Novel Interconnections in Lipid Metabolism Revealed by Overexpression of Sphingomyelin Synthase-1.

Gergana M Deevska, Patrick P Dotson, Alexander A Karakashian, Giorgis Isaac, Mark Wrona, Samuel B Kelly, Alfred H Merrill, Mariana N Nikolova-Karakashian

The Journal of biological chemistry 2017 Mar 24

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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 activity in vitro and 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 increases de novo ceramide 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. © 2017 by The American Society for Biochemistry and Molecular Biology, Inc.

Citation

Gergana M Deevska, Patrick P Dotson, Alexander A Karakashian, Giorgis Isaac, Mark Wrona, Samuel B Kelly, Alfred H Merrill, Mariana N Nikolova-Karakashian. Novel Interconnections in Lipid Metabolism Revealed by Overexpression of Sphingomyelin Synthase-1. The Journal of biological chemistry. 2017 Mar 24;292(12):5110-5122