Radioimmunoassay of the cellular protein p53 in mouse and human cell lines

We have developed quantitative radioimmunological solid phase assays for the host protein p53 from mouse cells and from human cells. The first assay, for mouse p53, depends on having two monoclonal antibodies reacting with different determinants on the p53 molecule. With this assay we have shown that SV40-transformed cells have approximately 100-fold more p53 than untransformed mouse cells and that other transformed cells have intermediate levels. Embryonal carcinoma cell lines have approximately 50-fold less p53 than SV40-transformed cells. This is in contrast to the high levels of incorporation of [35S]methionine into p53 in these cells and indicates that metabolic labelling is not a valid approach for measuring p53 levels. The second assay, for human p53, required a different approach and made use of the anti-p53 antibodies detected in the sera of some breast cancer patients. Human tumour cell lines contained amounts of p53 varying from the high level seen in SV40-transformed human fibroblasts down to less than one hundredth of this amount. Normal human cells showed low levels of p53. The data confirm that many, but not all, human tumour cell lines contain more p53 than normal cells.