Vitronectin enhances internalization of crocidolite asbestos by rabbit pleural mesothelial cells via the integrin αvβ5

Abstract

The mechanism by which pleural mesothelial cells, the likely progenitor cells of asbestos-induced mesothelioma, recognize and internalize crocidolite asbestos is unknown. Because incubation of asbestos fibers with serum increases their association with cells, we asked whether a protein coat on asbestos increased internalization of fibers via specific cellular receptors. Coating crocidolite with vitronectin, but not with fibronectin or other proteins, increased fiber internalization by rabbit pleural mesothelial cells, as measured by a new technique using fluorescence confocal microscopy. Receptors for vitronectin, αvβ3 and αvβ5, were identified on mesothelial cells. Inhibiting vitronectin receptors by plating cells on a vitronectin substrate or incubating cells with excess soluble vitronectin reduced internalization of vitronectin-coated crocidolite. Inhibition of αvβ5, but not αvβ3, with blocking antibodies similarly reduced internalization. In addition, αvβ5, but not αvβ3, showed immunocytochemical colocalization with fibers. Of biologic relevance, coating crocidolite with serum also increased internalization via αvβ5, an effect dependent on the vitronectin in serum. We conclude that pleural mesothelial cells recognize and internalize vitronectin- and serum-coated asbestos via the integrin αvβ5. Since integrins initiate some of the same signaling pathways as does asbestos, our findings may provide insights into the mechanisms of asbestos- induced biologic effects.