Effect of apolipoprotein(a) on plasminogen activation-induced apoptosis of vascular smooth muscle cells

Abstract number: P1353

Ho-Tin-Noé B., Meilhac O., Loyau S., Anglés-Cano E.

INSERM U460, France

Plasminogen activation may be inhibited by the athero-thrombogenic Lp(a) particle via a competitive interaction based on the homology between its apolipoprotein(a) (apo(a)) component and plasminogen. Pericellular proteolysis induced by plasminogen activation on vascular smooth muscle cells leads to cell detachment-induced apoptosis (Meilhac and al. this meeting). We have explored the possibility that apo(a) may interfere with this phenomenon. To investigate this hypothesis, we first determined the parameters of plasminogen binding and activation on rat VSMCs in culture with or without apo(a). Plasminogen and apo(a) binding were measured with specific antibodies and kinetics of plasminogen activation with a chromogenic substrate selective for plasmin. Our results show that plasminogen binds to membrane glycoproteins with an affinity similar to that found for other cell types (Kd = 1 µM), and is efficiently activated by membrane-bound t-PA expressed by VSMCs (Km = 90 nM). In contrast to plasminogen, recombinant apo(a) binds to VSMCs with a 270-fold higher affinity (Kd = 3.7 nM), and competes efficiently with plasminogen for cell binding inhibiting thereby the formation of plasmin (IC50 = 94 nM). Despite this inhibition, apo(a) does not protect VSMCs from cell detachment induced apoptosis. Surprisingly, at an intermediate concentration (100 nM), apo(a) potentates this phenomenon. This effect of apo(a) may involve apo(a) fragmentation by plasmin. We demonstrate such a fragmentation in in vitro experiments. Our results add further information to explain the athero-thrombogenic effect of apo(a).