Immunoassays (ELISA) of urokinase-type plasminogen activator (uPA): report of an EORTC/BIOMED-1 workshop.

The urokinase-type plasminogen activator (uPA) is considered to play a key role in the process of invasion and metastasis. In several independent studies, in a variety of cancer types (e.g. of the breast, colon, stomach, lung, ovary), high antigen levels of uPA in tumour extracts have been associated with rapid disease progression. In these studies, different sets of antibodies and standards (often as commercially available uPA ELISA kits) have been used. The standards provided with the different uPA ELISA kits are different from each other in both composition and source. In addition, the different uPA ELISA kits use antibodies which differ in specificity and affinity for the various forms of uPA including pro-uPA, HMW-uPA, LMW-uPA, the aminoterminal fragment (ATF) and complexes with inhibitors (PAI-1 and PAI-2) and the receptor (uPAR). Further, the composition of tumour tissue extraction buffers differ significantly among the published studies. Thus, it is not surprising that the ranges of cytosolic uPA levels reported differ considerably even when measured within the same tumour type. These discrepancies led the EORTC Receptor and Biomarker Study Group, in conjunction with the BIOMED-1 consortium on 'Clinical Relevance of Proteases in Tumour Invasion and Metastasis', to organise a workshop to study the characteristics associated with six different uPA immunoassays (ELISA) used in clinical studies reported in the literature. Although the absolute uPA antigen values measured with the respective uPA ELISA kits differed, high correlations were obtained for any two of the four uPA ELISA kits finally applied to sets of breast cancer cytosol preparations. The preparations used at present as standards in the various uPA ELISA kits are not representative of actual human breast cancer cytosols. Thus absolute standardisation is only possible by using a common reference sample (breast cancer cytosol) and similarly composed ELISA uPA kits. Then it will be possible to generate comparable data on clinical tissue as well as to check for batch-to-batch variations within particular ELISA kits.