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.
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This work was supported by National Institutes of Health (NIH) Grant R01AG019223 (to M. N. K.), American Heart Association (AHA) Postdoctoral Fellowship 11POST7650060 (to G. D.), AHA Postdoctoral Fellowship 10POST4300013 (to P. D.), and NIH Grant R01GM076217 (to A. H. M.).
We thank Dr. J. Holthuis (Utrecht University) for providing the SMS1 construct that was used as a reference and Dr. M. Mitov (University of Kentucky) for measuring the OCR in the Redox Metabolism Shared Resource Facility of the University of Kentucky Markey Cancer Center (supported by NCI, National Institutes of Health, Grant P30CA177558).
Deevska, Gergana M.; Dotson, Patrick P. II; Karakashian, Alexander A.; Isaac, Giorgis; Wrona, Mark; Kelly, Samuel B.; Merrill, Alfred H. Jr.; and Nikolova‑Karakashian, Mariana N., "Novel Interconnections in Lipid Metabolism Revealed by Overexpression of Sphingomyelin Synthase-1" (2017). Physiology Faculty Publications. 117.