Integrins are αβ heterodimeric receptors that mediate divalent cation-dependent cell-cell and cell-matrix adhesion through tightly regulated interactions with ligands. We have solved the crystal structure of the extracellular portion of integrin αVβ3 at 3.1 Å resolution. Its 12 domains assemble into an ovoid “head” and two “tails.” In the crystal, αVβ3 is severely bent at a defined region in its tails, reflecting an unusual flexibility that may be linked to integrin regulation. The main inter-subunit interface lies within the head, between a seven-bladed β-propeller from αV and an A domain from β3, and bears a striking resemblance to the Gα/Gβ interface in G proteins. A metal ion–dependent adhesion site (MIDAS) in the βA domain is positioned to participate in a ligand-binding interface formed of loops from the propeller and βA domains. MIDAS lies adjacent to a calcium-binding site with a potential regulatory function.

Rheumatoid arthritis (RA) is an inflammatory disease associated with intense angiogenesis and vascular expression of integrin αvβ3. Intra-articular administration of a cyclic peptide antagonist of integrin αvβ3 to rabbits with antigen-induced arthritis early in disease resulted in inhibition of synovial angiogenesis and reduced synovial cell infiltrate, pannus formation, and cartilage erosions. These effects were not associated with lymphopenia or impairment of leukocyte function. Furthermore, when administered in chronic, preexisting disease, the αvβ3 antagonist effectively diminished arthritis severity and was associated with a quantitative increase in apoptosis of the angiogenic blood vessels. Therefore, angiogenesis appears to be a central factor in the initiation and persistence of arthritic disease, and antagonists of integrin αvβ3 may represent a novel therapeutic strategy for RA.

Brain metastases are generally considered to be well demarcated from the surrounding brain parenchyma, although infiltrative growth patterns have been observed. We systemically investigated infiltration patterns and expression of adhesion molecules in a large and well-defined series of autopsy cases of brain metastases. Ninety-seven autopsy specimens from 57 brain metastasis patients (primary tumor: 27 lung cancer, 6 breast cancer, 8 melanoma, 2 colorectal cancer, 1 kidney cancer, and 13 other) were evaluated for patterns of invasion into surrounding brain parenchyma. Expression of integrins αv; cytoplasmic β3, αvβ3, αvβ5, αvβ6, and αvβ8; and of E and N cadherin were evaluated using immunohistochemistry. Three main invasion patterns were seen: well-demarcated growth (29/57, 51%), vascular co-option (10/57, 18%), and diffuse infiltration (18/57, 32%). There was no statistically significant association of invasion pattern with primary tumor type, although vascular co-option was most common in melanoma brain metastases (4/10). Invasion patterns of different brain metastases of the same patient were highly concordant (P < .001, chi-square test). Distance of infiltration from the main tumor mass ranged from 12.5 µm to 450 µm (median 56.2 µm) and was not significantly different between the vascular co-option and the diffuse infiltration groups. Levels of αvβ6 were significantly higher in the well-demarcated group than in the vascular co-option and the diffuse infiltration groups (P = .033, Kruskal-Wallis test). Expression of αvβ5 in tumor cells was higher in brain metastasis lesions previously treated with stereotactic radiosurgery (P = .034, chi-square test). Distinct invasion patterns of brain metastases into the brain parenchyma are not specific for primary tumor types, seem to be influenced by expression of αv integrin complexes, and may help to guide clinical decision-making.

It has previously been shown that the monoclonal antibody anti-Arc-1 dissociates Madin-Darby canine kidney (MDCK) epithelial cells and changes their morphology in vitro (Imhof, B.A., H.P. Vollmers, S.L. Goodman, and W. Birchmeier, 1983, Cell, 35:667-675). In this article we demonstrate that the anti-Arc-1 antibody recognizes an uvomorulin-like molecule on MDCK cells, i.e., it immunoprecipitates an 84-kD protein fragment from a tryptic digest of cell surfaces in the presence of Ca2+ (as does anti-uvomorulin antiserum). Furthermore, anti-uvomorulin antiserum prevents the binding of anti-Arc-1 to MDCK cells. The distribution of the Arc-1 antigen is also quite similar to that of uvomorulin: it is enriched at the cell-cell contacts both of MDCK cells and of cells in various canine tissues. In the intestinal epithelium the antigen could be further localized in the region of the junctional complex. To study the mechanism of action of the dissociating antibody, MDCK cells grown on Nuclepore filters in Boyden chambers were exposed to anti-Arc-1 from either the upper or lower compartment. It could be shown that the antibody interfered with cell adhesion only from the basolateral but not from the apical cell surface. Antibody action was inhibited in the presence of colchicine but not cytochalasin B. Furthermore, cell dissociation was prevented when the cellular cAMP level was raised. These findings indicate that the anti-Arc-1 antibody acts on a target below the tight junctions (possibly on the antigen located in the junctional complex), and they confirm that cytoskeleton and metabolic factors are actively involved in the maintenance of junctional integrity.