Bromocriptine/SKF38393 ameliorates islet dysfunction in the diabetic (db/db) mouse

Dysfunction of pancreatic islets plays a crucial role in the etiology of type II diabetes. Chronic hyperglycaemia or hyperlipidaemia may impair islet function. Previous studies by our laboratory have demonstrated that dopaminergic agonists ameliorated hyperglycaemia and hyperlipidaemia in obese and diabetic rodents. In the present study, we investigated the effect of a treatment with the dopamine D₂ /D₁ receptor agonists (bromocriptine/SKF38393, BC/SKF) on islet dysfunction in db/db mice. Our results show that a 2-week BC/SKF treatment markedly reduced hyperglycaemia and hyperlipidaemia, and significantly improved islet dysfunction demonstrated by an increase of secretagogue-stimulated insulin release from islets of db/db mice to levels observed in islets from lean mice. There was also a fourfold increase of insulin content in the pancreas of BC/SKF-treated db/db mice compared with that in untreated controls. The effect of BC/SKF on islet function cannot be mimicked in pair-fed animals. BC/SKF had no direct stimulatory effect on islet insulin secretion, suggesting BC/SKF treatment improved islet function via an indirect mechanism. This treatment markedly improved the abnormally elevated daily levels of corticosterone, blood glucose and plasma lipids, supporting the view that BC/SKF may affect the neuroendocrine system that in turn regulates peripheral metabolism and thereby improves islet function. Received 3 April 1998; accepted 27 April 1998     

The adhesion receptor GPR56 is activated by extracellular matrix collagen III to improve β-cell function

Aims

G-protein coupled receptor 56 (GPR56) is the most abundant islet-expressed G-protein coupled receptor, suggesting a potential role in islet function. This study evaluated islet expression of GPR56 and its endogenous ligand collagen III, and their effects on β-cell function.

Methods

GPR56 and collagen III expression in mouse and human pancreas sections was determined by fluorescence immunohistochemistry. Effects of collagen III on β-cell proliferation, apoptosis, intracellular calcium ([Ca²⁺]_i) and insulin secretion were determined by cellular BrdU incorporation, caspase 3/7 activities, microfluorimetry and radioimmunoassay, respectively. The role of GPR56 in islet vascularisation and innervation was evaluated by immunohistochemical staining for CD31 and TUJ1, respectively, in pancreases from wildtype (WT) and Gpr56^?/? mice, and the requirement of GPR56 for normal glucose homeostasis was determined by glucose tolerance tests in WT and Gpr56^?/? mice.

Results

Immunostaining of mouse and human pancreases revealed that GPR56 was expressed by islet β-cells while collagen III was confined to the peri-islet basement membrane and islet capillaries. Collagen III protected β-cells from cytokine-induced apoptosis, triggered increases in [Ca²⁺]_i and potentiated glucose-induced insulin secretion from WT islets but not from Gpr56^?/? islets. Deletion of GPR56 did not affect glucose-induced insulin secretion in vitro and it did not impair glucose tolerance in adult mice. GPR56 was not required for normal islet vascularisation or innervation.

Conclusion

We have demonstrated that collagen III improves islet function by increasing insulin secretion and protecting against apoptosis. Our data suggest that collagen III may be effective in optimising islet function to improve islet transplantation outcomes, and GPR56 may be a target for the treatment of type 2 diabetes.

     

Über die Wirkung blutzuckersenkender Sulfonylharnstoffe auf das isolierte Rattenzwerchfell

Summary The action of blood sugar depressing sulfonylureas on glucose and oxygen uptake, as well as on glycogen content and formation of C¹⁴O₂ from uniformly labelled C¹⁴-glucose was investigated in rat hemidiaphragms incubated in phosphate buffer. The following results were obtained: (1) Tolbutamide and Carbutamide increased the glucose uptake. (2) Tolbutamide decreased the glycogen-content. (3) Oxygen uptake as well as formation of C¹⁴O₂ were increased by Tolbutamide. (4) The action of Tolbutamide and insuline was equal with respect to glucose uptake but different as regarding the glycogen content, oxygen uptake and CO₂-formation.It is concluded that sulfonylureas increase glucose oxidation in the rat hemidiaphragm probably without increasing insulin sensitivity. To our knowledge this mechanism of action has hitherto not been described.     

Elevated kidney glucosyltransferase activity in genetic prediabetic mice

Summary Glucosyltransferase activity in the renal cortex of genetic diabetic KK mice was significantly increased at 40 days of age when compared to that of Swiss albino and F₁ hybrid mice. This increase in enzyme activity in the absence of glucose intolerance can be regarded as an earlier genetic marker for the diagnosis of diabetic microangiopathy.Acknowledgments. This research was supported in part by the General Research Support Grant No. RR-05398 from the General Research Support Branch, Division of Research Resources, National Institutes of Health, National Institutes of Health Training Grant No. 5-T01-AM-05617-06, Hope for Diabetics Foundation, Inc., and Pfizer Research Laboratories. Dr.R. A. Camerini-Davalos was a Career Scientist of the Health Research Council of the City of New York, and Drs.A. S. Reddi andC.A. Velasco were N. I. H. Trainees in Diabetes Mellitus during part of this study.     

Spatial learning impairments in PLB1Triple knock-in Alzheimer mice are task-specific and age-dependent

We recently generated an advanced mouse model of Alzheimer’s disease (AD) by targeted knock-in of single-copy mutated human amyloid precursor-protein (APP) and tau genes, crossed with a non-symptomatic presenilin (PS1A246E) over-expressing mouse line. These PLB1_Triple mice presented with age-dependent and AD-relevant phenotypes. Homozygous PLB1_Triple mice aged 4–12 months were assessed here in a battery of spatial learning tasks: Exp.1 radial-arm water maze (spatial reference and working memory) Exp.2 open-field water maze (spatial reference memory); Exp.3 home cage observation system with spatial learning (IntelliCage); Exp.4 spontaneous object recognition (SOR; novel object and spatial object shift). A separate test with high-expression transgenic APP mice matching the design of experiment 1 was also performed. Spatial deficits in PLB1_Triple mice were confirmed at 12, but not 4 months in both water maze tasks. PSAPP mice, by contrast, presented with severe yet non-progressive spatial learning deficits already at 4 months. During tests of spatial learning in SOR and IntelliCage, PLB1_Triple mice neither acquired the location of the water-rewarded corner, nor recognize novel or spatially shifted objects at 4 months, indicating these protocols to be more sensitive than the water maze. Collectively and in line with AD symptomatology, PLB1_Triple mice present with a graded and progressive age-dependent loss of spatial memory that can be revealed by the use of a battery of tasks. With the emergence of subtle deficits progressively increasing in severity, PLB1_Triple mice may offer a more patho-physiologically relevant model of dementia than aggressive expression models.     

Diffusion of d-glucose measured in the cytosol of a single astrocyte

Astrocytes interact with neurons and endothelial cells and may mediate exchange of metabolites between capillaries and nerve terminals. In the present study, we investigated intracellular glucose diffusion in purified astrocytes after local glucose uptake. We used a fluorescence resonance energy transfer (FRET)-based nano sensor to monitor the time dependence of the intracellular glucose concentration at specific positions within the cell. We observed a delay in onset and kinetics in regions away from the glucose uptake compared with the region where we locally super-fused astrocytes with the d-glucose-rich solution. We propose a mathematical model of glucose diffusion in astrocytes. The analysis showed that after gradual uptake of glucose, the locally increased intracellular glucose concentration is rapidly spread throughout the cytosol with an apparent diffusion coefficient (D _app) of (2.38 ± 0.41) × 10^?10 m² s^?1 (at 22–24 °C). Considering that the diffusion coefficient of d-glucose in water is D = 6.7 × 10^?10 m² s^?1 (at 24 °C), D _app determined in astrocytes indicates that the cytosolic tortuosity, which hinders glucose molecules, is approximately three times higher than in aqueous solution. We conclude that the value of D _app for glucose measured in purified rat astrocytes is consistent with the view that cytosolic diffusion may allow glucose and glucose metabolites to traverse from the endothelial cells at the blood–brain barrier to neurons and neighboring astrocytes.     

Efficacy of tumor cell vaccine after incorporating monophosphoryl lipid A (MPL) in tumor cell membranes containing tumor-associated ganglioside

Murine B16 melanoma expresses the ganglioside. GM₃. GM₃ shed from tumor cells is immunosuppressive and promotes tumor growth¹. Reduction or elimination of the shed GM₃ could be therapeutic, and the anti-GM₃ antibodies may reduce and clear the shed ganglioside. To test this hypothesis, mice were challenged with tumor cells, with or without inducing anti-GM₃ antibody response. Since gangliosides are poor immunogens and T-cell independent antigens, an adjuvant (monophosphoryl lipid A (MPL), a non-toxic lipid A ofSalmonella), directed against B-cells, was employed. MPL was incorporated onto liposomes and into the surface membrane of B16 mouse melanoma cells; both are rich in GM₃. C57BL/6J mice immunized with MPL-liposomes or MPL-B16 cells responded with elevated levels of anti-GM₃ IgM. Non-immunized mice or mice immunized with B16 cells alone or ganglioside GM₃ alone (without MPL) elicited poor anti-GM₃ IgM response, confirming the GM₃'s immunologic crypticity and MPL's immunopotentiating effect. MPL's immunopotentiating effect was improved by coupling it to melanoma cell membranes C57BL/6J mice were immunized with irradiated B16 alone or MPL alone or MPL-conjugated irradiated B16. After three weekly immunizations, each mouse received a challenge dose of viable syngeneic B16. Neither MPL alone nor B16 alone had a significant effect on tumor growth or host survival; however, administration of MPL-conjugated B16 cells significantly prevented tumor growth and prolonged survival. Our results indicate that MPL-incorporated B16 cells augment the anti-GM₃ IgM response, which may reverse GM₃-induced immunosuppression by eliminating tumor-derived GM₃, and restore immunocompetence.     

Effect of prostaglandins on skin tumorigenesis

Summary Concomitant administration of prostaglandins E₂ (PGE₂) and F₂ a(PGF₂ a) with a carcinogen, 3-methylcholanthrene (MCA) to mice for 2 months markedly enhanced the occurrence of squamous cell carcinomas. Only epidermal cell hyperplasia occurred in mice treated with MCA alone by that time. Radioactivity measurements and electron microscopic autoradiography revealed that prostaglandins stimulate DNA, RNA and protein synthesis in neoplastic cells. These findings indicate that PGE₂ and PGF₂ a can act as cocarcinogens on skin tumorigenesis.     

Der Einfluss von 2-Deoxy-D-glucose auf die Substratoxydation in Ascites-Tumorzellen

Summary The effect of the glycolytic inhibitor 2-deoxy-D-glucose on the oxidation of¹⁴C-labelled substrates of the tricarboxylic acid cycle was investigated in ascites carcinoma cells of the mouse. In the presence of high concentrations of glucose, deoxyglucose stimulates the oxidation of pyruvate C-2 and C-3 to CO₂, but not the oxidation of succinate C-1,4 to CO₂. While deoxyglucose causes, in the absence of glucose, an inhibition of the oxidation of exogenous palmitate C-1 to CO₂, it causes, in the presence of glucose, a stimulation of the palmitate oxidation. There is a possible correlation between the effect of deoxyglucose on the intracellular ATP metabolism and the effect of deoxyglucose on the palmitate oxidation.     

Veränderungen des Glucoseabbaus in Kaninchenaorten unter dem Einfluss von Biguaniden

Summary The influence of biguanides on glucoseoxidation was studied in intima-preparations of rabbit aortae. With high doses (500 µg/ml) an inhibition of CO₂-formation predominantly from position C-1 of glucose was found. With 4 µg/ml there was no significant change of CO₂-for-mation from glucose.