Regulation of intracellular protein stability by the ubiquitin-dependent proteasome system plays a crucial role in cell function. HO-1 (haem oxygenase) is a stress response protein, which confers cytoprotection against oxidative injury and provides a vital function in maintaining tissue homoeostasis. In the present study, we found a novel action of proteasome inhibitors MG132 and MG262 on HO-1 induction, and characterized the underlying mechanisms. MG132 (> or =0.1 microM) treatment resulted in a marked time- and concentration-dependent induction of the steady-state level of HO-1 mRNA in RAW264.7 macrophages, followed by a corresponding increase in HO-1 protein. Actinomycin D and cycloheximide inhibited MG132-responsive HO-1 protein expression, indicating a requirement for transcription and de novo protein synthesis. The involvement of signal pathways in MG132-induced HO-1 gene expression was examined using chemical inhibitors. Antioxidant N -acetylcysteine and SB203580, an antioxidant and inhibitor of p38 MAPK (mitogen-activated protein kinase), abolished MG132-inducible HO-1 expression. Furthermore, MG132 activated the p38 MAPK pathway. The half-life of HO-1 protein was prolonged by MG132, indicating that the upregulation of HO-1 by proteasome inhibitor is partially attributable to the inhibition of protein degradation. MG132 can ablate IkappaBalpha degradation and NF-kappaB (nuclear factor kappaB) activation induced by lipopolysaccharide, similar to the effect of another NF-kappaB inhibitor pyrrolidine dithiocarbamate. We found HO-1 upregulation by MG132 and pyrrolidine dithiocarbamate is unrelated to their inhibition of NF-kappaB, since leptomycin B, another NF-kappaB inhibitor, did not elicit similar induction of HO-1. Taken together, we found a novel effect of proteasome inhibitor on induction of HO-1 expression. This action is ascribed to the activation of the p38 MAPK pathway, but is not dependent on NF-kappaB inhibition.