Urea signaling in cultured murine inner medullary collecting duct (mIMCD3) cells involves protein kinase C, inositol 1,4,5-trisphosphate (IP₃), and a putative receptor tyrosine kinase

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'-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 (IP₃) release from mIMCD3 cells and induced tyrosine phosphorylation of the receptor tyrosine kinase-specific phospholipase C (PLC) isoform, PLC-γ. 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 PLC-γ activation, IP₃ release, and PKC activation. Urea-inducible PLC-γ activation, in conjunction with the genistein-sensitivity of urea-inducible Egr-1 expression suggest the possibility of a cell surface or cytoplasmic urea-sensing receptor tyrosine kinase.