Fumie Hamano

Identification and Characterization of a Novel Lysophosphatidic Acid Receptor, p2y5/LPA6

p2y5 is an orphan G protein-coupled receptor that is closely related to the fourth lysophosphatidic acid (LPA) receptor, LPA4. Here we report that p2y5 is a novel LPA receptor coupling to the G13-Rho signaling pathway. “LPA receptor-null” RH7777 and B103 cells exogenously expressing p2y5 showed [3H]LPA binding, LPA-induced [35S]guanosine 5′-3-O-(thio)triphosphate binding, Rho-dependent alternation of cellular morphology, and Gs/13 chimeric protein-mediated cAMP accumulation. LPA-induced contraction of human umbilical vein endothelial cells was suppressed by small interfering RNA knockdown of endogenously expressed p2y5. We also found that 2-acyl-LPA had higher activity to p2y5 than 1-acyl-LPA. A recent study has suggested that p2y5 is an LPA receptor essential for human hair growth. We confirmed that p2y5 is a functional LPA receptor and propose to designate this receptor LPA6.

LPA4/p2y9/GPR23 mediates rho-dependent morphological changes in a rat neuronal cell line.

Lysophosphatidic acid (LPA) is a potent lipid mediator that evokes a variety of biological responses in many cell types via its specific G protein-coupled receptors. In particular, LPA affects cell morphology, cell survival, and cell cycle progression in neuronal cells. Recently, we identified p2y9/GPR23 as a novel fourth LPA receptor, LPA4 (Noguchi, K., Ishii, S., and Shimizu, T. (2003) J. Biol. Chem. 278, 25600-25606). To assess the functions of LPA4 in neuronal cells, we used rat neuroblastoma B103 cells that lack endogenous responses to LPA. In B103 cells stably expressing LPA4, we observed Gq/11-dependent calcium mobilization, but LPA did not affect adenylyl cyclase activity. In LPA4 transfectants, LPA induced dramatic morphological changes, i.e. neurite retraction, cell aggregation, and cadherin-dependent cell adhesion, which involved Rho-mediated signaling pathways. Thus, our results demonstrated that LPA4 as well as LPA1 couple to Gq/11 and G12/13, whereas LPA4 differs from LPA1 in that it does not couple to Gi/o. Through neurite retraction and cell aggregation, LPA4 may play a role in neuronal development such as neurogenesis and neuronal migration.

LPA4 regulates blood and lymphatic vessel formation during mouse embryogenesis

Lysophosphatidic acid (LPA) is a potent lipid mediator with a wide variety of biological actions mediated through G protein-coupled receptors (LPA(1-6)). LPA(4) has been identified as a G(13) protein-coupled receptor, but its physiological role is unknown. Here we show that a subset of LPA(4)-deficient embryos did not survive gestation and displayed hemorrhages and/or edema in many organs at multiple embryonic stages. The blood vessels of bleeding LPA(4)-deficient embryos were often dilated. The recruitment of mural cells, namely smooth muscle cells and pericytes, was impaired. Consistently, Matrigel plug assays showed decreased mural cell coverage of endothelial cells in the neovessels of LPA(4)-deficient adult mice. In situ hybridization detected Lpa4 mRNA in the endothelium of some vasculatures. Similarly, the lymphatic vessels of edematous embryos were dilated. These results suggest that LPA(4) regulates establishment of the structure and function of blood and lymphatic vessels during mouse embryogenesis. Considering the critical role of autotaxin (an enzyme involved in LPA production) and Gα(13) in vascular development, we suggest that LPA(4) provides a link between these 2 molecules.

The G protein-coupled receptor T-cell death-associated gene 8 (TDAG8) facilitates tumor development by serving as an extracellular pH sensor.

Tumors often are associated with a low extracellular pH, which induces a variety of cellular events. However, the mechanisms by which tumor cells recognize and react to the acidic environment have not been fully elucidated. T-cell death-associated gene 8 (TDAG8) is an extracellular pH-sensing G protein-coupled receptor that is overexpressed in various tumors and tumor cell lines. In this report, we show that TDAG8 on the surface of tumor cells facilitates tumor development by sensing the acidic environment. Overexpression of TDAG8 in mouse Lewis lung carcinoma (LLC) cells enhanced tumor development in animal models and rendered LLC cells resistant to acidic culture conditions by increasing activation of protein kinase A and extracellular signal-regulated kinase in vitro. Moreover, shRNA-mediated knockdown of endogenous TDAG8 in NCI-H460 human non-small cell lung cancer cells reduced cell survival in an acidic environment in vitro as well as tumor development in vivo. Microarray analyses of tumor-containing lung tissues of mice injected with TDAG8-expressing LLC cells revealed up-regulation of genes related to cell growth and glycolysis. These results support the hypothesis that TDAG8 enhances tumor development by promoting adaptation to the acidic environment to enhance cell survival/proliferation. TDAG8 may represent a therapeutic target for arresting tumor growth.

Fatty acid remodeling by LPCAT3 enriches arachidonate in phospholipid membranes and regulates triglyceride transport

Polyunsaturated fatty acids (PUFAs) in phospholipids affect the physical properties of membranes, but it is unclear which biological processes are influenced by their regulation. For example, the functions of membrane arachidonate that are independent of a precursor role for eicosanoid synthesis remain largely unknown. Here, we show that the lack of lysophosphatidylcholine acyltransferase 3 (LPCAT3) leads to drastic reductions in membrane arachidonate levels, and that LPCAT3-deficient mice are neonatally lethal due to an extensive triacylglycerol (TG) accumulation and dysfunction in enterocytes. We found that high levels of PUFAs in membranes enable TGs to locally cluster in high density, and that this clustering promotes efficient TG transfer. We propose a model of local arachidonate enrichment by LPCAT3 to generate a distinct pool of TG in membranes, which is required for normal directionality of TG transfer and lipoprotein assembly in the liver and enterocytes. DOI: http://dx.doi.org/10.7554/eLife.06328.001

A comprehensive quantification method for eicosanoids and related compounds by using liquid chromatography/mass spectrometry with high speed continuous ionization polarity switching

Fatty acids and related metabolites, comprising several hundreds of molecular species, are an important target in disease metabolomics, as they are involved in various mammalian pathologies and physiologies. Selected reaction monitoring (SRM) analysis, which is capable of monitoring hundreds of compounds in a single run, has been widely used for comprehensive quantification. However, it is difficult to monitor a large number of compounds with different ionization polarity, as polarity switching requires a sub-second period per cycle in classical mass spectrometers. In the present study, we developed and evaluated a comprehensive quantification method for eicosanoids and related compounds by using LC/MS with high-speed continuous ionization polarity switching. The new method employs a fast (30ms/cycle) continuous ionization polarity switching, and differentiates 137 targets either by chromatography or by SRM transition. Polarity switching did not affect the lower limits of quantification, which ranged similarly from 0.5 to 200pg on column. Lipid extracts from mouse tissues were analyzed by this method, and 65 targets were quantitatively detected in the brain, including 6 compounds analyzed in the positive ion mode. We demonstrated that a fast continuous ionization polarity switching enables the quantification of a wide variety of lipid mediator species without compromising the sensitivity and reliability.

Fever Is Mediated by Conversion of Endocannabinoid 2-Arachidonoylglycerol to Prostaglandin E2.

Fever is a common response to inflammation and infection. The mechanism involves prostaglandin E2 (PGE2)-EP3 receptor signaling in the hypothalamus, which raises the set point of hypothalamic thermostat for body temperature, but the lipid metabolic pathway for pyretic PGE2 production remains unknown. To reveal the molecular basis of fever initiation, we examined lipopolysaccharides (LPS)-induced fever model in monoacylglycerol lipase (MGL)-deficient (Mgll −/−) mice, CB1 receptor-MGL compound-deficient (Cnr1 −/− Mgll −/−) mice, cytosolic phospholipase A2α (cPLA2α)-deficient (Pla2g4a −/−) mice, and diacylglycerol lipase α (DGLα)-deficient (Dagla −/−) mice. Febrile reactions were abolished in Mgll −/− and Cnr1 −/− Mgll −/− mice, whereas Cnr1 −/− Mgll +/+, Pla2g4a −/− and Dagla −/− mice responded normally, demonstrating that MGL is a critical enzyme for fever, which functions independently of endocannabinoid signals. Intracerebroventricular administration of PGE2 caused fever similarly in Mgll −/− and wild-type control mice, suggesting a lack of pyretic PGE2 production in Mgll −/− hypothalamus, which was confirmed by lipidomics analysis. Normal blood cytokine responses after LPS administration suggested that MGL-deficiency does not affect pyretic cytokine productions. Diurnal body temperature profiles were normal in Mgll −/− mice, demonstrating that MGL is unrelated to physiological thermoregulation. In conclusion, MGL-dependent hydrolysis of endocannabinoid 2-arachidonoylglycerol is necessary for pyretic PGE2 production in the hypothalamus.

Imaging of intracellular fatty acids by scanning X-ray fluorescence microscopy

Fatty acids are taken up by cells and incorporated into complex lipids such as neutral lipids and glycerophospholipids. Glycerophospholipids are major constituents of cellular membranes. More than 1000 molecular species of glycerophospholipids differ in their polar head groups and fatty acid compositions. They are related to cellular functions and diseases and have beenwell analyzed by mass spectrometry. However, intracellular imaging of fatty acids and glycerophospholipids has not been successful due to insufficient resolution using conventional methods. Here, we developed a method for labeling fatty acids with bromine (Br) and applied scanning X‐ray fluorescence microscopy (SXFM) to obtain intracellular Br mapping data with submicrometer resolution. Mass spectrometry showed that cells took up Br‐labeled fatty acids andmetabolized them mainly into glycerophospholipids in CHO cells. Most Br signals observed by SXFM were in the perinuclear region. Higher resolution revealed a spot‐like distribution of Br inthe cytoplasm. Thecurrentmethodenabledsuccessful visualizationof intracellular Br‐labeledfatty acids. Single‐element labeling combined with SXFM technology facilitates the intracellular imaging of fatty acids, whichprovides anewtool todeterminedynamic changes infattyacids andtheirderivatives at the single‐cell level.—A., Kita, Y., Ishizaka, Y., Yamauchi, K., Kohmura, Y., Lobinski, R., Shimizu, I., Shimizu, T. Imaging of intracellular fatty acids by scanning X‐ray fluorescence microscopy. FASEB J. 30, 4149–4158 (2016). www.fasebj.org

Monoacylglycerol lipase deficiency affects diet-induced obesity, fat absorption, and feeding behavior in CB1 cannabinoid receptor–deficient mice

Excess energy intake causes obesity, which leads to insulin resistance and various other complications of metabolic syndrome, including diabetes, atherosclerosis, dyslipidemia, and nonalcoholic fatty liver disease. Although recent studies have depicted altered lipid metabolism as an underlying feature, the detailed mechanisms are still unclear. Here we describe a possible role in high‐fat diet (HFD)‐induced obesity for monoacylglycerol lipase (MGL), an enzyme that is also known to hydrolyze the endocannabinoid 2‐arachidonoylglycerol in brain. MGL‐deficient [MGL‐knockout (KO)] mice fed a HFD gained less body weight than wild‐type mice and were protected from insulin resistance and hepatic steatosis. Food intake and energy expenditure were not altered in MGL‐KO mice, but blood triglyceride levels after oral olive oil gavage were suppressed, indicating a role for MGL in intestinal fat absorption. Experiments with cannabinoid receptor type 1 (CB1)/MGL double‐KO mice revealed that these phenotypes may include mechanisms that are independent of CB1‐receptor–mediated endocannabinoid functions. We also noted that MGL‐KO mice had less preference for HFD over normal chow diet. Oral but not intraperitoneal lipid administration strongly suppressed the appetites of MGL‐KO and CB1/MGL double‐KO mice, but not of wild‐type and CB1‐KO mice. Appetite suppression was reversed by vagotomy, suggesting involvement of MGL in the gut–brain axis regulation of appetite. Our results provide mechanistic insights of MGLs role in diet‐induced obesity, lipid metabolic disorder, and regulation of appetite.—Yoshida, K., Kita, Y., Tokuoka, S. M., Hamano, F., Yamazaki, M., Sakimura, K., Kano, M., Shimizu, T. Monoacylglycerol lipase deficiency affects diet‐induced obesity, fat absorption, and feeding behavior in CB1 cannabinoid receptor–deficient mice. FASEB J. 33, 2484–2497 (2019). www.fasebj.org