Integrin modulating factor-1: a lipid that alters the function of leukocyte integrins

Triggering through CD16 or phorbol esters enhances adhesion of NK cells to laminin via very late antigen 6

Very late antigens VLA-1, VLA-2, VLA-3, and VLA-6, belonging to the beta 1 subfamily of integrins, have been identified as receptors for different binding domains of laminin (LM). We have detected VLA-6, but not VLA-1 and VLA-2 on a subset (50-70%) of fresh peripheral blood CD3-, CD16+, CD56+ human natural killer (NK) cells by immunofluorimetric and biochemical analysis. Binding assays performed on LM-coated plates showed that 10-15% of NK cells spontaneously adhere to LM, and this adhesion is mediated by VLA-6. Activation of NK cells through CD16 triggering or by phorbol ester results in a rapid increase of adhesion to LM, which is still mediated by VLA-6. The enhanced adhesiveness is not associated with changes in beta 1 LM receptor expression, while it correlates with changes in the phosphorylation status of alpha 6 subunit. The expression of VLA-6 on NK cells and the modulation of its avidity by activating stimuli may be relevant for NK cell migration and tissue location during inflammation or immune response.

Adhesion molecules and tumor cell interaction with endothelium and subendothelial matrix

Cancer metastasis poses the greatest challenge to the eradication of malignancy. The majority of clinical and experimental evidence indicates that metastasis is a non-random, organ-specific process. Tumor cell interaction with endothelium and subendothelial matrix constitutes the most crucial factor in determining the organ preference of metastasis. A plethora of cell surface adhesion molecules, which encompass four major families (i.e., integrins, cadherins, immunoglobulins and selectins) and many other unclassified molecules, mediate tumor-host interactions. Adhesion molecules and adhesion processes are involved in most, if not all, of the intermediate steps of the metastatic cascade. Decreased E-cadherin expression and increased CD44 expression are clearly correlated with the acquisition of the invasive capacity of primary tumor cells. Similarly, altered expression pattern of many other adhesion molecules such as upregulated expression of the laminin receptors and depressed expression of fibronectin receptors (alpha 5 beta 1) appears to be involved in tumor cell invasion into the subendothelial matrix. Tumor cell-endothelium interactions involve several well-defined sequential steps that can be analyzed by the 'Docking and Locking' hypothesis at the molecular level. Tumor cell-matrix interactions are determined by the repertoire of adhesion receptors of tumor cells and the unique composition of organ-specific matrices. Our experimental data, together with others', suggest that the integrin alpha IIb beta 3 is one of the major players in these tumor-host interactions. Tumor-host interaction is a dynamic process which is constantly modulated by a host of factors including various cytokines, growth factors and arachidonate metabolites such as 12(S)-HETE. Delineation of the molecular mechanisms of tumor-host interactions may provide additional means to intervene in the metastatic process.

Signal transduction by integrin receptors for extracellular matrix: cooperative processing of extracellular information

Adhesion receptors allow cells to interact with a dynamic and information-rich environment of extracellular matrix molecules. The integrin family of adhesion receptors transduces signals from the extracellular matrix that regulate growth, gene expression and differentiation, as well as cell shape, motility and cytoskeletal architecture. Recent data support the hypothesis that integrins transduce signals cooperatively with other classes of adhesion receptors or with growth factor receptors. Furthermore, the ability of integrins to interact with the cytoskeleton appears to be fundamental to their mechanism for signal transduction.

Phorbol ester-induced promyelocytic leukemia cell adhesion to marrow stromal cells involves fibronectin specific alpha 5 beta 1 integrin receptors

The human promyelocytic cell line NB4 exhibited a weak adhesion capacity for bone marrow-derived stromal cells and their extracellular matrices (5-15% of adherent cells). Adhesion was enhanced by pulse-treatment of cells with phorbolester (PMA 10(-7) M). Adhesion was induced within minutes, was fibronectin-specific, and affected up to 100% of the treated cells. This biological response to PMA resulted from the activation of protein kinase C (PKC), since PKC inhibitors (staurosporine, sphingosine, CGP 41251, and calphostin C) prevented the phenomenon. Phenotypical analysis of integrin receptor expression (particularly FN receptors VLA-4 and VLA-5) at the membrane of untreated or PMA-treated cells revealed that PMA induced no significant modification of the level of expression of these receptors. However, inhibition studies carried out with anti-VLA monoclonal antibodies demonstrated that the FN-specific adhesion triggered by PKC involved the alpha 5 beta 1 FN-specific receptors (VLA-5). We showed that the binding of NB4 cells to fibronectin was RGD-dependent. PMA-induced adhesion was not correlated to phosphorylation of the VLA-5 receptor. These findings may partially explain the malignant behaviour of these cells: The loss of their capacity to adhere to stromal cells may arrest differentiation and explain the large number of leukemic cells in the circulation.

Adhesion cascades: diversity through combinatorial strategies

Cell adhesion occurs via a highly regulated set of sequential interactions. Prototypic components of a variety of adhesion cascades are discussed, including integrins, triggering molecules and lectin-carbohydrate interactions. Selectivity and efficiency are achieved by utilizing the right combination of multiple adhesion molecules and by their coordinated biochemical regulation.

Leukocyte-endothelial cell interactions

The movement of leukocytes from the blood circulation into organized lymphoid tissues or sites of inflammation requires cooperative interactions between signaling and adhesion molecules. Selectins mediate the initial rolling contacts of leukocytes with the endothelium. Following leukocyte activation, integrins strengthen adhesion and then direct migration beneath the endothelium. Unique combinations of signaling and adhesion molecules may regulate the subsets of leukocytes that are recruited into specific tissues.

Inside-out integrin signalling

Integrins are expressed by virtually all cells and play key roles in a range of cellular processes. Changes in the integrin surface repertoire provide a means of altering the strength and ligand preferences of cell adhesion. Recent research has examined the affinity modulation of integrins, a rapid and versatile mechanism of cell adhesion regulation. Studies with a prototype, alpha IIb beta 3, indicate that intracellular events influence the conformation and ligand-binding affinity of the extracellular domain of integrins. This 'inside-out' signal transduction appears to be mediated through the integrin cytoplasmic domains. In addition, in some cases affinity modulation of integrins may be cell-type specific. The clarification of the mechanisms of integrin affinity modulation should help explain rapid changes in cell adhesion that occur during cell migration, aggregation and the cell cycle.

Are beta 1 integrins involved in Xenopus gastrulation?

The molecular basis of vertebrate gastrulation is poorly understood. Work on urodele amphibians has implicated beta 1-containing integrins, but the limited information available for Xenopus indicates otherwise: peptides containing the RGD sequence do not inhibit gastrulation and induction of cell spreading in presumptive ectodermal cells by activin is not accompanied by an increase in synthesis of integrin beta 1. Here we report that beta 1-containing integrins are, nevertheless, the principal fibronectin receptors in the Xenopus gastrula, although their cell surface levels are low. Antibodies recognizing the external domain of the molecule can, unlike peptides containing the RGD site, block gastrulation when introduced into the blastocoel. These results allow us to propose a model to explain the role of integrin beta 1 in Xenopus gastrulation.

Characterization of a novel arachidonic acid-derived neutrophil chemoattractant

The oxidation of arachidonic acid in a superoxide-generating environment results in the formation of a potent chemoattractant that appears to be identical to a chemotactic material generated by hepatocytes when they metabolize alcohol. The product was extracted, chromatographed and characterized by physical methods including GC/MS. The physical properties are consistant with the parent structure: 19-hydroperoxy, 20-hydroxyarachidic acid. This novel saturated 20 carbon product, derived from arachidonic acid by free radical-generating reactions, may play a role in the neutrophilic infiltration observed during the course of acute alcoholic hepatitis.

CD31 expressed on distinctive T cell subsets is a preferential amplifier of beta 1 integrin-mediated adhesion

The CD31 (platelet endothelial cell adhesion molecule-1 [PECAM-1]/endothelial cell adhesion molecule [endoCAM]) molecule expressed on leukocytes, platelets, and endothelial cells is postulated to mediate adhesion to endothelial cells and thereby function in immunity, inflammation, and wound healing. We report the following novel features of CD31 which suggests a role for it in adhesion amplification of unique T cell subsets: (a) engagement of CD31 induces the adhesive function of beta 1 and beta 2 integrins; (b) adhesion induction by CD31 immunoglobulin G (IgG) monoclonal antibodies (mAbs) is sensitive, requiring only bivalent mAb; (c) CD31 mAb induces adhesion rapidly, but it is transient; (d) unique subsets of CD4+ and CD8+ T cells express CD31, including all naive (CD45RA+) CD8 T cells; and (e) CD31 induction is selective, inducing adhesive function of beta 1 integrins, particularly very late antigen-4, more efficiently than the beta 2 integrin lymphocyte function-associated antigen-1. Conversely, CD3 is more effective in inducing beta 2-mediated adhesion. Taken together, these findings indicate that unique T cell subsets express CD31, and CD31 has the capacity to induce integrin-mediated adhesion of T cells in a sensitive and selective fashion. We propose that, in collaboration with other receptors/ligands, CD31 functions in an "adhesion cascade" by amplifying integrin-mediated adhesion of CD31+ T cells to other cells, particularly endothelial cells.

Regulatory mechanisms in leukocyte adhesion: flexible receptors for sophisticated travelers

Unstimulated leukocytes spend extended periods circulating in the blood, punctuated by migration through lymphoid areas and peripheral tissues. During transit, strong cell-cell interactions control immune surveillance and specialized effector functions. The structures and mechanisms that allow this flexible adhesion and migration behavior are the subject of this review.

On the regulation of beta 2 integrins

The complex functions played by beta 2 integrins in mediating a large variety of adhesive interactions of leukocytes are highly regulated. This regulation results in transient adaptations/associations, permitting physical and functional recycling of these receptors during chemotaxis, phagocytosis and target-cell killing. The structural definition of these adaptations will lead not only to a better understanding of how these receptors are regulated in leukocytes but also shed valuable light on how these integrins integrate diverse extracellular signals into spatially and temporaly coordinated cellular responses.