Accumulation of soluble and nucleolar-associated p53 proteins following cellular stress

The tumor suppressor p53 is a nucleocytoplasmic shuttling protein that accu mulates in the nucleus of cells exposed to various cellular stresses. One i mportant role of nuclear p53 is to mobilize a stress response by transactiv ating target genes such as the p21(Waf1) gene. In this study, we investigat ed more closely the localization of p53 in cells following various stresses . Immunocytochemistry of fixed human fibroblasts treated with either UV lig ht, the kinase and transcription inhibitor DRB or the proteasome inhibitor MG132 revealed abundant p53 localized to the nucleus. When cells treated wi th UV or DRB were permeabilized prior to fixation to allow soluble proteins to diffuse, the nuclear p53 signal was abolished. However, in cells treate d with MG132, residual p53 localized to distinct large foci, Furthermore, n ucleolin co-localized with p53 to these foci, suggesting that these foci we re nucleolar structures. Interestingly, the MDM2 protein was found to co-lo calize with p53 to nucleolar structures following proteasome inhibition. Ou r results suggest that the p53 proteins accumulating in the nucleus followi ng UV-irradiation or blockage of transcription are freely soluble and, thus , should be able to roam the nucleus to ensure high occupancy of p53 bindin g sites. However, inhibition of proteasome activity may be a unique stress in that it leads to the sequestering of p53 proteins to the nucleolus, ther eby blunting the p53-mediated transactivation of target genes.