Urokinase-type plasminogen activator receptor induces conformational changes in the integrin alphaMbeta2 headpiece and reorientation of its transmembrane domains

Mitofusin-2 regulates leukocyte adhesion and β2 integrin activation

Neutrophils are critical for inflammation and innate immunity, and their adhesion to vascular endothelium is a crucial step in neutrophil recruitment. Mitofusin-2 (MFN2) is required for neutrophil adhesion, but molecular details are unclear. Here, we demonstrated that β₂ -integrin-mediated slow-rolling and arrest, but not PSGL-1-mediated cell rolling, are defective in MFN2-deficient neutrophil-like HL60 cells. This adhesion defect is associated with reduced expression of fMLP (N-formylmethionyl-leucyl-phenylalanine) receptor FPR1 as well as the inhibited β₂ integrin activation, as assessed by conformation-specific monoclonal antibodies. MFN2 deficiency also leads to decreased actin polymerization, which is important for β2 integrin activation. Mn²⁺ -induced cell spreading is also inhibited after MFN2 knockdown. MFN2 deficiency limited the maturation of β2 integrin activation during the neutrophil-directed differentiation of HL60 cells, which is indicated by CD35 and CD87 markers. MFN2 knockdown in β2-integrin activation-matured cells (CD87^high population) also inhibits integrin activation, indicating that MFN2 directly affects β2 integrin activation. Our study illustrates the function of MFN2 in leukocyte adhesion and may provide new insights into the development and treatment of MFN2 deficiency-related diseases.

A new class of orthosteric uPAR·uPA small-molecule antagonists are allosteric inhibitors of the uPAR·vitronectin interaction

The urokinase receptor (uPAR) is a GPI-anchored cell surface receptor that is at the center of an intricate network of protein-protein interactions. Its immediate binding partners are the serine proteinase urokinase (uPA), and vitronectin (VTN), a component of the extracellular matrix. uPA and VTN bind at distinct sites on uPAR to promote extracellular matrix degradation and integrin signaling, respectively. Here, we report the discovery of a new class of pyrrolone small-molecule inhibitors of the tight ∼1 nM uPAR·uPA protein-protein interaction. These compounds were designed to bind to the uPA pocket on uPAR. The highest affinity compound, namely 7, displaced a fluorescently labeled α-helical peptide (AE147-FAM) with an inhibition constant Ki of 0.7 μM and inhibited the tight uPAR·uPAATF interaction with an IC50 of 18 μM. Biophysical studies with surface plasmon resonance showed that VTN binding is highly dependent on uPA. This cooperative binding was confirmed as 7, which binds at the uPAR·uPA interface, also inhibited the distal VTN·uPAR interaction. In cell culture, 7 blocked the uPAR·uPA interaction in uPAR-expressing human embryonic kidney (HEK-293) cells and impaired cell adhesion to VTN, a process that is mediated by integrins. As a result, 7 inhibited integrin signaling in MDA-MB-231 cancer cells as evidenced by a decrease in focal adhesion kinase (FAK) phosphorylation and Rac1 GTPase activation. Consistent with these results, 7 blocked breast MDA-MB-231 cancer cell invasion with IC50 values similar to those observed in ELISA and surface plasmon resonance competition studies. Explicit-solvent molecular dynamics simulations show that the cooperativity between uPA and VTN is attributed to stabilization of uPAR motion by uPA. In addition, free energy calculations revealed that uPA stabilizes the VTNSMB·uPAR interaction through more favorable electrostatics and entropy. Disruption of the uPAR·VTNSMB interaction by 7 is consistent with the cooperative binding to uPAR by uPA and VTN. Interestingly, the VTNSMB·uPAR interaction was less favorable in the VTNSMB·uPAR·7 complex suggesting potential cooperativity between 7 and VTN. Compound 7 provides an excellent starting point for the development of more potent derivatives to explore uPAR biology.

Domain 2 of uPAR regulates single-chain urokinase-mediated angiogenesis through β1-integrin and VEGFR2

How single-chain urokinase (ScuPA) mediates angiogenesis is incompletely understood. ScuPA (≥4 nM) induces phosphorylated (p)ERK1/2 (MAPK44 and MAPK42) and pAkt (Ser(473)) in umbilical vein and dermal microvascular endothelial cells. Activation of pERK1/2 by ScuPA is blocked by PD-98059 or U-0126, and pAkt (Ser(473)) activation is inhibited by wortmannin or LY-294002. ScuPA (32 nM) or protease-inhibited two-chain urokinase stimulates pERK1/2 to the same extent, indicating that signaling is not dependent on enzymatic activity. ScuPA induces pERK1/2, but not pAkt (Ser(473)), in SIN1(-/-) cells, indicating that the two pathways are not identical. Peptides from domain 2 of the urokinase plasminogen activator receptor (uPAR) or domain 5 of high-molecular-weight kininogen compete with ScuPA for the induction of pERK1/2 and pAkt (Ser(473)). A peptide of the integrin-binding site on uPAR, a β1-integrin peptide that binds uPAR, antibody 6S6 to β1-integrin, tyrosine kinase inhibitors AG-1478 or PP3, and small interfering RNA knockdown of VEFG receptor 2, but not HER1-HER4, blocked ScuPA-induced pERK1/2 and pAkt (Ser(473)). ScuPA-induced endothelial cell proliferation was blocked by inhibitors of pERK1/2 and pAkt (Ser(473)), antibody 6S6, and uPAR or kininogen peptides. ScuPA initiated aortic sprouts and Matrigel plug angiogenesis in normal, but not uPAR-deficient, mouse aortae or mice, respectively, but these were blocked by PD-98059, LY-294002, AG-1478, or cleaved high-molecular-weight kininogen. In summary, this investigation indicates a novel, a nonproteolytic signaling pathway initiated by zymogen ScuPA and mediated by domain 2 of uPAR, β1-integrins, and VEGF receptor 2 leading to angiogenesis. Kininogens or peptides from it downregulate this pathway.

Ly6 family proteins in neutrophil biology

The murine Ly6 complex was identified 35 years ago using antisera to lymphocytes. With advances in mAb development, molecular cloning, and genome sequencing, >20 structurally related genes have been identified within this complex on chromosome 15. All members of the Ly6 family and their human homologues share the highly conserved LU domain and most also possess a GPI anchor. Interestingly, many Ly6 proteins are expressed in a lineage-specific fashion, and their expression often correlates with stages of differentiation. As a result, Ly6 proteins are frequently used as surface markers for leukocyte subset identification and targets for antibody-mediated depletion. Murine neutrophils display prominent surface expression of several Ly6 proteins, including Ly6B, Ly6C, and Ly6G. Although the physiology of most Ly6 proteins is not well understood, a role in neutrophil functions, such as migration, is recognized increasingly. In this review, we will provide an overview of the Ly6 complex and discuss, in detail, the specific Ly6 proteins implicated in neutrophil biology.

Small-molecule inhibition of the uPAR·uPA interaction: synthesis, biochemical, cellular, in vivo pharmacokinetics and efficacy studies in breast cancer metastasis

The uPAR·uPA protein-protein interaction (PPI) is involved in signaling and proteolytic events that promote tumor invasion and metastasis. A previous study had identified 4 (IPR-803) from computational screening of a commercial chemical library and shown that the compound inhibited uPAR·uPA PPI in competition biochemical assays and invasion cellular studies. Here, we synthesize 4 to evaluate in vivo pharmacokinetic (PK) and efficacy studies in a murine breast cancer metastasis model. First, we show, using fluorescence polarization and saturation transfer difference (STD) NMR, that 4 binds directly to uPAR with sub-micromolar affinity of 0.2 μM. We show that 4 blocks invasion of breast MDA-MB-231, and inhibits matrix metalloproteinase (MMP) breakdown of the extracellular matrix (ECM). Derivatives of 4 also inhibited MMP activity and blocked invasion in a concentration-dependent manner. Compound 4 also impaired MDA-MB-231 cell adhesion and migration. Extensive in vivo PK studies in NOD-SCID mice revealed a half-life of nearly 5h and peak concentration of 5 μM. Similar levels of the inhibitor were detected in tumor tissue up to 10h. Female NSG mice inoculated with highly malignant TMD-MDA-MB-231 in their mammary fat pads showed that 4 impaired metastasis to the lungs with only four of the treated mice showing severe or marked metastasis compared to ten for the untreated mice. Compound 4 is a promising template for the development of compounds with enhanced PK parameters and greater efficacy.

The leucocyte β2 (CD18) integrins: the structure, functional regulation and signalling properties

Leucocytes are highly motile cells. Their ability to migrate into tissues and organs is dependent on cell adhesion molecules. The integrins are a family of heterodimeric transmembrane cell adhesion molecules that are also signalling receptors. They are involved in many biological processes, including the development of metazoans, immunity, haemostasis, wound healing and cell survival, proliferation and differentiation. The leucocyte-restricted β2 integrins comprise four members, namely αLβ2, αMβ2, αXβ2 and αDβ2, which are required for a functional immune system. In this paper, the structure, functional regulation and signalling properties of these integrins are reviewed.

Overview: imaging in the study of integrins

Integrins play critical adhesion and signaling roles during development, wound healing, immunity, and cancer. Central to their function is a unique ability to dynamically modulate their adhesiveness and signaling properties through changes in conformation, both homo- and heterotypic protein-protein interactions and cellular distribution. Genetic, biochemical and structural studies have been instrumental in uncovering overall functions, describing ligand and regulatory protein interactions and elucidating the molecular architecture of integrins. However, such approaches alone are inadequate to describe how dynamic integrin behaviors are orchestrated in intact cells. To fill this void, a wide array of distinct light microscopy (largely fluorescence-based) imaging approaches have been developed and employed. Various microscopy technologies, including wide-field, optical sectioning (laser-scanning confocal, spinning-disk confocal, and multiphoton), TIRF and range of novel "Super-Resolution" techniques have been used in combination with diverse imaging modalities (such as IRM, FRET, FRAP, CALI, and fluorescence speckle imaging) to address distinct aspects of integrin function and regulation. This chapter provides an overview of these imaging approaches and how they have advanced our understanding of integrins.

Structures and interaction analyses of integrin αMβ2 cytoplasmic tails

Integrins are heterodimeric (α and β subunits) signal transducer proteins involved in cell adhesions and migrations. The cytosolic tails of integrins are essential for transmitting bidirectional signaling and also implicated in maintaining the resting states of the receptors. In addition, cytosolic tails of integrins often undergo post-translation modifications like phosphorylation. However, the consequences of phosphorylation on the structures and interactions are not clear. The leukocyte-specific integrin αMβ2 is essential for myeloid cell adhesion, phagocytosis, and degranulation. In this work, we determined solution structures of the myristoylated cytosolic tail of αM and a Ser phosphorylated variant in dodecylphosphocholine micelles by NMR spectroscopy. Furthermore, the interactions between non-phosphorylated and phosphorylated αM tails with β2 tail were investigated by NMR and fluorescence resonance energy transfer (FRET). The three-dimensional structures of the 24-residue cytosolic tail of αM or phosphorylated αM are characterized by an N-terminal amphipathic helix and a loop at the C terminus. The residues at the loop are involved in packing interactions with the hydrophobic face of the helix. 15N-1H heteronuclear single quantum coherence experiments identified residues of αM and β2 tails that may be involved in the formation of a tail-tail heterocomplex. We further examined interactions between myristoylated β2 tail in dodecylphosphocholine micelles with dansylated αM tail peptides by FRET. These studies revealed enhanced interactions between αM or phosphorylated αM tails with β2 tail with Kd values ∼5.2±0.6 and ∼4.4±0.7 μm, respectively. Docked structures of tail-tail complexes delineated that the αM/β2 interface at the cytosolic region could be sustained by a network of polar interactions, ionic interactions, and/or hydrogen bonds.

Association of Ang-2 with integrin beta 2 controls Ang-2/PDGF-BB-dependent upregulation of human peripheral blood monocyte fibrinolysis

Angiopoietin-2 (Ang-2), an angiogenic factor that is generally considered an autocrine factor for endothelial cells was shown in a previous study to upregulate peripheral blood monocyte fibrinolysis in concert with platelet-derived growth factor-BB (PDGF-BB). This upregulation of fibrinolysis was demonstrated to be due to upregulation of elements of the matrix metalloproteinase and serine protease fibrinolytic pathways. The manner in which Ang-2 interacts with monocytes was not elucidated though no expression of the angiopoietin receptor tyrosine kinase Tie-2 was found for monocytes. In this study Ang-2 was found to bind to integrin beta(2), and functional inhibition of integrin beta(2) eliminated Ang-2/PDGF-BB-mediated upregulation of monocyte fibrin invasion. Additionally, integrin beta(2) blockade significantly inhibited the Ang-2/PDGF-BB based increase in matrix metalloproteinase-9 (MMP-9) and membrane type-1-MMP (MT1-MMP). Furthermore, Ang-2/PDGF-BB-upregulated urokinase plasminogen-activator receptor (uPAR) was shown to be associated in complexes with integrin beta(2). In addition, Ang-2 was shown to upregulate PDGFR-beta expression in monocytes. Therefore several components of the mechanism via which the novel interaction of Ang-2 and PDGF-BB with monocytes occurs have been identified.

Urokinase-type plasminogen activator inhibits efferocytosis of neutrophils

RATIONALE

Phagocytosis of apoptotic cells, also called efferocytosis, plays an essential role in the resolution of inflammation. Urokinase-type plasminogen activator (uPA) is a multifunctional protein that has been implicated in inflammatory conditions, including pneumonia and severe infection, which are often accompanied by the development of acute lung injury. However, the role of uPA in modulating efferocytosis of apoptotic neutrophils has not been defined.

OBJECTIVES

To characterize the role of uPA in regulation of efferocytosis and to delineate the underlying mechanisms involved in this process.

METHODS

In vitro and in vivo phagocytosis, immunoprecipitation, and Western blotting assays.

MEASUREMENTS AND MAIN RESULTS

The phagocytosis of apoptotic neutrophils by macrophages was significantly inhibited by uPA. Mutant uPA lacking the growth factor domain and catalytically inactive uPA had similar inhibitory effects on efferocytosis, as did wild-type uPA. In contrast, absence of the kringle domain abrogated the ability of uPA to diminish efferocytosis. Both the α(V)β₃ integrin and vitronectin seemed to be involved in the inhibition of efferocytosis by uPA. Incubation of macrophages with uPA also diminished activation of the small GTPase Rac-1, which normally occurs during ingestion of apoptotic neutrophils. Under in vivo conditions in the lungs, uPA decreased the uptake of apoptotic neutrophils by alveolar macrophages.

CONCLUSIONS

Our data demonstrate a novel role for uPA in which it is able to diminish the uptake of apoptotic neutrophils by macrophages under both in vitro and in vivo conditions.

Factor XII stimulates ERK1/2 and Akt through uPAR, integrins, and the EGFR to initiate angiogenesis

Factor XII (FXII) and high molecular weight kininogen (HK) mutually block each other's binding to the urokinase plasminogen activator receptor (uPAR). We investigated if FXII stimulates cells by interacting with uPAR. FXII (3-62nM) with 0.05mM Zn(2+) induces extracellular signal-related kinase 1/2 (ERK1/2; mitogen-activated protein kinase 44 [MAPK44] and MAPK42) and Akt (Ser473) phosphorylation in endothelial cells. FXII-induced phosphorylation of ERK1/2 or Akt is a zymogen activity, not an enzymatic event. ERK1/2 or Akt phosphorylation is blocked upstream by PD98059 or Wortmannin or LY294002, respectively. An uPAR signaling region for FXII is on domain 2 adjacent to uPAR's integrin binding site. Cleaved HK or peptides from HK's domain 5 blocks FXII-induced ERK1/2 and Akt phosphorylation. A beta(1) integrin peptide that binds uPAR, antibody 6S6 to beta(1) integrin, or the epidermal growth factor receptor (EGFR) inhibitor AG1478 blocks FXII-induced phosphorylation of ERK1/2 and Akt. FXII induces endothelial cell proliferation and 5-bromo-2'deoxy-uridine incorporation. FXII stimulates aortic sprouting in normal but not uPAR-deficient mouse aorta. FXII produces angiogenesis in matrigel plugs in normal but not uPAR-deficient mice. FXII knockout mice have reduced constitutive and wound-induced blood vessel number. In sum, FXII initiates signaling mediated by uPAR, beta(1) integrin, and the EGFR to induce human umbilical vein endothelial cell proliferation, growth, and angiogenesis.

Regulation of cell signalling by uPAR

Urokinase-type plasminogen activator receptor (uPAR) expression is elevated during inflammation and tissue remodelling and in many human cancers, in which it frequently indicates poor prognosis. uPAR regulates proteolysis by binding the extracellular protease urokinase-type plasminogen activator (uPA; also known as urokinase) and also activates many intracellular signalling pathways. Coordination of extracellular matrix (ECM) proteolysis and cell signalling by uPAR underlies its important function in cell migration, proliferation and survival and makes it an attractive therapeutic target in cancer and inflammatory diseases. uPAR lacks transmembrane and intracellular domains and so requires transmembrane co-receptors for signalling. Integrins are essential uPAR signalling co-receptors and a second uPAR ligand, the ECM protein vitronectin, is also crucial for this process.

Participation of the urokinase receptor in neutrophil efferocytosis

The urokinase receptor (uPAR) plays an important role in regulation of fibronolysis, cell migration, and adhesion. In this study, we examined whether uPAR plays a role in modulating efferocytosis of neutrophils. Macrophages from uPAR(-/-) mice demonstrated enhanced ability to engulf viable wild-type (WT) neutrophils in vitro and in vivo in the lungs. The increased phagocytic activity of uPAR(-/-) macrophages was abrogated by incubation with soluble uPAR (suPAR), arginine-glycine-aspartic acid (RGD)-containing peptides, or anti-integrin antibodies. There was increased uptake of viable uPAR(-/-) neutrophils by WT macrophages. Incubation of uPAR(-/-) neutrophils with suPAR or anti-integrin antibodies diminished uptake by WT macrophages to baseline. Uptake of uPAR(-/-) neutrophils by uPAR(-/-) macrophages was not enhanced. However, incubation of uPAR(-/-) neutrophils or uPAR(-/-) macrophages, but not both, with suPAR enhanced the uptake of viable uPAR(-/-) neutrophils by uPAR(-/-) macrophages. The adhesion of WT neutrophils to uPAR(-/-) macrophages was higher than to WT macrophages. uPAR(-/-) neutrophils demonstrated increased adhesion to suPAR, which was abrogated by blocking of low-density lipoprotein related protein and integrins. Expression of uPAR on the surface of apoptotic neutrophils was reduced compared with levels on viable neutrophils. These results demonstrate a novel role for uPAR in modulating recognition and clearance of neutrophils.

Membrane-bound proteinase 3 and its receptors: relevance for the pathogenesis of Wegener's Granulomatosis

Wegener's Granulomatosis (WG) is a life-threatening autoimmune disease. A pathogenic role for anti-neutrophil cytoplasmic autoantibodies (ANCAs) by inducing necrotizing damage to the vessel wall has been strongly suggested by in vitro and in vivo experimental data. Proteinase 3 (PR3), a serine protease mainly stored in the azurophilic granules of neutrophils, has been identified as a major ANCA-antigen in WG. Elevated expression levels of membrane-bound PR3 (mPR3) has been observed in WG and some other chronic inflammatory diseases, suggesting a pathogenic role of mPR3 by allowing interaction with PR3-ANCA. Recent studies revealed CD177 as a receptor for mPR3 on the neutrophil membrane. However, we recently showed that CD177 negative neutrophils also express mPR3 and are susceptible to PR3-ANCA induced neutrophil activation. Therefore, it is of interest to further investigate the functional consequences of binding of mPR3 to CD177, to explore other binding partners for mPR3 on the neutrophil membrane, and to study the relevance of colocalization of these molecules for disease pathogenesis. This review gives updated information on the mechanism of mPR3 expression and the relevance of colocalization of mPR3 with other molecules on the neutrophil membrane for the pathophysiological events occurring in WG.