Tumor promoter phorbol ester reversibly modulates tyrosine dephosphorylatioin/ inactivation of protein kinase F_A/GSK‐3α in A431 cells

The signal transducrion 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 to ∼60% of the control level when cells were acutely treated with 1 μM tumor phorbol ester (TPA) at 37^oC for 30 min, as demonstrated by metabolic ³²P‐labeling the cells, followed by immunoprecipitation and two‐dimensional phosphoamino acid analysis and by immunodetection in an antikinase F_A/GSK‐3α immunoprecipitate kinase assay. Conversely, when cells were chronically treated with 1 μM TPA at 37°C for 24 h and processed under identical condetions, kinase F_A/GSK‐3α was found to be rephosphorylated on tyrosine residue and reactivated to ∼130% of the original control level. Taken together, the results provide initial evidence that the phosphotyrosine content and cellular activity of kinase F_A/GSK‐3α can be modulated in a reversible manner by short‐term and long‐term exposure of A431 cells to TPA. Since acute exposure of cells to TPA causes up‐regulation of cellular protein kinase C (PKC) activity and prolonged exposure to TPA causes down‐regulation of PKC, the results further suggest that the TPA‐mediated modulation of PKC may play a role in the regulation of tyrosine phosphorylation and concurrent activation of kinase F_A/GSK‐3α in cells, representing a new mode of signal transduction pathway for the regulation of this multisubstrate/multifunctional protein kinase in cells.