The Human Integrin α8β1 Functions as a Receptor for Tenascin, Fibronectin, and Vitronectin

Abstract

The integrin family of adhesion receptors consists of at least 21 heterodimeric transmembrane proteins that differ in their tissue distribution and ligand specificity. The recently identified ␣8 integrin subunit associates with ␤1 and is predominantly expressed in smooth muscle and other contractile cells in adult tissues, and in mesenchymal and neural cells during development. We now show that ␣8␤1 specifically localizes to focal contacts in cells plated on the extracellular matrix proteins fibronectin or vitronectin. In addition we show that human embryonic kidney cells (293), transfected with ␣8 cDNA, express ␣8␤1 on their surface and use this receptor for adhesion to fibronectin and vitronectin. Furthermore, ␣8␤1 binds to both fibronectin-and vitronectinSepharose and can be specifically eluted from either matrix protein by the arginine-glycine-aspartic acid (RGD)-containing peptide, GRGDSP. Because fibronectin and vitronectin adhesion appeared to be mediated by RGD, we examined additional RGD-containing proteins, including tenascin, fibrinogen, thrombospondin, osteopontin, and denatured collagen type I. We found that only tenascin was able to mediate adhesion of ␣8-transfected 293 cells. By using recombinant fragments of tenascin in adhesion assays, we were able to localize the ␣8␤1 binding domain of tenascin to the RGD-containing, third fibronectin type III repeat. These data strongly suggest that tenascin, fibronectin, and vitronectin are ligands for ␣8␤1 and that this integrin binds to the RGD site in each of these ligands through mechanisms that are distinct and separate from ␣5-and ␣v-containing integrins.Integrins are a class of cell adhesion glycoproteins composed of two noncovalently associated subunits, ␣ and ␤. Each subunit contains a large extracellular domain, a transmembrane domain, and a short cytoplasmic domain. Integrins are known to bind to a wide variety of extracellular matrix proteins, including fibronectin, vitronectin, collagens, and laminins. The specificity of protein binding is determined by particular combinations of ␣ and ␤ subunit pairing. The ligand binding site is formed by the extracellular domain of both subunits and requires the presence of divalent cations. Many integrins interact with ligands through the tripeptide arginine-glycine-aspartic acid (RGD).The ␣8 integrin subunit was originally identified by Bossy et al.(1) in the chick embryo nervous system and was shown to be a partner for ␤1. We have identified human ␣8, cloned and sequenced the cDNA, raised antibodies to the predicted cytoplasmic domain sequence, and determined its distribution in adult mammalian tissues (2). We found that ␣8 is predominantly expressed in a variety of visceral and vascular smooth muscle cells, kidney mesangial cells, and lung myofibroblasts (2).To gain insight into potential functions of ␣8␤1 in vivo, we sought to determine potential ligands. We tested various ligands for their ability to direct ␣8␤1 to focal contacts, to bind to ␣8␤1 by affinity chromatography, and to support adhesion of ␣8...

The Integrin α9β1 Binds to a Novel Recognition Sequence (SVVYGLR) in the Thrombin-cleaved Amino-terminal Fragment of Osteopontin

“…Since VCAM-1 does not contain an RGD, the attachment of ␣ 9 -transfected CHO cells and SW480 cells to recombinant VCAM-1 (8) is definitely RGD-independent. ␣ 9 ␤ 1 mediates adhesion to the third fibronectin type III repeat in tenascin-C, a repeat that does contain an RGD site that serves as a ligand binding site for the integrins ␣ v ␤ 3 (31), ␣ v ␤ 6 (31,32), and ␣ 8 ␤ 1 (33). However, the ␣ 9 -transfected SW480 cells we used to study interaction with tenascin-C do not express any of these integrins.…”

Radial migration of developing microglial cells in quail retina: A confocal microscopy study

“…However, microglial cells that ramify at L1 can then resume their radial migration to reach L2 (interface between sublayers within the IPL) and L3 (IPL-INL interface) by traversing the tenascinrich territory of the IPL. This would be explained by the presence in the tenascin molecule of domains with opposite functions, adhesive and antiadhesive Norenberg et al, 1995;Faissner, 1997;Ghert et al, 2001), and by the ability of this molecule to bind to different receptors (Schnapp et al, 1995;Mackie, 1997;Zacharias et al, 1999;Pesheva and Probstmeier, 2000). Therefore, tenascin is able to exert both inhibitory and stimulatory effects on cell migration (Faissner, 1997).…”

Expression of αv integrins and vitronectin receptor identity in breast cancer cells

“…The ,1 integrin in PI-positive focal adhesions on vitronectin is probably av,Bl, although the absence of a heterodimer-specific reagent for this integrin rules out definitive confirmation. However, apart from avfl, the only other cxJl integrin reported to bind vitronectin is a81I (Schnapp et al, 1995). Antibodies to a8 were not available for this study.…”