Regulatory Roles of Phospholipase A2 Enzymes and Bioactive Lipids in Mast Cell Biology

Abstract

Lipids play fundamental roles in life as an essential component of cell membranes, as a major source of energy, as a body surface barrier, and as signaling molecules that transmit intracellular and intercellular signals. Lipid mediators, a group of bioactive lipids that mediates intercellular signals, are produced via specific biosynthetic enzymes and transmit signals via specific receptors. Mast cells, a tissue-resident immune cell population, produce several lipid mediators that contribute to exacerbation or amelioration of allergic responses and also non-allergic inflammation, host defense, cancer and fibrosis by controlling the functions of microenvironmental cells as well as mast cell themselves in paracrine and autocrine fashions. Additionally, several bioactive lipids produced by stromal cells regulate the differentiation, maturation and activation of neighboring mast cells. Many of the bioactive lipids are stored in membrane phospholipids as precursor forms and released spatiotemporally by phospholipase A₂ (PLA₂) enzymes. Through a series of studies employing gene targeting and lipidomics, several enzymes belonging to the PLA₂ superfamily have been demonstrated to participate in mast cell-related diseases by mobilizing unique bioactive lipids in multiple ways. In this review, we provide an overview of our current understanding of the regulatory roles of several PLA₂-driven lipid pathways in mast cell biology.

Keywords: allergy; lipid mediator; mast cells; phospholipase A2; phospholipid; type 2 immunity.

Figure 1

PLA₂-driven lipid mediator pathways. Lysophospholipids (such as lysophosphatidylcholine (LPC), lysophosphatidylethanolamine (LPE), lysophosphatidylserine (LPS), lysophosphatidylglycerol (LPG), lysophosphatidylinositol (LPI) and LPA) and polyunsaturated fatty acids (such as AA, EPA and DHA) released from membrane phospholipids (PC, PE, PS, phosphatidylinositol (PI), phosphatidylglycerol (PG) and phosphatidic acid (PA)) by PLA₂ are metabolized by downstream enzymes into various lipid mediators, which in turn act on their specific receptors on target cells. In the PLA₂-lysophospholipid axis, various lysophospholipids are converted by autotaxin (ATX) into LPA, and alkyl-LPC is converted by LPC acyltransferase 2 (LPCAT2) into PAF. In the PLA₂-PUFA axis, ω6 AA released by PLA₂ is metabolized into prostanoids including PGD₂, PGE₂, PGF_2α, PGI₂, TXA₂, and 12-hydroxyheptadecatrenoic acid (12-HHT) via the COX pathway involving COX-1 or COX-2 and terminal PG synthases (PGDS, PGES, PGF_2α synthase (PGFS), PGI₂ synthase (PGIS) and TXA₂ synthase (TXAS)), or into LTB₄ and cysLTs (LTC₄, D₄ and E₄) via the 5-LOX pathway involving 5-LOX, its cofactor 5-LOX-activating protein (FLAP), and terminal LT synthases (LTA₄H and LTC₄S). Combined actions of 15-LOX and 5-LOX give rise to lipoxins (e.g., LXA₄), an AA-derived SPM. ω3 PUFAs (e.g., EPA and DHA) are metabolized by LOXs and/or CYP450s into various SPMs including hydroxy-EPA (e.g., 18-HEPE), hydroxy-DHA (e.g., 14- and 17-HDHAs), E- and D-series resolvins (e.g., RvE1-3 and RvD1-6), protectin D1 (PD1), maresins (e.g., MaR1-2), and ω3 epoxides (e.g., 17,18-EpETE).

Figure 2

The cPLA₂α-eicosanoid axis. (A) The role of cPLA₂α in generation of PGD₂ and LTs in IgE/antigen (Ag)-activated mast cells. Following FcεRI-dependent signaling that is linked to STIM1/ORAI1-driven Ca²⁺ influx, cPLA₂α translocates from the cytosol to the perinuclear, Golgi and ER membranes, where several downstream eicosanoid-biosynthetic enzymes are also located. The activity of cPLA₂α is augmented by phosphorylation at Ser⁵⁰⁵ by MAPKs, Ser⁷²⁷ by MAPK-activated protein kinases (MAPKAPKs), and possibly other kinases. The AA released from AA-containing phospholipids (PL-AA) by cPLA₂α is then metabolized into PGD₂ via the COX pathway involving COX-1 (or COX-2 if it is induced) and H-PGDS and into LTB₄ and LTC₄ via the 5-LOX pathway involving 5-LOX (which also translocates from the cytosol to the perinuclear membrane in response to Ca²⁺), FLAP (a cofactor that presents AA to 5-LOX), LTA₄H and LTC₄S. Alternatively, cPLA₂α-generated alkyl-LPC is acetylated by LPCAT2 to PAF. These reactions occur independently of degranulation and cytokine induction.

Figure 3

The mast cell-driven eicosanoid network in asthma. PGD₂ and LTs produced by activated mast cells act on various cell types including leukocytes, platelets, bronchial epithelial cells and smooth muscle cells to promote allergic responses. PGE₂, produced by various cells such as bronchial epithelial cells (but not mast cells), dampens allergic responses. NSAIDs (COX inhibitors) blocks PGE₂ production, thereby exacerbates asthma known as AERD. For details, please see the text.

Figure 4

The PAF-AH2-ω3 epoxide axis. (A) PAF-AH2-driven constitutive generation of ω3 epoxides in mast cells and their cell-autonomous role in augmentation of FcεRI signaling. PAF-AH2 directly hydrolyzes phospholipids with oxidized fatty acids (ω3 epoxides in this case) to release free ω3 epoxides (17,18-EpETE and 19,20-EpDPE), which act on the nuclear receptor PPARγ (probably indirectly) to suppress the expression of Srcin1, a Src-inhibitory protein. This Srcin1 downregulation eventually leads to increased activation of the FcεRI-proximal Src family kinases Fyn and Lyn, thereby ensuring optimal FcεRI signaling. (B) Suppression of pulmonary hypertension by PAF-AH2-derived ω3 epoxides. The ω3 epoxides released from lung mast cells acts on neighboring fibroblasts in a paracrine manner to prevent TGF-β-driven Smad2 phosphorylation, thereby attenuating perivascular fibrosis leading to amelioration of pulmonary hypertension.

Figure 5

Regulatory roles of sPLA₂s in mast cell biology. (A) The sPLA₂-III-driven paracrine PGD₂ circuit for proper mast cell maturation. sPLA₂-III secreted from immature mast cells hydrolyzes phospholipids in adjacent fibroblast membranes or possibly in mast cell-derived EVs to release AA. This AA is metabolized by fibroblastic COX/L-PGDS to PGD₂, which in turn acts on DP1 on mast cells to promote mast cell maturation. (B) Non-canonical action of sPLA₂-IIA on mast cells via shaping of the gut microbiota. sPLA₂-IIA, a potent bactericidal protein, is secreted from intestinal Paneth cells and modulates the gut microbiota. This event has systemic effects on immunity and metabolism, thereby secondarily affecting mast cell activation in distal tissues.