Effect of oral nitrite administration on gene expression of SNARE proteins involved in insulin secretion from pancreatic islets of male type 2 diabetic rats

Background

Nitrite stimulates insulin secretion from pancreatic β-cells; however, the underlying mechanisms have not been completely addressed. The aim of this study is to determine effect of nitrite on gene expression of SNARE proteins involved in insulin secretion from isolated pancreatic islets in Type 2 diabetic Wistar rats.

Methods

Three groups of rats were studied (n = 10/group): Control, diabetes, and diabetes + nitrite, which treated with sodium nitrite (50 mg/L) for 8 weeks. Type 2 diabetes was induced using a low-dose of streptozotocin (25 mg/kg) combined with high-fat diet. At the end of the study, pancreatic islets were isolated and mRNA expressions of interested genes were measured; in addition, protein expression of proinsulin and C-peptide in pancreatic tissue was assessed using immunofluorescence staining.

Results

Compared with controls, in the isolated pancreatic islets of Type 2 diabetic rats, mRNA expression of glucokinase (59%), syntaxin1A (49%), SNAP25 (70%), Munc18b (48%), insulin1 (56%), and insulin2 (52%) as well as protein expression of proinsulin and C-peptide were lower. In diabetic rats, nitrite administration significantly increased gene expression of glucokinase, synaptotagmin III, syntaxin1A, SNAP25, Munc18b, and insulin genes as well as increased protein expression of proinsulin and C-peptide.

Conclusion

Stimulatory effect of nitrite on insulin secretion in Type 2 diabetic rats is at least in part due to increased gene expression of molecules involved in glucose sensing (glucokinase), calcium sensing (synaptotagmin III), and exocytosis of insulin vesicles (syntaxin1A, SNAP25, and Munc18b) as well as increased expression of insulin genes.

Maternal Type 1 Diabetes Reduces Autoantigen-Responsive CD4+ T Cells in Offspring

Autoimmunity against pancreatic β-cell autoantigens is a characteristic of childhood type 1 diabetes (T1D). Autoimmunity usually appears in genetically susceptible children with the development of autoantibodies against (pro)insulin in early childhood. The offspring of mothers with T1D are protected from this process. The aim of this study was to determine whether the protection conferred by maternal T1D is associated with improved neonatal tolerance against (pro)insulin. Consistent with improved neonatal tolerance, the offspring of mothers with T1D had reduced cord blood CD4⁺ T-cell responses to proinsulin and insulin, a reduction in the inflammatory profile of their proinsulin-responsive CD4⁺ T cells, and improved regulation of CD4⁺ T cell responses to proinsulin at 9 months of age, as compared with offspring with a father or sibling with T1D. Maternal T1D was also associated with a modest reduction in CpG methylation of the INS gene in cord blood mononuclear cells from offspring with a susceptible INS genotype. Our findings support the concept that a maternal T1D environment improves neonatal immune tolerance against the autoantigen (pro)insulin.

Proinsulin-transferrin fusion protein as a novel long-acting insulin analog for the inhibition of hepatic glucose production

Proinsulin-transferrin (ProINS-Tf) fusion protein was evaluated for its in vivo pharmacokinetics, efficacy, and mechanism. Our previous studies have shown that ProINS-Tf was converted to active insulin-transferrin (INS-Tf) via the transferrin (Tf)-receptor-mediated pathway in hepatoma cells. We hypothesized that this fusion protein can be administered as a prodrug and be converted to a biologically active protein with specificity for the liver versus other insulin (INS)-sensitive tissues (muscle and adipose). Administration as an inactive prodrug with liver-specific action compared with other INS-sensitive tissues conceivably reduces negative side effects seen with other INS analogs. In this report, the data show that ProINS-Tf exhibited a slow, but sustained, in vivo hypoglycemic efficacy and long plasma half-life. The fusion protein showed activity in the liver, as evidenced by decreased expression of two key hepatic glucose production (HGP) enzymes, PEPCK and glucose-6-phosphatase, and increased glycogen levels under feeding conditions. Furthermore, the INS receptor (IR) phosphorylation (activation) in liver and muscle tissues was compared with postinjection of INS or ProINS-Tf. While INS activated IR in both the liver and muscle, ProINS-Tf only showed activation in the liver. Thus, ProINS-Tf fusion protein can potentially be administered as a prodrug with sustained Tf-mediated activation and selectivity in inhibiting HGP.

Proinsulin binds with high affinity the insulin receptor isoform A and predominantly activates the mitogenic pathway

Proinsulin is generally regarded as an inactive prohormone because of its low metabolic activity. However, proinsulin appears to regulate embryo development in animal models. In this study, we evaluated whether proinsulin may differentially bind to and activate the two insulin receptor (IR) isoforms (IR-A and IR-B), because IR-A is a relatively low-specificity receptor that is prevalent in fetal and cancer cells and is able to mediate the growth effects of IGF-II. Mouse R(-) fibroblasts devoid of IGF-I receptor (IGF-IR) and stably transfected with cDNA encoding either human IR-A or IR-B (R(-) /IR-A and R(-) /IR-B cells) were used. Three human cancer cell lines were also studied. We found that proinsulin stimulated phosphorylation of IR-A with an EC(50) of 4.5 ± 0.6 nm and displaced [(125)I]insulin from IR-A with a similar EC(50). In contrast, proinsulin EC(50) values for stimulation of IR-B phosphorylation and for [(125)I]insulin displacement from IR-B were approximately 7-fold higher. Proinsulin did not bind or activate IGF-IR or IR/IGF-IR hybrids. Via IR-A, proinsulin activated the ERK/p70S6K pathway to a similar degree as insulin but elicited a weaker Akt response. Despite its low metabolic activity, proinsulin was almost equipotent as insulin in inducing cell proliferation and migration in cells expressing various IR-A levels. In conclusion, proinsulin is a selective IR-A ligand and may induce biological effects through this IR isoform.

Regulation of cathepsin G reduces the activation of proinsulin-reactive T cells from type 1 diabetes patients

Autoantigenic peptides resulting from self-proteins such as proinsulin are important players in the development of type 1 diabetes mellitus (T1D). Self-proteins can be processed by cathepsins (Cats) within endocytic compartments and loaded to major histocompatibility complex (MHC) class II molecules for CD4⁺ T cell inspection. However, the processing and presentation of proinsulin by antigen-presenting cells (APC) in humans is only partially understood. Here we demonstrate that the processing of proinsulin by B cell or myeloid dendritic cell (mDC1)-derived lysosomal cathepsins resulted in several proinsulin-derived intermediates. These intermediates were similar to those obtained using purified CatG and, to a lesser extent, CatD, S, and V in vitro. Some of these intermediates polarized T cell activation in peripheral blood mononuclear cells (PBMC) from T1D patients indicative for naturally processed T cell epitopes. Furthermore, CatG activity was found to be elevated in PBMC from T1D patients and abrogation of CatG activity resulted in functional inhibition of proinsulin-reactive T cells. Our data suggested the notion that CatG plays a critical role in proinsulin processing and is important in the activation process of diabetogenic T cells.

Receptor-mediated activation of a proinsulin-transferrin fusion protein in hepatoma cells

A proinsulin-transferrin (ProINS-Tf) recombinant fusion protein was designed and characterized for the sustained release of an active form of insulin (INS) by hepatoma cells. During incubation with H4IIE hepatoma cells, a gradual decline of ProINS-Tf concentration, with a concomitant generation of the immuno-reactive insulin-transferrin (irINS-Tf), was detected in the culture medium by using INS- or proinsulin (ProINS)-specific radioimmunoassay (RIA) system. Further studies indicated that the conversion of ProINS-Tf to irINS-Tf was a transferrin receptor (TfR) mediated process that was pH-sensitive, and temperature- and microtubule-dependent. These results suggest that the conversion occurred during the slow recycling route of transferrin (Tf)-TfR pathway, possibly processed by proteases in the slow recycling compartments juxtaposed to the trans-Golgi network (TGN). ProINS-Tf exhibited little activity in the short-term promotion of glucose uptake in adipocytes, indicating that it was in an inactive form similar to ProINS. Stimulation of Akt phosphorylation by ProINS-Tf was detected only after prolonged incubation with H4IIE cells. On the other hand, ProINS-Tf pre-incubated with H4IIE cells for 24h acquired an immediate activity of stimulating Akt phosphorylation. Furthermore, ProINS-Tf elicited a strong activity in the inhibition of glucose production following 24h incubation with H4IIE cells. Based on these findings, we conclude that the Tf-TfR endocytosis and recycling pathway enables the conversion and release of ProINS-Tf in an active form of irINS-Tf. Results from this study suggest that the Tf-TfR pathway can be exploited for the design of prohormone-Tf fusion proteins as protein prodrugs for their sustained and targeted activation.

Proinsulin C-peptide and insulin: Limited pattern similarities of interest in inter-peptide interactions but no C-peptide effect on insulin and IGF-1 receptor signaling

The recently reported influence of proinsulin C-peptide on insulin prompted us to examine structural features of the C-peptide. Four sets of limited pattern similarities towards inter-chain end regions of insulin were noticed, involving secondary structure elements, binding residues and intra- as well as inter-peptide residue similarities. Using surface plasmon resonance, we examined insulin binding to truncated, soluble insulin receptor A and IGF-1 receptor, but C-peptide effects on these bindings were not detectable. Two forms of the insulin receptor, differing in activation of gene transcription with regards to (pre)proinsulin and glucokinase, respectively, were also uninfluenced by C-peptide. We conclude that the pattern similarities, if functional, reflect C-peptide interactions with molecules other than both insulin A and B receptors and IGF-1 receptors. Any such effects are of interest in relation to reported binding interactions between insulin and C-peptide.

Inhibition of platelet aggregation of a mutant proinsulin chimera engineered by introduction of a native Lys-Gly-Asp-containing sequence

An eight amino acid sequence, CAKGDWNC, from disintegrin barbourin, was introduced into an inactive human proinsulin molecule between the B28 and A2 sites to construct a chimeric, anti-thrombosis recombinant protein. The constructed Lys-Gly-Asp (KGD)-proinsulin gene was expressed in Escherichia coli and then purified. The KGD-proinsulin chimera protein inhibits human platelet aggregation, induced by ADP, with an IC50 value (molar concentration causing 50% inhibition of platelet aggregation) of 830 nM: and demonstrates also specific affinity to glycoprotein IIb/IIIa receptor. Its insulin receptor binding activity remains as low as 0.04% with native insulin as a control.

Human proinsulin C-peptide prevents proliferation of rat aortic smooth muscle cells cultured in high-glucose conditions

AIMS/HYPOTHESIS

Proinsulin C-peptide is involved in several biological activities. However, the role of C-peptide in vascular smooth muscle cells is unclear. We therefore investigated its effects, in vascular smooth muscle cells in high-glucose conditions.

METHODS

Rat aortic smooth muscle cells were cultured with 5.5 or 20 mmol/l glucose with or without C-peptide (1 to 100 nmol/l) for 3 weeks. Proliferation activities, the protein expression of platelet-derived growth factor (PDGF)-beta receptor, the phosphorylation of p42/p44 mitogen-activated protein (MAP) kinases, and glucose uptake were measured.

RESULTS

The proliferation activities increased approximately three-fold under high-glucose conditions (p<0.05). C-peptide suppressed hyperproliferation activities that were induced by high glucose. This happened in a dose-dependent manner from 1 to 100 nmol/l of C-peptide. C-peptide (10 and 100 nmol/l) inhibited the increased protein expression of PDGF-beta receptor and the phosphorylation of p42/p44 MAP kinases that had been induced by high glucose (p<0.05). Furthermore, 100 nmol/l of C-peptide augmented the impaired glucose uptake in the high-glucose conditions.

CONCLUSIONS/INTERPRETATION

These observations suggest that C-peptide could prevent diabetic macroangiopathy by inhibiting smooth muscle cell growth and ameliorating glucose utilisation in smooth muscle cells. C-peptide may thus be a novel agent for treating diabetic macroangiopathy in patients with type 1 and type 2 diabetes.

Locally born olfactory bulb stem cells proliferate in response to insulin-related factors and require endogenous insulin-like growth factor-I for differentiation into neurons and glia

After late embryogenesis, new neurons are continuously added to the olfactory bulb (OB) from stem cells located in the forebrain subventricular zone. Nonetheless, stem cells have not been described within the embryonic olfactory bulb. Here we report the isolation of local olfactory bulb stem cells from the embryonic day 12.5-14.5 mouse embryo. These cells were 99.2% nestin positive and proliferated extensively in culture to at least 150 cell doublings. Clonal analysis demonstrated that neurons (TuJ1(+)), astrocytes (GFAP(+)), and oligodendrocytes (O4(+)) could be generated from single-plated cells, indicating that they are multipotent. At least 90% of proliferating cells expressed insulin-like growth factor-I (IGF-I), (pro)insulin, and their cognate receptors; these growth factors collaborated with fibroblast growth factor-2 plus epidermal growth factor (EGF) to promote stem cell proliferation and sphere formation. Cells from Igf-I(-)/- mice, however, proliferated as extensively as did Igf-I(+/+) cells. Differentiation and survival of stem cell-generated neurons and glia showed strong dependence on exogenous IGF-I, but oligodendrocyte differentiation also required insulin at low concentration. Furthermore, the percentages of stem cell-generated neurons, astrocytes, and oligodendrocytes were markedly lower in the cultures prepared from the Igf-I(-)/- mice compared with those of Igf-I(+/+). Concordantly, lack of IGF-I resulted in abnormal formation of the olfactory bulb mitral cell layer and altered radial glia morphology. These results support the presence within the embryonic mouse olfactory bulb of stem cells with specific requirements for insulin-related growth factors for proliferation or differentiation. They demonstrate that IGF-I is an endogenous factor regulating the differentiation of stem and other precursor cells within the olfactory bulb.

Th2 dominance of T helper cell response to preproinsulin in individuals with preclinical type 1 diabetes

In human type 1 diabetes (T1D) autoantibodies to insulin precede clinical disease, while little is known about the contribution of insulin-specific T lymphocytes-in particular, T helper (Th) subsets. Here we have studied the in vivo primed cytokine response to preproinsulin in peripheral blood mononuclear cells (PBMCs) and two major Th cell subsets-CD45RO+ memory cells and CD45RA+ naive/resting cells-in 35 individuals with HLA-DRB1*04, DQB1*0302 diabetes risk marker: 12 patients with T1D, 12 autoantibody-positive (Ab+) individuals, and 11 healthy controls. Cytokine secretion (TNF-alpha, IFN-gamma, IL-2, IL-4, IL-5, and IL-10) was measured in the supernatants of the cultures stimulated with 21 overlapping preproinsulin peptides as well as proinsulin and insulin. In Ab+ individuals our results reveal higher IL-4 levels in CD45RO+ memory cells and higher IL-5 levels in CD45RA+ naive/resting cells, while higher IL-2 production was found in PBMCs. In contrast, in PBMCs of T1D patients higher IFN-gamma and IL-10 secretion was found. Our data delineate characteristic cytokine patterns in peripheral T lymphocytes from patients at different stages of the T1D development.

Unprocessed proinsulin promotes cell survival during neurulation in the chick embryo

We have chosen a vertebrate model accessible during neurulation, the chick, for analysis of endogenous insulin signaling and its contribution to early embryonic cell survival. Unlike rodents, humans and chickens have a single preproinsulin gene, facilitating its prepancreatic expression characterization. We show that in vivo interference with embryonic insulin signaling using antisense oligonucleotides against the insulin receptor increases apoptosis during neurulation. In contrast, high glucose administration does not increase the level of apoptosis in culture or in vivo. Exogenous insulin and, remarkably, proinsulin achieve similar survival protective effects at 10(-8) mol/l. The low abundant preproinsulin mRNA from the prepancreatic embryo is translated to a protein that remains as unprocessed proinsulin. This concurs with the absence of prohormone convertase 2 (PC2) in the embryo, whereas PC2 is present later in embryonic pancreas. A C-peptide--specific antibody stains proinsulin-containing neuroepithelial cells of the chick embryo in early neurulation, as well as other cells in mesoderm- and endoderm-derived structures in the 2.5-day embryo. We have determined by 5'-RACE (rapid amplification of cDNA ends), and confirmed by RNase protection assay, that prepancreatic and pancreatic proinsulin mRNA differ in their first exon, suggesting differential transcriptional regulation. All these data support the role of endogenous proinsulin in cell survival in the chick embryo during important pathophysiologic periods of early development.

Stimulation in vivo of expression of intra-abdominal adipose tissue plasminogen activator inhibitor Type I by proinsulin

AIMS/HYPOTHESIS

Impaired fibrinolytic system capacity secondary to increased plasminogen activator inhibitor type-1 expression has been suggested as a pathogenetic link between insulin resistance and increased cardiovascular risk in patients with Type II (non-insulin-dependent) diabetes mellitus, obesity, or both. In patients with syndromes of insulin resistance including those with Type II diabetes, precursors of insulin such as proinsulin can constitute more than 50% of insulin-like molecules in blood. The aim of this study was to determine whether proinsulin can increase plasminogen activator inhibitor type-1 expression in intra-abdominal adipose tissue in vivo, potentially contributing to the increased PAI-1 seen with insulin resistance.

METHODS

Lightly sedated normal rabbits were given intravenous proinsulin, insulin, or vehicle alone under euglycaemic clamp conditions with serial sampling of blood and assessment of PAI-1 expression in visceral fat.

RESULTS

Both proinsulin and insulin increased expression of plasminogen activator inhibitor type-1 in intra-abdominal adipose tissue, 5.3-fold (p = 0.006 vs control) and 2.5-fold (p = 0.031 vs control) respectively. PAI-1 inhibitor activity in blood peaked 3 h after administration of each, 5.1-fold, p = 0.020, and 3.4-fold, p = 0.004, respectively but did not change under control conditions.

CONCLUSION/INTERPRETATION

Hyperproinsulinaemia can contribute to increased expression of plasminogen activator inhibitor type-1 in intra-abdominal adipose tissue implicated in increasing PAI-1 activity in blood, impaired fibrinolysis, and accelerated atherogenesis typical of Type II diabetes.

Effect of insulin, proinsulin, and amylin on renin release from perfused rat kidney

To evaluate the possible role of insulin, proinsulin, and amylin in the renin-angiotensin system, the direct effect of these peptides on renin release was examined using perfused kidney of rats. Renin release was significantly increased from a basal value of 6.1 +/- 1.8 to a peak value of 10.1 +/- 2.3 ng angiotensin I (Ang I)/mL/h by 0.5 nmol insulin, from 6.0 +/- 1.7 to 16.7 +/- 4.5 ng Ang I/mL/h by 1 nmol insulin, and from 6.1 +/- 1.8 to 18.0 +/- 5.5 ng Ang I/mL/h by 8 nmol insulin. Renin release was not significantly changed by perfusion of 0.05 nmol proinsulin or amylin but significantly increased from a basal value of 6.1 +/- 1.7 to a peak value of 8.1 +/- 3.6 ng Ang I/mL/h by 1 nmol proinsulin, from 5.6 +/- 1.7 to 12.1 +/- 3.8 ng Ang I/mL/h by 8 nmol proinsulin, from 5.7 +/- 1.9 to 8.2 +/- 3.5 ng Ang I/mL/h by 1 nmol amylin, and from 5.2 +/- 2.0 to 12.4 +/- 3.3 ng Ang I/mL/h by 8 nmol amylin. The concentration of cyclic adenosine monophosphate in the effluent was significantly increased from a basal value of 5.1 +/- 1.6 to a peak value of 10.6 +/- 2.5 mmol/min by 8 nmol amylin but not altered by perfusion of insulin or proinsulin. The addition of 0.05 nmol proinsulin and 0.05 nmol of amylin on 0.5 nmol insulin significantly enhanced renin release. These results indicate that insulin may play an important physiologic role in the renin-angiotensin system and suggest that proinsulin and amylin may be involved in the genesis and development of hypertension through enhancement of insulin-stimulated renin release.

Inhibition of platelet aggregation of a mutant proinsulin molecule engineered by introduction of a native Arg-Gly-Asp sequence

A 13 amino acid sequence, CRVARGDWNDNYC, originated from disintegrin eristostatin, was introduced into an inactive human proinsulin molecule between the B29 and A2 sites to replace proinsulin C-peptide by molecular cloning techniques. The constructed Arg-Gly-Asp (RGD)-proinsulin gene was cloned into a temperature-inducible vector pBV220 and expressed in Escherichia coli. The expressed RGD-proinsulin was refolded and purified by Sephadex G50 and DEAE-Sephadex A25 separations. The chemical identity was confirmed by both amino acid composition and mass spectrometry analyses. This RGD-proinsulin showed an inhibitory activity of adenosine 5'-diphosphate-induced human platelet aggregation with an IC50 value of 200 nM. Its insulin receptor binding activity remained as low as 0.03% with native insulin as a control, and its insulin immune activity retained 27.6% compared with proinsulin.

Proinsulin stimulates growth of small intestinal crypt-like cells acting via specific receptors

The mechanisms that regulate cell turnover in the intestinal epithelium are incompletely understood. Here we tested the hypothesis that proinsulin, present in serum and pancreatic juice in picomolar concentrations, stimulates growth of the rat small intestinal crypt-like cell line IEC-6 under serum-free conditions. Proinsulin binding was assessed by competitive ligand binding studies. Proinsulin and insulin-like growth factor I (IGF-I) stimulated cell proliferation up to threefold above controls, with half-maximal action already in the picomolar range and with additive effects. In early confluent cell monolayers, proinsulin bound with higher affinity (IC50 1.3 +/- 0.05 nM) and capacity (87,200 +/- 2,500 receptors/cell) than IGF-I (4.0 +/- 0.6; 23,700 +/- 2,200, P < 0. 05). C-peptide competed with 10-fold lower affinity for binding of 125I-proinsulin but not for 125I-IGF-I or 125I-insulin, suggesting a specific binding epitope of the proinsulin molecule within or close to the C-peptide region. In contrast, insulin showed approximately 100-fold lower binding affinity and growth-promoting potency than proinsulin or IGF-I. We conclude that proinsulin stimulates growth of small intestinal crypt cells through specific binding and may play a physiological role in the regulation of intestinal epithelial cell proliferation.

Augmentation of arterial endothelial cell expression of the plasminogen activator inhibitor type-1 (PAI-1) gene by proinsulin and insulin in vivo

Diabetes mellitus is associated with an increased incidence and greater severity of primary atherosclerosis as well as restenosis after angioplasty for reasons not yet clear. We have shown previously that insulin and proinsulin-typically elevated in blood in patients with type II diabetes-increase plasma activity of plasminogen activator inhibitor type 1 (PAI-1)in vivo. Others have demonstrated that increased PAI-1 activity is associated with coronary heart disease. Accordingly, the present study was performed to identify sites of increased expression of the PAI-1 gene within the vessel wall. Equimolar concentrations of insulin, proinsulin, or vehicle alone as a control, were administered intravenously over 1 h to conscious rabbits that were kept euglycemic throughout by the use of glucose clamping. Within 3 h plasma PAI-1 activity increased from 1.15+/-1.34 to 11.33+/-4.30 AU/ml with insulin (mean+/-s.d., P=0.015) and from 2.83+/-0.74 to 15.43+/-4.70 AU/ml with proinsulin (P=0.035). This was found to be in contrast to the controls where the increase in plasma PAI-1 activity was of lesser degree (2.43+/-1.86 to 6.80+/-1.10 AU/ml, P=n.s., n=4 each). As judged from the results of in situ hybridization, the site of prominent aortic expression of the PAI-1 gene was the endothelium. Furthermore, expression increased further in this site after administration of insulin or proinsulin. As judged from results of immunohistochemistry, PAI-1 protein in the aorta was also prominent in endothelium. These results suggest that "hyper(pro)insulinemia", increases PAI-1 not only in blood but also in arterial endothelium. Thus, attenuation of vasculopathy and especially of restenosis after angioplasty in type II diabetes may be possible with somatic gene therapy targeting PAI-1 expression in endothelial cells.

A novel assay in vitro of human islet amyloid polypeptide amyloidogenesis and effects of insulin secretory vesicle peptides on amyloid formation

Human islet amyloid polypeptide (IAPP) is a 37-residue peptide that is co-secreted with insulin by the beta-cell and might be involved in the pathogenesis of non-insulin-dependent diabetes mellitus. We developed an improved assay in vitro based on the fluorescence of bound thioflavin T to study factors affecting amyloidogenesis. Monomeric IAPP formed amyloid fibrils, as detected by increased fluorescence and by electron microscopy. Fluorimetric analysis revealed that the initial rate of amyloid formation was: (1) proportional to the peptide monomer concentration, (2) maximal at pH 9.5, (3) maximal at 200 mMKCl, and (4) proportional to temperature from 4 to 37 degreesC. We found that 5-fold and 10-fold molar excesses of proinsulin inhibited fibril formation by 39% and 59% respectively. Insulin was somewhat more potent with 5-fold and 10-fold molar excesses inhibiting fibril formation by 69% and 73% respectively, whereas C-peptide had no effect at these concentrations. Thus at physiological ratios of IAPP to insulin, insulin and proinsulin, but not C-peptide, can retard amyloidogenesis. Because insulin resistance or hyperglycaemia increase the IAPP-to-insulin ratio, increased intracellular IAPP compared with insulin expression in genetically predisposed individuals might contribute to intracellular amyloid formation, beta-cell death and the genesis of non-insulin-dependent diabetes mellitus.

Differential effects of proinsulin C-peptide fragments on Na+, K+-ATPase activity of renal tubule segments

Proinsulin C-peptide has been shown to stimulate the activity of Na+ K+ ATPase of rat renal tubule segments. Thirty-six peptides and amino acids, corresponding to parts of the intact rat C-peptide and suitable controls were screened for capacity to stimulate Na+, K+-ATPase in an attempt to determine potential active sites in the C-peptide molecule. The carboxy-terminal tetra and penta peptides were found to elicit 92-103% of the intact molecule's activity, and the remaining segment, des-(27-31) C-peptide, did not possess stimulatory activity. Peptides from the middle C-peptide segment, however, centering around a GGPEAG sequence, stimulated Na+, K+-ATPase activity (36-80% of the intact molecule's effect) but this effect was not balanced by corresponding inactivity of other parts. Furthermore, it was paralleled by activity of a non-native dipeptide D-form. It is concluded that the latter effect and that of the middle segment may represent complex interactions other than the apparently specific effects of the C-terminal segment.

Introduction of apoptosis by high proinsulin and glucose in cultured human umbilical vein endothelial cells is mediated by reactive oxygen species

There is much evidence that diabetes and hyperglycaemia contribute to the impairment of endothelial function and induce severe changes in the proliferation, the adhesive and synthetic properties of endothelial cells. Induction of apoptosis could represent one mechanism to prevent the new accumulation of those vascular defects and to allow generation of vascular endothelium. In this study, we demonstrate that high concentrations of glucose or proinsulin induce apoptosis in human umbilical endothelial cells by three independent methods (DNA fragmentation, fluorescence activated cell sorting analysis, and morphology). The number of apoptotic cells was increased by glucose (30 mmol/l or proinsulin (100 nmol/l) from less than 10% to about 30%. Activation of protein kinase C (PKC) largely prevented the induction of apoptosis, whereas inhibition of PKC further increased the number of apoptotic cells. Similar changes as induced by glucose were also observed after incubation of the cells with the non-metabolisable 3-O-methylglucose. These findings indicate that hyperglycaemic conditions stimulate the induction of apoptosis in endothelial cells by a mechanism which is independent from the formation of diacylglycerol and the activation of PKC. The induction of apoptosis by the non-metabolisable glucose suggests that formation of oxygen derived radicals by autoxidative processes is involved and may lead to an activation of transcription factors such as nuclear transcription factor-kappaB (NF-kappaB) transferring the activation signal into the nucleus and leading to changes in gene expression necessary for induction of apoptosis.

Attenuation by gemfibrozil of expression of plasminogen activator inhibitor type 1 induced by insulin and its precursors

BACKGROUND

Insulin and its precursors found in increased plasma concentrations in non-insulin-dependent diabetes mellitus (NIDDM) augment synthesis of plasminogen activator inhibitor type 1 (PAI-1) in Hep G2 cells in vitro and in rabbit liver in vivo. Reduced endogenous fibrinolysis secondary to increased PAI-1 activity may exacerbate atherogenesis. Recently, the reduction of the coronary heart disease incidence in the Helsinki Heart Study has implicated favorable modulation of endogenous fibrinolysis by gemfibrozil.

METHODS AND RESULTS

In Hep G2 cells, 500 (700) mumol/L gemfibrozil decreased basal secretion of PAI-1 by 26% (43%) (P = .012 and P = .021, respectively) and attenuated insulin-induced (10 nmol/L) augmentation of PAI-1 in conditioned media by 61% (109%) (P = .010) within 24 hours. Inhibition was dependent on the duration of exposure (0 to 48 hours) and on the concentration of gemfibrozil (0 to 700 mumol/L) but not on the concentration of insulin (0.1 to 100 nmol/L). Gemfibrozil attenuated the augmentation of PAI-1 secretion induced by proinsulin (> 100%), by des(31,32)proinsulin (75%), and by des(64,65) proinsulin (77%) as well (10 nmol/L each). The specificity of these effects was confirmed by the unaltered levels of newly synthesized protein (metabolic labeling) and of total protein (both in conditioned media and cell lysates). Secretion of fibrinogen by Hep G2 cells was not affected by gemfibrozil. Changes in PAI-1 protein levels reflected modulation of PAI-1 gene expression as manifested by changes in levels of 3.2-kb PAI-1 mRNA (Northern blots).

CONCLUSIONS

Gemfibrozil attenuated the augmentation of synthesis and secretion of PAI-1 induced by insulin and its precursors directly and specifically. Accordingly, gemfibrozil may exert favorable therapeutic effects normalizing impaired fibrinolysis in patients with hyperinsulinemia such as NIDDM.

The role of insulin and proinsulin in the regulation of triglyceride metabolism

The mechanisms underlying the associations between abnormal glucose tolerance and myocardial infarction are poorly understood. It has often been suggested that an increased plasma insulin concentration is causally linked to many of the metabolic abnormalities that are associated with abnormal glucose tolerance, although this suggestion remains controversial. Recently it has been proposed that proinsulin and proinsulin-like molecules may also be involved in the atherogenic process. Both hyperinsulinaemia and insulin resistance are associated with fasting hypertriglyceridaemia and both increased VLDL production and increased plasma triglyceride concentrations commonly occur in association with abnormal glucose tolerance and atheromatous vascular disease. In order to study the effects of insulin, proinsulin and proinsulin-like molecules on hepatic triglyceride secretion we have undertaken experiments in vitro using the liver cell line HepG2. In conjunction with these in vitro experiments we have also studied, in vivo, the associations between insulin, proinsulin, proinsulin-like molecules and plasma triglyceride concentrations in subjects with both normal and abnormal glucose tolerance. Our results in vitro show that proinsulin and proinsulin-like molecules have similar and not different effects to insulin but are less biologically active. In vivo, our results show that concentrations of insulin, proinsulin and proinsulin-like molecules per se are not an important determinant of plasma triglyceride concentrations. Both abnormal NEFA suppression during an oral glucose tolerance test and increased central adiposity are closely linked to poor glucose tolerance and are the most important determinants of plasma triglyceride concentrations. Taken together these results suggest that it is not insulin nor proinsulin concentrations per se that are causally linked to hypertriglyceridaemia. We suggest that abnormal NEFA suppression plays an important part in the increase in risk of vascular disease associated with insulin resistance.

High affinity binding sites for proinsulin in human IM-9 lymphoblasts

Proinsulin and insulin binding in IM-9 lymphoblasts show curvilinear Scatchard plots, which may be explained by two binding sites, negative cooperativity of receptors, or both. Using flow-cytometric analysis of insulin binding, we were able to distinguish and separate two different IM-9 cell fractions. In both fractions, Scatchard plots for specific binding of insulin and proinsulin were linear, suggesting the presence of two distinct populations of receptors. Type 1 cells showed low capacity but high affinity of insulin binding (16,300 +/- 3,000 sites/cell; Kd 0.4 +/- 0.1 nmol/l). Proinsulin and insulin-like growth factor 1 (IGF-1) were significantly less potent in competition. MA-20, a specific antibody against human insulin receptors, inhibited insulin binding by 80%, while the specific antibody against human IGF-1 receptors, alpha IR-3, had no effect. Pretreatment with insulin decreased insulin binding by 90%. 125I-insulin displayed stepwise dissociation with the rate markedly enhanced by cold insulin. Type 2 cells exhibited significantly different binding characteristics with higher capacity but lower affinity of 125I-insulin binding (430,000 +/- 25,000 sites/cell, p < 0.001 vs type 1; Kd 2 +/- 0.4 nmol/l, p < 0.02 vs type 1). Proinsulin competed with similar potency for insulin binding, while IGF-1 was still less potent. 125I-proinsulin showed a significantly higher binding affinity than 125I-insulin (Kd 0.5 +/- 0.3 nmol/l, p < 0.05) with 50,000 +/- 10,000 binding sites/cell. C-peptide was able to compete for 125I-proinsulin, but not for 125I-insulin binding. MA-20 did not influence 125I-proinsulin binding, but inhibited 125I-insulin binding by 50%, whereas alpha IR-3 increased proinsulin binding 1.5-fold with no effect on insulin binding. Preincubation with insulin decreased insulin binding by 50% and proinsulin binding by 10%. The dissociation of 125I-proinsulin showed linear first-order kinetics and was not significantly accelerated by cold proinsulin. Furthermore, the tyrosine phosphorylation of a 65 kDa protein was stimulated to a significantly greater extent by proinsulin than by insulin, indicating activation of different signalling cascades. DNA analysis revealed that type 1 cells were predominantly in the G1 phase, whereas type 2 cells were in the S and G2 + M phases of the cell cycle. We conclude, that IM-9 lymphoblasts were separated by flow-cytometry into one fraction with typical insulin receptors and a second fraction with high affinity binding sites for proinsulin. High affinity proinsulin binding sites were distinguished from typical insulin receptors by: 1) higher affinity for proinsulin than insulin, 2) inhibition of proinsulin binding by C-peptide but not by the insulin receptor antibody MA-20, 3) non-co-operative first order dissociation kinetics of proinsulin binding, 4) resistance to down-regulation by insulin, and 5) differences in signal transduction.

Autocrine/paracrine role of insulin-related growth factors in neurogenesis: local expression and effects on cell proliferation and differentiation in retina

Early neurogenesis progresses by an initial massive proliferation of neuroepithelial cells followed by a sequential differentiation of the various mature neural cell types. The regulation of these processes by growth factors is poorly understood. We intend to understand, in a well-defined biological system, the embryonic chicken retina, the role of the insulin-related growth factors in neurogenesis. We demonstrate the local presence of signaling elements together with a biological response to the factors. Neuroretina at days 6-8 of embryonic development (E6-E8) expressed proinsulin/insulin and insulin-like growth factor I (IGF-I) mRNAs as well as insulin receptor and IGF type I receptor mRNAs. In parallel with this in vivo gene expression, E5 cultured neuroretinas synthesized and released to the medium a metabolically radiolabeled immunoprecipitable insulin-related peptide. Furthermore, insulin-related immunoreactive material with a HPLC mobility close to that of proinsulin was found in the E6-E8 vitreous humor. Exogenous chicken IGF-I, human insulin, and human proinsulin added to E6 cultured neuroretinas showed relatively close potencies stimulating proliferation, as determined by [methyl-3H]thymidine incorporation, with a plateau reached at 10(-8) M. These factors also stimulated neuronal differentiation, indicated by the expression of the neuron-specific antigen G4. Thus, insulin-related growth factors, interestingly including proinsulin, are present in the developing chicken retina and appear to play an autocrine/paracrine stimulatory role in the progression of neurogenesis.

Similar peptides from two beta cell autoantigens, proinsulin and glutamic acid decarboxylase, stimulate T cells of individuals at risk for insulin-dependent diabetes

BACKGROUND

Insulin (1) and glutamic acid decarboxylase (GAD) (2) are both autoantigens in insulin-dependent diabetes mellitus (IDDM), but no molecular mechanism has been proposed for their association. We have identified a 13 amino acid peptide of proinsulin (amino acids 24-36) that bears marked similarity to a peptide of GAD65 (amino acids 506-518) (G. Rudy, unpublished). In order to test the hypothesis that this region of similarity is implicated in the pathogenesis of IDDM, we assayed T cell reactivity to these two peptides in subjects at risk for IDDM.

MATERIALS AND METHODS

Subjects at risk for IDDM were islet cell antibody (ICA)-positive, first degree relatives of people with insulin-dependent diabetes. Peripheral blood mononuclear cells from 10 pairs of at-risk and HLA-DR matched control subjects were tested in an in vitro proliferation assay.

RESULTS

Reactivity to both proinsulin and GAD peptides was significantly greater among at-risk subjects than controls (proinsulin; p < 0.008; GAD; p < 0.018). In contrast to reactivity to the GAD peptide, reactivity to the proinsulin peptide was almost entirely confined to the at-risk subjects.

CONCLUSIONS

This is the first demonstration of T cell reactivity to a proinsulin-specific peptide. In addition, it is the first example of reactivity to a minimal peptide region shared between two human autoimmune disease-associated self antigens. Mimicry between these similar peptides may provide a molecular basis for the conjoint autoantigenicity of proinsulin and GAD in IDDM.

Comparison of the determination of insulin by a monoclonal antibody-based immunofluorometric assay and by radioimmunoassay

This paper describes an immunofluorometric assay (IFMA) for insulin and compares it with the classical radioimmunoassay (RIA). Monoclonal antibodies against insulin were produced and used to develop the IFMA. One, immobilized on microtiter plates, was used for capture, the other, labelled with Europium, was used as tracer antibody. The IFMA presents sensitivity to an amount of insulin of 3 pmol/l and acceptable values for intra- and interassay error. The IFMA presented superimposable curves for human insulin, Arg65/Gly66-split proinsulin and des-Lys64,Arg65, and no cross-reactivity with human proinsulin, Arg32/Glu33-split and des-Arg31,Arg32. The RIA showed 100% cross-reactivity with human proinsulin, 90% with Arg32/Glu33-split, 193% with Arg65/Gly66-split, 340% with des-Arg31,Arg32 and 170% with des-Lys64,Arg65. The assays were used to measure insulin in 300 serum samples from 50 subjects submitted to an oral glucose tolerance test (OGTT). Twenty were normal, 10 had impaired glucose tolerance and 20 non-insulin-dependent diabetes mellitus. The mean value (+/- SEM) obtained by IFMA was 166.7 +/- 12.1 pmol/l and the mean value obtained by RIA was 339.6 +/- 18.6, with a correlation of r = 0.80 (P < 0.01). Comparison of basal insulin levels of the different groups of individuals using IFMA or RIA led to the same conclusions. The area under the curve showed statistically significant differences only for the comparison between normal lean subjects and individuals with impaired glucose tolerance, when measured by RIA. Our data stress the importance of methodology definition when comparing insulin results.(ABSTRACT TRUNCATED AT 250 WORDS)

Induction of plasminogen activator inhibitor type-1 (PAI-1) by proinsulin and insulin in vivo

BACKGROUND

Fasting hyperinsulinemia (reflected by elevations in immunoreactive "insulin") is typical of patients with non-insulin-dependent diabetes mellitus (NIDDM) and is often associated with obesity and hypertension. The elevated concentrations detected are indicative not only of insulin but also of its immunologically cross-reactive precursors, including proinsulin. Fasting hyperinsulinemia appears to be associated with decreased fibrinolytic activity in blood, which results from increased activity of plasminogen activator inhibitor type-1 (PAI-1), a potential independent risk factor for coronary artery disease. Patients who were given proinsulin in a previous clinical study by others exhibited an increased incidence of cardiovascular events. Thus, a "proinsulin-PAI-1 axis" may predispose to coronary thrombosis. To define the possible presence of such an axis, this study was designed to determine whether insulin, its precursors, or both increase the concentrations of PAI-1 in rabbits in vivo.

METHODS AND RESULTS

Equimolar proinsulin (n = 10), insulin (n = 11), C-peptide (n = 4), or vehicle alone (n = 10) was administered intravenously over 1 hour to euglycemic, conscious rabbits. Plasma PAI-1 activity increased 3.8-fold with proinsulin (P = .002) and 3.6-fold with insulin (P = .002). By contrast, no increase occurred after C-peptide or vehicle was administered. The increased PAI-1 activity was shown to be attributable to PAI-1 protein by reverse fibrin autography. As judged from changes in mRNA in tissues, proinsulin and insulin increased PAI-1 gene expression within 3 hours by 2.1- and 2.1-fold, respectively, in aorta (P = .025 each) and by 1.9- and 2.4-fold in liver (P = .015 and P = .001), with return of values to baseline within 24 hours (n = 4 experiments in each case).

CONCLUSIONS

These results extend our previous observations from studies in vitro and suggest that hyperinsulinemia attributable to augmented concentrations of proinsulin and insulin in plasma increase plasma PAI-1 activity and may contribute to acceleration of atherosclerosis and impairment of coronary thrombolysis in patients with NIDDM.

Physiological concentrations of insulin induce cellular desensitization to the mitogenic effects of insulin-like growth factor I

Insulin and insulin-like growth factor I (IGF-I) are structurally related peptides capable of stimulating a variety of metabolic and mitogenic processes. In this study, we investigated the interaction between these peptides and their receptor-mediated pathways in an untransformed cell line. Cultured bovine fibroblasts specifically bound IGF-I and insulin, and each peptide could stimulate DNA synthesis and cell replication through its own receptor. Preincubation of bovine fibroblasts with concentrations of insulin that did not bind to the IGF-I receptor resulted in complete but reversible cellular desensitization to IGF-I-stimulated mitogenesis. Preincubation with as little as 0.1 nM insulin was sufficient to inhibit subsequent IGF-I action. Various insulin analogs produced desensitization in direct relation to the affinity of the insulin for the insulin receptor. Desensitization required > 4 h of cell exposure to insulin and was blocked in the presence of cycloheximide. Neither serum-stimulated mitogenesis nor IGF-I-stimulated glucose uptake were affected by insulin pretreatment. 125I-labeled IGF-I affinity cross-linking experiments indicated that preincubation with insulin did not affect labeling of the 130,000-M(r) alpha-subunit of the IGF-I receptor, but was associated with the loss of IGF-I- and insulin-inhibitive bands at M(r) = 100,000, 85,000, 58,000, and 34,000. These studies suggest that insulin, via interaction with insulin receptors on bovine fibroblasts, regulates IGF-I action at a step distal to IGF-I receptor binding and are consistent with desensitization occurring at an intracellular step in the mitogenic pathway shared by insulin and IGF-I.

Augmentation of the synthesis of plasminogen activator inhibitor type-1 by precursors of insulin. A potential risk factor for vascular disease

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.

The effects of subcutaneous human proinsulin on the production of 64/65 split proinsulin, glucose turnover and intermediary metabolism in non-insulin-dependent diabetic man

We have compared the effects of subcutaneously injected human proinsulin, insulin zinc suspension and inactive diluent (control) on glucose turnover, intermediary carbohydrate and lipid metabolism in non-insulin-dependent diabetic man. Six weight-matched (24.8 +/- 1.6 kg M-2) non-insulin-dependent diabetic subjects underwent 3 separate, randomized, 10 h isoglycemic clamps. Glucose turnover was measured using a primed continuous infusion of [6'6'2H2] glucose. Each subject received 0.35 U/kg of hormone or control made up to isovolumetric amounts. The mean blood glucose level of 7.3 +/- 0.8 mmol/l was similar at the start of each isoglycemic clamp. Incremental area under the curve proinsulin levels (1195 +/- 146 nmol/l) were about 21-fold higher, on a molar basis, than insulin (62.4 +/- 10 nmol/l). Des 64/65 split proinsulin increased in a parallel manner to intact proinsulin (r = 0.99, P < 0.0001) and comprised approximately 13% of the intact proinsulin concentration. Hepatic glucose production was suppressed similarly following proinsulin and insulin zinc injection. However, both proinsulin and insulin zinc had a significantly greater effect on suppression of hepatic glucose production compared to control (P = 0.01, P = 0.009, respectively). Metabolic clearance rate of glucose fell significantly during the control studies compared to insulin zinc or proinsulin injections (P < 0.05). Blood lactate, pyruvate and alanine concentrations were similar following control or hormone injections. However blood glycerol, 3-hydroxybutyrate and plasma-non-esterified fatty acids were suppressed significantly by proinsulin and insulin zinc compared to control injections. The conclusions were: (1) In overnight fasted hyperglycemic non-insulin-dependent subjects s.c. injections of proinsulin and insulin zinc can produce similar effects on glucose turnover, intermediary lipid and carbohydrate metabolism. (2) Similar carbohydrate intermediary metabolism profiles can be obtained following insulin zinc, proinsulin or control injections. (3) However lipolysis and ketogenesis were significantly suppressed by both hormones compared to control. (4) Subcutaneous proinsulin injection resulted in approximately 13% conversion to des 64/65 split proinsulin.

Assessment of proinsulin's effects on intermediary metabolism using the forearm technique in normal man

We have compared the effects of human proinsulin and insulin on forearm metabolism. Seven normal, non-obese subjects were infused with 386 pmol/kg per hour of proinsulin and 180 pmol/kg per hour of insulin using the euglycaemic clamp technique. Glucose appearance and utilization rates were quantified using a primed continuous infusion of [6',6'-2H2]glucose. Mean blood glucose was 4.1 +/- 0.1 and 4.1 +/- 0.2 mmol/l during proinsulin and insulin infusions respectively. Basal insulin concentrations increased from 0.02 +/- 0.01 to 0.25 +/- 0.03 nmol/l. The proinsulin infusion was chosen to give steady-state levels approximately 20-fold higher on a molar basis than those of insulin, based on previous findings that proinsulin has only 5% the biological potency of insulin. Basal proinsulin concentrations increased from 0.003 to 5.4 +/- 0.3 nmol/l. Hepatic glucose production was suppressed similarly during the last hour of each hormone infusion: 0.07 +/- 0.16 (proinsulin, P), and 0.01 +/- 0.13 (insulin, I) mg/kg per minute. Glucose disposal, however, was significantly increased during the final hour of the insulin infusion: 4.7 +/- 0.4 (I) and 3.4 +/- 0.2 (P) mg/kg per minute (P = 0.025). Net forearm glucose uptake (FGU) increased by a greater amount during insulin compared with proinsulin infusion: 1.44 +/- 0.02 (I) and 0.71 +/- 0.01 (P) mumol/100 ml forearm per minute (P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of human proinsulin on glucose turnover and intermediary metabolism in insulin-dependent-diabetes mellitus

We have compared the action of human proinsulin and insulin on glucose turnover, intermediary carbohydrate, and lipid metabolism in insulin-dependent-diabetic (IDDM) subjects. Six, young, weight-matched (23 +/- 2 kg-2) IDDM subjects underwent separate hyperinsulinemic euglycemic clamps. Three, low dose, iv infusions of both insulin and proinsulin were used to construct dose response curves. The proinsulin infusions were chosen to give steady state levels approximately or equal to 20-fold higher on a molar basis than insulin, based on previous findings that proinsulin has only 5-10% the biological potency of insulin. Hepatic glucose production, measured using [6'6'2H2]glucose, was suppressed equally by proinsulin and insulin at the three dose levels; (I1) 2.8 +/- 0.7 (P1) 3.3 +/- 0.6, (I2) 2.3 +/- 0.9 (P2) 3.3 +/- 1.1, (I3) -2.0 +/- 1.7 (P3) -1.1 +/- 0.6 mumol/kg min-1. Percentage elevation of glucose disposal was significantly increased during the insulin infusions compared to proinsulin; (I1) 132 +/- 12 (P1) 78 +/- 4 p < 0.01; (I2) 157 +/- 18 (P2) 104 +/- 14; P < 0.05; (I3) 242 +/- 23 (P3) 159 +/- 24 p = 0.02. Dose response curve analysis demonstrated that proinsulin stimulated glucose disposal approximately or equal to 3.7% whereas suppression of HGP was congruent to 5.7% compared to insulin. Proinsulin had a significantly weaker effect than insulin, at the lowest infusion dose, in percent suppression of plasma nonesterified fatty acids (I1 34 +/- 4, P1 14 +/- 15%; P < 0.05), blood glycerol (I1 47 +/- 4, P1 30 +/- 3%; P < 0.01) and 3-hydroxybutyrate levels (I1 81 +/- 7, P1 42 +/- 17%; P < 0.05). Proinsulin caused significant net reductions in blood lactate levels compared to insulin at each infusion dose; (P1) -130 +/- 34, (I1) -32 +/- 30 mumol/L (P < 0.05) (P2) -139 +/- 76 (I2) +8 +/- 65 mumol/L (P < 0.05) (P3) 48 +/- 60 (I3) 230 +/- 64 mumol/L (P < 0.05). We conclude that in IDDM: 1) proinsulin has a preferential effect on the liver compared to muscle, in terms of glucose handling; 2) proinsulin may have a different effect on lactate metabolism compared to insulin; 3) proinsulin at the lowest dose resulted in an inability to suppress lipolysis and ketogenesis; 4) glucose turnover can be underestimated using [6'6'2H2]glucose.

Stimulation by proinsulin of expression of plasminogen activator inhibitor type-I in endothelial cells

In patients with non-insulin-dependent diabetes mellitus, concentrations in plasma of insulin and its precursors, proinsulin and split proinsulin, are increased. Because increased concentrations of plasminogen activator inhibitor type-1 (PAI-1) occur also, we hypothesized that proinsulin and split proinsulin may augment endothelial cell PAI-1 expression, thereby potentially attenuating endogenous fibrinolysis and accelerating atherosclerosis. Proinsulin increased PAI-1 activity in conditioned media of endothelial cells as did split proinsulin, paralleled by increased expression of PAI-1 mRNA. These effects of proinsulin were not dependent on its conversion to insulin nor on its interactions with the insulin receptor. The proinsulin stimulation of PAI-1 expression was not attenuated by either anti-insulin receptor antibodies or a 100-fold excess of insulin. Furthermore, proinsulin-mediated increases in PAI-1 expression were not inhibited by a 500-fold excess of insulinlike growth factor I. In addition, inhibition of tyrosine kinase, which mediates many of the diverse effects of insulin and insulinlike growth factor I, did not attenuate the effect of proinsulin. These results indicate that proinsulin augments PAI-1 expression, potentially contributing to vasculopathy in patients with non-insulin-dependent diabetes mellitus.

Dose-response characteristics of human proinsulin and insulin in non-insulin-dependent diabetic humans

We compared the actions of human proinsulin and insulin on glucose turnover and on intermediary carbohydrate and lipid metabolism in non-insulin-dependent diabetes mellitus (NIDDM). Six diet-controlled weight-matched (25.4 +/- 1.0 kg/m2) NIDDM subjects underwent six separate isoglycemic clamps. Glucose turnover was measured using a primed continuous infusion of [6',6'-2H2]glucose. Each subject received three low-dose intravenous infusions of both insulin and proinsulin. Blood glucose was maintained at 6.7 +/- 0.3 mM during proinsulin and insulin infusion. Insulin (I) infusions gave steady-state levels of 0.12 +/- 0.001 (I1), 0.18 +/- 0.01 (I2), and 0.33 +/- 0.01 nM (I3). Steady-state proinsulin (P) levels were 2.5 +/- 0.1 (P1), 4.3 +/- 0.2 (P2), and 8.8 +/- 0.9 nM (P3). Hepatic glucose production was suppressed equally by proinsulin and insulin at all doses. The metabolic clearance rate of glucose was significantly increased during the insulin infusion compared with proinsulin. The use of [6',6'-2H2]glucose resulted in a mean underestimation of the glucose infusion rate of 10.0 +/- 4.0 and 6.0 +/- 2.5% during the two highest insulin and proinsulin doses, respectively. Proinsulin had a significantly weaker effect than insulin, at the lowest infusion dose, in percent suppression of plasma nonesterified fatty acids, blood glycerol, and beta-hydroxybutyrate levels (all P less than 0.05). Blood lactate levels were lower during the P1 (628 +/- 43 microM) and P2 (657 +/- 93 microM) infusions compared with I1 (776 +/- 60 microM) and I2 (878 +/- 44 microM; P less than 0.05, P less than 0.02), respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Biosynthetic human proinsulin. Review of chemistry, in vitro and in vivo receptor binding, animal and human pharmacology studies, and clinical trial experience

OBJECTIVE

To describe the rationale for the preclinical and clinical developmental course of human proinsulin (HPI), the second product after human insulin for the treatment of diabetes mellitus to be manufactured by DNA technology.

RESEARCH DESIGN AND METHODS

The relevant and available published and unpublished preclinical and clinical information generated on pork proinsulin and human proinsulin has been integrated to demonstrate how certain clinically attractive features of pork proinsulin (a soluble intermediate-acting and possibly hepatospecific insulin agonist) led to the development of HPI.

RESULTS

Clinical pharmacology studies demonstrated that HPI was definitely, although marginally, hepatospecific. More striking was the finding that the intrasubject/patient coefficient of variation of response to HPI was significantly less than that observed with NPH insulin. However, the fact that unique efficacy in controlled multicenter studies was not demonstrated suggested that these pharmacological features were not translated into clinical benefit. In one multicenter new patient study there were six myocardial infarctions, including two deaths, in patients treated for greater than or equal to 1 yr with HPI and none in the control group.

CONCLUSIONS

To obtain an independent review of the risks and benefits of HPI, in February 1988, Lilly convened a consultant group that examined all relevant information on HPI available. These experts shared our concerns about the safety of HPI in light of the failure to demonstrate unique efficacy. Accordingly, clinical trials with HPI were suspended in February 1988. Experience with HPI demonstrates the challenge associated with the development of new drugs in general and insulin agonists in particular.

Hypoglycemic potency and metabolic clearance rate of intravenously administered human proinsulin and metabolites

Since circulating proinsulin has been suggested to be important in the pathogenesis of noninsulin-dependent diabetes, and biosynthetic human proinsulin (HPI) may have a therapeutic role in patients with diabetes mellitus, the biological activity of proinsulin metabolites is of potential significance. Moreover, recent studies have suggested that the majority of circulating proinsulin immunoreactivity consists of metabolites. We, therefore, compared the blood glucose-lowering ability and MCR of the two proinsulin metabolites des-(31,32)HPI and des-(64,65)HPI with intact HPI in seven anesthetized dogs after an overnight fast. Intravenous bolus injections of 12.5 micrograms HPI/kg BW and equimolar amounts of des-(31,32)HPI and des-(64,65)HPI were given on three separate occasions. In addition to blood glucose, des-(31,33)HPI, des-(64,65)HPI, and HPI were measured using an insulin RIA and peptide-specific standard curves. Kinetic parameters were derived by fitting two exponentials to the respective decay curves. The MCR of HPI (3.3 +/- 0.1 ml/kg.min) was significantly lower (P less than 0.05) than that of des-(64,65)HPI (6.4 +/- 0.6 ml/kg.min), but was not significantly different from that of des-(31,32)HPI (3.8 +/- 0.4 ml/kg.min). The MCR of biosynthetic insulin (17.2 +/- 1.8 ml/kg.min), as measured in three of the dogs, was higher than that of HPI or the two metabolites. The blood glucose-lowering ability (defined as nadir glucose/fasting glucose, expressed as a percentage) of des-(64,65)HPI (49.3 +/- 5.0%) was significantly greater (P less than 0.05) than that of intact HPI (87 +/- 2.2%), and the glucose-lowering ability of des-(31,32)HPI (75.2 +/- 3.8%) was intermediate. In conclusion, HPI metabolites are more biologically active than intact HPI. The extent of in vivo conversion of proinsulin to metabolites may enhance the biological activity of proinsulin and, thus, have physiological, pathophysiological, and therapeutic significance.

Metabolic effects of combination glipizide and human proinsulin treatment in NIDDM

The effects of 3 mo of treatment with a combination of glipizide and human proinsulin were studied in a small group of closely monitored patients. The patients had non-insulin-dependent diabetes mellitus (NIDDM) and poor glucose regulation, despite maximal sulfonylurea therapy. This was a randomized double-blind placebo-controlled trial in which there were three treatment groups who received either 20 mg glipizide given 30 min before breakfast and dinner and human proinsulin given subcutaneously at bedtime (n = 5), glipizide and human proinsulin placebo (n = 5), or glipizide placebo and human proinsulin (n = 5). Glycemic regulation was assessed by measurements of 24-h plasma glucose profiles and glycosylated hemoglobin. Our observations demonstrate that the combination of glipizide plus human proinsulin was more effective than either agent alone in controlling overall glycemia in patients with NIDDM. The data support the concept of use of an agent during the day that has its major effects postprandially and another agent at bedtime that is relatively hepatospecific.

Regulation of testicular function by insulin and transforming growth factor-beta

Hyperinsulinism is associated with disorders of androgen production in humans. We have studied the effects of insulin and insulin-like growth factor-1 on androgen production in vitro using a crude preparation of mouse Leydig cells incubated with luteinizing hormone in a serum-free medium. We found a positive correlation between testosterone production and the luteinizing hormone dose over 3 hours. Exposure of the cells for 1 hour to insulin (1 micrograms/ml) prior to the addition of luteinizing hormone significantly augmented the amount of testosterone produced in response to the gonadotropin when added after this preincubation. In contrast, prior exposure of the cells to proinsulin (30 micrograms/ml), insulin-like growth factor-1 (30 ng/ml), or epidermal growth factor-1 (1 micrograms/ml) did not influence the testosterone response to luteinizing hormone. Transforming growth factor-beta reduced the testosterone response to luteinizing hormone. Transforming growth factor-beta (1,000 pg/ml) blocked the insulin augmentation of luteinizing hormone-stimulated testosterone production. We conclude that insulin has an endocrine effect on testosterone production by mouse Leydig cells in vitro. Furthermore, the Leydig cell response to insulin is itself sensitive to interaction with transforming growth factor-beta which may operate as part of the paracrine control of Leydig cell function.

Comparisons of insulin and biosynthetic human proinsulin actions in cultured hepatocytes. Kinetics and biologic potencies

The binding and biologic potencies as well as kinetics of action of human biosynthetic proinsulin (HPro) were studied in primary cultures of rat hepatocytes. HPro had 3% (on a molar basis) of the potency of porcine insulin for displacing (125I)-TyrA14-insulin from receptors. Maximally effective concentrations of insulin and HPro caused similar stimulations of 14C-glucose incorporation into glycogen and glycogen synthase activity. However, the dose response curve for HPro stimulation of glycogen synthase was shifted far to the right (EC50 = 4.1 +/- 1.1 nM) of that for insulin (.09 +/- .01). The relative biologic potency of HPro was approximately 3%. Biologically equivalent maximal doses of insulin (8.3 nM) and HPro (53.2 nM) stimulated glycogen synthase activity with similar time courses; half maximal between 15-30 min with maximal effects at 60 min. Deactivation of glycogen synthase upon removal of the hormone was very rapid for both hormones. The relative binding and biologic potencies of HPro compared to insulin in liver (approximately 3%) were similar to values previously seen in adipocytes. This fact, together with the similarity of kinetics of action, suggest that the in vivo hepatoselectivity of HPro is not a property of the target cell itself.

In vitro activity of biosynthetic human diarginylinsulin

In diarginylinsulin two arginine residues are located at the C-terminal end of the B-chain (ArgB31 and ArgB32). This accounts for a shift of the isoelectric point from pH 5.4 in native insulin to pH 7.0 in diarginylinsulin leading to pharmacodynamic characteristics of an intermediate acting insulin when administered s.c. as pH 4.0-5.0 solution. We have investigated insulin receptor binding and biological activity of biosynthetic human diarginylinsulin in human adipocytes and compared to native insulin and proinsulin. Association- and dissociation studies of insulin receptor binding revealed no differences for diarginylinsulin and native insulin. In competition studies under steady-state binding conditions, half-maximal displacement of tracer occurred at 352 +/- 33 pmol/l, 337 +/- 32 pmol/l and 3640 +/- 480 pmol/l for diarginylinsulin, insulin and proinsulin, respectively. The biologic potency of human diarginylinsulin was evaluated by the ability to stimulate D-glucose transport and by the assessment of the antilipolytic activity. Activation of D-glucose transport was half-maximal at 49.6 +/- 5.4 pmol/l (diarginylinsulin), 44.8 +/- 5.8 pmol/l (insulin) and at 476.7 +/- 134.3 pmol/l (proinsulin). Half-maximal inhibition of lipolysis occurred at 13.9 +/- 3.4 pmol/l, 15.4 +/- 2.9 pmol/l and 138.4 +/- 38.6 pmol/l, respectively. In conclusion, diarginylinsulin has almost identical insulin receptor binding characteristics and full biological activity in vitro compared to native insulin. This pharmacodynamically intermediate acting insulin preparation is therefore of potential therapeutical value.

Transcriptional induction of glucokinase gene by insulin in cultured liver cells and its repression by the glucagon-cAMP system

Primary cultures of rat hepatocytes were used to investigate the regulation of glucokinase gene expression by insulin and glucagon. Insulin added in physiological concentrations to the culture medium causes de novo induction of glucokinase mRNA. The induced plateau is reached 4 to 8 h after insulin addition, and the mRNA level remains high as long as insulin is present. Comparison of the potencies of insulin, proinsulin, and insulin-like growth factor I in this system indicates that induction by insulin is mediated via the insulin receptor. The magnitude of the insulin effect is independent of the extracellular glucose concentration. Run-on transcription assays with isolated nuclei show that the mRNA build up depends primarily on a specific stimulation of glucokinase gene transcription. Glucagon added to hepatocytes together with a supramaximal concentration of insulin prevents induction of glucokinase mRNA in a dose-dependent manner. The inhibitory effect of glucagon is mimicked by 8-(4-chlorophenylthio)-cAMP. The effect of this agent has also been tested in hepatocytes first induced for maximal glucokinase gene transcription by culture with insulin alone for 12 h. The transcriptional activity of the gene as measured by run-on assay was completely turned off within 30 min after addition of the cyclic nucleotide. Under these conditions, glucokinase mRNA decays rapidly, with an apparent half-life of 45 min. The mRNA degradation rate was similarly rapid after insulin withdrawal from induced cells. Thus, a cAMP-mediated repression mechanism is a key aspect in the regulation of glucokinase gene transcription in the hepatocyte. Insulin may act by relieving the gene from repression.

The characterization, regulation, and function of insulin receptors on osteoblast-like clonal osteosarcoma cell line

The properties and regulation of insulin receptors on monolayers of cultured clonal osteoblastic rat osteosarcoma UMR-106 cells and human osteosarcoma U20S cells were studied. Confluent cultures of UMR-106 cells bound lactoperoxidase-labeled, HPLC-purified [125I]A-14-monoiodinated insulin in a reversible, saturable, and specific manner. Binding was related inversely to the incubation temperature. Prolonged period of steady-state binding was achieved at all temperatures studied. Competition curves demonstrated half-maximal inhibition of [125I]insulin binding at an unlabeled insulin concentration of about 1 nM. Scatchard analysis of the binding data was curvilinear, suggesting negative cooperativity, and revealed that UMR-106 osteoblasts contained about 87,000 receptor sites per cell according to a two-site model. Bound [125I]insulin dissociated from osteoblasts with a t1/2 of about 15 minutes at 22 degrees C. The dissociation curve was multiexponential, and the addition of native insulin accelerated the dissociation of intact but not degraded [125I]insulin. Preincubation with 125 nM insulin for 1 h induced 70% loss of binding sites and reduced total insulin bound by 30%. When monolayers were treated with the lysosomotropic agent chloroquine, a 40% increase in cell-associated radioactivity that could not be dissociable in fresh buffer was observed. The use of an energy depleter, sodium fluoride, completely inhibited the effects of chloroquine. Similar results were obtained for human osteosarcoma U20S cells except that the number of receptor sites was far less than that of UMR-106 cells. Insulin increased collagen synthesis at a half-maximal concentration of 1 nM. To conclude, cultured rat and human osteoblasts possess insulin receptors that exhibit kinetic properties and specificity similar to those of other insulin target cells. Receptor-bound insulin is internalized and degraded by a chloroquine-sensitive, energy-requiring reaction. Insulin receptor on bone cells modulates the synthesis of collagen and this role may be important in bone homeostasis.

Insulin suppresses hepatitis B surface antigen expression in human hepatoma cells

The human hepatoma Hep3B cells contain integrated hepatitis B viral genome and continually secret hepatitis B surface antigen (HBsAg). The production of HBsAg (but not alpha-fetoprotein) was suppressed by addition of low concentrations (0.1-1 nM) of insulin into serum-free medium. In addition, the suppression of HBsAg production by insulin was paralleled with the decrease in HBsAg mRNA abundance. Insulin also cause a rapid rate of disappearance of HBsAg mRNA (t 1/2, 2 h) in Hep3B cells. The Hep3B cells carry specific receptor with high affinity for insulin (Kd = 1.8 nM). The receptor showed an insulin-dependent protein tyrosine kinase activity. The half-maximal insulin concentration for the activation of the receptor kinase was about 5 nM. Only very high concentrations of insulin-like growth factor I and human proinsulin can compete for the insulin receptor binding and suppress HBsAg production, this suggests that insulin may act through its receptor binding to suppress HBsAg expression in human hepatoma Hep3B cells.

Stimulation of protein synthesis in stage IV Xenopus oocytes by microinjected insulin

The effects of intracellular insulin on protein synthesis were examined in intact cells and isolated, undiluted cellular components. [35S]Methionine incorporation into protein was measured in Stage IV oocytes from Xenopus laevis maintained under paraffin oil. Radiolabel and insulin were introduced into the cytoplasm by microinjection. After a short delay (approximately 15 min), injected insulin stimulated the rate of methionine incorporation. Stimulation was dose-dependent, increasing with injected doses in the 7-50-fmol range. Neither proinsulin nor insulin-like growth factor 1 were as effective as insulin in stimulating protein synthesis; microinjected epidermal growth factor and the A and B chains of insulin were without effect. When oocyte surface membranes were removed under oil, the resulting cytoplasm-nucleus samples exhibited methionine incorporation rates that were comparable to those found in intact cells. Microinjection of insulin increased rates of methionine incorporation in cytoplasm-nucleus samples; the effects of external (prior to transfer to oil) and internal (microinjection in oil) insulin exposure were additive. Cytoplasm samples (nuclei and surface membranes removed under oil) also synthesized protein and responded to microinjected insulin. However, insulin responses were reduced relative to cells and to cytoplasm-nucleus samples. 125I-Insulin was degraded rapidly after microinjection into oocytes. Degradation occurred in both the nucleus and cytoplasm. Degradation was delayed by injecting bacitracin into the cells and delaying degradation increased the effectiveness of a low dose of injected insulin. Together, the data show that insulin can act at external, nuclear, and cytoplasmic sites to stimulate protein synthesis in Xenopus oocytes. The signaling pathway activated by internal insulin does not involve plasma membrane-generated second messengers and appears to be separate from that activated by external hormone. Finally, although microinjected insulin is degraded rapidly, it is the intact hormone rather than a degradation product that stimulates protein synthesis.

Receptor binding and biologic activity of biosynthetic human insulin and mini-proinsulin produced by recombinant gene technology

Human insulin and its precursor, mini-proinsulin, made with a new biosynthetic method, were tested for their receptor binding, biologic action, and antibody binding ability. The structure of mini-proinsulin is similar to that of proinsulin with a shortened C-peptide, B(1-29)-Ala-Ala-Lys-A(1-21) insulin. The ability of biosynthetic human insulin to bind to receptors, to stimulate 2-deoxyglucose uptake in isolated adipocytes, and to bind to insulin antibody was comparable to that of semisynthetic human insulin. The ability of mini-proinsulin to bind to insulin receptors and to stimulate 2-deoxyglucose uptake in adipocytes was 0.5 and 0.2% that of human insulin, whereas the corresponding abilities of proinsulin were 5 and 3%, respectively. Despite having less receptor binding and biologic activity, mini-proinsulin demonstrated higher affinity for the insulin antibody than did proinsulin. These results suggest that biosynthetic human insulin behaves similarly to semisynthetic human insulin in its receptor binding and biologic activity, and that the shortened C-peptide region reduces receptor binding by fixing or covering the N-terminal region of the A chain, which is important for receptor binding.

Reciprocal regulation of gene transcription by insulin. Inhibition of the phosphoenolpyruvate carboxykinase gene and stimulation of gene 33 in a single cell type

Two H4IIE hepatoma cell genes, phosphoenolpyruvate carboxykinase (PEPCK) and gene 33 (g33), are reciprocally regulated by insulin. Quantitation of mRNAPEPCK and mRNAg33 in total RNA isolated from cells treated with insulin showed a 7-fold increase in mRNAg33 amount and a 3-fold decrease of mRNAPEPCK. The cAMP analog 8-(4-chlorophenylthio)-cAMP induced mRNAPEPCK but had no effect on mRNAg33. The responses to various insulins and related molecules showed that the insulin receptor mediates the effects of physiologic concentrations of insulin on each of these genes. This inverse pattern of regulation by insulin was further characterized by determining the transcription rates of both genes in nuclei isolated at various times after the addition of insulin and 8-(4-chlorophenylthio)-cAMP to H4IIE cells. Insulin increased the rate of synthesis of mRNAg33 from 35 to 354 ppm and decreased the synthesis of mRNAPEPCK from 1175 to 109 ppm. These effects of insulin occurred rapidly and reached their maxima by 60 min. In both cases, greater effects were observed as insulin concentrations were increased from 10(-12) to 10(-8) M. Although the effects of insulin were concentration-dependent for both genes, the PEPCK gene was significantly more sensitive to low concentrations of insulin than was gene 33. The reciprocal effects of insulin on the synthesis of mRNAPEPCK and mRNAg33 in H4IIE cells provide a means of investigating how a hormone can exert opposing effects on two genes in the same cell.