Novel IκBα Proteolytic pathway in WEHI231 immature B cells

The Rel/NF-kappa B family of transcription factors is sequestered in the cytoplasm of most mammalian cells by inhibitor proteins belonging to the I kappa B family. Degradation of I kappa B by a phosphorylation-dependent ubiquitin-proteasome (inducible) pathway is believed to allow nuclear transport of active Rel/NF-kappa B dimers. Rel/NF-kappa B (a p50-c-Rel dimer) is constitutively nuclear in murine B cells, such as WEHI231 cells. In these cells, p50, c-Rel, and I kappa B alpha are synthesized at high levels but only I kappa B alpha is rapidly degraded. We have examined the mechanism of I kappa B alpha degradation and its relation to constitutive p50-c-Rel activation. We demonstrate that all I kappa B alpha is found complexed with c-Rel protein in the cytoplasm. Additionally, rapid I kappa B alpha proteolysis is independent of but coexistent with the inducible pathway and can be inhibited by calcium chelators and some calpain inhibitors. Conditions that prevent degradation of I kappa B alpha also inhibit nuclear p50-c-Rel activity. Furthermore, the half-life of nuclear c-Rel is much shorter than that of the cytoplasmic form, underscoring the necessity for its continuous nuclear transport to maintain constitutive p50-c-Rel activity. We observed that I kappa B beta, another NF-kappa B inhibitor, is also complexed with c-Rel but slowly degraded by a proteasome-dependent process in WEHI231 cells. In addition, I kappa B beta is basally phosphorylated and cytoplasmic. We thus suggest that calcium-dependent I kappa B alpha proteolysis maintains nuclear transport of a p50-c-Rel heterodimer which in turn activates the synthesis of I kappa B alpha, p50, and c-Rel to sustain this dynamic process in WEHI231 B cells.