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Signal-specific and phosphorylation-dependent RelB degradation: a potential mechanism of NF-kappa B control

Institute of Physiological Chemistry, Universität Ulm, Ulm, Baden-Württemberg, Germany
Oncogene (Impact Factor: 8.56). 01/2002; 20(56):8142-7. DOI: 10.1038/sj.onc.1204884
Source: PubMed

ABSTRACT RelB is an unusual member of the Rel/NF-kappaB family of transcription factors which are involved in oncogenic processes. Due to a relaxed control by the IkappaBs, the cytosolic NF-kappaB inhibitors, RelB is constitutively expressed in the nuclei of lymphoid cells. We show here that RelB is inducibly degraded upon activation of T cells in a fashion similar to the IkappaBs. However, RelB degradation differs from that of IkappaBs since it is not induced by TNFalpha but only by T cell receptor or TPA/ionomycin stimulation. Moreover, RelB degradation occurs in three steps: (i) after stimulation RelB is rapidly phosphorylated at amino acids Thr84 and Ser552 followed by (ii) an N-terminal cut and, finally, (iii) the complete degradation in the proteasomes. Since mutation of the two phosphoacceptor sites to non-acceptor sites abolished RelB phosphorylation in vivo and led to the stabilization of the mutated RelB(DM), site-specific phosphorylation appears to be a necessary prerequisite for RelB degradation. RelB is a crucial regulator of NF-kappaB-dependent gene expression. Thus, the signal-induced degradation of RelB should be an important control mechanism of NF-kappaB activity.

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    • "This result demonstrates qualitative differences between HGF and TNFα engagement of non-canonical NF-κB signaling molecules. Phosphorylation of Ser-552 of RelB is a potential tag for its degradation (Marienfeld et al., 2001), which could protect from kidney injury since RelB silencing in the kidney was shown to prevent renal ischemia/reperfusion injury in mice (Feng et al., 2009). However, we did not observe any loss of RelB following its phosphorylation by HGF in RPTEC, at least within the time limit of our experiments (data not shown). "
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    ABSTRACT: Inflammation induces the NF-κB dependent protein A20 in human renal proximal tubular epithelial cells (RPTEC), which secondarily contains inflammation by shutting down NF-κB activation. We surmised that inducing A20 without engaging the pro-inflammatory arm of NF-κB could improve outcomes in kidney disease. We showed that hepatocyte growth factor (HGF) increases A20 mRNA and protein levels in RPTEC without causing inflammation. Upregulation of A20 by HGF was NF-κB/RelA dependent as it was abolished by overexpressing IκBα or silencing p65/RelA. Unlike TNFα, HGF caused minimal IκBα and p65/RelA phosphorylation, with moderate IκBα degradation. Upstream, HGF led to robust and sustained AKT activation, which was required for p65 phosphorylation and A20 upregulation. While HGF treatment of RPTEC significantly increased A20 mRNA, it failed to induce NF-κB dependent, pro-inflammatory MCP-1, VCAM-1, and ICAM-1 mRNA. This indicates that HGF preferentially upregulates protective (A20) over pro-inflammatory NF-κB dependent genes. Upregulation of A20 supported the anti-inflammatory effects of HGF in RPTEC. HGF pretreatment significantly attenuated TNFα-mediated increase of ICAM-1, a finding partially reversed by silencing A20. In conclusion, this is the first demonstration that HGF activates an AKT-p65/RelA pathway to preferentially induce A20 but not inflammatory molecules. This could be highly desirable in acute and chronic renal injury where A20-based anti-inflammatory therapies are beneficial.
    Journal of Cellular Physiology 04/2012; 227(4):1382-90. DOI:10.1002/jcp.22851 · 3.87 Impact Factor
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    • "Domain swapping occurs as a consequence of the destabilization of the folded RelB dimerization domain, suggesting that domain stability is an important determinant of protein – protein interaction. In cells, decreased folding stability in both the aminoterminal and dimerization domains contributes to its degradation by the proteasome, which explains why the RelB homodimer does not exist in vivo (Marienfeld et al. 2001). Post-translational modification may also play a role in modulating dimerization propensity . "
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    ABSTRACT: The prosurvival transcription factor NF-kappaB specifically binds promoter DNA to activate target gene expression. NF-kappaB is regulated through interactions with IkappaB inhibitor proteins. Active proteolysis of these IkappaB proteins is, in turn, under the control of the IkappaB kinase complex (IKK). Together, these three molecules form the NF-kappaB signaling module. Studies aimed at characterizing the molecular mechanisms of NF-kappaB, IkappaB, and IKK in terms of their three-dimensional structures have lead to a greater understanding of this vital transcription factor system.
    Cold Spring Harbor perspectives in biology 09/2009; 1(3):a000075. DOI:10.1101/cshperspect.a000075 · 8.23 Impact Factor
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    ABSTRACT: The NF-kappaB-transcription factor RelB plays an important role during the development of lymphatic tissues, for the inflammatory response and during tumorigenesis. RelB acts as a key component of the alternative NF-kappaB signalling pathway. However, RelB also modulates the classical NF-kappaB signalling pathway by acting as transcriptional activator and repressor. Until now, the molecular mechanisms underlying this functional diversity are largely unknown. Yet, the previously described proteasomal RelB-degradation in activated T cells represents one mechanism for the regulation of RelB. Therefore, one aim of this dissertation was to prove the involvement of the degradation-inducing Lys48-linked ubiquitinylation within the process of RelB-degradation. Indeed, the inducible RelB-degradation was accompanied by an increasing polyubiquitinylation. Furthermore, a basal non-proteasomal ubiquitinylation of RelB was observed that could be linked to an increase of the transcriptional activity of RelB. The molecular mechanism underlying this positive ubiquitin-effect could not be solved completely, yet the obtained results point to alterations in cofactor-interactions by the ubiquitinylated RelB. The SUMOylation is another lysine-specific posttranslational modification that is able to affect cofactor-interactions and to antagonize the ubiquitinylation. Indeed, RelB-modifications by SUMO were observed. Functionally, the SUMOylation seems to antagonize RelB-ubiquitinylation as it appears to have a negative effect on RelB activity. In summary, the presented results display a new mechanism for regulating the transcriptional activity of RelB by ubiquitinylation and SUMOylation, which is unique among the NF-kappaB family.
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