Intracellular signaling mechanisms activated by cholecystokinin-regulating synthesis and secretion of digestive enzymes in pancreatic acinar cells

Organellar proteomics: analysis of pancreatic zymogen granule membranes

The zymogen granule (ZG) is the specialized organelle in pancreatic acinar cells for digestive enzyme storage and regulated secretion and has been a model for studying secretory granule functions. In an initial effort to comprehensively understand the functions of this organelle, we conducted a proteomic study to identify proteins from highly purified ZG membranes. By combining two-dimensional gel electrophoresis and two-dimensional LC with tandem mass spectrometry, 101 proteins were identified from purified ZG membranes including 28 known ZG proteins and 73 previously unknown proteins, including SNAP29, Rab27B, Rab11A, Rab6, Rap1, and myosin Vc. Moreover several hypothetical proteins were identified that represent potential novel proteins. The ZG localization of nine of these proteins was further confirmed by immunocytochemistry. To distinguish intrinsic membrane proteins from soluble and peripheral membrane proteins, a quantitative proteomic strategy was used to measure the enrichment of intrinsic membrane proteins through the purification process. The iTRAQ ratios correlated well with known or Transmembrane Hidden Markov Model-predicted soluble or membrane proteins. By combining subcellular fractionation with high resolution separation and comprehensive identification of proteins, we have begun to elucidate zymogen granule functions through proteomic and subsequent functional analysis of its membrane components.

Activation of p-ERK1/2 by nicotine in pancreatic tumor cell line AR42J: effects on proliferation and secretion

The objectives of the present study were to determine the effect of nicotine on MAPK signaling and on the proliferation of AR42J cells as well as to assess the relationship between MAPK activation and exocrine secretion in these cells. AR42J cells were incubated with nicotine and analyzed for the activation of MAPK by Western blot analysis using their respective antibodies and confirmed by immunohistochemistry. The effect of nicotine on cell proliferation was determined by the spectrophotometric method, and cell function was assessed by cholecystokinin (CCK)-stimulated amylase release into the culture medium. Nicotine at a dose of 100 microM induced phospho-ERK1/2 activation maximally in 3 min compared with untreated cells. Furthermore, immunofluorescence study confirmed the nicotine-induced increase in translocation of phospho-ERK1/2 to the nucleus. Activation of phospho-ERK1/2 was inhibited by an ERK1/2 pathway inhibitor but not by a nicotine receptor antagonist. At the same dose, there was significantly enhanced proliferation of AR42J cells until 72 h without toxic effect, as the percentage of lactate dehydrogenase release remained unchanged. Other MAPKs, c-Jun NH2-terminal kinase 1/2 and p38 MAPK, were not affected by nicotine treatment. At a nicotine dose of 100 microM, the CCK-stimulated release of amylase was maximal at 6 min, and, although a nicotinic receptor antagonist inhibited this response, it was not inhibited by the ERK1/2 pathway inhibitor. We conclude that nicotine treatment induced activation of ERK1/2 and increased the proliferation of AR42J cells. The data further indicate that MAPK signaling by nicotine is independent of the secretory response.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

Secretin differentially sensitizes rat pancreatic acini to the effects of supramaximal stimulation with caerulein

Supramaximal stimulation of the rat pancreas with CCK, or its analog caerulein, triggers acute pancreatitis and a number of pancreatitis-associated acinar cell changes including intracellular activation of digestive enzyme zymogens and acinar cell injury. It is generally believed that some of these various acinar cell responses to supramaximal secretagogue stimulation are interrelated and interdependent. In a recent report, Lu et al. showed that secretin, by causing generation of cAMP and activation of PKA, sensitizes acinar cells to secretagogue-induced zymogen activation, and, as a result, submaximally stimulating concentrations of caerulein can, in the presence of secretin, trigger intracellular zymogen activation. We found that secretin also sensitizes acinar cells to secretagogue-induced cell injury and to subapical F-actin redistribution but that it did not alter the caerulein concentration dependence of other pancreatitis-associated changes such as the induction of a peak plateau intracellular [Ca(2+)] rise, inhibition of secretion, activation of ERK1/2, and activation of NF-kappaB. The finding that secretin sensitizes acinar cells to both intracellular zymogen activation and cell injury is consistent with the concept that these two early events in pancreatitis are closely interrelated and, possibly, interdependent. On the other hand, the finding that, in the presence of secretin, caerulein can trigger subapical F-actin redistribution without inhibiting secretion challenges the concept that disruption of the subapical F-actin web is causally related to high-dose secretagogue-induced inhibition of secretion in pancreatic acinar cells.

Regulation of zymogen granule exocytosis by Ca2+, cAMP, and PKC in pancreatic acinar cells

The effect of cAMP and PKC on zymogen granule exocytosis was investigated by simultaneously measuring cytosolic Ca2+ concentration ([Ca2+]c) and individual zymogen granule exocytosis in isolated mouse pancreatic acini. When acinar cells were stimulated with acetylcholine (ACh, 10 microM), exocytic events were detected through granule-attached apical membranes with [Ca2+]c rise. Application of secretin, forskolin (an adenylate cyclase activator), or PMA (a PKC activator) alone did not elicit any [Ca2+]c rise or zymogen granule exocytosis, but co-stimulation with ACh led to exocytosis in that the total number of secreted granules increased markedly without a significant difference in [Ca2+]c rises. When we evoked exocytosis by [Ca2+]c ramps, pretreatment with forskolin or PMA elicited exocytosis at lower [Ca2+]c levels. These results indicate that PKC or cAMP alone could not directly elicit zymogen granule exocytosis, but that they increase the total releasable pool by rendering zymogen granules more sensitive to Ca2+.

Rho and Rac promote acinar morphological changes, actin reorganization, and amylase secretion

Supramaximal stimulation of isolated pancreatic acini with specific agonists such as CCK induces the formation of large basolateral blebs, redistributes filamentous actin, and inhibits secretion. Rho family small G proteins are well documented for their function in actin reorganization that determines cell shape and have been suggested to play a role in secretion. Here, we determined whether Rho and Rac are involved in the morphological changes, actin redistribution, and inhibition of amylase secretion induced by high concentrations of CCK. Introduction of constitutively active RhoV14 and RacV12 but not Cdc42V12 in mouse pancreatic acini by adenoviral vectors stimulated acinar morphological changes including basolateral protrusions, increased the total amount of F-actin, and reorganized the actin cytoskeleton. Dominant-negative RhoN19, Clostridium botulinum C3 exotoxin, which inhibits Rho, and dominant-negative RacN17 all partially blocked CCK-induced acinar morphological changes and actin redistribution. To study the correlation between actin polymerization and acinar shape changes, two marine toxins were employed. Jasplakinolide, a reagent that facilitates actin polymerization and stabilizes F-actin, stimulated acinar basolateral protrusions, whereas latrunculin, which sequesters actin monomers, blocked CCK-induced acinar blebbing. Unexpectedly, RhoV14, RacV12, and jasplakinolide all increased amylase secretion by CCK from 30 pM to 10 nM. The data suggest that Rho and Rac are involved in CCK-evoked changes in acinar morphology, actin redistribution, and secretion and that inhibition of secretion by high concentrations of CCK is not directly coupled to the changes in acinar morphology.

A role for Rho and Rac in secretagogue-induced amylase release by pancreatic acini

The actin cytoskeleton has long been implicated in protein secretion. We investigated whether Rho and Rac, known regulators of the cytoskeleton, are involved in amylase secretion by mouse pancreatic acini. Secretagogues, including cholecystokinin (CCK) and the acetylcholine analog carbachol, increased the amount of GTP-bound RhoA and Rac1 and induced translocation from cytosol to a membrane fraction. Immunocytochemistry revealed the translocation of Rho and Rac within the apical region of the cell. Expression by means of adenoviral vectors of dominant-negative Rho (RhoN19), dominant-negative Rac (RacN17), and Clostridium Botulinum C3 exotoxin, which ADP ribosylates and inactivates Rho, significantly inhibited amylase secretion by CCK and carbachol; inhibiting both Rho and Rac resulted in a greater reduction. This inhibitory effect of RhoN19 on CCK-induced amylase secretion was apparent in both the early and late phases of secretion, whereas RacN17 was more potent on the late phase of secretion. None of these three affected the basal Ca2+or the peak intracellular Ca2+concentration stimulated by CCK. Latrunculin, a marine toxin that sequesters actin monomers, time-dependently decreased the total amount of filamentous actin (F-actin) and dose-dependently decreased secretion by secretagogues without affecting Ca2+signaling. These data suggest that Rho and Rac are both involved in CCK-induced amylase release in pancreatic acinar cell possibly through an effect on the actin cytoskeleton.

Aging is associated with decreased pancreatic acinar cell regeneration and phosphatidylinositol 3-kinase/Akt activation

BACKGROUND & AIMS

The effects of aging on pancreatic acinar cell proliferation have not been clearly defined. Phosphatidylinositol 3-kinase (PI3K)-mediated phosphorylation of Akt is a critical step for proliferation of various cell types and insulin secretion from pancreatic endocrine cells; however, its role in acinar cell proliferation is not known. The purpose of this study was to (1) delineate the effects of aging on pancreatic regeneration after partial pancreatectomy (Px) and (2) define the involvement of the PI3K/Akt pathway in pancreatic regeneration.

METHODS

Following partial Px, pancreatic regeneration and activation of the PI3K pathway were compared in young and aged mice. Activation of the PI3K/Akt pathway was evaluated by Akt phosphorylation (pAkt). The role of the PI3K pathway in pancreatic regeneration after partial Px was assessed by effects of a pharmacologic PI3K inhibitor wortmannin or small interfering RNA (siRNA) to the p85alpha regulatory subunit. To confirm further the critical role of the PI3K/Akt pathway in pancreatic acinar cell proliferation, IGF-1-mediated cell proliferation was determined in cultured acinar cells pretreated with wortmannin or p85alpha siRNA.

RESULTS

Pancreatic regeneration and pAkt expression after partial Px were significantly decreased with aging. Treatment with wortmannin or p85alpha siRNA reduced pancreatic regeneration after partial Px. The IGF-1-mediated cell proliferation in vitro was completely blocked by wortmannin or p85alpha siRNA but not by the MEK/ERK inhibitor PD98059.

CONCLUSIONS

PI3K/Akt activation plays a critical role in the regeneration of pancreatic acini after resection. Furthermore, pancreatic regeneration is markedly attenuated in the aged pancreas most likely because of decreased PI3K/Akt activation.

JAK and STAT proteins are expressed and activated by IFN-gamma in rat pancreatic acinar cells

The development of acute pancreatitis (AP) is triggered by acinar events, but the subsequent extra-acinar events, particularly a distinct immune response, appear to determine its severity. Cytokines modulate this immune response and are derived not only from immunocytes but also from pancreatic acinar cells. We studied whether pancreatic acinar cells were also capable of responding to cytokines. The JAK/STAT-pathway represents the main effector for many cytokines. Therefore, expression and regulation of JAK and STAT proteins were investigated in rat pancreatic acinar cells. Western blotting showed expression of JAK1, JAK2, Tyk2, and STAT1, STAT2, STAT3, STAT5, STAT6. In addition, STAT1 was reversibly tyrosine-phosphorylated upon the procedure of acinar cell isolation. In contrast, STAT3-phosphorylation occurred spontaneously after pancreas removal and was not reversible within 8 h. STAT1 phosphorylation was also observed upon treatment with IFN-gamma but not upon EGF, TNF-alpha or IL-6, and inhibited by the JAK2-inhibitor AG-490. Immunohistochemistry revealed cytoplasmic expression of unphosphorylated STAT1 in untreated acinar cells and nuclear translocation of phosphorylated STAT1 following IFN-gamma-treatment. Interestingly, although CCK leads to the activation of multiple stress pathways in pancreatic acinar cells, we found no influence of CCK on phosphorylation of STAT1, STAT3, or STAT5 in the pancreas. In conclusion, our data provide further evidence that pancreatic acinar cells are able to interact with immune cells. Besides stimulating immune cells via cytokine secretion, acinar cells are in turn capable of responding to IFN-gamma via JAK2 and STAT1 which may have an impact on the development of AP.

Stable Golgi-Mitochondria Complexes and Formation of Golgi Ca2+ Gradients in Pancreatic Acinar Cells

We have determined the localization of the Golgi with respect to other organelles in living pancreatic acinar cells and the importance of this localization to the establishment of Ca(2+) gradients over the Golgi. Using confocal microscopy and the Golgi-specific fluorescent probe 6-((N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)hexanoyl)sphingosine, we found Golgi structures localizing to the outer edge of the secretory granular region of individual acinar cells. We also assessed Golgi positioning in acinar cells located within intact pancreatic tissue using two-photon microscopy and found a similar localization. The mitochondria segregate the Golgi from lateral regions of the plasma membrane, the nucleus, and the basal part of the cytoplasm. The Golgi is therefore placed between the principal Ca(2+) release sites in the apical region of the cell and the important Ca(2+) sink formed by the peri-granular mitochondria. During acetylcholine-induced cytosolic Ca(2+) signals in the apical region, large Ca(2+) gradients form over the Golgi (decreasing from trans- to cis-Golgi). We further describe a novel, close interaction of the peri-granular mitochondria and the Golgi apparatus. The mitochondria and the Golgi structures form very close contacts, and these contacts remain stable over time. When the cell is forced to swell, the Golgi and mitochondria remain juxtaposed up to the point of cell lysis. The strategic position of the Golgi (closer to release sites than the bulk of the mitochondrial belt) makes this organelle receptive to local apical Ca(2+) transients. In addition the Golgi is ideally placed to be preferentially supplied by ATP from adjacent mitochondria.

Inositol (1,4,5)-trisphosphate receptor links to filamentous actin are important for generating local Ca2+ signals in pancreatic acinar cells

We explored a potential structural and functional link between filamentous actin (F-actin) and inositol (1,4,5)-trisphosphate receptors (IP3Rs) in mouse pancreatic acinar cells. Using immunocytochemistry, F-actin and type 2 and 3 IP3Rs (IP3R2 and IP3R3) were identified in a cellular compartment immediately beneath the apical plasma membrane. In an effort to demonstrate that IP3R distribution is dependent on an intact F-actin network in the apical subplasmalemmal region, cells were treated with the actin-depolymerising agent latrunculin B. Immunocytochemistry indicated that latrunculin B treatment reduced F-actin in the basolateral subplasmalemmal compartment, and reduced and fractured F-actin in the apical subplasmalemmal compartment. This latrunculin-B-induced loss of F-actin in the apical region coincided with a reduction in IP3R2 and IP3R3, with the remaining IP3Rs localized with the remaining F-actin. Experiments using western blot analysis showed that IP3R3s are resistant to extraction by detergents, which indicates a potential interaction with the cytoskeleton. Latrunculin B treatment in whole-cell patch-clamped cells inhibited Ca2+-dependent Cl– current spikes evoked by inositol (2,4,5)-trisphosphate; this is due to an inhibition of the underlying local Ca2+ signal. Based on these findings, we suggest that IP3Rs form links with F-actin in the apical domain and that these links are essential for the generation of local Ca2+ spikes.

Aberrant localization of intracellular organelles, Ca2+ signaling, and exocytosis in Mist1 null mice

Ca2+ signaling and exocytosis are highly polarized functions of pancreatic acinar cells. The role of cellular architecture in these activities and the capacity of animals to tolerate aberrant acinar cell function are not known. A key regulator of acinar cell polarity is Mist1, a basic helix-loop-helix transcription factor. Ca2+ signaling and amylase release were examined in pancreatic acini of wild type and Mist1 null mice to gain insight into the importance of cellular architecture for Ca2+ signaling and regulated exocytosis. Mist1-/- acinar cells exhibited dramatically altered Ca2+ signaling with up-regulation of the cholecystokinin receptor but minimal effect upon expression of the M3 receptor. However, stimulation of inositol 1,4,5-trisphosphate production by cholecystokinin and carbachol was inefficient in Mist1-/- cells. Although agonist stimulation of Mist1-/- cells evoked a Ca2+ signal, often the Ca2+ increase was not in the form of typical Ca2+ oscillations but rather in the form of a peak/plateau-type response. Mist1-/- cells also displayed distorted apical-to-basal Ca2+ waves. The aberrant Ca2+ signaling was associated with mislocalization and reduced Ca2+ uptake by the mitochondria of stimulated Mist1-/- cells. Deletion of Mist1 also led to mislocalization of the Golgi apparatus and markedly reduced digestive enzyme content. The combination of aberrant Ca2+ signaling and reduced digestive enzyme content resulted in poor secretion of digestive enzymes. Yet, food consumption and growth of Mist1-/- mice were normal for at least 32 weeks. These findings reveal that Mist1 is critical to normal organelle localization in exocrine cells and highlight the critical importance of maintaining cellular architecture and polarized localization of cellular organelles in generating a propagating apical-to-basal Ca2+ wave. The studies also reveal the spare capacity of the exocrine pancreas that allows normal growth and development in the face of compromised exocrine pancreatic function.

Cleavage of SNAP-25 and VAMP-2 impairs store-operated Ca2+entry in mouse pancreatic acinar cells

We recently reported that store-operated Ca2+entry (SOCE) in nonexcitable cells is likely to be mediated by a reversible interaction between Ca2+channels in the plasma membrane and the endoplasmic reticulum, a mechanism known as “secretion-like coupling.” As for secretion, in this model the actin cytoskeleton plays a key regulatory role. In the present study we have explored the involvement of the secretory proteins synaptosome-associated protein (SNAP-25) and vesicle-associated membrane protein (VAMP) in SOCE in pancreatic acinar cells. Cleavage of SNAP-25 and VAMPs by treatment with botulinum toxin A (BoNT A) and tetanus toxin (TeTx), respectively, effectively inhibited amylase secretion stimulated by the physiological agonist CCK-8. BoNT A significantly reduced Ca2+entry induced by store depletion using thapsigargin or CCK-8. In addition, treatment with BoNT A once SOCE had been activated reduced Ca2+influx, indicating that SNAP-25 is needed for both the activation and maintenance of SOCE in pancreatic acinar cells. VAMP-2 and VAMP-3 are expressed in mouse pancreatic acinar cells. Both proteins associate with the cytoskeleton upon Ca2+store depletion, although only VAMP-2 seems to be sensitive to TeTx. Treatment of pancreatic acinar cells with TeTx reduced the activation of SOCE without affecting its maintenance. These findings support a role for SNAP-25 and VAMP-2 in the activation of SOCE in pancreatic acinar cells and show parallels between this process and secretion in a specialized secretory cell type.

Characterization of Gastrin-Induced Proangiogenic EffectsIn vivoin Orthotopic U373 Experimental Human Glioblastomas andIn vitroin Human Umbilical Vein Endothelial Cells

PURPOSE

This study aims to investigate the role of gastrin-17 (G17) on angiogenesis features in gliomas both in vitro and in vivo.

EXPERIMENTAL DESIGN

The influences of G17 and G17 receptor antagonists were characterized in vitro in terms of angiogenesis on human umbilical vein endothelial cell (HUVEC) tubulogenesis processes on Matrigel and in vivo with respect to U373 orthotopic glioma xenografts. The influence of phosphatidylinositol 3'-kinase, protein kinase C, and nuclear factor-kappaB inhibitors was characterized in vitro on G17-mediated HUVEC tubulogenesis. G17-mediated release of interleukin (IL)-8 from HUVECs and G17-induced modifications in nuclear factor-kappaB DNA binding activity were characterized by means of specific enzyme-linked immunosorbent assays. The influence of G17 on E- and P-selectin expression was determined by means of computer-assisted microscopy, whereas the influence of E- and P-selectin on HUVEC migration was approached by means of antisense oligonucleotides. The chemotactic influence of G17 and IL-8 on HUVEC migration was characterized by means of computer-assisted videomicroscopy with Dunn chambers.

RESULTS

Messenger RNAs for cholecystokinin (CCK)A, CCKB, and CCKC receptors were present in HUVECs and microvessels dissected from a human glioblastoma. Whereas G17 significantly increased the levels of angiogenesis in vivo in the U373 experimental glioma model and in vitro in the HUVECs, the CCKB receptor antagonist L365,260 significantly counteracted the G17-mediated proangiogenic effects. G17 chemoattracted HUVECs, whereas IL-8 failed to do so. IL-8 receptor alpha (CXCR1) and IL-8 receptor beta (CXCR2) mRNAs were not detected in these endothelial cells. Gastrin significantly (but only transiently) decreased the level of expression of E-selectin, but not P-selectin, whereas IL-8 increased the expression of E-selectin. Specific antisense oligonucleotides against E- and P-selectin significantly decreased HUVEC tubulogenesis processes in vitro on Matrigel.

CONCLUSIONS

The present study shows that gastrin has marked proangiogenic effects in vivo on experimental gliomas and in vitro on HUVECs. This effect depends in part on the level of E-selectin activation, but not on IL-8 expression/release by HUVECs.

Phosphatidylinositol 3-kinase regulates Ca2+ signaling in pancreatic acinar cells through inhibition of sarco(endo)plasmic reticulum Ca2+-ATPase

Calcium is a key mediator of hormone-induced enzyme secretion in pancreatic acinar cells. At the same time, abnormal Ca(2+) responses are associated with pancreatitis. We have recently shown that inhibition of phosphatidylinositol 3-kinase (PI3-kinase) by LY-294002 and wortmannin, as well as genetic deletion of PI3-kinase-gamma, regulates Ca(2+) responses and the Ca(2+)-sensitive trypsinogen activation in pancreatic acinar cells. The present study sought to determine the mechanisms of PI3-kinase involvement in Ca(2+) responses induced in these cells by CCK and carbachol. The PI3-kinase inhibitors inhibited both Ca(2+) influx and mobilization from intracellular stores induced by stimulation of acini with physiological and pathological concentrations of CCK, as well as with carbachol. PI3-kinase inhibition facilitated the decay of cytosolic free Ca(2+) concentration ([Ca(2+)](i)) oscillations observed in individual acinar cells. The PI3-kinase inhibitors decreased neither CCK-induced inositol 1,4,5-trisphosphate [Ins(1,4,5)P(3)] production nor Ins(1,4,5)P(3)-induced Ca(2+) mobilization, suggesting that the effect of PI3-kinase inhibition is not through Ins(1,4,5)P(3) or Ins(1,4,5)P(3) receptors. PI3-kinase inhibition did not affect Ca(2+) mobilization induced by thapsigargin, a specific inhibitor of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA). Moreover, SERCA blockade with thapsigargin abolished the effects of pharmacological and genetic PI3-kinase inhibition on [Ca(2+)](i) signals, suggesting SERCA as a downstream target of PI3-kinase. Both pharmacological PI3-kinase inhibition and genetic deletion of PI3-kinase-gamma increased the amount of Ca(2+) in intracellular stores during CCK stimulation. Finally, addition of the PI3-kinase product phosphatidylinositol 3,4,5-trisphosphate to permeabilized acini significantly attenuated Ca(2+) reloading into the endoplasmic reticulum. The results indicate that PI3-kinase regulates Ca(2+) signaling in pancreatic acinar cells through its inhibitory effect on SERCA.

Clinical endocrinology and metabolism. Cholecystokinin

Cholecystokinin (CCK) is a peptide hormone discovered in the small intestine. Together with secretin and gastrin, CCK constitutes the classical gut hormone triad. In addition to gallbladder contraction, CCK also regulates pancreatic enzyme secretion and growth, intestinal motility, satiety signalling and the inhibition of gastric acid secretion. CCK is, however, also a transmitter in central and intestinal neurons. Notably, CCK is the most abundant neuropeptide in the human brain. Owing to difficulties in developing accurate assays, knowledge about CCK secretion in disease is limited. Available data indicate, however, that proCCK is expressed in certain neuroendocrine tumours and sarcomas, whereas the secretion of CCK is impaired in celiac disease and bulimia nervosa. Stimulation with exogenous CCK has proved useful in diagnostic tests of gallbladder and pancreatic diseases, as well as medullary thyroid carcinomas.

Changes in IP3 receptor are associated with altered calcium response to cholecystokinin in diabetic rat pancreatic acini

OBJECTIVES

Pancreatic acini of diabetic rats release amylase less than normal acini on cholecystokinin (CCK) stimulation. Pancreatic enzyme secretion by CCK is closely related to the second messenger inositol 1,4,5-trisphosphate (IP3), which mobilizes intracellular calcium stores via the endoplasmic reticulum-located receptor IP3 (IP3R). Recently, we observed altered intracellular calcium response on CCK-8 stimulation in streptozotocin (STZ)- treated diabetic rat acini.

METHODS

To determine whether IP3R is involved in altered calcium response, we measured inositol phosphate (IP) formation and the expression and phosphorylation of type III IP3R protein in diabetic acini. Also, CCK receptor mRNA expression was examined to determine whether the changes in IP formation and IP3R protein phosphorylation in diabetic acini might result from the defect at the postreceptor level.

RESULTS

CCK-8-induced IP formation at all concentrations used was significantly reduced in diabetic acini, though IP formation was increased in a concentration-dependent manner. The expression of type III IP3R protein was significantly reduced in diabetic acini. Additionally, CCK-8-stimulated phosphorylation of type III IP3R protein was not observed in diabetic acini. However, the reduction of CCK receptor mRNA expression was not detected in diabetic acini.

CONCLUSION

Our results indicate that altered calcium response to CCK-8 in diabetic acini might be associated with a post-CCK receptor defect including the changes in IP formation, type III IP3R protein expression, and phosphorylation of type III IP3R protein.

Pharmacological characterization of rebamipide: its cholecystokinin CCK1 receptor binding profile and effects on Ca2+ mobilization and amylase release in rat pancreatic acinar cells

We previously reported that rebamipide (2-(4-chlorobenzoylamino)-3-[2(1H)-quinolinon-4-yl]-propionic acid) generated oscillations of intracellular Ca2+ concentration ([Ca2+]i) probably through the activation of cholecystokinin type 1 (CCK1) receptors in rat pancreatic acinar cells. Therefore, in the present study, we aimed to establish the pharmacological characteristics of rebamipide in rat pancreatic acinar cells. CCK-8S and rebamipide inhibited [125I]BH-CCK-8S binding to rat pancreatic acinar cell membranes with IC50 values of 3.13 nM and 37.7 microM, respectively. CCK-8S usually evoked [Ca2+]i oscillations at concentrations lower than 50 pM, and it induced biphasic [Ca2+]i increases at higher concentrations. In contrast to CCK-8S, rebamipide only induced [Ca2+]i oscillations at all the concentrations we used in this study. In addition, rebamipide was shown to inhibit high concentrations of CCK-8S-induced biphasic increases in [Ca2+]i, suggesting that rebamipide might be a partial agonist at cholecystokinin CCK1 receptors. Although rebamipide induced [Ca2+]i oscillations by activating the cholecystokinin CCK1 receptors, rebamipide did not cause amylase release and only inhibited CCK-stimulated amylase release reversibly and dose-dependently. However, rebamipide did not inhibit carbachol-, vasoactive intestinal polypeptide (VIP)-, and forskolin-induced amylase releases. These data indicate that rebamipide functions as a partial agonist for Ca2+ -mobilizing action, and it is also an antagonist for the amylase-releasing action of CCK.

Regulation of CCK-induced amylase release by PKC-delta in rat pancreatic acinar cells

PKC is known to be activated by pancreatic secretagogues such as CCK and carbachol and to participate along with calcium in amylase release. Four PKC isoforms, alpha, delta, epsilon, and zeta, have been identified in acinar cells, but which isoforms participate in amylase release are unknown. To identify the responsible isoforms, we used translocation assays, chemical inhibitors, and overexpression of individual isoforms and their dominant-negative variants by means of adenoviral vectors. CCK stimulation caused translocation of PKC-alpha, -delta, and -epsilon, but not -zeta from soluble to membrane fraction. CCK-induced amylase release was inhibited approximately 30% by GF109203X, a broad spectrum PKC inhibitor, and by rottlerin, a PKC-delta inhibitor, but not by Gö6976, a PKC-alpha inhibitor, at concentrations from 1 to 5 microM. Neither overexpression of wild-type or dominant-negative PKC-alpha affected CCK-induced amylase release. Overexpression of PKC-delta and -epsilon enhanced amylase release, whereas only dominant-negative PKC-delta inhibited amylase release by 25%. PKC-delta overexpression increased amylase release at all concentrations of CCK, but dominant-negative PKC-delta only inhibited the maximal concentration; both similarly affected carbachol and JMV-180-induced amylase release. Overexpression of both PKC-delta and its dominant-negative variant affected the late but not the early phase of amylase release. GF109203X totally blocked the enhancement of amylase release by PKC-delta but had no further effect in the presence of dominant-negative PKC-delta. These results indicate that PKC-delta is the PKC isoform involved with amylase secretion.

A Role of VAMP8/Endobrevin in Regulated Exocytosis of Pancreatic Acinar Cells

Despite our general understanding that members of the SNARE superfamily participate in diverse intracellular docking/fusion events, the physiological role of the majority of SNAREs in the intact organism remains elusive. In this study, through targeted gene knockout in mice, we establish that VAMP8/endobrevin is a major player in regulated exocytosis of the exocrine pancreas. VAMP8 is enriched on the membrane of zymogen granules and exists in a complex with syntaxin 4 and SNAP-23. VAMP8-/- mice developed normally but showed severe defects in the pancreas. VAMP8 null acinar cells contained three times more zymogen granules than control acinar cells. Furthermore, secretagogue-stimulated secretion was abolished in pancreatic fragments derived from VAMP8-/- mice. In addition, VAMP8-/- mice were partially resistant to supramaximal caerulein-induced pancreatitis. These results suggest a major physiological role of VAMP8 in regulated exocytosis of pancreatic acinar cells by serving as a v-SNARE of zymogen granules.

PKC-delta and -epsilon regulate NF-kappaB activation induced by cholecystokinin and TNF-alpha in pancreatic acinar cells

Although NF-kappaB plays an important role in pancreatitis, mechanisms underlying its activation remain unclear. We investigated the signaling pathways mediating NF-kappaB activation in pancreatic acinar cells induced by high-dose cholecystokinin-8 (CCK-8), which causes pancreatitis in rodent models, and TNF-alpha, which contributes to inflammatory responses of pancreatitis, especially the role of PKC isoforms. We determined subcellular distribution and kinase activities of PKC isoforms and NF-kappaB activation in dispersed rat pancreatic acini. We applied isoform-specific, cell-permeable peptide inhibitors to assess the role of individual PKC isoforms in NF-kappaB activation. Both CCK-8 and TNF-alpha activated the novel isoforms PKC-delta and -epsilon and the atypical isoform PKC-zeta but not the conventional isoform PKC-alpha. Inhibition of the novel PKC isoforms but not the conventional or the atypical isoform resulted in the prevention of NF-kappaB activation induced by CCK-8 and TNF-alpha. NF-kappaB activation by CCK-8 and TNF-alpha required translocation but not tyrosine phosphorylation of PKC-delta. Activation of PKC-delta, PKC-epsilon, and NF-kappaB with CCK-8 involved both phosphatidylinositol-specific PLC and phosphatidylcholine (PC)-specific PLC, whereas with TNF-alpha they only required PC-specific PLC for activation. Results indicate that CCK-8 and TNF-alpha initiate NF-kappaB activation by different PLC pathways that converge at the novel PKCs (delta and epsilon) to mediate NF-kappaB activation in pancreatic acinar cells. These findings suggest a key role for the novel PKCs in pancreatitis.

Feeding activates protein synthesis in mouse pancreas at the translational level without increase in mRNA

To determine the mechanism of meal-regulated synthesis of pancreatic digestive enzymes, we studied the effect of fasting and refeeding on pancreatic protein synthesis, relative mRNA levels of digestive enzymes, and activation of the translational machinery. With the use of the flooding dose technique with L-[3H]phenylalanine, morning protein synthesis in the pancreas of Institute for Cancer Research mice fed ad libitum was 7.9 +/- 0.3 nmol phenylalanine.10 min(-1).mg protein(-1). Prior fasting for 18 h reduced total protein synthesis to 70 +/- 1.4% of this value. Refeeding for 2 h, during which the mice consumed 29% of their daily food intake, increased protein synthesis to 117.3 +/- 4.9% of the control level. Pancreatic mRNA levels of amylase, lipases, trypsins, chymotrypsin, elastases, as well as those for several housekeeping genes tested were not significantly changed after refeeding compared with fasted mice. By contrast, the major translational control pathway involving Akt, mTOR, and S6K was strongly regulated by fasting and refeeding. Fasting for 18 h decreased phosphorylation of ribosomal protein S6 to almost undetectable levels, and refeeding highly increased it. The most highly phosphorylated form of the eIF4E binding protein (4E-BP1) made up the 14.6% of total 4E-BP1 in normally fed animals, was only 2.8% after fasting, and was increased to 21.4% after refeeding. This was correlated with an increase in the formation of the eIF4E-eIF4G complex after refeeding. By contrast, feeding did not affect eIF2B activity. Thus food intake stimulates pancreatic protein synthesis and translational effectors without increasing digestive enzyme mRNA levels.

Receptor biology and signal transduction in pancreatic acinar cells

PURPOSE OF REVIEW

Secretagogue receptors and their intracellular signaling pathways regulate pancreatic physiology and may be altered in pathophysiology. Therefore, understanding of the continued progress into their nature and function is relevant to both biology and disease.

RECENT FINDINGS

The major secretagogue receptors on acinar cells include those binding cholecystokinin and acetylcholine, whereas secretin receptors regulate duct cells. Two physical models of the cholecystokinin receptor and ligand binding have been proposed through extensive structure-activity studies. Receptor oligomerization has been described for both cholecystokinin and secretin receptors. Ca plays a central role in the control of digestive enzyme secretion and is largely mobilized from intracellular stores. Inositol trisphosphate has been joined by two other Ca-releasing messengers, cyclic ADP ribose and nicotinic acid adenine dinucleotide phosphate, in initiating and coordinating Ca signaling. Progress has also been made in determining the roles of specific organelles in Ca release. Ca triggers secretion, and knowledge of the function and regulation of the proteins involved in exocytosis is accumulating. Continuing advances have also been made in understanding the signaling pathways regulating protein synthesis and growth in adult pancreas. The protein kinase mammalian target of rapamycin and its downstream targets play a central role in protein synthesis, whereas the protein phosphatase calcineurin was recently reported to regulate pancreatic growth. Other signaling molecules include the MAP kinases, PKCs, cytoplasmic tyrosine kinases, and nitric oxide.

SUMMARY

The current findings reviewed here are illuminating the structure and function of receptors on pancreatic acinar and duct cells and the multiple intracellular signaling pathways that they initiate. Understanding of these mechanisms is contributing to knowledge of normal pancreatic functions and alterations in disease such as pancreatitis and pancreatic cancer.

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Calcineurin is required for translational control of protein synthesis in rat pancreatic acini

CCK increases the rate of net protein synthesis in rat pancreatic acini by activating initiation and elongation factors required for translation. The immunosuppressant FK506 inhibits the Ca2+-calmodulin-dependent phosphatase calcineurin in pancreatic acinar cells and blocks pancreatic growth induced by chronic CCK treatment. To test a requirement for calcineurin in the activation of the translational machinery stimulated by CCK, we evaluated the effects of FK506 on protein synthesis and on regulatory initiation and elongation factors in rat pancreatic acini in vitro. CCK acutely increased protein synthesis in acini from normal rats with a maximum increase at 100 pM CCK to 170 ± 11% of control. The immunosuppressant FK506 dose-dependently inhibited CCK-stimulated protein synthesis over the same concentration range that blocked calcineurin activity, as assessed by dephosphorylation of the calcineurin substrate calcium-regulated heat-stable protein of 24 kDa. Another immunosuppressant, cyclosporin A, inhibited protein synthesis, but its effects appeared more complex. FK506 also inhibited protein synthesis stimulated by bombesin and carbachol. FK506 did not significantly affect the activity of the initiation factor-2B, or the phosphorylation of the initiation factor-2α, ribosomal protein protein S6, or the mRNA cap binding protein eukaryotic initiation factor (eIF) 4E. Instead, blockade of calcineurin with FK506 reduced the phosphorylation of the eIF4E binding protein, reduced the formation of the eIF4F complex, and increased the phosphorylation of eukaryotic elongation factor 2. From these results, we conclude that calcineurin activity is required for protein synthesis, and this action may be related to an effect on the formation of the mRNA cap binding complex and the elongation processes.

Regulation of translation elongation and phosphorylation of eEF2 in rat pancreatic acini

While pancreatic protein synthesis and the initiation of translation are regulated by hormones and neurotransmiters, whether the elongation process is also regulated is unknown. Stimulatory doses of cholecystokinin (CCK) (100 pM), bombesin (10 nM), and carbachol (10 microM) increased elongation rates (measured as ribosomal half-transit time) in pancreatic acini in vitro. At the same time these secretagogues reduced elongation factor 2 (eEF2) phosphorylation, the main factor known to regulate elongation, and increased the phosphorylation of the eEF2 kinase. The mTOR inhibitor rapamycin reversed the dephosphorylation of eEF2 induced by CCK, as did treatment with the p38 MAPK inhibitor SB202190, the MEK inhibitor PD98059, and the phosphatase inhibitor calyculin A. Neither rapamycin, SB202190, PD98059 nor calyculin A had an effect on CCK mediated eEF2 kinase phosphorylation. Translation elongation in pancreatic acinar cells is likely regulated by eEF2 through the mTOR, p38, and MEK pathways, and modulated through PP2A.

Exocrine pancreatic secretion of phospholipid, menaquinone-4, and caveolin-1 in vivo

The exocrine pancreas releases secretory products essential for nutrient assimilation. In addition to digestive enzymes, the release of lipoprotein-like particles containing the membrane trafficking protein caveolin-1 from isolated pancreatic explants has been reported. The present study examined: (1) if gastrointestinal hormones induce the apical secretion of phospholipid in vivo and (2) a potential association of caveolin-1 and the lipid-soluble vitamin K analog menaquinone-4 (MK-4) with these structures. Analysis of isolated acinar cells, purified zymogen granules, and pancreatic juice collected in vivo indicated the presence a caveolin-1 immunoreactive protein that was acutely released in response hormone stimulation. Chloroform-extracted fractions of pancreatic juice also contained high concentrations of MK-4 that was secreted in parallel to protein and phospholipid. The presence of caveolin-1 and MK-4 in the phospholipid fraction of pancreatic juice places these molecules in the secretory pathway of exocrine cells and suggests a physiological role in digestive enzyme synthesis and/or processing.

Functional Mapping of Ca2+ Signaling Complexes in Plasma Membrane Microdomains of Polarized Cells

Many cells cluster signaling complexes in plasma membrane microdomains. Polarized secretory cells cluster all Ca2+ signaling proteins, including GPCRs, at the apical pole. The functional significance of such an arrangement is not known because of a lack of techniques for functional mapping of signaling complexes at plasma membrane patches. In the present work, we developed such a technique based on the use of two patch pipettes, a recording and a stimulating pipette (SP). Including 20% glycerol in the SP solution increased the viscosity and the hydrophobicity to prevent leakage and formation of tight seals on the plasma membrane. This allowed moving the SP between sites to stimulate multiple patches of the same cell and with the same agonist concentrations. Functional mapping of Ca2+ signaling in pancreatic acinar cells revealed that the M3, cholecystokinin, and bombesin signaling complexes at the apical pole are much more sensitive to stimulation than those at the basal pole. Furthermore, at physiological agonist concentrations, Ca2+ signals could be evoked only by stimulation of membrane patches at the apical pole. [Ca2+](i) imaging revealed that Ca2+ waves were invariably initiated at the site of apical membrane patch stimulation, suggesting that long range diffusion of second messengers is not obligatory to initiate and propagate apical-to-basal Ca2+ waves. The present studies reveal a remarkable heterogeneity in responsiveness of Ca2+ signaling complexes at membrane microdomains, with the most responsive complexes confined to the apical pole, probably to restrict the Ca2+ signals to the site of exocytosis and allow the polarized functions of secretory cells.

Noc2 is essential in normal regulation of exocytosis in endocrine and exocrine cells

Rab3 is a subfamily of the small GTP-binding protein Rab family and plays an important role in exocytosis. Several potential effectors of Rab3, including rabphilin3 and Rims (Rim1 and Rim2), have been isolated and characterized. Noc2 was identified originally in endocrine pancreas as a molecule homologous to rabphilin3, but its role in exocytosis is unclear. To clarify the physiological function of Noc2 directly, we have generated Noc2 knockout (Noc2(-/-)) mice. Glucose intolerance with impaired insulin secretion was induced in vivo by acute stress in Noc2(-/-) mice, but not in wild-type (Noc2(+/+)) mice. Ca(2+)-triggered insulin secretion from pancreatic isles of Noc2(-/-) mice was markedly impaired, but was completely restored by treatment with pertussis toxin, which inhibits inhibitory G protein Gi/o signaling. In addition, the inhibitory effect of clonidine, an alpha(2)-adrenoreceptor agonist, on insulin secretion was significantly greater in Noc2(-/-) islets than in Noc2(+/+) islets. Impaired Ca(2+)-triggered insulin secretion was rescued by adenovirus gene transfer of wild-type Noc2 but not by that of mutant Noc2, which does not bind to Rab3. Accordingly, Noc2 positively regulates insulin secretion from endocrine pancreas by inhibiting Gi/o signaling, and the interaction of Noc2 and Rab3 is required for the effect. Interestingly, we also found a marked accumulation of secretory granules in various exocrine cells of Noc2(-/-) mice, especially in exocrine pancreas with no amylase response to stimuli. Thus, Noc2, a critical effector of Rab3, is essential in normal regulation of exocytosis in both endocrine and exocrine cells.

Calcineurin mediates pancreatic growth in protease inhibitor-treated mice

CCK acts on pancreatic acinar cells to increase intracellular Ca(2+) leading to secretion of digestive enzymes and, in the long term, pancreatic growth. Calcineurin (CN) is a serine/threonine-specific protein phosphatase activated by Ca(2+) and calmodulin that recently has been shown to participate in the growth regulation of cardiac and skeletal myocytes. We therefore tested the effect of two different CN inhibitors, cyclosporine A (CsA) and FK506, on mouse pancreatic growth induced by oral administration of the synthetic protease inhibitor camostat, a known stimulator of endogenous CCK release. Mice were fed a powdered diet with or without 0.1% camostat. Pancreatic wet weight, protein, and DNA were increased in response to camostat in a time-dependent manner over 10 days in ICR mice but not in CCK-deficient mice. Both CsA (15 mg/kg) and FK506 (3 mg/kg) given twice daily blocked the increase in pancreatic wet weight and protein and DNA content induced by camostat. The increase in plasma CCK induced by camostat was not blocked by CsA or FK506. Camostat feeding also increased the relative amount of CN protein, whereas levels of MAPKs, ERKs, and p38 were not altered. In summary, 1) CCK released by chronic camostat feeding induces pancreatic growth in mice; 2) this growth is blocked by treatment with both CsA and FK506, indicating a role for CN; 3) CCK stimulation also increases CN protein. In conclusion, activation and possibly upregulation of CN may participate in regulation of pancreatic growth by CCK in mice.

Effects of secretagogues and bile acids on mitochondrial membrane potential of pancreatic acinar cells: comparison of different modes of evaluating DeltaPsim

In this study, we investigated the effects of secretagogues and bile acids on the mitochondrial membrane potential of pancreatic acinar cells. We measured the mitochondrial membrane potential using the tetramethylrhodamine-based probes tetramethylrhodamine ethyl ester and tetramethylrhodamine methyl ester. At low levels of loading, these indicators appeared to have a low sensitivity to the uncoupler carbonyl cyanide m-chlorophenylhydrazone, and no response was observed to even high doses of cholecystokinin. When loaded at high concentrations, tetramethylrhodamine methyl ester and tetramethylrhodamine ethyl ester undergo quenching and can be dequenched by mitochondrial depolarization. We found the dequench mode to be 2 orders of magnitude more sensitive than the low concentration mode. Using the dequench mode, we resolved mitochondrial depolarizations produced by supramaximal and by physiological concentrations of cholecystokinin. Other calcium-releasing agonists, acetylcholine, JMV-180, and bombesin, also produced mitochondrial depolarization. Secretin, which employs the cAMP pathway, had no effect on the mitochondrial potential; dibutyryl cAMP was also ineffective. The cholecystokinin-induced mitochondrial depolarizations were abolished by buffering cytosolic calcium. A non-agonist-dependent calcium elevation induced by thapsigargin depolarized the mitochondria. These experiments suggest that a cytosolic calcium concentration rise is sufficient for mitochondrial depolarization and that the depolarizing effect of cholecystokinin is mediated by a cytosolic calcium rise. Bile acids are considered possible triggers of acute pancreatitis. The bile acids taurolithocholic acid 3-sulfate, taurodeoxycholic acid, and taurochenodeoxycholic acid, at low submillimolar concentrations, induced mitochondrial depolarization, resolved by the dequench mode. Our experiments demonstrate that physiological concentrations of secretagogues and pathologically relevant concentrations of bile acids trigger mitochondrial depolarization in pancreatic acinar cells.

Morphological and functional changes of dissociated single pancreatic acinar cells: testing the suitability of the single cell as a model for exocytosis and calcium signaling

Isolated single pancreatic acinar cells have long been used as a model for studying many kinds of signaling processes due to their structural and functional polarities, but without significant validation. In this study, we examined the morphological and functional changes of dissociated single pancreatic acinar cells. Acutely isolated single cells showed a collapsed membrane potential and a much reduced secretion of zymogen granules in response to acetylcholine (ACh) stimulation, whereas clustered cells showed a much more negative membrane potential and potent exocytotic secretion. The isolated single cells became vertically flattened due to the loss of supporting adhesions with nearby cells, and the granule-attached luminal membrane was severely reduced versus that of clustered cells. However, polarized Ca(2+) signals and mitochondrial localizations were relatively well preserved in the isolated single cells, in that Ca(2+) release by ACh commenced at the indented luminal membrane. In clusters, the Ca(2+) release site was closest to the lumen where more than three cells met or at the tips of conical regions of the luminal membrane. These findings suggest that the dissociated single pancreatic acinar cells preserve an intact Ca(2+) signaling machinery but alter in shape and have impaired exocytotic functions and resting membrane potentials.

CCK-A receptor induction and P38 and NF-kappaB activation in acute pancreatitis

Bile-pancreatic duct ligation in rats excludes bile-pancreatic juice from the gut and induces acute pancreatitis. Bile-pancreatic juice exclusion from the gut results in increased plasma cholecystokinin (CCK) levels. CCK-A receptor-mediated exocrine pancreatic hyperstimulation is implicated in disease pathogenesis. In the present study, we show for the first time a progressive rise in CCK-A receptor protein expression in ligation-induced acute pancreatitis in rats. As CCK-A receptor induction could amplify CCK-mediated acinar hyperstimulation and exacerbate acinar cell stress with activation of the p38(MAPK) stress kinase pathway, we studied CCK-A receptor protein expression and p38(MAPK) activation in duct ligation-induced acute pancreatitis in rats. Compared to sham-operated controls, acute pancreatitis induced by bile-pancreatic duct ligation associates with a temporal increase in pancreatic CCK-A receptor protein expression, p38(MAPK) expression and activation, and NF-kappaB activation. These findings may have significance in the mechanism of disease pathogenesis in this experimental model.

Ethanol differentially regulates NF-kappaB activation in pancreatic acinar cells through calcium and protein kinase C pathways

Mechanisms of alcoholic pancreatitis remain unknown. Previously, we showed that ethanol feeding sensitizes rats to pancreatitis caused by CCK-8, at least in part, by augmenting activation of the proinflammatory transcription factor NF-kappaB. To elucidate the mechanism of sensitization, here we investigate the effect of ethanol on Ca(2+)- and PKC-mediated pathways of CCK-induced NF-kappaB activation using an in vitro system of rat pancreatic acini incubated with ethanol. Ethanol augmented CCK-8-induced activation of NF-kappaB, similar to our in vivo findings with ethanol-fed rats. In contrast, ethanol prevented NF-kappaB activation caused by thapsigargin, an agent that mobilizes intracellular Ca(2+) bypassing the receptor. Pharmacological analysis showed that NF-kappaB activation by thapsigargin but not by CCK-8 is mediated through the calcineurin pathway and that the inhibitory effect of ethanol on the thapsigargin-induced NF-kappaB activation could be through inhibiting this pathway. Ethanol augmented NF-kappaB activation induced by the phorbol ester PMA, a direct activator of PKC. Inhibitory analysis demonstrated that Ca(2+)-independent (novel and/or atypical) PKC isoforms are involved in NF-kappaB activation induced by both CCK-8 and PMA in cells treated and not treated with ethanol. The results indicate that ethanol differentially affects the Ca(2+)/calcineurin- and PKC-mediated pathways of NF-kappaB activation in pancreatic acinar cells. These effects may play a role in the ability of ethanol to sensitize pancreas to the inflammatory response and pancreatitis.

Phosphatidylinositide 3-kinase gamma regulates key pathologic responses to cholecystokinin in pancreatic acinar cells

BACKGROUND & AIMS

Early events in the pancreatic acinar cell critical for development of pancreatitis include activation of the transcription factor nuclear factor kappa B (NF-kappa B), abnormal Ca(2+) responses, and trypsinogen activation. Mechanisms underlying these responses, which can be studied in isolated pancreatic acini stimulated with supraphysiologic doses of cholecystokinin (CCK-8), remain poorly understood. We here report that these responses are regulated by phosphatidylinositide 3-kinase (PI3K) gamma.

METHODS

To inactivate PI3K, we used mice deficient in the catalytic PI3K gamma subunit p110 gamma as well as the PI3K inhibitors LY294002 and wortmannin. We measured Ca(2+) responses by using Fura-2, NF-kappa B-binding activity by electromobility shift assay, I kappa B degradation by Western blotting, and trypsinogen activation by fluorogenic assay.

RESULTS

CCK-induced intracellular Ca(2+) mobilization, Ca(2+) influx, trypsinogen, and NF-kappa B activation were all diminished in pancreatic acini isolated from p110 gamma(-/-) mice. Both in mouse and rat acini, these responses were inhibited by the PI3K inhibitors. The Ca(2+) signal and trypsinogen activation were similarly reduced in acini isolated from p110 gamma(-/-) and p110 gamma(+/-) mice compared with wild-type mice. By contrast, NF-kappa B activation was inhibited in p110 gamma(-/-) acini but not in p110 gamma(+/-) acini. These differences indicate that the mechanism of NF-kappa B regulation by PI3K gamma differs from those for the Ca(2+) and trypsinogen responses. CCK-induced responses in p110 gamma(-/-) acini were all further inhibited by LY294002, indicating the involvement of other PI3K isoform(s), in addition to PI3K gamma.

CONCLUSIONS

The results show that key pathologic responses of the pancreatic acinar cell are regulated by PI3K gamma and suggest an important role for this PI3K isoform in pancreatitis.

Hepatocyte growth factor activates several transduction pathways in rat pancreatic acini

The receptor of hepatocyte growth factor (HGF), c-met induces different physiological responses in several cell types. Little is known about the role of HGF in exocrine pancreas. However, abnormal HGF signaling has been strongly implicated in pancreatic tumorigenesis and association of HGF with pancreatitis has been demonstrated. We have studied the presence of c-met and activation of their intracellular pathways associated in rat pancreatic acini in comparison with cholecystokinin (CCK) and epidermal growth factor (EGF). C-met expression in rat exocrine pancreas was identified by immunohistochemistry and immunoprecipitation followed by Western analysis. Tyrosine phosphorylation of c-met is strongly stimulated as well as kinase pathways leading to ERK1/2 cascade. HGF, but not CCK or EGF, selectively caused a consistent increase in the amount of p85 regulatory subunit of PI3-K present in anti-phosphotyrosine immunoprecipitates. Downstream of PI3-K, HGF increased Ser473 phosphorylation of Akt selectively, as CCK or EGF did not affect it. HGF selectively stimulated tyrosine phosphorylation of phosphatase PTP1D. HGF failed to promote the well-known CCK effects in pancreatic acini such as amylase secretion and intracellular calcium mobilization. Although HGF shares activation of ERK1/2 with CCK, we demonstrate that it promotes the selective activation of intracellular pathways not regulated by CCK or EGF. Our results suggest that HGF is an in vivo stimulus of pancreatic acini and provide novel insight into the transduction pathways and effects of c-met/HGF in normal pancreatic acinar cells.

Physiological phenotyping of cholecystokinin-responsive rat taste receptor cells

The recent discovery that subsets of rat taste receptor cells (TRCs) express the peptide cholecystokinin (CCK) and that subsets of TRCs respond to CCK with altered potassium currents or elevated intracellular calcium via CCK-A receptor has lead to the hypothesis that CCK may play a novel signaling role within the taste bud, perhaps modifying tastant responses by co-transmission with a classic transmitter. To better understand this phenomenon, CCK-responsive TRCs were characterized for sensitivity to two bitter stimuli, quinine or caffeine, or to the neurotransmitter ACh using a ratiometric procedure with the calcium sensitive dye fura-2. In characterizing TRC responses to quinine, it was observed that quinine-induced elevations of intracellular calcium were not due to endogenous fluorescence of the quinine molecule. Most (60-70%) CCK-responsive cells were also sensitive to either bitter stimuli or to cholinergic stimulation. These data suggest that TRCs expressing CCK-receptors also express receptors to bitter stimuli and/or muscarinic receptors. They further support the notion of a putative modulatory role of CCK with convergence of multiple inputs occurring at the level of intracellular calcium.

Cholecystokinin rapidly stimulates CrkII function in vivo in rat pancreatic acini. Formation of CrkII-protein complexes

Crk belongs to a family of adapter proteins whose structure allows interaction with tyrosine-phosphorylated proteins and is therefore an important modulator of downstream signals, representing a convergence of the actions of numerous stimuli. Recently, it was demonstrated that cholecystokinin (CCK) induced tyrosine phosphorylation of proteins related to fiber stress formation in rat pancreatic acini. Here, we investigated whether CCK receptor activation signals through CrkII and forms complexes with tyrosine-phosphorylated proteins in rat pancreatic acini. We demonstrated that CCK promoted the transient formation of CrkII-paxillin and CrkII-p130Cas complexes with maximal effect at 1 min. Additionally, CCK decreased the electrophoretic mobility of CrkII. This decrease was time- and concentration-dependent and inversely related with its function. Carbachol and bombesin also decreased CrkII electrophoretic mobility, whereas epidermal growth factor, vasoactive intestinal peptide, secretin or pituitary adenylate cyclase-activating polypeptide had no effect. CCK-induced CrkII electrophoretic shift was dependent on the Src family of tyrosine kinases and occurred in the intact animal, suggesting a physiological role of CrkII mediating CCK actions in the exocrine pancreas in vivo.

Membrane targeting in secretion

Regulated secretion and exocytosis require the selective packaging of regulated secretory proteins in secretory storage organelles and the controlled docking and fusion of these organelles with the plasma membrane. Secretory granule biogenesis involves sorting of secretory proteins and membrane components both at the level of the trans-Golgi network and the immature secretory granule. Sorting is thought to be mediated by selective protein aggregation and the interaction of these proteins with specific membrane domains. There is now considerable interest in the understanding of the complex lipid-protein and protein-protein interactions at the trans-Golgi network and the granule membrane. A role for lipid microdomains and associated sorting receptors in membrane targeting and granule formation is vividly discussed for (neuro)endocrine cells. In exocrine cells, however, little has been known of granule membrane composition and membrane protein function. With the cloning and characterization of granule membrane proteins and their interactions at the inner leaflet of zymogen granules of pancreatic acinar cells, it is now possible to elucidate their function in membrane targeting and sorting of zymogens at the molecular level.

Signal transduction, calcium and acute pancreatitis

Evidence consistently suggests that the earliest changes of acute pancreatitis are intracellular, the hallmark of which is premature intracellular activation of digestive zymogens, accompanied by disruption of normal signal transduction and secretion. Principal components of physiological signal transduction include secretagogue-induced activation of G-protein-linked receptors, followed by generation of inositol 1,4,5-trisphosphate, nicotinic acid adenine dinucleotide phosphate and cyclic ADP-ribose. In response, calcium is released from endoplasmic reticulum terminals within the apical, granular pole of the cell, where calcium signals are usually contained by perigranular mitochondria, in turn responding by increased metabolism. When all three intracellular messengers are administered together, even at threshold concentrations, dramatic potentiation results in sustained, global, cytosolic calcium elevation. Prolonged, global elevation of cytosolic calcium is also induced by hyperstimulation, bile salts, alcohol and fatty acid ethyl esters, and depends on continued calcium entry into the cell. Such abnormal calcium signals induce intracellular activation of digestive enzymes, and of nuclear factor kappaB, as well as the morphological changes of acute pancreatitis. Depletion of endoplasmic reticulum calcium and mitochondrial membrane potential may contribute to further cell injury. This review outlines current understanding of signal transduction in the pancreas, and its application to the pathophysiology of acute pancreatitis.

The cholecystokinin system in the rat retina: receptor expression and in vivo activation of tyrosine phosphorylation pathways

High levels of endogenous cholecystokinin (CCK) are present in the rat retina and restricted to amacrine cells. Two types of CCK receptors exist but their expression and intracellular transduction pathways coupled in the rat retina are unknown. The aims of this study were to investigate CCK receptors expression in rat retina and to study downstream tyrosine phosphorylation pathways. For this purpose, total mRNA isolated from rat retina was subjected to RT-PCR analysis. Isolated rat retinas were incubated in presence of CCK. Soluble proteins in retinal homogenates were immunoprecipitated with anti-phoshpotyrosine or anti-p130(Cas) specific monoclonal antibodies and subjected to SDS-PAGE, followed by Western blotting analysis. Both types of CCK receptor mRNAs, A and B, are present in the rat retina. Incubation of retina with CCK induced a rapid increase in several phosphotyrosine-containing bands with molecular masses greater than 30 kDa. Western Blotting and immunoprecipitation with a specific monoclonal antibody identified one of the phosphotyrosine bands as the adapter protein p130(Cas). Tyrosine phosphorylation of p130(Cas) induced by CCK in rat retina was time and concentration dependent: CCK induced tyrosine phosphorylation of p130(Cas) occurred rapidly with the maximum effect observed at 2.5 min incubation with 1 microM CCK. Our data clearly identified CCK-A and CCK-B receptor mRNAs in the rat retina and demonstrated that they are functional, stimulating tyrosine phosphorylation pathways. Our results provide novel biochemical information to further understand the physiological role of CCK A and B receptors in rat retina.

Cholinergic receptor induction and JNK activation in acute pancreatitis

BACKGROUND

Cholecystokinin-A (CCK-A) and cholinergic receptor pathways, capable of activating stress kinases p38 mitogen-activated protein kinase (p38(MAPK)) and cJUN N-terminal kinase (JNK), are implicated in the pathogenesis of ligation-induced acute pancreatitis in rats. As ligation-induced acute pancreatitis in rats is associated with CCK-A receptor induction and p38(MAPK) activation, and as receptor induction could amplify acinar hyperstimulation and exacerbate cell stress, we tested the hypothesis that the cholinergic M3 receptor is induced and JNK is activated in this model.

METHODS

Cholinergic M3 receptor expression and JNK activation was compared in rats 1, 3, or 24 hours after sham operation or duct ligation.

RESULTS

Immunoblot analysis of pancreatic homogenates showed a time-dependent increase in cholinergic M3 receptor protein, total JNK, and phospho-JNK after duct ligation.

CONCLUSIONS

There is a rapid and progressive cholinergic M3 receptor induction and JNK activation in ligation-induced acute pancreatitis in rats. These findings may have significance in the mechanism of disease pathogenesis.

CCK-A receptor activates RhoA through G alpha 12/13 in NIH3T3 cells

Cholecystokinin (CCK) is a major regulator of pancreatic acinar cells and was shown previously to be capable of inducing cytoskeletal changes in these cells. In the present study, using NIH3T3 cells stably transfected with CCK-A receptors as a model cell, we demonstrate that CCK can induce actin stress fibers through a G13- and RhoA-dependent mechanism. CCK induced stress fibers within minutes similar to those induced by lysophosphatidic acid (LPA), the active component of serum. The effects of CCK were mimicked by active RhoV14 and blocked by dominant-negative RhoN19, Clostridium botulinum C3 transferase, and the Rho-kinase inhibitor Y-27632. CCK rapidly induced active Rho in cells as shown with a pull-down assay using the Rho binding domain of rhotekin and by a serum response element (SRE)-luciferase reporter assay. To evaluate the G protein mediating the action of CCK, cells were transfected with active alpha-subunits; Galpha13 and Galpha12 but not Galphaq induced stress fibers and in some cases cell rounding. A p115 Rho guanine nucleotide exchange factor (GEF) regulator of G protein signaling (RGS) domain known to interact with G12/13 inhibited active alpha12/13-and CCK-induced stress fibers, whereas RGS2 and RGS4, which are known to inhibit Gq, had no effect. Cotransfection with plasmids coding for the G protein alpha-subunit carboxy-terminal peptide from alpha13 and, to a lesser extent alpha12, also inhibited the effect of CCK, whereas the peptide from alphaq did not. These results show that in NIH3T3 cells bearing CCK-A receptors, CCK activates Rho primarily through G13, leading to rearrangement of the actin cytoskeleton.

Partial inhibition of SERCA is responsible for extracellular Ca2+ dependence of AlF-4-induced [Ca2+]i oscillations in rat pancreatic

AlF4-is known to generate oscillations in intracellular Ca2+ concentration ([Ca2+]i) by activating G proteins in many cell types. However, in rat pancreatic acinar cells, AlF4--evoked [Ca2+]i oscillations were reported to be dependent on extracellular Ca2+, which contrasts with the [Ca2+]i oscillations induced by cholecystokinin (CCK). Therefore, we investigated the mechanisms by which AlF4- generates extracellular Ca2+-dependent [Ca2+]i oscillations in rat pancreatic acinar cells. AlF4(-)-induced [Ca2+]i oscillations were stopped rapidly by the removal of extracellular Ca2+ and were abolished on the addition of 20 mM caffeine and 2 microM thapsigargin, indicating that Ca2+ influx plays a crucial role in maintenance of the oscillations and that an inositol 1,4,5-trisphosphate-sensitive Ca2+ store is also required. The amount of Ca2+ in the intracellular Ca2+ store was decreased as the AlF4--induced [Ca2+]i oscillations continued. Measurement of 45Ca2+ influx into isolated microsomes revealed that AlF4-directly inhibited sarco/endoplasmic reticulum Ca2+-ATPase (SERCA). The activity of plasma membrane Ca2+-ATPase during AlF4- stimulation was not significantly different from that during CCK stimulation. After partial inhibition of SERCA with 1 nM thapsigargin, 20 pM CCK-evoked [Ca2+]i oscillations were dependent on extracellular Ca2+. This study shows that AlF4- induces [Ca2+]i oscillations, probably by inositol 1,4,5-trisphosphate production via G protein activation but that these oscillations are strongly dependent on extracellular Ca2+ as a result of the partial inhibition of SERCA.

CRHSP-24 phosphorylation is regulated by multiple signaling pathways in pancreatic acinar cells

Ca2+-regulated heat-stable protein of 24 kDa (CRHSP-24) is a serine phosphoprotein originally identified as a physiological substrate for the Ca2+-calmodulin regulated protein phosphatase calcineurin (PP2B). CRHSP-24 is a paralog of the brain-specific mRNA-binding protein PIPPin and was recently shown to interact with the STYX/dead phosphatase protein in developing spermatids (Wishart MJ and Dixon JE. Proc Natl Acad Sci USA 99: 2112-2117, 2002). Investigation of the effects of phorbol ester (12-o-tetradecanoylphorbol-13-acetate; TPA) and cAMP analogs in 32P-labeled pancreatic acini revealed that these agents acutely dephosphorylated CRHSP-24 by a Ca2+-independent mechanism. Indeed, cAMP- and TPA-mediated dephosphorylation of CRHSP-24 was fully inhibited by the PP1/PP2A inhibitor calyculin A, indicating that the protein is regulated by an additional phosphatase other than PP2B. Supporting this, CRHSP-24 dephosphorylation in response to the Ca2+-mobilizing hormone cholecystokinin was differentially inhibited by calyculin A and the PP2B-selective inhibitor cyclosporin A. Stimulation of acini with secretin, a secretagogue that signals through the cAMP pathway in acini, induced CRHSP-24 dephosphorylation in a concentration-dependent manner. Isoelectric focusing and immunoblotting indicated that elevated cellular Ca2+ dephosphorylated CRHSP-24 on at least three serine sites, whereas cAMP and TPA partially dephosphorylated the protein on at least two sites. The cAMP-mediated dephosphorylation of CRHSP-24 was inhibited by low concentrations of okadaic acid (10 nM) and fostriecin (1 microM), suggesting that CRHSP-24 is regulated by PP2A or PP4. Collectively, these data indicate that CRHSP-24 is regulated by diverse and physiologically relevant signaling pathways in acinar cells, including Ca2+, cAMP, and diacylglycerol.

Role of gastrointestinal hormones in the proliferation of normal and neoplastic tissues

Gastrointestinal (GI) hormones are chemical messengers that regulate the physiological functions of the intestine and pancreas, including secretion, motility, absorption, and digestion. In addition to these well-defined physiological effects, GI hormones can stimulate proliferation of the nonneoplastic intestinal mucosa and pancreas. Furthermore, in an analogous fashion to breast and prostate cancer, certain GI cancers possess receptors for GI hormones; growth can be altered by administration of these hormones or by blocking their respective receptors. The GI hormones that affect proliferation, either stimulatory or inhibitory, include gastrin, cholecystokinin, gastrin-releasing peptide, neurotensin, peptide YY, glucagon-like peptide-2, and somatostatin. The effects of these peptides on normal and neoplastic GI tissues will be described. Also, future perspectives and potential therapeutic implications will be discussed.

Cholecystokinin-stimulated protein kinase C-delta kinase activation, tyrosine phosphorylation, and translocation are mediated by Src tyrosine kinases in pancreatic acinar cells

Protein kinase C-delta (PKC-delta) is involved in growth, differentiation, tumor suppression, and regulation of other cellular processes. PKC-delta activation causes translocation, tyrosine phosphorylation, and serine-threonine kinase activity. However, little is known about the ability of G protein-coupled receptors to activate these processes or the mediators involved. In the present study, we explored the ability of the neurotransmitter/hormone, CCK, to stimulate these changes in PKC-delta and explored the mechanisms. In rat pancreatic acini under basal conditions, PKC-delta is almost exclusively located in cytosol. CCK and TPA stimulated a rapid PKC-delta translocation to membrane and nuclear fractions, which was transient with CCK. CCK stimulated rapid tyrosine phosphorylation of PKC-delta and increased kinase activity. Using tyrosine kinase (B44) and a tyrosine phosphatase inhibitor (orthovanadate), changes in both CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation were shown to correlate with changes in its kinase activity but not translocation. Both PKC-delta tyrosine phosphorylation and activation occur exclusively in particulate fractions. The Src kinase inhibitors, SU6656 and PP2, but not the inactive related compound, PP3, inhibited CCK- and TPA-stimulated PKC-delta tyrosine phosphorylation and activation. In contrast, PP2 also had a lesser effect on CCK- but not TPA-stimulated PKC-delta translocation. CCK stimulated the association of Src kinases with PKC-delta, demonstrated by co-immunoprecipitation. These results demonstrate that CCKA receptor activation results in rapid translocation, tyrosine phosphorylation, and activation of PKC-delta. Stimulation of PKC-delta translocation precedes tyrosine phosphorylation, which is essential for activation to occur. Activation of Src kinases is essential for the tyrosine phosphorylation and kinase activation to occur and plays a partial role in translocation.