MIDKINE ENHANCES FIBRINOLYTIC-ACTIVITY OF BOVINE ENDOTHELIAL-CELLS

A hitherto unknown function of midkine (MK) was found in the regulatio n of fibrinolytic activity of vascular endothelial cells. Recombinant murine MR 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 in creased 6- and 4-fold, respectively. This effect was attributed to a m oderate upregulation of urokinase-type PA expression as well as to a s ignificant down-regulation of PA inhibitor-1 (PAI-1) expression. BAECs constitutively synthesized and secreted MK and its production was enh anced 2-fold with 1 mu M retinoic acid or 10 mu M retinol. It was foun d that MK served as a substrate for tissue transglutaminase. In the cu lture medium, MK existed as a transglutaminase-mediated complex of 36 kDa. Addition of anti-MR antibody to BAEC cultures resulted in a decre ase of basal PA activity and an increase of basal PAI-1 levels and att enuated the ability of retinol to enhance PA activity 50% and potentia ted the ability to increase PAI-1 levels 4-fold. Furthermore, MK and b asic fibroblast growth factor (bFGF) acted more than additively in enh ancing PA levels. We conclude that in BAECs MK is a novel autocrine fa ctor sustaining the fibrinolytic property. MK functions as a mediator of retinoid and cooperates with bFGF to enhance fibrinolytic activity of BAECs.