Pulmonary arterial hypertension - progress in understanding the disease and prioritizing strategies for drug development

Pulmonary arterial hypertension (PAH), at one time a largely overlooked disease, is now the subject of intense study in many academic and biotech groups. The availability of new treatments has increased awareness of the condition. This in turn has driven a change in the demographics of PAH, with an increase in the mean age at diagnosis. The diagnosis of PAH in more elderly patients has highlighted the need for careful phenotyping of patients and for further studies to understand how best to manage pulmonary hypertension associated with, for example, left heart disease. The breadth and depth of expertise focused on unravelling the molecular pathology of PAH has yielded novel insights, including the role of growth factors, inflammation and metabolic remodelling. The description of the genetic architecture of PAH is accelerating in parallel, with novel variants, such as those reported in potassium two-pore domain channel subfamily K member 3 (KCNK3), adding to the list of more established mutations in genes associated with bone morphogenetic protein receptor type 2 (BMPR2) signalling. These insights have supported a paradigm shift in treatment strategies away from simply addressing the imbalance of vasoactive mediators observed in PAH towards tackling more directly the structural remodelling of the pulmonary vasculature. Here, we summarize the changing clinical and molecular landscape of PAH. We highlight novel drug therapies that are in various stages of clinical development, targeting for example cell proliferation, metabolic, inflammatory/immune and BMPR2 dysfunction, and the challenges around developing these treatments. We argue that advances in the treatment of PAH will come through deep molecular phenotyping with the integration of clinical, genomic, transcriptomic, proteomic and metabolomic information in large populations of patients through international collaboration. This approach provides the best opportunity for identifying key signalling pathways, both as potential drug targets and as biomarkers for patient selection. The expectation is that together these will enable the prioritization of potential therapies in development and the evolution of personalized medicine for PAH.

Degradation of islet amyloid polypeptide by neprilysin

AIMS/HYPOTHESIS

A progressive loss of pancreatic beta cell function, a decrease in beta cell mass and accumulation of islet amyloid is characteristic of type 2 diabetes mellitus. The main constituent of islet amyloid is islet amyloid polypeptide (IAPP). In this study, we examined the ability of the peptidase neprilysin to cleave IAPP and prevent human IAPP-induced pancreatic beta cell toxicity.

METHODS

Neprilysin and a catalytically compromised neprilysin mutant were tested for their ability to inhibit human IAPP fibrillisation and human IAPP-induced pancreatic beta cell cytotoxicity. Degradation of human IAPP by neprilysin was followed by HPLC, and the degradation products were identified by MS.

RESULTS

Neprilysin prevented IAPP fibrillisation by cleaving IAPP at Arg(11)-Leu(12), Leu(12)-Ala(13), Asn(14)-Phe(15), Phe(15)-Leu(16), Asn(22)-Phe(23) and Ala(25)-Ile(26). It also appears to prevent human IAPP fibrillisation through a non-catalytic interaction. Neprilysin protected against beta cell cytotoxicity induced by exogenously added or endogenously produced human IAPP.

CONCLUSIONS/INTERPRETATION

The data presented support a potential therapeutic role for neprilysin in preventing type 2 diabetes mellitus. This study supports the hypothesis that extracellular human IAPP contributes to human IAPP-induced beta cell cytotoxicity. Whether human IAPP exerts its cytotoxic effect through a totally extracellular mechanism or through a cellular reuptake mechanism is unclear at this time.

Iron deficiency in pulmonary arterial hypertension: a potential therapeutic target

Iron deficiency is known to be common and detrimental in chronic left heart failure, where parenteral iron treatment has been shown to improve exercise capacity, New York Heart Association functional class and patient wellbeing. There is now increasing interest in the role of iron in the natural history of pulmonary arterial hypertension (PAH). Iron availability influences the pulmonary vasoconstrictor response to hypoxia and accumulating evidence indicates that iron deficiency is prevalent in idiopathic and heritable forms of PAH, iron status being related to exercise capacity, symptoms and poorer survival in patients with idiopathic PAH (IPAH). Potential mechanisms behind iron deficiency in IPAH include inhibition of dietary iron uptake by the master iron regulator hepcidin. High hepcidin levels underlie the anaemia of chronic disease. Possible stimuli of the observed high levels of hepcidin in IPAH include dysfunctional bone morphogenetic protein receptor type II signalling and inflammation. Iron status may influence outcomes through modulation of the pulmonary circulation as well as myocardial and skeletal muscle function. Two parallel studies, from our centre (Hammersmith Hospital, London, UK) and others in the UK and Amsterdam (the Netherlands), investigating the safety and potential benefit of iron supplementation in patients with PAH are currently under way.

Arsenic: toxicity, oxidative stress and human disease

Arsenic (As) is a toxic metalloid element that is present in air, water and soil. Inorganic arsenic tends to be more toxic than organic arsenic. Examples of methylated organic arsenicals include monomethylarsonic acid [MMA(V)] and dimethylarsinic acid [DMA(V)]. Reactive oxygen species (ROS)-mediated oxidative damage is a common denominator in arsenic pathogenesis. In addition, arsenic induces morphological changes in the integrity of mitochondria. Cascade mechanisms of free radical formation derived from the superoxide radical, combined with glutathione-depleting agents, increase the sensitivity of cells to arsenic toxicity. When both humans and animals are exposed to arsenic, they experience an increased formation of ROS/RNS, including peroxyl radicals (ROO•), the superoxide radical, singlet oxygen, hydroxyl radical (OH•) via the Fenton reaction, hydrogen peroxide, the dimethylarsenic radical, the dimethylarsenic peroxyl radical and/or oxidant-induced DNA damage. Arsenic induces the formation of oxidized lipids which in turn generate several bioactive molecules (ROS, peroxides and isoprostanes), of which aldehydes [malondialdehyde (MDA) and 4-hydroxy-nonenal (HNE)] are the major end products. This review discusses aspects of chronic and acute exposures of arsenic in the etiology of cancer, cardiovascular disease (hypertension and atherosclerosis), neurological disorders, gastrointestinal disturbances, liver disease and renal disease, reproductive health effects, dermal changes and other health disorders. The role of antioxidant defence systems against arsenic toxicity is also discussed. Consideration is given to the role of vitamin C (ascorbic acid), vitamin E (α-tocopherol), curcumin, glutathione and antioxidant enzymes such as superoxide dismutase, catalase and glutathione peroxidase in their protective roles against arsenic-induced oxidative stress.

Glucolipotoxicity age-dependently impairs beta cell function in rats despite a marked increase in beta cell mass

AIMS/HYPOTHESIS

Prolonged exposure of pancreatic beta cells to excessive levels of glucose and fatty acids, referred to as glucolipotoxicity, is postulated to contribute to impaired glucose homeostasis in patients with type 2 diabetes. However, the relative contribution of defective beta cell function vs diminished beta cell mass under glucolipotoxic conditions in vivo remains a subject of debate. We therefore sought to determine whether glucolipotoxicity in rats is due to impaired beta cell function and/or reduced beta cell mass, and whether older animals are more susceptible to glucolipotoxic condition.

METHODS

Wistar rats (2 and 6 months old) received a 72 h infusion of glucose + intravenous fat emulsion or saline control. In vivo insulin secretion and sensitivity were assessed by hyperglycaemic clamps. Ex vivo insulin secretion, insulin biosynthesis and gene expression were measured in isolated islets. Beta cell mass and proliferation were examined by immunohistochemistry.

RESULTS

A 72 h infusion of glucose + intravenous fat emulsion in 2-month-old Wistar rats did not affect insulin sensitivity, insulin secretion or beta cell mass. In 6-month-old rats by contrast it led to insulin resistance and reduced insulin secretion in vivo, despite an increase in beta cell mass and proliferation. This was associated with: (1) diminished glucose-stimulated second-phase insulin secretion and proinsulin biosynthesis; (2) lower insulin content; and (3) reduced expression of beta cell genes in isolated islets.

CONCLUSIONS/INTERPRETATION

In this in vivo model, glucolipotoxicity is characterised by an age-dependent impairment of glucose-regulated beta cell function despite a marked increase in beta cell mass.

The Lévy flight paradigm: random search patterns and mechanisms

Over recent years there has been an accumulation of evidence from a variety of experimental, theoretical, and field studies that many organisms use a movement strategy approximated by Lévy flights when they are searching for resources. Lévy flights are random movements that can maximize the efficiency of resource searches in uncertain environments. This is a highly significant finding because it suggests that Lévy flights provide a rigorous mathematical basis for separating out evolved, innate behaviors from environmental influences. We discuss recent developments in random-search theory, as well as the many different experimental and data collection initiatives that have investigated search strategies. Methods for trajectory construction and robust data analysis procedures are presented. The key to prediction and understanding does, however, lie in the elucidation of mechanisms underlying the observed patterns. We discuss candidate neurological, olfactory, and learning mechanisms for the emergence of Lévy flight patterns in some organisms, and note that convergence of behaviors along such different evolutionary pathways is not surprising given the energetic efficiencies that Lévy flight movement patterns confer.

Variational data assimilation with epidemic models

Mathematical modelling is playing an increasing role in developing an understanding of the dynamics of communicable disease and assisting the construction and implementation of intervention strategies. The threat of novel emergent pathogens in human and animal hosts implies the requirement for methods that can robustly estimate epidemiological parameters and provide forecasts. Here, a technique called variational data assimilation is introduced as a means of optimally melding dynamic epidemic models with epidemiological observations and data to provide forecasts and parameter estimates. Using data from a simulated epidemic process the method is used to estimate the start time of an epidemic, to provide a forecast of future epidemic behaviour and estimate the basic reproductive ratio. A feature of the method is that it uses a basic continuous-time SIR model, which is often the first point of departure for epidemiological modelling during the early stages of an outbreak. The method is illustrated by application to data gathered during an outbreak of influenza in a school environment.

The balance between proinsulin biosynthesis and insulin secretion: where can imbalance lead?

Insulin is stored in pancreatic beta-cells in beta-granules. Whenever insulin is secreted in response to a nutrient secretagogue, there is a complementary increase in proinsulin biosynthesis to replenish intracellular insulin stores. This specific nutrient regulation of proinsulin biosynthesis is predominately regulated at the translational level. Recently, a highly conserved cis-element in the 5'-untranslated region (UTR) of preproinsulin mRNA, named ppIGE, has been identified that is required for specific translational regulation of proinsulin biosynthesis. This ppIGE is also found in the 5'-UTR of certain other translationally regulated beta-granule protein mRNAs, including the proinsulin processing endopeptidases, PC1/3 and PC2. This provides a mechanism whereby proinsulin processing is adaptable to changes in proinsulin biosynthesis. However, relatively few beta-granules undergo secretion, with most remaining in the storage pool for approximately 5 days. Aged beta-granules are retired by intracellular degradation mechanisms, either via crinophagy and/or autophagy, as another long-term means of maintaining beta-granule stores at optimal levels. When a disconnection between insulin production and secretion arises, as may occur in type 2 diabetes, autophagy further increases to maintain beta-granule numbers. However, if this increased autophagy becomes chronic, autophagia-mediated cell death occurs that could then contribute to beta-cell loss in type 2 diabetes.

Exendin 4 controls insulin production in rat islet beta cells predominantly by potentiation of glucose-stimulated proinsulin biosynthesis at the translational level

AIMS/HYPOTHESIS

Ideally, a therapeutic insulin secretagogue should coordinately increase insulin production and insulin secretion to maintain islet beta cell secretory capacity. We compared the incretin mimetic exendin 4 and the sulfonylurea glibenclamide (known as glyburide in the USA and Canada) for their effects in upholding a balance between (pro)insulin biosynthesis and insulin secretion in pancreatic islets.

METHODS

Isolated rat islets were incubated for 1 or 16 h over a range of glucose concentrations (2.8-16.7 mmol/l) with or without exendin 4 (10 nmol/l) or glibenclamide (1 micromol/l). Islets were then analysed for preproinsulin mRNA expression by RNase protection and quantitative real-time RT-PCR assays. Proinsulin biosynthesis was analysed by metabolic pulse-radiolabelling, immunoprecipitation and PAGE. Insulin secretion and insulin content were analysed by radioimmunoassay.

RESULTS

Neither exendin 4 nor glibenclamide affected islet preproinsulin mRNA expression. However, exendin 4 significantly increased glucose-induced proinsulin biosynthesis at the translational level within 1 h, in marked contrast to glibenclamide, which inhibited proinsulin biosynthesis, especially at basal and intermediate glucose concentrations. Exendin 4 potentiated insulin secretion in a glucose-dependent manner, whereas glibenclamide stimulated insulin secretion independently of glucose. Exendin 4 better maintained rat islet insulin content compared with glibenclamide, which depleted intracellular stores of insulin in islet beta cells by 40% within 16 h.

CONCLUSIONS/INTERPRETATION

Exendin 4 maintains insulin stores and beta cell secretory capacity primarily by translational control of proinsulin biosynthesis in parallel to insulin secretion. Glibenclamide does not regulate insulin production in coordination with stimulated insulin secretion, and consequently depletes islet insulin stores, compromising secretory capacity. Thus, at the level of the beta cell, incretin mimetics have an advantage over sulfonylureas for treatment of type 2 diabetes.

Free radicals, metals and antioxidants in oxidative stress-induced cancer

Oxygen-free radicals, more generally known as reactive oxygen species (ROS) along with reactive nitrogen species (RNS) are well recognised for playing a dual role as both deleterious and beneficial species. The "two-faced" character of ROS is substantiated by growing body of evidence that ROS within cells act as secondary messengers in intracellular signalling cascades, which induce and maintain the oncogenic phenotype of cancer cells, however, ROS can also induce cellular senescence and apoptosis and can therefore function as anti-tumourigenic species. The cumulative production of ROS/RNS through either endogenous or exogenous insults is termed oxidative stress and is common for many types of cancer cell that are linked with altered redox regulation of cellular signalling pathways. Oxidative stress induces a cellular redox imbalance which has been found to be present in various cancer cells compared with normal cells; the redox imbalance thus may be related to oncogenic stimulation. DNA mutation is a critical step in carcinogenesis and elevated levels of oxidative DNA lesions (8-OH-G) have been noted in various tumours, strongly implicating such damage in the etiology of cancer. It appears that the DNA damage is predominantly linked with the initiation process. This review examines the evidence for involvement of the oxidative stress in the carcinogenesis process. Attention is focused on structural, chemical and biochemical aspects of free radicals, the endogenous and exogenous sources of their generation, the metal (iron, copper, chromium, cobalt, vanadium, cadmium, arsenic, nickel)-mediated formation of free radicals (e.g. Fenton chemistry), the DNA damage (both mitochondrial and nuclear), the damage to lipids and proteins by free radicals, the phenomenon of oxidative stress, cancer and the redox environment of a cell, the mechanisms of carcinogenesis and the role of signalling cascades by ROS; in particular, ROS activation of AP-1 (activator protein) and NF-kappaB (nuclear factor kappa B) signal transduction pathways, which in turn lead to the transcription of genes involved in cell growth regulatory pathways. The role of enzymatic (superoxide dismutase (Cu, Zn-SOD, Mn-SOD), catalase, glutathione peroxidase) and non-enzymatic antioxidants (Vitamin C, Vitamin E, carotenoids, thiol antioxidants (glutathione, thioredoxin and lipoic acid), flavonoids, selenium and others) in the process of carcinogenesis as well as the antioxidant interactions with various regulatory factors, including Ref-1, NF-kappaB, AP-1 are also reviewed.

Glucagon-like peptide-1 protects beta cells from cytokine-induced apoptosis and necrosis: role of protein kinase B

AIMS/HYPOTHESIS

The gut hormone glucagon-like peptide-1 (GLP-1) decreases beta cell apoptosis in a protein kinase B (PKB)-dependent fashion, and increases islet cell mass and function in vivo. In contrast, cytokines induce beta cell apoptosis, leading to decreased islet mass and type 1 diabetes. In the present study we used rat INS-1E beta cells and primary rat islet cells to examine the potential role of PKB as a mediator of the effect of GLP-1 on cytokine-induced apoptosis.

METHODS

Cell viability was determined by MTT assay, and apoptosis and necrosis by Hoechst 33342-propidium iodide staining. Immunoblot analysis was used to detect changes in protein expression, including active (phosphorylated) and total PKB, phosphorylated and total glycogen synthase kinase-3beta, activated caspase-3 and inducible nitric oxide synthase. Reactive oxygen species were determined by 1,7-dichlorofluorescein (DCF) analysis, and mutant forms of PKB were introduced into cells using adenoviral vectors.

RESULTS

Incubation of INS-1E cells with cytokines (IL-1beta, TNF-alpha and interferon-gamma; 10-50 ng/ml) for 18 h significantly decreased cell viability (by 44%, p<0.001), cell proliferation (by 80%, p<0.001), and activation of PKB (by 67%, p<0.001). Pre-treatment with exendin-4 (10(-7) mol/l), a long-acting GLP-1 receptor agonist, partially protected the cells against cytokine-induced toxicity (p<0.01) in association with a reduction in cytokine-induced inhibition of PKB phosphorylation (p<0.05). Exendin-4 pre-treatment did not change cell proliferation. Cytokine treatment increased apoptosis (by 156%, p<0.05) and necrosis (from undetectable to 2.6% of cells). These increases were both reduced by pre-treatment with exendin-4 (p<0.05-0.01). Furthermore, cytokine-induced apoptosis and necrosis were significantly increased in cells infected with kinase-dead PKB (p<0.05), and the protective effect of exendin-4 on both parameters was fully abolished in these cells. Similar changes were observed in primary islet cells. In parallel with these changes, exendin-4 decreased the cytokine-induced activation of caspase-3 (by 46%, p<0.05), and decreased levels of inducible nitric oxide synthase (by 71%, p<0.05) and reactive oxygen species (by 27%, p<0.05).

CONCLUSIONS/INTERPRETATION

The results of our study indicate that GLP-1 plays a protective role against cytokine-induced apoptosis and necrosis in beta cells through a PKB-dependent signalling pathway.

Intragranular targeting of syncollin, but not a syncollinGFP chimera, inhibits regulated insulin exocytosis in pancreatic beta-cells

Several proteins play a role in the mechanism of insulin exocytosis. However, these 'exocytotic proteins' have yet to account for the regulated aspect of insulin exocytosis, and other factors are involved. In pancreatic exocrine cells, the intralumenal zymogen granule protein, syncollin, is required for efficient regulated exocytosis, but it is not known whether intragranular peptides similarly influence regulated insulin exocytosis. Here, this issue has been addressed using expression of syncollin and a syncollin-green fluorescent protein (syncollinGFP) chimera in rat islet beta-cells as experimental tools. Syncollin is not normally expressed in beta-cells but adenoviral-mediated expression of both syncollin and syncollinGFP indicated that these were specifically targeted to the lumen of beta-granules. Syncollin expression in isolated rat islets had no effect on basal insulin secretion but significantly inhibited regulated insulin secretion stimulated by glucose (16.7 mM), glucagon-like peptide-1 (GLP-1) (10 nM) and glyburide (5 microM). Consistent with specific localization of syncollin to beta-granules, constitutive secretion was unchanged by syncollin expression in rat islets. Syncollin-mediated inhibition of insulin secretion was not due to inadequate insulin production. Moreover, secretagogue-induced increases in cytosolic intracellular Ca2+, which is a prerequisite for triggering insulin exocytosis, were unaffected in syncollin-expressing islets. Therefore, syncollin was most likely acting downstream of secondary signals at the level of insulin exocytosis. Thus, syncollin expression in beta-cells has highlighted the importance of intralumenal beta-granule peptide factors playing a role in the control of insulin exocytosis. In contrast to syncollin, syncollinGFP had no effect on insulin secretion, underlining its usefulness as a 'fluorescent tag' to track beta-granule transport and exocytosis in real time.

Glucagon-like peptide-1 prevents beta cell glucolipotoxicity

AIMS/HYPOTHESIS

We have provided evidence that glucagon-like peptide-1, a potential therapeutic agent in the treatment of diabetes, activates phosphatidylinositol-3 kinase/protein kinase B signalling in the pancreatic beta cell. Since this pathway promotes cell survival in a variety of systems, we tested whether glucagon-like peptide-1 protects beta cells against cell death induced by elevated glucose and/or non-esterified fatty acids.

METHODS

Human islets and INS832/13 cells were cultured at glucose concentrations of 5 or 25 mmol/l in the presence or absence of palmitate. Apoptosis was evaluated by monitoring DNA fragmentation and chromatin condensation. Wild-type and protein kinase B mutants were overexpressed in INS832/13 cells using adenoviruses. Nuclear factor-kappa B DNA binding was assayed by electrophoretic mobility shift assay.

RESULTS

In human pancreatic beta cells and INS832/13 cells, glucagon-like peptide-1 prevented beta cell apoptosis induced by elevated concentrations of (i) glucose (glucotoxicity), (ii) palmitate (lipotoxicity) and (iii) both glucose and palmitate (glucolipotoxicity). Overexpression of a dominant-negative protein kinase B suppressed the anti-apoptotic action of glucagon-like peptide-1 in INS832/13 cells, whereas a constitutively active protein kinase B prevented beta cell apoptosis induced by elevated glucose and palmitate. Glucagon-like peptide-1 enhanced nuclear factor-kappa B DNA binding activity and stimulated the expression of inhibitor of apoptosis protein-2 and Bcl-2, two anti-apoptotic genes under the control of nuclear factor-kappa B. Inhibition of nuclear factor-kappa B by BAY 11-7082 abolished the prevention of glucolipotoxicity by glucagon-like peptide-1.

CONCLUSIONS/INTERPRETATION

The results demonstrate a potent protective effect of glucagon-like peptide-1 on beta cell gluco-, lipo- and glucolipotoxicity. This effect is mediated via protein kinase B activation and possibly its downstream target nuclear factor-kappa B.

Fatty acid and phorbol ester-mediated interference of mitogenic signaling via novel protein kinase C isoforms in pancreatic beta-cells (INS-1)

It is possible that activation of protein kinase C (PKC) isoforms by free fatty acids (FFA) plays a role in the failure of pancreatic beta-cell mass expansion to compensate for peripheral insulin resistance in the pathogenesis of type-2 diabetes. The effect of lipid moieties on activation of conventional (PKC-alpha and -beta1), novel (PKC-delta) and atypical (PKC-zeta) PKC isoforms was evaluated in an in vitro assay, using biotinylated neurogranin as a substrate. Oleoyl-Coenzyme A (CoA) and palmitoyl-CoA, but not unesterified FFA, significantly increased the activity of all PKC isoforms (P< or =0.05), particularly that for PKC-delta. It was found that FFA (0.4 mM oleate/complexed to 0.5% bovine serum albumin) inhibited IGF-I-induced activation of protein kinase B (PKB) in the pancreatic beta-cell line (INS-1), but this was alleviated in the presence of the general PKC inhibitor (Gö6850; 1 microM). To further investigate whether conventional or novel PKC isoforms adversely affect beta-cell proliferation, the effect of phorbol ester (phorbol 12-myristate 13-acetate; PMA)-mediated activation of these PKC isoforms on glucose/IGF-I-induced INS-1 cell mitogenesis, and insulin receptor substrate (IRS)-mediated signal transduction was investigated. PMA-mediated activation of PKC (100 nM; 4 h) reduced glucose/IGF-I mediated beta-cell mitogenesis (>50%; P< or =0.05), which was reversible by the general PKC inhibitor Gö6850 (1 microM), indicating an effect of PKC and not due to a non-specific PMA toxicity. PMA inhibited IGF-I-induced activation of PKB, correlating with inhibition of IGF-I-induced association of IRS-2 with the p85 regulatory subunit of phosphatidylinositol-3 kinase. However, in contrast, PMA activated the mitogen-activated protein kinases, Erk1/2. Titration inhibition analysis using PKC isoform inhibitors indicated that these PMA-induced effects were via novel PKC isoforms. Thus, FFA/PMA-induced activation of novel PKC isoforms can inhibit glucose/IGF-I-mediated beta-cell mitogenesis, in part by decreasing PKB activation, despite an upregulation of Erk1/2. Thus, activation of novel PKC isoforms by long-chain acyl-CoA may well contribute to decreasing beta-cell mass in the pathogenesis of type-2 diabetes, similar to their inhibition of insulin signal transduction which causes insulin resistance.

Molecular insights into insulin action and secretion

Tightly co-ordinated control of both insulin action and secretion is required in order to maintain glucose homeostasis. Gene knockout experiments have helped to define key signalling molecules that affect insulin action, including insulin and insulin-like growth factor-1 (IGF-1) receptors, insulin receptor substrate (IRS) proteins and various downstream effector proteins. beta-cell function is also a tightly regulated process, with numerous factors (including certain signalling molecules) having an impact on insulin production, insulin secretion and beta-cell mass. While signalling molecules play important roles in insulin action and secretion under normal circumstances, abnormal insulin signalling in muscle, adipose tissue, liver and pancreas leads to insulin resistance and beta-cell dysfunction. In particular, the signalling protein IRS-2 may have a central role in linking these abnormalities, although other factors are likely to be involved.

Protein kinase Czeta activation mediates glucagon-like peptide-1-induced pancreatic beta-cell proliferation

Glucagon-like peptide-1 (GLP-1), an insulinotropic and glucoincretin hormone, is a potentially important therapeutic agent in the treatment of diabetes. We previously provided evidence that GLP-1 induces pancreatic beta-cell growth nonadditively with glucose in a phosphatidylinositol-3 kinase (PI-3K)-dependent manner. In the present study, we investigated the downstream effectors of PI-3K to determine the precise signal transduction pathways that mediate the action of GLP-1 on beta-cell proliferation. GLP-1 increased extracellular signal-related kinase 1/2, p38 mitogen-activated protein kinase (MAPK), and protein kinase B activities nonadditively with glucose in pancreatic beta(INS 832/13) cells. GLP-1 also caused nuclear translocation of the atypical protein kinase C (aPKC) zeta isoform in INS as well as in dissociated normal rat beta-cells as shown by immunolocalization and Western immunoblotting analysis. Tritiated thymidine incorporation measurements showed that the p38 MAPK inhibitor SB203580 suppressed GLP-1-induced beta-cell proliferation. Further investigation was performed using isoform-specific pseudosubstrates of classical (alpha, beta, and gamma) or zeta aPKC isoforms. The PKCzeta pseudosubstrate suppressed the proliferative action of GLP-1, whereas the inhibitor of classical PKC isoforms had no effect. Overexpression of a kinase-dead PKCzeta acting as a dominant negative protein suppressed GLP-1-induced proliferation. In addition, ectopic expression of a constitutively active PKCzeta mutant stimulated tritiated thymidine incorporation to the same extent as GLP-1, and the glucoincretin had no growth-promoting action under this condition. The data indicate that GLP-1-induced activation of PKCzeta is implicated in the beta-cell proliferative signal of the insulinotropic hormone. The results are consistent with a model in which GLP-1-induced PI-3K activation results in PKCzeta translocation to the nucleus, which may play a role in the pleiotropic effects (DNA synthesis, metabolic enzymes, and insulin gene expression) of the glucoincretin.

Gene and cell-replacement therapy in the treatment of type 1 diabetes: how high must the standards be set?

Recent advances in molecular and cell biology may allow for the development of novel strategies for the treatment and cure of type 1 diabetes. In particular, it is now possible to envisage restoration of insulin secretion by gene or cell-replacement therapy. The beta-cell is, however, remarkably sophisticated, and many of the features of this highly differentiated secretory cell will have to be faithfully mimicked in surrogate cells. In particular, insulin is normally secreted in a well-regulated fashion in rapid response to the metabolic needs of the individual and most specifically (but not exclusively) to changes in circulating levels of glucose. Such regulated secretion will be indispensable in order to avoid both hyper- and hypoglycemic episodes and depends on the ability of cells to store insulin in secretory granules before exocytosis in response to physiological stimuli. Furthermore, any newly created insulin-secreting cell will have to be able to adapt to alterations in insulin requirements that accompany changes with exercise, body weight, and aging. Fine tuning of insulin secretion over the longer term will also be important to avoid "clinical shifting" that could be caused by over-insulinization, including increased adiposity and cardiovascular disease. Finally, it will be necessary to ensure that newly created or implanted (surrogate) beta-cells are protected in some way from recognition by the immune system and in particular from autoimmune destruction.

A low-affinity Ca2+-dependent association of calmodulin with the Rab3A effector domain inversely correlates with insulin exocytosis

The stimulus-response coupling pathway for glucose-regulated insulin secretion has implicated a rise in cytosolic [Ca2+]i as a key factor to induce insulin exocytosis. However, it is unclear how elevated [Ca2+]i communicates with the pancreatic beta-cell's exocytotic apparatus. As Rab3A is a model protein involved in regulated exocytosis, we have focused on its role in regulating insulin exocytosis. By using a photoactivatable cross-linking synthetic peptide that mimics the effector domain of Rab3A and microsequence analysis, we found calmodulin to be a major Rab3A target effector protein in pancreatic beta-cells. Coimmunoprecipitation analysis from pancreatic islets confirmed a Rab3A-calmodulin interaction in vivo, and that it inversely correlated with insulin exocytosis. Calmodulin affected neither GTPase nor guanine nucleotide exchange activity of Rab3A. The calmodulin-Rab3A interaction was pH- and Ca2+-dependent, and it was preferential for GTP-bound Rab3A. However, Rab3A affinity for calmodulin was relatively low (Kd = 18-22 micromol/l at 10(-5) mol/l [Ca2+]) and competed by other calmodulin-binding proteins that had higher affinity (e.g., Ca2+/calmodulin-dependent protein kinase-2 [CaMK-2] [Kd = 300-400 nmol/l at 10(-5) mol/l [Ca2+]]). Moreover, the Ca2+ dependence of the calmodulin-Rab3A interaction (K0.5 = 15-18 micromol/l [Ca2+], maximal at 100 micromol/l [Ca2+]) was significantly lower compared with that of the calmodulin-CaMK-2 association (K0.5 = 40 micromol/l [Ca2+], maximal at 1 mmol/l [Ca2+]). The data suggested that a transient Rab3A-calmodulin interaction might represent a means of directing calmodulin to the cytoplasmic face of a beta-granule, where it can be subsequently transferred for activation of other beta-granule-associated calmodulin-binding proteins as local [Ca2+]i rises to promote insulin exocytosis.

Glycerol-stimulated proinsulin biosynthesis in isolated pancreatic rat islets via adenoviral-induced expression of glycerol kinase is mediated via mitochondrial metabolism

In this study, we examined whether adenoviral-mediated glycerol kinase (AdV-CMV-GlyK) expression in isolated rat pancreatic islets could introduce glycerol-induced proinsulin biosynthesis. In AdV-CMV-GlyK-infected islets, specific glycerol-induced proinsulin biosynthesis translation and insulin secretion were observed in parallel from the same islets. The threshold concentration of glycerol required to stimulate proinsulin biosynthesis was lower (0.25-0.5 mmol/l) than that for insulin secretion (1.0-1.5 mmol/l), reminiscent of threshold differences for glucose-stimulated proinsulin biosynthesis versus insulin secretion. The dose-dependent glycerol-induced proinsulin biosynthesis correlated with the rate of glycerol oxidation in AdV-CMV-GlyK-infected islets, indicating that glycerol metabolism was required for the response. However, glycerol did not significantly increase lactate output from AdV-CMV-GlyK-infected islets, but the dihydroxyacetone phosphate (DHAP) to alpha-glycerophosphate (alpha-GP) ratio significantly increased in AdV-CMV-GlyK-infected islets incubated at 2 mmol/l glycerol compared with that at a basal level of 2.8 mmol/l glucose (P < or = 0.05). The DHAP:alpha-GP ratio was unaffected in AdV-CMV-GlyK-infected islets incubated at 2 mmol/l glycerol in the added presence of alpha-cyanohydroxycinnaminic acid (alpha-CHC), an inhibitor of the plasma membrane and mitochondrial lactate/pyruvate transporter. However, alpha-CHC inhibited glycerol-induced proinsulin biosynthesis and insulin secretion in AdV-CMV-GlyK-infected islets (>75%; P = 0.05), similarly to glucose-induced proinsulin biosynthesis and insulin secretion in AdV-CMV-GlyK-infected and control islets. These data indicated that in AdV-CMV-GlyK-infected islets, the importance of mitochondrial metabolism of glycerol was required to generate stimulus-response coupling signals to induce proinsulin biosynthesis and insulin secretion.

Cooperativity between the preproinsulin mRNA untranslated regions is necessary for glucose-stimulated translation

Glucose regulates proinsulin biosynthesis via stimulation of the translation of the preproinsulin mRNA in pancreatic beta-cells. However, the mechanism by which this occurs has remained unclear. Using recombinant adenoviruses that express the preproinsulin mRNA with defined alterations, the untranslated regions (UTRs) of the preproinsulin mRNA were examined for elements that specifically control translation of the mRNA in rat pancreatic islets. These studies revealed that the preproinsulin 5'-UTR was necessary for glucose stimulation of preproinsulin mRNA translation, whereas the 3'-UTR appeared to suppress translation. However, together the 5'- and 3'-UTRs acted cooperatively to markedly increase glucose-induced proinsulin biosynthesis. In primary hepatocytes the presence of the preproinsulin 3'-UTR led to reduced mRNA levels compared with the presence of the SV40 3'-UTR, consistent with the presence of mRNA stability determinants in the 3'-UTR that stabilize the preproinsulin mRNA in a pancreatic beta-cell-specific manner. Translation of these mRNAs in primary hepatocytes was not stimulated by glucose, indicating that regulated translation of the preproinsulin mRNA occurs in a pancreatic beta-cell-specific manner. Thus, the untranslated regions of the preproinsulin mRNA play crucial roles in regulating insulin production and therefore glucose homeostasis by regulating the translation and the stability of the preproinsulin mRNA.

Differential activation of protein kinase B and p70(S6)K by glucose and insulin-like growth factor 1 in pancreatic beta-cells (INS-1)

It has been shown that IGF-1-induced pancreatic beta-cell proliferation is glucose-dependent; however, the mechanisms responsible for this glucose dependence are not known. Adenoviral mediated expression of constitutively active phosphatidylinositol 3-kinase (PI3K) in the pancreatic beta-cells, INS-1, suggested that PI3K was not necessary for glucose-induced beta-cell proliferation but was required for IGF-1-induced mitogenesis. Examination of the signaling components downstream of PI3K, 3-phosphoinositide-dependent kinase 1, protein kinase B (PKB), glycogen synthase kinase-3, and p70-kDa-S6-kinase (p70(S6K)), suggested that a major part of glucose-dependent beta-cell proliferation requires activation of mammalian target of rapamycin/p70(S6K), independent of phosphoinositide-dependent kinase 1/PKB activation. Adenoviral expression of the kinase-dead form of PKB in INS-1 cells decreased IGF-1-induced beta-cell proliferation. However, a surprisingly similar decrease was also observed in adenoviral wild type and constitutively active PKB-infected cells. Upon analysis of extracellular signal-regulated protein kinase 1 and 2 (ERK1/ERK2), an increase in ERK1/ERK2 phosphorylation activation by glucose and IGF-1 was observed in kinase-dead PKB-infected cells, but this phosphorylation activation was inhibited in the constitutively active PKB-infected cells. Hence, there is a requirement for the activation of both ERK1/ERK2 and mammalian target of rapamycin/p70(S6K) signal transduction pathways for a full commitment to glucose-induced pancreatic beta-cell mitogenesis. However, for IGF-1-induced activation, these pathways must be carefully balanced, because chronic activation of one (PI3K/PKB) can lead to dampening of the other (ERK1/2), reducing the mitogenic response.

Free fatty acid-induced inhibition of glucose and insulin-like growth factor I-induced deoxyribonucleic acid synthesis in the pancreatic beta-cell line INS-1

Pancreatic beta-cell mitogenesis is increased by insulin-like growth factor I (IGF-I) in a glucose-dependent manner. In this study it was found that alternative beta-cell nutrient fuels to glucose, pyruvate, and glutamine/leucine independently induced and provided a platform for IGF-I to induce INS-1 cell DNA synthesis in the absence of serum. In contrast, long chain FFA (>/=C(12)) inhibited 15 mM glucose-induced [(3)H]thymidine incorporation (+/-10 nM IGF-I) by 95% or more within 24 h above 0.2 mM FFA complexed to 1% BSA (K(0.5) for palmitate/1% BSA = 65-85 microM for 24 h; t(0.5) for 0.2 mM palmitate/1% BSA = approximately 6 h). FFA-mediated inhibition of glucose/IGF-I-induced ss-cell DNA synthesis was reversible, and FFA oxidation did not appear to be required, nor did FFA interfere with glucose metabolism in INS-1 cells. An examination of mitogenic signal transduction pathways in INS-1 cells revealed that glucose/IGF-I induction of early signaling elements in SH2-containing protein (Shc)- and insulin receptor substrate-1/2-mediated pathways leading to downstream mitogen-activated protein kinase and phosphoinositol 3'-kinase activation, were unaffected by FFA. However, glucose-/IGF-I-induced activation of protein kinase B (PKB) was significantly inhibited, and protein kinase Czeta was chronically activated by FFA. It is possible that FFA-mediated inhibition of ss-cell mitogenesis contributes to the reduction of beta-cell mass and the subsequent failure to compensate for peripheral insulin resistance in vivo that is key to the pathogenesis of obesity-linked diabetes.

Induction of endothelin-1 expression by glucose: an effect of protein kinase C activation

Enhanced actions or levels of endothelin-1 (ET-1), a potent vasoconstrictor, have been associated with decreased blood flow in the retina and peripheral nerves of diabetic animals and may be related to the development of pathologies in these tissues. Hyperglycemia has been postulated to increase ET-1 secretion in endothelial cells. We have characterized the mechanism by which elevation of glucose is increasing ET-1 mRNA expression in capillary bovine retinal endothelial cells (BREC) and bovine retinal pericytes (BRPC). Elevation of glucose, but not mannitol, from 5.5 to 25 mmol/l for 3 days increased membranous protein kinase C (PKC) activities and ET-1 mRNA in parallel levels by 2-fold in BREC and BRPC. These effects were reversed by decreasing glucose levels to 5.5 mmol/l for an additional 2 days. Glucose-induced ET-1 overexpression was inhibited by a general PKC inhibitor, GF109203X, and a mitogen-activated protein kinase kinase inhibitor, PD98059, but not by wortmannin, a phosphatidylinositol 3-kinase inhibitor. By immunoblot analysis, PKC-beta2 and -delta isoforms in BREC were significantly increased relative to other isoforms in the membranous fractions when glucose level was increased. Overexpression of PKC-beta1 and -delta isoforms but not PKC-zeta isoform by adenovirus vectors containing the respective cDNA enhanced in parallel PKC activities, proteins, and basal and glucose-induced ET-1 mRNA expression by at least 2-fold. These results showed that enhanced ET-1 expression induced by hyperglycemia in diabetes is partly due to activation of PKC-beta and -delta isoforms, suggesting that inhibition of these PKC isoforms may prevent early changes in diabetic retinopathy and neuropathy.

Induction of endothelin-1 expression by glucose: an effect of protein kinase C activation

Enhanced actions or levels of endothelin-1 (ET-1), a potent vasoconstrictor, have been associated with decreased blood flow in the retina and peripheral nerves of diabetic animals and may be related to the development of pathologies in these tissues. Hyperglycemia has been postulated to increase ET-1 secretion in endothelial cells. We have characterized the mechanism by which elevation of glucose is increasing ET-1 mRNA expression in capillary bovine retinal endothelial cells (BREC) and bovine retinal pericytes (BRPC). Elevation of glucose, but not mannitol, from 5.5 to 25 mmol/l for 3 days increased membranous protein kinase C (PKC) activities and ET-1 mRNA in parallel levels by 2-fold in BREC and BRPC. These effects were reversed by decreasing glucose levels to 5.5 mmol/l for an additional 2 days. Glucose-induced ET-1 overexpression was inhibited by a general PKC inhibitor, GF109203X, and a mitogen-activated protein kinase kinase inhibitor, PD98059, but not by wortmannin, a phosphatidylinositol 3-kinase inhibitor. By immunoblot analysis, PKC-beta2 and -delta isoforms in BREC were significantly increased relative to other isoforms in the membranous fractions when glucose level was increased. Overexpression of PKC-beta1 and -delta isoforms but not PKC-zeta isoform by adenovirus vectors containing the respective cDNA enhanced in parallel PKC activities, proteins, and basal and glucose-induced ET-1 mRNA expression by at least 2-fold. These results showed that enhanced ET-1 expression induced by hyperglycemia in diabetes is partly due to activation of PKC-beta and -delta isoforms, suggesting that inhibition of these PKC isoforms may prevent early changes in diabetic retinopathy and neuropathy.

"Spin-labelling" with the second hydrogen radioisotope: radicals formed from squalene by muonium addition

By radio-labelling with muonium (the second hydrogen radioisotope), a free radical species has been clearly identified in squalene (2,6,10,15,19,23-hexamethyl-2,6,10,14,18,22-tetracosahexaene ) with a muon-electron hyperfine coupling of 240.7 MHz, as measured by the muon spin rotation (MuSR) technique. The radical is undetectable in pure squalene due to its high viscosity and the large molecular size which leads to extreme line broadening, but its signals may be resolved on reducing the viscosity of the medium by dilution with diethyl ether. The potential of the MuSR method is thus demonstrated as a means for spin-labelling radical species such as are formed from cellular antioxidants; ESR spectroscopy is unsuitable for studies of radicals formed from this molecule, due to extreme line-broadening and spectral complexity.

IGF-I and GH post-receptor signaling mechanisms for pancreatic beta-cell replication

Certain nutrients, pharmacological agents and growth factors can stimulate pancreatic beta-cell proliferation; however, mitogenic signal transduction pathways in beta-cells have not been particularly well characterized. As a model system we have focussed on characterizing the signal transduction pathways immediately downstream of the IGF-I and GH receptors in beta-cells. The original idea was to gain an idea of important elements in mitogenic signaling pathways which might then be exploited to generate a marked increase in beta-cell proliferation. Such an approach could eventually reveal a means to increase the number of human pancreatic endocrine cells in vitro, in order to obtain an abundant source of beta-cells for routine transplantation therapy of type-I diabetes. However, in the course of our studies, we have also unveiled an unexpected insight into the pathogenesis of obesity-linked type-II diabetes. It has been observed that free fatty acids inhibit glucose- and glucose-dependent IGF-I/GH-induced beta-cell proliferation. We hypothesize that a gradual accumulation of intracellular fat in beta-cells during obesity can eventually lead to an inhibition of beta-cell mass expansion and hence failure to compensate for peripheral insulin resistance, so that type-II diabetes ensues.

Acute stimulation with long chain acyl-CoA enhances exocytosis in insulin-secreting cells (HIT T-15 and NMRI beta-cells)

Non-insulin-dependent diabetes mellitus is associated with, in addition to impaired insulin release, elevated levels of free fatty acids (FFA) in the blood. Insulin release is stimulated when beta-cells are acutely exposed to FFA, whereas chronic exposure may inhibit glucose-induced insulin secretion. In the present study we investigated the direct effects of long chain acyl-CoA (LC-CoA), the active intracellular form of FFA, on insulin exocytosis. Palmitoyl-CoA stimulated both insulin release from streptolysin-O-permeabilized HIT cells and fusion of secretory granules to the plasma membrane of mouse pancreatic beta-cells, as measured by cell capacitance. The LC-CoA effect was chain length-dependent, requiring chain lengths of at least 14 carbons. LC-CoA needed to be present to stimulate insulin release, and consequently there was no effect following its removal. The stimulatory effect was observed after inhibition of protein kinase activity and in the absence of ATP, even though both kinases and ATP, themselves, modulate exocytosis. The effect of LC-CoA was inhibited by cerulenin, which has been shown to block protein acylation. The data suggest that altered LC-CoA levels, resulting from FFA or glucose metabolism, may act directly on the exocytotic machinery to stimulate insulin release by a mechanism involving LC-CoA protein binding.

Correction of diet-induced hyperglycemia, hyperinsulinemia, and skeletal muscle insulin resistance by moderate hyperleptinemia

Human obesity and high fat feeding in rats are associated with the development of insulin resistance and perturbed carbohydrate and lipid metabolism. It has been proposed that these metabolic abnormalities may be reversible by interventions that increase plasma leptin. Up to now, studies in nongenetic animal models of obesity and in human obesity have concentrated on multiple injection therapy with mixed results. Our study sought to determine whether a sustained, moderate increase in plasma leptin, achieved by administration of a recombinant adenovirus containing the leptin cDNA (AdCMV-leptin) would be effective in reversing the metabolic abnormalities of the obese phenotype. Wistar rats fed a high-fat diet (HF) were heavier (P < 0.05), had increased fat mass and intramuscular triglycerides (mTG), and had elevated plasma glucose, insulin, triglyceride, and free fatty acids compared with standard chow-fed (SC) control animals (all P < 0.01). HF rats also had impaired glucose tolerance and were markedly insulin resistant, as demonstrated by a 40% reduction in insulin-stimulated muscle glucose uptake (P < 0.001). Increasing plasma leptin levels to 29.0 +/- 1.5 ng/ml (from 7.0 +/- 1.4 ng/ml, P < 0.001) for a period of 6 days decreased adipose mass by 40% and normalized plasma glucose and insulin levels. In addition, insulin-stimulated skeletal muscle glucose uptake was normalized in hyperleptinemic rats, an effect that correlated closely with a 60% (P < 0.001) decrease in mTG. Importantly, HF rats that received a control adenovirus containing the beta-galactosidase cDNA and were calorically matched to AdCMV-leptin-treated animals remained hyperglycemic, hyperinsulinemic, insulin resistant, and maintained elevated mTG. We conclude that a gene-therapeutic intervention that elevates plasma leptin moderately for a sustained period reverses diet-induced hyperglycemia, hyperinsulinemia, and skeletal muscle insulin resistance, and that these improvements are tightly linked to leptin-induced reductions in mTG.

Synaptotagmin III isoform is compartmentalized in pancreatic beta-cells and has a functional role in exocytosis

Synaptotagmin is involved in Ca2+-regulated secretion and has been suggested to serve as a general Ca2+ sensor on the membrane of secretory vesicles in neuronal cells. Insulin exocytosis from the pancreatic beta-cell is an example of a Ca2+-dependent secretory process. Previous studies of pancreatic beta-cells were unable to show presence of synaptotagmin I. We now present biochemical and immunohistochemical data showing that synaptotagmin III is present in pancreatic beta-cells as well as in the insulin-secreting cell line HIT-T15 and in rat insulinoma. By subcellular fractionation, we found synaptotagmin III in high-density fractions together with insulin and secretogranin I, indicating colocalization of synaptotagmin III and insulin in secretory granules. We could also show that blockade of synaptotagmin III by a specific antibody inhibited Ca2+-induced changes in beta-cell membrane capacitance, suggesting that synaptotagmin III is part of the functional protein complex regulating beta-cell exocytosis. The synaptotagmin III antibody did not affect the activity of the voltage-gated L-type Ca2+-channel. These findings are compatible with the view that synaptotagmin III, because of its distinct localization in the pancreatic beta-cell, functionally modulates insulin exocytosis. This indicates that synaptotagmin may have a general role in the regulation of exocytosis not only in neuronal cells but also in endocrine cells.

Regulation of endothelial constitutive nitric oxide synthase gene expression in endothelial cells and in vivo : a specific vascular action of insulin

BACKGROUND

The vasodilatory effect of insulin can be acute or increase with time from 1 to 7 hours, suggesting that insulin may enhance the expression of endothelial nitric oxide synthase (eNOS) in endothelial cells. The objective of the present study was to characterize the extent and signaling pathways by which insulin regulates the expression of eNOS in endothelial cells and vascular tissues.

METHODS AND RESULTS

Physiological concentrations of insulin (10(-10) to 10(-7) mmol/L) increased the levels of eNOS mRNA, protein, and activity by 2-fold after 2 to 8 hours of incubation in cultured bovine aortic endothelial cells. Insulin enhanced eNOS gene expression in microvessels isolated from Zucker lean rats but not from insulin-resistant Zucker fatty rats. Inhibitors of phosphatidylinositol-3 kinase (PI-3 kinase) decreased the effect of insulin on eNOS gene expression, but a general protein kinase C (PKC) inhibitor, GF109203X or PKCbeta isoform inhibitor, LY333531 enhanced eNOS expression. In contrast, PKC activators inhibited both the activation by insulin of PI-3 kinase and eNOS mRNA levels. Overexpression of PKCbeta isoform in endothelial cells inhibited the stimulation by insulin of eNOS expression and PI-3 kinase activities in parallel.

CONCLUSIONS

Insulin can regulate the expression of eNOS gene, mediated by the activation of PI-3 kinase, in endothelial cells and microvessels. Thus, insulin may chronically modulate vascular tone. The activation of PKC in the vascular tissues as in insulin resistance and diabetes may inhibit PI-3 kinase activity and eNOS expression and may lead to endothelial dysfunctions in these pathological states.

Interaction of the Ras-related protein associated with diabetes rad and the putative tumor metastasis suppressor NM23 provides a novel mechanism of GTPase regulation

Rad is the prototypic member of a new class of Ras-related GTPases. Purification of the GTPase-activating protein (GAP) for Rad revealed nm23, a putative tumor metastasis suppressor and a development gene in Drosophila. Antibodies against nm23 depleted Rad-GAP activity from human skeletal muscle cytosol, and bacterially expressed nm23 reconstituted the activity. The GAP activity of nm23 was specific for Rad, was absent with the S105N putative dominant negative mutant of Rad, and was reduced with mutations of nm23. In the presence of ATP, GDP.Rad was also reconverted to GTP.Rad by the nucleoside diphosphate (NDP) kinase activity of nm23. Simultaneously, Rad regulated nm23 by enhancing its NDP kinase activity and decreasing its autophosphorylation. Melanoma cells transfected with wild-type Rad, but not the S105N-Rad, showed enhanced DNA synthesis in response to serum; this effect was lost with coexpression of nm23. Thus, the interaction of nm23 and Rad provides a potential novel mechanism for bidirectional, bimolecular regulation in which nm23 stimulates both GTP hydrolysis and GTP loading of Rad whereas Rad regulates activity of nm23. This interaction may play important roles in the effects of Rad on glucose metabolism and the effects of nm23 on tumor metastasis and developmental regulation.

Stimulation of pancreatic beta-cell proliferation by growth hormone is glucose-dependent: signal transduction via janus kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) with no crosstalk to insulin receptor substrate-mediated mitogenic signalling

Mitogenic signal-transduction pathways have not been well defined in pancreatic beta-cells. In the glucose-sensitive rat beta-cell line, INS-1, glucose (6-18 mM) increased INS-1 cell proliferation (>20-fold at 15 mM glucose). Rat growth hormone (rGH) also induced INS-1 cell proliferation, but this was glucose-dependent in the physiologically relevant concentration range (6-18 mM glucose). The combination of rGH (10 nM) and glucose (15 mM) was synergistic, maximally increasing INS-1 cell proliferation by >50-fold. Moreover, glucose-dependent rGH-induced INS-1 cell proliferation was increased further by addition of insulin-like growth factor 1 (IGF-1; 10 nM) to >90-fold at 12 mM glucose. Glucose metabolism and phosphatidylinositol-3'-kinase (PI3'K) activation were necessary for both glucose- and rGH-stimulated INS-1 cell proliferation. Glucose (>3 mM) independently increased tyrosine-phosphorylation-mediated recruitment of growth-factor-bound protein 2 (Grb2)/murine sons of sevenless-1 protein (mSOS) and PI3'K to insulin receptor substrate (IRS)-1 and IRS-2, as well as SH2-containing protein (Shc) association with Grb2/mSOS and downstream activation of mitogen-activated protein kinase and 70 kDa S6 kinase. Glucose-induced IRS- and Shc-mediated signal transduction was enhanced further by the addition of IGF-1, but not rGH. In contrast, rGH was able to activate Janus kinase 2 (JAK2)/signal transducer and activator of transcription 5 (STAT5) signal transduction at glucose concentrations above 3 mM, but neither glucose independently, nor glucose with added IGF-1, were able to activate the JAK2/STAT5 signalling pathway. Thus rGH-mediated proliferation of beta-cells is directly via the JAK2/STAT5 pathway without engaging the Shc or IRS signal-transduction pathways, although activation of PI3'K may play an important permissive role in the glucose-dependent aspect of rGH-induced beta-cell mitogensis. The additive effect of rGH and IGF-1 on glucose-dependent beta-cell proliferation is therefore reflective of rGH and IGF-1 activating distinctly different mitogenic signalling pathways in beta-cells with minimal crosstalk between them.

Glucose or diabetes activates p38 mitogen-activated protein kinase via different pathways

Hyperglycemia can cause vascular dysfunctions by multiple factors including hyperosmolarity, oxidant formation, and protein kinase C (PKC) activation. We have characterized the effect of hyperglycemia on p38 mitogen-activated protein (p38) kinase activation, which can be induced by oxidants, hyperosmolarity, and proinflammatory cytokines, leading to apoptosis, cell growth, and gene regulation. Glucose at 16.5 mM increased p38 kinase activity in a time-dependent manner compared with 5.5 mM in rat aortic smooth muscle cells (SMC). Mannitol activated p38 kinase only at or greater than 22 mM. High glucose levels and a PKC agonist activated p38 kinase, and a PKC inhibitor, GF109203X, prevented its activation. However, p38 kinase activation by mannitol or tumor necrosis factor-alpha was not inhibited by GF109203X. Changes in PKC isoform distribution after exposure to 16.5 mM glucose in SMC suggested that both PKC-beta2 and PKC-delta isoforms were increased. Activities of p38 kinase in PKC-delta- but not PKC-beta1-overexpressed SMC were increased compared with control cells. Activation of p38 kinase was also observed and characterized in various vascular cells in culture and aorta from diabetic rats. Thus, moderate hyperglycemia can activate p38 kinase by a PKC-delta isoform-dependent pathway, but glucose at extremely elevated levels can also activate p38 kinase by hyperosmolarity via a PKC-independent pathway.

Changes in cytoplasmic ATP concentration parallels changes in ATP-regulated K+-channel activity in insulin-secreting cells

Changes in cytoplasmic ATP concentration were monitored in intact insulin-producing cells and correlated to changes in the activity of ATP-sensitive K+-channels (KATP channels). Luciferase was introduced into HIT M2.2 cells and whole pancreatic islets by transient expression of firefly (Photinus pyralis) luciferase cDNA. In transfected cells, extracellular addition of luciferin increased the luminescence signal to a maximum within 50-120 s. Addition of 1 microM of the mitochondrial uncoupler FCCP decreased the luminescence, an effect partly reversed upon withdrawal of the compound. High concentrations of glucose increased cytoplasmic free ATP concentration. Changes in the luminescence signal were accompanied by changes in activity of the ATP-sensitive K+-channel. Transfection per se did not deteriorate cell function, as verified by experiments showing similar changes in cytoplasmic free Ca2+-concentration, [Ca2+li, in both transfected and non-transfected cells. By measuring the cytoplasmic ATP concentration and KATP channel activity under similar experimental conditions, it was possible to establish, for the first time, a direct relationship between these two parameters. This indeed suggests that the cytoplasmic ATP concentration has a crucial role in the regulation of KATP channel activity under physiological conditions.

Cysteine string protein (CSP) is an insulin secretory granule-associated protein regulating beta-cell exocytosis

Cysteine string proteins (CSPs) are novel synaptic vesicle-associated protein components characterized by an N-terminal J-domain and a central palmitoylated string of cysteine residues. The cellular localization and functional role of CSP was studied in pancreatic endocrine cells. In situ hybridization and RT-PCR analysis demonstrated CSP mRNA expression in insulin-producing cells. CSP1 mRNA was present in pancreatic islets; both CSP1 and CSP2 mRNAs were seen in insulin-secreting cell lines. Punctate CSP-like immunoreactivity (CSP-LI) was demonstrated in most islets of Langerhans cells, acinar cells and nerve fibers of the rat pancreas. Ultrastructural analysis showed CSP-LI in close association with membranes of secretory granules of cells in the endo- and exocrine pancreas. Subcellular fractionation of insulinoma cells showed CSP1 (34/36 kDa) in granular fractions; the membrane and cytosol fractions contained predominantly CSP2 (27 kDa). The fractions also contained proteins of 72 and 70 kDa, presumably CSP dimers. CSP1 overexpression in INS-1 cells or intracellular administration of CSP antibodies into mouse ob/ob beta-cells did not affect voltage-dependent Ca2+-channel activity. Amperometric measurements showed a significant decrease in insulin exocytosis in individual INS-1 cells after CSP1 overexpression. We conclude that CSP is associated with insulin secretory granules and that CSP participates in the molecular regulation of insulin exocytosis by mechanisms not involving changes in the activity of voltage-gated Ca2+-channels.

Insulin-like growth factor I (IGF-I)-stimulated pancreatic beta-cell growth is glucose-dependent. Synergistic activation of insulin receptor substrate-mediated signal transduction pathways by glucose and IGF-I in INS-1 cells

Nutrients and certain growth factors stimulate pancreatic beta-cell mitogenesis, however, the appropriate mitogenic signal transduction pathways have not been defined. In the glucose-sensitive pancreatic beta-cell line, INS-1, it was found that glucose (6-18 mM) independently increased INS-1 cell proliferation (>20-fold at 15 mM glucose). Insulin-like growth factor I (IGF-I)-induced INS-1 cell proliferation was glucose-dependent only in the physiologically relevant concentration range (6-18 mM glucose). The combination of IGF-I and glucose was synergistic, increasing INS-1 cell proliferation >50-fold at 15 mM glucose + 10 nM IGF-I. Glucose metabolism and phosphatidylinositol 3'-kinase (PI 3'-kinase) activation were necessary for both glucose and IGF-I-stimulated INS-1 cell proliferation. IGF-I and 15 mM glucose increased tyrosine phosphorylation mediated recruitment of Grb2/mSOS and PI 3'-kinase to IRS-2 and pp60. Glucose and IGF-I also induced Shc association with Grb2/mSOS. Glucose (3-18 mM) and IGF-I, independently of glucose, activated mitogen-activated protein kinase but this did not correlate with IGF-I-induced beta-cell proliferation. In contrast, p70(S6K) was activated with increasing glucose concentration (between 6 and 18 mM), and potentiated by IGF-I in the same glucose concentration range which correlated with INS-1 cell proliferation rate. Thus, glucose and IGF-I-induced beta-cell proliferation were mediated via a signaling mechanism that was facilitated by mitogen-activated protein kinase but dependent on IRS-mediated induction of PI 3'-kinase activity and downstream activation of p70(S6K). The glucose dependence of IGF-I mediated INS-1 cell proliferation emphasizes beta-cell signaling mechanisms are rather unique in being tightly linked to glycolytic metabolic flux.

A specific increased expression of insulin receptor substrate 2 in pancreatic beta-cell lines is involved in mediating serum-stimulated beta-cell growth

Certain nutrients and growth factors can stimulate pancreatic beta-cell growth. However, the appropriate mitogenic signaling pathways in beta-cells have been relatively undefined. In this study, differential gene expression in NEDH rat insulinoma was compared with NEDH rat primary islet beta-cells. Differential mRNA display analysis revealed an elevated expression in insulinoma of VL30 transposons, S24 ribosomal protein, and cytochrome-C oxidaseVIIc that is typical for cells undergoing mitosis. A gene candidate approach revealed that mRNA levels of the oncogenes c-fos and c-jun were equivalently expressed in insulinoma and islet cells, as was the mRNA for the mitogenic signal transduction molecule insulin receptor substrate (IRS)-1. However, in contrast to that of IRS-1, IRS-2 gene expression was 60- to 70-fold higher in the insulinoma tissue compared with islets, which was reflected at the protein as well as the mRNA level. The specific elevated IRS-2 expression was a consistent observation across all rodent pancreatic beta-cell lines. To investigate whether IRS-2 was functional, serum-stimulated beta-cell proliferation was examined in isolated insulinoma cells. After a 48-h period of serum withdrawal, 24 h of serum refeeding rendered an 8- to 10-fold increase in [3H]thymidine incorporation into insulinoma cells. This serum-stimulated DNA synthesis was prevented by inhibitors of tyrosine protein kinase and phosphatidylinositol (PI) 3-kinase activities, as well as the activation of mitogen-activated protein (MAP) kinase and p70S6K. Examination of IRS-mediated signal transduction pathways indicated that after 10-15 min of serum refeeding, there was increased tyrosine phosphorylation of IRS-2 and pp60, and PI 3-kinase recruitment to IRS-2. Serum also increased the association of growth factor-bound protein 2/murine sons of sevenless 1 protein to a PI 3-kinase/IRS-2 protein complex. Moreover, serum also activated MAP-kinase (erk-1 and erk-2 isoforms) and 70 kD S6 kinase. Thus IRS-mediated signal transduction pathways are functional in pancreatic beta-cells. It is conceivable that IRS-2 expression in beta-cells contributes to maintaining the islet beta-cell population, complementary to observations in the IRS-2 knockout mouse in which beta-cell mass is markedly reduced.

Rabies in Zimbabwe: reservoir dogs and the implications for disease control

Using detailed field study observations of the side-striped jackal (Canis adustus) and a simple stochastic model of the transmission dynamics of the virus and host demography, we discuss the epidemiology of rabies virus infection in the jackal population of Zimbabwe. Of the two jackal species in Zimbabwe, the other being the black-backed jackal (Canis mesomelas), the bulk of notified rabies cases are in side-striped jackals. Specifically, we show that the side-striped jackal population itself does not seem able to support rabies infection endemically, i.e. without frequent reintroduction from outside sources of infection. We argue that this is probably because the overall average jackal population density is too low to maintain the chain of infection. This study suggests that the disease is regularly introduced to jackals by rabid dogs from populations associated with human settlements. Given the rapidly rising dog population in Zimbabwe, estimates are derived of the future incidence of jackal rabies based on different dog-vaccination scenarios.

A distinct difference in the metabolic stimulus-response coupling pathways for regulating proinsulin biosynthesis and insulin secretion that lies at the level of a requirement for fatty acyl moieties

The regulation of proinsulin biosynthesis in pancreatic beta-cells is vital for maintaining optimal insulin stores for glucose-induced insulin release. The majority of nutrient fuels that induce insulin release also stimulate proinsulin biosynthesis, but since insulin exocytosis and proinsulin synthesis involve different cellular mechanisms, a point of divergence in the respective metabolic stimulus-response coupling pathways must exist. A parallel examination of the metabolic regulation of proinsulin biosynthesis and insulin secretion was undertaken in the same beta-cells. In MIN6 cells, glucose-induced proinsulin biosynthesis and insulin release shared a requirement for glycolysis to generate stimulus-coupling signals. Pyruvate stimulated both proinsulin synthesis (threshold 0.13-0.2 mM) and insulin release (threshold 0.2-0.3 mM) in MIN6 cells, which was eliminated by an inhibitor of pyruvate transport (1 mM alpha-cyano-4-hydroxycinnamate). A combination of alpha-oxoisohexanoate and glutamine also stimulated proinsulin biosynthesis and insulin release in MIN6 cells, which, together with the effect of pyruvate, indicated that anaplerosis was necessary for instigating secondary metabolic stimulus-coupling signals in the beta-cell. A consequence of increased anaplerosis in beta-cells is a marked increase in malonyl-CoA, which in turn inhibits beta-oxidation and elevates cytosolic fatty acyl-CoA levels. In the beta-cell, long-chain fatty acyl moieties have been strongly implicated as metabolic stimulus-coupling signals for regulating insulin exocytosis. Indeed, it was found in MIN6 cells and isolated rat pancreatic islets that exogenous oleate, palmitate and 2-bromopalmitate all markedly potentiated glucose-induced insulin release. However, in the very same beta-cells, these fatty acids in contrast inhibited glucose-induced proinsulin biosynthesis. This implies that neither fatty acyl moieties nor beta-oxidation are required for the metabolic stimulus-response coupling pathway specific for proinsulin biosynthesis, and represent an early point of divergence of the two signalling pathways for metabolic regulation of proinsulin biosynthesis and insulin release. Therefore alternative metabolic stimulus-coupling factors for the specific control of proinsulin biosynthesis at the translational level were considered. One possibility examined was an increase in glycerophosphate shuttle activity and change in cytosolic redox state of the beta-cell, as reflected by changes in the ratio of alpha-glycerophosphate to dihydroxyacetone phosphate. Although 16.7 mM glucose produced a significant rise in the alpha-glycerophosphate/dihydroxyacetone phosphate ratio, 1 mM pyruvate did not. It follows that the cytosolic redox state and fatty acyl moieties are not necessarily involved as secondary metabolic stimulus-coupling factors for regulation of proinsulin biosynthesis. However, the results indicate that glycolysis and the subsequent increase in anaplerosis are indeed necessary for this signalling pathway, and therefore an extramitochondrial product of beta-cell pyruvate metabolism (that is upstream of the increased cytosolic fatty acyl-CoA) acts as a key intracellular secondary signal for specific control of proinsulin biosynthesis by glucose at the level of translation.

Chronic exposure to free fatty acid reduces pancreatic beta cell insulin content by increasing basal insulin secretion that is not compensated for by a corresponding increase in proinsulin biosynthesis translation

The pancreatic beta cell normally maintains a stable balance among insulin secretion, insulin production, and insulin degradation to keep optimal intracellular stores of the hormone. Elevated levels of FFA markedly enhance insulin secretion; however, the effects of FFA on insulin production and intracellular stores remain unclear. In this study, twofold elevation in total circulating FFA effected by infusion of lard oil and heparin into rats for 6 h under normoglycemic conditions resulted in a marked elevation of circulating insulin levels evident after 4 h, and a 30% decrease in pancreatic insulin content after a 6-h infusion in vivo. Adding 125 muM oleate to isolated rat pancreatic islets cultured with 5.6 mM glucose caused a 50% fall in their insulin content over 24 h, coupled with a marked enhancement of basal insulin secretion. Both effects of fatty acid were blocked by somatostatin. In contrast to the stimulatory effects of oleate on insulin secretion, glucose-induced proinsulin biosynthesis was inhibited by oleate up to 24 h, but was unaffected thereafter. This result was in spite of a two- to threefold oleate-induced increase in preproinsulin mRNA levels, underscoring the importance of translational regulation of proinsulin biosynthesis in maintaining beta cell insulin stores. Collectively, these results suggest that chronically elevated FFA contribute to beta cell dysfunction in the pathogenesis of NIDDM by significantly increasing the basal rate of insulin secretion. This increase in turn results in a decrease in the beta cell's intracellular stores that cannot be offset by commensurate FFA induction of proinsulin biosynthesis.

Epidemiology of communicable disease in small populations

We have recently shown how the outbreaks of measles virus infection in small isolated island communities exhibit an organised pattern in the distribution of epidemic sizes and distribution of epidemic durations. More conventional epidemiological analysis tends not to be useful in such highly intermittent dynamical regimes where there is frequent fade-out of infection. In this paper we discuss how our approach can be applied to an analysis of measles epidemics in highly vaccinated communities where susceptibles build up due to lack of vaccine uptake and also because of occasional vaccine failure.

On the critical behaviour of simple epidemics

We show how ideas and models which were originally introduced to gain an understanding of critical phenomena can be used to interpret the dynamics of epidemics of communicable disease in real populations. Specifically, we present an analysis of the dynamics of disease outbreaks for three common communicable infections from a small isolated island population. The strongly fluctuating nature of the temporal incidence of disease is captured by the model, and comparisons between exponents calculated from the data and from simulations are made. A forest-fire model with sparks is used to classify the observed scaling dynamics of the epidemics and provides a unified picture of the epidemiology which conventional epidemiological analysis is unable to reproduce. This study suggests that power-law scaling can emerge in natural systems when they are driven on widely separated time-scales, in accordance with recent analytic renormalization group calculations.