Urokinase (u-PA) is produced by collecting duct principal cells and is post-transcriptionally regulated by SV40 large-T, arginine vasopressin, and epidermal growth factor.

We have analyzed the expression and regulation of plasminogen activators (PA) in principal cells of the renal collecting duct. We used a rabbit principal cell line (RC.SVtsA58) infected with the temperature-sensitive SV40 strain tsA58. Transformed cells cultured at permissive temperature (33 degrees C) produced only tissue-type plasminogen activator (t-PA). Shifting the cells to nonpermissive temperature (39.5 degrees C) induced their differentiation and a marked increase in total fibrinolytic activity due to the induction of urokinase-type plasminogen activator (u-PA) synthesis and secretion. The effect on u-PA was post-transcriptional and it could be attributed to large-T inactivation at 39.5 degrees C since it was abolished by re-infecting the cells with wild-type SV40. Run-on assay and real-time RT-PCR of u-PA transcripts indicated that large-T altered post-transcriptional regulation. u-PA was also produced by primary cultures of collecting duct cells and was present in the rabbit urine. In the kidney, u-PA and its receptor (u-PAR) were almost exclusively expressed at the apex of collecting duct cells. We then analyzed the regulation of u-PA by arginine vasopressin (AVP) and epidermal growth factor (EGF), two key regulators of principal cell functions. We found that AVP and EGF, which have opposite hydro-osmotic effects in the collecting duct, also exhibited contrasted effects on u-PA synthesis in differentiated RC.SVtsA58 cells. EGF increased but AVP suppressed u-PA activity and protein, and these regulations occurred at post-transcriptional level. These results point to a physiological role of u-PA in principal cells of the renal collecting duct.