Tyrosin dephosphorylation and concurrent inactivation of protein kinase FA/GSK‐3α by genistein in A431 cells

Modulation 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 become tyrosine-dephosphorylated and inactivated down to less than 30% of control values in a concentration-dependent manner by 50-400 microM genistein (a specific tyrosine kinase inhibitor), as demonstrated by metabolic 32P-labeling of the cells followed by immunoprecipitation and two-dimensional phosphoamino acid analysis and by immunodetection in an antikinase FA/GSK-3 alpha immunoprecipitate kinase assay. Taken together, the results provide evidence that kinase FA/GSK-3 alpha may exist in a highly tyrosine-phosphorylated/activated state in resting cells which can be tyrosine-dephosphorylated and inactivated by extracellular stimulus and that tyrosine kinase(s) and/or tyrosine phosphatase(s) may play a role in the modulation of kinase FA/GSK-3 alpha activity in cells.