Nerve growth factor blocks thapsigargin-induced apoptosis at the level of the mitochondrion viaregulation of Bim

This study examined how the neurotrophin, nerve growth factor (NGF), protects PC12 cells against endoplasmic reticulum (ER) stress-induced apoptosis. ER stress was induced using thapsigargin (TG) that inhibits the sarcoplasmic/ER Ca²⁺-ATPase pump (SERCA) and depletes ER Ca²⁺ stores. NGF pre-treatment inhibited translocation of Bax to the mitochondria, loss of mitochondrial transmembrane potential, cytochrome c release, activation of caspases (-3, -7 and -9) and apoptosis induction by TG. Notably, TG also caused a marked induction of Bim_el mRNA and protein, and knockdown of Bim with siRNA protected cells against TG-induced apoptosis. NGF delayed the induction and increased the phosphorylation of Bim_el. NGF-mediated protection was dependent on phosphatidylinositol-3 kinase (PI3K) signalling since all above apoptotic events, including expression and phosphorylation status of Bim_el protein, could be reverted by the PI3K inhibitor LY294002. In contrast, NGF had no effect on the TG-mediated induction of the unfolded protein response (increased expression of Grp78, GADD34, splicing of XBP1 mRNA) or ER stress-associated pro-apoptotic responses (induction of C/EBP homologous protein [CHOP], induction and processing of caspase-12). These data indicate that NGF-mediated protection against ER stress-induced apoptosis occurs at the level of the mitochondria by regulating induction and activation of Bim and mitochondrial translocation of Bax.