Abstract 431: KLF15 Regulates the Circadian Susceptibility to Ischemia Reperfusion Injury in the Heart

Abstract

The onset and the injury of heart attacks both show a circadian rhythmicity with a peak during the sleep to active transition. Studies using cell type specific clock mutant demonstrated the intrinsic varying susceptibility of the cardiomyocytes to the ischemia reperfusion (I/R) injury in a time-of-the-day dependent fashion, however, the molecular mechanisms remain unclear. Transcription factor KLF15 expresses in a circadian fashion and is significantly reduced in ischemia cardiomyopathy in human and in mice, but its role in the cardiac I/R injury is unknown. Recently, we show that KLF15 coordinates the diurnal expression of catabolic genes in the heart during the active phase, suggesting it may also be involved in regulating the cardiac reactive oxygen species (ROS), which is not only a byproduct of catabolism but also the main injury during reperfusion. Here we show that cardiomyocyte specific KLF15 deficient mice have exaggerated I/R injury in vivo during the sleep to active transition when KLF15 expression is at peak, but not during the active to sleep transition, demonstrating its critical role in governing the circadian rhythmicity of myocardial injury. We found acute KLF15 deficiency in the primary cardiomyocytes is associated with increased ROS and exaggerated susceptibility to oxidative stress, which can be rescued by a MnSOD mimetic Tempol, but not by several other anti-oxidants. We further demonstrate that KLF15 deficiency in the cardiomyocytes in vitro and in vivo are both associated with increased mitochondrial acetylation, including MnSOD acetylation at Lysine 122, which leads to reduced MnSOD activity. There is no change in mitochondrial sirtuin SIRT3 level, the main deacetylase for mitochondrial acetylated proteins including MnSOD K122 , however its co-enzyme NAD + level and NAD + /NADH ratio are drastically reduced with KLF15 deficiency. Furthermore, NAD + precursor NMN rescues the increased susceptibility to ROS in KLF15 deficient cardiomyocytes and I/R injury in vivo. Finally, we show that KLF15 directly regulates nicotinamide phosphoribosyltransferase (NAMPT), the rate limiting enzyme of NAD + salvage pathway and the main NAD + determinant in the heart. Collectively, our results demonstrate that KLF15 coordinates cardiac metabolism and ROS clearance in a circadian fashion through mitochondrial NAD + ; loss of KLF15 expression in ischemic cardiomyopathy likely contributes to an enhanced susceptibility to additional I/R insult in a time-of-the-day dependent fashion.