Nucleolar p120 is expressed as a delayed early response gene and is inducible by DNA‐damaging agents

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Abstract
Regulation of the expression of the growth-related nucleolar p120 protein was examined in serum-deprived and stimulated nontransformed and SV40-transformed WI-38 human fibroblasts. In quiescent cells, transcriptional activity of the p120 gene was very low or undetectable, and the steady-state levels of the p120 mRNA and the p120 protein were also negligible. The transient expression of the p120 gene in the cell cycle was detected in middle G1-phase after the expression of the early response genes and before the expression of the DNA-synthesis genes. Protein synthesis was required for the induction of p120 expression in serumstimulated cells. The increased level of p120 mRNA in middle G1-phase was attributed to an increased transcription rate of the p120 gene, and not to a change in p120 mRNA stability. The calculated half-life of p120 mRNA was unchanged (1.8 ± 0.2 hr) in all four cell conditions tested; i.e., in middle G1- or S-phase cells and in exponentially growing normal or transformed cells. Transcription rate of the p120 gene was correlated with the steady-state levels of either p120 protein or p120 mRNA. A sharp increase in p120 mRNA level occurred in both normal and transformed cells treated with actinomycin D used to examine p120 mRNA stability. This induction of p120 mRNA expression was seen in early G1-phase, but not in quiescent cells, or in middle to late G1-phase when cells expressed the highest level of p120 mRNA. The same expression pattern was seen by treatment with chlorambucil, another DNA-damaging agent. The conclusions of these studies are that the expression of p120 (1) is serum inducible in a fashion characteristic of the delayed early response gene products, (2) requires the presence of newly synthesized proteins, (3) is regulated transcriptionally, and (4) can be induced by DNA-damaging agents.