Transformation-dependent activation of urokinase-type plasminogen activator by a plasmin-independent mechanism: involvement of cell surface membranes.
Transformed cells produce elevated levels of the urokinase-type plasminogen activator (u-PA), which has been linked with the invasive or migratory phenotype of these cells. The u-PA is secreted and normally maintained in the inactive, single-chain form (scu-PA) and it has been assumed that natural activation occurs via a plasmin-mediated cleavage converting scu-PA to the active, two-chain form (tcu-PA). We now demonstrate that secreted scu-PA in Rous sarcoma virus-transformed chicken embryo fibroblast (RSVCEF) cultures is activated by an endogenous, plasmin-independent mechanism. Normal CEFs and CEFs infected with a temperature-sensitive RSV mutant and incubated at the nonpermissive temperature do not activate scu-PA. Conditioned medium harvested from plasmin-free cultures of RSVCEFs contains active tcu-PA as determined by two independent methods. The scu-PA is progressively converted with time in culture and requires the presence of intact cells or a plasma membrane-enriched fraction. When added to RSVCEF cultures, a synthetic peptide corresponding to residues 20-41 of the growth factor domain of chicken u-PA blocks the conversion to tcu-PA, and scu-PA accumulates in the cultures. These results suggest that scu-PA is secreted by cells, becomes bound to a u-PA receptor, and is proteolytically converted to active tcu-PA by a catalytic mechanism on the surface of RSV-transformed fibroblasts.
Berkenpas
M B
MB
Department of Pathology, State University of New York, Stony Brook 11794.
Quigley
J P
JP
D000595:Amino Acid Sequence; D000818:Animals; D001358:Avian Sarcoma Viruses / Q000235:genetics*; D002462:Cell Membrane / Q000378:metabolism; D002471:Cell Transformation, Neoplastic; D002478:Cells, Cultured; D002642:Chick Embryo; D004789:Enzyme Activation; D004792:Enzyme Precursors / Q000378:metabolism*; D005341:Fibrinolysin / Q000502:physiology*; D007106:Immune Sera; D007531:Isoflurophate / Q000494:pharmacology; D007700:Kinetics; D008969:Molecular Sequence Data; D010455:Peptides / Q000138:chemical synthesis / Q000302:isolation & purification; D010960:Plasminogen Activators / Q000378:metabolism*; D014568:Urokinase-Type Plasminogen Activator / Q000378:metabolism*
D004792:Enzyme Precursors; D007106:Immune Sera; D010455:Peptides; D007531:Isoflurophate; D010960:Plasminogen Activators; D005341:Fibrinolysin; D014568:Urokinase-Type Plasminogen Activator