The Journal of Experimental Medicine
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Published online 1 May 2000.
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© The Rockefeller University Press, 0022-1007/2000/5/1535/ $5.00
The Journal of Experimental Medicine, Volume 191, Number 9, May 1, 2000 1535-1544


Original Article

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

James L. Younga, Galina K. Sukhovaa, Don Fosterb, Walter Kisielc, Peter Libbya, and Uwe Schönbecka
a From Cardiovascular Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Boston, Massachusetts 02115
b ZymoGenetics, Incorporated, Seattle, Washington 93102
c Department of Pathology, School of Medicine, University of New Mexico, Albuquerque, New Mexico 87131-5301

Correspondence to: Peter Libby, Cardiovascular Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, 221 Longwood Ave., LMRC 307, Boston, MA 02115. Tel:617-732-6628 Fax:617-732-6961 E-mail:plibby{at}rics.bwh.harvard.edu.

Interleukin-1ß–converting enzyme (ICE, caspase-1) regulates key steps in inflammation and immunity, by activating the proinflammatory cytokines interleukin (IL-)1ß 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-1ß 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-1ß 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-1ß, 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.

Key Words: caspase-1, interleukin 1ß, PI-9, smooth muscle cells, atherosclerosis


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