Dependence of induction of osteocalcin gene expression on the presence of wild‐type p53 in a murine osteosarcoma cell line

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Abstract
The p53 gene undergoes rearrangement in a high percentage of osteosarcomas, resulting in loss of its expression. A p53‐null murine osteosarcoma cell line F6 was transfected with either a wild‐type or a mutant p53 gene. Stably transfected cell lines were obtained, and their differentiation capabilities were compared in vitro with the parental cell line. Alkaline phosphatase and osteocalcin expression were measured as early and late differentiation markers, respectively. Induction of alkaline phosphatase expression was not affected by the presence of either p53 gene, whereas osteocalcin expression was seen in cells containing the wild‐type p53 gene but not in the parental p53‐null or mutant‐expressing cell lines. That the induction of osteocalcin was intrinsically dependent on the presence of wild‐type p53 was also indicated by the use of a temperature‐sensitive Val 135 p53 mutant at 32°C; predominant expression of p53 in the wild‐type conformation resulted in osteocalcin expression. While the wild‐type p53 gene could suppress tumor formation in vivo, the tumors expressing the mutant p53 gene grew two to three times as large as the tumors that did not express p53. Therefore, the absence of end‐point differentiation in bone due to p53 rearrangements may contribute to the maintenance of the tumorigenic phenotype in osteosarcomas.