LIPOPROTEIN(A) ISOFORMS DISPLAY DIFFERENCES IN AFFINITY FOR PLASMINOGEN-LIKE BINDING TO HUMAN MONONUCLEAR-CELLS

Binding of lipoprotein(a) (Lp(a)) to membrane proteins of the monocyte -macrophage cell lineage may be an important event in atheroma formati on. Since Lp(a) with distinct apolipoprotein(a) (apo(a)) isoforms may show differences in their affinity with regard to fibrin binding, the existence of such a functional behavior in the interaction of apo(a) i n Lp(a) with these cells was explored using the monocytic cell line TH P-1. Lp(a) preparations containing small size apo(a) isoforms (M-r=450 000 to 550 000) and high molecular mass isoforms (M-r greater than or equal to 700 000) were purified from plasmas containing >0.35 g/L of Lp(a) obtained from subjects (n=14) with cardiovascular atheroscleroti c disease. Binding of plasminogen to THP-1 cells was performed using t he method of radioisotopic dilution. For binding of Lp(a) to cells, th e THP-1 monocytic cells were incubated with varying concentrations of the different Lp(a) preparations; cells were then washed and the amoun t of Lp(a) bound was detected with a radiolabeled polyclonal antibody directed against apo(a). Binding due to kringle interactions with lysi ne residues was calculated by subtracting from the total bound the amo unt of Lp(a) bound (approximate to 10%) in the presence of 6-aminohexa noic acid. Analysis of data with the Langmuir equation indicated ident ical and independent (non-interacting) sites and allowed evaluation of the K-d. Binding isotherms of small size isoforms showed saturation a nd a high affinity (K-d=25.8+/-19 nmol/L) relative to that of plasmino gen (K-d=1750+/-760 nmol/L). A similar difference (K-d=17.5+/-7.9 nmol /L versus K-d=600+/-220 nmol/L) was found when binding experiments wer e performed with a fibrin surface. In contrast, binding isotherms of t he high molecular mass isoforms did not show saturation at the highest Lp(a) concentrations used, thus indicating a lower affinity. In concl usion, these results show that apo(a) isoforms may display polymorphis m-linked functional heterogeneity with regard to cell binding, which m ay explain the higher association with cardiovascular risk of small si ze isoforms. These qualitative differences in the binding of apo(a) is oforms to fibrin or cells may modulate the cardiovascular risk associa ted with high levels of Lp(a).