A role for phosphoinositides in tyrosine phosphorylation of p125 focal adhesion kinase in rat pancreatic acini

Group II p21-activated kinase, PAK4, is needed for activation of focal adhesion kinases, MAPK, GSK3, and β-catenin in rat pancreatic acinar cells

PAK4 is the only member of the Group II p21-activated kinases (PAKs) present in rat pancreatic acinar cells and is activated by gastrointestinal hormones/neurotransmitters stimulating PLC/cAMP and by various pancreatic growth factors. However, little is known of the role of PAK4 activation in cellular signaling cascades in pancreatic acinar cells. In the present study, we examined the role of PAK4's participation in five different cholecystokinin-8 (CCK-8)-stimulated signaling pathways (PI3K/Akt, MAPK, focal adhesion kinase, GSK3, and β-catenin), which mediate many of its physiological acinar-cell effects, as well as effects in pathophysiological conditions. To define PAK4's role, the effect of two different PAK4 inhibitors, PF-3758309 and LCH-7749944, was examined under experimental conditions that only inhibited PAK4 activation and not activation of the other pancreatic PAK, Group I PAK2. The inhibitors' effects on activation of these five signaling cascades by both physiological and pathophysiological concentrations of CCK, as well as by 12-O-tetradecanoylphobol-13-acetate (TPA), a PKC-activator, were examined. CCK/TPA activation of focal adhesion kinases(PYK2/p125^FAK) and the accompanying adapter proteins (paxillin/p130^CAS), Mek1/2, and p44/42, but not c-Raf or other MAPKs (JNK/p38), were mediated by PAK4. Activation of PI3K/Akt/p70s6K was independent of PAK4, whereas GSK3 and β-catenin stimulation was PAK4-dependent. These results, coupled with recent studies showing PAK4 is important in pancreatic fluid/electrolyte/enzyme secretion and acinar cell growth, show that PAK4 plays an important role in different cellular signaling cascades, which have been shown to mediate numerous physiological and pathophysiological processes in pancreatic acinar cells.NEW & NOTEWORTHY In pancreatic acinar cells, cholecystokinin (CCK) or 12-O-tetradecanoylphobol-13-acetate (TPA) activation of focal adhesion kinases (p125^FAK,PYK2) and its accompanying adapter proteins, p130CAS/paxillin; Mek1/2, p44/42, GSK3, and β-catenin are mediated by PAK4. PI3K/Akt/p70s6K, c-Raf, JNK, or p38 pathways are independent of PAK4 activation.

The p21-activated kinase, PAK2, is important in the activation of numerous pancreatic acinar cell signaling cascades and in the onset of early pancreatitis events

In a recent study we explored Group-1-p21-activated kinases (GP.1-PAKs) in rat pancreatic acini. Only PAK2 was present; it was activated by gastrointestinal-hormones/neurotransmitters and growth factors in a PKC-, Src- and small-GTPase-mediated manner. PAK2 was required for enzyme-secretion and ERK/1-2-activation. In the present study we examined PAK2's role in CCK and TPA-activation of important distal signaling cascades mediating their physiological/pathophysiological effects and analyzed its role in pathophysiological processes important in early pancreatitis. In rat pancreatic acini, PAK2-inhibition by the specific, GP.1.PAK-inhibitor, IPA-3-suppressed cholecystokinin (CCK)/TPA-stimulated activation of focal-adhesion kinases and mitogen-activated protein-kinases. PAK2-inhibition reversed the dual stimulatory/inhibitory effect of CCK/TPA on the PI3K/Akt/GSK-3β pathway. However, its inhibition did not affect PKC activation. PAK2-inhibition protected acini from CCK-induced ROS-generation; caspase/trypsin-activation, important in early pancreatitis; as well as from cell-necrosis. Furthermore, PAK2-inhibition reduced proteolytic-activation of PAK-2p34, which is involved in programmed-cell-death. To ensure that the study did not only rely in the specificity of IPA-3 as a PAK inhibitor, we used two other approaches for PAK inhibition, FRAX597 a ATP-competitive-GP.1-PAKs-inhibitor and infection with a PAK2-dominant negative(DN)-Advirus. Those two approaches confirmed the results obtained with IPA-3. This study demonstrates that PAK2 is important in mediating CCK's effect on the activation of signaling-pathways known to mediate its physiological/pathophysiological responses including several cellular processes linked to the onset of pancreatitis. Our results suggest that PAK2 could be a new, important therapeutic target to consider for the treatment of diseases involving deregulation of pancreatic acinar cells.

Role of phosphoinositide 3-kinase in the pathogenesis of acute pancreatitis

A large body of experimental and clinical data supports the notion that inflammation in acute pancreatitis has a crucial role in the pathogenesis of local and systemic damage and is a major determinant of clinical severity. Thus, research has recently focused on molecules that can regulate the inflammatory processes, such as phosphoinositide 3-kinases (PI3Ks), a family of lipid and protein kinases involved in intracellular signal transduction. Studies using genetic ablation or pharmacologic inhibitors of different PI3K isoforms, in particular the class I PI3Kδ and PI3Kγ, have contributed to a greater understanding of the roles of these kinases in the modulation of inflammatory and immune responses. Recent data suggest that PI3Ks are also involved in the pathogenesis of acute pancreatitis. Activation of the PI3K signaling pathway, and in particular of the class IB PI3Kγ isoform, has a significant role in those events which are necessary for the initiation of acute pancreatic injury, namely calcium signaling alteration, trypsinogen activation, and nuclear factor-κB transcription. Moreover, PI3Kγ is instrumental in modulating acinar cell apoptosis, and regulating local neutrophil infiltration and systemic inflammatory responses during the course of experimental acute pancreatitis. The availability of PI3K inhibitors selective for specific isoforms may provide new valuable therapeutic strategies to improve the clinical course of this disease. This article presents a brief summary of PI3K structure and function, and highlights recent advances that implicate PI3Ks in the pathogenesis of acute pancreatitis.

Integrin adhesion in regulation of lacrimal gland acinar cell secretion

The extracellular microenvironment regulates lacrimal gland acinar cell secretion. Culturing isolated rabbit lacrimal gland acinar cells on different extracellular matrix proteins revealed that laminin enhances carbachol-stimulated secretion to a greater extent than other extracellular matrix proteins investigated. Furthermore, immunofluorescence indicated that integrin subunits, potentially functioning as laminin receptors are present in acinar cells. Among these, the integrin alpha6 and beta1 subunit mRNA expression was also confirmed by RT-PCR and sequence analysis. Secretion assays, which measured beta-hexosaminidase activity released in the culture media, demonstrated that function-blocking integrin alpha6 and beta1 monoclonal antibodies (mAbs) induce a rapid, transient and dose-dependent secretory response in cultured cells. To determine the intracellular pathways by which integrin alpha6 and beta1 mAbs could induce secretion, selected second messenger molecules were inhibited. Although inhibitors of protein kinase C and IP(3)-induced Ca(2+) mobilization attenuated carbachol-stimulated secretion, no effect on integrin mAb-induced release was observed. In addition, protein tyrosine kinases do not appear to have a role in transducing signals arising from mAb interactions. Our data clearly demonstrate, though, that cell adhesion through integrins regulates secretion from lacrimal gland acinar cells. The fact that the integrin mAbs affect the cholinergic response differently and that the integrin beta1 mAb secretion, but not the alpha6, was attenuated by the phosphatase inhibitor, sodium orthovanadate, suggests that each subunit utilizes separate intracellular signaling pathways to induce exocytosis. The results also indicate that the secretory response triggered by the beta1 integrin mAb is generated through dephosphorylation events.

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.

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.

Rapid tyrosine phosphorylation of focal adhesion kinase, paxillin, and p130Cas by gastrin in human colon cancer cells

Although the expression of CCK(2) receptors is widely reported in human colorectal cancers, little is known on its role in mediating the proliferative effects of mature amidated gastrin (G17 amide) on colorectal cancers. The purpose of the present study was to determine the effects of G17 amide on tyrosine phosphorylation of focal adhesion kinase (FAK), paxillin, and p130 Crk-associated substrate (p130(Cas)) in Colo 320 cells, a human colorectal cancer cell line which expresses CCK(2) receptors. By immunoprecipitation and immunoblotting, an increase in tyrosine phosphorylation of FAK (tyrosine-397), paxillin (tyrosine-31), and p130(Cas) was detected in a time- and dose-dependent manner. Overexpression of CCK(2) receptors in Colo 320 cells (Colo 320 WT) by stable transfection with the human CCK(2) receptor cDNA resulted in an increased tyrosine phosphorylation of FAK, paxillin, and p130(Cas). After incubation with 1 microM L-365,260, a specific CCK(2) receptor antagonist, this increase was completely inhibited. Our results demonstrate that in human colon cancer cells, gastrin caused a rapid tyrosine phosphorylation of FAK, paxillin, and p130(Cas) by activation of CCK(2) receptor. The phosphorylation of these proteins might be important in mediating gastrin effects on proliferation, apoptosis, and metastasis.

Receptor biology and intracellular regulatory mechanisms in pancreatic acinar cells

Continuing progress is being made in understanding the regulation of pancreatic acinar cell function by receptor-activated intracellular signaling mechanisms. Knowledge of how ligands interact at the molecular level with their receptors and activate heterotrimeric G proteins is increasing. In addition to inositol trisphosphate, intracellular messengers include cyclic ADP ribose, nicotinic acid adenine dinucleotide phosphate, arachidonic acid, and diacylglycerol. Ca signaling involves the interaction of inositol trisphosphate, cyclic ADP ribose, and nicotinic acid adenine dinucleotide phosphate with distinct subcellular Ca stores. Ca signals ultimately induce exocytosis of zymogen granules and identification of the proteins involved on the granule and plasma membrane, and understanding of their roles is continuing. Other receptor-activated signaling pathways primarily regulate nonsecretory events. Considerable progress has been made in understanding how the mammalian target of rapamycin pathway regulates protein synthesis through translation factors and ribosomal proteins. Other pathways in acinar cells include the mitogen-activated protein kinases, the tyrosine kinases, and the transforming growth factor-beta-Smad pathways.

Src family kinase involvement in muscarinic receptor-induced tyrosine phosphorylation in differentiated SH-SY5Y cells

Muscarinic receptor-mediated changes in protein tyrosine phosphorylation were examined in differentiated human neuroblastoma SH-SY5Y cells. Treatment of differentiated cells with 1 mM carbachol caused rapid increases in the tyrosine phosphorylation of focal adhesion kinase (FAK), Cas, and paxillin. The src family kinase-selective inhibitor PP1 reduced carbachol-stimulated tyrosine phosphorylation of FAK, Cas, and paxillin by 50 to 75%. In contrast, carbachol-stimulated activation of ERK1/2 was unaffected by PP1. Src family kinase activation by carbachol was further demonstrated by increased carbachol-induced tyrosine phosphorylation of the src-substrate, p120, and tyrosine phosphorylation of the src family kinase activation-associated autophosphorylation site. Site-specific FAK phosphotyrosine antibodies were used to determine that the carbachol-stimulated increase in the autophosphorylation of FAK was unaffected by pretreatment with PP1, whereas the carbachol-stimulated increase in the src family kinase-mediated phosphotyrosine of FAK was completely blocked by pretreatment with PP1. In SH-SY5Y cell lines stably overexpressing Fyn, the phosphotyrosine immunoreactivity of FAK was 625% that of control cells. Thus, muscarinic receptors activate protein tyrosine phosphorylation in differentiated cells, and the tyrosine phosphorylation of FAK, Cas, and paxillin, but not ERK1/2, is mediated by a src family tyrosine kinase activated in response to stimulation of muscarinic receptors.