Unraveling algal lipid metabolism: Recent advances in gene identification

Microalgae are now the focus of intensive research due to their potential as a renewable feedstock for biodiesel. This research requires a thorough understanding of the biochemistry and genetics of these organisms’ lipid-biosynthesis pathways. Genes encoding lipid-biosynthesis enzymes can now be identified in the genomes of various eukaryotic microalgae. However, an examination of the predicted proteins at the biochemical and molecular levels is mandatory to verify their function. The essential molecular and genetic tools are now available for a comprehensive characterization of genes coding for enzymes of the lipid-biosynthesis pathways in some algal species. This review mainly summarizes the novel information emerging from recently obtained algal gene identification.     

Apoptosis-induced mitochondrial dysfunction causes cytoplasmic lipid droplet formation

A characteristic of apoptosis is the rapid accumulation of cytoplasmic lipid droplets, which are composed largely of neutral lipids. The proton signals from these lipids have been used for the non-invasive detection of cell death using magnetic resonance spectroscopy. We show here that despite an apoptosis-induced decrease in the levels and activities of enzymes involved in lipogenesis, which occurs downstream of p53 activation and inhibition of the mTOR signaling pathway, the increase in lipid accumulation is due to increased de novo lipid synthesis. This results from inhibition of mitochondrial fatty acid β-oxidation, which coupled with an increase in acyl-CoA synthetase activity, diverts fatty acids away from oxidation and into lipid synthesis. The inhibition of fatty acid oxidation can be explained by a rapid rise in mitochondrial membrane potential and an attendant increase in the levels of reactive oxygen species.     

Recent advances in lipid molecular design

The area of lipid molecular design is attracting widespread interest among numerous research groups worldwide. Diverse lipid assemblies in aqueous media, such as vesicles, bilayers and nanorods, offer new applications in chemical biology. Lipids with specifically tailored molecular architecture have been successfully employed as gene delivery vehicles, for controlled drug release and the preparation of supramolecular gels. Such molecular design of lipids, as well as their characterization upon membrane formation, offers an insight into the possible molecular basis of their properties. This in turn helps in the design of further generations of lipid systems with more predictable characteristics. Here, we present an overview of the current trends in lipid design and their utilization in various biochemical, physical and chemical applications.     

植物细胞质雄性不育及育性恢复研究进展

植物细胞质雄性不育是一种广泛存在于高等植物中的母性遗传性状。细胞质雄性不育不仅为研究核质互作提供了良好材料,同时也是植物杂种优势利用的重要基础,其分子机理是目前研究的重点。多种研究证据表明,线粒体基因与细胞质雄性不育密切相关。随着分子生物学和分子遗传学的不断发展,许多植物的恢复基因已经被定位和克隆,进一步阐明了植物细胞质雄性不育和育性恢复的分子机理。本文综述了近几年植物中细胞质雄性不育和育性恢复相关基因的研究进展,并探讨了细胞质雄性不育/育性恢复系统在育种方面的应用。     

昆虫糖脂代谢研究进展

肥胖症和糖尿病的日趋流行已经成为世界范围内的公共健康问题,其病因主要在于体内血糖/血脂含量升高引起的能量代谢紊乱。大量的证据表明,昆虫可以作为研究人类代谢疾病的理想模型,它不仅能合成与哺乳动物同源的糖脂代谢相关激素(如胰岛素样肽和脂动激素),而且还具有进化保守的代谢信号通路(如雷帕霉素靶蛋白信号通路)及相关器官与组织(如中肠和脂肪体)。本文主要介绍了昆虫糖脂代谢的过程与调控机制,重点涉及脂肪体和绛色细胞的生理功能、胰岛素样肽/脂动激素对血糖的拮抗调节、参与营养物质代谢的胰岛素-胰岛素样生长因子信号通路以及与类固醇激素合成相关的胆固醇代谢等内容,并结合最新研究成果对黑腹果蝇Drosophila melanogaster糖脂代谢相关基因及其功能进行了总结,以期为昆虫生理学和人类代谢疾病研究提供参考。     

Ezetimibe impairs transcellular lipid trafficking and induces large lipid droplet formation in intestinal absorptive epithelial cells

Ezetimibe inhibits Niemann-Pick C1-like 1 (NPC1L1) protein, which mediates intracellular cholesterol trafficking from the brush border membrane to the endoplasmic reticulum, where chylomicron assembly takes place in enterocytes or in the intestinal absorptive epithelial cells. Cholesterol is a minor lipid constituent of chylomicrons; however, whether or not a shortage of cholesterol attenuates chylomicron assembly is unknown. The aim of this study was to examine the effect of ezetimibe, a potent NPC1L1 inhibitor, on trans-epithelial lipid transport, and chylomicron assembly and secretion in enterocytes. Caco-2 cells, an absorptive epithelial model, grown onto culture inserts were given lipid micelles from the apical side, and chylomicron-like triacylglycerol-rich lipoprotein secreted basolaterally were analyzed after a 24-h incubation period in the presence of ezetimibe up to 50 μM. The secretion of lipoprotein and apolipoprotein B48 were reduced by adding ezetimibe (30% and 34%, respectively). Although ezetimibe allowed the cells to take up cholesterol normally, the esterification was abolished. Meanwhile, oleic acid esterification was unaffected. Moreover, ezetimibe activated sterol regulatory element-binding protein 2 by approximately 1.5-fold. These results suggest that ezetimibe limited cellular cholesterol mobilization required for lipoprotein assembly. In such conditions, large lipid droplet formation in Caco-2 cells and the enterocytes of mice were induced, implying that unprocessed triacylglycerol was sheltered in these compartments. Although ezetimibe did not reduce the post-prandial lipid surge appreciably in triolein-infused mice, the results of the present study indicated that pharmacological actions of ezetimibe may participate in a novel regulatory mechanism for the efficient chylomicron assembly and secretion.     

A new fluorescence-based method identifies protein phosphatases regulating lipid droplet metabolism

In virtually every cell, neutral lipids are stored in cytoplasmic structures called lipid droplets (LDs) and also referred to as lipid bodies or lipid particles. We developed a rapid high-throughput assay based on the recovery of quenched BODIPY-fluorescence that allows to quantify lipid droplets. The method was validated by monitoring lipid droplet turnover during growth of a yeast culture and by screening a group of strains deleted in genes known to be involved in lipid metabolism. In both tests, the fluorimetric assay showed high sensitivity and good agreement with previously reported data using microscopy. We used this method for high-throughput identification of protein phosphatases involved in lipid droplet metabolism. From 65 yeast knockout strains encoding protein phosphatases and its regulatory subunits, 13 strains revealed to have abnormal levels of lipid droplets, 10 of them having high lipid droplet content. Strains deleted for type I protein phosphatases and related regulators (ppz2, gac1, bni4), type 2A phosphatase and its related regulator (pph21 and sap185), type 2C protein phosphatases (ptc1, ptc4, ptc7) and dual phosphatases (pps1, msg5) were catalogued as high-lipid droplet content strains. Only reg1, a targeting subunit of the type 1 phosphatase Glc7p, and members of the nutrient-sensitive TOR pathway (sit4 and the regulatory subunit sap190) were catalogued as low-lipid droplet content strains, which were studied further. We show that Snf1, the homologue of the mammalian AMP-activated kinase, is constitutively phosphorylated (hyperactive) in sit4 and sap190 strains leading to a reduction of acetyl-CoA carboxylase activity. In conclusion, our fast and highly sensitive method permitted us to catalogue protein phosphatases involved in the regulation of LD metabolism and present evidence indicating that the TOR pathway and the SNF1/AMPK pathway are connected through the Sit4p-Sap190p pair in the control of lipid droplet biogenesis.     

Glucagon-like peptide-2 mobilization of intestinal lipid does not require canonical enterocyte chylomicron synthetic machinery

Background & aimsDietary triglycerides (TG) retained in the intestine after a meal can be mobilized many hours later by glucagon-like peptide-2 (GLP-2) in humans and animal models, despite the well-documented absence of expression of the GLP-2 receptor on enterocytes. In this study, we examined the site of GLP-2 action to mobilize intestinal lipids and enhance chylomicron production.MethodsIn mesenteric lymph duct-cannulated rats, we assessed GLP-2-stimulated lymph flow rate, TG concentration, TG output, and apoB48 abundance 5 h after an intraduodenal lipid bolus, in the presence of a validated GLP-2 antagonist or vehicle. Additionally, the same GLP-2-stimulated parameters were examined in the presence or absence of cis-Golgi disruption by Brefeldin A (BFA).ResultsCompared to placebo, GLP-2 administration increased lymph flow by 2.8-fold (P < 0.001), cumulative lymph volume by 2.69-fold (P < 0.001) and total TG output 2-fold (P = 0.015). GLP-2 receptor antagonism markedly diminished GLP-2's ability to stimulate lymph flow, cumulative lymph volume and total TG output, demonstrating the dependence of GLP-2 stimulation of lymph flow and TG output on its receptor activation. In contrast, disruption of the cis-Golgi apparatus with Brefeldin A did not diminish the GLP-2-response of lymph flow i.e., increased lymph flow by 2.7-fold (P = 0.001), lymph volume by 2.9-fold (P = 0.001), and total TG output i.e., increased by 2.5-fold (P = 0.003).ConclusionsGLP-2 mobilizes enteral lipid at a site distal to the Golgi, acting via its receptor. Since GLP-2 receptors are not expressed on enterocytes, GLP-2 likely mobilizes intestinal lipid residing extracellularly, either in the lamina propria or in the lymphatics.     

Recent advances in PTP1B signaling in metabolism and cancer

Protein tyrosine phosphorylation is one of the major post-translational modifications in eukaryotic cells and represents a critical regulatory mechanism of a wide variety of signaling pathways. Aberrant protein tyrosine phosphorylation has been linked to various diseases, including metabolic disorders and cancer. Few years ago, protein tyrosine phosphatases (PTPs) were considered as tumor suppressors, able to block the signals emanating from receptor tyrosine kinases. However, recent evidence demonstrates that misregulation of PTPs activity plays a critical role in cancer development and progression. Here, we will focus on PTP1B, an enzyme that has been linked to the development of type 2 diabetes and obesity through the regulation of insulin and leptin signaling, and with a promoting role in the development of different types of cancer through the activation of several pro-survival signaling pathways. In this review, we discuss the molecular aspects that support the crucial role of PTP1B in different cellular processes underlying diabetes, obesity and cancer progression, and its visualization as a promising therapeutic target.     

Regulation of lipid droplet homeostasis by hypoxia inducible lipid droplet associated HILPDA

Nearly all cell types have the ability to store excess energy as triglycerides in specialized organelles called lipid droplets. The formation and degradation of lipid droplets is governed by a diverse set of enzymes and lipid droplet-associated proteins. One of the lipid droplet-associated proteins is Hypoxia Inducible Lipid Droplet Associated (HILPDA). HILPDA was originally discovered in a screen to identify novel hypoxia-inducible proteins. Apart from hypoxia, levels of HILPDA are induced by fatty acids and adrenergic agonists. HILPDA is a small protein of 63 amino acids in humans and 64 amino acids in mice. Inside cells, HILPDA is located in the endoplasmic reticulum and around lipid droplets. Gain- and loss-of-function experiments have demonstrated that HILPDA promotes lipid storage in hepatocytes, macrophages and cancer cells. HILPDA increases lipid droplet accumulation at least partly by inhibiting triglyceride hydrolysis via ATGL and stimulating triglyceride synthesis via DGAT1. Overall, HILPDA is a novel regulatory signal that adjusts triglyceride storage and the intracellular availability of fatty acids to the external fatty acid supply and the capacity for oxidation.     

Regulation of metabolism and contraction by cytoplasmic calcium in the intestinal smooth muscle

Reduced pyridine nucleotides (PNred) and oxidized flavoproteins (FPox) were measured fluorometrically in the intestinal smooth muscle strip of guinea pig taenia caeci simultaneously with contractile tension. Cytoplasmic free Ca2+ levels ([Ca2+]cyt) were also measured by a fura-2-Ca2+ fluorescence technique. PNred, FPox, and [Ca2+]cyt increased during spontaneous contraction or upon the addition of high K+ or carbachol and decreased upon the removal of these stimulants. [Ca2+]cyt increased before the increase in muscle tension. PNred increased almost simultaneously with or immediately after the onset of contraction, while FPox increased before the initiation of contraction. Both PNred and FPox decreased a few seconds after the initiation of relaxation. In the K+-depolarized, Ca2+-depleted muscle, graded elevation of external Ca2+ increased PNred, FPox, and muscle tension. The sensitivity to Ca2+ was in the order of FPox greater than PNred greater than muscle tension. Changes in PNred were inhibited when glycolysis was inhibited by substitution of external glucose with oxaloacetate, pyruvate, or beta-hydroxybutylate, but not when oxidative phosphorylation was inhibited by N2 bubbling or by NaCN. In contrast to this, changes in the FPox were inhibited by N2 bubbling or NaCN, but not by the inhibition of glycolysis. These results suggest that an elevation of intracellular Ca2+ activates carbohydrate metabolism and contractile elements independently, resulting in the reduction of cytoplasmic pyridine nucleotides, oxidation of mitochondrial flavoproteins, and development of tension in the intestinal smooth muscle.