Interactions of intracellular mediators of amylase secretion in permeabilized pancreatic acini

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.

Synergistic effects of cAMP- and calcium-mediated amylase secretion in isolated pancreatic acini from cystic fibrosis mice

We evaluated pancreatic enzyme secretory response to secretagogues (cAMP- and Ca2+-mediating) involved in exocytosis and in chloride channel activation in an exon 10 knockout cystic fibrosis (CF) mouse model. Experiments were performed in isolated pancreatic acini from liquid-fed Cftr-/- mice (5-6 wk of age) and age-matched Cftr+/+ controls fed a solid or liquid diet. BrcAMP and forskolin alone induced higher amylase secretion (% initial amylase content) in the Cftr+/+ acini than carbachol (p < 0.05). Carbachol and BrcAMP or BrcAMP and forskolin, given in combination, produced additive effects on enzyme secretion in the Cftr+/+ acini. Ca2+- and cAMP-mediated amylase secretion in isolated pancreatic acini from the Cftr-/- mice was no different to that observed in the age- and diet-matched Cftr+/+ animals. However, Cftr-/- pancreatic acini showed a significantly greater amylase response to the combination of BrcAMP and carbachol than the sum of the individual responses in separate experiments (p < 0.05). The amylase response was not different in acini from solid-fed or liquid-fed Cftr+/+ controls. In summary, this study suggests that cystic fibrosis transmembrane conductance regulator is not essential for enzyme secretion as evidenced by no reduction in cAMP-mediated amylase secretion in Cftr-/- mice. The results in Cftr+/+ acini suggest pancreatic enzyme secretion is mediated via multiple intracellular pathways acting in parallel and probably converge at a distal step in the secretory process. However, Cftr-/- pancreatic acini exhibited a synergistic secretory response following stimulation by BrcAMP plus carbachol. The enhanced secretory response may partially contribute to the development of pancreatic dysfunction in CF patients by facilitating occlusion of digestive enzymes in the secretory canaliculus of the pancreatic acini.

Actin filament disassembly is a sufficient final trigger for exocytosis in nonexcitable cells

Although the actin cytoskeleton has been implicated in vesicle trafficking, docking and fusion, its site of action and relation to the Ca(2+)-mediated activation of the docking and fusion machinery have not been elucidated. In this study, we examined the role of actin filaments in regulated exocytosis by introducing highly specific actin monomer-binding proteins, the beta-thymosins or a gelsolin fragment, into streptolysin O-permeabilized pancreatic acinar cells. These proteins had stimulatory and inhibitory effects. Low concentrations elicited rapid and robust exocytosis with a profile comparable to the initial phase of regulated exocytosis, but without raising [Ca2+], and even when [Ca2+] was clamped at low levels by EGTA. No additional cofactors were required. Direct visualization and quantitation of actin filaments showed that beta-thymosin, like agonists, induced actin depolymerization at the apical membrane where exocytosis occurs. Blocking actin depolymerization by phalloidin or neutralizing beta-thymosin by complexing with exogenous actin prevented exocytosis. These findings show that the cortical actin network acts as a dominant negative clamp which blocks constitutive exocytosis. In addition, actin filaments also have a positive role. High concentrations of the actin depolymerizing proteins inhibited all phases of exocytosis. The inhibition overrides stimulation by agonists and all downstream effectors tested, suggesting that exocytosis cannot occur without a minimal actin cytoskeletal structure.

Nitric oxide participates in the regulation of pancreatic acinar cell secretion

The role of nitric oxide (NO) in the regulation of exocrine secretion was investigated in isolated rat pancreatic acini. NO synthase activity was detected in the extract of acini and purified by ion-exchange and 2',5'-ADP agarose chromatographies. Enzyme activity was determined by conversion of 3H-arginine to 3H-citrulline, by measurement of nitrite (a breakdown product of NO) and by generation of cyclic GMP. Treatment of acini with L-arginine increased nitrite as well as cyclic GMP and amylase release, which were prevented by the nitric oxide synthase inhibitors N-monomethyl-arginine [NMMA] and NG-nitro-L-arginine [NNA]. These nitric oxide inhibitors also blocked carbachol-induced amylase release as well as elevation of acinar cell cyclic GMP. NNA was a potent inhibitor of carbamylcholine-induced amylase release (est. Ki = 2.2 uM). Nitric oxide apparently participates significantly in the overall control of pancreatic acinar cell secretory function.

Effects of calyculin A on amylase release in streptolysin-O permeabilized acinar cells

The effects of the phosphatase inhibitors calyculin A and okadaic acid on amylase release from streptolysin-O permeabilized rat pancreatic acini were investigated. Both agents induced similar biphasic effects with moderate potentiation of calcium-stimulated amylase release at medium and strong inhibition at higher concentrations. Calyculin A was thirty times more potent than okadaic acid and at 100 nM totally inhibited calcium-induced amylase release while 3 microM okadaic acid reduced amylase release by 78%. 100nM calyculin A also completely inhibited GTP gamma S-potentiated amylase release and partially inhibited phorbol ester potentiated secretion. The data indicate that inhibition of a serine/threonine phosphatase, probably a type 1 phosphatase, leads to inhibition of calcium-induced amylase release in permeabilized pancreatic acini.

Effects of okadaic acid indicate a role for dephosphorylation in pancreatic stimulus-secretion coupling

Okadaic acid completely inhibits phosphatase 2A at nanomolar concentrations, while complete inhibition of type 1 phosphatases occurs at 1 microM. Phosphatase 2B is significantly inhibited only at concentrations > 1 microM. In rat pancreatic acini, 1 microM okadaic acid shifted the cholecystokinin (CCK) dose-response curve for stimulating amylase release to the right without reducing maximal secretion. At 3 microM, okadaic acid inhibited maximal CCK-induced amylase release to 78 +/- 7% of control, whereas the inactive analogue 1-Nor-okadaone had no effect. Three lines of evidence indicate that this inhibition by okadaic acid occurs at a late step in stimulus-secretion coupling: 1) intracellular Ca2+ signaling in response to agonist stimulation was not appreciably altered by okadaic acid; 2) stimulation with phorbol ester plus thapsigargin (thus by-passing receptor activation), which gave 85 +/- 4% of maximal CCK-induced amylase release, was inhibited 66 +/- 4% by 3 microM okadaic acid; and 3) Ca(2+)-induced amylase secretion in streptolysin O-permeabilized cells was also reduced by 85 +/- 7%. Two-dimensional polyacrylamide gel electrophoresis of 32P-labeled acini and autoradiography demonstrated that okadaic acid dose dependently increased overall protein phosphorylation. Correspondingly, okadaic acid also led to an inhibition of CCK-induced dephosphorylation. These results show that okadaic acid inhibits pancreatic acinar secretion at a step after generation of intracellular messengers and indicate a role for protein dephosphorylation in stimulus-secretion coupling.

Expression of the ras-related rab3a gene in insulinoma-derived cell lines

This study was designed to search for the expression of the small-molecular-weight GTP-binding protein rab3a in endocrine pancreatic cell lines. Total RNA was isolated from five different cell lines (RINm5F, RIN 104836, beta-TC1, HIT-15, and INRI-G9) and from whole rat brain. The expression of rab3a was analyzed by Northern blots. Similar as in brain two transcripts of 1300 and 1800 bp were detected in RIN-cells at low stringency conditions with the predominant signal at 1300 bp. At high stringency the stronger signal was at 1800 bp. When a 300 bp PstI fragment derived from the coding region of rab3a was utilized as probe the 1800 bp signal was predominant under each condition. Only a faint band at 1800 bp occurred in preparations from beta TC1-cells and no signal at all was found in HIT-15 and INRI-G9-cells. In conclusion, rab3a is expressed in rat insulin-releasing insulinoma-derived RIN-cells with a specific 1800 bp transcription product.

Protein kinase A modulates Ca(2+)- and protein kinase C-dependent amylase release in permeabilized rat pancreatic acini

The role of protein kinase A (PKA) in the release of amylase from permeabilized pancreatic acini was investigated. Addition of cyclic AMP (cAMP) to permeabilized acini resulted in a potentiation of Ca(2+)-dependent amylase release, shifting the Ca2+ dose/response curve leftwards. As with protein kinase C (PKC) activation, this is due to an increase in the time of active discharge. The effect of cAMP was shown to be blocked by two inhibitors of PKA, H89 and the PKI-(5-24)-peptide. At low concentration, cAMP synergizes from phorbol 12-myristate 13-acetate (PMA), while at optimal concentrations cAMP and PMA are additive. PKA and PKC appear to work via similar, but not identical mechanisms.

Activation of protein kinase C is not an absolute requirement for amylase release from permeabilized rat pancreatic acini

The effect of protein kinase C (PKC) on amylase discharge from streptolysin-O-permeabilized rat pancreatic acini was investigated. Addition of phorbol 12-myristate 13-acetate (PMA) to permeabilized cells potentiated Ca(2+)-stimulated release, but had no effect on discharge at non-stimulatory Ca2+ concentrations. PMA markedly shifted the Ca(2+)-concentration-dependence of amylase discharge to the left, by enhancing the time over which the permeabilized cells release. This effect was inhibited by both staurosporine and PKC-19-31-amide peptide inhibitor, indicating that the effect of PMA was due to its action on PKC. Staurosporine also partially inhibited amylase release at the optimal concentration of Ca2+; this effect was not replicated by the more specific PKC-19-31-amide peptide inhibitor and may be due to an effect on another second-messenger system. PKC appears to be an important modulator of release in pancreatic acini, but its activation is not an absolute requirement for Ca(2+)-dependent amylase discharge.