Low-affinity CCK-1 receptors inhibit bombesin-stimulated secretion in rat pancreatic acini--implication of the actin cytoskeleton

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 role of kinesin, dynein and microtubules in pancreatic secretion

The regulated secretion of pancreatic zymogens depends on a functional cytoskeleton and intracellular vesicle transport. To study the dynamics of tubulin and its motor proteins dynein and kinesin during secretion in pancreatic acinar cells, we infused rats with 0.1 mug/kg/h caerulein. Electron and fluorescence microscopy detected neither dynein nor kinesin at the apical secretory pole, nor on the surface of mature zymogen granules. After 30 min of secretagogue stimulation, kinesin and the Golgi marker protein 58 K were reallocated towards the apical plasma membrane and association of kinesin with tubulin was enhanced. Disruption of acinar cell microtubules had no effect on initial caerulein-induced amylase release but completely blocked secretion during a second stimulus. Our results suggest that mature zymogen granule exocytosis is independent of intact microtubules, kinesin and dynein. However, microtubule-dependent mechanisms seem to be important for the replenishment of secretory vesicles by redistribution of Golgi elements towards the apical cell pole.

Why does pancreatic overstimulation cause pancreatitis?

Many animal models are available to investigate the pathogenesis of pancreatitis, an inflammatory disorder of the pancreas. However, the secretagogue hyperstimulation model of pancreatitis is the most commonly used. Animals infused with high doses of cholecystokinin (CCK) exhibit hyperamylasemia, pancreatic edema, and acinar cell injury, which closely mimic pancreatitis in humans. Intra-acinar zymogen activation is an essential early event in the pathogenesis of secretagogue-induced pancreatitis. Early in the course of pancreatitis, lysosomal hydrolases colocalize with digestive zymogens and activate them. These activated zymogens then cause acinar cell injury and necrosis, a characteristic of pancreatitis. Besides being the site of initiation of injury in pancreatitis, acinar cells also synthesize and release cytokines and chemokines very early in the course of pancreatitis, which then attract and activate inflammatory cells and initiate the disease's systemic phase.

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.

Ethanol modifies the actin cytoskeleton in rat pancreatic acinar cells--comparison with effects of CCK

BACKGROUND

One of the early events leading to alcoholic pancreatitis seems to be the effect of ethanol on stimulus-secretion coupling. This study examines ethanol-induced modifications of filamentous actin (F-actin) content and localization in acini, the resulting alpha-amylase secretion and the role of protein kinase C (PKC) activity in these processes.

METHODS

Freshly isolated acini were treated with different concentrations of ethanol or cholecystokinin octapeptide (CCK-8) for different periods. F-actin was localized by confocal laser scanning microscopy; its quantity was determined fluorometrically, and the alpha-amylase secretion was measured.

RESULTS

Ethanol caused F-actin reorganization resembling the effects of supramaximal CCK-8 stimulation and of direct PKC activation by phorbol-12-myristate-13-acetate. The polyphasic time course of the F-actin content also resembled that under supramaximal CCK-8 stimulation and was counteracted by inhibition of PKC. The PKC inhibitor bisindolylmaleimide I did not increase the ethanol- induced alpha-amylase secretion, but the suboptimally CCK-8-stimulated secretion via high-affinity receptors.

CONCLUSION

Ethanol, like supramaximal CCK-8 concentrations, inhibits acinar secretion by reorganization of the actin cytoskeleton via PKC activation. This effect is suggested to be mediated by low-affinity CCK-A receptors. Together with the ethanol-induced stimulation of early steps of stimulus-secretion coupling, this may be a pancreas-damaging mechanism resembling that in experimental hyperstimulation pancreatitis.

Low affinity cholecystokinin receptor inhibits cholecystokinin- and bombesin-induced oscillations of cytosolic Ca2+ concentration

We have investigated whether low affinity cholecystokinin (CCK) receptors suppress agonist-induced rises of cytosolic free Ca(2+) concentration ([Ca(2+)]c) in pancreatic acinar cells by using properties of caffeine. A high concentration of caffeine (20 mM) completely blocked inositol 1,4,5-trisphosphate (InsP(3))-induced [Ca(2+)]c rises but spared the InsP(3)-independent long-lasting [Ca(2+)]c oscillations. In the presence of 20 mM caffeine, only high concentrations of CCK, but not bombesin or JMV-180, suppressed the caffeine-resistant CCK or bombesin-induced [Ca(2+)]c oscillations, indicating that low affinity CCK receptors inhibit agonist-induced [Ca(2+)]c oscillations. It could be one of the underlying mechanisms by which low affinity CCK receptors suppress secretion in pancreatic acinar cells.