A long-term sucrose-rich diet increases diacylglycerol content and membrane nPKCθ expression and alters glucose metabolism in skeletal muscle of rats

This study was designed to examine the possible metabolic pathways that could be involved in the increase of lipid storage in the gastrocnemius muscle and insulin resistance in rats fed a sucrose-rich diet (sucrose 63%, wt/wt) for 30 weeks. In the gastrocnemius muscle, malonyl-CoA, diacylglycerol (DAG), and long-chain acyl-CoA (LC ACoA) contents; novel protein kinase C θ(nPKCθ) and Glut4 protein mass; and hexokinase, GSa, and pyruvate dehydrogenase (PDH) and PDH kinase activities were analyzed. The results were compared with age-matched rats fed a control diet (starch 63%, wt/wt) during the same period. In the sucrose-rich–fed rats, 2 major findings are reported: (1) a significant increase of DAG levels and nPKCθ protein mass in the membrane fraction, accompanied by a high increase of both triglycerides and LC ACoA contents in the gastrocnemius muscle; and (2) a significant decrease of hexokinase activity without changes of Glut4 protein mass and an altered glucose oxidation (low PDH complex and high PDH kinase activities). These findings suggest that different metabolic pathways could contribute to the development of insulin resistance in the skeletal muscle of long-term sucrose-fed rats: (1) a high level of LC ACoA by their esterification to DAG could stimulate the increase and translocation of the nPKCθ isozyme to the cell membrane, which might interfere with insulin signaling pathways; (2) a decrease in the hexokinase activity alters glucose phosphorylation and could impair insulin-mediated glucose disposal; (3) a significant reduction of flux through PDH complex may limit glucose oxidation via the glucose-fatty acid cycle.