CELL CYCLE-DEPENDENT REGULATION OF NUCLEAR P53 TRAFFIC OCCURS IN ONE SUBCLASS OF HUMAN TUMOR-CELLS AND IN UNTRANSFORMED CELLS

We have analyzed the regulation of subcellular compartmentation of mut ant and wild-type (WT) p53 proteins as a function of the cell cycle us ing immunofluorescence microscopy and referring to different markers o f position in the cell cycle in different human cells expressing eithe r mutated (KHOS-240, A 431, and T47-D cells) or WT (WI 38 and MCF-7 ce lls) 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 ea rly postmitotic cells before the chromosomes have fully decondensed; t hey continue accumulating in this location without any obvious cytopla smic retention throughout the cell cycle until prophase, Such behavior is similar to that observed for the WT p53 associating with SV40 larg e T antigen in human WI 38 cells transformed by SV40, but it is in con trast to the behavior of the WT p53 protein present in both the untran sformed WI 38 and the tumor-derived MCF-7 cells. In these latter syste ms, the highest nuclear concentrations of the WT protein are always fo und in G(1) cells that still fail to exhibit a high rate of nuclear cy clin A; past the G(1)-S transition, the nuclear level of WT p53 tends to decrease, possibly to the benefit of cytoplasmic expression, wherea s that of cyclin A concomitantly increases, suggesting that the nuclea r accumulation of WT p53 becomes restricted during the phase of DNA re plication, As for Saos-2 cells stably transfected with the temperature -sensitive p53(Ala-143) mutant, they become arrested before the G(1)-S transition with a heavy pool of nuclear p53 at 32.5 degrees C, the te mperature at which the transcriptional activity of p53(Ala-143) is res tored, All these data are compatible with the presently acknowledged p rimary role for WT p53, which would be to brake transit through the G( 1)-S border possibly by directly transactivating the p21(cip1) protein .