Midkine enhances fibrinolytic activity of bovine endothelial cells

A hitherto unknown function of midkine (MK) was found in the regulation of fibrinolytic activity of vascular endothelial cells. Recombinant murine MK enhanced plasminogen activator (PA)/plasmin levels in bovine aortic endothelial cells (BAECs) in a dose- and time-dependent manner. After incubation with 10 ng/ml MK for 18 h, PA and plasmin levels increased 6- and 4-fold, respectively. This effect was attributed to a moderate upregulation of urokinase-type PA expression as well as to a significant down-regulation of PA inhibitor-1 (PAI-1) expression. BAECs constitutively synthesized and secreted MK and its production was enhanced 2-fold with 1 microM retinoic acid or 10 microM retinol. It was found that MK served as a substrate for tissue transglutaminase. In the culture medium, MK existed as a transglutaminase-mediated complex of 36 kDa. Addition of anti-MK antibody to BAEC cultures resulted in a decrease of basal PA activity and an increase of basal PAI-1 levels and attenuated the ability of retinol to enhance PA activity 50% and potentiated the ability to increase PAI-1 levels 4-fold. Furthermore, MK and basic fibroblast growth factor (bFGF) acted more than additively in enhancing PA levels. We conclude that in BAECs MK is a novel autocrine factor sustaining the fibrinolytic property. MK functions as a mediator of retinoid and cooperates with bFGF to enhance fibrinolytic activity of BAECs.