Biosynthesis of proinsulin and insulin in newborn rat pancreas. Interaction of glucose, cyclic AMP, somatostatin, and sulfonylureas on the (3H) leucine incorporation into immunoreactive insulin

GLP-1 receptor activation improves beta cell function and survival following induction of endoplasmic reticulum stress

Perturbation of endoplasmic reticulum (ER) homeostasis impairs insulin biosynthesis, beta cell survival, and glucose homeostasis. We show that a murine model of diabetes is associated with the development of ER stress in beta cells and that treatment with the GLP-1R agonist exendin-4 significantly reduced biochemical markers of islet ER stress in vivo. Exendin-4 attenuated translational downregulation of insulin and improved cell survival in purified rat beta cells and in INS-1 cells following induction of ER stress in vitro. GLP-1R agonists significantly potentiated the induction of ATF-4 by ER stress and accelerated recovery from ER stress-mediated translational repression in INS-1 beta cells in a PKA-dependent manner. The effects of exendin-4 on the induction of ATF-4 were mediated via enhancement of ER stress-stimulated ATF-4 translation. Moreover, exendin-4 reduced ER stress-associated beta cell death in a PKA-dependent manner. These findings demonstrate that GLP-1R signaling directly modulates the ER stress response leading to promotion of beta cell adaptation and survival.

Control of insulin gene expression by glucose

Northern-blot analysis was used to demonstrate that an increase in extracellular glucose concentration increased the content of preproinsulin mRNA 2.3-fold in the beta-cell line HIT T15. A probe for the constitutively expressed glyceraldehyde-3-phosphate dehydrogenase was used as a control. Mannoheptulose blocked this effect of glucose. A stimulatory effect on preproinsulin mRNA levels was also observed in response to mannose and to 4-methyl-2-oxopentanoate. However, galactose and arginine were ineffective. Glucagon, forskolin and dibutyryl cyclic AMP also elicited an increase in HIT-cell preproinsulin mRNA. The ability of the 5' upstream region of the preproinsulin gene to mediate the effect of glucose and other metabolites on transcription was studied by using a bacterial reporter gene technique. HIT cells were transfected with a plasmid, pOK1, containing the upstream region of the rat insulin-1 gene (-345 to +1) linked to chloramphenicol acetyltransferase (CAT). Co-transfection with a plasmid pRSV beta-gal containing beta-galactosidase driven by the Rous sarcoma virus promoter was used as a control for the efficiency of transfection; expression of CAT activity in transfected HIT cells was normalized by reference to expression of beta-galactosidase. Glucose caused a dose-dependent increase in expression of CAT activity, with a half-maximal effect at 5.5 mM and a maximum response of 4-fold. Mannoheptulose blocked this effect of glucose. Other metabolites (mannose, 4-methyl-2-oxopentanoate and leucine plus glutamine) were also able to increase insulin promoter-driven CAT expression, but galactose and arginine were ineffective. The stimulatory effect of glucose on CAT expression was not blocked by verapamil and was inhibited by increasing extracellular Ca2+ from 0.4 to 5 mM. Both dibutyryl cyclic AMP and forskolin caused an increase in insulin promoter-driven gene expression in the presence of 1 mM-glucose, but neither agent further increased the level of expression occurring in the presence of a maximally stimulating glucose concentration. The phorbol ester phorbol 12-myristate 13-acetate (PMA) also increased insulin promoter-driven CAT expression in the presence of 1 mM-, but not 11 mM-glucose. Staurosporine blocked the stimulatory effect not only of PMA but also of glucose and of dibutyryl cyclic AMP. We conclude that the 5' upstream region of the insulin gene contains sequences responsible for mediating the stimulatory effect of glucose on insulin-gene transcription.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of chloroquine on biosynthesis, release and degradation of insulin in isolated islets of rat pancreas

Insulin has been reported to degrade inside the islets and islet lysosomal proteases have been thought to take part. As chloroquine is regarded as a potent lysosomotropic agent, an attempt has been made to see whether chloroquine has an influence on intrainsular degradation of insulin. After preculture of collagenase-isolated rat islets at 11 mM glucose together with [3H]leucine for 3 days for labelling newly synthesized insulin, islets were cultured for 1 day at 2.2 or 22 mM glucose with or without 0.02 mM chloroquine. Afterwards, radioactivity was measured in the proinsulin/insulin fraction. For control, the influence of chloroquine during 3-h incubation of both freshly isolated and precultured islets was also studied. During the 1-day culture at 2.2 mM glucose, prelabelled insulin was degraded significantly and addition of chloroquine did not alter the amount of insulin degraded. At 22 mM glucose, no significant amount of insulin had been degraded. During the 3-h incubation of freshly isolated as well as precultured islets, chloroquine was found to inhibit significantly glucose-induced biosynthesis of insulin. Glucose-induced release of insulin, however, was not influenced by chloroquine. It is concluded that chloroquine does not influence glucose-induced release or intra-insular degradation of insulin, but it interferes with the biosynthesis of insulin.

Susceptibility of fetal rat endocrine pancreas to the diabetogenic action of cyproheptadine

The susceptibility of fetal endocrine pancreas to the diabetogenic action of cyproheptadine was investigated. Cyproheptadine (5 or 11 mg/kg) or water (control) was given orally once daily to pregnant rats on Days 13.5-20.5 or on Days 19.5-20.5 of gestation. Fetuses were obtained by cesarean section 24 hr after the last dose. Serum and pancreatic immunoreactive insulin and serum glucose from maternal and fetal animals were measured. Differences in maternal pancreatic insulin, serum insulin, and glucose between control and treated groups were not detected. In contrast, fetal pancreatic and serum insulin concentrations in animals exposed to 2 or 8 doses of cyproheptadine were less than 50% those of control. Drug treatment did not alter fetal pancreatic glucagon, pancreatic somatostatin, serum glucose, pancreas weight, or body weight. The drug-related depletion of fetal pancreatic insulin was reversible; the level returned to normal 3 days after cessation of the drug treatment. A similar depletion of fetal insulin was observed after 8 oral doses (11 mg/kg) of desmethylcyproheptadine, a metabolite which lacks the antiserotonin-antihistaminic properties of the parent compound. In vitro experiments showed that cyproheptadine inhibited the biosynthesis and release of insulin in fetal rat pancreas. These results indicate that cyproheptadine, when given to pregnant rats using a dose which produces no apparent effects in the maternal endocrine pancreas, causes abnormalities in the function of the insulin-secreting B cells in the fetal endocrine pancreas.

Glipizide: a second-generation sulfonylurea hypoglycemic agent. Pharmacology, pharmacokinetics and clinical use

Glipizide is a second-generation sulfonylurea in which the substitutions on the arylsulfonylurea nucleus are large, relatively nonpolar groups. This chemical change increases the intrinsic hypoglycemic activity of the molecule 100-fold on a weight basis compared to first-generation agents. In addition, the pharmacokinetic properties, spectrum and severity of side effects and metabolism of this agent are somewhat different from those of first-generation sulfonylureas. The most important component of the antidiabetic action of glipizide is its effect in potentiating insulin action. Glipizide-mediated increases in nutrient-stimulated insulin secretion may contribute to its antidiabetic action. The drug is effective in controlling the blood glucose in patients with noninsulin-dependent diabetes mellitus. It is at least as effective as and probably more effective than first-generation sulfonylureas in controlling hyperglycemia in diabetes. Glipizide is relatively free of serious side effects and is contraindicated principally in patients with significant liver or kidney disease.

The effect of maternal caffeine ingestion on pancreatic function in the neonatal rat

Pancreatic function was investigated in neonatal suckling offspring of caffeine-ingesting dams, with or without maternal sucrose supplementation, throughout pregnancy and lactation. In offspring of rats ingesting caffeine without sucrose supplementation, there was initial hyperinsulinaemia, followed by a progressive fall of plasma insulin to subnormal levels. This fall in plasma insulin coincided with depletion of pancreatic insulin stores. Both the fall in plasma insulin and depletion of pancreatic insulin stores were prevented by sucrose supplementation of caffeine-ingesting dams. Offspring of dams fed sucrose alone and control offspring also maintained pancreatic insulin stores and circulating insulin levels over the first 14 days of postnatal life. Pancreases from offspring of caffeine-exposed animals tested in vitro showed enhanced sensitivity of the insulin release process to glucose. This was reflected in the glucose concentration required to elicit half-maximal insulin release (2.4 +/- 0.2 mmol/l for caffeine offspring, 2.3 +/- 0.2 mmol/l for caffeine with sucrose, 3.8 +/- 0.3 mmol/l for sucrose and 4.1 +/- 0.3 mmol/l for control offspring, mean +/- SEM). In contrast, offspring of sucrose-supplemented (with or without caffeine) dams showed increased sensitivity of the proinsulin biosynthetic process to glucose, whereas offspring of dams ingesting caffeine alone showed no significant enhancement of the biosynthetic process compared with control offspring. Thus enhanced sensitivity of the insulin secretory process to glucose without a change in the sensitivity of the biosynthetic process in the offspring of the caffeine ingesting (non-sucrose supplemented) dams could explain the progressive depletion of pancreatic insulin stores and eventual hypoinsulinaemia seen in this group.

Zinc and insulin metabolism

A review of experimental studies of the effect of zinc nutrition on insulin metabolism is presented. In addition to a short introduction to the synthesis, secretion, and action of insulin, the effects of zinc deficiency-specifically on glucose tolerance, insulin secretion, insulin synthesis and storage, and on total insulin-like activity-are dealt with. The concentrations of zinc and chromium in serum, pancreas, and liver are compared to those of zinc-deficient animals and pair-fed controls.In contrast to pair-fed controls, zinc-deficient rats had unaltered proinsulin contents after glucose stimulation, but they showed a diminished glucose tolerance, lowered serum insulin content, and an elevated total insulin-like activity. The serum zinc concentration of the deficient animals was greatly reduced and did not change during glucose stimulation, whereas it rose in the case of the pair-fed controls. The serum chromium concentration increased in both groups in response to glucose stimulation. In the pancreas of the deficient animals, the zinc concentration was reduced 60% and it increased during the glucose tolerance test. In the liver there were no significant differences. The chromium concentrations were elevated in both the pancreas and liver of the zinc-deficient rats by 60 and 100%, respectively, and were not influenced by glucose injection.These studies show clearly that nutritional zinc deficiency influences insulin metabolism and action.

The relationship between somatostatin binding and cyclic AMP-stimulated protein kinase inhibition

Somatostatin binding and the ability to inhibit cyclic AMP stimulated protein kinase were investigated utilizing isolated pancreatic islets, anterior pituitary plasma membranes, adipocytes, erythrocyte ghosts, hepatic plasma membranes, and anterior pituitary secretion vesicles. Three types of response were observed. With type I response, somatostatin bound specifically to pancreatic islets and anterior pituitary secretion vesicles and inhibited cyclic AMP stimulated protein kinase. In type II response, adipocytes and anterior pituitary plasma membranes exhibited somatostatin binding but no effect of the ligand on the kinase. In erythrocyte membrane ghosts and hepatic plasma membranes, there was neither specific somatostatin binding nor protein kinase inhibition (type III response). The absence of somatostatin binding in erythrocytes or hepatic plasma membranes cannot be explained by degradation of the ligand per se. Secretory vesicles isolated from the anterior pituitary gland bind somatostatin with an average affinity which exceeds that observed in plasma membrane (for pituitary secretory vesicles Kd1 = 8.5 X 10(-8)M, Kd2 = 5.2 X 10(-7)M; for pituitary membranes Kd1 = 1.9 X 10(-8)M, Kd2 = 8.1 X 10(-7)M). The molar concentration of high affinity binding sites (Ro) for plasma membranes was 6.9 X 10(-10)M; for secretory vesicles 3.6 X 10(-9)M. Calculated in terms of somatostatin binding per U 5'nucleotidase activity, the binding for plasma membranes becomes 8.4 X 10(-14) mole/U 5'nucleotidase; secretory vesicles 4.4 X 10(-13) mole/U 5'nucleotidase. Thus, secretory vesicles are fivefold richer in high affinity receptor sites than plasma membranes. It is suggested that in order for somatostatin to act, both a receptor and an effector unit must be present. In the case of tissues secreting polypeptide hormones by granule extrusion, the secretory vesicle may possess both the receptor and the effector units. It is postulated that during the process of fusion of the plasma and secretory vesicle membranes, a high affinity binding site for somatostatin is incorporated into the plasma membrane, thereby allowing somatostatin to act at a specific locus in the cell in inhibiting hormone release.

Evidence that somatostatin inhibits proinsulin synthesis and insulin release by isolated pancreatic islets of the rat

Conflicting reports on the direction and magnitude of the effect of somatostatin on (pro)insulin synthesis prompted our investigation. Two assays for proinsulin synthesis were designed in which [4,5-3H]-L-leucine incorporation into proinsulin was normalized on the basis of postincubation insulin levels rather than on the number of islets incubated. Somatostatin at a concentration of 10 micrograms/ml inhibited 300 mg/dl glucose-stimulated proinsulin synthesis by 25% from 448 +/- 25 dpm/microunits insulin to 336 +/- 25 dpm/microunits insulin (disintegrations per minute in the proinsulin peak per microunit extractable insulin) (p less than 0.05). Glucagon (10 micrograms/ml) reversed the inhibitory effect of somatostatin on proinsulin synthesis from 336 +/- 25 dpm/microunits insulin to 480 +/- 44 dpm/microunits insulin (p less than 0.02). Somatostatin (10 micrograms/ml) had no significant effect on proinsulin synthesis in the presence of 70 mg/dl or 150 mg/dl glucose. Insulin release in 300 mg/dl glucose was inhibited 38% by 10 micrograms/ml somatostatin from 3.05 +/- 0.40 mU medium/mU tissue to 1.90 +/- 0.10 mU medium/mU tissue (p less than 0.01) over a 45-minute incubation period. These data suggest that somatostatin may act on glucose signal transduction on a level at which both insulin synthesis and secretion are affected. Further, the results are consistent with the hypothesis that cyclic AMP participates in mediating somatostatin effects on B-cell metabolism.

Regulation of insulin biosynthesis in vivo by glucose in the newborn rat

The effects of glucose on insulin biosynthesis in vivo were investigated in the 3-day-old rat. The extent of insulin synthesized was determined by following the incorporation of 3H-leucine into immunoreactive insulin (3H-IRI) and pancreatic proteins (3H-PROT). Blood glucose levels increased 2.3 +/- 0.2 g/liter and plasma insulin increased 208 +/- 10 muU/ml after an intraperitoneal injection of 3H-leucine plus glucose (2 g/kg, body wt) in neonates. Hyperglycemia resulted in a decrease of basal plasma specific radioactivity of leucine and 3H-PROT radioactivity by about 40%. Determination of the 3H-IRI/3H-PROT incorporation ratio shows that the glucose load induced a significant stimulation of insulin biosynthesis 120 min after the injections. The present data indicate that (1) short-term variations of blood glucose are efficient in the regulation of insulin biosynthesis, and (2) insulin biosynthesis in vivo is likely to operate at a basal level of blood glucose in the 3-day-old rat. The results suggested that the variations of blood glucose in a concentration range found under physiologic conditions may play a role in the regulation of insulin biosynthesis in vivo.

The effect of serum ionized calcium elevation on somatostatin inhibition of glucose-induced insulin release in humans

The effect of somatostatin on the insulin response to an acute intravenous glucose load was studied in five normal subjects before and after induction hypercalcaemia. In the normocalcaemic state, the insulin response to glucose was depressed by somatostatin. In the hypercalcaemic state, insulin responses to glucose in the presence of somatostatin, were partially restored and appeared to be related to the level of increment of serum ionized calcium. It is concluded that, in the human being, hypercalcaemia and somatostatin have opposite actions on glucose-stimulated insulin secretion.

Stimulation of gastrin secretion and synthesis in antral organ culture

The purpose of the present study was to examine stimulation of gastrin release and the synthesis of gastrin directly by measurement of incorporation of [(3)H]tryptophan into gastrin in rat antral mucosal explants maintained in organ culture. Gastrin synthesis and secretion were assessed simultaneously at intervals over the 24-h duration of explant culture. Antral mucosal explants from fed female Wistar rats (4-5 wk, 100-150 g) were cultured at 37 degrees C (95% O(2)/5% CO(2)) in medium containing 70% Trowell-T8 and 10% NCTC-135 without unlabeled tryptophan, 10% dialyzed fetal calf serum and [(3)H]tryptophan (100 muCi/ml). Antral tissue was harvested at regular intervals during 24-h culture periods. Incorporation of [(3)H]tryptophan into immunoreactive gastrin was determined by techniques utilizing double-antibody immunoprecipitation. Antral tissue protein synthesis was assessed by measurements of incorporation of [(3)H]tryptophan into tissue protein of cultured antral explants. In paired experiments, gastrin synthesis and secretion in the presence of dibutyryl cAMP (DBCAMP) were compared to those observed under control conditions. Gastrin and protein specific activity progressively increased with time. Gastrin specific activity at 30 min increased from 3.3+/-0.5 (SEM) to 55.2+/-10.6 fmol [(3)H]tryptophan/pmol gastrin (or from 1.57+/-0.48 to 26.28+/-5.05 pmol [(3)H]tryptophan/mug gastrin) at 24 h: specific activity of antral tissue protein at 30 min increased from 33.6+/-8.4 to 1,660+/-236 fmol [(3)H]tryptophan/mug protein at 16 h. Culturing of explants for 4 h in the presence of cycloheximide (100 mug/ml) inhibited both gastrin synthesis and protein synthesis by greater than 90 and 95%, respectively. DBCAMP (10 mM) significantly increased both the synthesis and secretion of antral gastrin when compared with control cultured explants. Results of these experiments provide direct demonstration of gastrin synthesis by rat antral mucosal explants in organ culture, indicate that both gastrin and total antral protein synthesis are inhibited by cycloheximide, and demonstrate DBCAMP-induced stimulation of both gastrin synthesis and secretion, suggesting the potentially important role of cyclic AMP in gastrin cell function.

Proinsulin and C-peptide: a review

The recent work on proinsulin and C-peptide has been reviewed with major emphasis on the most significant findings since 1972. Proinsulin has now been established as the biosynthetic precursor of insulin in all species examined, including man, with a preproinsulin as a possible precursor of the prohormone. The conversion of proinsulin which appears to occur exclusively in the pancreas leads to equimolar production of insulin and C-peptide. Although proinsulin has a direct biologic effect which is one-tenth as much as that of insulin, C-peptide has no biologic activity on homologous or heterologous tissue and no ability to modify the action of insulin and/or proinsulin. Previous work on proinsulin immunoassay suggested that this prohormone, but not C-peptide, cross-reacts with insulin antiserum. On the other hand, in the C-peptide immunoassay, proinsulin but not insulin cross-reacts with the antiserum. Up to this time, therefore, it has not been possible to immunoassay human proinsulin or C-peptide specifically. The very recent work from the laboratory of Heding, however, has brought about major advances in this area in which human C-peptide and proinsulin can be separated in the plasma by the use of Sepharose particles. With this recent major advancement, it is now possible to measure human C-peptide specifically. This measurement has been shown to be a useful tool for the assessment of beta-cell function in diabetic patients treated with insulin and in insulinoma patients in whom endogenous C-peptide secretion is not suppressed with exogenous insulin-induced hypoglycemia. With the use of a specific enzyme which degrades insulin but not proinsulin, postprandial plasma proinsulin values have been measured in a large number of subjects under a variety of physiologic and pathologic conditions. These results, which are comparable to those obtained by the more laborious column chromatography, could be summarized as follows: (1) proinsulin values in lean, young normal subjects do not vary greatly in response to insulin secretagogues; (2) proinsulin secretion in response to glucose results in a greater percentage of proinsulin in the older age group than in the younger group; (3) in lean adult and juvenile diabetic patients, the percentage of proinsulin is not excessive, whereas obese diabetics and pregnant diabetics appear to secrete relatively greater proinsulin than their diabetic controls; and (4) whereas most hyperinsulinemic states (Cusing's syndrome, adult-onset diabetics, acromegaly, and glucocorticoid therapy) are not associated with an increase in percentage of proinsulin, hyperinsulinemia of insulinoma, selected cases of functional hypoglycemia, and genetic hyperproinsulinemia are associated with a greater percentage of proinsulin. Identification of a possible new proinsulin intermediate(s) in these conditions deserves further investigation...