To determine the role of p53 protein on the cellular effects of amifostine, we used molecularly engineered HCT116 colon cancer cells in which the p53 gene was inactivated by targeted homologous recombination or p53 protein was degraded by high-level expression of papillomavirus E6 protein. Amifostine induced a G1 arrest and protected against paclitaxel toxicity in p53-proficient but not in p53-deficient cells. In the absence of p53 protein, amifostine enhanced the cytotoxicity of paclitaxel. In addition, treatment of HCT116 cells with amifostine alone resulted in apoptotic cell death. Compared with p53-deficient cells, p53-proficient cells exhibited low-level resistance to amifostine-induced apoptosis. Amifostine induced the expression of p53 protein in p53-proficient cells and the expression of p21 protein in both p53-proficient and -deficient cells. These findings indicate that amifostine-induced G1 arrest and cytoprotection are mediated via a pathway that is dependent on p53 protein and that amifostine-induced expression of p21 protein is not sufficient to sustain a G1 arrest or to mediate cytoprotection. In addition, these findings identify p53 protein as a mechanism of resistance to amifostine-induced apoptosis.

To study how the oncogenic process may involve effects on differentiation, we overexpressed an immortalizing oncogene in a developing tissue in transgenic mice. By use of a gene fusion of the aA-crystallin promoter to the viral immortalizing oncogene, polyoma large T antigen (PyLT), we created transgenic mice that express PyLT specifically in ocular lens. Expression of large T antigen during embryonic development led to a perturbation in lens development, specifically, an interference with the normal program of fiber cell differentiation. This resulted in microphthalmia, which persisted throughout the life of the animal. Histological analysis revealed impairment of cell elongation, denucleation, and mitotic senescence in both primary and secondary fiber cell differentiation. Strikingly, there was no evidence for hyperplasia or for tumor development in vivo, unlike the consequences of many immortalizing oncogenes on tissues in other transgenic mice. In vitro, however, the developmentally perturbed cells derived from the transgenic lens showed high proliferative capacity. Our results suggest that a primary effect of aberrant expression of an immortalizing gene is an interference with normal tissue development; however, this interference may not necessarily induce proliferation or lead to tumor formation.