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 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 serum-stimulated 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.
Fonagy
A
A
Department of Surgery, Lucille Parker Markey Cancer Center, University of Kentucky Chandler Medical Center, Lexington 40536, USA.
Swiderski
C
C
Freeman
J W
JW
D002453:Cell Cycle; D002460:Cell Line; D004249:DNA Damage; D015870:Gene Expression; D006657:Histones / Q000235:genetics; D006801:Humans; D066298:In Vitro Techniques; D009687:Nuclear Proteins / Q000235:genetics*; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic; D012359:tRNA Methyltransferases
D006657:Histones; D009687:Nuclear Proteins; D012333:RNA, Messenger; C065028:NOP2 protein, human; D012359:tRNA Methyltransferases