MECHANISM OF ACTION OF ANGIOSTATIC STEROIDS - SUPPRESSION OF PLASMINOGEN-ACTIVATOR ACTIVITY VIA STIMULATION OF PLASMINOGEN-ACTIVATOR INHIBITOR SYNTHESIS

Recently, a novel class of angiostatic steroids which block angiogenes is in several systems has been described. Since the elaboration of pro teases is believed to be an important component of angiogenesis, we te sted whether these steroids blocked the fibrinolytic response of endot helial cells to the angiogenic protein, basic fibroblast growth factor [bFGF]). Cultured bovine aortic endothelial (BAE) cells were incubate d with bFGF and/or medroxyprogesterone acetate (MPA), an angiostatic s teroid which has been shown to inhibit vascularization, collagenolysis , and tumor growth. When bFGF (3 ng/ml) was added to confluent monolay ers of BAE cells, plasminogen activator (PA) activity in the medium wa s increased threefold. In contrast, MPA at 10(-6) M, 10(-7) M, 10(-6) M, and 10(-9) M decreased PA levels in the medium by 83%, 83%, 75%, an d 39%, respectively. The stimulation of PA levels in BAE cells by bFGF (3 ng/ml) was abrogated by the presence of 10(-6) M MPA. This decreas e in PA activity was found to be mediated by a significant increase in plasminogen activator inhibitor type-1 (PAI-1) production. MPA, there fore, negated one of the important enzymatic activities associated wit h the angiogenic process. In contrast to the decreased levels of secre ted PA in cultures exposed simultaneously to MPA and bFGF, cell-associ ated PA levels remained high, consistent with earlier observations ind icating that PAI-1 does not inhibit cell-associated PA. Thus, angiosta tic steroids may exert their inhibitory effects on angiogenesis by inc reasing the synthesis of PAI-1. This, in turn, inhibits PA activity an d, therefore, plasmin generation, which is essential for the invasive aspect of angiogenesis.