Group VIB Ca2+-independent Phospholipase A2γ Promotes Cellular Membrane Hydrolysis and Prostaglandin Production in a Manner Distinct from Other Intracellular Phospholipases A2

Abstract

Although group VIA Ca 2؉ -independent phospholipase A 2 ␤ (iPLA 2 ␤) has been implicated in various cellular events, the functions of other iPLA 2 isozymes remain largely elusive. In this study, we examined the cellular functions of group VIB iPLA 2 ␥. Lentiviral transfection of iPLA 2 ␥ into HEK293 cells resulted in marked increases in spontaneous, stimulus-coupled, and cell deathassociated release of arachidonic acid (AA), which was converted to prostaglandin E 2 with preferred cyclooxygenase (COX)-1 coupling. Conversely, treatment of HEK293 cells with iPLA 2 ␥ small interfering RNA significantly reduced AA release, indicating the participation of endogenous iPLA 2 ␥. iPLA 2 ␥ protein appeared in multiple sizes according to cell types, and a 63-kDa form was localized mainly in peroxisomes. Electrospray ionization mass spectrometry of cellular phospholipids revealed that iPLA 2 ␥ and other intracellular PLA 2 enzymes acted on different phospholipid subclasses. Transfection of iPLA 2 ␥ into HCA-7 cells also led to increased AA release and prostaglandin E 2 synthesis via both COX-1 and COX-2, with a concomitant increase in cell growth. Immunohistochemistry of human colorectal cancer tissues showed elevated expression of iPLA 2 ␥ in adenocarcinoma cells. These results collectively suggest distinct roles for iPLA 2 ␤ and iPLA 2 ␥ in cellular homeostasis and signaling, a functional link between peroxisomal AA release and eicosanoid generation, and a potential contribution of iPLA 2 ␥ to tumorigenesis.Phospholipase A 2 (PLA 2 ) 1 catalyzes the hydrolysis of the sn-2 position of membrane glycerophospholipids to yield free fatty acids, including arachidonic acid (AA), and lysophospholipids. Mammalian PLA 2 enzymes have been classified into four major families, including secretory PLA 2 , cytosolic PLA 2 (cPLA 2 ), Ca 2ϩ -independent PLA 2 (iPLA 2 ), and platelet-activating factor acetyl hydrolases, each of which occurs as multiple isoforms (1, 2). Of these, the group IV cPLA 2 and group VI iPLA 2 families represent intracellular enzymes with a catalytic serine in their lipase consensus motif. Of the cPLA 2 family, cPLA 2 ␣, cPLA 2 ␤, and cPLA 2 ␦ possess a Ca 2ϩ -regulated C2 domain near the N terminus, and their function is Ca 2ϩ -dependent, whereas cPLA 2 ␥ is Ca 2ϩ -independent and membranebound (3-6). The regulatory roles for cPLA 2 ␣ in production of lipid mediators, including prostaglandins and leukotrienes, have been well documented in many studies, including gene targeting (7-10).Of the emerging iPLA 2 family, group VIA iPLA 2 ␤ has been implicated in various cellular events such as the basal fatty acid reacylation reaction (membrane remodeling) (11, 12), agonist-stimulated AA release and eicosanoid generation (13-16), cell proliferation (17), apoptosis (18 -21), exocytosis (22, 23), vascular relaxation (24), adipogenesis (25), and activation of store-operated channels and capacitative Ca 2ϩ influx (26). In iPLA 2 ␤ transgenic mice, the enzyme is activated during myocardial infarction and causes malignant ventr...

Calcium-independent Phospholipase A2γ Enhances Activation of the ATF6 Transcription Factor during Endoplasmic Reticulum Stress

“…Thus, the role of iPLA 2 ␥ in ER stress was not restricted to a single cell type. We and others have shown functional coupling of iPLA 2 ␥ with cyclooxygenases, as activation of iPLA 2 ␥ results in release of arachidonic acid and production of prostanoids (12,31). In this study, we demonstrated that the effect of iPLA 2 ␥ on tunicamycin-mediated activation of ATF6 and up-regulation of ER chaperones was, however, independent of prostanoids (Fig.…”

Phospholipase A2-catalyzed hydrolysis of plasmalogen phospholipids in thrombin-stimulated human platelets

“…Human iPLA 2 -γ has multiple alternative start sites for translation resulting in functional PLA 2 γ proteins with MW of 63-, 74-, 77-and 88-kDa [7]. Initially, the subcellular localization of iPLA 2 γ was assumed to be peroxisomal due to the presence of a C-terminal-SKL sequence [7] and recent studies have confirmed that the peroxisomal iPLA 2 γ is the 63-kDa protein [20,21]. In contrast, other recent studies have demonstrated higher molecular weight forms of iPLA 2 γ are present in rat heart mitochondria [22] and we have demonstrated that the 88-kDa iPLA 2 γ isoform is present in rabbit heart and kidney ER and mitochondria [23].…”

Genetic Ablation of Calcium-independent Phospholipase A2γ Leads to Alterations in Hippocampal Cardiolipin Content and Molecular Species Distribution, Mitochondrial Degeneration, Autophagy, and Cognitive Dysfunction

“…arachidonic acid at the sn-2 position) resulting in the generation of 2-arachidonoyl lysolipids, which serve as central nodes in eicosanoid generation and downstream signaling (31). In addition, previous studies demonstrated that iPLA 2 ␥ promotes agonist-stimulated release of arachidonic acid in HEK293 cells (45) and the selective release of arachidonic acid from plasmalogens in ventricular myocytes (46). Considering that both eicosanoids and lysolipids mediate multiple cellular signaling pathways, the observed neurologic pathologies due to iPLA 2 ␥ ablation are not unanticipated and likely result from the accumulation of lipid substrates for iPLA 2 ␥ within the mitochondria.…”