ISSN:0730-2312    

DOI:10.1002/jcb.240480102

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractIn human diabetes, inherent impaired insulin secretion can be exacerbated by desensitization of the β cell by chronic hyperglycemia. Interest in this phenomenon has generated extensive studies in genetic or experimentally induced diabetes in animals and in fully in vitro systems, with often conflicting results. In general, although chronic glucose causes decreased β‐cell response to this carbohydrate, basal response and response to alternate stimulating agents are enhanced. Glucose‐stimulated insulin synthesis can be increased or decreased depending on the system studied. Using a two‐compartment β‐cell model of phasic insulin secretion, a unifying hypothesis is described which can explain some of the apparent conflicting data. This hypothesis suggests that glucose‐desensitization is caused by an impairment in stimulation of a hypothetical potentiator singularly responsible for: (1) some of the characteristic phases of insulin secretion; (2) basal release; (3) potentiation of non‐glucose stimulators; and (4) apparent “recovery” from desensitization. Review of some of the pathways that regulate insulin secretion suggest that phosphoinositol metabolism and protein kinase‐C production are regulated similarly to the theoretical potentiator and their impairment is a major contributor to glucose desensiti

ISSN:0730-2312    

DOI:10.1002/jcb.240480103

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractIn the last few years several potential substrates of the insulin receptor tyrosine kinase have been identified, purified, and their cDNAs isolated. These putative substrates include: (1) pp15, a fatty acid‐binding protein; (2) pp120, a plasma membrane ecto‐ATPase; (3) pp42, a MAP serine/threonine kinase; (4) pp85, a subunit of the Type 1 phosphatidylinositol kinase; and (5) pp185, a phosphatidylinositol kinase binding protein. Although the tyrosine phosphorylation of several of these substrates correlates with the signalling capabilities of various mutant receptors, the role of these substrates in mediating any one of insulin's many biological responses is still unknown. In addition, recent data indicate that the tyrosine phosphorylation of pp42 may in fact be due to autophosphorylation, thereby removing it from the list of putative substrates of the insulin receptor kinase. Finally, the present review discusses the question of whether signalling occurs as a result of the tyrosine phosphorylation of substrates or via the formation of signalling comple

ISSN:0730-2312    

DOI:10.1002/jcb.240480104

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractInsulin receptors are disulfide‐linked oligotetramers composed of two heterodimers each containing a 130‐kDa α subunit and a 90‐kDa β subunit. Insulin binds to the extracellular α subunit, and in the process stimulates the autophosphorylation of the β subunit and the expression of tyrosine kinase activity. Studies combining the use of photoaffinity labeling and immunoprecipitation with anti‐peptide antibody have directly demonstrated that the cysteine‐rich domain, encoded by exon 3, in the α subunit is part of the insulin‐binding site of the receptor. Experiments with chimeric insulin receptors and chimeric insulin‐like growth factor I receptors have confirmed that the cysteine‐rich domain constitutes a part of the insulin‐binding site. In addition, results from these experiments suggest that the N‐terminal sequence, encoded by exon 2, in the α subunit also participates in insulin binding. In this review it is proposed that, assuming two insulin‐binding sites per each holoreceptor oligotetramer, each insulin‐binding domain may contain respectively two sub‐domains for hydrophobic and charge contact with insulin, and that high‐affinity binding would require the interaction of both subunits with the possibility of each subunit reciprocally co

ISSN:0730-2312    

DOI:10.1002/jcb.240480105

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractAlthough there is general agreement that insulin receptor tyrosine kinase activity mediates many of the actions of insulin, two types of studies suggest that non‐tyrosine kinase dependent pathways may also exist. First, both monoclonal and polyclonal antibodies to the receptor have been shown to mediate many of insulin's actions with little or no stimulation of receptor kinase. Second, insulin receptor mutants, with reduced or no tyrosine kinase activity, have been shown to mediate several actions of insulin. Non‐tyrosine kinase pathways that could signal insulin effects through the insulin receptor include non‐covalent activation of G proteins, phospholipase Cs, or docking proteins such as IRS‐1. Further studies on the chemical structures of phospholipids and their hydrolysis products involved in insulin action will be required to sort out the underlying mechanisms of insulin action via non‐tyrosine kinase dependent

ISSN:0730-2312    

DOI:10.1002/jcb.240480106

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractProtein‐tyrosine phosphatases (PTPases) play an important role in the regulation of insulin action by dephosphorylating the active (autophosphorylated) form of the insulin receptor and attenuating its tyrosine kinase activity. PTPases can also modulate post‐receptor signalling by catalyzing the dephosphorylation of cellular substrates of the insulin receptor kinase. Dramatic advances have recently been made in our understanding of PTPases as an extensive family of transmembrane and intracellular proteins that are involved in a number of pathways of cellular signal transduction. Identification of the PTPase(s) which act on various components of the insulin action cascade will not only enhance our understanding of insulin signalling but will also clarify the potential involvement of PTPases in the pathophysiology of insulin‐resistant disease states. This brief review provides a summary of reversible tyrosine phosphorlyation events in insulin action and available data on candidate PTPases in liver and skeletal muscle that may be involved in the regulation of insulin a

ISSN:0730-2312    

DOI:10.1002/jcb.240480107

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractClassical insulin and IGF‐1 receptors are α2β2heterotetrameric complexes synthesized from two identical αβ half‐receptor precursors [1,2]. Recent data strongly suggests, however, that nonidentical αβ half‐receptor precursors can assemble to generate hybrid holoreceptor species both in vivo and in vitro [3–6,41]. This review focuses primarily on two types of hybrid receptors. The first type is an insulin/IGF‐1 hybrid receptor generated by the association of an αβ insulin half‐receptor with an αβ IGF‐1 half‐receptor. The second type is one formed from a wildtype (kinase‐active) insulin or IGF‐1 αβ half‐receptor and a mutant (kinase‐inactive) insulin αβ half‐receptor. Although the functional properties of insulin/IGF‐1 hybrid receptors have not yet been completely defined, wildtype/mutant hybrid receptors are essentially substrate kinase inactive [6]. These data indicate that the mutant αβ half‐receptor exerts a transdominant inhibition upon the wildtype αβ half‐receptor within the α2β2holoreceptor complex. This defect in substrate kinase activity may contribute to the molecular defect underlying some syndromes of severe insulin resistance and diabetes. Heterozygous individuals expressing both wildtype and mutant tyrosine kinase‐defective insulin receptor precursors demonstrate varying degrees of insulin resistance and diabetes [7–11]. In addition, cell lines which express both endogenous wildtype and transfected kinase‐defective insulin receptors display markedly decreased insulin and IGF‐1 sensitivity and responsiveness [12–14]. Formation of hybrid receptors which results in premature termination of insulin signal transduction may be one mechanism underlying the observation that kinas

ISSN:0730-2312    

DOI:10.1002/jcb.240480108

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractGlucose transport into muscle cells occurs through facilitated diffusion mediated primarily by the GLUT1 and GLUT4 glucose transporters. These transporter proteins are controlled by acute and chronic exposure to insulin, glucose, muscle contraction, and hypoxia. We propose that acute responses occur through recruitment of pre‐formed glucose transporters from an intracellular storage site to the plasma membrane. In contrast, chronic control is achieved by changes in transporter biosynthesis and protein stability. Using subcellular fractionation of rat skeletal muscle, recruitment of GLUT4 glucose transporters to the plasma membrane is demonstrated by acute exposure to insulin in vivo. The intracellular pool appears to arise from a unique organelle depleted of transverse tubule, plasma membrane, or sarcoplasmic reticulum markers. In diabetic rats, GLUT4 content in the plasma membranes and in the intracellular pool is reduced, and incomplete insulin‐dependent GLUT4 recruitment is observed, possibly through a defective incorporation of transporters to the plasma membrane. The lower content of GLUT4 transporters in the muscle plasma membranes is reversed by restoration of normoglycemia with phlorizin treatment.In some muscle cells in culture, GLUT1 is the only transporter expressed yet they respond to insulin, suggesting that this transporter can also be regulated by acute mechanisms. In the L6 muscle cell line, GLUT1 transporter content diminishes during myogenesis and GLUT4 appears after cell fusion, reaching a molar ratio of about 1:1 in the plasma membrane. Prolonged exposure to high glucose diminishes the amount of GLUT1 protein in the plasma membrane by both endocytosis and reduced biosynthesis, and lowers GLUT4 protein content in the absence of changes in GLUT4 mRNA possibly through increased protein degradation. These studies suggest that the relative contribution of each transporter to transport activity, and the mechanisms by which glucose exerts control of the glucose transporters, will be key subjects of future investigati

ISSN:0730-2312    

DOI:10.1002/jcb.240480109

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractEarly postnatal mouse dorsal root ganglion neurons were found to express several glycosylphosphatidylinositol‐anchored (GPI) molecules from the immunoglobulin superfamily (neural cell adhesion molecule 120 kD isoform, F3, Thy1) whose expression is developmentally regulated. A hybrid cell line (ND26), made by fusing postmitotic rat dorsal root ganglion (DRG) neurons with the mouse neuroblastoma N18Tg2, could be induced to differentiate by manipulating the composition of the culture medium and expressed similar GPI molecules to DRG neurons. We used this model system to investigate the metabolism of GPI‐anchored molecules. We found that neural cell adhesion molecule 120 Kd isoform expression decreased upon differentiation, whereas the level of F3 and Thy1 increased, suggesting a role in neurite outgrowth processes.The ratio of molecules cleavable by exogenous phosphatidylinositol phospholipase C (PI‐PLC) was similar for all the GPI‐anchored molecules, which could mean that cell‐specific modifications of the basic anchoring structure determine the level of potentially releasable molecules. Measurements of spontaneous release indicated that this reflected the overall level of expression of these molecules by the ND26 cell line.Finally, we observed an effect of dibutyryl cAMP on the level of expression of F3 and Thy1 but not of N‐CAM. However, we could not detect any significant effect of nerve growth factor (NGF) either on the level of expression or on the amount of spontaneously release

ISSN:0730-2312    

DOI:10.1002/jcb.240480110

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractIsolated and cultured rat liver sinusoidal endothelial cells (LECs) retain the ability to specifically bind125I‐hyaluronan (HA) internalize it using a coated pit pathway [Biochem J, 257:875–884, 1989]. Here we have determined the effect of Ca+2on the binding and endocytosis of HA by LECs.125I‐HA binding to intact LECs at 4°C occurred both in the absence (10 mM EGTA) or the presence of physiologic concentrations of Ca+2(1.8 mM). However, the specific binding of125I‐HA to LECs increased linearly with increasing Ca+2concentrations. After permeabilization with the nonionic detergent digitonin, the Ca+2‐independent HA binding activity increased ∼743%, while the Ca+2‐dependent binding activity was enhanced only ∼46%. Therefore, the Ca+2‐dependent HA binding activity appears not to be intracellular, whereas the Ca+2‐independent HA receptor is found both inside LECs and on the cell surface. When LECs were allowed to endocytose125I‐HA at 37°C in 10 mM EGT A or in 1.8 mM Ca+2, no differences were seen in the extent or rate of endocytosis. When LECs were allowed to endocytose125I‐HA in the presence of 10mM Ca125, the amount of cell‐associated radiocctivity increased appoximately 20–50‐fold. However, this additional cell‐associated125I‐HA was not sensitive to hyperosmolarity and was removed by washing the cells in 10mM EGTA at 4°C. Therefore, the Ca+2‐dependent cell‐associated125I‐HA had accumulated on the cell surface and had not been internalized. From these studies we conclude the LECs have at least two trypes of specfic HA binding sites. One, the previously characterized HA receptor, is Ca+2‐indepoendent, localized both extracellularly and intracellular, and medicates he efficient binding and subsequent endocytosis of HA using a coated pit pathway. The other newly recognized HA binding activity is Ca+2‐dependent, localized wxtracellularly, and is not res

ISSN:0730-2312    

DOI:10.1002/jcb.240480111

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY