Summary
The urinary type plasminogen activator, urokinase (uPA) is localized on the cell surface
through the binding of a specific receptor, the uPA receptor (uPAR). The uPA localization
enhances plasmin formation on the cell surface and facilitates cell migration. The
cellular and tissue distribution of uPAR is not fully established. We have analyzed
uPAR expression in nine leukemic cell lines of distinct lineages and maturational
states and correlated this with expression of plasminogen receptors, tissue-type plasminogen
activator (tPA) receptors and LDL receptor-related protein (LRP). The most immature
and least differentiated cell line (an erythro-myeloid cell line) and cells of lymphoid
lineage, did not express uPAR, whereas cells differentiated along the myelo-monocytic
pathway displayed this receptor. Plasminogen and tPA receptors were expressed by all
leukemic cell lines and by all nucleated peripheral blood cells but B and T lymphocytes
were negative for cell surface expression of both uPAR and LRP while monocytes and
neutrophils were positive for expression of both uPAR and LRP. PMA stimulation induced
surface expression of uPAR in lymphocytes but did not induce expression of LRP by
these cells. In contrast, lymphoid cell lines were negative for uPAR expression even
after PMA stimulation, indicating differences in regulation of uPAR expression between
lymphocytes and lymphoid cell lines. The pattern of uPAR expression on leukemic cell
lines was also studied on bone marrow blast cells from leukemic patients. Only the
most mature myeloid cells expressed uPAR on their surfaces. In contrast, M3 leukemic
cells and other blast cells displaying lymphoid markers such as TdT (+) and/or CD2
(+) did not express intracellular or cell-surface associated uPAR, indicating an heterogeneity
among these promyelo-cytic cells and suggesting that uPAR may be a useful marker for
leukemia typing. Myeloid blast cells from some patients contained intracellular pools
of uPAR but displayed no receptor on the cell surface, suggesting that translocation
may be a mechanism regulating uPAR expression in these cells. The comparison of uPAR
expression between these cell lines and peripheral blood cells and it correlation
with plasminogen receptors, tPA receptors and LRP expression offers new insights regarding
potential mechanisms for regulation of uPA-uP-AR-mediated pericellular proteolysis.