The cytosolic aspartate aminotransferase (cAspAT) is a ubiquitous enzyme that displays liver-specific hormonal regulation. In the hepatoma cell line Fao, both the activity and the mRNA level of cAspAT are increased by glucocorticoids. This effect is potentiated by cAMP and inhibited by insulin. Using in vivo run-on experiments, we showed that these effectors act at the transcriptional level. A cAspAT gene fragment containing 2405 bp of the promoter was sequenced. Deletion fragments of this promoter were inserted upstream of the CAT gene, and the regulation of their activity was assayed following transfection in Fao cells. Stable transfection experiments established that the construct including the entire 2.405-kb fragment undergoes positive regulation by glucocorticoids and cAMP and negative regulation by insulin similar to the regulation of the endogenous gene. A physical separation of the positive and negative control elements is suggested by the fact that cAMP acted on the -682/-26-bp fragment (a 2-fold increase of the stimulation by dexamethasone), whereas the negative regulation by insulin (50% of the stimulation by dexamethasone) required the -1983/-1718-bp fragment. Both regions were required for maximal glucocorticoid activity (6-9-fold increase of CAT activity). We conclude that at least two regulatory regions, a proximal and a distal one, are required for full hormonal regulation of the cAspAT gene.

The basal expression and hormonal regulation of cytosolic aspartate aminotransferase (cAspAT) were investigated in the rat kidney. In adrenalectomized animals, the basal activity was highest in the renal cortex and in the inner stripe of the outer medulla (0.1-0.15 U/mg protein). The glucocorticoid analogue dexamethasone increased cAspAT activity about twofold in the cortex and in the inner stripe of the outer medulla but not in the papilla. A half-maximal increase in the activity was achieved at doses of approximately 5 micrograms/100 g body wt. The mineralocorticoid aldosterone did not modify the cAspAT activity. The cell specificity of the hormonal regulation was analyzed by in situ hybridization. In untreated adrenalectomized rats, a cAspAT cRNA probe labeled mainly the inner stripe of the outer medulla. After dexamethasone or hydrocortisone treatment, labeling was uniformly increased in this part of the medulla and was heterogeneously increased in the renal cortex. The specific increase in labeling within the cortex was shown to be confined to the distal convoluted tubule and the thick ascending limb. We conclude that, in addition to widespread basal expression, cAspAT is regulated by glucocorticoids in a highly cell-specific manner in the renal cortex. The enzyme may thus participate in the increased energy metabolism elicited by these hormones in these cells.

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