Concentrations of oxidized linoleic acid derived lipid mediators in the amygdala and periaqueductal grey are reduced in a mouse model of chronic inflammatory pain

The Octadecanoids: Synthesis and Bioactivity of 18-Carbon Oxygenated Fatty Acids in Mammals, Bacteria, and Fungi

The octadecanoids are a broad class of lipids consisting of the oxygenated products of 18-carbon fatty acids. Originally referring to production of the phytohormone jasmonic acid, the octadecanoid pathway has been expanded to include products of all 18-carbon fatty acids. Octadecanoids are formed biosynthetically in mammals via cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P450 (CYP) activity, as well as nonenzymatically by photo- and autoxidation mechanisms. While octadecanoids are well-known mediators in plants, their role in the regulation of mammalian biological processes has been generally neglected. However, there have been significant advancements in recognizing the importance of these compounds in mammals and their involvement in the mediation of inflammation, nociception, and cell proliferation, as well as in immuno- and tissue modulation, coagulation processes, hormone regulation, and skin barrier formation. More recently, the gut microbiome has been shown to be a significant source of octadecanoid biosynthesis, providing additional biosynthetic routes including hydratase activity (e.g., CLA-HY, FA-HY1, FA-HY2). In this review, we summarize the current field of octadecanoids, propose standardized nomenclature, provide details of octadecanoid preparation and measurement, summarize the phase-I metabolic pathway of octadecanoid formation in mammals, bacteria, and fungi, and describe their biological activity in relation to mammalian pathophysiology as well as their potential use as biomarkers of health and disease.

Lipid Alterations in Chronic Nonspecific Low Back Pain in the Chinese Population: A Metabolomic and Lipidomic Study

Chronic nonspecific low back pain (cNLBP) accounts for approximately 90% of low back pain cases, affecting 65-80% of the population and significantly impacting life quality and productivity. This condition also leads to substantial financial burden. Although there have been advancements, a comprehensive understanding of the underlying etiology of cNLBP remains elusive, resulting in less than optimal treatment outcomes. This study aimed to examine the correlation between lipid variations and the development of cNLBP. The cohort consisted of 26 healthy volunteers (HV group) and 30 cNLBP patients, with an assessment of metabolites and lipid composition in both groups. Metabolomic results revealed significant alterations in lipid-associated metabolites between the HV and cNLBP groups. Subsequent lipid analysis revealed that monoacylglycerols (MAGs) increased approximately 1.2-fold (p = 0.016), diacylglycerols (DAGs) increased approximately 1.4-fold (p = 0.0003), and phosphatidylserine (PS) increased approximately 1.4-fold (p = 0.011). In contrast, triacylglycerol (TAG) decreased to about 0.7-fold (p = 0.035) in the cNLBP group compared to the HV group. The contrasting trends in MAG/DAG and TAG levels indicated that the imbalance between MAG/DAG and TAG may have an impact on the development of cNLBP. This study has provided new insights into the relationship between the progression of cNLBP and specific lipids, suggesting that these lipids could serve as therapeutic targets for cNLBP.

Eicosanoid profiles in an arthritis model: Effects of a soluble epoxide hydrolase inhibitor

Rheumatoid arthritis is a common systemic inflammatory autoimmune disease characterized by damage to joints, inflammation and pain. It is driven by an increase of inflammatory cytokines and lipids mediators such as prostaglandins. Epoxides of polyunsaturated fatty acids (PUFAs) are lipid chemical mediators in a group of regulatory compounds termed eicosanoids. These epoxy fatty acids (EpFA) have resolutive functions but are rapidly metabolized by the soluble epoxide hydrolase enzyme (sEH) into the corresponding diols. The pharmacological inhibition of sEH stabilizes EpFA from hydrolysis, improving their half-lives and biological effects. These anti-inflammatory EpFA, are analgesic in neuropathic and inflammatory pain conditions. Nonetheless, inhibition of sEH on arthritis and the resulting effects on eicosanoids profiles are little explored despite the physiological importance. In this study, we investigated the effect of sEH inhibition on collagen-induced arthritis (CIA) and its impact on the plasma eicosanoid profile. We measured the eicosanoid metabolites by LC-MS/MS-based lipidomic analysis. The treatment with a sEH inhibitor significantly modulated 11 out of 69 eicosanoids, including increased epoxides 12(13)-EpODE, 12(13)-EpOME, 13-oxo-ODE, 15-HEPE, 20-COOH-LTB4 and decreases several diols 15,6-DiHODE, 12,13-DiHOME, 14,15-DiHETrE, 5,6-DiHETrE and 16,17-DiHDPE. Overall the inhibition of sEH in the rheumatoid arthritis model enhanced epoxides generally considered anti-inflammatory or resolutive mediators and decreased several diols with inflammatory features. These findings support the hypothesis that inhibiting the sEH increases systemic EpFA levels, advancing the understanding of the impact of these lipid mediators as therapeutical targets.

Oxidized linoleic acid metabolites regulate neuronal morphogenesis in vitro

Linoleic acid (LA, 18:2n-6) is an essential nutrient for optimal infant growth and brain development. The effects of LA in the brain are thought to be mediated by oxygenated metabolites of LA known as oxidized LA metabolites (OXLAMs), but evidence is lacking to directly support this hypothesis. This study investigated whether OXLAMs modulate key neurodevelopmental processes including axon outgrowth, dendritic arborization, cell viability and synaptic connectivity. Primary cortical neuron-glia co-cultures from postnatal day 0-1 male and female rats were exposed for 48h to the following OXLAMs: 1) 13-hydroxyoctadecadienoic acid (13-HODE); 2) 9-hydroxyoctadecadienoic acid (9-HODE); 3) 9,10-dihydroxyoctadecenoic acid (9,10-DiHOME); 4) 12(13)-epoxyoctadecenoic acid (12(13)-EpOME); 5) 9,10,13-trihydroxyoctadecenoic acid (9,10,13-TriHOME); 6) 9-oxo-octadecadienoic acid (9-OxoODE); and 7) 12,13-dihydroxyoctadecenoic acid (12,13-DiHOME). Axonal outgrowth, evaluated by Tau-1 immunostaining, was increased by 9-HODE, but decreased by 12,13-DiHOME in male but not female neurons. Dendrite arborization, evaluated by MAP2B-eGFP expression, was affected by 9-HODE, 9-OxoODE, and 12(13)-EpOME in male neurons and, by 12(13)-EpOME in female neurons. Neither cell viability nor synaptic connectivity were significantly altered by OXLAMs. Overall, this study shows select OXLAMs modulate neuron morphology in a sex-dependent manner, with male neurons being more susceptible.

The octadecanoids: an emerging class of lipid mediators

Oxylipins are enzymatic and non-enzymatic metabolites of mono- or polyunsaturated fatty acids that encompass potent lipid mediators including the eicosanoids and docosanoids. Previously considered of low interest and often dismissed as 'just fat', octadecanoid oxylipins have only recently begun to be recognized as lipid mediators in humans. In the last few years, these compounds have been found to be involved in the mediation of multiple biological processes related to nociception, tissue modulation, cell proliferation, metabolic regulation, inflammation, and immune regulation. At the same time, the study of octadecanoids is hampered by a lack of standardization in the field, a paucity of analytical standards, and a lack of domain expertise. These issues have collectively limited the investigation of the biosynthesis and bioactivity of octadecanoids. Here, we present an overview of the primary enzymatic pathways for the oxidative metabolism of 18-carbon fatty acids in humans and of the current knowledge of the major biological activity of the resulting octadecanoids. We also propose a systematic nomenclature system based upon that used for the eicosanoids in order to avoid ambiguities and resolve multiple designations for the same octadecanoid. The aim of this review is to provide an initial framework for the field and to assist in its standardization as well as to increase awareness of this class of compounds in order to stimulate research into this interesting group of lipid mediators.

Targeted dietary interventions to reduce pain in persistent post-traumatic headache among service members: Protocol for a randomized, controlled parallel-group trial

INTRODUCTION

Post-traumatic headache (PTH) is common after traumatic brain injury (TBI), especially among active-duty service members (SMs), affecting up to 35% of patients with chronic TBI. Persistent PTH is disabling and frequently unresponsive to treatment and is often migrainous. Here, we describe a trial assessing whether dietary modifications to increase n-3 eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) and reduce n-6 linoleic acid (LA), will alter nociceptive lipid mediators and result in clinical improvements in persistent PTH.

METHODS

This prospective, randomized, controlled trial tests the efficacy, safety, and biochemical effects of targeted, controlled alterations in dietary n-3 and n-6 fatty acids in 122 adult SMs and military healthcare beneficiaries with diagnosed TBI associated with actively managed persistent frequent (>8 /month) PTH with migraine. Following a 4-week baseline, participants are randomized to one of two equally intensive dietary regimens for 12 additional weeks: 1) increased n-3 EPA + DHA with low n-6 LA (H3L6); 2) usual US dietary content of n-3 and n-6 fatty acids (Control). During the intervention, participants receive diet arm-specific study oils and foods sufficient for 75% of caloric needs and comprehensive dietary counseling. Participants complete daily headache diaries throughout the intervention. Clinical outcomes, including the Headache Impact Test (HIT-6), headache hours per day, circulating blood fatty acid levels, and bioactive metabolites, are measured pre-randomization and at 6 and 12 weeks. Planned primary analyses include pre-post comparisons of treatment groups on clinical measures using ANCOVA and mixed-effects models. Similar approaches to explore biochemical and exploratory clinical outcomes are planned.

CLINICALTRIALS

gov registration: NCT03272399.

Oxidized linoleic acid metabolites maintain mechanical and thermal hypersensitivity during sub-chronic inflammatory pain

Inflammatory pain serves as a protective defense mechanism which becomes pathological when it turns into chronic inflammatory pain. This transition is mediated by a variety of peripheral mediators that sensitize nociceptors and increase pain perception in sensory neurons. Besides cytokines, chemokines and growth factors, accumulating evidence shows that oxidized lipids, such as eicosanoids and oxidized linoleic acid metabolites, contribute to this sensitization process. Most notably, the oxidized linoleic acid metabolite and partial TRPV1 agonist 9-HODE (hydroxyoctadecadienoic acid) was shown to be involved in this sensitization process. However, it is still unknown how some of the oxidized linoleic acid metabolites are synthesized in the inflammatory environment and in which phase of inflammation they become relevant. Here we show that the concentrations of oxidized linoleic acid metabolites, especially 9-HODE and 13-HODE, are significantly increased in inflamed paw tissue and the corresponding dorsal root ganglia in the sub-chronic phase of inflammation. Surprisingly, classical inflammatory lipid markers, such as prostaglandins were at basal levels in this phase of inflammation. Moreover, we revealed the cell type specific synthesis pathways of oxidized linoleic acid metabolites in primary macrophages, primary neutrophils and dorsal root ganglia. Finally, we show that blocking the most elevated metabolites 9-HODE and 13-HODE at the site of inflammation in the sub-chronic phase of inflammation, leads to a significant relief of mechanical and thermal hypersensitivity in vivo. In summary, these data offer an approach to specifically target oxidized linoleic acid metabolites in the transition of acute inflammatory pain to chronic inflammatory pain.

Dietary Interventions Are Beneficial for Patients with Chronic Pain: A Systematic Review with Meta-Analysis

Background

The standard Western diet is high in processed hyperpalatable foods that displace nutrient-dense whole foods, leading to inflammation and oxidative stress. There is limited research on how these adverse metabolic drivers may be associated with maladaptive neuroplasticity seen in chronic pain and whether this could be attenuated by a targeted nutritional approach. The aim of this study was to review the evidence for whole-food dietary interventions in chronic pain management.

Method

A structured search of eight databases was performed up to December 2019. Two independent reviewers screened studies and evaluated risk of bias by using the National Institutes of Health assessment tool for controlled or pre–post studies and the Joanna Briggs checklist for case reports. A meta-analysis was performed in Review Manager.

Results

Forty-three studies reporting on 48 chronic pain groups receiving a whole-food dietary intervention were identified. These included elimination protocols (n = 11), vegetarian/vegan diets (n = 11), single-food changes (n = 11), calorie/macronutrient restriction (n = 8), an omega-3 focus (n = 5), and Mediterranean diets (n = 2). A visual analog scale was the most commonly reported pain outcome measure, with 17 groups reporting a clinically objective improvement (a two-point or 33% reduction on the visual analog scale). Twenty-seven studies reported significant improvement on secondary metabolic measures. Twenty-five groups were included in a meta-analysis that showed a significant finding for the effect of diet on pain reduction when grouped by diet type or chronic pain type.

Conclusion

There is an overall positive effect of whole-food diets on pain, with no single diet standing out in effectiveness. This suggests that commonalities among approaches (e.g., diet quality, nutrient density, weight loss) may all be involved in modulating pain physiology. Further research linking how diet can modulate physiology related to pain (such as inflammation, oxidative stress, and nervous system excitability) is required.

Exploring the Lipidome: Current Lipid Extraction Techniques for Mass Spectrometry Analysis

In recent years, high-throughput lipid profiling has contributed to understand the biological, physiological and pathological roles of lipids in living organisms. Across all kingdoms of life, important cell and systemic processes are mediated by lipids including compartmentalization, signaling and energy homeostasis. Despite important advances in liquid chromatography and mass spectrometry, sample extraction procedures remain a bottleneck in lipidomic studies, since the wide structural diversity of lipids imposes a constrain in the type and amount of lipids extracted. Differences in extraction yield across lipid classes can induce a bias on down-stream analysis and outcomes. This review aims to summarize current lipid extraction techniques used for untargeted and targeted studies based on mass spectrometry. Considerations, applications, and limitations of these techniques are discussed when used to extract lipids in complex biological matrices, such as tissues, biofluids, foods, and microorganisms.

Eosinophils synthesize trihydroxyoctadecenoic acids (TriHOMEs) via a 15-lipoxygenase dependent process

Trihydroxyoctadecenoic acids (TriHOMEs) are linoleic acid-derived lipid mediators reported to be dysregulated in obstructive lung disease. In contrast to many other oxylipins, TriHOME biosynthesis in humans is still poorly understood. The association of TriHOMEs with inflammation prompted the current investigation into the ability of human granulocytes to synthesize the 16 different 9,10,13-TriHOME and 9,12,13-TriHOME isomers and of the TriHOME biosynthetic pathway. Following incubation with linoleic acid, eosinophils and (to a lesser extent) the mast cell line LAD2, but not neutrophils, formed TriHOMEs. Stereochemical analysis revealed that TriHOMEs produced by eosinophils predominantly evidenced the 13(S) configuration, suggesting 15-lipoxygenase (15-LOX)-mediated synthesis. TriHOME formation was blocked following incubation with the 15-LOX inhibitor BLX-3887 and was shown to be largely independent of soluble epoxide hydrolase and cytochrome P450 activities. TriHOME synthesis was abolished when linoleic acid was replaced with 13-HODE, but increased in incubations with 13-HpODE, indicating the intermediary role of epoxy alcohols in TriHOME formation. In contrast to eosinophils, LAD2 cells formed TriHOMEs having predominantly the 13(R) configuration, demonstrating that there are multiple synthetic routes for TriHOME formation. These findings provide for the first-time insight into the synthetic route of TriHOMEs in humans and expand our understanding of their formation in inflammatory diseases.

Pain Control by Targeting Oxidized Phospholipids: Functions, Mechanisms, Perspectives

Within the lipidome oxidized phospholipids (OxPL) form a class of chemically highly reactive metabolites. OxPL are acutely produced in inflamed tissue and act as endogenous, proalgesic (pain-inducing) metabolites. They excite sensory, nociceptive neurons by activating transient receptor potential ion channels, specifically TRPA1 and TRPV1. Under inflammatory conditions, OxPL-mediated receptor potentials even potentiate the action potential firing rate of nociceptors. Targeting OxPL with D-4F, an apolipoprotein A-I mimetic peptide or antibodies like E06, specifically binding oxidized headgroups of phospholipids, can be used to control acute, inflammatory pain syndromes, at least in rodents. With a focus on proalgesic specificities of OxPL, this article discusses, how targeting defined substances of the epilipidome can contribute to mechanism-based therapies against primary and secondary chronic inflammatory or possibly also neuropathic pain.

Oxidized Lipids in Persistent Pain States

Chemotherapy, nerve injuries, or diseases like multiple sclerosis can cause pathophysiological processes of persistent and neuropathic pain. Thereby, the activation threshold of ion channels is reduced in peripheral sensory neurons to normally noxious stimuli like heat, cold, acid, or mechanical due to sensitization processes. This leads to enhanced neuronal activity, which can result in mechanical allodynia, cold allodynia, thermal hyperalgesia, spontaneous pain, and may initiate persistent and neuropathic pain. The treatment options for persistent and neuropathic pain patients are limited; for about 50% of them, current medication is not efficient due to severe side effects or low response to the treatment. Therefore, it is of special interest to find additional treatment strategies. One approach is the control of neuronal sensitization processes. Herein, signaling lipids are crucial mediators and play an important role during the onset and maintenance of pain. As preclinical studies demonstrate, lipids may act as endogenous ligands or may sensitize transient receptor potential (TRP)-channels. Likewise, they can cause enhanced activity of sensory neurons by mechanisms involving G-protein coupled receptors and activation of intracellular protein kinases. In this regard, oxidized metabolites of the essential fatty acid linoleic acid, 9- and 13-hydroxyoctadecadienoic acid (HODE), their dihydroxy-metabolites (DiHOMEs), as well as epoxides of linoleic acid (EpOMEs) and of arachidonic acid (EETs), as well as lysophospholipids, sphingolipids, and specialized pro-resolving mediators (SPMs) have been reported to play distinct roles in pain transmission or inhibition. Here, we discuss the underlying molecular mechanisms of the oxidized linoleic acid metabolites and eicosanoids. Furthermore, we critically evaluate their role as potential targets for the development of novel analgesics and for the treatment of persistent or neuropathic pain.

Adipose tissue oxylipin profiles vary by anatomical site and are altered by dietary linoleic acid in rats

Dietary PUFA and their effects on adipose tissue have been well studied, but oxylipins, the oxygenated metabolites of PUFA, have been sparsely studied in adipose tissue. To determine the oxylipin profile and to examine their potential importance in various adipose sites, female and male rats were provided control, high linoleic acid (LA), or high LA and high α-linolenic acid (LA + ALA) diets for six weeks. Analysis of gonadal (GAT), mesenteric (MAT), perirenal (PAT), and subcutaneous adipose tissues (SAT) revealed higher numbers of oxylipins in MAT and SAT, primarily due to 20-22 carbon cytochrome P450 oxylipins, as well as metabolites of cyclooxygenase derived oxylipins. LA oxylipins made up 75-96% of the total oxylipin mass and largely determined the total relative amounts between depots (GAT > MAT > PAT > SAT). However, when the two most abundant LA oxylipins (TriHOMEs) were excluded, MAT had the highest mass of oxylipins and exhibited the most sex differences. These differences existed despite comparable PUFA composition between depots. Dietary LA increased oxylipins derived from n-6 PUFA, and the addition of ALA generally returned n-6 PUFA oxylipins to levels similar to control and elevated some n-3 oxylipins. These data on oxylipin profiles in adipose depots from different anatomical sites and the effects of diet and sex provide fundamental knowledge that will aid future studies investigating the physiological effects of adipose tissue.