Tumor promoter phorbol ester reversibly modulates 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 to approximately 60% of the control level when cells were acutely treated with 1 microM tumor promoter phorbol ester (TPA) at 37 degrees C 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 FA/GSK-3 alpha immunoprecipitate kinase assay. Conversely, when cells were chronically treated with 1 microM TPA at 37 degrees C for 24 h and processed under identical conditions, kinase FA/GSK-3 alpha was found to be rephosphorylated on tyrosine residue and reactivated to approximately 130% of the original control level. Taken together, the results provide initial evidence that the phosphotyrosine content and cellular activity of kinase FA/GSK-3 alpha 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 FA/GSK-3 alpha in cells, representing a new mode of signal transduction pathway for the regulation of this multisubstrate/multifunctional protein kinase in cells.