NAD Redox Imbalance Drives Diabetic Cardiomyopathy: Roles of oxidative stress and post‐translational modifications

Diabetes is a major risk factor of heart failure. Diabetes and heart failure both have been linked to NAD redox imbalance. However, the causal role of NAD redox state in the progression of diabetic cardiomyopathy (DC) has not been directly tested. We induced diabetes in wild type mice (WT) by streptozotocin (STZ) injections. Chronic diabetic stress to WT mice (16 weeks) led to gradual declines in cardiac function and lowered NAD/NADH ratio, suggesting the association of NAD redox imbalance in linking diabetic stress with cardiac dysfunction. To determine the causal role of NAD redox imbalance to drive DC, we used cardiac‐specific Ndufs4‐KO mice (cKO), which exhibit lowered cardiac NAD/NADH ratio without overt dysfunction. While insulin depletion and hyperglycemia were similar in diabetic control and cKO mice, cardiac dysfunctions were exacerbated in diabetic cKO mice. Cardiac fibrosis levels of diabetic control and cKO hearts were measured by trichrome staining. Although cardiac fibrosis was slightly elevated in diabetic hearts compared to non‐diabetic controls, but there was no difference between diabetic control and diabetic cKO hearts. Similar in vivo and histological observations were made in both diabetic male and female cohorts. The results suggest that the accelerated decline of cardiac function associated with the pre‐existed NAD redox imbalance is not be due to enhanced fibrosis and is independent of sex. We next analyzed acetylation‐dependent pathways to account for the accelerated decline of function in diabetic cKO hearts. NAD redox imbalance in diabetic cKO hearts promoted protein acetylation, including SOD2 acetylation (SOD2‐K68Ac). SOD2 acetylation inhibits its antioxidant function. We observed elevated protein oxidation levels in diabetic cKO hearts, while expression of genes regulating oxidative stress (Nox1, 2, 4) remained unchanged. The results suggest that NAD redox imbalance promotes oxidative stress by suppressing antioxidant pathway via protein acetylation. To gain further insights how NAD redox imbalance may regulate cardiomyocyte function, phosphorylation levels of myosin binding protein C (MyBPC) and troponin I (TnI) were examined. In diabetic cKO hearts, TnI phosphorylation was increased while MyBPC phosphorylation remained unchanged, suggesting that hyperacetylation in cKO hearts regulates cardiomyocyte contraction‐relaxation to promote cardiac dysfunction. To normalize NAD redox imbalance, we elevated NAD levels in diabetic cKO mice with cardiac‐specific NAMPT overexpression. NAMPT is the rate‐limiting enzyme in salvage pathway. Cardiac NAMPT expression slowed the accelerated decline of cardiac function in diabetic cKO hearts. Our findings collectively suggested that NAD redox imbalance drives DC and the NAD‐dependent pathways regulating DC progression warrant further investigations. Support or Funding Information 1. Scientist Development Grant (17SDG33330003), American Heart Association; 2. Seed Funding, Presbyterian Health Foundation of Oklahoma City