The current understanding of Atherosclerosis is that of a complex chronic inflammatory disease rather than a mere phenomenon of lipid deposition in the vascular wall. The inflammatory processes considered crucial to this prevalent human disease are orchestrated, in part, by the cytokine network. In previous work, the applicants and others associated enhanced expression of interleukin 18 (IL-18), a member of the IL-1 cytokine family, with pro-atherogenic functions. Notably, preliminary in vitro and in vivo studies revealed IL-18 as a prominent player in early rather than advanced atherosclerosis that mediates its atherogenic functions, at least in part, independent of interferon gamma (IFNgamma), its traditional downstream effector. To express biological activity IL-18, like IL-1b, requires maturation via caspase-1. This proposal aims to characterize pathways via which IL-18 promotes atherosclerosis and to determine whether these functions require interaction with other cytokines, namely IL-1beta, IL-12, and IFNgamma, or the activity of caspase-1. Finally, the hypothesis that IL-18 and caspase-1 provide novel therapeutic targets for this prevalent human disease will be tested in vivo.
Specific Aim I will elucidate pro-atherogenic processes via which IL-18 promotes atherosclerosis in vitro. Based on preliminary data this Specific Aim will focus on implications for IL-18 signaling in the pathogenic processes characterizing early atherogenesis, namely accumulation of inflammatory cells (adhesion, migration, proliferation) and affiliated extracellular matrix degradation. The experiments will further determine whether such IL-18-mediated proatherogenic functions depend on the activity of its converting enzyme caspase-1 or synergistic interactions with either IL- 1b (the other substrate of caspase- 1), IL- 12 (an established enhancer of IL- 18' s inflammatory functions), or IFNgamma (its traditional downstream effector). These studies will employ cultures of cell types implicated in atherosclerosis, namely endothelial and smooth muscle cells, mononuclear phagocytes, and T cells, of human or the respective gene-deficient murine origin to which endogenous as well as synthetic inhibitors, antibodies, and siRNA technique will be administered.
Specific Aim II will test the hypothesis that abrogation of IL-18 signaling or caspase-1 activity provides novel therapeutic targets for atherosclerosis in vivo. Initial studies administering anti-IL-18 antibodies to hypercholesterolemic apoE 4 mice with established atherosclerosis will definitively test the hypothesis that IL-18 is implicated in early rather than advanced atherogenesis. The role of caspase-1 in early and/or advanced atherogenesis will be tested in apoE/caspase-1 compound mutant mice as well as in apoE + mice (with or without established atherosclerotic lesions) that will be administered a synthetic caspase-1 inhibitor. Additional experiments will control for the dependency of IL-18's pro-atherogenic functions on the interaction with IL-lbeta, IL-12, or IFNgamma in this model and whether alternative pathways of IL-Ib or IL-18 activation can compensate caspase-1 deficiency. Finally, bone marrow reconstitution studies employing apoE/IL-18 receptor, apoE/caspase-1 as well as apoE/IFNgamma compound mutant mice will determine whether expression of these molecules in hematopoetic cells (T lymphocytes, mononuclear phagocytes) or cells of the vascular bedding (EC, SMC) promote atherosclerosis. Thus, the proposed study is aimed to obtain a comprehensive view of the functional role of the IL-18/caspase-1 cascade in atherosclerosis, thereby exploring the range of pro-atherogenic processes induced and testing the hypothesis that functional IL-18 and caspase-1 provide novel therapeutic targets in the treatment of atherosclerosis.
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