Regulation of insulin receptor function

Resistance to the biological actions of insulin contributes to the development of type 2 diabetes and risk of cardiovascular disease. A reduced biological response to insulin by tissues results from an impairment in the cascade of phosphorylation events within cells that regulate the activity of enzymes comprising the insulin signaling pathway. In most models of insulin resistance, there is evidence that this decrement in insulin signaling begins with either the activation or substrate kinase activity of the insulin receptor (IR), which is the only component of the pathway that is unique to insulin action. Activation of the IR can be impaired by post-translational modifications of the protein involving serine phosphorylation, or by binding to inhibiting proteins such as PC-1 or members of the SOCS or Grb protein families. The impact of these processes on the conformational changes and phosphorylation events required for full signaling activity, as well as the role of these mechanisms in human disease, is reviewed in this article. Received 3 August 2006; received after revision 1 December 2006; accepted 8 January 2007     

Dynamic insulin secretion from perifused rat pancreatic islets

Insulin secretion from isolated pancreatic islets of 8- to 12-day-old rats was investigated in a dynamic in vitro (perifusion) system. The aims of the study were (i) to describe a carefully controlled in vitro method to study the mechanism of insulin secretion and to analyse the effects and dynamic interactions of bioactive compounds on isolated rat pancreatic islets, (ii) to validate the method by comparing fundamental data on the functions of the islets obtained with this method to those collected with other techniques; and (iii) to find novel features of the control of insulin secretion. The method was carefully designed to maintain the functional capacity of the explanted cells. A functional standardization system was elaborated consisting of (i) analysis of the changes in the basal hormone secretion of the cells; (ii) evaluating responses to a standard, specific stimuli (50 mM glucose for 3 min); (iii) determining the alteration of the momentary size of the hormone pool with responses to KCl; and (iv) direct determination of the total intracellular hormone content from the extract of the column. The technique provides accurate quantitative data on the dynamic responses to biologically active compounds that act directly on the pancreatic islets. The islets maintained their full responsiveness for up to 7 days, and responses as close as in 1-min intervals could be distinguished. A linear dose-response relationship was found on the glucose-induced insulin release in case of 3-min stimulation with 4 and 500 mM of glucose (lin-log graph). Utilizing this method, we showed that no desensitization to glucose-induced insulin release can be observed if the responsiveness of the cells is properly maintained and the parameters of the stimulation are carefully designed. Exposure of the explanted islets to 10 μM acetylcholine or 30 mM arginine (Arg) induced a transitory elevation of insulin release similar in shape to that experienced after glucose stimulation. Norepinephrine (NE), dopamine (DA) and somatostatin (SS) did not induce any detectable alteration on the basal insulin secretion of the islets. However, 100 nM SS given together with 50 mM glucose, 30 mM Arg or 10 μM acetylcholine significantly reduced the insulin-releasing effect of these substances (by 75.5, 71.5 and 72.5%, respectively). At the same time, SS did not alter the insulin response of the islets to 100 mM elevation of K⁺ concentration. SS also inhibited glucose-induced insulin release in a dose-dependent way (ED₅₀ = 22 nM). A similar dose-dependent inhibitory effect on glucose-induced insulin release was found with NE (ED₅₀ = 89 nM) and DA (ED₅₀ = 2.2 μM). γ-Aminobutyric acid (GABA) did not influence insulin release under similar circumstances. Received 16 January 1998; received after revision 6 May 1998; accepted 8 May 1998     

Chronic gestational exposure to ethanol causes insulin and IGF resistance and impairs acetylcholine homeostasis in the brain

In fetal alcohol syndrome (FAS), cerebellar hypoplasia is associated with impaired insulin-stimulated survival signaling. This study characterizes ethanol dose-effects on cerebellar development, expression of genes required for insulin and insulin-like growth factor (IGF) signaling, and the upstream mechanisms and downstream consequences of impaired signaling in relation to acetylcholine (ACh) homeostasis. Pregnant Long Evans rats were fed isocaloric liquid diets containing 0%, 2%, 4.5%, 6.5%, or 9.25% ethanol from gestation day 6. Ethanol caused dose-dependent increases in severity of cerebellar hypoplasia, neuronal loss, proliferation of astrocytes and microglia, and DNA damage. Ethanol also reduced insulin, IGF-I, and IGF-II receptor binding, insulin and IGF-I receptor tyrosine kinase activities, ATP, membrane cholesterol, and choline acetyltransferase (ChAT) expression. In vitro studies linked membrane cholesterol depletion to impaired insulin receptor binding and insulin-stimulated ChAT. In conclusion, cerebellar hypoplasia in FAS is mediated by insulin/IGF resistance with attendant impairments in energy production and ACh homeostasis. Received 4 May 2006; received after revision 13 June 2006; accepted 20 June 2006     

C-peptide fragments stimulate glucose utilization in diabetic rats

Studies of C-peptide cellular effects show that not only the full-length native peptide but also specific C-terminal fragments are biologically active in in vitro systems. In the present study, the effect of five C-peptide fragments and the native peptide on whole-body glucose turnover was studied in streptozotocin diabetic rats using the insulin clamp technique. Insulin was infused intravenously at 18 pmol kg^–1 min^–1 for 90 min and blood glucose concentration was clamped at 8 and 4 mM in diabetic and non-diabetic animals. A steady state was reached during the last 30 min of the study period. Rat C-peptide II and fragments comprising residues 27–31 and 28–31 were effective in augmenting glucose turnover in diabetic rats (+100% to 150%), while no significant effects were seen for segments 1–26, 11–19 and 11–15. The metabolic clearance rate for glucose during infusion of C-peptide or fragments 27–31 and 28–31 in diabetic rats was similar to that seen in non-diabetic animals. We conclude that C-terminal tetra- and pentapeptides, but not fragments from the middle segment of C-peptide, are as effective as the full-length peptide in stimulating whole-body glucose turnover in diabetic rats.Received 18 December 2003; received after revision 19 January 2004; accepted 21 January 2004     

Dysfunctional insulin secretion in type 2 diabetes: role of metabolic abnormalities

Insulin secretion is finely tuned to the requirements of tissues by tight coupling to prevailing blood glucose levels. The normal regulation of insulin secretion is coupled to glucose metabolism in the pancreatic B cell, a major but not exclusive signal for secretion being closure of K⁺ATP (adenosine triphosphate)-dependent channels in the cell membrane through an increase in cytosolic ATP/adenosine diphosphate. Insulin secretion in type 2 diabetes is abnormal in several respects due to genetic causes but also due to the metabolic environment of the pancreatic B cells. This environment may be particularly important for the deterioration of insulin secretion which occurs with increasing duration of diabetes. Factors in the environment with potential importance include overstimulation, a negative effect of hyperglycemia per se (‘glucotoxicity’) as well as adverse effects of elevated fatty acids (‘lipotoxicity’). Elucidating the mechanisms behind these factors as well as their clinical importance will pave the way for treatment which could preserve B-cell function in type 2 diabetic patients. Received 4 October 1999; received after revision 1 November 1999; accepted 3 December 1999     

New insights into the metabolic and molecular basis for diabetic neuropathy

Diabetic polyneuropathy is the most common complication of diabetes mellitus. Several interactive pathogenetic mechanisms have been identified mainly in streptozotocin-induced diabetes in rats and have been ascribed to hyperglycemia. Over the last number of years it is becoming increasingly clear that diabetic neuropathy differs in type 1 and type 2 diabetes in humans and in murine models that more accurately mimic the human disorders. Beside hyperglycemia, attention is increasingly being paid to the pathogenetic roles of insulin and C-peptide deficiencies, particularly in type 1 diabetic neuropathy. There is now evidence to suggest that insulin and C-peptide deficiencies are mainly responsible for perturbations of neurotrophic factors and contribute to oxidative stress in diabetic nerve. This may also be true for apoptotic phenomena afflicting both the peripheral and central nervous systems in diabetes. The new data have lead to re-evaluations of pathogenetic components in this complex disorder, and their further exploration is likely to form a more refined basis for future therapeutic and preventive measures.Received 25 February 2003; received after revision 12 May 2003; accepted 19 May 2003     

Insulin activates Gαil,2 protein in rat hepatoma (HTC) cell membranes

Insulin action is initiated by binding to its cognate receptor, which then triggers multiple cellular responses by activating different signaling pathways. There is evidence that insulin receptor signaling may involve G protein activation in different target cells. We have studied the activation of G proteins in rat hepatoma (HTC) cells. We found that insulin stimulated binding of guanosine 5′-O-(3-thiotriphosphate) (GTP-γ-³⁵S) to plasma membrane proteins of HTC cells, in a dose-dependent manner. This effect was completely blocked by pertussis toxin treatment of the membranes, suggesting the involvement of G proteins of the Gα _i/Gα _o family. The expression of these Gα proteins was checked by Western blotting. Next, we used blocking antibodies to sort out the specific Gα protein activated by insulin stimulation. Anti-Gα _il,2 antibodies completely prevented insulin-stimulated GTP binding, whereas anti-Gα _o,i3 did not modify this effect of insulin on GTP binding. Moreover, we found physical association of the insulin receptor with Gα _i1,2 by copurification studies. These results further support the involvement of a pertussis toxin-sensitive G protein in insulin receptor signaling and provides some evidence of specific association and activation of Gα _i1,2 protein by insulin. These findings suggest that Gα _i1,2 proteins might be involved in insulin action. Received 23 September 1998; received after revision 23 November 1998; accepted 25 November 1998     

Separate functional features of proinsulin C-peptide

Proinsulin C-peptide influences a number of physiological parameters in addition to its well-established role in the parent proinsulin molecule. It is of interest as a candidate for future co-replacement therapy with insulin for patients with diabetes mellitus type 1, but specific receptors have not been identified and additional correlation with functional effects is desirable. Based on comparisons of 22 mammalian proinsulin variants, we have constructed analogues for activity studies, choosing phosphorylation of mitogen-activated protein kinases (MAPKs) in Swiss 3T3 fibroblasts for functional measurements. In this manner, we find that effective phosphorylation of MAPKs is promoted by the presence of conserved glutamic acid residues at positions 3, 11 and 27 of C-peptide and by the presence of helix-promoting residues in the N-terminal segment. Previous findings have ascribed functional roles to the C-terminal pentapeptide segment, and all results combined therefore now show the importance of different segments, suggesting that C-peptide interactions are complex or multiple.Received 2 May 2005; received after revision 9 June 2005; accepted 13 June 2005     

A role for <Emphasis Type="Italic">N</Emphasis>-acetylglucosamine as a nutrient sensor and mediator of insulin resistance

The ability to regulate energy balance at both the cellular and whole body level is an essential process of life. As western society has shifted to a higher caloric diet and more sedentary lifestyle, the incidence of type 2 diabetes (non-insulin-dependent diabetes mellitus) has increased to epidemic proportions. Thus, type 2 diabetes has been described as a disease of 'chronic overnutrition'. There are abundant data to support the relationship between nutrient availability and insulin action. However, there have been multiple hypotheses and debates as to the mechanism by which nutrient availability modulates insulin signaling and how excess nutrients lead to insulin resistance. One well-established pathway for nutrient sensing is the hexosamine biosynthetic pathway (HSP), which produces the acetylated aminosugar nucleotide uridine 5′-diphospho-N-acetylglucosamine (UDP-GlcNAc) as its end product. Since UDP-GlcNAc is the donor substrate for modification of nucleocytoplasmic proteins at serine and threonine residues with N-acetylglucosamine (O-GlcNAc), the possibility of this posttranslational modification serving as the nutrient sensor has been proposed. We have recently directly tested this model in adipocytes by examining the effect of elevated levels of O-GlcNAc on insulin-stimulated glucose uptake. In this review, we summarize the existing work that implicates the HSP and O-GlcNAc modification as nutrient sensors and regulators of insulin signaling. RID="*" ID="*"Corresponding author.     

The effect of cyproheptadine on insulin biosynthesis

Summary The effect of a potent antiserotonin-antithistaminic compound, cyproheptadine (CPH) on insulin biosynthesis was studied in pancreatic islets isolated from CPH-treated rats. Though insulin content of islets was markedly reduced in CPH-treated rats, the incorporation of radiolabeled leucine into proinsulin and insulin fractions was not affected with respect to the rate and amount. It is concluded that CPH may deplete insulin content of the islets through causing the leakage of insulin.     

Arachidonic acid signaling is involved in the mechanism of imidazoline-induced K<Subscript>ATP</Subscript> channel-independent stimulation of insulin secretion

The mechanism by which the novel, pure glucose-dependent insulinotropic, imidazoline derivative BL11282 promotes insulin secretion in pancreatic islets has been investigated. The roles of K_ATP channels, α₂-adrenoreceptors, the I₁-receptor-phosphatidylcholine-specific phospholipase (PC-PLC) pathway and arachidonic acid signaling in BL11282 potentiation of insulin secretion in pancreatic islets were studied. Using SUR1^(-/-) deficient mice, the previous notion that the insulinotropic activity of BL11282 is not related to its interaction with K_ATP channels was confirmed. Insulinotropic activity of BL11282 was not related to its effect on α₂-adrenoreceptors, I₁-imidazoline receptors or PC-PLC. BL11282 significantly increased [³H]arachidonic acid production. This effect was abolished in the presence of the iPLA₂ inhibitor, bromoenol lactone. The data suggest that potentiation of glucose-induced insulin release by BL11282, which is independent of concomitant changes in cytoplasmic free Ca²⁺ concentration, involves release of arachidonic acid by iPLA₂ and its metabolism to epoxyeicosatrienoic acids through the cytochrome P-450 pathway. Received 5 July 2007; received after revision 18 September 2007; accepted 20 September 2007     

Peutz-Jeghers syndrome: clinicopathology and molecular alterations

Peutz-Jeghers syndrome (PJS, OMIM 175200) is an unusual inherited intestinal polyposis syndrome associated with distinct peri-oral blue/black freckling [1–9]. Variable penetrance and clinical heterogeneity make it difficult to determine the exact frequency of PJS [4]. PJS is a cancer predisposition syndrome. Affected individuals are at high risk for intestinal and extra-intestinal cancers. In 1997, linkage studies mapped PJS to chromosome 19p [10, 11], and subsequently a serine/threonine kinase gene defect (LKB1) was noted in a majority of PJS cases [12, 13]. A phenotypically similar syndrome has been produced in an LKB1 mouse knockout model [14–18]. Several PJS kindred without LKB1 mutations have been described, suggesting other PJS loci [19–22]. The management of PJS is complex and evolving. New endoscopic technologies may improve management of intestinal polyposis. Identification of specific genetic mutations and their targets will more accurately assess the clinical course, and help gage the magnitude of cancer risk for affected individuals. Received 20 February 2006; received after revision 5 May 2006; accepted 15 June 2006     

α-Endosulfine, a new entity in the control of insulin secretion

ATP-dependent potassium (K_ATP) channels occupy a key position in the control of insulin release from the pancreatic β cell since they couple cell polarity to metabolism. These channels close when more ATP is produced via glucose metabolism. They are also controlled by sulfonylureas, a class of drugs used in type 2 diabetic patients for triggering insulin secretion from β cells that have lost part of their sensitivity to glucose. We have demonstrated the existence of endogenous counterparts to sulfonylureas which we have called ‘endosulfines.’ In this review, we describe the discovery, isolation, cloning, and biological features of the high-molecular-mass form, α-endosulfine, and discuss its possible role in the physiology of the β cell as well as in pathology. Received 1 February 1999; received after revision 26 March 1999; accepted 26 March 1999     

Endomannosidase processes oligosaccharides of α1-antitrypsin and its naturally occurring genetic variants in the Golgi apparatus

Endomannosidase provides an alternate glucose-trimming pathway in the Golgi apparatus. However, it is unknown if the action of endomannosidase is dependent on the conformation of the substrate. We have investigated the processing by endomannosidase of the α1-antitrypsin oligosaccharides and its disease-causing misfolded Z and Hong Kong variants. Oligosaccharides of wild-type and misfolded α1-antitrypsin expressed in castanospermine-treated hepatocytes or glucosidase II-deficient Phar 2.7 cells were selectively processed by endomannosidase and subsequently converted to complex type oligosaccharides as indicated by Endo H resistance and PNGase F sensitivity. Overexpression of endomannosidase in castanospermine-treated hepatocytes resulted in processing of all oligosaccharides of wild-type and variants of α1-antitrypsin. Thus, endomannosidase does not discriminate the folding state of the substrate and provides a back-up mechanism for completion of N-glycosylation of endoplasmic reticulum-escaped glucosylated glycoproteins. For exported misfolded glycoproteins, this would provide a pathway for the formation of mature oligosaccharides important for their proper trafficking and correct functioning. Received 18 April 2006; received after revision 12 June 2006; accepted 15 June 2006     

Production of butyrylcholinesterase by Caco-2 cells: lack of relationship with triglyceride production

Elevated levels of butyrylcholinesterase activity occur under a number of hypertriglyceridemic conditions, including diabetes and obesity. This study examines whether butyrylcholinesterase activity has a direct effect on triglyceride production, using Caco-2 cells, a human intestinal adenocarcinoma cell line. Caco-2 cells were incubated with 500 μM oleate to stimulate triglyceride production, and butyrylcholinesterase activity was measured in the cellular homogenate. Butyrylcholinesterase activity was approximately 3 × 10^-3 mmol/min per milligram protein. Although triglyceride production increased by almost five-fold after 18 h of stimulation with oleate, butyrylcholinesterase activity was not increased. Furthermore, inhibition of butyrylcholinesterase activity using 1 mM tetraisopropylpyrophosphoramide did not significantly affect triglyceride production or secretion. Human insulin (100 μU/ml) increased the production of butyrylcholinesterase without increasing triglyceride production. This demonstrates that stimulation of fatty acid production and butyrylcholinesterase activity occur by independent mechanisms and suggests that their correlation in hyperlipidemic conditions is not due to a direct relationship in production in situ. Received 23 April 2001; received after revision 25 May 2001; accepted 20 June 2001     

Platelet insulin receptor determination in non-insulin dependent diabetes mellitus

Summary The platelet membrane insulin receptors of healthy and non-insulin dependent (type 2) diabetic patients were studied. Receptor number and affinity proved to be decreased in type 2 diabetes mellitus. The changes in platelet insulin receptor characteristics are in good correlation with the alterations reported in other tissues or cells. The possible role of these phenomena in the pathogenesis of disturbed platelet function in diabetics needs further investigation.     

Proinsulin C-peptide and its analogues induce intracellular Ca2+ increases in human renal tubular cells

Based on the findings that proinsulin C-peptide binds specifically to cell membranes, we investigated the effects of C-peptide and related molecules on the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in human renal tubular cells using the indicator fura-2/AM. The results show that human C-peptide and its C-terminal pentapeptide (positions 27–31, EGSLQ), but not the des (27–31) C-peptide or randomly scrambled C-peptide, elicit a transient increase in [Ca²⁺]_i. Rat C-peptide and rat C-terminal pentapeptide also induce a [Ca²⁺]_i response in human tubular cells, while a human pentapeptide analogue with Ala at position 1 gives no [Ca²⁺]_i response, and those with Ala at positions 2–5 induce responses with different amplitudes. These results define a species cross-reactivity for C-peptide and demonstrate the importance of Glu at position 1 of the pentapeptide. Preincubation of cells with pertussis toxin abolishes the effect on [Ca²⁺]_i by both C-peptide and the pentapeptide. These results are compatible with previous data on C-peptide binding to cells and activation of Na⁺,K⁺ATPase. Combined, all data show that C-peptide is a bioactive peptide and suggest that it elicits changes in [Ca²⁺]_i via G-protein-coupled pathways, giving downstream enzyme effects. Received 13 May 2002; accepted 16 May 2002     

The use of microtiter plates for the simple and sensitive determination of insulin by an ELISA method

Summary A method of insulin determination using a commercially available ELISA kit was modified for use in microtiter plates. The adapted assay, based on the binding of procine anti-guinea pig insulin antibodies to microtiter plates and insulin-peroxidase conjugate as displacer, is sensitive between 0.5 and 30 ng/ml. Since it uses only 10–40 l of sample material it enables the determination of 5–100 pg of insulin. The rapid (5–6 h), automatable, reproducible and reliable assay makes it possible to determine many samples in a short time.     

Translation control of UCP2 synthesis by the upstream open reading frame

Uncoupling protein 2 (UCP2) belongs to a family of transporters of the mitochondrial inner membrane. In vivo low expression of UCP2 contrasts with a high UCP2 mRNA level, and induction of UCP2 expression occurs without change in mRNA level, demonstrating a translational control. The UCP2 mRNA is characterized by a long 5′ untranslated region (5′UTR), in which an upstream open reading frame (uORF) codes for a 36-amino-acid sequence. The 5′UTR and uORF have an inhibitory role in the translation of UCP2. The present study demonstrates that the 3′ region of the uORF is a major determinant for this inhibitory role. In this 3′ region, a single-base substitution that kept the codon sense unchanged significantly modified UCP2 translation, whereas some important amino acid changes had no effect. We discuss our results within the framework of the existing models explaining initiation of translation downstream of a uORF. Received 22 March 2006; received after revision 19 May 2006; accepted 8 June 2006 C. Hurtaud and C. Gelly contributed equally to this work.