Regulation of basal gastric acid secretion by the glycogen synthase kinase GSK3.
BACKGROUND
According to previous observations, basal gastric acid secretion is downregulated by phosphoinositol-3-(PI3)-kinase, phosphoinositide-dependent kinase (PDK1), and protein kinase B (PKBβ/Akt2) signaling. PKB/Akt phosphorylates glycogen synthase kinase GSK3. The present study explored whether PKB/Akt-dependent GSK3-phosphorylation modifies gastric acid secretion.


METHODS
Utilizing 2',7'-bis-(carboxyethyl)-5(6')-carboxyfluorescein (BCECF)-fluorescence, basal gastric acid secretion was determined from Na(+)-independent pH recovery (∆pH/min) following an ammonium pulse, which reflects H(+)/K(+)-ATPase activity. Experiments were performed in gastric glands from gene-targeted mice (gsk3 ( KI )) with PKB/serum and glucocorticoid-inducible kinase (SGK)-insensitive GSKα,β, in which the serines within the PKB/SGK phosphorylation site were replaced by alanine (GSK3α(21A/21A), GSK3β(9A/9A)).


RESULTS
The cytosolic pH in isolated gastric glands was similar in gsk3 ( KI ) and their wild-type littermates (gsk3 ( WT )). However, ∆pH/min was significantly larger in gsk3 ( KI ) than in gsk3 ( WT ) mice and ∆pH/min was virtually abolished by the H(+)/K(+)-ATPase inhibitor omeprazole (100 μM) in gastric glands from both gsk3 ( KI ) and gsk3 ( WT ). Plasma gastrin levels were lower in gsk3 ( KI ) than in gsk3 ( WT ). Both, an increase of extracellular K(+) concentration to 35 mM [replacing Na(+)/N-methyl-D: -glucamine (NMDG)] and treatment with forskolin (5 μM), significantly increased ∆pH/min to virtually the same value in both genotypes. The protein kinase A (PKA) inhibitor H89 (150 nM) and the H(2)-receptor antagonist ranitidine (100 μM) decreased ∆pH/min in gsk3 ( KI ) but not gsk3 ( WT ) and again abrogated the differences between the genotypes. The protein abundance of phosphorylated but not of total PKA was significantly larger in gsk3 ( KI ) than in gsk3 ( WT ).


CONCLUSIONS
Basal gastric acid secretion is enhanced by the disruption of PKB/SGK-dependent phosphorylation and the inhibition of GSK3. Thus, the inhibition of GSK3 participates in the signaling of PI3-kinase-dependent downregulation of basal gastric acid secretion.
Rotte
Anand
A
Department of Physiology, University of Tübingen, Gmelinstrasse 5, Tübingen, Germany.
Pasham
Venkanna
V
Eichenmüller
Melanie
M
Yang
Wenting
W
Qadri
Syed M
SM
Bhandaru
Madhuri
M
Lang
Florian
F
D000818:Animals; D003600:Cytosol / Q000378:metabolism; D015536:Down-Regulation; D005260:Female; D005744:Gastric Acid / Q000378:metabolism*; D005753:Gastric Mucosa / Q000378:metabolism; D055879:Gene Knock-In Techniques; D005838:Genotype; D038362:Glycogen Synthase Kinase 3 / Q000378:metabolism*; D000071679:Glycogen Synthase Kinase 3 beta; D006863:Hydrogen-Ion Concentration; D017874:Immediate-Early Proteins / Q000378:metabolism*; D008297:Male; D051379:Mice; D019869:Phosphatidylinositol 3-Kinases / Q000378:metabolism; D010766:Phosphorylation; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism*; D051057:Proto-Oncogene Proteins c-akt / Q000378:metabolism*; D015398:Signal Transduction; D000099184:Serum-Glucocorticoid Regulated Kinases
D038362:Glycogen Synthase Kinase 3; D000071679:Glycogen Synthase Kinase 3 beta; D017874:Immediate-Early Proteins; D019869:Phosphatidylinositol 3-Kinases; D017346:Protein Serine-Threonine Kinases; D051057:Proto-Oncogene Proteins c-akt; D000099184:Serum-Glucocorticoid Regulated Kinases; C000605507:Gsk3b protein, mouse; C448021:glycogen synthase kinase 3 alpha