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

The Important Role of p21-Activated Kinases in Pancreatic Exocrine Function

The p21-activated kinases (PAKs) are a conserved family of serine/threonine protein kinases, which are effectors for the Rho family GTPases, namely, Rac/Cdc42. PAKs are divided into two groups: group I (PAK1-3) and group II (PAK4-6). Both groups of PAKs have been well studied in apoptosis, protein synthesis, glucose homeostasis, growth (proliferation and survival) and cytoskeletal regulation, as well as in cell motility, proliferation and cycle control. However, little is known about the role of PAKs in the secretory tissues, including in exocrine tissue, such as the exocrine pancreas (except for islet function and pancreatic cancer growth). Recent studies have provided insights supporting the importance of PAKs in exocrine pancreas. This review summarizes the recent insights into the importance of PAKs in the exocrine pancreas by reviewing their presence and activation; the ability of GI hormones/neurotransmitters/GFs/post-receptor activators to activate them; the kinetics of their activation; the participation of exocrine-tissue PAKs in activating the main growth-signaling cascade; their roles in the stimulation of enzyme secretion; finally, their roles in pancreatitis. These insights suggest that PAKs could be more important in exocrine/secretory tissues than currently appreciated and that their roles should be explored in more detail in the future.

Cofilin activation in pancreatic acinar cells plays a pivotal convergent role for mediating CCK-stimulated enzyme secretion and growth

Introduction: The actin regulatory protein, cofilin plays a key signaling role in many cells for numerous cellular responses including in proliferation, development, motility, migration, secretion and growth. In the pancreas it is important in islet insulin secretion, growth of pancreatic cancer cells and in pancreatitis. However, there are no studies on its role or activation in pancreatic acinar cells. Methods: To address this question, we studied the ability of CCK to activate cofilin in pancreatic acinar cells, AR42J cells and CCK1-R transfected Panc-1 cells, the signaling cascades involved and its effect on enzyme secretion and MAPK activation, a key mediator of pancreatic growth. Results: CCK (0.3 and 100 nM), TPA, carbachol, Bombesin, secretin and VIP decreased phospho-cofilin (i.e., activate cofilin) and both phospho-kinetic and inhibitor studies of cofilin, LIM kinase (LIMK) and Slingshot Protein Phosphatase (SSH1) demonstrated these conventional activators of cofilin were not involved. Serine phosphatases inhibitors (calyculin A and okadaic acid), however inhibited CCK/TPA-cofilin activation. Studies of various CCK-activated signaling cascades showed activation of PKC/PKD, Src, PAK4, JNK, ROCK mediated cofilin activation, but not PI3K, p38, or MEK. Furthermore, using both siRNA and cofilin inhibitors, cofilin activation was shown to be essential for CCK-mediated enzyme secretion and MAPK activation. Conclusion: These results support the conclusion that cofilin activation plays a pivotal convergent role for various cell signaling cascades in CCK mediated growth/enzyme secretion in pancreatic acini.

Gastrointestinal hormones/neurotransmitters and growth factors can activate P21 activated kinase 2 in pancreatic acinar cells by novel mechanisms

P-21-activated kinases (PAKs) are serine/threonine kinases comprising six isoforms divided in two groups, group-I (PAK1-3)/group-II (PAK4-6) which play important roles in cell cytoskeletal dynamics, survival, secretion and proliferation and are activated by diverse stimuli. However, little is known about PAKs ability to be activated by gastrointestinal (GI) hormones/neurotransmitters/growth-factors. We used rat pancreatic acini to explore the ability of GI-hormones/neurotransmitters/growth-factors to activate Group-I-PAKs and the signaling cascades involved. Only PAK2 was present in acini. PAK2 was activated by some pancreatic growth-factors [EGF, PDGF, bFGF], by secretagogues activating phospholipase-C (PLC) [CCK, carbachol, bombesin] and by post-receptor stimulants activating PKC [TPA], but not agents only mobilizing cellular calcium or increasing cyclic AMP. CCK-activation of PAK2 required both high- and low-affinity-CCK1-receptor-state activation. It was partially reduced by PKC- or Src-inhibition, but not with PI3K-inhibitors (wortmannin, LY294002) or thapsigargin. IPA-3, which prevents PAK2 binding to small-GTPases partially inhibited PAK2-activation, as well as reduced CCK-induced ERK1/2 activation and amylase release induced by CCK or bombesin. This study demonstrates pancreatic acini, possess only one Group-I-PAK, PAK2. CCK and other GI-hormones/neurotransmitters/growth-factors activate PAK2 via small GTPases (CDC42/Rac1), PKC and SFK but not cytosolic calcium or PI3K. CCK-activation of PAK2 showed several novel features being dependent on both receptor-activation states, having PLC- and PKC-dependent/independent components and small-GTPase-dependent/independent components. These results show that PAK2 is important in signaling cascades activated by numerous pancreatic stimuli which mediate their various physiological/pathophysiological responses and thus could be a promising target for the development of therapies in some pancreatic disorders such as pancreatitis.

The Src kinase Yes is activated in pancreatic acinar cells by gastrointestinal hormones/neurotransmitters, but not pancreatic growth factors, which stimulate its association with numerous other signaling molecules

For growth factors, cytokines, G-protein-coupled receptors and numerous other stimuli, the Src Family of kinases (SFK) play a central signaling role. SFKs also play an important role in pancreatic acinar cell function including metabolism, secretion, endocytosis, growth and cytoskeletal integrity, although the specific SFKs involved are not fully known. In the present study we used specific antibodies for the SFK, Yes, to determine its presence, activation by pancreatic secretagogues or growth factors, and interaction with cellular signaling cascades mediated by CCK in which Yes participates in to cause acinar cell responses. Yes was identified in acini and secretagogues known to activate phospholipase C (PLC) [CCK, carbachol, bombesin] as well as post-receptor stimulants activating PKC [TPA] or mobilizing cellular calcium [thapsigargin/calcium ionophore (A23187)] each activated Yes. Secretin, which activates adenylate cyclase did not stimulate Yes, nor did pancreatic growth factors. CCK activation of Yes required both high- and low-affinity CCK(1)-receptor states. TPA-/CCK-stimulated Yes activation was completely inhibited by thapsigargin and the PKC inhibitor, GF109203X. CCK/TPA stimulated the association of Yes with focal adhesion kinases (Pyk2, FAK) and its autophosphorylated forms (pY397FAK, pY402Pyk2). Moreover, CCK/TPA stimulated Yes interacted with a number of other signaling proteins, including Shc, PKD, p130(Cas), PI3K and PTEN. This study demonstrates that in rat pancreatic acini, the SFK member Yes is expressed and activated by CCK and other gastrointestinal hormones/neurotransmitters. Because its activation results in the direct activation of many cellular signaling cascades that have been shown to mediate CCK's effect in acinar cell function our results suggest that it is one of the important pancreatic SFKs mediating these effects.

PKCθ activation in pancreatic acinar cells by gastrointestinal hormones/neurotransmitters and growth factors is needed for stimulation of numerous important cellular signaling cascades

The novel PKCθ isoform is highly expressed in T-cells, brain and skeletal muscle and originally thought to have a restricted distribution. It has been extensively studied in T-cells and shown to be important for apoptosis, T-cell activation and proliferation. Recent studies showed its presence in other tissues and importance in insulin signaling, lung surfactant secretion, intestinal barrier permeability, platelet and mast-cell functions. However, little information is available for PKCθ activation by gastrointestinal (GI) hormones/neurotransmitters and growth factors. In the present study we used rat pancreatic acinar cells to explore their ability to activate PKCθ and the possible interactions with important cellular mediators of their actions. Particular attention was paid to cholecystokinin (CCK), a physiological regulator of pancreatic function and important in pathological processes affecting acinar function, like pancreatitis. PKCθ-protein/mRNA was present in the pancreatic acini, and T538-PKCθ phosphorylation/activation was stimulated only by hormones/neurotransmitters activating phospholipase C. PKCθ was activated in time- and dose-related manner by CCK, mediated 30% by high-affinity CCK(A)-receptor activation. CCK stimulated PKCθ translocation from cytosol to membrane. PKCθ inhibition (by pseudostrate-inhibitor or dominant negative) inhibited CCK- and TPA-stimulation of PKD, Src, RafC, PYK2, p125(FAK) and IKKα/β, but not basal/stimulated enzyme secretion. Also CCK- and TPA-induced PKCθ activation produced an increment in PKCθ's direct association with AKT, RafA, RafC and Lyn. These results show for the first time the PKCθ presence in pancreatic acinar cells, its activation by some GI hormones/neurotransmitters and involvement in important cell signaling pathways mediating physiological responses (enzyme secretion, proliferation, apoptosis, cytokine expression, and pathological responses like pancreatitis and cancer growth).

Expression and localization of paxillin in rat pancreas during development

AIM: To investigate the expression and localization of paxillin in rat pancreas during development.

METHODS: Pancreata from Sprague Dawley rat fetuses, embryos, young animals, and adult animals were used in this study. Expression levels of paxillin in pancreata of different development stages were detected by reverse transcription polymerase chain reaction and Western blotting. To identify the cell location of paxillin in the developing rat pancreas, immunohistochemistry and double-immunofluorescent staining were performed using antibodies for specific cell markers and paxillin, respectively.

RESULTS: The highest paxillin mRNA level was detected at E15.5 (embryo day 15.5) following a decrease in the later developmental periods (P < 0.05 vs E18.5, P0 and adult, respectively), and a progressively increased paxillin protein expression through the transition from E15.5 to adult was detected. The paxillin positive staining was mainly localized in rat islets of Langerhans at each stage tested during pancreas development.

CONCLUSION: The dynamic expression of paxillin in rat pancreas from different stages indicates that paxillin might be involved in some aspects of pancreatic development.

The Src family kinase, Lyn, is activated in pancreatic acinar cells by gastrointestinal hormones/neurotransmitters and growth factors which stimulate its association with numerous other signaling molecules

Src family kinases (SFK) play a central signaling role for growth factors, cytokines, G-protein-coupled receptors and other stimuli. SFKs play important roles in pancreatic acinar cell secretion, endocytosis, growth, cytoskeletal integrity and apoptosis, although little is known of the specific SFKs involved. In this study we demonstrate the SFK, Lyn, is present in rat pancreatic acini and investigate its activation/signaling. Ca(2+)-mobilizing agents, cAMP-mobilizing agents and pancreatic growth factors activated Lyn. CCK, a physiological regulator of pancreatic function, rapidly activated Lyn. The specific SFK inhibitor, PP2, decreased Lyn activation; however, the inactive analogue, PP3, had no effect. Inhibition of CCK-stimulated changes in [Ca(2+)](i) decreased Lyn activation by 55%; GFX, a PKC inhibitor by 36%; and the combination by 95%. CCK activation of Lyn required stimulation of high and low affinity CCK(A) receptor states. CCK stimulated an association of Lyn with PKC-delta, Shc, p125(FAK) and PYK2 as well as with their autophosphorylated forms, but not with Cbl, p85, p130(CAS) or ERK 1/2. These results show Lyn is activated by diverse pancreatic stimulants. CCK's activation of Lyn is likely an important mediator of its ability to cause tyrosine phosphorylation of numerous important cellular mediators such as p125(FAK), PYK2, PKC-delta and Shc, which play central roles in CCK's effects on acinar cell function.

Adapter protein CRKII signaling is involved in the rat pancreatic acini response to reactive oxygen species

Recent studies demonstrate that reactive oxygen species (ROS) are important mediators of acute pancreatitis, whether induced experimentally or in necrotizing pancreatitis in humans; however, the cellular processes involved remain unclear. Adapter protein CrkII, plays a central role for convergence of cellular signals from different stimuli. Cholecystokinin (CCK), which induces pancreatitis, stimulates CrkII tyrosine phosphorylation and CrkII protein complexes, raising the possibility it can be important in the acinar cell responses to ROS. Therefore, our aim was to investigate whether CrkII signaling is involved in the biological response of rat pancreatic acini to H2O2 and the intracellular mediators implicated. Treatment of isolated rat pancreatic acini with H2O2 rapidly stimulates CrkII phosphorylation, measured as electrophoretic mobility shift and by using a phosphospecific antibody (pTyr221). Tyrosine kinase blocker B44 inhibits the higher phosphorylation state, demonstrating that it occurs mainly in tyrosine residues. H2O2-induced CrkII phosphorylation is time- and concentration-dependent, showing maximal effect with 3 mM H2O2 at 5 min. The intracellular pathways induced by H2O2 leading to CrkII tyrosine phosphorylation do not involve PKC, intracellular calcium, PI3-K or the actin cytoskeleton integrity. ROS generation clearly promotes the formation of protein complex CrkII-PYK2. In conclusion, ROS clearly affect the key adapter protein CrkII signaling by two ways: stimulation of CkII phosphorylation and a functional consequence: formation of CrkII-protein complexes. Because of its central role in activating more distal pathways, CrkII might likely play an important role in the ability of ROS to induce pancreatic cellular injury and pancreatitis.

Changes in tyrosine phosphorylation associated with true capacitation and capacitation-like state in boar spermatozoa

Capacitation is defined as a series of events that render boar sperm competent to fertilize, either in vivo or in vitro. Moreover, preliminary stages of cryopreservation of spermatozoa involving cooling to 5 degrees C have been shown to induce capacitation-like changes in boar spermatozoa. Capacitation of boar spermatozoa is accompanied by protein phosphorylation, however the relationship between both processes is poorly understood. Capacitation status was assessed by chlortetracycline (CTC) staining. Changes in protein tyrosine phosphorylation were examined in pre-cleared whole cell lysates using a specific anti-phosphotyrosine monoclonal antibody. Our results in boar spermatozoa show a significant positive correlation between p32 tyrosine phosphorylation levels and percentage of capacitated (CTC pattern B) spermatozoa. Moreover, incubation of boar spermatozoa with two unrelated tyrosine kinase inhibitors induces a significant reduction in the percentages of capacitated and acrosome-reacted (AR) boar spermatozoa and a reduction in the p32 tyrosine phosphorylation. In our conditions, cooling boar spermatozoa to 5 degrees C and rewarming to 39 degrees C in a noncapacitating medium results in similar CTC staining patterns to those obtained after incubation of boar sperm for 1 or 4 hr at 39 degrees C in a capacitating medium. However, cooled-rewarmed fails to induce an increase in p32 tyrosine phosphorylation in boar spermatozoa. Moreover, CTC staining patterns of cooled-rewarmed spermatozoa do not change after incubation with a tyrosine kinase inhibitor. In conclusion, our results show a direct relationship between capacitation and tyrosine phosphorylation and suggest that p32 tyrosine phosphorylation levels could be used as a marker of the true capacitation changes observed in boar spermatozoa. Moreover, our results show that true capacitation and capacitation-like changes induced after cooling involve alternative intracellular tyrosine phosphorylation pathways in boar spermatozoa.

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.