The epigenetic regulation of cell cycle and chromatin dynamic by sirtuins

The chromatin consists of a hierarchical and dynamical structure that is modulated during the different cell cycle stages in order to maintain genome integrity and preserve the genetic information coded in the DNA. The dynamic structure of the chromatin depends on the coordination of the different chromatin remodeling processes: histone modifications, chromatin remodeling enzymes/complexes, DNA methylation and chromatin architectural proteins (CAPs). Within the chromatin, the histone-mediated regulation responds mainly to the modification of its N-terminal domain or "tail. " Among the different histone modifications, acetylation at specific lysine residues (K) is one of the best characterized, and the acetylation of lysine 16 of histone H4 is the most frequently acetylated residue in eukaryotes. The acetylated form of H4K16 is an important mark of actively transcribed euchromatin from yeast to humans; whereas its non acetylated form is associated with gene silencing and heterochromatin regions. The dynamic of this histone modification is mainly governed by three enzymes: the histone acetyltransferases (HAT) MOF (males absent of the first), and the histone deacetylases (HDAC) SIRT1 and SIRT2. Therefore, both groups of enzymes are essential for the regulation of gene expression and chromatin organization in the nucleus, regulating the transition between transcriptionally active and inactive state of chromatin. SIRT1 and SIRT2 belong to the Class III of HDACs, termed as sirtuins, which are crucial for genomic integrity, adaptation to the environment and aging, among other functions. On one hand, SIRT2 is the only mammalian sirtuin located in the cytoplasm, which is known to shuttle to the nucleus during G2/M. Consistently, this HDAC has its main role in deacetylating H4K16Ac during G2-M. So far, the role of SIRT2 as the main H4K16Ac during mitosis has only been demonstrated by mammalian cell culture experiments or yeast studies. Therefore, for the first time, our study demonstrates the essential role of SIRT2 in regulating H4K16Ac levels during mitosis in vivo. As a matter of fact, our results support the function of SIRT2 in regulating chromatin dynamic by its involvement in the control not only of H4K16Ac levels, but also of H4K20me1-3 levels during the whole cell cycle. Notwithstanding, as happens with other sirtuin members, SIRT2 has also been shown to regulate and deacetylate non-histone substrates that govern cell cycle, stress response, cell survival and genome stability. Furthermore, one of the main roles of SIRT2 consists of modulating cell cycle progression and SIRT2 has been found to regulate diverse mitotic checkpoint proteins such as CDH1, CDC20, BubR1 and p53. Additionally, our results suggest that the chromatin histone Abstract patterns generated by SIRT2 during mitosis are essential in the control of cell cycle progression and attend to two complementary mechanisms: the deacetylation of both H4K16Ac and PR-Set7, the monomethyltrasferase of H4K20. We have found that SIRT2 is clearly involved in a mitotic checkpoint and regulate H4K20me1 deposition under stressful conditions, in order to preserve genome integrity. On the other hand, SIRT1 has been mainly involved in regulating heterochomatin formation and gene silencing by deacetylating histone and non-histone substrates. In fact, SIRT1 is involved in the maintenance of genome integrity due to its role in heterochromatin formation by deacetylating histone marks (H3K9Ac and H1K26Ac) and regulating heterochromatin related proteins such as HP1, Suv39h1 and Ezh2. In addition, SIRT1 also deacetylates H4K16Ac, H3K9Ac and H1K26Ac at specific promoters in order to control gene expression; and regulates non-histone proteins such as p53, FoxO factors, and Rb, among others, to specifically modulate the gene expression pattern. Nonetheless, SIRT1 has recently been implicated in cell cycle regulation throughout the control of Mcm10, the eukaryotic DNA initiation factor essential for S-phase progression. Accordingly, our study also demonstrate how SIRT1 may be involved in the regulation of cell cycle progression by modulating the expression of PR-Set7 and Suv4-20h2, the enzymes in charge of mono- and di-methylate H4K20, respectively. Altogether this evidences the role of sirtuins in preserving genome integrity by modulating chromatin dynamics and cell cycle progression from mitosis to S-phase.