Enhanced protein phosphorylation of carcinogen-initiated 10T1/2 cells accompanies their neoplastic transformation

Retinyl acetate (RAC) prevents neoplastic transformation of carcinogen-exposed C3H/10T1/2 C18 cells, and has allowed the isolation of cells having the properties expected of initiated cells. After removal of RAC from logarithmically growing initiated cells, cultures first become confluent and growth arrested, as occurs with their normal counterparts. This is followed by an increase in thymidine labelling index and finally by morphological transformation on days 15 and 23, respectively, after seeding and drug removal. The increase in labelling index is the earliest indication yet seen of transformation taking place in these cultures. Phosphorylation of three nonionic detergent-soluble proteins was observed to be correlated with the increase in labelling index. Using two-dimensional gel electrophoresis, a protein of 35 kd (pp35(1)) and one of 38 kd (pp38) were seen to become more heavily phosphorylated in association with the increase in labelling index. A second protein of 35 kd (pp35(2)) was only detected in isolated transformed cells and is phosphorylated in an alkali-resistant manner consistent with phosphorylation of tyrosine residues. Alkali-resistant phosphorylation of pp35(2)) could be eliminated in transformed cells by treatment with RAC. Phosphorylation of pp35(1)) appears to be a permanently acquired characteristic of these cells, and cannot be made to revert. Phosphorylation of pp38, as indicated by isoelectric point, is reduced by RAC treatment. These studies implicate changes in protein phosphorylation in loss of growth control, and suggest that the cancer chemopreventive action of retinoids may also be mediated at this level.