We show that cytosolic aspartate aminotransferase (cAspAT) is involved in adipocyte glyceroneogenesis, a regulated pathway that controls fatty acid homeostasis by promoting glycerol 3-phosphate formation for fatty acid re-esterification during fasting. cAspAT activity, as well as the incorporation of [14C]aspartate into the neutral lipid fraction of 3T3-F442A adipocytes was stimulated by the thiazolidinedione (TZD) rosiglitazone. Conversely, the ratio of fatty acid to glycerol released into the medium decreased. Regulation of cAspAT gene expression was specific to differentiated adipocytes and did not require any peroxisome proliferator-activated receptor γ (PPARγ)/retinoid X receptor-α direct binding. Nevertheless, PPARγ is indirectly necessary for both cAspAT basal expression and TZD responsiveness because they are, respectively, diminished and abolished by ectopic overexpression of a dominant negative PPARγ. The cAspAT TZD-responsive site was restricted to a single AGGACA hexanucleotide located at -381 to -376 bp whose mutation impaired the specific RORα binding. RORα ectopic expression activated the cAspAT gene transcription in absence of rosiglitazone, and its protein amount in nuclear extracts is 1.8-fold increased by rosiglitazone treatment of adipocytes. Finally, the amounts of RORα and cAspAT mRNAs were similarly increased by TZD treatment of human adipose tissue explants, confirming coordinated regulation. Our data identify cAspAT as a new member of glyceroneogenesis, transcriptionally regulated by TZD via the control of RORα expression by PPARγ in adipocytes.
J. Tordjman, S. Leroyer, G. Chauvet
Journal of Biological Chemistry