The histone acetyl transferase Tip60 as a regulator of tumor suppression
Tip60 is a histone acetyl transferase (HAT) and a cofactor of transcription, but also an interaction partner of the Mdm2 oncoprotein. The functional consequences of this interaction are only partially understood and were further explored in this study. We found that Tip60 expression leads to an incr...
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|Tip60 is a histone acetyl transferase (HAT) and a cofactor of transcription, but also an interaction partner of the Mdm2 oncoprotein. The functional consequences of this interaction are only partially understood and were further explored in this study. We found that Tip60 expression leads to an increase of Mdm2 and p53 protein levels, which is due to elevated protein half-lifes of these proteins. To explore the underlying mechanisms, the impact of Tip60 on the Mdm2-mediated ubiquitination of p53 was studied. However, we found that Tip60 expression does not affect p53-ubiquitination by Mdm2. Strikingly, Tip60 is capable of selectively inhibiting the Mdm2-mediated conjugation of the neural precursor cell expressed developmentally downregulated 8 (Nedd8) to p53, which had been shown to reduce its transcriptional activity. In contrast, the known Mdm2 antagonist p14arf preferentially blocked Ubiquitin conjugation by Mdm2. To identify underlying mechanisms, we studied the intracellular localization of Tip60 and Mdm2. Both proteins relocalized each other to the PML nuclear bodies, but a similar localization pattern was observed even in the absence of PML. Analysis of Tip60 deletion mutants revealed a stringent correlation between relocalization with Mdm2 and reduced neddylation of p53 and Mdm2. For both activities, the MYST domain but not the HAT domain of Tip60 was required. We propose that Tip60 can act as a selective antagonist to Mdm2-mediated neddylation. Hence, the two different E3-Ligase activities of Mdm2 can be regulated individually.
Furthermore, the effect of Tip60 on the transcriptional activity of p53 was investigated. Surprisingly, Tip60 overexpression did not result in a stimulation of p53 transcriptional activity, whereas the overexpression of the known Mdm2 antagonist p14arf did so. Studies of Tip60 knockdown showed that reduced Tip60 expression only mildly affects the p53-dependent transactivation of its target gene promoters under non-stress conditions.
Therefore, we investigated the role of Tip60 after ultraviolet (UV) irradiation. Immunoblot analysis after siRNA-mediated knockdown of the HTATIP gene (which encodes Tip60) expression showed that the cleavage of the poly(ADP-ribose) polymerase (PARP) upon UV irradiation was reduced, suggesting an impaired apoptotic response. However, the removal of p53 did not affect PARP cleavage, arguing that Tip60 enables UV induced apoptosis independently of p53. Instead, Tip60 knockdown led to reduced phosphorylation of histone 2AX (H2AX) and jun-N-terminal kinase (JNK), suggesting a role of Tip60 in the immediate early response to UV light exposure through JNK.
In this work, two additional roles of Tip60 have been unveiled that potentially contribute to tumor suppression. First, Tip60 can inhibit the Mdm2-mediated conjugation of the Nedd8 protein to p53, thereby possibly releaving it from transcriptional inhibition by Mdm2. Moreover, Tip60 apparently is required for the proper activation of the UV-induced JNK-signaling pathway, thereby ensuring an efficient response to DNA damage and apoptosis induction.