Showing papers on "Proinsulin published in 1994"
TL;DR: Long-term exposure to FFA inhibits glucose-induced insulin secretion and biosynthesis probably through a glucose fatty acid cycle.
Abstract: We tested effects of long-term exposure of pancreatic islets to free fatty acids (FFA) in vitro on B cell function. Islets isolated from male Sprague-Dawley rats were exposed to palmitate (0.125 or 0.25 mM), oleate (0.125 mM), or octanoate (2.0 mM) during culture. Insulin responses were subsequently tested in the absence of FFA. After a 48-h exposure to FFA, insulin secretion during basal glucose (3.3 mM) was severalfold increased. However, during stimulation with 27 mM glucose, secretion was inhibited by 30-50% and proinsulin biosynthesis by 3040%. Total protein synthesis was similarly affected. Conversely, previous palmitate did not impair a-ketoisocaproic acid (5 mM)-induced insulin release. Induction and reversibility ofthe inhibitory effect on glucose-induced insulin secretion required between 6 and 24 h. Addition of the carnitine palmitoyltransferase I inhibitor etomoxir (1 ,uM) partially reversed (by > 50%) FFA-associated decrease in secretory as well as proinsulin biosynthetic responses to 27 mM glucose. The inhibitory effect of previous palmitate was similar when co-culture was performed with 5.5, 11, or 27mM glucose. Exposure to palmitate or oleate reduced the production of 14CO2 from D-[U-14C1 glucose, and of "4CO2 from D-13,4-"CIglucose, both effects being reversed by etomoxir. Conclusions: long-term exposure to FFA inhibits glucose-induced insulin secretion and biosynthesis probably through a glucose fatty acid
697 citations
TL;DR: It is concluded that the oral glucose tolerance test can be used to derive estimates of the relative roles of insulin secretion and insulin resistance in population studies of glucose tolerance.
Abstract: The extent to which the oral glucose tolerance test can be used to estimate insulin secretion and insulin resistance has been evaluated by comparing glucose and insulin concentrations during an oral glucose tolerance test with specific measurements of insulin secretion and insulin resistance in 85 normoglycaemic subjects and 23 subjects with impaired glucose tolerance (IGT). Insulin secretion was measured by the first phase insulin response to intravenous glucose and insulin resistance by the insulin tolerance test which measures the decline of plasma glucose after the injection of a bolus of insulin. The best measure of insulin secretion was the ratio of the 30 min increment in insulin concentration to the 30 min increment in glucose concentration following oral glucose loading. This correlated with the first phase insulin release following intravenous glucose (r=0.61, p 0.05). Insulin resistance could be estimated by the fasting insulin, proinsulin, or split proinsulin concentrations. However, fasting split proinsulin appeared to discriminate best between insulin resistance (r = −0.53, p 0.05). Relative insulin resistance estimated by homeostasis model assessment (HOMA) also correlated well with insulin resistance (r= −0.57, p 0.05). We conclude that the oral glucose tolerance test can be used to derive estimates of the relative roles of insulin secretion and insulin resistance in population studies of glucose tolerance.
624 citations
TL;DR: The results are consistent with the hypothesis that precursors of insulin (proinsulin and proinsulin split products), known to be present in relatively high concentrations in plasma in patients with NIDDM and conditions characterized by insulin resistance, may directly stimulate PAI-1 synthesis, thereby attenuating fibrinolysis and accelerating atherogenesis.
Abstract: BACKGROUND Both vascular disease and elevated concentrations in plasma of plasminogen activator inhibitor type-1 (PAI-1) are prominent in patients with non-insulin-dependent diabetes mellitus (NIDDM). We and others have hypothesized that the increased PAI-1 may contribute to acceleration of atherosclerosis in this condition and in other states characterized by insulin resistance as well. Surprisingly, however, elevations of PAI-1 decrease when type II diabetic patients are treated with exogenous insulin, as do circulating concentrations of the precursor of insulin, proinsulin, in plasma. Accordingly, the increased PAI-1 in patients with NIDDM may reflect effects of precursors of insulin rather than or in addition to those of insulin itself. To assess this possibility directly, this study was performed to identify potential direct effects of proinsulin and proinsulin split products on synthesis of PAI-1 in liver cells, thought to be the major source of circulating PAI-1 in vivo. METHODS AND RESULTS Hep G2 cells (highly differentiated human hepatoma cells) were exposed to human proinsulin, des(31,32)proinsulin and des(64,65)proinsulin (split products of proinsulin), or C-peptide. Accumulation of PAI-1 in conditioned media increased in a time- and concentration-dependent fashion in response to the two des-intermediates [3.3-fold with des(31,32)proinsulin and 4.5-fold with des(64,65)proinsulin]. C-peptide elicited no increase. Stimulation was transduced at least in part by the insulin receptor as shown by inhibition of stimulation by insulin receptor antibodies, mediated at the level of PAI-1 gene expression as shown by the 2.2- to 2.9-fold increases in steady-state concentrations of PAI-1 mRNA, and indicative of newly synthesized protein as shown by results in metabolic labeling experiments. CONCLUSIONS Our results are consistent with the hypothesis that precursors of insulin (proinsulin and proinsulin split products), known to be present in relatively high concentrations in plasma in patients with NIDDM and conditions characterized by insulin resistance, may directly stimulate PAI-1 synthesis, thereby attenuating fibrinolysis and accelerating atherogenesis.
159 citations
TL;DR: Experimental evidence supports PC3 as the key endopeptidase that regulates proinsulin processing, and the more stringent Ca2+ and pH requirements of PC3 suggest it as the most likely enzyme to regulate proins insulin conversion, as well as initiate it.
Abstract: Pancreatic beta-cell dysfunction is a characteristic of non-insulin-dependent diabetes mellitus (NIDDM). An aspect of this dysfunction is that an increased proportion of proinsulin is secreted, but an actual beta-cell defect that leads to hyperproinsulinemia is unknown. Nevertheless, an impairment in beta-cell proinsulin conversion mechanism has been suggested as the most likely cause. Insulin is produced from its precursor molecule, proinsulin, by limited proteolytic cleavage at two dibasic sequences (Arg31, Arg32 and Lys64, Arg65). Two endopeptidase activities catalyze this cleavage: PC2 and PC3. PC2 endopeptidase cleaves predominately at Lys64, Arg65, and PC3 endopeptidase cleaves at Arg31, Arg32. The recent identification and characterization of these endopeptidases has enabled a better understanding of the human proinsulin-processing mechanism. In particular, experimental evidence suggests that the majority of human proinsulin processing is sequential. PC3 cleaves proinsulin first to generate a proinsulin conversion intermediate that is the preferred substrate of PC2. Both PC2 and PC3 activities are influenced by Ca2+ and pH, but the more stringent Ca2+ and pH requirements of PC3 suggest it as the most likely enzyme to regulate proinsulin conversion, as well as initiate it. When an increased demand is placed on the proinsulin-processing mechanism by a glucose-stimulated increase in proinsulin biosynthesis, there is a coordinate increase in PC3 biosynthesis (but not in PC2). This supports PC3 as the key endopeptidase that regulates proinsulin processing. In this perspective, the current concepts of the enzymology and regulation of proinsulin conversion at a molecular level are reviewed.(ABSTRACT TRUNCATED AT 250 WORDS)
152 citations
TL;DR: Results implicate a post-Golgi mechanism for the active removal of lysosomal hydrolases away from condensed granule contents during the storage process for regulated secretory proteins.
Abstract: In the beta-cells of pancreatic islets, insulin is stored as the predominant protein within storage granules that undergo regulated exocytosis in response to glucose. By pulse-chase analysis of radiolabeled protein condensation in beta-cells, the formation of insoluble aggregates of regulated secretory protein lags behind the conversion of proinsulin to insulin. Condensation occurs within immature granules (IGs), accounting for passive protein sorting as demonstrated by constitutive-like secretion of newly synthesized C-peptide in stoichiometric excess of insulin (Kuliawat, R., and P. Arvan. J. Cell Biol. 1992. 118:521-529). Experimental manipulation of condensation conditions in vivo reveals a direct relationship between sorting of regulated secretory protein and polymer assembly within IGs. By contrast, entry from the trans-Golgi network into IGs does not appear especially selective for regulated secretory proteins. Specifically, in normal islets, lysosomal enzyme precursors enter the stimulus-dependent secretory pathway with comparable efficiency to that of proinsulin. However, within 2 h after synthesis (the same period during which proinsulin processing occurs), newly synthesized hydrolases are fairly efficiently relocated out of the stimulus-dependent pathway. In tunicamycin-treated islets, while entry of new lysosomal enzymes into the regulated secretory pathway continues unperturbed, exit of nonglycosylated hydrolases from this pathway does not occur. Consequently, the ultimate targeting of nonglycosylated hydrolases in beta-cells is to storage granules rather than lysosomes. These results implicate a post-Golgi mechanism for the active removal of lysosomal hydrolases away from condensed granule contents during the storage process for regulated secretory proteins.
145 citations
TL;DR: The results suggest that any cell type might be engineered to produce mature, active, and stable insulin in the constitutive pathway of secretion.
141 citations
TL;DR: This review focuses on recent insights into the molecular nature of the processing machinery, and the granule Ca2+-dependent subtilisin-related endopeptidases which catalyse the initial rate-limiting step in the enzymic conversion of proinsulin.
Abstract: The insulin storage granule of the pancreatic beta cell is assembled within the trans Golgi network from around 50 or so gene products many of which are synthesized coordinately with the major component, proinsulin. An important contribution to our understanding of the regulation of this process has come from studies of the post-translational processing of proinsulin and of other proteins which are stored in the granule, particularly the processing enzymes themselves. The present review focusses on recent insights into the molecular nature of the processing machinery, and the granule Ca2+-dependent subtilisin-related endopeptidases which catalyse the initial rate-limiting step in the enzymic conversion of proinsulin.
118 citations
TL;DR: The data suggest that both PC1 and PC2 play a role in proinsulin processing in vivo, whereas PC2 is a likely candidate convertase participating in the processing of proglucagon, propancreatic polypeptide, and prosomatostatin in pancreatic islets.
Abstract: The immunocytochemical distribution and messenger RNA expression of the prohormone convertases PC1 and PC2 involved in the posttranslational processing of precursor proteins were analyzed in mouse and rat pancreatic islets. Immunocytochemical colocalization studies demonstrated a close association of insulin with both PC1 and PC2 in the adult mouse and rat pancreas. The coexpression of insulin with the prohormone convertases was further examined in rat pancreatic tumors induced by streptozotocin-nicotinamide treatment. These insulin-synthesizing tumors expressed PC1 and PC2, whereas insulin-silent adenomas did not. Colocalization studies demonstrated that only PC2, not PC1, colocalizes with glucagon, pancreatic polypeptide, and somatostatin. The highest levels of PC2-like immunoreactivity were observed in the glucagon-containing alpha-cells. Ontogeny studies carried out by in situ hybridization in mice showed the first detectable expression of the prohormone convertases in the pancreatic primordium at midgestation, starting for PC1 on embryonic day 11 and for PC2 on embryonic day 10. Enzyme expression was further confirmed by immunocytochemistry, which detected the presence of immunoreactive PC1- and PC2-like proteins on embryonic days 14 and 17, respectively. Taken together, our data suggest that both PC1 and PC2 play a role in proinsulin processing in vivo, whereas PC2 is a likely candidate convertase participating in the processing of proglucagon, propancreatic polypeptide, and prosomatostatin in pancreatic islets.
94 citations
TL;DR: It is concluded that sc rhIGF-I can reduce blood glucose effectively in selected patients with the type A phenotype of severe insulin resistance who have diabetes mellitus and can enhance insulin sensitivity, as assessed by a decrease in endogenous insulin levels, normalization of response to iv insulin, and a reduced steady state plasma glucose.
Abstract: Recombinant human (rh) insulin-like growth factor-I (IGF-I) is a potential therapy for individuals with severe insulin resistance, but its efficacy, mechanism of action, or duration of effect for these patients have not been explored fully. Two subjects with the type A phenotype of severe insulin resistance without insulin receptor mutations were investigated to assess insulin secretion, insulin action, and carbohydrate tolerance before and after 3-4 weeks of rhIGF-I treatment (100 micrograms/kg, sc, twice daily). Tests included 24-h glucose and insulin profile (modal day), standardized liquid meal with Sustacal, insulin tolerance test, insulin suppression test, and iv glucose tolerance test. In subject 1, the 24-h mean blood glucose level was 8.1 +/- 2.7 mmol/L before rhIGF-I treatment and fell to 4.2 +/- 0.9 mmol/L during rhIGF-I treatment. The pretreatment 24-h mean serum insulin level was 10,251 +/- 8,849 pmol/L and fell to 1533 +/- 1198 pmol/L. Fasting blood glucose fell from 4.4 to 3.4 mmol/L, and 2-h blood glucose after Sustacal administration fell from 10.3 to 5.3 mmol/L. Fasting serum insulin declined from 808 to 246 pmol/L, and the 2-h serum insulin level fell from 5,491 to 3,443 pmol/L. After bolus iv insulin injection (0.15 U/kg), glucose fell by 20% before rhIGF-I treatment and by 67% during rhIGF-I treatment. The steady state plasma glucose level was 18.2 +/- 0.7 before rhIGF-I and 10.8 +/- 0.1 mmol/L during rhIGF-I. In subject 2, fasting blood glucose fell from 12.0 to 7.4 mmol/L and 24-h mean blood glucose fell from 12.7 +/- 1.9 to 6.6 +/- 1.3 mmol/L. Twenty-four-hour mean serum insulin fell from 892 +/- 635 to 521 +/- 293 pmol/L, and first phase insulin secretion was restored during the iv glucose tolerance test. We conclude that sc rhIGF-I can reduce blood glucose effectively in selected patients with the type A phenotype of severe insulin resistance who have diabetes mellitus. rhIGF-I also can enhance insulin sensitivity, as assessed by a decrease in endogenous insulin levels, normalization of response to iv insulin, and a reduced steady state plasma glucose. The cellular mechanisms for these effects remain undefined.
94 citations
TL;DR: N nondiabetic subjects from a high-risk population for NIDDM are hyperinsulinemic and, further, do not secrete more proinsulin relative to insulin itself than do nondi diabetic Subjects from a low- risk population.
Abstract: Hyperinsulinemia and insulin resistance have been implicated as risk factors for the development of non-insulin-dependent diabetes mellitus (NIDDM). Recent data suggest that proinsulin may comprise a large proportion of immunoreactive insulin in subjects with NIDDM and possibly in those with impaired glucose tolerance (IGT) as well. Increased proinsulin concentrations are thought to be an early indicator of a failing pancreas. We examined proinsulin, insulin (using an assay that does not display appreciable cross-reactivity with proinsulin), and the fasting proinsulin:insulin ratio in 206 nondiabetic Mexican-American (a high-risk population for NIDDM) and 123 nondiabetic non-Hispanic white (a low-risk population for NIDDM) participants in the San Antonio Heart Study, a population-based study of diabetes and cardiovascular disease. Mexican-Americans had significantly higher fasting and 2-h proinsulin and insulin levels but similar fasting proinsulin:insulin ratios compared with non-Hispanic whites. After statistical adjustment for age, body mass index, waist-to-hip ratio, and glucose tolerance status, Mexican-Americans continued to have higher fasting and 2-h insulin and fasting and 2-h proinsulin concentrations but similar proinsulin:insulin ratios compared with non-Hispanic whites. The fasting proinsulin:insulin ratio was higher in 85 subjects with NIDDM compared with subjects with IGT or normal glucose tolerance (0.31, 0.09, and 0.07, respectively). Thus, nondiabetic subjects from a high-risk population for NIDDM are hyperinsulinemic (using an assay that does not cross-react with proinsulin) and, further, do not secrete more proinsulin relative to insulin itself than do nondiabetic subjects from a low-risk population.
91 citations
TL;DR: Results indicate that an increased level of synthesis of the amyloidogenic hIAPP is not sufficient to cause islet ameloid deposition, and the increased synthesis and storage of insulin in the islets of the transgenic mice are consistent with either a direct regulatory effect of IAPP on the β-cell or indirect stimulation of insulin production through IAPP-induced insulin resistance.
Abstract: Islet amyloid polypeptide (IAPP) is a secretory product of the pancreatic β-cell, which is the primary constituent of the islet amyloid that develops in type II diabetes. To study the role the inherent amyloidogenicity of human IAPP (hIAPP) plays in the formation of islet amyloid deposits and to investigate a possible hormonal role for IAPP, transgenic mice expressing hIAPP were developed. The transgene was composed of a fragment of an hIAPP cDNA linked to the rat insulin II promoter. One line of transgenic mice expressed the transgene and synthesized hIAPP in their pancreatic islets. IAPP-like immunoreactivity in pancreatic extracts and plasma were two- to threefold greater in the transgenic mice compared with nontransgenic control mice. Although plasma concentrations of immunoreactive insulin (IRI) and glucose were equal in transgenic and control mice, the pancreatic content of IRI was nearly twofold greater in the transgenic animals, and proinsulin mRNA was significantly elevated, suggesting increased rates of insulin biosynthesis. Pancreatic samples obtained from transgenic mice up to 19 months of age had no evidence of islet amyloid. These results indicate that an increased level of synthesis of the amyloidogenic hIAPP is not sufficient to cause islet amyloid deposition. However, the increased synthesis and storage of insulin in the islets of the transgenic mice are consistent with either a direct regulatory effect of IAPP on the β-cell or indirect stimulation of insulin production through IAPP-induced insulin resistance.
TL;DR: The results show that particularly nondiabetic individuals with the insulin resistance syndrome not only have hyperinsulinemia as a marker of insulin resistance, but also show an increase in proinsulin relative to insulin, which may reflect relative beta-cell failure or malfunction.
Abstract: Recent data suggest that proinsulin may be associated with increased cardiovascular risk factors in both diabetic and nondiabetic subjects. We examined the relation of insulin, proinsulin, and the fasting proinsulin/insulin ratio to a number of metabolic disorders believed to be related to the insulin resistance syndrome (low high density lipoprotein cholesterol and high triglyceride levels, hypertension, and impaired glucose tolerance). Proinsulin was measured by a RIA, and insulin was measured by a Linco RIA that does not cross-react with proinsulin. The increased fasting proinsulin/insulin ratio was significantly associated with hypertension, low high density lipoprotein cholesterol and high triglyceride levels, and impaired glucose tolerance in 423 nondiabetic subjects. The fasting proinsulin/insulin ratio increased significantly with the number of metabolic disorders (zero, 0.060; one, 0.086; two, 0.098; three, 0.177; four, 0.182; P < 0.001). The increased proinsulin/insulin ratio was also associated with a greater number of metabolic disorders in diabetic subjects. Our results show that particularly nondiabetic individuals with the insulin resistance syndrome not only have hyperinsulinemia as a marker of insulin resistance, but also show an increase in proinsulin relative to insulin, which may reflect relative beta-cell failure or malfunction.
TL;DR: Brefeldin A (BFA) was utilized to prevent protein export from the trans-Golgi network and restored granule formation and subsequent insulin production, providing biochemical confirmation that the generation of insulin occurs with IGs rather than in the TGN.
TL;DR: By quantitative immunoelectron microscopy and HPLC, the authors studied the effect of pH gradients, by ammonium chloride, on proinsulin conversion in the insulin-producing B-cells of the islets of langerhans.
Abstract: By quantitative immunoelectron microscopy and HPLC, we have studied the effect of disrupting pH gradients, by ammonium chloride, on proinsulin conversion in the insulin-producing B-cells of the islets of langerhans. Proinsulin content and pH in single secretory vesicles were measured on consecutive serial sections immunostained alternately with anti-proinsulin or anti-dinitrophenol (to reveal the pH-sensitive probe DAMP) antibodies. Radioactivity labeled proinsulin, proinsulin cleavage intermediates, and insulin were quantitated by HPLC analysis of extracts of islets treated in the same conditions. Cleavage at the C-peptide/A-chain junction is significantly less sensitive to pH gradient disruption than that of the B-chain/C-peptide junction, but the range of pH and proinsulin content in individual vesicles indicate that both cleavages occur in the same vesicle released from the TGN.
TL;DR: In this article, the first phase insulin response (FPIR) was measured in siblings of insulin-dependent diabetes mellitus patients, and they were divided into two groups according to their FPIR to i.v. glucose tolerance tests (IVGTTs).
Abstract: The objective of this study was to test whether levels of proinsulin immunoreactivity (PIM) relative to those of insulin immunoreactivity (IRI) or C-peptide are changed and related to subclinical beta-cell dysfunction in siblings of insulin-dependent diabetes mellitus (IDDM) patients. Twenty-three siblings, previously found positive for islet cell antibodies and/or insulin autoantibodies, were divided into 2 groups according to their first phase insulin response (FPIR) to i.v. glucose tolerance tests (IVGTTs) sequentially performed during an observation period of 2 yr. Eleven siblings had diminished FPIR on at least 1 occasion (group 1), whereas 12 siblings had a normal FPIR on all occasions studied (group 2). All underwent a further IVGTT (0.5 g glucose/kg BW), and serum samples were taken at 0, 1, 3, 6, 10, 20, 30, 40, 50, and 60 min. The 2 groups had comparable median age, female/male ratio, weight, height, fasting blood glucose, immunoreactive insulin, C-peptide, and insulin autoantibodies levels, but...
TL;DR: In obese subjects and those with subclinical diabetes, weight loss was associated with a reduction in insulin secretion rates, presumably as a result of improvements in insulin sensitivity, and in patients with overt diabetes and hyperglycemia, weight Loss improved β-cell responsiveness to glucose and was related with an increase in insulin clearance and a reduction of proinsulin immunoreactivity.
Abstract: Subjects with overt non-insulin-dependent diabetes mellitus (NIDDM) were studied in comparison to obese nondiabetic control subjects and patients with subclinical diabetes. Pancreatic insulin secretion rates were measured by deconvolution of peripheral C-peptide over a 24-h period while subjects consumed an isocaloric mixed diet. Subjects were then placed on caloric restriction for at least 6 weeks, during which time body weight fell by at least 10%. Refeeding with solid mixed meals was then resumed for at least 2 weeks until isocaloric intake was attained, and then the meal profiles were repeated. Before weight loss, insulin, C-peptide, and insulin secretion rates were significantly higher in subjects with subclinical diabetes than in the other two groups. Proinsulin concentrations were significantly greater in the two diabetic groups than in control subjects, but, when expressed as a percentage of the total insulin immunoreactivity, the differences were significant only in the group with overt diabetes. Weight loss because of hypocaloric feeding resulted in a significant increase in the rate of clearance of endogenously secreted insulin but did not affect the clearance of C-peptide. In obese subjects and those with subclinical diabetes, weight loss was associated with a reduction in insulin secretion rates, presumably as a result of improvements in insulin sensitivity. In patients with overt diabetes and hyperglycemia, weight loss improved beta-cell responsiveness to glucose and was associated with an increase in insulin clearance and a reduction in proinsulin immunoreactivity.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: As the biochemistry of proinsulin conversion is understood in increasingly fine molecular detail, it should become possible to make use of such unusual ratios to provide insight into lesions underlying altered beta-cell function in disease states.
Abstract: Proinsulin is converted to insulin in beta-cell granules. Conversion involves endoproteolytic cleavage at the two pairs of basic residues linking the insulin A- and B-chains to C-peptide. The sequence of events leading to complete conversion differs from one proinsulin species to the next. In man, the structure of the proinsulin molecule is such as to favour cleavage at the B-chain/C-peptide junction leading to the generation of des-31,32 split proinsulin as the predominant, naturally occurring conversion intermediate. Under normal circumstances, proinsulin conversion is largely completed before secretion, and neither the intact prohormone nor conversion intermediates are thus encountered in large quantities in the circulation. In some pathological situations, including non-insulin-dependent diabetes, insulinoma and familial hyperproinsulinaemia, unusually high ratios of des-31,32 split proinsulin and/or proinsulin to insulin have been reported. As we understand the biochemistry of proinsulin conversion in increasingly fine molecular detail, it should become possible to make use of such unusual ratios to provide insight into lesions underlying altered beta-cell function in disease states.
TL;DR: A relationship between birth weight and impaired glucose tolerance, NIDDM and the insulin resistance syndrome has now been observed in two populations in the UK, in Mexican Americans and in Pima Indians and is reproducible and applicable to widely differing populations.
Abstract: Improvements in the specificity and sensitivity of assays for insulin-related molecules in the circulation have proved to be necessary and informative in studies of the pathogenesis of non-insulin-dependent diabetes (NIDDM). Of particular interest has been the close relationship between increases in des 31,32 split proinsulin and susceptibility to loss of glucose tolerance and the insulin resistance syndrome. It is suggested that the analogy can be drawn between this measurement and the measurement of HbA1c. The amount of this partially processed precursor of insulin in the circulation indicates the degree of glucose stimulus applied to the beta cell combined with the inherent capacity of the insulin secretory system to respond. Further improvements of the sensitivity and specificity of the assay of proinsulin related molecules are desirable. Deterioration of the early insulin response to oral glucose is a major feature of the loss of glucose tolerance associated with the transition from normal to impaired glucose tolerance and to NIDDM. The extent to which this loss of insulin secretion reflects a major predisposing factor in the aetiology of this type of diabetes or is secondary to glucose toxicity or amyloid accumulation remains to be determined. A relationship between birth weight and impaired glucose tolerance, NIDDM and the insulin resistance syndrome has now been observed in two populations in the UK, in Mexican Americans and in Pima Indians. It is therefore reproducible and applicable to widely differing populations. Much further research is indicated to determine, amongst many questions, how much diabetes is associated with this link and what factors explain it.
TL;DR: Proinsulin not only is useful for the diagnosis of insulinoma, but it may have greater diagnostic accuracy than C-peptide and insulin.
Abstract: We used a newly developed immunochemiluminometric assay of proinsulin to determine its relative utility vis-a-vis C-peptide and insulin for the diagnosis of insulinoma. The evaluation was conducted in 20 consecutive patients with histologically confirmed insulinoma and 22 normal subjects who underwent a prolonged fast according to a standard protocol. Patients with insulinoma fasted to the point of demonstrating Whipple's triad; normal subjects fasted to 72 h. At the end of the prolonged fast, when the glucose value was 2.8 mmol/L or less (50 mg/dL), all three hormones had equal sensitivity (100%) in detecting insulinoma with no overlap with the values of normal subjects. When glucose levels were between 2.8 mmol/L (50 mg/dL) and 3.3 mmol/L (60 mg/dL) at the end of the prolonged fast, proinsulin was better than C-peptide and insulin in the diagnosis of insulinoma. The sensitivity was 90% for proinsulin and 85% for both C-peptide and insulin. Therefore, proinsulin not only is useful for the diagnosis of in...
TL;DR: The understanding of GLP‐1 receptor regulation and signal transduction will aid in the discovery of compounds that act as agonists of the GLP•1 receptor for potential use in the treatment of diabetes and will facilitate the understanding of its expression under normal and pathophysiological conditions.
01 Jan 1994
TL;DR: Fasting PIM/IRI and/or Pim/C-peptide ratio reflects subclinical beta-cell dysfunction in prediabetic subjects with evidence of immunological beta- cell assault and suggests that an elevated ratio may be an additional marker for later development of IDDM.
Abstract: The objective of this study was to test whether levels of proinsulin immunoreactivity (PIM) relative to those of insulin immunoreactivity (IRI) or C-peptide are changed and related to subclinical beta-cell dysfunction in siblings of insulin-dependent diabetes mellitus (IDDM) patients. Twenty-three siblings, previously found positive for islet cell antibodies and/or insulin autoantibodies, were divided into 2 groups according to their first phase insulin response (FPIR) to i.v. glucose tolerance tests (IVGTTs) sequentially performed during an observation period of 2 yr. Eleven siblings had diminished FPIR on at least 1 occasion (group 1), whereas 12 siblings had a normal FPIR on all occasions studied (group 2). All underwent a further IVGTT (0.5 g glucose/kg BW), and serum samples were taken at 0, 1, 3, 6, 10, 20, 30, 40, 50, and 60 min. The 2 groups had comparable median age, female/male ratio, weight, height, fasting blood glucose, immunoreactive insulin, C-peptide, and insulin autoantibodies levels, but group 1 had significantly higher islet cell antibodies levels. Fasting median PIM/IRI and PIM/C-peptide ratios were 2- to 3-fold higher in group 1 [10.5% (range, 1.8-93.8%) vs. 5.2% (range, 1.9-14.3%) and 3.3% (range, 0.4-23.1%) vs. 1.3% (range, 0.7-2.6%; P < 0.05]. Fasting PIM/C-peptide ratios correlated inversely with FPIRs (rs = -0.68; P < 0.01). During glucose stimulation, maximal responses of IRI and C-peptide were 4-fold lower in group 1, and the time of maximal responses of IRI and C-peptide occurred later in group 1 than in group 2. In contrast, no difference in maximal responses of PIM was found, but the time of maximal responses of PIM occurred later in group 1. Nine of 11 siblings in group 1 presented with IDDM 1-28 months after the test, compared to none in group 2. In group 1 a paradoxical inhibitory response of PIM was observed during the first 6 min of the IVGTT. These data indicate that fasting PIM/IRI and/or PIM/C-peptide ratio reflects subclinical beta-cell dysfunction in prediabetic subjects with evidence of immunological beta-cell assault and suggests that an elevated ratio may be an additional marker for later development of IDDM.
TL;DR: The insulin-degrading enzyme (IDE) and the multicatalytic proteinase (MCP) can be isolated as components of a cytosolic proteolytic complex.
TL;DR: In the diabetic Psammomys obesus, the pancreatic capacity for storage of insulin may be limited; the metabolic consequences of this limitation are amplified by increased secretory demand secondary to insulin resistance, thus facilitating the establishment of hyperglycemia, which may in itself further exacerbate pancreatic dysfunction.
Abstract: Patients with noninsulin-dependent diabetes mellitus exhibit increased proportions of plasma proinsulin and proinsulin conversion intermediates. We used hyperinsulinemic diabetic and nondiabetic Psammomys obesus to study the possible relationship between steady state pancreatic insulin stores and the proportion of proinsulin-related peptides in the plasma and pancreas. Insulin-like peptides were separated by reverse phase HPLC and identified by pulse-chase experiments. A marked increase in the proportions of proinsulin and proinsulin conversion intermediates in the plasma and pancreas of diabetic nonfasted Psammomys was associated with 90% reduction in insulin stores of the pancreas. After a 16- to 20-h fast, the depletion of pancreatic insulin in the diabetic animals was partially corrected, and the proinsulin/insulin ratio was normalized. In contrast, nondiabetic Psammomys showed only 50% reduction in pancreatic insulin stores under nonfasting conditions, with no change in the proinsulin/insulin ratio. ...
TL;DR: In this article, the expression of PC1 and PC2 in islet beta and non-beta cells was evaluated by autofluorescence-activated flow cytometry to separate beta cells from non-Beta cells, and conversion endoprotease levels were analysed by Western blotting.
Abstract: PC1 (PC3) and PC2, members of the mammalian family of proprotein convertases homologous to the yeast Kex2 gene product, are both expressed in pancreatic islets of Langerhans. Recent studies have suggested that PC1 and PC2 are responsible for the conversion of proinsulin to insulin and connecting peptide (C-peptide) in the islet beta cells. However, the insulin-secreting beta cells are not the only cells present in these complex micro-organs, prompting us to evaluate the expression of PC1 and PC2 in islet beta and non-beta cells. Rat islet cells were sorted by autofluorescence-activated flow cytometry to separate beta cells from non-beta cells, and conversion endoprotease levels were analysed by Western blotting. The immunolabel ratio of PC1/PC2 in beta cells was 2.6. Non-beta cells displayed much lower levels of PC1 than beta cells, but twice as much PC2 (PC1/PC2 = 0.05). Post-translational modification of the convertases themselves was found to differ between the cell types. In particular, a 75 kDa precursor form of PC2 (pro-PC2) was found to accumulate in beta cells, whereas only the fully processed 67 kDa form was detected in the non-beta cells. Finally, the quantification of PC1 and PC2 and their precursor forms in transformed cells (insulin-producing beta-TC and glucagon-producing alpha-TC) showed that transformation appeared to be accompanied by unusually high levels of the precursors.
TL;DR: First-degree relatives of NIDDM patients, who have a high risk of developing diabetes, do not exhibit elevated levels of fasting or glucose-stimulated proinsulin as long as their fasting glucose levels remain normal.
Abstract: OBJECTIVE To examine proinsulin and insulin levels in first-degree relatives of patients with non-insulin-dependent diabetes mellitus (NIDDM). RESEARCH DESIGN AND METHODS Comparison of insulin and proinsulin responses to an oral glucose tolerance test in four groups of individuals: 1) 31 patients with newly diagnosed NIDDM treated with diet alone, 2) 34 first-degree relatives of NIDDM patients with impaired glucose tolerance (IGT), 3) 26 relatives with normal glucose tolerance (NGT), and 4) 30 subjects without a family history of diabetes. RESULTS Both fasting and post-glucose levels of proinsulin were elevated in patients with diabetes, but not in the relatives with IGT or NGT. Levels of true insulin were highest in the diabetic group, followed by the subjects with IGT, and were lowest among relatives with NGT. Proinsulin levels correlated with glucose levels, suggesting that hyperglycemia is the main stimulus for increased proinsulin secretion. CONCLUSIONS First-degree relatives of NIDDM patients, who have a high risk of developing diabetes, do not exhibit elevated levels of fasting or glucose-stimulated proinsulin as long as their fasting glucose levels remain normal.
TL;DR: In these studies, the effects of glucose on PC2 and PC3 biosynthesis in freshly isolated islets from normal and hyperglycemic (ob/ob) mice are compared and inhibition of PC2 biosynthesis by glucose in normal islet non β‐cells may obscure stimulation ofPC2 biosynthetic activity innormal islet β-cells.
TL;DR: In vitro experiments suggest that high levels of insulin and IGF-I stimulate MC-mediated oxidation of LDL, an effect that is potentially atherogenic.
TL;DR: In conclusion, IGF-I appears to correct partially some metabolic sequelae of severe insulin resistance and may, hence, be used as a new therapeutic agent.
Abstract: Severe insulin resistance type A is due to mutations in the insulin receptor gene and is characterized by glucose intolerance or diabetes mellitus, despite extreme hyperinsulinemia, virilization and acanthosis nigricans. At present, there is no therapy for this condition. Recently, we showed that glucose levels in three such patients are promptly lowered by an i.v. bolus of recombinant human insulin-like growth factor I (rhIGF-I). In the present study, we investigated two of these rare patients again and determined fasting and postprandial glucose, insulin, C-peptide, proinsulin and lipid levels during five control, five treatment and three wash-out days while on a constant diet. Treatment consisted of 2 x 150 micrograms rhIGF-I/kg sc per day, which elevated total IGF-I levels 4.5-fold above the control. Fasting glucose levels (days 1-5) in the two patients were 9.6 +/- 1.3 and 9.2 +/- 1.2 mmol/l, respectively, and fell to 4.4 +/- 0.4 and 5.1 +/- 0.5 mmol/l on treatment days 8-10. Fasting insulin (2950 +/- 450 and 690 +/- 125 pmol/l), C-peptide (2217 +/- 183 and 1317 +/- 235 pmol/l) and proinsulin control levels (125 +/- 35 and 66 +/- 0 pmol/l) also decreased by approximately 65% during rhIGH-I treatment, as did the respective postprandial levels. Lipid levels hardly changed at all. In conclusion, IGF-I appears to correct partially some metabolic sequelae of severe insulin resistance and may, hence, be used as a new therapeutic agent.
TL;DR: In H23 cells an unusually large amount of proinsulin is diverted to the constitutive pathway, where it is partially converted to des-31,32-split proins insulin before release, which results in rapid secretion of newly synthesized products that is insensitive to secretagogues.
TL;DR: It is proposed that the methyl esters of succinic acid and related molecules may represent new tools with which to bypass defects in glucose transport, phosphorylation and further catabolism and, hence, to stimulate both proinsulin biosynthesis and insulin release in NIDDM.
Abstract: Impairment of glucose-induced insulin secretion in non-insulin-dependent diabetes mellitus (NIDDM) may be caused by GLUT 2 underexpression in the pancreatic beta cell, a mutation of the glucokinase gene, glucose 6-phosphatase overactivity, FAD-linked glycerophosphate dehydrogenase deficiency, a mitochondrial DNA defect and/or a secondary phenomenon of so-called glucotoxicity possibly involving glycogen accumulation in the beta-cell. It is proposed that the methyl esters of succinic acid and related molecules may represent new tools with which to bypass these defects in glucose transport, phosphorylation and further catabolism and, hence, to stimulate both proinsulin biosynthesis and insulin release in NIDDM.