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Stabilization and Reactivation of the p53 Tumor Suppressor Protein in Nontumorigenic Revertants of HeLa Cervical Cancer Cells¹

Program in Cancer Biology, Division of Public Health, Fred Hutchinson Cancer Research Center, Seattle, Washington 98104-2092 [M. A., L. J., B. H., H. Z., A. M. M.], and Division of Toxicology and Center for Environmental Health Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 [M. A., Y. H., B. H., H. Z., A. M. M.]

Abstract

We demonstrated previously that loss of in vitro transformation and in vivo tumorigenicity in two independent revertant clones of HeLa cells (designated HA and HF) resulted from dominant-acting genetic changes. Analysis of the p53 tumor suppressor gene revealed stabilization and at least partial restoration of wild-type p53 transactivation properties pathways in both revertants of HPV-induced cell transformation. The half-lives of the p53 protein and both of the HA and HF clones were increased 4 fold compared with the parental HeLa cells (16, 17, and 4 min, respectively). The levels of E6 viral protein expression were similar in the three cell lines, whereas the levels of the ubiquitin ligase protein, E6 associated protein (E6-AP), were elevated in the revertants. Western blot analysis of immunoaffinity-purified p53 demonstrated that stabilization of p53 in the revertants was correlated with a reduction in the in vivo formation of complexes involving the E6 oncoprotein and p53. Stabilization of p53 function in the revertants did not result from mutations in either the p53 or E6-AP genes. Despite the observed stabilization and restoration of p53 transactivation function in the revertants, exposure of the revertants to DNA-damaging agents did not result in elevated levels of p21^waf-1 protein and failed to induce growth arrest in the G₁ phase of the cell cycle. However, p53-independent induction of p21^waf-1 protein also failed to induce the G₁ phase of the cell cycle. Thus, restoration of wild-type p53 transactivation activity in the HA and HF revertants is insufficient to induce G₁ arrest and reversion from HPV-induced cell transformation in our model system.

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