Insulin-like growth factor-1-mediated protection from neuronal apoptosis is linked to phosphorylation of the pro-apoptotic protein BAD but not to inhibition of cytochrome c translocation in rat cerebellar neurons

Cerebellar granule neurons cultured in the presence of serum and depolarizi ng potassium concentrations undergo apoptosis when switched to serum-free m edium containing physiological potassium concentrations but remain viable a fter serum deprivation alone. Here, we show that potassium deprivation is a ssociated with the dephosphorylation of the BCL-2-related BAD protein. Expo sure to insulin-like growth factor-1 (IGF-1) inhibits both apoptosis and de phosphorylation of BAD. Both effects of IGF-1 do not depend on protein synt hesis but are nullified by the phosphatidylinositol-3 kinase inhibitors, wo rtmannin and LY294002. In contrast to the treatment with cycloheximde, IGF- 1 does not block the translocation of cytochrome c from mitochondria to the cytosol. Further, dephosphorylation of BAD alone does not appear to be suf ficient to trigger apoptosis, since inhibition of protein synthesis by cycl oheximide prevents apoptosis, but not BAD dephosphorylation, after potassiu m deprivation. These results suggest the coexistence of two parallel pathwa ys, protein synthesis-dependent cytochrome c translocation and protein synt hesis-independent dephosphorylation of BAD, both of which have to be activa ted to induce neuronal apoptosis. (C) 2000 Elsevier Science Ireland Ltd. Al l rights reserved.