IB-is an inducible nuclear protein that interacts with nuclear factor-B (NF-B) via its carboxyl-terminal ankyrin-repeats. Previous studies using an NF-B reporter have shown that IB-inhibits the activity of NF-B. In the present study, we dissected the aminoterminal region of IB-, which shows no homology to any other proteins. Indirect immunofluorescence studies demonstrated the presence of a bipartite nuclear localization signal spanning amino acids 163-178. Using GAL4 fusion proteins, we found that internal fragments containing amino acids 329 -402 possessed intrinsic transcriptional activation activity. Interestingly, the activity was not detected in GAL4 fusion proteins of the full-length IB-. On the other hand, the GAL4-dependent transcriptional activity was generated by co-expression of the GAL4-NF-B p50 subunit fusion protein and the full-length IB-, neither of which exhibited the activity on their own. A new splicing variant, IB-(D), with a deletion of amino acids 236 -429, was found to lack transactivation activity. Forced expression of IB-, but not IB-(D), augmented interleukin-6 production, indicating the functional significance of the transactivation activity. In contrast, tumor necrosis factor-␣ production was inhibited by expression of IB-, highlighting the dual functions of this molecule. These results indicate that IB-harbors latent transcriptional activation activity, and that the activity is expressed upon interaction with the NF-B p50 subunit. In addition to the inhibitory activity on NF-B-mediated transcription, the transcriptional activation activity of IBshould be crucial for the regulation of inflammation.

Activation of nuclear factor-kappaB (NF-kappaB), a prominent cellular response to bacterial endotoxin or other microbial products, must be strictly regulated because excessive activation leads to overproduction of cytotoxic cytokines that culminates in septic shock. During screening for genes up-regulated upon inflammation, we identified a new member of the IkappaB family proteins with the ankyrin-repeats. This protein, designated IkappaB-zeta, is hardly detectable in resting cells, but is strongly induced upon stimulation by lipopolysaccharide, which stimulates cells through the Toll-like receptor 4. Interleukin-1beta stimulation also results in the strong induction of IkappaB-zeta, but tumor necrosis factor-alpha does not. In contrast to IkappaB-alpha or IkappaB-beta, IkappaB-zeta localizes in the nucleus, where it inhibits NF-kappaB activity. NF-kappaB activity is essential for the induction of IkappaB-zeta, but is not sufficient. Thus, this protein is a new anti-inflammatory protein, which is specifically induced upon inflammation to regulate NF-kappaB activity.

Activation of nuclear factor-kappaB (NF-kappaB), a prominent cellular response to bacterial endotoxin or other microbial products, must be strictly regulated because excessive activation leads to overproduction of cytotoxic cytokines that culminates in septic shock. During screening for genes up-regulated upon inflammation, we identified a new member of the IkappaB family proteins with the ankyrin-repeats. This protein, designated IkappaB-zeta, is hardly detectable in resting cells, but is strongly induced upon stimulation by lipopolysaccharide, which stimulates cells through the Toll-like receptor 4. Interleukin-1beta stimulation also results in the strong induction of IkappaB-zeta, but tumor necrosis factor-alpha does not. In contrast to IkappaB-alpha or IkappaB-beta, IkappaB-zeta localizes in the nucleus, where it inhibits NF-kappaB activity. NF-kappaB activity is essential for the induction of IkappaB-zeta, but is not sufficient. Thus, this protein is a new anti-inflammatory protein, which is specifically induced upon inflammation to regulate NF-kappaB activity.