Enhanced generation of Alzheimer's amyloid-β following chronic exposure to phorbol ester correlates with differential effects on alpha and epsilon isozymes of protein kinase C

Alzheimer's amyloid precursor protein (APP) sorting and processing are modulated through signal transduction mechanisms regulated by protein phosphorylation. Notably, protein kinase C (PKC) appears to be an important component in signaling pathways that control APP metabolism. PKCs exist in at least 11 conventional and unconventional isoforms, and PKCα and PKCε isoforms have been specifically implicated in controlling the generation of soluble APP and amyloid-β (Aβ) fragments of APP, although identification of the PKC substrate phospho-state-sensitive effector proteins remains challenging. In the current study, we present evidence that chronic application of phorbol esters to cultured cells in serum-free medium is associated with several phenomena, namely: (i) PKCα down-regulation; (ii) PKCε up-regulation; (iii) accumulation of APP and/or APP carboxyl-terminal fragments in the trans Golgi network; (iv) disappearance of fluorescence from cytoplasmic vesicles bearing a green fluorescent protein tagged form of APP; (v) insensitivity of soluble APP release following acute additional phorbol application; and (vi) elevated cellular APP mRNA levels and holoprotein, and secreted Aβ. These data indicate that, unlike acute phorbol ester application, which is accompanied by lowered Aβ generation, chronic phorbol ester treatment causes differential regulation of PKC isozymes and increased Aβ generation. These data have implications for the design of amyloid-lowering strategies based on modulating PKC activity.