Esterified Oxylipins: Do They Matter?

Abstract

Oxylipins are oxygenated metabolites of fatty acids that share several similar biochemical characteristics and functions to fatty acids including transport and trafficking. Oxylipins are most commonly measured in the non-esterified form which can be found in plasma, free or bound to albumin. The non-esterified form, however, reflects only one of the possible pools of oxylipins and is by far the least abundant circulating form of oxylipins. Further, this fraction cannot reliably be extrapolated to the other, more abundant, esterified pool. In cells too, esterified oxylipins are the most abundant form, but are seldom measured and their potential roles in signaling are not well established. In this review, we examine the current literature on experimental oxylipin measurements to describe the lack in reporting the esterified oxylipin pool. We outline the metabolic and experimental importance of esterified oxylipins using well established roles of fatty acid trafficking in non-esterified fatty acids and in esterified form as components of circulating lipoproteins. Finally, we use mathematical modeling to simulate how exchange between cellular esterified and unesterified pools would affect intracellular signaling.. The explicit inclusion of esterified oxylipins along with the non-esterified pool has the potential to convey a more complete assessment of the metabolic consequences of oxylipin trafficking.

Keywords: esterification; lipoprotein; metabolism; oxylipin.

Figure 1

PRISMA Table for Study Identification. PRISMA table shows the process for identifying records to include in our survey of publications, reproduced from the 2020 PRISMA guidelines [25]. Records were pulled from PubMed using “plasma”, “oxylipin”, “oxylipid”, and oxylipin class names (DiHETE, DiHETrE, DiHOME, EpDPE, EpETE, EpETrE, EpOME, HDoHE, HEPE, HETE, HODE, HpETE, KETE, KODE, resolvin, maresin, protectin, EET, EEQ, EEP, DHET, OXO-ODE, OXO-ETE), carried out in humans, limited to the year 2020, and published in English.

Figure 2

Extant data confirm active role for esterification. An expression of the common model is shown demonstrating the role of intracellular esterification of EpETrE (A); components modeled here are shown color coded for (A,B). Input into the green compartment (black arrow) represents novel EpETrE synthesis. Esterification into PLs is a two-step process: thioloization into coA-EpETrE (red compartment), and acyltransferase activity with lyosophospholipids to create a phospholipid-EpETrE (purple compartment). Disposal of EpETrE occurs as sEH activity from the cytoplasm (green compartment) and is represented by the blue arrow. Red dots represent directly observed data. (B): To demonstrate the relative role of esterification versus ‘inactivation’ of EpETrE by sEH hydrolysis and other means, a model was constructed from extant radioactive isotope incorporation data from Karara et al. Model fit of the data using Bayesian Information Criteria is good and it conforms to the conclusions of Karara et al. The model indicates epoxide sequestration in membrane phospholipids, not disposal of free (e.g., by sEH), are the major route for signal termination. Rate constants and associated half-lives are reported in Table 2. CoA, coenzyme A; DiHETrE, dihydroxyeicosatrienoic acid; EpETrE, epoxyeicosatrienoic acid; PL, phospholipid; PLA₂, phospholipase A-2; sEH, soluble epoxide hydrolase.

Figure 3

Comparison of non-recycling systems to recycling systems. To graphically illustrate the potential consequences of sequestering EpETrEs into membrane phospholipids, we developed a simulation based on the model in Figure 2. We simulated both basal EpETrE synthesis at 1 U/min and an adaptive response to stimulate EpETrE production, as a 100-fold increase in EpETrE production to 100 U/minute beginning at time = 0 and lasting for 1 min (EpETrE R_appearance). Model (A) represents the system with no recycling in which all of the fractional clearance (FCR = 0.21) is from the cytosolic NEOx to permanent disposal, as would be the case if EpETrE disposal occurs entirely in the cytosol by sEH-mediated hydrolysis to vicinal diols. Model (B) represents the system with recycling, in which the fractional clearance from the cytosol is divided as 0.19 pools/min to coA-EpETrE and 0.02 pool/min to sEH and other disposal. The total fraction clearance from the cytosol is identical, however most occurs by re-acylation into membrane phospholipids and only a fraction by sEH. Panel (C) overlays NEOx from both systems, the non-recycling (solid) and recycling (dashed) after subtracting out background differences in steady state from each. This facilitates visualization of the effect recycling has on signal extension. The simulated models are theoretical in nature and concentrations are in arbitrary units (U).

Figure 4

Effect of repeated stimuli on non-recycling (model A) and recycling systems (model B). To illustrate the impact of successive intervals of EpETrE appearance, the simulations in Figure 3 were modified to simulate repeated pulsed appearance of EpETrE (100 U over 1 min) every 20 min (EpETrE R_appearance). This feature demonstrates how a recycling system (model B) is better organized for signal amplification in response to successive stimuli. As in Figure 3, concentrations are in arbitrary units (U).