1. Introduction
Resolvins from the D-series are bioactive oxygenated metabolites of docosahexaenoic acid (22:6n-3; DHA) that were discovered in mice exudate cells treated with aspirin and DHA and were termed resolvins due to their role in dampening and promoting resolution of inflammation processes [
1]. Resolvins from the D-series (RvD) are biosynthesized by the action of 15-LOX on DHA to produce 17S-hydroperoxydocosahexaenoic acid that is converted to various types of resolvin D (RvD): RvD
1, RvD
2, RvD
3, RvD
4, RvD
5 and RvD
6 by the action of 5-LOX [
1,
2]. RvD
1 has therapeutic effects such as slowing the progression of osteoarthritis in joints, preventing neuronal dysfunction in Parkinson's disease, and increasing efferocytosis in the elderly [
3]. RvD
2 promotes subcellular localized healing, regenerative, and protective effects in burn wounds, such as keratinocyte restoration, muscle regeneration, tissue necrosis restriction, tumor growth inhibition and clearing cellular debris in mice [
3,
4]. RvD
3 has powerful anti-inflammatory effects on leukocytes, decreases the levels of pro-inflammatory cytokines (MCP-1, IL-6, and keratinocyte chemoattractant protein), and eicosanoids (LTB
4, PGD
2 and TxB
2), inhibits neutrophil transmigration and enhances macrophage absorption of microbial particles [
5]. RvD
4 protects organs in cases of ischemia kidney injury [
3,
6]. RvD
5 improves phagocytosis by increasing neutrophil and macrophage movement, modulates TNF-α and NF-κB in blood, synovial fluid and exudates discharged after a hemorrhage and in the late stages of coagulation prolongs the agglutination process and reduces the likelihood of bleeding [
7]. RvD
6 increases nerve regeneration, stimulates hepatocyte growth factor genes specifically as upstream regulators and a gene network involved in axon growth and suppression of neuropathic pain, indicating a novel function of this lipid mediator to maintain cornea integrity and homeostasis after injury [
2,
8].
In general, resolvins produced from DHA have demonstrated promising therapeutic advantages in terms of cell damage reduction, oxidative stress inhibition, and tumor growth suppression [
9]. In mammals, there is growing evidence that resolvins may assist in the resolution of acute inflammation and potently suppress inflammatory and neuropathic pain. Although it is unknown whether this is the case in fish, a study conducted by Ruyter and colleagues showed that greater dietary DHA levels lead to higher concentrations of resolvins in plasma, which may have health benefits in fish [
10]. Neuronal deficiencies and developmental issues in larvae have been reported in fish fed with a diet scarce in DHA [
11], hence the possible link between resolvins in neuronal function of fish.
Different approaches have been reported for the determination of resolvins in different kind of samples. Enzyme immunoassay (EIA) kits has been used for determining the production of RvD
1 in human and fish plasma [
10,
12] with high sensitivity. However, it is also well known that EIA is prone to cross-reactivity, which in turn cause an overestimation of the levels of specific resolvins. In addition, EIA are limited to just one type of resolvin per commercial kit (e.g., either RvD
1 or RvD
2) which makes the technique remarkably expensive when different resolvins are considered.
High performance liquid chromatography with UV diode array detection (HPLC-DAD) and gas chromatography coupled to mass spectrometry (GC-MS) have been used as alternative techniques to validate the results of liquid chromatography mass spectrometry in tandem mode (LC-MS/MS). For instance, enzymatically generated RvD
1 was determined by LC-MS/MS analysis and further evidence of its positive identification was obtained using HPLC-DAD to confirm the presence of a conjugated tetraene structure within RvD
1 that is responsible for its characteristic triplet chromophore at a λ
max = 301 nm. A subsequent GC-MS analysis was performed after derivatizing RvD
1 with diazomethane to its corresponding trimethylsilyl derivative [
9,
13]. In a similar way, actively phagocytosing poly-morphonuclear neutrophils were converted to RvD
2 and determined by LC-MS/MS analysis, followed by a subsequent GC-MS analysis of derivatized RvD
2 to validate the LC-MS/MS determination [
9]. The main drawback associated with HPLC-DAD is the potential coelution of isomeric resolvins with similar spectrum which may hinder their discrimination, while the most evident disadvantage GC-MS is that is restricted to thermally stable volatile compounds, generally prepared by time-consuming derivatization process [
14,
15,
16].
Resolvins are commonly found in biological fluids and organs at extremely low concentrations, including peripheral blood, cerebral fluid, placenta, synovial fluids, urine, sputum, spleen, lymph nodes, cell cultures and others [
17]. As a result, successful extraction is essential to wash and clean up the sample followed by drying up and reconstitution in a small volume to improve the concentration of analyte. An overview of the literature indicated that solid phase extraction (SPE) is arguably the most popular extraction method for the analysis of resolvins in different kind of samples and LC-MS/MS the preferred quantitative technique for its outstanding sensitivity. A recent article has proposed a cumbersome methodology that combines liquid-liquid extraction (LLE) with chloroform and acetate-water buffered at pH 4 followed by µ-SPE with methanol-water buffered at pH 4 and final LC-MS/MS analysis for the quantification of resolvins in human keratinocyte cell lysates to obtain recoveries around 42 and 64% [
18]. Unfortunately, SPE alone (not to mention combined with LLE) is a time consuming and complicated method that requires multiple steps and different solvents prior to LC-MS/MS analysis. Our current SPE protocol for the analysis of resolvins (RvD
1, RvD
2) in cell culture requires a total of seven different solvents (six solvents for SPE and one solvent for final reconstitution) [
14]. Poor recovery, reproducibility, and insufficient cleaning of sample extracts are some of the drawbacks commonly associated to SPE [
19]. Different SPE adsorbent materials and LLE have been compared to propose a suitable method for the extraction of RvD
1 from human endothelial cells and further determination by LC-MS, and the best results were obtained using LLE with a solution of methanol containing the internal standard [
20]. Although it seems a promising approach, this LLE protocol was validated incorrectly by using spiked plasma samples instead of human endothelial cells. Furthermore, methanol is generally used for protein removal, therefore LLE with methanol for biological samples requires multiple and time-consuming centrifugation steps to ensure complete precipitation. For instance, centrifugation times of 45 min for metabolome [
21] and 70 min for RvD
1 analysis [
20] followed by drying methanol with a stream of nitrogen that is a lengthy operation prior to final reconstitution and LC-MS/MS analysis.
Overall, sample preparation for the analysis of multiple compounds (e.g., resolvins) is the major bottleneck in analytical laboratories. The aim of this study is to propose a simple and rapid LLE procedure to quantify the temporal production of released resolvins (RvD1, RvD2, RvD3, RvD4, RvD5) into Leibovitz's L-15 complete medium by head kidney cells from Atlantic salmon exposed to DHA and further liquid chromatography triple quadrupole mass spectrometry (LC-MS/MS). To our knowledge, this is the first validated LLE procedure for quantifying biosynthesized D-series resolvins by cell cultures.
3. Materials and Methods
3.1. Reagents
Resolvin D1 (RvD1, 95%), resolvin D2 (RvD2, 95%), resolvin D3 (RvD3, 95%), resolvin D4 (RvD4, 95%), resolvin D5 (RvD5, 95%), deuterated resolvin D1 (RvD1-d5, 95%), deuterated resolvin D2 (RvD2-d5, 95%) and deuterated resolvin D3 (RvD3-d5, 95%). Acetonitrile (99.8 %) and formic acid (98 %) were purchased from SigmaAldrich (St. Louis, MO, USA). 2-propanol (HPLC grade, 99.9 %) from Merck (Darmstadt, Germany). Chloroform (HPLC grade, 99.8%) was obtained from Merck (Darmstadt, Germany). A Millipore Milli-Q system was used to produce ultra-pure water 18 MΩ (Millipore, Milford, USA). Cis-4,7,10,13,16,19-docosahexaenoic acid (DHA, ≥98%) were purchased from Sigma–Aldrich (Oslo, Norway). Leibovitz`s L-15 medium from Sigma-Aldrich (St. Louis, MO, USA). Fetal bovine serum (FBS, cat# 14-801F) was from BioWhittaker (Petit Rechain, Belgium). The glutaMaxTM 100× (Gibco-BRL, cat# 35056) was from Gibco-BRL (CergyPontoise, France).
3.2. Head Kidney Cells
For each fish, the head kidneys were directly sampled and added PBS at 5⁰C and then cut with a scissor and squeezed through a 40 µM Falcon cell strainer. The cells were transferred to tubes and centrifuged in a Hettich Zentrifugen, 320 R, at 400 × g for 5 min at 4⁰C. Cell pellets were resuspended in PBS and layered carefully on top of equal amounts of diluted Percoll in density 1.08 g/mL. The tubes were centrifuged at 800 × g for 30 min at 4⁰C. The cell layer in the interface containing the head kidney leukocytes was collected and the cells were pelleted by centrifugation, 400 × g for 5 min at 4◦C. An additional washing step in PBS was performed. The cells were counted using a Bürker chamber and 0.4% trypan blue solution and the viability was above 85%.
3.3. Cell Cultures
A complete L-15 (cL-15) medium was supplemented with 10% foetal bovine serum (FBS), 2% pen/strep, 2% glutamaxTM100× and used to prepare cL-15 solutions containing DHA that was diluted to a concentration 50 μM and a control solution containing ethanol (the solvent used to dissolve the DHA). Approximately 1×107 salmon head kidney cells were cultured into each well (control and DHA). The cell culture plates were incubated in a normal atmosphere incubator (Sanyo Electric Company Ltd. Osaka, Japan) at 9⁰C for 6, 12 and 24 h under dark conditions. The two suspensions of cells (control and DHA) were prepared in pentaplicate. The head kidney cells were centrifuged at 50 × g for 5 min at 4◦C, the medium collected and stored at -80⁰C until extraction followed by LC-MS/MS analysis.
3.4. Optimal Concentrations of the Internal Standards
A 3k factorial design, where 3 represents the number of concentration levels (low, medium, high) and k the number of factors (analyte and internal standard), was used to study variations in the response factor (RF) when the concentrations of both, the internal standards (IS) and the analytical resolvins are varied between 15-45 ng/mL. The optimal IS concentrations should yield a stable RF over the explored analytical range.
3.5. Extraction protocol
Two successive aliquots of acetonitrile (500 μL) containing the mixture of internal standards at the concentration levels indicated in
Table 1 and chloroform (500 μL) were added successively into an Eppendorf tube containing 200 μL of the mixture of resolvins at the concentration levels of every experiment in
Table 1. The Eppendorf tube was vortex-mixed for 30 sec (Bandelin RK 100 ultra mixer, Berlin, Germany), centrifuged at 1620×g for 3 min (Eppendorf AG centrifuge, Hamburg, Germany), the top phase was removed, and the extraction procedure repeated in the remaining phase using acetonitrile without internal standards and chloroform. After removing the chloroform phase, the remaining solution was vacuum-dried at room temperature (Labconco vacuum drier system, Kansas, MO), diluted to 50 μL with methanol, transferred to an autosampler vial and submitted to LC-MS/MS analysis.
3.6. Analytical Performance
The parameters used to assess the analytical performance of the extraction method in conjunction with LC-MS/MS were selectivity, limit of detection (LOD), limit of quantification (LOQ), calibration range and recovery. The selectivity of the method was evaluated by comparing the chromatograms obtained after injection of L-15 medium samples with and without the analytes. The calibration curves for the resolvins in L-15 medium were prepared between 0-50 ng/mL and extracted as described above by using the optimal concentration of internal standards suggested by the 3
k factorial design. The linearity was judged by computing both, variance ratio lack-of-fit to pure error and the coefficient of regression, as suggested by the Analytical Method Committee [
28] and the International Council for Harmonisation guidance for validation of analytical procedures [
29]. The ratio standard deviation (σ) to slope (φ) of the regression curves for every resolvin was used to determine the LOD (3.3×σ/φ) and LOQ (10×σ/φ) as described elsewhere [
30]. The percentage of recovery was assessed by comparing the degree of agreement between experimental and nominal concentrations as acknowledged by the ICH [
29,
30].
3.7. Liquid Chromatography Mass Spectrometry
An Agilent ultra-high performance liquid chromatography (UHPLC) coupled to a 6495 QQQ triple quadrupole (Agilent Technologies, Waldbronn, Germany) with an electrospray ionization (ESI) interface and iFunnel ionization was used to quantify the eicosanoids. The UHPLC system was equipped with a Zorbax RRHD Eclipse Plus C18, 95Å, 2.1 × 50 mm, 1.8 µm chromatographic column. The mobile phase delivered at 0.4 mL/min in gradient mode consisted of ultra-pure water with 0.1 % formic acid (solution A) and an equal volume mixture of acetonitrile and methanol with 0.1 % formic acid (solution B). The solvent gradient was as follows: solution A was reduced from 60 to 5 % from 0.00 to 4.00 min, kept at 5 % between 4.00 and 5.50 min, increased to 60 % between 5.50 and 5.51 min and kept at 60 % between 5.51 and 10.00-min. Mass spectrometric detection was performed by multiple reactions monitoring (MRM) in negative mode. The monitored transitions in percentage of ion counts (%) were: m/z 375→ 141 for RvD1 and RvD2; m/z 375→147 for RvD3; m/z 375→101 for RvD4; m/z 359→199 for RvD5; m/z 380→ 141 for RvD1-d5; m/z 380 → 141 for RvD2-d5; m/z 380→147 for RvD3-d5. The ESI parameters were gas temperature (120◦C), gas flow rate (19 L/min), nebulizer pressure (20 psi), sheath gas temperature (300◦C), sheath gas flow (10 L/min), capillary voltage (3500V) and nozzle voltage (2000V). The integration of the chromatograms was performed using the MassHunter Qualitative Navigator software (version 10.0). The levels of resolvins were estimated by means of the internal standards and expressed in ng/ mL units.
3.8. Statistics
Statgraphics Centurion XV Version 15.2.11 (StatPoint Technologies, Inc. Warrenton, VA, USA) was used for the statistical analyses.