The nuclear sirtuin SIRT6 plays a role in regulating cardiac branched‐chain amino acid catabolism
Authors: Samant Sadhana , Pillai Vinodkumar , Bao Riyue , Gupta Mahesh P. 
Published: April 2020

IntroductionTo derive energy, heart uses multiple substrates such as fatty acids, glucose, amino acids, ketones and lactic acid. Defective catabolism of branched‐chain amino acids (BCAAs), which include valine, leucine and isoleucine, is implicated in coronary heart disease and in cardiac ischemia‐reperfusion injury. BCAAs are primarily catabolized in extra‐hepatic tissues, which include muscle, kidney, brain and adipose. Enzymes involved in BCAA catabolism are highly expressed in the heart. Abnormal elevation in BCAA‐levels is linked to impaired lipid metabolism, development of insulin‐resistance and suppression of glucose metabolism. Increased levels of BCAAs in plasma and urine are considered as a better indicator of the pre‐diabetic state with impaired insulin sensitivity. Under physiological conditions, BCAAs maintain glucose homeostasis by stimulating insulin secretion. The NAD+‐dependent enzyme, SIRT6 plays a critical role to maintain cellular glucose and lipid homeostasis. This chromatin‐bound sirtuin is a key epigenetic regulator of glucose homeostasis. This study was undertaken to explore the role of SIRT6 in BCAA metabolism in the heart tissue.Methods and ResultsRNAseq analysis of the heart tissue from wild type and global SIRT6 knockout mice revealed that the expression of genes regulating BCAA catabolism were significantly affected with loss of SIRT6. We also investigated expression of the genes of BCAA pathway, in neonatal rat cardiomyocytes infected for 72 hours with adenoviruses for null or rat SIRT6‐shRNA‐(KO) or for human SIRT6‐over‐expression. The expression of RNA in these cells was measured by RNAseq and qPCR analyses, and proteins by western blotting. SIRT6 deficiency in cardiomyocytes and in the heart tissue resulted in a general downregulation of the genes involved in BCAA catabolism. One of the rate‐limiting step in BCAA catabolism is decarboxylation of Branched‐chain α‐keto acid (BCKA), which is catalyzed by the BCK‐dehydrogenase (BCKDH) complex. The BCKDH complex is activated by dephosphorylation by the phosphatase Pp2cm (Ppm1k). In the mouse and human failing hearts, Ppm1k deficiency has been linked to dysfunctional BCAA catabolism. We found nearly 2 fold downregulation of Ppm1k in SIRT6‐deficient hearts and cardiomyocytes, however, no change was observed in expression of other enzymes of the BCKDH complex. Interestingly, with SIRT6 over‐expression in cardiomyocytes, gene expression of other members of the BCAA catabolism pathway, including Ppm1K, was restored to normalcy. These results were confirmed by RT‐qPCR analysis of the heart tissue. Moreover, we found that the promoter for Ppm1k gene harbors binding sites for NF‐κB p65 and c‐Jun transcription factors, which are known targets for SIRT6.ConclusionOur results indicate that SIRT6 transcriptionally regulates the BCAA catabolic pathway in the mouse heart, and its loss could contribute to development of heart failure.Support or Funding InformationNIH RO1 HL136712, NIH RO1 HL143488