Different patterns of proteins are secreted by the pig pancreas when stimulated by secretin alone or in combination with caerulein

Rat pancreas secretes particulate ecto-nucleotidase CD39

In exocrine pancreas, acini release ATP and the excurrent ducts express several types of purinergic P2 receptors. Thereby, ATP, or its hydrolytic products, might play a role as a paracrine regulator between acini and ducts. The aim of the present study was to elucidate whether this acinar-ductal signalling is regulated by nucleotidase(s), and to characterize and localize one of the nucleotidases within the rat pancreas. Using RT-PCR and Western blotting we show that pancreas expresses the full length ecto-nucleoside triphosphate diphosphohydrolase, CD39. Immunofluorescence shows CD39 localization on basolateral membranes of acini and intracellularly. In small intercalated/ interlobular ducts, CD39 immunofluorescence was localized on the luminal membranes, while in larger ducts it was localized on the basolateral membranes. Upon stimulation with cholecystokinin-octapeptide-8 (CCK-8), acinar CD39 relocalizes in clusters towards the lumen and is secreted. As a result, pancreatic juice collected from intact pancreas stimulated with CCK-8 contained nucleotidase activity, including that of CD39, and no detectable amounts of ATP. Anti-CD39 antibodies detected the full length (78 kDa) CD39 in pancreatic juice. This CD39 was confined only to the particulate and not to the soluble fraction of CCK-8-stimulated secretion. No CD39 activity was detected in secretion stimulated by secretin. The role of secreted particulate, possibly microsomal, CD39 would be to regulate intraluminal ATP concentrations within the ductal tree. In conclusion, we show a novel inducible release of full length particulate CD39, and propose its role in the physiological context of pancreatic secretion.

Identification of a glycosylphosphatidylinositol-anchored glycoprotein with nucleoside phosphatase activity on the membrane of pig pancreatic zymogen granules

The molecular events between the second messenger-mediated triggering of regulated exocytosis and the subsequent fusion of the secretory granules with the apical plasma membrane are unclear. The glycoprotein GP-2, the most abundant of the very few proteins of the pancreatic zymogen granule membrane has been cloned and sequenced in dog and rat, but no (enzymatic) function has so far been ascribed to it. Nucleoside phosphatase activities associated with the pig zymogen granule membrane were recently assumed to be related to GP-2. To identify the protein(s) carrying these activities we have used a novel combination of native and denaturing one- and two-dimensional polyacrylamide gel electrophoresis in the detergents CHAPS, Triton X-100 or SDS. Histochemical examination on the gels and incubation with lectins and phosphatidylinositol phospholipase-C have allowed characterization of the protein with the nucleoside di- and tri-phosphatase activities. SDS-PAGE of the single protein spot with nucleoside phosphatase activity excised from Triton X-100 2-dimensional gels showed the presence of 92 kDa and 67 kDa glycoproteins. The isolated protein had an isoelectric point of 5.2, formed high molecular weight complexes, was shown to be glycosylphosphatidylinositol-anchored and contained complex carbohydrate structures. It hydrolyses di- and tri-phosphate nucleotides in dependence of the glycosylphosphatidylinositol anchor and is sensitive to non-mitochondrial diphosphohydrolase inhibitors. In summary, this paper identifies GP-2 as a nucleoside phosphatase within the zymogen granule membrane, suggesting it may be involved in energy-requiring processes on the cytosolic side of the granules.

Epidermal growth factor induces the differential release of GP2 and amylase from AR4-2J cells

Epidermal growth factor (EGF) stimulates secretion of glycoprotein 2 (GP2) in a time-and concentration-dependent manner from the AR4-2J pancreatoma cell line. Cell differentiation induced by dexamethasone treatment for 3 d, however, did not significantly alter either basal or EGF-stimulated GP2 release. Basal and EGF-stimulated GP2 release were similarly unaffected by caerulein, which promotes amylase secretion by a regulated route. A brief exposure to cycloheximide profoundly blocked EGF-evoked GP2 secretion. Furthermore, EGF-stimulated GP2 release was not accompanied by significant alterations in intracellular ionic calcium levels, in contrast to the stimulatory actions of caerulein. We conclude that EGF-stimulated release of GP2 occurs via a novel secretory pathway that is neither regulated nor constitutive as currently defined.

Reconstitution in vitro of the pH-dependent aggregation of pancreatic zymogens en route to the secretory granule: implication of GP-2

Regulated secretory proteins are thought to be sorted in the trans-Golgi network (TGN) via selective aggregation. To elucidate the biogenesis of the secretory granule in the exocrine pancreas, we reconstituted in vitro the conditions of pH and ions believed to exist in the TGN using the end product of this sorting process, the zymogen granule contents. Protein aggregation was dependent on pH (acidic) and on the presence of cations (10 mM Ca2+, 150 mM K+) to reproduce the pattern of proteins found in the granule. The constitutive secretory protein IgG was excluded from these aggregates. Zymogen aggregation correlated with the relative proportion of the major granule membrane protein GP-2 in the assay. These results show that the glycosylphosphatidylinositol-anchored protein GP-2 co-aggregates with zymogens in the acidic environment believed to exist in the pancreatic TGN, and thus suggest that GP-2 would function as a membrane anchor for zymogen aggregates, facilitating their entrapment in budding vesicles directed towards the regulated secretory pathway.

In resting conditions, the pancreatic granule membrane protein GP-2 is secreted by cleavage of its glycosylphosphatidylinositol anchor

GP-2 is the major membrane protein of the exocrine pancreatic secretory granule. It is an integral protein which is anchored by a phosphatidylinositolglycan. In addition to being present in the soluble contents of the granule, GP-2 is also actively secreted by the pancreas. Although 93% of the GP-2 in the resting secretions of anaesthetized rats could be pelleted, Triton X-114 phase extraction showed that 70% of this GP-2 had lost its hydrophobic properties. Proteases have been postulated to release GP-2 from the membrane, but phospholipases also have the capacity to release the protein from the membrane by hydrolysis of its peculiar glycosylphosphatidylinositol membrane anchor. These studies show the presence of inositol 1,2-(cyclic)monophosphate on the secreted hydrophilic GP-2, confirming the involvement of an endogenous phospholipase C in the solubilization of GP-2 by the exocrine pancreas. It is therefore concluded that most of the GP-2 secreted by the pancreas of anaesthetized rats under resting conditions is released from the membrane by a phospholipase C which hydrolyses the phosphodiester bond linking GP-2 to its diradylglycerol anchor.

Secretory pathways in animal cells: with emphasis on pancreatic acinar cells

Studies over the past three decades have clearly established the existence of at least two distinct pathways for the intracellular transport and release of secretory proteins by animal cells. These have been identified as the regulated and constitutive pathways. Many observations have indicated that in certain cells, such as those of the exocrine pancreas and parotid glands at least, these pathways coexist in the same cells. Although the general scheme of protein transport within these pathways is well established, many fundamental aspects of intracellular transport remain to be unraveled. How are proteins transported through the endoplasmic reticulum? How are the transitional vesicles formed and what are the underlying mechanisms involved in their fusion with the cis-Golgi cisterna? Even the general mode of transfer through the Golgi stack is debated: Is there a diffusion through the stack by flow through intercisternal tubules and openings or is there a vesicle transfer system where membrane quanta hop from one cisterna to the other? What is the fate of secretory proteins in the trans-Golgi area and by what mechanisms is a fraction of newly synthesized molecules of a given secretory protein released spontaneously while the majority of such nascent molecules are diverted into a secretory granule compartment? In this review, we have examined these and other aspects of intracellular transport of secretory proteins using pancreatic acinar cells as our reference model and we present some evidence to support the existence of a paragranular pathway of secretion associated with secretory granule maturation.

Secretion of gamma-glutamyltranspeptidase by the pancreas: evidence for a membrane shedding process during exocytosis

Gamma Glutamyltranspeptidase (GGT) is a membrane-bound enzyme involved in glutathione metabolism. It is present in rat exocrine pancreas at a level which is only exceeded by the kidney. It has been previously shown that most of the enzyme activity is located in the apical area of the acinar cell, more precisely at the level of zymogen granules and plasma membrane. The aim of the present study was to examine the secretory behavior of that enzyme. Under resting conditions, in vivo, high levels of GGT were found in pancreatic juice and its level was not related to protein concentration. Under secretin infusion, a relatively constant level of GGT was released, and again, there was no correlation between enzyme activity and protein concentration. Following a bolus injection of caerulein, an analog of cholecystokinin, marked and concomitant rises in protein and GGT levels were observed. Ultracentrifugation, as well as gel filtration on Sepharose 4B, demonstrated that the enzyme was not released in a soluble form. This observation is in agreement with in vitro determinations on isolated zymogen granules showing that GGT is totally associated with the ZG membrane and undetect-able in the content of these organelles. The present data show that 1 degree GGT is released from the rat pancreas acinar cells in a particulate form; 2 degree GGT release is elicited by hormonal stimulation coinciding with the exocytotic release of secretory proteins. Our observations lead us to propose that in rat pancreas, ZG membrane fragments are released along with secretory proteins during exocytosis.

A competition enzyme-linked immunosorbent assay (ELISA) for the measurement of pancreatic GP-2 glycoprotein

A competition enzyme-linked immunosorbent assay has been developed for the quantitative detection of soluble and membrane-bound GP-2, a glycoprotein which is confined to the exocrine pancreas. Zymogen granule membranes fixed to microtiter plates with poly-L-lysine were used as the source of antigen. Detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS] was added to the assay in order to reveal all the antigens, more particularly in membranous samples. Presence of detergent at concentrations as high as 0.5% did not interfere with any particular steps of the ELISA. This competition ELISA can detect 10 ng of GP-2 and will be useful for measuring soluble as well as membrane GP-2 in order to elucidate its role in the secretory process of the pancreas as well as in certain pathologies such as cystic fibrosis.

The major protein of pancreatic zymogen granule membranes (GP-2) is anchored via covalent bonds to phosphatidylinositol

GP-2, the major integral protein characteristic of the pancreatic zymogen granule membrane can be released from the membrane by the action of a phosphatidylinositol specific phospholipase C (PI-PLC). In a hydrophobic/hydrophilic phase separation system using the non-ionic detergent Triton X-114, the membrane-bound form of the protein went from the detergent phase into the hydrophilic phase upon action of the phospholipase. PI-PLC solubilization of GP-2 unmasked an antigenic determinant similar to the cross-reacting determinant of the trypanosome variant surface glycoproteins. This determinant being a distinctive feature of the glycan moiety of phosphatidyl-inositol anchored membrane proteins, it established the glycosyl-phosphatidyl-inositol nature of the GP-2 membrane anchor. Since soluble GP-2 is also found in the contents of the granule and is secreted intact into the pancreatic juice, it is likely that one of the mechanisms responsible for its release could be a specific phospholipase. GP-2 is the first glycosyl-phosphatidyl-inositol-anchored protein that is integral to the membrane of an organelle and not located at the surface of the cell.

Pathways of secretion in the exocrine pancreas: the status of resting secretion

In the last decade the concept of two distinct pathways of secretion in the exocrine pancreas has slowly emerged. According to this concept, one pathway is involved in stimulated (regulated) conditions and another under resting (constitutive) conditions. This hypothesis was elaborated at first from the comparison of the specific radioactivities of secretory proteins released by the gland under resting and stimulated conditions. Analysis of the protein composition of the juice released under these two physiological conditions further supported that hypothesis. More recent studies compared the kinetic of accumulation of newly synthesized proteins in zymogen granule and their release in the gland lumen. The latter results are in agreement with a model in which secretory proteins are channelled in two separate pathways, one regulated, and one constitutive. Essentially, the constitutive pathway would correspond to a paragranular route in which the proteins would be immediately secreted instead of being stored in zymogen granules. In addition, some of the proteins released in the juice under "resting" conditions are associated to microvesicles. The term "microvesicular secretion" is used to designate that type of secretion.