Shin-Ichi Fukuoka

Pancreatic Dysfunction in Cystic Fibrosis Occurs as a Result of Impairments in Luminal ph, Apical Trafficking of Zymogen Granule Membranes, and Solubilization of Secretory Enzymes

Recent progress in understanding the luminal biochemistry of regulated pancreatic exocrine secretion, including acid-base interactions between acinar and duct cells and pH-dependent processes that regulate membrane trafficking (endocytosis) at the apical plasma membrane, have led to the development of in vitro models of cystic fibrosis in the rat exocrine pancreas. Based on investigations in these model systems, a unifying hypothesis is presented that proposes that pancreatic dysfunction in cystic fibrosis occurs as a result of progressive acidification of the acinar and duct lumen, which leads to secondary defects in (i) apical trafficking of zymogen granule membranes and (ii) solubilization of secretory (pro)enzymes. By directly acidifying the pH of the acinar lumen in cholescystokinin-stimulated acini, the early cytological findings observed in cystic fibrosis, including (i) massive dilatation of the acinar lumen, (ii) decreased appearance of zymogen granules, (iii) loss of the apical pole of the acinar cell, and (iv) persistent aggregation of secretory (pro)enzymes released into the luminal space, have been reproduced in primary cultures of pancreatic tissue.

Role of the GP2/THP family of GPI-anchored proteins in membrane trafficking during regulated exocrine secretion

Identification and characterization of the GPY THP family of GPI-anchored membrane proteins associated with apical secretory membranes suggest that this new class of GPI-linked proteins plays a critical role in regulated protein secretion and ion transport in polarized epithelial cells in pancreas, liver, lung, kidney, and gastrointestinal tract. Based on recent information obtained from the world literature and from our own investigations we present the following two hypotheses capable of unifying previously diverse observations. Hypothesis 1 is that formation of GP2 tetramers in the acidic milieu of the trans-Golgi network (TGN) organizes a GP2iproteoglycan (PG) matrix tightly associated with the luminal surface of zymogen granule (ZG) membranes, and proposes that this matrix functions in (a) membrane sorting during granule assembly in the TGN, (b) inactivation of ZG membranes during the storage phase of secretion, and (c) regulated trafficking of ZG membranes from the apical plasma membrane (APM) after exocytosis. Hypothesis 2 is that the acinar lumen constitutes a distinct physiologic compartment for coupled biochemical reactions between acinar and duct cells. Because the acidic pH of the TGN plays a critical role in condensation of secretory proteins, alkalinization of the acinar lumen is required for (a) neutralization of the acidic pH of exocytic contents and (b) solubilization of aggregated (pro)enzymes. Further alkalinization appears to be required for pH-dependent release of the GP2/PG matrix from the APM, a process that may regulate internalization of ZG membranes for reuse during secretion. Taken together, the two hypotheses suggest that luminal factors including acid-base interactions and matrix assembly and disassembly processes perform critical functions during regulated storage and release of pancreatic (pro)enzymes. The requirement that coupling reactions be coordinated through the actions of separate hormones [cholecystokinin (CCK) and secretin] on divergent epithelial cells (acinar and duct cells, respectively) provides a new appreciation for the importance of combined CCK and secretin stimulation during pancreatic secretion in response to food intake.

Pancreatic enzyme secretion mediated by novel peptide: monitor peptide hypothesis

A new model is proposed for pancreatic enzyme secretion in response to food protein intake in rats. We have found a novel peptide in rat bile-pancreatic juice, which exhibits a trypsin-sensitive, cholecystokinin (CCK)-releasing activity. The amino acid sequence of the peptide purified from rat bile-pancreatic juice is very similar to that of a conservative region in pancreatic secretory trypsin inhibitor (PSTI). The peptide loses its CCKreleasing activity during trypsin digestion, but food protein intake prevents this trypsin digestion. Results of a reconstitution experiment indicate that pancreatic enzyme secretion in respoqse to food protein intake only occurs as a result of interaction between trypsin and our purified peptide. Also, a peptidespecific antibody abolished the response. These findings lead us to hypothesize that this peptide acts as an intraduodenal mediator for CCK release in response to food protein intake.

Elevation of plasma CCK concentration after intestinal administration of a pancreatic enzyme secretion-stimulating peptide purified from rat bile-pancreatic juice: Analysis with N-terminal region specific radioimmunoassay

The rat plasma cholecystokinin (CCK) concentration was measured after intestinal administration of a peptide purified from rat bile-pancreatic juice, which has a stimulatory effect on pancreatic enzyme secretion. The plasma CCK concentration was measured by means of a radioimmunoassay using CCK-8 N-terminal specific antibody, OAL-656. In experimental rats with protease-free intestines, intraduodenal infusion of 10 micrograms of the purified peptide, which stimulates pancreatic enzyme secretion 2.0-2.5 fold, induced a significant increase in the plasma CCK level. Furthermore, after removal of CCK from the plasma by immunoabsorption with an OAL-656-bound Sepharose 4B column, the stimulatory effect of the plasma on pancreatic enzyme secretion was abolished when it was injected intravenously into recipient rats. It was concluded that this peptide stimulates the release of CCK in the intestine and that this is responsible at least in part for the pancreatic enzyme secretion-stimulating activity of the peptide.

Expression of Bcl-2 and PCNA in duct cells after pancreatic duct ligation in rats

Obstruction of the pancreatic duct induces acinar cell deletion followed by duct proliferation and interstitial fibrosis. Apoptosis has been reported to be involved in the induction of acinar cell deletion after pancreatic duct ligation (PDL) in rats, however, the mechanism of pancreatic duct cell proliferation is still unknown. We hypothesized that Bcl-2 (antiapoptosis protein) and PCNA (cell cycle-related protein) could be involved in the mechanism of pancreatic duct cell proliferation after PDL. In PDL rats, acinar cells decreased in number and disappeared completely after duct ligation and duct-lining cells increased in number and formed duct-tubular complexes. Immunohistochemical study showed that PCNA expression appeared in the ductules and centroacinar cells from early stages after duct ligation and that Bcl-2 expression in duct cells, which was faint in normal pancreas, increased significantly when acinar cells were diminishing. Western blotting demonstrated that Bcl-2 was detected as a single band at 26 kDa, and the intensity of Bcl-2 in PDL rats was approximately ninefold stronger than in normal pancreas. Expression of Bcl-2 and PCNA after pancreatic duct ligation may be related to the prevention of apoptosis and cell proliferation of pancreatic duct cells in rats.

Competition of a growth stimulating-/cholecystokinin (CCK) releasing-peptide (monitor peptide) with epidermal growth factor for binding to 3T3 fibroblasts

The growth stimulating-/cholecystokinin (CCK) releasing-peptide (monitor peptide) is a peptide purified from rat bile-pancreatic juice on the basis of its stimulatory activity toward pancreatic enzyme secretion. Its multiple functions and peptide sequence suggested that it is distinct from epidermal growth factor (EGF). However, we found that the peptide competes with [125I]-EGF in the binding to Swiss 3T3 fibroblast cells to almost the same extent as unlabeled EGF does. [125I]-EGF binding was inhibited by 50% by the peptide at 82.8 ng/ml and by unlabeled EGF at 71.4 ng/ml. This suggests that the growth stimulating effect of the peptide on 3T3 fibroblasts is mediated via the EGF receptor, and also suggests that the partial homologous sequence between monitor peptide and EGF is required for the receptor binding, or that the EGF receptor has a broad ligand specificity.

Growth stimulating activity on 3T3 fibroblasts of the molecular weight 6,500-peptide purified from rat pancreatic juice.

Growth stimulating activity of the molecular weight 6,500-peptide purified from rat pancreatic juice was measured on 3T3 fibroblasts. This peptide was reported to be a cholecystokinin-releasing peptide and to stimulate pancreatic enzyme secretion in the rat small intestine in response to food intake. Incorporation of [3H]thymidine and [35S]methionine into 3T3 was significantly stimulated and the cell number was also increased after 24-48 hr incubation with 10-100 ng/ml of the peptide. The increase in [3H]thymidine incorporation was dose-related and started 12 hr after the incubation, a peak being reached 24 hr after the incubation. These results show that this peptide exhibits growth stimulating activity to the mammalian cells, and suggest that the peptide might have a physiological effect in vivo.

CHARACTERIZATION AND FUNCTIONAL EXPRESSION OF THE CDNA ENCODING HUMAN BRAIN QUINOLINATE PHOSPHORIBOSYLTRANSFERASE

Mammalian quinolinate phosphoribosyltransferase (QPRTase) (EC 2.4.2.19) is a key enzyme in catabolism of quinolinate, an intermediate in the tryptophan-nicotinamide adenine dinucleotide (NAD) pathway. Quinolinate acts as a most potent endogenous exitotoxin to neurons. Elevation of quinolinate levels in the brain has been linked to the pathogenesis of neurodegenerative disorders. As the first step to elucidate molecular basis underlying the quinolinate metabolism, the cDNA encoding human brain QPRTase was cloned and characterized. Utilizing partial amino acid sequences obtained from highly purified porcine kidney QPRTase, a human isolog was obtained from a human brain cDNA library. The cDNA encodes a open reading frame of 297 amino acids, and shares 30 to 40% identity with those of bacterial QPRTases. To confirm that the cDNA clone encodes human QPRTase, its functional expression was studied in a bacterial host. Introduction of the human cDNA into a QPRTase defective (nadC) E. coli strain brought about an abrupt increase in QPRTase activity and allowed the cells to grow in the absence of nicotinic acid. It is concluded that the cloned cDNA encodes human QPRTase which is functional beyond the phylogenic boundary.

Localization of pancreatic enzyme secretion-stimulating activity and trypsin inhibitory activity in zymogen granule of the rat pancreas

The intracellular localization of pancreatic enzyme secretion-stimulating activity in rat pancreas was investigated. We found and purified a pancreatic enzyme secretion-stimulating peptide from rat bile/pancreatic juice. The peptide is trypsin-sensitive (showing temporary trypsin inhibitory activity), and it is hypothesized that it acts as a trypsin-sensitive mediator in the feedback regulation of diet-induced pancreatic enzyme secretion. The zymogen granule fraction was purified 5-fold by ultracentrifugation by the Percoll density gradient method. The purity of the zymogen granule fraction was determined from the specific amylase activity and electron microscopic morphology. The specific enzyme activities of amylase and trypsin and the trypsin inhibitory activity increased in parallel during the purification, and the pancreatic enzyme secretion-stimulating activity was also localized in the zymogen granule fraction. These results suggest that the pancreatic enzyme secretion-stimulating peptide originates from the acinar cells, and that it is secreted through exocytosis of zymogen granules into the small intestine, its ratio to trypsin thus remaining constant. This idea supports our hypothesis that the stimulating peptide acts as a mediator for the feedback regulation of pancreatic enzyme secretion by trypsin.

Identification of a novel protein complex containing annexin A4, rabphilin and synaptotagmin

Rabphilin is a synaptic vesicle‐associated protein proposed to play a role in regulating neurotransmitter release. Here we report the isolation and identification of a novel protein complex containing rabphilin, annexin A4 and synaptotagmin 1. We show that the rabphilin C2B domain interacts directly with the N‐terminus of annexin A4 and mediates the co‐complexing of these two proteins in PC12 cells. Analyzing the cellular localisation of these co‐complexing proteins we find that annexin A4 is located on synaptic membranes and co‐localises with rabphilin at the plasma membrane in PC12 cells. Given that rabphilin and synaptotagmin are synaptic vesicle proteins involved in neurotransmitter release, the identification of this complex suggests that annexin A4 may play a role in synaptic exocytosis.