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We have previously described that the NFκB pathway is upregulated during differentiation of glioblastoma stem-like cells (GSCs) which keeps differentiating GSCs in a proliferative astrocytic precursor state. However, extracellular signals and cellular mediators of this pathway are not clear yet. Here, we show that TLR4 is a key factor to promote NFκB activation in differentiating GSCs. TLR4 is upregulated during differentiation of GSCs and promotes transcriptional activation of NFκB as determined by luciferase-reporter assays and expression of NFκB target genes. Downregulation of TLR4 by shRNAs or blockade with anti-TLR4 specific antibodies drastically inhibited NFκB activity which promoted further differentiation and reduced proliferation of GSCs. We found that hyaluronic acid (HA), a main component of brain extracellular matrix, triggers the TLR4-NFκB pathway in differentiating GSCs. Moreover, HA is synthesized and released by GSCs undergoing differentiation and leads to transcriptional activation of NFκB, which is inhibited following downregulation of TLR4 or blockade of HA synthesis. Thus, we have demonstrated that during the process of differentiation, GSCs upregulate TLR4 and release the TLR4 ligand HA, which activates the TLR4-NFκB signaling pathway. This strategy may efficiently be used by differentiating GSCs to maintain their proliferative potential and consequently their tumorigenic capacity.

NLRP2 has been shown to inhibit the NF-kappaB signaling pathway, and thus may contribute to modulate the inflammatory response, where NF-kappaB plays a major role. In this study, we report that expression of NLRP2 is induced upon differentiation of CD34+ hemopoietic progenitors into granulocyte or monocyte/macrophages. We also found that NLRP2 was up-regulated following differentiation of mesenchymal stem cells toward adipocytes. Notably, stimulation of HEK293T cells with TNF-alpha or overexpression of the p65 subunit of NF-kappaB resulted in up-regulation of NLRP2 and the formation of NF-kappaB-NLRP2 promoter complexes. Moreover, ectopic expression of p65 but not of other transcriptional regulators induced transactivation of the NLRP2 promoter. Thus, NLRP2 may control NF-kappaB activation through a regulatory loop. Nucleotide changes within the NACHT domain of other NLRP proteins have been associated with hereditary fever syndromes and chronic inflammatory diseases. We identified five single nucleotide polymorphisms present in the NACHT domain of NLRP2 by sequencing genomic DNA from 319 healthy controls. The frequencies of the rare alleles varied between 0.2 and 10%. Of note, one of these variants, I352S was unable to block the transcriptional activity of NF-kappaB and the formation of NF-kappaB-DNA-binding complexes following stimulation with TNF-alpha. Overall, our findings provide molecular insight into the expression of NLRP2 by NF-kappaB and suggest that a polymorphism within the NACHT domain of NLRP2 may contribute to the amplification of inflammatory responses due to a reduction of inhibitory signals on the NF-kappaB pathway.

Caspase activating and recruitment domain 8 (CARD8) potently inhibits NF-kappaB signaling, which plays a key role in inflammation, and may contribute to avoid a pathologic activation of NF-kappaB; however, the transcriptional mechanisms regulating CARD8 expression and the relevance of this protein in inflammatory diseases are poorly understood. We found a NF-kappaB-binding element within the human CARD8 promoter that was required for transcriptional activity in response to TNF-alpha and the p65 subunit of NF-kappaB. Moreover, TNF-alpha and overexpression of p65 induced the formation of NF-kappaB-CARD8 promoter complexes. Thus, CARD8 may control NF-kappaB activation through a regulatory loop. To study the relevance of CARD8 in chronic inflammatory disorders, we functionally characterized a deleterious polymorphism (p.C10X) and studied its association with rheumatoid arthritis (RA). Transfection of cell lines with the allelic variants of CARD8 revealed that full-length (CARD8-L) but not truncated (CARD8-S) protein inhibits NF-kappaB transcriptional activity, and abrogates the binding of NF-kappaB to its consensus site. Furthermore, in contrast to the full-length protein, CARD8-S did not modify the expression of NF-kappaB target genes (cIAP, A1), in response to TNF-alpha. We analyzed the p.C10X polymorphism in 200 patients with RA, and found that homozygous carriers of the CARD8-S allele have higher disease activity score (p = 0.014), more extra-articular manifestations (p = 0.03), and a lower probability of clinical remission (p = 0.03) than the CARD8-L allele carriers. Overall, our findings provide molecular insight into the expression of CARD8 by NF-kappaB, and suggest that a deleterious polymorphism of CARD8 may help predict the severity of RA.

Nod2, a member of the Apaf1/Nod protein family, confers responsiveness to bacterial products and activates NF-kappaB, a transcription factor that plays a central role in innate immunity. Recently, genetic variation in Nod2 has been associated with susceptibility to Crohn's disease. Here, we report that expression of Nod2 is induced upon differentiation of CD34(+) hematopoietic progenitor cells into granulocyte or monocyte/macrophages. In peripheral blood cells, the highest levels of Nod2 were observed in CD14(+) (monocytes), CD15(+) (granulocytes), and CD40(+)/CD86(+) (dendritic cells) cell populations. Notably, stimulation of myeloblastic and epithelial cells with bacterial lipopolysaccharide or TNFalpha resulted in up-regulation of Nod2. A search for consensus sites within the Nod2 promoter revealed a NF-kappaB binding element that was required for transcriptional activity in response to TNFalpha. Moreover, ectopic expression of p65 induced transactivation, whereas that of dominant-negative IkappaBalpha blocked the transcriptional activity of the Nod2 promoter. Upon stimulation with TNFalpha or lipopolysaccharide, both p50 and p65 subunits of NF-kappaB were bound to the Nod2 promoter. Thus, Nod2 expression is enhanced by proinflammatory cytokines and bacterial components via NF-kappaB, a mechanism that may contribute to the amplification of the innate immune response and susceptibility to inflammatory disease.