In this study, male Wistar rats fed with 20% fructose drinking water for 12 weeks developed MS animal model escorted with increment of body weight and body weight gain as eminent features. Several studies have been reported an association between chronic fructose consumption and the development of obesity and MS [12, 18]. This could be ascribed to high calorie intake and boosted lipogenesis observed in chronic fructose consumption conditions [12, 19].
In our observations, co-administration of NAC resulted in a remarkable reduction of final body weight and body weight gain. In an in vitro study, NAC treatment to cultured adipocytes repressed lipid accumulation and ROS production [20]. Also, in experimental models of diet induced obesity, NAC supplementation exhibited weight reduction effect [21, 22]. Ma et al. [21] observed that NAC administration prevent lipid accumulation in brown adipose tissue which has a prominent role in thermogenesis and in mobilizing lipids utilization. Also, in their study, they revealed augmentation of thermogenic genes expression in NAC treated animals suggesting that NAC treatment may enhance energy expenditure. Furthermore, Shen et al. [22] attributed the reduction of body weight with NAC treatment to a vicious circle of suppressed OS, and increased motor activity, which aids to reduce body fat and weight.
This model also, showed IR as evidenced by hyperglycemia, hyperinsulinemia and HOMA-IR index with concomitant reduction in QUICKI index. In addition, OGTT and ITT emphasized our previous findings. The IR could be attributed to reduced adiponectin expression observed in MS [23]. Lihn et al. [24] reported that diminished expression of adiponectin has been associated with IR in animal studies indicating a role for hypoadiponectinaemia in relation to IR. Also, it has been demonstrated that hidden inflammation and adipocyte hypertrophy, lessened regenerative potential of fat progenitor cells, and impaired renewal of fat depots could be mechanisms of IR [25].
In our study, we observed an amendment of glycemic control and IR with NAC supplementation. Accumulating experimental evidence indicated that NAC promoted adiponectin gene expression, resulting in reduced hyperglycemia and hyperinsulinemia, and amelioration of IR [21, 26]. The principal mechanisms by which adiponectin improve insulin sensitivity seems to be through augmented fatty acid oxidation and suppression of hepatic glucose production [24]. Also, NAC could ameliorate IR through down-regulation of intracellular ROS and direct free radical scavenger actions [27].
In the present investigation, chronic fructose consumption encouraged OS as evidenced by increased MDA levels and decreased TAC levels. It has been long established that MS and IR have been linked to OS. Both clinical and experimental evidences displayed the relation of MS and obesity with boosting of OS [4, 28]. Also, recently Bilginoglu [29] reported an increase serum, cardiac and aortic tissues OS markers with a decrement of antioxidants levels in rats with MS. Heightening of OS in MS could be due to improper rise of free fatty acids which trigger ROS production [30]. Also, fat buildup lead to lowering of anti-oxidative enzymes which elicit ROS formation [31]. In addition, repression of P53, which is a protector genome, reported in MS. This reduces the expression of anti-oxidant enzymes; glutathione peroxidase and superoxide dismutase consequently triggers cellular ROS production, leading to tissue oxidative injury [32]. Moreover, it was found that most of diseases associated with MS as IR, hypertension, obesity, dyslipidemia might impede mitochondrial homeostasis and cause mitochondrial OS [33].
The present study confirmed the effectiveness of NAC in opposing free radical mediated oxidative insult produced by chronic fructose consumption in MS induced rats. This was evident from the reduction of MDA and the increment of TAC levels in animals co-administered NAC and fructose. NAC has been reported to scavenge free radicals, replenish reduced glutathione and inhibit its depletion, and block lipid peroxidation [34]. It can also reinstate the pro-oxidant/antioxidant imbalance via its metal-chelation activity [35]. Furthermore, NAC may be a salutary candidate to handle the mitochondrial alteration and OS. It has been reported that NAC supplementation resulted in up-regulation of mitochondrial silent information regulator 3 protein which lessens mitochondrial injury and maintain its homeostasis [36].
Hypertension is a main character of MS affecting about up to 85% of MS patients [37]. In the current investigation, both SBP and DBP are increased with chronic fructose consumption. As described in several studies, chronic fructose administration induced an early rise in blood pressure [38, 39]. This could be attributed to repression of NO bioavailability observed in IR which in turn, resulted in endothelial dysfunction and impairment of NO dependent vasodilatation [40]. Also, boosting of renal expression of renin with consequent activation of angiotensin II and aldosterone formation reported in fructose fed animals [41]. In addition, the incorporated feedbacks from afferent nerves, ROS and increased metabolic hormones such as leptin work centrally to encourage sympathetic outputs and further increase blood pressure. Moreover, amendment in sodium transporter expression and activity throughout the kidney promote plasma volume expansion which is responsible, at least in part, for the noticed hypertension [42].
Also, in our results, concurrent treatment with NAC was reported to lower elevated arterial blood pressure. This effect of NAC may be mediated by its ability to restore NO bioactivity which in turn, aids in normalization of the arterial blood pressure [43]. In addition, clinical evidence has suggested the ability of NAC to suppress the sympathetic stimulation [44]. Hence, help in blood pressure restoration in MS animals. Furthermore, NAC has been proven to restrain renin expression in the renal tissue [45]. Finally, according to Krause et al. [46], NAC by its antioxidant activity and glutathione supply could improve hypertension in cases of increased OS associated with elevated blood pressure.
In our observations, dyslipidemia with increased atherogenic index was reported in fructose induced MS animals. The results herein were in resemblance with those obtained previously by Ghibu et al. [47] who encountered dyslipidemia in rats fed high fructose diet. It has been demonstrated that high fructose intake has a lipogenic effect via de novo lipogenesis in the liver and accumulation of lipids in liver and elevated their blood values. On the long run, this excess fat intracellularly and systematically induces OS and inflammation which in turn progress forward to IR and high basal glycemia [48]. Also, the IR developed by hypertriglycerdima leads to ongoing lipolysis with more and more fatty acids and glycerol. Then, they both enter the adipose tissue to form triglycerides surpassing to a vicious circle of more triglycerides to be formed [12]. In addition, recently in a novel polygenic rat model of MS, obesity, and diabetes, Han et al. [49] reported that gene expressions involved in lipid metabolism were dysregulated with the key proteins participating in pathogenesis of dyslipidemia and IR were up-regulated.
The co-administration of NAC revealed partial recovery of lipid profile as evidenced by normalization of total cholesterol, triglycerides, HDL-cholesterol and atherogenic index and decrement of LDL-cholesterol confirming the improvement of dyslipidemia, which is supported by previous studies [21, 50, 51]. The lipid-lowering action of NAC in our MS animal model of hyperlipidemia can be partially ascribed to the suppression of mRNA expression of lipogenic related enzymes [52]. Also, in an experimental mouse model of high-sucrose diet feeding, NAC hampered the metabolic shifting in cardiac tissue, promoting fatty acid oxidation [53]. In addition, the upkeep of the normal structure of lipoprotein receptors is pivotal for their function, enabling the cellular uptake of serum lipids from the blood. On the other hand, ROS oxidize lipoproteins and prohibit lipid intracellular uptake [50]. It is possible that the decreased serum cholesterol levels in rats fed NAC-supplementation are due to the anti-oxidative effects of NAC.
CT-1 is elevated in the myocardium and plasma of heart failure patients [54], and it has been proven to be related to hypertension, cardiac hypertrophy, and fibrosis both in patients [54–56] and experimental models [57]. Interestingly, CT-1 is up-regulated in cardiac fibroblasts and cardiomyocytes in response to metabolic, humoral, mechanical and hypoxic stress [58]. The existing data showed up-regulation of CT-1 expression in both heart and aorta in MS rats. In agreement, we demonstrated an increased cardiac interstitial fibrosis in those animals. In the thoracic aorta, increased thickness of TM with deposition of connective tissue was reported.
In line with these reports, López-Andrés et al. [57] verified that CT-1 treatment increased left ventricular volumes and induced myocardial dilatation and myocardial fibrosis meanwhile, in aorta, arterial stiffness, vascular media thickness, collagen and fibronectin content were increased by CT-1 treatment. Also, it has been proposed that CT-1 accelerates the development of atherosclerotic lesions by stimulating the inflammasome, foam cell formation and collagen-1 production in vascular smooth muscle cells [59]. In addition, in an in-vitro study, it was found that CT-1 induces the proteolytic potential in human aortic endothelial cells by up-regulating matrix metalloproteinase-1 expression thus, may play an important role in the pathophysiology of atherosclerosis and plaque instability [60]. On the other hand, decreased arterial stiffness, media thickness and vascular wall fibrosis were demonstrated in CT-1-null mice [61].
The increase in relative heart weight in our MS animals was reported previously by Wu et al. [62] who showed an increase in the heart weight/ body weight ratio and myocardial hypertrophy in high fructose intake mice model. Furthermore, Yan et al. [63] demonstrated that intima to media thickness ratio was increased in the thoracic aorta of MS model.
The aortic stiffness reported in MS may be ascribed to thoracic aorta perivascular adipose tissue dysfunction observed in MS that through interplay between TNFα and NADPH-oxidase 2 causing aortic stiffness [64]. Also, Martinez-Martinez [65] observed that CT-1 could up-regulate cardiac galectin-3 which, in turn, mediates the proinflammatory and profibrotic myocardial effects of CT-1.
In our experiment, NAC co-treatment resulted in down-regulation of CT-1 in the heart and aorta. Also, relative heart weight was decreased by NAC supplementation. Similar results observed by Jia et al. [66] who reported that cardiac fibroblast proliferation, collagen Ι and CT-1 overexpression induced by isoprenaline stimulation were effectively abrogated by NAC treatment.
The beneficial effects of NAC against cardiac hypertrophy and aortic stiffness were reported previously [67–69]. It has been deduced that reducing OS by NAC in pressure overload may prevent electrical remodeling and ameliorate hypertrophy in epicardial myocytes [67]. Also, down-regulation of CT-1 either in vivo or in vitro was shown to be a mechanism of cardioprotection in hypertrophied heart [68]. In addition, Wu et al. [69] demonstrated that NAC treatment could prevent pyroptosis which is a cellular mechanism for the pro-atherosclerotic plaque formation in human aortic endothelial cells. Furthermore, mouse and human hypertrophic cardiomyopathy were antagonized by NAC treatment through stimulation of miR-29a expression and suppression of pro-fibrotic gene TGFβ expression and secretion [70]. Also, it has been demonstrated that NAC could inhibit the decrease in collagen I/III ratio which play a role in cardiac extracellular matrix composition and cardiac hypertrophy [71].
Our histopathological findings confirmed the biochemical results. In the present investigation, histopathological examination of the heart of the MS group revealed marked degeneration of the cardiomyocytes, interstitial fibrosis and infiltration of inflammatory cells which is in agreement with the findings observed by Putakala et al. [72] who found fibrosis, degenerative changes, neutrophil infiltration and fat deposition in chronically fructose fed animals. Also, variable degrees of collagen fibers proliferation could be detected in routinely H&E stained heart specimens of high fructose diet-induced MS rats [73]. Furthermore, inflammatory cell infiltration and mast cell activation close to the blood vessels, and degeneration of myofibrils in cardiomyoctes with intercalated discs were reported by Acikel Elmas et al. [74]. This could be attributed to mast cell infiltration of the cardiac tissue which release chemokines, pro-inflammatory cytokines, histamine and proteases in MS conditions [75]. Collagen deposition in the heart of MS models was proved to be due to fructose intake which in turn caused release of ROS with subsequent promoting inflammasome followed by fibrosis [76]. NAC co-administration in our study resulted in remarkable improvement of the histopathological appearance of the cardiac tissues. This could be ascribed to down-regulation of OS which may directly or indirectly improve cardiac pathology as evidenced by our results.
The present data showed a robust positive correlation between both cardiac and aortic CT-1 expression and basal glycemia, SBP, DBP, total cholesterol, triglycerides and LDL-cholesterol and a negative correlation with HDL- cholesterol in MS and MS + NAC groups.
A reduction in serum CT-1 levels after weight-loss timetable and a strong association of decreased CT-1 with lowering of cholesterol levels were demonstrated previously [77]. In this study, CT-1 was suggested to be an indicator for the diagnosis of MS in overweight/obese children population. In a study done by Gkaliagkousi et al. [78], CT-1 levels was positively correlated with blood pressure and the indices of arterial stiffness. In another study, positive correlation between plasma CT-1 and basal glycemia, SBP and DBP were reported [79]. Also, they observed a positive association between CT-1 and arterial damage (increase intima-media thickness). In addition, in obese children, CT-1 transcript levels were reduced after lifestyle interference [80]. Furthermore, Anik Ilhan et al. [81] demonstrated that CT-1 levels were found to be positively correlated with DBP and triglyceride levels in MS women with polycystic ovaries.
Increased expression of CT-1 in both heart and aorta are strongly related to the intensity of several parameters associated with cardiometabolic risk factors as observed in our correlation study. These observations suggested the potential involvement of CT-1 in cardiovascular injury and diseases.