Sites of urokinase-type plasminogen activator expression and distribution of its receptor in the normal human kidney

The urokinase-type plasminogen activator (uPA) is secreted into the urine at high concentrations and both the uPA protein and mRNA are present in human renal tissue. Normal kidney tissue also expresses the receptor for uPA. Neither the precise sites of uPA mRNA expression, nor the distribution of the uPA-receptor antigen, have been elucidated in the human kidney. In the present study, the sites of uPA mRNA expression were identified by in situ hybridization, and the cellular localization of both uPA and uPA-receptor was determined by immunohistochemical analysis. High-level uPA mRNA expression was restricted to epithelial cells of the convoluted proximal tubules and the thick ascending limb of Henle's loop (the straight part of the distal tubule). However, uPA immunoreactivity was not confined to sites of uPA mRNA expression, but was present in all segments of the tubular epithelium. Tubular epithelial cells also exhibited a consistent immunoreactivity with uPA-receptor antibody, indicative of a co-localization of the uPA antigen and its receptor in the uriniferous epithelium. We propose that the uPA antigen expression in nephron segments lacking demonstrable endogenous uPA synthesis may be the result of a uPA-receptor-mediated uptake of uPA.