Bringing the human pancreas into focus: new paradigms for the understanding of Type 1 diabetes

Type 1 diabetes affects increasingly large numbers of people globally (including at least half a million children under the age of 14 years) and it remains an illness with life-long and often devastating consequences. It is surprising, therefore, that the underlying aetiology of Type 1 diabetes remains poorly understood. This is largely because the cellular and molecular processes leading to the loss of β cells in the pancreas have rarely been studied at, or soon after, the onset of disease. Where such studies have been undertaken, a number of surprises have emerged which serve to challenge conventional wisdom. In particular, it is increasingly understood that the process of islet inflammation (insulitis) is much less florid in humans than in certain animal models. Moreover, the profile of immune cells involved in the inflammatory attack on β cells is variable and this variation occurs at the level of individual patients. As a result, two distinct profiles of insulitis have now been defined that are differentially aggressive and that might, therefore, require specifically tailored therapeutic approaches to slow the progression of disease. In addition, the outcomes are also different in that the more aggressive form (termed 'CD20Hi') is associated with extensive β-cell loss and an early age of disease onset (<7 years), while the less aggressive profile (known as 'CD20Lo') is associated with later onset (>13 years) and the retention of a higher proportion of residual β cells. In the present review, these new findings are explained and their implications evaluated in terms of future therapies.

The impact of anti-inflammatory cytokines on the pancreatic β-cell

Considerable efforts have been invested to understand the mechanisms by which pro-inflammatory cytokines mediate the demise of β-cells in type 1 diabetes but much less attention has been paid to the role of anti-inflammatory cytokines as potential cytoprotective agents in these cells. Despite this, there is increasing evidence that anti-inflammatory molecules such as interleukin (IL)-4, IL-10 and IL-13 can exert a direct influence of β-cell function and viability and that the circulating levels of these cytokines may be reduced in type 1 diabetes. Thus, it seems possible that targeting of anti-inflammatory pathways might offer therapeutic potential in this disease. In the present review, we consider the evidence implicating IL-4, IL-10 and IL-13 as cytoprotective agents in the β-cell and discuss the receptor components and downstream signaling pathways that mediate these effects.

Sirtuin 3 regulates mouse pancreatic beta cell function and is suppressed in pancreatic islets isolated from human type 2 diabetic patients

AIMS/HYPOTHESIS

Sirtuin (SIRT)3 is a mitochondrial protein deacetylase that regulates reactive oxygen species (ROS) production and exerts anti-inflammatory effects. As chronic inflammation and mitochondrial dysfunction are key factors mediating pancreatic beta cell impairment in type 2 diabetes, we investigated the role of SIRT3 in the maintenance of beta cell function and mass in type 2 diabetes.

METHODS

We analysed changes in SIRT3 expression in experimental models of type 2 diabetes and in human islets isolated from type 2 diabetic patients. We also determined the effects of SIRT3 knockdown on beta cell function and mass in INS1 cells.

RESULTS

SIRT3 expression was markedly decreased in islets isolated from type 2 diabetes patients, as well as in mouse islets or INS1 cells incubated with IL1β and TNFα. SIRT3 knockdown in INS1 cells resulted in lowered insulin secretion, increased beta cell apoptosis and reduced expression of key beta cell genes. SIRT3 knockdown also blocked the protective effects of nicotinamide mononucleotide on pro-inflammatory cytokines in beta cells. The deleterious effects of SIRT3 knockdown were mediated by increased levels of cellular ROS and IL1β.

CONCLUSIONS/INTERPRETATION

Decreased beta cell SIRT3 levels could be a key step in the onset of beta cell dysfunction, occurring via abnormal elevation of ROS levels and amplification of beta cell IL1β synthesis. Strategies to increase the activity or levels of SIRT3 could generate attractive therapies for type 2 diabetes.

Expression of the enteroviral capsid protein VP1 in the islet cells of patients with type 1 diabetes is associated with induction of protein kinase R and downregulation of Mcl-1

AIMS/HYPOTHESIS

Immunohistochemical staining reveals that the enteroviral capsid protein VP1 is present at higher frequency in the insulin-containing islets of patients with recent-onset type 1 diabetes than in controls. This is consistent with epidemiological evidence suggesting that enteroviral infection may contribute to the autoimmune response in type 1 diabetes. However, immunostaining of VP1 is not definitive since the antibody widely used to detect the protein (Clone 5D8/1) might also cross-react with additional proteins under some conditions. Therefore, we sought to verify that VP1 immunopositivity correlates with additional markers of viral infection.

METHODS

Antigen immunoreactivity was examined in formalin-fixed, paraffin-embedded, pancreases from two different collections of type 1 diabetes and control cases: a historical collection from the UK and the nPOD (network of Pancreatic Organ donors with Diabetes) cohort from the USA.

RESULTS

VP1 immunoreactivity was present in ~20% of insulin-containing islets of both cohorts under stringent conditions but was absent from insulin-deficient islets. The presence of VP1 was restricted to beta cells but only a minority of these contained the antigen. The innate viral sensor, protein kinase R (PKR) was upregulated selectively in beta cells that were immunopositive for VP1. The anti-apoptotic protein myeloid cell leukaemia sequence-1 (Mcl-1) was abundant in beta cells that were immunonegative for VP1 but Mcl-1 was depleted in cells containing VP1.

CONCLUSIONS/INTERPRETATION

The presence of immunoreactive VP1 within beta cells in type 1 diabetes is associated with a cellular phenotype consistent with the activation of antiviral response pathways and enhanced sensitivity to apoptosis. However, definitive studies confirming whether viral infections are causal to beta cell loss in human diabetes are still awaited.

Expression of endoplasmic reticulum stress markers in the islets of patients with type 1 diabetes

AIMS/HYPOTHESIS

Endoplasmic reticulum (ER) stress may play a role in cytokine-mediated beta cell death in type 1 diabetes, but it remains controversial whether ER stress markers are present in islets from type 1 diabetic individuals. Therefore, we evaluated by immunostaining the expression of markers of the three main branches of the ER stress response in islets from 13 individuals with and 15 controls without type 1 diabetes (eight adults and seven children).

METHODS

Antibodies against the ER stress markers C/EBP homologous protein (CHOP), immunoglobulin heavy chain (BIP) and X-box binding protein 1 (XBP-1) were validated using HeLa cells treated with the ER stressor thapsigargin. These antibodies were then used to stain serial sections of paraffin-embedded pancreas from type 1 diabetic and non-diabetic individuals; samples were also immunostained for CD45, insulin and glucagon. Immunostaining intensities of the ER stress markers were quantified using a software-based, unbiased quantitative approach.

RESULTS

Islets from individuals with type 1 diabetes showed increased levels of CHOP and, at least for insulitis-positive and beta cell-containing islets, BIP. XBP-1 expression was not, however, increased.

CONCLUSIONS/INTERPRETATION

Islet cells from individuals with type 1 diabetes display a partial ER stress response, with evidence of the induction of some, but not all, components of the unfolded protein response.

Histone deacetylases 1 and 3 but not 2 mediate cytokine-induced beta cell apoptosis in INS-1 cells and dispersed primary islets from rats and are differentially regulated in the islets of type 1 diabetic children

AIMS/HYPOTHESIS

Histone deacetylases (HDACs) are promising pharmacological targets in cancer and autoimmune diseases. All 11 classical HDACs (HDAC1-11) are found in the pancreatic beta cell, and HDAC inhibitors (HDACi) protect beta cells from inflammatory insults. We investigated which HDACs mediate inflammatory beta cell damage and how the islet content of these HDACs is regulated in recent-onset type 1 diabetes.

METHODS

The rat beta cell line INS-1 and dispersed primary islets from rats, either wild type or HDAC1-3 deficient, were exposed to cytokines and HDACi. Molecular mechanisms were investigated using real-time PCR, chromatin immunoprecipitation and ELISA assays. Pancreases from healthy children and children with type 1 diabetes were assessed using immunohistochemistry and immunofluorescence.

RESULTS

Screening of 19 compounds with different HDAC selectivity revealed that inhibitors of HDAC1, -2 and -3 rescued INS-1 cells from inflammatory damage. Small hairpin RNAs against HDAC1 and -3, but not HDAC2, reduced pro-inflammatory cytokine-induced beta cell apoptosis in INS-1 and primary rat islets. The protective properties of specific HDAC knock-down correlated with attenuated cytokine-induced iNos expression but not with altered expression of the pro-inflammatory mediators Il1α, Il1β, Tnfα or Cxcl2. HDAC3 knock-down reduced nuclear factor κB binding to the iNos promoter and HDAC1 knock-down restored insulin secretion. In pancreatic sections from children with type 1 diabetes of recent onset, HDAC1 was upregulated in beta cells whereas HDAC2 and -3 were downregulated in comparison with five paediatric controls.

CONCLUSIONS/INTERPRETATION

These data demonstrate non-redundant functions of islet class I HDACs and suggest that targeting HDAC1 and HDAC3 would provide optimal protection of beta cell mass and function in clinical islet transplantation and recent-onset type 1 diabetic patients.

Immunohistochemical analysis of the relationship between islet cell proliferation and the production of the enteroviral capsid protein, VP1, in the islets of patients with recent-onset type 1 diabetes

AIMS/HYPOTHESIS

The enteroviral capsid protein, VP1, was recently shown to be present in some beta cells in more than 60% of patients with recent-onset type 1 diabetes but in very few age-matched controls. The rate of proliferation of islet cells was also markedly increased in the type 1 diabetic patients. As it has been suggested that enteroviruses replicate most efficiently in proliferating cells, we have investigated whether VP1 is preferentially present in proliferating beta cells in type 1 diabetes.

METHODS

Combined immunoperoxidase and immunofluorescence staining was used to record the presence of enteroviral VP1, insulin and Ki67 in the islets of recent-onset type 1 diabetic patients.

RESULTS

From a total of 1,175 islets, 359 (30.5%) contained insulin. VP1-producing endocrine cells were found in 72 islets (6.1% of total), all of which retained insulin. Ki67(+) endocrine cells were present in 52 (4.4%) islets, with 44 (84.6%) of these being insulin-positive. Overall, 28 of 1,175 (2.4%) islets contained both Ki67(+) cells and VP1(+) cells. Dual positivity of these markers accounted for 38.9% of the total VP1(+) islets and 53.8% of the total Ki67(+) islets. No individual islet cells were dual-positive for Ki67 and VP1.

CONCLUSIONS/INTERPRETATION

Ki67(+) cells were frequently observed in islets that also contained VP1(+) cells, suggesting that the factors facilitating viral replication may also drive islet cell proliferation. However, in an individual cell, VP1 production does not require concurrent beta cell proliferation.

Evidence of increased islet cell proliferation in patients with recent-onset type 1 diabetes

AIMS/HYPOTHESIS

In adults, the rate of beta cell replication is normally very low, but recent evidence suggests that it may increase during insulitis. We therefore studied tissue from donors with recent-onset type 1 diabetes to establish whether islet cell proliferation is increased during the disease process.

METHODS

Paraffin-embedded pancreatic sections from ten donors with recent-onset type 1 diabetes and a range of relevant controls were stained by immunohistochemical techniques with antibodies against the proliferation markers Ki67 and minichromosome maintenance protein-2 (MCM-2). A combination staining technique involving immunoperoxidase and immunofluorescence methods was developed to quantify the numbers of alpha and beta cells with Ki67-positive nuclei and to investigate the relationship between insulitis and islet cell proliferation.

RESULTS

In non-diabetic control donors, only 1.1 +/- 0.3% (mean +/- SEM) of islets contained one or more Ki67(+) islet cells, whereas this proportion was increased markedly in recent-onset type 1 diabetes (10.88 +/- 2.5%; p < 0.005). An equivalent increase in Ki67(+) staining occurred in alpha and beta cells and was correlated positively with the presence of insulitis. A significant increase in the labelling of islet cells from type 1 diabetic donors was also seen when MCM-2 staining was employed. Increased islet cell proliferation was not evident in three donors with longer duration type 1 diabetes or in ten type 2 diabetic donors.

CONCLUSIONS/INTERPRETATION

Alpha and beta cells undergo a marked increase in proliferation during the progression of type 1 diabetes in humans. The results imply that islet cell proliferation is re-initiated in response to the autoimmune attack associated with type 1 diabetes.

The prevalence of enteroviral capsid protein vp1 immunostaining in pancreatic islets in human type 1 diabetes

AIMS/HYPOTHESIS

Evidence that the beta cells of human patients with type 1 diabetes can be infected with enterovirus is accumulating, but it remains unclear whether such infections occur at high frequency and are important in the disease process. We have now assessed the prevalence of enteroviral capsid protein vp1 (vp1) staining in a large cohort of autopsy pancreases of recent-onset type 1 diabetic patients and a range of controls.

METHODS

Serial sections of paraffin-embedded pancreatic autopsy samples from 72 recent-onset type 1 diabetes patients and up to 161 controls were immunostained for insulin, glucagon, vp1, double-stranded RNA activated protein kinase R (PKR) and MHC class I.

RESULTS

vp1-immunopositive cells were detected in multiple islets of 44 out of 72 young recent-onset type 1 diabetic patients, compared with a total of only three islets in three out of 50 neonatal and paediatric normal controls. vp1 staining was restricted to insulin-containing beta cells. Among the control pancreases, vp1 immunopositivity was also observed in some islets from ten out of 25 type 2 diabetic patients. A strong correlation was established between islet cell vp1 positivity and PKR production in insulin-containing islets of both type 1 and type 2 diabetic patients, consistent with a persistent viral infection of the islets.

CONCLUSIONS/INTERPRETATION

Immunoreactive vp1 is commonly found in the islets of recent-onset type 1 diabetes patients, but only rarely in normal paediatric controls. vp1 immunostaining was also observed in some islets of type 2 diabetes patients, suggesting that the phenomenon is not restricted to type 1 diabetes patients.

Analysis of islet inflammation in human type 1 diabetes

The immunopathology of type 1 diabetes (T1D) has proved difficult to study in man because of the limited availability of appropriate samples, but we now report a detailed study charting the evolution of insulitis in human T1D. Pancreas samples removed post-mortem from 29 patients (mean age 11.7 years) with recent-onset T1D were analysed by immunohistochemistry. The cell types constituting the inflammatory infiltrate within islets (insulitis) were determined in parallel with islet insulin content. CD8(+) cytotoxic T cells were the most abundant population during insulitis. Macrophages (CD68(+)) were also present during both early and later insulitis, although in fewer numbers. CD20(+) cells were present in only small numbers in early insulitis but were recruited to islets as beta cell death progressed. CD138(+) plasma cells were infrequent at all stages of insulitis. CD4(+) cells were present in the islet infiltrate in all patients but were less abundant than CD8(+) or CD68(+) cells. Forkhead box protein P3(+) regulatory T cells were detected in the islets of only a single patient. Natural killer cells were detected rarely, even in heavily inflamed islets. The results suggest a defined sequence of immune cell recruitment in human T1D. They imply that both CD8(+) cytotoxic cells and macrophages may contribute to beta cell death during early insulitis. CD20(+) cells are recruited in greatest numbers during late insulitis, suggesting an increasing role for these cells as insulitis develops. Natural killer cells and forkhead box protein P3(+) T cells do not appear to be required for beta cell death.

Structural requirements for the cytoprotective actions of mono-unsaturated fatty acids in the pancreatic beta-cell line, BRIN-BD11

BACKGROUND AND PURPOSE

Exposure of pancreatic beta-cells to long-chain free fatty acids leads to differential responses according to the chain length and degree of unsaturation. In particular, long-chain saturated molecules such as palmitate (C16:0) cause apoptosis, whereas equivalent mono-unsaturated species (for example, palmitoleate (C16:1)) are not overtly toxic. Moreover, mono-unsaturates exert a powerful cytoprotective response against a range of proapoptotic stimuli. However, the structural requirements that determine cytoprotection have not been determined and form the basis of the present study.

EXPERIMENTAL APPROACH

BRIN-BD11 and INS-1 beta-cells were exposed either to the saturated fatty acid palmitate, or to serum withdrawal, to mediate cytotoxicity. The protective effects of a wide range of mono-unsaturated fatty acid derivatives were tested in cytotoxicity assays. Effector caspase activity was also measured and correlated with viability.

KEY RESULTS

The cytotoxic actions of palmitate were inhibited dose-dependently by long-chain mono-unsaturated fatty acids with a defined potency order C18:1>C16:1>>C14:1. The configuration of the double bond was also important with cis forms being more potent than trans forms. Alkylated mono-unsaturated fatty-acid derivates were also cytoprotective, although their efficacy declined as the alkyl chain length increased. Cytoprotection was achieved rapidly on addition of mono-unsaturates and correlated with a rapid and dramatic inhibition of caspase-3/7 activity in palmitate-treated cells.

CONCLUSIONS AND IMPLICATIONS

The data reveal the structural requirements that dictate the cytoprotective actions of mono-unsaturated fatty acids in pancreatic beta-cells. Metabolic activation is not required and the data point at the potential involvement of a fatty acid receptor in mediating cytoprotection.

Inhalation of glutamic acid decarboxylase 65-derived peptides can protect against recurrent autoimmune but not alloimmune responses in the non-obese diabetic mouse

Systemic administration of islet-derived antigens has been shown to protect against diabetes in the non-obese diabetic (NOD) mouse by the induction of antigen-specific regulatory T cells. Bystander regulation to related and unrelated islet-derived antigens (intramolecular and intermolecular recognition) in this context is recognized. We tested if intranasal administration of glutamic acid decarboxylase 65 (GAD 65)-derived peptides could protect against both autoimmune and, through bystander regulation, alloimmune responses in a NOD mouse model. Spontaneously diabetic female NOD mice underwent islet transplantation from either C57Bl/6 or NOD islet donors. Islet recipients were treated with intranasal GAD 65-derived peptides or control (ovalbumin) peptide pre- and post-transplantation. In-vitro analysis of the effect of inhalation was defined using lymph node proliferation assays and supernatant analysis for cytokines. GAD 65-derived peptide inhalation resulted in significant protection against recurrent autoimmune disease, with the generation of an interleukin (IL)-10-producing immune phenotype in a syngeneic islet transplant model. This phenotype, however, was not robust enough to protect against alloimmune responses. Inhalation of GAD-derived peptides induces an immunoregulatory response that protects against recurrent autoimmune, but not alloimmune responses in the NOD mouse.

Pre-incubation with interleukin-4 mediates a direct protective effect against the loss of pancreatic beta-cell viability induced by proinflammatory cytokines

Loss of pancreatic beta-cells in type I diabetes is associated with an increase in T helper 1 (Th1) proinflammatory cytokines in the islet milieu, with a concomitant reduction in Th2 anti-inflammatory cytokines. In animal models, manoeuvres designed to polarize Th1 responses towards Th2, particularly involving interleukin (IL)-4, have been shown to protect against insulitis and diabetes. The aim of this study was to determine whether IL-4 can exert a direct effect on beta-cell viability. The rat pancreatic beta-cell line, BRIN-BD11, was used. IL-4R mRNA expression was assayed by reverse transcription-polymerase chain reaction and DNA sequencing and protein expression measured using anti-IL-4R antibodies and confocal microscopy. Cells were pretreated in vitro with IL-4, incubated with IL-1beta and interferon (IFN)-gamma and DNA fragmentation and nitrite production analysed by flow cytometry and Griess assay, respectively. Expression of type I (IL-4R alpha and common gamma-chain) and type II (IL-4R alpha, IL-13R alpha-1) IL-4R mRNA transcripts, together with cell surface expression of IL-4R, was demonstrated. Pre-incubation with IL-4 reduced significantly cell death induced by IL-1beta alone or by a combination of IL-1beta and IFN-gamma, although this was not accompanied by a reduced production of nitrite. The protective effect of IL-4 was not seen when all three cytokines were added simultaneously. These results demonstrate, for the first time, expression of IL-4 receptor components on rat pancreatic beta-cells and reveal a direct protective effect on the loss of viability mediated by proinflammatory cytokines when beta-cells are pre-incubated with IL-4.

Imidazoline receptors: new targets for antihyperglycaemic drugs

In both animal models of Type 2 diabetes and in man, it has been evident for many years that certain imidazoline drugs can stimulate insulin secretion and improve glycaemia. This suggests that they may be useful new reagents for use in the management of Type 2 diabetes. However, despite their promise, no imidazoline compound has yet come into clinical use as an effective therapeutic agent in diabetes. This should not be taken as evidence of a flaw in the basic hypothesis, but derives, in part, from continuing ignorance about the molecular characteristics of imidazoline binding proteins, and the precise structure-activity relationships of their ligands. In this review, the mode of action of antihyperglycaemic imidazoline compounds is considered, and the possibility discussed that these agents may interact with a unique subtype of imidazoline binding site associated with ATP-sensitive potassium channels. The functional consequences of this interaction are summarised together with evidence that the binding site may actually lie within the channel complex. Additional data implicating the participation of alpha2-adrenoceptors in some actions of imidazolines are evaluated, and examples of relevant drugs having encouraging therapeutic profiles are highlighted. The possibility that some anti-diabetic imidazoline reagents may exert extra-pancreatic effects is also considered. Overall, the article aims to highlight important developments within the field but also draws attention to those areas where controversy remains.

Evidence that protein kinase Cdelta is not required for palmitate-induced cytotoxicity in BRIN-BD11 beta-cells

Chronic exposure of pancreatic beta-cells to saturated fatty acids leads to loss of viability, an effect that has been implicated in the process of beta-cell 'lipotoxicity' associated with the progression of type 2 diabetes. The mechanisms involved are unknown but recent evidence has implicated the delta isoform of protein kinase C (PKCdelta) in mediating fatty acid toxicity. We have investigated this proposition in the clonal insulin-secreting cell line, BRIN-BD11. BRIN-BD11 cells were found to undergo apoptosis when exposed to palmitate and this response was attenuated by the purportedly selective inhibitor of PKCdelta, rottlerin. However, activation of PKCdelta with the phorbol ester, phorbol-12-myristate-13-acetate (PMA), failed to promote cell death and down-regulation of PKCdelta did not prevent the cytotoxic effects of palmitate. Moreover, rottlerin remained effective as a blocker of the palmitate response in cells depleted of PKCdelta. Since rottlerin can inhibit various other kinases in addition to PKCdelta, a range of additional kinase inhibitors was also tested. Of these, only the putative Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) inhibitor, KN-62, was found to inhibit palmitate-induced cell death. However, this effect was not reproduced by a more selective pseudo-substrate inhibitor of CaM kinase II. Therefore, the present results reveal that palmitate induces cell death in BRIN-BD11 cells and suggest that this may involve the activation of a rottlerin (and KN-62)-sensitive kinase. However, it is clear that PKCdelta is not required for this response.

Superoxide, nitric oxide, peroxynitrite and cytokine combinations all cause functional impairment and morphological changes in rat islets of Langerhans and insulin secreting cell lines, but dictate cell death by different mechanisms

We have shown that nitric oxide treatment for 30-90 min causes inhibition of insulin secretion, DNA damage and disturbs sub-cellular organization in rat and human islets of Langerhans and HIT-T15 cells. Here rat islets and beta-cell lines were treated with various free radical generating systems S-nitrosoglutathione (nitric oxide), xanthine oxidase plus hypoxanthine (reactive oxygen species), 3-morpholinosydnonimine (nitric oxide, super-oxide, peroxynitrite, hydrogen peroxide) and peroxynitrite and their effects over 4 h to 3 days compared with those of the cytokine combination interleukin-1beta, tumour necrosis factor-alpha and interferon-gamma. End points examined were de novo protein synthesis, cellular reducing capacity, morphological changes and apoptosis by acridine orange cytochemistry, DNA gel electrophoresis and electron microscopy. Treatment (24-72 h) with nitric oxide, superoxide, peroxynitrite or combined cytokines differentially decreased redox function and inhibited protein synthesis in rat islets of Langerhans and in insulin-containing cell lines; cytokine effects were arginine and nitric oxide dependent. Peroxynitrite gave rare apoptosis in HIT-T15 cells and superoxide gave none in any cell type, but caused the most beta cell-specific damage in islets. S-nitroso-glutathione was the most effective agent at causing DNA laddering or chromatin margination characteristic of apoptotic cell death in insulin-containing cells. Cytokine-induced apoptosis was observed specifically in islet beta cells, combined cytokine effects on islet function and death most resembled those of the mixed radical donor SIN-1.

Differential effects of genistein on apoptosis induced by fluoride and pertussis toxin in human and rat pancreatic islets and RINm5F cells

Clonal pancreatic beta-cell lines have been used widely for the study of the factors involved in the regulation of apoptosis but it has not been firmly established that the response of normal islets mirrors that found in transformed beta-cells. In the present work, the role of pertussis toxin (Ptx)-sensitive G-proteins in the control of beta-cell apoptosis was studied in isolated rat and human islets of Langerhans and compared with the clonal beta-cell line, RINm5F. Annexin-V and deoxycarboxyfluoroscein diacetate staining was used to identify viable, apoptotic and necrotic cells directly, under fluorescence illumination. Treatment of human and rat islet cells with the G-protein activator fluoride (NaF; 5 mM) caused a marked increase in apoptosis that was further potentiated in islets pretreated with Ptx. The tyrosine kinase inhibitor genistein (100 microM) also increased islet cell apoptosis and the combination of 100 microM genistein and 5 mM NaF did not lead to any diminution of the apoptotic response. This latter effect was quite different from that seen in RINm5F cells where the combination of 100 microM genistein and 5 mM NaF resulted in much less apoptosis than was observed with either agent alone. In islets treated with a lower concentration of genistein (25 microM; that did not, itself, increase cell death), the drug attenuated NaF-induced apoptosis and also blocked the enhancement mediated by Ptx. These results revealed that human (and rat) islets are equipped with a Ptx-sensitive pathway that may be regulated by tyrosine phosphorylation and is anti-apoptotic. However, they also define conditions under which marked differences in response between RINm5F cells and normal islets were observed and they suggest that care should be taken when extrapolating data obtained with clonal cell lines to the situation in normal islet cells.

Islet glutamic acid decarboxylase modified by reactive oxygen species is recognized by antibodies from patients with type 1 diabetes mellitus

The generation of an autoimmune response against islet beta-cells is central to the pathogenesis of type 1 diabetes mellitus, and this response is driven by the stimulation of autoreactive lymphocytes by components of the beta-cells themselves. Reactive oxygen species (ROS) have been implicated in the beta-cell destruction which leads to type 1 diabetes and may modify beta-cell components so as to enhance their immunogenicity. We investigated the effects of oxidation reactions catalysed by copper or iron on the major beta-cell autoantigen glutamic acid decarboxylase (GAD). Lysates of purified rat islets were exposed to copper or iron sulphate with or without hydrogen peroxide or ascorbic acid. Immunostaining showed that these treatments generated high molecular weight covalently linked aggregates containing GAD. These are not formed by intermolecular disulphide bonds between cysteine residues since they cannot be resolved into monomeric form when electrophoresed under extreme reducing conditions. There was no modification of insulin or pro-insulin by ROS. The same oxidative changes to GAD could be induced in viable islet cells treated with copper sulphate and hydrogen peroxide, and thus the modifications are not an artefact of the catalysed oxidation of cell-free lysates. Sera from patients with type 1 diabetes and stiffman syndrome containing GAD antibodies reacted predominantly with the highest molecular weight modified protein band of GAD: normal human sera did not precipitate GAD. Thus, oxidatively modified aggregates of GAD react with serum antibodies of type 1 diabetes patients and some SMS patients: this is consistent with oxidative modifications of autoantigens being relevant to the pathogenesis of type 1 diabetes.

Imidazoline binding sites in the endocrine pancreas: can they fulfil their potential as targets for the development of new insulin secretagogues?

A variety of compounds containing an imidazoline ring have the ability to stimulate insulin secretion. Many of these also improve glycaemia in experimental models of type 2 diabetes and in man, suggesting that this class may be useful in the development of new orally active anti-diabetic drugs. However, the mechanisms by which imidazolines promote insulin secretion have not been clarified. The response does not appear to be due to the binding of ligands to either of the two major types of "imidazoline receptor" defined by pharmacological criteria (I1 and I2 sites) but may result from interaction with a novel imidazoline binding site. One such site has been identified in association with the ATP-sensitive potassium (K(ATP)) channel in the beta-cell and has been designated "I3". Electrophysiological and biochemical evidence suggest that the I3 site may be intrinsic to the ion-conducting pore component, Kir6.2, of the K(ATP) channel, but the effects of imidazoline ligands on insulin secretion can be dissociated from the regulation of Kir6.2. Indeed, there is increasing evidence that some imidazolines can control exocytosis directly, both in beta-cells and in pancreatic alpha-cells. Thus, it is proposed that a further imidazoline binding site is primarily responsible for control of hormone secretion. Evidence is reviewed which suggests that this site occupies a central position within an amplification pathway that also mediates the effects of cAMP in the beta-cell. Characterisation of this site should provide the stimulus for the design of new insulin secretagogues that are devoid of K(ATP) channel-blocking properties.

Characterization of a KATP channel-independent pathway involved in potentiation of insulin secretion by efaroxan

Efaroxan, like several other imidazoline reagents, elicits a glucose-dependent increase in insulin secretion from pancreatic beta-cells. This response has been attributed to efaroxan-mediated blockade of KATP channels, with the subsequent gating of voltage-sensitive calcium channels. However, increasing evidence suggests that, at best, this mechanism can account for only part of the secretory response to the imidazoline. In support of this, we now show that efaroxan can induce functional changes in the secretory pathway of pancreatic beta-cells that are independent of KATP channel blockade. In particular, efaroxan was found to promote a sustained sensitization of glucose-induced insulin release that persisted after removal of the drug and to potentiate Ca2+-induced insulin secretion from electropermeabilized islets. To investigate the mechanisms involved, we studied the effects of the efaroxan antagonist KU14R. This agent is known to selectively inhibit insulin secretion induced by efaroxan, without altering the secretory response to glucose or KCl. Surprisingly, however, KU14R markedly impaired the potentiation of insulin secretion mediated by agents that raise cAMP, including the adenylate cyclase activator, forskolin, and the phosphodiesterase inhibitor isobutylmethyl xanthine (IBMX). These effects were not accompanied by any reduction in cAMP levels, suggesting an antagonistic action of KU14R at a more distal point in the pathway of potentiation. In accord with our previous work, islets that were exposed to efaroxan for 24 h became selectively desensitized to this agent, but they still responded normally to glucose. Unexpectedly, however, the ability of either forskolin or IBMX to potentiate glucose-induced insulin secretion was severely impaired in these islets. By contrast, the elevation of cAMP was unaffected by culture of islets with efaroxan. Taken together, the data suggest that, in addition to effects on the KATP channel, imidazolines also interact with a more distal component that is crucial to the potentiation of insulin secretion. This component is not required for Ca2+-dependent secretion per se but is essential to the mechanism by which cAMP potentiates insulin release. Overall, the results indicate that the actions of efaroxan at this distal site may be more important for control of insulin secretion than its effects on the KATP channel.

Effects of imidazoline binding site ligands on the growth and viability of clonal pancreatic beta-cells

Pancreatic beta-cells express imidazoline binding sites which play a role in the regulation of insulin secretion, but it is not known whether ligands for these sites also affect other aspects of beta-cell physiology. In the present study, we have investigated the effects of a range of imidazoline reagents on the growth and viability of clonal pancreatic beta-cells (RINm5F and HIT-T15). Three imidazoline compounds (idazoxan, phentolamine and antazoline) were found to cause marked inhibition of beta-cell growth in a time and concentration dependent manner. Idazoxan was the most potent of these with as little as 0.5 microM causing a significant decrease in beta-cell viability (EC50 approximately 10 microM). All three imidazolines also decreased the viability of clonal beta-cells in parallel with their inhibitory effects on cell growth. These effects were not reproduced by any of a wide-range of other imidazoline compounds, including effective insulin secretagogues such as efaroxan and RX821002. The effects of the three ligands did not correlate with their relative potencies for binding to any of the well-characterised imidazoline binding sites nor to alpha2-adrenoceptors. In addition, the inhibitory responses were not antagonised by other imidazoline binding site ligands. The inhibitory effects of idazoxan on the growth of RINm5F and HIT-T15 beta-cells required as little as 3-h exposure to the imidazoline and were not readily reversible when the reagent was removed. Reductions in growth rate were accompanied by marked alterations in the morphology of the cells, which could be detected before loss of viability. Cells exposed to phentolamine showed the characteristic features of apoptosis in that the nuclei were condensed (as judged by acridine orange staining) and electrophoresis of DNA revealed the presence of oligonucleosomal fragmentation. These changes could not be detected in cells exposed to idazoxan despite the more profound reduction in viability induced by this agent. We conclude that a sub-group of imidazoline compounds can exert profoundly detrimental effects on the growth and viability of clonal beta-cells but that these effects do not correlate with their binding affinity at imidazoline binding sites or alpha2-adrenoceptors.

Dissociation between Fas expression and induction of apoptosis in human islets of Langerhans

There is increasing evidence that inappropriate induction of apoptosis in pancreatic beta-cells may precede the development of type 1 diabetes in animal models and in man. One mechanism by which this has been proposed to occur involves up-regulation of the death receptor Fas on beta-cells, resulting in apoptosis of the Fas-bearing beta-cells upon ligation of the receptor. We have examined this hypothesis in isolated human islets of Langerhans and show that--in contrast to data obtained with rodent beta-cells--expression of Fas per se is not sufficient to allow induction of apoptosis upon addition of agonistic anti-Fas serum.

Extraction of active clonidine-displacing substance from bovine lung and comparison with clonidine-displacing substance extracted from other tissues

Crude methanolic clonidine-displacing substance (CDS) extracted from bovine lung competed for radioligand binding from alpha2-adrenoceptors and I2-sites present in rat brain membranes, and from I1-sites present in rat brain and kidney membranes. There was no difference in the competition of [3H]clonidine binding to alpha2-adrenoceptors present in either rat or rabbit brain membranes by the crude CDS extract and therefore either tissue could be used to estimate the number of units of CDS present in extracts. Further purification by reverse phase high performance liquid chromatography (RP-HPLC), with UV detection, of extracts obtained from bovine lung, brain and rat brain exhibited similar three-peak profiles, previously reported. Corresponding fractions competed for radioligand binding to alpha2-adrenoceptors present in rat brain membranes, eluting between 19 and 23 min, which corresponded with the middle peak of the three-peaks. Therefore, we propose the CDS-like material eluting from all these tissues to be similar. Interestingly, CDS extracted from bovine adrenal glands under the same conditions showed a similar three-peak profile, but did not repeat the displacement of binding just at 19-23 min, but at every time point after 4 min. This suggests this tissue could represent a source of CDS in this species.

Characterization of monoamine oxidase isoforms in human islets of Langerhans

In this paper, we describe the characterization of the expression of monoamine oxidase (MAO) in whole pancreas and in isolated islets of Langerhans from human. Classical monamine oxidase activity assays reveal that both isoforms A & B are present in human pancreas. Two complementary approaches indicated that both MAO A and B are expressed in isolated islet: RT-PCR using specific primers revealed amplification products with the expected size for MAO-A and MAO-B: two peptides corresponding to MAO A (approximately 61 kDa) and B (approximately 55 kDa) were detected using a polyclonal anti MAO-A/MAO-B antiserum. Western blotting and subsequent densitometric analysis indicate that whole and endocrine pancreas express the two isoforms with different relative proportions. Islets appear to express almost twice as much MAO protein as whole pancreas, in near equal proportions of the two isoforms, whereas whole pancreas expresses more MAO-A than the B isoform. The expression of MAO A and B in islets could be the first step toward the characterization of the functional properties of these enzymes in the endocrine pancreas.

Effect of the new imidazoline derivative S-22068 (PMS 847) on insulin secretion in vitro and glucose turnover in vivo in rats

We have investigated the possible mechanisms underlying the antihyperglycaemic effect of the imidazoline derivative S-22068. In vitro, in the presence of 5 mmol/l glucose, S-22068 (100 micromol/l) induced a significant and sustained increase in insulin secretion from isolated, perifused, rat islets and a marked sensitization to a subsequent glucose challenge (10 mmol/l). S-22068 (100 micromol/l was able to antagonize the stimulatory effect of diazoxide on 86Rb efflux from preloaded islets incubated in the presence of 20 mmol/l glucose. Experiments were also performed to investigate whether S-22068 can alter glucose turnover and peripheral insulin sensitivity in vivo in mildly diabetic rats and obese, insulin resistant, Zucker rats. Neither glucose production nor individual tissue glucose utilization was modified by S-22068 in either group of rats. Similar results were obtained whether the studies were performed under basal conditions or during euglycaemic/hyperinsulinemic clamps. The results suggest that S-22068 exerts part of its antihyperglycaemic effect by promoting insulin secretion without alteration of peripheral insulin sensitivity.

Imidazolines and pancreatic hormone secretion

A range of imidazoline derivatives are known to be effective stimulators of insulin secretion, and this response correlates with closure of ATP-sensitive potassium channels in the pancreatic beta-cell. However, mounting evidence indicates that potassium channel blockade may form only part of the mechanism by which imidazolines exert their effects on insulin secretion. Additionally, it remains unclear whether members of this class of drugs can bind directly to potassium channel components and whether occupation of a single binding site accounts for their functional activity. This review considers recent developments in the field and highlights evidence that does not fit readily with the concept that a single mechanism of action is sufficient to mediate the effects of imidazolines on pancreatic hormone secretion.

Affinity isolation of imidazoline binding proteins from rat brain using 5-amino-efaroxan as a ligand

We have employed an amino derivative of the imidazoline ligand, efaroxan, to isolate imidazoline binding proteins from solubilised extracts of rat brain, by affinity chromatography. A number of proteins were specifically retained on the affinity column and one of these was immunoreactive with an antiserum raised against the ion conducting pore component of the ATP-sensitive potassium channel. Patch clamp experiments confirmed that, like its parent compound, amino-efaroxan blocks ATP-sensitive potassium channels in human pancreatic beta-cells and can stimulate the insulin secretion from these cells. The results reveal that a member of the ion conducting pore component family is strongly associated with imidazoline binding proteins in brain and in the endocrine pancreas.

Natural killer cells in relapsing-remitting MS: effect of treatment with interferon beta-1B

OBJECTIVE

To determine the effect of treatment with interferon beta-1b (IFN-beta) on natural killer (NK) cell function and phenotype in relapsing-remitting MS (RRMS) patients, and their relationship to disease activity assessed both clinically and with serial MRI.

BACKGROUND

NK cells may play a role in the immunopathogenesis of MS. Previously the authors reported a positive relationship between mean NK cell functional activity (FA) and total number of active lesions on MRI in a serial study of RRMS. Cycles in NK cell FA over time created a series of peaks and valleys, and a significant relationship has been identified between the valleys and the appearance of active lesions on MRI or onset of clinical attacks. The development of valleys in NK cell FA before the appearance of active lesions on MRI was statistically significant.

METHODS

The authors studied the effect of alternate-day therapy with 8.0 mIU (high dose [HD]) or 1.6 mIU (low dose [LD]) IFN-beta on NK cell FA, assessed by an in vitro 51Cr release K-562 target cell assay, and phenotype determination in RRMS patients.

RESULTS

Treatment with HD IFN-beta results in an inverse relationship between mean NK cell FA and total number of active lesions on MRI over 2 years. A stronger inverse relationship was found in those patients who did not develop neutralizing antibodies to IFN (HD-) compared with a positive relationship in those who did (HD +). Treatment with IFN-beta did not affect the cyclic nature of NK cell FA, mean NK cell FA, variability around the mean, mean length of the cycle, time spent in valleys and peaks, or the significant relationship between the appearance of active lesions on MRI/onset of clinical attacks and valleys in NK cell FA. In contrast, treatment with HD but not LD IFN-beta did result in a significant reduction in CD57+ (a cell surface marker for subsets of NK cells) peripheral blood lymphocytes (PBL) compared with placebo. This effect, which originated largely from the HD- group of patients, developed shortly after treatment was initiated and was maintained throughout the study.

CONCLUSIONS

RRMS patients with higher mean NK cell FA may be not only at greater risk for the development of active lesions but also may be more likely to respond to IFN-beta. Development of neutralizing antibodies to IFN-beta could interfere with this effect. This effect may be mediated through an action on a CD57+ subset of PBL.

Activation of protein kinase C modulates alpha2-adrenergic signalling in rat pancreatic islets

Treatment of rat pancreatic islets with 4beta-phorbol-myristate-acetate (PMA) caused a significant reduction in the ability of the alpha2-adrenoceptor agonist noradrenaline to inhibit glucose-induced insulin secretion. This effect was most evident when low concentrations of the catecholamine were used (less than 1 microM) and was lost when the noradrenaline concentration was increased to 10 microM. The effect was probably mediated by activation of protein kinase C, because the ability of PMA to desensitise islets to noradrenaline was prevented by a selective inhibitor of calcium-dependent isoforms of the enzyme, Gö6976. The response to PMA was reproduced when islet protein kinase C was activated by a receptor-mediated mechanism involving incubation with the muscarinic agonist carbachol. In parallel with desensitisation of the inhibitory control of insulin secretion by noradrenaline, PMA treatment also reduced the ability of a low concentration of noradrenaline (0.1 microM) to inhibit islet cAMP formation. The loss of sensitivity to catecholamine, induced by PMA in rat islets, was not caused by any change in the levels of alpha2-adrenoceptor expression or in their ligand-binding affinity. It was, however, associated with a marked increase in the extent of phosphorylation of members of the Gi/Go, family of pertussis toxin-sensitive G proteins in PMA-treated islets. Immunoprecipitation of Gi alpha2 and Galpha o from 32P-labelled islets after treatment with PMA revealed that both G proteins are substrates for protein kinase C. Overall, the results indicate that activation of protein kinase C leads to phosphorylation of islet Gi and Go causing their uncoupling from alpha2-adrenoceptors. We propose that this mechanism may form an important component of a physiological system designed to limit the tendency for catecholamines to inhibit insulin secretion under conditions in which the parasympathetic innervation of the islets is activated.

'Seeing through a glass darkly': casting light on imidazoline 'I' sites

Although imidazoline sites have been the subject of research for several years, there is still controversy about their structure, diversity and physiology. The I1 site is thought to exist principally as a binding site and is widely purported to play a role in controlling systemic blood pressure, although this is still unclear. The majority of I2 sites are widely accepted as being allosteric sites on monoamine oxidase; however, even with selective ligands, their exact function remains to be determined. A putative I3 site modulates insulin secretion and could represent the first functional site to be pharmacologically defined with selective agonists and antagonists. The structure and relevance of the proposed endogenous ligand 'clonidine-displacing substance' remains elusive. A potential candidate for this substance is agmatine; however, although it is capable of displacing bound clonidine from imidazoline sites, it lacks the functionality ascribed to the clonidine-displacing substance. In this review, Richard M. Eglen and colleagues assess our knowledge of imidazoline sites in the light of recent data.

Characterisation of new efaroxan derivatives for use in purification of imidazoline-binding sites

The insulin secretagogue activity of certain imidazoline compounds is mediated by a binding site associated with ATP-sensitive K+ (K(ATP)) channels in the pancreatic beta-cell. We describe the effects of a series of structural modifications to efaroxan on its activity at this site. Substitution of amino-, nitro- or azide- groups onto the 5-position of the benzene ring of efaroxan did not significantly affect the functional interaction of the ligand with the islet imidazoline binding site. Modification of the imidazoline ring to an imidazole to generate 2-(2-ethyl-2,3-dihydrobenzo[b]furan-2-yl)-1H-imidazole (KU14R) resulted in loss of secretagogue activity. Indeed, this reagent appeared to act as an imidazoline antagonist since it blocked the secretory responses to imidazoline compounds and also inhibited the blockade of beta-cell K(ATP) channels by efaroxan in patch clamp experiments. Application of KU14R alone resulted in a modest reduction in K(ATP) channel opening, suggesting that it may display weak partial agonism, at least in patch-clamp experiments.

A role for natural killer cells in the immunopathogenesis of multiple sclerosis

Seventeen relapsing-remitting (R/R) multiple sclerosis (MS) patients and age/sex matched controls were studied every 6 weeks for 2 years. Disease activity, determined both clinically and by serial MRI, was correlated with natural killer (NK) cell functional activity (FA) and phenotype. Mean NK cell FA is significantly lower in MS patients, compared to controls (P < 0.001), while variability around the means is significantly greater (P < 0.01). The spectrum of mean NK cell FA, observed in the patient cohort, along with cyclical nature of the FA and phenotype over time, observed in both patients and controls, may begin to explain the discrepant results reported in previous studies. In R/R MS, there is a significant correlation between reductions (valleys) in NK cell FA and the development of active lesions on MRI, new (P < 0.001) or enlarging (P = 0.05). More importantly, a significant number of active lesions, new (P = 0.01) and enlarging (P = 0.02), are preceded by a reduction in NK cell FA. The correlation between the onset of clinical attacks and valleys of NK cell FA is also significant (P = 0.002). When taken together, the results suggest that reductions (valleys) in NK cell FA represent periods of susceptibility for the development of active lesions on MRI and clinical attacks. A significant positive correlation is also identified between mean NK cell FA for each R/R MS patient and total number of active MRI lesions developed by that patient over the 2 years (P = 0.001). The results would suggest that R/R MS patients with a higher mean NK cell FA are at greater risk for the development of active lesions. These results support the proposal that NK cells may play a role in the immunopathogenesis of R/R MS.

Effector systems involved in the insulin secretory responses to efaroxan and RX871024 in rat islets of Langerhans

One component of the mechanism by which imidazoline compounds promote insulin secretion involves closure of ATP-sensitive K+ channels in the beta-cell plasma membrane. Recently, however, it has also been proposed that these compounds may exert important effects on more distal effector systems. In the present work, we have investigated the contribution played by protein kinases A and C to the insulin secretory responses of isolated rat islets of Langerhans treated with efaroxan and RX871024 (1-phenyl-2-(imidazolin-2-yl) benzimidazole). Removal of extracellular Ca2+ or blockade of voltage-sensitive Ca2+ channels prevented stimulation of insulin secretion by efaroxan, confirming a critical role for increased Ca2+ influx in the secretory response. By contrast, inhibition of protein kinases A or C failed to alter efaroxan-induced insulin secretion. RX871024 dose-dependently increased insulin secretion from cultured islets incubated with 20 mM glucose. This effect was unaffected by modulation of protein kinase C, but was significantly attenuated by a selective inhibitor of protein kinase A (Rp-cAMPs). Measurements of cAMP revealed that RX871024 increased the islet cAMP content by more than 3-fold; reaching values similar in magnitude to those elicited by 50 microM 3-isobutyl-1-methyl xanthine. The results reveal that neither protein kinase A nor protein kinase C is obligatory for stimulation of insulin secretion by imidazolines. However, they suggest that a rise in cAMP may contribute to the amplified secretory response observed when cultured islets are incubated with RX871024 in the presence of a stimulatory glucose concentration.

Sigma receptor ligands and imidazoline secretagogues mediate their insulin secretory effects by activating distinct receptor systems in isolated islets

The effects of two potent sigma receptor agonists (+)-3-PPP ((R)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine) and DTG (N,N'-di-(o-tolyl)guanidine) on the insulin secretory responses in rat islets of Langerhans were investigated. Both sigma receptor ligands were able to potentiate the insulin secretory response of islets incubated at 6 mM glucose, in a dose-dependent manner and were also able to reverse the effects of diazoxide on insulin release. When islets were treated with efaroxan, a well-characterised imidazoline insulin secretagogue, and either (+)-3-PPP or DTG together, there was an unexpected and profound absence of stimulation of insulin release as compared to when islets were incubated with each compound alone. Experiments performed with islets where there was desensitization of DTG/sigma receptor or efaroxan/imidazoline binding site mediated responses suggest that at least two distinct receptor systems appear to be involved. The complex interactions of these two classes of drug require further investigation.

Human islets of Langerhans express Fas ligand and undergo apoptosis in response to interleukin-1beta and Fas ligation

IDDM results from a progressive loss of pancreatic beta-cells that, in humans, may be triggered by a combination of genetic and environmental factors. Recently, attention has been focused on the hypothesis that the loss of beta-cells is initiated by inappropriate induction of apoptosis. We now demonstrate that human islets of Langerhans undergo apoptosis upon exposure to interleukin-1beta. The cytokine also sharply increases the number of cells that enter apoptosis on treatment with a stimulatory anti-Fas antibody. Western blotting and immunocytochemistry clearly show for the first time that human pancreatic beta-cells normally express Fas ligand. The results suggest that human islet cells are primed to undergo apoptosis by interleukin-1beta and that this involves the close association between cell-surface Fas and its ligand.

Clotrimazole and efaroxan stimulate insulin secretion by different mechanisms in rat pancreatic islets

It is now well established that the imidazoline insulin secretagogue efaroxan mediates its effects by inducing closure of ATP-sensitive potassium channels in the pancreatic beta-cell, leading to membrane depolarisation, Ca2+ influx and increased insulin secretion. However, a recent study has shown that efaroxan may also act as a blocker of a second class of potassium channel (the Kmaxi channel) in red blood cells, raising the possibility that its effects in islets could be mediated by interactions with both types of channel. Since the antimycotic imidazole compound clotrimazole is a highly potent blocker of Kmaxi channels, we have studied the effects of this drug on insulin secretion. Clotrimazole stimulated insulin secretion from rat islets of Langerhans incubated in the presence of 6 mM glucose, in a dose-dependent manner. Experiments performed at different glucose concentrations showed that the actions of clotrimazole were most prominent at low glucose concentrations whereas it did not enhance secretion beyond the rate induced by 20 mM glucose. The insulinotropic action of clotrimazole was temperature dependent but was independent of extracellular calcium. Clotrimazole appeared to block ATP-sensitive potassium channels in islets since, like efaroxan and glibencamide, it was able to prevent the inhibitory effects of diazoxide on glucose-induced insulin secretion. However, neither the direct stimulatory effect of clotrimazole on insulin release nor the abilty of clotrimazole to reverse the inhibitory actions of diazoxide was sensitive to blockade by the imidazoline secretagogue antagonist KU14R. Overall, the results suggest that clotrimazole exerts an insulinotropic effect in pancreatic beta-cells that is distinct from the actions of imidazoline secretagogues such as efaroxan. Clotrimazole can increase insulin secretion at sub-maximal glucose concentrations by an action which appears to be independent of membrane ion channel events.

Insulin secretagogues with an imidazoline structure inhibit arginine-induced secretion from isolated glucagon secretion from isolated rat islets of Langerhans

It is well documented that imidazoline compounds such as efaroxan and phentolamine act as potent insulin secretagogues both in vivo and in vitro, an effect which is mediated principally by blockade of ATP-sensitive potassium channels in the pancreatic B-cell. However, little is known about the effects of these drugs on the secretion of other pancreatic hormones and, in the present work, we have investigated the effects of selective imidazoline compounds on glucagon release from isolated rat islets of Langerhans. None of several imidazoline compounds tested (efaroxan, phentolamine, idazoxan, antazoline) affected glucagon secretion from islets incubated with 4 mM glucose. However, when the rate of glucagon release was stimulated by L-arginine (20 mM) efaroxan caused a rapid, sustained and dose-dependent inhibition of the secretory response (EC50 approximately 30 microM). This effect was seen under both static incubation and islet perifusion conditions. Antazoline and phentolamine also inhibited arginine-induced glucagon secretion, whereas idazoxan (an imidazoline which does not affect insulin secretion) failed to alter glucagon release. The inhibitory effects of imidazolines on glucagon release were not secondary to changes in insulin secretion. Taken together, the results indicate that pancreatic A-cells express functional imidazoline receptors which can regulate the secretory activity of the cells.

Interactions between imidazoline compounds and sulphonylureas in the regulation of insulin secretion

1. Imidazoline alpha 2-antagonist drugs such as efaroxan have been shown to increase the insulin secretory response to sulphonylureas from rat pancreatic B-cells. We have investigated whether this reflects binding to an islet imidazoline receptor or whether alpha 2-adrenoceptor antagonism is involved. 2. Administration of (+/-)-efaroxan or glibenclamide to Wistar rats was associated with a transient increase in plasma insulin. When both drugs were administered together, the resultant increase in insulin levels was much greater than that obtained with either drug alone. 3. Use of the resolved enantiomers of efaroxan revealed that the ability of the compound to enhance the insulin secretory response to glibenclamide resided only in the alpha 2-selective-(+)-enantiomer; the imidazoline receptor-selective-(-)-enantiomer was ineffective. 4. In vitro, (+)-efaroxan increased the insulin secretory response to glibenclamide in rat freshly isolated and cultured islets of Langerhans, whereas (-)-efaroxan was inactive. By contrast, (+)-efaroxan did not potentiate glucose-induced insulin secretion but (-)-efaroxan induced a marked increase in insulin secretion from islets incubated in the presence of 6 mM glucose. 5. Incubation of rat islets under conditions designed to minimize the extent of alpha 2-adrenoceptor signalling (by receptor blockade with phenoxybenzamine; receptor down-regulation or treatment with pertussis toxin) abolished the capacity of (+)- and (+/-)-efaroxan to enhance the insulin secretory response to glibenclamide. However, these manoeuvres did not alter the ability of (+/-)-efaroxan to potentiate glucose-induced insulin secretion. 6. The results indicate that the enantiomers of efaroxan exert differential effects on insulin secretion which may result from binding to effector sites having opposite stereoselectivity. Binding of (-)-efaroxan (presumably to imidazoline receptors) results in potentiation of glucose-induced insulin secretion, whereas interaction of (+)-efaroxan with a second site leads to selective enhancement of sulphonylurea-induced insulin release.

Evidence that the ability of imidazoline compounds to stimulate insulin secretion is not due to interaction with sigma receptors

Recent studies have suggested that a variety of ion channels possess a binding site for ligands such as phencyclidine (PCP), dizocilpine and certain sigma ligands and that some imidazoline compounds can also bind to this site. We have investigated whether interaction with this binding site could account for the ability of imidazolines to stimulate insulin secretion from rat islets. Neither PCP nor dizocilpine shared the insulin secretory activity of the imidazoline efaroxan in rat islets suggesting that they do not have similar actions in the pancreatic B-cell. Further, we were able to define a new antagonist, KU14R (2(2-ethyl 2,3-dihydro-2-benzofuranyl)-2-imidazole), which selectively blocks the insulin secretory response to imidazolines. The results suggest that imidazolines do not stimulate insulin secretion by causing physical blockade of the K(+)-ATP channel in pancreatic B-cells and show that their effects are not reproduced by PCP or sigma receptor ligands.

The effect of the putative endogenous imidazoline receptor ligand, clonidine-displacing substance, on insulin secretion from rat and human islets of Langerhans

1. The effects of a rat brain extract containing clonidine-displacing substance (CDS), a putative endogenous imidazoline receptor ligand, on insulin release from rat and human isolated islets of Langerhans were investigated. 2. CDS was able to potentiate the insulin secretory response of rat islets incubated at 6 mM glucose, in a dose-dependent manner. The magnitude of this effect was similar to that in response to the well-characterized imidazoline secretagogue, efaroxan. 3. CDS, like other imidazoline secretagogues, was also able to reverse the inhibitory action of diazoxide on glucose-induced insulin release, in both rat and human islets. 4. These effects of CDS on secretion were reversed by the imidazoline secretagogue antagonists, RX801080 and the newly defined KU14R, providing the first evidence that imidazoline-mediated actions of CDS can be blocked by specific imidazoline antagonists. 5. The effects of CDS on insulin secretion were unaffected when the method of preparation involved centri-filtration through a 3,000 Da cut-off membrane or when the extract was treated with protease. These results confirm that the active principle is of low molecular weight and is not a peptide. 6. Overall, the data suggest that CDS behaves as a potent endogenous insulin secretagogue acting at the islet imidazoline receptor.

Evidence for the involvement of cGMP and protein kinase G in nitric oxide-induced apoptosis in the pancreatic B-cell line, HIT-T15

Intracellular production of nitric oxide (NO) is thought to mediate the pancreatic B-cell-directed cytotoxicity of cytokines in insulin-dependent diabetes mellitus, and recent evidence has indicated that this may involve induction of apoptosis. A primary effect of NO is to activate soluble guanylyl cyclase leading to increased cGMP levels and this effect has been demonstrated in pancreatic B-cells, although no intracellular function has been defined for islet cGMP. Here we demonstrate that the NO donor, GSNO, induces apoptosis in the pancreatic B-cell line HIT-T15 in a dose- and time-dependent manner. This response was significantly attenuated by micromolar concentrations of a specific inhibitor of soluble guanylyl cyclase, ODQ, and both 8-bromo cGMP (100 microM) and dibutyryl cGMP (300 microM) were able to fully relieve this inhibition. In addition, incubation of HIT-T15 cells with each cGMP analogue directly promoted cell death in the absence of ODQ. KT5823, a potent and highly selective inhibitor of cGMP-dependent protein kinase (PKG), abolished the induction of cell death in HIT cells in response to either GSNO or cGMP analogues. This effect was dose-dependent over the concentration range of 10-250 nM. Overall, these data provide evidence that the activation of apoptosis in HIT-T15 cells by NO donors is secondary to a rise in cGMP and suggest that the pathway controlling cell death involves activation of PKG.

Differential expression of alpha 2-adrenoceptor subtypes in purified rat pancreatic islet A- and B-cells

In the endocrine pancreas, alpha 2-adrenoceptor stimulation reduces glucose-induced insulin secretion from islet B-cells. There is, however, controversy with regard to the effects of alpha 2-agonists on islet A-cell function. In this paper, we have used RNA samples prepared from whole rat islets and from FACS-purified rat A- and B-cells to study alpha 2-adrenoceptor subtype expression. RNase protection assays detected transcripts encoding alpha 2a and alpha 2b subtypes in the RNA pool of rat islets. Reverse transcription-PCR revealed that transcripts for all three alpha 2-adrenoceptor subtypes are present in rat islet cells in purified A-cell RNA. In contrast, RT-PCR of islet B-cell RNA yielded products corresponding to alpha 2a and alpha 2c, with no evidence for the presence of alpha 2b. Thus, the results reveal that both islet cell types express more than one receptor subtype and suggest that the distribution of the subtypes may differ between rat islet A- and B-cells.

Heterotrimeric G-proteins are implicated in the regulation of apoptosis in pancreatic beta-cells

Recent studies have provided evidence that apoptosis of pancreatic beta-cells is important in the early etiology of both type I and type II diabetes mellitus. The mechanisms responsible for induction of apoptosis are unknown, but we present evidence that the signal transduction pathway controlling the process in pancreatic beta-cells is regulated by G-proteins. We have employed the global G-protein activator fluoride and show that this agent induces apoptosis in clonal RINm5F pancreatic beta-cells and also in the cells of normal rat islets of Langerhans. The process is time and concentration dependent and may reflect the formation of AIF4- since it was inhibited by the aluminum chelator deferoxamine. Induction of apoptosis by fluoride was confirmed by acridine orange staining of cell nuclei, by electron-microscopic examination of chromatin condensation, and by oligonucleosomal degradation of DNA. The involvement of G-proteins was confirmed by culture of beta-cells in the presence of pertussis toxin (PTX) prior to exposure to fluoride. PTX did not affect the extent of cell death under control conditions but it consistently, and markedly, enhanced the response to fluoride. The results demonstrate that apoptosis can be induced in pancreatic beta-cells by sustained activation of a G-protein-dependent signaling pathway(s) and they further suggest that a pertussis toxin-sensitive G-protein is involved in attenuation of the response. Treatment of RINm5F pancreatic beta-cells with dibutyrylcAMP resulted in a dose-dependent, saturable increase in cell death, suggesting that a sustained rise in intracellular cAMP may form part of the effector system controlling apoptosis.

Expression of alpha 2- and beta-adrenoceptor subtypes in human islets of Langerhans

Sequences from cDNA molecules encoding alpha 2-adrenoceptor subtype genes were subcloned into prokaryotic vectors and riboprobes generated to hybridise selectively with each of the human alpha 2C2-, alpha 2C4- and alpha 2C10-adrenoceptor subtype mRNA species. The riboprobes were labelled with either 32P or digoxigenin and used to study the expression of alpha 2-adrenoceptor subtypes in sections of human pancreas, in isolated human islets of Langerhans and in clonal HIT-T15 pancreatic beta-cells. Using a ribonuclease protection assay protocol, expression of mRNA species encoding both alpha 2 C2 and alpha 2 C10 was demonstrated in preparations of isolated human islets of Langerhans. mRNA encoding alpha 2C4 was also detected in human islet RNA, using reverse transcription coupled with the polymerase chain reaction. In situ hybridisation was then employed to examine the distribution of each alpha 2-adrenoceptor subtype in sections of human pancreas. All three subtypes of alpha 2-adrenoceptor mRNA were identified in sections of formalin-fixed, paraffin-embedded human pancreas using riboprobes labelled with digoxigenin. Although some labelling of the three alpha 2-adrenoceptor mRNA subtypes was seen in the islets, the labelling was most intense in the exocrine tissue of the pancreas for each receptor subtype. The specificity of the digoxigenin-labelled RNA probes was confirmed in several control tissues and by in situ hybridisation studies using sense probes in the pancreas. The integrity of the pancreas sections was confirmed by in situ hybridisation with an antisense riboprobe derived from human insulin cDNA. The results demonstrate that multiple alpha 2-adrenoceptor subtypes are expressed in human pancreas. Both the exocrine and endocrine cells express more than one receptor subtype, although the islets stain less intensely than the bulk of the tissue suggesting that the islet cells may have lower levels of expression than the acinar tissue. The presence of alpha 2-adrenoceptor subtype mRNA species in pancreatic beta-cells was confirmed by Northern blotting of RNA extracted from the clonal beta-cell line, HIT-T15. Transcripts encoding each of the three cloned alpha 2-adrenoceptor subtypes were detected in HIT-T15 cells. Hybridisation of sections of human pancreas with oligodeoxynucleotide probes designed to hybridise with beta 2-adrenoceptor mRNA revealed expression of this species in islet beta-cells but not in the exocrine tissue of the pancreas.

A specific inhibitor of cytosolic phospholipase A2 activity, AACOCF3, inhibits glucose-induced insulin secretion from isolated rat islets

In pancreatic beta-cells, arachidonic acid accumulation results primarily from phospholipid hydrolysis by phospholipase A2, and activation of this enzyme has been shown to accompany glucose-induced insulin secretion. Inhibitors of phospholipase A2 attenuate the secretory response, although the compounds used to date have not discriminated between cytosolic and secretory isoforms of phospholipase A2. In this work, the specific cytosolic phospholipase A2 inhibitor, AACOCF3, caused a dose-dependent inhibition of glucose-induced insulin secretion from isolated rat islets and this response was significantly relieved by exogenous arachidonic acid. The results suggest that, despite the low levels of expression of cytosolic phospholipase A2 in rat islets, this enzyme contributes to the control of glucose-induced insulin secretion in rat pancreatic beta-cells.