Estrogen-related receptor γ regulates hepatic triglyceride metabolism through phospholipase A2 G12B.

Hypersecretion of hepatic very LDL (VLDL)-associated triglyceride (TG) is the hallmark of hypertriglyceridemia. The estrogen-related receptor γ (ERRγ), an orphan nuclear receptor, plays crucial roles in the regulation of metabolic homeostasis, including TG formation in the liver. It remains unclear whether ERRγ regulates hepatic VLDL-TG secretion. We demonstrated that knockdown of ERRγ impairs hepatic VLDL-TG secretion in mice, whereas overexpression of ERRγ favors the secretion, indicating a novel role of ERRγ in hepatic TG metabolism. We found that ERRγ transcriptionally regulates the expression of PLA2G12B by binding to the promoter region of the Pla2g12b gene. In Pla2g12b-null mice, ERRγ fails to regulate hepatic VLDL-TG secretion. There is an apparent accumulation of large lipid droplets in the liver of Pla2g12b-null mice. These data suggest that ERRγ is a novel regulator of hepatic VLDL-TG secretion, which is mediated through the action on PLA2G12B.-Chen, L., Wu, M., Zhang, S., Tan, W., Guan, M., Feng, L., Chen, C., Tao, J., Chen, L., Qu, L. Estrogen-related receptor γ regulates hepatic triglyceride metabolism through phospholipase A2 G12B.

Group X secreted phospholipase A(2) induces lipid droplet formation and prolongs breast cancer cell survival.

BACKGROUND: Alterations in lipid metabolism are inherent to the metabolic transformations that support tumorigenesis. The relationship between the synthesis, storage and use of lipids and their importance in cancer is poorly understood. The human group X secreted phospholipase A2 (hGX sPLA2) releases fatty acids (FAs) from cell membranes and lipoproteins, but its involvement in the regulation of cellular FA metabolism and cancer is not known. RESULTS: Here we demonstrate that hGX sPLA2 induces lipid droplet (LD) formation in invasive breast cancer cells, stimulates their proliferation and prevents their death on serum deprivation. The effects of hGX sPLA2 are shown to be dependent on its enzymatic activity, are mimicked by oleic acid and include activation of protein kinase B/Akt, a cell survival signaling kinase. The hGX sPLA2-stimulated LD biogenesis is accompanied by AMP-activated protein kinase (AMPK) activation, up-regulation of FA oxidation enzymes and the LD-coating protein perilipin 2, and suppression of lipogenic gene expression. Prolonged activation of AMPK inhibited hGX sPLA2-induced LD formation, while etomoxir, an inhibitor of FA oxidation, abrogated both LD formation and cell survival. The hGX sPLA2-induced changes in lipid metabolism provide a minimal immediate proliferative advantage during growth under optimal conditions, but they confer to the breast cancer cells a sustained ability to resist apoptosis during nutrient and growth factor limitation. CONCLUSION: Our results identify hGX sPLA2 as a novel modulator of lipid metabolism that promotes breast cancer cell growth and survival by stimulating LD formation and FA oxidation.

Group X secretory phospholipase A2 enhances TLR4 signaling in macrophages.

Secretory phospholipase A(2)s (sPLA(2)) hydrolyze glycerophospholipids to liberate lysophospholipids and free fatty acids. Although group X (GX) sPLA(2) is recognized as the most potent mammalian sPLA(2) in vitro, its precise physiological function(s) remains unclear. We recently reported that GX sPLA(2) suppresses activation of the liver X receptor in macrophages, resulting in reduced expression of liver X receptor-responsive genes including ATP-binding cassette transporters A1 (ABCA1) and G1 (ABCG1), and a consequent decrease in cellular cholesterol efflux and increase in cellular cholesterol content (Shridas et al. 2010. Arterioscler. Thromb. Vasc. Biol. 30: 2014-2021). In this study, we provide evidence that GX sPLA(2) modulates macrophage inflammatory responses by altering cellular cholesterol homeostasis. Transgenic expression or exogenous addition of GX sPLA(2) resulted in a significantly higher induction of TNF-α, IL-6, and cyclooxygenase-2 in J774 macrophage-like cells in response to LPS. This effect required GX sPLA(2) catalytic activity, and was abolished in macrophages that lack either TLR4 or MyD88. The hypersensitivity to LPS in cells overexpressing GX sPLA(2) was reversed when cellular free cholesterol was normalized using cyclodextrin. Consistent with results from gain-of-function studies, peritoneal macrophages from GX sPLA(2)-deficient mice exhibited a significantly dampened response to LPS. Plasma concentrations of inflammatory cytokines were significantly lower in GX sPLA(2)-deficient mice compared with wild-type mice after LPS administration. Thus, GX sPLA(2) amplifies signaling through TLR4 by a mechanism that is dependent on its catalytic activity. Our data indicate this effect is mediated through alterations in plasma membrane free cholesterol and lipid raft content.

Production of vascular endothelial growth factors from human lung macrophages induced by group IIA and group X secreted phospholipases A2.

Angiogenesis and lymphangiogenesis mediated by vascular endothelial growth factors (VEGFs) are main features of chronic inflammation and tumors. Secreted phospholipases A(2) (sPLA(2)s) are overexpressed in inflammatory lung diseases and cancer and they activate inflammatory cells by enzymatic and receptor-mediated mechanisms. We investigated the effect of sPLA(2)s on the production of VEGFs from human macrophages purified from the lung tissue of patients undergoing thoracic surgery. Primary macrophages express VEGF-A, VEGF-B, VEGF-C, and VEGF-D at both mRNA and protein level. Two human sPLA(2)s (group IIA and group X) induced the expression and release of VEGF-A and VEGF-C from macrophages. Enzymatically-inactive sPLA(2)s were as effective as the active enzymes in inducing VEGF production. Me-Indoxam and RO092906A, two compounds that block receptor-mediated effects of sPLA(2)s, inhibited group X-induced release of VEGF-A. Inhibition of the MAPK p38 by SB203580 also reduced sPLA(2)-induced release of VEGF-A. Supernatants of group X-activated macrophages induced an angiogenic response in chorioallantoic membranes that was inhibited by Me-Indoxam. Stimulation of macrophages with group X sPLA(2) in the presence of adenosine analogs induced a synergistic increase of VEGF-A release and inhibited TNF-alpha production through a cooperation between A(2A) and A(3) receptors. These results demonstrate that sPLA(2)s induce production of VEGF-A and VEGF-C in human macrophages by a receptor-mediated mechanism independent from sPLA(2) catalytic activity. Thus, sPLA(2)s may play an important role in inflammatory and/or neoplastic angiogenesis and lymphangiogenesis.

Secretory phospholipase A2 and airway inflammation and hyperresponsiveness.

Phospholipases mediate the release of arachidonic acid from membrane phospholipids, enabling the subsequent metabolism to potent inflammatory mediator products of cyclooxygenase and lipoxygenase enzymes, such as prostaglandins and leukotrienes. Cytosolic phospholipase A2 has long been recognized as important, but newly characterized are secreted A2 isoenzymes. These secretory phospholipases are released into the extracellular compartment on cell activation. Elevated levels have been found in allergic patients after allergen challenge. Earlier investigations in a mouse asthma model utilizing airway challenges with allergen showed an important role for cysteinyl leukotrienes in the airway remodeling process. Utilizing secretory phospholipase knockout mice, group X deficiency significantly diminished the airway goblet cell metaplasia, mucus hypersecretion, increased airway smooth muscle mass, and subepithelial fibrosis observed in wild type mice after allergen challenge. The mechanism is likely through impaired generation of cysteinyl leukotrienes in the knockout mice. Recent human investigation in patients with exercise induced bronchoconstriction is supportive of a role of secretory phospholipase, directing attention to these enzymes as particularly attractive pharmacologic targets in asthma.

Macrophage secretory phospholipase A2 group X enhances anti-inflammatory responses, promotes lipid accumulation, and contributes to aberrant lung pathology.

Secreted phospholipase A2 group X (sPLA(2)-X) is one of the most potent enzymes of the phospholipase A(2) lipolytic enzyme superfamily. Its high catalytic activity toward phosphatidylcholine (PC), the major phospholipid of cell membranes and low-density lipoproteins (LDL), has implicated sPLA(2)-X in chronic inflammatory conditions such as atherogenesis. We studied the role of sPLA(2)-X enzyme activity in vitro and in vivo, by generating sPLA(2)-X-overexpressing macrophages and transgenic macrophage-specific sPLA(2)-X mice. Our results show that sPLA(2)-X expression inhibits macrophage activation and inflammatory responses upon stimulation, characterized by reduced cell adhesion and nitric oxide production, a decrease in tumor necrosis factor (TNF), and an increase in interleukin (IL)-10. These effects were mediated by an increase in IL-6, and enhanced production of prostaglandin E(2) (PGE(2)) and 15-deoxy-Delta12,14-prostaglandin J(2) (PGJ(2)). Moreover, we found that overexpression of active sPLA(2)-X in macrophages strongly increases foam cell formation upon incubation with native LDL but also oxidized LDL (oxLDL), which is mediated by enhanced expression of scavenger receptor CD36. Transgenic sPLA(2)-X mice died neonatally because of severe lung pathology characterized by interstitial pneumonia with massive granulocyte and surfactant-laden macrophage infiltration. We conclude that overexpression of the active sPLA(2)-X enzyme results in enhanced foam cell formation but reduced activation and inflammatory responses in macrophages in vitro. Interestingly, enhanced sPLA(2)-X activity in macrophages in vivo leads to fatal pulmonary defects, suggesting a crucial role for sPLA(2)-X in inflammatory lung disease.

Human group III phospholipase A2 suppresses adenovirus infection into host cells. Evidence that group III, V and X phospholipase A2s act on distinct cellular phospholipid molecular species.

Of 10 mammalian secreted phospholipase A(2) (sPLA(2)) enzymes identified to date, group V and X sPLA(2)s, which are two potent plasma membrane-acting sPLA(2)s, are capable of preventing host cells from being infected with adenovirus, and this anti-viral action depends on the conversion of phosphatidylcholine (PC) to lysophosphatidylcholine (LPC) in the host cell membrane. Here, we show that human group III sPLA(2), which is structurally more similar to bee venom PLA(2) than to other mammalian sPLA(2)s, also has the capacity to inhibit adenovirus infection into host cells. Mass spectrometry (MS) demonstrated that group III sPLA(2) hydrolyzes particular molecular species of PC to generate LPC in human bronchial epithelial cells. Remarkably, in addition to the catalytically active sPLA(2) domain, the N-terminal, but not C-terminal, domain unique to this enzyme was required for the anti-adenovirus effect. To our knowledge, this is the first demonstration that the biological action of group III sPLA(2) depends on its N-terminal domain. Finally, our MS analysis provided additional and novel evidence that group III, V and X sPLA(2)s target distinct phospholipid molecular species in cellular membranes.