Summary
When plasminogen binds to cell surfaces, its activation is markedly enhanced compared
to soluble plasminogen. Although several distinct molecules may contribute to plasminogen
binding to a given cell type, the subset of plasminogen receptors responsible for
enhancing plasminogen activation expose a carboxyl-terminal lysine on the cell surface
and are sensitive to proteolysis by carboxypeptidase B (CpB). To distinguish this
subset of plasminogen receptors from plasminogen-binding proteins that are not profibrinolytic,
we treated intact U937 monocytoid cells and peripheral blood monocytes with CpB to
remove exposed carboxyl-terminal lysines, and subjected the membrane proteins to two-dimensional
gel electrophoresis followed by ligand blotting with 125 I-plasminogen. Western blotting was performed with antibodies against previously characterized
candidate plasminogen receptors to identify plasminogen-binding proteins on the two-dimensional
ligand blots. Densitometry of autoradiograms of the 125 I-plasminogen ligand blots of U937 cell membranes revealed that membraneassociated
α-enolase, actin and annexin II showed minimal changes in 125 I-plasminogen binding following CpB treatment of intact cells, suggesting that these
proteins are not accessible to CpB on the U937 cell surface and most likely do not
serve as profibrinolytic plasminogen receptors on U937 cells. In contrast, densitometry
of autoradiograms of 125 I-plasminogen ligand blots of monocyte membranes revealed that 125 I-plasminogen binding to α-enolase was reduced 71% by treatment of intact cells with
CpB, while binding to annexin II was reduced 14%. Thus, a portion of membrane-associated
α-enolase and annexin II expose carboxyl terminal lysines that are accessible to CpB
on the peripheral blood monocyte surface, suggesting that these molecules may serve
as profibrinolytic plasminogen receptors on monocytes. Our data suggest that U937
cells and peripheral blood monocytes have distinct sets of molecules that constitute
the population of cell surface profibrinolytic plasminogen-binding proteins. Furthermore,
our data suggest that while several plasminogen-binding proteins with carboxyl terminal
lysines are associated with cell membranes, only a small subset of these proteins
expose a carboxyl terminal lysine that is accessible to CpB on the cell surface.
The abbreviations used are : 2D, two-dimensional; 2D-PAGE, two-dimensional polyacrylamide gel electrophoresis;
BSA, bovine serum albumin; CpB, carboxypeptidase B; EACA, є-aminocaproic acid; HBSS,
Hanks’ Balanced Salt Solution supplemented with 20 mM HEPES; HBSS-BSA, HBSS with 0.1%
bovine serum albumin; HRP, horseradish peroxidase; IEF, isoelectric focusing; PBS,
phosphate buffered saline; PMSF, phenylmethylsulfonyl fluoride; PVDF, polyvinylidene
difluoride; SDS, sodium dodecyl sulfate; SDSPAGE, sodium dodecyl sulfate polyacrylamide
gel electrophoresis; TBST, Tris buffered saline with 0.1% Tween 20; uPAR, urokinase-type
plasminogen activator receptor.
Keywords Plasminogen - receptor - fibrinolysis - U937 - monocyte