The insulin receptor substrate 2 (Irs2) branch of the insulin/insulin-like growth factor-signaling cascade prevents diabetes in mice because it promotes  cell replication, function, and survival, especially during metabolic stress. Because exendin-4 (Ex4), a long acting glucagon-like peptide 1 receptor agonist, has similar effects upon  cells in rodents and humans, we investigated whether Irs2 signaling was required for Ex4 action in isolated  cells and in Irs2 ؊/؊ mice. Ex4 increased cAMP levels in human islets and Min6 cells, which promoted Irs2 expression and stimulated Akt phosphorylation. In wild type mice Ex4 administered continuously for 28 days increased  cell mass 2-fold. By contrast, Ex4 failed to arrest the progressive  cell loss in Irs2 ؊/؊ mice, which culminated in fatal diabetes; however, Ex4 delayed the progression of diabetes by 3 weeks by promoting insulin secretion from the remaining islets. We conclude that some short term therapeutic effects of glucagon-like peptide 1 receptor agonists can be independent of Irs2, but its long term effects upon  cell growth and survival are mediated by the Irs2 branch of the insulin/insulin-like growth factor signaling cascade.Diabetes mellitus is a complex disorder that arises from various causes, including dysregulated glucose sensing and impaired insulin secretion (maturity-onset diabetes of youth, MODY), autoimmune-mediated  cell destruction (type 1), or insufficient compensation for peripheral insulin resistance (type 2) (1). Type 2 diabetes is the most prevalent form. It usually occurs at middle age and afflicts more than 30 million people over the age of 65 but is appearing with greater frequency in children and adolescents (2). Dysregulated insulin signaling exacerbated by chronic hyperglycemia promotes a cohort of systemic disorders, including dyslipidemia, hypertension, cardiovascular disease, and female infertility (3, 4). The search for strategies to promote  cell function and regeneration has lead to the discovery that glucagon-like peptide-1 (GLP1) 2 receptor agonists increase insulin biosynthesis and secretion from  cells, inhibit glucagon secretion from ␣-cells, and promote peripheral insulin sensitivity and satiety in type 2 diabetics (5-9). During a meal, GLP1 is secreted into the circulation from L cells located in the intestine (10); however, GLP1 is quickly inactivated by circulating dipeptidyl-peptidase IV, which diminishing its usefulness as an injectable therapeutic. Compounds that inhibit dipeptidyl-peptidase IV or GLP1 homologs like exendin-4 (Ex4) that are not degraded by dipeptidyl-peptidase IV display improved therapeutic efficacy (11-16). Administration of Ex4 to rodents or humans with type 2 diabetes increases first-phase insulin secretion and increases  cell mass, which can compensate for peripheral insulin resistance (8,9,17,18). Recently, a synthetic Ex4 called Exenatide (Byetta, Amylin/Lily) has gained Food and Drug Administration approval as an injectable treatment for type 2 diabetes (15). Because Exenatide is the...
Leptin signals the status of body energy stores via the leptin receptor (LR), a member of the Type I cytokine receptor family. Type I cytokine receptors mediate intracellular signaling via the activation of associated Jak family tyrosine kinases. Although their COOH-terminal sequences vary, alternatively spliced LR isoforms (LRaLRd) share common NH 2 -terminal sequences, including the first 29 intracellular amino acids. The so-called long form LR (LRb) activates Jak-dependent signaling and is required for the physiologic actions of leptin. In this study, we have analyzed Jak activation by intracellular LR sequences under the control of the extracellular erythropoeitin (Epo) (Epo receptor/LRb chimeras). We show that Jak2 is the requisite Jak kinase for signaling by the LRb intracellular domain and confirm the requirement for the Box 1 motif for Jak2 activation. A minimal LRb intracellular domain for Jak2 activation includes intracellular amino acids 31-48. Although the sequence requirements for intracellular amino acids 37-48 are flexible, intracellular amino acids 31-36 of LRb play a critical role in Jak2 activation and contain a loose homology motif found in other Jak2-activating cytokine receptors. The failure of short form sequences to function in Jak2 activation reflects the absence of this motif.Leptin is a 16-kDa adipocyte-derived hormone that communicates the status of body energy stores to the central nervous system, regulating appetite, metabolic rate, and neuroendocrine function (1, 2). Leptin mediates these effects by binding and activating a cell surface leptin receptor (LR) 1 ; the structure of leptin is homologous to that of the IL-6 family of cytokines, and the LR is a member of the IL-6 receptor family of class I cytokine receptors (3). Alternative splicing of RNA from a single LR gene produces multiple LR isoforms that share a common ligand-binding extracellular domain (4, 5). LRe lacks a transmembrane domain and is secreted. LRa-d each contain the same transmembrane domain and 29 membrane-proximal amino acids including the highly conserved, proline-rich Box 1 sequence that is required for Jak kinase activation by cytokine receptors. The number and identity of the subsequent amino acids varies among murine LRa-d, as well as the three human LR isoforms. LRb, which is highly conserved across species, contains a 282-amino acid extension (total 301-amino acid intracellular tail), robustly activates intracellular signaling, and is required to mediate the physiologic actions of leptin. Murine LRa, LRc, and LRd are the "short forms" of the leptin receptor with unclear physiological roles; these receptors contain 5, 3, and 11 amino acid extensions for 34-, 32-, and 40-amino acid intracellular tails, respectively.
Leptin is an adipocyte-derived hormone that communicates the status of body energy stores to the brain to regulate feeding and energy balance. The inability of elevated leptin levels to adequately suppress feeding in obesity suggests attenuation of leptin action under these conditions; the activation of feedback circuits due to high leptin levels could contribute to this leptin resistance. Using cultured cells exogenously expressing the long form of the leptin receptor (LRb) or an erythropoietin receptor/LRb chimera, we show that chronic stimulation results in the attenuation of LRb signaling and the establishment of a state in which the receptor is refractory to reactivation. Mutation of LRb Tyr1138 (the site that recruits signal transducer and activator of transcription 3) alleviated this feedback inhibition, suggesting that signal transducer and activator of transcription 3 mediates the induction of a feedback inhibitor, such as suppressor of cytokine signaling 3 (SOCS3), during chronic LRb stimulation. Indeed, manipulation of the expression or activity of the LRb-binding tyrosine phosphatase, SH2-domain containing phosphatase-2, by overexpression of wild-type and dominant negative isoforms or RNA interference-mediated knockdown did not alter the attenuation of LRb signals. In contrast, SOCS3 overexpression repressed LRb signaling, whereas RNA interference-mediated knockdown of SOCS3 resulted in increased LRb signaling that was not attenuated during chronic ligand stimulation. These data suggest that Tyr1138 of LRb and SOCS3 represent major effector pathways for the feedback inhibition of LRb signaling. Furthermore, we show that mice expressing an LRb isoform mutant for Tyr1138 display increased activity of the leptin-dependent growth and immune axes, suggesting that Tyr1138-mediated feedback inhibition may regulate leptin sensitivity in vivo.
Jak family tyrosine kinases mediate signaling by cytokine receptors to regulate diverse biological processes. Although Jak2 and other Jak kinase family members are phosphorylated on numerous sites during cytokine signaling, the identity and function of most of these sites remains unknown. Using tandem mass spectroscopic analysis of activated Jak2 protein from intact cells, we identified Tyr 221 and Tyr 570 as novel sites of Jak2 phosphorylation. Phosphorylation of both sites was stimulated by cytokine treatment of cultured cells, and this stimulation required Jak2 kinase activity. While we observed no gross alteration of signaling upon mutation of Tyr 221 , Tyr 570 lies within the inhibitory JH2 domain of Jak2, and mutation of this site (Jak2 Y570F ) results in constitutive Jak2-dependent signaling in the absence of cytokine stimulation and enhances and prolongs Jak2 activation during cytokine stimulation. Mutation of Tyr 570 does not alter the ability of SOCS3 to bind or inhibit Jak2, however. Thus, the phosphorylation of Tyr 570 in vivo inhibits Jak2-dependent signaling independently of SOCS3-mediated inhibition. This Tyr 570 -dependent mechanism of Jak2 inhibition likely represents an important mechanism by which cytokine function is regulated.Type I cytokines mediate a plethora of physiologic processes, ranging from hematopoietic and immune functions (such as those mediated by erythropoietin [EPO] and the interleukins [ILs]) to growth and neuroendocrine responses (such as those mediated by growth hormone and leptin) (12,14,16,23). These actions are mediated by the activation of cytokine receptor proteins found on the surface of target cells. Cytokine receptors each contain an extracellular domain that recognizes its specific cytokine ligand, a single transmembrane domain, and an intracellular domain that, although devoid of enzymatic activity, transmits intracellular signals by means of an associated Jak family tyrosine kinase. Ligand binding activates the associated intracellular Jak kinase, resulting in the tyrosine phosphorylation of the Jak kinase and the intracellular domain of the cytokine receptor. These tyrosine phosphorylation events mediate the recruitment of downstream signaling molecules that contain phosphotyrosine-binding SH2 domains (such as STAT proteins) (12, 16); tyrosine phosphorylation may also mediate other regulatory events during cytokine signaling (8, 31).The Jak kinase family contains four members: Jak1 to Jak3 and Tyk2 (12, 16). Of these, Jak1, Jak2, and Tyk2 are ubiquitously expressed, while Jak3 is found predominantly in immune and hematopoietic tissues. Jak kinases are composed of four conserved domains. The NH 2 -terminal FERM domain is required for interaction with cytokine receptors (24, 30), while the adjacent SH2-like fold has no known function. The COOH-terminal portion of Jak kinases contains a kinase-like JH2 domain that is devoid of enzymatic activity but that inhibits the activity of the COOH-terminal JH1 tyrosine kinase domain (10,19,22,28,29).Our laboratory studies s...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2025 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.