Urokinase and urokinase receptor expression in somatic cell hybrids

To study the mechanisms that control cell-bound urokinase (u-PA) activity we characterised the expression of u-PA and its cellular receptor (u-PAR) in somatic cell hybrids between mouse L fibroblasts that do not produce u-PA and human HT1080 fibrosarcoma cells that express high levels of u-PA and u-PAR. Although the hybrids possessed a number of copies of the human u-PA and u-PAR genes comparable to that of HT1080 cells, they had levels of human u-PA mRNA and u-PAR mRNA significantly lower than HT1080 cells. Accordingly, the levels of u-PA and u-PAR were lower in the hybrids than in HT1080 cells. However, the hybrid cells surprisingly had a 2- to 6-fold higher u-PA-binding capacity than HT1080 cells. Some hybrid cells expressed glycosilation variants of u-PAR. In addition, in the hybrids, that express very low levels of u-PA, u-PAR was predominantly in the ligand-binding, three-domain form, whereas HT1080 cells, that have a high u-PA level, possessed higher levels of the cleaved, two-domain form of u-PAR devoid of ligand-binding activity. This indicates that the u-PA-binding capacity is controlled not only by u-PAR expression but also depends on receptor affinity and cleavage by u-PA or plasmin