Summary
There is now ample evidence that the proteolytic action of urokinase (UK) is potentiated
by a specific cell surface receptor. The present study was undertaken to assess the
role of UK as a modulator of its receptor. GEO colonic cells, which secrete relatively
low levels of UK (≃0.1 nM/72 h per 106 cells) and display approximately 104 receptors per cell, 10% of which are "tagged" with the endogenous plasminogen activator
(PA), was selected for the study. A 90% reduction in the specific binding of radioactive
DFP-UK was observed for cells cultivated in the presence of two-chain (TC) UK (M
r 55,000). This only partly reflected occupation of the receptors with UK supplied
in the culture medium, since the specific binding of the radioligand was still reduced
by 60% after an acid pretreatment, which dissociates receptor-bound UK. The reduction
in radioactive DFP-UK binding to cells treated with high molecular weight UK, either
in the single or two-chain form, was both concentration and time dependent. Maximum
reductions (70%) were achieved by treatment of the cells for 24 h with 1 nM of the
plasminogen activator. In contrast, low molecular weight UK, which lacks part of the
UK A chain, had no effect on ligand binding. Attenuation of radioactive DFP-UK binding
to UK treated GEO cells was a consequence of a 60% reduction in the number of binding
sites. Treatment of GEO cells with an antibody, which blocks the binding of endogenous
UK to its receptor, augmented radioactive DFP-UK binding by two-fold. These data indicate
that for one colonic cell line, at least, UK down-regulates its own binding site subsequent
to it being bound to the receptor.
