Reversible tyrosine phosphorylation/dephosphorylation of proline-directed protein kinase FA/glycogen synthase kinase-3alpha in A431 cells.

Modulation of protein kinase FA/glycogen synthase kinase-3alpha (kinase FA/GSK-3alpha) by reversible tyrosine phosphorylation/dephosphorylation was investigated. In addition to genistein, other protein tyrosine kinase (PTK) inhibitors, such as tyrphostin A47 and B42, also could induce tyrosine dephosphorylation and inactivation of kinase FA/GSK-3alpha in A431 cells, and this process was found to be reversible. Pretreatment of the cells with 100 microM orthovanadate, a protein tyrosine phosphatase (PTP) inhibitor, could diminish significantly the effects of PTK inhibitors on both enzyme activity and phosphotyrosine content of the kinase, suggesting that the PTK inhibitors induced tyrosine dephosphorylation/inactivation of this kinase is mediated by orthovanadate-sensitive PTP(s) in A431 cells. Moreover, the phosphotyrosine moiety of kinase FA/GSK-3alpha was found to be highly turned over in resting cells. Interestingly, we found that the less active, tyrosine-dephosphorylated form of kinase FA/GSK-3alpha immunoprecipitated from genistein-treated cells was able to reactivate partially with concomitant rephosphorylation of tyrosine residue in vitro. Taken together, these findings demonstrate that tyrosine phosphorylation and concomitant activation of kinase FA/GSK-3alpha can be carried out both in vitro and in vivo and an in vivo phosphatase activity may function in antagonism to PTK activation of kinase FA/GSK-3alpha.