Activation of nuclear factor kappa B and bcl-x survival gene expression bynerve growth factor requires tyrosine phosphorylation of I kappa B alpha

NGF has been shown to support neuron survival by activating the transcripti on factor nuclear factor-kappaB (NF kappaB). We investigated the effect of NGF on the expression of Bcl-xL, an anti-apoptotic Bcl-2 family protein. Tr eatment of rat pheochromocytoma PC12 cells, human neuroblastoma SH-SY5Y cel ls, or primary rat hippocampal neurons with NGF (0.1-10 ng/ml) increased th e expression of bcl-xL mRNA and protein. Reporter gene analysis revealed a significant increase in NF kappaB activity after treatment with NGF that wa s associated with increased nuclear translocation of the active NF kappaB p 65 subunit. NGF-induced NF kappaB activity and Bcl-xL expression were inhib ited in cells overexpressing the NF kappaB inhibitor, I kappaB alpha. Unlik e tumor necrosis factor-alpha (TNF-alpha), however, NGF-induced NF kappaB a ctivation occurred without significant degradation of I kappa Bs determined by Western blot analysis and time-lapse imaging of neurons expressing gree n fluorescent protein-tagged I kappaB alpha. Moreover, in contrast to TNF-a lpha, NGF failed to phosphorylate I kappaB alpha at serine residue 32, but instead caused significant tyrosine phosphorylation. Overexpression of a Y4 2F mutant of I kappaB alpha potently suppressed NFG-, but not TNF-alpha -in duced NF kappaB activation. Conversely, overexpression of a dominant negati ve mutant of TNF receptor-associated factor-6 blocked TNF-alpha-, but not N GF-induced NF kappaB activation. We conclude that NGF and TNF-alpha induce different signaling pathways in neurons to activate NF kappaB and bcl-x gen e expression.