Oxylipin Response to Acute and Chronic Exercise: A Systematic Review

Oxylipins are oxidized compounds of polyunsaturated fatty acids that play important roles in the body. Recently, metabololipidomic-based studies using advanced mass spectrometry have measured the oxylipins generated during acute and chronic physical exercise and described the related physiological effects. The objective of this systematic review was to provide a panel of the primary exercise-related oxylipins and their respective functions in healthy individuals. Searches were performed in five databases (Cochrane, PubMed, Science Direct, Scopus and Web of Science) using combinations of the Medical Subject Headings (MeSH) terms: “Humans”, “Exercise”, “Physical Activity”, “Sports”, “Oxylipins”, and “Lipid Mediators”. An adapted scoring system created in a previous study from our group was used to rate the quality of the studies. Nine studies were included after examining 1749 documents. Seven studies focused on the acute effect of physical exercise while two studies determined the effects of exercise training on the oxylipin profile. Numerous oxylipins are mobilized during intensive and prolonged exercise, with most related to the inflammatory process, immune function, tissue repair, cardiovascular and renal functions, and oxidative stress.

Plasma oxylipin levels can be altered in some disease states and are influenced by nutritional status and mental and physiological stressors [2,[6][7][8][9][10]. Acute and chronic exercise have a strong effect on inflammation and immune function, and oxylipins may be involved at a regulatory level [2,[10][11][12][13]. This potential linkage has generated interest in evaluating the effect of varying exercise workloads on oxylipin generation from COX, LOX, and CYP enzyme systems, and the interactive effects with different forms of nutritional support [10,11]. This interest has been fueled by advances in mass spectrometry (MS) and bioinformatics support that have allowed an ever increasing number of oxylipins to be of oxylipins to be measured [2]. Additionally, oxylipins are not stored but are generated by enzymatic systems in response to various types of stressors, providing a scaffold to effectively evaluate the influences of stressor doses, nutrition, obesity, medications, and other factors [2,7,8,10].
The scientific area of exercise and oxylipins is emergent, but enough studies have been published to systematically tabulate the types of oxylipins generated during different exercise workloads. The aim of this systematic review was to summarize oxylipin responses to acute and chronic exercise by their enzymatic pathways and to provide insights into potential physiological effects. The conclusions derived from this review will provide an evidence-based framework for future investigations.

Results
After searching the literature, 1749 documents were identified ( Figure 1). Nine papers were included in the final analysis after excluding duplicates and studies that did not meet the inclusion criteria ( Table 1). The main reasons for exclusion were the type of study participants (animal-based, children or elderly individuals, and those with a pathology) and the lack of focus on exercise-oxylipin effects.    [21] 0 0 1 0 1 2 Poor A scoring system was used to rank the studies for quality of research design, analysis methods, statistical support, and novelty (Table 5). Two studies were classified as having excellent quality [14,15], three as good [16][17][18], one as fair [19], and three as poor [20][21][22]. The detailed score of each study is shown in Table 1. Table 2 summarizes the main findings and the study design of the nine selected articles. Seven studies focused on oxylipin responses to acute exercise [14][15][16][17]19,20,22] and two studies on chronic physical training [18,21]. Two studies used acute resistance exercise [16,17], one study used a graded, maximal treadmill test [19], and six studies used varying levels of acute or chronic cardiorespiratory exercise [14,15,18,[20][21][22].
Nieman et al. [22] described large-fold increases in LA-directed hydroxyoctadecadienoic acids (9-HODE and 13-HODE), and dihydroxyoctadecenoic acids (9,10-DiHOME and 12,13-DiHOME) in 19 male cyclists after a 75 km cycling protocol. The same author in two other more recent studies, using the same 75 km cycling protocol, reported large-fold increases in plasma levels of 43 of 45 [14] and 64 of 67 [15] oxylipins. Most of the oxylipins were from AA, EPA, and DHA fatty acid substrates, with oxidation through the COX, LOX, and CYP pathways.

Discussion
This systematic review provided an overview of the oxylipins that are altered with chronic exercise training or that increase after acute resistance and cardiorespiratory exercise in healthy individuals. Acute exercise induces changes in a high number of oxylipins, especially after prolonged and intensive exercise, and are generated by COX, LOX, CYP and non-enzymatic pathways from multiple fatty acid substrates (Figure 2). The specific roles of oxylipins during and after stressful levels of exercise are still being investigated, and may include regulation of inflammatory and immune system processes, vascular function, and kidney function [1][2][3]6,[23][24][25][26]. This systematic review showed that the number of oxylipins generated and the fold increase is dependent on the exercise mode and workload. Plasma levels of oxylipins, even after prolonged and intensive exercise, are close to pre-exercise levels within 5 h of recovery.

Exercise-Related Oxylipin Formation
The release of PUFAs from cell membranes is stimulated by a group of enzymes identified as phospholipase A 2 (PLA 2 ) [1]. The PLA 2 enzymes hydrolyze the phospholipids into fatty acids and lysophospholipids. This process may be activated when the cell is stimulated by several types of signaling pathways including mitogen-activated protein kinases (MAPKs) and extracellular signal-regulated kinases (ERK), transcriptional activators (e.g., nuclear factor-kappa B), pro-inflammatory cytokines, and other inflammatory stimuli [14,[27][28][29][30][31]. These signaling pathways can be activated by exercise-induced muscle cell membrane injury and metabolic processes [32][33][34] increasing the release of PUFAs, oxylipin generation, and the inflammatory response.
Although this area of scientific endeavor is emerging, available data indicate that ω-6 and ω-3 oxylipin production is dependent on the intensity and duration of physical exercise. This observation is similar to what we have reported with plasma metabolites in general [52]. Data from the studies included in this systematic review showed small-fold changes in plasma levels of oxylipins after a single low intensity, short duration bout of exercise in contrast to large-fold changes after prolonged and intensive exercise (Tables 2 and 3). The data also indicate that the increase of pro-inflammatory oxylipins tend to occur early in exercise recovery (TXs, PGs, HETEs and HODEs) with an increase of SPMs later in recovery [2,10]. Moreover, the studies also support that most oxylipins return to near pre-exercise levels within five hours of recovery [14][15][16][17]22]. These findings have implications for evaluating the influence of drugs and nutritional supplements on exercise-induced changes in oxylipins.
The chronic effect of physical exercise on the COX, LOX, and CYP pathways and oxylipin generation is largely unknown, and studies published thus far have numerous study design limitations [18,21]. Reported changes in plasma oxylipins with exercise training are variable and of small magnitude. Larger long-term exercise training studies are needed to confirm whether or not the typical anti-inflammatory response is supported through corresponding changes in pro-and anti-inflammatory oxylipins. Limited data indicate that obese compared to normal weight individuals have higher plasma levels of pro-inflammatory oxylipins and other related biomarkers [10,13].

Matrix
Six studies included in this review used blood samples (serum or plasma) for oxylipin analysis [14][15][16]19,20,22], with two using urine samples [18,21] and one using muscle biopsy samples [17]. More needs to be learned about the influence of the sample matrix on exercise-induced changes in oxylipins. The acute exercise-induced oxylipin response appeared to be comparable across serum, plasma, and muscle sample matrices. Urine samples may be more useful in long-term [53], chronic exercise training studies, with serum and plasma samples preferred for acute exercise studies due to the transient appearance of oxylipins [2,53]. The oxylipin changes following acute resistance exercise in plasma and muscle biopsy samples were somewhat comparable, but more investigation in this area is needed.

Limitations
Research in the area of exercise and oxylipins is relatively new, and only six studies included in this review included a large number of oxylipins measured with sensitive MS platforms. Thus, the conclusions drawn in this systematic review may change as more studies are published using a wider array of exercise modalities and workloads.

Materials and Methods
This study was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines [54]. The systematized data extraction and studies selection were performed using the free standardized electronic tool State of the Art through Systematic Review (StArt) [55].

Search Strategy
The electronic search was performed in March 10th, 2020, using the MeSH terms selected: "Humans", "Exercise", "Physical Activity", "Sports", "Oxylipins", "Lipid Mediators", according to the PICOS model (Table 4) [54]. A new search was performed in May 25th, 2020, using the same terms to an update. The articles were retrieved from the databases: PubMed (via National Library of Medicine), Web of Science, SCOPUS (Elsevier), Cochrane, and Science Direct. Moreover, the search strategy was adjusted using the databases features to retrieve only human studies and English language. After the extraction from the databases, the article selection process included these steps: (1) titles and abstracts were independently examined for relevance by two researchers (EFS and CDS); (2) the full text of potentially eligible studies was reviewed. A third independent researcher (CAS) verified the inclusion process in order to solve any disagreement between the two main researchers. References of the included studies were checked for any additional relevant papers. Figure 1 represents the flow diagram of papers through the study selection process.

Inclusion and Exclusion Criteria
Studies were evaluated according to the listed criteria: (1) human adults and healthy population as the total or part of the sample; (2) analysis of an oxylipins list in serum, plasma, urine or muscle biopsy samples using MS; and (3) exercise as the main factor inducing changes in metabolism. Article were excluded if they focused on only one metabolite, were non-English, did not include an appropriate analysis method, included children, elderly individuals, individuals with any pathology or risk factor (such as hypertension, dyslipidemia, smoking/alcoholism and obesity), included animals only or in vitro models.

Data Extraction
The following data were extracted from the selected studies using procedures previously reported by our group [52]: name of the first author and year of publication, characteristics of participants and groups (i.e., population, sample size, groups, gender, age, physical activity level), research design elements (i.e., type of research, exercise mode), exercise intensity and duration, enzymatic pathway analyzed, metabololipidomics procedures (i.e., analytical platform, metabolite data), matrix, and summary comments. In addition, the oxylipins determined after physical exercise were summarized in scheme and table. Only data from the placebo or non-dietary intervention groups or phases were used.

Studies Quality Assessment
The quality of the studies was assessed using an adjusted scoring system created specifically for metabolomics studies [52] (see Table 5). The studies were classified according to the scores as excellent , good (8-6), fair , and poor (<4). Table 5. Score setting adjusted for lipidomics studies quality assessment.

Section
Maximum Score Aspects Score Attribution

Conclusions
Recent improvements in mass spectrometry and bioinformatic procedures have advanced scientific understanding of oxylipins, their physiological roles, and the effects of lifestyle interventions including exercise. Exercise-induced oxylipin production during exercise has only recently been described, and the science in this area should advance greatly during the next decade. The number of oxylipins and the magnitude of increase during acute exercise bouts are directly related to the overall workload. The accumulation of oxylipins in plasma is relatively short-lived, with levels returning close to pre-exercise levels within five hours even after about three hours of intense exercise. Although physiological roles during exercise and recovery remain to be determined, data from ancillary studies suggest widespread regulatory effects centered around inflammation and vascular function. The enzyme systems involved with oxylipin generation are complex with multiple regulatory controls, and future research, such as the National Institutes of Health project, 'Molecular Transducers of Physical Activity in Humans Consortium (MoTrPAC) [56], will better define responses to chronic exercise training, moderate-versus high-intensity exercise, muscle damage, sports nutrition, and drug interventions, and varying exercise modalities.