Mammalian cells manufacture fatty acids de novo using a distinct pathway to synthesize fatty acids from acetyl and malonyl esters of CoA catalyzed by the dimer of FASN (EC

This article is available online at http://www.jlr.org Mammalian cells manufacture fatty acids de novo using a distinct pathway to synthesize fatty acids from acetyl and malonyl esters of CoA catalyzed by the dimer of FASN (EC 2.3.1.85) ( 1 ). FASN is absolutely essential for production of de novo fatty acids in mammals ( 2 ). The biological requirement for FASN is highlighted by the fact that FASN is detected in most normal human tissues and that homozygous deletion of FASN in the mouse results in embryonic lethality ( 3, 4 ). The importance of the de novo pathway is underscored by the complex biological functions of its fatty acid products, including biosynthesis of phospholipids, energy storage via esterifi cation into triglycerides (TAGs), use as energy substrates for -oxidation, providing both endocrine and nuclear hormone signaling ligands, and for critical posttranslational modifi cations of proteins ( 5–12 ). Perhaps the most demanding setting for FASN catalysis is during lactation, when extremely high quantities of de novo fatty acids are transmitted to the offspring as substrate for the growth of the young ( 13–15 ). Regulation of the principle enzymes of de novo fatty acid synthesis, ATP citrate lyase (ACLY) (EC 2.3.3.8), acetyl-CoA carboxylase (ACC) (EC 6.4.1.2), and FASN, has been shown to occur at the level of enzyme gene expression ( 16 ), translation/degradation ( 17–19 ), phosphorylation ( 20–22 ), assembly into multimeric complexes ( 23–25 ), and allosteric activation/inhibition ( 18, 23, 26, 27 ). Recently, evidence for the control of de novo fatty acid synthesis by small effector proteins has emerged. Thyroid hormone responsive protein “Spot 14” (THRSP) (Spot14, S14) and only family Abstract Thyroid hormone responsive protein Spot 14 has been consistently associated with de novo fatty acid synthesis activity in multiple tissues, including the lactating mammary gland, which synthesizes large quantities of medium chain fatty acids (MCFAs) exclusively via FASN. However, the molecular function of Spot14 remains undefi ned during lactation. Spot14-null mice produce milk defi cient in total triglyceride and de novo MCFA that does not sustain optimal neonatal growth. The lactation defect was rescued by provision of a high fat diet to the lactating dam. Transgenic mice overexpressing Spot14 in mammary epithelium produced total milk fat equivalent to controls, but with signifi cantly greater MCFA. Spot14-null dams have no diminution of metabolic gene expression, enzyme protein levels, or intermediate metabolites that accounts for impaired de novo MCFA. When [ 13 C] fatty acid products were quantifi ed in vitro using crude cytosolic lysates, native FASN activity was 1.6-fold greater in control relative to Spot14-null lysates, and add back of Spot14 partially restored activity. Recombinant FASN catalysis increased 1.4-fold and C = 14:0 yield was enhanced 4-fold in vitro following addition of Spot14. These fi ndings implicate Spot14 as a direct protein enhancer of FASN catalysis in the mammary gland during lactation when maximal MCFA production is needed. —Rudolph, M. C., E. A. Wellberg, A. S. Lewis, K. L. Terrell, A. L. Merz, N. K. Maluf, N. J. Serkova, and S. M. Anderson. Thyroid hormone responsive protein Spot14 enhances catalysis of fatty acid synthase in lactating mammary epithelium. J. Lipid Res. 2014. 55: 1052–1065.