Auranofin (2,3,4,6-tetra-O-acetyl-1-thio-beta-D-glucopyranosato-S-[triethylphosphine] gold) is a gold(I)-containing antirheumatic drug that possesses anti-inflammatory properties. The pharmacological activity of this drug is associated with its ability to induce heme oxygenase-1 (HO-1). However, the mechanism underlying auranofin-mediated HO-1 induction remains unclear. We investigated the action of auranofin on activation of nuclear factor erythroid 2-related factor 2 (Nrf2), an activator of HO-1. Auranofin elevated cellular levels of Nrf2 by increasing protein stability but not transcriptional activation. Coimmunoprecipitation and Western blot analysis indicated that auranofin inhibited Nrf2 degradation by inducing the dissociation of the Nrf2 / Kelch-like ECH-associated protein 1 (Keap1) complex, which resulted in nuclear accumulation of Nrf2. In addition, auranofin treatment activated cellular Rac1 and induced inducible nitric oxide synthase (iNOS) expression. An inhibitor of Rac1 (NSC23766) blocked the iNOS induction as well as Nrf2 activation and HO-1 expression. N(G)-nitro-L-arginine methyl ester and aminoguanidine, inhibitors of iNOS, diminished the auranofin-induced Nrf2 activation and HO-1 expression. Phosphorylation of mitogen-activated protein kinases (MAPKs) was increased by auranofin treatment, and inhibitors of MAPKs partially diminished the Nrf2 activation. A chromatin immunoprecipitation assay showed that the Nrf2 activated by auranofin was involved in transactivation of the HO-1 gene. These findings indicate that auranofin leads to HO-1 upregulation by activating Keap1/Nrf2 signaling via Rac1/iNOS induction and MAPK activation.