Summary
Because of its structural homology with plasminogen, the apolipoprotein(a) [apo(a)]
component of the athero-thrombogenic lipoprotein(a) [Lp(a)] particle inhibits plasminogen
binding and activation onto fibrin as well as the subsequent fibrinolysis. In a similar
manner, apo(a) may also interfere with plasmin(ogen)-induced cell detachment and apoptosis
of adherent cells. To investigate this hypothesis, we studied the effect of a recombinant
apo(a) [r-apo(a)] on plasminogen activation-induced apoptosis of vascular smooth muscle
cells (VSMCs) and fibroblasts-like CHO-K1 cells. We demonstrate for the first time
that apo(a) displays a concentration-dependent biphasic, enhancing/preventing effect
on plasmin(ogen) induced cell detachment of VSMCs and CHO-K1 cells. Our results show
that r-apo(a) binds to these cells with higher affinity than plasminogen [Kd = 0.9 ± 0.2 µM for plasminogen, Kd = 1.77 ± 0.34 nM for r-apo(a)] in a lysine-dependent manner. At high r-apo(a)/plasminogen
ratios, their competitive interaction results in a partial inhibition of plasminogen
activation by cell-bound t-PA. As a consequence, r-apo(a) prevents plasmin(ogen)-induced
cell detachment and apoptosis. Surprisingly,at low r-apo(a)/plasminogen ratios,an
enhancement in plasmin(ogen)-induced cell detachment and apoptosis was observed. This
effect was shown to be “plasmin-selective” as r-apo(a) was unable to potentiate cell
detachment induced by human neutrophil elastase and trypsin. Altogether these data
are consistent with a new mechanism of apo(a)/plasmin(ogen) interactions that may
contribute to the atherothrombogenic potential of Lp(a).
Keywords
Apolipoprotein(a) - cell detachment-induced apoptosis - plasminogen - vascular smooth
muscle cells - CHO-K1 cells