Dual Notch signaling in proinflammatory macrophage activation

Recently, Xu et al. published an interesting article in which they demonstrate that in response to inflammatory signals in alcoholic steatohepatitis (ASH), macrophages, but not Kupffer cells, are reprogrammed to become proinflammatory (M1) macrophages through dual Notch signaling pathways, in which the NOTCH1 intracellular domain (NICD1) translocates into the nucleus and mitochondria to up-regulate the expression of M1 genes (e.g., inducible nitric oxide synthase, NOS2) and mitochondrial respiratory complex-related genes (e.g., pyruvate dehydrogenase phosphatase). This work also demonstrates that Notch-mediated activation of mitochondrial oxidative phosphorylation (OXPHOS) increases adenosine triphosphate (ATP) production through the electron transport chain complexes, where the majority of mitochondrial reactive oxygen species (mtROS) are generated. mtROS also stimulate nuclear factor-jB and stabilize hypoxia-inducible factor-1a (HIF-1a), which regulate the transcription of NOS2. Thus, these results suggest the existence of a pathway wherein inhibition of Notch signaling may be a potential therapeutic target for ameliorating ASH (Fig. 1). Notch is a transmembrane receptor that mediates cell-cell communication, which coordinates the development of multicellular organisms. Mechanistically, Notch signals are transduced through c-secretasemediated cleavage of Notch receptors after interaction with one of five canonical Notch ligands composed of the Jagged (Jagged 1 and 2) and Delta-like ligands (DIl1, DIl3, and DIl4), resulting in the nuclear translocation of NICD. After association with DNAbinding recombination signal-binding protein J (RBPJ), also known as CSL or CBF1, NICD forms an activator complex that induces the transcription of Notch target genes such as hairy/enhancer of split (HES) or hairy/enhancer-of-split-related with YRPW motif (HEY). Alternatively, a noncanonical Notch signaling pathway may be triggered by noncanonical signals, such as lipopolysaccharide (LPS) and interferonc. In this pathway, a distinct set of genes are upregulated by crosstalk with several signaling pathways including Wnt, transforming growth factor-b, HIF1a, and toll-like receptor (TLR). Recently, literature on the mechanistic involvement of Notch in signaling pathways has expanded, especially in relation to macrophage differentiation. One study reports that Notch-RBPJ signaling augments TLR4-mediated expression of M1 macrophageassociated genes, such as NOS2 and interleukin-12 (IL-12), by promoting the synthesis of interferon regulatory factor 8, suggesting that the integration of these two different signaling pathways regulates macrophage plasticity and polarization. Xu et al. suggest another plausible mechanism whereby M1 macrophage polarization is induced by the coupling of LPS/Notchmediated direct up-regulation of M1 gene transcription with indirect mtROS-mediated amplification of M1 gene expression. Moreover, this study not only validates the enhanced glucose oxidation by mitochondria as a key metabolic feature of M1 macrophages but also links mitochondrial OXPHOS-mediated ROS to augmented M1 gene expression, by demonstrating that Notch-induced mtROS generation activates HIF1a and nuclear factor-jB to induce inflammatory responses of M1 macrophages. As reported, activation of HIF-1a and nuclear factor-jB in M1 macrophages may be due to mtROS-mediated inhibition of prolyl hydroxylase and c-Src-mediated tyrosine phosphorylation of IjB, respectively. Furthermore, Xu et al. demonstrated that NICD1 localization into mitochondrial DNA is a key molecular mechanism for mitochondrial respiration and mtROS production because NICD1 binding to the promoter region of mitochondrial DNA increases the expression of electron transport chain components, such as reduced nicotinamide adenine dinucleotide dehydrogenase, cytochrome b, cytochrome c, and ATP synthase, thereby stimulating OXPHOS activity. However, Xu et al. did not demonstrate whether Notch activation changes the constant mtROS/ATP ratio by increasing OXPHOS activation, and this question should be addressed. Therefore, it will be important to investigate whether Notch1-mediated reprogramming is a