β-amyloid and ionophore A23187 evoke tau hyperphosphorylation by distinct intracellular pathways: Differential involvement of the calpain/protein kinase C system

SH‐SY‐5Y human neuroblastoma cells were treated with 22 μM of a synthetic peptide corresponding to amino acid residues 25–35 of β‐amyloid (βA) or 3 μM calcium ionophore A23187 in culture medium containing 1.8 mM extracellular calcium. Both agents increased tau immunoreactivity towards antibodies (PHF‐1, ALZ‐50) that recognize epitopes common with paired helical filaments (PHFs) and towards an antibody (5E2) that recognized a phosphate‐independent tau epitope. However, only ionophore increased immunoreactivity with an additional phosphate‐dependent antibody (AT‐8) that recognized an epitope of tau when phosphorylated, and induced a corresponding decrease in immunoreactivity towards an additional antibody (Tau‐1) that recognizes the same site when that site is not phosphorylated. Moreover, the ionophore‐mediated increase in PHF‐1 was blocked by EGTA, by the calpain inhibitor calpeptin and by the PKC inhibitor H7, while that evoked by βA treatment was not inhibited by any of these treatments. Since ionophore‐mediated calpain activation induces proteolytic PKC activation, we further examined the influence of PKC inhibition on βA and ionophore‐mediated PHF‐1 induction. Antisense oligonucleotide‐mediated downregulation of PKCϵ in a stable transfectant SH‐SY‐5Y subclone diminished the ionophore‐mediated, but not the βA‐mediated, increase in PHF‐1 immunoreactivity. These data indicate specific differences in the intracellular cascade of events invoked by βA and ionophore A23187. Moreover, although βA invoked calcium influx in these cells, our findings further suggest that the induction of tau hyperphosphorylation by βA may not be due to calcium influx. J. Neurosci. Res. 49:759–768, 1997. © 1997 Wiley‐Liss, Inc.