Despite certain structural and biochemical similarities, differences exist in the function of the
NF-kappaB (
nuclear factor kappaB) inhibitory
proteins IkappaBalpha (inhibitory kappaBalpha) and
IkappaBbeta. The functional disparity arises in part from variance at the level of gene regulation, and in particular from the substantial induction of
IkappaBalpha, but not
IkappaBbeta, gene expression post-
NF-kappaB activation. In the present study, we probe the differential effects of IL (interleukin)-1beta on induction of
IkappaBalpha and perform the first characterization of the human
IkappaBbeta promoter. A consensus
NF-kappaB-binding site, capable of binding
NF-kappaB both in vitro and in vivo, is found in the
IkappaBbeta gene 5' flanking region. However, the
IkappaBbeta promoter was not substantially activated by pro-inflammatory
cytokines, such as IL-1beta and tumour
necrosis factor alpha, that are known to cause strong activation of
NF-kappaB. Furthermore, in contrast with
IkappaBalpha,
NF-kappaB activation did not increase expression of endogenous
IkappaBbeta as assessed by analysis of
mRNA and
protein levels. Unlike kappaB-responsive promoters,
IkappaBbeta promoter-bound p65 inefficiently recruits
RNA polymerase II, which stalls at the promoter. We present evidence that this stalling is likely due to the absence of
transcription factor IIH engagement, a prerequisite for
RNA polymerase II phosphorylation and transcriptional initiation. Differences in the conformation of promoter-bound
NF-kappaB may underlie the variation in the ability to engage the basal transcriptional apparatus at the
IkappaBbeta and kappaB-responsive promoters. This accounts for the differential expression of IkappaB family members in response to
NF-kappaB activation and furthers our understanding of the mechanisms involved in
transcription factor activity and
IkappaBbeta gene regulation.