A20 inhibits NF-κB activation downstream of multiple Map3 kinases and interacts with the IκB signalosome

A20, a TNF inducible gene, inhibits TNF-mediated apoptosis as well as NF-KB induced by this cytokine. Reporter assay experiments revealed that A20 is a very effective inhibitor of NF-KB signaling induced by TRAFs and several Map3 kinases, including NIK, MEKK1, COT, and TAK1. Similarly, the NF-kappaB inducing activity of TAX, an activator of the I kappaB kinase complex, is also abrogated by A20. Inhibition of NF-kappaB is specific as A20 has no effect on TNF-alpha -induced JNK activation. These results suggest that the molecular target of A20 is more distal to the receptor than TRAFs as previously proposed. A20 inhibits NF-KB-dependent transcription without a concomitant decrease in nuclear NT-KB DNA binding activity or nuclear translocation of p65. This apparent discrepancy between transcriptional readout and gel shift experiments is observed with a variety of stimuli, including expression of IKK beta. Therefore, in addition to the phosphorylation Of I kappaB, another signal is needed for transcriptional activation of NF-KB. A20 inhibits this non-redundant signal. The observation that A20 associates with IKK alpha and is phosphorylated upon IKK beta co-expression may suggest that A20 interferes with some aspects of signalosome function.