Mark Anstadt (Committee Member), Khalid Elased (Committee Co-chair), Paul Koles (Committee Member), Mariana Morris (Committee Co-chair), Christopher Wyatt (Committee Member)
Doctor of Philosophy (PhD)
Insulin resistance and kidney disease are intimately linked and both independently tied to dysregulation of the renin angiotensin system (RAS). The purpose of this research was to better investigate Angiotensin Converting Enzyme (ACE) 2 as a possible motif in the pathophysiology of these two disease processes. The project was divided into two parts. Part 1 investigated ACE2 KO mice as a model of albuminuria. Part 2 investigated how the lack of ACE2 enzyme affected glucose homeostasis and subsequent albuminuria.
Part 1: The Role of ACE2 in the Progression of Albuminuria. To determine if global loss of ACE2 leads to spontaneous development of albuminuria in mice, we followed male wild type (WT) and knockout (KO) mice until 20 weeks of age. Basic metabolic parameters and blood pressure were measured in addition to urinary parameters, and histological evaluation. We found ACE2 KO mice only differed from their ACE2 WT mice littermates in terms of increased water consumption which correlated with increased urine output. Systolic blood pressure was also found to be mildly increased during the dark period. Urinary albumin was similar between WT and KO mice as was total protein and sodium excretion. Despite the lack of ACE2 in KO mice, urinary angiotensin (Ang) II and Ang-(1-7) were found to be equal to that of ACE2 WT mice. Reduction-oxidation (redox) studies suggested elevated oxidative stress in ACE2 KO kidneys though histopathology did not reveal any major changes at the tissue level. To determine the role of ACE2 in a model of kidney disease, mice were chronically infused with Ang II. Both WT and KO mice groups had a similar pressor response, but ACE2 deficient mice showed higher levels of kidney damage with respect to albuminuria and proteinuria. In concordance, integrity of the glomerular filtration barrier was also diminished. Urinary Ang II increased significantly in KO mice while Ang-(1-7) levels were raised equally. Renal pathology revealed greater mesangial expansion and renal fibrosis in following Ang II infusion as well as elevated redox status vs. the Ang II infused WT mice. To determine the role of the major ACE2 product Ang-(1-7), mice deficient in ACE2 were infused with Mas receptor inhibitor, A779. The Mas receptor blockade did not result in significant phenotypic or urinary differences compared to control groups. Urinary Ang levels were also unchanged but ACE2 activity levels increased. This was linked to increased NADPH oxidase (Nox) activity underscoring the role of the Mas receptor in Nox regulation. Overall, a paucity of histological change was also observed despite increased Nox activity. Together, these data suggest a lack of ACE2 does not precipitate albuminuria under normal conditions. The similar urinary Ang levels point towards alternative pathways to Ang II degradation and Ang-(1-7) synthesis which may influence Nox activity. Finally, ACE2 may work as a compensatory enzyme to protect the kidney during times of Ang II excess.
Part 2: The Role of ACE2 in Glucose Homeostasis. To determine the role of ACE2 in obesity-related insulin resistance, ACE2 null mice were fed a high fat (HF) diet for 12 weeks. Body weight and fat were continuously monitored and ACE2 KO mice gained weight at the same rate as WT mice littermates. Terminal fasting glucose and glucose tolerance were found to be impaired in ACE2 KO mice despite similar weights and body fat. Plasma metabolic hormones and adipokines were compared between groups. ACE2 KO mice showed decreased plasma insulin concentration as well as higher cholesterol levels. Pancreas ACE2 activity was elevated in control mice on a HF diet, suggesting a protective role of ACE2 in the endocrine pancreas. Next we investigated obesity-associated insulin resistance of ACE2 null mice as a model for diabetic albuminuria. The level of insulin resistance and circulating blood glucose was not sufficient ...
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