Inactivation and dephosphorylation of protein kinase Bα (PKBα) promoted by hyperosmotic stress

To study the role of protein kinase B (PKB) in response to cellular stress, we examined PKB alpha activity following different stress treatments. Hyperosmotic but not chemical stress resulted in inactivation of PKB alpha and prevented activation by pervanadate and mitogens. Hyperosmotic shock did not affect the MAP kinase pathway, suggesting that this inhibitory effect was specific for PKB, Our data further indicate that down-regulation occurs via dephosphorylation of Thr308 and Ser473, the major regulatory phosphorylation sites of PKB alpha, Indeed, calyculin A, which inhibits protein phosphatases 1 and 2A, effectively blocked hyperosmotic stress-mediated inactivation (dephosphorylation) of PKB alpha, High osmolarity did not affect phosphatidylinositol 3-kinase activity but led to a marked increase in PI(3,4,5)P-3 and a decrease in PI(3,4)P-2 formation after pervanadate stimulation, suggesting that hyperosmotic stress has an inhibitory effect on a phosphatidylinositol 5-phosphatase which converts PI(3,4,5)P-3 into PI(3,4)P-2. Immunofluorescence studies revealed that membrane translocation, a prerequisite for PKB activation, was not affected by hyperosmotic stress, Our results indicate that hyperosmotic stress can act at two levels: (i) inhibition of phosphorylation of Thr308 and Ser473 by upstream kinases and (ii) by promoting rapid dephosphorylation of these regulatory sites.