We report here on gene transfer studies designed to investigate the function of the aeD1 integrin and its ‘role in transformation. Transfection of the human a5 and p, cDNAs into transformed Chinese hamster ovary (Cl-IO) cells followed by methotrexate-induced amplification yielded clonal cell lines overexpressing this fibronectin receptor. The overexpressors deposited more fibronectin in their extracellular matrix and migrated less than control cells. In addition, they showed reduced saturation density and educed ability to grow in soft agar. The overexpressor cells, unlike the control CHO cells, were nontumorigenic when injected subcutaneously into nude mice. The results indicate that extracellular matrix recognition by the a& integrin plays a role in the control of cell proliferation and suggest that a reduction of this fibronectin receptor may be responsible for the acquisition of anchorage independence by transformed cells. tered upon transformation. Thus, the level of some integrins that include a& is reduced in transformed cells (Plantefaber and Hynes, 1989). Moreover, the cytoplasmic domain of the 0, subunit has been found to be phosphorylated in cells transformed by tyrosine kinase oncogenes (Hirst et al., 1988), and there is evidence suggesting that tyrosine phosphorylation might reduce cytoskeletal association and fibronectin binding by integrins (Tapley et al., 1989). Finally, the PI integrins are diffusely distributed at the surface of transformed cells, rather than being concentrated at extracellular matrix contact sites as they are in normal cells (Giancotti et al., 1988a; Chen et al., 1988; Roman et al., 1989). This latter finding in particular suggests a connection between integrins and the defective extracellular matrix and cytoskeleton of transformed cells.

To study the significance of the a& integrin in transformed cells, we have overexpressed this integrin from cDNA in a Chinese hamster ovary (CHO) cell line. The CHO cells show a transformed morphology (Hsie and Puck, 1971; Leader et al., 1983), deposit little fibronectin into their extracellular matrix (Quade and McDonald, 1988), and are tumorigenic in vivo (Esko et al., 1988). As a result of the overexpression of the a& receptor, the CHO cells accumulated a fibronectin matrix, became less migratory, reacquired features of normal growth control in culture, and lost the ability to form tumors when injected subcutaneously into nude mice. Introduction