Heme released from heme-binding proteins on internal hemorrhage, hemolysis, myolysis, or other cell damage is highly toxic due to oxidative and proinflammatory effects. Complex formation with hemopexin, the high-affinity heme-binding protein in plasma and cerebrospinal fluid, dampens these effects and is suggested to facilitate cellular heme metabolism. Using a ligand-affinity approach, we purified the human hemopexin-heme receptor and identified it as the low-density lipoprotein receptor-related protein (LRP)/CD91, a receptor expressed in several cell types including macrophages, hepatocytes, neurons, and syncytiotrophoblasts. Binding experiments, including Biacore analysis, showed that hemopexin-heme complex formation elicits the high receptor affinity. Uptake studies of radio-labeled hemopexin-heme complex in LRP/CD91-expressing COS cells and confocal microscopy of the cellular processing of fluorescent hemopexin-heme complex established the ability of LRP/CD91 to mediate hemopexin-heme internalization resulting in cellular heme uptake and lysosomal hemopexin degradation. Uptake of hemopexin-heme complex induced LRP/CD91-dependent heme-oxygenase 1 mRNA transcription in cultured monocytes. In conclusion, hemopexin-heme complexes are removed by a receptor-mediated pathway showing striking similarities to the CD163-mediated haptoglobin-hemoglobin clearance in macrophages. Furthermore, the data indicate a hitherto unknown role of LRP/CD91 in inflammation. (Blood. 2005; 106:2572-2579)

Angiostatin, a proteolytic fragment of plasminogen, is a potent antagonist of angiogenesis and an inhibitor of endothelial cell migration and proliferation. To determine whether the mechanism by which angiostatin inhibits endothelial cell migration and/or proliferation involves binding to cell surface plasminogen receptors, we isolated the binding proteins for plasminogen and angiostatin from human umbilical vein endothelial cells. Binding studies demonstrated that plasminogen and angiostatin bound in a concentration-dependent, saturable manner. Plasminogen binding was unaffected by a 100-fold molar excess of angiostatin, indicating the presence of a distinct angiostatin binding site. This finding was confirmed by ligand blot analysis of isolated human umbilical vein endothelial cell plasma membrane fractions, which demonstrated that plasminogen bound to a 44-kDa protein, whereas angiostatin bound to a 55-kDa species. Amino-terminal sequencing coupled with peptide mass fingerprinting and immunologic analyses identified the plasminogen binding protein as annexin II and the angiostatin binding protein as the α/β-subunits of ATP synthase. The presence of this protein on the cell surface was confirmed by flow cytometry and immunofluorescence analysis. Angiostatin also bound to the recombinant α-subunit of human ATP synthase, and this binding was not inhibited by a 2,500-fold molar excess of plasminogen. Angiostatin’s antiproliferative effect on endothelial cells was inhibited by as much as 90% in the presence of anti-α-subunit ATP synthase antibody. Binding of angiostatin to the α/β-subunits of ATP synthase on the cell surface may mediate its antiangiogenic effects and the down-regulation of endothelial cell proliferation and migration.

A unifying feature of many neurodegenerative disorders is the accumulation of polyubiquitinated protein inclusions in dystrophic neurons, e.g. containing α-synuclein, which is suggestive of an insufficient proteasomal activity. We demonstrate that α-synuclein and 20 S proteasome components co-localize in Lewy bodies and show that subunits from 20 S proteasome particles, in contrast to subunits of the 19 S regulatory complex, bind efficiently to aggregated filamentous but not monomeric α-synuclein. Proteasome binding to insoluble α-synuclein filaments and soluble α-synuclein oligomers results in marked inhibition of its chymotrypsin-like hydrolytic activity through a non-competitive mechanism that is mimicked by model amyloid-Aβ peptide aggregates. Endogenous ligands of aggregated α-synuclein like heat shock protein 70 and glyceraldehyde-6-phosphate dehydrogenase bind filaments and inhibit their anti-proteasomal activity. The inhibitory effect of amyloid aggregates may thus be amenable to modulation by endogenous chaperones and possibly accessible for therapeutic intervention.