Protein kinase C and parietal cell function

Gastric acid, calcium absorption, and their impact on bone health

Calcium balance is essential for a multitude of physiological processes, ranging from cell signaling to maintenance of bone health. Adequate intestinal absorption of calcium is a major factor for maintaining systemic calcium homeostasis. Recent observations indicate that a reduction of gastric acidity may impair effective calcium uptake through the intestine. This article reviews the physiology of gastric acid secretion, intestinal calcium absorption, and their respective neuroendocrine regulation and explores the physiological basis of a potential link between these individual systems.

Revisiting the parietal cell

The parietal cell is responsible for secreting concentrated hydrochloric acid into the gastric lumen. To fulfill this task, it is equipped with a broad variety of functionally coupled apical and basolateral ion transport proteins. The concerted scientific effort over the last years by a variety of researchers has provided us with the molecular identity of many of these transport mechanisms, thereby contributing to the clarification of persistent controversies in the field. This article will briefly review the current model of parietal cell physiology and ion transport in particular and will update the existing models of apical and basolateral transport in the parietal cell.

Cell biology of acid secretion by the parietal cell

Acid secretion by the gastric parietal cell is regulated by paracrine, endocrine, and neural pathways. The physiological stimuli include histamine, acetylcholine, and gastrin via their receptors located on the basolateral plasma membranes. Stimulation of acid secretion typically involves an initial elevation of intracellular calcium and/or cAMP followed by activation of a cAMP-dependent protein kinase cascade that triggers the translocation and insertion of the proton pump enzyme, H,K-ATPase, into the apical plasma membrane of parietal cells. Whereas the H,K-ATPase contains a plasma membrane targeting motif, the stimulation-mediated relocation of the H,K-ATPase from the cytoplasmic membrane compartment to the apical plasma membrane is mediated by a SNARE protein complex and its regulatory proteins. This review summarizes the progress made toward an understanding of the cell biology of gastric acid secretion. In particular we have reviewed the early signaling events following histaminergic and cholinergic activation, the identification of multiple factors participating in the trafficking and recycling of the proton pump, and the role of the cytoskeleton in supporting the apical pole remodeling, which appears to be necessary for active acid secretion by the parietal cell. Emphasis is placed on identifying protein factors that serve as effectors for the mechanistic changes associated with cellular activation and the secretory response.

Protein kinase C-alpha attenuates cholinergically stimulated gastric acid secretion of rabbit parietal cells

(1) The phorbolester 12-O-tetradecanoyl phorbol-13-acetate (TPA), an activator of protein kinase C (PKC), inhibits cholinergic stimulation of gastric acid secretion. We observed that this effect strongly correlated with the inhibition of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) activity in rabbit parietal cells. (2) The aim of this study was to specify the function of PKC-alpha in cholinergically stimulated H(+) secretion. PKC-alpha represents the only calcium-dependent PKC isoenzyme that has been detected in rabbit parietal cells. (3) Gö 6976, an inhibitor of calcium-dependent PKC, concentration-dependently antagonized the inhibitory effect of TPA, and, therefore, revealed the action of PKC-alpha on carbachol-induced acid secretion in rabbit parietal cells. (4) TPA exerted no additive inhibition of carbachol-stimulated acid secretion if acid secretion was partially inhibited by the potent CaMKII inhibitor 1-[N,O-bis(5-isoquinolinsulfonyl)-N-methyl-L-tyrosyl]-4-phenyl-piperazine (KN-62). (5) Since both kinase modulators, TPA and KN-62, affected no divergent signal transduction pathways in the parietal cell, an in vitro model has been used to study if PKC directly targets CaMKII. CaMKII purified from parietal cell-containing gastric mucosa of pig, was transphosphorylated by purified cPKC containing PKC-alpha up to 1.8 mol P(i) per mol CaMKII in vitro. The autonomy site of CaMKII was not transphosphorylated by PKC. (6) The phosphotransferase activity of the purified CaMKII was in vitro inhibited after transphosphorylation by PKC if calmodulin was absent during transphosphorylation. Attenuation of CaMKII activity by PKC showed strong similarity to the downregulation of CaMKII by basal autophosphorylation. (7) Our results suggest that PKC-alpha and CaMKII are closely functionally linked in a cholinergically induced signalling pathway in rabbit parietal cells. We assume that in cholinergically stimulated parietal cells PKC-alpha transinhibits CaMKII activity, resulting in an attenuation of acid secretion.

Different actions of protein kinase C isoforms alpha and epsilon on gastric acid secretion

1. The phorbol ester TPA, an activator of protein kinase C (PKC), inhibits cholinergic stimulation of gastric acid secretion but increases basal H(+) secretion. 2. Since these contradictory findings suggest the action of different PKC isozymes we analysed the role of calcium-dependent PKC-alpha, and calcium-independent PKC-epsilon in gastric acid secretion. 3. Inhibition of PKC-alpha by the indolocarbazole Gö 6976 revealed that about 28% of carbachol-induced acid secretion was inhibited by PKC-alpha. In the presence of Gö 6976 approximately 64% of the carbachol-induced signal transduction is mediated by Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), and 14% is conveyed by PKC-epsilon as deduced from the inhibition with the bisindolylmaleimide Ro 31-8220. 4. Inhibition of carbachol-induced acid secretion by TPA was accompanied by a decrease in CaMKII activity. 5. The stimulation of basal acid secretion by TPA was biphasic with a peak at a very low concentration (10 pM), resulting in an activation of the calcium-sensor CaMKII. The activation was determined with a phosphospecific polyclonal antibody against active CaMKII. The TPA-induced increase of H(+) secretion was sensitive to the cell-permeable Ca(2+)-chelator BAPTA/AM, Ro 31-8220, and the CaMKII-inhibitor KN-62, but not to Gö 6976. 6. Since TPA induced the translocation of PKC-epsilon but not of PKC-alpha in resting parietal cells, PKC-epsilon seems to be at least responsible for an initial elevation of free intracellular calcium to initiate TPA-induced acid secretion. 7. Our data indicate the different roles of two PKC isoforms: PKC-epsilon activation appears to facilitate cholinergic stimulation of H(+)-secretion likely by increasing intracellular calcium. In contrast, PKC-alpha activation attenuates acid secretion accompanied by a down-regulation of CaMKII activity.

Modulation of acid secretion in common bile duct ligation-related gastropathy in Wistar rats

BACKGROUND

Portal hypertensive gastropathy is associated with fundic gland atrophy, resulting in a decrease in chief and parietal cells, and diminished acid secretion.

METHODS

Acid secretion by isolated parietal cells was measured (acridine orange retention), along with the levels of various second messengers (intracellular Ca(2+), cyclic adenosine monophosphate and protein kinase C) in the common bile duct, ligated portal hypertensive rats and compared with sham-operated controls.

RESULTS

There was a significant decrease in the response of isolated parietal cells to the secretagogues histamine and carbachol. This resulted in the blunted acid secretion in the common bile duct ligated group. In addition, all the second messengers studied were significantly decreased as compared with the sham-operated controls.

CONCLUSION

These results suggest that the blunted acid secretory response in the portal hypertensive rat is caused by an alteration in the intracellular signal transduction mechanism.

Effects of phorbol ester treatment on dibutyryl cyclic adenosine-5' monophosphate- and carbachol-stimulated aminopyrine accumulation in isolated rat parietal cells

BACKGROUND

The functional role of the intracellular diacylglycerol/protein kinase C second-messenger pathway in the regulation of gastric acid secretion and the effects on the involved inositotrisphosphate/Ca2+/calmodulin system are not well understood, and contradictory data have been reported. We therefore evaluated the effects of phorbol ester treatment (tetradecanoylphorbol-12,13-acetate (TPA)) on dibutyryl cyclic adenosine-5' monophosphate (dBcAMP)- and carbachol-stimulated aminopyrine (AP) accumulation in comparison with intracellular alterations of the phospholipase C/inostol phosphate signal transduction pathway in isolated rat gastric parietal cells.

METHODS

[14C]AP accumulation was determined as an indirect measure of gastric acid secretion. Inositolphosphate second-messenger activation was investigated with [3H]inositolmonophosphate release in [3H]-myoinositol prelabeled rat gastric parietal cells.

RESULTS

TPA at a low concentration of 5 nM caused a small (45%) but significant increase in carbachol (0.1 mM)-stimulated AP accumulation, which was dose-dependently inhibited by higher concentrations of TPA with corresponding shifts in the dose-response curve for carbachol-stimulated AP accumulation. AP uptake stimulated by dBcAMP (0.1 mM) and the synergistic stimulatory effect induced by carbachol together with dBcAMP were inhibited by TPA at all concentrations investigated. In the presence of increasing concentrations of the calcium ionophore ionomycin (10(-8)-10(-5) M) TPA at 5 nM increased AP accumulation (AP ratio was 4.02 with 5 nM TPA versus 1.23 in the absence of TPA; P < 0.05), indicating that phorbol ester stimulates AP uptake in rat parietal cells. Simultaneous investigation of [14C]AP accumulation and [3H]inositol monophosphate release showed that inhibitory effects of TPA on carbachol- and carbachol plus dBcAMP-stimulated cells are mediated by an inhibition of the receptor/G-protein/phospholipase C interaction, leading to a reduction of inositolphosphate release. The costimulation of rat parietal cells with dBcAMP, ionomycin, and TPA (5 nM) did not reproduce the synergistic effects of carbachol together with dBcAMP on AP accumulation, suggesting that carbachol-stimulated AP uptake seems to be additionally mediated by a still unknown pathway independent of intracellular calcium release or protein kinase C activation.

Protein kinase C inhibits cyclic adenosine monophosphate generation by histamine and truncated glucagon like peptide 1 in the human gastric cancer cell line HGT-1

The HGT-1 gastric cancer cell line was used to determine the actions of protein kinase C on the stimulation of adenylate cyclase by the human histamine H2 receptor, and the receptors for gastric inhibitory polypeptide and truncated glucagon like peptide 1 (TGLP-1). Suspensions of HGT-1 cells were preincubated with the activator of protein kinase C, 12-O-tetradecanoylphorbol 13-acetate (TPA, 100 nmol/l), for 10 minutes. The subsequent cyclic adenosine monophosphate (AMP) response to 0.5 mmol/l histamine or 100 nmol/l TGLP-1 was reduced by comparison with control cells preincubated in the absence of TPA. The cyclic AMP response to 100 nmol/l gastric inhibitory polypeptide was enhanced by preincubation with TPA, while the responses to cholera toxin and forskolin were unaffected. Preincubation with pertussis toxin prevented the enhancement of the gastric inhibitory polypeptide response by TPA, suggesting an involvement of an inhibitory guanine nucleotide regulatory subunit of the Gi class, but did not change the inhibition of histamine stimulation. In conclusion, activation of protein kinase C produces a specific inhibition of the effects of histamine and TGLP-1 on adenylate cyclase activity in a human gastric cancer cell line by acting at a site close to their receptors.

Protein kinase C inhibits stimulation of adenylate cyclase by the histamine H2 receptor in rat parietal cells

The action of protein kinase C on the stimulation of adenylate cyclase activity by the histamine H2 receptor was investigated in rat parietal cells. Protein kinase C was activated by preincubating cells with 12-O-tetradecanoylphorbol 13-acetate (TPA), and adenylate cyclase activity was measured in sonicated extracts. TPA (100 nM) inhibited adenylate cyclase activity stimulated by histamine (100 nM-500 microM). This effect was related to the concentration of TPA. TPA (100 nM) enhanced the stimulation of adenylate cyclase activity by forskolin (100 microM) but had no effect on the stimulation by NaF (10 mM). In conclusion, protein kinase C inhibits stimulation of adenylate cyclase by the histamine H2 receptor. This action could be mediated by changes in the number of affinity of histamine H2 receptors or in the coupling of the receptor to the stimulatory guanine nucleotide regulatory subunit Gs.

Interaction of signal transduction pathways in mediating acid secretion by rat parietal cells

To examine the role of protein kinase C (PKC) on the acid secretory activity of isolated rat parietal cells, histamine-and dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP)-stimulated [14C]aminopyrine accumulation was determined in the presence of agents that redistribute PKC activity to plasma membranes. Phorbol 12-myristate 13-acetate (PMA), 1-oleoyl-2-acetylglycerol (OAG), and phospholipase C inhibited, in a dose-dependent fashion, histamine- and DBcAMP-stimulated [14C]aminopyrine accumulation. Because PKC inhibitor 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H-7) reversed the effect, the results suggest that inhibition of histamine- or DBcAMP-stimulated [14C]aminopyrine accumulation induced by PMA, OAG, or phospholipase C was caused by increased activity of PKC in plasma membrane. To determine where in the cascade of events PKC inhibits acid secretion, histamine-, cholera toxin-, and forskolin-stimulated [14C]aminopyrine accumulation was measured with or without PMA. Because the percent of inhibition by PMA of [14C]aminopyrine accumulation was similar with the three secretagogues, the results suggest that PKC inhibits acid secretion at a point beyond adenosine 3',5'-cyclic monophosphate (cAMP) production. This was supported by the fact that PMA had no effect on histamine-stimulated production of cAMP and by the finding that activation of PKC had the same effect on histamine- or DBcAMP-stimulated [14C]aminopyrine accumulation. Histamine and DBcAMP inhibited PKC activity, suggesting a reciprocal interaction between PKC and histamine-triggered signal transduction pathway.

Effect of phorbol esters on the distribution and total activity of protein kinase C in the perfused rat heart

1. The perfused rat heart was treated with the tumour-promoter and protein kinase C activator, phorbol 12-myristate 13-acetate and the distribution of protein kinase C activity between cytosolic and particulate fractions determined. 2. Phorbol ester treatment led to a rapid loss of protein kinase C activity from the cytosol (t0.5 = 2 min) with a corresponding translocation into the particulate fraction. Translocated protein kinase C activity was tightly bound to the particulate fraction, could only be extracted with buffers containing 2% Triton X-100 and could therefore be misinterpreted as being down-regulated. 3. Claims of rapid down-regulation of protein kinase C activity by phorbol esters need to be supported by rigorous procedures for extraction of the particulate material.