Multiple hemopoietic defects and lymphoid hyperplasia in mice lacking the transcriptional activation domain of the c-Rel protein

NF-κB/Rel Transcription Factors: c-Rel Promotes Airway Hyperresponsiveness and Allergic Pulmonary Inflammation

The NF-κB/Rel family of transcription factors induces many genes involved in immune and inflammatory responses. Mice with germline deletions of individual NF-κB/Rel subunits have different phenotypes, suggesting that the NF-κB/Rel transcription factors have different functions. We tested whether c-Rel promotes allergic asthma using a murine model of allergen-induced pulmonary inflammation and airway hyperresponsiveness. Our investigation focused on c-Rel, which is expressed in lymphoid cells and is important for lymphocyte activation. In response to allergen sensitization and challenge, c-Rel-deficient mice did not develop increases in pulmonary inflammation, bronchoalveolar lavage fluid eosinophilia, or total serum IgE. c-Rel deficiency also prevented the induction of airway hyperresponsiveness. Allergen-treated wild-type mice had increased DNA binding to an NF-κB consensus site. Chemokine expression was altered in allergen-treated c-Rel-deficient mice. Monocyte chemoattractant protein-1, which is regulated by NF-κB, was decreased in allergen-treated c-Rel-deficient mice relative to wild-type controls. The increase in NF-κB/Rel transcription factors after allergen challenge in wild-type mice and the decrease in allergen reactivity found in c-Rel-deficient mice indicate that c-Rel promotes allergic inflammation. Alteration of pulmonary chemokine expression in c-Rel-deficient mice may inhibit allergen-induced pulmonary inflammation and airway hyperresponsiveness.

Genetic approaches in mice to understand Rel/NF-kappaB and IkappaB function: transgenics and knockouts

Rel/NF-kappaB transcription factors have been implicated in regulating a wide variety of genes important in cellular processes that include cell division, cell survival, differentiation and immunity. Here genetic models in which various Rel/NF-kappaB and IkappaB proteins have either been over-expressed or deleted in mice will be reviewed. Although expressed fairly ubiquitously, homozygous disruption of individual Rel/NF-kappaB genes generally affects the development of proper immune cell function. One exception is rela, which is essential for embryonic liver development. The disruption of genes encoding the individual subunits of the IkappaB kinase, namely IKKalpha and IKKbeta, has demonstrated that IKKbeta transmits the response to most common NF-kappaB inducing agents, whereas IKKalpha has an unexpected role in keratinocyte differentiation. Future studies will no doubt focus on the effect of multiple gene disruptions of members of this signaling pathway, on tissue-specific disruptions of these genes, and on the use of these mice as models for human diseases.

Aberrant rel/nfkb genes and activity in human cancer

Rel/NF-kappaB transcription factors are key regulators of immune, inflammatory and acute phase responses and are also implicated in the control of cell proliferation and apoptosis. Remarkable progress has been made in understanding the signal transduction pathways that lead to the activation of Rel/NF-kappaB factors and the consequent induction of gene expression. Evidence linking deregulated Rel/NF-kappaB activity to oncogenesis in mammalian systems has emerged in recent years, consistent with the acute oncogenicity of the viral oncoprotein v-Rel in animal models. Chromosomal amplification, overexpression and rearrangement of genes coding for Rel/NF-kappaB factors have been noted in many human hematopoietic and solid tumors. Persistent nuclear NF-kappaB activity was also described in several human cancer cell types, as a result of constitutive activation of upstream signaling kinases or mutations inactivating inhibitory IkappaB subunits. Studies point to a correlation between the activation of cellular gene expression by Rel/NF-kappaB factors and their participation in the malignant process. Experiments implicating NF-kappaB in the control of the apoptotic response also support a role in oncogenesis and in the resistance of tumor cells to chemotherapy. This review focuses on the status of the rel, nfkb and ikb genes and their activity in human tumors and their association with the onset or progression of malignancies.

Rel/NF-kappaB can trigger the Notch signaling pathway by inducing the expression of Jagged1, a ligand for Notch receptors

Jagged1 belongs to the DSL family of ligands for Notch receptors that control the proliferation and differentiation of various cell lineages. However, little is known about the transcription factors that regulate its expression. Here, we show that Jagged1 is a Rel/NF-kappaB-responsive gene. Both c-Rel and RelA induced jagged1 gene expression, whereas a mutant defective for transactivation did not. Importantly, jagged1 transcripts were also upregulated by endogenous NF-kappaB activation and this effect was inhibited by a dominant mutant of IkappaBalpha, a physiological inhibitor of NF-kappaB. Cell surface expression of Jagged1 in c-Rel-expressing cell monolayers led to a functional interaction with lymphocytes expressing the Notch1/TAN-1 receptor. This correlated with the initiation of signaling downstream of Notch, as evidenced by increased levels of HES-1 transcripts in co-cultivated T cells and of CD23 transcripts in co-cultivated B cells. Consistent with its Rel/NF-kappaB-dependent induction, Jagged1 was found to be highly expressed in splenic B cells where c-Rel is expressed constitutively. These results demonstrate that c-Rel can trigger the Notch signaling pathway in neighboring cells by inducing jagged1 gene expression, and suggest a role for Jagged1 in B-cell activation, differentiation or function. These findings also highlight the potential for an interplay between the Notch and NF-kappaB signaling pathways in the immune system.

Functional redundancy of the nuclear factor kappa B inhibitors I kappa B alpha and I kappa B beta

The transcription factor NF-kappaB is sequestered in the cytoplasm by the inhibitor proteins of the IkappaB family. Each member of the IkappaB exhibits structural and biochemical similarities as well as differences. In an effort to address the functional redundancy of two closely related IkappaB molecules, IkappaBalpha and IkappaBbeta, we generated knock-in mice by replacing the IkappaBalpha gene with the IkappaBbeta gene. The knock-in mice do not express IkappaBalpha, but express a T7-tagged IkappaBbeta under the promoter and regulatory sequence of ikba. Unlike the IkappaBalpha-deficient mice, which display severe postnatal developmental defects and die by postnatal day 8, homozygous knock-in mice survive to adulthood, are fertile, and exhibit no apparent abnormalities. Furthermore, thymocytes and embryonic fibroblasts from the knock-in animals exhibit an inducible NF-kappaB response similar to that of wild-type animals. These results indicate that IkappaBalpha and IkappaBbeta share significant similarities in their biochemical activity, and that they acquired their different functions from divergent expression patterns during evolution.