EXPLORING THE ROLE OF ELEVATED LIPID PEROXIDATION AND GLUTATHIONE PEROXIDASE 4 IN METABOLIC SYNDROME AND CARDIAC REMODELING IN OBESITY
Katunga, Lalage Adalaide
Cardiovascular disease continues to be a leading cause of global mortality. Metabolic perturbations including obesity, hyperlipidemia and type II diabetes arising from a western style diet and a more sedentary lifestyle are significant contributors to this phenomenon. Diabetic patients are at a significantly higher risk of developing heart disease. Approximately 50% of diabetic patients suffer from cardiomyopathy and they are twice as likely to develop congestive heart failure. Importantly, this cardiomyopathy is often independent of hypertension and coronary artery disease, suggesting that factors inherent to the metabolic disease are pathogenic to the heart. Mitochondrial dysfunction is a mechanism that is has been postulated to have an etiological role in the development of diabetic cardiomyopathy and heart failure. Studies on diabetic hearts reveal these mitochondria have increased reactive oxygen species (ROS) emission, reduced ATP production and oxidative damage. At a gross scale, an early histological finding in the hearts of a sub-set of both diabetic and obese patients is increased collagen deposition and an infiltration of fibroblasts which over time leads to reduced cardiac compliance/ relaxation and ultimately diastolic dysfunction. Lipid peroxides and reactive aldehyde derivatives (LPPs) are derived from ROS peroxidation of cellular polyunsaturated fatty acids and have been linked to cardiometabolic disease. The objective of this study was to test the hypothesis that LPPs underlie cardiometabolic derangements in obesity. This hypothesis was tested using glutathione peroxidase 4 haploinsufficent (GPx4+/- ) mice. GPx4 is one of the few enzymes specialized to neutralize lipid peroxides. GPx4 +/- mice were fed a high fat, high sucrose (HFHS) diet to investigate cardiometabolic derangements in a model of increased LPP production. Our research revealed that greater carbonyl stress, exacerbated glucose intolerance, dyslipidemia, and liver steatosis occurred in GPx4 +/- mice compared to wild-type (WT) on a high fat high sucrose diet (HFHS). Although normotensive, cardiac hypertrophy was evident with obesity, and cardiac fibrosis was more pronounced in obese GPx4 +/- mice. Mitochondrial dysfunction manifesting as decreased fat oxidation capacity and increased reactive oxygen species was also present in obese GPx4 +/- but not WT hearts, along with up-regulation of pro-inflammatory and pro-fibrotic genes. Biochemical analysis was also performed on samples of human atrial myocardium revealed patients with diabetes and hyperglycemia exhibited significantly less GPx4 enzyme and greater HNE-adducts in their hearts, compared with age-matched non-diabetic patients. The pathogenic role of LPPs was further investigated using carnosinol (CAR), an LPP scavenger that preferentially binds and sequesters reactive aldehydes. In this follow-on study, CAR was administered via drinking water (40mg/kg) starting mid-way through the HFHS diet. In WTs, CAR improved whole body glucose tolerance, blood lipid profiles and insulin sensitivity. CAR treatment also mitigated HFHS- diet induced cardiac hypertrophy and hepatic steatosis. Levels of 4-HNE adducts were reduced in the cardiac mitochondria, liver and whole heart following treatment. Mitochondrial oxidative phosphorylation efficiency (OxPhos) in the heart was also improved with CAR in WT mice fed HFHS diet, although no effect on mitochondria in the GPx4+/- mice was observed. Collectively, the data presented in this dissertation provide evidence that lipid peroxides and their reactive aldehyde derivatives are a distinct form of oxidative stress that has an etiological role in human cardio-metabolic disease. It establishes that GPx4 is an adaptive response to oxidative stress in obesity, and that deficiency in this important enzyme accelerates the pathogenic effect of the condition. Furthermore, it provides support for the development of a novel class of aldehyde scavenging compounds to ameliorate the cardio-metabolic disease and associated comorbidities common with obesity.