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.

Requirement for receptor‐bound urokinase in plasmin‐dependent cellular conversion of latent TGF‐β to TGF‐β

“…In this regard several groups have demonstrated that surface localization of plasminogen and prouPA promotes their activation and affords relative protection against plasma proteinase inhibitors (Stephens et al, 1989;Ellis et al, 1990). More recently Stephens et al (1991) observed that twochain uPA on human RCBA promyelomonocytic leukemia cells was protected from inactivation by alpha2macroglobulin (azM). As a2M is believed to be a scavenger for TGF-fl as well (O'Connor-McCourt et al, 1987;Huang et al, 19881, cell-surface localization of the activation complex for LTGF-p may have the additional advantage of prolonging the local half-life of TGF-p.…”

The urokinase plasminogen activator receptor in blood from healthy individuals and patients with cancer

“…uPA activity may also be lost from the cell-surface by plasmin cleavage at Lys135-Lys136 in the A-chain of uPA, leaving only the so-called amino terminal fragment (ATF) bound on the cell. The general plasma proteinase inhibitor, a2-macroglobulin, inactivates soluble uPA relatively slowly, but uPA bound to uPAR is apparently protected by a steric effect (46), thus allowing plasminogen activation to occur on the surface of cells even in the presence of serum (29).…”

Regulation and interactions in the activation of cell-associated plasminogen

“…a 2 -Macroglobulin binds plasminogen activators and their inhibitor complexes as well as plasmin. Cell-bound plasmin is protected from a 2 -macroglobulin [85]. The discovery that plasmin solubilizes the fibrin network led to the utilization of plasminogen activators as thrombolytic agents in cardiovascular diseases and ischemic stroke [19,86].…”