Hyperosmotic urea increases transcription and synthesis of Egr-1 in murine inner medullary collecting duct (mIMCD3) cells

It was previously shown that when cells of renal epithelial origin are exposed to hyperosmotic urea in concentrations unique to the renal medulla, abundance of mRNA encoding the immediate-early gene product Egr-1 is up- regulated. This phenomenon appears restricted to cells of renal epithelial origin. In the present study, the newly isolated murine renal inner medullary cell line mIMCD3 was used to determine whether the urea-induced increase in Egr-1 mRNA abundance is associated with increased expression of functional protein product, and whether this increase is transcriptionally mediated. In Western analysis, urea (200 mM) increased Egr-1 immunoreactivity 3-fold relative to sham-treated cells. [³⁵S]Methionine pulse-labeling followed by immunoprecipitation confirmed that this increased immunoreactivity was associated with increased de novo Egr-1 protein synthesis. Electrophoretic mobility shift assay demonstrated that urea treatment induced a commensurate increase in specific DNA binding activity for the Egr-1 consensus sequence. In addition, the increase in Egr-1 mRNA expression accompanying urea treatment was a consequence of enhanced transcription, as determined by nuclear run-off assay. Taken together, these data indicate that hyperosmotic urea increases both Egr-1 transcription and new protein synthesis in renal epithelial cells in culture, and that this newly synthesized Egr-1 is a functional DNA-binding protein. To our knowledge, this is the first example of urea-inducible gene transcription. In addition, Egr-1 represents the first eukaryotic transcription factor transcriptionally activated by a hyperosmotic stressor.