Hyperactivation of mitogen-activated protein kinase increases phospho-tau immunoreactivity within human neuroblastoma: Additive and synergistic influence of alteration of additional kinase activities

Mitogen-activated protein (MAP) kinase phosphorylates tau in cell-free anal yses, but whether or not it does so within intact cells remains controversi al. In the present study, microinjection of MAP kinase into SH-SY-5Y human neuroblastoma cells increased tau immunoreactivity toward the phosphodepend ent antibodies PHF-1 and AT-8. In contrast, treatment with a specific inhib itor of MAP kinase (PD98059) did not diminish "basaI" levels of these immun oreactivities in otherwise untreated cells. These findings indicate that hy peractivation of MAP kinase increases phospho-tau levels within cells, desp ite that MAP kinase apparently does not substantially influence intracellul ar tau phosphorylation under normal conditions. These findings underscore t hat results obtained following inhibition of kinase activities do not neces sarily provide an indication of the consequences accompanying hyperactivati on of that same kinase. Several studies conducted in cell-free systems indi cate that exposure of tau to multiple kinases can have synergistic effects on the nature and extent of tau phosphorylation. We therefore examined whet her or not such effects could be demonstrated within these cells. Site-spec ific phospho-tau immunoreactivity was increased in additive and synergistic manners by treatment of injected cells with TPA (which activates PKC), cal cium ionophore (which activates calcium-dependent kinases), and wortmannin (which inhibits PIP3 kinase). Alteration in total tau levels was insufficie nt to account for the full extent of the increase in phospho-tau immunoreac tivity. These additional results indicate that multiple kinase activities m odulate the influence of MAP kinase on tau within intact cells.