Abstract Body (Do not enter title and authors here): Coronary microvascular dysfunction (CMD), which is associated with diabetic cardiomyopathy, Takotsubo cardiomyopathy, andheart failure with preserved ejection fraction (HFpEF), is understudied. CMD is characterized by impaired endothelial-dependent vasodilation, but detailed mechanisms have yet to be elucidated.
Nuclear Sirtuin 6 (SIRT6) plays essential roles in gene transcriptional, stress tolerance, DNA repair, inflammation, and aging. SIRT6 is strongly associated with cardiovascular pathologies, but how SIRT6 regulates endothelial metabolisms and homeostasis under metabolic stress and the underlying mechanism remains poorly understood. It might be because global Sirt6 knockout mice are perinatally lethal caused by hypoglycemia, suggesting the essential role of SIRT6 in glucose metabolism.
In our preliminary studies, we generated inducible global Sirt6 knockout mice by crossing with Sirt6 f/f mice with CAG-cre (Sirt6 ^{f/f, CAG}), and mice were viable with normal glucose levels. However, they showed impaired endothelial-dependent dilation (EDD) and impaired coronary flow reserve (CFR), an index clinically used to diagnose CMD. It suggests that deletion of Sirt6 might cause EC dysfunction because Sirt6 is reported to protect EC from premature senescence and oxidative stress by sustaining high eNOS levels. Surprisingly, when we studied non-inducible Sirt6 endothelial-specific knockout (Sirt6 ^{f/f, tie-2 cre}) and inducible Sirt6 endothelial-specific knockout (Sirt6 f/f, ^{Cdh5-cre/ERT2}) and wild-type (WT) mice, Sirt6 ^{f/f, Tie-2} and Sirt6 ^{f/f, Cadh5} mice do not phenocopy the inducible global SIRT6 knockout mice, they had normal EDD and CFR. When the mice were fed a high fat and high sugar (HFHS) diet, the Sirt6 ^{f/f, Tie-2}and Sirt6 ^{f/f, Cadh5} had impaired EDD, suggesting Sirt 6 functioned differently in the mice fed with chow diet or HFHS diet.
We hypothesize Sirt 6 deficiency causes coronary endothelial dysfunction and contributes to CMD; activating Sirt6 will ameliorate CMD. EDD was assessed using myography (DMT). Myocardial blood flow (MBF) was measured by Doppler. Our preliminary data show that the mediator of coronary vasodilation switched from NO to H₂O₂ in the Sirt6 knockout mice with impaired EDD. Interestingly, when the mice fed on HFHS were treated with Sirt 6 activator MDL-800, the coronary microvascular function was improved, and the blood glucose level was decreased. The underlying mechanism and the pathways involved will be elucidated.