Free fatty acids regulate insulin secretion from pancreatic beta cells through GPR40

Role of GPR40 in fatty acid action on the beta cell line INS-1E

GPR40 is a G protein-coupled receptor expressed preferentially in beta cells, that has been implicated in mediating free fatty acid-stimulated insulin release. GPR40 RNAi impaired the ability of palmitic acid (PA) to increase both insulin secretion and intracellular calcium ([Ca2+]i). The PA-dependent [Ca2+]i increase was attenuated by inhibitors of Galphaq, PLC, and SERCA. Thus GPR40 activates the Galphaq pathway, leading to release of Ca2+ from the ER. Yet the GPR40-dependent [Ca2+]i rise was dependent on extracellular Ca2+ and elevated glucose, and was blocked by inhibition of L-type calcium channels (LTCC) or opening of the K(ATP) channel; this suggests that GPR40 promotes Ca2+ influx through up-regulation of LTCC pre-activated by glucose and membrane depolarization. Taken together, the data indicate that GPR40 mediates the increase in [Ca2+]i and insulin secretion through the Galphaq-PLC pathway, resulting in release of Ca2+ from the ER and leading to up-regulation of Ca2+ influx via LTCC.

Oleic acid interacts with GPR40 to induce Ca2+ signaling in rat islet beta-cells: mediation by PLC and L-type Ca2+ channel and link to insulin release

It has long been thought that long-chain free fatty acids (FFAs) stimulate insulin secretion via mechanisms involving their metabolism in pancreatic beta-cells. Recently, it was reported that FFAs function as endogenous ligands for GPR40, a G protein-coupled receptor, to amplify glucose-stimulated insulin secretion in an insulinoma cell line and rat islets. However, signal transduction mechanisms for GPR40 in beta-cells are little known. The present study was aimed at elucidating GPR40-linked Ca(2+) signaling mechanisms in rat pancreatic beta-cells. We employed oleic acid (OA), an FFA that has a high affinity for the rat GPR40, and examined its effect on cytosolic Ca(2+) concentration ([Ca(2+)](i)) in single beta-cells by fura 2 fluorescence imaging. OA at 1-10 microM concentration-dependently increased [Ca(2+)](i) in the presence of 5.6, 8.3, and 11.2 mM, but not 2.8 mM, glucose. OA-induced [Ca(2+)](i) increases at 11.2 mM glucose were inhibited in beta-cells transfected with small interfering RNA targeted to rat GPR40 mRNA. OA-induced [Ca(2+)](i) increases were also inhibited by phospholipase C (PLC) inhibitors, U73122 and neomycin, Ca(2+)-free conditions, and an L-type Ca(2+) channel blocker, nitrendipine. Furthermore, OA increased insulin release from isolated islets at 8.3 mM glucose, and it was markedly attenuated by PLC and L-type Ca(2+) channel inhibitors. These results demonstrate that OA interacts with GPR40 to increase [Ca(2+)](i) via PLC- and L-type Ca(2+) channel-mediated pathway in rat islet beta-cells, which may be link to insulin release.

The short-term effect of fatty acids on glucagon secretion is influenced by their chain length, spatial configuration, and degree of unsaturation: studies in vitro

The influence of fatty acids on beta cell function has been well established whereas little is known about the role of fatty acids on alpha cell function. The aim of our study was to investigate the short-term effects of chain length, spatial configuration, and degree of unsaturation of fatty acids on glucagon secretion from isolated mouse islets and alpha tumor cell 1 clone 6 cells (alpha TC1-6 cells). Glucagon release was measured with different saturated and unsaturated fatty acids as well as cis and trans isomers of fatty acids at low and high glucose. Palmitate (0.1-0.5 mmol/L) immediately stimulated glucagon release in a dose-dependent manner from both isolated islets and alpha TC 1-6 cells. The longer chain length of saturated fatty acids, the higher glucagon responses were obtained. The average fold increase in glucagon to saturated fatty acids (0.3 mmol/L) compared to control was octanoate 1.5, laurate 2.0, myristate 2.9, palmitate 5.4, and stearate 6.2, respectively. Saturated fatty acids were more effective than unsaturated fatty acids in stimulating glucagon secretion. At an equimolar concentration, trans-fatty acids were more potent than their cis isomers. Fatty acids immediately stimulate glucagon secretion from isolated mouse islets pancreatic alpha cells. The chain length, spatial configuration, and degree of unsaturation of fatty acids influence the glucagonotropic effect.

Suppressing actions of butyrate on growth hormone (GH) secretion induced by GH-releasing hormone in rat anterior pituitary cells

We assessed the inhibitory effects of butyrate on the growth hormone (GH) secretion in order to investigate the cellular mechanisms in rat somatotrophs. Isolated anterior pituitary cells were cultured in DMEM for several hours, either in the presence (1, 3, or 10mM) or absence of butyrate, and then stimulated with 10(-7)M GHRH for 30 min, in the presence of butyrate at the concentrations used for the previous culture. The increase in GHRH-induced GH release was significantly reduced in a time-dependent and concentration-dependent manner in the cells previously cultured with butyrate. GH content (the sum of GH released into the medium induced by GHRH stimulation and the GH remaining in the cells after stimulation) was reduced by the culture of cells in the presence of butyrate, which was also inversely dependent on the concentrations used for the culture. Simultaneous addition of an L-type Ca(2+) channel blocker, nifedipine (10 pM), to the medium during 10(-9)M GHRH stimulation significantly reduced the stimulated GH release, which was further significantly decreased by a simultaneous addition of 10 mM butyrate. Butyrate blunted the GHRH (10(-9)M)-induced increase in cellular cyclic AMP and calcium ion concentrations, the activity of protein kinases (A and C), and GHmRNA expression. The expression of mRNA for GPR 41 and 43, known as receptors for short-chain fatty acids, was confirmed in the anterior pituitary cells. These findings suggest that butyrate inhibits GHRH-induced GH release as well as GH production, and the cellular inhibitory actions of butyrate occur in diverse cellular signaling pathways of rat somatotrophs.

Mechanisms of oleic acid deterioration in insulin secretion: role in the pathogenesis of type 2 diabetes

Obesity is highly associated with type 2 diabetes where free fatty acids (FFAs) may be a trigger factor. To examine this hypothesis, in this study, we investigated the role of FFAs in the pathogenic development of type 2 diabetes. The release of insulin, the expression of preproinsulin (PPI), glucose transporter2 (GLUT2) and pancreatic duodenal homeobox-1 (PDX-1), and levels of intracellular free Ca++([Ca++]i) were measured in rat pancreatic islets treated with or without high concentrations of FFA (0.1 and 1.0 mM oleic acid) for 24 h. In comparison with untreated control, islets exposed to oleic acid showed an increase in basal insulin release and a decrease in glucose induced insulin secretion (GSIS). Elevated expression of PPI, PDX-1 and GLUT2 was also observed after treatment of the islets with oleic acid, which may partially contribute to the increased basal insulin secretion. Moreover, [Ca++]i levels increased after oleic acid exposure, which most likely accounts for the decrease of GSIS. Our findings, thus strongly suggest, that the increased levels of basal insulin secretion involved in glucose sensing, insulin producing and insulin secreting induced by high levels of FFAs may cause hyperinsulinemia in patients with type 2 diabetes, and thus long-term hyperinsulinemia could desensitize insulin receptors. We hypothesize that hyperinsulinemia may be a primary and independent event in the pathogenesis of diabetes. If proven, it may be possible to create novel and effective approaches for the prevention and treatment of type 2 diabetes.

Eicosapentaenoic acid suppresses cell proliferation in MCF-7 human breast cancer xenografts in nude rats via a pertussis toxin-sensitive signal transduction pathway

The type and content of dietary PUFAs have profound influences on the growth rate of transplantable human breast cancers in immunodeficient rodents. Diets enriched in linoleic acid (LA), an (n-6) fatty acid, stimulate tumor growth, whereas dietary fats containing (n-3) fatty acids slow such growth. Interactions between LA and (n-3) fatty acids capable of regulating cell proliferation in solid tumors in vivo are not yet well defined. Here we tested the hypothesis that plasma eicosapentaenoic acid (EPA), an (n-3) fatty acid, suppresses cell proliferation in MCF-7 human breast cancer xenografts via a pertussis toxin-sensitive reduction of intratumor cAMP, LA uptake, and formation of the mitogen 13-hydroxyoctadecadienoic acid (13-HODE) from LA. Plasma fatty acid uptake and 13-HODE release were determined in control and EPA-treated xenografts from arteriovenous differences measured during perfusion in situ. Intratumor cAMP, extracellular signal-regulated kinase p44/p42 (ERK1/2) phosphorylation, and [3H]thymidine incorporation (TTI) were measured in tumors freeze-clamped at the end of the perfusions. Arterial blood containing EPA caused significant decreases (P < 0.05) in cAMP, uptake of SFA, monounsaturated fatty acids, and (n-6) PUFA, 13-HODE formation, ERK1/2 phosphorylation, and TTI in MCF-7 xenografts. These effects of EPA were reversed by the addition of either pertussis toxin or 8-bromoadenosine-cAMP to the EPA-containing arterial blood. Addition of 13-HODE to the EPA-containing arterial blood restored phosphorylated ERK1/2 and TTI but not FA uptake. The results suggest that EPA regulates cell proliferation in MCF-7 xenografts via a novel inhibitory G protein-coupled, (n-3) FFA receptor-mediated signal transduction pathway.

Glucose-induced lipogenesis in pancreatic beta-cells is dependent on SREBP-1

High concentrations of glucose induce de novo fatty acid synthesis in pancreatic beta-cells and chronic exposure of elevated glucose and fatty acids synergize to induce accumulation of triglycerides, a phenomenon termed glucolipotoxicity. Here we investigate the role of sterol-regulatory element binding proteins in glucose-induced lipogenesis in the pancreatic beta-cell line INS-1E. We show that glucose induces SREBP-1c expression and SREBP-1 activity independent of insulin secretion and signaling. Using adenoviral expression of SREBP-1c and a SREBP-mutant we show that lipogenic gene expression, de novo fatty acid synthesis and lipid accumulation are induced primarily through sterol-regulatory elements (SREs) and not E-Boxes. Adenoviral expression of a dominant negative SREBP compromises glucose induction of some lipogenic genes and significantly reduces glucose-induction of de novo fatty acid synthesis. Thus, we demonstrate for the first time that SREBP activity is necessary for full glucose induction of de novo fatty acid synthesis in pancreatic beta-cells.

Effect of a cyclohexenonic long-chain fatty alcohol on calcium mobilization

Cyclohexenonic long-chain fatty alcohols constitute a family of synthetic compounds with trophic, secretagogue and antioxidant properties. Despite their multiple biological actions in neuronal and non-neuronal tissues, the intracellular mechanisms underlying CFA activity remain unknown. In the present study, we show that 3-(15-hydroxypentadecyl)-2,4,4-trimethyl-2-cyclohexen-1-one (tCFA15) directly mobilizes Ca(2+) in the pituitary neural lobe synaptosomes and in primary sensory neurons from dorsal root ganglia. This effect is dependent on the presence of extracellular Ca(2+), but does not involve transmembrane voltage-operated calcium channels. Using a combination of pharmacological agents that block or deplete intracellular Ca(2+) stores, our results suggest the implication of a calcium induced-calcium release mechanism evoked by tCFA15-induced Ca(2+) influx. To our knowledge, these findings constitute the first attempt towards the comprehension of the biological actions of cyclohexenonic long-chain fatty alcohols at a molecular level.

Identification of N-arachidonylglycine, U18666A, and 4-androstene-3,17-dione as novel insulin Secretagogues

The glucose-induced insulin secretion is fine-tuned by numerous factors. To systematically identify insulinotropic factors, we optimized a primary beta-cell-based functional assay to monitor intracellular Ca2+ flux ([Ca2+]i). By this assay system, we successfully identified several insulinotropic peptides including cholecystokinin, gastrin releasing peptide, vasopressin, and oxytocin from tissue extracts. Screening of an assortment of chemical compounds, we determined three novel insulin secretagogues: N-arachidonylglycine (NAGly), 3beta-(2-diethylamino-ethoxy) androstenone hydrochloride (U18666A), and 4-androstene-3,17-dione. The NAGly increased [Ca2+]i through stimulation of the voltage-dependent Ca2+ channels and it was dependent on extracellular glucose level. On the other hand, U18666A and 4-androstene-3,17-dione increased [Ca2+]i in the presence of K ATP channel opener diazoxide while it was inhibited by the presence of Ca2+ channel blocker nitrendipine, suggesting that their effects are independent of K ATP channel. These unique features will be useful for further development of insulinotropic factors and drugs for treating type 2 diabetes.

Peroxisome proliferator-activated receptor alpha (PPARalpha) potentiates, whereas PPARgamma attenuates, glucose-stimulated insulin secretion in pancreatic beta-cells

Fatty acids (FAs) are known to be important regulators of insulin secretion from pancreatic beta-cells. FA-coenzyme A esters have been shown to directly stimulate the secretion process, whereas long-term exposure of beta-cells to FAs compromises glucose-stimulated insulin secretion (GSIS) by mechanisms unknown to date. It has been speculated that some of these long-term effects are mediated by members of the peroxisome proliferator-activated receptor (PPAR) family via an induction of uncoupling protein-2 (UCP2). In this study we show that adenoviral coexpression of PPARalpha and retinoid X receptor alpha (RXRalpha) in INS-1E beta-cells synergistically and in a dose- and ligand-dependent manner increases the expression of known PPARalpha target genes and enhances FA uptake and beta-oxidation. In contrast, ectopic expression of PPARgamma/RXRalpha increases FA uptake and deposition as triacylglycerides. Although the expression of PPARalpha/RXRalpha leads to the induction of UCP2 mRNA and protein, this is not accompanied by reduced hyperpolarization of the mitochondrial membrane, indicating that under these conditions, increased UCP2 expression is insufficient for dissipation of the mitochondrial proton gradient. Importantly, whereas expression of PPARgamma/RXRalpha attenuates GSIS, the expression of PPARalpha/RXRalpha potentiates GSIS in rat islets and INS-1E cells without affecting the mitochondrial membrane potential. These results show a strong subtype specificity of the two PPAR subtypes alpha and gamma on lipid partitioning and insulin secretion when systematically compared in a beta-cell context.

Effects of a westernized lifestyle on the association between fasting serum nonesterified fatty acids and insulin secretion in Japanese men

The effects of the prolonged elevation of nonesterified fatty acid (NEFA) levels on insulin secretion have been controversial and thought to be sex-specific. To investigate the association between a westernized lifestyle and the effects of NEFA on insulin secretion in Japanese men, we examined 67 nondiabetic Japanese-American men and 220 nondiabetic native Japanese men who underwent a 75-g oral glucose tolerance test (OGTT). Most Japanese Americans we surveyed are genetically identical to Japanese living in Japan, but their lifestyle is more westernized. Sets of multiple regression analyses were performed to evaluate the relationship between the sum of the immunoreactive insulin (IRI) levels during the OGTT ((Sigma)IRI) and clinical parameters. Japanese Americans had higher levels of fasting IRI, (Sigma)IRI, and a higher insulin resistance index (homeostasis model assessment for insulin resistance [HOMA-IR]) than native Japanese, whereas there were no significant differences in fasting NEFA and triglyceride levels. A multiple regression analysis adjusted for age, fasting triglycerides, and body mass index (BMI) demonstrated that the fasting NEFA level was an independent determinant of the (Sigma)IRI only in Japanese-American men ( P = .001), but not in native Japanese men ( P = .054). Even when HOMA-IR was included in models instead of BMI, the NEFA level was a significant variable of (Sigma)IRI only in Japanese Americans ( P < .001), and not in native Japanese ( P = .098). In addition, a multiple regression analysis adjusted for age, fasting triglycerides, and BMI demonstrated that the fasting NEFA level was the only independent determinant of (Sigma)C-peptide in Japanese-American men ( P = .041). In conclusion, NEFA seems to be associated with insulin secretion independent of obesity or HOMA-IR. A westernized lifestyle may increase the effects of serum fasting NEFA levels on total insulin secretion after a glucose load in Japanese men.

Fat storage in pancreas and in insulin-sensitive tissues in pathogenesis of type 2 diabetes

Obesity is associated with increased storage of lipids in nonadipose tissues like skeletal muscle, liver, and pancreatic beta cells. These lipids constitute a continuous source of long-chain fatty acyl CoA (LC-CoA) and derived metabolites like diacylglycerol and ceramide, acting as signalling molecules on protein kinases activities (in particular, the family of PKCs), ion channel, gene expression, and protein acylation. In skeletal muscle, the increase in LC-CoA and diacylglycerol translocates and activates specific protein kinase C (PKC) isoforms, which will phosphorylate IRS-1 on serine, preventing its phosphorylation on tyrosine and association with PI3 kinase. This interrupts the insulin signalling pathway leading to the stimulation of glucose transport. In pancreatic beta cells, short-term excess of fatty acids or LC-CoA activates PKC and also directly stimulates insulin exocytosis. Long-term exposure to free fatty acids (FFA) leads to an increased basal and blunted glucose-stimulated insulin secretion by affecting gene expression, increase in K(ATP) channel activity, and uncoupling of the mitochondria. In addition, the saturated FFA palmitate increases cell death by apoptosis via increase in ceramide synthesis.

Impact of apolipoprotein A5 polymorphisms on insulin sensitivity and beta-cell function

OBJECTIVES

Alternation in lipid metabolism can affect both insulin sensitivity and beta-cell function. Apolipoprotein A5 (APOA5) is an important determinant of lipid metabolism. The impact of the APOA5 gene on insulin sensitivity and beta-cell function has not been examined.

METHODS

We examined the influence of 2 amino acid polymorphisms (V150M and G182C) in the APOA5 gene on insulin sensitivity and beta-cell function in 67 glucose-tolerant white subjects. Insulin sensitivity index (ISI) and first- and second-phase insulin responses (1stIR and 2ndIR) were assessed using a hyperglycemic clamp technique.

RESULTS

We identified 59 VV and 8 VM subjects, and none had either the GC or CC genotype. Although no association was found with fasting lipid profile and plasma glucose concentrations during oral glucose tolerance test, the V150M was associated with higher 1stIR (P = 0.0010) and 2ndIR (P = 0.0016) and lower ISI (P = 0.0135). The associations of this polymorphism with 1stIR (P = 0.0081) and 2ndIR (P = 0.0087) were independent of sex, age, and body mass index, but not ISI.

CONCLUSIONS

The V150M polymorphism had an independent influence on 1stIR and 2ndIR. Although the biologic consequence of this polymorphism remains to be determined, the V150M polymorphism in the APOA5 gene is a genetic marker for beta-cell function.

Bile acids promote glucagon-like peptide-1 secretion through TGR5 in a murine enteroendocrine cell line STC-1

Bile acids play essential roles in the absorption of dietary lipids and in the regulation of bile acid biosynthesis. Recently, a G protein-coupled receptor, TGR5, was identified as a cell-surface bile acid receptor. In this study, we show that bile acids promote glucagon-like peptide-1 (GLP-1) secretion through TGR5 in a murine enteroendocrine cell line STC-1. In STC-1 cells, bile acids promoted GLP-1 secretion in a dose-dependent manner. As STC-1 cells express TGR5 mRNA, we examined whether bile acids induce GLP-1 secretion through TGR5. RNA interference experiments showed that reduced expression of TGR5 resulted in reduced secretion of GLP-1. Furthermore, transient transfection of STC-1 cells with an expression plasmid containing TGR5 significantly enhanced GLP-1 secretion, indicating that bile acids promote GLP-1 secretion through TGR5 in STC-1 cells. Bile acids induced rapid and dose-dependent elevation of intracellular cAMP levels in STC-1 cells. An adenylate cyclase inhibitor, MDL12330A, significantly suppressed bile acid-promoted GLP-1 secretion, suggesting that bile acids induce GLP-1 secretion via intracellular cAMP production in STC-1 cells.

The FFA receptor GPR40 links hyperinsulinemia, hepatic steatosis, and impaired glucose homeostasis in mouse

Obesity is typically associated with elevated levels of free fatty acids (FFAs) and is linked to glucose intolerance and type 2 diabetes. FFAs exert divergent effects on insulin secretion from beta cells: acute exposure to FFAs stimulates insulin secretion, whereas chronic exposure impairs insulin secretion. The G protein-coupled receptor GPR40 is selectively expressed in beta cells and is activated by FFAs. We show here that GPR40 mediates both acute and chronic effects of FFAs on insulin secretion and that GPR40 signaling is linked to impaired glucose homeostasis. GPR40-deficient beta cells secrete less insulin in response to FFAs, and loss of GPR40 protects mice from obesity-induced hyperinsulinemia, hepatic steatosis, hypertriglyceridemia, increased hepatic glucose output, hyperglycemia, and glucose intolerance. Conversely, overexpression of GPR40 in beta cells of mice leads to impaired beta cell function, hypoinsulinemia, and diabetes. These results suggest that GPR40 plays an important role in the chain of events linking obesity and type 2 diabetes.

Free fatty acids inhibit serum deprivation-induced apoptosis through GPR120 in a murine enteroendocrine cell line STC-1

Free fatty acids (FFAs) provide an important energy source and also act as signaling molecules. FFAs are known to exert a variety of physiological responses via their G protein-coupled receptors (GPCRs), such as the GPR40 family. Recently, we identified a novel FFA receptor, GPR120, that promotes secretion of glucagon-like peptide-1 (Hirasawa, A., Tsumaya, K., Awaji, T., Katsuma, S., Adachi, T., Yamada, M., Sugimoto, Y., Miyazaki, S., and Tsujimoto, G. (2005) Nat. Med. 11, 90-94). Here we showed that FFAs inhibit serum deprivation-induced apoptosis of murine enteroendocrine STC-1 cells, which express two types of GPCRs, GPR120 and GPR40, for unsaturated long chain FFA. We first found that linolenic acid potently activated ERK and Akt/protein kinase B (Akt) in STC-1 cells. ERK kinase inhibitors significantly reduced the anti-apoptotic effects of linolenic acid. Inhibitors for phosphatidylinositol 3-kinase (PI3K), a major target of which is Akt, significantly reduced the anti-apoptotic effects. Transfection of STC-1 cells with the dominant-negative form of Akt also inhibited the anti-apoptotic effect. These results suggested that the activation of ERK and PI3K-Akt pathways is required for FFA-induced anti-apoptotic effects on STC-1 cells. Transient transfection of STC-1 cells with GPR120 cDNA, but not GPR40 cDNA, enhanced inhibition of caspase-3 activation. RNA interference experiments showed that reduced expression of GPR120, but not GPR40, resulted in reduced ERK activation and reduced effects of FFAs on caspase-3 inhibition. Collectively, these results demonstrated that FFAs promote the activation of ERK and PI3K-Akt pathways mainly via GPR120, leading to the anti-apoptotic effect of STC-1 cells.

A family of fatty acid binding receptors

The family of G protein-coupled receptors (GPCRs) serves as the target for almost a third of currently marketed drugs, and provides the predominant mechanism through which extracellular factors transmit signals to the cell. The discovery of GPCRs with no known ligand has initiated a frenzy of research, with the aim of elucidating the physiological ligands for these "orphan" receptors and revealing new drug targets. The GPR40 family of receptors, tandemly located on chromosome 19q13.1, exhibit 30-40% homology to one another and diverse tissue distribution, yet all are activated by fatty acids. Since agonists of GPR40 are medium to longchain fatty acids and those for GPR41 and 43 are short-chain fatty acids, the family clearly provides an intriguing example of how the ligand specificity, patterns of expression, and function of GPCRs can diverge through evolution. Here we summarize the identification, structure, and pharmacology of the receptors and speculate on the respective physiological roles that the GPR40 family members may play.

Pancreatic β-Cell Lipoprotein Lipase Independently Regulates Islet Glucose Metabolism and Normal Insulin Secretion

Lipid and glucose metabolism are adversely affected by diabetes, a disease characterized by pancreatic beta-cell dysfunction. To clarify the role of lipids in insulin secretion, we generated mice with beta-cell-specific overexpression (betaLPL-TG) or inactivation (betaLPL-KO) of lipoprotein lipase (LPL), a physiologic provider of fatty acids. LPL enzyme activity and triglyceride content were increased in betaLPL-TG islets; decreased LPL enzyme activity in betaLPL-KO islets did not affect islet triglyceride content. Surprisingly, both betaLPL-TG and betaLPL-KO mice were strikingly hyperglycemic during glucose tolerance testing. Impaired glucose tolerance in betaLPL-KO mice was present at one month of age, whereas betaLPL-TG mice did not develop defective glucose homeostasis until approximately five months of age. Glucose-simulated insulin secretion was impaired in islets isolated from both mouse models. Glucose oxidation, critical for ATP production and triggering of insulin secretion mediated by the ATP-sensitive potassium (KATP) channel, was decreased in betaLPL-TG islets but increased in betaLPL-KO islets. Islet ATP content was not decreased in either model. Insulin secretion was defective in both betaLPL-TG and betaLPL-KO islets under conditions causing calcium-dependent insulin secretion independent of the KATP channel. These results show that beta-cell-derived LPL has two physiologically relevant effects in islets, the inverse regulation of glucose metabolism and the independent mediation of insulin secretion through effects distal to membrane depolarization.

GPR40 gene Arg211His polymorphism may contribute to the variation of insulin secretory capacity in Japanese men

GPR40 is a member of the G-protein-coupled receptors. Recent studies suggest that GPR40 is highly expressed in pancreatic beta cells and insulin-secreting cell lines, and that fatty acids increase intracellular calcium concentration and amplify glucose-stimulated insulin secretion by activating GPR40. Despite identification of the Arg211His polymorphism in the GPR40 gene, there have been no clinical studies concerning this polymorphism. The present study was performed to investigate the effects of the GPR40 gene Arg211His polymorphism on clinical and metabolic parameters, including serum insulin level, in 327 healthy Japanese men, using the TaqMan polymerase chain reaction method. Serum insulin level, homeostasis model of insulin resistance (HOMA-IR), and beta-cell function (HOMA-beta were significantly different (P = .0075, .0152, and .0039, respectively) and were lowest in Arg/Arg homozygotes and highest in His/His homozygotes, although plasma glucose and serum lipids were not significantly different. Multiple regression analyses showed that serum insulin level, HOMA-IR, and HOMA- beta were significantly correlated with this polymorphism after adjusting for age and body mass index. After Bonferroni's correction for multiple comparisons was made, only HOMA- beta was significantly different among the 3 genotypes. These results suggest that the Arg211His polymorphism in the GPR40 gene may contribute to the variation of insulin secretory capacity in Japanese men.

Lysophosphatidylcholine enhances glucose-dependent insulin secretion via an orphan G-protein-coupled receptor

A lysophospholipid series, such as lysophosphatidic acid, lysophosphatidylserine, and lysophosphatidylcholine (LPC), is a bioactive lipid mediator with diverse physiological and pathological functions. LPC has been reported to induce insulin secretion from pancreatic beta-cells, however, the precise mechanism has remained elusive to date. Here we show that an orphan G-protein-coupled receptor GPR119 plays a pivotal role in this event. LPC potently enhances insulin secretion in response to high concentrations of glucose in the perfused rat pancreas via stimulation of adenylate cyclase, and dose-dependently induces intracellular cAMP accumulation and insulin secretion in a mouse pancreatic beta-cell line, NIT-1 cells. The Gs-protein-coupled receptor for LPC was identified as GPR119, which is predominantly expressed in the pancreas. GPR119-specific siRNA significantly blocked LPC-induced insulin secretion from NIT-1 cells. Our findings suggest that GPR119, which is a novel endogenous receptor for LPC, is involved in insulin secretion from beta-cells, and is a potential target for anti-diabetic drug development.

Oleate promotes the proliferation of breast cancer cells via the G protein-coupled receptor GPR40

Evidence from epidemiological studies and animal models suggests a link between high levels of dietary fat intake and risk of breast cancer. In addition, obesity, in which circulating lipids are elevated, is associated with increased risk of various cancers. Relative to this point, we previously showed that oleate stimulates the proliferation of breast cancer cells and that phosphatidylinositol 3-kinase plays a role in this process. Nonetheless, questions remain regarding the precise mechanism(s) by which oleate promotes breast cancer cell growth. Pharmacological inhibitors of the GTP-binding proteins G(i)/G(o), phospholipase C, Src, and mitogenic-extracellular signal-regulated kinase 1/2 (MEK 1/2) decreased oleate-induced [3H]thymidine incorporation in the breast cancer cell line MDA-MB-231. In addition, oleate caused a rapid and transient rise in cytosolic Ca2+ and an increase in protein kinase B phosphorylation. Overexpressing in these cells the G protein-coupled receptor GPR40, a fatty acid receptor, amplified oleate-induced proliferation, whereas silencing the GPR40 gene using RNA interference decreased it. Overexpressing GPR40 in T47D and MCF-7 breast cancer cells that are poorly responsive to oleate allowed a robust proliferative action of oleate. The data indicate that the phospholipase C, MEK 1/2, Src, and phosphatidylinositol 3-kinase/protein kinase B signaling pathways are implicated in the proliferative signal induced by oleate and that these effects are mediated at least in part via the G protein-coupled receptor GPR40. The results suggest that GPR40 is implicated in the control of breast cancer cell growth by fatty acids and that GPR40 may provide a link between fat and cancer.

Three's company: two or more unrelated receptors pair with the same ligand

Intercellular communication relies on signal transduction mediated by extracellular ligands and their receptors. Although the ligand-receptor interaction is usually a two-player event, there are selective examples of one polypeptide ligand interacting with more than one phylogenetically unrelated receptor. Likewise, a few receptors interact with more than one polypeptide ligand, and sometimes with more than one coreceptor, likely through an interlocking of unique protein domains. Phylogenetic analyses suggest that for certain triumvirates, the matching events could have taken place at different evolutionary times. In contrast to a few polypeptide ligands interacting with more than one receptor, we found that many small nonpeptide ligands have been paired with two or more plasma membrane receptors, nuclear receptors, or channels. The observation that many small ligands are paired with more than one receptor type highlights the utilitarian use of a limited number of cellular components during metazoan evolution. These conserved ligands are ubiquitous cell metabolites likely favored by natural selection to establish novel regulatory networks. They likely possess structural features useful for designing agonistic and antagonistic drugs to target diverse receptors.

Rad51 siRNA delivered by HVJ envelope vector enhances the anti-cancer effect of cisplatin

BACKGROUND

Every cancer therapy appears to be transiently effective for cancer regression, but cancers gradually transform to be resistant to the therapy. Cancers also develop machineries to resist chemotherapy. Short interfering RNA (siRNA) has been evaluated as an attractive and effective tool for suppressing a target protein by specifically digesting its mRNA. Suppression of the machineries using siRNA may enhance the sensitivity to chemotherapy in cancers when combined with an effective delivery system.

METHODS

To enhance the anti-cancer effect of chemotherapy, we transferred siRNA against Rad51 into various human cancer cells using the HVJ (hemagglutinating virus of Japan, Sendai virus) envelope vector in the presence or absence of cis-diamminedichloroplatinum(II) (CDDP, cisplatin). The inhibition of cell growth was assessed by a modified MTT assay, counting cell number, or fluorescence-activated cell sorting (FACS) analysis after Annexin V labeling. The synthetic Rad51 siRNA was also introduced into subcutaneous tumor masses of HeLa cells in SCID mice with or without intraperitoneal injection of CDDP, and tumor growth was monitored.

RESULTS

When synthetic Rad51 siRNA was delivered into HeLa cells using the HVJ envelope vector, no Rad51 transcripts were detected on day 2, and Rad51 protein completely disappeared for 4 days after siRNA transfer. When HeLa cells were incubated with 0.02 microg/ml CDDP for 3 h after siRNA transfer, the number of colonies decreased to approximately 10% of that with scrambled siRNA. The sensitivity to CDDP was enhanced in various human cancer cells, but not in normal human fibroblasts. When Rad51 siRNA was delivered into tumors using the HVJ envelope vector, the Rad51 transcript level was reduced to approximately 25%. Rad51 siRNA combined with CDDP significantly inhibited tumor growth when compared to siRNA or CDDP alone.

CONCLUSIONS

Rad51 siRNA could enhance the sensitivity to CDDP in cancer cells both in vitro and in vivo. Our results suggest that the combination of CDDP and Rad51 siRNA will be an effective anti-cancer protocol.

Perspective: emerging evidence for signaling roles of mitochondrial anaplerotic products in insulin secretion

The importance of mitochondrial biosynthesis in stimulus secretion coupling in the insulin-producing beta-cell probably equals that of ATP production. In glucose-induced insulin secretion, the rate of pyruvate carboxylation is very high and correlates more strongly with the glucose concentration the beta-cell is exposed to (and thus with insulin release) than does pyruvate decarboxylation, which produces acetyl-CoA for metabolism in the citric acid cycle to produce ATP. The carboxylation pathway can increase the levels of citric acid cycle intermediates, and this indicates that anaplerosis, the net synthesis of cycle intermediates, is important for insulin secretion. Increased cycle intermediates will alter mitochondrial processes, and, therefore, the synthesized intermediates must be exported from mitochondria to the cytosol (cataplerosis). This further suggests that these intermediates have roles in signaling insulin secretion. Although evidence is quite good that all physiological fuel secretagogues stimulate insulin secretion via anaplerosis, evidence is just emerging about the possible extramitochondrial roles of exported citric acid cycle intermediates. This article speculates on their potential roles as signaling molecules themselves and as exporters of equivalents of NADPH, acetyl-CoA and malonyl-CoA, as well as alpha-ketoglutarate as a substrate for hydroxylases. We also discuss the "succinate mechanism," which hypothesizes that insulin secretagogues produce both NADPH and mevalonate. Finally, we discuss the role of mitochondria in causing oscillations in beta-cell citrate levels. These parallel oscillations in ATP and NAD(P)H. Oscillations in beta-cell plasma membrane electrical potential, ATP/ADP and NAD(P)/NAD(P)H ratios, and glycolytic flux are known to correlate with pulsatile insulin release. Citrate oscillations might synchronize oscillations of individual mitochondria with one another and mitochondrial oscillations with oscillations in glycolysis and, therefore, with flux of pyruvate into mitochondria. Thus citrate oscillations may synchronize mitochondrial ATP production and anaplerosis with other cellular oscillations.

Development of HVJ Envelope Vector and Its Application to Gene Therapy

To create a highly efficient vector system that is minimally invasive, we initially developed liposomes that contained fusion proteins from the hemagglutinating virus of Japan (HVJ; Sendai virus). These HVJ-liposomes delivered genes and drugs to cultured cells and tissues. To simplify the vector system and develop more efficient vectors, the next approach was to convert viruses to non-viral vectors. Based on this concept, we recently developed the HVJ envelope vector. HVJ with robust fusion activity was inactivated, and exogenous DNA was incorporated into the viral envelope by detergent treatment and centrifugation. The resulting HVJ envelope vector introduced plasmid DNA efficiently and rapidly into both cultured cells in vitro and organs in vivo. Furthermore, proteins, synthetic oligonucleotides, and drugs have also been effectively introduced into cells using the HVJ envelope vector. The HVJ envelope vector is a promising tool for both ex vivo and in vivo gene therapy experiments. Hearing impairment in rats was prevented and treated by hepatocyte growth factor gene transfer to cerebrospinal fluid using HVJ envelope vector. For cancer treatment, tumor-associated antigen genes were delivered efficiently to mouse dendritic cells to evoke an anti-cancer immune response. HVJ envelope vector fused dendritic cells and tumor cells and simultaneously delivered cytokine genes, such as IL-12, to the hybrid cells. This strategy successfully prevented and treated cancers in mice by stimulating the presentation of tumor antigens and the maturation of T cells. For human gene therapy, a pilot plant to commercially produce clinical grade HVJ envelope vector has been established.

Studies of relationships between variation of the human G protein-coupled receptor 40 Gene and Type 2 diabetes and insulin release

AIMS

Recently, a novel human G protein-coupled receptor 40 (GPR40), which is predominantly expressed in pancreatic islets, was shown to mediate an amplifying effect of long-chain fatty acids on glucose-induced insulin secretion. The present aim was to examine the coding region of GPR40 for variation and to assess whether identified variants confer an increased risk of Type 2 diabetes or altered insulin release.

METHODS

Mutation analysis was performed in 43 patients with Type 2 diabetes, 18 normal glucose-tolerant subjects, and 3 maturity-onset of diabetes in the young (MODY) X patients using direct sequencing. Genotyping was performed using polymerase chain reaction (PCR)-generated primer extension products analysis by high throughput chip-based mass spectrometry (MALDI-TOF). The potential impact of GPR40 mutations on [(3)H]-myo-inositol turnover was estimated in COS-7 cells after stimulation with various concentrations of 5,8,11-eicosatriynoic acid.

RESULTS

Two nucleotide substitutions, an Arg211His polymorphism and a rare Asp175Asn mutation, were identified. Both variants showed EC(50) values similar to the wild type. However, the maximal efficacy of the rare Asp175Asn was 39% lower compared with the wild type (P = 0.01). The Arg211His polymorphism had a similar allele frequency among 1384 Type 2 diabetic patients [MAF%; 23.4 (95% CI: 21.8-25.0)] and 4424 middle-aged glucose-tolerant subjects [24.1% (23.2-25.0)]. A genotype-quantitative trait study of 5597 non-diabetic, middle-aged subjects from the Inter99 cohort showed no significant differences in oral glucose tolerance test (OGTT)-derived estimates of insulin release between carriers of various GPR40 genotypes.

CONCLUSIONS

Variations in the coding region of GPR40 do not appear to be associated with Type 2 diabetes or insulin release alterations.

Islet adaptation to insulin resistance: mechanisms and implications for intervention

Insulin sensitivity and insulin secretion are reciprocally related such that insulin resistance is adapted by increased insulin secretion to maintain normal glucose and lipid homeostasis. The relation between insulin sensitivity and secretion is curvilinear and mathematically best described as a hyperbolic relation. Several potential mediators have been suggested to be signals for the beta cells to respond to insulin resistance such as glucose, free fatty acids, autonomic nerves, fat-derived hormones and the gut hormone glucagon-like peptide-1 (GLP-1). Failure of these signals or of the pancreatic beta cells to adequately adapt insulin secretion in relation to insulin sensitivity results in inappropriate insulin levels, impaired glucose intolerance (IGT) and type 2 diabetes. Therefore, treatment of IGT and type 2 diabetes should aim at restoring the normal relation between insulin sensitivity and secretion. Such treatment includes stimulation of insulin secretion (sulphonylureas, repaglinide and nateglinide) and insulin sensitivity (metformin and thiazolidinediones), as well as treatment aimed at supporting the signals mediating the islet adaptation (cholinergic agonists and GLP-1). Both, for correct understanding of diabetes pathophysiology and for development of novel treatment modalities, therefore, the non-linear inverse relation between insulin sensitivity and secretion needs to be acknowledged.

Identification and characterization of an endogenous chemotactic ligand specific for FPRL2

Chemotaxis of dendritic cells (DCs) and monocytes is a key step in the initiation of an adequate immune response. Formyl peptide receptor (FPR) and FPR-like receptor (FPRL)1, two G protein-coupled receptors belonging to the FPR family, play an essential role in host defense mechanisms against bacterial infection and in the regulation of inflammatory reactions. FPRL2, the third member of this structural family of chemoattractant receptors, is characterized by its specific expression on monocytes and DCs. Here, we present the isolation from a spleen extract and the functional characterization of F2L, a novel chemoattractant peptide acting specifically through FPRL2. F2L is an acetylated amino-terminal peptide derived from the cleavage of the human heme-binding protein, an intracellular tetrapyrolle-binding protein. The peptide binds and activates FPRL2 in the low nanomolar range, which triggers intracellular calcium release, inhibition of cAMP accumulation, and phosphorylation of extracellular signal-regulated kinase 1/2 mitogen-activated protein kinases through the G(i) class of heterotrimeric G proteins. When tested on monocytes and monocyte-derived DCs, F2L promotes calcium mobilization and chemotaxis. Therefore, F2L appears as a new natural chemoattractant peptide for DCs and monocytes, and the first potent and specific agonist of FPRL2.

N-Formylated humanin activates both formyl peptide receptor-like 1 and 2

We have discovered that humanin (HN) acts as a ligand for formyl peptide receptor-like 1 (FPRL1) and 2 (FPRL2). This discovery was based on our finding that HN suppressed forskolin-induced cAMP production in Chinese hamster ovary (CHO) cells expressing human FPRL1 (CHO-hFPRL1) or human FPRL2 (CHO-hFPRL2). In addition, we found that N-formylated HN (fHN) performed more potently as a ligand for FPRL1 than HN: in CHO-hFPRL1 cells, the effective concentration for the half-maximal response (EC(50)) value of HN was 3.5nM, while that of fHN was 0.012nM. We demonstrated by binding experiments using [(125)I]-W peptide that HN and fHN directly interacted with hFPRL1 on the membrane. In addition, we found that HN and fHN showed strong chemotactic activity for CHO-hFPRL1 and CHO-hFPRL2 cells. HN is known to have a protective effect against neuronal cell death. Our findings contribute to the understanding of the mechanism behind HN's function.

Synthesis and biological activity of novel alpha-substituted beta-phenylpropionic acids having pyridin-2-ylphenyl moiety as antihyperglycemic agents

We previously reported the identification of novel oximes having 5-benzyl-2,4-thiazolidinedione with antihyperglycemic activity. We now report the synthesis and biological activity of a novel series of oximes and amides having alpha-substituted-beta-phenylpropionic acids. In this series, we obtained potent PPAR alpha/gamma dual agonist (S)-9d, with which activation of PPAR alpha and PPAR gamma was considerably more potent than that of the reference compounds GW9578 22 and rosiglitazone 3, respectively. This means (S)-9d is of the strongest class of PPAR alpha/gamma dual agonists. In the course of this study, we also obtained 8h, which indicated potent plasma glucose lowering effect in spite of weak PPAR alpha/gamma agonistic activity.

Free fatty acids regulate gut incretin glucagon-like peptide-1 secretion through GPR120

Diabetes, a disease in which the body does not produce or use insulin properly, is a serious global health problem. Gut polypeptides secreted in response to food intake, such as glucagon-like peptide-1 (GLP-1), are potent incretin hormones that enhance the glucose-dependent secretion of insulin from pancreatic beta cells. Free fatty acids (FFAs) provide an important energy source and also act as signaling molecules in various cellular processes, including the secretion of gut incretin peptides. Here we show that a G-protein-coupled receptor, GPR120, which is abundantly expressed in intestine, functions as a receptor for unsaturated long-chain FFAs. Furthermore, we show that the stimulation of GPR120 by FFAs promotes the secretion of GLP-1 in vitro and in vivo, and increases circulating insulin. Because GLP-1 is the most potent insulinotropic incretin, our results indicate that GPR120-mediated GLP-1 secretion induced by dietary FFAs is important in the treatment of diabetes.

Functional expression of TREK-2 in insulin-secreting MIN6 cells

Insulin secretion from pancreatic beta cells is partly regulated by cell membrane potential. Background K+ channels that stabilize the resting membrane potential would suppress excitability and insulin secretion. Recent studies show that members of the two-pore domain K+ (K2P) channel family behave as background K+ channels in many excitable cells. Therefore, the expression of K2P channels was studied in insulin-secreting MIN6 cells. Reverse transcriptase PCR showed that, among nine K2P channels tested, TASK-1, TASK-2, TASK-3, TREK-2, and TRESK-2 were expressed in MIN6 cells. Cell-attached recordings on MIN6 cells revealed five types of K+ channels that were open at rest. Two were ATP-sensitive and Ca2+-activated K+ channels, as judged by their sensitivity to ATP and Ca2+, respectively, and single-channel conductance. Among five K2P channels, only TREK-2 could be clearly identified in MIN6 cells. The molecular identity of two other K+ channels is not yet known. TREK-2 in MIN6 cells was activated by arachidonic acid, membrane stretch, and low pH solution (pH 5.8). Arachidonic acid increased Ba2+-sensitive whole-cell current in MIN6 cell. These results suggest that TREK-2 contributes to the background K+ conductance in MIN6 cells, and may regulate depolarization-induced secretion of insulin.

Current status of the E23K Kir6.2 polymorphism: implications for type-2 diabetes

The ATP-sensitive potassium (KATP) channel couples membrane excitability to cellular metabolism and is a critical mediator in the process of glucose-stimulated insulin secretion. Increasing numbers of KATP channel polymorphisms are being described and linked to altered insulin secretion indicating that genes encoding this ion channel could be susceptibility markers for type-2 diabetes. Genetic variation of KATP channels may result in altered beta-cell electrical activity, glucose homeostasis, and increased susceptibility to type-2 diabetes. Of particular interest is the Kir6.2 E23K polymorphism, which is linked to increased susceptibility to type-2 diabetes in Caucasian populations and may also be associated with weight gain and obesity, both of which are major diabetes risk factors. This association highlights the potential contribution of both genetic and environmental factors to the development and progression of type-2 diabetes. In addition, the common occurrence of the E23K polymorphism in Caucasian populations may have conferred an evolutionary advantage to our ancestors. This review will summarize the current status of the association of KATP channel polymorphisms with type-2 diabetes, focusing on the possible mechanisms by which these polymorphisms alter glucose homeostasis and offering insights into possible evolutionary pressures that may have contributed to the high prevalence of KATP channel polymorphisms in the Caucasian population.

Palmitate stimulation of glucagon secretion in mouse pancreatic alpha-cells results from activation of L-type calcium channels and elevation of cytoplasmic calcium

We have investigated the short-term effects of the saturated free fatty acid (FFA) palmitate on pancreatic alpha-cells. Palmitate (0.5 or 1 mmol/l bound to fatty acid-free albumin) stimulated glucagon secretion from intact mouse islets 1.5- to 2-fold when added in the presence of 1-15 mmol/l glucose. Palmitate remained stimulatory in islets depolarized with 30 mmol/l extracellular K(+) or exposed to forskolin, but it did not remain stimulatory after treatment with isradipine or triacsin C. The stimulatory action of palmitate on secretion correlated with a 3.5-fold elevation of intracellular free Ca(2+) when applied in the presence of 15 mmol/l glucose, a 40% stimulation of exocytosis (measured as increases in cell capacitance), and a 25% increase in whole-cell Ca(2+) current. The latter effect was abolished by isradipine, suggesting that palmitate selectively modulates l-type Ca(2+) channels. The effect of palmitate on exocytosis was not mediated by palmitoyl-CoA, and intracellular application of this FFA metabolite decreased rather than enhanced Ca(2+)-induced exocytosis. The stimulatory effects of palmitate on glucagon secretion were paralleled by a approximately 50% inhibition of somatostatin release. We conclude that palmitate increases alpha-cell exocytosis principally by enhanced Ca(2+) entry via l-type Ca(2+) channels and, possibly, relief from paracrine inhibition by somatostatin released by neighboring delta-cells.

Mitochondrial superoxide: production, biological effects, and activation of uncoupling proteins

Mitochondria are potent producers of cellular superoxide, from complexes I and III of the electron transport chain, and mitochondrial superoxide production is a major cause of the cellular oxidative damage that may underlie degradative diseases and aging. This superoxide production is very sensitive to the proton motive force, so it can be strongly decreased by mild uncoupling. Superoxide and the lipid peroxidation products it engenders, including hydroxyalkenals such as hydroxynonenal, are potent activators of proton conductance by mitochondrial uncoupling proteins such as UCP2 and UCP3, although the mechanism of activation has yet to be established. These observations suggest a hypothesis for the main, ancestral function of uncoupling proteins: to cause mild uncoupling and so diminish mitochondrial superoxide production, hence protecting against disease and oxidative damage at the expense of a small loss of energy. We review the growing evidence for this hypothesis, in mitochondria, in cells, and in vivo. More recently evolved roles of uncoupling proteins are in adaptive thermogenesis (UCP1) and perhaps as part of a signaling pathway to regulate insulin secretion in pancreatic beta cells (UCP2).

Fatty acid binding proteins--the evolutionary crossroads of inflammatory and metabolic responses

Fatty acid binding proteins (FABPs) are members of a highly conserved family of proteins with the task of protecting a cell's delicate lipid balance. Yet they fail when faced with metabolic or inflammatory stress, turning the cytosol into an inhospitable environment with less than ideal outcomes. This review will focus on how FABPs direct lipid traffic and simultaneously control inflammatory and metabolic pathways under the pressures of the Metabolic Syndrome.

Postgenomic characterization of G-protein-coupled receptors

G-protein-coupled receptors (GPCRs) constitute one of the largest families of membrane-spanning proteins. Their importance in drug development has been proven over and over again. Therefore, they remain one of the most significant groups of molecules to be characterized. In the postgenomic era, the methods used for the characterization of GPCRs have dramatically changed: the predicted orphan receptors are now often used to ascertain the ligands (reverse pharmacology), whereas, in the past, the bioactive ligand was used to identify the receptor (classic approach). In this review, we will give an overview of the recent postgenomic functional assays that are frequently used to link the orphan GPCR of both vertebrate and invertebrate organisms with their ligands.

A glance at G-protein-coupled receptors for lipid mediators: a growing receptor family with remarkably diverse ligands

A plethora of lipid-like molecules known to act as intracellular second messengers are now recognized to signal cells through plasma membrane 7 transmembrane G-protein-coupled receptors (GPCRs). This has been the result of a decade-long genetic hunt for novel sequences encoding 7 transmembrane receptor proteins and the efforts to pair novel sequences with biologically active substances of (partly) unknown molecular mechanism of action. Identification of novel GPCR ligand pairs represents the first step to shed more light into the mode of action of novel cellular signaling molecules in human health and disease and might represent a fruitful source for the development of new drugs, judged on the successful history of GPCR as drug targets. Since 2000, more than 16 reports became available on lipid mediators--as diverse as lysophospholipids, arachidonic acid metabolites, short-, medium-, and long-chain fatty acids as well as steroid-like molecules--exerting their effects as extracellular mediators via rhodopsin-like family GPCRs. These reports have opened new avenues for research in human lipid receptor physiology and pharmacology. Here, the current knowledge on the recently deorphanized lipid receptors, including their isolation, expression pattern, function, and possible physiological or pathological roles will be reviewed.

Identification of a G Protein-coupled Receptor Specifically Responsive to β-Alanine

We isolated a cDNA encoding an orphan G protein-coupled receptor, TGR7, which has been recently reported to correspond to MrgD. To search for ligands for TGR7, we screened a series of small molecule compounds by detecting the Ca2+ influx in Chinese hamster ovary cells expressing TGR7. Through this screening, we found that beta-alanine at micromolar doses specifically evoked Ca2+ influx in cells expressing human, rat, or mouse TGR7. A structural analogue, gamma-aminobutyric acid, weakly stimulated cells expressing human or rat TGR7, but another analogue, glycine, did not. In addition, beta-alanine decreased forskolin-stimulated cAMP production in cells expressing TGR7, suggesting that TGR7 couples with G proteins Gq and Gi. In guanosine 5'-O-3-thiotriphosphate binding assays conducted using a membrane fraction of cells expressing TGR7, beta-alanine specifically increased the binding of guanosine 5'-O-3-thiotriphosphate. When a fusion protein composed of TGR7 and green fluorescent protein was expressed in cells, it localized at the plasma membrane but internalized into the cytoplasm after treatment with beta-alanine. In addition, we found that beta-[3H]alanine more efficiently bound to TGR7-expressing cells than to control cells. From these results, we concluded that TGR7 functioned as a specific membrane receptor for beta-alanine. Quantitative PCR analysis revealed that TGR7 mRNA was predominantly expressed in the dorsal root ganglia in rats. By in situ hybridization and immunostaining, we confirmed that TGR7 mRNA was co-expressed in the small diameter neurons with P2X3 and VR1, both in rat and monkey dorsal root ganglia. Our results suggest that TGR7 participates in the modulation of neuropathic pain.

A role for the malonyl-CoA/long-chain acyl-CoA pathway of lipid signaling in the regulation of insulin secretion in response to both fuel and nonfuel stimuli

The malonyl-CoA/long-chain acyl-CoA (LC-CoA) model of glucose-induced insulin secretion (GIIS) predicts that malonyl-CoA derived from glucose metabolism inhibits fatty acid oxidation, thereby increasing the availability of LC-CoA for lipid signaling to cellular processes involved in exocytosis. For directly testing the model, INSr3 cell clones overexpressing malonyl-CoA decarboxylase in the cytosol (MCDc) in a tetracycline regulatable manner were generated, and INS(832/13) and rat islets were infected with MCDc-expressing adenoviruses. MCD activity was increased more than fivefold, and the malonyl-CoA content was markedly diminished. This was associated with enhanced fat oxidation at high glucose, a suppression of the glucose-induced increase in cellular free fatty acid (FFA) content, and reduced partitioning at elevated glucose of exogenous palmitate into lipid esterification products. MCDc overexpression, in the presence of exogenous FFAs but not in their absence, reduced GIIS in all beta-cell lines and in rat islets. It also markedly curtailed the stimulation of insulin secretion by other fuel and nonfuel secretagogues. In the absence of MCDc overexpression, the secretory responses to all types of secretagogues were amplified by the provision of exogenous fatty acids. In the presence of exogenous FFAs, the fatty acyl-CoA synthetase inhibitor triacsin C reduced secretion in response to glucose and nonfuel stimuli. The data show the existence of important links between the metabolic coupling factor malonyl-CoA, the partitioning of fatty acids, and the stimulation of insulin secretion to both fuel and nonfuel stimuli.

Existence of GPR40 functioning in a human breast cancer cell line, MCF-7

GPR40, which has recently been identified as a G-protein-coupled cell-surface receptor for long-chain fatty acids, was assessed in a human breast cancer cell line (MCF-7). We detected GPR40 mRNA by RT-PCR and found that oleate and linoleate, but not palmitate or stearate, caused an increase in cellular Ca(2+) concentrations, which was partially blocked by the pertussis toxin (PTX) treatment. We examined the expression of GPR40 mRNA by quantitative RT-PCR in the relation to cell number. It was significantly increased at the beginning and at the end of cell proliferation. These results indicate the possibility that GPR40 for long-chain fatty acids may be involved in cellular function such as cell proliferation, providing a new perspective for the action of long-chain fatty acids on mammary epithelial cells.

THEIDENTIFICATION OFLIGANDS ATORPHANG-PROTEINCOUPLEDRECEPTORS

The completion of the human genome sequencing project has identified approximately 720 genes that belong to the G-protein coupled receptor (GPCR) superfamily. Approximately half of these genes are thought to encode sensory receptors. Of the remaining 360 receptors, the natural ligand has been identified for approximately 210 receptors, leaving 150 so-called orphan GPCRs with no known ligand or function. The identification of ligands active at orphan GPCRs has been achieved through the development of a number of experimental approaches, including the screening of putative small molecule and peptide ligands, reverse pharmacology, and the use of bioinformatics to predict candidate ligands. In this review, we discuss the methodologies developed for the identification of ligands at orphan GPCRs and include examples of their successful application.

Saturated fatty acids suppress adrenocorticotropic hormone (ACTH) release from rat anterior pituitary cells in vitro

We studied whether fatty acids modify adrenocorticotropic hormone (ACTH) release induced by stimulation with corticotropin-releasing hormone (CRH) from rat anterior pituitary cells. Stimulation with CRH (0.01-100 nmol/l) significantly and concentration-dependently increased ACTH release, which was synergistically enhanced by the simultaneous stimulation with 1 nmol/l arginine-vasopressin. Addition of saturated fatty acids (butyrate, caprylate, laurate, palmitate and stearate) in a medium at 1 mmol/l, despite effects on the basal release, significantly reduced the ACTH release induced by CRH (1 nmol/l) stimulation. Caprylate suppressed ACTH release in a concentration-dependent manner. However, unsaturated C18 and C20 fatty acids (oleate, linolate, linolenate and arachidonate) at 1 mmol/l significantly increased the basal release, but none of them suppressed CRH (1 nmol/l)-induced ACTH release. In the presence of caprylate (1 mmol/l), CRH (1 nmol/l)-stimulated increase in cellular calcium ion concentration was diminished. From these results we conclude that saturated fatty acids have a suppressing effect on CRH-induced ACTH increase in primary cultured rat anterior pituitary cells.

Familial hyperinsulinemic hypoglycemia caused by a defect in the SCHAD enzyme of mitochondrial fatty acid oxidation

Inappropriately elevated insulin secretion is the hallmark of persistent hyperinsulinemic hypoglycemia of infancy (PHHI), also denoted congenital hyperinsulinism. Causal mutations have been uncovered in genes coding for the beta-cell's ATP-sensitive potassium channel and the metabolic enzymes glucokinase and glutamate dehydrogenase. In addition, one hyperinsulinemic infant was recently found to have a mutation in the gene encoding short-chain 3-hydroxyacyl-CoA dehydrogenase (SCHAD), an enzyme participating in mitochondrial fatty acid oxidation. We have studied a consanguineous family with severe neonatal hypoglycemia due to increased insulin levels and where well-established genetic causes of hyperinsulinism had been eliminated. A genome-wide, microsatellite-based screen for homozygous chromosomal segments was performed. Those regions that were inherited in accordance with the presupposed model were searched for mutations in genes encoding metabolic enzymes. A novel, homozygous deletion mutation was found in the gene coding for the SCHAD enzyme. The mutation affected RNA splicing and was predicted to lead to a protein lacking 30 amino acids. The observations at the molecular level were confirmed by demonstrating greatly reduced SCHAD activity in the patients' fibroblasts and enhanced levels of 3-hydroxybutyryl-carnitine in their blood plasma. Urine metabolite analysis showed that SCHAD deficiency resulted in specific excretion of 3-hydroxyglutaric acid. By the genetic explanation of our family's cases of severe hypoglycemia, it is now clear that recessively inherited SCHAD deficiency can result in PHHI. This finding suggests that mitochondrial fatty acid oxidation influences insulin secretion by a hitherto unknown mechanism.

Leptin and the control of respiratory gene expression in muscle

Leptin plays a central role in the regulation of fatty acid homeostasis, promoting lipid storage in adipose tissue and fatty acid oxidation in peripheral tissues. Loss of leptin signaling leads to accumulation of lipids in muscle and loss of insulin sensitivity secondary to obesity. In this study, we examined the direct and indirect effects of leptin signaling on mitochondrial enzymes including those essential for peripheral fatty acid oxidation. We assessed the impact of leptin using the JCR:LA-cp rat, which lacks functional leptin receptors. The activities of marker mitochondrial enzymes citrate synthase (CS) and cytochrome oxidase (COX) were similar between wild-type (+/?) and corpulent (cp/cp) rats. In contrast, several tissues showed variations in the fatty acid oxidizing enzymes carnitine palmitoyltransferase II (CPT II), long-chain acyl-CoA dehydrogenase (LCAD) and 3-hydroxyacyl-CoA dehydrogenase (HOAD). It was not clear if these changes were due to loss of leptin signaling or to insulin insensitivity. Consequently, we examined the effects of leptin on cultured C(2)C(12) and Sol8 cells. Leptin (3 days at 0, 0.2, or 2.0 nM) had no direct effect on the activities of CS, COX, or fatty acid oxidizing enzymes. Leptin treatment did not affect luciferase-based reporter genes under the control of transcription factors involved in mitochondrial biogenesis (nuclear respiratory factor-1 (NRF-1), nuclear respiratory factor-2 (NRF-2)) or fatty acid enzyme expression (peroxisome proliferator-activated receptors (PPARs)). These studies suggest that leptin exerts only indirect effects on mitochondrial gene expression in muscle, possibly arising from insulin resistance.

Palmitate-induced Ca2+-signaling in pancreatic beta-cells

Free fatty acids (FFA) have been proposed to participate in the regulation of insulin release from pancreatic beta-cells (beta-cells). As a rise in cytosolic free Ca2+ ([Ca(2+)]i) is a key event for the stimulation of insulin secretion, the effects of saturated FFA on [Ca2+]i were investigated. Palmitate was used as a reference compound and [Ca2+]i was measured in single fura-2 loaded HIT-T15 and in primary mouse beta-cells. Stimulation of single beta-cells with palmitate (100 microM) caused either repetitive Ca2+ transients or a plateau-like rise in [Ca2+]i. In HIT-T15 and in mouse beta-cells, the number of palmitate-responsive cells, and the amplitude of the palmitate-induced Ca2+-signals were dependent on the extracellular glucose concentration. In Ca2+-free medium palmitate (100 microM) caused only 1 or 2 Ca2+ transients indicating mobilization of Ca2+ from internal stores. Withdrawal of external Ca2+, the addition of voltage-sensitive Ca2+ channel (VSCC) blockers, as well as the K(ATP)-channel opener diazoxide (100 microM) reversibly blocked the palmitate-induced cytosolic Ca2+ responses. This demonstrates that Ca2+ influx through VSCC of the L-type coupled to membrane depolarization through closure of K(ATP)-channels are crucial for a sustained Ca2+-signal in response to palmitate. Methyl palmoxirate (100 microM) and 2-bromopalmitate (100 microM), which both inhibit transport of acyl-CoA into the mitochondria, reversibly blocked the palmitate-induced Ca2+-signals in HIT-T15 as well as in primary mouse beta-cells. By contrast, cerulenin (100 microM), an inhibitor of protein acylation, had no effect on the palmitate-induced changes in [Ca2+]i, which suggests that mitochondrial palmitate metabolism is required for eliciting the Ca2+-signals. Simultaneous measurement of [Ca2+]i and the mitochondrial membrane potential (DeltaPsi) revealed palmitate-induced depolarization of DeltaPsi which demonstrates that palmitate does not enhance mitochondrial ATP production. Therefore mitochondrial signals other than ATP appear to be generated from palmitate metabolism that underly the palmitate-induced Ca2+-signals in pancreatic beta-cells.

Continued discovery of ligands for G protein-coupled receptors

G protein-coupled receptors are under intense scrutiny as potential targets of drug research, which stems mostly from the sheer size and diversity of this receptor family as well as the recognized high levels of specificity and sensitivity attainable by drugs targeting these receptors. The continued discovery of genes encoding G protein-coupled receptors has provided an extensive reserve of potential therapeutic targets. However, testing experimental therapeutic agents at these receptors requires a high degree of receptor characterization, beginning with the identity of an endogenous ligand. Often, low levels of sequence identity of a newly identified receptor to previously characterized receptors preclude the prompt identification of a ligand. In such cases, innovative techniques commonly referred to as reverse pharmacology have been employed to ascertain the ligand's identity for these "orphan" receptors. To date over 30 endogenous ligands, both novel and previously known, have been paired with orphan G protein-coupled receptors. Here, we briefly summarize the recent identification of neuropeptides W and B and carboxylic acid anions for their respective receptors GPR7, GPR8 and GPR40, GPR41, GPR43.