MG132 proteasome inhibitor modulates proinflammatory cytokines production and expression of their receptors in U937 cells: involvement of nuclear factor-κB and activator protein-1

In response to inflammatory stimuli, monocytes/macrophages secrete greater quantities of the proinflammatory cytokines tumour necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and IL-6. The inflammatory process and the innate immune response are related to the activation of several transcription factors, such as nuclear factor κB (NF-κB) and activator protein 1 (AP-1). The proteasome is a multimeric protease complex, which plays a vital role in several cellular functions, including the regulation of transcription factors like NF-κB. In this study, we used the human monocyte cell line U937 stimulated with lipopolysaccharide (LPS) and phorbol 12-myristate 13-acetate (PMA) as a model to investigate the in vitro effects of MG132, a proteasome inhibitor, on the release of TNF-α, IL-1β and IL-6 and on the expression of their membrane and soluble receptors TNF-R1, IL-1R1 and IL-6R. We also analysed the effects of MG132 on the activation of NF-κB and AP-1 and on the IκB molecule. MG132 significantly inhibited the secretion of those proinflammatory cytokines. MG132 increased the release of the soluble receptors TNF-R1 and IL-1R1 from U937 cells and decreased their cell-surface expression. MG132 also increased IL-6R cell-surface expression and decreased its release. Proteasome inhibition also led to an increase in LPS+PMA-induced AP-1 activation and the attenuation of LPS+PMA-induced IκB degradation, resulting in the abolition of NF-κB activation. Our experiments strongly suggest that the proteasome is an important factor in the regulation of proinflammatory cytokines and their receptors.