Pseudomonas aeruginosa stimulates nuclear sphingosine-1-phosphate generation and epigenetic regulation of lung inflammatory injury

Sphingolipid-Induced Bone Regulation and Its Emerging Role in Dysfunction Due to Disease and Infection

The human skeleton is a metabolically active system that is constantly regenerating via the tightly regulated and highly coordinated processes of bone resorption and formation. Emerging evidence reveals fascinating new insights into the role of sphingolipids, including sphingomyelin, sphingosine, ceramide, and sphingosine-1-phosphate, in bone homeostasis. Sphingolipids are a major class of highly bioactive lipids able to activate distinct protein targets including, lipases, phosphatases, and kinases, thereby conferring distinct cellular functions beyond energy metabolism. Lipids are known to contribute to the progression of chronic inflammation, and notably, an increase in bone marrow adiposity parallel to elevated bone loss is observed in most pathological bone conditions, including aging, rheumatoid arthritis, osteoarthritis, and osteomyelitis. Of the numerous classes of lipids that form, sphingolipids are considered among the most deleterious. This review highlights the important primary role of sphingolipids in bone homeostasis and how dysregulation of these bioactive metabolites appears central to many chronic bone-related diseases. Further, their contribution to the invasion, virulence, and colonization of both viral and bacterial host cell infections is also discussed. Many unmet clinical needs remain, and data to date suggest the future use of sphingolipid-targeted therapy to regulate bone dysfunction due to a variety of diseases or infection are highly promising. However, deciphering the biochemical and molecular mechanisms of this diverse and extremely complex sphingolipidome, both in terms of bone health and disease, is considered the next frontier in the field.

The Sphingolipid-Modulating Drug Opaganib Protects against Radiation-Induced Lung Inflammation and Fibrosis: Potential Uses as a Medical Countermeasure and in Cancer Radiotherapy

Fibrosis is a chronic pathology resulting from excessive deposition of extracellular matrix components that leads to the loss of tissue function. Pulmonary fibrosis can follow a variety of diverse insults including ischemia, respiratory infection, or exposure to ionizing radiation. Consequently, treatments that attenuate the development of debilitating fibrosis are in desperate need across a range of conditions. Sphingolipid metabolism is a critical regulator of cell proliferation, apoptosis, autophagy, and pathologic inflammation, processes that are all involved in fibrosis. Opaganib (formerly ABC294640) is the first-in-class investigational drug targeting sphingolipid metabolism for the treatment of cancer and inflammatory diseases. Opaganib inhibits key enzymes in sphingolipid metabolism, including sphingosine kinase-2 and dihydroceramide desaturase, thereby reducing inflammation and promoting autophagy. Herein, we demonstrate in mouse models of lung damage following exposure to ionizing radiation that opaganib significantly improved long-term survival associated with reduced lung fibrosis, suppression of granulocyte infiltration, and reduced expression of IL-6 and TNFα at 180 days after radiation. These data further demonstrate that sphingolipid metabolism is a critical regulator of fibrogenesis, and specifically show that opaganib suppresses radiation-induced pulmonary inflammation and fibrosis. Because opaganib has demonstrated an excellent safety profile during clinical testing in other diseases (cancer and COVID-19), the present studies support additional clinical trials with this drug in patients at risk for pulmonary fibrosis.

Ubiquitin ligase CHFR mediated degradation of VE-cadherin through ubiquitylation disrupts endothelial adherens junctions

Vascular endothelial cadherin (VE-cadherin) expressed at endothelial adherens junctions (AJs) is vital for vascular integrity and endothelial homeostasis. Here we identify the requirement of the ubiquitin E3-ligase CHFR as a key mechanism of ubiquitylation-dependent degradation of VE-cadherin. CHFR was essential for disrupting the endothelium through control of the VE-cadherin protein expression at AJs. We observe augmented expression of VE-cadherin in endothelial cell (EC)-restricted Chfr knockout (ChfrΔEC) mice. We also observe abrogation of LPS-induced degradation of VE-cadherin in ChfrΔEC mice, suggesting the pathophysiological relevance of CHFR in regulating the endothelial junctional barrier in inflammation. Lung endothelial barrier breakdown, inflammatory neutrophil extravasation, and mortality induced by LPS were all suppressed in ChfrΔEC mice. We find that the transcription factor FoxO1 is a key upstream regulator of CHFR expression. These findings demonstrate the requisite role of the endothelial cell-expressed E3-ligase CHFR in regulating the expression of VE-cadherin, and thereby endothelial junctional barrier integrity.

Altered Smooth Muscle Cell Histone Acetylome by the SPHK2/S1P Axis Promotes Pulmonary Hypertension

BACKGROUND

Epigenetic regulation of vascular remodeling in pulmonary hypertension (PH) is poorly understood. Transcription regulating, histone acetylation code alters chromatin accessibility to promote transcriptional activation. Our goal was to identify upstream mechanisms that disrupt epigenetic equilibrium in PH.

METHODS

Human pulmonary artery smooth muscle cells (PASMCs), human idiopathic pulmonary arterial hypertension (iPAH):human PASMCs, iPAH lung tissue, failed donor lung tissue, human pulmonary microvascular endothelial cells, iPAH:PASMC and non-iPAH:PASMC RNA-seq databases, NanoString nCounter, and cleavage under targets and release using nuclease were utilized to investigate histone acetylation, hyperacetylation targets, protein and gene expression, sphingolipid activation, cell proliferation, and gene target identification. SPHK2 (sphingosine kinase 2) knockout was compared with control C57BL/6NJ mice after 3 weeks of hypoxia and assessed for indices of PH.

RESULTS

We identified that Human PASMCs are vulnerable to the transcription-promoting epigenetic mediator histone acetylation resulting in alterations in transcription machinery and confirmed its pathological existence in PH:PASMC cells. We report that SPHK2 is elevated as much as 20-fold in iPAH lung tissue and is elevated in iPAH:PASMC cells. During PH pathogenesis, nuclear SPHK2 activates nuclear bioactive lipid S1P (sphingosine 1-phosphate) catalyzing enzyme and mediates transcription regulating histone H3K9 acetylation (acetyl histone H3 lysine 9 [Ac-H3K9]) through EMAP (endothelial monocyte activating polypeptide) II. In iPAH lungs, we identified a 4-fold elevation of the reversible epigenetic transcription modulator Ac-H3K9:H3 ratio. Loss of SPHK2 inhibited hypoxic-induced PH and Ac-H3K9 in mice. We discovered that pulmonary vascular endothelial cells are a priming factor of the EMAP II/SPHK2/S1P axis that alters the acetylome with a specificity for PASMC, through hyperacetylation of histone H3K9. Using cleavage under targets and release using nuclease, we further show that EMAP II-mediated SPHK2 has the potential to modify the local transcription machinery of pluripotency factor KLF4 (Krüppel-like factor 4) by hyperacetylating KLF4 Cis-regulatory elements while deletion and targeted inhibition of SPHK2 rescues transcription altering Ac-H3K9.

CONCLUSIONS

SPHK2 expression and its activation of the reversible histone H3K9 acetylation in human pulmonary artery smooth muscle cell represent new therapeutic targets that could mitigate PH vascular remodeling.

Pulmonary Pathogen-Induced Epigenetic Modifications

Epigenetics generally involves genetic control by factors other than our own DNA sequence. Recent research has focused on delineating the mechanisms of two major epigenetic phenomena: DNA methylation and histone modification. As epigenetics involves many cellular processes, it is no surprise that it can also influence disease-associated gene expression. A direct link between respiratory infections, host cell epigenetic regulations, and chronic lung diseases is still unknown. Recent studies have revealed bacterium- or virus-induced epigenetic changes in the host cells. In this review, we focused on respiratory pathogens (viruses, bacteria, and fungi) induced epigenetic modulations (DNA methylation and histone modification) that may contribute to lung disease pathophysiology by promoting host defense or allowing pathogen persistence.

A targeted metabolomics approach for sepsis-induced ARDS and its subphenotypes

BACKGROUND

Acute respiratory distress syndrome (ARDS) is etiologically and clinically a heterogeneous disease. Its diagnostic characteristics and subtype classification, and the application of these features to treatment, have been of considerable interest. Metabolomics is becoming important for identifying ARDS biology and distinguishing its subtypes. This study aimed to identify metabolites that could distinguish sepsis-induced ARDS patients from non-ARDS controls, using a targeted metabolomics approach, and to identify whether sepsis-induced direct and sepsis-induced indirect ARDS are metabolically distinct groups, and if so, confirm their metabolites and associated pathways.

METHODS

This study retrospectively analyzed 54 samples of ARDS patients from a sepsis registry that was prospectively collected from March 2011 to February 2018, along with 30 non-ARDS controls. The cohort was divided into direct and indirect ARDS. Metabolite concentrations of five analyte classes (energy metabolism, free fatty acids, amino acids, phospholipids, sphingolipids) were measured using liquid chromatography-tandem mass spectrometry and gas chromatography-mass spectrometry by targeted metabolomics.

RESULTS

In total, 186 metabolites were detected. Among them, 102 metabolites could differentiate sepsis-induced ARDS patients from the non-ARDS controls, while 14 metabolites could discriminate sepsis-induced ARDS subphenotypes. Using partial least-squares discriminant analysis, we showed that sepsis-induced ARDS patients were metabolically distinct from the non-ARDS controls. The main distinguishing metabolites were lysophosphatidylethanolamine (lysoPE) plasmalogen, PE plasmalogens, and phosphatidylcholines (PCs). Sepsis-induced direct and indirect ARDS were also metabolically distinct subgroups, with differences in lysoPCs. Glycerophospholipid and sphingolipid metabolism were the most significant metabolic pathways involved in sepsis-induced ARDS biology and in sepsis-induced direct/indirect ARDS, respectively.

CONCLUSION

Our study demonstrated a marked difference in metabolic patterns between sepsis-induced ARDS patients and non-ARDS controls, and between sepsis-induced direct and indirect ARDS subpheonotypes. The identified metabolites and pathways can provide clues relevant to the diagnosis and treatment of individuals with ARDS.

Pseudomonas Aeruginosa Theft Biofilm Require Host Lipids of Cutaneous Wound

OBJECTIVE

This work addressing complexities in wound infection, seeks to test the reliance of bacterial pathogen Pseudomonas aeruginosa (PA) on host skin lipids to form biofilm with pathological consequences.

BACKGROUND

PA biofilm causes wound chronicity. Both CDC as well as NIH recognizes biofilm infection as a threat leading to wound chronicity. Chronic wounds on lower extremities often lead to surgical limb amputation.

METHODS

An established preclinical porcine chronic wound biofilm model, infected with PA or Pseudomonas aeruginosa ceramidase mutant (PA ∆Cer ), was used.

RESULTS

We observed that bacteria drew resource from host lipids to induce PA ceramidase expression by three orders of magnitude. PA utilized product of host ceramide catabolism to augment transcription of PA ceramidase. Biofilm formation was more robust in PA compared to PA ∆Cer . Downstream products of such metabolism such as sphingosine and sphingosine-1-phosphate were both directly implicated in the induction of ceramidase and inhibition of peroxisome proliferator-activated receptor (PPAR)δ, respectively. PA biofilm, in a ceram-idastin-sensitive manner, also silenced PPARδ via induction of miR-106b. Low PPARδ limited ABCA12 expression resulting in disruption of skin lipid homeostasis. Barrier function of the wound-site was thus compromised.

CONCLUSIONS

This work demonstrates that microbial pathogens must co-opt host skin lipids to unleash biofilm pathogenicity. Anti-biofilm strategies must not necessarily always target the microbe and targeting host lipids at risk of infection could be productive. This work may be viewed as a first step, laying fundamental mechanistic groundwork, toward a paradigm change in biofilm management.

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.

The Sphingosine Kinase 2 Inhibitor Opaganib Protects Against Acute Kidney Injury in Mice

Introduction

Acute kidney injury (AKI) is a common multifactorial adverse effect of surgery, circulatory obstruction, sepsis or drug/toxin exposure that often results in morbidity and mortality. Sphingolipid metabolism is a critical regulator of cell survival and pathologic inflammation processes involved in AKI. Opaganib (also known as ABC294640) is a first-in-class experimental drug targeting sphingolipid metabolism that reduces the production and activity of inflammatory cytokines and, therefore, may be effective to prevent and treat AKI.

Methods

Murine models of AKI were used to assess the in vivo efficacy of opaganib including ischemia-reperfusion (IR) injury induced by either transient bilateral occlusion of renal blood flow (a moderate model) or nephrectomy followed immediately by occlusion of the contralateral kidney (a severe model) and lipopolysaccharide (LPS)-induced sepsis. Biochemical and histologic assays were used to quantify the effects of oral opaganib treatment on renal damage in these models.

Results

Opaganib suppressed the elevations of creatinine and blood urea nitrogen (BUN), as well as granulocyte infiltration into the kidneys, of mice that experienced moderate IR from transient bilateral ligation. Opaganib also markedly decreased these parameters and completely prevented mortality in the severe renal IR model. Additionally, opaganib blunted the elevations of BUN, creatinine and inflammatory cytokines following exposure to LPS.

Conclusion

The data support the hypotheses that sphingolipid metabolism is a key mediator of renal inflammatory damage following IR injury and sepsis, and that this can be suppressed by opaganib. Because opaganib has already undergone clinical testing in other diseases (cancer and Covid-19), the present studies support conducting clinical trials with this drug with surgical or septic patients at risk for AKI.

Recent Progress in the Development of Opaganib for the Treatment of Covid-19

The Covid-19 pandemic driven by the SARS-CoV-2 virus continues to exert extensive humanitarian and economic stress across the world. Although antivirals active against mild disease have been identified recently, new drugs to treat moderate and severe Covid-19 patients are needed. Sphingolipids regulate key pathologic processes, including viral proliferation and pathologic host inflammation. Opaganib (aka ABC294640) is a first-in-class clinical drug targeting sphingolipid metabolism for the treatment of cancer and inflammatory diseases. Recent work demonstrates that opaganib also has antiviral activity against several viruses including SARS-CoV-2. A recently completed multinational Phase 2/3 clinical trial of opaganib in patients hospitalized with Covid-19 demonstrated that opaganib can be safely administered to these patients, and more importantly, resulted in a 62% decrease in mortality in a large subpopulation of patients with moderately severe Covid-19. Furthermore, acceleration of the clearance of the virus was observed in opaganib-treated patients. Understanding the biochemical mechanism for the anti-SARS-CoV-2 activity of opaganib is essential for optimizing Covid-19 treatment protocols. Opaganib inhibits three key enzymes in sphingolipid metabolism: sphingosine kinase-2 (SK2); dihydroceramide desaturase (DES1); and glucosylceramide synthase (GCS). Herein, we describe a tripartite model by which opaganib suppresses infection and replication of SARS-CoV-2 by inhibiting SK2, DES1 and GCS. The potential impact of modulation of sphingolipid signaling on multi-organ dysfunction in Covid-19 patients is also discussed.

Opaganib in COVID-19 pneumonia: Results of a randomized, placebo-controlled Phase 2a trial

Background

Opaganib, an oral sphingosine kinase-2 inhibitor with antiviral and anti-inflammatory properties, was shown to inhibit severe acute respiratory syndrome coronavirus 2 replication in vitro. We thus considered that opaganib could be beneficial for moderate to severe coronavirus disease 2019 (COVID-19) pneumonia. The objective of the study was to evaluate the safety of opaganib and its effect on supplemental oxygen requirements and time to hospital discharge in COVID-19 pneumonia hospitalized patients requiring supplemental oxygen.

Methods

This Phase 2a, randomized, double-blind, placebo-controlled study was conducted between July and December 2020 in 8 sites in the United States. Forty-two enrolled patients received opaganib (n = 23) or placebo (n = 19) added to standard of care for up to 14 days and were followed up for 28 days after their last dose of opaganib/placebo.

Results

There were no safety concerns arising in this study. The incidence of ≥Grade 3 treatment-emergent adverse events was 17.4% and 33.3% in the opaganib and placebo groups, respectively. Three deaths occurred in each group. A numerical advantage for opaganib over placebo was observed in in this nonpowered study reflected by total supplemental oxygen requirement from baseline to Day 14, the requirement for supplemental oxygen for at least 24 hours by Day 14, and hospital discharge.

Conclusions

In this proof-of-concept study, hypoxic, hospitalized patients receiving oral opaganib had a similar safety profile to placebo-treated patients, with preliminary evidence of benefit for opaganib as measured by supplementary oxygen requirement and earlier hospital discharge. These findings support further evaluation of opaganib in this population.

Sphingosine kinase 1 regulates lysyl oxidase through STAT3 in hyperoxia-mediated neonatal lung injury

INTRODUCTION

Neonatal lung injury as a consequence of hyperoxia (HO) therapy and ventilator care contribute to the development of bronchopulmonary dysplasia (BPD). Increased expression and activity of lysyl oxidase (LOX), a key enzyme that cross-links collagen, was associated with increased sphingosine kinase 1 (SPHK1) in human BPD. We, therefore, examined closely the link between LOX and SPHK1 in BPD.

METHOD

The enzyme expression of SPHK1 and LOX were assessed in lung tissues of human BPD using immunohistochemistry and quantified (Halo). In vivo studies were based on Sphk1^-/- and matched wild type (WT) neonatal mice exposed to HO while treated with PF543, an inhibitor of SPHK1. In vitro mechanistic studies used human lung microvascular endothelial cells (HLMVECs).

RESULTS

Both SPHK1 and LOX expressions were increased in lungs of patients with BPD. Tracheal aspirates from patients with BPD had increased LOX, correlating with sphingosine-1-phosphate (S1P) levels. HO-induced increase of LOX in lungs were attenuated in both Sphk1^-/- and PF543-treated WT mice, accompanied by reduced collagen staining (sirius red). PF543 reduced LOX activity in both bronchoalveolar lavage fluid and supernatant of HLMVECs following HO. In silico analysis revealed STAT3 as a potential transcriptional regulator of LOX. In HLMVECs, following HO, ChIP assay confirmed increased STAT3 binding to LOX promoter. SPHK1 inhibition reduced phosphorylation of STAT3. Antibody to S1P and siRNA against SPNS2, S1P receptor 1 (S1P₁) and STAT3 reduced LOX expression.

CONCLUSION

HO-induced SPHK1/S1P signalling axis plays a critical role in transcriptional regulation of LOX expression via SPNS2, S1P₁ and STAT3 in lung endothelium.

A Comprehensive Review on the Interplay between Neisseria spp. and Host Sphingolipid Metabolites

Sphingolipids represent a class of structural related lipids involved in membrane biology and various cellular processes including cell growth, apoptosis, inflammation and migration. Over the past decade, sphingolipids have become the focus of intensive studies regarding their involvement in infectious diseases. Pathogens can manipulate the sphingolipid metabolism resulting in cell membrane reorganization and receptor recruitment to facilitate their entry. They may recruit specific host sphingolipid metabolites to establish a favorable niche for intracellular survival and proliferation. In contrast, some sphingolipid metabolites can also act as a first line defense against bacteria based on their antimicrobial activity. In this review, we will focus on the strategies employed by pathogenic Neisseria spp. to modulate the sphingolipid metabolism and hijack the sphingolipid balance in the host to promote cellular colonization, invasion and intracellular survival. Novel techniques and innovative approaches will be highlighted that allow imaging of sphingolipid derivatives in the host cell as well as in the pathogen.

Potential drug development and therapeutic approaches for clinical intervention in COVID-19

While the vaccination is now available to many countries and will slowly dissipate to others, effective therapeutics for COVID-19 is still illusive. The SARS-CoV-2 pandemic has posed an unprecedented challenge to researchers, scientists, and clinicians and affected the wellbeing of millions of people worldwide. Since the beginning of the pandemic, a multitude of existing anti-viral, antibiotic, antimalarial, and anticancer drugs have been tested, and some have shown potency in the treatment and management of COVID-19, albeit others failed to leave any positive impact and a few also became controversial as they showed mixed clinical outcomes. In the present article, we have brought together some of the candidate therapeutic drugs being repurposed or used in the clinical trials and discussed their clinical efficacy and safety for COVID-19.

The regulatory effect of acetylation of HMGN2 and H3K27 on pyocyanin-induced autophagy in macrophages by affecting Ulk1 transcription

Pyocyanin (PYO) is a major virulence factor secreted by Pseudomonas aeruginosa, and autophagy is a crucial homeostatic mechanism for the interaction between the pathogens and the host. It remains unknown whether PYO leads to autophagy in macrophages by regulating histone acetylation. The high mobility group nucleosomal binding domain 2 (HMGN2) has been reported to regulate the PYO‐induced autophagy and oxidative stress in the epithelial cells; however, the underlying molecular mechanism has not been fully elucidated. In this study, PYO was found to induce autophagy in macrophages, and the mechanism might be correlated with the up‐regulation of HMGN2 acetylation (HMGN2ac) and the down‐regulation of H3K27 acetylation (H3K27ac) by modulation of the activities of acetyltransferases and deacetylases. Moreover, we further demonstrated that the up‐regulated HMGN2ac enhances its recruitment to the Ulk1 promoter, while the down‐regulation of H3K27ac reduces its recruitment to the Ulk1 promoter, thereby promoting or inhibiting the transcription of Ulk1. In conclusion, HMGN2ac and H3K27ac play regulatory roles in the PYO‐induced autophagy in macrophages.

Nuclear Sphingosine-1-phosphate Lyase Generated ∆2-hexadecenal is A Regulator of HDAC Activity and Chromatin Remodeling in Lung Epithelial Cells

Sphingosine-1-phosphate (S1P), a bioactive lipid mediator, is generated from sphingosine by sphingosine kinases (SPHKs) 1 and 2 and is metabolized to ∆2-hexadecenal (∆2-HDE) and ethanolamine phosphate by S1P lyase (S1PL) in mammalian cells. We have recently demonstrated the activation of nuclear SPHK2 and the generation of S1P in the nucleus of lung epithelial cells exposed to Pseudomonas aeruginosa. Here, we have investigated the nuclear localization of S1PL and the role of ∆2-HDE generated from S1P in the nucleus as a modulator of histone deacetylase (HDAC) activity and histone acetylation. Electron micrographs of the nuclear fractions isolated from MLE-12 cells showed nuclei free of ER contamination, and S1PL activity was detected in nuclear fractions isolated from primary lung bronchial epithelial cells and alveolar epithelial MLE-12 cells. Pseudomonas aeruginosa-mediated nuclear ∆2-HDE generation, and H3/H4 histone acetylation was attenuated by S1PL inhibitors in MLE-12 cells and human bronchial epithelial cells. In vitro, the addition of exogenous ∆2-HDE (100-10,000 nM) to lung epithelial cell nuclear preparations inhibited HDAC1/2 activity, and increased acetylation of Histone H3 and H4, whereas similar concentrations of S1P did not show a significant change. In addition, incubation of ∆2-HDE with rHDAC1 generated five different amino acid adducts as detected by LC-MS/MS; the predominant adduct being ∆2-HDE with lysine residues of HDAC1. Together, these data show an important role for the nuclear S1PL-derived ∆2-HDE in the modification of HDAC activity, histone acetylation, and chromatin remodeling in lung epithelial cells.

The Functional Role of Sphingosine Kinase 2

Sphingosine-1-phosphate (S1P) is a bioactive lipid molecule that is present in all eukaryotic cells and plays key roles in various extracellular, cytosolic, and nuclear signaling pathways. Two sphingosine kinase isoforms, sphingosine kinase 1 (SPHK1) and sphingosine kinase 2 (SPHK2), synthesize S1P by phosphorylating sphingosine. While SPHK1 is a cytoplasmic kinase, SPHK2 is localized to the nucleus, endoplasmic reticulum, and mitochondria. The SPHK2/S1P pathway regulates transcription, telomere maintenance, mitochondrial respiration, among many other processes. SPHK2 is under investigation as a target for treating many age-associated conditions, such as cancer, stroke, and neurodegeneration. In this review, we will focus on the role of SPHK2 in health and disease.

Lysophospholipids in Lung Inflammatory Diseases

The lysophospholipids (LPLs) belong to a group of bioactive lipids that play pivotal roles in several physiological and pathological processes. LPLs are derivatives of phospholipids and consist of a single hydrophobic fatty acid chain, a hydrophilic head, and a phosphate group with or without a large molecule attached. Among the LPLs, lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are the simplest, and have been shown to be involved in lung inflammatory symptoms and diseases such as acute lung injury, asthma, and chronic obstructive pulmonary diseases. G protein-coupled receptors (GPCRs) mediate LPA and S1P signaling. In this chapter, we will discuss on the role of LPA, S1P, their metabolizing enzymes, inhibitors or agonists of their receptors, and their GPCR-mediated signaling in lung inflammatory symptoms and diseases, focusing specially on acute respiratory distress syndrome, asthma, and chronic obstructive pulmonary disease.

NOX4 Mediates Pseudomonas aeruginosa-Induced Nuclear Reactive Oxygen Species Generation and Chromatin Remodeling in Lung Epithelium

Pseudomonas aeruginosa (PA) infection increases reactive oxygen species (ROS), and earlier, we have shown a role for NADPH oxidase-derived ROS in PA-mediated lung inflammation and injury. Here, we show a role for the lung epithelial cell (LEpC) NOX4 in PA-mediated chromatin remodeling and lung inflammation. Intratracheal administration of PA to Nox4flox/flox mice for 24 h caused lung inflammatory injury; however, epithelial cell-deleted Nox4 mice exhibited reduced lung inflammatory injury, oxidative stress, secretion of pro-inflammatory cytokines, and decreased histone acetylation. In LEpCs, NOX4 was localized both in the cytoplasmic and nuclear fractions, and PA stimulation increased the nuclear NOX4 expression and ROS production. Downregulation or inhibition of NOX4 and PKC δ attenuated the PA-induced nuclear ROS. PA-induced histone acetylation was attenuated by Nox4-specific siRNA, unlike Nox2. PA stimulation increased HDAC1/2 oxidation and reduced HDAC1/2 activity. The PA-induced oxidation of HDAC2 was attenuated by N-acetyl-L-cysteine and siRNA specific for Pkc δ, Sphk2, and Nox4. PA stimulated RAC1 activation in the nucleus and enhanced the association between HDAC2 and RAC1, p-PKC δ, and NOX4 in LEpCs. Our results revealed a critical role for the alveolar epithelial NOX4 in mediating PA-induced lung inflammatory injury via nuclear ROS generation, HDAC1/2 oxidation, and chromatin remodeling.

CFTR Correctors and Antioxidants Partially Normalize Lipid Imbalance but not Abnormal Basal Inflammatory Cytokine Profile in CF Bronchial Epithelial Cells

A deficiency in cystic fibrosis transmembrane conductance regulator (CFTR) function in CF leads to chronic lung disease. CF is associated with abnormalities in fatty acids, ceramides, and cholesterol, their relationship with CF lung pathology is not completely understood. Therefore, we examined the impact of CFTR deficiency on lipid metabolism and pro-inflammatory signaling in airway epithelium using mass spectrometric, protein array. We observed a striking imbalance in fatty acid and ceramide metabolism, associated with chronic oxidative stress under basal conditions in CF mouse lung and well-differentiated bronchial epithelial cell cultures of CFTR knock out pig and CF patients. Cell-autonomous features of all three CF models included high ratios of ω-6- to ω-3-polyunsaturated fatty acids and of long- to very long-chain ceramide species (LCC/VLCC), reduced levels of total ceramides and ceramide precursors. In addition to the retinoic acid analog fenretinide, the anti-oxidants glutathione (GSH) and deferoxamine partially corrected the lipid profile indicating that oxidative stress may promote the lipid abnormalities. CFTR-targeted modulators reduced the lipid imbalance and oxidative stress, confirming the CFTR dependence of lipid ratios. However, despite functional correction of CF cells up to 60% of non-CF in Ussing chamber experiments, a 72-h triple compound treatment (elexacaftor/tezacaftor/ivacaftor surrogate) did not completely normalize lipid imbalance or oxidative stress. Protein array analysis revealed differential expression and shedding of cytokines and growth factors from CF epithelial cells compared to non-CF cells, consistent with sterile inflammation and tissue remodeling under basal conditions, including enhanced secretion of the neutrophil activator CXCL5, and the T-cell activator CCL17. However, treatment with antioxidants or CFTR modulators that mimic the approved combination therapies, ivacaftor/lumacaftor and ivacaftor/tezacaftor/elexacaftor, did not effectively suppress the inflammatory phenotype. We propose that CFTR deficiency causes oxidative stress in CF airway epithelium, affecting multiple bioactive lipid metabolic pathways, which likely play a role in CF lung disease progression. A combination of anti-oxidant, anti-inflammatory and CFTR targeted therapeutics may be required for full correction of the CF phenotype.

My Journey in Academia as a Lipid Biochemist

An overview of Prof. Viswanathan Natarajan's journey in academia as a mentor, teacher, and lipid scientist for nearly 50 years is presented. As a graduate student, Dr. Natarajan interrogated biosynthesis and catabolism of phospholipids in the developing brain; however, in the last five decades, he has been investigating the role of sphingolipids and sphingolipid-metabolizing enzymes in pulmonary endothelial cells, epithelial cells, and fibroblasts under normal conditions and during various lung pathologies such as sepsis, asthma, pulmonary hypertension, idiopathic pulmonary fibrosis, bronchopulmonary dysplasia, and lung cancer. His recent work on sphingosine-1-phosphate and lysophosphatidic acid metabolism in pre-clinical animal models has identified small molecule inhibitors in the signaling pathways that could have therapeutic potential in ameliorating pulmonary fibrosis, hypoxia-induced pulmonary hypertension, lung cancer, and bronchopulmonary dysplasia. Future research in bioactive lipids in combination with OMICS should unravel the importance of various lipid mediators as modulators of cell function under normal and pathological conditions.

Sphingosine 1-phosphate in sepsis and beyond: Its role in disease tolerance and host defense and the impact of carrier molecules

Sphingosine 1-phosphate (S1P) is an important immune modulator responsible for physiological cellular responses like lymphocyte development and function, positioning and emigration of T and B cells and cytokine secretion. Recent reports indicate that S1P does not only regulate immunity, but can also protect the function of organs by inducing disease tolerance. S1P also influences the replication of certain pathogens, and sphingolipids are also involved in pathogen recognition and killing. Certain carrier molecules for S1P like serum albumin and high density lipoproteins contribute to the regulation of S1P effects. They are able to associate with S1P and modulate its signaling properties. Similar to S1P, both carrier molecules are also decreased in sepsis patients and likely contribute to sepsis pathology and severity. In this review, we will introduce the concept of disease tolerance and the involvement of S1P. We will also discuss the contribution of S1P and its precursor sphingosine to host defense mechanisms against pathogens. Finally, we will summarize current data demonstrating the influence of carrier molecules for differential S1P signaling. The presented data may lead to new strategies for the prevention and containment of sepsis.

Genome-Scale Metabolic Modeling for Unraveling Molecular Mechanisms of High Threat Pathogens

Pathogens give rise to a wide range of diseases threatening global health and hence drawing public health agencies' attention to establish preventative and curative solutions. Genome-scale metabolic modeling is ever increasingly used tool for biomedical applications including the elucidation of antibiotic resistance, virulence, single pathogen mechanisms and pathogen-host interaction systems. With this approach, the sophisticated cellular system of metabolic reactions inside the pathogens as well as between pathogen and host cells are represented in conjunction with their corresponding genes and enzymes. Along with essential metabolic reactions, alternate pathways and fluxes are predicted by performing computational flux analyses for the growth of pathogens in a very short time. The genes or enzymes responsible for the essential metabolic reactions in pathogen growth are regarded as potential drug targets, as a priori guide to researchers in the pharmaceutical field. Pathogens alter the key metabolic processes in infected host, ultimately the objective of these integrative constraint-based context-specific metabolic models is to provide novel insights toward understanding the metabolic basis of the acute and chronic processes of infection, revealing cellular mechanisms of pathogenesis, identifying strain-specific biomarkers and developing new therapeutic approaches including the combination drugs. The reaction rates predicted during different time points of pathogen development enable us to predict active pathways and those that only occur during certain stages of infection, and thus point out the putative drug targets. Among others, fatty acid and lipid syntheses reactions are recent targets of new antimicrobial drugs. Genome-scale metabolic models provide an improved understanding of how intracellular pathogens utilize the existing microenvironment of the host. Here, we reviewed the current knowledge of genome-scale metabolic modeling in pathogen cells as well as pathogen host interaction systems and the promising applications in the extension of curative strategies against pathogens for global preventative healthcare.

Increased activation of HDAC1/2/6 and Sp1 underlies therapeutic resistance and tumor growth in glioblastoma

BACKGROUND

Glioblastoma is associated with poor prognosis and high mortality. Although the use of first-line temozolomide can reduce tumor growth, therapy-induced stress drives stem cells out of quiescence, leading to chemoresistance and glioblastoma recurrence. The specificity protein 1 (Sp1) transcription factor is known to protect glioblastoma cells against temozolomide; however, how tumor cells hijack this factor to gain resistance to therapy is not known.

METHODS

Sp1 acetylation in temozolomide-resistant cells and stemlike tumorspheres was analyzed by immunoprecipitation and immunoblotting experiments. Effects of the histone deacetylase (HDAC)/Sp1 axis on malignant growth were examined using cell proliferation-related assays and in vivo experiments. Furthermore, integrative analysis of gene expression with chromatin immunoprecipitation sequencing and the recurrent glioblastoma omics data were also used to further determine the target genes of the HDAC/Sp1 axis.

RESULTS

We identified Sp1 as a novel substrate of HDAC6, and observed that the HDAC1/2/6/Sp1 pathway promotes self-renewal of malignancy by upregulating B cell-specific Mo-MLV integration site 1 (BMI1) and human telomerase reverse transcriptase (hTERT), as well as by regulating G2/M progression and DNA repair via alteration of the transcription of various genes. Importantly, HDAC1/2/6/Sp1 activation is associated with poor clinical outcome in both glioblastoma and low-grade gliomas. However, treatment with azaindolyl sulfonamide, a potent HDAC6 inhibitor with partial efficacy against HDAC1/2, induced G2/M arrest and senescence in both temozolomide-resistant cells and stemlike tumorspheres.

CONCLUSION

Our study uncovers a previously unknown regulatory mechanism in which the HDAC6/Sp1 axis induces cell division and maintains the stem cell population to fuel tumor growth and therapeutic resistance.

Sphingosine 1-Phosphate in Malaria Pathogenesis and Its Implication in Therapeutic Opportunities

Sphingosine 1-Phosphate (S1P) is a bioactive lipid intermediate in the sphingolipid metabolism, which exist in two pools, intracellular and extracellular, and each pool has a different function. The circulating extracellular pool, specifically the plasma S1P is shown to be important in regulating various physiological processes related to malaria pathogenesis in recent years. Although blood cells (red blood cells and platelets), vascular endothelial cells and hepatocytes are considered as the important sources of plasma S1P, their extent of contribution is still debated. The red blood cells (RBCs) and platelets serve as a major repository of intracellular S1P due to lack, or low activity of S1P degrading enzymes, however, contribution of platelets toward maintaining plasma S1P is shown negligible under normal condition. Substantial evidences suggest platelets loss during falciparum infection as a contributing factor for severe malaria. However, platelets function as a source for plasma S1P in malaria needs to be examined experimentally. RBC being the preferential site for parasite seclusion, and having the ability of trans-cellular S1P transportation to EC upon tight cell-cell contact, might play critical role in differential S1P distribution and parasite growth. In the present review, we have summarized the significance of both the S1P pools in the context of malaria, and how the RBC content of S1P can be channelized in better ways for its possible implication in therapeutic opportunities to control malaria.

Post-translational modifications of S1PR1 and endothelial barrier regulation

Sphingosine-1-phosphate receptor-1 (S1PR1), a G-protein coupled receptor that is expressed in endothelium and activated upon ligation by the bioactive lipid sphingosine-1-phosphate (S1P), is an important vascular-barrier protective mechanism at the level of adherens junctions (AJ). Loss of endothelial barrier function is a central factor in the pathogenesis of various inflammatory conditions characterized by protein-rich lung edema formation, such as acute respiratory distress syndrome (ARDS). While several S1PR1 agonists are available, the challenge of arresting the progression of protein-rich edema formation remains to be met. In this review, we discuss the role of S1PRs, especially S1PR1, in regulating endothelial barrier function. We review recent findings showing that replenishment of the pool of cell-surface S1PR1 may be crucial to the effectiveness of S1P in repairing the endothelial barrier. In this context, we discuss the S1P generating machinery and mechanisms that regulate S1PR1 at the cell surface and their impact on endothelial barrier function.

Angiocrine Sphingosine-1-Phosphate Activation of S1PR2-YAP Signaling Axis in Alveolar Type II Cells Is Essential for Lung Repair

Lung alveolar epithelium is composed of alveolar type I (AT1) and type II (AT2) cells. AT1 cells mediate gas exchange, whereas AT2 cells act as progenitor cells to repair injured alveoli. Lung microvascular endothelial cells (LMVECs) play a crucial but still poorly understood role in regulating alveolar repair. Here, we studied the role of the LMVEC-derived bioactive lipid sphingosine-1-phosphate (S1P) in promoting alveolar repair using mice with endothelial-specific deletion of sphingosine kinase 1 (Sphk1), the key enzyme promoting S1P generation. These mutant lungs developed airspace-enlargement lesions and exhibited a reduced number of AT1 cells after Pseudomonas-aeruginosa-induced lung injury. We demonstrated that S1P released by LMVECs acted via its receptor, S1PR2, on AT2 cells and induced nuclear translocation of yes-associated protein (YAP), a regulator of AT2 to AT1 transition. Thus, angiocrine S1P released after injury acts via the S1PR2-YAP signaling axis on AT2 cells to promote AT2 to AT1 differentiation required for alveolar repair.

Lipid Mediators Regulate Pulmonary Fibrosis: Potential Mechanisms and Signaling Pathways

Idiopathic pulmonary fibrosis (IPF) is a progressive lung disease of unknown etiology characterized by distorted distal lung architecture, inflammation, and fibrosis. The molecular mechanisms involved in the pathophysiology of IPF are incompletely defined. Several lung cell types including alveolar epithelial cells, fibroblasts, monocyte-derived macrophages, and endothelial cells have been implicated in the development and progression of fibrosis. Regardless of the cell types involved, changes in gene expression, disrupted glycolysis, and mitochondrial oxidation, dysregulated protein folding, and altered phospholipid and sphingolipid metabolism result in activation of myofibroblast, deposition of extracellular matrix proteins, remodeling of lung architecture and fibrosis. Lipid mediators derived from phospholipids, sphingolipids, and polyunsaturated fatty acids play an important role in the pathogenesis of pulmonary fibrosis and have been described to exhibit pro- and anti-fibrotic effects in IPF and in preclinical animal models of lung fibrosis. This review describes the current understanding of the role and signaling pathways of prostanoids, lysophospholipids, and sphingolipids and their metabolizing enzymes in the development of lung fibrosis. Further, several of the lipid mediators and enzymes involved in their metabolism are therapeutic targets for drug development to treat IPF.

S1P and plasmalogen derived fatty aldehydes in cellular signaling and functions

Long-chain fatty aldehydes are present in low concentrations in mammalian cells and serve as intermediates in the interconversion between fatty acids and fatty alcohols. The long-chain fatty aldehydes are generated by enzymatic hydrolysis of 1-alkyl-, and 1-alkenyl-glycerophospholipids by alkylglycerol monooxygenase, plasmalogenase or lysoplasmalogenase while hydrolysis of sphingosine-1-phosphate (S1P) by S1P lyase generates trans ∆2-hexadecenal (∆2-HDE). Additionally, 2-chloro-, and 2-bromo- fatty aldehydes are produced from plasmalogens or lysoplasmalogens by hypochlorous, and hypobromous acid generated by activated neutrophils and eosinophils, respectively while 2-iodofatty aldehydes are produced by excess iodine in thyroid glands. The 2-halofatty aldehydes and ∆2-HDE activated JNK signaling, BAX, cytoskeletal reorganization and apoptosis in mammalian cells. Further, 2-chloro- and 2-bromo-fatty aldehydes formed GSH and protein adducts while ∆2-HDE formed adducts with GSH, deoxyguanosine in DNA and proteins such as HDAC1 in vitro. ∆2-HDE also modulated HDAC activity and stimulated H3 and H4 histone acetylation in vitro with lung epithelial cell nuclear preparations. The α-halo fatty aldehydes elicited endothelial dysfunction, cellular toxicity and tissue damage. Taken together, these investigations suggest a new role for long-chain fatty aldehydes as signaling lipids, ability to form adducts with GSH, proteins such as HDACs and regulate cellular functions.

S1P/S1P Receptor Signaling in Neuromuscolar Disorders

The bioactive sphingolipid metabolite, sphingosine 1-phosphate (S1P), and the signaling pathways triggered by its binding to specific G protein-coupled receptors play a critical regulatory role in many pathophysiological processes, including skeletal muscle and nervous system degeneration. The signaling transduced by S1P binding appears to be much more complex than previously thought, with important implications for clinical applications and for personalized medicine. In particular, the understanding of S1P/S1P receptor signaling functions in specific compartmentalized locations of the cell is worthy of being better investigated, because in various circumstances it might be crucial for the development or/and the progression of neuromuscular diseases, such as Charcot-Marie-Tooth disease, myasthenia gravis, and Duchenne muscular dystrophy.

Genetic deletion of Sphk2 confers protection against Pseudomonas aeruginosa mediated differential expression of genes related to virulent infection and inflammation in mouse lung

BACKGROUND

Pseudomonas aeruginosa (PA) is an opportunistic Gram-negative bacterium that causes serious life threatening and nosocomial infections including pneumonia. PA has the ability to alter host genome to facilitate its invasion, thus increasing the virulence of the organism. Sphingosine-1- phosphate (S1P), a bioactive lipid, is known to play a key role in facilitating infection. Sphingosine kinases (SPHK) 1&2 phosphorylate sphingosine to generate S1P in mammalian cells. We reported earlier that Sphk2-/- mice offered significant protection against lung inflammation, compared to wild type (WT) animals. Therefore, we profiled the differential expression of genes between the protected group of Sphk2-/- and the wild type controls to better understand the underlying protective mechanisms related to the Sphk2 deletion in lung inflammatory injury. Whole transcriptome shotgun sequencing (RNA-Seq) was performed on mouse lung tissue using NextSeq 500 sequencing system.

RESULTS

Two-way analysis of variance (ANOVA) analysis was performed and differentially expressed genes following PA infection were identified using whole transcriptome of Sphk2-/- mice and their WT counterparts. Pathway (PW) enrichment analyses of the RNA seq data identified several signaling pathways that are likely to play a crucial role in pneumonia caused by PA such as those involved in: 1. Immune response to PA infection and NF-κB signal transduction; 2. PKC signal transduction; 3. Impact on epigenetic regulation; 4. Epithelial sodium channel pathway; 5. Mucin expression; and 6. Bacterial infection related pathways. Our genomic data suggests a potential role for SPHK2 in PA-induced pneumonia through elevated expression of inflammatory genes in lung tissue. Further, validation by RT-PCR on 10 differentially expressed genes showed 100% concordance in terms of vectoral changes as well as significant fold change.

CONCLUSION

Using Sphk2-/- mice and differential gene expression analysis, we have shown here that S1P/SPHK2 signaling could play a key role in promoting PA pneumonia. The identified genes promote inflammation and suppress others that naturally inhibit inflammation and host defense. Thus, targeting SPHK2/S1P signaling in PA-induced lung inflammation could serve as a potential therapy to combat PA-induced pneumonia.

Diverse Facets of Sphingolipid Involvement in Bacterial Infections

Sphingolipids are constituents of the cell membrane that perform various tasks as structural elements and signaling molecules, in addition to regulating many important cellular processes, such as apoptosis and autophagy. In recent years, it has become increasingly clear that sphingolipids and sphingolipid signaling play a vital role in infection processes. In many cases the attachment and uptake of pathogenic bacteria, as well as bacterial development and survival within the host cell depend on sphingolipids. In addition, sphingolipids can serve as antimicrobials, inhibiting bacterial growth and formation of biofilms. This review will give an overview of our current information about these various aspects of sphingolipid involvement in bacterial infections.