Background: IκB, a cytoplasmic inhibitor of NF-κB, has been reported to be degraded via the proteasome. However, we recently found that long-term incubation with PIs (PS-341 or MG132) induces IκBa degradation via an alternative pathway, lysosome, which results in NF-κB activation that confers resistance to PI-induced lung cancer cell death. To enhance the anti-cancer effi cacy of PIs, elucidation of the regulatory mechanism of PI-induced IκBa degradation is necessary. Methods: NCI-H157 and A549 lung cancer cells were used. The expression level of light chain 3B (LC3B, macroautophagy marker), lysosome-associated membrane protein 2a (Lamp2a, the receptor for chaperone-mediated autophagy), or nuclear factor erythroid 2-related factor 2 (Nrf2) was determined by real-time PCR and Western blot analysis. siRNAs targeting LC3B and adenovirus vector expressing dominant-negative Nrf2 (Ad-DN-Nrf2) were used to inhibit LC3B induction and Nrf2 activation, respectively.KEAP1 (Nrf2 inhibitor) expression was determined by Western blot analysis. Results: PIs increased the protein level of LC3B, but not Lamp2a in both lung cancer cells. LC3B induction was through de novo protein synthesis. Pretreatment with cycloheximide (inhibitor of protein biosynthesis) or Knock-down of LC3B expression by transfection with siRNAs blocked PI-induced IκBa degradation. PIs up-regulated Nrf2via both de novo protein synthesis and down-regulation of KEAP1, which was responsible for IκBa degradation. Overexpression of dominant-negative Nrf2 suppressed PI-induced LC3B up-regulation and subsequent IκBa degradation. Conclusions: Collectively, our data demonstrate that PIs induces LC3B up-regulation via Nrf2 activation, which mediates IκBa degradation via lysosome.