Secretory leucoprotease inhibitor prevents lipopolysaccharide-induced IκBα degradation without affecting phosphorylation or ubiquitination

Abstract

Secretory leucoprotease inhibitor (SLPI) is a non-glycosylated protein produced by epithelial cells, macrophages, and neutrophils and was initially identified as a serine protease inhibitor of the neutrophil proteases elastase and cathepsin G. In addition to its antiprotease activity, SLPI has been shown to exhibit anti-inflammatory properties including down-regulation of tumor necrosis factor-α expression by lipopolysaccharide (LPS) in monocytes, inhibition of NF-κB activation by IgG immune complexes in a rat model of acute lung injury, and prevention of human immunodeficiency virus infectivity in monocytic cells via as yet unidentified mechanisms. In this report we have shown that SLPI prevents LPS-induced NF-κB activation by inhibiting degradation of IκBα without affecting the LPS-induced phosphorylation and ubiquitination of IκBα. We have also demonstrated that SLPI prevents LPS-induced interleukin-1 receptor-associated kinase and IκBβ degradation. In addition, we have demonstrated that oxidized SLPI, a variant of SLPI that has diminished antiprotease activity, cannot prevent LPS-induced NF-κB activation or Inhibitor κB α/β degradation indicating that the anti-inflammatory effect of SLPI on the LPS-signaling pathway is dependent on its antiprotease activity. These results suggest that SLPI may be inhibiting proteasomal degradation of NF-κB regulatory proteins, an effect that is dependent on the antiprotease activity of SLPI.