Function

Much research has focused on cPLA₂-α because of its central role in the initiation of arachidonic acid metabolism. cPLA₂-α enzymes catalyze the hydrolysis of the sn-2 position of membrane glycerophospholipids, leading to the production of free fatty acids and lysophospholipids (Fig. 1). If the esterified fatty acid is arachidonic acid (AA), this is converted to  prostaglandins (PGs) by  cyclooxygenases (COX), hydroxyeicosatetraenoic acids (HETEs) by lipoxygenases (LOX), or epoxyeicosatrienoic acids (EETs) by P450-dependent epoxygenase (EOX). By binding to  G-protein-coupled receptors, these eicosanoids are central mediators of  inflammation and could be mitogenic. The other reaction products of cPLA₂-α action, lysophospholipids, are also biologically active via binding to  G-protein-coupled receptors and, in some settings, are the precursors of platelet-activating factor.

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A direct role for sPLA₂ in supplying arachidonic acid to downstream enzymes for eicosanoid synthesis has now been demonstrated in vivo. sPLA₂ enzymes have been implicated in a number of biological processes, such as  inflammation and host defense. sPLA₂-IIA is found in inflammatory fluids, tissue exudates, or serum and is believed to propagate inflammation in response to pro-inflammatory cytokines including interleukin-1,  interleukin-6, and tumor necrosis factor. A gene deletion experiment in mice shows that knockout of the sPLA₂-V gene reduces eicosanoid production in response to inflammatory stimuli. There are also reports describing the necessity of sPLA₂-V in eicosanoid production in murine mast cells.

Biological roles for the other cPLA₂ enzymes including the newly identified cPLA₂-δ, cPLA₂-ε, and cPLA₂-ξ have not yet been clearly defined. The Ca²⁺-independent iPLA2 appears to play a role in phospholipid remodeling, regulation of store-operated calcium channels,  apoptosis, and release of arachidonic acid.

Regulation

For cPLA₂-α, Ca²⁺ binding to the N-terminal C2 domain is required for translocation of the enzyme from the cytosol to the endoplasmic reticulum, Golgi apparatus, and perinuclear membranes – where cPLA₂-α associates with the preferred substrate, arachidonoyl phospholipids, resulting in eicosanoid biosynthesis, as well as brings close proximity to eicosanoid-producing enzymes such as COX and LOX. cPLA₂-α activity is also regulated by phosphorylation on serine residues with protein kinases including MAPK. The main site of cPLA₂-α phosphorylation is Ser-505, and the replacement of Ser-505 with Ala abolishes agonist-stimulated arachidonate release. A change in cPLA₂-α conformation due to phosphorylation increases phospholipid-binding affinity and AA release.

Annexin 1 (ANX1, also known as lipocortin I and calpactin II) and annexin 2 (ANX2, also known as lipocortin II and calpactin I) are naturally inhibitors of cPLA₂-α in various cell types. There are 13 members in annexin family, and the inhibition of cPLA₂-α appears to be specific and not a general phenomenon for all ANXs. Although the exact mechanism for the inhibition of cPLA₂-α is not clear, it is unlikely due to competition with cPLA₂-α for substrate. Some in vitro studies have shown that secreted phospholipase A₂ can also be inhibited by annexins due to substrate sequestration by annexins.

A homeodomain-interacting protein kinase-2 (HIPK2), also a corepressor for homeodomain transcriptional factors, is capable to retrain cPLA₂-α gene expression through interacting with histone deacetylase-1.

Some studies suggest that sPLA₂ can mediate indirect activation of cPLA₂-α via mobilization of calcium and/or  MAP kinase-mediated phosphorylation. There are two possible mechanisms underlying this mediation. It is possible that sPLA₂ binds to the external cell surface resulting in the release of fatty acid and lysophospholipid. Arachidonic acid released in this manner will be metabolized to HETE and/or PGE₂. These metabolites and/or lysophospholipid may in turn activate cPLA₂-α by mobilization of calcium intracellularly and/or by activation of the  MAP kinase pathway. Alternatively, sPLA₂ could exert its action on cPLA₂-α by binding to heparan sulfate proteoglycans (e.g., glypican) leading to internalization.

Cancer Relevance

Eicosanoid pathway is activated in many types of cancers and contributes to disease progression by promoting cell proliferation,  motility,  invasion, and  angiogenesis. As the predominant source of intracellular AA for eicosanoid synthesis, cPLA₂-α is induced and/or activated in a range of human tumor types, such as the colon and prostate. An increase in cPLA₂-α causes the activation of oncogenic protein kinase B (i.e., AKT) in colon and prostate cancer cells. Inversely, the activated AKT promotes stabilization of cPLA₂-α. In  prostate cancer, ANX1 and ANX2, the negative regulators of cPLA₂-α, are lost and secreted and sPLA₂-IIA, the potential positive regulator of cPLA₂-α, is increased in prostate cancer. Thus, in addition to  COX and LOX enzymes, PLA₂ enzymes are dysregulated and could contribute to the pathogenesis of cancer. Considering the fact that (i) COX and LOX inhibitors suppress the production of PGs or HETEs only, (ii) platelet-activating factors produced from the remaining lysophospholipid after AA cleavage by cPLA₂-α are oncogenic in the breast and colon and can stimulate angiogenesis and  NF-kB, and (iii) blockade of PLA₂ enzymes is expected to block AA supply to both COX and LOX pathways as well as the production of platelet-activating factors, PLA₂ enzymes represent potential targets for the treatment of cancer.