PGE2 Produced by Exogenous MSCs Promotes Immunoregulation in ARDS Induced by Highly Pathogenic Influenza A through Activation of the Wnt-β-Catenin Signaling Pathway

Abstract

Acute respiratory distress syndrome is an acute respiratory failure caused by cytokine storms; highly pathogenic influenza A virus infection can induce cytokine storms. The innate immune response is vital in this cytokine storm, acting by activating the transcription factor NF-κB. Tissue injury releases a danger-associated molecular pattern that provides positive feedback for NF-κB activation. Exogenous mesenchymal stem cells can also modulate immune responses by producing potent immunosuppressive substances, such as prostaglandin E2. Prostaglandin E2 is a critical mediator that regulates various physiological and pathological processes through autocrine or paracrine mechanisms. Activation of prostaglandin E2 results in the accumulation of unphosphorylated β-catenin in the cytoplasm, which subsequently reaches the nucleus to inhibit the transcription factor NF-κB. The inhibition of NF-κB by β-catenin is a mechanism that reduces inflammation.

Keywords: NF-κB; PGE2; acute respiratory distress syndrome; infectious disease; influenza virus; mesenchymal stem cell; β-catenin.

Figure 1

Cytokine Storm and ALI/ARDS Induced by Influenza Virus Infection. When a highly pathogenic influenza A virus infects alveolar epithelial cells, macrophages recognize this virus using TLR 7, which involves several adaptor molecules and transcription factors, such as MyD88, and activates IRF7 and NF-κB. Influenza A virus can also be recognized by RIG 1 via the adaptor molecule IPD-I and activate IRF3 and NF-κB. The activation of IRF-7 and IRF-3 causes their translocation into the nucleus. This produces interferon type I, while NF-κB acts as a transcription factor to induce pro-inflammatory cytokines’ production, including IL-6, TNFα, and pro-IL-1β. Macrophages also present virus peptides on the cell surface through MHC class II and activate lymphocyte CD4 T cells, which differentiate into Th1. Activated lymphocyte T cells then release various pro-inflammatory cytokines and chemoattractants. All of these mechanisms result in a cytokine storm. The cytokine storm then leads to ARDS, characterized by necrosis, tissue damage, an influx of leucocytes, and blood vessel dilation, causing edema fluids to accumulate in the alveolar, characterized by tissue hypoxia and hypoxemia. The damaged tissue releases HMGB1 and binds with RAGE, activating the intracellular signaling pathway and resulting in NF-κB activation and pro-inflammatory gene expression induction; thus, the cytokine storm continues. We created this figure using the BioRender online app and license.

Figure 2

Communication of MSCs in Damaged Tissue and the Immunosuppression Property of MSCs. Tissue injury, inflammation, and hypoxia factors cue a signal that mobilizes MSCs to the damaged tissue. Once MSCs enter the microenvironment of the injured tissue, IFNγ, together with pro-inflammatory cytokines, such as TNFα, IL-1α, and IL-1β, the occurrence of a burst of chemokine, and the expression of adhesion molecules, stimulate MSCs to release large amounts of immunosuppressive factors. These immunosuppressive factors then promote the activation of immune cells, which induce regulatory cells and inhibit cytokine production. This figure was created using the BioRender online app and license.

Figure 3

PGE2 Promotes Immunoregulation through Cross-Regulation between β-catenin and NF-κB by Activating the Wnt-β-catenin Signaling Pathways. (A) Without the Wnt ligand, β-catenin is bound by GSK-3β, Axin, and APC. This binding-stimulated phosphorylation of β-catenin by GSK-3β leads to the ubiquitination of β-catenin, which is further degraded by proteasomes. (B) When the Wnt ligand binds to the Fz receptor and the LRP co-receptor, activation of the canonical Wnt signal occurs, followed by activation of Dvl and inhibition of GSK-3β. To regulate gene transcription, β-catenin accumulates in the nucleus and binds to T cell factor and lymphoid enhancer factors (TCF/LEF). (C) PGE2 stimulates a double signaling cascade through the EP2 receptor via PI3K and protein kinase Akt activation involving the free G-protein βγ subunit and the direct association of the G-protein αs subunit to Axin. Following PGE2 activation in these two pathways, unphosphorylated β-catenin accumulates in the cytoplasm, then enters the nucleus and inhibits the transcription factor of NF-κB (D). The activity of β-catenin can inhibit NF-κB via two possible mechanisms—by stabilizing IκBα or by binding to NF-κB subunit p50—which prevents its transport from the cytoplasm to the nucleus and also occurs in the nucleus. Both mechanisms can cause a decrease in the transcriptional activity of NF-κB. The inhibition of NF-κB by β-catenin activation reduces inflammation. This figure was created using the BioRender online app and license.

Figure 4

Immunoregulation Mechanisms of Exogenous MSCs on Virus-Induced Acute Lung Injury. A single-stranded RNA of the highly pathogenic influenza A virus binding with the host initiates several signaling transductions. IKKβ is then activated, which can lead to the phosphorylation and degradation of Iκβ, causing the active transcription factor NF-κB to release. NF-κB encodes pro-inflammatory cytokines and chemokine attractants, which lead to ARDS. The injured cells release HMGB1, which has a high affinity to RAGE ligands. HMGB1 bonds with RAGE, and this cues a signal to activate NF-κB, which releases adhesion molecules and pro-inflammatory cytokines. CXCL12 (SDF-1) regulation increases in the inflammation niche due to pro-inflammatory cytokine stimulation. CXCL12 (SDF-1) interacts with its ligand CXCR4, which is expressed by exogenous MSCs and starts the homing process. Active biomolecules of endogenous PGE2 are then produced in large amounts, which leads to the phosphorylation of GSK-3 such that β-catenin accumulates in the cytoplasm. These accumulations inhibit NF-κB via the activation of Iκβ, and as a consequence, the release of pro-inflammatory cytokines and chemokines is inhibited. This figure was created using the BioRender online app and license.