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-jB activation, favoring the expression of genes implicated in the inflammatory response, such as the cytokines TNF-a and IL-6, or enzymes, such as iNOS. Delta4 seems to be the most effective ligand to induce Notch activation and increasing NF-jB transcriptional activity in macrophages. We show that Notch1 signaling promotes NF-jB translocation to the nucleus and DNA binding by increasing both phosphorylation of the IjB kinase a/b complex and the expression of some NF-jB family members. Treatment of macrophages with the c-secretase inhibitor DAPT, which prevents the cleavage and activation of Notch receptors, inhibits all these processes, diminishing NF-jB activity following LPS stimulation. Additionally, we show that the active intracellular Notch fragment can directly interact with TNF-a and iNOS promoters. Our results suggest that Notch signaling results in an amplification of the macrophage-dependent inflammatory response by enhancing NF-jB signaling.Key words: Macrophages . NF-kB . Notch
IntroductionMacrophages are essential cells for the innate immune response. They discriminate between pathogens and self through signals triggered by TLR, which recognize different pathogens' components, such as LPS, lipoproteins, or dsRNA, among others [1]. Activation of most TLR on the macrophage surface triggers a complex signaling pathway, which involves NF-kB activation (reviewed in [2]). In the classical NF-kB pathway, a ternary IkB kinase (IKK) complex, formed by IKK-a, IKK-b, and NF-kB essential modulator, is responsible for inducing IkB phosphorylation, allowing the release of sequestered cytoplasmic NF-kB from IkB and its translocation to the nucleus. Once in the nucleus, NF-kB controls the expression of multiple genes implicated in the inflammatory response, including cytokines, effector enzymes such as iNOS and COX-2, and adhesion molecules [2].Notch proteins encompass a family of transmembrane receptors composed of an extracellular subunit linked to a transmembrane and intracellular subunit via heterodimerization domains [3]. Ligand binding induces proteolytic cleavage of the transmembrane and intracellular receptor subunit by several proteases, including g-secretase [4], allowing the release of the intracellular domain of Notch (NIC), which then translocates to the nucleus and converts the CBF1 factor from a repressor to a transcriptional activator. Some NIC target genes have been characterized, including basic-helix-loop-helix transcription factors belonging to
2556the hairy/enhancer of split (HES) gene family [3]. Although some CBF1-independent Notch signaling can occur, its mechanism of action is not well characterized yet [5].Notch signaling is an evolutionarily conserved pathway that controls different aspects of tissue development and homeostasis [6]. In cells o...