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

The signal transduction mechanism of protein kinase FA/GSK-3 alpha by tyrosine phosphorylation in A431 cells was investigated. Kinase FA/GSK-3 alpha 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 FA/GSK-3 alpha 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 FA/GSK-3 alpha activity in cells, suggesting an involvement of serine/threonine dephosphorylation in the modulation of tyrosine phosphorylation and activation of protein kinase FA/GSK-3 alpha, representing a new mode of signal transduction pathway for the regulation of this multisubstrate protein kinase in cells.