Expression of the human urokinase‐type plasminogen activator receptor in E. coli and Chinese hamster ovary cells: Purification of the recombinant proteins and generation of polyclonal antibodies in chicken

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Abstract
The receptor for urokinase‐type plasminogen activator (uPAR) may contribute to the invasive and metastatic capacity of tumor cells by focusing the serine protease urokinase‐type plasminogen activator (uPA) to the cell surface. uPA activates plasminogen to plasmin which in turn degrades extracellular matrix proteins or activates other proteases. Mature uPAR is a heavily glycosylated protein of about 284 amino acids attached to the plasma membrane via a glycosyl‐phosphatidylinositol (GPI) anchor. A set of different polyclonal uPAR antibodies has been generated in order to investigate the role of uPAR in tumor spreading in more detail. For this purpose, uPAR (lacking the GPI anchor) was expressed in E. coli and Chinese hamster ovary (CHO) cells. Recombinant uPAR from E. coli (corresponding to amino acids 1–284 of human uPAR) was expressed with an N‐terminal histidine‐tag insertion and purified by nickel chelate affinity chromatography. Soluble uPAR, synthesized by CHO cells (corresponding to amino acids 1–277 of human uPAR), was isolated by ligand (uPA) affinity chromatography. Expression in E. coli led to a nonglycosylated form of uPAR, whereas uPAR produced by CHO cells seemed to be glycosylated to a similar extent as the naturally occurring human form of uPAR (as analyzed by sodium dodecyl sulfate‐polyacrylamide gel electrophoresis). Prior to immunization the N‐termini of the recombinant uPAR variants were determined by amino acid sequence analysis. Polyclonal antibodies were generated in chickens and purified from egg yolk. The reaction patterns of these antibodies were analyzed by Western blot analyses and flow cytofluorometry.