Leptin does not affect adipocyte glucose metabolism: studies in fresh and cultured adipocytes

Leptin, the 16-kd hormone produced by white fat cells, regulates energy homeostasis, satiety, and multiple sites in the neuroendocrine system. Leptin receptors have been identified in the central nervous system (CNS) and are widespread in peripheral tissues, including fat. Given the association between insulin resistance and obesity, it is important to establish whether leptin has additional effects on peripheral insulin action and glucose metabolism. This study examined whether leptin has a direct autocrine/paracrine action on glucose metabolism in both freshly isolated and 24-hour cultured rat fat cells. Freshly isolated rat adipocytes were incubated for 30 minutes with 200 ng/mL recombinant murine leptin. Thereafter, basal and insulin-stimulated (10(-8) mol/L) glucose transport, glycolysis-Krebs oxidation and lipogenesis ([6-14C]glucose conversion to [14C]O2 and to [14C]triglyceride), and lipolysis were measured. Upon leptin exposure, no statistical differences were detected in glucose transport or metabolism. Increasing the leptin concentration to 1 to 2 microg/mL or prolonging the duration of preincubation with the fat cells to 60 minutes before the metabolic assays did not alter the results. Finally, using two disparate fat cell culture methods with differing substrate additions (pyruvate and high or low glucose concentrations), there was no effect of 24-hour exposure to leptin (200 ng/mL) on basal and insulin-stimulated glucose transport or lipogenesis. We conclude that leptin does not modulate basal or insulin-stimulated glucose metabolism in isolated and cultured fat cells in vitro. However, in vivo, higher pericellular leptin concentrations, as well as other cellular or soluble serum factors, may exist that might lead to a physiologically relevant autocrine action of leptin.