Antibodies against carbamylated proteins: prevalence and associated disease characteristics in Belgian patients with rheumatoid arthritis or other rheumatic diseases

Objectives: Anti-carbamylated protein antibodies (anti-CarP) are reported to be associated with increased disease activity and with more severe joint damage in rheumatoid arthritis (RA) patients. The present study investigated the presence of anti-CarP in various rheumatic diseases, and their specific clinical significance in RA, in Belgian rheumatology patients.Method: We tested sera from 254 RA patients, 56 healthy controls, and 153 patients with different rheumatic conditions: juvenile idiopathic arthritis (JIA), axial spondyloarthritis, systemic sclerosis, and Sjögren's syndrome (SS). An in-house enzyme-linked immunosorbent assay was used to detect immunoglobulin G antibodies against carbamylated foetal calf serum.Results: Anti-CarP were detected in 88 RA patients (34.6%), of whom 82% were also positive for anti-citrullinated protein antibodies (ACPAs) and 81% were also rheumatoid factor (RF) positive. Of note, 11 anti-CarP single-positive patients were detected (4.3%). The previously reported association with joint erosions was not detected. However, in ACPA- and RF-negative RA patients, the presence of anti-CarP was associated with higher disease activity and disability. Fifteen per cent of JIA patients and 30% of SS patients also tested positive for anti-CarP and their antibody levels did not differ significantly from those of anti-CarP-positive RA patients. Anti-CarP levels were, however, significantly higher in ACPA- or RF-positive patients.Conclusion: Anti-CarP antibodies were detected in the sera of a cohort of Belgian RA patients. Moreover, they were also detected in primary SS patients and in JIA patients. In the seronegative subset of RA patients, anti-CarP antibodies showed prognostic value.

Glucose-Dependent Insulinotropic Peptide Prevents Serum Deprivation-Induced Apoptosis in Human Bone Marrow-Derived Mesenchymal Stem Cells and Osteoblastic Cells

Human bone marrow-derived mesenchymal stem cells (hBMSC) are able to differentiate into cells of connective tissue lineages, including bone and cartilage. They are therefore considered as a promising tool for the treatment of bone degenerative diseases. One of the major issues in regenerative cell therapy is the biosafety of fetal bovine serum used for cell culture. Therefore, the development of a culture medium devoid of serum but preserving hBMSC viability will be of clinical value. The glucose-dependent insulinotropic peptide (GIP) has an anti-apoptotic action in insulin-producing cells. Interestingly, GIP also exerts beneficial effects on bone turnover by acting on osteoblasts and osteoclasts. We therefore evaluated the ability of GIP to prevent cell death in osteoblastic cells cultured in serum-free conditions. In hBMSC and SaOS-2 cells, activation of the GIP receptor increased intracellular cAMP levels. Serum deprivation induced apoptosis in SaOS-2 and hBMSC that was reduced by 30 and 50 %, respectively, in the presence of GIP. The protective effect of GIP involves activation of the adenylate cyclase pathway and inhibition of caspases 3/7 activation. These findings demonstrate that GIP exerts a protective action against apoptosis in hBMSC and suggest a novel approach to preserve viability of hBMSC cultured in the absence of serum.

Oleate Abrogates Palmitate-Induced Lipotoxicity and Proinflammatory Response in Human Bone Marrow-Derived Mesenchymal Stem Cells and Osteoblastic Cells

Osteoporosis is a metabolic bone disease associated with unequilibrated bone remodeling resulting from decreased bone formation and/or increased bone resorption, leading to progressive bone loss. In osteoporotic patients, low bone mass is associated with an increase of bone marrow fat resulting from accumulation of adipocytes within the bone marrow. Marrow adipocytes are active secretory cells, releasing cytokines, adipokines and free fatty acids (FA) that influence the bone marrow microenvironment and alter the biology of neighboring cells. Therefore, we examined the effect of palmitate (Palm) and oleate (Ole), 2 highly prevalent FA in human organism and diet, on the function and survival of human mesenchymal stem cells (MSC) and MSC-derived osteoblastic cells. The saturated FA Palm exerted a cytotoxic action via initiation of endoplasmic reticulum stress and activation of the nuclear factor κB (NF-κB) and ERK pathways. In addition, Palm induced a proinflammatory response, as determined by the up-regulation of Toll-like receptor 4 expression as well as the increase of IL-6 and IL-8 expression and secretion. Moreover, we showed that MSC-derived osteoblastic cells were more sensitive to lipotoxicity than undifferentiated MSC. The monounsaturated FA Ole fully neutralized Palm-induced lipotoxicity by impairing activation of the pathways triggered by the saturated FA. Moreover, Ole promoted Palm detoxification by fostering its esterification into triglycerides and storage in lipid droplets. Altogether, our data showed that physiological concentrations of Palm and Ole differently modulated cell death and function in bone cells. We therefore propose that FA could influence skeletal health.

Suppressor of cytokine signalling-3 expression inhibits cytokine-mediated destruction of primary mouse and rat pancreatic islets and delays allograft rejection

AIMS/HYPOTHESIS

The pro-inflammatory cytokines IL-1 and IFNgamma are critical molecules in immune-mediated beta cell destruction leading to type 1 diabetes mellitus. Suppressor of cytokine signalling (SOCS)-3 inhibits the cytokine-mediated destruction of insulinoma-1 cells. Here we investigate the effect of SOCS3 in primary rodent beta cells and diabetic animal models.

METHODS

Using mice with beta cell-specific Socs3 expression and a Socs3-encoding adenovirus construct, we characterised the protective effect of SOCS3 in mouse and rat islets subjected to cytokine stimulation. In transplantation studies of NOD mice and alloxan-treated mice the survival of Socs3 transgenic islets was investigated.

RESULTS

Socs3 transgenic islets showed significant resistance to cytokine-induced apoptosis and impaired insulin release. Neither glucose-stimulated insulin release, insulin content or glucose oxidation were affected by SOCS3. Rat islet cultures transduced with Socs3-adenovirus displayed reduced cytokine-induced nitric oxide and apoptosis associated with inhibition of the IL-1-induced nuclear factor-kappaB and mitogen-activated protein kinase (MAPK) pathways. Transplanted Socs3 transgenic islets were not protected in diabetic NOD mice, but showed a prolonged graft survival when transplanted into diabetic allogenic BALB/c mice.

CONCLUSIONS/INTERPRETATION

SOCS3 inhibits IL-1-induced signalling through the nuclear factor-kappaB and MAPK pathways and apoptosis induced by cytokines in primary beta cells. Moreover, Socs3 transgenic islets are protected in an allogenic transplantation model. SOCS3 may represent a target for pharmacological or genetic engineering in islet transplantation for treatment of type 1 diabetes mellitus.

Induction of nuclear factor-kappaB and its downstream genes by TNF-alpha and IL-1beta has a pro-apoptotic role in pancreatic beta cells

AIMS/HYPOTHESIS

IL-1beta and TNF-alpha contribute to pancreatic beta cell death in type 1 diabetes. Both cytokines activate the transcription factor nuclear factor-kappaB (NF-kappaB), but recent observations suggest that NF-kappaB blockade prevents IL-1beta + IFN-gamma- but not TNF-alpha + IFN-gamma-induced beta cell apoptosis. The aim of the present study was to compare the effects of IL-1beta and TNF-alpha on cell death and the pattern of NF-kappaB activation and global gene expression in beta cells.

METHODS

Cell viability was measured after exposure to IL-1beta or to TNF-alpha alone or in combination with IFN-gamma, and blockade of NF-kappaB activation or protein synthesis. INS-1E cells exposed to IL-1beta or TNF-alpha in time course experiments were used for IkappaB kinase (IKK) activation assay, detection of p65 NF-kappaB by immunocytochemistry, real-time RT-PCR and microarray analysis.

RESULTS

Blocking NF-kappaB activation protected beta cells against IL-1beta + IFNgamma- or TNFalpha + IFNgamma-induced apoptosis. Blocking de novo protein synthesis did not increase TNF-alpha- or IL-1beta-induced beta cell death, in line with the observations that cytokines induced the expression of the anti-apoptotic genes A20, Iap-2 and Xiap to a similar extent. Microarray analysis of INS-1E cells treated with IL-1beta or TNF-alpha showed similar patterns of gene expression. IL-1beta, however, induced a higher rate of expression of NF-kappaB target genes putatively involved in beta cell dysfunction and death and a stronger activation of the IKK complex, leading to an earlier translocation of NF-kappaB to the nucleus.

CONCLUSIONS/INTERPRETATION

NF-kappaB activation in beta cells has a pro-apoptotic role following exposure not only to IL-1beta but also to TNF-alpha. The more marked beta cell death induced by IL-1beta is explained at least in part by higher intensity NF-kappaB activation, leading to increased transcription of key target genes.

Global profiling of coxsackievirus- and cytokine-induced gene expression in human pancreatic islets

AIMS/HYPOTHESIS

It is thought that enterovirus infections initiate or facilitate the pathogenetic processes leading to type 1 diabetes. Exposure of cultured human islets to cytolytic enterovirus strains kills beta cells after a protracted period, suggesting a role for secondary virus-induced factors such as cytokines.

METHODS

To clarify the molecular mechanisms involved in virus-induced beta cell destruction, we analysed the global pattern of gene expression in human islets. After 48 h, RNA was extracted from three independent human islet preparations infected with coxsackievirus B5 or exposed to interleukin 1beta (50 U/ml) plus interferon gamma (1,000 U/ml), and gene expression profiles were analysed using Affymetrix HG-U133A gene chips, which enable simultaneous analysis of 22,000 probe sets.

RESULTS

As many as 13,077 genes were detected in control human islets, and 945 and 1293 single genes were found to be modified by exposure to viral infection and the indicated cytokines, respectively. Four hundred and eighty-four genes were similarly modified by the cytokines and viral infection.

CONCLUSIONS/INTERPRETATION

The large number of modified genes observed emphasises the complex responses of human islet cells to agents potentially involved in insulitis. Notably, both cytokines and viral infection significantly (p<0.02) increased the expression of several chemokines, the cytokine IL-15 and the intercellular adhesion molecule ICAM-1, which might contribute to the homing and activation of mononuclear cells in the islets during infection and/or an early autoimmune response. The present results provide novel insights into the molecular mechanisms involved in viral- and cytokine-induced human beta cell dysfunction and death.

Global profiling of double stranded RNA- and IFN-gamma-induced genes in rat pancreatic beta cells

AIMS/HYPOTHESIS

Viral infections and local production of IFN-gamma might contribute to beta-cell dysfunction/death in Type 1 Diabetes. Double stranded RNA (dsRNA) accumulates in the cytosol of viral-infected cells, and exposure of purified rat beta cells to dsRNA (tested in the form of polyinosinic-polycytidylic acid, PIC) in combination with IFN-gamma results in beta-cell dysfunction and apoptosis. To elucidate the molecular mechanisms involved in PIC + IFN-gamma-effects, we determined the global profile of genes modified by these agents in primary rat beta cells.

METHODS

FACS-purified rat beta cells were cultured for 6 or 24 h in control condition or with IFN-gamma, PIC or a combination of both agents. The gene expression profile was analysed in duplicate by high-density oligonucleotide arrays representing 5000 full-length genes and 3000 EST's. Changes of greater than or equal to 2.5-fold were considered as relevant.

RESULTS

Following a 6- or 24-h treatment with IFN-gamma, PIC or IFN-gamma and PIC, we observed changes in the expression of 51 to 189 genes. IFN-gamma modified the expression of MHC-related genes, and also of genes involved in beta-cell metabolism, protein processing, cytokines and signal transduction. PIC affected preferentially the expression of genes related to cell adhesion, cytokines and dsRNA signal transduction, transcription factors and MHC. PIC and/or IFN-gamma up-regulated the expression of several chemokines and cytokines that could contribute to mononuclear cell homing and activation during viral infection, while IFN-gamma induced a positive feedback on its own signal transduction. PIC + IFN-gamma inhibited insulin and GLUT-2 expression without modifying pdx-1 mRNA expression.

CONCLUSION/INTERPRETATION

This study provides the first comprehensive characterization of the molecular responses of primary beta cells to dsRNA + IFN-gamma, two agents that are probably present in the beta cell milieu during the course of virally-induced insulitis and Type 1 Diabetes. Based on these findings, we propose an integrated model for the molecular mechanisms involved in dsRNA + IFN-gamma induced beta-cell dysfunction and death.

Effects of D-mannoheptulose upon D-glucose metabolism in tumoral pancreatic islet cells

D-[3H]mannoheptulose was recently reported to be poorly taken up by tumoral pancreatic islet cells of the RINm5F and INS-1 lines. We have now investigated the effects of D-mannoheptulose upon D-glucose metabolism in these two cell lines. D-mannoheptulose (1.0-10.0 mM) only caused a minor decrease of D-glucose metabolism in RINm5F cells, whether at low (1.1 mM) or higher (8.3 mM) D-glucose concentration. A comparable situation was found in INS-1 cells examined after more than 20 passages. In both cases, however, the hexaacetate ester of D-mannoheptulose (5.0 mM) efficiently inhibited D-glucose metabolism. In the INS-1 cells, the relative extent of the inhibitory action of D-mannoheptulose upon D-glucose metabolism increased from 12.4 +/- 2.6 to 38.3 +/- 3.8% as the number of passages was decreased from more than 20 to 13-15 passages, the latter percentage remaining lower, however, than that recorded in INS-I cells also examined after 13-15 passages but exposed to D-mannoheptulose hexaacetate (66.9 +/- 2.2%). These findings when compared to our recent measurements of D-[3H]mannoheptulose uptake, reinforce the view that the entry of the heptose into cells and, hence, its inhibitory action on D-glucose metabolism are dictated by expression of the GLUT2 gene.

Uptake of D-mannoheptulose by normal and tumoral pancreatic islet cells

D-mannoheptulose was recently proposed as a possible tool to label preferentially insulin-producing cells in the pancreatic gland. In the present study, D-[3H]-mannoheptulose uptake by rat pancreatic islets or dispersed islet cells was found to represent a time-related and temperature-sensitive process inhibited by cytochalasin B. This mould metabolite also inhibited the efflux of D-[3H]-mannoheptulose from prelabelled islets. After 60 min incubation at 37 degrees C, the apparent intracellular distribution space of the tritiated heptose was close to or somewhat higher than that of D-[5-3H]glucose and close to 50% of the intracellular 3HOH space. It was further enhanced by D-glucose and a high concentration of 10 mM of D-mannoheptulose. The uptake of D-[3H]mannoheptulose was much lower however than that of D-[3H]mannoheptulose hexaacetate. As judged from the fate of D-mannoheptulose hexa[2-14C]acetate, the latter ester was efficiently hydrolyzed in the islet cells. The internalization of D-[3H]mannoheptulose (or its ester) coincided with the generation of tritiated acidic metabolites, reflecting phosphorylation of the heptose. The situation found in normal islet cells sharply differed from that found in tumoral islet cells of either the RINm5F or INS-1 line, in which the apparent distribution space of D-[3H]mannoheptulose represented only about 3 and 9%, respectively, of the intracellular 3HOH space. These results indicate that the entry of D-mannoheptulose into islet cells represents a carrier-mediated process, possibly mediated at the intervention of GLUT2 and, hence, provide further support to the possible use of a suitable D-mannoheptulose analog as a tool for the preferential labelling of insulin-producing cells in the pancreatic gland.

Glycogen accumulation in cultured tumoral or normal pancreatic islet and acinar cells

Tumoral pancreatic islet cells of either the RINm5F or INS-1 cell lines, when cultured in the presence of 30.0 mM D-glucose, accumulate about 50 times more glycogen than tumoral pancreatic acinar cells of the AR42J line cultured under the same experimental conditions. Expressed per nl of intracellular water, the glycogen content of the RINm5F or INS-1 cells is even higher than that found in rat pancreatic islets also cultured under the same experimental conditions. Moreover, at variance with normal islet cells, tumoral islet cells do not require to be exposed to a high concentration of D-glucose to accumulate large amounts of glycogen. Based on these findings, it is proposed that the labelling of the glycogen pool, e.g. by 11C-labelled D-glucose or 2-deoxy-2-[18F]fluoro-D-glucose, may allow identification and localization of insulinomas in the pancreatic gland by a non-invasive imaging procedure.

Possible participation of an islet B-cell calcium-sensing receptor in insulin release

The calcium-sensing receptor gene was recently shown to be expressed in rat pancreatic islets and purified islet B-cells. In this study, we investigated the possible role of this receptor in the regulation of insulin release from isolated rat pancreatic islets. Poly-L-arginine (0.2-0.3 microM) and poly-L-lysine (0.03-0.1 microM) increased insulin output evoked by D-glucose (8.3 mM). This positive effect faded out at higher concentrations of the basic peptides. Likewise, the release of insulin evoked by 8.3 mM D-glucose was significantly lower at high (1.0 mM) than low (0.05-0.1 mM) concentrations of neomycin. The insulinotropic action of Ba2+ in Ca2+-deprived islets was potentiated in rats pretreated with pertussis toxin. However, Gd3+ inhibited insulin release evoked by D-glucose in islets prepared from normal rats or animals pretreated with pertussis toxin and incubated in the absence or presence of either theophylline or forskolin. Gd3+ (0.3 mM) failed to affect effluent radioactivity from islets prelabeled with myo-[2-3H]inositol and cyclic AMP net production in islets incubated in the absence or presence of forskolin. Gd3+ decreased, however, 45Ca efflux from prelabeled islets perifused in the absence or presence of extracellular Ca2+. It is speculated that a negative insulinotropic action mediated by the calcium-sensing receptor, and possibly attributable to a fall in cytosolic Ca2+ concentration, may prevent excessive insulin secretion in pathological situations of hypercalcemia.

Expression of the calcium-sensing receptor in pancreatic islet B-cells

Exposure of pancreatic islet B-cells to D-glucose and many other insulinotropic agents results in an increase of cytoplasmic calcium concentration, which triggers the exocytosis of secretroy granules. Previous studies have demonstrated that calcium itself, at concentrations ranging from 2 to 18 mM, is able to induce a dose-related stimulation of insulin secretion, even in the absence of any other secretagogue. It was recently demonstrated that parathyroid cells and several other cell types, whether involved or not in calcium homeostasis, sense extracellular calcium through a G-protein coupled calcium-sensing receptor (CaSR). In the present study, the presence of the receptor in islet pancreatic B-cells was scrutinized. Using reverse transcriptase-polymerase chain reaction and Northern blot analysis, we demonstrate the expression of the CaSR in purified rat pancreatic islet B-cells. The nucleotide sequences of the rt-PCR products demonstrated more than 99% homology with the rat kidney CaSR complementary DNA. A specific 5.3 kb transcript of the CaSR was expressed in normal pancreatic B-cells as well as in tumoral insulin-secreting cells. In pancreatic islets, the physiological role of the CaSR in the regulation of insulin release could involve the sensing of endogenous ligands other than calcium.

Cytotoxic action of 2-deoxy-D-glucose tetraacetate in tumoral pancreatic islet cells

Tumoral insulin-producing cells of the RINm5F line were cultured for 8-96 h in the absence or presence of 2-deoxy-D-glucose (0.15-0.80 mM) or its tetraacetate ester (0.08-0.80 mM). Despite the fact that over a short incubation of 120 min the utilization of D-[5-3H]glucose and oxidation of D-[U-14C]glucose were not more markedly decreased by 2-deoxy-D-glucose tetraacetate than by the unesterified glucose analogue, the growth of the tumoral cells, as assessed by either the generation of formazan from 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide or direct cell counting, was more severely affected by the ester than by 2-deoxy-D-glucose. At a high concentration (0.80 mM), the ester even decreased the cell number below its initial value. No restoration of cell growth was observed when the cells were exposed for only 8 h to 2-deoxy-D-glucose tetraacetate (0.80 mM) and then further cultured for 64 h in the absence of the ester. These findings indicate that such an ester acts as a powerful cytostatic and cytotoxic agent in this tumoral cell line.

D-glucose metabolism in BRIN-BD11 islet cells

A novel insulin-secreting cell line, BRIN-BD11, was recently established following electrofusion of RINm5F cells with NEDH rat pancreatic islet cells. In the present study, D-glucose metabolism was compared in BRIN-BD11 and RINm5F cells. The concentration dependency of D[5-3H]glucose utilization displayed a comparable pattern in the two cell lines, but the absolute values were lower in BRIN-BD11 than RINm5F cells. Except in the case of D-[1-14C]glucose, the ratio between 14C labeled D-glucose oxidation and D-[5-3H]glucose utilization was higher, however, in BRIN-BD11 than RINm5F cells. Moreover, BRIN-BD11 cells were less affected than RINm5F cells by a rise in D-glucose concentration, in terms of the inhibitory action of the hexose upon oxidative variables, such as oxidative glycolysis, pyruvate decarboxylation, and oxidation of glucose-derived acetyl residues in the Krebs cycle. The total energy yield from D-glucose catabolism appeared similar, however, in BRIN-BD11 and RINm5F cells. These findings extend the knowledge that BRIN-BD11 cells display an improved metabolic and secretory behavior, when considering the difference otherwise found between normal and tumoral islet cells.

The coupling of metabolic to secretory events in pancreatic islets. Glucose-induced changes in mitochondrial redox state

Mitochondrial NAD+, NADH, NADP+ and NADPH were measured in dispersed pancreatic islet cells incubated in the absence or presence of D-glucose and then exposed for 20 s to 0.5 mg/ml digitonin. The latter treatment resulted in the full release of lactate dehydrogenase without any detectable loss of glutamate dehydrogenase. The permeabilized cells were separated from the incubation medium by centrifugation through an oil layer and their content in pyridine nucleotides measured by a radioisotopic procedure coupled to the classical cycling technique. Relative to basal value, D-glucose, in concentrations of 2.8 and 16.7 mM, caused a concentration-related increase in both the NADH/NAD+ and NADPH/NADP+ ratio. These findings provide the first direct evidence for the induction of a more reduced mitochondrial redox state in glucose-stimulated pancreatic islets.

Relevance of lactate dehydrogenase activity to the control of oxidative glycolysis in pancreatic islet B-cells

The activities of hexokinase isoenzymes, lactate dehydrogenase, cytosolic NAD-linked glycerophosphate dehydrogenase, mitochondrial FAD-linked glycerophosphate dehydrogenase, and glutamate dehydrogenase were measured in homogenates of rat purified pancreatic B and non-B islet cells. In B cell homogenates, the maximal activity of hexokinase and glucokinase was one to two orders of magnitude lower than that of lactate dehydrogenase. The activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase was also much lower than that of the cytosolic NAD-linked glycerophosphate dehydrogenase . A comparable hierarchy in the activity of these enzymes was observed in non-B islet cells. These findings reinforce the view that the preferential stimulation of oxidative glycolysis observed in insulin-producing cells, when exposed to high concentrations of D-glucose, is attributable to a Ca2+-induced activation of the mitochondrial FAD-linked glycerophosphate dehydrogenase, rather than to saturation of the catalytic activity of lactate dehydrogenase.

FAD-glycerophosphate dehydrogenase activity in lymphocytes of type-2 diabetic patients and their relatives

The activities of FAD-linked glycerophosphate dehydrogenase (m-GDH), glutamate dehydrogenase (GlDH), glutamate-pyruvate transaminase (GPT) and glutamate-oxalacetate transaminase (GOT) were measured in purified populations of CD3+ lymphocytes from 55 control subjects, 62 type-2 diabetics and 50 non-diabetic relatives of the latter patients. The activity of m-GDH was measured by both a radioisotopic procedure and colourimetric technique. As judged from these measurements and relative to the paired value for GlDH, the incidence of abnormally low m-GDH activity was significantly higher in type-2 diabetics than in control subjects. Moreover, the paired ratio in reaction velocity between the colourimetric and radioisotopic assay of m-GDH was abnormally high in patients with low m-GDH activity. Low m-GDH activity often coincided with increased GPT activity in plasma or high GPT/GOT ratio in lymphocytes. No obvious clustering of these anomalies was found in relatives of diabetic patients. These findings suggest that an inherited or acquired genomic defect of m-GDH in lymphocytes, and possibly in pancreatic B-cells, may participate to the pathogenesis of non-insulin-dependent diabetes mellitus.

Ontogeny of FAD-linked glycerophosphate dehydrogenase in rat pancreatic islets

The activities of the mitochondrial FAD-linked glycerophosphate dehydrogenase (m-GDH), glutamate dehydrogenase, alpha-ketoglutarate dehydrogenase, glutamate-pyruvate transaminase (GPT) and glutamate-oxaloacetate transaminase were measured in islet and liver homogenates from fetal, neonatal, adult male, adult female, pregnant and lactating rats. Either parallel or dissociated ontogenic changes were observed in islet and liver homogenates. The activity of islet m-GDH was slightly, albeit not significantly, lower in neonates than in adult rats, comparable in male and female adult animals, unaffected by pregnancy, and increased during lactation. It was much higher in fetal or adult islets cultured for 7 days than in freshly isolated islets from adult rats. In cultured islets from adult rats, the increase in m-GDH activity coincided with a dramatic decrease of GPT activity, a situation the mirror image of that found in several animal models of non-insulin-dependent diabetes mellitus. The intrinsic properties of m-GDH, as judged by comparison of measurements made by either a radioisotopic or a colorimetric procedure, were not identical in islet and liver homogenates and differed between fetal and adult islets, suggesting the existence of distinct iso-enzymes. These findings illustrate adaptive changes of islet enzymes, with exclusive or partial mitochondrial location, in ontogenic situations characterized by a remodelling of fuel homeostasis.

Metabolism of succinic acid methyl esters in myocytes

The metabolism of the dimethyl ester of succinic acid (SAD) was examined in myocytes. When expressed in terms of CO(2) output, the oxidation of SAD (10 mM) only represented 30-40% of that of an equimolar concentration of D-glucose. Except for a modest decrease in D-[5-(3)H]glucose utilization, SAD failed to affect the catabolism of exogenous D-glucose. SAD also failed, like D-glucose, to augment O(2) consumption by the myocytes. These findings indicate that SAD is less efficiently metabolized in myocytes than in hepatocytes or pancreatic islets. It is nevertheless argued that the methyl esters of succinic acid could be efficiently used as nutrients by myocytes in situations of ATP depletion.

Impaired activity of rat pancreatic islet mitochondrial glycerophosphate dehydrogenase in protein malnutrition

In rats that received a low protein isocaloric diet (protein content of the diet: 8 instead of 20%) during fetal life and thereafter up to the time of sacrifice at 12-13 weeks of age, a low plasma insulin concentration, a decreased insulin content of isolated pancreatic islets, and an impaired secretory response of the islets to either D-glucose or the association of L-leucine and L-glutamine coincided, in islet homogenates, with a low activity of the mitochondrial glycerophosphate dehydrogenase and an abnormally high ratio between glutamate-alanine and glutamate-aspartate transaminase activities. Opposite enzymatic changes were found in liver extracts of the same rats. No obvious change in these hormonal, secretory, and enzymatic variables were observed when the period of protein deficiency was restricted to fetal life. These findings support the view that, in protein malnutrition, an impaired activity of pancreatic B-cell mitochondrial glycerophosphate dehydrogenase contributes, possibly in association with other enzymatic anomalies, to the perturbation of islet function.

Metabolism of succinic acid methyl esters in neural cells

The metabolism and metabolic effects of succinic acid methyl esters were examined in both NG108-15 mouse neuroblastoma x rat glioma hybrid cells and normal rat brain cells. The conversion of the dimethyl ester of 14C-labeled succinic acid (10 mM) to 14CO2 only represented 5% or less of that found at an equimolar concentration of D-[U- 14C]glucose. Neither the monomethyl nor the dimethyl ester of succinic acid exerted any significant effect upon the metabolism of D-glucose. Likewise, D-glucose (10 mM) failed to significantly affect the oxidation of the dimethyl ester of either [1,4- 14C]succinic acid or [2,3- 14C]succinic acid. It is concluded that, at variance with the situation recently documented in rat pancreatic islets and hepatocytes, the methyl esters of succinic acid are poorly metabolized in neural cells.

Deleterious effect of formycin A on tumor islet cells

The adenosine analogue formycin A was recently introduced as a potent insulin secretagogue in normal pancreatic islet cells. In the present study, formycin A was found to inhibit insulin release and to exert a cytotoxic effect in tumor islet cells of the RINm5F line. The latter effect was concentration-related in the 10-100 microM range of formycin A concentrations, not rapidly reversed, and not reproduced by adenosine. This study thus reveals that formycin A may display cytotoxic potential in the same range of concentrations in which it causes a progressive increase of glucose-stimulated insulin secretion in normal pancreatic islets.

Activity of cytosolic and mitochondrial enzymes participating in nutrient catabolism of normal and tumoral islet cells

The aim of the present study is to compare normal and tumoral pancreatic islet cells in terms of both the activity of selected cytosolic and mitochondrial enzymes participating to nutrient catabolism and the intrinsic properties of FAD-glycerophosphate dehydrogenase. The activity of the glycolytic enzymes hexokinase and lactate dehydrogenase was higher in tumoral (RINm5F) than normal islet cells. The opposite was seen for glutamate decarboxylase, glutamate-oxaloacetate transaminase, glutamate-pyruvate transaminase, glutamate dehydrogenase, 2-ketoglutarate dehydrogenase and FAD-glycerophosphate dehydrogenase (m-GDH). These findings are consistent with the high rates of glycolysis and protein synthesis seen in tumoral islet cells compared with normal islet cells, which favour mitochondrial oxidative events associated with the catabolism of D-glucose and amino acids. The intrinsic catalytic properties of m-GDH were comparable, albeit not identical, in normal and tumoral islet cells. Since a deficiency of m-GDH in pancreatic islets may represent a contributing factor in the pathogenesis of non-insulin-dependent diabetes, it is proposed that RINm5F cells may readily yield sufficient islet m-GDH for purification and further gene cloning.

Insulinotropic action of glutamic acid dimethyl ester

Glutamic acid dimethyl ester (GME; 3.0-10.0 mM) enhanced insulin release evoked by 6.0-8.3 mM D-glucose, 1.0-10.0 mM L-leucine, or 5.0-10.0 mM 2-amino-bicyclo(2,2,1)heptane-2-carboxylic acid, causing a shift to the left of the sigmoidal relationship between insulin output and D-glucose concentration. In the absence of D-glucose, GME also unmasked the insulinotropic potential of glibenclamide. In islets exposed to L-leucine, the insulinotropic action of GME coincided with an early fall and later increase in 86Rb outflow and augmentation of 45Ca outflow from prelabeled islets. The measurement of O2 uptake, NH4+ output, production of 14CO2 from islets prelabeled with [U-14C]palmitate, generation of 14C-labeled amino acids and 14CO2 from the dimethyl ester of either L-[1-14C]glutamic acid or L-[U-14C]glutamic acid, and D-[2-14C]glucose as well as D-[6-14C]glucose oxidation in the presence or absence of GME indicated that the latter ester was efficiently converted to L-glutamate and its further metabolites. The overall gain in O2 uptake represented the balance between GME oxidation and its sparing action on the catabolism of endogenous fatty acids and exogenous D-glucose. It is proposed that GME might represent a new tool to bypass beta-cell defects in D-glucose transport, phosphorylation, and further metabolism and, hence, to stimulate insulin release in experiments conducted in animal models of non-insulin-dependent diabetes mellitus.

Pancreatic islet response to dicarboxylic acid esters in rats with type 2 diabetes: enzymatic, metabolic and secretory aspects

This study aimed to compare the metabolic and secretory responses of pancreatic islets from animals with non-insulin-dependent diabetes to D-glucose with the effects of the methyl esters of succinic acid (SME) and glutamic acid (GME). The insulin secretory response to D-glucose was impaired in islets from rats with diabetes which was either inherited (Goto-Kakizaki (GK) rats) or acquired (streptozotocin-treated (STZ) rats). This coincided with a preferential alteration of oxidative relative to total glycolysis in intact islets and a selective defect of FAD-linked mitochondrial glycerophosphate dehydrogenase (m-GDH) in islet homogenates. This enzymatic defect was also found in purified B cells from STZ rats. It contrasted both with unaltered activities of glutamate dehydrogenase and succinate dehydrogenase in the islets of diabetic animals and with a normal or even increased activity of m-GDH in the livers of GK and STZ rats. The oxidation of [1,4-14C]SME and [U-14C]GME appeared decreased in islets of GK or STZ animals when compared with control rats, but no significant difference between control and diabetic rats was observed when the oxidative data were expressed relative to the rate of [U-14C]GME hydrolysis. Nevertheless, the absolute values for insulin release evoked by a non-metabolized analogue of L-leucine (BCH), by SME and by the association of BCH with either SME or GME were invariably lower in islets of GK and STZ rats than in those of control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired FAD-glycerophosphate dehydrogenase activity in islet and liver homogenates of fa/fa rats

The mitochondrial FAD-linked enzyme glycerophosphate dehydrogenase plays a key role in the pancreatic B-cell glucose sensing device. In the present study, the activity of this enzyme was examined in islets of fa/fa rats in which inherited diabetes mellitus is associated with obesity, hyperinsulinism and severe insulin resistance. The specific activity of both FAD-linked glycerophosphate dehydrogenase and glutamate dehydrogenase were decreased in islet and liver homogenates prepared from fa/fa, as compared to Fa/Fa, rats, this coinciding with a low ratio between glutamateoxalacetate and glutamate-pyruvate transaminase activity in both islet and liver extracts, islet hyperplasia, hyperinsulinemia and hepatic steatosis in the hyperglycemic fa/fa rats. It is speculated that a low activity of FAD-linked glycerophosphate dehydrogenase in the pancreatic B-cell may participate to the perturbation of glucose homeostasis in fa/fa rats, like in other animal models of non-insulin-dependent diabetes mellitus.

Secretory, biosynthetic, respiratory, cationic, and metabolic responses of pancreatic islets to palmitate and oleate

Palmitate and oleate (0.5 to 1.0 mM) caused a time- and concentration-related augmentation of insulin release evoked by D-glucose (6.0 to 16.7 mM) in rat isolated pancreatic islets. This contrasted with an inhibitory action of the fatty acids upon L-[4-3H]phenylalanine incorporation into TCA-precipitable material, but coincided with an increased biosynthesis of proinsulin relative to that of other islet peptides. The failure of palmitate to cause an immediate increase in insulin output at a low glucose concentration (6.0 mM) coincided with an unchanged rate of O2 uptake over a 10- to 15-min exposure to this fatty acid. Over prolonged incubation (90 min), however, both palmitate and oleate (1.0 mM) stimulated 45Ca net uptake by islets exposed to 6.0 mM D-glucose. Like their insulinotropic effect, the time course for the oxidation of [U-14C]palmitate and [U-14C]oleate was characterized by a progressive buildup in 14CO2 production rate. Moreover, palmitate and oleate decreased D-[5-3H]glucose conversion to 3HOH and D-[U-14C]glucose conversion to radioactive acidic metabolites over short (30 min) but not prolonged (120 min) incubation periods. The two fatty acids also interfered with the generation of 14CO2 from islets prelabeled with [U-14C]palmitate, but not L-[U-14C]glutamine. It is concluded that, at least during prolonged exposure to either palmitate or oleate, the secretory, cationic, and metabolic response to these fatty acids displays features comparable to those usually found in islets stimulated by nutrient secretagogues.

Enzymatic, metabolic and secretory patterns in human islets of type 2 (non-insulin-dependent) diabetic patients

Islets were isolated by automatic digestion from non-diabetic cadaveric organ donors and from Type 2 (non-insulin-dependent) diabetic subjects. The activity of FAD-glycerophosphate dehydrogenase, but not that of either glutamate dehydrogenase, glutamate-oxalacetate transaminase or glutamate-pyruvate transaminase, was lower in Type 2 diabetic patients than control subjects. Hexokinase, glucokinase and glutamate decarboxylase activities were also measured in islets from control subjects. The utilization of D-[5-3H]glucose, oxidation of D-[6-14C]glucose and release of insulin evoked by D-glucose were all lower in Type 2 diabetic patients than control subjects. The secretory response to the combination of L-leucine and L-glutamine appeared less severely affected. Islets from Type 2 diabetic patients may thus display enzymatic, metabolic and secretory anomalies similar to those often observed in animal models of Type 2 diabetes, including a deficiency of beta-cell FAD-linked glycerophosphate dehydrogenase, the key enzyme of the glycerol phosphate shuttle.

Hexose metabolism in pancreatic islets: succinate dehydrogenase activity in islet homogenates

Succinate dehydrogenase activity was measured in rat pancreatic islet homogenates incubated in the presence of [1,4-14C]succinate, the reaction velocity being judged through the generation of 14CO2 in the auxiliary reactions catalysed by pig heart fumarase and chicken liver NADP-malate dehydrogenase. In the presence of 1.0 mM succinate, the reaction velocity averaged 5.53 +/- 0.44 pmol min-1 microgram-1 islet protein. The Km for succinate was close to 0.4 mM and the enzymic activity was restricted to mitochondria. These kinetic results indicate that, under the present experimental conditions, the activity of succinate dehydrogenase does not vastly exceed that of either NAD-isocitrate dehydrogenase or the 2-ketoglutarate dehydrogenase complex, at least when the latter enzymes are activated by ADP and/or Ca2+. Nevertheless, the activity of succinate dehydrogenase is sufficient to account for the increase in O2 uptake evoked in intact islets by the monomethyl ester of succinic acid. It could become a rate-limiting step of the Krebs cycle in models of B-cell dysfunction.

Deficient activity of FAD-linked glycerophosphate dehydrogenase in islets of GK rats

In pancreatic islet extracts of rats with hereditary non-insulin-dependent diabetes mellitus (GK rats), the activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase, as measured by either a radioisotopic or colorimetric procedure, only represented 30 to 40% of that found in control rats. This decrease in enzymic activity was not attributable to any sizeable change in either islet DNA content or the relative contribution of insulin-producing beta cells to total islet mass. It contrasted with a normal activity of other mitochondrial dehydrogenases and hexokinase isoenzymes. It coincided, however, with an increased activity of glutamate-pyruvate transaminase, as already observed in adult rats injected with streptozotocin during the neonatal period. The decreased activity of islet FAD-linked glycerophosphate dehydrogenase also contrasted with an increased activity of the same enzyme in the liver of GK, as compared to control rats. In the light of these findings and recent metabolic data collected in intact islets of GK rats, it is proposed that a deficiency of beta-cell FAD-linked glycerophosphate dehydrogenase, the key enzyme of the glycerol phosphate shuttle, may represent a cause of inherited non-insulin-dependent diabetes.

Effect of streptozotocin and nicotinamide upon FAD-glycerophosphate dehydrogenase activity and insulin release in purified pancreatic B-cells

Purified rat pancreatic insulin-producing B-cells, which display a 12-fold higher activity of FAD-linked glycerophosphate dehydrogenase than other islet endocrine cells, were exposed for 30 min to 2 mM streptozotocin and subsequently cultured for 2 days in the absence or presence of 2 mM nicotinamide. Streptozotocin decreased by 54% the number of B-cells and, in surviving cells, lowered by 75% the activity of FAD-linked glycerophosphate dehydrogenase, whilst failing to affect that of glutamate dehydrogenase. This coincided with a 42-51% reduction of insulin secretion, when expressed relative to either the DNA or hormonal content of surviving cells. After exposure to streptozotocin, the presence of nicotinamide in the culture medium reduced cell death by 44% and also reduced the deleterious effects of streptozotocin upon both the enzymic and secretory activities of surviving cells. These findings indicate that the decreased activity of FAD-linked glycerophosphate dehydrogenase previously documented in pancreatic islets from streptozotocin-injected rats, as well as the protective effect of nicotinamide thereupon, are not attributable solely to changes in the number of B-cells but also to an altered enzymic activity in surviving B-cells. The latter anomaly may account, in part at least, for an impaired B-cell secretory response to D-glucose.

Respiratory, ionic, and functional effects of succinate esters in pancreatic islets

The methyl esters of succinic acid were introduced a few years ago as new potent insulin secretagogues. In the present study, they were found to increase O2 uptake by rat islets incubated in the absence or presence of D-glucose; to decrease 86Rb outflow from prelabeled islets; to stimulate biosynthetic activity in the islets, with a preferential effect on the synthesis of proinsulin; to inhibit 45Ca efflux from prelabeled islets perifused in the absence of extracellular Ca2+ but to augment 45Ca net uptake and to cause a biphasic stimulation of 45Ca outflow in islets incubated or perifused in the presence of extracellular Ca2+; and to evoke a biphasic stimulation of insulin release. The insulinotropic action of these methyl esters coincided with a shift to the left of the sigmoidal relationship between insulin output and D-glucose concentration, was concentration related in the 2-10 mM range, failed to be duplicated by succinic acid, displayed both Ca2+ dependency and resistance to a lowering of extracellular pH, and was operative in the absence of D-glucose whether or not the islets were stimulated by non-nutrient secretagogues. It is concluded that the respiratory, cationic, biosynthetic, and secretory responses of the islets to succinate methyl esters display the characteristic features usually encountered in the process of nutrient-stimulated insulin release.

Increased activity of FAD-linked glycerophosphate dehydrogenase in pancreatic islets of BB rats

In several animal models of non-insulin-dependent diabetes, a decreased activity of FAD-linked glycerophosphate dehydrogenase was recently documented in pancreatic islet, but not liver, homogenates. The present study reveals that, on the contrary, the activity of the same mitochondrial enzyme is increased in islet, but not liver or spleen, homogenates of BB, as compared to BW, rats examined before the onset of severe hyperglycemia in this animal model of autoimmune insulin-dependent diabetes.

Streptozotocin-induced FAD-glycerophosphate dehydrogenase suppression in pancreatic islets. Relationship with the severity and duration of hyperglycaemia and resistance to insulin or riboflavin treatment

In vitro, streptozotocin (1.0-2.0 mM) fails to exert any immediate effect on the activity of FAD-glycerophosphate dehydrogenase in either pancreatic islet homogenate or freshly isolated intact islets. However, when injected in vivo, streptozotocin (40 mg/kg body weight) lowers the specific activity of the FAD-linked enzyme in islet homogenates within 24 h, whilst causing little change in 2-ketoglutarate dehydrogenase and increasing glutamate dehydrogenase islet activity. In animals which became frankly hyperglycaemic as the result of the injection of streptozotocin, the activity of islet FAD-glycerophosphate dehydrogenase, measured 2 weeks after administration of the B-cell cytotoxic agent, was decreased to 10-20% of its control value. Neither insulin treatment nor riboflavin supplementation affected this enzymic defect. Even when the animals injected with streptozotocin remained virtually euglycaemic, the activity of islet FAD-glycerophosphate dehydrogenase was markedly decreased. This coincided with a preferential impairment of aerobic glycolysis, as judged from the ratio between D-[3,4-14C]glucose oxidation and D-[5-3H] glucose utilization by the islets. It is proposed, therefore, that the administration of sub-diabetogenic amounts of streptozotocin to adult rats represents an alternative and easier approach to the study of B-cell dysfunction in this model of type 2 (non-insulin-dependent) diabetes than does streptozotocin injection in neonatal rats.

Hexose metabolism in pancreatic islets. Regulation of NAD-isocitrate dehydrogenase activity

D-Glucose causes a preferential stimulation of mitochondrial oxidative events relative to glycolysis in pancreatic islets. The possible participation of a Ca(2+)-induced activation of NAD-isocitrate dehydrogenase in this process was investigated. The activity of the enzyme in rat islet homogenates was measured through the generation of either NADH or 2-ketoglutarate. In the absence of Ca2+ and ADP, half-maximal velocities were recorded at isocitrate and NAD+ concentrations close to 1.2 and 0.5 mM, respectively. At isocitrate concentrations in the 0.15-1.5 mM range, ADP (1.0 mM) markedly increased the reaction velocity recorded in the absence of Ca2+ and conferred to the enzyme the property of being activated by Ca2+, with a Ka for Ca2+ somewhat below 1.0 microM. From these data and by comparison with the activity of 2-ketoglutarate dehydrogenase, it is proposed that activation of NAD-isocitrate dehydrogenase by such factors as ADP and Ca2+ may be required in order to match, in nutrient-stimulated islets, the rates of 2-ketoglutarate generation and oxidative decarboxylation.

Long term in vitro effects of streptozotocin, interleukin-1, and high glucose concentration on the activity of mitochondrial dehydrogenases and the secretion of insulin in pancreatic islets

When cultured mouse pancreatic islets were exposed for 30 min to streptozotocin (STZ; 1.8 mM) and then maintained for 7 days in tissue culture, they displayed a decreased secretory response to D-glucose and an impairment of both FAD-linked glycerophosphate dehydrogenase and NAD-dependent 2-ketoglutarate dehydrogenase specific activities, with little change in either NAD-linked glycerophosphate dehydrogenase or glutamate dehydrogenase activity. The enzymatic defect was not reproduced by prolonged exposure of either rat islets to interleukin-1 (10 U/ml) or mouse islets to a high concentration of D-glucose (28 mM). In the former, but not latter, situation, the secretory response to D-glucose was again impaired. These findings reveal that STZ, but not all beta-cytotoxic agents, lowers the activity of selected islet mitochondrial dehydrogenases. Such enzymatic defects, especially the suppression of FAD-linked glycerophosphate dehydrogenase, may explain the preferential alteration of the B-cell metabolic and secretory responses to D-glucose, as previously observed in islets of adult rats injected with STZ during the neonatal period.

Enzymic and metabolic anomalies in islets of diabetic rats: relationship to B cell mass

A preferential impairment of the pancreatic B cell secretory response to D-glucose occurs in adult rats injected with streptozotocin during the neonatal period. Three possible explanations for such a preferential defect were investigated in the present study. First, the time course for 3-O-methyl-D-glucose uptake by islets suggested that the anomaly in hexose transport was mainly attributable to a decrease in the space accessible to the D-glucose analog commensurate with the decrease in B cell mass, rather than to a delayed equilibration of hexose concentration across the B cell plasma membrane. Second, the activity of glucose-6-phosphatase was found to be equally low in islets from diabetic and control rats, ruling out the futile cycling between D-glucose and D-glucose 6-phosphate as a cause for the preferential alteration of the secretory response to the hexose. Third, the activity of flavine adenine dinucleotide-linked glycerophosphate dehydrogenase was found to be decreased to a greater relative extent than the B cell mass. This coincided with an impaired generation of 3HOH from L-[2-3H] glycerol in intact islets. It is proposed, therefore, that an altered circulation in the glycerol phosphate shuttle may play a major role in the impaired process of glucose-stimulated insulin release in this model of noninsulin-dependent diabetes.

Study of hexose transport, glycerol phosphate shuttle and Krebs cycle in islets of adult rats injected with streptozotocin during the neonatal period

At 3-4 degrees C, the transport of 3-O-methyl-D-glucose (30 mM) was severely impaired in islets prepared from adult rats injected with streptozotocin during the neonatal period. However, at 37 degrees C, the first and second phase of glucose-stimulated insulin release were decreased to the same relative extent in perifused islets of diabetic, as compared to control, animals. Moreover, the time-related increase in the oxidative response of the islets to 16.7 mM D-glucose was less pronounced in diabetic than control rats. The activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase in islet homogenates of diabetic rats only represented one-fifth of that found in control rats, whereas the activity of the cytosolic NAD-glycerophosphate dehydrogenase was comparable in both types of rats. This coincided with the fact that a rise in D-glucose concentration from 2.8 to 16.7 mM failed to increase significantly L-[2-3H]glycerol conversion to 3HOH in islets from diabetic rats, in contrast to the situation found in control animals. The activity of 2-ketoglutarate dehydrogenase in islet homogenates when expressed per microgram protein was not different in control and diabetic rats. Likewise, the ratio between D-[6-14C]glucose oxidation and D-[3,4-14C]glucose oxidation and the capacity of either a non-metabolized analog of L-leucine or 3-phenylpyruvate to preferentially stimulated D-[6-14C]glucose oxidation relative to D-[5-3H]glucose utilization were both unaffected in islets from diabetic rats. These findings argue against the existence of a primary defect in the Krebs cycle of diabetic rats. It is proposed that, despite an obvious alteration of the hexose transport system in the islet cells of diabetic rats, the preferential impairment of the B-cell secretory response to D-glucose, as distinct from other secretagogues, in this model of non-insulin-dependent diabetes is mainly attributable to the low activity of FAD-linked glycerophosphate dehydrogenase, resulting in a decreased metabolic flow through the glycerol phosphate shuttle and a reduced rate of aerobic glycolysis.

Intrinsic properties of FAD-linked glycerophosphate dehydrogenase in islets from normal and streptozotocin-induced diabetic rats

An acquired or inherited deficiency of FAD-linked glycerophosphate dehydrogenase activity in the pancreatic islet B-cell was recently proposed to represent a far-from-uncommon contributing factor in the pathogenesis of non-insulin-dependent diabetes mellitus. In the present study, it was investigated whether the postulated genomic defect coincides with the biosynthesis of an enzymic protein with altered catalytic properties and might concern an isoenzyme distinct from that found in extrapancreatic tissues. The activity of FAD-linked glycerophosphate dehydrogenase, as measured by either a radioisotopic or colorimetric procedure, was indeed severely decreased in islets from rats injected with streptozotocin. The intrinsic properties of the enzyme were preserved, however, as judged from the affinity for L-glycerol-3-phosphate, the ratio in reaction velocity using either FAD or iodonitrotetrazolium as electron acceptor and the activation of the enzyme by Ca2+. When the same kinetic parameters were compared in islet, liver and spleen homogenates from normal rats, significant differences were observed, however, between these three tissues, suggesting the possible existence of distinct isoenzymes.

Hexose metabolism in pancreatic islets. Glucose-induced and Ca(2+)-dependent activation of FAD-glycerophosphate dehydrogenase

A rise in extracellular D-glucose concentration causes in pancreatic islets a preferential stimulation of aerobic, relative to total, glycolysis. The possible participation in such a phenomenon of a glucose-induced and Ca(2+)-dependent activation of FAD-glycerophosphate dehydrogenase was investigated. In islet homogenates, the activity of the mitochondrial and Ca(2+)-responsive FAD-glycerophosphate dehydrogenase was about two orders of magnitude lower than that of the cytosolic and Ca(2+)-insensitive NAD-glycerophosphate dehydrogenase. In islet mitochondria, Ca2+ increased the affinity of the FAD-glycerophosphate dehydrogenase for L-glycerol 3-phosphate, but did not affect the maximal reaction velocity. In the presence of 0.1 mM-L-glycerol 3-phosphate, the Ka for Ca2+ was close to 0.1 microM. When intact islets were preincubated in the presence of both D-glucose and Ca2+, the activity of FAD-glycerophosphate dehydrogenase measured in intact mitochondria incubated in the presence of 1.2 microM-Ca2+ was higher than that recorded under the same conditions in islets preincubated in the absence of D-glucose and/or Ca2+. These findings support the view that, in islets exposed to a high concentration of D-glucose, a Ca(2+)-induced activation of mitochondrial FAD-glycerophosphate dehydrogenase favours the transfer of reducing equivalents by the glycerol phosphate shuttle, and hence accounts, in part at least, for the preferential stimulation of aerobic glycolysis.

Streptozotocin-induced suppression of FAD-linked glycerophosphate dehydrogenase in pancreatic islets of adult rats

Injection of streptozotocin (30-40 mg/kg body weight) to adult rats caused within 4-6 days a sizeable decrease in the activity of FAD-linked glycerophosphate dehydrogenase in pancreatic islets, with little change in either glutamate dehydrogenase or 2-oxoglutarate dehydrogenase activity. The severity of the enzymatic defect was related to that of the diabetic state, although a decreased enzymic activity was also observed in islets from virtually normoglycemic animals examined 2-3 weeks after streptozotocin injection. The administration of nicotinamide prior to that of streptozotocin prevented the change in enzymic activity. It is proposed that the enzymatic defect, rather than being attributable to a genomic effect of streptozotocin, may reflect the preferential impairment of a subpopulation of pancreatic B-cells.

Impairment of glycerol phosphate shuttle in islets from rats with diabetes induced by neonatal streptozocin

In islets from adult rats injected with streptozocin during the neonatal period, the oxidative and secretory responses to D-glucose are more severely affected than those evoked by L-leucine. A possible explanation for such a preferential defect was sought by comparing the rate of aerobic glycolysis, taken as the sum of D-[3,4-14C]glucose conversion to labeled CO2, pyruvate, and amino acid, with the total glycolytic flux, as judged from the conversion of D-[5-3H]glucose to 3H2O. A preferential impairment of aerobic relative to total glycolysis was found in islets from diabetic rats incubated at either low or high D-glucose concentration. This coincided in islet mitochondria of diabetic rats with a severe decrease in both the basal (no-Ca2+) generation of 3H2O from L-[2-3H]glycerol-3-phosphate and the Ca2(+)-induced increment in [3H]glycerophosphate detritiation. The mitochondria of diabetic rats were also less efficient than those of control animals in generating 14CO2 from [1-14C]-2-ketoglutarate. The diabetes-induced alteration of 2-ketoglutarate dehydrogenase in islet mitochondria was less marked, however, than that of the FAD-linked glycerophosphate dehydrogenase and was not associated with any change in responsiveness to Ca2+. Sonicated islet mitochondria of diabetic rats displayed normal to slightly elevated glutamate dehydrogenase activity. We propose, therefore, that the preferential impairment of the oxidative and secretory responses of islet cells to D-glucose in this experimental model of diabetes may be at least partly attributable to an altered transfer of reducing equivalents into the mitochondria as mediated by the glycerol phosphate shuttle.

Hexose metabolism in pancreatic islets. Regulation of aerobic glycolysis and pyruvate decarboxylation

1. D-Glucose (0.5-16.7 mM) preferentially stimulates aerobic glycolysis and D-[3,4-14C]glucose oxidation, relative to D-[5-3H]glucose utilization in rat pancreatic islets, the concentration dependency of such a preferential effect displaying a sigmoidal pattern. 2. Inorganic and organic calcium antagonists, as well as Ca2+ deprivation, only cause a minor decrease in the ratio between D-[3,4-14C]glucose oxidation and D-[5-3H]glucose utilization in islets exposed to a high concentration of the hexose (16.7 mM). 3. Non-glucidic nutrient secretagogues such as 2-aminobicyclo[2,2,1]heptane-2-carboxylate (BCH), 2-ketoisocaproate and 3-phenylpyruvate fail to stimulate aerobic glycolysis and D-[3,4-14C]glucose oxidation in islets exposed to 6.0 mM D-glucose. Nevertheless, BCH augments [1-14C]pyruvate and [2-14C]pyruvate oxidation. 4. The glucose-induced increment in the paired ratio between D-[3,4-14C]glucose oxidation and D-[5-3H]glucose utilization is impaired in the presence of either cycloheximide or ouabain. 5. These findings suggest that the preferential effect of D-glucose upon aerobic glycolysis and pyruvate decarboxylation is not attributable solely to a Ca(2+)-induced activation of FAD-linked glycerophosphate dehydrogenase and/or pyruvate dehydrogenase, but may also involve an ATP-modulated regulatory process.

Hexose metabolism in pancreatic islets. Participation of Ca2(+)-sensitive 2-ketoglutarate dehydrogenase in the regulation of mitochondrial function

A rise in extracellular D-glucose concentration results in a preferential and Ca2(+)-dependent stimulation of mitochondrial oxidative events in pancreatic islet cells. The possible participation of Ca2(+)-dependent mitochondrial dehydrogenases, especially 2-ketoglutarate dehydrogenase, in such an unusual metabolic situation was explored in intact islets, islet homogenates and isolated islet mitochondria. In intact islets exposed to a high concentration of D-glucose, the removal of extracellular Ca2+ impaired D-[6-14C]glucose oxidation whilst failing to affect the cytosolic or mitochondrial ATP/ADP ratios. In islet homogenates, the activity of 2-ketoglutarate dehydrogenase displayed exquisite Ca2(+)-dependency, the presence of Ca2+ causing a 10-fold increase in affinity for 2-ketoglutarate. In intact islet mitochondria, the oxidation of 2-[1-14C]ketoglutarate also increased as a function of extramitochondrial Ca2+ availability. Moreover, prior stimulation of intact islets by D-glucose resulted in an increased capacity of mitochondria to oxidize 2-[1-14C]ketoglutarate. The absence of extracellular Ca2+ during the initial stimulation of intact islets impaired but did not entirely suppress such a memory phenomenon. It is proposed that the mitochondrial accumulation of Ca2+ in nutrient-stimulated islets indeed accounts, in part at least, for the preferential stimulation of mitochondrial oxidative events in this fuel-sensor organ.

Hexose metabolism in pancreatic islet cells: the coupling between hexose phosphorylation and mitochondrial respiration

The possible relevance of D-glucose phosphorylation by mitochondria-bound hexokinase to the control of respiration was examined in mitochondria prepared from either tumoral pancreatic islet cells (RINm5F line) or normal rat liver. In both systems, ATP generated by mitochondria exposed to ADP and succinate could serve as a substrate for the phosphorylation of D-glucose. However, after exposure to exogenous ADP in the presence of succinate, only mitochondria isolated from RINm5F cells displayed a sizeable increase in O2 consumption in response to a subsequent administration of D-glucose. In this respect, the discrepancy between mitochondria from islet cells and liver, respectively, was found to be attributable to the much lower hexokinase activity, relative to respiratory rate, in liver than in RINm5F cell mitochondria. It is speculated that the coupling between hexose phosphorylation and respiration in islet cells may prime the mitochondria to generate ATP during the early metabolic and secretory response to a rise in extracellular D-glucose concentration.

Stimulus-secretion coupling of arginine-induced insulin release: comparison with histidine-induced insulin release

L-Histidine, when tested at a 10-mM concentration, caused a rapid and sustained stimulation of insulin release from rat islets exposed to either D-glucose (7.0 or 8.3 mM) or L-leucine (10.0 mM). The stimulation of insulin release could not be ascribed to an increase in oxygen uptake, to the generation of histamine from L-histidine, or to its participation in a transglutaminase-catalyzed reaction. Like other cationic amino acids, however, L-histidine rapidly accumulated in islet cells, increased 86Rb outflow from prelabeled islets perifused in the presence or absence of extracellular Ca2+, and stimulated the entry of Ca2+ into islet cells. Yet, the amount of exogenous L-histidine present in the islet cells with a positively charged side chain was estimated to be below the threshold value required for stimulation of insulin release by fully ionized cationic amino acids, such as L-arginine. Hence, the present findings argue against the view that the insulinotropic action of cationic amino acids is solely attributable to the accumulation of these positively charged molecules inside the islet B cell with subsequent depolarization of the plasma membrane.

Ketone bodies and islet function: 86Rb handling and metabolic data

The metabolism of ketone bodies was investigated in rat pancreatic islets incubated in the absence or presence of D-glucose. The generation of 14CO2 from 3-14C-labeled ketone bodies, the interconversion of D-(-)-beta-hydroxybutyrate and acetoacetate (AcAc), the reciprocal effects of ketone bodies and D-glucose on their respective catabolism, and the influence of these exogenous nutrients on the output of 14CO2 from islets preincubated with either L-[U-14C]glutamine or [U-14C]palmitate provided an estimation of the nutrient-induced changes in O2 uptake that was in fair agreement with the observed modifications of islet respiration. There was a close correlation between such changes and the corresponding values for insulin output. Because the stimulation of insulin release by ketone bodies also coincided with a decrease in 86Rb outflow from prelabeled islets, these findings suggest that the insulinotropic action of ketone bodies is causally linked to their catabolism through an increase in ATP generation rate and a subsequent decrease in K+ conductance. A complementary participation of changes in mitochondrial redox state to stimulus-secretion coupling is considered, however, in the light of comparisons between the effects of D-(-)-beta-hydroxybutyrate and AcAc, respectively, on mitochondrial NADH generation, 45Ca net uptake, and D-[6-14C]glucose oxidation.

Stimulus-secretion coupling of arginine-induced insulin release: comparison with lysine-induced insulin secretion

L-Lysine, like-L-arginine, L-ornithine, or L-homoarginine, accumulated in rat pancreatic islets and stimulated 86Rb efflux, 45Ca uptake and efflux, and insulin release in islets exposed to D-glucose (7.0 mM). The effect of L-lysine differed from that of the other cationic amino acids by such features as the absence of a threshold concentration for stimulation of insulin release, a much lesser sensitivity of the secretory response to intracellular acidification, and the stimulation of 86Rb net uptake over 60 min of incubation. This coincided with the fact that even in the absence of another exogenous nutrient, L-lysine was well oxidized, augmented NH4+ production, increased both the ATP content and ATP/ADP ratio, caused a time-related decrease in 86Rb fractional outflow, and provoked either a transient (10 mM L-lysine) or sustained (20 mM L-lysine) stimulation of insulin secretion. It is proposed, therefore, that the functional response of the pancreatic B-cell to L-lysine involves not only a biophysical mechanism similar to that responsible for the insulinotropic action of L-homoarginine, but also a significant, albeit modest, metabolic component, which reflects the capacity of L-lysine to act as a fuel in islet cells.