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ML Massa, MI Borelli, H Del Zotto, and JJ Gagliardino

We correlated the changes in glucose-induced insulin secretion with those observed in glucose metabolism and hexokinase/glucokinase activity in islets from normal sucrose-fed hamsters. Blood glucose and insulin levels were measured in normal male hamsters fed with (S5) or without (C5) 10% sucrose in the drinking water for 5 weeks. Isolated islets (collagenase digestion) from both groups of animals were used to study insulin secretion, (14)CO(2) and (3)H(2)O production from D-[U-(14)C]-glucose and D-[5-(3)H]-glucose respectively, with 3.3 or 16.7 mM glucose in the medium, and hexokinase/glucokinase activity (fluorometric assay) in islet homogenates. Whereas S5 and C5 animals had comparable normal blood glucose levels, S5 showed higher insulin levels than C5 hamsters (2.3+/-0.1 vs 0.6+/-0.03 ng/ml, P<0.001). Islets from S5 hamsters released significantly more insulin than C5 islets in the presence of low and high glucose (3.3 mM glucose: 0.77+/-0.04 vs 0.20+/-0.06 pg/ng DNA/min, P<0.001; 16.7 mM glucose: 2.77+/-0.12 vs 0.85+/-0.06 pg/ng DNA/min, P<0.001) and produced significantly higher amounts of (14)CO(2) and (3)H(2)O at both glucose concentrations ((14)CO(2): 3.3 mM glucose: 0.27+/-0.01 vs 0.18+/-0.01, P<0.001; 16.7 mM glucose: 1.44+/-0.15 vs 0.96+/-0.08, P<0.02; (3)H(2)O: 3.3 mM glucose: 0.31+/-0.02 vs 0.15+/-0.01, P<0.001; 16.7 mM glucose: 1.46+/-0.20 vs 0.76+/-0.05 pmol glucose/ng DNA/min, P<0.005). The hexokinase K(m) and V(max) values from S5 animals were significantly higher than those from C5 ones (K(m): 100.14+/-7.01 vs 59.90+/- 3.95 microM, P<0.001; V(max): 0.010+/-0.0005 vs 0.008+/- 0.0006 pmol glucose/ng DNA/min, P<0.02). Conversely, the glucokinase K(m) value from S5 animals was significantly lower than in C5 animals (K(m): 15.31+/-2.64 vs 35.01+/-1.65 mM, P<0.001), whereas V(max) figures were within a comparable range in both groups (V(max): 0.048+/-0.009 vs 0.094+/-0.035 pmol glucose/ng DNA/min, not significant). The glucose phosphorylation ratio measured at 1 and 100 mM (hexokinase/glucokinase ratio) was significantly higher in S5 (0.26+/-0.02) than in C5 animals (0.11+/-0.01, P<0.005), and it was attributable to an increase in the hexokinase activity in S5 animals. In conclusion, sucrose administration increased the hexokinase/glucokinase activity ratio in the islets, which would condition the increase in glucose metabolism by beta-cells, and in beta-cell sensitivity and responsiveness to glucose. These results support the concept that increased hexokinase rather than glucokinase activity causes the beta-cell hypersensitivity to glucose, hexokinase being metabolically more active than glucokinase to up-regulate beta-cell function.

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JJ Gagliardino, H Del Zotto, L Massa, LE Flores, and MI Borelli

The aim of this work was to study the possible relationship between pancreatic duodenal homeobox-1 (Pdx-1) and islet neogenesis-associated protein (INGAP) during induced islet neogenesis. Pregnant hamsters were fed with (S) and without (C) sucrose, and glycemia, insulin secretion in vitro, and pancreas immunomorphometric parameters were measured in their 7-day-old offspring. S offspring had significantly lower glycemic levels than C animals. Insulin release in response to increasing glucose concentrations in the incubation medium (2-16 mM glucose) did not increase in pancreata from either C or S offspring. However, pancreata from S offspring released more insulin than those from C animals. In S offspring, beta-cell mass, beta-cell replication rate and islet neogenesis increased significantly, with a simultaneous decrease in beta-cell apoptotic rate. INGAP- and Pdx-1-positive cell mass also increased in the islets and among acinar and duct cells. We found two subpopulations of Pdx-1 cells: INGAP-positive and INGAP-negative. Pdx-1/INGAP-positive cells did not stain with insulin, glucagon, somatostatin, pancreatic polypeptide, or neurogenin 3 antibodies. The increment of Pdx-1/INGAP-positive cells represented the major contribution to the Pdx-1 cell mass increase. Such increments varied among pancreas subsectors: ductal>insular>extrainsular. Our results suggested that INGAP participates in the regulation of islet neogenesis, and Pdx-1/INGAP-positive cells represent a new stem cell subpopulation at an early stage of development, highly activateable in neogenesis.

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B Maiztegui, M I Borelli, M L Massa, H Del Zotto, and J J Gagliardino

Administration of a sucrose-rich diet (SRD) to normal hamsters induces an insulin-resistant state and a significant increase of insulin secretion and β-cell mass. Islets isolated from these animals had a marked increase in glucose metabolism and glucose-induced insulin secretion, at both low and high glucose concentrations. They also presented increased hexokinase (HK) activity, without measurable changes in glucokinase (GK) activity. In this study we measured HK and GK activity in homogenates of islets isolated from normal control and SRD-fed hamsters, as well as in their particulate and cytosolic fractions. We also measured transcription rate (mRNA by reverse transcriptase PCR) and expression levels (Western blotting) of both enzymes in these islets. We found an increase in HK activity and expression levels, without measurable changes in HK mRNA level in SRD-fed animals. Whereas a similar GK activity was measured in homogenates of islets isolated from both groups, such activity was significantly higher in the cytosolic fraction of SRD islets. On the other hand, GK transcription rate and expression level were similar in both experimental groups. Our results suggest that the increased β-cell secretory response to low glucose can be partly ascribed to an increased activity of islet HK consecutive to an enhanced expression of the enzyme, while the enhanced response to high glucose could be due to changes in GK compartmentalization.

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H Del Zotto, CL Gomez Dumm, S Drago, A Fortino, GC Luna, and JJ Gagliardino

The aim of the present study was to clarify the mechanisms by which a sucrose-rich diet (SRD) produces an increase in the pancreatic beta-cell mass in the rat. Normal Wistar rats were fed for 30 weeks either an SRD (SRD rats; 63% wt/wt), or the same diet but with starch instead of sucrose in the same proportion (CD rats). We studied body weight, serum glucose and triacylglycerol levels, endocrine tissue and beta-cell mass, beta-cell replication rate (proliferating cell nuclear antigen; PCNA), islet neogenesis (cytokeratin immunostaining) and beta-cell apoptosis (propidium iodide). Body weight (g) recorded in the SRD rats was significantly (P<0.05) larger than that of the CD group (556.0+/-8.3 vs 470.0+/-13.1). Both serum glucose and triacylglycerol levels (mmol/l) were also significantly higher (P<0.05) in SRD than in CD rats (serum glucose, 8.11+/-0.14 vs 6.62+/-0.17; triacylglycerol, 1.57+/-0.18 vs 0.47+/-0.04). The number of pancreatic islets per unit area increased significantly (P<0.05) in SRD rats (3.29+/-0.1 vs 2.01+/-0.2). A significant increment (2.6 times) in the mass of endocrine tissue was detected in SRD animals, mainly due to an increase in the beta-cell mass (P=0.0025). The islet cell replication rate, measured as the percentage of PCNA-labelled beta cells increased 6.8 times in SRD rats (P<0.03). The number of apoptotic cells in the endocrine pancreas decreased significantly (three times) in the SRD animals (P=0.03). The cytokeratin-positive area did not show significant differences between CD and SRD rats. The increase of beta-cell mass induced by SRD was accomplished by an enhanced replication of beta cells together with a decrease in the rate of beta-cell apoptosis, without any evident participation of islet neogenesis. This pancreatic reaction was unable to maintain serum glucose levels of these rats at the level measured in CD animals.

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LE Flores, ME Garcia, MI Borelli, H Del Zotto, ME Alzugaray, B Maiztegui, and JJ Gagliardino

The aim of the present study was to test the possible presence and expression of islet neogenesis-associated protein (INGAP) in islet cells of normal adult hamsters. Pancreata from normal male Syrian hamsters were removed to perform the following studies. (i) Western blot analysis using the cytosolic fraction from homogenates of isolated islets, exocrine tIssue and whole pancreas, and rabbit INGAP-specific antibody. (ii) Immunohistochemical identification of INGAP-positive cells in fixed sections of intact pancreata, fresh and 72 h cultured islets (isolated by collagenase digestion), and smears of exocrine pancreatic cells, using the same INGAP-specific antibody and streptavidin-biotin complex. (iii) RT-PCR using total RNA extracted from isolated islets and from exocrine tIssue as template, and a specific pair of primers. (iv) Control of the sequence of the PCR products. INGAP protein was identified by Western blot in the cytosolic fraction of homogenates from fresh isolated islets, exocrine cells and whole fresh pancreas. INGAP-immunopositive cells were observed in duct, exocrine and islet cells in either fixed intact or digested pancreatic tIssue. INGAP mRNA was identified in samples of total RNA from fresh and cultured isolated islets and from exocrine cells. Our data demonstrate that INGAP is present and expressed in islets and in exocrine pancreatic cells of normal hamsters. The ubiquitous localization of INGAP suggests its possible role in the physiological process of islet growth and its protective effect upon streptozotocin-induced diabetes.

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B Maiztegui, M I Borelli, M A Raschia, H Del Zotto, and J J Gagliardino

β-Cell mass, hexokinase/glucokinase (HK/GK) activity, glucose metabolism and insulin secretion were studied in the islets of rats with fructose-induced insulin resistance (IR). Normal male Wistar rats were fed a standard commercial diet and water without (control, C) or with 10% fructose-rich diet (FRD) for 3 weeks. Blood glucose (strips), triglyceride (commercial kit), and insulin (RIA) levels were measured at the time of death. Glucose-induced insulin release, glucose metabolism (14CO2 and 3H2O production from d-[U-14C]- and d-[5-3H]-glucose) and HK/GK activity (G-6-P production), transcription (RT-PCR), protein expression (Western blot), and cellular compartmentalization were measured in isolated islets (collagenase digestion). FRD rats presented normoglycemia but impaired glucose tolerance, hypertriglyceridemia, hyperinsulinemia, and increased HOMA-IR index. In these rats, β-cell mass decreased significantly by 33%, with a 44% increase in the percentage of apoptotic cells. Glucose-induced insulin release and islet glucose metabolism were higher in FRD rats. While GK activity (total and cytosolic fraction) and protein expression were significantly higher in FRD islets, HK showed no change in any of these parameters. Our results demonstrate that the changes induced by dietary-induced IR upon β-cell function and mass are strongly conditional on the nutrient model used. In our model (intact animals with impaired glucose tolerance), GK activity increases through mechanisms previously shown only in vitro or under highly hyperglycemic conditions. Such an increase plays a pivotal role in the adaptive increased release of insulin in response to IR, even in the presence of marked β-cell mass reduction.

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H Del Zotto, L Massa, R Rafaeloff, GL Pittenger, A Vinik, G Gold, A Reifel-Miller, and JJ Gagliardino

The possible relationship between changes in islet cell mass and in islet neogenesis-associated protein (INGAP)-cell mass induced by sucrose administration to normal hamsters was investigated. Normal hamsters were given sucrose (10% in drinking water) for 5 (S8) or 21 (S24) weeks and compared with control (C) fed hamsters. Serum glucose and insulin levels were measured and quantitative immunocytochemistry of the endocrine pancreas was performed. Serum glucose levels were comparable among the groups, while insulin levels were higher in S hamsters. There was a significant increase in beta-cell mass (P<0.02) and in beta-cell 5-bromo-2'-deoxyuridine index (P<0.01), and a significant decrease in islet volume (P<0.01) only in S8 vs C8 hamsters. Cytokeratin (CK)-labelled cells were detected only in S8 hamsters. INGAP-positive cell mass was significantly larger only in S8 vs C8 hamsters. Endocrine INGAP-positive cells were located at the islet periphery ( approximately 96%), spread within the exocrine pancreas ( approximately 3%), and in ductal cells (<1%) in all groups. INGAP positivity and glucagon co-localization varied according to topographic location and type of treatment. In C8 hamsters, 49.1+/-6. 9% cells were INGAP- and glucagon-positive in the islets, while this percentage decreased by almost half in endocrine extra-insular and ductal cells. In S8 animals, co-expression increased in endocrine extra-insular cells to 36.3+/-9.5%, with similar figures in the islets, decreasing to 19.7+/-6.9% in ductal cells. INGAP-positive cells located at the islet periphery also co-expressed CK. In conclusion, a significant increase of INGAP-positive cell mass was only observed at 8 weeks when neogenesis was present, suggesting that this peptide might participate in the control of islet neogenesis. Thus, INGAP could be a potentially useful tool to treat conditions in which there is a decrease in beta-cell mass.

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H Del Zotto, M I Borelli, L Flores, M E García, C L Gómez Dumm, A Chicco, Y B Lombardo, and J J Gagliardino

This study aimed to determine the relative importance of different functional and morphological pancreatic changes induced by the chronic administration of a sucrose-rich diet (SRD) to maintain normal glucose homeostasis. Male Wistar rats were fed either sucrose (SRD) or starch (CD) for 6 and 12 months. At both periods, serum glucose and triacylglycerol levels were significantly higher (P<0.05; paired and unpaired Student’s t-test) in SRD rats. Serum insulin levels were significantly lower in SRD only at 12 months. At 6 months, the insulin secretion dose–response curve in SRD rats showed a shift to the left that was no longer observed at 12 months, when SRD islets decreased their response to 16 mM glucose. At 6 months, SRD rats showed a significant increase in β-cell volume density (Vvi) and islet cell replication rate, together with a decrease in β-cell apoptotic rate. Changes were not detected in the percentage of PDX-1- and islet neogenesis associated protein (INGAP)-positive cells. Conversely, at 12 months, there was a significant decrease in β-cell Vvi and in the percentage of PDX-1-positive cells; the islet cell replication rate was not modified, and the number of apoptotic β-cells increased significantly. No signs of increased neogenesis or INGAP-positive cells were recorded at any period in SRD rats. Our results show that SRD rats are unable to develop functional and morphological pancreatic reactive changes sufficient to maintain normal glucose and triacylglycerol levels for a long period. Such failure could be ascribed to their inability to increase the rate of neogenesis and of INGAP production.