Pancreastatin increases free cytosolic Ca2+ in rat hepatocytes, involving both pertussis-toxin-sensitive and -insensitive mechanisms

Functional Gly297Ser Variant of the Physiological Dysglycemic Peptide Pancreastatin Is a Novel Risk Factor for Cardiometabolic Disorders

Pancreastatin (PST), a chromogranin A-derived potent physiological dysglycemic peptide, regulates glucose/insulin homeostasis. We have identified a nonsynonymous functional PST variant (p.Gly297Ser; rs9658664) that occurs in a large section of human populations. Association analysis of this single nucleotide polymorphism with cardiovascular/metabolic disease states in Indian populations (n = 4,300 subjects) displays elevated plasma glucose, glycosylated hemoglobin, diastolic blood pressure, and catecholamines in Gly/Ser subjects as compared with wild-type individuals (Gly/Gly). Consistently, the 297Ser allele confers an increased risk (∼1.3-1.6-fold) for type 2 diabetes/hypertension/coronary artery disease/metabolic syndrome. In corroboration, the variant peptide (PST-297S) displays gain-of-potency in several cellular events relevant for cardiometabolic disorders (e.g., increased expression of gluconeogenic genes, increased catecholamine secretion, and greater inhibition of insulin-stimulated glucose uptake) than the wild-type peptide. Computational docking analysis and molecular dynamics simulations show higher affinity binding of PST-297S peptide with glucose-regulated protein 78 (GRP78) and insulin receptor than the wild-type peptide, providing a mechanistic basis for the enhanced activity of the variant peptide. In vitro binding assays validate these in silico predictions of PST peptides binding to GRP78 and insulin receptor. In conclusion, the PST 297Ser allele influences cardiovascular/metabolic phenotypes and emerges as a novel risk factor for type 2 diabetes/hypertension/coronary artery disease in human populations.

Pancreastatin is an endogenous peptide that regulates glucose homeostasis

Pancreastatin (PST) is a regulatory peptide containing 49 amino acids, first isolated from porcine pancreas. Intracellular and extracellular processing of the prohormone Chromogranin A (Chga) results various bioactive peptides of which PST has dysglycemic activity. PST regulates glucose, lipid, and protein metabolism in liver and adipose tissues. It also regulates the secretion of leptin and expression of leptin and uncoupling protein 2 in adipose tissue. In Chga knockout mice, PST induces gluconeogenesis in the liver. PST reduces glucose uptake in mice hepatocytes and adipocytes. In rat hepatocytes, PST induces glycogenolysis and glycolysis and inhibits glycogen synthesis. In rat adipocytes, PST inhibits lactate production and lipogenesis. These metabolic effects are confirmed in humans. In the dual signaling mechanism of PST receptor, mostly PST activates Gαq/11 protein leads to the activation of phospholipase C β3-isoform, therefore increasing cytoplasmic free calcium and stimulating protein kinase C. PST inhibits the cell growth in rat HTC hepatoma cells, mediated by nitric oxide and cyclic GMP production. Elevated levels of PST correlating with catecholamines have been found in gestational diabetes and essential hypertension. Rise in the blood PST level in Type 2 diabetes suggests that PST is a negative regulator of insulin sensitivity and glucose homeostasis.

Reprint of: Metabolic effects and mechanism of action of the chromogranin A-derived peptide pancreastatin

Pancreastatin is one of the regulatory peptides derived from intracellular and/or extracellular processing of chromogranin A, the soluble acidic protein present in the secretory granules of the neuroendocrine system. While the intracellular functions of chromogranin A include formation and maturation of the secretory granule, the major extracellular functions are generation of biologically active peptides with demonstrated autocrine, paracrine or endocrine activities. In this review, we will focus on the metabolic function of one of these peptides, pancreastatin, and the mechanisms underlying its effects. Many different reported effects have implicated PST in the modulation of energy metabolism, with a general counterregulatory effect to that of insulin. Pancreastatin induces glycogenolysis in liver and lipolysis in adipocytes. Metabolic effects have been confirmed in humans. Moreover, naturally occurring human variants have been found, one of which (Gly297Ser) occurs in the functionally important carboxy-terminus of the peptide, and substantially increases the peptide's potency to inhibit cellular glucose uptake. Thus, qualitative hereditary alterations in pancreastatin's primary structure may give rise to interindividual differences in glucose and lipid metabolism. Pancreastatin activates a receptor signaling system that belongs to the seven-spanning transmembrane receptor coupled to a Gq-PLCβ-calcium-PKC signaling pathway. Increased pancreastatin plasma levels, correlating with catecholamines levels, have been found in insulin resistance states, such as gestational diabetes or essential hypertension. Pancreastatin plays important physiological role in potentiating the metabolic effects of catecholamines, and may also play a pathophysiological role in insulin resistance states with increased sympathetic activity.

Metabolic effects and mechanism of action of the chromogranin A-derived peptide pancreastatin

Pancreastatin is one of the regulatory peptides derived from intracellular and/or extracellular processing of chromogranin A, the soluble acidic protein present in the secretory granules of the neuroendocrine system. While the intracellular functions of chromogranin A include formation and maturation of the secretory granule, the major extracellular functions are generation of biologically active peptides with demonstrated autocrine, paracrine or endocrine activities. In this review, we will focus on the metabolic function of one of these peptides, pancreastatin, and the mechanisms underlying its effects. Many different reported effects have implicated PST in the modulation of energy metabolism, with a general counterregulatory effect to that of insulin. Pancreastatin induces glycogenolysis in liver and lipolysis in adipocytes. Metabolic effects have been confirmed in humans. Moreover, naturally occurring human variants have been found, one of which (Gly297Ser) occurs in the functionally important carboxy-terminus of the peptide, and substantially increases the peptide's potency to inhibit cellular glucose uptake. Thus, qualitative hereditary alterations in pancreastatin's primary structure may give rise to interindividual differences in glucose and lipid metabolism. Pancreastatin activates a receptor signaling system that belongs to the seven-spanning transmembrane receptor coupled to a Gq-PLCbeta-calcium-PKC signaling pathway. Increased pancreastatin plasma levels, correlating with catecholamines levels, have been found in insulin resistance states, such as gestational diabetes or essential hypertension. Pancreastatin plays important physiological role in potentiating the metabolic effects of catecholamines, and may also play a pathophysiological role in insulin resistance states with increased sympathetic activity.

eNOS, nNOS, cGMP and protein kinase G mediate the inhibitory effect of pancreastatin, a chromogranin A-derived peptide, on growth and proliferation of hepatoma cells

Pancreastatin (PST), a chromogranin A-derived peptide, has an anti-insulin metabolic effect and inhibits growth and proliferation by producing nitric oxide (NO) in HTC rat hepatoma cells. When NO production is blocked, a proliferative effect prevails due to the activation a Galphaq/11-phospholipase C-beta (PLC-beta) pathway, which leads to an increase in [Ca2+]i, protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) activation. The aim of the present study was to investigate the NO synthase (NOS) isoform that mediates these effects of PST on HTC hepatoma cells and the possible roles of cyclic GMP (cGMP) and cGMP-dependent protein kinase. DNA and protein synthesis in response to PST were measured as [3H]-thymidine and [3H]-leucine incorporation in the presence of various pharmacological inhibitors: N-monomethyl-L-arginine (NMLA, nonspecific NOS inhibitor), L-NIO (endothelial nitric oxide synthase (eNOS) inhibitor), espermidine (neuronal nitric oxide synthase (nNOS) inhibitor), LY83583 (guanylyl cyclase inhibitor), and KT5823 (protein kinase G inhibitor, (PKG)). L-NIO, similarly to NMLA, reverted the inhibitory effect of PST on hepatoma cell into a stimulatory effect on growth and proliferation. Nevertheless, espermidine also prevented the inhibitory effect of PST, but there was no stimulation of growth and proliferation. When guanylyl cyclase activity was blocked, there was again a reversion of the inhibitory effect into a stimulatory action, suggesting that the effect of NO was mediated by the production of cGMP. PKG inhibition prevented the inhibitory effect of PST, but there was no stimulatory effect. Therefore, the inhibitory effect of PST on growth and proliferation of hepatoma cells may be mainly mediated by eNOS activation. In turn, the effect of NO may be mediated by cGMP, whereas other pathways in addition to PKG activation seem to mediate the inhibition of DNA and protein synthesis by PST in HTC hepatoma cells.

Pancreastatin, a chromogranin A-derived peptide, inhibits DNA and protein synthesis by producing nitric oxide in HTC rat hepatoma cells

BACKGROUND/AIMS

Pancreastatin, a chromogranin A-derived peptide, has a counter-regulatory effect on insulin action. We have previously characterized pancreastatin receptor and signalling in rat liver and HTC hepatoma cells. A G alpha(q/11)-PLC-beta pathway leads to an increase in [Ca2+]i, PKC and mitogen activated protein kinase (MAPK) activation. These data suggested that pancreastatin might have a role in growth and proliferation, similar to other calcium-mobilizing hormones.

METHODS

DNA and protein synthesis were measured as [3H]-thymidine and [3H]-leucine incorporation. Nitric oxide (NO) was determined by the Griess method and cGMP production was quantified by enzyme-linked immunoassay.

RESULTS

Contrary to the expected results, we have found that pancreastatin inhibits protein and DNA synthesis in HTC hepatoma cells. On the other hand, when the activity of NO synthase was inhibited by N-monomethyl-L-arginine (NMLA), the inhibitory effect of pancreastatin on DNA and protein synthesis was not only reverted, but a dose-dependent stimulatory effect was observed, probably due to MAPK activation, since it was prevented by PD98059. These data strongly suggested the role of NO in the inhibitory effect of pancreastatin on protein and DNA synthesis, which is overcoming the effect on MAPK activation. Moreover, pancreastatin dose-dependently increased NO production in parallel to cyclic guanosine monophosphate (cGMP). Both effects were prevented by NMLA. Finally, an indirect effect of pancreastatin through the induction of apoptosis was ruled out.

CONCLUSIONS

Therefore, the NO and the cGMP produced by the NO-activated guanylate cyclase may mediate the dose-dependent inhibitory effect of pancreastatin on growth and proliferation in HTC hepatoma cells.

Pancreastatin, a chromogranin A-derived peptide, activates Galpha(16) and phospholipase C-beta(2) by interacting with specific receptors in rat heart membranes

Pancreastatin (PST) is one of the chromogranin A (CGA)-derived peptides with known biological activity. It has a general inhibitory effect on secretion in many exocrine and endocrine systems including the heart atrium. Besides, a role of PST as a counter-regulatory peptide of insulin action has been proposed in the light of its effects on glucose and lipid metabolism in the liver and adipose tissue, where receptors and signaling have been described. Galpha(q/11) pathway seems to mediate PST action. Since PST has been shown to function as a typical calcium-dependent hormone, and increased plasma levels have been found in essential hypertension correlating with catecholamines, we sought to study its possible interaction and signaling in heart membranes. Here, we are characterizing specific PST binding sites and signaling in rat heart membranes. We have found that PST receptor has a K(d) of 0.5 nM and a B(max) of 34 fmol/mg of protein. The PST binding is inhibited by guanine nucleotides, suggesting the functional coupling of the receptor with GTP binding proteins (G proteins). Moreover, PST dose-dependently increases GTP binding to rat heart membranes. Finally, we have studied PST signaling-effector system by measuring phospholipase C (PLC) activity using blocking antibodies against different G proteins and PLC isoforms. We have found that PST stimulates PLCbeta(2)>PLCbeta(1)>PLCbeta(3) by activating Galpha(16) in rat heart membranes. These data suggest that PST may modulate the cardiac function.

Characterization of pancreastatin receptor and signaling in rat HTC hepatoma cells

Pancreastatin, a chromogranin A-derived peptide widely distributed throughout the neuroendocrine system, has a general inhibitory effect on endocrine secretion and a counterregulatory effect on insulin action. We have recently described the cross-talk of pancreastatin with insulin signaling in rat hepatoma cells (HTC), where it inhibits insulin action and signaling through the serine phosphorylation of the insulin receptor, thereby impairing tyrosine kinase activity. Here, we have characterized pancreastatin receptors and signaling in HTC cells. The pancreastatin effector systems were studied by determining phospholipase C activity in HTC membranes and mitogen-activated protein kinase (MAPK) phosphorylation activity in HTC cells. Binding studies with radiolabeled pancreastatin showed a population of high affinity binding sites, with a B(max) of 8 fmol/mg protein and a K(d) of 0.6 nM. Moreover, we assessed the coupling of the receptor with a G protein system by inhibiting the binding with guanine nucleotide and by measuring the GTP binding to HTC membranes. We found that pancreastatin receptor was coupled with a G alpha(q/11) protein which activates phospholipase C-beta(1) and phospholipase C-beta(3), in addition to MAPK via both beta gamma and alpha(q/11).

Affinity purification of pancreastatin receptor-Gq/11 protein complex from rat liver membranes

Pancreastatin, a chromogranin A derived peptide, exerts a glycogenolytic effect on the hepatocyte. This effect is initiated by binding to membrane receptors which are coupled to pertussis toxin insensitive G proteins belonging to the Gq/11 family. We have recently solubilized active pancreastatin receptors from rat liver membranes still functionally coupled to G proteins. Here, we have purified pancreastatin receptors by a two-step procedure. First, pancreastatin receptors with their associated Gq/11 regulatory proteins were purified from liver membranes by lectin absorption chromatography on wheat germ agglutinin immobilized on agarose. A biotinylated rat pancreastatin analog was tested for binding to liver membranes before using it for affinity purification. Unlabeled biotinylated rat pancreastatin competed for 125I-labeled [Tyr0]PST binding to solubilized receptors with a Kd = 0.27 nM, comparable to that of native pancreastatin. The biotinylated analog was immobilized on streptavidin-coated Sepharose beads and used to further affinity purify wheat germ agglutinin eluted receptor material. Specific elution at low pH showed that the receptor protein was purified as an 80-kDa protein in association with a G protein of the q/11 family, as demonstrated by specific immunoblot analysis. The specificity of the receptor band was assessed by chemical cross-linking of the purified material followed by SDS-PAGE and autoradiography. In conclusion, we have purified pancreastatin receptor as a glycoprotein of 80 kDa physically associated with a Gq/11 protein.

Characterization of pancreastatin receptors and signaling in adipocyte membranes

Pancreastatin (PST), a chromogranin A derived peptide with an array of effects in different tissues, has a role as a counterregulatory hormone of insulin action in hepatocytes and adipocytes, regulating glucose, lipid and protein metabolism. We have previously characterized PST receptors and signaling in rat hepatocytes, in which PST functions as a calcium-mobilizing hormone. In the present work we have studied PST receptors as well as the signal transduction pathways generated upon PST binding in adipocyte membranes. First, we have characterized PST receptors using radiolabeled PST as a ligand. Analysis of binding data indicated the existence of one class of binding sites, with a B(max) of 5 fmol/mg of protein and a K(d) of 1 nM. In addition, we have studied the G protein system that couples the PST receptor by gamma-(35)S-GTP binding studies. We have found that two G protein systems are involved, pertussis toxin-sensitive and -insensitive respectively. Specific anti-G protein alpha subtype sera were used to block the effect of pancreastatin receptor activation. Galpha(q/11) and to a lesser extent Galpha(i1,2) are activated by PST in rat adipocyte membranes. On the other hand, adenylate cyclase activity was not affected by PST. Finally, we have studied the specific phospholipase C isoform that is activated in response to PST. We have found that PST receptor is coupled to PLC-beta(3) via Galpha(q/11) activation in adipocyte membranes.

G protein G alpha q/11 and G alpha i1,2 are activated by pancreastatin receptors in rat liver: studies with GTP-gamma 35S and azido-GTP-alpha-32P

In the liver, pancreastatin exerts a glycogenolytic effect through interaction with specific receptors, followed by activation of phospholipase C and guanylate cyclase. Pancreastatin receptor seems to be coupled to two different G protein systems: a pertussis toxin-insensitive G protein that mediates activation of phospholipase C, and a pertussis toxin sensitive G protein that mediates the cyclic GMP production. The aim of this study was to identify the specific G protein subtypes coupling pancreastatin receptors in rat liver membranes. GTP binding was determined by using gamma-35S-GTP; specific anti-G protein alpha subtype sera were used to block the effect of pancreastatin receptor activation. Activation of G proteins was demonstrated by the incorporation of the photoreactive GTP analogue 8-azido-alpha-32P-GTP into liver membranes and into specific immunoprecipitates of different Galpha subunits from soluble rat liver membranes. Pancreastatin stimulation of rat liver membranes increases the binding of gamma-35S-GTP in a time- and dose-dependent manner. Activation of the soluble receptors still led to the pancreastatin dose-dependent stimulation of gamma-35S-GTP binding. Besides, WGA semipurified receptors also stimulates GTP binding. The binding was inhibited by treatment with anti-Galphaq/11 (85%) and anti-Galphai1,2 (15%) sera, whereas anti-Galphao,i3 serum failed to affect the binding. Finally, pancreastatin stimulates GTP photolabeling of particulate membranes. Moreover, it specifically increased the incorporation of 8-azido-alpha-32P-GTP into Galphaq/11 and Galpha, but not into Galphao,i3 from soluble rat liver membranes. In conclusion, pancreastatin stimulation of rat liver membranes led to the activation of Galphaq/11 and Galphai1,2 proteins. These results suggest that Galphaq/11 and Galphai1,2 may play a functional role in the signaling of pancreastatin receptor by mediating the production of IP3 and cGMP respectively.

Pancreastatin inhibits insulin action in rat adipocytes

Pancreastatin (PST), a regulatory peptide with a general inhibitory effect on secretion, is derived from chromogranin A, a glycoprotein present throughout the neuroendocrine system. We have previously demonstrated the counterregulatory role of PST on insulin action in rat hepatocytes. Here, we are reporting the PST effects on rat adipocytes. PST dose dependently inhibits basal and insulin-stimulated glucose transport, lactate production, and lipogenesis, impairing the main metabolic actions of insulin in adipocytes. These effects were observed in a wide range of insulin concentrations, leading to a shift to the right in the dose-response curve. Maximal effect was observed at 10 nM PST, and the IC50 value was approximately 1 nM. Moreover, PST has a lipolytic effect in rat adipocytes (ED50 0.1 nM), although it was completely inhibited by insulin. In contrast, PST dose dependently stimulated protein synthesis and enhanced insulin-stimulated protein synthesis. In summary, these data show the lipokinetic effect of PST and the inhibitory effect of PST on insulin metabolic action within a range of physiological concentrations. Therefore, these results give new pathophysiological basis for the association of PST with insulin resistance.

Pancreastatin activates beta3 isoform of phospholipase C via G(alpha)11 protein stimulation in rat liver membranes

Pancreastatin (PST) receptors have been recently shown to mediate activation of phospholipase C (PLC) in rat liver membranes. There is evidence that the G protein that links pancreastatin receptor with PLC-beta is pertussis toxin-insensitive and belongs to the G(alpha)q family. Here, we have employed blocking antisera to sort out the specific PLC-beta isoform as well as the specific G(alpha) subunit activated by PST receptor in rat liver membranes. The presence of different PLC-beta isoforms was checked by immunoblot analysis. Only PLC-beta4 was not detected, whereas PLC-beta1, beta2 and beta3 were abundant in rat liver membranes. However, only anti-PLC-beta3 serum was able to block the PST receptor response. We also checked the expression of G(alpha)q and Galpha11 in rat liver membranes by immunoblot. Even though both isoforms were present. only anti-Galpha11 serum was able to block the PST receptor response. In order to check the specificity of the blocking antisera, we employed them to block the effect of ADP and thrombin stimulating PLC activity in platelet membranes, a system lacking Galpha11. Anti-G(alpha)q but not anti-Galpha11 sera were able to block the agonist stimulated PLC activity. These data suggest that PST receptor response is mediated by the activation of the beta3 isoform of PLC via Galpha11 protein stimulation in rat liver membranes.

Pancreastatin receptor is coupled to a guanosine triphosphate-binding protein of the G(q/11)alpha family in rat liver membranes

Pancreastatin (PST), a recently discovered regulatory peptide derived from chromogranin A, has been shown to have a glycogenolytic effect in the hepatocyte that is mediated by increasing intracellular calcium. Our previous studies on pancreastatin signaling suggested that PST receptor is coupled to some G proteins in the plasma membrane of the hepatocyte. The nature of this interaction was investigated using antisera against G(q/11)alpha by different approaches. Indirect evidence of a pertussis toxin (PT)-insensitive G protein of the family of G(q/11)alpha was obtained by measuring high-affinity guanosine triphosphatase (GTPase) activity in soluble rat liver membranes. PST increased GTPase activity in a dose-dependent manner. This effect was only slightly inhibited by PT pretreatment of the membranes, whereas anti-G(q/11)alpha antisera blocked most of the PST-stimulated GTPase activity. The selective association of the PST receptor with this G protein was further studied by the coelution in wheat germ agglutinin semipurification of the receptor and by immunoprecipitation of the G protein-PST receptor complexes using G-protein-specific antisera. A G protein of the family of G(q/11)alpha was found to be associated with the semipurified PST receptor. Moreover, anti-G(q/11)alpha antisera immunoprecipitated most PST-binding activity (95%), bringing down most of the specific G protein, whereas anti-G(il,2)alpha and -G(o,i3)alpha failed to immunoprecipitate the PST-binding activity. Finally, the coupling of the PST receptor with the effector phospholipase C was disrupted by blocking with G(q/11)alpha antisera, suggesting that a G protein of the family of G(q/11)alpha is a signal mediator from PST receptors to phospholipase C activation in rat liver membranes.

Modifications of intracellular calcium release channels and calcium mobilization following 70% hepatectomy

The aim of this study was to investigate the properties of ryanodine and IP3 receptors in regenerating liver following 70% hepatectomy, and to evaluate the hepatic Ca2+ distribution and mobilization during this process. Specific [3H]ryanodine and [3H]IP3 binding to hepatic smooth endoplasmic reticulum membranes, as well as subcellular Ca2+ determination by atomic absorption flame photometry and Ca2+ mobilization by INDO-1 AM spectrofluorescence in hepatocytes, was performed in regenerating livers after surgical 70% hepatectomy. Incorporation of 14C amino acids into proteins and of 32P into phospholipids was done in subcellular fractions. Ryanodine receptor Kd presented a dramatic increase after 12 h of surgery and remained high up to 2 days of treatment. IP3 receptor Bmax showed a significant augmentation starting at 6 h after hepatectomy and returning to normal values after 1 week. Cytosolic total calcium content decreased from 12 h until 4 days after hepatectomy whereas the microsomal and mitochondrial total calcium increased at 1 and 2-4 days of liver regeneration, which coincided with the differential turnover of proteins and phospholipids in these fractions. ATP-induced Ca2+ transients in hepatocytes of 24-h-hepatectomized rats confirmed the altered sensitivity of the ryanodine receptor toward its ligand, since 10 times more ryanodine was necessary to alter the ATP-induced Ca2+ transient. The data support the notion that the calcium release channels are targets of mechanisms of metabolic control during the proliferative response following 70% hepatectomy and might be part of the modified intracellular Ca2+ dynamics during liver regeneration.

Solubilization and molecular characterization of active pancreastatin receptors from rat liver membranes

Pancreastatin receptors were solubilized from rat liver membranes with the nonionic detergent Triton X-100. Binding of a iodinated analog of rat pancreastatin ([125I-Tyr0]pancreastatin) to the soluble fraction was time dependent, saturable, and reversible. Scatchard analysis of binding under equilibrium conditions indicated that the soluble extracts contained a single class of pancreastatin-binding sites, with a binding capacity of 14 fmol/mg protein and a Kd of 0.3 nM. As observed with membrane-bound receptors, binding of [125I]pancreastatin to soluble extracts was inhibited by guanine nucleotides with the following rank order of potency: guanyl-5'-yl-imidodiphosphate > GTP > GDP > GMP, indicating that the soluble receptors are functionally linked to G proteins. Molecular analysis of the soluble pancreastatin receptor by covalent cross-linking to [125I]pancreastatin using disuccinimidyl suberate and further identification on SDS-PAGE indicated a single band of 85,000 Mr. Gel filtration of soluble extracts on Sephacryl S-300 revealed two molecular components with binding abilities (Mr 80,000 and 170,000). The higher molecular mass component was more sensitive to guanine nucleotides, and covalent cross-linking of both components to [125I]pancreastatin and further SDS-PAGE analysis revealed again a single band of 85,000 Mr, suggesting an association of the receptor with a G protein. Moreover, direct evidence that a Gq was present in the same chromatographic fraction was obtained by specific immunodetection. The soluble receptor is a glycoprotein that can be specifically bound to the wheat-germ agglutinin lectin. We conclude that we solubilized active pancreastatin receptors from rat liver membranes, and these results support the conclusion that the liver pancreastatin receptor consists of a 80,000 Mr glycoprotein associated with G proteins.

The role of intracellular Ca2+ in the regulation of gluconeogenesis

A hypothesis for the hormonal regulation of gluconeogenesis, in which increases in cytosolic free-Ca2+ levels ([Ca2+]i) play a major role, is presented. This hypothesis is based on the observation that gluconeogenic hormones evoke a common pattern of Ca2+ redistribution, resulting in increases in [Ca2+]i. Current concepts of hormonally evoked Ca2+ fluxes are presented and discussed. It is suggested that the increase in [Ca2+]i is functionally linked to stimulation of gluconeogenesis. The stimulation of gluconeogenesis is accomplished in two ways: (1) by increasing the activities of the Krebs cycle and the electron-transfer chain, thereby supplying adenosine triphosphates (ATP) and reducing equivalents to the process; and (2) by stimulating the activities of key gluconeogenic enzymes, such as pyruvate carboxylase. The hypothesis presents a conceptual framework that ties together two interrelated manifestations of hormone action: signal transduction and metabolism.

Effects of pancreastatin and somatostatin on secretagogues-induced rise in intracellular free calcium in single rat pancreatic islet cells

Pancreastatin (PST) is known to inhibit glucose-stimulated insulin release both in vivo and in vitro, but it has not been determined whether PST acts directly on pancreatic B-cells and no study has been reported on the effect of PST on the intracellular free Ca2+ concentration ([Ca2+]i) in pancreatic islet cells. In the present study, by using the dissociated rat pancreatic B-cells, we examined the effects of PST on the increase in [Ca2+]i induced by several insulin secretagogues, and compared them with those of somatostatin (SRIF). PST (1-100 nM) dose-dependently inhibited the glucose-induced rise in [Ca2+]i in single pancreatic islet cells. SRIF (10 nM) also suppressed the glucose-induced rise in [Ca2+]i. These demonstrated direct inhibitory actions of PST and SRIF on the pancreatic B-cells. Acetylcholine (ACh, 10 microM) with 5.5 mM glucose induced a biphasic increase in [Ca2+]i in single islet cells. SRIF (10 nM) suppressed the second phase in [Ca2+]i increase without affecting the first phase. In contrast, PST (100 nM) had no effect on the ACh-induced response. Gastric inhibitory polypeptide (100 nM) with 5.5 mM glucose induced a rise in [Ca2+]i in single islet cells. SRIF inhibited this increase, but PST did not. Both PST and SRIF failed to affect the sustained rise in [Ca2+]i evoked by excess K+. These results suggest that PST and SRIF suppress the glucose-induced insulin secretion at least partly by inhibiting the rise in [Ca2+]i in pancreatic B-cells. Furthermore, PST may suppress the glucose-induced rise in [Ca2+]i via a mechanism different from that of SRIF.

Pancreastatin action in the liver: dual coupling to different G proteins

Pancreastatin is a 49 amino acid peptide first isolated, purified and characterized from porcine pancreas. Its biological activity in different tissues can be assigned to the C-terminal part of the molecule. Pancreastatin has a prohormonal precursor, chromogranin A, which is a glycoprotein present in neuroendocrine cells, including the endocrine pancreas. We have been interested in pancreastatin action in the liver. We found that pancreastatin has a glycogenolytic effect in the hepatocyte both in vivo and in vitro. We then studied and characterized the specific pancreastatin receptor in the rat liver plasma membrane, as well as the specific signal transduction. This receptor appears to be coupled to two different G proteins. A pertussis toxin-insensitive G proteins leads to the activation of phospholipase C, and therefore mediates the glycogenolytic effect in the liver by increasing cytoplasmic free calcium and stimulating protein kinase C. The role of cyclic GMP in the action of pancreastatin is not known yet, although it seems to regulate negatively the activation of phospholipase C. The precise mechanism by which pancreastatin stimulates guanylate cyclase activity remains to be studied.

Plasma pancreastatin-like immunoreactivity correlates with plasma norepinephrine levels in essential hypertension

Pancreastatin (PST), a 49 amino acid peptide originally isolated from porcine pancreas, is derived from chromogranin A (Cg A), an acidic protein co-released with catecholamines from sympathetic nerve terminals and chromaffin cells. Extracellular processing of Cg A yields PST as well as other biological active peptides. Measurement of Cg A and PST-like immunoreactivity (PST-LI) has been used to investigate patients with pheochromocytoma and other neuroendocrine neoplasia. Some studies have found increased plasma norepinephrine (NE) levels in essential hypertension. We therefore measured venous plasma PST-LI and catecholamines in patients with essential hypertension. We employed a radioimmunoassay developed with commercially available reagents for measuring plasma PST-like immunoreactivity, and HPLC with electrochemical detection for measurement of plasma catecholamines. The correlation of PST-LI with epinephrine (E) was very weak. However, its correlation with NE was highly significant. Thus, venous plasma PST-LI immunoreactivity may reflect sympathetic nerve activity in essential hypertension.

Pancreastatin activates protein kinase C by stimulating the formation of 1,2-diacylglycerol in rat hepatocytes

We describe here the stimulation by pancreastatin of 1,2-diacylglycerol production and protein kinase C activity in liver plasma membrane and isolated hepatocytes. The dose-dependency for the stimulation of both processes was similar to the recently described pattern of glucose output and cytosolic Ca2+ transients produced by pancreastatin. The time course of diacylglycerol production at 30 degrees C showed a rapid increase within 5 min, reaching a maximum at 10 min. Protein kinase C from hepatocytes was dependent on Ca2+ and phosphatidylserine. Neither the pancreastatin-stimulated diacylglycerol production nor the activation of protein kinase C was affected by pretreatment with pertussis toxin. However, the presence of GTP partially inhibited this pancreastatin stimulation of 1,2-diacylglycerol in a dose-dependent manner, although GTP alone stimulates diacylglycerol accumulation. This inhibitory effect of GTP on pancreastatin stimulation of diacylglycerol synthesis was completely abolished by the pretreatment with pertussis toxin. In conclusion, this study provides evidence that pancreastatin stimulates the formation of 1,2-diacylglycerol by a pertussis-toxin-independent mechanism, which may be responsible for the pancreastatin activation of protein kinase C.

Pancreastatin activates pertussis toxin-sensitive guanylate cyclase and pertussis toxin-insensitive phospholipase C in rat liver membranes

We have recently found the calcium dependent glycogenolytic effect of a pancreastatin on rat hepatocytes and the mobilization of intracellular calcium. To further investigate the mechanism of action of pancreastatin on liver we have studied its effect on guanylate cyclase, adenylate cyclase, and phospholipase C, and we have explored the possible involvement of GTP binding proteins by measuring GTPase activity as well as the effect of pertussis toxin treatment of plasma liver membranes on the pancreastatin stimulated GTPase activity and the production of cyclic GMP and myo-inositol 1,4,5-triphosphate. Pancreastatin stimulated GTPase activity of rat liver membranes about 25% over basal. The concentration dependency curve showed that maximal stimulation was achieved at 10(-7)M pancreastatin (EC50 = 3 nM). This stimulation was partially inhibited by treatment of the membranes with pertussis toxin. The effect of pancreastatin on guanylate cyclase and phospholipase C were examined by measuring the production of cyclic GMP and myo-inositol 1,4,5-triphosphate respectively. Pancreastatin increased the basal activity of guanylate cyclase to a maximum of 2.5-fold the unstimulated activity at 30 degrees C, in a time- and dose-dependent manner, reaching the maximal stimulation above control with 10(-7) M pancreastatin at 10 min (EC50 = 0.6 nM). This effect was completely abolished when rat liver membranes had been ADP-ribosylated with pertussis toxin. On the other hand, adenylate cyclase activity was not affected by pancreastatin. Phospholipase C activity of rat liver membranes was rapidly stimulated (within 2-5 min) at 30 degrees C by 10(-7) M pancreastatin, reaching a maximum at 15 min.(ABSTRACT TRUNCATED AT 250 WORDS)

Pancreastatin inhibits insulin-stimulated glycogen synthesis but not glycolysis in rat hepatocytes

The effect of rat pancreastatin on glycogen synthesis and glycolysis rate was studied in insulin-stimulated rat hepatocytes. We have determined the incorporation of [U-14C]glucose into glycogen as a measurement of the rate of glycogen synthesis; and the production of lactate as a measurement of the rate of glycolysis. Rat pancreastatin by itself did not affect either the rate of glycogen synthesis or glycolysis in rat hepatocytes from 6 h fasted rats. However, pancreastatin inhibited about 45% the insulin-stimulated glycogen synthesis whereas it enhanced the rate of glycolysis of insulin-stimulated hepatocytes about 25%. These effects were found to be dependent on pancreastatin concentration from 10(-11) M to 10(-7) M. Maximal effect was achieved at 10(-8) M and the half-maximal effect was observed at 0.3 nM. Pancreastatin decreased the rate of glycogen synthesis in a wide range of insulin concentrations (10(-12) - 10(-8) M). However, the effect on insulin-stimulated glycolysis was only observed at high concentrations of pancreastatin and insulin. These results suggest a role of pancreastatin in the possible mechanisms involved in insulin resistance.