A role for Ca2+ in mediating hormone-induced biphasic pepsinogen secretion from the chief cell determined by luminescent and fluorescent probes and X-ray microprobe

The role of Sonic Hedgehog as a regulator of gastric function and differentiation

The Hedgehog (Hh) genes play a key role in the regulation of embryonic development and govern processes such as cell differentiation, cell proliferation, and tissue patterning. In vertebrate embryos, Hh gene expression regulates correct formation of limbs, skeleton, muscles, and organs including stomach. In the adult, the Hh pathway functions in tissue repair and regeneration, along with maintenance of stem cells. Sonic Hedgehog (Shh) signaling has been extensively studied for its role in developmental and cancer biology. Recent advances in the field of gastroenterology show that in the stomach, Shh is responsible for proper differentiation of the gastric glands. The aberrant activity of the Shh signaling pathway leads to an altered gastric differentiation program and loss of gastric acid secretion that is the predominant function of the stomach. In this chapter, we review the most recent findings that reveal the role of Shh as a regulator of gastric function and differentiation and how this signaling is dysregulated during the development of gastric cancer in response bacterial infection.

Cholinergic agonist-induced pepsinogen secretion from murine gastric chief cells is mediated by M1 and M3 muscarinic receptors

Muscarinic cholinergic mechanisms play a key role in stimulating gastric pepsinogen secretion. Studies using antagonists suggested that the M3 receptor subtype (M3R) plays a prominent role in mediating pepsinogen secretion, but in situ hybridization indicated expression of M1 receptor (M1R) in rat chief cells. We used mice that were deficient in either the M1 (M1R-/-) or M3 (M3R-/-) receptor or that lacked both receptors (M(1/3)R-/-) to determine the role of M1R and M3R in mediating cholinergic agonist-induced pepsinogen secretion. Pepsinogen secretion from murine gastric glands was determined by adapting methods used for rabbit and rat stomach. In wild-type (WT) mice, maximal concentrations of carbachol and CCK caused a 3.0- and 2.5-fold increase in pepsinogen secretion, respectively. Maximal carbachol-induced secretion from M1R-/- mouse gastric glands was decreased by 25%. In contrast, there was only a slight decrease in carbachol potency and no change in efficacy when comparing M3R-/- with WT glands. To explore the possibility that both M1R and M3R are involved in carbachol-mediated pepsinogen secretion, we examined secretion from glands prepared from M(1/3)R-/- double-knockout mice. Strikingly, carbachol-induced pepsinogen secretion was nearly abolished in glands from M(1/3)R-/- mice, whereas CCK-induced secretion was not altered. In situ hybridization for murine M1R and M3R mRNA in gastric mucosa from WT mice revealed abundant signals for both receptor subtypes in the cytoplasm of chief cells. These data clearly indicate that, in gastric chief cells, a mixture of M1 and M3 receptors mediates cholinergic stimulation of pepsinogen secretion and that no other muscarinic receptor subtypes are involved in this activity. The development of a murine secretory model facilitates use of transgenic mice to investigate the regulation of pepsinogen secretion.

Stimulation of pepsinogen release from chief cells by Helicobacter pylori: evidence for a role of calcium and calmodulin

To define the mechanisms by which Helicobacter pylori stimulates pepsinogen secretion, the in vitro release of pepsinogen was studied using a preparation of pig chief cell monolayers. Helicobacter pylori induced a time- and concentration-dependent release of pepsinogen into the medium, with about a three-fold increase in pepsinogen secretion over controls found after 45 min of incubation. 3x10(7) H. pylori produced 50% of the maximal response found at a H. pylori count of 2x10(8). The action of H. pylori did not depend on the presence of the vacuolating toxin (vacA) and the cytotoxin-associated protein (cagA). Dibutyryl-cAMP and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate also markedly stimulated pepsinogen secretion and enhanced the stimulatory effect of H. pylori. Helicobacter pylori-stimulated pepsinogen release was inhibited by lanthanum and the calmodulin antagonist W-7, but not by the L-type Ca2+ channel blocker nifedipine, TMB-8, an agent that blocks the release of Ca2+ from intracellular stores, the protein kinase C inhibitor staurosporine and the protein kinase A inhibitor H-8. It is suggested that H. pylori directly stimulates pepsinogen release from gastric chief cells and that this effect is mediated via the calcium/calmodulin messenger branch.

Effects of dibutyryl guanosine 3',5'-cyclic monophosphate and sodium nitroprusside in pepsinogen secretion from guinea pig chief cells with respect to intracellular Ca2+

Both Ca2+ and adenosine 3',5'-cyclic monophosphate act as intracellular second messengers in pepsinogen secretion from chief cells. However, the role of intracellular guanosine 3',5'-cyclic monophosphate (cGMP) in this process has not been defined. Although dibutyryl cGMP (dbcGMP), a membrane-permeable derivative of cGMP, has been shown to inhibit pepsinogen secretion only stimulated by cholecystokinin (CCK), the intracellular mechanism of this effect remains unclear. We evaluated the role of intracellular cGMP in pepsinogen secretion from monolayer cultured guinea pig chief cells using dbcGMP and sodium nitroprusside, both of which increase intracellular cGMP. Dibutyryl cGMP and sodium nitroprusside have now been shown to inhibit pepsinogen secretion induced by not only CCK octapeptide but also carbamylcholine chloride and ionomycin in a dose-dependent manner. Furthermore, dbcGMP reduced the increase in intracellular free Ca2+ concentration induced by carbamylcholine chloride, CCK octapeptide, and ionomycin. These results suggest that intracellular cGMP may inhibit pepsinogen secretion by reducing the intracellular free Ca2+ concentration.

Protein kinase C alpha-, beta- and gamma-subspecies in basal granulated cells of rat duodenal mucosa

Protein kinase C [cPKC: alpha, beta (beta I, beta II), gamma], a Ca(2+)- and phospholipid-dependent enzyme, has been thought to play a critical role in the synthesis and secretion of gut hormones in gastrointestinal mucosa. However, the localization of PKC has not yet been clarified at the cellular level in the gastrointestinal epithelium. The present study was made to identify cPKC-containing cells immunohistochemically in the rat duodenal epithelium by light and electron microscopy and by confocal laser scanning microscopy. Special attention was paid to the demonstration of cPKC in basal granulated cells. By light microscopy, some duodenal epithelial cells were demonstrated to be immunopositive for PKC alpha-, beta- and gamma-subspecies. Their distribution and incidence were almost similar to those of cells stained by the silver impregnation method of Grimelius. By electron microscopy, profiles of secretory granules were found at the basal region of the PKC-immunopositive epithelial cells. When the cells were double-immunostained for gastrin, serotonin or somatostatin and for PKC alpha-, beta- or gamma-subspecies, these gut hormones and PKC subspecies were shown to colocalize as examined by confocal laser scanning microscopy. These findings show that cPKC (alpha, beta, gamma) is present in basal granulated cells such as G-, EC- and D-cells, presumably playing some important role in regulation of gut hormones, including their synthesis and/or secretion.

Receptor-operated Ca2+ signaling and crosstalk in stimulus secretion coupling

In the cells of higher eukaryotic organisms, there are several messenger pathways of intracellular signal transduction, such as the inositol 1,4,5-trisphosphate/Ca2+ signal, voltage-dependent and -independent Ca2+ channels, adenylate cyclase/cyclic adenosine 3',5'-monophosphate, guanylate cyclase/cyclic guanosine 3',5'-monophosphate, diacylglycerol/protein kinase C, and growth factors/tyrosine kinase/tyrosine phosphatase. These pathways are present in different cell types and impinge on each other for the modulation of the cell function. Ca2+ is one of the most ubiquitous intracellular messengers mediating transcellular communication in a wide variety of cell types. Over the last decades it has become clear that the activation of many types of cells is accompanied by an increase in cytosolic free Ca2+ concentration ([Ca2+]i) that is thought to play an important part in the sequence of events occurring during cell activation. The Ca2+ signal can be divided into two categories: receptor- and voltage-operated Ca2+ signal. This review describes and integrates some recent views of receptor-operated Ca2+ signaling and crosstalk in the context of stimulus-secretion coupling.

Ca2+-ATPase in mucous and oxyntico-peptic cells of the fowl proventriculus

Calcium adenosine triphosphatase (Ca(2+)-ATPase) was localized by means of histo-and ultracytochemistry in the secretory cells of the proventriculus of the domestic fowl. The mucous cells exhibited plasmalemmal-associated enzyme activity on the external aspect of the basolateral cell membrane. Intracellularly, the luminal aspect of Golgi-membranes and of secretory vesicle membranes reacted positively for Ca(2+)-ATPase activity, as did the apical cytosol and the matrix of lysosomes. Oxyntico-peptic cells were characterized by apical and apico-lateral plasmalemmal activity and by an organelle-associated distributional pattern similar to that in the mucous cells. In addition, Ca(2+)-ATPase was associated either with the matrix of mitochondria or with tubuli of the rough-surfaced endoplasmic reticulum. The results are discussed with respect to messenger and effector functions of calcium in the process of proventricular mucus secretion. In addition, Ca(2+)-ATPase distributional patterns in the oxyntico-peptic cell are related to the unique structure and function of these cells.

Gastric chief cells: receptors and signal-transduction mechanisms

Elucidation of receptors and mediators regulating gastric pepsinogen secretion has lagged behind understanding of the factors that control acid secretion. During the past decade, as a consequence of the development of in vitro models for studying the control of pepsinogen secretion at the cellular level, much information about chief cell receptors and signal-transduction mechanisms has been obtained, including the identification and characterization of receptors for secretin, vasoactive intestinal polypeptide, cholinergic agonists, gastrin, cholecystokinin, peptide YY, and cholera toxin. Moreover, these cell preparations have permitted secretagogue-induced changes in chief-cell calcium concentration, protein kinase C distribution, and phosphoinositide and cyclic nucleotide content to be measured and related to changes in pepsinogen secretion. This article reviews these advances, discusses areas of uncertainty and controversy, and indicates areas for future investigation.

Cellular distribution of gastric chief cell protein kinase C activity: differential effects of diacylglycerol, phorbol esters, carbachol, and cholecystokinin

Stimulation of chief cells with carbachol or cholecystokinin (CCK) results in the production of inositol trisphosphate (IP3) and diacylglycerol (DAG). Although IP3 increases cell calcium concentration, thereby stimulating pepsinogen secretion, the role of DAG and its target, protein kinase C (PKC), is less clear. To examine the relation between the cellular distribution of PKC activity and pepsinogen secretion, we determined PKC activity in cytosolic and membrane fractions from dispersed chief cells from guinea pig stomach. To validate our assay, we studied the actions of the phorbol ester PMA. PMA caused a rapid, dose-dependent, 6-fold increase in pepsinogen secretion and membrane-associated PKC activity. Similarly, dose-response curves for pepsinogen secretion and the increase in membrane-associated PKC activity induced by a membrane-permeant DAG (1-oleoyl-2-acetylglycerol) were superimposable. In contrast, CCK (0.1 nM to 1.0 microM) and carbachol (0.1 microM to 1.0 mM) caused a 4-fold increase in pepsinogen secretion, but did not alter the distribution of PKC activity. These results indicate that in gastric chief cells, PMA- and DAG-induced pepsinogen secretion is accompanied by increased membrane-associated PKC activity. However, the cellular distribution of PKC activity is not altered by CCK or carbachol.

Secretagogue-induced Ca2+ oscillations in isolated canine gastric chief cells

Agonist-induced changes in cytoplasmic free Ca2+ concentration [( Ca2+]i) of isolated canine gastric chief cells were evaluated by microspectrofluorometry of superfused fura-2 loaded cells. Application of high concentrations of carbachol (CCh, 10(-5) M) or cholecystokinin octapeptide (10(-8) M) resulted in biphasic Ca2+ mobilization comprising an initial large transient followed by a small sustained elevation above the prestimulation level. Submaximal concentrations of CCh (10(-6) M) or cholecystokinin (10(-9) M) led to either a transient series of large amplitude Ca2+ spike(s) or a higher frequency of sustained Ca2+ oscillations of smaller amplitude. Cholecystokinin at 10(-10) M induced only sustained Ca2+ oscillations. Elimination of Ca2+ from the medium had no immediate effect on oscillations indicating an intracellular source of Ca2+. Thus the Ca2+ signalling mode in chief cells is dependent on agonist concentrations.

The role of Ca2+ in the time-dependent pepsinogen secretion of frog oesophageal peptic glands stimulated by bombesin

Time- and dose-related stimulation of pepsinogen secretion by bombesin was studied in perifused dispersed peptic glands from the oesophagus of the American bullfrog Rana catesbeiana. The dose response to bombesin was monophasic between 10(-10) and 10(-7) M, with an EC50 of 10(-9) M. Time-dependent secretion was closely monitored at 1-2 min intervals. Though there was overlap, we could discriminate an early response at approximately 2 min (phase I) and a delayed or sustained response at greater than or equal to 2 min (phase II) on the basis of responses in the presence and absence of external Ca2+. Phase I was relatively independent of external [Ca2+] and coincided with 45Ca efflux following a dose-dependent increase in cytosolic [Ca2+], measured by Fura-2AM. Phase II was sustained at approximately 80% of control at an external [Ca2+] of 1-5 microM, but was eliminated by adding 0.5-1 mM EGTA. Bombesin caused a sustained Ca2+ influx and, when this was prevented by EGTA, the response to successive stimulations by bombesin and by acetylcholine was greatly attenuated. The phorbol ester, 12-O-tetradecanoyl phorbol 13-acetate, which stimulates secretion at high concentrations, was used as background at a threshold concentration of 10(-7) M, which did not by itself stimulate secretion. At this concentration, 12-O-tetradecanoyl phorbol 13-acetate potentiated the responses to bombesin and to acetylcholine. These results define the different Ca2+ dependencies of the immediate and sustained secretory responses to bombesin, but indicate a complex relationship of stimulation responses to Ca2+ homeostasis in various agonist-sensitive Ca2+ pools.

Cholinergic desensitization of pepsinogen secretion and calcium mobilization of dispersed guinea pig chief cells

When dispersed chief cells from guinea pig stomach were first incubated with carbachol, washed, and then reincubated with carbachol in fresh incubation solution, the stimulation of pepsinogen secretion and the rise in intracellular calcium concentration during the second incubation were reduced. Carbachol did not cause residual enzyme secretion, but the same range of concentrations that causes enzyme secretion caused desensitization that was rapid, temperature dependent, and reversible with time. Preincubation with carbachol caused approximately a 65% reduction in enzyme secretion stimulated during a subsequent incubation with this agonist, but the potency of carbachol was unaffected. Prior exposure to carbachol also reduced subsequent stimulation caused by cholecystokinin (CCK-8), gastrin I, ionophore A23187, or 12-O-tetradecanoylphorbol 13-acetate but did not alter stimulation by any agonist that increases cellular cAMP. Carbachol pretreatment of Fura-loaded chief cells caused a threefold increase in the EC50 for carbachol-stimulated [Ca2+]i and approximately a 30% reduction in the maximal rise in [Ca2+]i in response to carbachol or CCK-8. Inhibition of [N-methyl-3H] scopolamine binding by carbachol following carbachol pretreatment indicated that modulation of receptor affinity or number did not account for functional desensitization. These data indicate that carbachol causes heterologous desensitization of pepsinogen secretion stimulated by agonists that mobilize cellular Ca2+ or activate protein kinase C through a postreceptor action and suggest that an attenuated rise in chief cell calcium is one mechanism mediating the desensitization of enzyme secretion.