Significance: Increased endothelial permeability and inflammation are two major hallmarks of the life-threatening conditions such as acute respiratory distress syndrome and sepsis. There is a growing consensus in the field that the Rho family of small guanosine triphosphates are critical regulators of endothelial function at both physiological and pathological states. A basal level of reactive oxygen species (ROS) is essential for maintaining metabolic homeostasis, vascular tone, and angiogenesis; however, excessive ROS generation impairs endothelial function and promotes lung inflammation. In this review, we will focus on the role of Rho in control of endothelial function and also briefly discuss a nexus between ROS generation and Rho activation during endothelial dysfunction. Recent Advances: Extensive studies in the past decades have established that a wide range of barrier-disruptive and proinflammatory agonists activate the Rho pathway that, ultimately, leads to endothelial dysfunction via disruption of endothelial barrier and further escalation of inflammation. An increasing body of evidence suggests that a bidirectional interplay exists between the Rho pathway and ROS generation during endothelial dysfunction. Rac, a member of the Rho family, is directly involved in ROS production and ROS, in turn, activate RhoA, Rac, and Cdc42. Critical Issues: A precise mechanism of interaction between ROS generation and Rho activation and its impact on endothelial function needs to be elucidated. Future Directions: By employing advanced molecular techniques, the sequential cascades in the Rho-ROS crosstalk signaling axis need to be explored. The therapeutic potential of the Rho pathway inhibitors in endothelial-dysfunction associated cardiopulmonary disorders needs to be evaluated.

Circulating and cell membrane phospholipids undergo oxidation caused by enzymatic and non-enzymatic mechanisms. As a result, a diverse group of bioactive oxidized phospholipids generated in these conditions have both beneficial and harmful effects on the human body. Increased production of oxidized phospholipid products with deleterious effects is linked to the pathogenesis of various cardiopulmonary disorders such as atherosclerosis, thrombosis, acute lung injury (ALI), and inflammation. It has been determined that the contrasting biological effects of lipid oxidation products are governed by their structural variations. For example, full-length products of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphorylcholine oxidation (OxPAPC) have prominent endothelial barrier protective and anti-inflammatory activities while most of the truncated oxidized phospholipids induce vascular leak and exacerbate inflammation. The extensive studies from our group and other groups have demonstrated a strong potential of OxPAPC in mitigating a wide range of agonist-induced lung injuries and inflammation in pulmonary endothelial cell culture and rodent models of ALI. Concurrently, elevated levels of truncated oxidized phospholipids are present in aged mice lungs that potentiate the inflammatory agents-induced lung injury. On the other hand, increased levels of full length OxPAPC products accelerate ALI recovery by facilitating production of anti-inflammatory lipid mediator, lipoxin A4, and other molecules with anti-inflammatory properties. These findings suggest that OxPAPC-assisted lipid program switch may be a promising therapeutic strategy for treatment of acute inflammatory syndromes. In this review, we will summarize the vascular-protective and deleterious aspects of oxidized phospholipids and discuss their therapeutic potential including engineering of stable analogs of oxidized phospholipids with improved anti-inflammatory and barrier-protective properties.

Extracellular histones released into the circulation following trauma, sepsis, and ARDS may act as potent damage-associated molecular pattern signals leading to multiple organ failure. Endothelial cell (EC) dysfunction caused by extracellular histones has been demonstrated in vitro and in vivo; however, precise mechanistic details of histone-induced EC dysfunction and exacerbation of ongoing inflammation remain poorly understood. This study investigated the role of extracellular histones in exacerbating preexisting endothelial dysfunction and acute lung injury. Histone subunits H3 and H4, but not H1, H2A, or H2B, induced permeability in human pulmonary EC. H3 and H4 at concentrations above 30 µg/mL caused EC inflammation reflected by activation of the NF-κB pathway, transcriptional activation, and release of cytokines and chemokines including IL-6 and IL-8, and increased mRNA and protein expression of EC adhesion molecules VCAM-1 and ICAM-1. Pharmacological inhibitors targeting Toll-like receptor TLR4 but not TLR2/6, blocked histone-induced EC dysfunction. H3 and H4 also strongly augmented EC permeability and inflammation caused by Gram-negative and Gram-positive bacterial particles, endotoxin, and TNFα. Heparin blocked histone-induced augmentation of EC inflammation caused by endotoxin and TNFα. Injection of histone in mouse models of lung injury caused by bacterial wall lipopolysaccharide (LPS) and heat-killed Staphylococcus aureus (HKSA) augmented ALI parameters: increased protein content, cell count, and inflammatory cytokine secretion in bronchoalveolar lavage fluid. Important clinical significance of these findings is in the demonstration that even a modest increase in extracellular histone levels can act as a severe exacerbating factor in conjunction with other EC barrier disruptive or proinflammatory agents.

Endothelial dysfunction has been attributed as one of the major complications in COVID-19 patients, a global pandemic that has already caused over 4 million deaths worldwide. The dysfunction of endothelial barrier is characterized by an increase in endothelial permeability and inflammatory responses, and has even broader implications in the pathogenesis of acute respiratory syndromes such as ARDS, sepsis and chronic illnesses represented by pulmonary arterial hypertension and interstitial lung disease. The structural integrity of endothelial barrier is maintained by cytoskeleton elements, cell-substrate focal adhesion and adhesive cell junctions. Agonist-mediated changes in endothelial permeability are directly associated with reorganization of actomyosin cytoskeleton leading to cell contraction and opening of intercellular gaps or enhancement of cortical actin cytoskeleton associated with strengthening of endothelial barrier. The role of actin cytoskeleton remodeling in endothelial barrier regulation has taken the central stage, but the impact of microtubules in this process remains less explored and under-appreciated. This review will summarize the current knowledge on the crosstalk between microtubules dynamics and actin cytoskeleton remodeling, describe the signaling mechanisms mediating this crosstalk, discuss epigenetic regulation of microtubules stability and its nexus with endothelial barrier maintenance, and overview a role of microtubules in targeted delivery of signaling molecules regulating endothelial permeability and inflammation.

Extracellular histones released from injured or dying cells following trauma and other severe insults can act as potent damage‐associated molecular patterns. In fact, elevated levels of histones are present in human circulation in hyperinflammatory states such as acute respiratory distress syndrome and sepsis. The molecular mechanisms owing to histone‐induced pathologies are at the very beginning of elucidating. However, neutralization of histones with antibodies, histone‐binding or histone‐degrading proteins, and heparan sulfates have shown promising therapeutic effects in pre‐clinical acute respiratory distress syndrome and sepsis models. Various cell types undergoing necrosis and apoptosis or activated neutrophils forming neutrophil extracellular traps have been implicated in excessive release of histones which further augments tissue injury and may culminate in multiple organ failure. At the molecular level, an uncontrolled inflammatory cascade has been considered as the major event; however, histone‐activated coagulation and thrombosis represent additional pathologic events reflecting coagulopathy. Furthermore, epigenetic regulation and chemical modifications of circulating histones appear to be critically important in their biological functions as evidenced by increased cytotoxicity associated with citrullinated histone. Herein, we will briefly review the current knowledge on the role of histones in acute respiratory distress syndrome and sepsis, and discuss the future potential of anti‐histone therapy for treatment of these life‐threatening disorders.

Tissue acidification resulting from dysregulated cellular bioenergetics accompanies various inflammatory states. GPR68, along with other members of proton-sensing G protein-coupled receptors, responds to extracellular acidification and has been implicated in chronic inflammation-related diseases such as ischemia, cancer, and colitis. The present study examined the role of extracellular acidification on human pulmonary endothelial cell (EC) permeability and inflammatory status per se and investigated potential synergistic effects of acidosis on endothelial dysfunction caused by bacterial lipopolysaccharide (LPS, Klebsiella pneumoniae). Results showed that medium acidification to pH 6.5 caused a delayed increase in EC permeability illustrated by a decrease in transendothelial electrical resistance and loss of continuous VE-cadherin immunostaining at cell junctions. Likewise, acidic pH induced endothelial inflammation reflected by increased mRNA and protein expression of EC adhesion molecules VCAM-1 and ICAM-1, upregulated mRNA transcripts of tumor necrosis factor-α, IL-6, IL-8, IL-1β, and CXCL5, and increased secretion of ICAM-1, IL-6, and IL-8 in culture medium monitored by ELISA. Among the GPCRs tested, acidic pH selectively increased mRNA and protein expression of GPR68, and only the GPR68-specific small molecule inhibitor OGM-8345 rescued acidosis-induced endothelial permeability and inflammation. Furthermore, acidic pH exacerbated LPS-induced endothelial permeability and inflammatory response in cultured lung macrovascular as well as microvascular endothelial cells. These effects were suppressed by OGM-8345 in both EC types. Altogether, these results suggest that GPR68 is a critical mediator of acidic pH-induced dysfunction of human pulmonary vascular endothelial cells and mediates the augmenting effect of tissue acidification on LPS-induced endothelial cell injury.