Glucagon-like peptide-1 stimulates human insulin promoter activity in part through cAMP-responsive elements that lie upstream and downstream of the transcription start site

Glucagon-like peptide-1 (GLP-1) is a peptide hormone secreted from the enteroendocrine L-cells of the gut and which acts primarily to potentiate the effects of glucose on insulin secretion from pancreatic beta-cells. It also stimulates insulin gene expression, proinsulin biosynthesis and affects the growth and differentiation of the islets of Langerhans. Previous studies on the mechanisms whereby GLP-1 regulates insulin gene transcription have focused on the rat insulin promoter. The aim of this study was to determine whether the human insulin promoter was also responsive to GLP-1, and if so to investigate the possible role of cAMP-responsive elements (CREs) that lie upstream (CRE1 and CRE2) and downstream (CRE3 and CRE4) of the transcription start site. INS-1 pancreatic beta-cells were transfected with promoter constructs containing fragments of the insulin gene promoter placed upstream of the firefly luciferase reporter gene. GLP-1 was found to stimulate the human insulin promoter, albeit to a lesser degree than the rat insulin promoter. Mutagenesis of CRE2, CRE3 and CRE4 blocked the stimulatory effect of GLP-1 while mutagenesis of CRE1 had no effect. Analysis of nuclear protein binding to the four CREs showed that, while they share some proteins, each CRE site is unique. Stimulation of transcription by GLP-1 through CRE2, CRE3 and CRE4 resulted in altered protein binding that was different for each of the CRE sites involved. Collectively, these data show that the four human CREs are not simply multiple copies of the rat CRE site and further emphasise that the human insulin promoter is distinct from the rodent promoter.

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.

The orphan G protein-coupled receptor GPR40 is activated by medium and long chain fatty acids

GPR40 is a member of a subfamily of homologous G protein-coupled receptors that include GPR41 and GPR43 and that have no current function or ligand ascribed. Ligand fishing experiments in HEK293 cells expressing human GPR40 revealed that a range of saturated and unsaturated carboxylic acids with carbon chain lengths greater than six were able to induce an elevation of [Ca(2+)](i), measured using a fluorometric imaging plate reader. 5,8,11-Eicosatriynoic acid was the most potent fatty acid tested, with a pEC(50) of 5.7. G protein coupling of GPR40 was examined in Chinese hamster ovary cells expressing the G alpha(q/i)-responsive Gal4-Elk1 reporter system. Expression of human GPR40 led to a constitutive induction of luciferase activity, which was further increased by exposure of the cells to eicosatriynoic acid. Neither the constitutive nor ligand-mediated luciferase induction was inhibited by pertussis toxin treatment, suggesting that GPR40 was coupled to G alpha(q/11.) Expression analysis by quantitative reverse transcription-PCR showed that GPR40 was specifically expressed in brain and pancreas, with expression in rodent pancreas being localized to insulin-producing beta-cells. These data suggest that some of the physiological effects of fatty acids in pancreatic islets and brain may be mediated through a cell-surface receptor.

Insulin sensitisation in the treatment of Type 2 diabetes

Type 2 diabetes is reaching epidemic proportions worldwide, fueled by the increasing prevalence of obesity as many populations adopt a western lifestyle. Secondary complications affecting both the microvascular and macrovascular systems are responsible for premature mortality in Type 2 diabetes, with two thirds or more dying of cardiovascular disease. Two interacting metabolic defects, insulin resistance and beta-cell dysfunction are present in Type 2 diabetes. It is now recognised that insulin resistance is central to a cluster of metabolic abnormalities--called the insulin resistance syndrome--that are responsible for the excess of cardiovascular disease. Older antidiabetic agents such as the sulfonylureas, metformin and insulin are more effective than lifestyle modification in reducing microvascular complications of Type 2 diabetes, but overall do not reduce cardiovascular risk. Metformin, although no more effective as a glucose-lowering agent than sulfonylureas or insulin, does significantly reduce cardiovascular disease, probably as a result of its weak insulin-sensitising action. The newly-marketed thiazolidinedione insulin-sensitising antidiabetic agents also improve multiple biomarkers of cardiovascular risk, suggesting that novel approaches to insulin sensitisation will not only provide effective long-term glycaemic control but improve cardiovascular outcomes in Type 2 diabetes. Multiple therapeutic targets within the insulin signalling cascade are being explored, together with follow-up compounds to the first generation thiazolidinediones. These initiatives, together with developments in beta(3)-adrenoceptor agonists, 11 beta-hydroxysteroid dehydrogenase Type 1 inhibitors and modulators of the glucagon-like peptide 1 axis, all of which also potentially enhance insulin sensitivity, are critically evaluated.

Reproducibility of targeted gene expression measurements in human islets of Langerhans

The expression of 47 genes involved in the biosynthesis and secretion of insulin, apoptosis, and cellular stress was evaluated in isolated human islets using cDNA probes arrayed on nitrocellulose membranes. Isolated human islets were cultured for four days, or one month, with glucose present at a concentration of either 5.5 or 16.7 mmol/L. Extracted islet total RNA was used to generate [32P]dATP-labelled complex cDNA targets and hybridised with immobilised cDNA arrays. The positive expression of 45 mRNA transcripts in isolated human islets was documented. The coefficient of variance for relative levels of expression of transcripts was <25% for 9, 25-50% for 22, and 50-100% for 10, indicating good reproducibility between islet preparations from five different human pancreas donors. This study demonstrates the utility of nitrocellulose-based cDNA arrays for a focused reproducible analysis of gene expression changes in human islets of Langerhans.

Anorectic, thermogenic and anti-obesity activity of a selective orexin-1 receptor antagonist in ob/ob mice

A single dose of the orexin-1 (OX1) receptor antagonist 1-(2-methylbenzoxazol-6-yl)-3-[1,5] naphthyridin-4-yl urea hydrochloride (SB-334867-A) reduces orexin-A-induced feeding and natural feeding in Sprague Dawley rats. In this study, the anti-obesity effects of SB-334867-A were determined in genetically obese (ob/ob) mice dosed with SB-334867-A (30 mg/kg, i.p.) once daily for 7 days, and then twice daily for a further 7 days. SB-334867-A reduced cumulative food intake and body weight gain over 14 days. Total fat mass gain, determined by Dual Emission X-ray Absorptiometry, was reduced, while gain in fat-free mass was unchanged. Fasting (5 h) blood glucose was also reduced at the end of the study, with a trend to reduced plasma insulin. Interscapular brown adipose tissue (BAT) weight was reduced, the tissue was noticeably darker in colour and quantitative PCR (TaqMan) analysis of this tissue showed a trend to an increase in uncoupling protein-1 mRNA expression, suggesting that SB-334867-A might stimulate thermogenesis. This was confirmed in a separate study in which a single dose of SB-334867-A (30 mg/kg, i.p.) increased metabolic rate over 4 h in ob/ob mice. OX1 receptor mRNA was detected in BAT, and its expression was increased by 58% by treatment with SB-334867-A. This is the first demonstration that OX1 receptor antagonists have potential as both anti-obesity and anti-diabetic agents.

Effects of chronic murine and human leptin infusion on plasma leptin and corticosterone levels and energy balance in lean Zucker rats

AIM

To clarify whether centrally delivered leptin can access the circulation and to determine to what extent the effects of i.c.v. h-leptin and m-leptin on body weight and plasma corticosterone are due to reduced food intake.

METHODS

Male lean Zucker rats were infused i.c.v. with recombinant m-leptin or h-leptin (42 microg/day) for 7 days. Terminal plasma leptin levels were measured using selective r-leptin, m-leptin and h-leptin RIA. Plasma h-leptin and corticosterone levels were determined on days 0, 2, 4 and 6 of h-leptin infusion. Interscapular brown adipose tissue weight and UCP-1 mRNA expression (an indicator of thermogenic capacity) were also measured.

RESULTS

The terminal plasma leptin level was elevated (from 2.2 +/- 0.4 to 42.7 +/- 20.2 ng/ml) in the h-leptin-treated lean rats to levels similar to those in vehicle i.c.v. infused fa/fa rats (72.2 +/- 4.7 ng/ml), but this was only detectable when the h-leptin radioimmunoabsorbent assay (RIA) was used. Further, both m-leptin and h-leptin infusions in lean rats elevated terminal plasma corticosterone (352 +/- 37 and 389 +/- 55 ng/ml, respectively) to levels similar to those in i.c.v. rats (386 +/- 62 ng/ml), whereas diet-restriction by pair-feeding, with the h-leptin group, in lean rats had no effect (207 +/- 45 ng/ml). The increase in plasma corticosterone level coincided with the maximum hypophagic effects of leptin and preceded the appearance and sustained elevation of exogenous human leptin in the circulation. Both m-leptin and h-leptin i.c.v. infusion reduced body weight gain (3% and 4%, respectively, compared to pair-fed group) and increased UCP-1 expression (11-fold and 16-fold, respectively) in lean rats. However, h-leptin elicited an earlier effect than m-leptin on body weight, manifested as an earlier reduction in food intake and greater increase in UCP-1 expression. h-Leptin also elicited a greater reduction in body weight gain than did pair-feeding.

CONCLUSIONS

Intracerebroventricular-infused m-leptin or h-leptin was detected in the circulation. Furthermore, m-leptin and h-leptin elevated plasma corticosterone levels and h-leptin caused some weight loss in lean rats independently of its suppression of food intake. The elevation of corticosterone levels in the lean rats may be a mechanism whereby they resist excessive weight loss in response to leptin.

Leptin receptor long-form signalling in a human liver cell line

Expression of the long form of the leptin receptor was detected by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blotting in the human liver cell line WRL68. Leptin (50-200 nM) significantly increased tyrosine phosphorylation of STAT cytoplasmic transcription factors STAT3 and STAT5b in a dose-dependent manner and produced a gel-shift with STAT3- and STAT5-specific oligonucleotides. WRL68 cells therefore provide the first human in vitro hepatocyte system in which to study leptin receptor-mediated signalling and to elucidate the role of leptin in liver.

Fatty acids inhibit leptin signalling in BRIN-BD11 insulinoma cells

The effect of treatment with a 0.03% fatty acid (FA) cocktail on leptin-receptor-mediated STAT (signal transducers and activators of transcription) activation in the rat insulinoma cell line BRIN-BD11 was investigated. Leptin (10 nM) stimulated the tyrosine phosphorylation of STAT3 and STAT5b. Acute treatment with FAs prevented leptin-stimulated STAT3 tyrosine phosphorylation and significantly raised basal STAT5 phosphorylation. A chronic treatment (5 days) of BRIN-BD11 cells with FAs similarly attenuated leptin-stimulated STAT tyrosine phosphorylation. Chronic FA treatment also attenuated prolactin-stimulated STAT5b tyrosine phosphorylation but not interleukin-6-stimulated STAT3 tyrosine phosphorylation, suggesting that the effect is receptor/ligand specific. TaqMan analysis of gene expression following chronic FA treatment showed neither a decrease in the amount of leptin receptor (Ob-R) mRNA, nor an increase in the negative regulators of STAT signalling, SOCS3 (suppressors of cytokine signalling) or cytokine inducible sequence (CIS). These data demonstrate that FAs modulate leptin and prolactin signalling in beta-cells, implying that high levels of circulating FAs present in obese individuals affect the action of selective cytokines in beta-cell function.

Anti-obesity drugs: a critical review of current therapies and future opportunities

The last 25 years have seen a great increase in the incidence of obesity, both in the Western world and in developing third world countries. Despite the seeming inexorable progression of this disease, there have been limited advances in the pharmacotherapy of this condition. Of the newest introductions to the obesity drug portfolio, orlistat, which acts to prevent dietary fat absorption, and sibutramine, which seems to affect both arms of the energy balance equation, were the first new chemical entities to be introduced for the treatment of obesity in 30 years. In this article, we review these and other agents available in various countries for the treatment of obesity. Perhaps more importantly, we have focussed on areas of potential productivity in the future. The huge recent increase in our knowledge in this area has largely stemmed from discovery research at the genomics level. Over the last 5 or so years, this impetus in obesity research has provided us with exciting new drug targets involved in the regulation of feeding behaviour and cellular mechanisms involved in energy expenditure. Compared with the last 25 years, the future offers more hope.

A selective orexin-1 receptor antagonist reduces food consumption in male and female rats

A variety of evidence implicates the orexins, especially orexin-A, in the regulation of food intake, but it has not been established whether this effect is mediated by the orexin-1 or orexin-2 receptor. In the present study, a selective orexin-1 receptor antagonist, 1-(2-methylbenzoxazol-6-yl)-3-[1,5]naphthyridin-4-yl urea hydrochloride (SB-334867-A), was administered intraperitoneally to rats under various conditions, and food consumption was subsequently measured over 24 h. In male rats, a single dose of SB-334867-A (30 mg/kg, i.p.) given during the light phase reduced both orexin-A-induced food intake (7 nmol, i.c.v.) and feeding stimulated by an overnight fast for 4 h. When given at the start of the dark phase, food consumption was reduced in both male and female rats over 24 h. Daily injections at the start of the dark phase for 3 days reduced natural feeding in male rats over 24 h on days one and three. These findings demonstrate direct inhibition of orexin-A induced food intake with a selective orexin-1 receptor antagonist. Furthermore, the suppression of nocturnal feeding and food intake stimulated by an overnight fast supports other evidence that orexin-A is involved in the regulation of natural feeding and suggests that orexin-1 receptor antagonists could be useful in the treatment of obesity.

Expression of melanin-concentrating hormone receptors in insulin-producing cells: MCH stimulates insulin release in RINm5F and CRI-G1 cell-lines

Melanin-concentrating hormone (MCH) is a hypothalamic orexigenic peptide. Recently, an orphan G-protein-coupled receptor (SLC-1) was identified that binds MCH with high affinity. Here, we demonstrate the mRNA expression of this receptor in insulin-producing cells including CRI-G1 and RINm5F cells, and in rat islets of Langerhans. Immunofluorescence studies in CRI-G1 and RINm5F cell-lines demonstrated cell-surface expression of the receptor. Rat MCH significantly stimulated insulin secretion in both cell-lines. The potency and the efficacy of MCH were significantly increased in the simultaneous presence of forskolin, suggesting that MCH may amplify the insulinotropic effect of cyclic AMP elevating stimuli. Salmon MCH, which differs from rat/human MCH by six amino acids, was less efficacious than rat/human MCH in stimulating insulin release. The data provide evidence for the expression of MCH receptors in insulin producing cells. The insulinotropic effect of MCH may contribute to the regulation of metabolism and energy balance by this peptide.

Insulin stimulates pancreatic-duodenal homoeobox factor-1 (PDX1) DNA-binding activity and insulin promoter activity in pancreatic beta cells

Pancreatic-duodenal homoeobox factor-1 (PDX1) is a homoeodomain transcription factor that plays an important role in linking glucose metabolism in pancreatic beta cells to the regulation of insulin gene transcription. Our previous results indicated that glucose activates PDX1 DNA-binding activity and insulin promoter activity via a stress-activated signalling pathway involving phosphatidylinositol 3-kinase (PtdIns 3-kinase) and stress-activated protein kinase 2 (SAPK2/p38). The present study was undertaken to determine the effects of other metabolizable and non-metabolizable nutrients. The results indicate that non-metabolizable nutrients, with the exception of 2-deoxyglucose, had no effect. Metabolizable nutrients that could stimulate calcium uptake and insulin release were shown to activate both PDX1 and the insulin promoter. The possible role of insulin acting via an autoregulatory loop was therefore examined. Insulin was shown to potently activate PDX1 DNA-binding activity and insulin promoter activity. The effects of insulin were inhibited by the PtdIns 3-kinase inhibitors wortmannin and LY294002 and by the SAPK2 inhibitor SB203580, suggesting that its effects were mediated via activation of PtdIns 3-kinase and SAPK2. Further support for the insulin-mediated activation of SAPK2 came from the observation that both glucose and insulin stimulated the phosphorylation of SAPK2. These results suggest that both glucose and insulin stimulate PDX1 DNA-binding activity and insulin promoter activity via a pathway involving PtdIns 3-kinase and SAPK2.

Therapeutic index for rosiglitazone in dietary obese rats: separation of efficacy and haemodilution

1. The blood glucose-lowering efficacy of rosiglitazone (RSG) and the mechanisms of associated weight gain were determined in dietary obese rats (DIOs). DIO and chow-fed rats received RSG 0.3-30 mg kg-1 daily for 21 days. 2. In DIOs, plasma glucose and insulin concentrations were reduced by RSG at dosages of 3 and 10 mg kg-1, respectively. Homeostasis model assessment (HOMA) indicated the threshold for a reduction of insulin resistance was 1 mg kg-1. Neither glucose nor insulin levels were affected by treatment in chow-fed rats. 3. RSG 0.3 mg kg-1 lowered free fatty acids (FFAs) in DIOs, whereas for plasma triglycerides (TGs), the threshold was 3 mg kg-1. By contrast, the threshold for reducing packed red cell volume (PCV) and increasing cardiac mass was 10 mg kg-1. Thus, the therapeutic index for RSG in DIOs was >3 and < or = 10. 4. Energy intake and weight gain increased in treated DIOs (by 20% and 50 g, at 30 mg kg-1) and chow-fed rats (by 25% and 35 g, at 30 mg kg-1). In DIOs, these increases coincided with falls in plasma leptin (40% lower at 30 mg kg-1) and insulin (43% lower at 30 mg kg-1). By contrast, in chow-fed rats, weight gain and hyperphagia occurred without changes in either leptin or insulin. However, reductions in FFAs below 0.4 - 0.3 mM were associated with hyperphagia and weight gain in DIO and chow-fed rats. 5. We conclude that increased energy intake and body weight did not attenuate the improved metabolism evoked by RSG in DIO rats, and that insulin action was enhanced at a dose >3 fold below the threshold for causing haemodilution and cardiac hypertrophy in DIO rats.

Hypothalamic orexin expression: modulation by blood glucose and feeding

Orexins (hypocretins), novel peptides expressed in specific neurons of the lateral hypothalamic area (LHA), stimulate feeding when injected intracerebroventricularly. We investigated their role in feeding in the rat by measuring hypothalamic prepro-orexin mRNA levels under contrasting conditions of increased hunger. Prepro-orexin mRNA levels increased significantly after 48 h of fasting (by 90-170%; P < 0.05) and after acute (6 h) hypoglycemia when food was withheld (by 90%; P < 0.02). By contrast, levels were unchanged during chronic food restriction, streptozotocin-induced diabetes, hypoglycemia when food was available, voluntary overconsumption of palatable food, or glucoprivation induced by systemic 2-deoxy-D-glucose. Orexin expression was not obviously related to changes in body weight, insulin, or leptin, but was stimulated under conditions of low plasma glucose in the absence of food. Orexins may participate in the short-term regulation of energy homeostasis by initiating feeding in response to falls in glucose and terminating it after food ingestion. The LHA is known to contain neurons that are stimulated by falls in circulating glucose but inhibited by feeding-related signals from the viscera; orexin neurons may correspond to this neuronal population.

Effects of single and chronic intracerebroventricular administration of the orexins on feeding in the rat

Two novel hypothalamic neuropeptides, orexin-A and -B, are suggested to regulate feeding. A single intracerebroventricular injection of orexin-A (23.4 nmol), administered 3 h into the light phase, increased feeding in satiated rats and prolonged feeding in fasted rats; it also increased feeding when given 6 h into, but not at the start of, the dark phase. An 8-day intracerebroventricular infusion with orexin-A (18 nmol/day) increased daytime feeding on days 2 and 8, but nocturnal feeding was reduced and there was no change in 24 h intake. Orexin-B had no effects. These results demonstrate a circadian variation in feeding responses to orexin-A.

Hypothalamic NPY status during positive energy balance and the effects of the NPY antagonist, BW 1229U91, on the consumption of highly palatable energy-rich diet

We have studied the hypothalamic activity of the neuropeptide Y (NPY) system in dietary-induced obese male Wistar rats and examined whether the NPY antagonist, BW1229U91, can inhibit the hyperphagia during positive energy balance associated with feeding rats an energy-rich, highly palatable diet. Rats given a highly palatable, high-fat diet became obese after 8 weeks and exhibited hyperinsulinemia and hyperleptinemia, as compared to lean rats fed on standard pellet laboratory diet. Hypothalamic NPY mRNA concentrations were significantly reduced by approximately 70% in dietary-obese rats compared with lean controls, and the former were hypersensitive to intracerebroventricular injections of NPY, possibly as a result of NPY receptor up-regulation. Intracerebroventricular injections of BW 1229U91, that inhibits food intake in starved rats, did not alter food intake in either control or obese rats fed either standard pellet diet or the highly palatable diet, respectively. We conclude that dietary-obese rats have underactive hypothalamic NPYergic neurons compared to lean controls, possibly as a result of increased plasma concentrations of leptin and/or insulin that directly inhibit the NPY neuronal activity. The lack of effect of BW1229U91 on the increased caloric intake of dietary-obese rats suggests that the hyperphagia is not NPY-driven and supports the data indicating reduced synaptic activity of the hypothalamic NPY system.

Lactation suppresses diurnal rhythm of serum leptin

Rats consume most of their daily food intake at night; serum leptin levels and adipose tissue leptin mRNA content are elevated at night in non-lactating rats fed ad libitum. Lactation induces massive hyperphagia with most food still consumed at night, but the nocturnal increase in leptin secretion was not observed in lactating rats. Thus the link between nocturnal food intake and increased serum leptin is broken during lactation and the hypoleptinaemia may be an important factor promoting the hyperphagia of lactation.

Acute hyperleptinemia does not modify insulin sensitivity in vivo in the rat

Insulin resistance is associated with hyperleptinemia, whilst exposure of hepatoma cells and isolated adipocytes to high concentrations of leptin has been demonstrated to result in attenuated insulin response and a reduced suppression of gluconeogenesis. To determine the acute metabolic effects of hyperleptinemia, we measured whole body glucose uptake (WBU) and hepatic glucose production rate (HGP) in rats using the euglycemic hyperinsulinemic clamping technique. Anesthetised male rats received recombinant murine leptin (1 microg/min) or vehicle into the jugular vein for 90 min. After 30 min of leptin infusion, insulin was infused to a level of 70 microU/ml and a variable-rate glucose infusion was adjusted to maintain blood glucose levels to 4-4.5 mmol/l. Glucose infusion rates during clamping were not different between leptin-infused and control rats, and there were no significant effects on the HPR or WBU measured using [6-(3)H]glucose under basal or clamped conditions. In summary, our data demonstrate that acute hyperleptinemia in normal weight Wistar rats does not appear to reduce insulin sensitivity, in vivo, or to affect HPR under clamp conditions.

ob gene expression and secretion of leptin following differentiation of rat preadipocytes to adipocytes in primary culture

Expression of the ob gene and production of leptin have been examined on differentiation of rat fibroblastic preadipocytes to adipocytes in primary culture. Preadipocytes were obtained from the inguinal fat pad of suckling rats, and following differentiation the cells contained lipid droplets and the mRNAs for both lipoprotein lipase and adipsin were detected by Northern blotting. ob mRNA was not, however, detected on Northern blots, but analysis by RT-PCR indicated that the ob gene was expressed, particularly after differentiation. Measurement of leptin in the culture medium by ELISA showed that the ob gene product was secreted by adipocytes from approximately 4 days after the induction of differentiation. Leptin production was sustained over a 2-week period with a peak at 8-10 days post-induction. Dexamethasone stimulated leptin production, while an inhibition was observed with the beta-adrenoceptor agonist isoprenaline. These results demonstrate that following the differentiation of fibroblastic preadipocytes to adipocytes in primary culture, leptin is secreted with the cells responding to stimuli which regulate production of the hormone.

The thiazolidinedione insulin sensitiser, BRL 49653, increases the expression of PPAR-gamma and aP2 in adipose tissue of high-fat-fed rats

The effects of the thiazolidinedione insulin sensitiser BRL 49653 on plasma leptin concentrations and on epididymal fat OB, PPAR-gamma and aP2 mRNA expression were examined in high-fat-fed and high-carbohydrate-fed adult Wistar rats. Diets were given for 4 weeks, with BRL 49653 (10 micromol/kg/day) administered by oral gavage for the last 4 days. Treatment with BRL 49653 reduced plasma leptin concentrations in high-fat-fed rats from 2.34 +/- 0.19 (n=9) to 1.42 +/- 0.09 (n=9) ng/ml (p<0.001). Plasma leptin was unaffected by BRL 49653 in the high-carbohydrate-fed rats. There was no difference in OB mRNA expression between high-fat-fed and high-carbohydrate-fed rats, with or without treatment. PPAR-gamma and aP2 mRNA expression were significantly increased in the high-fat-fed rats treated with BRL 49653 (p < 0.01 and p < 0.001 respectively), but not in carbohydrate-fed rats.

Molecular characterization of erythrocyte glucose-6-phosphate dehydrogenase deficiency in Al-Ain District, United Arab Emirates

In a cross-sectional study, the activity, electrophoretic mobility and genotypes of glucose-6-phosphate dehydrogenase (G6PD) were determined among healthy, UAE national school boys from Al-Ain District in the United Arab Emirates, The prevalence of G6PD deficiency in this population sample was 11%. The majority of G6PD-deficient subjects were descendants of Omani, Baluchi or Yemeni migrants. Of 18 deficient subjects, 16 had an enzyme activity of < 10% of normal while 2 had an activity of just above 10%. Electrophoresis was performed on 166 samples and showed that, apart from deficient samples, all had the normal mobility of G6PD type B. Of the 18 deficient subjects, 14 had the B type mobility of G6PD Mediterranean and 4 had the A type mobility of G6PD A-. Genotyping demonstrated that 10 had the Mediterranean mutation while 3 had the A- mutation, consistent with their electrophoretic mobility. Another 3 had the G6PD Aures mutation, recently described as polymorphic in Algeria and Spain. The mutations in the remaining 2 subjects have not yet been identified.

Cleavable signal peptides are rarely found in bacterial cytoplasmic membrane proteins (review)

Most proteins destined for secretion are synthesized with amino-terminal extensions, known as signal peptides, which play a vital role in their translocation across the membrane bordering the cytoplasm. Following translocation across the eukaryotic endoplasmic reticulum (ER) membrane or the bacterial cytoplasmic membrane, signal peptides are proteolytically removed from the preproteins. The process of membrane protein assembly can be likened to that of protein export in that it involves the translocation of portions of proteins across membranes. Moreover, the topological similarities between eukaryotic ER and plasma membrane proteins and bacterial cytoplasmic membrane proteins suggest that the mechanisms of membrane protein assembly may, like those of protein export, share fundamental similarities in eukaryotic and bacterial cells. However, whilst many of the ER and plasma membrane proteins of higher eukaryotes are synthesized with cleavable signal peptides, the same is true of only very few bacterial cytoplasmic membrane proteins. This fact is not widely appreciated, probably because certain exceptional (signal peptide-containing) bacterial membrane proteins, such as the major coat protein of bacteriophage M13, have been the subject of extensive investigations. In this review we highlight this anomaly and discuss it within the general context of membrane protein topology.

Pharmacological interference with phospholipase A2 activity reveals mechanistic differences between glucose and glyceraldehyde induced insulin release: implication for coupling of glucose metabolism to phospholipase A2 activity

Prostaglandin E2 levels in isolated rat islets were increased from 64 +/- 11 pg/30 islets when incubated in medium containing 2 mM glucose to 115 +/- 9 pg/30 islets in medium containing 20 mM glucose. In contrast, glyceraldehyde (10 mM) reduced prostaglandin E2 levels to 29 +/- 6 pg/30 islets. Inhibition of glucose metabolism by mannoheptulose (10 mM) abolished the stimulatory effect of glucose on prostaglandin E2 levels and inhibited glucose-induced insulin release. The cyclooxygenase inhibitor, flurbiprofen (20 microM), did not affect insulin release caused by glucose or glyceraldehyde. In the presence of 1 mg/ml bovine serum albumin, insulin secretion induced by 20 mM glucose (6.9 +/- 1.1% of islet insulin content) was reduced by the lipoxygenase inhibitor BW755 C (20 microM) to 3.1 +/- 0.6%, and by the phospholipase A2 inhibitor, p-bromophenacyl bromide (10 microM), to 2.1 +/- 0.8%. In the absence of bovine serum albumin the inhibitory action of BW755 C and p-bromophenacyl bromide on glucose-induced insulin release was significantly more pronounced. These drugs whether in the presence or absence of bovine serum albumin, did not affect glyceraldehyde-stimulated insulin secretion. Glyceraldehyde (10 mM), potentiated glucose-induced insulin release in the presence of 2-8 mM glucose, but not for 10-20 mM glucose. Although the phospholipase A2 activator, melittin, initiated insulin release in the presence of 2 mM glucose and enhanced 10 mM glyceraldehyde-stimulated insulin secretion it had no effect on 20 mM glucose-induced insulin release. These two stimulatory effects of melittin on insulin release were totally abolished by p-bromophenacyl bromide.(ABSTRACT TRUNCATED AT 250 WORDS)

TnblaM: a transposon for directly tagging bacterial genes encoding cell envelope and secreted proteins

A transposon, TnblaM, designed for the direct selection of bacterial mutants with insertions in genes encoding cell envelope and secreted proteins, was constructed and subcloned into plasmid and bacteriophage lambda delivery vectors. TnblaM is a spectinomycin-resistant derivative of Tn5 with an unexpressed open reading frame encoding mature beta-lactamase (BlaM) at its left end. Therefore, when it inserts into genes in the correct orientation and reading frame, gene fusions encoding hybrid proteins are generated. By introducing TnblaM into bacterial cells and selecting ampicillin-resistant (ApR) colonies, the subset of isolates producing extracytoplasmic BlaM, and hence containing TnblaM inserted in genes encoding secreted proteins and cell envelope proteins, can be directly selected. TnblaM, like TnphoA, can therefore be used to preferentially mutagenise genes encoding extracytoplasmic proteins, but it has the advantage over TnphoA that the desired mutants can be isolated by direct selection (as ApR colonies) rather than by phenotypic screening. Isolates in which TnblaM occupies sites in the chromosome from which it can transpose at high frequency are readily identifiable, and constitute TnblaM donors, with which to simply and efficiently generate rare types of insertion mutants. Moreover, the ApR selection that is used with TnblaM can be fine-tuned to obtain blaM fusions to poorly or well-expressed genes.

Beta-lactamase as a probe of membrane protein assembly and protein export

The enzyme TEM beta-lactamase constitutes a versatile gene-fusion marker for studies on membrane proteins and protein export in bacteria. The mature form of this normally periplasmic enzyme displays readily detectable and distinctly different phenotypes when localized to the bacterial cytoplasm versus the periplasm, and thus provides a useful alternative to alkaline phosphatase for probing the topology of cytoplasmic membrane proteins. Cells producing translocated forms of beta-lactamase can be directly selected as ampicillin-resistant colonies, and consequently a beta-lactamase fusion approach can be used for positive selection for export signals, and for rapid assessment of whether any protein expressed in Escherichia coli inserts into the bacterial cytoplasmic membrane. The level of ampicillin resistance conferred on a cell by an extracytoplasmic beta-lactamase derivative depends on its level of expression, and therefore a beta-lactamase fusion approach can be used to directly select for increased yields of any periplasmic or membrane-bound gene products expressed in E. coli.

Starvation decreases insulin secretion, prostaglandin E2 production and phospholipase A2 activity in rat pancreatic islets

Pancreatic islets, isolated from rats starved for 48 h, secreted significantly less insulin in the presence of 2 mmol glucose/l than islets of fed controls. In contrast, the insulin secretory response of islets from fed and starved rats to a challenge of 20 mmol glucose/l was similar. Concentrations of prostaglandin E2 (PGE2) in islets from starved rats incubated with 2 mmol glucose/l were significantly lower compared with those in control islets obtained from fed animals. Although glucose (20 mmol/l) stimulated PGE2 production in islets from starved and fed rats by 2.7- and 1.6-fold respectively, the concentrations achieved were the same as a consequence of the different prestimulated concentrations. Incubation with [14C]arachidonic acid of sonicated islet preparations from fed rats and separation of metabolites generated by high-pressure liquid chromatography, indicated the biosynthesis of a number of cyclo-oxygenase- and lipoxygenase-derived compounds, including 6-keto-PGF1 alpha, PGF2 alpha, PGE2 and 12-hydroxyeicosatetraenoic acid. Metabolism of arachidonic acid to cyclo-oxygenase-derived compounds occurred with the same efficiency, but production of lipoxygenase-derived compounds was reduced by 50% in sonicated islets from starved compared with fed rats. Activity of phospholipase A2 of islets from starved rats was significantly less than that measured in islets from fed rats, although the degree of stimulation by 20 mmol glucose/l was the same in both types of islet. These alterations in the phospholipase A2/arachidonic acid cascade may contribute to the diminished insulin secretory response of islets from starved rats to relatively low concentrations of glucose.

Opiate-prostaglandin interactions in the regulation of insulin secretion from rat islets of Langerhans in vitro

The inadequate insulin secretory response to glucose stimulation in non-insulin dependent diabetes has been attributed to many factors including high PGE2 levels blunting the secretory response, and to the existence of inhibitory opiate activity in vivo. The purpose of the present work was to see if there was a connection between these two independent theories. Radioimmunoassayable PGE2 in islets of Langerhans was found to be proportional to islet number and protein content and was typically 4 to 5pg/micrograms islet protein. Indomethacin (2.8 X 10(-5) M), sodium salicylate (1.25 X 10(-3) M) and chlorpropamide (7.2 X 10(-5) M) all lowered islet PGE2 levels and stimulated insulin release in vitro. Dynorphin (1-13), stimulated insulin release at a concentration of 6 X 10(-9) M, while lowering islet PGE2. Conversely, at a higher concentration, (6 X 10(-7) M), dynorphin had no stimulatory effect on insulin secretion and did not lower PGE2 levels in islets or in the incubation media. The stimulatory effects of dynorphin and sodium salicylate on insulin secretion were blocked by exogenous PGE2 (10(-5) M). PGE2 at a lower concentration (10(-9) M) did not exert any inhibitory effect on dynorphin- or sodium salicylate-induced insulin release. This concentration of exogenous PGE2 stimulated insulin release in the presence of 6mM glucose. Results from these experiments suggest that since an opioid peptide can lower endogenous PGE2 production in islets and since the stimulatory effects of the opioid peptide are reversed by exogenous PGE2 there may be interactions between these two modulators of insulin secretion.

Effect of a hypoglycaemic agent M&B 39890A on glucagon secretion in isolated rat islets of Langerhans

Administration of the compound M&B 39890A lowered serum glucose levels significantly (p less than 0.001) in genetically obese mice, while no effect on serum insulin levels was observed. In in vitro experiments with isolated rat islets of Langerhans M&B 39890A inhibited arginine-stimulated glucagon release at all concentrations tested (0.5, 5.0 and 50 mumol/l). Insulin secretion was not inhibited by M&B 39890A (0.5 and 5.0 mumol/l), but was slightly decreased at 50 mumol/l. M&B 39890A (5 mumol/l) also inhibited glucagon secretion in vitro in the presence of 2 mmol/l, 6 mmol/l and 20 mmol/l glucose, while exerting no effect on insulin secretion. These results suggest that the hypoglycaemic action of M&B 39890A may be due to its direct and selective effect on glucagon secretion; this appears to operate by a mechanism different to that of glucose.