Role of Tissue Inhibitor of Metalloproteinase 4 (TIMP4) and TIMP3 in Vascular Diseases

  • Author / Creator
    Hu, Mei
  • Atherosclerosis and aortic aneurysms are two major vascular diseases, and extracellular matrix (ECM) plays an important role in both as a non-cellular component of the aortic wall that also regulates vascular cell behavior and function. Atherosclerosis is a major contributor to cardiovascular complications and has been significant healthcare burden worldwide. It is a chronic inflammatory disease characterized by accumulation of lipids and ECM in the arterial wall. Tissue inhibitor of metalloproteinases (TIMPs) can impact atherosclerotic plaque deposition by regulating vascular ECM turnover, infiltration of inflammatory cells, and plaque stability. Aortic aneurysm is a permanent localized dilation of the vessel characterized by the degradation of aortic ECM, elevated matrix metalloproteinases (MMPs) activities and localized inflammatory responses. TIMPs play an important regulatory role in the pathogenesis of aortic aneurysms through modulating MMP activities. Both thoracic and abdominal aortic aneurysms (TAAs & AAAs) progress irreversibly with a high risk of fatal dissection or rupture, and invasive surgical repair is currently the only treatment available due to the lack of drug-based treatment.
    TIMPs include 4 family members. Abnormal expression of TIMP1, TIMP2 and TIMP3 has been found in patients with atherosclerosis. However, the role of TIMP4 in atherosclerosis has not been reported so far. Therefore, we first investigated the role of TIMP4 in atherosclerosis. Mice lacking low-density lipoprotein receptor (Ldlr) and Timp4 (Ldlr-/-/Timp4-/-) were fed high fat (HFD) or regular laboratory diet. After 3 or 6 months, HFD-fed male and female Ldlr-/-/Timp4-/- mice exhibited higher plaque density in the abdominal aorta (but not in the thoracic aorta) compared to Ldlr-/- mice. Although plasma lipid and cholesterol levels were lower in Ldlr-/-/Timp4-/--HFD mice, cholesterol content in the abdominal aorta was higher along with elevated inflammatory cytokines, MMP activities, macrophage-like smooth muscle cells (SMCs) in Ldlr-/-/Timp4-/--HFD compared to Ldlr-/--HFD mice. In vitro, oxidized LDL markedly increased CD68 expression, reduced SMC markers, increased lipid uptake and reduced cholesterol efflux protein ABCA1 in Ldlr-/-/Timp4-/- compared to Ldlr-/- SMCs from abdominal, but not thoracic aorta. TIMP4 expression in the abdominal aorta and its corresponding SMCs was ~2-fold higher than in the thoracic aorta and thoracic SMCs; TIMP4 levels decreased following HFD. Timp4-deficiency in bone marrow-derived macrophages did not alter their foam cell formation capacity in response to oxidized LDL treatment.
    Considering the link between atherosclerosis and AAA, I then investigated the susceptibility of Timp4-deficient abdominal aorta to AAA and the role of atherosclerosis in this susceptibility. I established a hypercholesterolemic aneurysm model in Ldlr-/- or Ldlr-/-/Timp4-/- mice by using HFD and Angiotensin II (Ang II). We found despite more plaque deposition in the abdominal aorta, the incidence and severity of AAA in Ldlr-/-/Timp4-/- mice was much less than in Ldlr-/-mice.
    Accordingly, I next investigated if Timp4-deficiency reduced susceptibility to aortic aneurysm by subjecting the Timp4-/- mice to a different model of aortic aneurysm (peri-adventitial elastase-induced-AAA or TAA). I also used Timp3-/- mice in parallel to compare the impact of TIMP4 loss to the absence of another TIMP in these models. I found that loss of TIMP3, but not TIMP4, exacerbated TAA and AAA severity in males and females. Timp3-/- mice exhibited more increased proteinase activity and SMC phenotypic switching post-AAA and -TAA compared to the parallel WT and Timp4-/- groups. Timp3-/- mice exhibited increased infiltration of inflammatory cells in the media post-AAA, but in the adventitia post-TAA. Consistent with this observation, Timp3-/- mice showed impaired intimal barrier integrity following AAA, but a greater adventitial neovascularization post-TAA. Timp4-/- and WT mice responded similarly to TAA and AAA induction. In healthy human aorta specimens, TIMP3 was detected in media and intima in the abdominal aorta, and additionally in the adventitia in the thoracic aorta; and decreased in these regions in AAA and TAA specimens. TIMP4 levels remained unchanged in these specimens.
    In summary, this research has demonstrated that TIMP4 protects against plaque deposition in the abdominal aorta independent of plasma cholesterol levels. However, loss of TIMP4 did not increase susceptibility to aortic aneurysm in the hyperlipidemic model or the elastase model indicating that Timp4-deficiency increases susceptibility to atherosclerosis but not to aortic aneurysm, whereas Timp3-deficiency exacerbated aortic aneurysm, highlighting the disease-specific function of different TIMPs in the vasculature.

  • Subjects / Keywords
  • Graduation date
    Spring 2023
  • Type of Item
  • Degree
    Doctor of Philosophy
  • DOI
  • License
    This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.