AMP-activated protein kinase: a physiological off switch for murine gastric acid secretion

AMPK activation promotes gastroprotection through mutual interaction with the gaseous mediators H2S, NO, and CO

Activation of 5' adenosine monophosphate-activated protein kinase (AMPK) stimulates production of the gaseous mediators nitric oxide (NO) and carbon monoxide (CO), which are involved in mucosal defense and gastroprotection. As AMPK itself has gastroprotective effects against several gastric ulcer etiologies, in the present study, we aimed to elucidate whether AMPK may also prevent ethanol-induced injury and play a key role in the associated gastroprotection mediated by hydrogen sulfide (H₂S), NO, and CO. Mice were pretreated with AICAR (20 mg/kg, an AMPK activator) alone or with 50% ethanol. Other groups were pretreated with respective gaseous mediator inhibitors PAG, l-NAME, or ZnPP IX 30 min prior to AICAR, or with gaseous mediator donors NaHS, Lawesson's reagent and l-cysteine (H₂S), SNP, l-Arginine (NO), Hemin, or CORM-2 (CO) 30 min prior to ethanol with or without compound C (10 mg/kg, a non-selective AMPK inhibitor). H₂S, nitrate/nitrite (NO₃^-/NO₂^-), bilirubin levels, GSH and MDA concentration were evaluated in the gastric mucosa. The gastric mucosa was also collected for histopathological analysis and AMPK expression assessment by immunohistochemistry. Pretreatment with AICAR attenuated the ethanol-induced injury and increased H₂S and bilirubin levels but not NO₃^-/NO₂^- levels in the gastric mucosa. In addition, inhibition of H₂S, NO, or CO synthesis exacerbated the ethanol-induced gastric damage and inhibited the gastroprotection by AICAR. Pretreatment with compound C reversed the gastroprotective effect of NaHS, Lawesson's reagent, l-cysteine, SNP, l-Arginine, CORM-2, or Hemin. Compound C also reversed the effect of NaHS on H₂S production, SNP on NO₃^-/NO₂^- levels, and Hemin on bilirubin levels. Immunohistochemistry revealed that AMPK is present at basal levels mainly in the gastric mucosa cells, and was increased by pretreatment with NaHS, SNP, and CORM-2. In conclusion, our findings indicate that AMPK activation exerts gastroprotection against ethanol-induced gastric damage and mutually interacts with H₂S, NO, or CO to facilitate this process.

Regulation of ion channels and transporters by AMP-activated kinase (AMPK)

The energy-sensing AMP-activated kinase AMPK ensures survival of energy-depleted cells by stimulating ATP production and limiting ATP utilization. Both energy production and energy consumption are profoundly influenced by transport processes across the cell membane including channels, carriers and pumps. Accordingly, AMPK is a powerful regulator of transport across the cell membrane. AMPK regulates diverse K(+) channels, Na(+) channels, Ca(2+) release activated Ca(2+) channels, Cl(-) channels, gap junctional channels, glucose carriers, Na(+)/H(+)-exchanger, monocarboxylate-, phosphate-, creatine-, amino acid-, peptide- and osmolyte-transporters, Na(+)/Ca(2+)-exchanger, H(+)-ATPase and Na(+)/K(+)-ATPase. AMPK activates ubiquitin ligase Nedd4-2, which labels several plasma membrane proteins for degradation. AMPK further regulates transport proteins by inhibition of Rab GTPase activating protein (GAP) TBC1D1. It stimulates phosphatidylinositol 3-phosphate 5-kinase PIKfyve and inhibits phosphatase and tensin homolog (PTEN) via glycogen synthase kinase 3β (GSK3β). Moreover, it stabilizes F-actin as well as downregulates transcription factor NF-κB. All those cellular effects serve to regulate transport proteins.

Activation of AMPK inhibits cholera toxin stimulated chloride secretion in human and murine intestine

Increased intestinal chloride secretion through chloride channels, such as the cystic fibrosis transmembrane conductance regulator (CFTR), is one of the major molecular mechanisms underlying enterotoxigenic diarrhea. It has been demonstrated in the past that the intracellular energy sensing kinase, the AMP-activated protein kinase (AMPK), can inhibit CFTR opening. We hypothesized that pharmacological activation of AMPK can abrogate the increased chloride flux through CFTR occurring during cholera toxin (CTX) mediated diarrhea.

Chloride efflux was measured in isolated rat colonic crypts using real-time fluorescence imaging. AICAR and metformin were used to activate AMPK in the presence of the secretagogues CTX or forskolin (FSK). In order to substantiate our findings on the whole tissue level, short-circuit current (SCC) was monitored in human and murine colonic mucosa using Ussing chambers. Furthermore, fluid accumulation was measured in excised intestinal loops.

CTX and forskolin (FSK) significantly increased chloride efflux in isolated colonic crypts. The increase in chloride efflux could be offset by using the AMPK activators AICAR and metformin. In human and mouse mucosal sheets, CTX and FSK increased SCC. AICAR and metformin inhibited the secretagogue induced rise in SCC, thereby confirming the findings made in isolated crypts. Moreover, AICAR decreased CTX stimulated fluid accumulation in excised intestinal segments.

The present study suggests that pharmacological activation of AMPK effectively reduces CTX mediated increases in intestinal chloride secretion, which is a key factor for intestinal water accumulation. AMPK activators may therefore represent a supplemental treatment strategy for acute diarrheal illness.

Regulation of Orai1/STIM1 by the kinases SGK1 and AMPK

STIM and Orai isoforms orchestrate store operated Ca2+ entry (SOCE) and thus cytosolic Ca2+ fluctuations following stimulation by hormones, growth factors and further mediators. Orai1 is a target of Nedd4-2, an ubiquitin ligase preparing several plasma membrane proteins for degradation. Phosphorylation of Nedd4-2 by the serum and glucocorticoid inducible kinase SGK1 leads to the binding of Nedd4-2 to the protein 14-3-3 thus preventing its interaction with Orai1. Nedd4-2 is activated by the energy sensing AMP activated kinase AMPK. Thus, SGK1 disrupts and AMPK fosters degradation of Orai1. New synthesis of both, Orai1 and STIM1, is stimulated by the transcription factor NF-κB (nuclear factor kappa B), which binds to the respective promoter regions of the genes encoding STIM1 and Orai1. SGK1 upregulates and AMPK presumably downregulates NF-κB and thus de novo synthesis of Orai1 and STIM1 proteins. The regulation by SGK1 links SOCE to the signaling of a wide variety of hormones and growth factors, the AMPK dependent regulation of Orai1 and STIM1 may serve to limit inadequate activation of SOCE following energy depletion, which is otherwise expected to activate SOCE by depletion of intracellular Ca2+ stores due to impairment of the ATP consuming sarco/endoplasmatic reticulum Ca2+ ATPase SERCA.

Enhanced Ca²⁺ entry and Na+/Ca²⁺ exchanger activity in dendritic cells from AMP-activated protein kinase-deficient mice

In dendritic cells (DCs), chemotactic chemokines, such as CXCL12, rapidly increase cytosolic Ca(2+)concentrations ([Ca(2+)](i)) by triggering Ca(2+) release from intracellular stores followed by store-operated Ca(2+) (SOC) entry. Increase of [Ca(2+)](i) is blunted and terminated by Ca(2+) extrusion, accomplished by K(+)-independent Na(+)/Ca(2+) exchangers (NCXs) and K(+)-dependent Na(+)/Ca(2+) exchangers (NCKXs). Increased [Ca(2+)](i) activates energy-sensing AMP-activated protein kinase (AMPK), which suppresses proinflammatory responses of DCs and macrophages. The present study explored whether AMPK participates in the regulation of DC [Ca(2+)](i) and migration. DCs were isolated from AMPKα1-deficient (ampk(-/-)) mice and, as control, from their wild-type (ampk(+/+)) littermates. AMPKα1, Orai1-2, STIM1-2, and mitochondrial calcium uniporter protein expression was determined by Western blotting, [Ca(2+)](i) by Fura-2 fluorescence, SOC entry by inhibition of endosomal Ca(2+) ATPase with thapsigargin (1 μM), Na(+)/Ca(2+) exchanger activity from increase of [Ca(2+)](i), and respective whole-cell current in patch clamp following removal of extracellular Na(+). Migration was quantified utilizing transwell chambers. AMPKα1 protein is expressed in ampk(+/+) DCs but not in ampk(-/-) DCs. CXCL12 (300 ng/ml)-induced increase of [Ca(2+)](i), SOC entry, Orai 1 protein abundance, NCX, and NCKX were all significantly higher in ampk(-/-) DCs than in ampk(+/+) DCs. NCX and NCKX currents were similarly increased in ampk(-/-) DCs. Moreover, CXCL12 (50 ng/ml)-induced DC migration was enhanced in ampk(-/-) DCs. AMPK thus inhibits SOC entry, Na(+)/Ca(2+) exchangers, and migration of DCs.

Role of activation of 5'-adenosine monophosphate-activated protein kinase in gastric ulcer healing in diabetic rats

BACKGROUND

The potential utility of 5'-adenosine monophosphate-activated protein kinase (AMPK)-activating agents, such as metformin, in inducing angiogenesis, could be a promising approach to promote healing of gastric ulcers complicated by diabetes mellitus. The aim of the present study was to assess the effect of a drug that activates AMPK, namely metformin, in gastric ulcer healing in streptozotocin-induced diabetic rats.

METHODS

Forty male Wistar albino rats were made diabetic by intraperitoneal (i.p.) streptozotocin injection and 10 rats were injected i.p. by a single dose of physiological saline. Six weeks following streptozotocin or saline injection, gastric ulcers were induced by serosal application of acetic acid. Three days after acetic acid application, rats were divided into group 1 (nondiabetic control), group 2 (streptozotocin-injected rats), groups 3-5 (streptozotocin-injected rats treated with metformin or metformin and an inhibitor of AMPK, namely compound C or pioglitazone) for 7 days following acetic acid application.

RESULTS

Administration of metformin, but not pioglitazone, resulted in a significant decrease in the gastric ulcer area, a significant increase in epithelial regeneration assessed histologically, a significant increase in the number of microvessels in the ulcer margin, a significant increase in gastric vascular endothelial growth factor concentration and gastric von Willebrand factor as well as a significant increase in gastric phospho-AMPK. Compound C, an inhibitor of AMPK, blocked metformin-induced changes in assessed parameters suggesting that the effect of metformin was mediated mainly through activation of AMPK.

CONCLUSION

Our results suggest the feasibility of a novel treatment strategy, namely drugs activating AMPK, for patients in whom impairment of ulcer healing constitutes a secondary complication of diabetes mellitus.

Ca2+ activated K+ channel Kca3.1 as a determinant of gastric acid secretion

The Ca(2+) activated K(+) channel K(ca)3.1 is expressed in a variety of tissues. In the gastric gland it is expressed in the basolateral cell membrane. To determine the functional significance of K(ca)3.1 activity for gastric acid secretion, gastric acid secretion was determined in isolated glands from gene targeted mice lacking functional K(ca)3.1 (K(ca)3.1(-/-)) and from their wild type littermates (K(ca)3.1(+/+)). According to BCECF-fluorescence cytosolic pH in isolated gastric glands was similar in K(ca)3.1(-/-) and K(ca)3.1(+/+) mice. Na(ca)-independent pH recovery (ΔpH/min) following an ammonium pulse, a measure of H(ca)/K(ca) ATPase activity, was, however, significantly faster in K(ca)3.1(-/-) than in K(ca)3.1(+/+) mice. Accordingly, the luminal pH was significantly lower and the acid content significantly higher in K(ca)3.1(-/-) than in K(ca)3.1(+/+) mice. The abundance of mRNA encoding H(ca)/K(ca) ATPase and KCNQ1 was similar in both genotypes. Increase of extracellular K(ca) concentrations to 35 mM (replacing Na(ca)/NMDG) and treatment with histamine (100 μM) significantly increased ΔpH/min to a larger extent in K(ca)3.1(+/+) than in K(ca)3.1(-/-) mice and dissipated the differences between the genotypes. Carbachol (100 μM) increased ΔpH/min in both genotypes but did not abolish the difference between K(ca)3.1(-/-) and K(ca)3.1(+/+) mice. In K(ca)3.1(+/+) mice the K(ca)3.1 opener DCEBIO (100 μM) did not significantly alter basal ΔpH/min but significantly blunted ΔpH/min in the presence of carbachol. In conclusion, K(ca)3.1 activity suppresses carbachol stimulated gastric acid secretion.

Regulation of gastric acid secretion by the serum and glucocorticoid inducible kinase isoform SGK3

BACKGROUND

The serum and glucocorticoid inducible kinase isoform SGK3 is ubiquitously expressed and has been shown to participate in the regulation of cell survival and transport. Similar to SGK1 and protein kinase B (PKB/Akt) isoforms, SGK3 may phosphorylate glycogen synthase kinase (GSK) 3α,β, which has recently been shown to participate in the regulation of basal gastric acid secretion. The present study thus explored the role of SGK3 in the regulation of gastric acid secretion.

METHODS

Experiments were performed in isolated glands from gene-targeted mice lacking functional SGK3 (sgk3-/-) or from their wild-type littermates (sgk3+/+). Utilizing 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, acetoxymethyl ester (BCECF) fluorescence, gastric acid secretion was determined from Na(+)-independent pH recovery (∆pH/min) following an ammonium pulse, which reflects H+/K+ adenosine triphosphatase (ATP) ase activity.

RESULTS

Cytosolic pH in isolated gastric glands was similar in sgk3-/- and sgk3+/+ mice. ∆pH/min was, however, significantly larger in sgk3-/- than in sgk3+/+ mice. In both genotypes, ∆pH/min was virtually abolished in the presence of the H(+)/K(+) ATPase inhibitor omeprazole (100 μM) and SCH28080 (500 nM). Increase of extracellular K+ concentrations to 35 mM (replacing Na+/NMDG) or treatment with 5 μM forskolin increased ∆pH/min in sgk3+/+ mice to a larger extent than in sgk3-/- mice and abrogated the differences between genotypes. The protein kinase A inhibitor H89 (150 nM) decreased ∆pH/min to similarly low values in both genotypes.

CONCLUSIONS

SGK3 suppresses gastric acid secretion, an effect presumably mediated by the stimulation of protein kinase A with the subsequent activation of K+ channels.

Update on the mechanisms of gastric acid secretion

Acid-related disorders represent a major healthcare concern. In recent years, our understanding of the physiologic processes underlying gastric acid secretion has improved notably. The identity of several apical ion transport proteins, which are necessary for acid secretion to take place, has been resolved. The recent developments have uncovered potential therapeutic targets for the treatment of acid-related disorders. This brief review provides an update on the mechanisms of gastric acid secretion, with a particular focus on apical ion transport.

Gastric secretion

PURPOSE OF REVIEW

This review summarizes the past year's literature regarding the regulation of gastric exocrine and endocrine secretion at the central, peripheral, and cellular levels.

RECENT FINDINGS

Gastric acid secretion is an intricate and dynamic process that is regulated by neural (efferent and afferent), hormonal (e.g., gastrin), and paracrine (e.g., histamine, ghrelin, somatostatin) pathways as well as mechanical (e.g., distension) and chemical (e.g., protein, glutamate, coffee, and ethanol) stimuli. Secretion of hydrochloric acid by the parietal cell involves recruitment and fusion of HK-adenosine triphosphatase (HK-ATPase)-containing cytoplasmic tubulovesicles with the apical membrane with subsequent electroneutral transport of hydronium ions in exchange for potassium; the source of the latter is the potassium channel, KCNQ1. Concomitantly, chloride exits via the cystic fibrosis transmembrane regulator. Inhibition of the HK-ATPase by proton pump inhibitors leads to a compensatory hypergastrinemia which, if prolonged, results in parietal and enterochromaffin-like cell hyperplasia. The clinical consequence is rebound acid secretion which may induce dyspeptic symptoms in healthy individuals and exacerbate reflux symptoms in patients with gastroesophageal reflux disease.

SUMMARY

We continue to make progress in our understanding of the regulation of gastric acid secretion in health and disease. A better understanding of the pathways and mechanisms regulating acid secretion should lead to improved management of patients with acid-induced disorders as well as those who secrete too little acid.

Regulation of gastric acid secretion by PKB/Akt2

Pharmacological inhibition of phosphoinositol 3 kinase (PI3K) and partial deficiency of phosphoinositide dependent kinase PDK1 have previously been shown to enhance basal gastric acid secretion. PI3K/PDK1 dependent signaling involves activation of protein kinase B/Akt, which may thus be similarly involved in the regulation of gastric acid secretion. To test that hypothesis, gastric acid secretion was determined in isolated glands from gene targeted mice lacking functional Akt2 (akt2(-/-)) or from their wild type littermates (akt2(+/+)). According to BCECF-fluorescence cytosolic pH in isolated gastric glands was similar in akt2(-/-) and akt2(+/+) mice. Na(+)-independent pH recovery (DeltapH/min) following an ammonium pulse, a measure of H(+)/K(+) ATPase activity, was, however, significantly faster in akt2(-/-) than in akt2(+/+) mice. In both genotypes, DeltapH/min was virtually abolished by H(+)/K(+) ATPase inhibitor omeprazole (100 muM). Increase of extracellular K(+) concentrations to 35 mM (replacing Na(+)) increased DeltapH/min to a significantly larger extent in akt2(+/+) than in akt2(-/-) mice and dissipated the differences between the genotypes. Similarly, treatment with 5 muM forskolin enhanced DeltapH/min significantly only in akt2(+/+) mice and abolished the differences between the genotypes. Conversely, protein kinase A inhibitor H89 (50 nM) decreased DeltapH/min to similarly low values in both genotypes. In conclusion, Akt2 suppresses gastric acid secretion and contributes to or even accounts for the inhibition of gastric acid secretion by PI3K.

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.