Phorbol ester and mitogens stimulate human fibroblast secretions of plasmin-activatable plasminogen activator and protease nexin, an antiactivator/antiplasmin.

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Abstract
Tumor-promoting phorbol esters were reported to greatly increase plasminogen activator (PA) activity produced in numerous cell types. Many of these studies employed a widely used fibrinolysis assay for PA activity that involves large-scale dilution of cell lysates or conditioned medium (CM) into buffer containing plasminogen and the plasmin substrate 125I-fibrin. Apparently phorbol ester and the mitogens epidermal growth factor (EGF) and thrombin all stimulate secretion of PA activity in human foreskin fibroblast cultures. These effects are not observed in a modified fibrinolysis assay employing undiluted conditioned culture medium unless the medium is first treated at pH 3, which inactivates the secreted protease inhibitor, protease nexin (PN). A direct assay for plasminogen activator activity based on cleavage of 125I-plasminogen indicates that conditioned culture medium contains little if any active plasminogen activator either before or after treatment of the cultures with phorbol ester or EGF. Phorbol ester and mitogens do stimulate secretion of an inactive PA that can be activated by plasmin and PN, which inhibits both the activated form of the PA and plasmin. Secretions of the inactive PA and PN are further correlated in that release of both is stimulated most by phorbol ester, somewhat less by EGF, and least by thrombin. Significantly, these effects are not accompanied by increases in total protein secretion. Fibroblasts may secrete PA in an inactive form in the presence of PN to confine PA activity to an as yet undefined location or event.