Title: p90 and UHRF1, Two Novel Regulators of the p53 Signaling Pathway
Description: To ensure proper and differentiated regulation of stress response pathways, the p53 tumor suppressor calls for an intricate network of control of activation and fine tuning of transcription activity, which is offered largely through post-translational modifications. Accumulating evidence supports the indispensability of acetylation in the activation of p53 function and indicates modulation of cell fate decision; however the underlying molecular mechanisms are not well understood and identification of the regulatory mechanisms controlling p53 acetylation remains an important step in furthering the understanding of p53 regulation in vivo. In this study we identify p90 and UHRF1 as two novel members of the p53 regulatory network upstream of TIP60-mediated p53 acetylation. Through biochemical purification, p90 was identified as a unique regulator for p53. p90 (also called CCDC8, coiled-coil domain containing 8) interacts with p53 both in vitro and in vivo. Depletion of p90 by RNAi has no obvious effect on p53 stability or p53-mediated activation of p21, but specifically abrogates PUMA activation.

Moreover, p90 also interacts with the TIP60 acetyltransferase and stimulates TIP60-dependent Lys120 acetylation of p53, therefore enhancing the apoptotic response of p53. These data reveal p90 as an upstream regulator of the Tip60-p53 interaction and demonstrate that p90 is specifically required for p53-mediated apoptosis upon DNA damage. We also report that the epigenetic regulator UHRF1 (ubiquitin-like with PHD and RING finger domains 1) interacts with TIP60 and induces degradation-independent ubiquitination of TIP60. Moreover, UHRF1 markedly suppresses the ability of TIP60 to acetylate p53. In contrast, RNAi-mediated inactivation of UHRF1 increases endogenous p53 acetylation and significantly augments p53-mediated apoptosis. To elucidate the mechanisms of this regulation, we found that the interaction between TIP60 and p53 is severely inhibited in the presence of UHRF1, suggesting that UHRF1 modulates TIP60-mediated functions in both K120 acetylation-dependent and -independent manners.

Consistent with this notion, UHRF1 knockdown promotes activation of p21 and PUMA but not HDM2. These findings demonstrate that UHRF1 is a critical negative regulator of TIP60 and suggest that UHRF1-mediated effects on p53 may contribute, at least in part, to its role in tumorigenesis. This study provides insight for understanding the regulation of p53 acetylation and cell fate decision. Both p90 and UHRF1 are previously unidentified members of the p53 regulatory network. Although both function upstream of the TIP60-p53 interplay, they act through distinct and opposing mechanisms to dynamically regulate TIP60-mediated effects on p53 in vivo.