Generation and Activation of Multiple Dimeric Transcription Factors within the NF-κB Signaling System

Abstract

The NF-B signaling pathway regulates the activity of multiple dimeric transcription factors that are generated from five distinct monomers. The availabilities of specific dimers are regulated during cell differentiation and organ development and determine the cell's responsiveness to inflammatory or developmental signals. An altered dimer distribution is a hallmark of many chronic diseases. Here, we reveal that the cellular processes that generate different NF-B dimers are highly connected through multiple cross-regulatory mechanisms. First, we find that steady-state expression of RelB is regulated by the canonical pathway and constitutive RelA activity. Indeed, synthesis control of RelB is the major determinant of noncanonical NF-B dimer activation. Second, processing, not synthesis, of p100 and p105 is mechanistically linked via competitive dimerization with a limited pool of RelA and RelB. This homeostatic cross-regulatory mechanism determines the availability of the p50-and p52-containing dimers and also of the noncanonical IB p100. Our results inform a wiring diagram to delineate NF-B dimer formation that emphasizes that inflammatory and developmental signaling cannot be considered separately but are highly interconnected.

Crosstalk via the NF-κB signaling system

“…Interestingly, nfkb1 -/-mice were also shown to have inguinal lymph node defects and derived MEF showed attenuated RelB activation in response to LTβR stimulation (Lo et al, 2006). As described earlier, rela -/-mice also exhibited an early organogenic defect with complete absence of lymph nodes in new born mice (Alcamo et al, 2002), and RelB:p52 dimer activation by LTβR signaling was shown to be defective in rela -/-MEF (Basak et al, 2008). As opposed to the commonly held view of distinct canonical and non-canonical signaling axes, these studies indicated the potential involvement of multiple cross-regulatory mechanisms, whose details may best be presented and characterized within network description of the NF-κB signaling system.…”

“…LTβR stimulation was shown to induce nuclear accumulation of RelB:p52 via the NIK/IKK1 pathway and the phosphorylation and processing of de novo synthesized, rather than preexisting p100 protein (Mordmuller et al, 2003;Senftleben et al, 2001;Xiao et al, 2001). Indeed, a first phase of RelA activity was proposed to induce p100 synthesis to amplify RelB:p52 dimer activation (Dejardin et al, 2002;Muller and Siebenlist, 2003), but recent evidence indicates that RelA-responsive expression of RelB is more important in this regard (Basak et al, 2008). Signaling through multiple other TNF receptor superfamily members, such as BAFFR, CD40R, BCMA, TAC1 or RANK, was also reported to activate the RelB:p52 dimer via the non-canonical pathway.…”

“…Important lymphoid chemokine genes, such as BLC and SLC, were shown to be RelB target genes (Basak et al, 2008;Schneider et al, 2004) via a proposed variant kappaB site that recruits RelB-but not RelA-containing dimers upon LTβR stimulation (Bonizzi et al, 2004). Homozygous knockin mice expressing a non-activatable IKK1 variant (ikk1 AA/AA ) show impaired lymphoid organogenesis (Senftleben et al, 2001) due to stromal cell defects (Bonizzi et al, 2004).…”

“…We have recently explored the functional interconnectedness of these two pathways by systematically characterizing the mechanisms that control the generation of each of the relevant NF-κB dimers and thereby determine their availability for stimulus-responsive activation (Basak et al, 2008). The interdependencies between the components of the NF-κB signaling system could be represented using a wiring diagram (Fig.…”

“…On the other hand, feedback inhibition mediated by the re-synthesized IκBs are capable of terminating RelA responses. In addition, it was shown that the RelB-containing dimers could upregulate NFκB2 and RelB mRNA synthesis (Basak et al, 2008), forming a positive feedback loop in the pathway. Finally, p50 and p52 compete with each other for binding to a limited pool of RelA and RelB proteins.…”