The phosphoprotein p53 is down-regulated post-transcriptionally during embryogenesis in vertebrates

Antisense RNA and p53 regulation in induced murine cell differentiation

p53 expression is strongly modulated during the process of induced differentiation, at the same time as both cell cycle and genetic expression become modulated, giving rise to a commitment to terminal differentiation. We took advantage of two murine cell lines inducible for differentiation, an erythroleukemia and a melanoma cell line, to outline common features of the regulation of p53 expression during the differentiation process. We found that p53 mRNA decreased early after induced differentiation and that regulation was controlled at a posttranscriptional level. Our data showed that this regulation affects p53 pre-mRNA maturation. Because, in both systems used, actinomycin D treatment abolished the inducer-mediated decrease of p53 mRNA, we looked for induced RNAs potentially involved in this process. Using different parts of the p53 gene and flanking regions as probes, we identified three RNA species whose expression is modulated during induced differentiation. A first species is made of high molecular weight RNAs that accumulate in the nuclear compartment and seem to represent antisense transcripts of the p53 gene. A second species, 1.3-kb long, was found to accumulate in the nucleus of induced MEL cells and was homologous to a restricted part of the first intron of the p53 gene due to the presence of a B1 repetitive element in an antisense orientation with respect to the p53 pre-messenger RNA. Finally, a family of B2-containing RNAs was observed in both cytoplasmic and nuclear compartments. The variation in the amounts of sense and antisense RNAs, respectively, suggested an interesting speculative model for the maturation of B2-containing pre-messenger RNAs.

Elevated p53 RNA expression correlates with incomplete osteogenic differentiation of radiation-induced murine osteosarcomas

An important role for the p53 gene in osteogenic sarcomas has been imputed by identification of somatically acquired gene alterations in human osteosarcomas and by the development of osteosarcomas in p53 transgenic mice. To study the involvement of p53 in radiation-induced osteosarcomagenesis, we have investigated gene alterations and expression of p53 in radiation-induced murine osteosarcomas and tumor-derived cell lines. Eighteen of 31 tumors and 8 of 9 cells lines showed alterations in the p53 gene region, or elevated levels of p53 RNA. Expression of the osteoblast marker gene bone gla protein was substantially reduced in tumors which simultaneously showed high steady-state levels of p53 RNA. Our data indicate that p53, in addition to its function in regulating DNA synthesis, may be involved in the control of osteogenic differentiation in osteosarcomagenesis.