Glucocorticoids inhibit transcription and expression of the UT-A urea transporter gene

Dexamethasone treatment increases urea excretion and decreases urea permeability and urea transporter UT-A1 protein abundance in the inner medullary collecting duct (IMCD) of adrenalectomized rats. We examined the effect of dexamethasone treatment for 3 days on the abundance of several UT-A mRNA transcripts in rat renal medulla. By Northern blot analysis, a significant decrease in mRNA expression was observed in the inner medulla of dexamethasone-treated rats compared with controls for UT-A1 (71%), UT-A3 (75%), and UT-A3b (75%), but not for UT-A2. We then tested the effect of 100 nM dexamethasone on the activity of promoter I in the UT-A gene, using LLC-PK₁-GR101 cells that express the glucocorticoid receptor. Dexamethasone significantly decreased the activity of rat UT-A promoter I (72%) but did not affect UT-A promoter II. Deletion analysis and site-directed mutagenesis demonstrated that sequences between -423 and -244 are important for this inhibition and that a 10-bp sequence at -363, which binds a nuclear protein in a gel shift assay, is necessary for basal promoter activity. The specific factors involved in repression of UT-A promoter I activity by glucocorticoids remain to be determined.