Insulin Suppresses PDK4 Expression in Skeletal Muscle Independent of Plasma FFA

Starvation and experimental diabetes induce a stable increase in pyruvate dehydrogenase kinase (PDK) activity in skeletal muscle, which is largely due to a selective upregulation of PDK4 expression. Increased free fatty acid (FFA) level has been suggested to be responsible for the upregulation. Since these metabolic states are also characterized by insulin deficiency, the present study was designed to examine whether insulin has a significant effect to regulate PDK mRNA expression in rat skeletal muscle. In Study 1 overnight fasted rats received an infusion of saline or insulin for 5 hours (n=6 for each). During the insulin infusion, plasma glucose was clamped at basal levels (euglycemic hyperinsulinemic clamp). A third group (n=6) received Intralipid infusion during the clamp to prevent a fall in plasma FFA. PDK2 mRNA level in gastrocnemius muscle was not altered by insulin or FFA (i.e., Intralipid infusion). In contrast, PDK4 mRNA level was decreased 72% by insulin (P<0.05), and Intralipid infusion prevented only 20% of the decrease. PDK4 protein level was decreased 20% by insulin (P<0.05) but this effect was not altered by Intralipid infusion. In Study 2 overnight fasted rats were refed or received an infusion of saline or nicotinic acid (NA, 30 µmol/h) for 5 hours (n=5 for each). During the refeeding and the NA infusion, plasma FFA levels were similarly (i.e., 60-70% vs. saline control) lowered. Muscle PDK4 mRNA level decreased 77% after the refeeding (P<0.05) but not after the NA infusion. In conclusion, the present data indicate that insulin had a profound effect to suppress PDK4 expression in skeletal muscle and that, in contrary to previous suggestions, circulating FFA had little impact on PDK4 mRNA expression, at least within 5 hours. 3 Lipid oxidation is generally increased in individuals with obesity or type 2 diabetes (3,4,17). Increased lipid oxidation suppresses glucose oxidation, and this substrate competition mechanism may play a major role in the development of insulin resistance (i.e., decreased insulin's action on glucose uptake) in skeletal muscle (20,21,27). One major target for this regulation is the activity of pyruvate dehydrogenase (PDH) complex that catalyzes a key irreversible step of glucose oxidation, i.e., the conversion of pyruvate to acetyl CoA. The activity of PDH complex is regulated acutely by the products of FFA oxidation, i.e., acetyl CoA and NADH. In addition, PDH complex is inhibited by phosphorylation of its PDH component by pyruvate dehydrogenase kinase (PDK). Increased PDK activity has been suggested …