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Reports on the enzyme cytosolic aspartate aminotransferase which displays liver-specific hormonal regulation. Increase of the activity and mRNA level of cAspAT by glucocorticoids; Insertion of deletion fragments; Stable transfection experiments; Physical separation of the positive and negative control elements.

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. [ABSTRACT FROM AUTHOR]

Fenofibrate, a peroxisome proliferator-activated receptor α (PPARα) activator, increases the expression of the cytosolic aspartate aminotransferase (cAspAT) gene in human liver cells, which may partially explain the increase of this enzyme in the serum of individuals undergoing fenofibrate treatment. Conversely, in rodents, fenofibrate represses the expression of the cAspAT gene. We compared the mechanisms of fenofibrate action in human and rat hepatoma cells. Transfection assays of the wild-type and mutated rat promoters in Fao and H4IIEC3 cells established a critical role for sequences similar to nuclear receptor responsive elements in the −404/−366 bp region. Nuclear proteins bound to these sequences and the amounts of protein bound were decreased by fenofibrate treatment, probably accounting for the decreased gene expression. Pharmacological studies confirmed the involvement of PPARα. However, this receptor did not bind directly to these sequences. The human promoter was cloned and the regulatory region localized between −2663/−706 bp. Co-transfection assays suggested that, in humans, the PPARα was also involved in the increase in expression of the cAspAT gene due to fibrates, without the presence of a canonical PPAR responsive element. [Copyright &y& Elsevier]