Sub-lethal levels of arsenic induce upregulation of stress proteins. We here report for the first time that inorganic arsenic activates the transcription factor Nrf2, which controls the expression of oxidative stress-induced proteins. Treatment of cultured MC3T3-E1 osteoblasts with arsenite or arsenate induced increase of Nrf2, followed by transcriptional activation of target genes encoding HO-1, Prx 1, and A 170. We found that arsenate (200-800 muM) only slightly increased the normal 60 kDa A 170 protein but markedly increased higher molecular mass forms of A 170 (HMM-A170) that appeared as smeared bands. Arsenate also markedly increased ubiquitin-conjugated cellular proteins, suggesting that HMM-A170 was one of the poly-ubiquitinated proteins. Arsenite (50100 muM) also induced accumulation of HMM-A170 and ubiquitin-conjugated proteins. These results provide the first direct evidence that toxic arsenics impair the normal function of A 170. Our findings provide a potential diagnostic tool for monitoring biotoxicity in cells and tissues in response to arsenic compounds. (C) 2003 Elsevier Science (USA). All rights reserved.