Inactivation of NF-κB-Dependent Cell Survival, a Novel Mechanism for the Proapoptotic Function of c-Abl

Using a system that expresses a constitutively kinase-active c-Abl protein [c-Abl(KA)], we identified the protein IκBα as a novel substrate of c-Abl. This kinase-substrate relationship is not only confirmed at the level of endogenous proteins but is also supported by a physical interaction between the two proteins. Interestingly, the association of c-Abl with IκBα, which is detectable in the form of nonphosphorylated proteins, is remarkably enhanced by an inducible binding of tyrosine-phosphorylated IκBα to the c-Abl SH2 domain. In contrast to the serine 32/34 phosphorylation that triggers ubiquitination and degradation of IκBα, c-Abl-mediated phosphorylation at tyrosine 305 is associated with an increase of the IκBα protein stability. Significantly, this activity is not shared by the oncogenic Bcr-Abl, because it is unique to the nuclear c-Abl. We also demonstrate that c-Abl targets the nuclear subpopulation of IκBα for phosphorylation and induces it to accumulate in the nucleus. As a consequence, NF-κB transcription activity is abolished, leading to an increased cellular sensitivity to the induction of apoptosis. The functional importance of c-Abl-mediated IκBα phosphorylation is highlighted by a loss of response of the IκBα(Y305F) protein to c-Abl-mediated regulation. Using cells expressing the c-Abl(KA) protein under the control of an inducible promoter, we demonstrate inactivation of the NF-κB-dependent cell survival pathway as one of the mechanisms for c-Abl-mediated apoptosis.