Dichotomous Role of Tumor Necrosis Factor in Pulmonary Barrier Function and Alveolar Fluid Clearance

Abstract

Alveolar-capillary leak is a hallmark of the acute respiratory distress syndrome (ARDS), a potentially lethal complication of severe sepsis, trauma and pneumonia, including COVID-19. Apart from barrier dysfunction, ARDS is characterized by hyper-inflammation and impaired alveolar fluid clearance (AFC), which foster the development of pulmonary permeability edema and hamper gas exchange. Tumor Necrosis Factor (TNF) is an evolutionarily conserved pleiotropic cytokine, involved in host immune defense against pathogens and cancer. TNF exists in both membrane-bound and soluble form and its mainly -but not exclusively- pro-inflammatory and cytolytic actions are mediated by partially overlapping TNFR1 and TNFR2 binding sites situated at the interface between neighboring subunits in the homo-trimer. Whereas TNFR1 signaling can mediate hyper-inflammation and impaired barrier function and AFC in the lungs, ligand stimulation of TNFR2 can protect from ventilation-induced lung injury. Spatially distinct from the TNFR binding sites, TNF harbors within its structure a lectin-like domain that rather protects lung function in ARDS. The lectin-like domain of TNF -mimicked by the 17 residue TIP peptide- represents a physiological mediator of alveolar-capillary barrier protection. and increases AFC in both hydrostatic and permeability pulmonary edema animal models. The TIP peptide directly activates the epithelial sodium channel (ENaC) -a key mediator of fluid and blood pressure control- upon binding to its α subunit, which is also a part of the non-selective cation channel (NSC). Activity of the lectin-like domain of TNF is preserved in complexes between TNF and its soluble TNFRs and can be physiologically relevant in pneumonia. Antibody- and soluble TNFR-based therapeutic strategies show considerable success in diseases such as rheumatoid arthritis, psoriasis and inflammatory bowel disease, but their chronic use can increase susceptibility to infection. Since the lectin-like domain of TNF does not interfere with TNF's anti-bacterial actions, while exerting protective actions in the alveolar-capillary compartments, it is currently evaluated in clinical trials in ARDS and COVID-19. A more comprehensive knowledge of the precise role of the TNFR binding sites versus the lectin-like domain of TNF in lung injury, tissue hypoxia, repair and remodeling may foster the development of novel therapeutics for ARDS.

Keywords: COVID-19; TNF lectin-like domain; TNF receptor; acute respiratory distress syndrome; epithelial sodium channel.

FIGURE 1

Tumor necrosis factor receptors, TNF ligands, TNF-soluble TNF receptor complexes and TNF lectin-like activity [Adapted from Horiuchi et al. (2010)]. sTNF, soluble TNF; mTNF, transmembrane TNF; sTNFR, soluble TNF receptor; TACE, TNF Alpha Converting Enzyme; TNF, tumor necrosis factor; TNFR1, membrane-associated TNF receptor 1; TNFR2, membrane-associated TNF receptor 2.

FIGURE 2

TNF Receptor-1 and TNF Receptor-2 signaling pathways [Adapted from Brenner et al. (2015) and Conrad et al. (2016)]. cIAP, Cellular Inhibitor of Apoptosis Protein; FADD, Fas-associated protein with death domain; FLIP_L, FLICE-like inhibitory protein; IKK, IκB kinase; JNK, c-Jun NH2-terminal kinase; LUBAC, linear ubiquitin chain assembly complex; MLKL, mixed lineage kinase domain-like; mTNF, membrane TNF; NEMO, NF-κB essential modulator; NF-κB, Nuclear factor kappa B; NIK, NF-κB-inducing kinase; RIPK1, Receptor-interacting serine/threonine-protein kinase 1; sTNF, soluble TNF; TAK1, transforming growth factor-β Transforming growth factor beta-activated kinase 1; TNF, tumor necrosis factor; TNFR1, TNF receptor 1; TNFR2, TNF receptor 2; TRAF2, TNF receptor-associated factor 2; TRADD, TNFRSF1A Associated via death domain.

FIGURE 3

Natural and pharmacological inhibitors of TNF and TNF receptor expression and bioactivity. AP-1, Activator protein 1; AQP5, Aquaporin 5; ASK1, Apoptosis signal-regulating kinase 1; ASM, acid sphingomyelinase; cIAP1, Cellular Inhibitor of Apoptosis Protein 1; ENaC, epithelial sodium channel; FADD, Fas-associated protein with death domain; ICAM-1, intercellular adhesion molecule-1; IL, Interleukin; JNK, c-Jun NH2-terminal kinase; MAPK, mitogen-activated protein kinase; MEKK, mitogen-activated protein Kinase/ERK kinase kinase; mRNA, messenger RNA; NF-κB, Nuclear factor kappa B; NIK, NF-κB-inducing kinase; sTNF, soluble TNF; TACE, TNF converting enzyme; TJs, tight junctions; TNF, tumor necrosis factor; TNFR1, TNF receptor 1; TNFR2, TNF receptor 2; TRAF2, TNF receptor-associated factor 2; TRADD, TNFRSF1A Associated via death domain; XIAP, X-Linked inhibitor of apoptosis.

FIGURE 4

Barrier-protective mechanism mediated by the TNF lectin-like domain in pneumolysin-treated human lung microvascular endothelial cells [adapted from Czikora et al. (2017)]. Pneumolysin forms pores in cholesterol-containing cell membranes, which increases Ca²⁺ -influx (Lucas et al., 2012b), which mobilizes calmodulin to activate CaMKII, which in turn phosphorylates its substrate filamin A. Phosphorylated filamin A (Alli et al., 2015) then promotes stress fiber formation and enhances permeability of capillaries. Activation of ENaC by TIP peptide or of the non-selective cation channel, (NSC) either by TIP peptide (binding to ENaC-α) or MitTx (binding to ASIC1a), inhibits pneumolysin-mediated CaMKII activation and subsequent hyperpermeability. ENaC activation moreover promotes PGE2 generation and eNOS activation, resulting in increased barrier-protective NO production. ASIC1a, acid-sensing ion channel 1a; CaMKII, Calcium/calmodulin- dependent protein kinase II; ENaC, epithelial sodium channel.

FIGURE 5

Divergent actions of TNF receptor binding sites versus the lectin-like domain in acute lung injury Adapted from Hamacher et al. (2018). Note that the TNF receptor binding sites for TNF receptor type 1 and TNF receptor type 2 are in the regions between two TNF homotrimers and are not identical, but in part overlapping. ↑, upregulation; ↓, downregulation; ASM, acid sphingomyelinase; ENaC, Epithelial sodium channel; ICAM-1, intercellular adhesion molecule 1; Na⁺/K⁺ -ATPase, sodium–potassium adenosine triphosphatase; VCAM-1, vascular cell adhesion molecule-1; ROS, reactive oxygen species.