Macrophage foam cell lipoprotein(a)/apoprotein(a) receptor. Cell-surface localization, dependence of induction on new protein synthesis, and ligand specificity.

Abstract

Understanding the interaction of the atherogenic lipoprotein, lipoprotein(a) [Lp(a)], with macrophages may provide important insight into the physiology and pathophysiology of this lipoprotein. We have recently shown that cholesterol loading of macrophages, such as occurs in atheroma foam cells, leads to marked upregulation of a novel receptor activity for native Lp(a) and its plasminogen-like protein component, apoprotein(a) [apo(a)]. We show here that the Lp(a)/apo(a) receptor activity on cholesterol-loaded macrophages is trypsin sensitive, indicating that a cell-surface protein is involved and that the upregulation by cholesterol loading requires new protein synthesis. Ligand studies revealed that the foam cell receptor activity recognizes Lp(a) containing both small and large isoforms of apo(a) as well as rhesus monkey Lp(a), which contains an inactive kringle-4 37 (K4 37 ) lysine-binding domain. Elastase degradation products of plasminogen did not compete for 12 Elevated plasma levels of Lp(a) occur in «=20% of the adult Caucasian population, and there appears to be an association between elevated Lp(a) levels and the occurrence of coronary atherosclerosis.1 -2 Furthermore, cholesterolfed apo(a) transgenic mice develop significantly more atherosclerosis than nontransgenic controls.3 Despite several important hypotheses and in vitro observations regarding possible roles of Lp(a) in atherosclerosis, 47 neither the mechanism of Lp(a) atherogenicity nor the normal physiological roles of the lipoprotein are definitively known. Nonetheless, the interaction of Lp(a) with macrophages is thought to be important, since cholesteryl ester-filled macrophages (foam cells) are a prominent feature of atherosclerotic lesions. 810 In fact, apo(a) has been found to colocalize with foam cells in these lesions. © 1994 American Heart Association, Inc.interaction. Consistent with these data, the degradation of 125 I-r-apo(a) was completely blocked by an anti-Lp(a) polyclonal antibody that does not cross-react with plasminogen. Furthermore, the multiple sialic residues of apo(a) are also not involved in receptor interaction, since desialylated r-apo(a) interacted with foam cells as well as native r-apo(a Previous studies have shown that cultured macrophages internalize and degrade native Lp(a) and apo(a) poorly. 1215 Recent work from our laboratory, however, revealed an Lp(a) receptor activity, different from known lipoprotein receptors, that can be induced in macrophages by cholesterol loading. 16 The interaction of Lp(a) with cholesterol-loaded macrophages is solely dependent on the apo(a) moiety of Lp(a), since lipid-free recombinant apo(a) [r-apo(a)] but not apo(a)-free Lp(a-) is recognized by the foam cell receptor activity. 16In the present study, we investigated whether a cell-surface protein is involved in the foam cell Lp(a)/ apo(a) receptor activity and whether upregulation induced by cholesterol loading requires new protein synthesis. Furthermore, we explored several key properties of ligands that are need...

The Binding Activity of the Macrophage Lipoprotein(a)/Apolipoprotein(a) Receptor Is Induced by Cholesterol via a Post-translational Mechanism and Recognizes Distinct Kringle Domains on Apolipoprotein(a)

“…The r-apo(a) constructs were iodinated by the IODO-BEAD method (Pierce) as described (18). Briefly, 500 g of each construct was reacted with 200 Ci of carrier-free 125 I and one IODO-BEAD for 4 min.…”

“…Ref. (35) and below) or degradation (17,18). For the binding assay, the cells were cooled for 1 h at 4°C before the addition of ligand and then incubated with …”

“…Another possible clue to the potential atherogenicity of Lp(a) comes from the observation that Lp(a) and apo(a) are often physically in contact with cholesterol-loaded macrophages (foam cells) (13), which are prominent components of atherosclerotic lesions (14 -16). In previous work, we have demonstrated that mouse peritoneal and human monocyte-derived macrophages have a receptor activity that can bind, internalize, and lysosomally degrade Lp(a) (17,18). Importantly, the receptor activity is induced by cholesterol loading (17) and down-regulated by interferon-␥ (19), suggesting possible roles in atherosclerosis and inflammation.…”

“…Importantly, the receptor activity is induced by cholesterol loading (17) and down-regulated by interferon-␥ (19), suggesting possible roles in atherosclerosis and inflammation. The receptor recognizes the apo(a) moiety of Lp(a) and interacts similarly with Lp(a) and a 17-kringle recombinant apo(a) construct (17,18,20). We have shown that this foam cell receptor is distinct from the LDL receptor, the scavenger receptor, the LDL receptor-related protein, and plasminogen receptors (17,18); furthermore, antibodies against the macrophage MAC-1 receptor do not block interaction of foam cells with apo(a).…”

“…The receptor recognizes the apo(a) moiety of Lp(a) and interacts similarly with Lp(a) and a 17-kringle recombinant apo(a) construct (17,18,20). We have shown that this foam cell receptor is distinct from the LDL receptor, the scavenger receptor, the LDL receptor-related protein, and plasminogen receptors (17,18); furthermore, antibodies against the macrophage MAC-1 receptor do not block interaction of foam cells with apo(a). 2 To further elucidate how the foam cell Lp(a)/apo(a) receptor interacts with apo(a), the present study was designed to determine which sites on apo(a) bind to the receptor.…”