Pathophysiological significance of integrin expression by vascular endothelial cells

The vitronectin receptor plays a role in the adhesion of human cytotrophoblast cells to endothelial cells

During placental development in higher primates trophoblast cells invade maternal blood vessels and migrate along the luminal surface of endothelium. In the present study, the adherence of human cytotrophoblast cells to endothelial cells has been characterized to test the hypothesis that vitronectin receptors (alpha(v) integrins) play a role in intra-luminal trophoblast migration. Adherence was measured using a quantitative fluorescence-based assay and was found to increase in a time-dependent fashion up to about 2 h after which it leveled off. Adhesion was detectable at 4 degrees C but was greatly reduced compared to that seen at 37 degrees C. Adhesion was partially blocked by antibodies against alpha(v)beta3/beta5 integrin, beta1 integrin and by antibodies against P-selectin. Antibodies against beta3 integrin subunits had no effect. Adhesion was reduced by galactose-6-phosphate and fructose-6-phosphate. Flow cytometric analysis revealed alpha(v) integrin on the surface of cytotrophoblast and endothelial cells. Beta1 integrin was detected on the surface of endothelial cells and on cytokine-stimulated cytotrophoblast cells. Beta3 and beta5 integrins were not detected on the surface of either cell type, although beta3 was detected using permeabilized endothelial cells. These results raise the possibility that alpha(v) integrins expressed by both cytotrophoblast cells and endothelial cells, and P-selectin expressed by endothelial cells, may be important in facilitating trophoblast adhesion and migration along the uterine microvasculature.

Tumor necrosis factor-alpha stimulates attachment of small cell lung carcinoma to endothelial cells

Tumor cell attachment to endothelial cells (ECs) is an important step in the metastasis of small cell lung carcinoma (SCLC). Tumor necrosis factor-alpha (TNF-alpha) stimulation of ECs increases the attachment of some malignant cell types to ECs by affecting the expression of cell adhesion molecules (CAMs). Similarly, the inhibition of EC protein kinase C (PKC) and tyrosine kinase (TK) pathways modulates TNF-alpha-mediated effects on CAM expression. We hypothesized that TNF-alpha would increase SCLC attachment to ECs by affecting CAM expression through activation of PKC and TK pathways. To test this hypothesis, human umbilical vein endothelial cells (HUVECs) were stimulated with TNF-alpha (0 to 500 U/mL) for variable time periods (1 to 24 hours), and the attachment of H82 cells (an SCLC cell line) to the HUVECs was quantified. TNF-alpha stimulation of the HUVECs increased H82 attachment from 28.1% +/- 1.6% to 48.8% +/- 1.7% (P < .05). Preincubation of HUVECs with the PKC inhibitors bis-indolylmaleimide (BIN) or calphostin C or the TK inhibitors genistein or herbimycin A (HMA) blocked the TNF-alpha-induced increase in H82 cell attachment. The addition of antibodies to vitronectin (Vn) or beta1-integrin to TNF-alpha-activated HUVECs before the addition of the H82 cells also significantly decreased H82 attachment, whereas the addition of antibodies to E-selectin, P-selectin, vascular cell adhesion molecule (VCAM), intercellular adhesion molecule (ICAM), neural cell adhesion molecule (NCAM), sialyl-Lewis(x), fibronectin (Fn), alpha(v)-integrin, alpha3-integrin, alpha4-integrin, or alpha5-integrin had no effect on SCLC attachment. In summary, the TNF-alpha-mediated increase in SCLC attachment to ECs appears to be mediated by the activation of EC PKC and TK pathways as well as through effects on the function or expression of EC Vn and beta1 integrin.

Design of a new class of orally active fibrinogen receptor antagonists

The integrin receptor recognition sequence Arg-Gly-Asp was successfully used as a template from which to develop a series of potent, selective, orally active, peptide-based fibrinogen receptor antagonists with a long duration of action. Simple modifications centered on the Arg and Gly residues quickly led to a modified peptide (1) with significantly enhanced ability to inhibit in vitro platelet aggregation. Substitution of the guanidino group in 1 by piperidine provided 3, which showed not only a further increase in potency but also a modest degree of oral efficacy. Finally, exploration of the nature of the C-terminal amino acid, with respect to its side-chain functionality and the carboxy terminus, yielded a group of molecules that showed excellent in vitro potency for inhibiting platelet aggregation, excellent integrin selectivity, a high level of oral efficacy, and an extended duration of action.

Regulation of endothelial cell and platelet receptor-ligand binding by the 12- and 15-lipoxygenase monohydroxides, 12-, 15-HETE and 13-HODE

In previous studies, we reported that vascular wall cells such as endothelial cells metabolize linoleic acid to 13-hydroxyoctadecadienoic acid (13-HODE) via the 15-lipoxygenase pathway. Endothelial cell 13-HODE levels vary inversely with endothelial cell reactivity to platelets, which, in turn, varies directly with the expression of the vitronectin receptor (VnR) on the apical surface of endothelial cells. We and others have also found that tumour cell adhesivity is dependent, in part, upon the relative amounts of intracellular 13-HODE and the arachidonic acid monohydroxide(s), 12- and/or 15-hydroxyeicosatetraenoic acids (12-, 15-HETE). In addition, we and others have found that platelet adhesivity is dependent upon the intraplatelet level of its major lipoxygenase metabolite, 12-HETE. Finally, we have demonstrated that 13-HODE and VnR co-localize in nonadhesive endothelial cells but dissociate following endothelial cell injury, at which time, the VnR relocates on the endothelial cell apical surface. These data suggest to us that lipoxygenase-derived monohydroxides regulate the ability of various receptors to recognize their specific ligands. The latter data also suggest that these monohydroxides act directly by a physiochemical mechanism. The present study supports this possibility. Thus, we demonstrate that 13-HODE downregulates VnR binding with vitronectin (Vn) > fibronectin (Fn) > fibrinogen (Fgn), whereas 12- and 15-HETE upregulate specific VnR/ligand binding, using purified VnR/liposomes and purified ligands in an adhesion assay; and that 12- and 15-HETE upregulate GPIIb/IIIa:liposome binding of Fgn > Fn > Vn. We conclude that cell-specific monohydroxides influence cell-specific receptor-ligand binding directly through a physiochemical mechanism.

Recent advances in the immunology of xenotransplantation

The transplantation of tissue and organs between individuals of different species, that is xenotransplantation, engenders a variety of severe immune responses. Xenogeneic immune responses mediated by naturally occurring antibodies and complement lead to hyperacute and acute vascular rejection of vascularized organ grafts and may also cause vascular rejection of cell and tissue grafts. Under some circumstances, however, a vascularized organ graft may evade humoral rejection despite the presence of antidonor antibodies in the circulation of the recipient; this condition is called accommodation. Xenogeneic immune responses mediated by T-lymphocytes and natural killer cells may cause acute cellular rejection. The extent to which cellular rejection of xenografts resembles cellular rejection of allografts remains to be determined. New insights into the molecular mechanisms underlying the immune responses to xenotransplantation have shed new light on the pathogenesis of immunological disease and have allowed the development of specific immunomodulatory strategies that may facilitate clinical application of xenotransplantation.

Embryonic angiogenesis: a review

Supply with nutrients is essential from early embryonic stages onwards. Therefore, circulatory organs form the first functioning organ system. With the exception of the heart, this system is at first formed by only one cell type, the endothelial cell. Emergence, behavior, and differentiation of endothelial cells are discussed in this review. At first, endothelial cells develop from angioblasts (primary angiogenesis/angioblastic development), later they develop from preexisting endothelial cells (secondary angiogenesis/angiotrophic growth). The composition of the extracellular matrix may promote or inhibit angiogenesis. Various growth factors which can be bound to the extracellular matrix may have been found, but only two of them (VEGF, P1GF) seem to influence endothelial cell behavior directly. Heterogeneity and organ-typical differentiation of endothelial cells seem to be dependent on cell-cell signaling within each organ.

A perspective on xenograft rejection and accommodation

There is increasing interest in the potential clinical application of xenotransplantation. This interest derives in part from the need to identify a more abundant source of organs for transplantation and in part from rapid progress in understanding the cellular and molecular events that contribute to xenograft rejection. Recent areas of progress include the characterization of xenoreactive antibodies which would initiate the rejection of porcine organs transplanted into primates. These antibodies consist predominantly of IgM and their binding is characterized by high avidity and surprising uniformity. Xenoreactive antibodies recognize porcine glycoproteins of the integrin family; the determinants residing on N-linked substitutions. The predominant substitution has a terminal alpha Gal residue. Antibody binding initiates activation of complement through the classical pathway triggering a number of effector mechanisms. These mechanisms may include loss of heparan sulfate from endothelial cells mediated by C5a and xenoreactive antibody; a change in endothelial cell shape mediated by C5b-7 or the membrane-attack complex; procoagulant changes mediated by the membrane-attack complex; and neutrophil adhesion mediated by iC3b. If hyperacute rejection is prevented by the depletion of xenoreactive antibody and/or the inhibition of complement, acute vascular rejection may be seen some days later. Acute vascular rejection is characterized by prominent evidence of thrombosis and neutrophil infiltration. The cause of acute vascular rejection is unknown, but may reflect profound alterations in the function of endothelial cells lining blood vessels in the graft. In some cases, when recipients of xenografts are modified by depletion of xenoreactive antibodies, acute vascular rejection does not occur; rather, a process called accommodation allows the xenograft to survive despite the return to the circulation of xenoreactive antibodies and complement. The mechanism for accommodation is not known. New therapeutic strategies including the development of specific immunoabsorbants, identification of preferred donor animals expressing low levels of antigen and the development of transgenic donor animals expressing human complement regulatory proteins are among the strategies which may bring xenotransplantation closer to the clinical arena.

Adhesion molecules and their role in cancer metastasis

This article describes various adhesion molecules and reviews evidence to support a mechanistic role for adhesion molecules in the process of cancer metastasis. A variety of evidence supports the involvement of specific adhesion molecules in metastasis. 1. For example, some cancer cells metastasize to specific organs, irrespective of the first organ encountered by the circulating cancer cells. This ability to colonize a specific organ has been correlated with the preferential adhesion of the cancer cells to endothelial cells derived from the target organ. This suggests that cancer cell/endothelial cell adhesion is involved in cancer cell metastasis and that adhesion molecules are expressed on the endothelium in an organ-specific manner. 2. Further, inclusion of peptides that inhibit cell adhesion, such as the YIGSR- or RGD-containing peptides, is capable of inhibiting experimental metastasis. 3. Metastasis can be enhanced by acute or chronic inflammation of target vessels, or by treatment of animals with inflammatory cytokines, such as interleukin-1. In vitro, cancer cell/endothelial cell adhesion can be enhanced by pretreating the endothelial cell monolayer with cytokines, such as interleukin-1 or tumor necrosis factor-alpha. This suggests that, in addition to organ-specific adhesion molecules, a population of inducible endothelial adhesion molecules is involved and is relevant to metastasis. 4. Further support for this model is found in the comparison to leukocyte/endothelial adhesion during leukocyte trafficking. Convincing evidence exists, both in vivo and in vitro, to demonstrate an absolute requirement for leukocyte/endothelial adhesion before leukocyte extravasation can occur. The relevance of this comparison to metastasis is reinforced by the observation that some of the adhesion molecules involved in leukocyte/endothelial adhesion are also implicated in cancer cell/endothelial adhesion. The involvement of adhesion molecules suggests a potential therapy for metastasis based on interrupting adhesive interactions that would augment other treatments for primary tumors.

Identification of a role of the vitronectin receptor and protein kinase C in the induction of endothelial cell vascular formation

When cultured on a basement membrane substratum, endothelial cells undergo a rapid series of morphological and functional changes which result in the formation of histotypic tube-like structures, a process which mimics in vivo angiogenesis. Since this process is probably dependent on several cell adhesion and cell signaling phenomena, we examined the roles of integrins and protein kinase C in endothelial cell cord formation. Polyclonal antisera directed against the entire vitronectin (alpha v beta 3) and fibronectin (alpha 5 beta 1) receptors inhibited cord formation. Subunit-specific monoclonal antibodies to alpha v, beta 3, and beta 1 integrin subunits inhibited cord formation, while monoclonal antibodies to alpha 5 did not, which implicated the vitronectin receptor, and not the fibronectin receptor, in vascular formation. Protein kinase C inhibitors inhibited cord formation, while phorbol 12-myristate 13-acetate (PMA) caused endothelial cells to form longer cords. Since the vitronectin receptor has been shown to be phosphorylated in an in vitro system by protein kinase C, the possible functional link between the vitronectin receptor and protein kinase C during cellular morphogenesis was examined. The vitronectin receptor was more highly phosphorylated in cord-forming endothelial cells on basement membrane than in monolayer cells on vitronectin. Furthermore, this phosphorylation was inhibited by protein kinase C inhibitors, and PMA was required to induce vitronectin receptor phosphorylation in endothelial cells cultured on vitronectin. Colocalization studies were also performed using antisera to the vitronectin receptor and antibodies to protein kinase C. Although no strict colocalization was found, protein kinase C was localized in the cytoskeleton of endothelial cells initially plated on basement membrane or on vitronectin, and it translocated to the plasma membrane of C-shaped cord-forming cells on basement membrane. Thus, both the vitronectin receptor and protein kinase C play a role in in vitro cord formation.

Functional and morphological changes induced by tunicamycin in dividing and confluent endothelial cells

Cultured bovine aortic endothelial cells treated with tunicamycin, an inhibitor of glycoprotein synthesis, developed a concentration-dependent inhibition of N-acetylglucosamine-1-phosphate transferase activity, and this inhibition was correlated with a substantial decrease in [3H]mannose incorporation by the cells. Endothelial cells were very sensitive to tunicamycin, and changes in their morphology occurred as a result of the inhibition of glycoprotein synthesis. The cells became elongated, the surface irregular, roughened, and granular, and there was an increase in the interstitial space between the cells. Electron dense material was accumulated within and dilated the rough endoplasmic reticulum, and the distribution of the glycoproteins laminin and fibronectin throughout the endothelial cell monolayer was modified. These morphological changes coincided with functional impairment with the permeability of endothelial cell monolayers to both 125I-albumin and [3H]inulin being increased by treatment with tunicamycin (10(-6) M) for 24 h. These results indicate that the synthesis of glycoproteins is crucial for cell-cell adhesion and the functional properties of the endothelial lining of blood vessels.