Inhibition of IκB-α and IκB-β Proteolysis by Calpain Inhibitor I Blocks Nitric Oxide Synthesis

Lipopolysaccharide (LPS) stimulates the induction of the inducible isoform of nitric oxide synthase (iNOS) in part by inducing the nuclear translocation of the transcription factor nuclear factor-κ B (NF-κB). LPS induces ubiquination and phosphorylation of the IκB inhibitory subunit of NF-κB. Subsequently, the ubiquitin–proteasome multicatalytic enzyme complex catalyzes the proteolytic degradation of IκB with resultant nuclear translocation of NF-κB. Our results demonstrate that the proteasome inhibitor calpain inhibitor I dose-dependently inhibited LPS-induced nitric oxide synthesis in RAW macrophages. The inhibitor was found to block iNOS transcription and protein translation as noted by Northern analysis and Western blotting, respectively. LPS stimulated rapid proteolytic degradation of IκB-α which was inhibited by approximately 50% in the presence of calpain inhibitor I. In contrast, LPS induced the delayed proteolytic degradation of IκB-β which was almost totally inhibited by calpain inhibitor I. Calpain inhibitor I also decreased the LPS-induced nuclear translocation of NF-κB. These results demonstrate that the ubiquitin–proteasome complex has an important role in induction of iNOS in response to stimuli which act via the NF-κB/IκB signal transduction pathway. Furthermore, the results suggest that the ubiquitin–proteasome complex is important in the degradation of IκB-β as well as IκB-α. Finally, we have demonstrated that there is a marked difference in the extent of proteolysis of IκB-α and IκB-β when the ubiquitin–proteasome complex is inhibited with calpain inhibitor I.