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C Svensson

ABSTRACT

This study was undertaken to investigate the effects of growth hormone (GH) on the in vitro maturation of the metabolism of fetal rat islets. For this purpose fetal islets were obtained from 21-day-old fetuses by mild collagenase digestion of the pancreas and cultured in RPMI 1640 supplemented with 10% fetal calf serum. After one day the medium was changed and supplemented with 1% fetal calf serum with or without GH (1 μg/ml, human recombinant) and the islets cultured for another two days. Islets were then studied with regard to insulin secretion, (pro)insulin and total protein biosynthesis, glucose utilization and oxidation, thymidine incorporation, insulin and DNA contents and the contents of mRNAs for either insulin, adenine nucleotide translocator or cytochrome b. In addition, the activities of glucose phosphorylating enzymes and succinate-cytochrome c reductase were measured. Islets treated with GH showed increased insulin secretion in response to glucose, increased rates of glucose oxidation and utilization, increased thymidine incorporation and increased activities of succinate cytochrome c reductase and glucose phosphorylation at high glucose concentrations. There were, however, no changes in (pro)insulin and total protein biosynthesis, contents of insulin and DNA or the contents of any of the mRNAs. These combined data show that fetal β-cells are sensitive to growth hormone with respect to glucose metabolism, insulin release and DNA replication. The increased rates of islet glucose phosphorylation may reflect glucokinase activity and explain part of the increased insulin responsiveness to glucose of the fetal rat β-cell. These observations suggest that GH is of physiological significance for the maturation of the fetal β-cell.

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H Shibata, M Kanzaki, T Takeuchi, J-i Miyazaki, and I Kojima

ABSTRACT

Activin A stimulates insulin secretion in pancreatic β-cells by a calcium-dependent mechanism. The present study was conducted to further characterize the effects of activin A in two glucose-responsive insulinoma cell lines, MIN6 and HIT-T15 cells. In HIT-T15 cells, activin A evoked an increase in cytoplasmic free calcium concentration, stimulated insulin secretion, maintained glucose responsiveness of the cells and inhibited DNA synthesis. However, activin A did not have any effect in MIN6 cells. Measurement of 125I-labeled activin A binding in MIN6 cells revealed that the number of binding sites was markedly reduced, suggesting that the refractoriness was due, at least partly, to the reduced numbers of the activin receptor. Stable transfectants of MIN6 cells that overexpressed the type II activin receptor were then developed. The transfected cells (MIN6-ActR cells) expressed ten times more 125I-labeled activin A-binding sites than parental cells and the apparent K d was 1·15 nm, which was nearly identical to that in parental cells. Affinity cross-linking in MIN6-ActR cells showed that a 90 kDa type II receptor as well as a 52 kDa protein, presumably follistatin, was markedly labeled with 125I-labeled activin A. Although MIN6-ActR cells expressed significant numbers of activin receptors, activin A did not induce immediate calcium-dependent responses in these cells. In contrast, activin A was capable of inducing long-term effects in MIN6-ActR cells; thus, reduction of the glucose concentration in culture medium from 25 to 5·5 mm for 4 days resulted in a remarkable loss of insulin response to glucose stimulation but this decline in response to glucose was prevented by the addition of activin A during culture. In addition, activin A inhibited DNA synthesis in MIN6-ActR cells. Hence, although activin A did not induce calcium-dependent responses, it evoked some calcium-independent effects in MIN6-ActR cells. Taken together, activin A elicits various effects in β-cells by both calcium-dependent and -independent mechanisms.

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B Lee, PG Bradford, and SG Laychock

The inositol 1,4,5-trisphosphate receptor (InsP3R) is an intracellular Ca2+ channel that plays a role in the regulation of insulin secretion. In rat isolated pancreatic islets the expression of types I, II and III InsP3R mRNA was identified by reverse transcriptase-polymerase chain reaction and confirmed by cDNA cloning and sequencing. The islet ratios of types I, II and III InsP3R mRNA to beta-actin mRNA were 0.08 +/- 0.02, 0.08 +/- 0.03 and 0.25 +/- 0.04 respectively. Types I, II and III InsP3R mRNA were also expressed in rat (RINm5F) and mouse (betaHC9) pancreatic beta-cell lines, and rat cerebellum. Type III InsP3R mRNA was quantitatively the most abundant form in rat islets and RINm5F cells. In betaHC9 cells, types II and III InsP3R mRNA were expressed at similar levels, and in much greater abundance than type I mRNA. Type III was the least abundant InsP3R mRNA in cerebellum. Culture of betaHC9 cells for 5 days at 2.8 and 25 mM glucose, or RINm5F cells for 7 days at 5.5 and 20 mM glucose, resulted in significantly enhanced expression of type III, but not types I and II, InsP3R mRNA in the cells at the higher glucose concentrations. During short-term (0.5-2 h) incubations, betaHC9 cell type III InsP3R mRNA levels increased in response to glucose in a time- and concentration-dependent manner. Actinomycin D inhibited the glucose response. Alpha-ketoisocaproic acid also stimulated betaHC9 cell type III InsP3R mRNA expression in a concentration-dependent manner, whereas 2-deoxyglucose and 3-O-methylglucose were without effect. The different levels of expression of mRNA for three InsP3R isoforms in islets and insulinoma cells, and the influence of glucose and alpha-ketoisocaproic acid on the expression of type III mRNA, suggests that nutrient metabolism plays a role in the regulation of this gene and that the function of InsP3R subtypes may be unique with each playing a distinct role in beta-cell signal transduction and insulin secretion.

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B Gallwitz, M Witt, U R Fölsch, W Creutzfeldt, and W E Schmidt

ABSTRACT

Glucagon-like peptide-1(7–36)amide (GLP-1(7–36) amide) and gastric inhibitory polypeptide (GIP), peptides of the glucagon family, stimulate insulin secretion in vitro and in vivo. They possess high N-terminal sequence homology. Binding studies with 125I-labelled GIP and 125I-labelled GLP-1(7– 36)amide were performed in RINm5F insulinoma cells to investigate receptor specificity and to compare both receptors directly. Both binding sites were highly ligand-specific: GIP did not bind to the GLP-1(7–36)amide receptor and vice versa. Both peptides increased intracellular cyclic AMP levels; GLP-1(7– 36)amide was 100-fold more potent in stimulating cyclic AMP production when compared with GIP. At ranges of 1–10 nmol GLP-1(7–36)amide/1 and 0·1–10 GIP/1, corresponding to submaximal binding concentrations, the hormones showed an additive effect on cyclic AMP production. The N-terminal portion of GIP was important for binding, as GIP(1–30) showed almost full binding and biological activity. GIP(17–42) bound in a concentration-dependent manner with approximately 500-fold lower potency than GIP. At concentrations of up to 10 μmol GIP(17–42)/1 no stimulation of cyclic AMP was observed.

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B. Gallwitz, W. E. Schmidt, J. M. Conlon, and W. Creutzfeldt

ABSTRACT

Glucagon-like peptide-1(7–36)amide (GLP-1(7–36)amide) is a potent stimulator of insulin secretion. Receptors for this hormone have been found on different insulinoma-derived cell lines, e.g. the RINm5F cell line which is derived from a radiation-induced rat insulinoma. To characterize the part of the GLP-1(7–36)amide molecule that is responsible for binding to its receptor on RINm5F cells, binding studies with synthetic C-terminal (GLP-1(21–36)amide) and synthetic N-terminal (GLP-1(7–25)) GLP-1 fragments were carried out. GLP-1(21–36)amide showed dose-dependent binding to the GLP-1(7–36)amide receptor but was approximately 1500 times less potent in inhibiting binding of 125I-labelled GLP-1(7–36)amide than the intact hormone. GLP-1(7–25) at concentrations up to 10 μmol/l did not inhibit binding of label. Neither fragment changed intracellular cyclic AMP concentrations, in contrast to GLP-1(7–36)amide which increased intracellular cyclic AMP. GLP-1(21–36)amide, however, acted as a weak partial antagonist of GLP-1(7–36)amide with respect to GLP-1(7–36)amide-dependent stimulation of cyclic AMP production.

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S. K. Sharma, C. Austin, A. Howard, G. Lo, C. G. Nicholl, and S. Legon

ABSTRACT

Gastric inhibitory peptide (GIP) is a 42 amino acid gastrointestinal peptide which inhibits gastric acid secretion and stimulates pancreatic insulin secretion in the presence of glucose. Here we report the sequence of the cDNA encoding the rat GIP precursor. PreproGIP was 144 amino acids in length and comprised the GIP peptide itself, N- and C-terminal flanking peptides of 22 and 59 amino acids respectively and a typical hydrophobic signal peptide. The sequence indicated that GIP is released from its precursor by cleavage at single arginine residues. The C-terminal flanking peptide may have an important function since it was well conserved and contained a region of 16 amino acids with only a single, conservative replacement. Rat GIP mRNA was found in the duodenum and jejunum. Levels of GIP mRNA in the duodenum were increased twofold after a period of 2 days of starvation. There was no detectable expression of the GIP gene in other parts of the gastrointestinal tract or in other endocrine tissues. However, in pancreatic mRNA preparations, a larger mRNA was detected after low stringency hybridization. This could represent a further member of this gene family.

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N. S. Berrow, G. Milligan, and N. G. Morgan

ABSTRACT

Inhibition of insulin secretion from rat islets of Langerhans is known to involve at least one pertussis toxin-sensitive guanine-nucleotide binding (G) protein. We have used antisera raised against unique antigenic determinants of different members of the family of pertussis toxin-sensitive G proteins to identify these proteins in rat islets. Antiserum SG1, which recognizes both Gi1 and Gi2, reacted with an islet protein having an approximate M r of 40 000. Antiserum IlC (Gi1 specific) failed to recognize any islet proteins, suggesting that Gi2 is present in much greater amounts than Gi1. Indeed, Gi1 levels were below the detection limit of a sensitive immunogold/silver-staining method, indicating that it may be absent from the cells of rat islets.

Two different antisera were used to identify Go-like G proteins in rat islet homogenates. Both antisera reacted with a protein band which, under appropriate conditions, could be resolved to reveal two separate proteins of M r 39–40 000. Thus, at least two molecular forms of Go are present in rat islets.

Subcellular fractionation indicated that all three G proteins identified in this study (Gi2 and two forms of Go) are localized to islet membranes. No immunoreactivity could be detected in the cytosolic fraction.

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NM Morton, V Emilsson, P de Groot, AL Pallett, and MA Cawthorne

Leptin is a cytokine secreted from adipose tissue at a rate commensurate with the size of the body's fat stores. In addition to its anorectic and thermogenic central actions, leptin is known to act on peripheral tissues, including the pancreatic beta-cell where it inhibits insulin secretion and reduces insulin transcript levels. However, the role of leptin signalling through its full-length receptor, OB-Rb, in the beta-cell remains unclear. In the present study, we show that leptin activates a signal transducer and activator of transcription (STAT)3 signalling mechanism in pancreatic islets and in a rat model of the pancreatic beta-cell, RINm5F. Leptin induced DNA binding to a STAT consensus oligonucleotide and resulted in transcriptional activation from STAT reporter constructs in a manner consistent with STAT3 activation. Western blot analysis confirmed activation of STAT3 in RINm5F and isolated rat islets. Conditions that mimic increased metabolic activity resulted in attenuation of leptin-mediated STAT DNA binding but had no significant effect on STAT3 tyrosine phosphorylation in RINm5F cells. In addition, leptin activated the mitogen activated protein (MAP) kinase pathway in RINm5F cells. The present study provides a framework for OB-Rb signalling mechanisms in the programming of the beta-cell by leptin and suggests that increased metabolic activity may modulate this function.

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K J Parker, P M Jones, C H Hunton, S J Persaud, C G Taylor, and S L Howell

ABSTRACT

The liberation of arachidonic acid (AA), by phospholipase A2 (PLA2), is the rate-limiting step in a number of cell signalling pathways. In the pancreatic β-cell, AA itself is thought to participate in the regulation of insulin secretion. Recently a Ca2+-sensitive, AA-selective cytosolic PLA2 (type IV cPLA2) has been isolated from the human monocyte U937 cell line. Although the DNA sequence of this enzyme implies a molecular weight of 85 kDa, the protein migrates with a molecular weight of 100-110 kDa on SDS-PAGE. In many cell types, cPLA2s which are reactive towards antibodies raised against the type IV cPLA2 have been shown to hydrolyse AA from membrane glycerophospholipids. Using a polyclonal antibody raised against a recombinant form of type IV cPLA2, we have detected an immunoreactive protein with a molecular weight of 93·5 kDa in rat islets of Langerhans. Furthermore, we have detected similar immunoreactive proteins in insulin-secreting β-cell lines and have shown co-expression of type IV cPLA2 immunoreactivity and insulin immunoreactivity in rat pancreatic β-cells. Under non-stimulatory conditions the 93·5 kDa immunoreactive protein detected in rat islets of Langerhans was located predominantly in the cytosolic fraction. We have shown that immunoprecipitation of the rat immunoreactive protein from rat islet homogenates significantly decreases the total dithiothreitol/β-mercaptoethanol-insensitive PLA2 activity by 56·4±7% This provides further evidence that the immunoreactive rat protein is a type IV cPLA2 and is responsible for a large component of the PLA2 activity in rat islets of Langerhans. It is possible that, in the rat β-cell, type IV cPLA2 couples the increase in intracellular Ca2+, brought about by insulin secretagogues, to the liberation of AA and the subsequent release of insulin.

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J H Jaggar, E A Harding, B J Ayton, and M J Dunne

ABSTRACT

The hyperglycaemia-inducing sulphonamide diazoxide has been previously shown to mediate its effects upon insulin secretion by opening K+ channels and hyperpolarizing the β-cell membrane. The target site has been characterized as the ATP-regulated K+ (K+ ATP) channel protein. In the present study, a detailed investigation of interactions of diazoxide and another K+ channel opener, cromakalim, with K+ ATP channels has been performed in individual insulin-secreting cells using patchclamp techniques. In agreement with previous studies, diazoxide and cromakalim were found to be effective only when ATP was present upon the inside face of the plasma membrane. The ability of both diazoxide and cromakalim to open channels was, however, found to diminish with time following isolation of inside-out patches. Within seconds of forming the recording configuration, the actions of both compounds were potent, and were found to decline steadily as the number of operational channels decreased ('run-down'). In open cells, where the plasma membrane remains partially intact, the rate of run-down was significantly reduced, and effects of channel openers were recorded up to 80 min following cell permeabilization. We also demonstrated that in the absence of ATP, but in the presence of ADP, both diazoxide and cromakalim were able to open K+ ATP channels. Interestingly, once the effects of diazoxide and cromakalim on K+ ATP channels in the presence of ATP were lost, both compounds opened channels in the presence of ADP. One implication of these data is that the actions of diazoxide and cromakalim involve regulatory proteins associated with the ion channel; this molecule is able to bind ATP, ADP and possibly other cytosolic nucleotides.