title: Novel Interconnections in Lipid Metabolism Revealed by Overexpression of Sphingomyelin Synthase-1.
abstract: This study investigates the consequences of elevating sphingomyelin synthase 1 (SMS1) activity, which generates the main mammalian sphingolipid, sphingomyelin. HepG2 cells stably transfected with SMS1 (HepG2-SMS1) exhibit elevated enzyme activityin vitroand increased sphingomyelin content (mainly C22:0- and C24:0-sphingomyelin) but lower hexosylceramide (Hex-Cer) levels. HepG2-SMS1 cells have fewer triacylglycerols than controls but similar diacylglycerol acyltransferase activity, triacylglycerol secretion, and mitochondrial function. Treatment with 1 mm palmitate increasesde novoceramide synthesis in both cell lines to a similar degree, causing accumulation of C16:0-ceramide (and some C18:0-, C20:0-, and C22:0-ceramides) as well as C16:0- and C18:0-Hex-Cers. In these experiments, the palmitic acid is delivered as a complex with delipidated BSA (2:1, mol/mol) and does not induce significant lipotoxicity. Based on precursor labeling, the flux through SM synthase also increases, which is exacerbated in HepG2-SMS1 cells. In contrast, palmitate-induced lipid droplet formation is significantly reduced in HepG2-SMS1 cells. [14C]Choline and [3H]palmitate tracking shows that SMS1 overexpression apparently affects the partitioning of palmitate-enriched diacylglycerol between the phosphatidylcholine and triacylglycerol pathways, to the benefit of the former. Furthermore, triacylglycerols from HepG2-SMS1 cells are enriched in polyunsaturated fatty acids, which is indicative of active remodeling. Together, these results delineate novel metabolic interactions between glycerolipids and sphingolipids.
authors.affiliation: From the Department of Physiology, University of Kentucky, Lexington, Kentucky 40536.
authors.forename: Gergana M
authors.initials: GM
authors.lastname: Deevska
authors.affiliation: From the Department of Physiology, University of Kentucky, Lexington, Kentucky 40536.
authors.forename: Patrick P
authors.initials: PP
authors.lastname: Dotson
authors.affiliation: From the Department of Physiology, University of Kentucky, Lexington, Kentucky 40536.
authors.forename: Alexander A
authors.initials: AA
authors.lastname: Karakashian
authors.affiliation: Pharmaceutical Discovery and Life Sciences, Waters Corporation, Milford, Massachusetts 01757, and.
authors.forename: Giorgis
authors.initials: G
authors.lastname: Isaac
authors.affiliation: Pharmaceutical Discovery and Life Sciences, Waters Corporation, Milford, Massachusetts 01757, and.
authors.forename: Mark
authors.initials: M
authors.lastname: Wrona
authors.affiliation: the School of Biology and Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332.
authors.forename: Samuel B
authors.initials: SB
authors.lastname: Kelly
authors.affiliation: the School of Biology and Petit Institute for Bioengineering and Bioscience, Georgia Institute of Technology, Atlanta, Georgia 30332.
authors.forename: Alfred H
authors.initials: AH
authors.lastname: Merrill
authors.affiliation: From the Department of Physiology, University of Kentucky, Lexington, Kentucky 40536, mariana.karakashian@uky.edu.
authors.forename: Mariana N
authors.initials: MN
authors.lastname: Nikolova-Karakashian
mesh_terms: D002518:Ceramides / Q000378:metabolism; D004075:Diglycerides / Q000378:metabolism; D005227:Fatty Acids / Q000378:metabolism; D056945:Hep G2 Cells; D022781:Hepatocytes / Q000378:metabolism; D006801:Humans; D050356:Lipid Metabolism; D008565:Membrane Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D009419:Nerve Tissue Proteins / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D017855:Transferases (Other Substituted Phosphate Groups) / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D014280:Triglycerides / Q000378:metabolism; D015854:Up-Regulation
chemical_list: C011439:1,2-diacylglycerol; D002518:Ceramides; D004075:Diglycerides; D005227:Fatty Acids; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D014280:Triglycerides; C455440:SGMS1 protein, human; D017855:Transferases (Other Substituted Phosphate Groups)
keywords: ceramide; diacylglycerol; hepatocytes; lipid droplets; lipid metabolism; palmitic acid; phosphatidylcholine; phospholipid; sphingomyelin synthase; triacylglycerol