Increased fat accumulation in liver may link insulin resistance with subcutaneous abdominal adipocyte enlargement, visceral adiposity, and hypoadiponectinemia in obese individuals

Original Research Communications Juraj Koska, Norbert Stefan, Paska A Permana, Christian Weyer, Mina Sonoda, Clifton Bogardus, Steven R Smith, Denis R Joanisse, Tohru Funahashi, Jonathan Krakoff, and Joy C Bunt ABSTRACT Background: Enlargement of adipocytes from subcutaneous abdominal adipose tissue (SAT), increased intrahepatic lipid content (IHL), intramyocellular lipid content (IMCL), and low circulating adiponectin concentrations are associated with insulin resistance. Objective: Because adiponectin increases fat oxidation in skeletal muscle and liver, and the expression of the adiponectin gene in SAT is inversely associated with adipocyte size, we hypothesized that hypoadiponectinemia links hypertrophic obesity with insulin resistance via increased IMCL and IHL. Design: Fifty-three obese Pima Indians with a mean (앐SD) age of 27 앐 8 y, body fat of 35 앐 5%, and normal glucose regulation (normal fasting and 2-h glucose concentration per WHO 1999 criteria) underwent euglycemic-hyperinsulinemic clamp, biopsies of SAT and vastus lateralis muscle, and magnetic resonance imaging of the abdomen. Results: Adipocyte diameter (AD) correlated positively with body fat (P 쏝 0.0001) and IHL (estimated from magnetic resonance imaging intensity of liver; P ҃ 0.047). No association was found between AD and plasma adiponectin or IMCL. Plasma adiponectin negatively correlated with type II IMCL (IIA, P ҃ 0.004; IIX, P ҃ 0.009) or IHL (P ҃ 0.02). In a multivariate analysis, plasma adiponectin, AD, and visceral adipose tissue (VAT) independently predicted IHL. Low insulin-mediated glucose disposal was associated with low plasma adiponectin (P ҃ 0.02) and high IHL (P ҃ 0.0003), SAT (P ҃ 0.02), and VAT (P ҃ 0.04). High IHL was the only predictor of reduced insulin-mediated suppression of hepatic glucose production (P ҃ 0.02) and the only independent predictor of insulin-mediated glucose disposal in a multivariate analysis. Conclusions: Increased lipid content in the liver may independently link hypoadiponectinemia, hypertrophic obesity, and increased visceral adiposity with peripheral and hepatic insulin resistance. Am J Clin Nutr 2008;87:295–302. KEY WORDS Obesity, adipocyte size, adiponectin, intrahepatic fat, intramyocellular fat, insulin action INTRODUCTION Obesity is characterized by a growing adipose tissue mass associated with increases in adipocyte size and number (1). It has been known for a few decades that obese people with enlarged subcutaneous adipocytes (ie, with hypertrophic obesity) are, on average, more hyperinsulinemic than those with a similar degree of adiposity but relatively smaller adipocytes (2). More recent analyses have shown that the enlargement of subcutaneous abdominal adipocytes is associated with reduced insulinmediated glucose disposal independent of overall adiposity (3, 4). It has been hypothesized that enlargement of subcutaneous adipocytes is a sign of reduced adipogenic capacity of subcutaneous adipose tissue (SAT), which in turn leads to increased accumulation of fat in visceral adipose tissue (VAT), skeletal muscle, and liver and a subsequent worsening of insulin action and glucose tolerance (5). In support of this hypothesis, implantation of adipose tissue in lipodystrophic mice (mice with no 1 From the Obesity and Diabetes Clinical Research Section, NIDDK/NIH/ DHHS, Phoenix, AZ (JK, NS, CW, CB, JK, and JCB); Carl T Hayden Veterans Affairs Medical Center, Phoenix, AZ (PAP); the Department of Internal Medicine and Molecular Science, Graduate School of Medicine, Osaka University, Osaka, Japan (MS and TF); Pennington Biomedical Research Center, Baton Rouge, LA (SRS); and the Department of Kinesiology, Laval University, Quebec, Canada (DRJ). 2 Supported by an intramural research program of the NIDDK/NIH/DHHS (to JK, NS, PAP, CW, CB, JK, and JCB) and a new investigator award from the Fonds de la Recherche en Santé du Québec (to DRJ). 3 Reprints not available. Address correspondence to J Krakoff, ODCRS/ NIDDK/NIH/DHHS; 4212 N 16th Street, Phoenix, AZ 85016. E-mail: . Received June 8, 2007. Am J Clin Nutr 2008;87:295–302. Printed in USA. © 2008 American Society for Nutrition 295 Increased fat accumulation in liver may link insulin resistance with subcutaneous abdominal adipocyte enlargement, visceral adiposity, and hypoadiponectinemia in obese individuals1–3 296 KOSKA ET AL SUBJECTS AND METHODS Subjects Subjects were at least 3 quarters Pima Indian, between 18 and 45 y of age, nonsmokers at the time of the study, and healthy on the basis of medical history, physical examination, and routine laboratory tests. The protocol was approved by the Tribal Council of the Gila River Indian Community and by the Institutional Review Board of the National Institute of Diabetes and Digestive and Kidney Diseases, and all subjects provided written informed consent before participation. The female subjects were studied during the follicular phase of the menstrual cycle. Methods All subjects were admitted to a clinical research unit and were placed on a weight-maintaining diet (containing 50% of energy as carbohydrates, 30% as fat, and 20% as protein). Body composition was measured by dual-energy X-ray absorptiometry (DPX-L; Lunar Radiation, Madison, WI) (16) to confirm obesity status [percentage body fat (BF) 욷25 in men and 욷30 in women]. At least 3 d after admission and after a 12-h overnight fast, the subjects underwent a 2-h 75-g oral-glucose-tolerance test to exclude impaired glucose regulation or diabetes according Accepted for publication September 13, 2007. to the World Health Organization 1999 criteria. Plasma glucose concentrations were determined by the glucose oxidase method (Beckman Instruments, Fullerton, CA). Plasma insulin concentrations were measured with an automated immunoassay (Access; Beckman Instruments). Blood samples for the measurement of fasting plasma adiponectin concentrations were drawn at 0700 with prechilled syringes, transferred into prechilled EDTAcoated tubes, and immediately placed on ice. All tubes were centrifuged at 4 °C within several minutes of collection and were stored at Ҁ70 °C until assayed. Total plasma adiponectin concentrations were determined with the use of a validated sandwich enzyme-linked immunosorbent assay that uses an adiponectinspecific antibody (intraassay and interassay CVs of 3.3% and 7.4%, respectively). Insulin action was assessed at physiologic insulin concentrations during a hyperinsulinemic-euglycemic glucose clamp (17). Briefly, after the subjects fasted overnight, a primed (30 ␮Ci) continuous (0.3 ␮Ci/min) 3-[3H]glucose infusion was started to determine endogenous glucose production (EGP). Two hours after the isotope infusion was started, a primed continuous intravenous insulin infusion was administered for 100 min at a constant rate of 40 mU䡠mҀ2 body surface area䡠minҀ1. Blood samples for measurement of 3-[3H] glucose specific activity were collected from the end of the basal period to and every (...truncated)