Prolonged exposure of human pancreatic islets to high glucose concentrations in vitro impairs the beta-cell function

Sensitivity of human pancreatic islets to peroxynitrite-induced cell dysfunction and death

Nitric oxide and peroxynitrite (generated by the reaction of nitric oxide with the superoxide anion) may both be mediators of beta-cell damage in early insulin-dependent diabetes mellitus. We observed that acute exposure of primary cultured human pancreatic islets to peroxynitrite results in a significant decrease in glucose oxidation and islet retrieval. DNA strand breaks in single human and rat islet cells are detectable after acute peroxynitrite exposure, followed by a decrease in islet cell survival after 1 h and 24 h. Cell death appeared to occur via a toxic cell death mechanism (necrosis) rather than apoptosis, as suggested by vital staining and ultrastructural evidence of early membrane and organelle degradation, mitochondrial swelling and loss of matrix. This study demonstrates for the first time that cultured human pancreatic islets are susceptible to the noxious effects of peroxynitrite.

Cytokines activate the nuclear factor kappa B (NF-kappa B) and induce nitric oxide production in human pancreatic islets

We studied the ability of cytokines to activate the nuclear transcription factor NF-kappaB in human pancreatic islets and the putative role of NF-kappaB for cytokine-induced NO production. Brief exposure (20 min) of human islets of Langerhans to a combination of interleukin-1beta + interferon-gamma + tumor necrosis factor-alpha induced a 2.6-fold increase in nuclear NF-kappaB activity in gel shift analysis. This increase was prevented by the NF-kappaB inhibitor, pyrrolidine dithiocarbamate (PDTC), which also counteracted NO production by human islets exposed for 14 h to the cytokine combination. High concentrations of interleukin-1beta alone (150 and 250 U/ml) increased NF-kappaB nuclear binding but failed to induce NO formation in human islets. The present data are the first to demonstrate that cytokines activate NF-kappaB in primary adult human pancreatic islets and suggest that activation of NF-kappaB may be a necessary but not sufficient signal for cytokine-induced iNOS expression in human islets of Langerhans.

Nitric oxide donors decrease the function and survival of human pancreatic islets

Nitric oxide (NO) has been proposed as a possible mediator of beta-cell damage in human IDDM. This hypothesis is based on in vitro studies with rodent pancreatic islets. In the present study we examined whether human beta-cells are affected by NO. In view of species differences in beta-cell sensitivity to damaging agents, rat islets were investigated in parallel. Isolated islets were exposed for 90 min to different concentrations of three chemically unrelated NO donors, SIN-1, GSNO or RBS. At the end of this incubation, human insulin release was mostly similar in control and NO-treated islets but, 48 h later, islet retrieval, islet DNA and insulin content, and glucose-induced insulin release were markedly lower in islets exposed to NO donors. Rat islets were already inhibited during the initial 90 min; 48 h later their loss in beta-cell function was similar to that in human islets. Nicotinamide or succinic acid monomethyl ester partially protected against SIN-1 induced islet cell loss, but not against the functional inhibition of human pancreatic islets. Exposure of human or rat islets to RBS was associated with significant DNA strand breakage, as judged by the comet assay (single cell gel electrophoresis) and by ultrastructural signs of cell damage. DNA damage was more severe in rat islet cells exposed to similar amounts of RBS. It is concluded that NO donors can damage human pancreatic islets, an effect paralleled by induction of nuclear DNA strand breaks.

Chronic exposure of betaTC-6 cells to supraphysiologic concentrations of glucose decreases binding of the RIPE3b1 insulin gene transcription activator

We have shown previously that chronic exposure of HIT-T15 cells to supraphysiologic glucose concentrations causes decreased insulin gene transcription and decreased binding activities of two beta-cell specific transcription factors, STF-1 and the RIPE3b1 activator, and have suggested that these events may provide a mechanism for glucose toxicity on beta-cell function. However, this contention can be criticized because it is not clear whether these observations are unique to the HIT-T15 cell or generalizable to other beta-cell lines and the islet. Therefore, we cultured betaTC-6 cells for up to 41 wk in either 11.1 or 0.8 mM glucose. We observed a passage-dependent decrease in insulin content and insulin mRNA levels in betaTC-6 cells chronically cultured in 11.1 mM glucose. Cells chronically cultured in 0.8 mM glucose had higher insulin mRNA levels than cells chronically cultured in 11.1 mM glucose. The relative activity of a chloramphenicol acetyl transferase (CAT) reporter gene controlled by the 5' regulatory region of the human insulin gene was decreased in late passage betaTC-6 cells chronically cultured in 11.1 mM glucose, but was preserved in late passages of cells chronically cultured in 0.8 mM glucose. Electromobility shift assays demonstrated that binding of a specific nuclear protein that recognizes the RIPE3b1 binding site of the insulin gene was markedly diminished in late passage cells chronically exposed to 11.1 mM glucose, whereas binding activities of STF-1 and ICE activators were unchanged. RIPE3b1 binding activity was preserved in late passage cells chronically exposed to 0.8 mM glucose. Mutation of the RIPE3b1 binding site almost completely abolished insulin gene transcription as well as binding activity. We conclude that chronic exposure of betaTC-6 cells to high glucose concentrations paradoxically decreases insulin gene transcription, in part, by decreasing activity of the trans-activating factor which binds to the RIPE3b1 sequence. This study uniquely demonstrates that altered binding to the RIPE3b1 sequence mediates glucose toxicity in betaTC-6 cells, thus reinforcing the importance of this cis-acting element in the regulation of insulin gene transcription. We conclude that the phenomenon of glucose toxicity decreasing binding of transcription factors and thereby reducing insulin gene expression is not a feature solely of HIT-T15 cells and may be demonstrable generally in beta-cell lines.

An integrated view of beta-cell dysfunction in type-II diabetes

Type-II (non-insulin-dependent) diabetes mellitus (NIDDM) is a heterogeneous disease resulting from insulin resistance and beta-cell dysfunction. beta-Cell dysfunction in Type-II diabetes is characterized by a specific lack of first-phase glucose-induced insulin secretion. This defect is readily reversible upon normalization of blood glucose levels. Chronic hyperglycemia itself is harmful to the beta-cell and affects both insulin biosynthesis and exocytosis. No unique intracellular defect has been demonstrated to be responsible for all common forms of the disease. However, mutations of the glucokinase gene have been identified in maturity onset diabetes in the young, a particular form of NIDDM.

Effects of prolonged glucose infusion on insulin secretion, clearance, and action in normal subjects

It was the aim of this study to determine whether prolonged hyperglycemia can produce "glucose toxicity" in normal human subjects. To this end, plasma glucose was clamped at approximately 5, approximately 8.8, and approximately 12.6 mM for 68 h in healthy volunteers. Rates of insulin secretion (by deconvolution of plasma C-peptide) and rates of insulin clearance [area under curve (AUC) 24 h insulin secretion/AUC 24 h insulin] were determined. Pre- and posthyperglycemia glucose turnover was measured (with [6,6-2H2]glucose) during euglycemic-hyperinsulinemic clamping to assess peripheral (muscle) and hepatic insulin action. Hyperglycemia (approximately 12.6 mM) for 68 h was associated with significant reductions in rates of insulin secretion (-35%, P < 0.05), insulin clearance (-57%, P < 0.05), glucose infusion rates needed to maintain hyperglycemia (-36%, P < 0.05), and insulin-stimulated glucose uptake (-55%, P < 0.01). No significant changes were seen during approximately 8.8 mM hyperglycemia or during euglycemia. These data showed that 12.6 mM hyperglycemia, but not 8.8 mM hyperglycemia or euglycemia, was associated with reduced insulin secretion, insulin clearance, and peripheral (muscle) insulin action. We concluded that 1) in normal subjects, desensitization to glucose involving beta-cells and muscle developed at plasma glucose concentrations between approximately 9 and approximately 12 mM, and 2) these effects were partially compensated for by a decrease in insulin clearance.

Survival and function of islets during culture

Cell and tissue culture techniques have improved considerably since the first attempts to maintain explants of animal tissue in vitro. The two major developments that have allowed these improvements are the ability to produce continuous cell lines, thus allowing reproducible results to be obtained, and the definition of media for different cell types, thereby reducing the need for supplements of serum and other extraneous extracts. The requirements of islets in culture have been more difficult to define, largely because islets do not proliferate in culture and proliferation rate cannot therefore be used to measure the suitability of the medium. Further difficulties arise because islets are highly metabolically active "mini-organelles." Although many studies have been undertaken to try and optimize media for the culture islets of Langerhans, the media most commonly used are commercially available media developed for other cell types. There remains ample scope for further refinement of the composition of islet culture media, with the possibility of different media for islets from different species.

Human and rat beta cells differ in glucose transporter but not in glucokinase gene expression

Glucose homeostasis is controlled by a glucose sensor in pancreatic beta-cells. Studies on rodent beta-cells have suggested a role for GLUT2 and glucokinase in this control function and in mechanisms leading to diabetes. Little direct evidence exists so far to implicate these two proteins in glucose recognition by human beta-cells. The present in vitro study investigates the role of glucose transport and phosphorylation in beta-cell preparations from nondiabetic human pancreata. Human beta-cells differ from rodent beta-cells in glucose transporter gene expression (predominantly GLUT1 instead of GLUT2), explaining their low Km (3 mmol/liter) and low VMAX (3 mmol/min per liter) for 3-O-methyl glucose transport. The 100-fold lower GLUT2 abundance in human versus rat beta-cells is associated with a 10-fold slower uptake of alloxan, explaining their resistance to this rodent diabetogenic agent. Human and rat beta-cells exhibit comparable glucokinase expression with similar flux-generating influence on total glucose utilization. These data underline the importance of glucokinase but not of GLUT2 in the glucose sensor of human beta-cells.

Differences in the expression of heat-shock proteins and antioxidant enzymes between human and rodent pancreatic islets: implications for the pathogenesis of insulin-dependent diabetes mellitus

BACKGROUND

It has previously been observed that the insulin-producing cells of human pancreatic islets are more resistant to alloxan-, streptozotocin-, nitroprusside-, or cytokine-induced injury than those of mouse and rat islets.

MATERIALS AND METHODS

Human pancreatic islets were obtained from heart-beating organ donors. The expression of the stress proteins heat shock protein 70 (hsp70) and heme oxygenase and the anti-apoptosis gene bcl-2 was determined in isolated rat, mouse, and human islets, either cultured in vitro or transplanted under the kidney capsule of nude mice, using immunoblot analysis. Rat and human islet sensitive hydrogen peroxide was assess by glucose oxidation measurements. Isolated islets were also analyzed for their catalase and superoxide dismutase activities, and the islet cell levels of reduced glutathione were determined in response to hydrogen peroxide and nitroprusside. Programmed cell death in human and rat islets in response to streptozotocin was evaluated using TUNEL staining.

RESULTS

Cultured human islets expressed higher contents of hsp70 than mouse and rat islets at basal conditions. Also after 4 weeks under the kidney capsule of normoglycemic mice, the hsp70 levels were higher in human islets than in rat islets. The expression of another stress protein, heme oxygenase (HO), was strongly increased in cultured rat islets, but was not affected in human islets. Expression of the bcl-2 gene could not be detected in human islets. In spite of this, 0.5 mM streptozotocin induced apotosis in rat but not in human islet cells. Hydrogen peroxide (0.1 and 0.4 mM) decreased glucose oxidation rates in rat but not in human islets. The levels of reduced glutathione were moderately decreased in human and rat islet cells and sharply decreased in mouse islet cells in response to hydrogen peroxide. Moreover, the activities of catalase and superoxide dismutase (SOD) were markedly lower in mouse islets than in human islets. The activity of catalase was lower in rat islets than in human islets.

CONCLUSION

Human islets differ clearly from mouse and rat islets in their increased expression of hsp70, catalase, and SOD, which may explain the increased resistance of human islets to beta cell toxins.

Impairment of glucose-induced insulin secretion in human pancreatic islets transplanted to diabetic nude mice

Hyperglycemia-induced beta-cell dysfunction may be an important component in the pathogenesis of non-insulin-dependent diabetes mellitus. However, most available data in this field were obtained from rodent islets. To investigate the relevance of this hypothesis for human beta-cells in vivo, human pancreatic islets were transplanted under the renal capsule of nude mice. Experimental groups were chosen so that grafted islets were exposed to either hyper- or normoglycemia or combinations of these for 4 or 6 wk. Grafts of normoglycemic recipients responded with an increased insulin release to a glucose stimulus during perfusion, whereas grafts of hyperglycemic recipients failed to respond to glucose. The insulin content of the grafts in the latter groups was only 10% of those observed in controls. Recipients initially hyperglycemic (4 wk), followed by 2 wk of normoglycemia regained a normal graft insulin content, but a decreased insulin response to glucose remained. No ultrastructural signs of beta-cell damage were observed, with the exception of increased glycogen deposits in animals hyperglycemic at the time of killing. It is concluded that prolonged exposure to a diabetic environment induces a long-term secretory defect in human beta-cells, which is not dependent on the size of the islet insulin stores.

Cycad toxin-induced damage of rodent and human pancreatic beta-cells

Environmental toxins may be risk factors for some forms of diabetes mellitus and neurodegenerative diseases. The medicinal and food use of seed from the cycad plant (Cycas spp.), which contains the genotoxin cycasin, is a proposed etiological factor for amyotrophic lateral sclerosis/Parkinsonism-dementia complex (ALS/PDC), a prototypical neurodegenerative disease found in the western Pacific. Patients with ALS/PDC have a very high prevalence of glucose intolerance and diabetes mellitus (in the range of 50-80%). We investigated whether the cycad plant toxin cycasin (methylazoxymethanol (MAM) beta-D-glucoside) or the aglycone MAM are toxic in vitro to mouse or human pancreatic islets of Langerhans. Mouse pancreatic islets treated for 6 days with cycasin impaired the beta-cell insulin response to glucose, but this effect was reversible after a further 4 days in culture without the toxin. When mouse islets were exposed for 24 hr to MAM/MAM acetate (MAMOAc; 0.1-1.0 mM), there was a dose-dependent impairment in insulin release and glucose metabolism, and a significant decrease in islet insulin and DNA content. At higher MAM/MAMOAc concentrations (1.0 mM), widespread islet cell destruction was observed. Glucose-induced insulin release remained impaired even after removal of MAM and a further culturing for 4 days without the toxin. MAM damages islets by two possible mechanisms: (a) nitric oxide generation, as judged by increased medium nitrite accumulation; and (b) DNA alkylation, as judged by increased levels of O6-methyldeoxyguanosine in cellular DNA. Incubation of mouse islets with hemin (10 or 100 microM), a nitric oxide scavenger, or nicotinamide (5-20 mM) protected beta-cells from a decrease in glucose oxidation by MAM. In separate studies, a 24 hr treatment of human beta-islet cells with MAMOAc (1.0 mM) produced a significant decrease in both insulin content and release in response to glucose. In conclusion, the present data indicate that cycasin and its aglycone MAM impair both rodent and human beta-cell function which may lead to the death of pancreatic islet cells. These data suggest that a "slow toxin" may be a common aetiological factor for both diabetes mellitus and neurodegenerative disease.

Rapid deposition of amyloid in human islets transplanted into nude mice

Human islets of Langerhans were transplanted to the subcapsular space of the kidneys of nude mice which were either normoglycaemic or made diabetic with alloxan. After 2 weeks, the transplants were processed for light and electron microscopical analyses. In all transplants, islet amyloid polypeptide (IAPP)-positive cells were found with highest frequency in normoglycaemic animals. IAPP-positive amyloid was seen in 16 out of 22 transplants (73%), either by polarisation microscopy after Congo red staining or by immune electron microscopy. At variance with previous findings of amyloid deposits exclusively in the extracellular space of islets of non-insulin-dependent diabetic patients, the grafted islets contained intracellular amyloid deposits as well. There was no clear difference in occurrence of amyloid between diabetic and non-diabetic animals. The present study indicates that human islets transplanted into nude mice very soon present IAPP-positive amyloid deposits. This technique may provide a valuable model for studies of the pathogenesis of islet amyloid and its impact on islet cell function.

SAM prevents impairment of glucose-stimulated insulin secretion caused by hexose deprivation or starvation

Succinic acid monomethyl ester (SAM) was recently proposed as an insulinotropic tool in non-insulin-dependent diabetes mellitus. Three models were now used to investigate whether SAM protects the B-cell against the impairment of glucose-stimulated insulin release caused by either glucose deprivation or starvation. In the first model, preincubation of the islets for 180 min at low glucose concentration in the presence of SAM prevented the decrease in the secretory response to D-glucose otherwise observed during a subsequent incubation. In the second model, an impaired secretory response to D-glucose was observed after 3-day culture at low (2.8 or 5.6 mM) as distinct from high (11.1 mM) hexose concentration and the presence of SAM in the culture medium again protected against this anomaly. In the third model, the infusion of SAM for 3 days to starved rats restored the secretory potential of isolated islets to a level comparable to that otherwise found in fed rats. Thus, during glucose deprivation or starvation, SAM is indeed able to maintain B-cell responsiveness to D-glucose.

Diabetes enhancement and increased islet antigen expression following neonatal injections of glucose and arginine in non-obese diabetic mice

Modulation of beta-cell antigens at birth may affect the course of type I diabetes. Since the functional state of beta cells modulates antigen expression, we investigated whether neonatal injections of glucose and arginine (G-A) influence diabetes in non-obese diabetic (NOD) mice. Two groups of 90 mice (45 female, 45 male) were injected for the first 6 days of life with G-A or saline. To determine whether these injections influenced beta cell functional maturation, isolated islets were characterized according to insulin response to glucose or arginine. Modulation of antigens for islet-cell autoantibodies (ICA antigens) was analyzed by indirect immunofluorescence using ICA-positive human sera. Variations of pancreatic glutamic acid decarboxylase 67-kD (GAD 67) mRNA were evaluated by polymerase chain reaction (PCR), hybridization with a 32P-labeled probe, and densitometry of the autoradiographic bands. Female NOD mice treated with G-A displayed diabetes earlier and with a higher incidence (P < .01) than control mice, whereas the diabetes incidence was not statistically modified in G-A-treated male NOD mice. Insulitis was more severe (P < .03) in 2-month-old G-A-treated female NOD mice than in control mice, but was not statistically modified in male NOD mice. In both sexes, ICA antigens and GAD 67 mRNA were higher in G-A-treated mice than in control mice (P < .01). Islets isolated after neonatal G-A injections exhibited improved insulin sensitivity to both stimuli (P < .01).(ABSTRACT TRUNCATED AT 250 WORDS)

Major species differences between humans and rodents in the susceptibility to pancreatic beta-cell injury

The ability of beta cells to endure assaults may be relevant in the development of insulin-dependent diabetes mellitus. This study examines the susceptibility of human pancreatic islets to agents that are cytotoxic for rodent beta cells--i.e., sodium nitroprusside (NP, a nitric oxide donor), streptozotocin (SZ), or alloxan. After 5-8 days in tissue culture, human or rodent islets were exposed for 14 h to NP (50-200 microM) or for 30 min to SZ or alloxan (1-3 mM). Glucose oxidation by human islets was not reduced by NP, but there was a dose-dependent inhibition in rat (40-90% inhibition; P < 0.001) and mouse (10-60% inhibition; P < 0.05) islet glucose oxidation. Glucose (16.7 mM)-induced insulin release by human islets was not impaired after a 30-min exposure to SZ or alloxan, at concentrations that inhibited insulin release from rat (30-80% inhibition; P < 0.001) or mouse (10-70% inhibition; P < 0.05) islets. The viability of human beta cells purified by flow cytometry was not affected by SZ or alloxan (5 mM), as judged 1 or 4 days after a 10-min exposure and subsequent culture; these conditions were cytotoxic for rat beta cells, with 65-95% (P < 0.01) dead beta cells after 4 days. Human islets transplanted under the kidney capsule of nude mice were not affected by in vivo alloxan exposure, as suggested by preserved graft morphology and insulin content, whereas the endogenous beta cells of the transplanted mice were severely damage (80% decrease in pancreatic insulin content and morphological signs of beta-cell destruction). Thus human beta cells are resistant to NP, SZ, or alloxan at concentrations that decrease survival and function of rat or mouse beta cells. These marked interspecies differences emphasize the relevance of repair and/or defense mechanisms in beta-cell destruction and raise the possibility that such differences may also be present among individuals of the same species.

Improved functional survival of human islets of Langerhans in three-dimensional matrix culture

The current study evaluates functional survival of human islets maintained in tissue culture for up to 4 wk in suspension media (CMRL-1066 with supplements) and contrasts these results with immobilizing three-dimensional matrices (agarose or alginate). The absolute number and volume of islets retrieved from agarose is significantly higher after two and four wk of culture compared to conventional free-floating media. In vitro function of islets, assessed by insulin/DNA content, insulin secretion into the culture media over 24 h and glucose-theophylline stimulated insulin release in a dynamic perifusion system, was not significantly different between free-floating and matrix preserved islets. In vivo islet function was evaluated by the effectiveness for reversal of insulin-dependent diabetes mellitus by transplantation of the islets under the kidney capsule of nude mice. Although adequate insulin responses to glucose were seen after culture in conventional or matrix media, only agarose embedded islets were consistently able to induce normoglycemia in diabetic recipients after 14 days of culture. Additional transplantation experiments defined the threshold level required to reverse diabetes to be between 1,000 and 1,500 agarose preserved islets. Our data suggest improved engraftment of human islets after agarose culture. This culture method may be of benefit for the accumulation of functionally competent human islets, thus facilitating the implementation of clinical protocols that utilize freshly isolated islets from multiple donors without the need for cryopreservation.

Aberrant function and long-term survival of mouse beta cells exposed in vitro to high glucose concentrations

In vitro culture of murine Langerhans islets usually ends in islet death after 1-3 wk. Given contradictory published data, we studied the influence of glucose on the function and survival of islets from DBA/2 mice. Islets were cultured on plastic microwells, using 1, 2, or 11 g/l glucose concentrations. Using our routine technique, insulin secretion was evaluated after islet incubation for 15 min in basal medium [(sIS), 1 g/l glucose], followed by 15 min in stimulating medium [(sIS), 3 g/l glucose, 20 mM/l arginine, 5 mM/l theoophylline]. Insulin secretion of islets cultured in 1 g/l glucose remained stable and normal over a period of 2 mo [Day 7: bIS, 6.3 +/- 3.1 microU/50 microliters; sIS, 16.6 +/- 6.8 microU/50 microliters. Day 60: bIS, 6.0 +/- 4.0 microU/50 microliters; sIS, 21.3 +/- 10.5 microU/50 microliters]. Islet morphology also remained normal. Islets cultured in 2 g/l glucose showed elevated insulin response under basal and stimulating conditions during 2-3 wk, followed by a dramatic drop in insulin secretion [Day 7: bIS, 19.5 +/- 5.7 microU/50 microliters; sIS, 80.9 +/- 10.7 microU/50 microliters. Day 60: bIS, 5.4 +/- 5.0 microU/50 microliters; sIS, 2.7 +/- 1.4 microU/50 microliters]. Severe morphologic alterations appeared rapidly and islet destruction was nearly complete by 60 days. At 11 g/l glucose, functional and morphological islet alterations were accelerated [Day 7: bIS, 10.3 +/- 2.7 microU/50 microliters; sIS, 18.8 +/- 4.9 microU/50 microliters. Day 21: bIS and sIS almost undetectable].(ABSTRACT TRUNCATED AT 250 WORDS)

Glucose induces oscillatory Ca2+ signalling and insulin release in human pancreatic beta cells

Mechanisms of pulsatile insulin release in man were explored by studying the induction of oscillatory Ca2+ signals in individual beta cells and islets isolated from the human pancreas. Evidence was provided for a glucose-induced closure of ATP-regulated K+ channels, resulting in voltage-dependent entry of Ca2+. The observation of step-wise increases of capacitance in response to depolarizing pulses suggests that an enhanced influx of Ca2+ is an effective means of stimulating the secretory activity of the isolated human beta cell. Activation of muscarinic receptors (1-10 mumol/l carbachol) and of purinergic P2 receptors (0.01-1 mumol/l ATP) resulted in repetitive transients followed by sustained elevation of the cytoplasmic Ca2+ concentration ([Ca2+]i). Periodic mobilisation of intracellular calcium was seen also when injecting 100 mumol/l GTP-gamma-S into beta cells hyperpolarized to -70 mV. Individual beta cells responded to glucose and tolbutamide with increases of [Ca2+]i, manifested either as large amplitude oscillations (frequency 0.1-0.5/min) or as a sustained elevation. Glucose regulation was based on sudden transitions between the basal and the two alternative states of raised [Ca2+]i at threshold concentrations of the sugar characteristic for the individual beta cells. The oscillatory characteristics of coupled cells were determined collectively rather than by particular pacemaker cells. In intact pancreatic islets the glucose induction of well-synchronized [Ca2+]i oscillations had its counterpart in 2-5 min pulses of insulin. Each of these pulses could be resolved into regularly occurring short insulin transients. It is concluded that glucose stimulation of insulin release in man is determined by the number of beta cells entering into a state with Ca(2+)-induced secretory pulses.

Regulation of GAD expression in islets of Langerhans occurs both at the mRNA and protein level

The expression of the autoantigen glutamate decarboxylase in islets of Langerhans was investigated under different culture conditions, which affect the functional activity of the beta-cell. Using immunoprecipitations and analyses of enzyme activity, an increase in glutamate decarboxylase was detected in rat islets cultured at a glucose concentration of 11 mmol/l compared with those cultured at 5.6 mmol/l glucose. To determine whether the change was induced at the level of mRNA expression, total RNA was extracted from rat islets cultured at 5.6 or 11 mmol/l glucose, reverse transcribed and amplified by the polymerase chain reaction. Comparative quantitation in a phosphor imager revealed a significantly higher (82%, P < 0.005) content of glutamate decarboxylase mRNA in islets cultured at 11 mmol/l glucose. In parallel, human recombinant interleukin-1 beta, and diazoxide were tested for their effects on the expression of glutamate decarboxylase. Islets cultured at 11 mmol/l glucose in the presence of 40 U/ml of interleukin-1 beta, showed a 63% decrease (P < 0.005) in enzyme activity compared with those cultured at 11 mmol/l glucose alone, and similar decreases were noted on analysis of glutamate decarboxylase biosynthesis and mRNA. Islets cultured at 11 mmol/l glucose in the presence of 22.5 mg/ml diazoxide exhibited a significant reduction in enzyme activity (59%; P < 0.001) compared with those cultured at 11 mmol/l glucose only. This reduction, however, was not accompanied by a decrease in the content of glutamate decarboxylase mRNA.(ABSTRACT TRUNCATED AT 250 WORDS)

Variable regulation by insulin of insulin gene expression in HIT-T15 cells

Glucose and insulin are generally considered to express opposite effects on insulin synthesis and secretion from pancreatic islets. For the most part this generalization has arisen from short-term experiments. Our studies focused on the chronic, long-term effects of variable insulin concentrations on insulin gene expression and secretion in cultures of HIT-T15 cells. From passage 70, HIT cells were split and passed weekly for 25 weeks in media containing either (A) 11.1 mmol/l glucose with no insulin added; (B) 11.1 mmol/l glucose with insulin added to maintain a level of approximately 4,000 microU/ml; (C) 0.8 mmol/l glucose with no insulin added; (D) 0.8 mmol/l glucose with insulin added to maintain a level of approximately 4,000 microU/ml; and (E) 0.8 mmol/l glucose with progressively less insulin added over time to mimic the gradual decrease in media insulin levels found in condition A. Our data indicate that during chronic passing of HIT cells, addition of exogenous insulin led to preserved levels of insulin mRNA, insulin content and insulin secretion in cells cultured in media containing 11.1 mmol/l glucose concentration. However, in media containing 0.8 mmol/l glucose concentration, addition of insulin diminished the levels of insulin mRNA, insulin content and insulin secretion. Nonetheless, in all cases exogenously added insulin sustained greater levels of insulin mRNA, insulin content and insulin secretion than the instance wherein media containing a high concentration of glucose only was used.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytokines suppress human islet function irrespective of their effects on nitric oxide generation

Cytokines have been proposed as inducers of beta-cell damage in human insulin-dependent diabetes mellitus via the generation of nitric oxide (NO). This concept is mostly based on data obtained in rodent pancreatic islets using heterologous cytokine preparations. The present study examined whether exposure of human pancreatic islets to different cytokines induces NO and impairs beta-cell function. Islets from 30 human pancreata were exposed for 6-144 h to the following human recombinant cytokines, alone or in combination: IFN-gamma (1,000 U/ml), TNF-alpha (1,000 U/ml), IL-6 (25 U/ml), and IL-1 beta (50 U/ml). After 48 h, none of the cytokines alone increased islet nitrite production, but IFN-gamma induced a 20% decrease in glucose-induced insulin release. Combinations of cytokines, notably IL-1 beta plus IFN-gamma plus TNF-alpha, induced increased expression of inducible NO synthase mRNA after 6 h and resulted in a fivefold increase in medium nitrite accumulation after 48 h. These cytokines did not impair glucose metabolism or insulin release in response to 16.7 mM glucose, but there was an 80% decrease in islet insulin content. An exposure of 144 h to IL-1 beta plus IFN-gamma plus TNF-alpha increased NO production and decreased both glucose-induced insulin release and insulin content. Inhibitors of NO generation, aminoguanidine or NG-nitro-L-arginine, blocked this cytokine-induced NO generation, but did not prevent the suppressive effect of IL-1 beta plus IFN-gamma plus TNF-alpha on insulin release and content. In conclusion, isolated human islets are more resistant to the suppressive effects of cytokines and NO than isolated rodent islets. Moreover, the present study suggests that NO is not the major mediator of cytokine effects on human islets.

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.

Succinic acid monomethyl ester protects rat pancreatic islet secretory potential against interleukin-1 beta (IL-1 beta) without affecting glutamate decarboxylase expression or nitric oxide production

Rat pancreatic islets exposed to interleukin-1 beta (IL-1 beta) in the presence of succinic acid monomethyl ester (SAM) have a higher insulin release in response to glucose and higher glucose oxidation rates, as compared to islets exposed to IL-1 beta alone. These beneficial effects of SAM were not accompanied by any decrease in IL-1 beta-induced nitric oxide (NO) production nor inhibition of aconitase activity. Moreover, SAM did not increase biosynthesis of glutamate decarboxylase. SAM apparently improves beta-cell function mostly by increasing the capacity of these cells to endure NO exposure and partial blockage of the Krebs cycle.

Nicotinamide decreases nitric oxide production and partially protects human pancreatic islets against the suppressive effects of combinations of cytokines

It has been recently reported that human pancreatic islets in tissue culture produce nitric oxide (NO) and show a decreased function when exposed for 6 days to combinations of cytokines (interleukin-1 beta (IL-1 beta) + tumor necrosis factor-alpha (TNF-alpha) + interferon-gamma (IFN-gamma). Here we study the effects of nicotinamide (Nic; 10 or 20 mmol/l) on these deleterious effects of cytokines (50 U/ml IL-1 beta + 1000 U/ml TNF-alpha + 1000 U/ml IFN-gamma). Islets were isolated from 8 human pancreata at the Central Unit of the beta-Cell Transplant, Brussels, sent to Uppsala and, after 3-5 days in culture, exposed for 6 additional days to the cytokines and/or Nic. The cytokines induced a 6-fold increase in islet NO production (P < 0.001), and this effect was partially counteracted by Nic (50-60% decrease in NO production; P < 0.001). The cytokines severely decreased the islet insulin content and glucose-induced insulin release (16.7 mmol/l glucose; 90% decrease; P < 0.001). Both these effects of cytokines were partially counteracted by Nic, especially at the highest concentration (20 mmol/l; 2-4-fold increase compared to islets exposed to cytokines alone; P < 0.01). Nic by itself did not affect the insulin content or insulin release by control islets. In conclusion, the present data indicate that Nic counteracts the deleterious effects of cytokines on human pancreatic islets. This effect of Nic may be relevant for the beneficial effects of the drug in early IDDM.

Nicotinamide is a potent inducer of endocrine differentiation in cultured human fetal pancreatic cells

The effects of nicotinamide (NIC) on human fetal and adult endocrine pancreatic cells were studied in tissue culture. Treatment of the fetal cells with 10 mM NIC resulted in a twofold increase in DNA content and a threefold increase in insulin content. This was associated with the development of beta cell outgrowths from undifferentiated epithelial cell clusters and an increase in the expression of the insulin, glucagon, and somatostatin genes. DNA synthesis was stimulated only in the undifferentiated cells. Half-maximal doses for the insulinotropic and mitogenic effects of NIC were 5-10 and 1-2 mM, respectively. Islet-like cell clusters cultured with NIC responded to glucose stimulation with a biphasic increase in insulin release (fourfold peak), whereas control cells were unresponsive to glucose. Both control and NIC-treated cells developed into functional islet tissue after transplantation into athymic nude mice. As compared with adult islets, the insulinotropic action of NIC could only be demonstrated in the fetal cells. Our results indicate that NIC induces differentiation and maturation of human fetal pancreatic islet cells. This model should be useful for the study of molecular mechanisms involved in beta cell development.

Chronic exposure of HIT cells to high glucose concentrations paradoxically decreases insulin gene transcription and alters binding of insulin gene regulatory protein

Chronically culturing HIT-T15 cells in media containing high glucose concentrations leads to decreased insulin mRNA levels, insulin content, and insulin secretion. These changes can be prevented by culturing the cells in media containing lower glucose levels (Robertson, R. P., H.-J. Zhang, K. L. Pyzdrowski, and T. F. Walseth. 1992. J. Clin. Invest. 90:320-325). The mechanism of this seemingly paradoxical phenomenon was examined by transiently transfecting HIT cells with a chloramphenicol acetyl transferase (CAT) reporter gene controlled by the 5'-regulatory domain of the human insulin gene (INSCAT). Early passages of HIT cells readily expressed INSCAT, whereas late passages of cells chronically cultured in 11.1 mM glucose expressed only 28.7 +/- 2.3% (mean +/- SEM) of the CAT activity expressed in early passages. In contrast, late passages of HIT cells chronically cultured in 0.8 mM glucose retained the ability to express the INSCAT reporter gene to 69.6 +/- 10.0% of the CAT activity observed in early passages. The decrease in INSCAT expression in late passages of cells serially cultured in 11.1 mM glucose was associated with the inability to form a specific nuclear protein-DNA complex with the CT motifs of the human insulin promoter. Formation of this specific protein-DNA complex was preserved in late passages of HIT cells when serially cultured in 0.8 mM glucose. Mutations of the CT motifs caused markedly diminished CAT activity in all passages examined. These data indicate that chronic exposure of the beta cell to high glucose concentrations can paradoxically decrease insulin gene transcription, in part, by altering the ability of a regulatory protein (GSTF) to interact with the insulin gene promoter. This provides a potential mechanism for glucotoxic effects on the beta cell at the level of the insulin gene.