SIRT7 activates p53 by enhancing PCAF-mediated MDM2 degradation to arrest the cell cycle

Sirtuin 7 (SIRT7), an NAD+-dependent deacetylase. Here, we report that SIRT7 is required for p53-dependent cell-cycle arrest during glucose deprivation. SIRT7 directly interacts with and deacetylates PCAF, which promotes PCAF binding to MDM2 and consequential degrades MDM2 to enhance p53 stability.

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Our laboratory has studied the biological functions of SIRT7 in recent years and found some interesting phenomena. SIRT7 involves in DNA damage and responses in multiple ways. For example, SIRT7 regulates DNA damage repair by deactivating ATM when DNA damage repair has been completed [1]. In addition, SIRT7, as a therapeutic target, is degraded when treatment with anti-neoplastic agent 5-fluorouracil (5-FU) in human colorectal cancer cells, by which raises a potential on promoting therapeutic efficiency when combination with irradiation and 5-FU [2]. Using SIRT7+/+ and SIRT7-/- mouse embryonic fibroblasts as a model system, we also identified and quantified that thousands acetylation sites in several hundred proteins are changed between SIRT7-/- and SIRT7+/+ cells. Therefore, these proteins were considered as putative SIRT7 substrates and were carried forward for further analysis in response to DNA damage stress [3]. Moreover, we further identified that SIRT7 also promotes gluconeogenesis by modulating glucose-6-phosphatase (G6PC) expression to maintain blood glucose homeostasis upon glucose starvation [4]. These amazing stories give us inspiration to further explore the functions of SIRT7 in human cancer cells. 

Given the key roles of SIRT7 in glucose sensing and signally [5], we thus hypothesized that SIRT7 might be involved in regulating cell-cycle arrest under energy limitation. We first performed to test the cell cycle arrest during glucose deprivation in HCT116 SIRT7-intact cells or SIRT7-KO cells. Surprisingly, glucose deprivation-induced p21 expression and cell cycle arrest were greatly suppressed in SIRT7-KO cells. Based on several previous studies, it demonstrates the indispensable roles of p53 in regulating glucose starvation-induced cell cycle arrest [6], we thus aimed to investigate whether there is a connection between SIRT7 and p53 during glucose deprivation. Although SIRT7 has been reported to interact and deacetylate p53 in mouse cardiomyocytes [7], unfortunately, our data and other studies did not find evidence showing that SIRT7 deacetylates p53 in vivo or in vitro [8, 9]. Thereafter, we tried efforts to investigate how SIRT7 affects p53 activity. Unexpectedly, we observed that SIRT7 enhances p53 stability but not at transcription levels. 

In order to understand the mechanism underlying SIRT7 enhances p53 stability, we decided to validate whether SIRT7 regulates p53-related E3 ligases. We were luckily to find that SIRT7 could promote degradation of murine double minute (MDM2), a well-known p53 E3 ligase, by the ubiquitin–proteasome pathway. However, there was no a direct evidence to show SIRT7 deacetylates MDM2, thus we considered whether SIRT7 regulates MDM2 degradation indirectly. As we known, p300/CBP-associated factor (PCAF) plays a role in ubiquitinating MDM2 and downregulating MDM2 levels [10]. We thus began to explore whether SIRT7 regulates PCAF by a deacetylation modification. Indeed, SIRT7-regulated MDM2 degradation by deacetylating PCAF at K720 site and deacetylated PCAF is able to form a complex with MDM2, which increases p53 stability and activation. Our study first reveals the mechanism by which SIRT7 regulates p53 through PCAF-mediated MDM2 degradation (Figure 1). 

                                     Figure1
  Figure 1. A schematic model showing how SIRT7 controls p53 activation in response to glucose deprivation

Our journey was full of bumpy roads, we got it eventually. We have to thank the master students Ya-Fei  who started this project with full passion and hard work. We would like to appreciate other my group members that commented on this study with enthusiasm.

Link to the paper: https://www.nature.com/article...

Blogpost written by Ya-fei Lu and Wei-Guo Zhu. 

REFERENCES 

1 Tang M, Li Z, Zhang C, Lu X, Tu B, Cao Z et al. SIRT7-mediated ATM deacetylation is essential for its deactivation and DNA damage repair. Science advances 2019; 5: eaav1118. 

2 Tang M, Lu X, Zhang C, Du C, Cao L, Hou T et al. Downregulation of SIRT7 by 5-fluorouracil induces radiosensitivity in human colorectal cancer. Theranostics 2017; 7: 1346-1359. 

3 Zhang C, Zhai Z, Tang M, Cheng Z, Li T, Wang H et al. Quantitative proteome-based systematic identification of SIRT7 substrates. Proteomics 2017; 17.

4 Jiang L, Xiong J, Zhan J, Yuan F, Tang M, Zhang C et al. Ubiquitin-specific peptidase 7 (USP7)-mediated deubiquitination of the histone deacetylase SIRT7 regulates gluconeogenesis. The Journal of biological chemistry 2017; 292: 13296-13311.

5 Chalkiadaki A, Guarente L. Sirtuins mediate mammalian metabolic responses to nutrient availability. Nature reviews Endocrinology 2012; 8: 287-296.

6 Jones RG, Plas DR, Kubek S, Buzzai M, Mu J, Xu Y et al. AMP-activated protein kinase induces a p53-dependent metabolic checkpoint. Molecular cell 2005; 18: 283-293.

7 Vakhrusheva O, Smolka C, Gajawada P, Kostin S, Boettger T, Kubin T et al. Sirt7 increases stress resistance of cardiomyocytes and prevents apoptosis and inflammatory cardiomyopathy in mice. Circulation research 2008; 102: 703-710. 

8 Barber MF, Michishita-Kioi E, Xi Y, Tasselli L, Kioi M, Moqtaderi Z et al. SIRT7 links H3K18 deacetylation to maintenance of oncogenic transformation. Nature 2012; 487: 114-118. 

9 Michishita E, Park JY, Burneskis JM, Barrett JC, Horikawa I. Evolutionarily conserved and nonconserved cellular localizations and functions of human SIRT proteins. Molecular biology of the cell 2005; 16: 4623-4635.

10 Linares LK, Kiernan R, Triboulet R, Chable-Bessia C, Latreille D, Cuvier O et al. Intrinsic ubiquitination activity of PCAF controls the stability of the oncoprotein Hdm2. Nature cell biology 2007; 9: 331-338.

 

 

 

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Wei-Guo Zhu

professor, Shenzhen University School of Medicine

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