Leptin Induces Insulin-like Signaling That Antagonizes cAMP Elevation by Glucagon in Hepatocytes

Abstract

Although many effects of leptin are mediated through the central nervous system, leptin can regulate metabolism through a direct action on peripheral tissues, such as fat and liver. We show here that leptin, at physiological concentrations, acts through an intracellular signaling pathway similar to that activated by insulin in isolated primary rat hepatocytes. This pathway involves stimulation of phosphatidylinositol 3-kinase (PI3K) binding to insulin receptor substrate-1 and insulin receptor substrate-2, activation of PI3K and protein kinase B (AKT), and PI3K-dependent activation of cyclic nucleotide phosphodiesterase 3B, a cAMP-degrading enzyme. One important function of this signaling pathway is to reduce levels of cAMP, because leptin-mediated activation of both protein kinase B and phosphodiesterase 3B is most marked following elevation of cAMP by glucagon, and because leptin suppresses glucagon-induced cAMP elevation in a PI3K-dependent manner. There is little or no expression of the long form leptin receptor in primary rat hepatocytes, and these signaling events are probably mediated through the short forms of the leptin receptor. Thus, leptin, like insulin, induces an intracellular signaling pathway in hepatocytes that culminates in cAMP degradation and an antagonism of the actions of glucagon. Leptin (OB)1 is a 16-kDa protein secreted primarily from adipocytes (1-3). Rodents that are defective in leptin synthesis, the ob/ob mice, or leptin receptor function, the db/db mice, Zucker fa/fa rats, and Koletsky rats, are obese and develop hyperinsulinemia and insulin resistance similar to the metabolic abnormalities associated with type 2 diabetes. Leptin suppresses food intake (4 -7) and increases thermogenesis (8) and metabolic rate (9). These responses appear to be mediated mainly through the central nervous system, because they can be achieved by intracerebroventricular injections of leptin (10, 11). Leptin has also been shown to have a wide repertoire of peripheral effects, some of which are mediated through the central nervous system, and others that are induced through a direct action on target tissues. The latter include direct inhibition of insulin secretion and gene expression in pancreatic ␤-cells, stimulation of fatty acid oxidation in adipocytes, and stimulation of angiogenesis and T-cell proliferation (12-16).Molecular cloning of the leptin receptors (OB-R) has revealed that they are single membrane-spanning receptors with homology to members of the cytokine receptor superfamily (10, 17). The different leptin receptors arise from alternative splicing, and although the extracellular domains of these splice variants are identical, differences are apparent in the intracellular signaling domain. The splice variants containing transmembrane domains can be divided into two groups: one group that has short 32-97 amino acid residue intracellular domains (OB-Ra, -Rc, -Rd, -Rf, and a unique form expressed in hematopoietic cells, termed Rg here) and another group (OB-Rb) that has a long 302-residue intrace...

Adipocyte-selective Reduction of the Leptin Receptors Induced by Antisense RNA Leads to Increased Adiposity, Dyslipidemia, and Insulin Resistance

“…With a specific antibody recognizing several variants of leptin receptor (15), we analyzed OBR expression at the protein level in different tissues of the TKO-OBR mice. Although this antibody has been characterized previously (15), we further demonstrated its specificity by comparing its immunoactivity with that of another leptin receptor antibody recognizing different epitopes (K20, Santa Cruz Biotechnology, CA). The results in Fig.…”

“…The adipocytes were subsequently treated with leptin or insulin for 10 min before being harvested for analysis of leptin signaling. The analysis of PI3-kinase was carried out with a protocol described previously in which we used an anti-phosphotyrosine (pY20) antibody (Transduction Laboratory) to precipitate the PI3-kinase activity (15). The analysis of phospho-STAT3 and phospho-MAPK was based on Western blot analysis with an anti-phospho-STAT3 (pY705) antibody and an anti-phospho-MAPK (pT202/pY204) antibody, respectively (Cell Signaling).…”

“…These studies point to the possibility of a peripheral effect of leptin on body weight regulation. Indeed, many recent studies have suggested that at the cellular level leptin can play important physiological roles in many systems, such as adipocytes, hepatocytes, skeletal muscle, Tlymphocytes, pancreatic ␤-cells, and vascular endothelial cells (11,(15)(16)(17)(18)(19)(20)(21). In this study, we aim to create a genetic model to investigate whether leptin might play a role in regulating the peripheral functions (such as those of adipocytes) in vivo, in addition to its actions in the central nervous system.…”

Effect of high-fat diet feeding on leptin receptor expression in white adipose tissue in rats: depot- and sex-related differential response

“…However, recent studies have suggested that leptin can play important physiological roles in an autocrine/paracrine way, acting in different types of cells such as adipocytes, hepatocytes, T-lymphocytes, pancreatic cells, etc. [9,10,24,28]. In fact, the role of leptin on white adipose tissue (WAT) has been reported to be essential in modulating the adipocytes' metabolic function [25], up-regulating fat oxidation [13] and decreasing lipogenesis [27].…”

Novel mechanisms of the regulation of Protein kinase B in adipocytes; implications for Protein kinase A, Epac, Phosphodiesterases 3 and 4

“…In skeletal muscle, PKB has also been shown to be negatively regulated by cAMP and PKA signalling [21]. In hepatocytes, the ability of insulin and leptin to activate PKB was potentiated by glucagon via increases in cAMP and PKA [22]. Thus, PKB seems to be a target not only for insulin action, but also for multiple cAMP signalling pathways, resulting in an increase or decrease of its kinase activity.…”