Sequential induction of NF-κB/Rel family proteins during B-cell terminal differentiation

The NF-kappa B/Rel family of at least five transcription factor polypeptides is thought to function both as a developmental regulator in B cells and as a rapid response system in all cells. To examine this notion in more detail, we determined the protein contents of both the inducible and constitutive NF-kappa B/Rel activities in a pre-B-cell line, 70Z/3, and a mature B-cell line, WEHI 231. NF-kappa B p50/p65 is the major inducible nuclear complex after lipopolysaccharide or phorbol myristate acetate treatment of 70Z/3 cells. The constitutive and inducible complexes in WEHI 231 cells are mainly composed of p50 and Rel. The constitutive or induced activities are all sensitive to I kappa B-alpha, but this inhibitor is very short-lived in WEHI 231 cells, suggesting that the balance between synthesis and degradation of I kappa B-alpha determines whether a particular cell lineage has constitutive activity. A patterned expression of the NF-kappa B/Rel activator proteins emerges from an analysis of other B-lineage cell lines and splenic B cells: mainly p50 and p65 in pre-B (and non-B) cells, a predominance of Rel and p50 in mature B cells, and expression of p52 and RelB in plasmacytoma lines. This ordered pattern of regulators may reflect the requirement for expression of different genes during terminal B-cell differentiation because different combinations of NF-kappa B/Rel family members preferentially activate distinct kappa B sites in reporter constructs

c-Rel-Dependent Priming of Naive T Cells by Inflammatory Cytokines

SummaryThe intrinsic refractoriness of naive T cells for cytokine production is counteracted by cells of the innate immune system. Upon sensing danger via Toll-like receptors, these cells upregulate T cell costimulatory molecules and secrete cytokines that enhance T cell activation. We show that cytokine-mediated priming of naive T cells requires the NF-κB family member c-Rel. In resting naive cells c-Rel is associated primarily with IκBβ, an inhibitory molecule that is not effectively degraded by TCR signals. Exposure of T cells to proinflammatory cytokines, TNF-α and IL-1β, shifts c-Rel to IκBα-associated complexes that are readily targeted by the TCR. As a consequence, IL-2 and IFN-γ mRNA are produced more quickly, and at higher levels, in cytokine-primed T cells. This mechanism does not operate in effector T cells where cytokine gene expression is c-Rel-independent. We propose that c-Rel plays a crucial role as a target of innate signals in T cells

NF-kB inhibitor blocks B cell development at two checkpoints

Members of the NF-kB transcription factor family are differentially expressed in the B cell lineage. Disruption of individual or two NF-kB subunits exhibits distinct defects in B lymphocyte development, activation, and survival. However, the role each NF-kB plays during B cell development has been obscured by molecular compensation. To address this issue, a trans-dominant form of IkBα was transduced into bone marrow cells to act as a pan-inhibitor of NF-kB using a retroviral system. While the development of T-lymphocytes and myeloid cell lineages was not grossly affected by the transduced IkBα gene, a significant reduction in the number and percentage of B lineage cells was apparent in IkBα transduced chimeric mice. IkBα expression decreased the percentage of pre-B and immature B cell subsets in the bone marrow and further impaired the development of follicular mature B cells and marginal zone B cells in the periphery. Introduction of the Bcl-X transgene completely restored the pre-B and immature B cell pool in the bone marrow. However, despite a significant improvement of overall viability of the B cell lineage, Bcl-X expression was insufficient to overcome the maturation block resulting from NF-kB inhibition. Together, our study suggests that NF-kB activity is required for two distinct checkpoints during B cell development: one is for pre-B/immature B cell viability, the other is to provide both survival and maturation signals to ensure the proper development of follicular mature B cells

The candidate proto-oncogene bcl-3 encodes a transcriptional coactivator that activates through NF-κB p50 homodimers

The candidate proto-oncogene bcl-3 encodes a protein that shares structural features with IκB-α and other proteins that bind to members of the Rel protein family. Here, we show that in contrast to the inhibitory activity of IκB-α, the bcl-3 gene product superactivates NF-κB p50 homodimer-mediated gene expression both in vivo and in vitro. BCL-3 protein can, as well, selectively associate with p50 homodimers in the presence of DNA containing a κB motif. These results strongly suggest that BCL-3 can act as a transcriptional coactivator, acting through DNA-bound p50 homodimers

The p65 subunit of NF-κB regulates IκB by two distinct mechanisms

Transcription factor NF-κB (p50/p65) is generally localized to the cytoplasm by its inhibitor IκB. Overproduced IκB, free from NF-κB, is rapidly degraded. Overexpression of p65 increases endogenous IκB protein in both carcinoma and lymphoid cells by two mechanisms: protein stabilization and increased transcription of IκB mRNA. In contrast, p65Δ, a naturally occurring splice variant, fails to markedly augment IκB protein levels. Both overexpressed p65 and coexpressed p50 are cytoplasmic, whereas p65Δ is partly nuclear, indicating that the IκB induced by p65 can maintain NF-κB in the cytoplasm. Thus, p65 and IκB are linked in an autoregulatory loop, ensuring that NF-κB is held in the cytoplasm until cells are specifically induced to translocate it to the nucleus

Opposing Roles for NF-κB/Rel Factors p65 and c-Rel in the Modulation of Neuron Survival Elicited by Glutamate and Interleukin-1β

The nuclear transcription factors NF-kappaB/Rel have been shown to function as key regulators of either cell death or survival in neuronal cells. Here, we investigated whether selective activation of diverse NF-kappaB/Rel family members might lead to distinct effects on neuron viability. In both cultured rat cerebellar granule cells and mouse hippocampal slices, we examined NF-kappaB/Rel activation induced by two opposing modulators of cell viability: 1) interleukin-1beta (IL-1beta), which promotes neuron survival and 2) glutamate, which can elicit toxicity. IL-1beta produced a prolonged stimulation of NF-kappaB/Rel factors by inducing both IkappaBalpha and IkappaBbeta degradation. Glutamate produced a delayed and transient activation of NF-kappaB/Rel, which was associated with a brief loss of IkappaBalpha. Moreover, IL-1beta activated the p50, p65, and c-Rel subunits of NF-kappaB/Rel, whereas glutamate activated only the p50 and p65 proteins. The inhibition of NF-kappaB/Rel protein expression by antisense oligonucleotides in cerebellar granule cells showed that p65 was involved in glutamate-mediated cell death, whereas c-Rel was essential for IL-1beta-preserved cell survival. Furthermore, the depletion of c-Rel in cultured neurons as well as in the hippocampus from the c-Rel(-/-) mouse converted the IL-1beta effect into toxicity. These findings suggest that, within a single neuron, the balance between cell death and survival in response to external stimuli may rely on the activation of distinct NF-kappaB/Rel proteins

A Novel Mechanism of TRAF Signaling Revealed by Structural and Functional Analyses of the TRADD–TRAF2 Interaction

AbstractTRAF proteins are major mediators for the cell activation, cell survival, and antiapoptotic functions of the TNF receptor superfamily. They can be recruited to activated TNF receptors either by direct interactions with the receptors or indirectly via the adaptor protein TRADD. We now report the structure of the TRADD-TRAF2 complex, which is highly distinct from receptor–TRAF2 interactions. This interaction is significantly stronger and we show by an in vivo signaling assay that TRAF2 signaling is more readily initiated by TRADD than by direct receptor–TRAF2 interactions. TRADD is specific for TRAF1 and TRAF2, which ensures the recruitment of cIAPs for the direct inhibition of caspase activation in the signaling complex. The stronger affinity and unique specificity of the TRADD–TRAF2 interaction are crucial for the suppression of apoptosis and provide a mechanistic basis for the perturbation of TRAF recruitment in sensitizing cell death induction