Soluble urokinase receptor released from human carcinoma cells: a plasma parameter for xenograft tumour studies

The urokinase plasminogen activator receptor (uPAR) plays a critical role i n urokinase-mediated plasminogen activation and thereby in the process lead ing to invasion and metastasis. Soluble urokinase receptor (suPAR) is relea sed from tumours, and in cancer patients the blood level of soluble recepto r is increased. Using an enzyme-linked, immunosorbent assay (ELISA)-specifi c for the human urokinase receptor. release of soluble receptor was measure d in cultures of human breast carcinoma cells, in tumour extracts and in pl asma from mice with xenografted human tumours. Soluble human urokinase rece ptor (shuPAR) was released into culture supernatant during the growth of th e human breast cancer cell line MDA-MB-231 BAG, and the level of shuPAR in conditioned medium determined by ELISA was a linear function of both viable cell number and time of incubation. Western blotting showed that the form of shuPAR measured by ELISA in conditioned medium consisted virtually exclu sively of the three-domain full-length protein, while uPAR in cell lysates consisted of full-length uPAR as well as the domains (2+3) cleavage product . shuPAR was also released into the plasma of nude mice during growth of MD A-MB-231 BAG, MDA-MB-435 BAG and HCT 116 cells as subcutaneously xenografte d tumours. Western blotting demonstrated that the shuPAR released from the xenografted human tumours into plasma consisted of the three-domain full-le ngth protein, despite the finding of some cleaved uPAR in detergent extract s of tumour tissue. The levels of shuPAR determined by ELISA in the plasma of host mice during the growth of xenografted cell lines were highly correl ated with tumour volume. (C) 1999 Cancer Research Campaign.