The Serpin Proteinase Inhibitor 9 Is an Endogenous Inhibitor of Interleukin 1β–Converting Enzyme (Caspase-1) Activity in Human Vascular Smooth Muscle Cells

Abstract

Interleukin-1β–converting enzyme (ICE, caspase-1) regulates key steps in inflammation and immunity, by activating the proinflammatory cytokines interleukin (IL-)1β and IL-18, or mediating apoptotic processes. We recently provided evidence for the regulation of caspase-1 activity via an endogenous inhibitor expressed by human vascular smooth muscle cells (SMCs) (Schönbeck, U., M. Herzberg, A. Petersen, C. Wohlenberg, J. Gerdes, H.-D. Flad, and H. Loppnow. 1997. J. Exp. Med. 185:1287–1294). However, the molecular identity of this endogenous inhibitor remained undefined. We report here that the serine proteinase inhibitor (serpin) PI-9 accounts for the endogenous caspase-1 inhibitory activity in human SMCs and prevents processing of the enzyme's natural substrates, IL-1β and IL-18 precursor. Treatment of SMC lysates with anti–PI-9 antibody abrogated the caspase-1 inhibitory activity and coprecipitated the enzyme, demonstrating protein–protein interaction. Furthermore, PI-9 antisense oligonucleotides coordinately reduced PI-9 expression and promoted IL-1β release. Since SMCs comprise the majority of cells in the vascular wall, and because IL-1 is implicated in atherogenesis, we tested the biological validity of our in vitro findings within human atheroma in situ. The unaffected arterial wall contains abundant and homogeneously distributed PI-9. In human atherosclerotic lesions, however, PI-9 expression correlated inversely with immunoreactive IL-1β, supporting a potential role of the endogenous caspase-1 inhibitor in this chronic inflammatory disease. Thus, our results provide new insights into the regulation of this enzyme involved in immune and inflammatory processes of chronic inflammatory diseases, and point to an endogenous antiinflammatory action of PI-9, dysregulated in a prevalent human disease.

A novel isoform of pro-interleukin-18 expressed in ovarian tumors is resistant to caspase-1 and -4 processing

“…Thus, it is likely that the alternate splicing, leading to the generation of D3pro-IL-18, may represent a regulatory mechanism that inhibits the production of biologically active IL-18 in some ovarian tumors. In this context, it should be outlined that similar to the IL-1b/IL-1R system, the IL-18/IL-18R system also has several natural inhibitors which may negatively regulate the IL-18 biological activity, protecting the organism from unappropriate IL-18-mediated damage (Novick et al, 1999;Kim et al, 2000;Young et al, 2000;Xiang and Moss, 2001;Bufler et al, 2002). Thus, a soluble IL-18 binding protein (BP), which prevent binding of mature IL-18 to its receptor, and inhibits IL-18 biological activity has been identified Figure 6 D3pro-IL-18 binds to caspase-1.…”

The Granzyme B Inhibitor, Protease Inhibitor 9, Is Mainly Expressed by Dendritic Cells and at Immune-Privileged Sites

“…PI9 binds to and inhibits the activity of the serine protease elastase (21) although with less efficiency than granzyme B (association rate constant K a is 1.7 ϫ 10 6 M Ϫ1 s Ϫ1 and 1.5 ϫ 10 5 M Ϫ1 s Ϫ1 for complex formation of PI9 with granzyme B (12) and elastase (21), respectively). Although it is not an effective caspase inhibitor (11), PI9 has been reported to inhibit caspase 1 (22,23) and caspase 4 (11) in vitro.…”

Control of granzymes by serpins

“…A subsequent investigation suggested indirectly that SERPINB9 controls caspase-1 activity in vascular smooth muscle cells in vivo during atherogenesis, 60 a function that may be linked to proinflammatory and interleukin-1-responsive elements in the SERPINB9 promoter. 55 Perhaps at sites of rampant inflammation, such as atheroma, there is enough interleukin-1 to raise SERPINB9 levels sufficiently to overcome weak kinetics and allow inhibition of caspase-1, thus modulating the inflammatory response.…”