title: Alpha 2-macroglobulin restricts plasminogen activation to the surface of RC2A leukemia cells.
abstract: Human RC2A myelomonocytic leukemia cells are able to activate the prourokinase (pro-u-PA) they secrete so that active u-PA is present both in serum-free conditioned medium from these cells, as well as on the cell surface. When the cells are grown in serum-containing medium, no u-PA activity can be found in the medium but active u-PA is found bound to the cell surface where it can generate bound plasmin. This distribution of u-PA activity was shown to be, first, the net result of slow inactivation of free active u-PA by serum inhibitor(s) and simultaneous rapid uptake of u-PA onto the cell surface. Binding to cells was at least six times faster than inactivation by 10% serum. The principal serum inhibitor of u-PA was identified as alpha 2-macroglobulin (alpha 2M), and prior inactivation of u-PA by purified human alpha 2M was also shown to prevent uptake of u-PA activity onto cells. Second, although endogenous u-PA could form covalent complexes with purified alpha 2M in the culture medium of RC2A cells, covalent alpha 2M complexes were not formed by u-PA on the cell surface; the u-PA taken up in this compartment was protected against alpha 2M inhibition. u-PA anchored to plastic surfaces via monoclonal antibodies to the amino-terminal region of u-PA was also protected against alpha 2M, suggesting that the protection of cell surface u-PA results from a steric effect. These results provide evidence as to how the active u-PA produced by leukemia cells can contribute to proteolytic activity on their cell surface in the presence of serum inhibitors.
authors.affiliation: Department of Virology, University of Helsinki, Finland.
authors.forename: R W
authors.initials: RW
authors.lastname: Stephens
authors.forename: H
authors.initials: H
authors.lastname: Tapiovaara
authors.forename: T
authors.initials: T
authors.lastname: Reisberg
authors.forename: J
authors.initials: J
authors.lastname: Bizik
authors.forename: A
authors.initials: A
authors.lastname: Vaheri
mesh_terms: D000911:Antibodies, Monoclonal; D001693:Biological Transport, Active; D004789:Enzyme Activation; D006801:Humans; D007700:Kinetics; D007951:Leukemia, Myeloid / Q000378:metabolism*; D010960:Plasminogen Activators / Q000378:metabolism; D011994:Recombinant Proteins; D014407:Tumor Cells, Cultured / Q000378:metabolism; D014568:Urokinase-Type Plasminogen Activator / Q000037:antagonists & inhibitors / Q000276:immunology / Q000378:metabolism*; D000511:alpha-Macroglobulins / Q000378:metabolism*
chemical_list: D000911:Antibodies, Monoclonal; D011994:Recombinant Proteins; D000511:alpha-Macroglobulins; D010960:Plasminogen Activators; D014568:Urokinase-Type Plasminogen Activator; C067551:saruplase