Selected antihyperlipidemic drugs and estrogen prevented hyperlipidemia, aortic tissue inflammatory and oxidative stress, in ovariectomized rats fed with thermoxidized oil diets. Deep fat frying with palm and Soybean oil initiated chemical reactions that resulted in the generation of oxidized products while also reducing the antioxidant contents of the oil [11]. These reactive products can activate inflammatory processes that culminate in diminished vascular integrity and cardiovascular health. These effects, coupled with estrogen withdrawal in postmenopausal women significantly increase the risk for CVD [30, 31].
Chronic consumption of thermoxidized palm/soya oil in postmenopausal rats has been reported to cause atherogenic dyslipidaemia through the elevation of total cholesterol, triglycerides and LDL-c, and decreasing the HDL-c [32, 33, 34]. In this study, palm and soya oils supplemented diet fed to ovariectomized rats significantly increased atherogenic dyslipidemia. Treatment with antihyperlipidemic drugs and estradiol reversed indices of atherogenic dyslipidemia in ovariectomized rats fed with palm and soya oil supplemented diets. The association between hyperlipidemia and atherosclerosis including the ensuing incidence of cardio-cerebrovascular diseases is well established in literature [32, 35, 36]. ETD, a major component of the therapeutic regimen is widely used in the treatment of the postmenopausal hormonal deficiency syndrome. The effect of ETD on serum and lipoprotein lipids are characterized by an increase in the lipid constituents of high-density lipoproteins (HDL) and, usually, a decrease in low density lipoproteins (LDL). LDL-c reduction by these treatments show that they offer significant protection against atherosclerosis [37, 38]. According to the European Society of Cardiology (ESC) and European atherosclerosis society (EAS) guidelines for the management of dyslipidemia, 1 mmol/L reduction in LDL cholesterol level corresponds to a one-fifth reduction in the risk of atherosclerotic cardiovascular disease (ASCVD) [39]. When evaluated for other lipid parameters, ATV, EZE + ATV and ETD treatments decreased the TG levels, while ATV increased the HDL-c levels. The mechanisms underlying the lipid-controlling effect of these antihyperlipidemic drugs have been widely described in the literature [39]. ATV inhibits HMG-CoA reductase while EZE inhibits dietary and biliary cholesterol uptake in the intestine [39]. In OVX + TPO rats, ATV decreased the TC levels significantly while ATV and EZE + ATV significantly decreased TG and VLDL-c levels. Estradiol reduces LDL cholesterol levels and increases HDL cholesterol levels in postmenopausal women with normal or elevated baseline lipid levels [40, 41]. The reduction in LDL cholesterol levels is probably a result of accelerated conversion of hepatic cholesterol to bile acids and increased expression of LDL receptors on cell surfaces, resulting in augmented clearance of LDL from the plasma [40, 41].
Search of the literature show evidence of increased cardiovascular risk and incidence of adverse events associated with decrease in the HDL/LDL cholesterol ratio [42, 43]. TSO and TPO decreased the HDL/LDL ratio albeit insignificantly while also elevating the AI and CRI significantly (p < 0.05). Atherogenic index is a strong marker for assessing the risk of atherosclerosis and coronary heart disease in vessels. As the AI increases, the risk of atherogenesis increases [44, 45]. Estradiol and the antihyperlipidemic drugs employed in this study significantly decreased the AI and CRI. ATV, EZE + ATV and ETD administration significantly decreased the AI and/or CRI. This decrease in the atherogenic indices demonstrates the beneficial effect of early administration of the antihyperlipidemic drugs and estrogen in the reduction or prevention of atherogenesis. In rats fed with TSO diet, ATV treatment significantly increased the HDL/LDL cholesterol ratio by approximately three folds. An increase in the HDL/LDL cholesterol ratio is an indicator of low circulating LDL in the plasma, which is beneficial in the prevention of atherosclerotic vascular disease [39, 46]. In rats fed with TPO diet, ATV administration significantly decreased the AI and CRI, while ETD and EZE + ATV decreased AI significantly.
Aortic nitric oxide was examined as an indicator of vascular integrity in this study. In both TPO and TSO fed ovariectomized rats, aortic nitric oxide levels were reduced. This effect was reversed in antihyperlipidemic, and estrogen treated rats. Diminished NO is associated with endothelial dysfunction and predisposes the vessel to atherosclerosis [47, 48]. The elevation of NO through the up regulation of endothelial nitric oxide synthase (eNOS) plays an important role in preventing or reversing endothelial dysfunction associated with hypertension, atherosclerosis and other cardiovascular diseases [49, 50]. Estrogen and the combination of ATV and EZE reportedly increased aortic nitric oxide levels, possibly via the up-regulation of eNOS as earlier demonstrated for the individual drugs [51, 52].
TNF-α-activated signaling contributes to vascular dysfunction, development and progression of atherosclerosis [53]. The pro-atherogenic effects of TNF-α on the endothelium includes reactive oxygen species (ROS) production while increased TNF-α levels impair endothelium-dependent vasodilation in a NO-dependent manner [53]. Evaluation of TNF-α levels in the tissues of the thoracic aorta revealed a significant increase in TNF-α levels of TSO and TPO fed rats. This indicates that chronic consumption of thermoxidised oils results in diminished endothelial and, by extension, vascular integrity. Prolonged increase in aortic TNF-α concentration initiates expression of pro-inflammatory, pro-atherogenic proteins that contribute to increased recruitment and transmigration of circulating leukocytes into the vascular wall. These processes are deleterious to vascular health and contribute to atherogenesis [53]. In both TSO and TPO fed rats, the antihyperlipidemic drugs and estradiol decreased TNF-α levels significantly. Results from this study suggest that inhibition of TNF-α signaling is a possible mechanism for the prevention of endothelial dysfunction and atherosclerosis progression by these drugs.
The role of oxidative stress in aortic damage progression has been widely described in the literature [54, 55]. It has been reported to involve the generation of ROS and reactive nitrogen species (RNS) through different pathways such as mitochondrial xanthine oxidase, NADPH oxidase (Nox) or endothelial nitric oxide synthase (eNOS). The thermal oxidation of edible oils decreases the antioxidant content and increases lipid peroxidation and generation of ROS [31]. Oxidative stress occurs when the cellular antioxidant system is overwhelmed by pro-oxidant molecules. Vascular oxidative stress is one of the leading causes of cardiovascular diseases, including atherosclerosis. Elevated oxidative stress results in vasoconstriction, vascular remodeling, inflammation, and fibrosis [56, 57]. MDA is a product of lipid peroxidation and a marker of oxidative stress while GSH, catalase and SOD are important components of the antioxidant system. Lipid peroxidation is associated with the endothelial dysfunction involved in atherosclerosis and plays an important role in the mechanism of immune response to vascular injury [58]. Results from this study suggest that the chronic consumption of TSO and TPO diet elevates oxidative stress through the generation of ROS and the deficiency of the antioxidant system, indicated by the significant increase in aortic MDA levels, as well as the significant decrease in the GSH, SOD and catalase in the untreated OVX + TSO and OVX + TPO rats. ETD appears to reduce the levels of lipid peroxidation and generation of ROS in both TSO and TPO fed rats, as shown by a significant decrease in MDA levels. On the other hand, Catalase and SOD levels were significantly increased by these treatments as well as the ATV and EZE + ATV treatments. Similarly, aortic GSH level was significantly increased by EZE + ATV treatment. Estrogen modulate genes involved in regulation of vascular tone and response to vascular injury. Estrogen binding to the vascular receptor stimulate expression of genes for enzymes that participate in vasodilation such as Nitric oxide synthase and prostacyclin synthase [59]. This result indicates a replenishment of the antioxidant system by the respective treatments. Replenishing the antioxidant system helps to combat oxidative stress, thus reducing or preventing tissue damage. Increase in the expression of SOD has been reported to offer vascular protection including the inhibition of vascular hypertrophy [60].
In rats fed with TPO diet, ATV, EZE + ATV and ETD treatments offered protection against oxidative stress, as shown by the significant decrease in aortic tissue MDA levels while ATV, EZE + ATV and ETD treatments replenished the antioxidant system as shown by the significant increases in one or more of aortic tissue concentrations of GSH, CAT, and SOD.
Histological evaluation of the ascending aorta of the untreated OVX + TSO rats showed evidence of atherosclerotic changes. Accumulation of numerous bloods cells was observed in the luminal aspect of the aortic wall, indicating inflammation in the aorta. The process of vascular inflammation includes stasis of blood flow and vascular congestion in the lumen of the aorta, leukocytes margination and adhesion along the endothelium, diapedesis and phagocytosis [61].
Further examination revealed hypertrophy of the tunica media and increased peri-adventitial fat in the aorta of the untreated OVX + TSO. Hypertrophy is a form of cellular adaptation that involves increase in cell size in response to external stimulus such as prolonged inflammation and mechanical stress. Aortic medial hypertrophy impacts on the elasticity of the vessel and can result in blood pressure elevation and CVD [6]. Although the peri-adventitial adipose tissue has been found to offer protection to the vessels, persistent increase in overweight/obese subjects result in adipocytes dysfunction. Dysfunctional adipocytes are implicated in the immune response and vascular smooth muscle cells proliferation involved in the progression of atherosclerosis [62]. Examination of the thoracic aorta of OVX + TSO rats revealed recanalization of the aorta along with arterial medial hypertrophy and increased peri-adventitia fat. This pattern of atherosclerotic changes was also observed in the ascending and thoracic aorta of rats fed with TPO, as shown by by the accumulation of red blood cells in the intimal part of the ascending aorta coupled, with increased peri-adventitia fat, and recanalization of the descending aorta, with tunica media hypertrophy. Examination of the aorta of rats in all the treated groups of both TSO and TPO rats revealed restoration to normal histoarchitecture.