Effects of aging and type 2 diabetes on cardiac structure and function: Underlying mechanisms

This study characterized long-term changes in cardiac structure and function in a high-fat diet/streptozotocin mouse model of aging and type 2 diabetes mellitus (T2D) and how the intersection of both conditions alters plasma metabolomics. Furthermore, we wanted to assess the possible mechanistic roles played by oxidative stress, increases in arginase activity and pro-inflammatory cytokines levels. For this purpose, 38, 13-month-old C57BL/6 male mice were utilized. Control animals (n=13) were fed a regular chow for a period of 10 months. T2D (n=25) animals were provided a single injection of streptozotocin (75 mg/kg/IP) and fed a 60% high fat diet (HFD) for the same period of time. To track changes in cardiac structure and function serial echocardiography was performed. At the end of the study, blood samples were collected via a cardiac puncture and plasma samples frozen. Hearts were sectioned and allocated for either rapid freezing (biochemical studies) or fixed in formalin for histological analysis. Biochemical assays using blood and/or tissue samples included determinations for arginase activity, pro-inflammatory cytokines, protein carbonylation, glucose tolerance tests, Western blots and plasma metabolomics. Glucose tolerance tests confirmed the development of T2D in the HFD animal group. Serial echo yielded moderate increases in cardiac mass with aging that were greater in T2D animals. Fractional shortening (figure) significantly decreased by 16 months of age in T2D animals yielding a value of 28% at 23 months vs. 40% in aged controls. Ejection fraction also significantly decreased by 16 months of age in T2D animals yielding 55% at 23 months vs. aged animals (70%). Western blots of myocardium samples documented increases in fibronectin, type III collagen and smooth muscle actin with T2D which were confirmed by immunohistochemistry. Significant increases in plasma protein carbonyls, TGF-β1, TNF-α, IL1β and IFN-γ occurred with aging and further increased with T2D. Untargeted metabolomics and cheminformatics revealed clear differences in the plasma metabolome of the HFD mice vs. controls. Multiple lipid metabolites linked to insulin resistance were dysregulated, including C16:0 and C18:0 acyl carnitines (up-modulated) and saturated and unsaturated phosphocholine-containing lipids (up and down-modulated). In summary, we document the development of cardiac hypertrophy with aging that is aggravated by T2D. Systolic dysfunction occurred as a late age associated event with T2D. Increases in OS markers and pro-inflammatory cytokines occurred with aging and were aggravated with T2D. These changes were accompanied by increases in myocardial and blood arginase activity and TGF-β1 levels which may account for the development of fibrosis and may be associated with altered plasma metabolomics.