Free PKC catalytic subunits (PKM) phosphorylate tau via a pathway distinctfrom that utilized by intact PKC

Protein kinase C (PKC) is reversibly activated at the plasma membrane: by t he generation of diacylglycerol (DAG) coupled with the release of Ca2+ from intracellular stores. PKC is also irreversibly activated by calpain-mediat ed PKC cleavage of the regulatory and catalytic subunits; resultant free PK C catalytic subunits are termed "PKM''. Unlike PKC, PKM is co-factor-indepe ndent, remains active following diffusion away from the membrane, and can t heoretically phosphorylate targets inaccessible to, and inappropriate for, PKC. We examined the downstream consequences of PKC activation by the phorb ol ester TPA and by ionophore A23187-mediated calcium influx (which experim entally correspond to DAG-mediated and calpain-mediated activation, respect ively) on phosphorylation of the microtubule-associated protein tau. Both m ethods increased phospho-tau immunoreactivity, and neither was inhibited by lithium or olomoucin (inhibitors of tan kinases GSK-3 beta and cdk5, respe ctively). The TPA-mediated increase, and not the ionophore-mediated increas e, was blocked by co-treatment with the mitogen-activated protein (MAP) kin ase kinase inhibitor PD98059. These findings indicate that PKC phosphorylat es tau via the MAP kinase pathway, but that PKM can bypass this requirement , therefore demonstrating that distinct intracellular pathways can be media ted by PKC and PKM. PKM generation may therefore trigger one or more additi onal pathways contributing to tau phosphorylation following inappropriate c alcium influx. (C) 1999 Elsevier Science B.V. All rights reserved.