BACKGROUNDCurrently it is unclear whether lipid accumulation occurs in a particular sequence and its relationship with whole body insulin resistance (IR). This study aimed to answer this question.

METHODSMale Sprague-Dawley (SD) rats were fed on a normal or a high-fat diet for 20 weeks. Serum triglycerides (TG), serum free fatty acids (FFA), fasting plasma glucose (FPG), and liver and skeletal muscle TG were measured. The glucose infusion rate (GIR) and mRNA levels of acetyl-CoA carboxylase (ACC) and carnitine palmitoyltransferase-1 (CPT-1) in the liver and skeletal muscle were determined at different stages.

RESULTSCompared with rats fed on the normal diet, serum FFA was not significantly increased in rats fed on the high-fat diet until 20 weeks. In contrast, liver TG was significantly increased by the high-fat diet by four weeks (20-fold; P < 0.01), and remained elevated until the end of the study. However, skeletal muscle TG was not significantly increased by the high-fat diet until 20 weeks (10.6-fold; P < 0.01), and neither was the FPG. The GIR was significantly reduced (1.6-fold; P < 0.01) by the high-fat diet after 8 weeks. The mRNA levels of ACC gradually increased over time and CPT-1 decreased over time, in both the liver and skeletal muscle in rats fed the high-fat diet.

CONCLUSIONSLipid accumulation in the liver occurs earlier than lipid accumulation in the skeletal muscle. Fatty liver may be one of the early markers of whole body IR. Changes in the gene expression levels of ACC and CPT-1 may have important roles in the process of IR development.

Acetyl-CoA Carboxylase ; genetics ; Animals ; Blood Glucose ; analysis ; Carnitine O-Palmitoyltransferase ; genetics ; Fatty Acids, Nonesterified ; blood ; Fatty Liver ; etiology ; Insulin Resistance ; Lipid Metabolism ; Liver ; metabolism ; Male ; Muscle, Skeletal ; metabolism ; RNA, Messenger ; analysis ; Rats ; Rats, Sprague-Dawley ; Triglycerides ; metabolism

Mammals have two major isoforms of acetyl-CoA carboxyase (ACC). The 275 kDa beta-form (ACC beta) is predominantly in heart and skeletal muscle while the 265 kDa alpha-form (ACC alpha) is the major isoform in lipogenic tissues such as liver and adipose tissue. ACC alpha is thought to control fatty acid oxidation by means of the ability of malonyl-CoA to inhibit carnitine palmitoyl-CoA transferase-1 (CPT-1), which is a rate-limiting enzyme of fatty acid oxidation in mitochondria. Previously, it was reported that MyoD and other muscle regulating factors (MRFs) up-regulate the expression of ACC beta by interactions between these factors and several cis-elements of ACC beta promoter. We described here that ACC beta expression mediated by MRFs is regulated by retinoic acids. Endogenous expression of ACCb in differentiated H9C2 myotube was significantly increased by retinoic acid treatment. However, on transient transfection assay in H9C2 myoblast, ACC beta promoter activity was suppressed by RXRa and more severely by RAR alpha. These effects on ACCb expression in myoblasts and myotubes by RXR alpha and RAR alpha seem to be mediated by their interactions with MRFs because no consensus sequence for RXR alpha and RAR alpha has been found in ACC beta promoter and retinoic acid receptors did not affect this promoter activities by itself. In transient transfection in NIH3T3 fibroblast, the activation of ACC beta promoter by MyoD, main MRF in myoblast, was significantly suppressed by RAR alpha and to a less extent by RXR alpha while the RXR alpha drastically augmented the activation by MRF4, major MRF in myotube. These results explained that retinoic acids differentially affected the action of MRFs according to their types and RXR alpha specially elevates the expression of muscle specific genes by stimulating the action of MRF4.
3T3 Cells ; Acetyl-CoA Carboxylase/genetics/*metabolism ; Animals ; Cell Differentiation ; Cells, Cultured ; Gene Expression Regulation, Enzymologic/drug effects ; Mice ; MyoD Protein/*metabolism ; Myoblasts/drug effects/metabolism ; Myogenic Regulatory Factors/*metabolism ; Promoter Regions (Genetics)/drug effects ; Receptors, Retinoic Acid/genetics/*metabolism ; Trans-Activation (Genetics) ; Transcription Factors/genetics/*metabolism ; Tretinoin/pharmacology