The p53 tumor suppressor gene is a transcription factor and functions as an integrator of stress response signals that regulate cell fate by inducing or repressing the transcription of genes that regulate cell cycle progression or that lead to the activation of apoptosis. Phosphorylation of p53 is one mechanism that is believed to regulate p53 accumulation and activation. Phosphorylation-specific antibodies were used to characterize the phosphorylation of human p53 in A549 cells at 12 known modification sites to three DNA damaging agents: ultraviolet light, ionizing radiation (IR), and adriamycin. In A549 UV light induced apoptosis while IR primarily induced cell cycle arrest. DNA damage induced phosphorylation at most monitored sites, but the time of onset, the strength, and the duration of the signals differed dramatically between agents. Phosphorylation of Thr18 was more potent after IR than UV or adriamycin; in contrast, phosphorylation of Ser37, Ser46 and Ser392 was more efficient after exposure to UV light. Analysis of phosphorylation-site mutant p53s indicated that phosphorylation of Ser33 and 46 are important for the activation of apoptosis. Recently we have demonstrated that the homeodomain interacting protein kinase-2 (HIPK2) binds to and activates p53 by directly phosphorylating it at Ser46. HIPK2 co-localises with p53 and PML-3 into the nuclear bodies and is activated after UV irradiation. Antisense inhibition of HIPK2 expression reduces the UV-induced apoptosis. Furthermore, HIPK2 and p53 cooperate in the activation of p53-dependent transcription and apoptotic pathways. Unlike phosphorylation, p53 invariably undergoes acetylation in cells exposed to a variety of stress-inducing agents including hypoxia, anti-metabolites, nuclear export inhibitor and actinomycin D treatment. Invivo, p53 acetylation is mediated by the p300 and CBP acetyltransferases. Overexpression of either p300 or CBP, but not an acetyltransferase-deficient mutant, efficiently induces specific p53 acetylation at Lys382. In contrast, MDM2, a negative regulator of p53, actively suppresses p300/CBP-mediated p53 acetylation invivoand invitro. This inhibitory activity of MDM2 on p53 acetylation is in turn abrogated by tumor suppressor p19(ARF), indicating that regulation of acetylation is a central target of the p53-MDM2-p19(ARF) feedback loop. Inhibition of deacetylation promotes p53 stability, suggesting that acetylation plays a positive role in the accumulation of p53 protein in stress response. The tumor suppressor p53 induces cellular senescence in response to oncogenic signals. Expression of oncogenic Ras in primary human cells results in activation of p53 and a permanent G1 arrest. We have found that in Ras-expressing cells, p53 is modified on Ser33 and Ser46, two sites phosphorylated by the p38 kinase. The activity of the p38 kinase is regulated by the p53-inducible phosphatase Wip1, thus creating a potential feedback loop. Expression of oncogenic ras suppresses Wip1 mRNA induction, leaving p53 in an activated, phosphorylated state. Retrovirus-mediated Wip1 overexpression reduces p53 phosphorylation at these sites, rescues cells from senescence, and leads to cellular transformation in a p53-dependent manner. The Wip1 gene is located at chromosome 17q22/q23 and is amplified in breast tumor cell lines and primary breast tumors. These findings suggest that Wip1 over-expression may contribute to the development of human cancer by inactivating wild-type p53.
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