Summary
Apolipoprotein(a), the plasminogen-like component of lipoprotein(a), is transformed
into fragments by polymorphonuclear neutrophils (PMNs) elastase. Since stimulated
PMNs express urokinase-type plasminogen activator (uPA), we sought to investigate
the relevance of apo(a) fragmentation on plasminogen activation by neutrophils. Freshly
isolated human PMNs stimulated by a 10 kringle recombinant apo(a), r-apo(a), activate
plasminogen in a specific and saturable manner (Km = 476 ± 42 nM, Vmax = 896 ± 18
pmol min-1). This activation is prevented by amiloride, an inhibitor of u-PA, and
ɛ-aminocaproic acid, ɛ-ACA, a lysine analogue that blocks plasminogen binding to PMNs.
Stimulation of PMNs by apo(a) results in the formation of elastase-derived apo(a)
fragments. These fragments produce a concentration-dependent decrease in the formation
of plasmin. Addition of elastase inhibitors to PMNs prevented degradation of apo(a)
and partially restored the formation of plasmin. In a similar manner, isolated r-apo(a)
fragments were able to produce a 100% decrease in plasmin generation as compared to
intact r-apo(a). These data indicate that apo(a) fragments produce a more pronounced
inhibition in the generation of cellbound plasmin by uPA than the parent apo(a). These
effects of apo(a) and its fragments were neutralised by a monoclonal antibody directed
against the lysine-binding site of apo(a). This mechanism may be of biological relevance
to the effects of Lp(a) in conditions where PMNs accumulate and release elastase,
i.e. thrombus lysis and inflammatory lesions.
Keywords
Neutrophils - apolipoprotein(a) - elastase - plasminogen - proteolysis