UNIPROT:P01275 - FACTA Search

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Query: UNIPROT:P01275 (glucagon)
26,492 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

In cultured rat hepatocytes, the gluconeogenic key enzyme, phosphoenolpyruvate carboxykinase (PCK), is induced by glucagon via elevation of cyclic 3',5' adenosine monophosphate (cAMP). The proinflammatory cytokine, interleukin-6 (IL-6), which in the liver together with IL-1beta and tumor necrosis factor alpha triggers the acute-phase response, had been shown to attenuate the glucagon-induced increase in PCK gene transcription, messenger (mRNA) levels, and enzyme activity. The molecular mechanism of this inhibition was investigated in the present study. Glucagon increased cyclic cAMP and PCK mRNA levels to a transient maximum twofold and fivefold, respectively. The increases were attenuated by IL-6. Forskolin, which stimulates adenylate cyclase activity, increased cAMP and PCK mRNA levels 1.6-fold and fivefold, respectively. However, IL-6 attenuated the forskolin-stimulated increase in PCK mRNA but not the increase in cAMP. This showed that IL-6 inhibited PCK mRNA increase in part by the attenuation of cAMP increase, but also beyond cAMP formation. This was confirmed in experiments in which PCK mRNA levels were increased by the nonhydrolyzable cAMP-analogue, chlorophenylthio (CPT)-cAMP. The increase in PCK mRNA was again attenuated by IL-6. In pertussis toxin- and in isobutylmethylxanthine-treated hepatocytes, IL-6 still inhibited the glucagon-stimulated increase in cAMP, indicating that IL-6 did not activate an inhibitory G-protein or phosphodiesterase, which could cause the impairment of cAMP increase. To demonstrate whether the inhibition of PCK gene expression by IL-6 beyond cAMP might be caused by the inhibition of the activation of the PCK gene promoter by cAMP, cultured rat hepatocytes were transfected with a luciferase reporter gene construct under the control of a PCK gene promoter fragment (base -979 to base +32). Luciferase activity was determined after stimulation of the cells with CPT-cAMP in the absence or presence of IL-6. CPT-cAMP increased luciferase activity by 1.7-fold, which was inhibited in the presence of IL-6. It is concluded that IL-6 had a dual inhibitory effect on the stimulation of PCK gene expression by glucagon. It inhibited the increase in cAMP at a site before cAMP formation by adenylate cyclase and at a site after cAMP formation, the activation of the PCK gene promoter by cAMP.
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PMID:Mechanism of the impairment of the glucagon-stimulated phosphoenolpyruvate carboxykinase gene expression by interleukin-6 in rat hepatocytes: inhibition of the increase in cyclic 3',5' adenosine monophosphate and the downstream cyclic 3',5' adenosine monophosphate action. 921 54

Gene expression of aldolase B, an important enzyme for glucose and fructose metabolism, is regulated by hormones. We examined direct effects of major hormones on aldolase B gene expression in rat primary cultured hepatocytes, in comparison with those on the gene expression of phospho(enol)pyruvate carboxykinase (PEPCK), a key enzyme for gluconeogenesis. Insulin, dexamethasone, and high concentration of glucose increased aldolase B mRNA abundance in the hepatocytes. Glucagon strongly suppressed aldolase B gene expression, and this hormone canceled the stimulative effects of insulin, dexamethasone, and high concentration of glucose. Epinephrine and thyroxine slightly reduced aldolase B mRNA abundance, but these hormones did not cancel the stimulative effects of insulin and dexamethasone. To the contrary, expression of PEPCK gene was suppressed by insulin, dexamethasone, and high concentration of glucose, and remarkably induced by glucagon. Glucagon rapidly suppressed aldolase B gene expression at the transcriptional level. Forskolin and dibutyryl cAMP mimicked the suppressive effect of glucagon on aldolase B gene expression. These results suggest that glucagon may be a key regulator of aldolase B gene transcription through a cAMP/protein kinase A-signaling pathway.
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PMID:Hormonal regulation of aldolase B gene expression in rat primary cultured hepatocytes. 947 4

From video imaging of fura 2-loaded baby hamster kidney (BHK) cells stably expressing the cloned human glucagon receptor, we found the Ca2+ response to glucagon to be specific, dose dependent, synchronous, sensitive to pertussis toxin, and independent of Ca2+ influx. Forskolin did not elicit a Ca2+ response, but treatment with a protein kinase A inhibitor, the Rp diastereomer of 8-bromoadenosine-3',5'-cyclic monophosphothioate, resulted in a reduced glucagon-mediated Ca2+ response as well as Ca2+ oscillations. The specific phospholipase C inhibitor U-73122 abolished the Ca2+ response to glucagon, and a modest twofold increase in inositol trisphosphate (IP3) production could be observed after stimulation with glucagon. In BHK cells coexpressing glucagon and muscarinic (M1) acetylcholine receptors, carbachol blocked the rise in intracellular free Ca2+ concentrations in response to glucagon, whereas glucagon did not affect the carbachol-induced increase in Ca2+. Furthermore, carbachol, but not glucagon, could block thapsigargin-activated increases in intracellular free Ca2+ concentration. These results indicate that, in BHK cells, glucagon receptors can activate not only adenylate cyclase but also a second independent G protein-coupled pathway that leads to the stimulation of phospholipase C and the release of Ca2+ from IP3-sensitive intracellular Ca2+ stores. Finally, we provide evidence to suggest that cAMP potentiates the IP3-mediated effects on intracellular Ca2+ handling.
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PMID:Glucagon-mediated Ca2+ signaling in BHK cells expressing cloned human glucagon receptors. 961 Nov 20

Cyclic AMP potentiates glucose-stimulated insulin release by actions predominantly at a site, or sites, distal to the elevation of the cytosolic free Ca2+ concentration ([Ca2+]i). Glucose also acts at a site, or sites, distal to the elevation of [Ca2+]i via the ATP-sensitive K+ channel (K+ATP channel)-independent signaling pathway. Accordingly, using rat pancreatic islets, we studied the location of the action of cAMP and its interaction with the glucose pathway. Forskolin, an activator of adenylyl cyclase, raised intracellular cAMP levels and enhanced KCl-induced (Ca2+ -stimulated) insulin release in the presence, but not in the absence, of glucose. Thus, cAMP has no direct effect on Ca2+ -stimulated insulin release. The interaction between cAMP and glucose occurs at a step distal to the elevation of [Ca2+]i because forskolin enhancement of KCl-induced insulin release, in the presence of glucose, was demonstrated in the islets treated with diazoxide, a K+ATP channel opener. The enhancement of insulin release was not associated with any increase in [Ca2+]i. Furthermore, the interaction between cAMP and glucose was unequivocally observed even under stringent Ca2+ -free conditions, indicating the Ca2+ -independent action of cAMP. This action of cAMP is physiologically relevant, because not only forskolin but also glucagon-like peptide 1, glucose-dependent insulinotropic polypeptide, and pituitary adenylyl cyclase activating polypeptide exerted similar actions. In conclusion, the cAMP/protein kinase A pathway has no direct effect on Ca2+ -stimulated insulin exocytosis. Rather, it strongly potentiates insulin release by increasing the effectiveness of the K+ATP channel-independent action of glucose.
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PMID:cAMP enhances insulin secretion by an action on the ATP-sensitive K+ channel-independent pathway of glucose signaling in rat pancreatic islets. 1033 4

Although phospholipase A(2) (PLA(2)) is of importance for insulin secretion, it is not established how it relates to other signalling mechanisms. This study examined the crosstalk between PLA(2) and the cyclic AMP (cAMP)-protein kinase A (PKA) pathway in isolated rat islets. Forskolin, IBMX, and dbcAMP reduced [(3)H]arachidonic acid ([(3)H]AA) efflux from prelabelled islets during PLA(2) activation by mellitin or cholecystokinin (CCK-8), while efflux induced by carbachol was unaffected. The PKA inhibitor myrPKI(14-22) prevented this reduction of CCK-8-induced efflux. Glucagon-like peptide-1 (GLP-1), gastric inhibitory polypeptide (GIP), and vasoactive intestinal polypeptide (VIP) diminished CCK-8-induced efflux. Also in the absence of Ca(2+), forskolin/IBMX and dbcAMP reduced CCK-8-induced efflux. In parallel with effects on [(3)H]AA, the expected additive insulin secretion induced by mellitin or CCK-8 in combination with forskolin or GLP-1, respectively, was reduced. In conclusion, the cAMP-PKA pathway restrains both Ca(2+)-dependent and Ca(2+)-independent PLA(2) activation, indicating a regulating crosstalk between these two pathways.
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PMID:The cyclic AMP-protein kinase A pathway restrains islet phospholipase A(2) activation. 1069 7

Since many isoforms of adenylyl cyclase and adenosine 3', 5'-monophosphate (cAMP) phosphodiesterase have been cloned, it is likely that receptors of each hormone have a specific combination of these isoforms. Types I, III and VIII adenylyl cyclases are reported to be stimulated by Ca(2+)-calmodulin, type I phosphodiesterase by Ca(2+)-calmodulin, but types IV and VII (cAMP-specific) phosphodiesterases by Co2+. In the present study, we examined different effects of Ca2+ and Co2+ on hormone-induced cAMP response in the isolated perfused rat liver.The removal of Ca2+ from the perfusion medium (0 mM CaCl(2 ) + 0.5 mM EGTA) did not affect glucagon (0.1 nM)-responsive cAMP but reduced secretin (1 nM)-, vasoactive intestinal polypeptide (VIP, 1-10 nM)- and forskolin (1 microM)-responsive cAMP considerably. The addition of 1 mM CoCl2 reduced glucagon- and secretin-responsive cAMP considerably, forskolin-responsive cAMP partly, did not affect 1 nM VIP-responsive cAMP, but enhanced 10 nM VIP-responsive cAMP. Forskolin- and VIP-responsive cAMP was greater in the combination (0 mM CaCl(2) + 0.5 mM EGTA + 3 mM CoCl2) than in the Ca(2+)-free perfusion alone. These results suggest that secretin, VIP1 and VIP2 receptors are linked to Ca(2+)-calmodulin-sensitive adenylyl cyclase; glucagon receptor to Ca(2+)-calmodulin-insensitive adenylyl cyclase; VIP1 receptor to Ca(2+)-calmodulin-dependent phosphodiesterase; glucagon, secretin and VIP2 receptors to cAMP-specific phosphodiesterase, respectively, in the rat liver.
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PMID:Hormone-specific combinations of isoforms of adenylyl cyclase and phosphodiesterase in the rat liver. 1125 14

The PICM-19 fetal liver cell line was isolated from the primary culture and spontaneous differentiation of pig epiblast cells, i.e. embryonic stem cells. PICM-19 cells were induced to differentiate into mostly ductular formations by culturing at pH 7.6-7.8. The ductules were functionally assayed by treatment with cAMP inducing agents and bioactive peptides reported to influence the secretory activity of liver bile ductules. The secretory response of the cells was assessed by qualitative or quantitative measurement of the cross-sectional area of the ductal lumens and the appearance of biliary canaliculi in between PICM-19 cells that had formed monolayers instead of ducts. Forskolin (10 microM) and 8-bromoadenosine 3':5'-cyclic monophosphate (bcAMP; 2 mM) stimulated fluid transport and expansion of ductal structures in 15-20 min and stimulated the appearance and expansion of biliary canaliculi in 30-60 min. Cholera toxin (50 ng/ml) stimulates fluid transport in both ductules and canaliculi in 1-2 h, while 8-bromoguanosine 3':5'-cyclic monophosphate (bcGMP; 2 mM) stimulated only biliary canaliculi in 2 h. Glucagon (1.4 nM) produced a similar response in 5-10 min in ductal structures only, but the response was transitory and was almost completely reversed within 30 min. Secretin (100 pM) and vasoactive intestinal peptide (75 pM) produced a sustained response with maximal ductal lumen expansion occurring in 5-10 min and neither had an immediate effect on canaliculi. Somatostatin (0.5 microM) and gastrin (1 microM) caused marked reduction or disappearance of ductal lumens in 30-60 min, but was ineffective in reversing secretin (100 nM)-induced duct distension. Application of the adrenergic agonists, epinephrine, isoproterenol, and phenylephrine (100 microM), resulted in the complete shrinkage of ductal lumens in 20-30 min. A shift to pH 7.0-7.2 resulted in almost complete reduction of ductal lumens, while a shift to pH 7.8-8.0 resulted in expansion, although not full expansion, of the ductal lumens. PICM-19 bile duct cultures were positive for cytokeratin-7, aquaporin-1 and aquaporin-9 by Western blot analysis. The amounts of these proteins increased in the cultures as differentiation proceeded over time. Transmission electron microscopy revealed that the ductal structures were usually sandwiched between SIM mouse, thioguanine- and ouabain-resistant (STO) feeder cells that had produced a collagen matrix. Also, the ductular PICM-19 cells possessed cilia, probably occurring as a single cilium in each cell, that projected into the lumens of the ducts. The results indicated that the in vitro-produced ductal structures of the PICM-19 cell line are a functional model for biliary epithelium.
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PMID:The PICM-19 cell line as an in vitro model of liver bile ductules: effects of cAMP inducers, biopeptides and pH. 1209 33

Glucose-induced insulin secretion from isolated, perifused rat islets is pulsatile with a period of about 5-10 min, similar to the insulin oscillations that are seen in healthy humans but which are impaired in Type II diabetes. We evaluated the pattern of enhancement by the potent incretin, glucagon-like peptide 1 (GLP-1). GLP-1 increased the amplitude of pulses and the magnitude of insulin secretion from the perifused islets, without affecting the average time interval between pulses. Forskolin and the phosphodiesterase inhibitor isobutylmethylxanthine had the same effect, suggesting that the effect was due to elevated cAMP levels. The possibility that cAMP might enhance the amplitude of pulses by reducing phosphofructo-2-kinase (PFK-2) activity was eliminated when the liver isoform of PFK-2 was shown to be absent from beta-cells. The possibility that cAMP enhanced pulsatile secretion, at least in part, by stimulating lipolysis was supported by the observations that added oleate had a similar effect on secretion, and that the incretin effect of GLP-1 was inhibited by the lipase inhibitor orlistat. These data show that the physiological incretin GLP-1 preserves and enhances normal pulsatile insulin secretion, which may be essential in proposed therapeutic uses of GLP-1 or its analogues.
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PMID:Glucagon-like peptide 1 and fatty acids amplify pulsatile insulin secretion from perifused rat islets. 1235 35

Isolated rat islets were exposed to cAMP-elevating agents and/or nutrients. Insulin exocytosis subsequently triggered by a depolarizing concentration of K(+) or a stimulatory concentration of glucose was employed as an index of time-dependent potentiation (TDP). Stimulatory concentrations (>or=5.5 mM) of glucose caused TDP, and 6 micro M forskolin (an activator of adenylyl cyclase) significantly enhanced it (3.1-fold at most). Forskolin produced an 8.0-fold increase in islet cell cAMP; however, it returned to the baseline after washout by the time of stimulation of exocytosis. Two millimoles of dibutyryl cAMP (a cAMP analog), 0.1 mM isobutylmethylxanthine (a phosphodiesterase inhibitor), and 100 nM glucagon-like peptide-1 (an incretin hormone) also enhanced glucoseinduced TDP. The time-dependent effect of cAMP was not attenuated by protein kinase A inhibitors (200 micro M adenosine 3',5'-cyclic monophosphothioate, Rp isomer, and 10 micro M H89). Although glucose-induced TDP was attenuated by NaN(3) (a mitochondrial poison) and cerulenin (an inhibitor of protein acylation), cAMP enhancement of it was unaffected by these agents. In conclusion, cAMP time-dependently stimulates insulin exocytosis, provided the extracellular glucose concentration is equivalent to or higher than ambient plasma levels. Protein kinase A, mitochondrial metabolism, and protein acylation are not involved in this cAMP action. Incretin stimulation of insulin exocytosis may occur in part via this mechanism.
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PMID:Time-dependent stimulation of insulin exocytosis by 3',5'-cyclic adenosine monophosphate in the rat islet beta-cell. 1239 13

We investigated intracellular Ca(2+) ([Ca(2+)](i)) oscillations evoked by glucagon-like peptide 1 (GLP-1) in relation to the ryanodine receptor (RyR) and Ca(2+)-induced Ca(2+)release (CICR) mechanism in pancreatic B cell HIT. GLP-1 produced [Ca(2+)](i) oscillations in the cells, both in media with and without Ca(2+), an effect inhibited by ruthenium red and mimicked by 8-Br-cAMPS. In addition, the GLP-1-evoked [Ca(2+)](i) rise was initiated at the local intercellular peripheral cytoplasm, and a resultant expansion of the intercellular space was also observed. Caffeine induced [Ca(2+)](i) elevation in the medium with or without Ca(2+), an effect inhibited by ruthenium red. GLP-1-evoked [Ca(2+)](i) oscillations were also enhanced by IBMX, and eliminated by Rp-8-Br-cAMPS or 20 microM H-89 treatments whereas they were unaffected by 2 microM H-89 treatment. Forskolin caused a transient elevation in [Ca(2+)](i) that was reduced by Rp-8-Br-cAMPS, 2 microM or 20 microM H-89. Our results indicate that GLP-1 initially generated a local [Ca(2+)](i) elevation at the peripheral cytoplasm, subsequently producing [Ca(2+)](i) oscillations that were inhibited by ruthenium red, involving ryanodine-sensitive and cAMP-activated CICR mechanisms. The cytoplasmic levels of cAMP as well as local Ca(2+) might be responsible for [Ca(2+)](i) oscillations.
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PMID:Involvement of the ryanodine-sensitive Ca2+ store in GLP-1-induced Ca2+ oscillations in insulin-secreting HIT cells. 1246 36

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