner. Mouse embryonic fibroblast cells lacking Nrf2 exhibited elevated ROS production and apoptosis, which were markedly further increased by Cr(VI), suggesting a protective role of Nrf2 against Cr(VI) toxicity. Protection by Nrf2 correlated with induction of cytoprotective genes Ho-1 and Nqo1. Induction of the genes by Cr(VI) involved inhibition of ubiquitination of Nrf2 and accumulation of Nrf2 into the nucleus. In the nucleus, treatment with Cr(VI), but not phenolic antioxidant tert-butylhydroquinone, librates Nrf2 from the Nrf2/Keap1 association and recruits Nrf2 to the antioxidant response elements (ARE) located in the enhancers of Ho-1 and Nqo1. Activation of Nrf2 by Cr(VI) was accompanied by the nuclear translocation and deubiquitination of Keap1 implicating recycling of Keap1 in Nrf2 signaling. Thus, protection against Cr(VI) toxicity involves a transcriptional signaling loop that includes activation of Nrf2 by the toxic metal, transcription of ARE-driven genes, and reduction of ROS production.