REVERSIBLE TYROSINE PHOSPHORYLATION DEPHOSPHORYLATION OF PROLINE-DIRECTED PROTEIN-KINASE FA/GLYCOGEN SYNTHASE KINASE-3-ALPHA IN A431 CELLS/

Modulation of protein kinase FA/glycogen synthase kinase-3 alpha (kina se FA/GSK-3 alpha) by reversible tyrosine phosphorylation/dephosphoryl ation was investigated. In addition to genistein, other protein tyrosi ne kinase (PTK) inhibitors, such as tyrphostin A47 and B42, also could induce tyrosine dephosphorylation and inactivation of kinase FA/GSK-3 alpha in A431 cells, and this process was found to be reversible. Pre treatment of the cells with 100 mu M 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 de phosphorylation/inactivation of this kinase is mediated by orthovanada te-sensitive PTP(s) in A431 cells. Moreover, the phosphotyrosine moiet y of kinase FA/GSK-3 alpha was found to be highly turned over in resti ng cells. Interestingly, we found that the less active, tyrosine-depho sphorylated form of kinase FA/GSK-3 alpha immunoprecipitated from geni stein-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 act ivation of kinase FA/GSK-3 alpha 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-3 alpha. (C) 1997 Wiley-Liss, Inc.