Phosphorylation of the fibronectin receptor complex in cells transformed by oncogenes that encode tyrosine kinases.

Abstract

The fibronectin (FN) receptor in avian cells has been characterized previously as a complex of three membrane glycoproteins of about Mr 160,000, Mr 140,000, and Mr 120,000 (simply termed protein band 1, band 2, and band 3, respectively). Monoclonal antibodies to the band 3 protein of the complex prevent FN and laminin binding both in vivo and in vitro and enable the detection of the receptor proteins in the plasma membrane and in adhesion plaques. Association of the FN receptor proteins with the adhesion-plaque protein talin also has been reported. We now find that the band 2 and band 3 proteins in the complex are phosphorylated in Rous sarcoma virus-transformed chicken cells but not in normal chicken cells. Phosphorylation occurs predominantly on tyrosine and is accompanied by a reorganization ofthe receptor complex in the membrane of the transformed cells. Whereas normal cells contain the FN receptor in focal contacts and cellular processes between cells, v-src-transformed cells exhibit a more diffuse distribution of this receptor. In addition to the viral v-src oncogene, cells transformed by other avian oncogenes that also encode tyrosine kinases (v-.fs, v-erbB, and v-yes) also express the receptor complex proteins in the phosphorylated state regardless of whether the transforming protein is detectable in adhesion plaques. These results suggest that the altered FN and laminin receptor proteins may contribute to the transformed phenotype, but their significance and role in the transformed state remain to be established.Transformation by Rous sarcoma virus (RSV) is initiated and maintained by the protein product of the viral v-src gene (1, 2). This protein, designated pp60src, possesses an inherent tyrosine kinase activity essential for transformation and is situated along the cytoplasmic face of the plasma membrane and concentrated within cell-substratum adhesion sites (3). These latter sites are termed focal contacts or adhesion plaques and are associated either directly or indirectly with several functions that are altered in RSV-transformed cells. Focal contacts mediate not only the physical associations of cells to each other and to substrata (via extracellular molecules) but also serve as organizing centers for attachment of extracellular matrix protein and intracellular focal points for stress-fiber termination (4-10). These sites are very specialized regions of the plasma membrane, and proteins within these structures are logical targets for such RSV-induced alterations as decreased cellular adhesiveness, loss of cell surface fibronectin (FN), abnormal cell migration, dissolution of stress-fiber bundles, and ultimately the rounded transformed cell morphology.Little is known of the molecular composition and complexity of adhesion plaques; however, several proteins have been identified as constituents of these structures (7,(11)(12)(13)(14)(15)(16).Of these proteins, vinculin has received considerable attention because it was shown to contain increased levels of phosphotyrosine in RSV-transformed...

A mutant form of the rho protein can restore stress fibers and adhesion plaques in v-src transformed fibroblasts

“…Among these are paxillin, talin, subunits of the integrin receptor, the focal adhesion kinase p125 FAK and vinculin (Sefton et al, 1981;Hirst et al, 1986;Pasquale et al, 1986;Turner, 1991;Guan and Shalloway, 1992).…”

Fibronectin receptor overexpression and loss of transformed phenotype in a stable variant of the K562 cell line.

“…The potential role of the FNR in neoplasia was suggested by the findings that the FNR is phosphorylated in v-src transformed cells (Hirst et al, 1986); that transforming agents affect the structure, function, and expression levels of the alpha5betal FNR (Plantefaber and Hynes, 1989;Symington et al, 1989); and that glycine-arginine-glycine-aspartate-serine (GRGDS) peptide or anti-beta1 antibodies can prevent pulmonary metastases of B16 melanoma cells (Humphries et al, 1986). Our interest in the potential role of the FNR in tumor metastasis prompted efforts to generate alpha5betal expression variants of established tumorigenic cell lines.…”

Quantitative switch in integrin expression accompanies differentiation of F9 cells treated with retinoic acid

“…This down-regulation in the level of phosphorylation could play a functional role in the phenotypic conversion of F9 to RA cells, especially if there were differential switching between the two integrins. Hirst et al (1986) also have shown that integrin phosphorylation occurs, raising the possibility that interactions with talin or other cytoplasmic proteins might be modulated by phosphorylation changes to affect function. If this model were correct in the present context, retinoic acid would lead to altered integrin expression on the cell surface but the importance of that alteration, phenotypically, would be the alteration of organization of integrins with the cytoskeleton, setting up a differential ability to form focal contacts.…”