Targeting vascular remodeling in abdominal aortic aneurysm To identify novel treatment strategies and drug candidates

Sammanfattning: Abdominal aortic aneurysm (AAA) is a degenerative weakening of the aortic wall, mainly affecting elderly men with a prevalence of 4.4-7.7 %. AAA is characterized by medial and adventitial inflammatory cell infiltration associated with vascular remodeling of the extracellular matrix proteins such as collagen and elastin and with phenotypic modulation and loss of vascular smooth muscle cells (VSMCs). Although much research has been performed, the precise cellular and molecular pathways behind these processes are still poorly understood. The overall aim of this thesis was to target signaling pathways that affect vascular remodeling of AAA to potentially identify novel strategies and drug candidates for future treatment of aneurysmal diseases. In order to develop our understanding of the pathophysiology of AAA, we used the angiotensin (Ang) II-induced AAA animal model and human biopsies taken at end-stage of disease to recapitulate key aspects of disease formation.Innate immune receptors such as toll-like receptors (TLRs) are known to regulate immunological processes leading to the formation and progression of vascular disease including AAA. In paper I, we aimed to investigate the role of TLR signaling under the control of the TRIF adaptor protein in the formation of AAA. Human, aneurysmal aortas displayed increased expression of TLR3 and TLR4 in surface of macrophages and T lymphocytes. AngII-induced aneurysm formation was attenuated in mice lacking the Trif gene  (ApoE-/-Triʃ-/-), and these knockout mice presented with a more intact medial layer together with a reduced inflammatory response by macrophages and T lymphocytes and reduced levels of pro-inflammatory cytokines, chemokines, and proteases. Our results suggest an involvement of TRIF in the pathophysiology of AAA.Current management of AAA fully depends on imaging and surgical techniques, and drug-based therapies are still mostly ineffective. In paper II, we aimed to investigate the potential protective role of the tyrosine kinase inhibitor imatinib on the molecular mechanism involved in AAA formation. In AngII-infused ApoE-/-mice, 10 mg/kg imatinib per day affected several key features important in aneurysmal formation, including  preservation of the medial layer of the VSMCs, reduced infiltrations of CD3ε-positive T lymphocytes, and reduced gene expression of mast cell chymase, resulting in decreased aortic diameter and vessel wall thickness. These results highlight the importance of the tyrosine kinase inhibitor imatinib as a potential drug in the treatment of pathological vascular inflammation and remodeling in conditions such as AAA.In paper III, we aimed to investigate the role of adiponectin in experimentally induced AAA formation in mice. In mice with elevated adiponectin levels, AAA development was inhibited, and this was associated with reduced inflammatory cell infiltration, reduced medial degeneration of VSMCs and of elastin in the aortic vessel wall together with an improved systemic cytokine profile and the attenuation of periaortic adipose tissue (PVAT) inflammation. These results support the protective effect of adiponectin in the remodeling occurring in the aortic wall and in the prevention of AAA.In paper IV, we performed a descriptive study investigating the composition of PVAT adjacent to the aneurysmal aorta. We used immunohistochemistry to identify neutrophils, macrophages, mast cells, and T lymphocytes surrounding necrotic adipocytes in PVAT together with increased gene expression of IL-6 and cathepsin K and S. We also determined the concentrations of pro-inflammatory ceramides in PVAT and found an association to T lymphocytes. These results suggest that inflamed adipose tissue might be a source of proinflammatory cells and mediators that contribute to aortic wall degeneration.