Phosphorylation of IkappaB-beta is necessary for neuronal survival

Cerebellar granule neurons undergo apoptosis when switched from culture medium containing depolarizing levels of potassium (high potassium or HK) to nondepolarizing medium (low potassium or LK). We showed that in healthy neurons maintained in HK medium, IkappaB-beta is phosphorylated at a novel site, Tyr-161. LK-induced neuronal apoptosis is accompanied by a decrease in the extent of IkappaB-beta phosphorylation at this residue. Tyr-161 shares similarity to the consensus sequence for phosphorylation by the nonreceptor tyrosine kinases Abl and Arg. Arg phosphorylates Tyr-161 differentially in vitro, and LK treatment does cause a down-regulation of Arg activity. Moreover, treatment of neurons with two structurally distinct and highly selective Abl inhibitors, PD173955 and Gleevec, blocks HK-induced phosphorylation of IkappaB-beta at Tyr-161 and induces neuronal apoptosis. Overexpression of wild-type IkappaB-beta blocks LK-induced apoptosis, but this effect is abolished when Arg is pharmacologically inhibited. On the other hand, forced overexpression of IkappaB-beta in which Tyr-161 is mutated inhibits survival in HK demonstrating the importance of this residue to neuronal survival. Phosphorylation of IkappaB-beta enhances its association with p65/RelA causing an increase in NF-kappaB DNA binding activity. Our results identified IkappaB-beta phosphorylation as a key event in neuronal survival and provided a mechanism by which this is mediated.