The primary goal of this study was to evaluate the effect of RSG on parameters of cardiac structure and function, in the setting of diets high in corn oil (n-6 fatty acids) or fish oil (n-3 fatty acids) in an aging mouse model of hyperglycemia and insulin resistance. The novel findings of the present investigation were that corn oil supplementation with RSG resulted in (i) reduced fractional shortening and increased left ventricular and vascular hypertrophy, (ii) increased pro-inflammatory TNF-α and IL-6 with decreased anti-inflammatory IL-10 in the LV particularly by corn oil and (iii) up-regulation of ANP, BNP and fibronectin expressions. Interestingly, simultaneous supplementation of fish oil with RSG prevented these effects in the obese diabetic mice. Together, our data suggest that RSG provided in a corn oil diet exerts negative effects on LV remodeling, while fish oil supplementation prevents corn oil + RSG-mediated inflammation and cardiac dysfunction.
LV remodeling is defined as the structural and functional changes that occur in the LV in response to stress or injury, characterized by an increase in LV mass [36
]. Among the molecular pathways involved in LV hypertrophy, myocardial stretch, oxidative stress, and cytokine activation pathways have been the best characterized [37
]. Corn oil is primarily composed of linoleic acids (>55%), and its consumption accounts for 70% of the dietary polyunsaturated fatty acids consumed. Moreover, corn oil is a significant contributor to the obesity epidemic [38
]. We observed increased LV mass in the corn oil + RSG obese aging mice, consistent with a negative impact of corn oil on RSG action. Of note, the dose used in the present investigation was higher than human doses in order to achieve a translational dose in mice [23
]. The increase in LV mass was attenuated when fish oil was added, indicating that fish oil counters the negative effects of the corn oil. As observed in clinical studies that RSG administration with fat intake did not alter the systemic bioavailability [39
], similarly we noticed glucose lowering pharmacological effect in corn oil and fish oil fed mice in the present animal study.
TNF-α and IL-6 increase during heart failure [40
], as well as in obese and type 2 diabetic patients [41
]. At the same time, IL-10, a pleiotropic anti-inflammatory cytokine that exerts vasculoprotective effects in numerous animal models, decreases [42
]. Oxidative stress, which tips the balance between oxygen free radicals and antioxidant defense mechanisms, also increases in heart failure [32
]. Antioxidant treatment with catalase, prevents hypertrophy induced by TNF-α [43
]. We demonstrated that fish oil + RSG in aging diabetic mice reduced LV hypertrophy, inflammatory levels, and oxidative stress induced by corn oil + RSG.
Adiponectin plasma levels are negatively correlated to LV remodeling [30
]. Elevated plasma adiponectin with fish oil supplementation is due to the activation of PPAR-α in adipose tissue [44
]. Adiponectin-deficient mice have enhanced LV hypertrophy that can be rescued by adenovirus-mediated delivery of adiponectin [45
], and Tao et al. have recently confirmed that adiponectin is indispensable for RSG-mediated cardioprotection in a mouse MI model [14
]. The balance between beneficial and detrimental effects of RSG, therefore, is context dependent. Low adiponectin levels associate with high levels of the inflammatory mediators C-reactive protein and IL-6 [46
]. In this study, we show increased adiponectin levels in fish oil + RSG treated aging mice that explain the attenuation of LV hypertrophy.
Chronic corn oil (n-6 fatty acids) supplementation results in age-associated obesity, inflammation, and hyperglycemia [25
]. Although RSG treatment controlled hyperglycemia and increased lean mass in the corn oil fed mice, corn oil + RSG also up-regulated mRNA expression of hypertrophy genes (ANP, BNP and fibronectin). Similar results have been reported in Sprague–Dawley rats, as RSG treatment induced eccentric LV hypertrophy despite positive effects on serum glucose levels [13
]. Genomic analysis of db/db
mouse hearts demonstrated transcriptional changes with diabetes that were exacerbated by RSG treatment [47
]. Also the evidence that, RSG-induced LV hypertrophy is related to volume-overload due to higher sodium and water retention by the kidney [48
]. In contrast, studies in a mouse reperfusion model showed a protective effect of RSG, but this benefit can be explained by the acute treatment timing [49
]. Therefore, length of treatment and edema evaluation are important considerations, providing compelling rationale to study further in detail.
Fish oil increases plasma adiponectin levels, suppresses inflammation, and prevents cardiac remodeling and dysfunction in a pressure overload rat model [21
]. Fish oil also lowers triglycerides and plasma free fatty acids that reduce CVD risk, especially in overweight and T2D patients [51
]. Kuda et al. recently showed that fish oil + RSG exerted additive effect in the prevention of obesity, adipocyte hypertrophy, low-grade adipose tissue inflammation, dyslipidemia and insulin resistance [52
]. RSG also reduced triglycerides in our aging diabetic fish oil fed mice, consistent with previous reports from other groups [53
]. In the clinic, elderly patients with T2D receive the same pharmacological therapy as the younger adult cohort [54
]. More than 20% of the population over 65 years old has diabetes [55
]. Our studies provide persuasive motivation to study effects of drugs in different age groups of animals to predict usage related safety and toxicity aspects. Therefore, future studies that investigate effects of chronic RSG treatment in aging animals and humans are warranted. In addition, studies that investigate the ability of fish oil supplementation to prevent or limit adverse effects of RSG superimposed on a corn oil diet are needed. Further studies are also warranted to delineate the diverse effects of individual fish oil components, including EPA and DHA, in the RSG-mediated LV hypertrophy that occurs in aging insulin resistant mice.