Stimulation of mono- and diacylglycerol lipase activities by gangliosides in chicken neuronal cultures

Regulation of triglyceride metabolism in medaka (Oryzias latipes) hepatocytes by Neu3a sialidase

Fish store triglycerides (TGs) in the liver, muscle, and adipose tissue and TGs constitute an energy source upon metabolic demand. The liver generally plays important roles in lipid metabolism. Recent studies have suggested the possibility of hepatic lipid metabolic regulation by ganglioside in mammals; however, ganglioside-mediated regulation of lipid metabolism is unclear in fish. This study aimed to clarify the role of ganglioside in fish TG metabolism, with particular reference to Neu3a, a ganglioside-specific sialidase expressed in the fish liver. Under fasting conditions, there was a decrease in hepatic TG contents, and neu3a mRNA level was significantly up-regulated in the medaka liver. To determine the role of Neu3a in hepatic lipid metabolism, Neu3a stable transfectants were generated using fish liver Hepa-T1 cells. After treating Neu3a cells with oleic acid, reduction of TG was detected in comparison with the mock cells. Furthermore, lipase activity was greater in Neu3a cells than in mock cells. To examine which ganglioside regulates these events, alterations of ganglioside composition in Neu3a cells were analyzed. Neu3a cells exhibited increased level of lactosylceramide (LacCer), a Neu3 enzymatic product originating from GM3. In addition, exposure of LacCer toward Hepa-T1 cells resulted in an increase of neutral lipase activity. The present results suggest that Neu3a up-regulation in medaka under fasting condition accelerates hepatic TG degradation for energy production via GM3 desialylation.

Endocannabinoids and their actions

Endocannabinoids are a new class of lipid mediators, which includes amides and esters of long-chain polyunsaturated fatty acids. Anandamide (I) and 2-arachidonoylglycerol (II) are the main endogenous agonists of cannabinoid receptors, able to mimic several pharmacological effects of delta 9-tetrahydrocannabinol (III), the active principle of Cannabis sativa preparations such as hashish and marijuana. The pathways leading to the synthesis and release of anandamide and 2-arachidonoylglycerol from neuronal and nonneuronal cells are rather uncertain. Instead, evidence has accumulated showing that the activity of these compounds at their specific receptors is limited by cellular uptake through a specific membrane transporter, followed by intracellular degradation by a fatty acid amide hydrolase. Here, the endocannabinoids and the endocannabinoid-like compounds most relevant for human physiology will be discussed, along with the synthetic and degradative pathways of anandamide and 2-arachidonoylglycerol and their molecular targets on the cell surface. The main actions of the endocannabinoids in human cells and tissues will also be reviewed, focusing on the activities most recently discovered in the central nervous system and in the periphery.

Effects of glycosaminoglycans and glycosphingolipids on cytosolic phospholipases A2 from bovine brain

Two forms of Ca(2+)-independent cytosolic phospholipase A2 activity (110 kDa and 39 kDa) were found in bovine brain. They were separated by Sephadex G-75 column chromatography. The 110 kDa phospholipase A2 was much more active with phosphatidylethanolamine and was not affected by glycosaminoglycans, whereas the 39 kDa phospholipase A2 was much more active with ethanolamine plasmalogen and was markedly inhibited by glycosaminoglycans. Heparan sulphate was the most potent inhibitor, followed by chondroitin sulphate, hyaluronic acid and heparin. Gangliosides, especially the GM3 ganglioside, but not other glycosphingolipids, inhibited the activity of the 39 kDa phospholipase A2 in a dose-dependent manner. The heat-inactivation profiles of the 110 kDa and 39 kDa phospholipases A2 provide further evidence for the differences between these cytosolic enzymes. Interactions between glycosaminoglycans, gangliosides and phospholipases A2 may be involved in the maintenance of membrane function.