High levels of c-myb expression are necessary for the proliferation of hema
topoietic precursor cells whereas do,vn-regulation of c-myb is required for
terminal differentiation; this down-regulation occurs through a conditiona
l block to transcriptional elongation in intron I, We previously observed t
hat cAMP analogs prevented the late down-regulation of c-myb during hexamet
hylene bisacetamide (HMBA)-induced differentiation of murine erythroleukemi
a (MEL) cells and blocked differentiation; this correlated with the inducti
on of NF-kappa B (p50/RelB) complexes which were shown to bind to NF-kappa
B recognition sites flanking the transcriptional pause site of c-myb. We no
w selected stably-transfected MEL cells which overexpressed p50, RelB or bo
th at levels similar to those induced by cAMP to determine whether these NF
-kappa B proteins regulate c-myb expression in intact cells, We demonstrate
that transcriptionally active NF-kappa B (p50/RelB) complexes, but not p50
or RelB alone, prevented the early and late down-regulation of c-myb mRNA
and increased c-myb transcriptional elongation in HMBA-induced MEL cells. T
he increase in c-myb expression was sufficient to block erythroid different
iation and allow continuous proliferation of cells in the presence of HMBA,
Steady-state c-myb mRNA levels in untreated cells were not affected by ove
rexpression of NF-kappa B, suggesting that p50/RelB specifically modulated
the efficiency of transcriptional attenuation during MEL cell differentiati
on.