N-terminal determinants of I kappa B alpha necessary for the cytoplasmic regulation of c-Rel

Mechanisms of proinflammatory cytokine-induced biphasic NF-kappaB activation

The transcription factor NF-kappaB regulates genes involved in innate and adaptive immune response, inflammation, apoptosis, and oncogenesis. Proinflammatory cytokines induce the activation of NF-kappaB in both transient and persistent phases. We investigated the mechanism for this biphasic NF-kappaB activation. Our results show that MEKK3 is essential in the regulation of rapid activation of NF-kappaB, whereas MEKK2 is important in controlling the delayed activation of NF-kappaB in response to stimulation with the cytokines TNF-alpha and IL-1alpha. MEKK3 is involved in the formation of the IkappaBalpha:NF-kappaB/IKK complex, whereas MEKK2 participates in assembling the IkappaBbeta:NF-kappaB/IKK complex; these two distinct complexes regulate the proinflammatory cytokine-induced biphasic NF-kappaB activation. Thus, our study reveals a novel mechanism in which different MAP3K and IkappaB isoforms are involved in specific complex formation with IKK and NF-kappaB for regulating the biphasic NF-kappaB activation. These findings provide further insight into the regulation of cytokine-induced specific and temporal gene expression.

Interaction between hnRNPA1 and IkappaBalpha is required for maximal activation of NF-kappaB-dependent transcription

Transcriptional activation of NF-kappaB is mediated by signal-induced phosphorylation and degradation of its inhibitor, IkappaBalpha. NF-kappaB activation induces a rapid resynthesis of IkappaBalpha which is responsible for postinduction repression of transcription. Following resynthesis, IkappaBalpha translocates to the nucleus, removes template bound NF-kappaB, and exports NF-kappaB to the cytoplasm in a transcriptionally inactive form. Here we demonstrate that IkappaBalpha interacts directly with another nucleocytoplasmic shuttling protein, hnRNPA1, both in vivo and in vitro. This interaction requires one of the N-terminal RNA binding domains of hnRNPA1 and the C-terminal region of IkappaBalpha. Cells lacking hnRNPA1 are defective in NF-kappaB-dependent transcriptional activation, but the defect in these cells is complemented by ectopic expression of hnRNPA1. hnRNPA1 expression in these cells increased the amount of IkappaBalpha degradation, compared to that of the control cells, in response to activation by Epstein-Barr virus latent membrane protein 1. Thus in addition to regulating mRNA processing and transport, hnRNPA1 also contributes to the control of NF-kappaB-dependent transcription.