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-alpha expression by lipopolysaccharide (LPS) in monocytes, inhibition of NF-kappaB 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-kappaB activation by inhibiting degradation of IkappaBalpha without affecting the LPS-induced phosphorylation and ubiquitination of IkappaBalpha. We have also demonstrated that SLPI prevents LPS-induced interleukin-1 receptor-associated kinase and IkappaBbeta degradation. In addition, we have demonstrated that oxidized SLPI, a variant of SLPI that has diminished antiprotease activity, cannot prevent LPS-induced NF-kappaB activation or Inhibitor kappaB alpha/beta 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-kappaB regulatory proteins, an effect that is dependent on the antiprotease activity of SLPI.
Highlights
Secretory leucoprotease inhibitor (SLPI)1 is an 11.7-kDa non-glycosylated protein, which is expressed by epithelial cells, macrophages, and neutrophils [1,2,3]
These results suggest that SLPI may be inhibiting proteasomal degradation of NF-B regulatory proteins, an effect that is dependent on the antiprotease activity of SLPI
The antiprotease activity of SLPI appears to be required for this mechanism of inhibition because oxidized SLPI cannot prevent LPS-induced NF-B activation nor prevent IRAK, IB␣, and IB degradation
Summary
Secretory leucoprotease inhibitor (SLPI)1 is an 11.7-kDa non-glycosylated protein, which is expressed by epithelial cells, macrophages, and neutrophils [1,2,3]. These results suggest that SLPI may be inhibiting proteasomal degradation of NF-B regulatory proteins, an effect that is dependent on the antiprotease activity of SLPI. In this report we show that SLPI inhibits LPS-induced NF-B activation in U937 cells by preventing degradation of IB␣ but without affecting the phosphorylation or ubiquitination of IB␣.
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