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

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 α8 integrin subunit associates with β1 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 α8β1 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 α8 cDNA, express α8β1 on their surface and use this receptor for adhesion to fibronectin and vitronectin. Furthermore, α8β1 binds to both fibronectin- and vitronectin-Sepharose 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 α8-transfected 293 cells. By using recombinant fragments of tenascin in adhesion assays, we were able to localize the α8β1 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 α8β1 and that this integrin binds to the RGD site in each of these ligands through mechanisms that are distinct and separate from α5-and αv-containing integrins. 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 α8 integrin subunit associates with β1 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 α8β1 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 α8 cDNA, express α8β1 on their surface and use this receptor for adhesion to fibronectin and vitronectin. Furthermore, α8β1 binds to both fibronectin- and vitronectin-Sepharose 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 α8-transfected 293 cells. By using recombinant fragments of tenascin in adhesion assays, we were able to localize the α8β1 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 α8β1 and that this integrin binds to the RGD site in each of these ligands through mechanisms that are distinct and separate from α5-and αv-containing integrins.