Okadaic acid, a serine/threonine phosphatase inhibitor, induces tyrosine dephosphorylation/inactivation of protein kinase F_A/GSK-3α in A431 cells

The signal transduction mechanism of protein kinase F_A/GSK-3α by tyrosine phosphorylation in A431 cells was investigated. Kinase F_A/GSK-3α was found to exist in a highly tyrosine-phosphorylated/activated state in resting cells but could be tyrosine-dephosphorylated and inactivated down to less than 15% of control values in a concentration-dependent manner by 50-400 nM okadaic acid (a specific inhibitor of protein phosphatase types 1 and 2A), as demonstrated by metabolic ³²P labeling the cells, followed by immunoprecipitation and two-dimensional phosphoamino acid analysis and by immunodetection in an anti-kinase F_A/GSK-3α immunoprecipitate kinase assay. Taken together, the results provide initial evidence that serine/threonine phosphatase(s) may play a role involved in the modulation of kinase F_A/GSK-3α activity in cells, suggesting an involvement of serine/ threonine dephosphorylation in the modulation of tyrosine phosphorylation and activation of protein kinase F_A/GSK-3α, representing a new mode of signal transduction pathway for the regulation of this multisubstrate protein kinase in cells.