Abstract
Dietary obesity after middle age is associated with insulin resistance and oxidative stress and is a major risk factor for the development of cardiovascular diseases. However it remains unclear about the effects of high-fat diet (HFD) on vascular oxidative status and endothelial function, and both of them are interactive and pivotal to the development of obesity-associated cardiovascular disease, such as atherosclerosis. Nox2 is a major source of vascular reactive oxygen species (ROS); however its role in diet-induced metabolic disorders and vascular dysfunction is unclear. Therefore, the objective of this PhD project is to investigate the role of Nox2 and oxidative stress in HFD-induced metabolic disorders and endothelial dysfunction in mice. Male C57BL/6J wild-type (WT) mice and Nox2 knockout (KO) mice at 7 month of age were fed with a HFD or a normal chow diet (NCD) for 16 weeks. Body weight, blood pressure (BP) and fasting blood samples were taken before and after diet intervention. Mice were sacrificed for assessment of aortic function ex vivo. Compared to NCD-fed mice, WT mice fed with HFD showed significant increases in body weight, fasting glucose and insulin levels with elevated lipids profiles and BP. These changes were accompanied by significant increases in nicotinamide adenine dinucleotide phosphate (NADPH)-dependent ROS production by aortas and a decrease in endothelium-dependent vessel relaxation and this was reversed back to control levels by adding superoxide scavenger tiron. Compared to WT mice, Nox2 KO mice fed with HFD has no significant increase in fasting glucose with less augmented lipids profiles and unaltered BP. The aortic NADPH-dependent ROS production remained unchanged and the endothelium-dependent vessel relaxation was well preserved. In a parallel study, HFD-fed WT mice were supplied with a Nox2 inhibitor apocynin in drinking water. Apocynin treatment significantly attenuated the HFD-induced overweight, the high lipids levels and the high BP, but had no significant effect on blood glucose levels. Apocynin treatment also preserved the endothelium-dependent vessel relaxation to acetylcholine in HFD group. To further investigate the relationship between HFD and the pathogenesis of atherosclerosis, we fed HFD to apolipoprotein E (apoE) KO mice at age of 6 weeks for 10 weeks. The HFD promoted obesity, hyperglycaemia, hyperinsulinemia, type 2 diabetes-like insulin resistance and accelerated the accumulation of low-density lipoprotein in apoE KO mice. HFD also attenuated the endothelium-dependent relaxation responses of aortas accompanied with higher BP in apoE KO mice. In conclusion, HFD increases the levels of ROS production in the vasculature. Activated Nox2 and induced oxidative stress by diet-induced obesity contribute to metabolic disorders including impaired glucose and lipids metabolism, and endothelial dysfunction. Deletion or inhibition of Nox2 attenuates these detrimental pathological progresses. Therefore, Nox2 and Nox2-derived ROS play a key role in mediating dietary obesity-related metabolic disorders and endothelial dysfunction after middle-age.