NUCLEOLAR P120 IS EXPRESSED AS A DELAYED EARLY RESPONSE GENE AND IS INDUCIBLE BY DNA-DAMAGING AGENTS

Regulation of the expression of the growth-related nucleolar p 120 protein was examined in serum-deprived and stimulated nontransformed and SV40-transformed WI-38 human fibroblasts. In quiescent cells, transcriptional activity of the p 120 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 G(1)-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 serum-stimulated cells. The increased level of p120 mRNA in middle G(1)-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 G(1)- 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 G(1)-phase, but not in quiescent cells, or in middle to late G(1)-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. (C) 1995 Wiley-Liss, Inc.