Urea Signaling in Cultured Murine Inner Medullary Collecting Duct (mIMCD3) Cells Involves Protein Kinase C, Inositol 1,4,5-Trisphosphate (IP

Urea, in concentrations unique to the renal medulla, increases transcription and protein expression of several immediate–early genes (IEGs) including the zinc finger-containing transcription factor, Egr-1 . In the present study, the proximal 1.2 kb of the murine Egr-1 5 9 -flanking sequence conferred urea-responsiveness to a heterologous luciferase reporter gene when transiently transfected into renal medullary mIMCD3 cells, and this effect was comparable with that of the extremely potent immediate-early gene inducer, O -tetradecanoylphorbol 13-acetate (TPA). Urea inducibility of Egr-1 expression was protein kinase C (PKC)-dependent because staurosporine and calphostin C abrogated the urea effect, and down-regulation of PKC through chronic TPA treatment inhibited both urea-inducible Egr-1 protein expression and gene transcription. In addition, hyperosmotic urea increased inositol 1,4,5-trisphosphate (IP 3 ) release from mIMCD3 cells and induced tyrosine phosphorylation of the receptor tyrosine kinase-specific phospholipase C (PLC) isoform, PLCg . Importantly, urea-inducible Egr-1 expression was strongly genistein-sensitive, to a much greater extent than the comparable TPA-inducible Egr-1 expression. These data suggest that urea-inducible Egr-1 expression is a consequence of sequential PLCg activation, IP 3 release, and PKC activation. Urea-inducible PLCg activation, in conjunction with the genistein-sensitivity of ureainducible Egr-1 expression suggest the possibility of a cell surface or cytoplasmic urea-sensing receptor tyrosine kinase. ( J. Clin. Invest. 1996. 97:1884–1889.)