Molecular Basis for the Interaction of Low Density Lipoprotein Receptor-related Protein 1 (LRP1) with Integrin αMβ2

Abstract

The LDL receptor-related protein 1 (LRP1) is a large endocytic receptor that controls macrophage migration in part by interacting with ␤ 2 integrin receptors. However, the molecular mechanism underlying LRP1 integrin recognition is poorly understood. Here, we report that LRP1 specifically recognizes ␣ M ␤ 2 but not its homologous receptor ␣ L ␤ 2 . The interaction between these two cellular receptors in macrophages is significantly enhanced upon ␣ M ␤ 2 activation by LPS and is mediated by multiple regions in both LRP1 and ␣ M ␤ 2 . Specifically, we find that both the heavy and light chains of LRP1 are involved in ␣ M ␤ 2 binding. Within the heavy chain, the binding is mediated primarily via the second and fourth ligand binding repeats. For ␣ M ␤ 2 , we find that the ␣ M -I domain represents a major LRP1 recognition site. Indeed, substitution of the I domain of the ␣ L ␤ 2 receptor with that of ␣ M confers the ␣ L ␤ 2 receptor with the ability to interact with LRP1. Furthermore, we show that residues 160 EQLKKSKTL 170 within the ␣ M -I domain represent a major LRP1 recognition site. Given that perturbation of this specific sequence leads to altered adhesive activity of ␣ M ␤ 2 , our finding suggests that binding of LRP1 to ␣ M ␤ 2 could alter integrin function. Indeed, we further demonstrate that the soluble form of LRP1 (sLRP1) inhibits ␣ M ␤ 2 -mediated adhesion of cells to fibrinogen. These studies suggest that sLRP1 may attenuate inflammation by modulating integrin function.The LDL receptor-related protein 1 (LRP1) 3 is a large endocytic receptor that recognizes numerous ligands (1, 2). LRP1 is synthesized as a single chain 600-kDa precursor that is processed by furin to generate a 515-kDa heavy chain and an 85-kDa light chain (3). The heavy chain contains four clusters of LDL receptor type A or "ligand binding" repeats, which are responsible for recognizing most of the ligands that bind to LRP1. The LRP1 light chain includes the transmembrane and cytoplasmic domain, which contains two NPxY motifs, and two dileucine repeats. The second NPxY motif overlaps with a YxxL motif, and along with the two dileucine repeats, contributes to LRP1 endocytosis (4).In addition to binding soluble ligands, such as ␣ 2 macroglobulin-protease complexes (5) or various serpin-enzyme complexes (6), LRP1 also associates with other transmembrane proteins. These include the amyloid precursor protein (7, 8), the platelet-derived growth factor receptor-␤ (9 -12), and ␤ 2 integrins (13,14). The association of these cell surface transmembrane proteins with LRP1 alters their trafficking and functional properties. In the case of ␤ 2 integrins, Spijkers et al. (13) found that in U937 cells, ␣ M ␤ 2 co-precipitates with LRP1, which regulates its cell adhesion properties. Cao et al. (14) found that LRP1 co-localizes with ␣ M ␤ 2 at the trailing edge of migrating macrophages and further demonstrated that macrophage migration depends upon a coordinated effort between LRP1 and ␣ M ␤ 2 along with tissue plasminogen activator and its inhibitor, ...

LRP-1–CD44, a New Cell Surface Complex Regulating Tumor Cell Adhesion

“…Indeed, Spijkers and colleagues have described a complex formation between LRP-1 and ␤2-integrin by using recombinant proteins and showed that LRP-1 inhibition is associated with impaired ␤2-dependent adhesion to endothelial cells (51). In our study, the use of LRP-1 minireceptors revealed that the fourth ligand binding cluster of LRP-1 is involved in CD44 binding, as previously mentioned for ␤2-integrins (46,51). LRP-1 was also described to mediate the uptake of integrin-containing complexes in macrophages to facilitate the cellular detachment at the rear of the cell (7).…”

“…Cao and colleagues have proposed an exciting molecular model in macrophages in which LRP-1 facilitates cell detachment at the trailing edge by mediating the internalization of integrin-containing adhesion complexes (7). Cooperation at the cell surface between LRP-1 and ␤2-integrin was also reported to mediate the adhesion of leukocytes (46,51). The main distribution of LRP-1 at the leading edge and at the rear of the cell could therefore orchestrate cell polarization and support directional migration of various cell types (7,8).…”

“…LRP-1 is able to interact with other membrane-anchored proteins, including the amyloid protein precursor, the urokinase-type plasminogen activator receptor (uPAR), the platelet-derived growth factor receptor ␤ (PDGFR-␤), and some integrins, to regulate their trafficking and the associated intracellular signals (3,6,10,37,46,51). This leads to a variety of cellular responses, such as modulating blood-brain-barrier integrity (60), contributing to vasculature protection (3)(4)(5), or regulating cellular migration under various physiopathological conditions (7,8,12,29).…”

Metalloproteinase-mediated Shedding of Integrin β2 Promotes Macrophage Efflux from Inflammatory Sites

“…Mac-1 deficient macrophages (mice lacking integrin ␣M) did not exit the peritoneal cavity in the accelerated exiting model (17), although we observed more rapid exiting in the same model with mice lacking integrin ␤2. Outside-in signaling (29) and binding to low density lipoprotein receptor-related protein (LRP)-1, which is required for migration (30), are both impaired in the absence of integrin ␣M. Thus, although Mac-1 null mice still express ␣X␤2 that could mediate fibrin binding, their macrophages would be impaired in "outside-in" signaling from fibrin.…”

Midkine, a middle manager of β2 integrins

“…3 Relevant to the current study, low-density lipoprotein receptorrelated protein 1 (LRP-1) has previously been described as being associated with the I-domain of the a-chain of the b 2 integrin macrophage antigen-1 (Mac-1) 5 and being required for integrin clustering and adhesion of the monocytic cell line U937. 6 Using pharmacologic inhibition, the authors provide evidence that LRP-1 is the receptor for MK on neutrophils because blockade of this receptor inhibited the MK-mediated LFA-1 high-affinity conformation and neutrophil adhesion in vitro.…”