Modulation of acid secretion by phosphoinositide 3‐kinase in gastric parietal cells

The role of phosphoinositide 3‐kinase (PI3K) in stimulating acid secretion by parietal cells has been examined. Previous studies demonstrated that gastric glands treated with histamine (H) + the PI3K inhibitor, LY294002 (H+LY), had enhanced secretory response and cAMP levels compared to glands stimulated by H alone. Here we examine mechanisms by which LY, and inhibition of PI3K, might potentiate acid secretion. LY did not potentiate stimulation by forskolin, IBMX, or dbcAMP. The difference between the action of LY on histamine vs forskolin, IBMX, or dbcAMP is the deployment of the G‐protein coupled histamine receptor (GPCR). Thus we propose that PI3K is activated during parietal cell stimulation via the GPCR and that PI3K modulates an inhibitory pathway. Knowing that PKB (also known as AKT) is a common downstream effector of PI3K we examined the role of PKB. Western blot showed i) PKB was present in cultured gastric glands, ii) PKB was phosphorylated at Ser473, and iii) phosphorylation of Ser473 was diminished by LY. Wild‐type Akt (WT‐AKT), dominant negative Akt (DN‐AKT) and a constitutively active Akt (Myr‐AKT) were expressed (kind gifts from K. Walsh, Mol. Cell Biol., 7:, 1999). Treatment of both WT and DN‐AKT infected glands with H+LY showed diminished phosphorylation of Ser473. Decreased phosphorylation was not observed in glands infected with Myr‐AKT. AP assays revealed increased acid secretion in glands infected with DN‐AKT. Taken together we propose the histamine GPCR activates PI3K which leads to activation of PKB. Active PKB is then able to alter PDE activity and lower the intracellular [cAMP]. Support by NIH.