Leptin revisited: its mechanism of action and potential for treating diabetes

Abstract

Since the discovery of leptin in 1994, we now have a better understanding of the cellular and molecular mechanisms underlying its biological effects. In addition to its established anti-obesity effects, leptin exerts antidiabetic actions that are independent of its regulation of body weight and food intake. In particular, leptin can correct diabetes in animal models of type 1 and type 2 diabetes. In addition, long-term leptin replacement therapy improves glycaemic control, insulin sensitivity and plasma triglycerides in patients with severe insulin resistance due to lipodystrophy. These results have spurred enthusiasm for the use of leptin therapy to treat diabetes. Here, we review the current understanding of the glucoregulatory functions of leptin, emphasizing its central mechanisms of action and lessons learned from clinical studies, and discuss possible therapeutic applications of leptin in the treatment of type 1 and type 2 diabetes.

Figure 1. Neuronal Leptin Receptor Activation and Inactivation

A. In the resting state, the long form leptin receptor (LepRb) exists as a homodimer at the plasma membrane. The tyrosine kinases Janus Kinase (JAK2) and Src-Family-Kinases (SFKs), and the JAK2-binding protein SH2B, are constitutively associated with membrane-proximal regions of LepRb^,. B. Upon ligand (leptin) binding to LepRb (1:1 stoichiometry), the two receptor subunits undergo a conformational change resulting in transphosphorylation and transactivation of JAK2 and SFKs proteins^,. SH2B enhances JAK2 enzymatic activity. Activated JAK2 and possibly SFK enzymes, then phosphorylate LepRb tyrosine (Tyr) residues. Three cytoplasmic Tyr residues of the murine LepRb acts as binding sites for SH2-domain containing proteins. Specifically, SHP2 principally binds to pY985, STAT5 to pY1077 and STAT3 to pY1338^,. LepRb-bound SHP2, STAT3 and STAT5 proteins then become phosphorylated by JAK2 and SFKs. Additional downstream signalling proteins and pathways include PI3K-Akt; GRB2-Erk1/2^,,; p90RSK; p70RSK and ribosomal S6 proteins; and the forkhead box-containing protein O1 (FoxO1) ^,. These enzymes and pathways ultimately exert regulation of cellular processes, including transcriptional control (socs3^,, pomc^,, cFos, carboxypeptidase E (cpe), as well as agrp and npy ), translational control^–, and neuronal activity and firing^,. C. LepRb is inactivated at proximal steps by PTP1B/PTPN1, a tyrosine phosphatase that directly dephosphorylates JAK2. Furthermore, SOCS3 acts to inhibit JAK2 activity by binding directly to JAK2 or indirectly by first binding to Tyr985 or Tyr 1077 of LepRb^,,. Finally, yet unidentified protein tyrosine phosphatases are predicted to directly dephosphorylate LepRb and SFKs.

Figure 2. Cellular Mechanisms Causing CNS Leptin Resistance in High-Fat-Diet (HFD)-Induced Obesity in Rodents

A. Leptin normally inhibits fat accumulation and body weight gain by entering the brain to decrease caloric intake and increase energy expenditure. In rodents, consumption of a HFD causes hyperleptinemia, hypothalamic endoplasmic reticulum (ER) stress and production of pro-inflammatory cytokines, ultimately leading to neuronal leptin resistance and diminished anti-obesity actions of leptin. HFD-induced leptin resistance causes obesity by a defect in leptin transport across the blood-brain barrier (BBB), leptin receptor signaling in LepRb-expressing neurons and/or in down-stream circuits. B. Leptin activated pSTAT3 immunohistochemistry is reduced in the hypothalamic arcuate nucleus (ARH) of HFD mice for 10 weeks. pSTAT3 is a functional marker for LepRb signalling in LepRb-expressing (1^st order) neurons. 3v= 3^rd ventricle; ME=median eminence. C. Leptin normally enters most parts of the brain and reaches its target neurons via transport across the blood-brain-barrier (BBB). The ARH however is in close anatomical proximity to the median eminence (ME), a circumventricular organ (CVO) with fenestrated capillaries (red circles), suggesting that leptin may reach its 1^st-order LepRb-expressing neurons within the ARH without actively being transported across the BBB^,. The ARH 1^st-order neurons include the POMC and AgRP neurons (lower inserts). D. The HFD-induced hyperleptinemia, ER stress and/or inflammation causes leptin resistance within POMC and AgRP neurons^,. Because activation of pSTAT3 is diminished in these neurons^,, by exclusion, the signalling defect must be located at a step upstream of STAT3 phosphorylation. HFD mice have increased ARH expression of negative regulators of proximal LepRb-signaling in HFD mice, namely PTP1B, SOCS3 and TC-PTP^,. Additional possibilities that may explain neuronal leptin resistance are HFD-induced inhibition of LepRb surface expression or increased expression of yet-to-be-identified LepRb-PTPases.

Figure 3. Key Milestones in the Discovery of Leptin’s Anti-Diabetic Actions

Figure 4. CNS Neuronal Mediators, Efferent Pathways, and Peripheral Mechanisms Underlying the Anti-Diabetic Actions of Leptin

A. Schematic model of central neuronal pathways and efferent processes whereby leptin exerts its anti-diabetic actions in insulin-resistance and obese mice (e.g. genetically modified db/db animals). ARH POMC-expressing neurons are currently the best candidates responsible for mediating the improved glucose control by leptin in the db/db Type 2 diabetic rodent models, although AgRP-expressing neurons and other hypothalamic and extra-hypothalamic neurons are also likely to play important roles. These neurons act via axonal projections on downstream neurocircuits (e.g. the melanocortin system) to engage efferent pathways. This may include regulation of sympathetic (SNS) and parasympathetic (PNS) branches of the autonomic nervous system, ultimately affecting muscle glucose uptake, pancreatic glucagon production and/or hepatic glucose production. B. ARH leptin-target neurons, including POMC neurons, produce and secrete a number of different molecules from axon terminals, including the POMC-polypeptide-derived neuropeptides; alpha-melanocortin stimulating hormone (α-MSH) and β-Endorphin. Cocaine and amphetamine-regulated transcript (Cart) and Nesfatin-1 is also co-expressed with POMC peptides. Finally, these ARH neurons are heterogeneous with regard to neurotransmitter phenotype; different subpopulations produce glutamate, GABA and acetylcholine (ACh). Combined, these neurotransmitters and neuropeptides are the candidate effector-molecules to mediate glycemic control by leptin in the db/db model.