After incubation of confluent monolayer cultures of human HT-1080
fibrosarcoma cells with purified native human
plasminogen in
plasminogen-depleted serum-containing medium, bound
plasmin activity could be specifically eluted from the cells with
tranexamic acid, an analogue of
lysine.
Dexamethasone reduced the amount of recoverable bound
plasmin activity in a dose-dependent manner.
Dexamethasone was also found to induce a time- and dose-dependent decrease in the ability of the cells to bind added
plasmin. Untreated HT-1080 cells bound added
plasmin with a high capacity (600,000 molecules bound per cell), and this decreased to an undetectable level
after treatment with 100 nM
dexamethasone. The kinetics of the loss of
plasmin binding by the
dexamethasone-treated
sarcoma cells, a clear decrease after 4 h, correlated with those for the loss of cell-bound
urokinase (
u-PA) activity.
Plasmin was not, however, bound to the active site of
u-PA: an anti-catalytic
monoclonal antibody to
u-PA had no effect on
plasmin binding. Other
glucocorticoids, such as
hydrocortisone and
corticosterone, had a similar effect to
dexamethasone on
plasmin binding to HT-1080 cells. The effect of
glucocorticoids on the
plasmin receptor seemed to occur at least partly via a decrease in the affinity for
plasmin, since the Kd for
plasmin with untreated cells was 5.4 x 10(-9) M, and with cells treated with 5 nM
dexamethasone, the Kd value for
plasmin was 1.2 x 10(-7) M. These results show that
glucocorticoids induce down-regulation of
plasmin receptors on the surface of HT-1080 cells: a novel mechanism, in addition to the known effects of
glucocorticoids on
u-PA and PA inhibitors, by which human
tumor cells may be disarmed of their pericellular proteolytic activity.