Growth regulation of a cellular tumour antigen, p53, in nontransformed cells

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Abstract
Many transformed cells in culture have been found to express elevated levels of a cellular tumour antigen, termed p531–9. This protein has also been implicated in the regulation of cellular growth10–12. For these reasons experiments were designed to examine the expression of p53 as quiescent cultures of nontrans formed 3T3 fibroblasts were stimulated to reenter the cell cycle. Synchronous populations of cells were obtained by releasing a culture from density-dependent inhibition of growth with the addition of fresh serum13. Steady-state levels of p53 protein and mRNA were measured as a function of time after addition of serum to quiescent cultures and the rate of synthesis of p53 protein was analysed at a number of time points. The results, reported here, demonstrate an increase in the synthesis and steady-state levels of p53 protein and mRNA prior to DNA synthesis in late G1 and suggest a role for p53 in the progression of cells from a growth-arrested state to an actively dividing state.