Isomer-Specific Antidiabetic Properties of Conjugated Linoleic Acid Improved Glucose Tolerance, Skeletal Muscle Insulin Action, and UCP-2 Gene Expression J.W. Ryder, 1 C.P. Portocarrero, 2 X.M. Song, 1 L. Cui, 3 M. Yu, 1 T. Combatsiaris, 3 D. Galuska, 1 D.E. Bauman, 4 D.M. Barbano, 5 M.J. Charron, 3 J.R. Zierath, 1 and K.L. Houseknecht 2 Conjugated linoleic acid (CLA) isomers have a number of beneficial health effects, as shown in biomedical studies with animal models. Previously, we reported that a mixture of CLA isomers improved glucose toler- ance in ZDF rats and activated peroxisome proliferator– activated receptor (PPAR)- response elements in vitro. Here, our aim was to elucidate the effect(s) of specific CLA isomers on whole-body glucose tolerance, insulin action in skeletal muscle, and expression of genes im- portant in glucose and lipid metabolism. ZDF rats were fed either a control diet (CON), one of two CLA supple- mented diets (1.5% CLA) containing differing isoforms of CLA (47% c9,t11; 47.9% c10,t12, 50:50; or 91% c9,t11, c9,t11 isomers), or were pair-fed CON diet to match the intake of 50:50. The 50:50 diet reduced adiposity and improved glucose tolerance compared with all other ZDF treatments. Insulin-stimulated glucose transport and glycogen synthase activity in skeletal muscle were improved with 50:50 compared with all other treat- ments. Neither phosphatidlyinositol 3-kinase activity nor Akt activity in muscle was affected by treatment. Uncoupling protein 2 in muscle and adipose tissue was upregulated by c9,t11 and 50:50 compared with ZDF controls. PPAR- mRNA was downregulated in liver of c9,t11 and pair-fed ZDF rats. Thus, the improved glu- cose tolerance in 50:50 rats is attributable to, at least in part, improved insulin action in muscle, and CLA effects cannot be explained simply by reduced food intake. Diabetes 50:1149 –1157, 2001 C onjugated linoleic acids (CLAs), geometric and positional isomers of linoleic acid, are potent cancer preventative agents in animal models of chemical-induced carcinogenesis (1). CLA iso- mers occur naturally in foods, are highest in ruminant meats and milks, and may be manipulated by animal husbandry (2,3). Recently, we reported that a mixture of dietary CLA isomers normalized impaired glucose toler- ance and prevented/delayed the development of hypergly- cemia in obese ZDF rats (4). ZDF rats spontaneously develop diabetes at 7–12 weeks due to -cell decompen- sation; the development of hyperglycemia is secondary to obesity, because food restriction can prevent hyperglyce- mia in this model (5). We and others (4,6,7) have observed modest to significant reductions in food intake and/or adiposity with dietary CLA supplementation. Furthermore, data are emerging that different isomers of CLA are re- sponsible for the antiobesity and anticancer effects (6,8,9). In addition to antiobesity therapies, pharmaceutical agents that improve insulin-stimulated glucose uptake and utilization by skeletal muscle, such as the thiazolidine- diones (TZDs), are able to improve whole-body glucose homeostasis and prevent/improve the hyperglycemic state in ZDF rats (10 –12). It is possible that dietary CLA may improve glucose homeostasis by multiple mechanisms in ZDF rats. Thus, the aims of this work were to determine 1) which CLA isomer(s) is responsible for the antidiabetic effects, 2) if the CLA-induced antidiabetic effects are due to a reduction in food intake, 3) whether CLA exerts an antidiabetic effect by improving insulin action in skeletal muscle, and 4) whether CLA regulates expression of genes important in glucose and lipid homeostasis. At least a portion of the CLA antidiabetic effects appears to be via activation of peroxisome proliferator–activated receptor (PPAR)- (4). We reported induction of aP2 gene expression in adipose tissue of CLA-treated ZDF rats and activation PPAR- response elements by CLA in vitro (4). It is not surprising that CLA can activate PPARs; the crys- tal structure of PPAR ligand binding domains reveals a large ligand binding pocket that explains the promiscuity of this receptor for multiple ligands, including fatty acids (13). PPAR- ligands, such as TZD, are potent insulin- sensitizing drugs that impact whole-body glucose utiliza- From the 1 Department of Clinical Physiology, Karolinska Institute, Stockholm Sweden; the 2 Department of Animal Sciences, Purdue University, West Lafayette, Indiana; the 3 Department of Biochemistry, Yeshiva University, Bronx; and the Departments of 4 Animal Science and 5 Food Science, Cornell University, Ithaca, New York. Address correspondence and reprint requests to Karen L. Houseknecht, Discovery Pharmaceuticals, Pfizer Global Research and Development-Groton Laboratories, MS 8220-2239, Eastern Point Rd., Groton, CT 06340-8002. E-mail: karen_l_houseknecht@groton.pfizer.com. Received for publication 27 September 2000 and accepted in revised form 9 February 2001. J.W.R. and C.P.P contributed equally to this work. K.L.H. is employed by Pfizer Inc. (Groton, CT), which manufactures and markets pharmaceuticals related to the treatment of diabetes and its compli- cations. BAT, brown adipose tissue; CLA, conjugated linoleic acid; CON, control diet; ECL, enhanced chemiluminescence; EDL, extensor digitorum longus; G-6-P, glucose-6-phosphate; KHB, Krebs-Henseleit buffer; NEFA, nonesterified fatty acid; PI, phosphatidylinositol; PPAR, peroxisome proliferator–activated receptor; RT-PCR, reverse transcriptase–polymerase chain reaction; SSC, sodium chloride–sodium citrate; TBST, Tris-buffered saline plus Tween; TG, triglyceride; TZD, thiazolidinedione; UCP, uncoupling protein. DIABETES, VOL. 50, MAY 2001 1149