Insulin does not mediate glucose stimulation of proinsulin biosynthesis

Molecular mechanism of the anti-diabetic activity of an identified oligosaccharide from Rosa canina

Background and purpose

Because of the high prevalence, diabetes is considered a global health threat. Hence, the need for effective, cheap, and comfortable therapies are highly felt. In previous study, a novel oligosaccharide with strong anti-diabetic activity in the crude extract of Rosa canina fruits, from the rosacea family, was identified. The present study was designed to ensure its efficacy using in vivo and in vitro studies.

Experimental approach

Crude extract and its purified oligosaccharide were prepared from corresponding herb. Adult male Wistar rats were randomly divided into four groups of 10 each, as follows: group 1, healthy control rats given only sterile normal saline; group 2, diabetic control rats received sterile normal saline; group 3, diabetic rats treated with crude extract of Rosa canina (40% w/v) by oral gavage for 8 weeks; group 4, diabetic rats treated with purified oligosaccharide of Rosa canina (2 mg/kg) by oral gavage for 8 weeks. After treatment, body weight, fasting blood glucose, serum insulin levels and islet beta-cell repair and proliferation were investigated. The possible cytoprotective action of oligosaccharide was evaluated in vitro. The effect of oligosaccharide on apoptosis and insulin secretion in cell culture media were examined. Real-time PCR was used to determine the expression level of some glucose metabolism-related regulator genes.

Findings / Results

In the animal model of diabetes, the insulin levels were increased significantly due to the regeneration of beta-cells in the islands of langerhans by the purified oligosaccharide. In vitro cell apoptosis examination showed that high concentration of oligosaccharide increased cell death, while at low concentration protected cells from streptozotocin-induced apoptosis. Molecular study showed that the expression of Ins1 and Pdx1 insulin production genes were increased, leading to increased expression of insulin-dependent genes such as Gck and Ptp1b. On the other hand, the expression of the Slc2a2 gene, which is related to the glucose transporter 2, was significantly reduced due to insulin concentrations.

Conclusion and implications

The purified oligosaccharide from Rosa canina was a reliable anti-diabetic agent, which acted by increasing insulin production in beta-cells of the islands of Langerhans.

The dynamic plasticity of insulin production in β-cells

BACKGROUND

Although the insulin-producing pancreatic β-cells are quite capable of adapting to both acute and chronic changes in metabolic demand, persistently high demand for insulin will ultimately lead to their progressive dysfunction and eventual loss. Recent and historical studies highlight the importance of 'resting' the β-cell as a means of preserving functional β-cell mass.

SCOPE OF REVIEW

We provide experimental evidence to highlight the remarkable plasticity for insulin production and secretion by the pancreatic β-cell alongside some clinical evidence that supports leveraging this unique ability to preserve β-cell function.

MAJOR CONCLUSIONS

Treatment strategies for type 2 diabetes mellitus (T2DM) targeted towards reducing the systemic metabolic burden, rather than demanding greater insulin production from an already beleaguered β-cell, should be emphasized to maintain endogenous insulin secretory function and delay the progression of T2DM.

The Pancreatic β-Cell: A Bioenergetic Perspective

The pancreatic β-cell secretes insulin in response to elevated plasma glucose. This review applies an external bioenergetic critique to the central processes of glucose-stimulated insulin secretion, including glycolytic and mitochondrial metabolism, the cytosolic adenine nucleotide pool, and its interaction with plasma membrane ion channels. The control mechanisms responsible for the unique responsiveness of the cell to glucose availability are discussed from bioenergetic and metabolic control standpoints. The concept of coupling factor facilitation of secretion is critiqued, and an attempt is made to unravel the bioenergetic basis of the oscillatory mechanisms controlling secretion. The need to consider the physiological constraints operating in the intact cell is emphasized throughout. The aim is to provide a coherent pathway through an extensive, complex, and sometimes bewildering literature, particularly for those unfamiliar with the field.

Direct autocrine action of insulin on β-cells: does it make physiological sense?

In recent years there has been a growing interest in the possibility of a direct autocrine effect of insulin on the pancreatic β-cell. Indeed, there have been numerous intriguing articles and several eloquent reviews written on the subject (1-3); however, the concept is still controversial. Although many in vitro experiments, a few transgenic mouse studies, and some human investigations would be supportive of the notion, there exist different insights, other studies, and circumstantial evidence that question the concept. Therefore, the idea of autocrine action of insulin remains a conundrum. Here we outline a series of thoughts, insights, and alternative interpretations of the available experimental evidence. We ask, how convincing are these, and what are the confusing issues? We agree that there is a clear contribution of certain downstream elements in the insulin signaling pathway for β-cell function and survival, but the question of whether insulin itself is actually the physiologically relevant ligand that triggers this signal transduction remains unsettled.

The molecular mechanisms of pancreatic β-cell glucotoxicity: recent findings and future research directions

It is well established that regular physiological stimulation by glucose plays a crucial role in the maintenance of the β-cell differentiated phenotype. In contrast, prolonged or repeated exposure to elevated glucose concentrations both in vitro and in vivo exerts deleterious or toxic effects on the β-cell phenotype, a concept termed as glucotoxicity. Evidence indicates that the latter may greatly contribute to the pathogenesis of type 2 diabetes. Through the activation of several mechanisms and signaling pathways, high glucose levels exert deleterious effects on β-cell function and survival and thereby, lead to the worsening of the disease over time. While the role of high glucose-induced β-cell overstimulation, oxidative stress, excessive Unfolded Protein Response (UPR) activation, and loss of differentiation in the alteration of the β-cell phenotype is well ascertained, at least in vitro and in animal models of type 2 diabetes, the role of other mechanisms such as inflammation, O-GlcNacylation, PKC activation, and amyloidogenesis requires further confirmation. On the other hand, protein glycation is an emerging mechanism that may play an important role in the glucotoxic deterioration of the β-cell phenotype. Finally, our recent evidence suggests that hypoxia may also be a new mechanism of β-cell glucotoxicity. Deciphering these molecular mechanisms of β-cell glucotoxicity is a mandatory first step toward the development of therapeutic strategies to protect β-cells and improve the functional β-cell mass in type 2 diabetes.

beta-Cells at the crossroads: choosing between insulin granule production and proliferation

Pancreatic beta-cells are the sole source of insulin, the major hormonal regulator of glycaemia. In physiological and pathological conditions with increased insulin demand, beta-cells adjust their insulin output either through increased insulin secretory granule (ISG) biogenesis and secretion, or hyperplasia. Failure of these compensatory mechanisms eventually results in hyperglycaemia and diabetes mellitus. Both of these major adaptive behaviours are positively regulated by several extrinsic factors, such as glucose, GLP-1, insulin and growth hormones (GH). Still unclear, however, it is how beta-cells in response to these stimuli opt for one or the other strategy at a given time. Here we review recent advances concerning the factors and pathways that enhance ISG biogenesis and beta-cell replication, and propose the existence of 'switch factors' that play a key role in regulating the shift between these two adaptive responses.

Insulin signaling in the pancreatic beta-cell

The appropriate function of insulin-producing pancreatic beta-cells is crucial for the regulation of glucose homeostasis, and its impairment leads to diabetes mellitus, the most common metabolic disorder in man. In addition to glucose, the major nutrient factor, inputs from the nervous system, humoral components, and cell-cell communication within the islet of Langerhans act together to guarantee the release of appropriate amounts of insulin in response to changes in blood glucose levels. Data obtained within the past decade in several laboratories have revitalized controversy over the autocrine feedback action of secreted insulin on beta-cell function. Although insulin historically has been suggested to exert a negative effect on beta-cells, recent data provide evidence for a positive role of insulin in transcription, translation, ion flux, insulin secretion, proliferation, and beta-cell survival. Current insights on the role of insulin on pancreatic beta-cell function are discussed.

In vitro proliferation of cells derived from adult human beta-cells revealed by cell-lineage tracing

OBJECTIVE

Expansion of insulin-producing beta-cells from adult human islets could alleviate donor shortage for cell-replacement therapy of diabetes. A major obstacle to development of effective expansion protocols is the rapid loss of beta-cell markers in the cultured cells. Here, we report a genetic cell-lineage tracing approach for following the fate of cultured beta-cells.

RESEARCH DESIGN AND METHODS

Cells dissociated from isolated human islets were infected with two lentiviruses, one expressing Cre recombinase under control of the insulin promoter and the other, a reporter cassette with the structure cytomegalovirus promoter-loxP-DsRed2-loxP-eGFP.

RESULTS

Beta-cells were efficiently and specifically labeled by the dual virus system. Label(+), insulin(-) cells derived from beta-cells were shown to proliferate for a maximum of 16 population doublings, with an approximate doubling time of 7 days. Isolated labeled cells could be expanded in the absence of other pancreas cell types if provided with medium conditioned by pancreatic non-beta-cells. Analysis of mouse islet cells by the same method revealed a much lower proliferation of labeled cells under similar culture conditions.

CONCLUSIONS

Our findings provide direct evidence for survival and dedifferentiation of cultured adult human beta-cells and demonstrate that the dedifferentiated cells significantly proliferate in vitro. The findings confirm the difference between mouse and human beta-cell proliferation under our culture conditions. These findings demonstrate the feasibility of cell-specific labeling of cultured primary human cells using a genetic recombination approach that was previously restricted to transgenic animals.

Protein profiling of pancreatic islets

The insulin-producing beta cell in the islet of Langerhans is central in glucose homeostasis. Its dysfunction is part of the pathogenesis of both Type 1 and 2 diabetes mellitus. In both forms of the disease, there is a cytotoxic component either induced by cytokines, as in Type 1 diabetes, or by elevated levels of glucose and fatty acids, as in Type 2 diabetes. To find the mechanisms responsible for the cytotoxic effects of these compounds proteomic approaches with 2D gel electrophoresis and surface-enhanced laser desorption/ionization time-of-flight mass spectrometry have been undertaken. In this article, we describe these methods, and other methodological aspects of protein profiling of pancreatic islets, and summarize the results obtained with these methods.

Pancreatic islet cell therapy for type I diabetes: understanding the effects of glucose stimulation on islets in order to produce better islets for transplantation

While insulin replacement remains the cornerstone treatment for type I diabetes mellitus (T1DM), the transplantation of pancreatic islets of Langerhans has the potential to become an important alternative. And yet, islet transplant therapy is limited by several factors, including far too few donor pancreases. Attempts to expand mature islets or to produce islets from stem cells are far from clinical application. The production and expansion of the insulin-producing cells within the islet (so called beta cells), or even creating cells that secrete insulin under appropriate physiological control, has proven difficult. The difficulty is explained, in part, because insulin synthesis and release is complex, unique, and not entirely characterized. Understanding beta-cell function at the molecular level will likely facilitate the development of techniques to manufacture beta-cells from stem cells. We will review islet transplantation, as well as the mechanisms underlying insulin transcription, translation and glucose stimulated insulin release.

Modulation of insulin secretion by fatty acyl analogs

The secretagogue, the incretin-like, and the suppressive activities of long-chain fatty acids (LCFAs) in modulating insulin secretion in vivo and in cultured islets were simulated here by beta,beta'-tetramethyl-hexadecanedioic acid (M16) and alpha,alpha'-tetrachloro-tetradecanedioic acid (Cl-DICA). M16, but not Cl-DICA, serves as a substrate for ATP-dependent CoA thioesterification but is not further metabolized. M16, but not Cl-DICA, acted as a potent insulin secretagogue in islets cultured in basal but not high glucose. Short-term exposure to M16 or Cl-DICA resulted in activation of glucose- but not arginine-stimulated insulin secretion. Long-term exposure to M16, but not to Cl-DICA, resulted in suppression of glucose-, arginine-, and K(+)-stimulated insulin secretion; inhibition of glucose-induced proinsulin biosynthesis; and depletion of islets insulin. beta-Cell mass and islet ATP content remained unaffected. Hence, nonmetabolizable LCFA analogs may highlight discrete LCFA metabolites and pathways involved in modulating insulin secretion, which could be overlooked due to the rapid turnover of natural LCFA.

Regulation of insulin secretion and proinsulin biosynthesis by succinate

Succinate stimulates insulin secretion and proinsulin biosynthesis. We studied the effects of reduced nicotinamide adenine dinucleotide phosphate (NADPH)-modulating pathways on glucose- and succinate-stimulated insulin secretion and proinsulin biosynthesis in the rat and the insulin-resistant Psammomys obesus. Disruption of the anaplerotic pyruvate/malate shuttle by phenylacetic acid inhibited glucose- and succinate-stimulated insulin secretion and succinate-stimulated proinsulin biosynthesis in both species. In contrast, phenylacetic acid failed to inhibit glucose-stimulated proinsulin biosynthesis in P. obesus islets. Inhibition of the NADPH-consuming enzyme neuronal nitric oxide synthase (nNOS) with l-N(G)-nitro-l-arginine methyl ester or with N(G)-monomethyl-l-arginine(G) doubled succinate-stimulated insulin secretion in rat islets, suggesting that succinate- and nNOS-derived signals interact to regulate insulin secretion. In contrast, nNOS inhibition had no effect on succinate-stimulated proinsulin biosynthesis in both species. In P. obesus islets, insulin secretion was not stimulated by succinate in the absence of glucose, whereas proinsulin biosynthesis was increased 5-fold. Conversely, under stimulating glucose levels, succinate doubled insulin secretion, indicating glucose-dependence. Pyruvate ester and inhibition of nNOS partially mimicked the permissive effect of glucose on succinate-stimulated insulin secretion, suggesting that anaplerosis-derived signals render the beta-cells responsive to succinate. We conclude that beta-cell anaplerosis via pyruvate carboxylase is important for glucose- and succinate-stimulated insulin secretion and for succinate-stimulated proinsulin biosynthesis. In P. obesus, pyruvate/malate shuttle dependent and independent pathways that regulate proinsulin biosynthesis coexist; the latter can maintain fuel stimulated biosynthetic activity when the succinate-dependent pathway is inhibited. nNOS signaling is a negative regulator of insulin secretion, but not of proinsulin biosynthesis.

Identification of insulin signaling elements in human beta-cells: autocrine regulation of insulin gene expression

Although many studies using rodent islets and insulinoma cell lines have been performed to determine the role of insulin in the regulation of islet function, the autocrine effect of insulin on insulin gene expression is still controversial, and no consensus has yet been achieved. Because very little is known about the insulin signaling pathway in human islets, we used single-cell RT-PCR to profile the expression of genes potentially involved in the insulin signaling cascade in human beta-cells. The detection of mRNAs for insulin receptor (IR)A and IRB; insulin receptor substrate (IRS)-1 and IRS-2; phosphoinositide 3-kinase (PI3K) catalytic subunits p110alpha, p110beta, PI3KC2alpha, and PI3KC2gamma; phosphoinositide-dependent protein kinase-1; protein kinase B (PKB)alpha, PKBbeta, and PKBgamma in the beta-cell population suggests the presence of a functional insulin signaling cascade in human beta-cells. Small interfering RNA-induced reductions in IR expression in human islets completely suppressed glucose-stimulated insulin gene expression, suggesting that insulin regulates its own gene expression in human beta-cells. Defects in this regulation may accentuate the metabolic dysfunction associated with type 2 diabetes.

Psammomys obesus, a model for environment-gene interactions in type 2 diabetes

Type 2 diabetes is characterized by insulin resistance and progressive beta-cell failure. Deficient insulin secretion, with increased proportions of insulin precursor molecules, is a common feature of type 2 diabetes; this could result from inappropriate beta-cell function and/or reduced beta-cell mass. Most studies using tissues from diabetic patients are retrospective, providing only limited information on the relative contribution of beta-cell dysfunction versus decreased beta-cell mass to the "beta-cell failure" of type 2 diabetes. The gerbil Psammomys obesus is a good model to address questions related to the role of insulin resistance and beta-cell failure in nutritionally induced diabetes. Upon a change from its natural low-calorie diet to the calorie-rich laboratory food, P. obesus develops moderate obesity associated with postprandial hyperglycemia. Continued dietary load, superimposed on its innate insulin resistance, results in depletion of pancreatic insulin stores, with increased proportions of insulin precursor molecules in the pancreas and the blood. Inadequate response of the preproinsulin gene to the increased insulin needs is an important cause of diabetes progression. Changes in beta-cell mass do not correlate with pancreatic insulin stores and are unlikely to play a role in disease initiation and progression. The major culprit is the inappropriate insulin production with depletion of insulin stores as a consequence. Similar mechanisms could operate during the evolution of type 2 diabetes in humans.

Mitochondrial regulation of insulin production in rat pancreatic islets

AIMS/HYPOTHESIS

The study was designed to identify the key metabolic signals of glucose-stimulated proinsulin gene transcription and translation, focusing on the mechanism of succinate stimulation of insulin production.

METHODS

Wistar rat islets were incubated in 3.3 mmol/l glucose with and without esters of different mitochondrial metabolites or with 16.7 mmol/l glucose. Proinsulin biosynthesis was analysed by tritiated leucine incorporation into newly synthesised proinsulin. Preproinsulin gene transcription was evaluated following transduction with adenoviral vectors expressing the luciferase reporter gene under the control of the rat I preproinsulin promoter. Steady-state preproinsulin mRNA was determined using relative quantitative PCR. The mitochondrial membrane potential was measured by microspectrofluorimetry using rhodamine-123.

RESULTS

Succinic acid monomethyl ester, but not other mitochondrial metabolites, stimulated preproinsulin gene transcription and translation. Similarly to glucose, succinate increased specific preproinsulin gene transcription and biosynthesis. The inhibitor of succinate dehydrogenase (SDH), 3-nitropropionate, abolished glucose- and succinate-stimulated mitochondrial membrane hyperpolarisation and proinsulin biosynthesis, indicating that stimulation of proinsulin translation depends on SDH activity. Partial inhibition of SDH activity by exposure to fumaric acid monomethyl ester abolished the stimulation of preproinsulin gene transcription, but only partially inhibited the stimulation of proinsulin biosynthesis by glucose and succinate, suggesting that SDH activity is particularly important for the transcriptional response to glucose.

CONCLUSIONS/INTERPRETATION

Succinate is a key metabolic mediator of glucose-stimulated preproinsulin gene transcription and translation. Moreover, succinate stimulation of insulin production depends on its metabolism via SDH. The differential effect of fumarate on preproinsulin gene transcription and translation suggests that these processes have different sensitivities to metabolic signals.

Reduced beta-cell mass and altered glucose sensing impair insulin-secretory function in betaIRKO mice

Pancreatic beta-cell-restricted knockout of the insulin receptor results in hyperglycemia due to impaired insulin secretion, suggesting that this cell is an important target of insulin action. The present studies were undertaken in beta-cell insulin receptor knockout (betaIRKO) mice to define the mechanisms underlying the defect in insulin secretion. On the basis of responses to intraperitoneal glucose, approximately 7-mo-old betaIRKO mice were either diabetic (25%) or normally glucose tolerant (75%). Total insulin content was profoundly reduced in pancreata of mutant mice compared with controls. Both groups also exhibited reduced beta-cell mass and islet number. However, insulin mRNA and protein were similar in islets of diabetic and normoglycemic betaIRKO mice compared with controls. Insulin secretion in response to insulin secretagogues from the isolated perfused pancreas was markedly reduced in the diabetic betaIRKOs and to a lesser degree in the nondiabetic betaIRKO group. Pancreatic islets of nondiabetic betaIRKO animals also exhibited defects in glyceraldehyde- and KCl-stimulated insulin release that were milder than in the diabetic animals. Gene expression analysis of islets revealed a modest reduction of GLUT2 and glucokinase gene expression in both the nondiabetic and diabetic mutants. Taken together, these data indicate that loss of functional receptors for insulin in beta-cells leads primarily to profound defects in postnatal beta-cell growth. In addition, altered glucose sensing may also contribute to defective insulin secretion in mutant animals that develop diabetes.

Glucose-induced translational control of proinsulin biosynthesis is proportional to preproinsulin mRNA levels in islet beta-cells but not regulated via a positive feedback of secreted insulin

Proinsulin biosynthesis is regulated in response to nutrients, most notably glucose. In the short term (</=2h) this is due to increases in the translation of pre-existing mRNA. However, prolonging glucose stimulation (24 h) also increases preproinsulin mRNA levels. It has been proposed that secreted insulin from the pancreatic beta-cell regulates its own synthesis through a positive autocrine feedback mechanism. Here the comparative contributions of translation and mRNA levels on the levels of proinsulin biosynthesis were examined in isolated pancreatic islets. Also, the autocrine role of insulin upon four beta-cell functions (insulin secretion, proinsulin translation, preproinsulin mRNA levels, and total protein synthesis) was investigated in parallel. The results showed that proinsulin biosynthesis is regulated, in the short term (1 h), solely at the level of translation, through an approximately 6-fold increase in response to glucose (2.8 mm versus 16.7 mm glucose). In the longer term, when preproinsulin mRNA levels have increased approximately 2-fold, a corresponding increase was observed in the fold response of proinsulin translation to a stimulatory glucose concentration (>/=10-fold). Importantly, neither exogenously added nor secreted insulin were found to play any role in regulating insulin secretion, proinsulin translation, preproinsulin mRNA levels, or total protein synthesis. The results presented here indicate that long term nutritional state sets the preproinsulin mRNA level in the beta-cell at which translation control regulates short term changes in rates of proinsulin biosynthesis in response to glucose, but this is not mediated by any autocrine effect of insulin.