RND3 Potentiates Proinflammatory Activation through NOTCH Signaling in Activated Macrophages.

Macrophage activation is a complex process with multiple control elements that ensures an adequate response to the aggressor pathogens and, on the other hand, avoids an excess of inflammatory activity that could cause tissue damage. In this study, we have identified RND3, a small GTP-binding protein, as a new element in the complex signaling process that leads to macrophage activation. We show that RND3 expression is transiently induced in macrophages activated through Toll receptors and potentiated by IFN-γ. We also demonstrate that RND3 increases NOTCH signaling in macrophages by favoring NOTCH1 expression and its nuclear activity; however, Rnd3 expression seems to be inhibited by NOTCH signaling, setting up a negative regulatory feedback loop. Moreover, increased RND3 protein levels seem to potentiate NFκB and STAT1 transcriptional activity resulting in increased expression of proinflammatory genes, such as Tnf-α, Irf-1, or Cxcl-10. Altogether, our results indicate that RND3 seems to be a new regulatory element which could control the activation of macrophages, able to fine tune the inflammatory response through NOTCH.

Absence of Notch1 in murine myeloid cells attenuates the development of experimental autoimmune encephalomyelitis by affecting Th1 and Th17 priming.

Inhibition of Notch signalling in T cells attenuates the development of experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. Growing evidence indicates that myeloid cells are also key players in autoimmune processes. Thus, the present study evaluates the role of the Notch1 receptor in myeloid cells on the progression of myelin oligodendrocyte glycoprotein (MOG)35-55 -induced EAE, using mice with a myeloid-specific deletion of the Notch1 gene (MyeNotch1KO). We found that EAE progression was less severe in the absence of Notch1 in myeloid cells. Thus, histopathological analysis revealed reduced pathology in the spinal cord of MyeNotch1KO mice, with decreased microglia/astrocyte activation, demyelination and infiltration of CD4+ T cells. Moreover, these mice showed lower Th1 and Th17 cell infiltration and expression of IFN-γ and IL-17 mRNA in the spinal cord. Accordingly, splenocytes from MyeNotch1KO mice reactivated in vitro presented reduced Th1 and Th17 activation, and lower expression of IL-12, IL-23, TNF-α, IL-6, and CD86. Moreover, reactivated wild-type splenocytes showed increased Notch1 expression, arguing for a specific involvement of this receptor in autoimmune T cell activation in secondary lymphoid tissues. In summary, our results reveal a key role of the Notch1 receptor in myeloid cells for the initiation and progression of EAE.

The Tetraspanin TSPAN33 Controls TLR-Triggered Macrophage Activation through Modulation of NOTCH Signaling.

The involvement of NOTCH signaling in macrophage activation by Toll receptors has been clearly established, but the factors and pathways controlling NOTCH signaling during this process have not been completely delineated yet. We have characterized the role of TSPAN33, a tetraspanin implicated in a disintegrin and metalloproteinase (ADAM) 10 maturation, during macrophage proinflammatory activation. Tspan33 expression increases in response to TLR signaling, including responses triggered by TLR4, TLR3, and TLR2 activation, and it is enhanced by IFN-γ. In this study, we report that induction of Tspan33 expression by TLR and IFN-γ is largely dependent on NOTCH signaling, as its expression is clearly diminished in macrophages lacking Notch1 and Notch2 expression, but it is enhanced after overexpression of a constitutively active intracellular domain of NOTCH1. TSPAN33 is the member of the TspanC8 tetraspanin subgroup more intensely induced during macrophage activation, and its overexpression increases ADAM10, but not ADAM17, maturation. TSPAN33 favors NOTCH processing at the membrane by modulating ADAM10 and/or Presenilin1 activity, thus increasing NOTCH signaling in activated macrophages. Moreover, TSPAN33 modulates TLR-induced proinflammatory gene expression, at least in part, by increasing NF-κB-dependent transcriptional activity. Our results suggest that TSPAN33 represents a new control element in the development of inflammation by macrophages that could constitute a potential therapeutic target.

DLK1 is a novel inflammatory inhibitor which interferes with NOTCH1 signaling in TLR-activated murine macrophages.

Delta-like protein 1 (DLK1) is a noncanonical ligand that inhibits NOTCH1 receptor activity and regulates multiple differentiation processes. In macrophages, NOTCH signaling increases TLR-induced expression of key pro-inflammatory mediators. We have investigated the role of DLK1 in macrophage activation and inflammation using Dlk1-deficient mice and Raw 264.7 cells overexpressing Dlk1. In the absence of Dlk1, NOTCH1 expression is increased and the activation of macrophages with TLR3 or TLR4 agonists leads to higher production of IFN-ß and other pro-inflammatory cytokines, including TNF-α, IL-12, and IL-23. The expression of key proteins involved in IFN-ß signaling, such as IRF3, IRF7, IRF1, or STAT1, as well as cRel, or RelB, which are responsible for the generation of IL-12 and IL-23, is enhanced in Dlk1 KO macrophages. Consistently, Dlk1 KO mice are more sensitive to LPS-induced endotoxic shock. These effects seem to be mediated through the modulation of NOTCH1 signaling. TLR4 activation reduces DLK1 expression, whereas increases NOTCH1 levels. In addition, DLK1 expression diminishes during differentiation of human U937 cells to macrophages. Overall, these results reveal a novel role for DLK1 as a regulator of NOTCH-mediated, pro-inflammatory macrophage activation, which could help to ensure a baseline level preventing constant tissue inflammation.

Characterization of a proximal Sp1 response element in the mouse Dlk2 gene promoter.

BACKGROUND: DLK2 is an EGF-like membrane protein, closely related to DLK1, which is involved in adipogenesis. Both proteins interact with the NOTCH1 receptor and are able to modulate its activation. The expression of the gene Dlk2 is coordinated with that of Dlk1 in several tissues and cell lines. Unlike Dlk1, the mouse Dlk2 gene and its locus at chromosome 17 are not fully characterized. RESULTS: The goal of this work was the characterization of Dlk2 mRNA, as well as the analysis of the mechanisms that control its basal transcription. First, we analyzed the Dlk2 transcripts expressed by several mouse cells lines and tissues, and mapped the transcription start site by 5' Rapid Amplification of cDNA Ends. In silico analysis revealed that Dlk2 possesses a TATA-less promoter containing minimal promoter elements associated with a CpG island, and sequences for Inr and DPE elements. Besides, it possesses six GC-boxes, considered as consensus sites for the transcription factor Sp1. Indeed, we report that Sp1 directly binds to the Dlk2 promoter, activates its transcription, and regulates its level of expression. CONCLUSIONS: Our results provide the first characterization of Dlk2 transcripts, map the location of the Dlk2 core promoter, and show the role of Sp1 as a key regulator of Dlk2 transcription, providing new insights into the molecular mechanisms that contribute to the expression of the Dlk2 gene.

Cooperation of adenosine with macrophage Toll-4 receptor agonists leads to increased glycolytic flux through the enhanced expression of PFKFB3 gene.

Macrophages activated through Toll receptor triggering increase the expression of the A(2A) and A(2B) adenosine receptors. In this study, we show that adenosine receptor activation enhances LPS-induced pfkfb3 expression, resulting in an increase of the key glycolytic allosteric regulator fructose 2,6-bisphosphate and the glycolytic flux. Using shRNA and differential expression of A(2A) and A(2B) receptors, we demonstrate that the A(2A) receptor mediates, in part, the induction of pfkfb3 by LPS, whereas the A(2B) receptor, with lower adenosine affinity, cooperates when high adenosine levels are present. pfkfb3 promoter sequence deletion analysis, site-directed mutagenesis, and inhibition by shRNAs demonstrated that HIF1α is a key transcription factor driving pfkfb3 expression following macrophage activation by LPS, whereas synergic induction of pfkfb3 expression observed with the A(2) receptor agonists seems to depend on Sp1 activity. Furthermore, levels of phospho-AMP kinase also increase, arguing for increased PFKFB3 activity by phosphorylation in long term LPS-activated macrophages. Taken together, our results show that, in macrophages, endogenously generated adenosine cooperates with bacterial components to increase PFKFB3 isozyme activity, resulting in greater fructose 2,6-bisphosphate accumulation. This process enhances the glycolytic flux and favors ATP generation helping to develop and maintain the long term defensive and reparative functions of the macrophages.

Notch1 upregulates LPS-induced macrophage activation by increasing NF-kappaB activity.

Macrophages present different Notch receptors and ligands on their surface. Following macrophage activation by LPS or other TLR ligands, Notch1 expression is upregulated. We report here that Notch signaling increases both basal and LPS-induced NF-kappaB activation, favoring the expression of genes implicated in the inflammatory response, such as the cytokines TNF-alpha and IL-6, or enzymes, such as iNOS. Delta4 seems to be the most effective ligand to induce Notch activation and increasing NF-kappaB transcriptional activity in macrophages. We show that Notch1 signaling promotes NF-kappaB translocation to the nucleus and DNA binding by increasing both phosphorylation of the IkappaB kinase alpha/beta complex and the expression of some NF-kappaB family members. Treatment of macrophages with the gamma-secretase inhibitor DAPT, which prevents the cleavage and activation of Notch receptors, inhibits all these processes, diminishing NF-kappaB activity following LPS stimulation. Additionally, we show that the active intracellular Notch fragment can directly interact with TNF-alpha and iNOS promoters. Our results suggest that Notch signaling results in an amplification of the macrophage-dependent inflammatory response by enhancing NF-kappaB signaling.