Transcriptional elongation of c-myb is regulated by NF-kappa B (p50/RelB)

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.