Ability of oncogenically transformed cells to grow without anchorage correlates with phosphorylation of a group of cell surface membrane proteins.

Anchorage-independent growth in vitro is strongly correlated with cellular malignancy in vivo and it has been shown that retinoic acid (RA; a vitamin A analog) inhibits anchorage-independent growth of a wide variety of oncogenically transformed cells (RA-sensitive cells). We report here that decreased or lack of phosphorylation of a group of low molecular weight (20-30 kD) cell surface membrane proteins, particularly one of Mr 28 kD, correlates strongly with RA-induced loss of anchorage-independent growth of RA-sensitive cells. Our studies also show that this group of proteins are not phosphorylated in non-transformed cells which do not grow in an anchorage-independent manner. Analysis of [35S]methionine-labeled proteins revealed that these polypeptides are present in both RA-treated and untreated cell surface membranes. This suggests that modulation of phosphorylation rather than lack of synthesis of these proteins is correlated with anchorage regulation of cells. V8 protease mapping of the 28 kD phosphoprotein from transformed cells, irrespective of their origin or of transforming agents, revealed complete fragment homology. Furthermore, the 28 kD phosphoprotein was found to be phosphorylated exclusively at threonine residues. The data obtained from this study suggest that the ability of cells to grow without anchorage is correlated with the phosphorylation of a group of cell surface membrane proteins and RA inhibits anchorage-independent growth by interfering with the phosphorylation rather than synthesis of these proteins.