Caspase-Mediated Proteolytic Activation of Calcineurin in Thapsigargin-Mediated Apoptosis in SH-SY5Y Neuroblastoma Cells

We previously demonstrated a loss in calmodulin (CaM)-dependent protein kinase activity in SH-SY5Y cells undergoing thapsigargin-mediated apoptosis, (K. M. McGinnis et al., 1998, J. Biol. Chem. 273, 19993–20000). Here we demonstrate that the large subunit of the CaM-dependent protein phosphatase 2B (calcineurin) is fragmented during SH-SY5Y cell apoptosis to a major fragment of 45 kDa in a caspase inhibitor-sensitive manner. A 45-kDa fragment was also produced when purified calcineurin was digested with recombinant caspase-3. The major cleavage site was identified to be DFGD* G386ATAA, which removes the C-terminal CaM-binding and autoinhibitory regions from the catalytic domain. Phosphatase activity increased progressively with caspase-3 digestion, coupled with the eventual loss of CaM-dependency. Calcineurin-mediated dephosphorylation of NFATc was also detected in thapsigargin-treated cells. Last, calcineurin inhibitors FK506 and cypermethrin provided partial protection against thapsigargin-mediated apoptosis, suggesting that calcineurin overactivation contributes to thapsigargin-induced apoptosis.