The acetyltransferase p300 and its relative CREB-binding protein (CBP), originally discovered as transcriptional co-activators, have more recently been implicated in tumour suppression. Our view of how widespread their functions are during this process is in continual flux as more and more of their targets are identified. The latest update comes from two papers in EMBO Journal and Nature that now implicate them in regulating p53 stability and aiding DNA repair.

p53 is acetylated in response to several environmental insults, including hypoxia and oxidative stress, emphasizing the importance of this modification in the upregulation of p53. Ito and colleagues show that acetylation of p53 accompanies the kinetics of its stabilization, leading them to ask what factors might mediate this acetylation. Previous studies indicate that p300 and CBP can acetylate p53 in vitro, making them prime candidates. To confirm that this is physiologically relevant, the authors overexpressed p300 or CBP in vivo and showed that both could induce p53 acetylation.

How does acetylation promote p53 activity? One possibility is that it contributes to protein stability — a possibility that that authors tested and confirmed. In the presence of a deacetylase inhibitor, trichostatin A (TSA), they found that the half-life of p53 increased markedly.

Next, the authors asked whether factors that negatively regulate p53 activity might do so by interfering with this acetylation step. They looked to see whether mouse double minute 2 (MDM2) — an inhibitor of p53 activity — affects the acetylation of p53. Overexpression of MDM2, they showed, reduced p53 acetylation in a dose-dependent manner. As p300 protein levels remained constant throughout this experiment, they concluded that MDM2 has a direct effect on p53 acetylation. This was confirmed by the fact that addition of TSA blocked this inhibition. And importantly, p19Arf, an inhibitor of MDM2, restores p53 acetylation.

The conclusion is that p53 stability is regulated by balancing its acetylation, mediated by p300 and CBP, and deacetylation, promoted by MDM2. The authors speculate that reversible acetylation, by inhibiting ubiquitylation, might have a more general role in regulating protein stability — how widespread remains to be seen.

In a second paper, Hasan and colleagues describe a function for p300 in DNA repair. Fibroblasts lacking p300 show a severely reduced ability to synthesize DNA, an observation that led Hasan and colleagues to investigate whether p300 acts as a cofactor for DNA synthesis. Through immunoprecipitation experiments from nuclear HeLa cell extracts, they found that p300 forms a complex with proliferating cell nuclear antigen (PCNA) — an essential processivity factor for DNA synthesis — that does not depend on the S phase of the cell cycle. Next, using an in vitro DNA synthesis assay, they showed that this complex could induce DNA synthesis. To confirm that p300 is a cofactor for DNA synthesis in vivo, they conducted chromatin immunoprecipitations, and showed that p300 associates with newly synthesized DNA after ultraviolet treatment of the cells. They then narrowed down the reason for this association, and showed that p300 is involved in DNA repair. This role was supported by the observation that p300 also binds XPA, another DNA repair factor.

The authors propose that, as an acetyltransferase, p300 “might change the structure of the chromatin adjacent to DNA lesions, inducing chromatin changes that facilitate PCNA function and DNA repair synthesis, although the p300 complex is unlikely to affect DNA synthesis directly”.

Together, these papers add to the transcriptional repertoire of p300 and CBP, indicating that they have more diverse functions in the cell, including DNA repair and protein stability, both of which might contribute to their role as tumour suppressors.