Abstract EC103: A Novel Nuclear Role For REDD1 In Cardiac Insulin Sensitivity And Resistance

Abstract

Insulin resistance (IR) is a hallmark of type II diabetes (TIID) and causes cardiac dysfunction independent of hypertension and coronary artery disease, or diabetic cardiomyopathy (DC). Impaired metabolism and energetics are a major cause of DC, however the molecular mechanisms underlying these defects remain ill-defined. Our unbiased RNA sequencing (RNA-Seq) studies identified ‘regulated in development and DNA damage (REDD)1’ as a potential critical regulator of cardiac insulin sensitivity, as a 1.5-fold increase in REDD1 expression was observed in the hearts of mice treated with acute insulin (1 hour (h)) versus vehicle (n=3). REDD1 is an insulin-sensitive, negative regulator of mTORC1 and global REDD1 deletion leads to whole body IR. Interestingly, cardiac REDD1 is also upregulated in several models of IR. Thus, we hypothesized that while REDD1 is critical for cardiac insulin sensitivity, sustained elevated levels contribute to IR via chronic inhibition of mTORC1. Our findings confirm that acute insulin stimulation or chronic high fat diet (HFD) (2 to 16 weeks) induce cardiac REDD1 expression in vivo (1.7-fold±0.1, n=6). In neonatal rat ventricular myocytes (NRVM) or Hap1 cells, insulin (1 h) or palmitate (24 h) also induce REDD1 expression (1.3-fold±0.1, n=3 or 1.6-fold±0.1, n=9, respectively), which indeed inhibits mTORC1. Notably, insulin treatment following HFD or palmitate, does not further enhance REDD1. In addition, subcellular fractionation shows that REDD1 is uniquely detected in nuclear and chromatin fractions and increases with insulin (1.5-fold±0.1, n=6). We confirmed REDD1 nuclear localization via microscopy and chromatin binding via chromatin immunoprecipitation-deep sequencing (ChIP-Seq) (n=5, pooled). In addition, we determined that REDD1 primarily binds the transcription start sites of metabolic genes and increases with insulin, corresponding to reduced gene expression. Overall, our findings suggest that insulin-inducible cardiac REDD1 exists as part of negative feedback loops that prevent the overactivation of mTORC1 and metabolic gene expression, contributing to insulin sensitivity. Further, sustained fat-induced REDD1 expression, may drive IR via chronic inhibition of mTORC1 and metabolic gene expression.