Functional Metabolomics Reveals Novel Active Products in the DHA Metabolome

Abstract

Endogenous mechanisms for successful resolution of an acute inflammatory response and the local return to homeostasis are of interest because excessive inflammation underlies many human diseases. In this review, we provide an update and overview of functional metabolomics that identified a new bioactive metabolome of docosahexaenoic acid (DHA). Systematic studies revealed that DHA was converted to DHEA-derived novel bioactive products as well as aspirin-triggered forms of protectins (AT-PD1). The new oxygenated DHEA-derived products blocked PMN chemotaxis, reduced P-selectin expression and platelet-leukocyte adhesion, and showed organ protection in ischemia/reperfusion injury. These products activated cannabinoid receptor (CB2 receptor) and not CB1 receptors. The AT-PD1 reduced neutrophil (PMN) recruitment in murine peritonitis. With human cells, AT-PD1 decreased transendothelial PMN migration as well as enhanced efferocytosis of apoptotic human PMN by macrophages. The recent findings reviewed here indicate that DHEA oxidative metabolism and aspirin-triggered conversion of DHA produce potent novel molecules with anti-inflammatory and organ-protective properties, opening the DHA metabolome functional roles.

Keywords: DHEA; aspirin; neutrophil; protectins; resolvins; specialized pro-resolving mediator.

Figure 1

The role of chemical mediators in the acute inflammatory response. At the site of tissue injury or bacterial invasion, both exogenous and endogenous chemical mediators are liberated. Classic endogenous mediators such as prostaglandins and leukotrienes dilate vasculatures, enhance permeability of capillaries, increase blood flow, and stimulate the recruitment of neutrophils (PMNs) to form inflammatory exudate. Novel chemical mediators are produced in the evolution and resolution of the exudate that regulate tissue responses (see text for details). The black arrow denotes leukocyte traffic from venules and the dashed arrow denotes exogenous, i.e., bacterial components and chemoattractants.

Figure 2

The fate of DHA in its new metabolome in vivo. Via esterification, DHA enters phospholipids and triglycerides that can circulate or reside in cell membranes. Autooxidation of DHA can produce neuroisoprostanes, the DHA form of isoprostanes. Enzymatic conversion in local inflammatory exudates can generate the specialized pro-resolving lipid mediators (SPMs), including D-series resolvins, protectins, and maresins.

Figure 3

DHA bioactive metabolome. DHA is the precursor to D-series resolvin (RvD), aspirin-triggered resolvin D (AT-RvD), maresins (MaR), protectin D1 (PD1), and aspirin-triggered protectin D1 (AT-PD1). Another metabolic fate of DHA is conversion to docosahexaenoyl ethanolamide (DHEA). See text for details on the recently identified novel series of bioactive oxygenated DHEA-derived products including 15-HEDPEA and 10,17S-diHDHEA.

Figure 4

Novel pathways and oxygenation products from DHEA. DHEA is first converted to 17-HpDHEA by 15-LOX. Next, 17-HpDHEA is partially reduced to the oxide radical, which reacts with the vicinal double bond at the 16-position to yield the 16(17)-epoxide radical (see text and Yang et al., 2011). Non- or low stereospecific addition of oxygen to the intermediate leads to formation of two types of peroxide radical diastereomers; their further reduction produces 13-HEDPEA and 15-HEDPEA. Via a LOX-related mechanism, 17-HpDHEA is converted to 10, 17S-diHDHEA or directly reduced to 17S-HDHEA. 10, 17S-diHDHEA, and 15-HEDPEA block PMN chemotaxis, reduce P-selectin expression and aggregation of platelets and leukocytes, and show organ protection in ischemia/reperfusion injury. Although these new products directly act on recombinant CB2 receptors in vitro, the activation of CB2 in vivo and/or additional receptors in vivo by 10,17S-diHDHEA and 15-HEDPEA remains of interest.

Figure 5

Proposed biosynthesis scheme for protectin D1 (PD1) and aspirin-triggered protectin D1 (AT-PD1). DHA is first converted to 17S-HpDHA intermediate by 15-LOX. Then PD1 is generated through 16S, 17S-epoxide. In the aspirin-triggered pathway, 17R-HpDHA is generated from DHA via acetylated-COX, and converted to AT-PD1 via a 16R, 17R-containing epoxide intermediate. Both PD1 and AT-PD1 reduce leukocytes and PMN infiltration in peritonitis stimulated by either TNF-α or zymosan A. PD1 and AT-PD1 each enhance human macrophage efferocytosis of apoptotic human PMNs and limit human PMN transmigration across human endothelial cells, the defining pro-resolving bioactions.