Abrogation of wild-type p53 mediated growth-inhibition by nuclear exclusion.

We used clone 6 cells (rat embryo fibroblasts transformed by the temperature sensitive mutant p53val135 and an activated H-ras-gene (Michalovitz et al., 1990)), growth arrested at 32 degrees C, as a model to analyse whether and how transformed cells, growth-arrested by an overexpressed wild-type p53, might overcome p53-mediated growth inhibition. When clone 6 cells were kept at 32 degrees C for about 2 weeks, foci of cells appeared which grew temperature-independent. Analysis of individual clones of such cell demonstrated that the ectopically expressed tsp53-gene had not been altered by an additional mutation, but that the tsp53 in these cells at 32 degrees C had lost its ability to upregulate expression of the p53 target genes waf1 and mdm2. This loss of p53-specific transactivation correlated with nuclear exclusion of the tsp53 at 32 degrees C, which was most likely mediated by cytoplasmic retention of the tsp53 protein via short-lived anchor proteins. Cytoplasmic retention of the tsp53 at 32 degrees C was also observed in PC12 pheochromocytoma cells ectopically expressing tsp53val135, there occurring without specific selection. Also in these cells nuclear exclusion of the tsp53 correlated with loss of p53 mediated growth inhibition. Nuclear exclusion of p53 thus might serve as an epigenetic mechanism to eliminate the growth-inhibitory function of p53.