Expression Of Proinsulin Research Articles

Upstream AUGs in embryonic proinsulin mRNA control its low translation level.

Proinsulin is expressed prior to development of the pancreas and promotes cell survival. Here we study the mechanism affecting the translation efficiency of a specific embryonic proinsulin mRNA. This transcript shares the coding region with the pancreatic form, but presents a 32 nt extended leader region. Translation of proinsulin is markedly reduced by the presence of two upstream AUGs within the 5' extension of the embryonic mRNA. This attenuation is lost when the two upstream AUGs are mutated to AAG, leading to translational efficiency similar to that of the pancreatic mRNA. The upstream AUGs are recognized as initiator codons, because expression of upstream ORF is detectable from the embryonic transcript, but not from the mutated or the pancreatic mRNAs. Strict regulation of proinsulin biosynthesis appears to be necessary, since exogenous proinsulin added to embryos in ovo decreased apoptosis and generated abnormal developmental traits. A novel mechanism for low level proinsulin expression thus relies on upstream AUGs within a specific form of embryonic proinsulin mRNA, emphasizing its importance as a tightly regulated developmental signal.

Tetracycline-regulated secretion of human insulin in transfected primary myoblasts

A mechanism for safely regulating transgene expression will be necessary for gene therapy approaches to endocrine disorders. In this study, a two-plasmid tetracycline-inducible system was used to regulate expression of human proinsulin (hppI1) and a mutated proinsulin construct (hppI4, allowing cleavage by furin) in primary rat soleus myoblasts. In hppI1 and hppI4 transient transfections, the presence of 0.01 and 0.1 μg/ml tetracycline for 48 h inhibited pro/insulin secretion to 19–27% and 7–12%, respectively, compared to tetracycline untreated myoblasts. Following a 48 h tetracycline incubation (1.0 μg/ml), pro/insulin secretion in hppI1 and hppI4 transfected myoblasts was reduced to <4% of that in cells incubated without tetracycline. Pro/insulin secretion equivalent to that of untreated cells was restored following tetracycline withdrawal and incubation for a further 72 h. Conversion of proinsulin to insulin in transfected myoblasts was <1% for hppI1 and >45% for hppI4. In conclusion, regulated insulin secretion has been achieved in a dose-dependent and reversible manner in primary myoblasts.

Acceleration of type 1 diabetes mellitus in proinsulin 2-deficient NOD mice.

Accumulating evidence favors a role for proinsulin as a key autoantigen in diabetes. In the mouse, two proinsulin isoforms coexist. Most studies point to proinsulin 2 as the major isoform recognized by T cells in the NOD mouse. We studied mice in which a null proinsulin 2 mutation was transferred from proinsulin 2-deficient 129 mice onto the NOD background along with 16 genetic markers (including I-A(g7) MHC molecule) associated with diabetes. Intercross mice from the fourth backcross generation showed that proinsulin 2(-/-) mice develop accelerated insulitis and diabetes. The high prevalence of anti-insulin autoantibodies in proinsulin 2(-/-) mice indicates that diabetes acceleration relates to altered recognition of proinsulin. The prevalence of anti-glutamic acid decarboxylase autoantibodies and of sialitis is not increased in proinsulin 2(-/-) mice. We give evidence that proinsulin 2 expression leads to silencing of T cells specific for an epitope shared by proinsulin 1 and proinsulin 2. In the human, alleles located in the VNTR region flanking the insulin gene control beta cell response to glucose and proinsulin expression in the thymus and are key determinants of diabetes susceptibility. Proinsulin 2(-/-) NOD mice provide a model to study the role of thymic expression of insulin in susceptibility to diabetes.

Acceleration of type 1 diabetes mellitus in proinsulin 2–deficient NOD mice

Accumulating evidence favors a role for proinsulin as a key autoantigen in diabetes. In the mouse, two proinsulin isoforms coexist. Most studies point to proinsulin 2 as the major isoform recognized by T cells in the NOD mouse. We studied mice in which a null proinsulin 2 mutation was transferred from proinsulin 2–deficient 129 mice onto the NOD background along with 16 genetic markers (including I-Ag7 MHC molecule) associated with diabetes. Intercross mice from the fourth backcross generation showed that proinsulin 2–/– mice develop accelerated insulitis and diabetes. The high prevalence of anti-insulin autoantibodies in proinsulin 2–/– mice indicates that diabetes acceleration relates to altered recognition of proinsulin. The prevalence of anti–glutamic acid decarboxylase autoantibodies and of sialitis is not increased in proinsulin 2–/– mice. We give evidence that proinsulin 2 expression leads to silencing of T cells specific for an epitope shared by proinsulin 1 and proinsulin 2. In the human, alleles located in the VNTR region flanking the insulin gene control β cell response to glucose and proinsulin expression in the thymus and are key determinants of diabetes susceptibility. Proinsulin 2–/– NOD mice provide a model to study the role of thymic expression of insulin in susceptibility to diabetes.

Development of new expression vector based on Pseudorabies virus amplicons: application to human insulin expression

Pseudorabies virus (PrV), a herpesvirus from the Alphaherpesvirinae subfamily, is suitable for amplicon vector replication and packaging into virions, with helper virus for trans replication and cleavage-packaging functions. PrV amplicon vectors were developed in a bacterial plasmid construction using PrV ori s and pac signals as the required cis elements. Human insulin cDNA was then cloned in the amplicon vector for human proinsulin expression. In the same construction, green fluorescent protein was used as a marker. PrV amplicons may have several advantages over herpes simplex virus type 1 (HSV1) amplicons in human gene therapy because it can infect human cells in vitro and in vivo, it is not pathogenic for primates and there is no pre-existing immunity and risk of recombination with latent PrV as occurs with HSV1.

Molecular target structures in alloxan-induced diabetes in mice

Type 1 diabetes results from irreversible damage of insulin-producing β-cells. In laboratory animals, diabetes can be induced with alloxan (ALX), a 2,4,5,6-tetraoxopyrimidine. ALX is a potent generator of reactive oxygen species (ROS), which can mediate β-cell toxicity. However, the initial lesions on essential β-cell structures are not known. In this study, we report that the glucose transporter 2 (GLUT2) and glucokinase (GK) are target molecules for ALX. Ex vivo, a gradual decrement of both GLUT2 and GK mRNA expression was found in islets isolated from ALX-treated C57BL/6 mice. This reduction was more pronounced for GLUT2 than for GK. The mRNA expression of β-actin was also slightly affected with time after ALX exposure, the proinsulin mRNA, however, remained unaffected as well as the pancreatic total insulin content. Pretreatment with D-glucose (D-G) protected the mRNA expression of GLUT2 and GK against ALX toxicity and prevented diabetes. Yet, in these euglycemic mice, an impaired oral glucose tolerance persisted. Pretreatment with 5-thio-D-glucose (5-T-G) failed to prevent ALX diabetes, administration of zinc sulfate (Zn 2+)-enriched drinking water, however, reduced ALX-induced hyperglycemia. In conclusion, ALX exerted differential toxicity on β-cell structures similar to in vitro results reported from this laboratory. Furthermore, the present results differ from those reported for the diabetogen streptozotocin (STZ). Injections of multiple low doses (MLD) of STZ reduced GLUT2 expression only, but failed to affect expression of GK and proinsulin as well as β-actin as internal control. MLD-STZ diabetes was prevented by pretreatment with both D-G and 5-T-G and administration of Zn 2+-enriched drinking water. Apparently, ALX and MLD-STZ exert diabetogenicity by different pathways requiring different interventional schedules for prevention.

Human type 2 diabetes: morphological evidence for abnormal beta-cell function.

The exact nature of the beta-cell defect in type 2 diabetic patients is still unclear. beta-Cell mass reduction has been reported but remains controversial. A preliminary study of a large series of patients has demonstrated that in most, the beta-cell defect is not related to a decreased beta-cell mass. Amyloid deposits are observed in the islets of some type 2 diabetic patients but also in normoglycemic subjects. Because it has been claimed that these deposits interfere with beta-cell function, we evaluated in situ the effect of insular amyloid deposits on beta-cell transcription and translation. Pancreases were obtained at autopsy from 28 normoglycemic patients and 41 type 2 diabetic patients. Staining with hemaluneosin and Congo red was used to analyze the general features of the islets and the presence of amyloid deposits, respectively. Immunohistochemistry for proinsulin was performed with an antibody recognizing the junction between B-chain and C-peptide, thus specifically labeling the Golgi area where proinsulin is produced. In seven patients, we evaluated insulin gene transcription by in situ hybridization of proinsulin mRNA combined with Congo red staining, and we evaluated insulin storage by double immunostaining for insulin and amylin. In many type 2 diabetic patients, the islets appeared entirely normal. Amyloid deposits were found in 57% of diabetic subjects and 33% of normoglycemic age-matched control subjects. The percentage of amyloid-infiltrated islets varied from 0.4 to 74%. beta-Cells from amyloid-containing islets still had specific Golgi proinsulin labeling. In obese type 2 diabetic patients, the number of beta-cells with abnormal expression of proinsulin in the whole cytoplasm was significantly higher than in normoglycemic control subjects. Proinsulin mRNA was significantly reduced in islets with amyloid deposits when compared with amyloid-free islets, but the mean reduction did not exceed 16%. Insulin was still present in the beta-cells of amyloid-containing islets, and its amount, estimated by measurement of the insulin-labeling optical density, was not statistically different from that in amyloid-free islets. In conclusion, even in amyloid-containing islets, beta-cells maintain active insulin transcription and translation and normal insulin storage. Taking into account that in most cases only a small proportion of islets are infiltrated by amyloid, the limited reduction in proinsulin mRNA is unlikely to play a major role in the pathogenesis of diabetes.

Increased production of human proinsulin in the periplasmic space of Escherichia coli by fusion to DsbA

The production of human proinsulin in its disulfide-intact, native form in Escherichia coli requires disulfide bond formation and the periplasmic space is the favourable compartment for oxidative folding. However, the secretory expression of proinsulin is limited by its high susceptibility to proteolysis and by disulfide bond formation, which is rate-limiting for proinsulin folding. In this report we describe a method for the production of high amounts of soluble, native human proinsulin in E. coli. We fused proinsulin to the C-terminus of the periplasmic disulfide oxidoreductase DsbA via a trypsin cleavage site. As DsbA is the main catalyst of disulfide bond formation in E. coli, we expected increased yields of proinsulin by intra- or intermolecular catalysis of disulfide bond formation. In the context of the fusion protein, proinsulin was found to be stabilised, probably due to an increased solubility and faster disulfide bond formation. To increase the yield of DsbA-proinsulin in the periplasm, several parameters were optimised, including host strains and cultivation conditions, and in particular growth medium composition and supplement of low molecular weight additives. We obtained a further, about three-fold increase in the amount of native DsbA-proinsulin by addition of l-arginine or ethanol to the culture medium. The maximum yield of native human proinsulin obtained from the soluble periplasmic fraction after specific cleavage of the fusion protein with trypsin was 9.2 mg g −1, corresponding to 1.8% of the total cell protein.

Yeast secretory expression of insulin precursors

Since the 1980s, recombinant human insulin for the treatment of diabetes mellitus has been produced using either the yeast Saccharomyces cerevisiae or the prokaryote Escherichia coli. Here, development of the insulin secretory expression system in S. cerevisiae and its subsequent optimisation is described. Expression of proinsulin in S. cerevisiae does not result in efficient secretion of proinsulin or insulin. However, expression of a cDNA encoding a proinsulin-like molecule with deletion of threonine(B30) as a fusion protein with the S. cerevisiae alpha-factor prepro-peptide (leader), followed either by replacement of the human proinsulin C-peptide with a small C-peptide (e.g. AAK), or by direct fusion of lysine(B29) to glycine(A1), results in the efficient secretion of folded single-chain proinsulin-like molecules to the culture supernatant. The secreted single-chain insulin precursor can then be purified and subsequently converted to human insulin by tryptic transpeptidation in organic aqueous medium in the presence of a threonine ester. The leader confers secretory competence to the insulin precursor, and constructed (synthetic) leaders have been developed for efficient secretory expression of the insulin precursor in the yeasts S. cerevisiae and Pichia pastories. The Kex2 endoprotease, specific for dibasic sites, cleaves the leader-insulin precursor fusion protein in the late secretory pathway and the folded insulin precursor is secreted to the culture supernatant. However, the Kex2 endoprotease processing of the pro-peptide-insulin precursor fusion protein is incomplete and a significant part of the pro-peptide-insulin precursor fusion protein is secreted to the culture supernatant in a hyperglycosylated form. A spacer peptide localised between the leader and the insulin precursor has been developed to optimise Kex2 endoprotease processing and insulin precursor fermentation yield.

Effect of Jindangwon on streptozotocin-induced diabetes

The inhibitory effects of the traditional herbal medicine Jindangwon (JDW) on streptozotocin (ST)-induced diabetic mellitus were studied using the ST-treated diabetic model. Glucokinase activity of pancreatic islets was severely impaired by ST treatment. However, when ST-treated islets were treated with 1 mg/ml of JDW, the enzyme activities of glucokinase and hexokinase were protected, glucose-6-phosphatase was not. When the effects of JDW on ST-induced ATP/ADP ratio of islets were assayed, JDW was effective in restoring of ATP/ADP ratio. In addition, ST decreased the enzyme activities of PDH, while JDW had a protective effect on the enzyme. ST-induced cGMP accumulation was significantly inhibited by JDW treatment. Furthermore, ST-induced nitrite formation was significantly inhibited by JDW treatment. JDW also showed the suppressed nitrite production in ST-treated pancreatic islet cells. When the islets (200/condition) were treated with ST (5 mM for 30 min), and then JDW was added to the ST-treated cells, 1.0 mg/ml of JDW showed the activated and recovered aconitase activity in pancreatic islet cells. When the effect of ST on the gene expression of pancreatic GLUT2 and glucokinase were examined, the level of GLUT2 and glucokinase mRNA in pancreatic islets was significantly decreased. However, JDW protected and improved the expression of protein and genes, indicating that JDW is effective on ST-induced inhibition of gene expression of GLUT2, glucokinase and proinsulin in islets. These results suggested that JDW is effective in this model to treat ST-induced diabetes.

Adenovirus-mediated transfer of a modified human proinsulin gene reverses hyperglycemia in diabetic mice.

The human proinsulin cDNA was introduced into a replication-defective adenovirus and was found to confer proinsulin expression to a hepatocyte (H4-II-E) cell line upon infection. A second virus was constructed in which the dibasic prohormone convertase recognition sequence was mutated to a tetrabasic furin cleavage site. Cells infected with this virus synthesized both proinsulin and mature insulin. Gel filtration chromatography, competition of insulin binding, and activation of the insulin receptor kinase activity demonstrated that this mature insulin was functionally identical to that of authentic processed insulin. Injection of these viral constructs into the external jugular vein of mice resulted in insulin gene expression in the liver. Expression from the mutated proinsulin virus dramatically improved the glycemic state of diabetic mice. However, the effects of the viral infection were transient, being maximal at approximately 5-7 days and returning to steady-state levels by 14-21 days. These data demonstrate that somatic cell insulin gene delivery by the use of recombinant adenovirus can be used to transiently reverse the diabetic state in mice.

GLUT2 in pancreatic islets: crucial target molecule in diabetes induced with multiple low doses of streptozotocin in mice.

In mice, diabetes can be induced by multiple low doses of streptozotocin (MLD-STZ), i.e., 40 mg/kg body wt on each of 5 consecutive days. In this model, diabetes develops only when STZ induces both beta-cell toxicity and T-cell-dependent immune reactions. The target molecule(s) of MLD-STZ-induced beta-cell toxicity are not known, however. In this study, we report that GLUT2 is a target molecule for MLD-STZ toxicity. Ex vivo, a gradual decrement of both GLUT2 protein and mRNA expression was found in pancreatic islets isolated from MLD-STZ-treated C57BL/6 male mice, whereas mRNA expression of beta-actin, glucokinase, and proinsulin remained unaffected. Significant reduction of both GLUT2 protein and mRNA expression was first noted 1 day after the third STZ injection, clearly preceding the onset of hyperglycemia. The extent of reduction increased with the number of STZ injections administered and increased over time, after the last, i.e., fifth, STZ injection. The STZ-induced reduction of GLUT2 protein and mRNA was not due to an essential loss of beta-cells, because ex vivo, not only the total RNA yield and protein content in isolated islets, but also proinsulin mRNA expression, failed to differ significantly in the differently treated groups. Furthermore, islets isolated from MLD-STZ-treated donors responded to the nonglucose secretagogue arginine in a pattern similar to that of solvent-treated donors. Interestingly, the MLD-STZ-induced reduction of both GLUT2 protein and mRNA was prevented by preinjecting mice with 5-thio-D-glucose before each STZ injection. Apparently, GLUT2 is a crucial target molecule of MLD-STZ toxicity, and this toxicity seems to precede the immune reactions against beta-cells.

The expression and characterization of human recombinant proinsulin- like growth factor II and a mutant that is defective in the O- glycosylation of its E domain

The expression and characterization of human recombinant proinsulin- like growth factor II and a mutant that is defective in the O- glycosylation of its E domain

Differential expression of rat pancreatic islet beta-cell glucose transporter (GLUT 2), proinsulin and islet amyloid polypeptide genes after prolonged fasting, insulin-induced hypoglycaemia and dexamethasone treatment.

The question posed by these studies was whether chronic adaptive changes in glucose-stimulated insulin secretion are accompanied by comparable changes in islet Beta-cell glucose transporter (GLUT 2) gene expression. Control, fasted (3-day), insulin-injected hypoglycaemic (5-day), and dexamethasone-treated (4-day) rats (n = 5 for each condition), were studied. After fasting significant decrements in proinsulin mRNA/microgram RNA (-32%, p < 0.05) and islet amyloid polypeptide mRNA/microgram RNA (-44%, p < 0.05) were observed, while there was no change in GLUT 2 mRNA/microgram RNA (-13%, p > 0.05). After insulin-induced hypoglycaemia, decrements in proinsulin mRNA/microgram RNA (-49%, p < 0.01) and islet amyloid polypeptide mRNA/microgram RNA (-44%, p < 0.01) were also observed, with no change in islet GLUT 2 mRNA/microgram RNA (-18%, p > 0.05). Dexamethasone treatment resulted in a marked stimulatory effect on proinsulin mRNA/microgram RNA (+236%, p < 0.001) and islet amyloid polypeptide mRNA/microgram RNA (+221%, p < 0.01), while again there was no change in islet GLUT 2 mRNA/microgram RNA (+0.3%, p > 0.05). Quantitative immunoblot analysis with a GLUT 2 specific antibody revealed no change in islet GLUT 2 protein with fasting, but a small decrease (-39 +/- 11%) in islet GLUT 2/microgram protein after insulin-induced hypoglycaemia. These results do not support the hypothesis that chronic changes in glucose-stimulated insulin secretion are accompanied by changes in GLUT 2 expression. In contrast to the lack of correlation with GLUT 2, there was a striking correlation between proinsulin and islet amyloid polypeptide mRNAs for all experimental conditions (r = 0.974, p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Somatic gene therapy for diabetes with an immunological safety system for complete removal of transplanted cells.

To develop somatic gene therapy for diabetes, we studied an animal model with proinsulin-producing fibroblasts with an immunological safety system. Cultured mouse fibroblasts of the Ltk- cell line were transfected first with the efficient human proinsulin expression vector pBMG-Neo-Ins. Initially, 2 x 10(6) cells with a proinsulin-production rate of 91 ng.24 h-1.10(6) cells-1 were transplanted i.p. into streptozocin-induced diabetic C3H mice. The blood glucose concentrations improved between the first and the 28th day, but the animals died of hypoglycemia between the 29th and 46th days. The proinsulin-producing Ltk- cells were further transfected with a second plasmid, pHEBo-CD8.2, encoding BALB/c mouse T-cell differentiation antigen. The CD8.2 allotype is different from CD8.1 allotype by only one amino acid substitution and should be only slightly antigenic to the recipient C3H mice. Somatic gene therapy with these doubly transfected cells followed by the consecutive administration of a monoclonal antibody to CD8.2 resulted in an initial decrease of blood glucose concentrations followed by the permanent recurrence of hyperglycemia, thus proving the complete removal of the transplanted cells. Cultured fibroblasts were thus proven capable of supplying sufficient proinsulin to lower the blood glucose concentrations in diabetic animals. The immunological safety system with a combination of artificial expression of cell surface antigen and the administration of the specific monoclonal antibody was an effective safety system for somatic gene therapy.

High level expression of proinsulin in the yeast, Saccharomyces cerevisiae

Human proinsulin (PI) has been expressed to a high level (100 mg/liter) as a human Superoxide dismutase-PI fusion protein in the yeast, Saccharomyces cerevisiae. At the junction of the two proteins is a methionine residue, allowing PI to be released from the fusion by reaction with cyanogen bromide. The fusion is expressed using a regulated, hybrid promoter containing the regulatory region of the alcohol dehydrogenase II promoter and the 3' end of a glyceraldehyde-3-phosphate dehydrogenase promoter, allowing the recombinant yeast cells to be stably maintained. Production of the fusion protein is induced by growth in medium lacking a fermentable carbon source. The heterologous fusion protein is probably insoluble within the cell, since electron microscopy reveals the presence of ‘inclusion bodies’. In a cell-free extract the fusion protein is also insoluble, but can be solubilized with sodium dodecyl sulfate, and cleaved with cyanogen bromide. The PI that is produced contains incorrect disulfide bonds. After sulfitolysis, the product can be easily purified, renatured, and processed to yield insulin.

Expression of a β-Lactamase Preproinsulin Fusion Protein in Escherichia Coli

The level of expression of a β-lactamase human preproinsulin fusion protein1 in Escherichia coli was increased by plasmid and host manipulations. The initial expression level (0.01% of total protein) was increased two- to three-fold by replacing the p-lactamase promoter with two strong promoters: trp from Serratia marcescens and lacUV5 from E. coli. Replacement of the β-lactamase ribosome binding site (RBS) with a trp RBS enhanced the yield to a total of 0.16% of the total cell protein. Proinsulin was unstable, with a half-life of 14 minutes at 37°C. Proinsulin expression was increased three-fold (to about 0.5% of the total cell protein) by incubating the host at 30°C rather than 37°C.