SELECTIVE ACTIVATION BY BRYOSTATIN-1 DEMONSTRATES UNIQUE ROLES FOR PKC-EPSILON IN NEURITE EXTENSION AND TAU-PHOSPHORYLATION

Phorbol esters such as 12-O-tetradeonyl phorbol-13 acetate (TPA) induc e a time-dependent biphasic effect on protein kinase C (PKC)-mediated events by fostering translocation of cytosolic (latent) PKC to the pla sma membrane (where it is activated). Continued treatment, however, de pletes the cell's entire PKC complement and induces a functional stake of PKC inhibition. Previous studies from several laboratories have de monstrated that long-term TPA treatment, like treatment with PKC inhib itors, induces neuronal differentiation. Bryostatin-l also induces tra nslocation and overall downregulation of PKC following long-term treat ment, yet, unlike TPA or PKC inhibitors, does not induce neuronal diff erentiation, promoting controversy regarding the role of PKC inhibitio n in neuronal differentiation. We demonstrate herein that, despite ove rall downregulation in human neuroblastoma cells, membrane-associated levels of one PKC isoform (PKC epsilon) are actually increased followi ng long-term bryostatin-l treatment. Since previous studies have impli cated this PKC isoform in phosphorylation of the microtubule-associate d protein tau and in neuritogenesis, we examined the consequences of l ong-term bryostatin treatment on these phenomena. Treatment with 25 n- 100 Icl bryostatin-l for 72 h increased tau phosphorylation and inhibi ted neuritogenesis, By contrast, treatment with either TPA or the PKC inhibitor staurosporine did not induce tau phosphorylation and induced neurite elaboration. Bryostatin-l antagonized neurite induction by st aurosporine. These findings provide additional evidence for a unique r ole of PKC epsilon in the regulation of tan phosphorylation and neuron al differentiation,, and demonstrate that bryostatin-l can function un der cel tain conditions as a selective PKC epsilon activator even foll owing long-term treatment. (C) 1997 Published by Elsevier Science Ltd on behalf of ISDN.