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

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.