p100 Deficiency Is Insufficient for Full Activation of the Alternative NF-κB Pathway: TNF Cooperates with p52-RelB in Target Gene Transcription

Background: Constitutive activation of the alternative NF-kappa B pathway leads to marginal zone B cell expansion and disorganized spleen microarchitecture. Furthermore, uncontrolled alternative NF-kappa B signaling may result in the development and progression of cancer. Here, we focused on the question how does the constitutive alternative NF-kappa B signaling exert its effects in these malignant processes. Methodology/Principal Findings: To explore the consequences of unrestricted alternative NF-kappa B activation on genome-wide transcription, we compared gene expression profiles of wild-type and NF-kappa B2/p100-deficient (p100(-/-)) primary mouse embryonic fibroblasts (MEFs) and spleens. Microarray experiments revealed only 73 differentially regulated genes in p100(-/-) vs. wild-type MEFs. Chromatin immunoprecipitation (ChIP) assays showed in p100(-/-) MEFs direct binding of p52 and RelB to the promoter of the Enpp2 gene encoding ENPP2/Autotaxin, a protein with an important role in lymphocyte homing and cell migration. Gene ontology analysis revealed upregulation of genes with anti-apoptotic/proliferative activity (Enpp2/Atx, Serpina3g, Traf1, Rrad), chemotactic/locomotory activity (Enpp2/Atx, Ccl8), and lymphocyte homing activity (Enpp2/Atx, Cd34). Most importantly, biochemical and gene expression analyses of MEFs and spleen, respectively, indicated a marked crosstalk between classical and alternative NF-kappa B pathways. Conclusions/Significance: Our results show that p100 deficiency alone was insufficient for full induction of genes regulated by the alternative NF-kappa B pathway. Moreover, alternative NF-kappa B signaling strongly synergized both in vitro and in vivo with classical NF-kappa B activation, thereby extending the number of genes under the control of the p100 inhibitor of the alternative NF-kappa B signaling pathway.