Urokinase plasminogen activator receptor-deficient mice demonstrate reduced hyperoxia-induced lung injury

Urokinase-Type Plasminogen Activator Receptor (uPAR) in Inflammation and Disease: A Unique Inflammatory Pathway Activator

The urokinase-type plasminogen activator receptor (uPAR) is a unique protease binding receptor, now recognized as a key regulator of inflammation. Initially, uPA/uPAR was considered thrombolytic (clot-dissolving); however, recent studies have demonstrated its predominant immunomodulatory functions in inflammation and cancer. The uPA/uPAR complex has a multifaceted central role in both normal physiological and also pathological responses. uPAR is expressed as a glycophosphatidylinositol (GPI)-linked receptor interacting with vitronectin, integrins, G protein-coupled receptors, and growth factor receptors within a large lipid raft. Through protein-to-protein interactions, cell surface uPAR modulates intracellular signaling, altering cellular adhesion and migration. The uPA/uPAR also modifies extracellular activity, activating plasminogen to form plasmin, which breaks down fibrin, dissolving clots and activating matrix metalloproteinases that lyse connective tissue, allowing immune and cancer cell invasion and releasing growth factors. uPAR is now recognized as a biomarker for inflammatory diseases and cancer; uPAR and soluble uPAR fragments (suPAR) are increased in viral sepsis (COVID-19), inflammatory bowel disease, and metastasis. Here, we provide a comprehensive overview of the structure, function, and current studies examining uPAR and suPAR as diagnostic markers and therapeutic targets. Understanding uPAR is central to developing diagnostic markers and the ongoing development of antibody, small-molecule, nanogel, and virus-derived immune-modulating treatments that target uPAR.

Emerging role of cellular senescence in normal lung development and perinatal lung injury

Cellular senescence is a status of irreversible growth arrest, which can be triggered by the p53/p21cip1 and p16INK4/Rb pathways via intrinsic and external factors. Senescent cells are typically enlarged and flattened, and characterized by numerous molecular features. The latter consists of increased surfaceome, increased residual lysosomal activity at pH 6.0 (manifested by increased activity of senescence-associated beta-galactosidase [SA-β-gal]), senescence-associated mitochondrial dysfunction, cytoplasmic chromatin fragment, nuclear lamin b1 exclusion, telomere-associated foci, and the senescence-associated secretory phenotype. These features vary depending on the stressor leading to senescence and the type of senescence. Cellular senescence plays pivotal roles in organismal aging and in the pathogenesis of aging-related diseases. Interestingly, senescence can also both promote and inhibit wound healing processes. We recently report that senescence as a programmed process contributes to normal lung development. Lung senescence is also observed in Down Syndrome, as well as in premature infants with bronchopulmonary dysplasia and in a hyperoxia-induced rodent model of this disease. Furthermore, this senescence results in neonatal lung injury. In this review, we briefly discuss the molecular features of senescence. We then focus on the emerging role of senescence in normal lung development and in the pathogenesis of bronchopulmonary dysplasia as well as putative signaling pathways driving senescence. Finally, we discuss potential therapeutic approaches targeting senescent cells to prevent perinatal lung diseases.

Mitofusin-2 regulates leukocyte adhesion and β2 integrin activation

Neutrophils are critical for inflammation and innate immunity, and their adhesion to vascular endothelium is a crucial step in neutrophil recruitment. Mitofusin-2 (MFN2) is required for neutrophil adhesion, but molecular details are unclear. Here, we demonstrated that β₂ -integrin-mediated slow-rolling and arrest, but not PSGL-1-mediated cell rolling, are defective in MFN2-deficient neutrophil-like HL60 cells. This adhesion defect is associated with reduced expression of fMLP (N-formylmethionyl-leucyl-phenylalanine) receptor FPR1 as well as the inhibited β₂ integrin activation, as assessed by conformation-specific monoclonal antibodies. MFN2 deficiency also leads to decreased actin polymerization, which is important for β2 integrin activation. Mn²⁺ -induced cell spreading is also inhibited after MFN2 knockdown. MFN2 deficiency limited the maturation of β2 integrin activation during the neutrophil-directed differentiation of HL60 cells, which is indicated by CD35 and CD87 markers. MFN2 knockdown in β2-integrin activation-matured cells (CD87^high population) also inhibits integrin activation, indicating that MFN2 directly affects β2 integrin activation. Our study illustrates the function of MFN2 in leukocyte adhesion and may provide new insights into the development and treatment of MFN2 deficiency-related diseases.

The Contribution of the Urokinase Plasminogen Activator and the Urokinase Receptor to Pleural and Parenchymal Lung Injury and Repair: A Narrative Review

Pleural and parenchymal lung injury have long been characterized by acute inflammation and pathologic tissue reorganization, when severe. Although transitional matrix deposition is a normal part of the injury response, unresolved fibrin deposition can lead to pleural loculation and scarification of affected areas. Within this review, we present a brief discussion of the fibrinolytic pathway, its components, and their contribution to injury progression. We review how local derangements of fibrinolysis, resulting from increased coagulation and reduced plasminogen activator activity, promote extravascular fibrin deposition. Further, we describe how pleural mesothelial cells contribute to lung scarring via the acquisition of a profibrotic phenotype. We also discuss soluble uPAR, a recently identified biomarker of pleural injury, and its diagnostic value in the grading of pleural effusions. Finally, we provide an in-depth discussion on the clinical importance of single-chain urokinase plasminogen activator (uPA) for the treatment of loculated pleural collections.

Predictive value of soluble urokinase plasminogen activator receptor, soluble ST2, and IL-33 in bronchopulmonary dysplasia

BACKGROUND

Bronchopulmonary dysplasia (BPD) remains an important complication of preterm births. The soluble form of ST2 (sST2), interleukin-33 (IL-33), and soluble form of the urokinase plasminogen activator receptor (suPAR) have attracted increasing attention as biomarkers for different diseases. The aim of the current study was to assess the predictive value of plasma sST2, IL-33, and suPAR levels in patients with risk of BPD development.

METHODS

A total of 38 babies were studied prospectively on delivery to the neonatal intensive care unit. Serum levels of IL-33, sST2, and suPAR were measured using enzyme-linked immunosorbent assay. Serum samples were collected from umbilical cord (at the time of delivery, termed CB) and peripheral blood (on day 14, termed PB).

RESULTS

Levels of suPAR (PB-suPAR) and sST2 (PB-sST2) in the peripheral blood of the BPD group were significantly higher than the corresponding levels in the non-BPD group (P < 0.001, P = 0.028, respectively. There was a statistically significant correlation between PB-suPAR levels and the severity of BPD (P < 0.001)) when the suPAR results were analyzed using the receiver operating characteristic curve.

CONCLUSION

PB-suPAR and PB-sST2 levels are sensitive and specific independent predictive biomarkers in preterm babies with BPD.

AMP-activated protein kinase activation by 5-aminoimidazole-4-carbox-amide-1-β-D-ribofuranoside (AICAR) reduces lipoteichoic acid-induced lung inflammation

Adenosine monophosphate-activated protein (AMP)-activated kinase (AMPK) is a highly conserved kinase that plays a key role in energy homeostasis. Activation of AMPK was shown to reduce inflammation in response to lipolysaccharide in vitro and in vivo. 5-Aminoimidazole-4-carbox-amide-1-β-D-ribofuranoside (AICAR) is intracellularly converted to the AMP analog ZMP, which activates AMPK. Lipoteichoic acid (LTA) is a major component of the cell wall of Gram-positive bacteria that can trigger inflammatory responses. In contrast to lipopolysaccharide, little is known on the effects of AMPK activation in LTA-triggered innate immune responses. Here, we studied the potency of AMPK activation to reduce LTA-induced inflammation in vitro and in lungs in vivo. Activation of AMPK in vitro reduced cytokine production in the alveolar macrophage cell line MH-S. In vivo, AMPK activation reduced LTA-induced neutrophil influx, as well as protein leak and cytokine/chemokine levels in the bronchoalveolar space. In conclusion, AMPK activation inhibits LTA-induced lung inflammation in mice.

Effect of insulin-like growth factor blockade on hyperoxia-induced lung injury

Insulin-like growth factor (IGF)-1 is increased in different models of acute lung injury, and is an important determinant of survival and proliferation in many cells. We previously demonstrated that treatment of mice with IGF-1 receptor-blocking antibody (A12) improved early survival in bleomycin-induced lung injury. We have now examined whether administration of A12 improved markers of lung injury in hyperoxia model of lung injury. C57BL/6 mice underwent intraperitoneal administration of A12 or control antibody (keyhole limpet hemocyanin [KLH]), then were exposed to 95% hyperoxia for 88-90 hours. Mice were killed and bronchoalveolar lavage (BAL) and lung tissue were obtained for analysis. Hyperoxia caused a significant increase in IGF levels in BAL and lung lysates. Peripheral blood neutrophils expressed IGF-1R at baseline and after hyperoxia. BAL neutrophils from hyperoxia-treated mice and patients with acute lung injury also expressed cell surface IGF-1R. A12-treated mice had significantly decreased polymorphonuclear cell (PMN) count in BAL compared with KLH control mice (P = 0.02). BAL from A12-treated mice demonstrated decreased PMN chemotactic activity compared with BAL from KLH-treated mice. Pretreatment of PMNs with A12 decreased their chemotactic response to BAL from hyperoxia-exposed mice. Furthermore, IGF-1 induced a dose-dependent chemotaxis of PMNs. There were no differences in other chemotactic cytokines in BAL, including CXCL1 and CXCL2. In summary, IGF blockade decreased PMN recruitment to the alveolar space in a mouse model of hyperoxia. Furthermore, the decrease in BAL PMNs was at least partially due to a direct effect of A12 on PMN chemotaxis.

The receptor for urokinase regulates TLR2 mediated inflammatory responses in neutrophils

The urokinase-type plasminogen activator receptor (uPAR), a glycosylphosphatidylinositol (GPI) anchored membrane protein, regulates urokinase (uPA) protease activity, chemotaxis, cell-cell interactions, and phagocytosis of apoptotic cells. uPAR expression is increased in cytokine or bacteria activated cell populations, including macrophages and monocytes. However, it is unclear if uPAR has direct involvement in the response of inflammatory cells, such as neutrophils and macrophages, to Toll like receptor (TLR) stimulation. In this study, we found that uPAR is required for optimal neutrophil activation after TLR2, but not TLR4 stimulation. We found that the expression of TNF-α and IL-6 induced by TLR2 engagement in uPAR-/- neutrophils was less than that in uPAR+/+ (WT) neutrophils. Pretreatment of neutrophils with PI-PLC, which cleaves GPI moieties, significantly decreased TLR2 induced expression of TNF-α in WT neutrophils, but demonstrated only marginal effects on TNF-α expression in PAM treated uPAR-/- neutrophils. IκB-α degradation and NF-κB activation were not different in uPAR-/- or WT neutrophils after TLR2 stimulation. However, uPAR is required for optimal p38 MAPK activation after TLR2 engagement. Consistent with the in vitro findings that uPAR modulates TLR2 engagement induced neutrophil activation, we found that pulmonary and systemic inflammation induced by TLR2, but not TLR4 stimulation is reduced in uPAR-/- mice compared to WT counterparts. Therefore, our data suggest that neutrophil associated uPAR could be a potential target for treating acute inflammation, sepsis, and organ injury related to severe bacterial and other microbial infections in which TLR2 engagement plays a major role.

CD44 is protective during hyperoxia-induced lung injury

Patients with acute lung injury or respiratory distress syndrome often require supplemental oxygen to maintain tissue oxygenation; however, this treatment can cause or worsen lung inflammation. CD44 is a transmembrane adhesion molecule that is present on a wide variety of cell types, including leukocytes and parenchymal cells, and is an important player in leukocyte trafficking. The aim of this study was to determine the role of CD44 during hyperoxia-induced (> 95% oxygen) acute lung injury. Whereas all wild-type mice survived the 72-hour observation period, 37.5% of CD44 knockout (KO) mice died. CD44 deficiency was associated with a profound influx of neutrophils into the bronchoalveolar space, in the presence of similar or even lower neutrophil numbers in lung parenchyma, suggesting that CD44 is important for containing neutrophils in the pulmonary interstitium during hyperoxia. In addition, CD44 deficiency resulted in increased IL-6 and keratinocyte-derived chemokine release into bronchoalveolar lavage fluid (BALF). CD44 KO mice further displayed evidence for increased vascular leak and injury of type II respiratory epithelial cells. CD44 protected against bronchial epithelial cell death, as shown by increased epithelial cell necrosis and a trend toward increased BALF nucleosome levels in CD44 KO mice. CD44 can bind and internalize hyaluronic acid (HA), which acts proinflammatory. Concentrations of HA increased in BALF from CD44 KO but not wild-type mice during hyperoxia. These data suggest that CD44 protects against hyperoxia-induced lung injury and mortality by a mechanism that at least in part relies on its ability to clear HA from the bronchoalveolar space.