Fibroblast-conditioned medium contains cell surface proteins required for cell attachment and spreading

PROLIFERATION in diploid fibroblasts is limited to cells that are attached and spread on a substrate. The attachment and spreading activities of normal cells are enhanced in the presence of media conditioned by the growth of diploid fibroblasts 1,2 , and a cell surface protein (LETS/CSP) 3 extracted from normal cells enhances the attachment of transformed cells 4,5 . We report here that the activities present in conditioned medium are due to several high molecular weight glycoproteins (‘released proteins’). In addition, we find that the same activities can be extracted (‘extracted proteins’) from the surfaces of whole cells. The results establish that growth related biological activities are transferred intercellularly through the release and absorption of cell surface components.