Impact of plasma 5-hydroxyindoleacetic acid, a serotonin metabolite, on clinical outcome in septic shock, and its effect on vascular permeability

Control of inflammatory lung injury and repair by metabolic signaling in endothelial cells

PURPOSE OF REVIEW

Sepsis-induced inflammatory lung injury includes acute lung injury (ALI) and acute respiratory distress syndrome (ARDS). There are currently no effective treatments for ALI/ARDS, but clinical outcomes could be improved by inhibiting lung injury and/or promoting post-sepsis vascular repair. In this review, we describe studies of endothelial cell metabolic pathways in sepsis-induced ALI/ARDS and vascular repair and identify areas of research that deserve attention in future studies. We also describe studies of metabolic interventions that aim to inhibit ALI/ARDS and/or promote post-sepsis vascular repair, including those that target endothelial cell metabolites, endothelial cell metabolic signaling pathways, and endothelial cell metabolism.

RECENT FINDINGS

Endothelial cells are integral to both the injury and repair phases of ALI/ARDS. During the injury phase of ALI/ARDS, lung endothelial cell survival decreases, and lung endothelial cell-to-endothelial cell (EC-EC) junctions are weakened. During the repair phase after sepsis-induced lung injury, lung endothelial cell proliferation and lung EC-EC junction reannealing occur. These crucial aspects of ALI/ARDS and post-sepsis vascular repair, that is, endothelial cell viability, growth, and junction integrity, are controlled by a myriad of metabolites and metabolic signaling pathways in endothelial cells.

SUMMARY

Metabolic signaling pathways in endothelial cells represent a novel class of putative targets for the prevention and treatment of sepsis-induced inflammatory lung injury. Therapies that target metabolic signaling in endothelial cells are currently being explored as potential treatments for sepsis-induced inflammatory lung injury.

Dissecting the mediating role of inflammatory factors in the interaction between metabolites and sepsis: insights from bidirectional Mendelian randomization

Sepsis, a life-threatening condition, involves complex interactions among metabolic alterations, inflammatory mediators, and host responses. This study utilized a bidirectional Mendelian randomization approach to investigate the causal relationships between 1400 metabolites and sepsis, and the mediating role of inflammatory factors. We identified 36 metabolites significantly associated with sepsis (p < 0.05), with AXIN1, FGF-19, FGF-23, IL-4, and OSM showing an inverse association, suggesting a protective role, while IL-2 exhibited a positive correlation, indicating a potential risk factor. Among these metabolites, Piperine and 9-Hydroxystearate demonstrated particularly interesting protective effects against sepsis. Piperine's protective effect was mediated through its interaction with AXIN1, contributing to a 16.296% reduction in sepsis risk. This suggests a potential pathway where Piperine influences sepsis outcomes by modulating AXIN1 levels. 9-Hydroxystearate also exhibited a protective role against sepsis, mediated through its positive association with FGF-19 and negative association with IL-2, contributing 9.436% and 12.565%, respectively, to its protective effect. Experimental validation confirmed significantly elevated IL-2 levels and reduced FGF-19, AXIN1, piperine, and 9-hydroxyoctadecanoic acid levels in sepsis patients compared to healthy controls. Piperine levels positively correlated with AXIN1, while 9-hydroxyoctadecanoic acid levels negatively correlated with IL-2 and positively correlated with FGF-19, supporting the Mendelian randomization findings. Our findings provide insights into the molecular mechanisms of sepsis, highlighting the unique roles and contributions of specific metabolites and their interactions with inflammatory mediators. This study enhances our understanding of sepsis pathophysiology and opens avenues for targeted therapeutic interventions and biomarker development for sepsis management. However, further research is essential to validate these pathways across diverse populations and fully explore the roles of these metabolites in sepsis.

Association of tryptophan pathway metabolites with mortality and effectiveness of nutritional support among patients at nutritional risk: secondary analysis of a randomized clinical trial

Tryptophan is an essential amino acid and is the precursor of many important metabolites and neurotransmitters. In malnutrition, the availability of tryptophan is reduced, potentially putting patients at increased risks. Herein, we investigated the prognostic implications of the tryptophan metabolism in a secondary analysis of the Effect of Early Nutritional Support on Frailty, Functional Outcomes, and Recovery of Malnourished Medical Inpatients Trial (EFFORT), a randomized, controlled trial comparing individualized nutritional support to usual care in patients at risk for malnutrition. Among 238 patients with available measurements, low plasma levels of metabolites were independently associated with 30-day mortality with adjusted hazard ratios (HR) of 1.77 [95% CI 1.05-2.99, p 0.034] for tryptophan, 3.49 [95% CI 1.81-6.74, p < 0.001] for kynurenine and 2.51 [95% CI 1.37-4.63, p 0.003] for serotonin. Nutritional support had more beneficial effects on mortality in patients with high tryptophan compared to patients with low tryptophan levels (adjusted HR 0.61 [95% CI 0.29-1.29] vs. HR 1.72 [95% CI 0.79-3.70], p for interaction 0.047). These results suggest that sufficient circulating levels of tryptophan might be a metabolic prerequisite for the beneficial effect of nutritional interventions in this highly vulnerable patient population.

Serotonin Signaling and the Hyperpermeable Endothelial Barrier in Sepsis: Clues to a Molecular Mechanism

Sepsis is characterized by a severe systemic inflammatory response caused by hyperpermeability of the endothelial barrier resulting microvascular leakage, which is a leading factor to multiorgan failure. In sepsis, the hyperpermeable endothelial cells contribute to the activation of platelets, which release numerous mediators that affect coagulation, inflammatory response and are believed to directly or indirectly affect the integrity of the endothelial barrier. One such mediator is serotonin (5-hydroxytryptamine, 5-HT), a signaling molecule which mediates a number of cellular functions including regulation of cytoskeletal dynamics associated with barrier function of endothelial cells. The actions of 5-HT are mediated by different types of receptors and terminated via an uptake mechanism of a 5-HT transporter (SERT) on the platelet and endothelial cell. Earlier studies revealed unexpected discoveries concerning the impact of 5-HT signaling on the permeability of the endothelial barrier. These findings have been supported by the clinical reports on the anti-inflammatory property of 5-HT reuptake inhibitor, SSRIs in treating sepsis-related morbidity and mortality. This review focuses on a wide-range of literature to pinpoint cellular and molecular mechanisms that mediate 5-HT-induced microvascular injury in sepsis pathogenesis.

GPR35 promotes neutrophil recruitment in response to serotonin metabolite 5-HIAA

Rapid neutrophil recruitment to sites of inflammation is crucial for innate immune responses. Here, we reveal that the G-protein-coupled receptor GPR35 is upregulated in activated neutrophils, and it promotes their migration. GPR35-deficient neutrophils are less recruited from blood vessels into inflamed tissue, and the mice are less efficient in clearing peritoneal bacteria. Using a bioassay, we find that serum and activated platelet supernatant stimulate GPR35, and we identify the platelet-derived serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) as a GPR35 ligand. GPR35 function in neutrophil recruitment is strongly dependent on platelets, with the receptor promoting transmigration across platelet-coated endothelium. Mast cells also attract GPR35+ cells via 5-HIAA. Mice deficient in 5-HIAA show a loss of GPR35-mediated neutrophil recruitment to inflamed tissue. These findings identify 5-HIAA as a GPR35 ligand and neutrophil chemoattractant and establish a role for platelet- and mast cell-produced 5-HIAA in cell recruitment to the sites of inflammation and bacterial clearance.

Impact of Plasma 5 Hydroxyindoleacetic Acid, a Serotonin Metabolite, on Clinical Severity in Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is characterized by dysregulated vascular permeability. The clinical outcomes remain poor, and the disease burden is widespread. We demonstrated that plasma 5-hydroxyindoleacetic acid (5-HIAA), a serotonin metabolite, is a pivotal severity indicator of ARDS. Serotonin is an effector of cellular contraction and a modulator of vascular permeability. Plasma 5-HIAA levels were significantly elevated in severe ARDS cases with shock status (p = 0.047) and positively correlated with SOFA (p < 0.0001) and APACHE-II score (p < 0.0001). In the longitudinal analysis, plasma 5-HIAA levels were also a strong independent predictor of mortality rate (p = 0.005). This study indicates that plasma 5-HIAA is a biomarker of ARDS severity and highlights the importance of evaluating vascular leakage levels for ARDS treatment.