Cholecystokinin octapeptide inhibits Ca2+-dependent amylase secretion from permeabilized pancreatic acini by blocking the MgATP-dependent priming of exocytosis

Ca²⁺-regulated secretory granule exocytosis in pancreatic and parotid acinar cells

Protein secretion from acinar cells of the pancreas and parotid glands is controlled by G-protein coupled receptor activation and generation of the cellular messengers Ca(2+), diacylglycerol and cAMP. Secretory granule (SG) exocytosis shares some common characteristics with nerve, neuroendocrine and endocrine cells which are regulated mainly by elevated cell Ca(2+). However, in addition to diverse signaling pathways, acinar cells have large ∼1 μm diameter SGs (∼30 fold larger diameter than synaptic vesicles), respond to stimulation at slower rates (seconds versus milliseconds), demonstrate significant constitutive secretion, and in isolated acini, undergo sequential compound SG-SG exocytosis at the apical membrane. Exocytosis proceeds as an initial rapid phase that peaks and declines over 3 min followed by a prolonged phase that decays to near basal levels over 20-30 min. Studies indicate the early phase is triggered by Ca(2+) and involves the SG proteins VAMP2 (vesicle associated membrane protein2), Ca(2+)-sensing protein synatotagmin 1 (syt1) and the accessory protein complexin 2. The molecular details for regulation of VAMP8-mediated SG exocytosis and the prolonged phase of secretion are still emerging. Here we review the known regulatory molecules that impact the sequential exocytic process of SG tethering, docking, priming and fusion in acinar cells.

CD44 is the signaling component of the macrophage migration inhibitory factor-CD74 receptor complex

The macrophage migration inhibitory factor (MIF) receptor (CD74) was cloned recently, but the signaling mechanism is not evident. We hypothesized that signaling requires an additional molecule such as CD44, which activates nonreceptor tyrosine kinases. We utilized the CD74- and CD44-deficient COS-7/M6 cell to create stable transfectants expressing CD74, CD44, and a truncated CD44 lacking its intracytoplasmic signaling domain. CD74 alone mediated MIF binding; however, MIF-induced ERK1 and ERK2 kinase phosphorylation required the coexpression of full-length CD44. MIF binding was associated with the serine phosphorylation of CD74 and CD44. Investigations that used siRNA or kinase inhibitors indicate that MIF-induced ERK1 and ERK2 activation through CD44 required the Src tyrosine kinase. Studies of CD74, CD44, and CD74-CD44 transformants and corresponding mutant cells showed that CD74 and CD44 were necessary for MIF protection from apoptosis. These data establish CD44 as an integral member of the CD74 receptor complex leading to MIF signal transduction.

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.

Dominant negative Rab3D inhibits amylase release from mouse pancreatic acini

Rab3 proteins are believed to play an important role in regulated exocytosis and previous work has demonstrated the presence of Rab3D on pancreatic zymogen granules. To further understand the function of Rab3D in acinar cell exocytosis, adenoviral constructs were prepared encoding hemagglutinin-tagged wild type Rab3D and three mutant forms, N135I and T36N (both deficient in guanine nucleotide binding) and Q81L (deficient in GTP hydrolysis), which also expressed enhanced green fluorescent protein driven by a separate promoter. When isolated mouse pancreatic acini were cultured with 5 x 10(6) pfu/ml adenovirus, nearly 100% of acini were infected as visualized by expression of green fluorescent protein. Cultured acini showed a biphasic dose-response to cholecystokinin (CCK); basal amylase secretion was 1.8 +/- 0.3%/30 min, peak release was 7.3 +/- 0.2%/30 min at 30 pm CCK and reduced secretion was observed at higher CCK concentrations. Control beta-galactosidase virus infection had no effect on either basal or CCK-induced secretion in the titer range from 0.5 to 10 x 10(6) pfu/ml. While the expression of Rab3D and Rab3D Q81L had no effect on amylase secretion, Rab3D N135I and T36N functioned as dominant negative mutants and inhibited CCK-induced amylase release by 40-50% at all points on the CCK dose-response curve from 3 to 300 pm. Inhibition was stronger during the first 5 min (71 +/- 5%) than over 30 min (36%+/-5%). Similar inhibition was found using other agonists including bombesin, carbachol, and cAMP. Localization of adenoviral expressed Rab protein showed wild type Rab3D localized to zymogen granules. The two dominant negative mutants did not localize to granules and were primarily in the basolateral region of the cell. Since both dominant negative Rab3D mutants had no effect on intracellular calcium increase induced by CCK, it is unlikely that they acted at receptors or transmembrane signaling. These results suggest that Rab3D plays an important role in regulating the terminal steps of acinar exocytosis and that this effect is greatest on the early phase of amylase release.

The cholecystokinin analogues JMV-180 and CCK-8 stimulate phospholipase C through the same binding site of CCK(A) receptor in rat pancreatic acini

1. This study was designed to address the controversy related to the involvement of phospholipase C in the signalling pathway linked to CCK(A) receptor stimulation by the cholecystokinin analogue JMV-180, a full agonist for amylase release, in rat pancreatic acini. 2. JMV-180 was shown to stimulate phospholipase C by measuring the incorporation of [(32)P]-orthophosphoric acid ([(32)P]-Pi) into phosphatidic acid (PtdOH) and phosphatidylinositol (PtdIns). Both responses elicited by JMV-180 were time and concentration dependent. Maximal effects elicited by JMV-180 were 39.08+/-0.72 and 8.02+/-0.40% for the labelling of [(32)P]-PtdIns and [(32)P]-PtdOH, respectively, as compared to the maximal effects of CCK-8, a full agonist of the CCK(A) receptor. 3. [(32)P]-Pi incorporation into PtdOH and PtdIns was sensitive to lithium, demonstrating that both responses are a consequence of phospholipase C activation. However, since lithium blocks the phosphoinositide cycle by an uncompetitive mechanism, its effect was only apparent at high concentrations of CCK-8 (>10 pM), which elicited stimuli above 20 and 60% of the maximal [(32)P]-PtdOH and [(32)P]-PtdIns labelling, respectively. 4. JMV-180 inhibited the incorporation of [(32)P]-Pi into PtdOH and PtdIns as stimulated by CCK-8, down to its own maximal effect. The estimated IC(50) values for the inhibition curves were not significantly different from those calculated assuming the same single binding site for both agonists. These results indicated that the well established role of JMV-180 as a partial agonist for CCK(A) receptor-linked signalling responses, also applies for the stimulation of phospholipase C. 5. The comparison of CCK-8 and JMV-180 dose-response curves of amylase release to those of PtdIns and PtdOH labelling with [(32)P]-Pi showed the existence of an amplification mechanism between phospholipase C and amylase release for both agonists. 6. In conclusion, we show that JMV-180, as well as CCK-8, stimulate phospholipase C upon interaction with the same binding site at the CCK(A) receptor in rat pancreatic acini.

Sequential-replenishment mechanism of exocytosis in pancreatic acini

Here we report exocytosis of zymogen granules, as examined by multiphoton excitation imaging in intact pancreatic acini. Cholecystokinin induces Ca 2+ oscillations that trigger exocytosis when the cytosolic Ca 2+ concentration exceeds 1 microM. Zymogen granules fused with the plasma membrane maintain their Omega-shaped profile for an average of 220 s and serve as targets for sequential fusion of granules that are located within deeper layers of the cell. This secondary exocytosis occurs as rapidly as the primary exocytosis and accounts for most exocytotic events. Granule-granule fusion does not seem to precede primary exocytosis, indicating that secondary fusion events may require a plasma-membrane factor. This sequential-replenishment mechanism of exocytosis allows the cell to take advantage of a large supply of fusion-ready granules without needing to transport them to the plasma membrane.

A tetanus toxin sensitive protein other than VAMP 2 is required for exocytosis in the pancreatic acinar cell

The neurotoxin sensitivity of regulated exocytosis in the pancreatic acinar cell was investigated using streptolysin-O permeabilized pancreatic acini. Treatment of permeabilized acini with botulinum toxin B (BoNT/B) or botulinum toxin D (BoNT/D) had no detectable effect on Ca(2+)-dependent amylase secretion but did result in the complete cleavage of VAMP 2. In comparison, tetanus toxin (TeTx) treatment both significantly inhibited Ca(2+)-dependent amylase secretion and cleaved VAMP 2. These results indicate that regulated exocytosis in the pancreatic acinar cell requires a tetanus toxin sensitive protein(s) other than VAMP 2.

Intracellular regulatory mechanisms in pancreatic acinar cellular function

The intracellular mechanisms regulating pancreatic acinar cell function are more complex than previously realized. This is probably due in part to the need to match the biosynthetic and secretory functions of the cells. Much information is available on how secretagogue receptors acutely couple through heterotrimeric G proteins to increase intracellular messengers, particularly cytoplasmic free Ca(2+), although details are still being worked out. Less is known about how Ca(2+) signals to induce fusion of zymogen granules with the apical plasma membrane. Investigation has focused on the proteins of the zymogen granule membrane, and several novel proteins have recently been identified. In addition, understanding of the three MAP kinase cascades, the mTOR-p70S6 kinase pathway, and the focal adhesion kinase pathway in acinar cells is increasing. The functions of these pathways in acini have been linked to mitogenesis, protein synthesis, and regulation of the cytoskeleton.