This article is available online at http://www.jlr.org create a group of diseases classifi ed as lipotoxicities. These diseases include nonalcoholic fatty liver disease, type 2 diabetes, and metabolic cardiomyopathy ( 2 ).The major storage form of lipids within the body is triglyceride (TG), a lipid that is believed to be biologically inert ( 3 ). Diacylglycerol (DAG) acyl transferase 1 (DGAT1) is one of two enzymes that catalyze the fi nal step in TG synthesis. Dgat1 belongs to a gene family that includes acylCoA:cholesterol acyltransferases 1 and 2 ( 4 ). DGAT1 has been overexpressed in mice to elucidate the function of this gene in the development of metabolic disease. In skeletal muscle, overexpression of DGAT1 increased TG stores but reduced DAG and ceramide, increased FA oxidation, and improved insulin sensitivity ( 5 ); this mimics the biology of chronic exercise and is sometimes referred to as the "athlete paradox" ( 6, 7 ). Overexpression of DGAT1 in adipose tissue using an AP2 promoter led to greater obesity but not insulin resistance in C57BL6 mice ( 8 ). FVB mice, however, still had insulin resistance ( 9 ), a fi nding that might refl ect genotype differences or variation in macrophage DGAT1 expression with the AP2 promoter. Overexpression of DGAT1 in the liver ( 10 ) and heart ( 11 ) increased TG content of those tissues but did not cause insulin resistance or heart dysfunction. Similarly, overexpression of DGAT1 in macrophages ameliorated FA-induced infl ammation and high-fat feeding-induced insulin resistance ( 12 ). These data support the hypothesis that conversion of intermediary toxic lipids to TG via DGAT1 can be a detoxifying process ( 3 ).Lipotoxic cardiomyopathy models have been created in which lipid oxidation is insufficient to balance lipid uptake, leading to increased accumulation of TG, free FA Abstract Accumulation of excess lipids is associated with heart failure. The effects of transgenic expression of diacylglycerol acyl transferase 1 (DGAT1) in cardiomyocytes is controversial. We explored whether mice expressing DGAT1 via the myosin heavy chain (MHC) promoter develop heart dysfunction with aging or after crossing with mice overexpressing peroxisome proliferator-activated receptor ␥ (PPAR ␥ ) in the heart. MHC-DGAT1 transgenic mice had increased heart triglyceride but no evidence of heart dysfunction, even up to age 12 months. The MHC-DGAT1 transgene improved heart dysfunction and survival of MHC-PPAR ␥ -expressing transgenic mice. Both diacylglycerol and ceramide levels in the heart were reduced by this cross, as were the levels of several mRNAs of genes involved in lipid metabolism. There were fewer large lipid droplets in MHC-DGAT1×MHC-PPAR ␥ mice compared with MHC-PPAR ␥ , but total lipid content was not changed. Therefore, overexpression of DGAT1 is not toxic to the heart but reduces levels of toxic lipids and improves lipotoxic cardiomyopathy. Moreover, the benefi cial effects of DGAT1 illustrate the interrelationship of several lipid metabolic pathways and the diffi culty of...