Post-translational processing of the leukocyte integrin alpha 4 beta 1

Aberrant regulation of the integrin very late antigen-4 in systemic lupus erythematosus

Integrin very late antigen-4 (VLA4) is induced during inflammation and can regulate monocyte migration. It has been implicated in atherogenesis, a significant concern in systemic lupus erythematosus (SLE). The aim of this study was to define VLA4 expression in SLE monocytes. Flow cytometry, reverse transcription polymerase chain reaction, Western blotting, and immunohistochemistry staining with confocal microscopy were used to evaluate VLA4 expression in SLE patients and controls. We found elevated expression of VLA4 in SLE patients with significantly increased VLA4 staining intracellularly compared to control. Exposure of control monocytes to SLE sera or immune complexes led to increased intracellular expression, and immune complexes were capable of driving redistribution of surface VLA4 to the cytoplasm. Therefore, VLA4 was found to be subject to complex regulation with SLE sera driving both RNA expression and redistribution of protein. Stimulation of SLE monocytes with a VLA4 ligand induced significant TNFα expression, confirming a functional effect. This behavior may contribute to increased atherosclerosis and monocyte infiltrates in end organs.

Acquisition of resistance toward HYD1 correlates with a reduction in cleaved α4 integrin expression and a compromised CAM-DR phenotype

We recently reported that the β1 integrin antagonist, referred to as HYD1, induces necrotic cell death in myeloma cell lines as a single agent using in vitro and in vivo models. In this article, we sought to delineate the determinants of sensitivity and resistance toward HYD1-induced cell death. To this end, we developed an HYD1 isogenic resistant myeloma cell line by chronically exposing H929 myeloma cells to increasing concentrations of HYD1. Our data indicate that the acquisition of resistance toward HYD1 correlates with reduced levels of the cleaved α4 integrin subunit. Consistent with reduced VLA-4 (α4β1) expression, the resistant variant showed ablated functional binding to fibronectin, VCAM-1, and the bone marrow stroma cell line HS-5. The reduction in binding of the resistant cell line to HS-5 cells translated to a compromised cell adhesion-mediated drug resistant phenotype as shown by increased sensitivity to melphalan- and bortezomib-induced cell death in the bone marrow stroma coculture model of drug resistance. Importantly, we show that HYD1 is more potent in relapsed myeloma specimens than newly diagnosed patients, a finding that correlated with α4 integrin expression. Collectively, these data indicate that this novel d-amino acid peptide may represent a good candidate for pursuing clinical trials in relapsed myeloma and in particular patients with high levels of α4 integrin. Moreover, our data provide further rationale for continued preclinical development of HYD1 and analogues of HYD1 for the treatment of multiple myeloma and potentially other tumors that home and/or metastasize to the bone.

The relative influence of metal ion binding sites in the I-like domain and the interface with the hybrid domain on rolling and firm adhesion by integrin alpha4beta7

We examined the effect of conformational change at the beta(7) I-like/hybrid domain interface on regulating the transition between rolling and firm adhesion by integrin alpha(4)beta(7). An N-glycosylation site was introduced into the I-like/hybrid domain interface to act as a wedge and to stabilize the open conformation of this interface and hence the open conformation of the alpha(4) beta(7) headpiece. Wild-type alpha(4)beta(7) mediates rolling adhesion in Ca(2+) and Ca(2+)/Mg(2+) but firm adhesion in Mg(2+) and Mn(2+). Stabilizing the open headpiece resulted in firm adhesion in all divalent cations. The interaction between metal binding sites in the I-like domain and the interface with the hybrid domain was examined in double mutants. Changes at these two sites can either counterbalance one another or be additive, emphasizing mutuality and the importance of multiple interfaces in integrin regulation. A double mutant with counterbalancing deactivating ligand-induced metal ion binding site (LIMBS) and activating wedge mutations could still be activated by Mn(2+), confirming the importance of the adjacent to metal ion-dependent adhesion site (ADMIDAS) in integrin activation by Mn(2+). Overall, the results demonstrate the importance of headpiece allostery in the conversion of rolling to firm adhesion.

Physical association and functional interaction between beta1 integrin and CD98 on human T lymphocytes

CD98 is a cell surface protein previously characterized as a cell activation marker, an amino acid transporter, and has recently been implicated in integrin-related functions. Integrins are cell surface proteins, important for homotypic cell aggregation, cell adhesion, and coactivation of T lymphocytes. We have previously shown that the anti-CD98 mAb 80A10, when coimmobilized with anti-CD3 mAb OKT3, is able to mediate human T cell coactivation that is inhibited by anti-beta1 integrin specific mAb 18D3. These results indicated a functional association of CD98 and beta1 integrin signaling but left open the question of a physical association. We now show the induction of homotypic aggregation through CD98 among human T cells and this aggregation was inhibited by anti-beta1 integrin mAb. Therefore, CD98-dependent lymphocyte proliferation and adhesion may involve integrins. Competitive binding assays and fluorescence colocalization analysis suggested that CD98 and beta1 integrin were physically associated. Differential extraction techniques and immunoprecipitations provided the first evidence that the alpha4beta1 integrin and CD98 are specifically associated on human T lymphocytes.

Regulation of integrin function by CD47 ligands. Differential effects on alpha vbeta 3 and alpha 4beta1 integrin-mediated adhesion

We examined the regulation of alpha4beta1 integrin function in melanoma cells and T cells by ligands of CD47. A CD47 antibody (B6H12) that inhibited alphavbeta3-mediated adhesion of melanoma cells induced by CD47-binding peptides from thrombospondin-1 directly stimulated alpha4beta1-mediated adhesion of the same cells to vascular cell adhesion molecule-1 and N-terminal regions of thrombospondin-1 or thrombospondin-2. B6H12 also stimulated alpha4beta1- as well as alpha2beta1- and alpha5beta1-mediated adhesion of CD47-expressing T cells but not of CD47-deficient T cells. alpha4beta1 and CD47 co-purified as a detergent-stable complex on a CD47 antibody affinity column. CD47-binding peptides based on C-terminal sequences of thrombospondin-1 also specifically enhanced adhesion of melanoma cells and T cells to alpha4beta1 ligands. Unexpectedly, activation of alpha4beta1 function by the thrombospondin-1 CD47-binding peptides also occurred in CD47-deficient T cells. CD47-independent activation of alpha4beta1 required the Val-Val-Met (VVM) motif of the peptides and was sensitive to inhibition by pertussis toxin. These results indicate that activation of alpha4beta1 by the CD47 antibody B6H12 and by VVM peptides occurs by different mechanisms. The antibody directly activates a CD47-alpha4beta1 complex, whereas VVM peptides may target an unidentified Gi-linked receptor that regulates alpha4beta1.

Identification of a novel structural variant of the alpha 6 integrin

The alpha(6) integrin is a 140-kDa (nonreduced) laminin receptor. We have identified a novel 70-kDa (nonreduced) form of the alpha(6) integrin called alpha(6)p for the latin word parvus, meaning small. The variant was immunoprecipitated from human cells using four different alpha(6)-specific monoclonal antibodies but not with alpha(3) or alpha(5) antibodies. The alpha(6)p integrin contained identical amino acid sequences within exons 13--25, corresponding to the extracellular "stalk region" and the cytoplasmic tail of the alpha(6) integrin. The light chains of alpha(6) and alpha(6)p were identical as judged by alpha(6)A-specific antibodies and electrophoretic properties. The alpha(6)p variant paired with either beta(1) or beta(4) subunits and was retained on the cell surface three times longer than alpha(6). Reverse transcription/polymerase chain reaction analysis revealed a single polymerase chain reaction product. The alpha(6)p variant was found in human prostate (DU145H, LnCaP, PC3) and colon (SW480) cancer cell lines but not in normal prostate (PrEC), breast cancer (MCF-7), or lung cancer (H69) cell lines or a variant of a prostate carcinoma cell line (PC3-N). Protein levels of alpha(6)p increased 3-fold during calcium-induced terminal differentiation in a normal mouse keratinocyte model system. A novel form of the alpha(6) integrin exists on cell surfaces that contains a dramatically altered extracellular domain.

Expression of a soluble functional form of the integrin alpha4beta1 in mammalian cells

The integrin alpha4beta1(VLA4) has been expressed as a soluble, active, heterodimeric immunoglobulin fusion protein. cDNAs encoding the extracellular domains of the human alpha4 and beta1 subunits were fused to the genomic DNA encoding the human gamma1 immunoglobulin Fc domain and functional integrin fusion protein was expressed as a secreted, soluble molecule from a range of mammalian cell lines. Specific mutations were introduced into the Fc region of the molecules to promote alpha4beta1 heterodimer formation. The soluble alpha4beta1-Fc fusion protein exhibited divalent cation dependent binding to VCAM-1, which was blocked by the appropriate function blocking antibodies. The apparent Kd for VCAM-1 binding were similar for both the soluble and native forms of alpha4beta1. In addition, the integrin-Fc fusion was shown to stain cells expressing VCAM-1 on their surface by FACs analysis. This approach for expressing soluble alpha4beta1 should be generally applicable to a range of integrins.

1,25-Dihydroxyvitamin D3 suppresses the expression of the VCAM-1 receptor, VLA-4 in human leukemic HL-60 cells

Very late antigen-4 (VLA-4) is the complex with alpha4 and beta1 integrins, which is the receptors to fibronectin and VCAM-1. We evaluate the effect of 1,25(OH)2D3 on the expression of VLA-4 in human leukemic HL-60, U937 cells and human melanoma A375 cells. Flow cytometric analysis demonstrate that the expression of alpha4 integrin is negatively regulated in the cell lines we studied. The expression of beta1 integrin is also decreased in HL-60 and U937 cells. The mRNA expression of alpha4 integrin is significantly decreased by the treatment with 1,25(OH)2D3, whereas 1,25(OH)2D3 does not alter the expression of beta1 mRNA. The adhesion assay demonstrate that the number of adherent cells treated with 1, 25(OH)2D3 is significantly lower than that untreated on VCAM-1-coated wells. Because VCAM-1 is highly expressed in the endothelial cells, it is possible that 1,25(OH)2D3 prevents the attachment of the cells from the endothelial cells in vivo.

Identification of ligand binding sites on integrin alpha4beta1 through chemical cross-linking

We have used chemical cross-linking to identify sequences in integrin alpha4beta1 that are involved in its interactions with ligands. A recently described leucine-aspartic acid-valine (LDV)-based small molecule inhibitor of alpha4beta1 (BIO-1494), that contained a single reactive amino group for targeting the cross-linking, was used for these studies. The specificity of the interaction was defined by (i) the ability to block the interaction with a competitive inhibitor lacking the reactive group, (ii) the absolute requirement of divalent cations for cross-linking, and (iii) the lack of cross-linking to the functionally related integrin alpha4beta7. With ANB-NOS as the cross-linker, only the beta1 chain was labeled with BIO-1494, while with the more flexible cross-linker DSS both the alpha4 and beta1 chains were modified. Similar results were obtained when cross-linking was performed on K562 cells expressing alpha4beta1 but not on K562 cells expressing alpha2beta1. The site of cross-linking on the beta1 chain was localized by CNBr peptide mapping within residues 130-146, a region that contains the putative metal binding site DXSXS and for which analogous data had been generated with RGD binding to integrin alphaIIbbeta3. The striking similarity between the data we generated for an LDV ligand and published data for the RGD family supports the notion of a common ligand binding pocket formed by both integrin chains. The cross-linking strategy developed here should serve as a useful tool for studying alpha4beta1 function.

Animal membrane receptors and adhesive molecules

This review summarizes some important data, principles, opinions, commentaries, and modern methodology concerning the receptor structure, interactions, signaling and receptor-mediated cell functions. Three sections give a brief overview of the signaling synergy, multivariant signaling, and some reactions in phosphorylation networks. A concise report about the cytotoxic reaction of NK cells represents an example of multistage recognition reaction, involving differently acting receptors, changes in affinities of cell-cell interactions, and secretion of regulatory and cytotoxic molecules. Some interesting trends in receptor engineering, including antibody molecules as a special receptor phenomenon are mentioned in the final section.

Integrin-mediated entry into S phase of human gastric adenocarcinoma cells

The integrins are a family of integral membrane receptors that participate in binding to various extracellular and cell surface proteins during adhesion, migration, and homing of normal and neoplastic cells. In this study, we characterized the involvement of integrins in mediating the growth of an adhesion-dependent gastric adenocarcinoma line, ST2. This line was distinguished and selected for study based on its inability to grow when suspended in soft agar or plated on poly(2-hydroxyethyl methacrylate)-coated dishes. ST2 cells arrested in G0/G1 of the cell cycle when deprived of adhesion to substrate. Using purified matrix components, collagen was found to be highly active in promoting beta 1 integrin-mediated cell attachment and spreading. Subsequent to spreading on collagen, the cells were released from G0/G1 block and progressed into S phase. Monoclonal antibodies to alpha 2 or beta 1 integrin blocked the reinduction of both cell spreading and entry into S phase. These studies suggest that during the metastatic process, integrin receptor interaction with the insoluble matrix may be an important step leading to proliferation of some tumors.

Integrin alpha 6 expression in human prostate carcinoma cells is associated with a migratory and invasive phenotype in vitro and in vivo

Cell adhesion and migration are important features in tumor invasion, being mediated in part by integrins (extracellular matrix receptors). Integrins are significantly decreased in human prostate cancer. An exception is alpha 6 integrin (laminin receptor) which persists during prostate tumor progression. We have selected high (DU-H) and low (DU-L) expressors of alpha 6 integrin from a human prostate tumor cell line, DU145, to assess experimentally the importance of alpha 6 integrin in tumor invasion. DU-H cells exhibited a four-fold increased expression of alpha 6 integrin on the surface compared to DU-L cells. Both cell types contained similar amounts of alpha 3 and alpha 5 integrin. The DU-H cells contained alpha 6 subunits complexed with both the beta 1 and beta 4 subunits whereas DU-L cells contained alpha 6 complexed only with beta 4. DU-H cells were three times more mobile on laminin as compared to DU-L, but adhered similarly on laminin. Adhesion and migration were inhibited with anti-alpha 6 antibody. Each subline was injected intraperitoneally into SCID mice to test its invasive potential. Results showed greater invasion of DU-H compared to DU-L cells, with increased expression of alpha 6 integrin on the tumor at the areas of invasion. These data suggest that alpha 6 integrin expression is advantageous for prostate tumor cell invasion.

Homotypic leukocyte aggregation triggered by a monoclonal antibody specific for a novel epitope expressed by the integrin beta 1 subunit: conversion of nonresponsive cells by transfecting human integrin alpha 4 subunit cDNA

The monoclonal antibody 33B6 was found to be specific for the beta 1 integrin subunit. Treatment of leukocytes with this antibody induced a vigorous homotypic aggregation that had similar physiologic conditions as aggregation induced by a monoclonal antibody specific for the alpha 4 subunit. Expression of a beta 1 subunit on the cell surface was not sufficient for mAb 33B6-mediated aggregation to occur, since cells of the K562 erythroleukemia line failed to respond even though they expressed the beta 1 subunit and the 33B6 epitope. However, after transfection with cDNA encoding the alpha 4 subunit, K562 cells acquired the ability to aggregate in response to mAb 33B6 binding. By contrast, mAb 33B6 blocked cell binding to the endothelial surface protein vascular cell adhesion molecule-1 and the extracellular matrix protein fibronectin. These results suggest that the beta 1 epitope defined by mAb 33B6 may play a novel role in regulating leukocyte adhesive interactions.