A cis-element in the 3′-untranslated region of IκB-ζ mRNA governs its stimulus-specific expression

IκB-ζ, an essential transcriptional regulator in inflammatory reactions, is induced by microbial substances that stimulate Toll-like receptors and interleukin (IL)-1β but not by tumor necrosis factor (TNF)-α, via specific mRNA stabilization. Here, we attempted to identify a cis-element in IκB-ζ mRNA that confers the specific induction. To evaluate the activities of various fragments in the post-transcriptional regulation, we constructed unique reporter plasmids, in which a fragment was inserted downstream of a destabilized luciferase cDNA transcribed by the SV40 early enhancer/promoter. In NIH3T3 cells, a reporter plasmid harboring IκB-ζ mRNA exhibited elevated luciferase activity following stimulation with lipopolysaccharide or IL-1β, but not TNF-α, indicating the stimulus-specificity. We found that a 165-nucleotide fragment in the 3′-untranslated region conferred the specific induction. Stimulus-specific induction of IκB-ζ was observed by transfection of full-length IκB-ζ mRNA, but not of a mRNA without the fragment. Thus, this sequence is essential for the stimulus-specific induction of IκB-ζ via a post-transcriptional regulatory mechanism.