Original Article Unraveling the Temporal Pattern of Diet-Induced Insulin Resistance in Individual Organs and Cardiac Dysfunction in C57BL/6 Mice So-Young Park, 1 You-Ree Cho, 1 Hyo-Jeong Kim, 1 Takamasa Higashimori, 1 Cheryl Danton, 1 Mi-Kyung Lee, 1 Asim Dey, 2 Beverly Rothermel, 2 Young-Bum Kim, 3 April Kalinowski, 4 Kerry S. Russell, 4 and Jason K. Kim 1,5 Type 2 diabetes is a heterogeneous disease characterized by insulin resistance and altered glucose and lipid metab- olism in multiple organs. To understand the complex series of events that occur during the development of obesity- associated diabetes, we examined the temporal pattern of changes in insulin action and glucose metabolism in indi- vidual organs during chronic high-fat feeding in C57BL/6 mice. Insulin-stimulated cardiac glucose metabolism was significantly reduced after 1.5 weeks of high-fat feeding, and cardiac insulin resistance was associated with blunted Akt-mediated insulin signaling and GLUT4 levels. Insulin resistance in skeletal muscle, adipose tissue, and liver developed in parallel after 3 weeks of high-fat feeding. Diet-induced whole-body insulin resistance was associated with increased circulating levels of resistin and leptin but unaltered adiponectin levels. High-fat feeding caused insu- lin resistance in skeletal muscle that was associated with significantly elevated intramuscular fat content. In con- trast, diet-induced hepatic insulin resistance developed before a marked increase in intrahepatic triglyceride lev- els. Cardiac function gradually declined over the course of high-fat feeding, and after 20 weeks of high-fat diet, cardiac dysfunction was associated with mild hyperglycemia, hy- perleptinemia, and reduced circulating adiponectin levels. Our findings demonstrate that cardiac insulin resistance is an early adaptive event in response to obesity and develops before changes in whole-body glucose homeostasis. This suggests that obesity-associated defects in cardiac func- tion may not be due to insulin resistance per se but may be attributable to chronic alteration in cardiac glucose and lipid metabolism and circulating adipokines. Diabetes 54: 3530 –3540, 2005 T ype 2 diabetes has reached epidemic propor- tions, affecting 170 million people globally, and cardiovascular disease (CVD) is the leading cause of mortality in diabetes (1,2). The inci- dence of obesity, which increases the risk for development of type 2 diabetes and CVD, also continues to rise rapidly worldwide (3,4). The apparent triangular relationship of obesity, diabetes, and CVD may be interconnected by insulin resistance and altered glucose and lipid metabo- lism in response to insulin (5,6). Using genetic animal models of obesity, such as Zucker diabetic rats and leptin-deficient (ob/ob) mice, as well as diet-intervention models, many previous studies have reported that obese animals develop insulin resistance in skeletal muscle, adipose tissue, and liver (7–10). The mechanism underly- ing obesity-mediated insulin resistance involves the tissue- specific accumulation of fat and fatty acid metabolites and their deleterious effects on insulin signaling and glucose transport activity (11–13). An alternative mechanism is that adipocytes produce a host of metabolic hormones and inflammatory cytokines (adipokines), including resistin, adiponectin, leptin, tumor necrosis factor-, and interleu- kin-6, and that the dysregulated production of adipokines alters whole-body insulin sensitivity (14 –18). Thus, the underlying mechanism by which obesity causes insulin resistance remains unclear. The heart is a constitutively energy-demanding organ, and normal cardiac function is dependent on a constant rate of ATP resynthesis by mitochondrial oxidative phos- phorylation and, to a much lesser extent, glycolysis (19). Although mitochondrial lipid oxidation is the principal energy source, the maintenance of glucose utilization is necessary for normal cardiac function (20,21). This impor- tant role of cardiac glucose metabolism and insulin action was recently demonstrated in mice with cardiac-specific ablation of GLUT4 or the insulin receptor that developed cardiac hypertrophy and other phenotypes resembling the diabetic heart (22–24). Furthermore, studies using isolated perfused heart preparations, cultured cardiomyocytes, and positron emission tomography have uniformly shown in- sulin resistance in human and animal models of diabetic heart (25,26). In an important finding, cardiac insulin resistance was associated with diabetes independent of From the 1 Department of Internal Medicine, Section of Endocrinology and Metabolism, Yale University School of Medicine, New Haven, Connecticut; the 2 Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas; the 3 Division of Endocrinology, Diabetes, and Metabo- lism, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts; the 4 Department of Internal Medicine, Section of Cardiology, Yale University School of Medicine, New Haven, Connecticut; and the 5 Yale Mouse Metabolic Phenotyping Center, Yale University School of Medicine, New Haven, Connecticut. Address correspondence and reprint requests to Prof. Jason K. Kim, Pennsylvania State University College of Medicine, Department of Cellular and Molecular Physiology (H166), 500 University Dr., Room C4600D, Hershey, PA 17033-0850. E-mail: jason.kim@psu.edu. Received for publication 11 March 2005 and accepted in revised form 6 September 2005. S.-Y.P. is currently affiliated with the Department of Physiology, Yeungnam University College of Medicine, Yeungnam, South Korea. 2-[ 14 C]DG, 2-deoxy-D-[1- 14 C]glucose; AMPK, AMP-activated protein kinase; CVD, cardiovascular disease; GSK-3, glycogen synthase kinase-3; HGP, hepatic glucose production; 1 H-MRS, 1 H-magnetic resonance spectroscopy. © 2005 by the American Diabetes Association. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. 3530 DIABETES, VOL. 54, DECEMBER 2005