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Mol. Cell. Biol., Jan 1998, 19-29, Vol 18, No. 1
S Miyamoto, BJ Seufzer and SD Shumway
The Rel/NF-kappaB family of transcription factors is sequestered in the
cytoplasm of most mammalian cells by inhibitor proteins belonging to the
IkappaB family. Degradation of IkappaB by a phosphorylation- dependent
ubiquitin-proteasome (inducible) pathway is believed to allow nuclear
transport of active Rel/NF-kappaB dimers. Rel/NF-kappaB (a p50- c-Rel
dimer) is constitutively nuclear in murine B cells, such as WEHI231 cells.
In these cells, p50, c-Rel, and IkappaB alpha are synthesized at high
levels but only IkappaB alpha is rapidly degraded. We have examined the
mechanism of IkappaB alpha degradation and its relation to constitutive
p50-c-Rel activation. We demonstrate that all IkappaB alpha is found
complexed with c-Rel protein in the cytoplasm. Additionally, rapid IkappaB
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 IkappaB 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 IkappaB beta, another NF-kappaB
inhibitor, is also complexed with c-Rel but slowly degraded by a
proteasome-dependent process in WEHI231 cells. In addition, IkappaB beta is
basally phosphorylated and cytoplasmic. We thus suggest that
calcium-dependent IkappaB alpha proteolysis maintains nuclear transport of
a p50-c-Rel heterodimer which in turn activates the synthesis of IkappaB
alpha, p50, and c-Rel to sustain this dynamic process in WEHI231 B cells.
Copyright © 1998, American Society for Microbiology
Novel IkappaB alpha proteolytic pathway in WEHI231 immature B cells
Department of Human Oncology, University of Wisconsin-Madison, 53792, USA. miyamoto@humonc.wisc.edu
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