Apolipoprotein M Protects Against Lipopolysaccharide-Induced Acute Lung Injury via Sphingosine-1-Phosphate Signaling

High-density lipoproteins and COVID-19: preparing the next pandemic

High-density lipoproteins (HDLs) are heterogeneous particles with pleiotropic functions including anti-inflammatory and anti-infectious effects. In clinical studies, lower HDL-associated cholesterol (HDL-C) concentration has been associated with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection, severity, and mortality. A reduction in the number of HDL particles, particularly small ones has been observed with alterations in their protein and lipid composition impairing their functions. These observations have supported HDL supplementation with promising results in small preliminary studies. This review summarizes available evidence to better understand the two-way interaction between HDLs and Coronavirus disease 2019 (COVID-19) and guide future HDL-based therapies for preparing for the next pandemic.

Black phosphorus nanomaterials mediate size-dependent acute lung injury by promoting macrophage polarization

Black phosphorus nanomaterials (BPNM) exhibit excellent properties and potential applications in electronics, but workers may face inhalation exposure during BPNM production. In addition, there is a lack of biosafety assessments regarding respiratory exposure to BPNM of different sizes. In this study, we investigated the lung toxicity in mice exposed to 5, 50, 500 μg/kg of black phosphorus quantum dots (BPQDs) and black phosphorus nanosheet (BPNS) via single tracheal instillation. The average diameter of the BPQDs and BPNS were 13.48 ± 4.82 nm and 325.53 ± 165.17 nm, respectively. Twenty-four hours after exposure, both BPQDs and BPNS caused acute lung injury, characterized by bronchial wall thickening, alveolar collapse, increased lamellar bodies, and immune cell infiltration. BPNS exposure resulted in reduced gene expression of pulmonary surfactant proteins Spb and Spc. Additionally, both BPQDs and BPNS increased inflammatory factors gene expression and induced lung macrophages polarization, with BPNS demonstrating a more significant effect. This study is the first to show that BPQDs and BPNS induced acute lung injury and inflammation in mice, with BPNS being more toxic. These findings are crucial for enhancing the biosafety assessment of BPNM and advancing technologies to improve the safety of nanomaterials.

S1PR3-driven positive feedback loop sustains STAT3 activation and keratinocyte hyperproliferation in psoriasis

Psoriasis is a chronic inflammatory skin disorder characterized by hyperproliferation of keratinocytes and persistent inflammation. Although persistent activation of signal transducer and activator of transcription 3 (STAT3) is implicated in its pathogenesis, the mechanisms underlying the sustained STAT3 activation remain poorly understood. Here, we identify sphingosine-1-phosphate receptor 3 (S1PR3) as a critical regulator of STAT3 activation and psoriasis pathogenesis, orchestrating a self-amplifying circuit that sustains keratinocyte hyperproliferation and chronic inflammation. S1PR3 expression is markedly elevated in psoriatic lesions and correlates with disease severity. Using genetic and pharmacological approaches, we reveal a novel S1PR3-Src-STAT3 signaling axis that drives both early and prolonged STAT3 activation in keratinocytes. Mechanistically, S1PR3 operates through Gαi/PKA-mediated Src activation, enhancing STAT3 phosphorylation and subsequent transcriptional activity. Importantly, we reveal a previously unrecognized positive feedback loop wherein activated STAT3 directly upregulates S1PR3 expression, perpetuating inflammation and hyperproliferation. Genetic deletion of S1pr3 in mice or pharmacological inhibition of S1PR3 significantly attenuates psoriasis-like skin inflammation, decreasing epidermal hyperplasia, dermal angiogenesis, and inflammatory mediator production. These findings provide new insights into the molecular mechanisms underlying psoriasis and identify S1PR3 as a promising therapeutic target. Our study suggests that disrupting the S1PR3-STAT3 feedback loop may offer a novel strategy for treating psoriasis and potentially other chronic inflammatory diseases driven by persistent STAT3 activation.

The adipocyte apolipoprotein M is negatively associated with inflammation

Obesity is associated with the development of local adipose tissue (AT) and systemic inflammation. Most adipokines are upregulated with obesity and have pro-inflammatory properties. Few are downregulated and possess beneficial anti-inflammatory effects. The apolipoprotein M (APOM) is an adipokine whose expression is low during obesity and associated with a metabolically healthy AT. Here, the role of adipose-derived APOM on obesity-associated AT inflammation was investigated by measuring the expression of pro-inflammatory genes in human and mouse models. In 300 individuals with obesity, AT APOM mRNA level was negatively associated with plasma hs-CRP. The inflammatory profile was assessed in Apom-/- and WT mice fed a normal chow diet (NCD), or a high-fat diet (HFD) to induce AT inflammation. After HFD, mice had a higher inflammatory profile in AT and liver, and a 50% lower Apom gene expression compared with NCD-fed mice. Apom deficiency was associated with a higher inflammatory signature in AT compared with WT mice but not in the liver. Adeno-associated viruses encoding human APOM were used to induce APOM overexpression: in vivo, in WT mice AT prior to HFD; in vitro, in human adipocytes which conditioned media was applied to ThP-1 macrophages. The murine AT overexpressing APOM gene had a reduced inflammatory profile. The macrophages treated with APOM-enriched media from adipocytes exhibited lower IL6 and MCP1 gene expression compared with macrophages treated with control media, independently of S1P. Our study highlights the protective role of adipocyte APOM against obesity-induced AT inflammation.

How do sphingosine-1-phosphate affect immune cells to resolve inflammation?

Inflammation is an important immune response of the body. It is a physiological process of self-repair and defense against pathogens taken up by biological tissues when stimulated by damage factors such as trauma and infection. Inflammation is the main cause of high morbidity and mortality in most diseases and is the physiological basis of the disease. Targeted therapeutic strategies can achieve efficient toxicity clearance at the inflammatory site, reduce complications, and reduce mortality. Sphingosine-1-phosphate (S1P), a lipid signaling molecule, is involved in immune cell transport by binding to S1P receptors (S1PRs). It plays a key role in innate and adaptive immune responses and is closely related to inflammation. In homeostasis, lymphocytes follow an S1P concentration gradient from the tissues into circulation. One widely accepted mechanism is that during the inflammatory immune response, the S1P gradient is altered, and lymphocytes are blocked from entering the circulation and are, therefore, unable to reach the inflammatory site. However, the full mechanism of its involvement in inflammation is not fully understood. This review focuses on bacterial and viral infections, autoimmune diseases, and immunological aspects of the Sphks/S1P/S1PRs signaling pathway, highlighting their role in promoting intradial-adaptive immune interactions. How S1P signaling is regulated in inflammation and how S1P shapes immune responses through immune cells are explained in detail. We teased apart the immune cell composition of S1P signaling and the critical role of S1P pathway modulators in the host inflammatory immune system. By understanding the role of S1P in the pathogenesis of inflammatory diseases, we linked the genomic studies of S1P-targeted drugs in inflammatory diseases to provide a basis for targeted drug development.

Receptor-dependent effects of sphingosine-1-phosphate (S1P) in COVID-19: the black side of the moon

Severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) infection leads to hyper-inflammation and amplified immune response in severe cases that may progress to cytokine storm and multi-organ injuries like acute respiratory distress syndrome and acute lung injury. In addition to pro-inflammatory cytokines, different mediators are involved in SARS-CoV-2 pathogenesis and infection, such as sphingosine-1-phosphate (S1P). S1P is a bioactive lipid found at a high level in plasma, and it is synthesized from sphingomyelin by the action of sphingosine kinase. It is involved in inflammation, immunity, angiogenesis, vascular permeability, and lymphocyte trafficking through G-protein coupled S1P receptors. Reduction of the circulating S1P level correlates with COVID-19 severity. S1P binding to sphingosine-1-phosphate receptor 1 (S1PR1) elicits endothelial protection and anti-inflammatory effects during SARS-CoV-2 infection, by limiting excessive INF-α response and hindering mitogen-activated protein kinase and nuclear factor kappa B action. However, binding to S1PR2 opposes the effect of S1PR1 with vascular inflammation, endothelial permeability, and dysfunction as the concomitant outcome. This binding also promotes nod-like receptor pyrin 3 (NLRP3) inflammasome activation, causing liver inflammation and fibrogenesis. Thus, higher expression of macrophage S1PR2 contributes to the activation of the NLRP3 inflammasome and the release of pro-inflammatory cytokines. In conclusion, S1PR1 agonists and S1PR2 antagonists might effectively manage COVID-19 and its severe effects. Further studies are recommended to elucidate the potential conflict in the effects of S1P in COVID-19.

S1PR1 attenuates pulmonary fibrosis by inhibiting EndMT and improving endothelial barrier function

BACKGROUND

Idiopathic pulmonary fibrosis (IPF) is a chronic fatal disease of unknown etiology. Its pathological manifestations include excessive proliferation and activation of fibroblasts and deposition of extracellular matrix. Endothelial cell-mesenchymal transformation (EndMT), a novel mechanism that generates fibroblast during IPF, is responsible for fibroblast-like phenotypic changes and activation of fibroblasts into hypersecretory cells. However, the exact mechanism behind EndMT-derived fibroblasts and activation is uncertain. Here, we investigated the role of sphingosine 1-phosphate receptor 1 (S1PR1) in EndMT-driven pulmonary fibrosis.

METHODS

We treated C57BL/6 mice with bleomycin (BLM) in vivo and pulmonary microvascular endothelial cells with TGF-β1 in vitro. Western blot,flow cytometry, and immunofluorescence were used to detect the expression of S1PR1 in endothelial cells. To evaluate the effect of S1PR1 on EndMT and endothelial barrier and its role in lung fibrosis and related signaling pathways, S1PR1 agonist and antagonist were used in vitro and in vivo.

RESULTS

Endothelial S1PR1 protein expression was downregulated in both in vitro and in vivo models of pulmonary fibrosis induced by TGF-β1 and BLM, respectively. Downregulation of S1PR1 resulted in EndMT, indicated by decreased expression of endothelial markers CD31 and VE-cadherin, increased expression of mesenchymal markers α-SMA and nuclear transcription factor Snail, and disruption of the endothelial barrier. Further mechanistic studies found that stimulation of S1PR1 inhibited TGF-β1-mediated activation of the Smad2/3 and RhoA/ROCK1 pathways. Moreover, stimulation of S1PR1 attenuated Smad2/3 and RhoA/ROCK1 pathway-mediated damage to endothelial barrier function.

CONCLUSIONS

Endothelial S1PR1 provides protection against pulmonary fibrosis by inhibiting EndMT and attenuating endothelial barrier damage. Accordingly, S1PR1 may be a potential therapeutic target in progressive IPF.

Nuclear SPHK2/S1P induces oxidative stress and NLRP3 inflammasome activation via promoting p53 acetylation in lipopolysaccharide-induced acute lung injury

A bulk of evidence identified that macrophages, including resident alveolar macrophages and recruited macrophages from the blood, played an important role in the pathogenesis of acute respiratory distress syndrome (ARDS). However, the molecular mechanisms of macrophages-induced acute lung injury (ALI) by facilitating oxidative stress and inflammatory responses remain unclear. Herein, we noticed that the levels of mitochondrial reactive oxygen species (mtROS), SPHK2 and activated NLRP3 inflammasome were higher in peripheral blood mononuclear cells (PBMCs) of ARDS patients than that in healthy volunteers. Similar observations were recapitulated in LPS-treated RAW264.7 and THP-1 cells. After exposure to LPS, the SPHK2 enzymatic activity, NLRP3 inflammasome activation and mtROS were significantly upregulated in macrophages. Moreover, knockdown SPHK2 via shRNA or inhibition SPHK2 could prominently decrease LPS-induced M1 macrophage polarization, oxidative stress and NLRP3 inflammasome activation. Further study indicated that upregulated SPHK2 could increase nuclear sphingosine-1-phosphate (S1P) levels and then restrict the enzyme activity of HDACs to facilitate p53 acetylation. Acetylation of p53 reinforced its binding to the specific region of the NLRP3 promoter and drove expression of NLRP3. In the in vivo experiments, it was also observed that treating with Opaganib (ABC294640), a specific SPHK2 inhibitor, could observably alleviate LPS-induced ALI, evidencing by lowered infiltration of inflammatory cells, increased M2 macrophages polarization and reduced oxidative damage in lung tissues. Besides, SPHK2 inhibition can also decrease the accumulation of acetylated p53 protein and the activation of NLRP3 inflammasome. Taken together, our results demonstrated for the first time that nuclear S1P can regulate the acetylation levels of non-histone protein through affecting HDACs enzyme activities, linking them to oxidative stress and inflammation in response to environmental signals. These data provide a theoretical basis that SPHK2 may be an effective therapeutic target of ARDS.

MiR-363-3p/S1PR1 axis inhibits sepsis-induced acute lung injury via the inactivation of NF-κB signaling

Infection-associated inflammation and coagulation are critical pathologies in sepsis-induced acute lung injury (ALI). This study aimed to investigate the effects of microRNA-363-3p (miR-363-3p) on sepsis-induced ALI and explore the underlying mechanisms. A cecal ligation and puncture-induced septic mouse model was established. The results of this study suggested that miR-363-3p was highly expressed in lung tissues of septic mice. Knockdown of miR-363-3p attenuated sepsis-induced histopathological damage, the inflammation response and oxidative stress in lung tissues. Furthermore, knockdown of miR-363-3p reduced the formation of platelet-derived microparticles and thrombin generation in blood samples of septic mice. Downregulation of miR-363-3p suppressed sphingosine-1-phosphate receptor 1 (S1PR1) expression in lung tissues and subsequently inactivated the nuclear factor kappa-B ligand (NF-κB) signaling. A luciferase reporter assay confirmed that miR-363-3p directly targeted the 3'-UTR of the mouse S1pr1 mRNA. Collectively, our study suggests that inactivation of NF-κB signaling is involved in the miR-363-3p/S1PR1 axis-mediated protective effect on septic ALI.

JTE-013 Alleviates Inflammatory Injury and Endothelial Dysfunction Induced by Sepsis In Vivo and In Vitro

BACKGROUND

Nowadays, there is no approved targeted agent for lung injury induced by sepsis. S1PR2 is confirmed to be a promising diagnosis and treatment target. JTE-013 as S1PR2 antagonists may be an agent of great potential. In this research, we sought to determine the functional role of JTE-013 in lung injury induced by sepsis.

MATERIALS AND METHODS

Seventy-two rats were assigned into normal group, sepsis model group and JTE-013 group. The animal model of lung injury induced by sepsis was constructed by cecal ligation and puncture. The human pulmonary microvascular endothelial cells (HPMECs) were divided into control, LPS and LPS + JTE-013 group. HPMECs induced by LPS served as the cell model of lung injury induced by sepsis. HE staining assay was performed for assessment of the pathological condition and Evans blue was applied for assessment of pulmonary tissue permeability. Wet/dry ratio was measured as indicators of pulmonary edema degree and neutrophil count was measured as indicators of infection status. The levels of inflammatory factors were detected by corresponding kits, cell survival by CCK-8 assay and protein expression level by western blot.

RESULTS

S1PR2 was highly expressed in vivo model of lung injury induced by sepsis. It was observed that JTE-013 as antagonist of S1PR2 alleviated the lung tissue injury, endothelial dysfunction and pulmonary edema induced by sepsis. In addition, JTE-013 reduced neutrophil count and levels of inflammatory factors. Moreover, results confirmed that JTE-013 enhanced cell viability and mitigated inflammatory response in cell model of sepsis.

CONCLUSIONS

Overall, JTE-013 as an antagonist of S1PR2 could relieve inflammatory injury and endothelial dysfunction induced by sepsis in vivo and vitro, resulting in attenuation of lung injury. These findings elucidated that JTE-013 may be a promising targeted agent for lung injury induced by sepsis.

Propofol promotes migration, alleviates inflammation, and apoptosis of lipopolysaccharide-induced human pulmonary microvascular endothelial cells by activating PI3K/AKT signaling pathway via upregulating APOM expression

Propofol (PRO), a clinical potent intravenous anesthetic, plays a significant role in relieving inflammatory diseases by repressing the release of inflammatory cytokines. The present study was aimed to reveal a novel mechanism by which PRO alleviates acute lung injury (ALI). Lipopolysaccharide (LPS) was utilized to induce human pulmonary microvascular endothelial cells (HPMECs) so as to simulate the microenvironment of ALI, and the expression of apolipoprotein M (APOM) was examined with western blotting. Then, APOM was silenced and profopol was used to treat the LPS-injured HPMECs. The cell viability, migration, and apoptosis were respectively observed after the processes of cell counting kit-8, wound healing, transwell, and TUNEL assay. Meanwhile, the inflammatory response was detected by determining the contents of inflammatory cytokines. Subsequently, the relationship between phosphoinositide-3 kinase (PI3K)/protein kinase B (AKT) signaling pathway and PRO was analyzed by western blotting. PI3K/AKT inhibitor LY294002 was employed to evaluate whether the effects of PRO on LPS-challenged HPMECs injury were mediated by this pathway. Results revealed that APOM was notably downregulated in HPMECs after LPS exposure. PRO treatment promoted cell proliferation and migration while alleviated inflammation and apoptosis of LPS-treated HPMECs, which was reversed by APOM-downregulation. PRO brought about the upregulation of proteins in PI3K/AKT signaling pathway, and LY294002 intervention further accentuated the impacts of APOM-knockdown on LPS-challenged HPMECs injury. To conclude, PRO promotes migration and alleviates inflammation and apoptosis of LPS-treated HPMECs by PI3K/AKT signaling pathway via upregulating APOM, which laid an experimental foundation for the future study and clinical application of PRO.

Barrier maintenance by S1P during inflammation and sepsis

Sphingosine 1-phosphate (S1P) is a multifaceted lipid signaling molecule that activates five specific G protein-coupled S1P receptors. Despite the fact that S1P is known as one of the strongest barrier-enhancing molecules for two decades, no medical application is available yet. The reason for this lack of translation into clinical practice may be the complex regulatory network of S1P signaling, metabolism and transportation.In this review, we will provide an overview about the physiology and the network of S1P signaling with the focus on endothelial barrier maintenance in inflammation. We briefly describe the physiological role of S1P and the underlying S1P signaling in barrier maintenance, outline differences of S1P signaling and metabolism in inflammatory diseases, discuss potential targets and compounds for medical intervention, and summarize our current knowledge regarding the role of S1P in the maintenance of specialized barriers like the blood-brain barrier and the placenta.

Increased expression levels of inflammatory cytokines and adhesion molecules in lipopolysaccharide‑induced acute inflammatory apoM‑/‑ mice

Apolipoprotein M (apoM) may serve a protective role in the development of inflammation. Nuclear factor‑κB (NF‑κB) and its downstream factors (including a number of inflammatory cytokines and adhesion molecules) are essential for the regulation of inflammatory processes. In the present study, the importance of apoM in lipopolysaccharide (LPS)‑induced acute inflammation and its potential underlying mechanisms, were investigated using an apoM‑knockout mouse model. The levels of inducible nitric oxide synthase (iNOS), NF‑κB, interleukin (IL)‑1β, intercellular adhesion molecule 1 (ICAM‑1) and vascular cell adhesion protein 1 (VCAM‑1) were detected using reverse transcription‑quantitative PCR and western blotting. The serum levels of IL‑6 and IL‑10 were detected using Luminex technology. The results demonstrated that the protein levels of iNOS, NF‑κB, IL‑1β, ICAM‑1 and VCAM‑1 were significantly increased in apoM‑/‑ mice compared with those in apoM+/+ mice. In addition, two‑way ANOVA revealed that the interaction between apoM and LPS had a statistically significant effect on a number of factors, including the mRNA expression levels of hepatic iNOS, NF‑κB, IL‑1β, ICAM‑1 and VCAM‑1. Notably, the effects of apoM and 10 mg/kg LPS on the levels of IL‑6 and IL‑10 were the opposite of those induced by 5 mg/kg LPS, which could be associated with the dual anti‑ and pro‑inflammatory effects of IL‑6 and IL‑10. Collectively, the results of the present study revealed that apoM is an important regulator of inflammatory cytokine and adhesion molecule production in LPS‑induced inflammation, which may consequently be associated with the severity of inflammation. These findings indicated that the anti‑inflammatory effects of apoM may partly result from the inhibition of the NF‑κB pathway.

Comprehensive lipidomics in apoM-/- mice reveals an overall state of metabolic distress and attenuated hepatic lipid secretion into the circulation

Apolipoprotein M (apoM) participates in both high-density lipoprotein and cholesterol metabolism. Little is known about how apoM affects lipid composition of the liver and serum. In this study, we systemically investigated the effects of apoM on liver and plasma lipidomes and how apoM participates in lipid cycling, via apoM knockout in mice and the human SMMC-7721 cell line. We used integrated mass spectrometry-based lipidomics approaches to semiquantify more than 600 lipid species from various lipid classes, which include free fatty acids, glycerolipids, phospholipids, sphingolipids, glycosphingolipids, cholesterol, and cholesteryl esters (CEs), in apoM-/- mouse. Hepatic accumulation of neutral lipids, including CEs, triacylglycerols, and diacylglycerols, was observed in apoM-/- mice; while serum lipidomic analyses showed that, in contrast to the liver, the overall levels of CEs and saturated/monounsaturated fatty acids were markedly diminished. Furthermore, the level of ApoB-100 was dramatically increased in the liver, whereas significant reductions in both ApoB-100 and low-density lipoprotein (LDL) cholesterol were observed in the serum of apoM-/- mice, which indicated attenuated hepatic LDL secretion into the circulation. Lipid profiles and proinflammatory cytokine levels indicated that apoM-/- leads to hepatic steatosis and an overall state of metabolic distress. Taken together, these results revealed that apoM knockout leads to hepatic steatosis, impaired lipid secretion, and an overall state of metabolic distress.

HDL-S1P protects endothelial function and reduces lung injury during sepsis in vivo and in vitro

OBJECTIVE

In sepsis, the protection of the vascular endothelium is essential and the maintenance of its function is critical to prevent further deterioration. High-density lipoprotein (HDL)-associated sphingosine-1-phosphate (S1P) is a bioactive lipid in plasma and its role in sepsis has not been extensively studied. This study aimed to investigate the effects of HDL-S1P on sepsis in cellular and animal models, as well as human plasma samples.

MEASUREMENTS

We established an animal model of sepsis with different severities achieved by caecal ligation and puncture (CLP) and lipopolysaccharide (LPS) injection, and then explored the relationship between HDL-S1P and lung endothelial dysfunction in vivo. To determine the effects of HDL-S1P in the pulmonary endothelium of septic rats, we then injected HDL-S1P into septic rats to find out if it can reduce the lung injury caused by sepsis. Further, we explored the mechanism in vitro by studying the role of S1P-specific receptor agonists and inhibitors in LPS-stimulated human umbilical vein endothelial cells. We also explored the relationship between plasma HDL-S1P content and sepsis severity in septic patients by analysing their plasma samples.

RESULTS

HDL-S1P concentrations in plasma were negatively correlated with endothelial functional damage in sepsis, both in the animal model and in the septic patients in our study. In vivo, HDL-S1P injection significantly reduced pulmonary oedema and endothelial leakage in septic rats. In vitro, cell experiments showed that HDL-S1P effectively protected the proliferation and migration abilities of endothelial cells, which could be partly explained by its biased activation of the S1P receptor 1.

CONCLUSION

Our study preliminary explored the function of HDL-S1P in sepsis in cellular and animal models, as well as human subjects. The results indicate HDL-S1P protected endothelial functions in septic patients. Thus, it has therapeutic potential and can be used for the clinical treatment of sepsis.

High-Density Lipoproteins Are Bug Scavengers

Lipoproteins were initially defined according to their composition (lipids and proteins) and classified according to their density (from very low- to high-density lipoproteins—HDLs). Whereas their capacity to transport hydrophobic lipids in a hydrophilic environment (plasma) is not questionable, their primitive function of cholesterol transporter could be challenged. All lipoproteins are reported to bind and potentially neutralize bacterial lipopolysaccharides (LPS); this is particularly true for HDL particles. In addition, HDL levels are drastically decreased under infectious conditions such as sepsis, suggesting a potential role in the clearance of bacterial material and, particularly, LPS. Moreover, "omics" technologies have unveiled significant changes in HDL composition in different inflammatory states, ranging from acute inflammation occurring during septic shock to low-grade inflammation associated with moderate endotoxemia such as periodontal disease or obesity. In this review, we will discuss HDL modifications associated with exposure to pathogens including bacteria, viruses and parasites, with a special focus on sepsis and the potential of HDL therapy in this context. Low-grade inflammation associated with atherosclerosis, periodontitis or metabolic syndrome may also highlight the protective role of HDLs in theses pathologies by other mechanisms than the reverse transport of cholesterol.

Apolipoprotein M: Research Progress and Clinical Perspective

Apolipoprotein M (apoM) was first identified and characterized to the apolipoprotein family in 1999. Human apoM gene is located in a highly conserved segment in the major histocompatibility complex (MHC) class III locus on chromosome 6 and codes for an about 23 kDa protein that structurally belongs to the lipocalin superfamily. ApoM is selectively expressed in hepatocytes and in the tubular epithelium of kidney. In human plasma, apoM is mainly confined to the high-density lipoprotein (HDL) particles, but it may also occur in other lipoprotein classes, such as in the triglyceride-rich particles after fat intake. It has been demonstrated that apoM is critical for the formation of HDL, notably pre-beta HDL1. The antiatherogenic function of HDL is well established, and its ability to promote cholesterol efflux from foam cells in the atherosclerotic lesions is generally regarded as one of the key mechanisms behind this protective function. However, HDL could also display a variety of properties that may affect the complex atherosclerotic processes by other mechanisms, thus being involved in processes related to antioxidant defense, immune system, and systemic effects in septicemia, which may be partly contributed via its apolipoproteins and/or phospholipids. Moreover, it has been demonstrated that apoM functions as a natural carrier of sphingosin-1-phosphate (S1P) in vivo which may be related to its antiatherosclerotic and protective effects on endothelial cell barrier and anti-inflammatory properties. These may also provide a link between the diverse effects of HDL.

Apolipoprotein M/sphingosine-1-phosphate: novel effects on lipids, inflammation and kidney biology

PURPOSE OF REVIEW

In 2011, the crystal structure of apolipoprotein M (apoM) and its capacity to bind sphingosine-1-phosphate (S1P) was characterized. Since then, a variety of studies has increased our knowledge on apoM biology and functionality. From being an unknown and hardly significant player in overall metabolism, apoM has gained significant interest.

RECENT FINDINGS

Key discoveries in the last 2 years have indicated that the apoM/S1P complex has important roles in lipid metabolism (affecting triglyceride turnover), inflammation (a marker of severe sepsis and potentially providing anti-inflammatory signaling) and kidney biology (potential to protect against immunoglobulin A nephropathy).

SUMMARY

Several studies suggest a potential for apoM/S1P as biomarkers for inflammation, sepsis and nephropathy. Also, a novel chaperone is characterized and could have potential as a drug for treatment in inflammation and nephropathy.

The signalling roles of sphingosine-1-phosphate derived from red blood cells and platelets

Sphingosine-1-phosphate (S1P) is an essential, bioactive lysophospholipid mediator that regulates various physiological functions such as lymphocyte trafficking, inflammation and behavioural characteristics of the vascular system. S1P signalling is mediated via a family of five GPCRs, which are expressed in various cell types and tissues. S1P concentration is maintained in a gradient through the activity of S1P degrading enzymes, and this gradient is critical for lymphocyte egress. To exert its extracellular signalling roles, S1P must be secreted out of the cells by protein transporters. The recent discovery of S1P transporters has shed light on the sources of S1P. However, these transporters still need to be clarified as they are important in defining the S1P gradient for lymphocyte recirculation and the source of S1P for maintenance of blood vessels. Here, we review the current understanding of S1P sources, highlighting the roles of S1P transporters with an emphasis on haematopoietic cells as a major source of circulatory S1P.

Photobiomodulation prevents DNA fragmentation of alveolar epithelial cells and alters the mRNA levels of caspase 3 and Bcl-2 genes in acute lung injury

Acute respiratory distress syndrome (ARDS) and acute lung injury (ALI) are defined as pulmonary inflammation that could occur from sepsis and lead to pulmonary permeability and alveolar edema making them life-threatening diseases. Photobiomodulation (PBM) properties have been widely described in the literature in several inflammatory diseases; although the mechanisms of action are not always clear, this could be a possible treatment for ARDS/ALI. Thus, the aim of this study was to evaluate the mRNA levels from caspase-3 and BCL-2 genes and DNA fragmentation in lung tissue from Wistar rats affected by ALI and subjected to photobiomodulation by exposure to a low power infrared laser (808 nm; 100 mW; 3.571 W cm-2; four points per lung). Adult male Wistar rats were randomized into 6 groups (n = 5, for each group): control, PBM10 (10 J cm-2, 2 J and 2 seconds), PBM20 (20 J cm-2, 5 J and 5 seconds), ALI, ALI + PBM10 and ALI + PBM20. ALI was induced by intraperitoneal Escherichia coli lipopolysaccharide injection. Lung samples were collected and divided for mRNA expression of caspase-3 and Bcl-2 and DNA fragmentation quantifications. Data show that caspase-3 mRNA levels are reduced and Bcl-2 mRNA levels increased in ALI after low power infrared laser exposure when compared to the non-exposed ALI group. DNA fragmentation increased in inflammatory infiltrate cells and reduced in alveolar cells. Our research shows that photobiomodulation can alter relative mRNA levels in genes involved in the apoptotic process and DNA fragmentation in inflammatory and alveolar cells after lipopolysaccharide-induced acute lung injury. Also, inflammatory cell apoptosis is part of the photobiomodulation effects induced by exposure to a low power infrared laser.