Mechanisms of Aortic Aneurysm Formation

The effects of IL-1β on smooth muscle cell phenotype during aneurysm formation

Summary : Although the etiology of formation of aneurysms is poorly understood, there is evidence that local inciting events of unknown nature lead to upregulation of inflammatory cytokines, leukocyte infiltration into the aortic wall, smooth muscle cell (SMC) proliferation, upregulation of matrix metalloproteinases (MMPs) by leukocytes and SMCs, and subsequent destruction of the aortic wall. MMP-2 produced by mesenchymal SMCs and MMP-9 produced by macrophages play a role in aneurysm formation. In late stages of aneurysm progression, SMCs undergo apoptosis, thereby eliminating the cells that are the primary source of extracellular matrix proteins necessary for stabilization of the blood vessel, thus increasing the risk of aneurysm rupture.

Vascular SMCs have the ability to undergo profound changes in phenotype after pathological stimuli and during atherosclerosis. The nature of changes in SMC phenotype typically involves coordinated repression of SMC marker genes and production of MMPs and their inhibitors. We have observed that the pro-inflammatory cytokine IL-1β (more than any other cytokine tested) profoundly increases the expression of MMP-2, -9 in vitro while simultaneously suppressing the expression of all SMC marker genes including SM α-actin. However, little is known regarding the effect of local inflammatory cytokines on SMC phenotypic switching and their role in aneurysm formation in vivo.

This poject tests the hypothesis that IL-1β within aortic aneurysms induces an inflammatory phenotype in SMCs characterized by marked suppression of SMC marker genes, activation of MMPs and inflammatory mediators, and that this process plays a critical role in mediating degradation of the aortic wall matrix and aneurysm formation. Our rationale for focusing on IL-1β is as follows: 1) Tissue from human aortic aneurysms has demonstrated a profound upregulation of IL-1β. 2) Preliminary data suggests that IL-1β, compared to other cytokines, induces the most profound phenotypic switching in SMCs as noted above. 3) The availability of IL-1β and IL-1R knockout (KO) mice will allow this hypothesis to be tested. The studies described will provide a template for future studies testing other cytokines present in human aortic aneurysm tissue.

Model: Our lab utilizes a mouse aortic aneurysm model as shown below. A) Elastase perfusion of the infrarenal abdominal aorta. B) An explanted abdominal aortic aneurysm 14 days after elastase perfusion. C) Significant development of aortic aneurysm (aortic diameter) occurs after 14 days.

Smooth muscle cell phenotypic switching during experimental aneurysm formation is KLF4 dependent

Summary: Recent evidence has suggested an important role for transforming growth factor-beta (TGF-β) during aneurysm development. Mutations of the TGF-β receptors, TGFBR1 and TGFBR2, have been causally linked to familial aneurysm syndromes. Furthermore, mouse models of Marfan's syndrome deficient in fibrillin are characterized by increased TGF-β and can be inhibited by specific antibodies to TGF-β.

We have identified and cloned a novel protein, Kruppel Like Factor-4 (KLF4), to be a key effector that binds to the TGF-β control element and represses TGF-β dependent increases in SMC marker genes in vivo. Recent studies using a conditional knockout KLF4 mouse created by Gary Owens have shown that wire injury of the carotid artery results in delayed repression of SMC marker genes and enhanced neointimal formation. Moreover, evidence suggests that KLF4 plays a key role in activating inflammatory genes through myocardin-dependent inhibition of NFkB. Thus, KLF4 represses SMC marker genes and may alter both SMC and leukocyte responses to vascular injury/ inflammation as related to atherosclerosis. However, nothing is known regarding the role of KLF4 in regulating SMC phenotypic switching in vivo during aneurysm formation.

This project tests the hypothesis that SMCs within aortic aneurysms undergo phenotypic switching to an inflammatory phenotype characterized by marked suppression of SMC marker genes and activation of MMPs and that this process is dependent on KLF4 which plays a critical role in mediating matrix degradation in aneurysm formation.