Electrophoretic and cytological evidence for heterogeneity of pancreatic acinar cell responsiveness to carbachol, caerulein and secretin

Agonist-induced vesiculation of the Golgi apparatus in pancreatic acinar cells

BACKGROUND & AIMS

The pancreatic acinar cell is known to regulate exocytosis, total protein synthesis, and secretory protein transport in response to a secretory stimulus. Whether secretory vesicle formation also is regulated is unclear. In this study, we determined whether agonist stimulation induces morphologic alterations in the acinar cell Golgi apparatus, and we evaluated the role of the vesicle severing protein dynamin.

METHODS

Changes in Golgi structural integrity by examining the distribution of various Golgi and TGN lipid and protein markers in live and fixed cells on stimulation with cholecystokinin were noted in a primary pancreatic acinar cell model. Multiple dynamin reagents were used to examine the distribution and function of this molecular pinchase in resting and stimulated cells.

RESULTS

Regulated secretion in acinar cells induced (1) marked fragmentation of the trans-Golgi network (TGN) that corresponded temporally with an increase in cytoplasmic calcium whereas pre-TGN compartments of the Golgi and regions of the TGN involved in the generation of constitutively trafficking vesicles were unaffected by agonist, and (2) significant recruitment of dynamin to the acinar cell Golgi apparatus that appeared to potentiate fragmentation of the TGN.

CONCLUSIONS

These results suggest that the TGN is a dynamic organelle that fragments in response to cholecystokinin stimulation, a process that may contribute to zymogen granule formation.

Resting (basal) secretion of proteins is provided by the minor regulated and constitutive-like pathways and not granule exocytosis in parotid acinar cells

Resting secretion of salivary proteins by the parotid gland is sustained in situ between periods of eating by parasympathetic stimulation and has been assumed to involve low level granule exocytosis. By using parotid lobules from ad libitum fed rats stimulated with low doses of carbachol as an in vitro analog of resting secretion, we deduce from the composition of discharged proteins that secretion does not involve granule exocytosis. Rather, it derives from two other acinar export routes, the constitutive-like (stimulus-independent) pathway and the minor regulated pathway, which responds to low doses of cholinergic or beta-adrenergic agonists (Castle, J. D., and Castle, A. M. (1996) J. Cell Sci. 109, 2591-2599). The protein composition collected in vitro mimics that collected from cannulated ducts of glands given low level stimulation in situ. Analysis of secretory trafficking along the two pathways of resting secretion has indicated that the constitutive-like pathway may pass through endosomes after diverging from the minor regulated pathway at a brefeldin A-sensitive branch point. The branch point is deduced to be distal to a common vesicular budding event by which both pathways originate from immature granules. Detectable perturbation of neither pathway in lobules was observed by wortmannin addition, and neither serves as a significant export route for lysosomal procathepsin B. These findings show that parotid acinar cells use low capacity, high sensitivity secretory pathways for resting secretion and reserve granule exocytosis, a high capacity, low sensitivity pathway, for massive salivary protein export during meals. An analogous strategy may be employed in other secretory cell types.

Demonstration of acetylcholinesterase molecular forms in a continuous tubular lysosomal system of rat pancreatic acinar cells

By applying the highly sensitive cytochemical Gautron's technique, we were able to reveal AChE activity in rat pancreatic acinar cells, particularly at the level of a complex membrane-bound network formed by tubules with varicosities located around the nuclei and close to the basolateral membrane. The Golgi apparatus was devoid of cytochemical reaction beside the trans-Golgi network cisternae, which showed a positive reaction. The RER of some acinar cells also presented a signal, demonstrating their capability of synthesizing AChE. Immunogold using a specific anti-AChE antibody yielded similar results. Double-labeling experiments corroborated the presence of enzyme cytochemical and immunocytochemical signals in the same lysosomal tubular network. Biochemical sedimentation assays confirmed the presence of AChE in acinar cells, which exists as two globular molecular forms, G(1) and G(4). These results were obtained with pancreatic tissue in situ as well as with isolated acinar cells maintained in culture and devoid of neural elements. The existence of a continuous tubular lysosomal network containing AChE is in agreement with previous reports on acinar and other cell types, and supports a more general hypothesis on dynamic continuities among cell structures. Whether AChE is being secreted by the acinar cells or internalized through this endo-lysosomal system was not defined. However, the capability of the acinar cells to synthesize AChE and to channel it through a tubular system is a good indication that the cells can modulate their cholinergic stimulation for optimal secretion of digestive enzymes.

Heterogeneous distribution of plasma membrane glycoconjugates in pancreatic acinar cells

Flow-cytometric studies of lectin binding to individual acinar cells have been carried out in order to analyse the distribution of membrane glycoconjugates in cells from different areas of the pancreas: duodenal lobule (head) and splenic lobule (body and tail). The following fluoresceinated lectins were used: wheat germ agglutinin (WGA), Tetragonolobus purpureus agglutinin (TP) and concanavalin A (Con A), which specifically bind to N-acetyl D-glucosamine and sialic acid, L-fucose and D-mannose, respectively. In both pancreatic areas, two cell populations (R1 and R2) were identified according to the forward scatter (size). On the basis of their glycoconjugate pattern, R1 cells displayed higher density of WGA and TP receptors than R2 cells throughout the pancreas. Although no difference in size was found between the cells from duodenal and splenic lobules, N-acetyl D-glucosamine and/or sialic acid and L-fucose residues were more abundant in plasma membrane cell glycoconjugates from the duodenal lobule. The results provide evidence for biochemical heterogeneity among individual pancreatic cells according to the distribution of plasma membrane glycoconjugates.

Cell-specific expression of the ZPK gene in adult mouse tissues

ZPK is a recently identified human putative protein kinase gene that encodes an unusual serine/threonine kinase containing two potential leucine zipper motifs similar to those found in transcription factors as well as in members of the newly discovered mixed-lineage family of protein kinases. To study the normal biological function of ZPK, we have isolated a mouse ZPK cDNA and examined the pattern of ZPK mRNA expression in adult mouse tissues by Northern blot and in situ hybridization analyses. The predicted open reading frame of this cDNA encodes an 888-amino-acid protein that shares 95% overall identity with its human counterpart. By Northern blot analysis, we detected expression of ZPK mRNA in the brain of adult mice, but not in any other tissue tested. In situ hybridization analysis of mouse brain sections revealed specific association of ZPK mRNA with neuronal cell populations, primarily in the hippocampus, the cerebral cortex, and the Purkinje cell layer of the cerebellum. Interestingly, a remarkable pattern of cell-type-specific expression was also found in the epithelial compartment of various organ systems, including stomach, small intestine, liver, and pancreas, as well as in the seminiferous tubules of mature testes. Taken together, these observations suggest that ZPK could play a role in development, function, and maintenance of a variety of specialized cells.

Heterogeneity and contact-dependent regulation of amylase release by individual acinar cells

We have used a reverse hemolytic plaque assay to investigate the amylase release of single and aggregated pancreatic acinar cells. We have found that a minority of single acinar cells released detectable amounts of amylase under basal conditions and were modestly stimulated, in a dose-dependent manner, during a 30-min exposure to concentrations of carbamylcholine (CCh) ranging from 10(-8) to 10(-5) M. This stimulation was largely accounted for by the recruitment of additional secreting cells, rather than by a significant increase in their individual secretory output. We have also observed that aggregates comprising two to five acinar cells secreted more frequently and released more amylase than single acinar cells in the presence of each of the CCh concentrations tested. Under both basal conditions and following CCh stimulation, the proportion of secreting aggregates and their amylase output increased linearly with the aggregate size. Under basal conditions as well as in the presence of secretagogue concentrations in the 10(-8) - 10(-7) M range, individual cells contributed similarly to amylase secretion whether they were single or part of aggregates. By contrast, following stimulation by 10(-6) - 10(-5) M CCh, aggregated cells showed a much higher average secretion than single cells. Investigating the mechanism of this contact-dependent effect, we found that 10(-3) M heptanol did not significantly modify the secretion of single cells and markedly promoted the basal amylase release of acinar cell pairs. This effect was associated with a marked reduction in gap junctional communication between acinar cells, as evaluated by microinjection of Lucifer yellow, and was not observed during exposure to high concentrations of CCh, which also reduced junctional communication. These data show that pancreatic acinar cells are intrinsically heterogeneous in their ability to release amylase and that their basal as well as stimulated secretion are promoted by the establishment of direct intercellular contacts. Our experiments also suggest that junctional coupling contributes to the contact-dependent mechanism which enhances the recruitment of secreting cells and their individual output. These observations strengthen the view that direct interactions between acinar cells are essential in the control of pancreatic secretion.

[Detailed analysis of human pancreatic secretions collected by retrograde catheterization. Parallel or non-parallel excretion of digestive enzymes?]

Pancreatic juices were collected by selective reverse catherism of the chief pancreatic duct in two patients, one free from pancreatic disease and the other having a pancreas cancer. They were analysed in detail especially in order to get information on the mechanism of enzyme excretion. The variations of the digestive enzyme activities (amylase, lipase, trypsin, chymotrypsin, carboxypeptidase A and B) were not superimposable among them or with the fluctuations of the protein concentration in the pancreatic juice samples. These results agree with a non-parallel enzyme-excretion mechanism by the pancreas. However deep electrophoresis analyses of pancreatic juice samples showed that the ratio of each digestive enzyme concentration remained almost constant in the same patient. This observation disagrees with the above conclusion and suggests that the data obtained by using classical methods for estimating digestive enzyme activities have to be considered prudently. By another way, two main significant differences were reported by analysing the ionic composition of the pancreatic juice samples following their origin. The pancreatic juice samples of the patient having a pancreas cancer had a lower and more variable Na+ concentration than those coming from the patient who was free from pancreas disease. They had a HCO3- concentration which was almost constant, contrary to what was observed for the pancreatic juice secreted by the other patient.

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.

Effects of 5-fluorouracil on protein synthesis and secretion of the rat parotid gland

100 mg/kg of FU were injected intraperitoneally once daily for three days. Animals were anaesthetized with 50 mg/kg of sodium pentobarbital before cannulation of the parotid duct. The total volume, amylase and protein content of the saliva were determined after stimulation with either 5 mg/kg pilocarpine or 5 mg/kg isoproterenol in FU-treated, pair-fed, and control animals. Saliva from FU-treated animals was significantly lower (p less than 0.05) in volume, amylase and protein content than that of both control groups. SDS, anionic and cationic gel electrophoresis of parotid saliva revealed no qualitative changes in the types of proteins secreted. FU reduced the total glandular amylase per unit DNA in both unstimulated and isoproterenol-stimulated parotids (p less than 0.05). Decreased protein synthesis may be the mechanism underlying the depleted secretory protein stores because the contents of isolated secretory granules from experimental glands contained less radiolabelled protein than those of either control group, and whole-gland homogenates had marked reductions in the activities of three lysosomal enzymes and in total RNA content. The secretory granules of experimental animals contained less labelled protein than those of controls, but experimental animals secreted a greater proportion of their total glandular radiolabelled secretory protein into saliva relative to amylase, suggesting that newly synthesized secretory proteins were preferentially secreted.

Morphometric study of the rat exocrine pancreas after diversion of bile and pancreatic juice from the intestine

The effects of 6 h of diversion of bile and pancreatic juice from the intestine on the ultrastructure of rat pancreatic acinar cells were studied by morphometric procedures. After diversion the volume of zymogen granules decreased markedly, representing a reduction of 77%. The volume of autophagic vacuoles increased twofold after diversion. The volume of rough endoplasmic reticulum and Golgi apparatus was not different from that of control. It has been proved by electron microscopic morphometry that the degree of reduction of zymogen granules was similar to that after stimulation by exogenous cholecystokinin or introduction of trypsin inhibitor to the duodenum. This suggests that hypersecretion caused by diversion is due to failure of a negative feedback regulation normally induced by proteases found in pancreatic juice. An increased number of autophagic vacuoles suggests an increase of organelle turnover due to heightened secretory activity of acinar cells.

Presence of two distinct acinar cell populations in human pancreas based on their antigenicity

The immunohistochemical localization of ABH- and Lewis (Le)-related blood group antigens, including CA 19-9, a sialylated Lea antigen, was examined using monoclonal antibodies (MoAbs) in 18 normal human pancreases and compared with ABH blood group antigenicity of the individuals. Acinar cells expressed ABH, Leb, Ley, and in some cases, Lex antigen in various proportions, but not Lea and CA 19-9. The reactivity of Leb and Ley was similar with regard to cellular localization and specificity. In all specimens but one, the distribution of Leb (and Ley) and H antigens on the one hand, and of A or B antigens on the other, showed a reciprocal relationship, in that one group of acini expressed Leb (and Ley) and H antigens, but lacked any A or B antigens (type 1 acinar cell); another group of acinar cells had A or B antigens, but expressed neither Leb (Ley) or H antigens (type 2 acinar cell). In ductal cells, four of eight individuals with blood group A, two of three with blood group B, and five of six with blood group O expressed the appropriate antigen, while the remainder did not. Lea antigen was expressed primarily by centroacinar and terminal ductular and ductal cells of medium-sized ducts of all specimens, whereas Leb was present in the cells of small and large ducts in all but four cases. The reactivity of ductal and ductular cells to Lex was negative, except for one case. MoAb-Ley and MoAb 19-9 reacted only with a few ductal cells in six (33%) and 12 cases (67%), respectively. There was no relationship in the expression of Le-related antigens between acinar and ductal/ductular cells; nor were there any sex difference with regard to the binding patterns of any antibodies. However, age appeared to influence the reactivity of some antibodies with acinar cells. Islet cells did not react with any of the antibodies. The results indicate that, although the antigenicity of epithelial cells can be affected by the host blood group types, there might be several regulatory systems for expression of blood group antigens in a cell-specific pattern.