The Interplay of Notch Signaling and STAT3 in TLR-Activated Human Primary Monocytes

Abstract

The highly conserved Notch signaling pathway essentially participates in immunity through regulation of developmental processes and immune cell activity. In the adaptive immune system, the impact of the Notch cascade in T and B differentiation is well studied. In contrast, the function, and regulation of Notch signaling in the myeloid lineage during infection is poorly understood. Here we show that TLR signaling, triggered through LPS stimulation or in vitro infection with various Gram-negative and -positive bacteria, stimulates Notch receptor ligand Delta-like 1 (DLL1) expression and Notch signaling in human blood-derived monocytes. TLR activation induces DLL1 indirectly, through stimulated cytokine expression and autocrine cytokine receptor-mediated signal transducer and activator of transcription 3 (STAT3). Furthermore, we reveal a positive feedback loop between Notch signaling and Janus kinase (JAK)/STAT3 pathway during in vitro infection that involves Notch-boosted IL-6. Inhibition of Notch signaling by γ-secretase inhibitor DAPT impairs TLR4-stimulated accumulation of NF-κB subunits p65 in the nucleus and subsequently reduces LPS- and infection-mediated IL-6 production. The reduced IL-6 release correlates with a diminished STAT3 phosphorylation and reduced expression of STAT3-dependent target gene programmed death-ligand 1 (PD-L1). Corroborating recombinant soluble DLL1 and Notch activator oxaliplatin stimulate STAT3 phosphorylation and expression of immune-suppressive PD-L1. Therefore we propose a bidirectional interaction between Notch signaling and STAT3 that stabilizes activation of the transcription factor and supports STAT3-dependent remodeling of myeloid cells toward an immuno-suppressive phenotype. In summary, the study provides new insights into the complex network of Notch regulation in myeloid cells during in vitro infection. Moreover, it points to a participation of Notch in stabilizing TLR-mediated STAT3 activation and STAT3-mediated modulation of myeloid functional phenotype through induction of immune-suppressive PD-L1.

The Role of Notch Signaling in Macrophages during Inflammation and Infection: Implication in Rheumatoid Arthritis?

“…Consistently, in gram-positive and gram-negative bacterial infection models, IL-6 was noted to induce the expression of DLL-1 in primary human monocytes through activation of transcription factor STAT-3. In turn, DLL-1 was shown to enhance IL-6 production and subsequently activate STAT-3 [46]. Of course, Notch signaling seems to directly regulate IL-6 expression.…”

“…Specifically, lipopolysaccharide (LPS) in gram-negative bacteria binds to TLR-4, which in turn mediates expression of pro-inflammatory mediators through NF-κB and microtubule-associated protein (MAP) kinase signaling. The mediators through autocrine and paracrine processes activate downstream signaling pathways such as Janus kinase/signal transducers and activators of transcription (JAK/STAT) pathway, which ultimately induce more modulation of the immune response [46]. Dysregulation in PRRs signaling system has been linked to increasing the susceptibility to various bacterial and viral infections [47,48] and the development of inflammatory and autoimmune diseases.…”

COVID-19 in the heart and the lungs: could we “Notch” the inflammatory storm?

“…In macrophages, Notch1 directly binds to interleukin (IL)-6 promoter in response to interferon (IFN)-γ and positively regulates IL-6 production [52]. It is important to underline that IL-6, in turn, increases the expression of the Notch ligand Dll1, thus amplifying the Notch signalling and establishing a positive feedback loop that promotes the further production of IL-6 [21]. In Th cells, Notch signalling triggered by Dll1,4 ligands promotes the production of inflammatory Th1/Th17 cytokines, whereas Jagged1 suppresses IL-6-induced Th17 activation [49] (Fig.…”

PD-L1 is overexpressed on breast cancer stem cells through notch3/mTOR axis

“…28 Likewise, PD-L1 was upregulated through a notch in human primary monocytes. 29 In this study, we showed for the first time that Notch signaling pathway is critical for PD-L1 expression on breast CSCs.…”