Cell cycle-dependent regulation of nuclear p53 traffic occurs in one subclass of human tumor cells and in untransformed cells.

Abstract

We have analyzed the regulation of subcellular compartmentation of mutant and wild-type (WT) p53 proteins as a function of the cell cycle using immunofluorescence microscopy and referring to different markers of position in the cell cycle in different human cells expressing either mutated (KHOS-240, A 431, and T47-D cells) or WT (WI 38 and MCF-7 cells) p53. The mutant p53 proteins present in the KHOS-240, A 431, and T47-D tumor-derived cell lines enter very rapidly in the nucleus in early postmitotic cells before the chromosomes have fully decondensed; they continue accumulating in this location without any obvious cytoplasmic retention throughout the cell cycle until prophase. Such behavior is similar to that observed for the WT p53 associating with SV40 large T antigen in human WI 38 cells transformed by SV40, but it is in contrast to the behavior of the WT p53 protein present in both the untransformed WI 38 and the tumor-derived MCF-7 cells. In these latter systems, the highest nuclear concentrations of the WT protein are always found in G1 cells that still fail to exhibit a high rate of nuclear cyclin A; past the G1-S transition, the nuclear level of WT p53 tends to decrease, possibly to the benefit of cytoplasmic expression, whereas that of cyclin A concomitantly increases, suggesting that the nuclear accumulation of WT p53 becomes restricted during the phase of DNA replication. As for Saos-2 cells stably transfected with the temperature-sensitive p53Ala-143 mutant, they become arrested before the G1-S transition with a heavy pool of nuclear p53 at 32.5 degrees C, the temperature at which the transcriptional activity of p53Ala-143 is restored. All these data are compatible with the presently acknowledged primary role for WT p53, which would be to brake transit through the G1-S border possibly by directly transactivating the p21cip1 protein.