C-13 NMR-STUDY OF THE EFFECTS OF LEPTIN TREATMENT ON KINETICS OF HEPATIC INTERMEDIARY METABOLISM

The recent discovery of leptin receptors in peripheral tissue raises q uestions about which of leptin's biological actions arise from direct effects of the hormone on extraneural tissues and what intracellular m echanisms are responsible for leptin's effects on carbohydrate and lip id metabolism. The present study is focused on the action of leptin on hepatic metabolism. Nondestructive C-13 NMR methodology was used to f ollow the kinetics of intermediary metabolism by monitoring flux of C- 13-labeled substrate through several multistep pathways. In perfused l iver from either ob/ob or lean mice, we found that acute treatment wit h leptin in vitro modulates pathways controlling carbohydrate flux int o C-13-labeled glycogen, thereby rapidly enhancing synthesis by an ins ulin-independent mechanism. Acute treatment of ob/ob liver also caused a rapid stimulation of long-chain fatty acid synthesis from C-13-labe led acetyl-CoA by the de novo synthesis route. Chronic leptin treatmen t in vivo induced homeostatic changes that resulted in a tripling of t he rate of glycogen synthesis via the gluconeogenic pathway;from [2-C- 13] pyruvate in ob/ob mouse liver perfused in the absence of the hormo ne. Consistent with the C-13 NMR results, leptin treatment of the ob/o b mouse in vivo resulted insignificantly increased hepatic glycogen sy nthase activity. Chronic treatment with leptin in vivo exerted the opp osite effect of acute treatment in vitro and markedly decreased hepati c de novo synthesis of fatty acids in ob/ob mouse liver. In agreement with the C-13 NMR findings, activities of hepatic acetyl-CoA carboxyla se and fatty acid synthase were significantly reduced by chronic treat ment of the ob/ob mouse with leptin. Our data represent a demonstratio n of direct effects of leptin in the regulation of metabolism in the i ntact functioning liver.