Ghrelin system in pancreatic AR42J cells: its ligand stimulation evokes calcium signalling through ghrelin receptors

Ghrelin inhibits IKKβ/NF-κB activation and reduces pro-inflammatory cytokine production in pancreatic acinar AR42J cells treated with cerulein

BACKGROUND

Previous studies have provided conflicting results regarding whether the serum ghrelin concentration can reflect the severity of acute pancreatitis (AP). The present study examined the correlation between the serum ghrelin concentration and AP severity in animal models and investigated whether altered ghrelin expression in pancreatic acinar cells influences IKKβ/NF-κB signaling and pro-inflammatory cytokine production.

METHODS

Mild or severe AP was induced in rats by intraperitoneal injection of cerulein or retrograde cholangiopancreatic duct injection of sodium taurocholate, respectively. After successful model induction, serum ghrelin, tumor necrosis factor-α (TNF-α), and interleukin-6 (IL-6) concentrations were determined by enzyme-linked immunosorbent assay, and IKKβ/NF-κB activation was assessed by immunohistochemistry. Subsequently, stable overexpression or knockdown of ghrelin in AR42J cells was achieved by lentiviral transfection. After transfected cells and control cells were treated with cerulein for 24 h, the TNF-α and IL-1β levels in the supernatants were determined by enzyme-linked immunosorbent assay, and the expression levels of p-p65, IKKβ, and p-IKKβ were detected by Western blotting.

RESULTS

In rat AP models, AP severity was correlated with increased IKKβ/NF-κB activation, pro-inflammatory cytokine production, and ghrelin secretion. The levels of pro-inflammatory cytokines TNF-α and IL-1β as well as IKKβ/NF-κB signaling activity were increased upon knockdown of ghrelin in the AP acinar cell model and decreased with ghrelin overexpression.

CONCLUSIONS

Serum ghrelin is related to the severity of AP. Ghrelin may play a protective role in the pathogenesis of AP by inhibiting the pro-inflammatory cytokines and the activation of the IKKβ/NF-κB signaling pathway.

Effects of Ghrelin miRNA on Inflammation and Calcium Pathway in Pancreatic Acinar Cells of Acute Pancreatitis

OBJECTIVES

The study investigated the effects of endogenous targeted inhibition of ghrelin gene on inflammation and calcium pathway in an in vitro pancreatic acinar cell model of acute pancreatitis.

METHODS

Lentiviral expression vector against ghrelin gene was constructed and transfected into AR42J cells. The mRNA and protein expression of each gene were detected by reverse transcription polymerase chain reaction, Western blotting, or enzyme-linked immunosorbent assay. The concentration of intracellular calcium ([Ca]i) was determined by calcium fluorescence mark probe combined with laser scanning confocal microscopy.

RESULTS

Compared with the control group, cerulein could upregulate mRNA and protein expression of inflammatory factors, calcium pathway, ghrelin, and [Ca]i. mRNA and protein expression of inflammatory factors increased significantly in cells transfected with ghrelin miRNA compared with the other groups. Intracellular calcium and expression of some calcium pathway proteins decreased significantly in cells transfected with ghrelin miRNA compared with the other groups.

CONCLUSIONS

Targeted inhibition of ghrelin gene in pancreatic acinar cells of acute pancreatitis can upregulate the expression of the intracellular inflammatory factors and alleviate the intracellular calcium overload.

Molecular Ghrelin System in the Pancreatic Acinar Cells: The Role of the Polypeptide, Caerulein and Sensory Nerves

Ghrelin (GHRL) is an endogenous ligand for the growth hormone secretagogue receptor (GHS-R). Experimental studies showed that GHRL protects the stomach and pancreas against acute damage, but the effect of GHRL on pancreatic acinar cells was still undetermined. Aim: To investigate the effect of GHRL and caerulein on the functional ghrelin system in pancreatic acinar cells taking into account the role of sensory nerves (SN). Methods: Experiments were carried out on isolated pancreatic acinar cells and AR42J cells. Before acinar cells isolation, GHRL was administered intraperitoneally at a dose of 50 µg/kg to rats with intact SN or with capsaicin deactivation of SN (CDSN). After isolation, pancreatic acinar cells were incubated in caerulein-free or caerulein containing solution. AR42J cells were incubated under basal conditions and stimulated with caerulein, GHRL or a combination of the above. Results: Incubation of isolated acinar cells with caerulein inhibited GHS-R and GHRL expression at the level of mRNA and protein in those cells. Either in rats with intact SN or with CDSN, administration of GHRL before isolation of acinar cells increased expression of GHRL and GHS-R in those cells and reversed the caerulein-induced reduction in expression of those parameters. Similar upregulation of GHS-R and GHRL was observed after administration of GHRL in AR42J cells. Conclusions: GHRL stimulates its own expression and expression of its receptor in isolated pancreatic acinar cells and AR42J cells on the positive feedback pathway. This mechanism seems to participate in the pancreatoprotective effect of GHRL in the course of acute pancreatitis.

Analysis of expression and structure of the rat GH-secretagogue/ghrelin receptor (Ghsr) gene: roles of epigenetic modifications in transcriptional regulation

In the current study, to elucidate the molecular basis of cell type-specific expression of the GH-secretagogue/ghrelin receptor type 1A (GHSR1A), we characterized the structure and putative promoter region of the rat Ghsr gene. We identified an alternative 5'-untranslated first exon that contains multiple transcription start sites, and confirmed a 200-bp sequence proximal to this exon to be sufficient for basal promoter activity. A promoter-associated CpG island conserved across different species was found to be hypomethylated in Ghsr1a-expressing cell lines, while being heavily methylated in non-expressing cells. In cells with low or absent Ghsr1a expression, treatment with demethylating agents activated Ghsr1a transcription. Chromatin immunoprecipitation assays demonstrated Ghsr1a-expressing cells to display active histone modifications, whereas repressive modifications were present exclusively in other cell types. These results suggest epigenetic modifications at GHSR to play important roles in determining GHSR1A expression and abundance, and therefore the consequent sensitivity of cells to ghrelin.

PDZ-domain containing-2 (PDZD2) drives the maturity of human fetal pancreatic progenitor-derived islet-like cell clusters with functional responsiveness against membrane depolarization

We recently reported the isolation and characterization of a population of pancreatic progenitor cells (PPCs) from early trimester human fetal pancreata. The PPCs, being the forerunners of adult pancreatic cell lineages, were amenable to growth and differentiation into insulin-secreting islet-like cell clusters (ICCs) upon stimulation by adequate morphogens. Of note, a novel morphogenic factor, PDZ-domain containing-2 (PDZD2) and its secreted form (sPDZD2) were ubiquitously expressed in the PPCs. Our goals for this study were to evaluate the potential role of sPDZD2 in stimulating PPC differentiation and to establish the optimal concentration for such stimulation. We found that 10(-9)M sPDZD2 promoted PPC differentiation, as evidenced by the upregulation of the pancreatic endocrine markers (PDX-1, NGN3, NEURO-D, ISL-1, NKX 2.2, NKX 6.1) and INSULIN mRNA. Inhibited endogenous production of sPDZD2 suppressed expression of these factors. Secreted PDZD2 treatment significantly elevated the C-peptide content of the ICCs and increased the basal rate of insulin secretion. However, they remained unresponsive to glucose stimulation, reflected by a minimal increase in GLUT-2 and GLUCOKINASE mRNA expression. Interestingly, sPDZD2 treatment induced increased expression of the L-type voltage-gated calcium channel (Ca(v)1.2) in the ICCs, triggering calcium ion influx under KCl stimulation and conferring an ability to secrete insulin in response to KCl. Pancreatic progenitor cells from 10- and 13-week fetal pancreata showed peak expression of endogenous sPDZD2, implying that sPDZD2 has a specific role in islet development during the first trimester. In conclusion, our data suggest that sPDZD2 promotes functional maturation of human fetal PPC-derived ICCs, thus enhancing its transplanting potentials.

Regulation of ghrelin structure and membrane binding by phosphorylation

The peptide hormone ghrelin requires Ser-3 acylation for receptor binding, orexigenic and anti-inflammatory effects. Functions of desacylghrelin are less well understood. In vitro kinase assays reveal that the evolutionarily conserved Ser-18 in the basic C-terminus is an excellent substrate for protein kinase C. Circular dichroism reveals that desacylghrelin is approximately 12% helical in aqueous solution and approximately 50% helical in trifluoroethanol. Ser-18-phosphorylation, Ser-18-Ala substitution, or Ser-3-acylation reduces the helical character in trifluoroethanol to approximately 24%. Both ghrelin and desacylghrelin bind to phosphatidylcholine:phosphatidylserine sucrose-loaded vesicles in a phosphatidylserine-dependent manner. Phosphoghrelin and phosphodesacylghrelin show greatly diminished phosphatidylserine-dependent binding. These results are consistent with binding of ghrelin and desacylghrelin to acidic lipids via the basic face of an amphipathic helix with Ser-18 phosphorylation disrupting both helical character and membrane binding.

Complete loss of functional G protein-coupled receptors in mouse pancreatic acinar cell MPC-83

AIM: To study G-protein-coupled receptor (GPCR) in mouse exocrine pancreatic acinar cell line MPC-83.

METHODS: Cytoplasmic calcium concentration in mouse exocrine pancreatic acinar cell line MPC-83 was measured both before and after stimulation with GPCR agonists.

RESULTS: Acetylcholine (ACh, 25 nmol/L) and cholecystokinin (CCK, 5 pmol/L) induced regular calcium oscillations in freshly isolated normal mouse pancreatic acinar cells. When GPCR agonists CCK (1 μmol/L), vasopressin (VP, 1 μmol/L), substance P (5 μmol/L), histamine (10 μmol/L), phenylephrine (PE, 10 μmol/L) and ACh 100 μmol/L were added to MPC-83 cells, no change of intracellular calcium concentration was detected. After 100 nmol/L dexamethasone pre-treatment of MPC-83 cells for 72 h, MPC-83 cell proliferation was reduced, but still no calcium increases were detected after stimulation with GPCR agonists 1 μmol/L CCK, 1 μmol/L VP and 5 μmol/L substance P.

CONCLUSION: The pancreatic acinar tumor cell line MPC-83 from Kunming mice have completely lost their functional GPCR which are normally present in pancreatic acinar cells, and these cells are in extreme de-differentiation state.

The effect of luminal ghrelin on pancreatic enzyme secretion in the rat

Ghrelin, a 28-amino-acid peptide produced predominantly by oxyntic mucosa has been reported to affect the pancreatic exocrine function but the mechanism of its secretory action is not clear. The effects of intraduodenal (i.d.) infusion of ghrelin on pancreatic amylase outputs under basal conditions and following the stimulation of pancreatic secretion with diversion of pancreato-biliary juice (DPBJ) as well as the role of vagal nerve, sensory fibers and CCK in this process were determined. Ghrelin given into the duodenum of healthy rats at doses of 1.0 or 10.0 microg/kg increased pancreatic amylase outputs under basal conditions or following the stimulation of pancreatic secretion with DPBJ. Bilateral vagotomy as well as capsaicin deactivation of sensory fibers completely abolished all stimulatory effects of luminal ghrelin on pancreatic exocrine function. Pretreatment with lorglumide, a CCK(1) receptor blocker, reversed the stimulation of amylase release produced by intraduodenal application of ghrelin. Intraduodenal ghrelin at doses of 1.0 or 10.0 microg/kg increased plasma concentrations of CCK and ghrelin. In conclusion, ghrelin given into the duodenum stimulates pancreatic enzyme secretion. Activation of vagal reflexes and CCK release as well as central mechanisms could be implicated in the stimulatory effect of luminal ghrelin on the pancreatic exocrine functions.

The ghrelin system in acinar cells: localization, expression, and regulation in the exocrine pancreas

OBJECTIVES

Ghrelin and its receptor are expressed abundantly in the stomach and pituitary. Recently, a ghrelin system, consisting of both ligand and receptor, has also been found to exist in the endocrine cells of pancreatic islets. This ghrelin system may play a role in regulating insulin secretion and glucose homeostasis. The aim of the present study was to investigate whether a functional ghrelin system also exists in the exocrine pancreas.

METHODS

Precise localization and expression of ghrelin and its receptor in rat pancreatic acinar cells were examined by immunocytochemistry and Western blot, whereas messenger RNA levels were examined by semiquantitative reverse transcription-polymerase chain reaction. The roles of physiological and pathophysiological conditions, such as gastric acid inhibition, starvation, and acute pancreatitis, in regulation of ghrelin and its receptor were also examined.

RESULTS

Both ghrelin and its receptor were detected, at both protein and messenger RNA levels, in the acinar cells of the exocrine pancreas. Ghrelin receptor expression was up-regulated by gastric acid inhibition and down-regulated by acute pancreatitis, whereas levels remained unchanged after food deprivation. In contrast, ghrelin expression did not exhibit significant changes in any condition.

CONCLUSIONS

Our data indicate that a ghrelin system exists in the acinar cells of the exocrine pancreas. This system is subject to regulation by physiological and pathophysiological stimuli and may thus regulate exocrine functions by paracrine and/or autocrine mechanisms.

Ghrelin acts on the dorsal vagal complex to stimulate pancreatic protein secretion

Ghrelin receptors are present in the central nervous system. We hypothesized that ghrelin released from the stomach acts as an endocrine substance and stimulates brain stem vagovagal circuitry to evoke pancreatic secretion. In an in vivo anesthetized rat model, an intravenous infusion of ghrelin at doses of 5, 10, and 25 nmol increased pancreatic protein secretion from a basal level of 125 +/- 6 to 186 +/- 8, 295 +/- 12, and 356 +/- 11 mg/h, respectively. Pretreatment with atropine or hexamethonium or an acute vagotomy, but not a perivagal application of capsaicin, completely abolished pancreatic protein secretion responses to ghrelin. In conscious rats, an intravenous infusion of ghrelin at a dose of 10 nmol resulted in a 2.2-fold increase in pancreatic protein secretion over basal volume. Selective ablation of the area postrema abolished pancreatic protein secretion stimulated by intravenous infusion of ghrelin but did not alter the increase in pancreatic protein secretion evoked by diversion of bile-pancreatic juice. Immunohistochemical staining showed a marked increase in the number of c-Fos-expressing neurons in the area postrema, nucleus of the solitary tract, and dorsal motor nucleus of the vagus after an intravenous infusion of ghrelin in sham-lesioned rats; selective ablation of the area postrema eliminated this increase. In conclusion, ghrelin stimulates pancreatic secretion via a vagal cholinergic efferent pathway. Circulating ghrelin gains access to the brain stem vagovagal circuitry via the area postrema, which represents the primary target on which peripheral ghrelin may act as an endocrine substance to stimulate pancreatic secretion.

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Characterization of turkey and chicken ghrelin genes, and regulation of ghrelin and ghrelin receptor mRNA levels in broiler chickens

Ghrelin, a peptide hormone produced by the stomach in mammals, stimulates growth hormone release and food intake. Recently, ghrelin was identified and characterized in chicken proventriculus and shown to stimulate growth hormone release but inhibit feed intake. The purpose of this work was to identify and further characterize the ghrelin gene in chickens and in turkeys. Using molecular cloning techniques we have sequenced cDNAs corresponding to chicken (White Leghorn) and turkey ghrelin mRNAs. A total of 844 (chicken) or 869 (turkey) bases including the complete coding regions (CDS), and the 5'- and 3'-untranslated regions (UTRs) were determined. Nucleotide sequence (CDS) predicted a 116 amino acid precursor protein (preproghrelin) for both the chicken and the turkey that demonstrated complete conservation of an N-terminal 'active core' (GSSF) including a serine (position 3 of the mature hormone) known to be a modification (acylation) site important for ghrelin bioactivity. Additional nucleotide sequence was found in the 5'-UTRs of both Leghorn and turkey cDNAs that was not present in broilers or the red jungle fowl. The turkey ghrelin gene, sequenced from genomic DNA templates, contained five exons and four introns, a structure similar to mammalian and chicken ghrelin genes. Ghrelin was highly expressed in proventriculus with much lower levels of expression in other tissues such as pancreas, brain, and intestine. RT-PCR was used to quantify ghrelin mRNA levels relative to 18S rRNA in 3-week-old male broiler chickens. The level of ghrelin mRNA increased in proventriculus in response to fasting but did not decline with subsequent refeeding. Plasma ghrelin levels did not change significantly in response to fasting or refeeding and did not appear to reflect changes in proventriculus ghrelin mRNA levels. Ghrelin mRNA levels declined in broiler pancreas after a 48 h fast and increased upon refeeding. Expression of the gene encoding the receptor for ghrelin (growth hormone secretagogue receptor, GHS-R) and a variant form was detected in a variety of tissues collected from 3-week-old male broiler chickens possibly suggesting autocrine/paracrine effects. These results offer new information about the avian ghrelin and ghrelin receptor genes and the potential role that this system might play in regulating feed intake and energy balance in poultry.

Neurohormonal control of exocrine pancreatic secretion

PURPOSE OF REVIEW

Investigations into the neural and hormonal control of pancreatic exocrine function have led to many exciting discoveries over the past year. This review seeks to identify those articles that further our understanding into the complex relation of the varying factors regulating pancreatic secretion.

RECENT FINDINGS

Major findings include the new insights into the regulation of the pancreas through receptor-mediated mechanisms, investigations of pancreatic exocytosis, impairment of pancreatic exocrine function by insulin deficiency, the effects of surgical interventions for the treatment of chronic pancreatitis on pancreatic exocrine function, how exocrine function is altered by the cause of acute pancreatitis, and clinical observations relating to management of pancreatic disease and investigations of pancreatic function testing.

SUMMARY

Over the past year, substantial new information has been published on the neurohormonal control of pancreatic exocrine function. These data provide insights into the physiology and pathophysiology of pancreatic secretion and diseases of exocrine insufficiency.