Anchorage-independent growth of p53-knockout dermal fibroblasts is reversed by wild-type p53

Background: p53 is a 393-residue nuclear phosphoprotein. Mutation of p53 occurs in over half of all human cancers and thus is a crucial step in the process of cell transformation and tumorigenesis. Since tumorigenesis is a multistep process, it generally requires the mutation of certain key oncogenes and/or tumor-suppressor genes. Using p53-deficient mice, we can investigate the p53-dependent mechanisms leading to tumorigenesis. Objective: To examine the unique anchorage-independent growth characteristics of dermal Fibroblasts isolated from p53-deficient mice. Methods: The growth characteristics of highly confluent cultured dermal Fibroblasts from wild-type (p53(-/-)) and p53-deficient (p53(-/-)) mice were compared by DNA fragmentation assay, colony formation in soft agar, and overexpression of a wild-type p53 transgene in p53-deficient cells. Results: p53(-/-) fibroblasts have a growth rate dramatically higher than p53(+/+) cells and detach from plastic cultureware at high density. The detachment of p53(-/-) cells is not due to apoptosis. Furthermore, these cells have the capacity to grow in soft agar-a hallmark of cell trans formation-and this anchorage-independent growth can be reversed by the introduction of a wild-type p53 transgene. Conclusion: Dermal fibroblasts isolated from p53-deficient mice show anchorage-independent growth. Therefore, the absence of p53 is sufficient for the initiation of cell transformation in this cell type and establishes this model system as an excellent tool to dissect the molecular steps involved in oncogenesis.