Regulation and function of p38 protein kinase in isolated canine gastric parietal cells

Anti-proliferative Effect of C3 Exoenzyme in Fibroblasts is Mediated by c-Jun Phosphorylation

The ADP-ribosyltransferase C3 exoenzyme from C. botulinum selectively inactivates Rho and is therefore often used as an inhibitor for investigations on Rho signaling. Previous studies of our group revealed that C3 inhibited cell proliferation in HT22 cells accompanied by increased transcriptional activities of Sp1 and c-Jun and reduced levels of cyclin D1, p21 and phosphorylated p38. By use of a p38α-deficient and a p38α-expressing control cell line, the impact of p38 on C3-mediated inhibition of cell proliferation and alterations on MAPK signaling was studied by growth kinetic experiments and Western blot analyses. The cell growth of p38α-expressing cells was impaired by C3, while the p38α-deficient cells did not exhibit any C3-induced effect. The activity of the MKK3/6-p38 MAPK signaling cascade as well as the phosphorylation of c-Jun and JNK was reduced by C3 exclusively in the presence of p38α. Moreover, the activity of upstream MAPKKK TAK1 was lowered in the p38α-expressing cells. These results indicated a resistance of p38α-deficient cells to C3-mediated inhibition of cell growth. This anti-proliferative effect was highly associated with the decreased activity of c-Jun and upstream p38 and JNK MAPK signaling as a consequence of the absence of p38α in these cells.

C3 exoenzyme impairs cell proliferation and apoptosis by altering the activity of transcription factors

C3 exoenzyme from C. botulinum is an ADP-ribosyltransferase that inactivates selectively RhoA, B, and C by coupling an ADP-ribose moiety. Rho-GTPases are involved in various cellular processes, such as regulation of actin cytoskeleton, cell proliferation, and apoptosis. Previous studies of our group with the murine hippocampal cell line HT22 revealed a C3-mediated inhibition of cell proliferation after 48 h and a prevention of serum-starved cells from apoptosis. For both effects, alterations of various signaling pathways are already known, including also changes on the transcriptional level. Investigations on the transcriptional activity in HT22 cells treated with C3 for 48 h identified five out of 48 transcription factors namely Sp1, ATF2, E2F-1, CBF, and Stat6 with a significantly regulated activity. For validation of identified transcription factors, studies on the protein level of certain target genes were performed. Western blot analyses exhibited an enhanced abundance of Sp1 target genes p21 and COX-2 as well as an increase in phosphorylation of c-Jun. In contrast, the level of p53 and apoptosis-inducing GADD153, a target gene of ATF2, was decreased. Our results reveal that C3 regulates the transcriptional activity of Sp1 and ATF2 resulting downstream in an altered protein abundance of various target genes. As the affected proteins are involved in the regulation of cell proliferation and apoptosis, thus the C3-mediated anti-proliferative and anti-apoptotic effects are consequences of the Rho-dependent alterations of the activity of certain transcriptional factors.

Functional role of bone morphogenetic protein-4 in isolated canine parietal cells

Bone morphogenetic protein (BMP)-4 is an important regulator of cellular growth and differentiation. Expression of BMP-4 has been documented in the gastric mucosa. We reported that incubation of canine parietal cells with EGF for 72 h induced both parietal cell morphological transformation and inhibition of H(+)/K(+)-ATPase gene expression through MAPK-dependent mechanisms. We explored the role of BMP-4 in parietal cell maturation and differentiation. Moreover, we investigated if BMP-4 modulates the actions of EGF in parietal cells. H(+)/K(+)-ATPase gene expression was examined by Northern blots and quantitative RT-PCR. Acid production was assessed by measuring the uptake of [(14)C]aminopyrine. Parietal cell apoptosis was quantitated by Western blots with anti-cleaved caspase 3 antibodies and by counting the numbers of fragmented, propidium iodide-stained nuclei. MAPK activation and Smad1 phosphorylation were measured by Western blots with anti-phospho-MAPK and anti-phospho-Smad1 antibodies. Parietal cell morphology was examined by immunohistochemical staining of cells with anti-H(+)/K(+)-ATPase alpha-subunit antibodies. BMP-4 stimulated Smad1 phosphorylation and induced H(+)/K(+)-ATPase gene expression. BMP-4 attenuated EGF-mediated inhibition of H(+)/K(+)-ATPase gene expression and blocked EGF induction of both parietal cell morphological transformation and MAPK activation. Incubation of cells with BMP-4 enhanced histamine-stimulated [(14)C]aminopyrine uptake. BMP-4 had no effect on parietal cell apoptosis, whereas TGF-beta stimulated caspase-3 activation and nuclear fragmentation. In conclusion, BMP-4 promotes the induction and maintenance of a differentiated parietal cell phenotype. These findings may provide new clues for a better understanding of the mechanisms that regulate gastric epithelial cell growth and differentiation.

Phosphorylation of ezrin on threonine 567 produces a change in secretory phenotype and repolarizes the gastric parietal cell

Phosphorylation of the membrane-cytoskeleton linker protein ezrin has been functionally linked to acid secretion and vesicle recruitment to the apical secretory membrane in gastric parietal cells. Phosphorylation of the conserved T567 residue of ezrin has been shown to alter the N/C oligomerization of ezrin and promote the formation of actin-rich surface projections in other cells. To test the importance of T567 as a regulatory site for ezrin in parietal cell activation, we incorporated wild-type (WT) and mutant forms of ezrin, including the nonphosphorylatable T567A mutation and a mutant mimicking permanent phosphorylation, T567D. All ezrin constructs included C-terminal cyan-fluorescent protein (CFP) and were incorporated into adenoviral constructs for efficient introduction into cultured parietal cells from rabbit stomach. Fluorescence microscopy was used to localize CFP-ezrin and monitor morphological responses. Accumulation of a weak base (aminopyrine) was used to monitor receptor-mediated acid secretory response of the cultured cells. Similar to endogenous ezrin, WT and T567A CFP-ezrin localized heavily to apical membrane vacuoles with considerably lower levels associated with the surrounding basolateral membrane. Interestingly, H,K-ATPase within cytoplasmic tubulovesicles was incorporated into the apical vacuoles along with WT and T567A mutant ezrin. In these parietal cells secretagogue stimulation produced a striking vacuolar expansion associated with HCl secretion and the secretory phenotype. Expression of T567D CFP-ezrin was quite different, being rarely associated with apical vacuoles. T567D was more typically localized to the basolateral membrane, often associated with long spikes and fingerlike projections. Moreover, the cells did not display secretagogue-dependent morphological changes and, to our surprise, H,K-ATPase was recruited to the T567D CFP-ezrin-enriched basolateral projections. We conclude that T567 phosphorylation, which is probably regulated through Rho signaling pathway, may direct ezrin to membrane-cytoskeletal activity at the basolateral membrane and away from apical secretory activity. The large basolateral expansion is predicted to recruit membranes from sources not normally targeted to that surface.

Role of Rho in rabbit parietal cell

Rho is known as an important regulator of actin microfilament formation. We were led to study it because a dynamic rearrangement of actin filaments occurs during activation of gastric acid secretion. In order to use specific probes, the rabbit gastric gland culture system was employed and the various genes were expressed using adenovirus vector. When the constitutive active mutant of Rho (RhoAV14) was expressed, histamine- or carbachol-stimulated acid secretion monitored by (14)C-aminopyrine accumulation was inhibited. Conversely, expression of C3 toxin, the specific inhibitor of Rho, and expression of G(12/13)-specific regulator of G-protein signaling domain, the specific inhibitor of G(12/13) which is considered to be an upstream mediator of Rho, both potentiated acid secretion stimulated by the agonists. F-actin staining of parietal cell expressing RhoAV14 revealed that the microfilament supporting the intracellular canaliculi (not on the basolateral membrane) almost disappeared. No clear changes in the intracellular localization of Rho were observed during stimulation of parietal cell. In resting glands, the endogenous active form of Rho was relatively high, and it decreased during histamine stimulation. The finding that any treatment which inhibit Rho augment acid secretion whereas those that activate Rho inhibit secretion strongly suggests that the Rho-pathway conducts a negatively regulating signal in parietal cell activation, possibly via site-specific regulation of actin microfilaments.

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.

p38 mitogen-activated protein kinase inhibits calcium-dependent chloride secretion in T84 colonic epithelial cells

We have previously shown that Ca(2+)-dependent Cl(-) secretion across intestinal epithelial cells is limited by a signaling pathway involving transactivation of the epidermal growth factor receptor (EGFR) and activation of ERK mitogen-activated protein kinase (MAPK). Here, we have investigated a possible role for p38 MAPK in regulation of Ca(2+)-dependent Cl(-) secretion. Western blot analysis of T(84) colonic epithelial cells revealed that the muscarinic agonist carbachol (CCh; 100 microM) stimulated phosphorylation and activation of p38 MAPK. The p38 inhibitor SB-203580 (10 microM) potentiated and prolonged short-circuit current (I(sc)) responses to CCh across voltage-clamped T(84) cells to 157.4 +/- 6.9% of those in control cells (n = 21; P < 0.001). CCh-induced p38 phosphorylation was attenuated by the EGFR inhibitor tyrphostin AG-1478 (0.1 nM-10 microM) and by the Src family kinase inhibitor PP2 (20 nM-2 microM). The effects of CCh on p38 phosphorylation were mimicked by thapsigargin (TG; 2 microM), which specifically elevates intracellular Ca(2+), and were abolished by the Ca(2+) chelator BAPTA-AM (20 microM), implying a role for intracellular Ca(2+) in mediating p38 activation. SB-203580 (10 microM) potentiated I(sc) responses to TG to 172.4 +/- 18.1% of those in control cells (n = 18; P < 0.001). When cells were pretreated with SB-203580 and PD-98059 to simultaneously inhibit p38 and ERK MAPKs, respectively, I(sc) responses to TG and CCh were significantly greater than those observed with either inhibitor alone. We conclude that Ca(2+)-dependent agonists stimulate p38 MAPK in T(84) cells by a mechanism involving intracellular Ca(2+), Src family kinases, and the EGFR. CCh-stimulated p38 activation constitutes a similar, but distinct and complementary, antisecretory signaling pathway to that of ERK MAPK.

A novel regulatory mechanism of the mitochondrial Ca2+ uniporter revealed by the p38 mitogen-activated protein kinase inhibitor SB202190

It is widely acknowledged that mitochondrial Ca2+ uptake modulates the cytosolic [Ca2+] ([Ca2+]c) acting as a transient Ca2+ buffer. In addition, mitochondrial [Ca2+] ([Ca2+]M) regulates the rate of respiration and may trigger opening of the permeability transition pore and start apoptosis. However, no mechanism for the physiological regulation of mitochondrial Ca2+ uptake has been described. We show here that SB202190, an inhibitor of p38 mitogen-activated protein (MAP) kinase, strongly stimulates ruthenium red-sensitive mitochondrial Ca2+ uptake, both in intact and in permeabilized HeLa cells. The [Ca2+]M peak induced by agonists was increased about fourfold in the presence of the inhibitor, with a concomitant reduction in the [Ca2+]c peak. The stimulation occurred fast and was rapidly reversible. In addition, experiments in permeabilized cells perfused with controlled [Ca2+] showed that SB202190 stimulated mitochondrial Ca2+ uptake by more than 10-fold, but only in the physiological [Ca2+]c range (1-4 mM). Other structurally related p38 MAP kinase inhibitors (SB203580, PD169316, or SB220025) produced little or no effect. Our data suggest that in HeLa cells, a protein kinase sensitive to SB202190 tonically inhibits the mitochondrial Ca2+ uniporter. This novel regulatory mechanism may be of paramount importance to modulate mitochondrial Ca2+ uptake under different physiopathological conditions.

Regulation and function of COX-2 gene expression in isolated gastric parietal cells

We examined expression, function, and regulation of the cyclooxygenase (COX)-2 gene in gastric parietal cells. COX-2-specific mRNA was isolated from purified (>95%) canine gastric parietal cells in primary culture and measured by Northern blots using a human COX-2 cDNA probe. Carbachol was the most potent inducer of COX-2 gene expression. Gastrin and histamine exhibited minor stimulatory effects. Carbachol-stimulated expression was inhibited by intracellular Ca(2+) chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM (90%), protein kinase C (PKC) inhibitor GF-109203X (48%), and p38 kinase inhibitor SB-203580 (48%). Nuclear factor (NF)-kappaB inhibitor 1-pyrrolidinecarbodithioic acid inhibited carbachol-stimulated expression by 80%. Similar results were observed in the presence of adenoviral vector Ad.dom.neg.IkappaB, which expresses a repressor of NF-kappaB. Addition of SB-203580 with Ad.dom.neg.IkappaB almost completely blocked carbachol stimulation of COX-2 gene expression. We examined the effect of carbachol on PGE(2) release by enzyme-linked immunoassay. Carbachol induced PGE(2) release. Ad.dom.neg.IkappaB, alone or with SB-203580, produced, respectively, partial (70%) and almost complete (>80%) inhibition of carbachol-stimulated PGE(2) production. Selective COX-2 inhibitor NS-398 blocked carbachol-stimulated PGE(2) release without affecting basal PGE(2) production. In contrast, indomethacin inhibited both basal and carbachol-stimulated PGE(2) release. Carbachol induces COX-2 gene expression in the parietal cells through signaling pathways that involve intracellular Ca(2+), PKC, p38 kinase, and activation of NF-kappaB. The functional significance of these effects seems to be stimulation of PGE(2) release.

p38 MAP kinase modulates liver cell volume through inhibition of membrane Na+ permeability

In hepatocytes, Na+ influx through nonselective cation (NSC) channels represents a key point for regulation of cell volume. Under basal conditions, channels are closed, but both physiologic and pathologic stimuli lead to a large increase in Na+ and water influx. Since osmotic stimuli also activate mitogen-activated protein (MAP) kinase pathways, we have examined regulation of Na+ permeability and cell volume by MAP kinases in an HTC liver cell model. Under isotonic conditions, there was constitutive activity of p38 MAP kinase that was selectively inhibited by SB203580. Decreases in cell volume caused by hypertonic exposure had no effect on p38, but increases in cell volume caused by hypotonic exposure increased p38 activity tenfold. Na+ currents were small when cells were in isotonic media but could be increased by inhibiting constitutive p38 MAP kinase, thereby increasing cell volume. To evaluate the potential inhibitory role of p38 more directly, cells were dialyzed with recombinant p38alpha and its upstream activator, MEK-6, which substantially inhibited volume-sensitive currents. These findings indicate that constitutive p38 activity contributes to the low Na+ permeability necessary for maintenance of cell volume, and that recombinant p38 negatively modulates the set point for volume-sensitive channel opening. Thus, functional interactions between p38 MAP kinase and ion channels may represent an important target for modifying volume-sensitive liver functions.

The antihyperglycemic drug alpha-lipoic acid stimulates glucose uptake via both GLUT4 translocation and GLUT4 activation: potential role of p38 mitogen-activated protein kinase in GLUT4 activation

The cofactor of mitochondrial dehydrogenase complexes and potent antioxidant alpha-lipoic acid has been shown to lower blood glucose in diabetic animals. alpha-Lipoic acid enhances glucose uptake and GLUT1 and GLUT4 translocation in 3T3-L1 adipocytes and L6 myotubes, mimicking insulin action. In both cell types, insulin-stimulated glucose uptake is reduced by inhibitors of p38 mitogen-activated protein kinase (MAPK). Here we explore the effect of alpha-lipoic acid on p38 MAPK, phosphatidylinositol (PI) 3-kinase, and Akt1 in L6 myotubes. alpha-Lipoic acid (2.5 mmol/l) increased PI 3-kinase activity (31-fold) and Akt1 (4.9-fold). Both activities were inhibited by 100 nmol/l wortmannin. alpha-Lipoic acid also stimulated p38 MAPK phosphorylation by twofold within 10 min. The phosphorylation persisted for at least 30 min. Like insulin, alpha-lipoic acid increased the kinase activity of the alpha (2.8-fold) and beta (2.1-fold) isoforms of p38 MAPK, measured by an in vitro kinase assay. Treating cells with 10 micromol/l of the p38 MAPK inhibitors SB202190 or SB203580 reduced the alpha-lipoic acid-induced stimulation of glucose uptake by 66 and 55%, respectively. In contrast, SB202474, a structural analog that does not inhibit p38 MAPK, was without effect on glucose uptake. In contrast to 2-deoxyglucose uptake, translocation of GLUT4myc to the cell surface by either alpha-lipoic acid or insulin was unaffected by 20 micromol/l of SB202190 or SB203580. The results suggest that inhibition of 2-deoxyglucose uptake in response to alpha-lipoic acid by inhibitors of p38 MAPK is independent of an effect on GLUT4 translocation. Instead, it is likely that regulation of transporter activity is sensitive to these inhibitors.

Molecular mechanisms for the antiapoptotic action of gastrin

Gastrin (G17) has a CCK-B receptor-mediated growth-promoting effect on the AR42J rat acinar cell line. We examined whether G17 inhibits apoptosis induced by serum withdrawal of AR42J cells and CHO-K1 cells stably expressing CCK-B receptors (CHO-K1/CCK-B cells). Cellular apoptosis was measured by flow cytometry and the terminal deoxynucleotidyltransferase-mediated dUTP-FITC nick end-labeling method. Serum withdrawal induced AR42J and CHO-K1/CCK-B cell apoptosis. Addition of 10 nM G17 reversed these effects. We examined the action of G17 (10 nM) on phosphorylation and activation of protein kinase B/Akt, a kinase known to promote cell survival. Akt phosphorylation and activation were measured by kinase assays and Western blots with an anti-phospho-Akt antibody. G17 stimulated Akt phosphorylation and activation. G17 induction of Akt phosphorylation was inhibited by the phosphoinositide 3-kinase (PI 3-kinase) inhibitors LY-294002 (10 microM) and wortmannin (200 nM) but not by the mitogen-activated protein kinase kinase 1 inhibitor PD-98059 (50 microM). To study the role of p38 kinase in G17 signaling to Akt, we examined the effect of G17 on p38 kinase activation and phosphorylation using kinase assays and Western blots with an anti-phospho-p38 kinase antibody. G17 induced p38 kinase activity at doses and with kinetics similar to those observed for Akt induction. The p38 kinase inhibitor SB-203580 inhibited G17 induction of Akt phosphorylation and activation at a concentration (10 microM) 10-fold higher than necessary to block p38 kinase (1 microM), suggesting the possible involvement of kinase activities other than p38 kinase. Transduction of AR42J cells with the adenoviral vector Adeno-dn Akt, which overexpresses an inhibitor of Akt, reversed the antiapoptotic action of G17. In conclusion, G17 promotes AR42J cell survival through the induction of Akt via PI 3-kinase and SB-203580-sensitive kinase activities.

Gastric secretion

This article summarizes data published during the past year that improve our understanding of the mechanisms by which various neurotransmitters, paracrine agents, and hormones regulate gastric acid secretion and are themselves regulated. The main stimulants of acid secretion are histamine, gastrin, and acetylcholine. The main inhibitor is somatostatin, which exerts a tonic restraint on parietal, enterochromaffin-like (ECL), and gastrin cells. Histamine, released from ECL cells, stimulates the parietal cell directly via H(2) receptors and indirectly via H(3) receptors coupled to inhibition of somatostatin secretion. Gastrin, acting via gastrin/cholecystokinin-B (CCK-B), now termed CCK(2), receptors on ECL cells activates histidine decarboxylase, releases histamine, and induces ECL hypertrophy and hyperplasia. The latter might be responsible for the rebound hyperacidity observed after withdrawal of long-term antisecretory therapy. The neurotransmitter pituitary adenylate cyclase-activating polypeptide stimulates histamine secretion from isolated ECL cells, but its physiologic role, if any, is not known. Acetylcholine, released from gastric postganglionic intramural neurons, stimulates the parietal cell directly via muscarinic M(3) receptors and indirectly by inhibiting somatostatin secretion. Although infection with H. pylori is associated with increased basal and stimulated acid outputs in patients with duodenal ulcer, most people infected with the organism are asymptomatic and have pangastritis with decreased acid output. In the latter, eradication of the bacterium leads to an increase in gastric acidity and is associated with a two-to threefold increase in gastroesophageal reflux.