Sirtuins, a family of protein deacetylases, promote cellular homeostasis by mediating communication between cells and environment. The enzymatic activity of the mammalian sirtuin SIRT7 targets acetylated lysine in the N‐terminal tail of histone H3 (H3K18Ac), thus modulating chromatin structure and transcriptional competency. SIRT7 deletion is associated with reduced lifespan in mice through unknown mechanisms. Here, we show that SirT7‐knockout mice suffer from partial embryonic lethality and a progeroid‐like phenotype. Consistently, SIRT7‐deficient cells display increased replication stress and impaired DNA repair. SIRT7 is recruited in a PARP1‐dependent manner to sites of DNA damage, where it modulates H3K18Ac levels. H3K18Ac in turn affects recruitment of the damage response factor 53BP1 to DNA double‐strand breaks (DSBs), thereby influencing the efficiency of non‐homologous end joining (NHEJ). These results reveal a direct role for SIRT7 in DSB repair and establish a functional link between SIRT7‐mediated H3K18 deacetylation and the maintenance of genome integrity.

The establishment of the epigenetic mark H4K20me1 (monomethylation of H4K20) by PR-Set7 during G 2 /M directly impacts S-phase progression and genome stability. However, the mechanisms involved in the regulation of this event are not well understood. Here we show that SirT2 regulates H4K20me1 deposition through the deacetylation of H4K16Ac (acetylation of H4K16) and determines the levels of H4K20me2/3 throughout the cell cycle. SirT2 binds and deacetylates PR-Set7 at K90, modulating its chromatin localization. Consistently, SirT2 depletion significantly reduces PR-Set7 chromatin levels, alters the size and number of PR-Set7 foci, and decreases the overall mitotic deposition of H4K20me1. Upon stress, the interaction between SirT2 and PR-Set7 increases along with the H4K20me1 levels, suggesting a novel mitotic checkpoint mechanism. SirT2 loss in mice induces significant defects associated with defective H4K20me1-3 levels. Accordingly, SirT2-deficient animals exhibit genomic instability and chromosomal aberrations and are prone to tumorigenesis. Our studies suggest that the dynamic cross-talk between the environment and the genome during mitosis determines the fate of the subsequent cell cycle.

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Conquering obesity has become a major socioeconomic challenge. Here, we show that reduced expression of the miR-25-93-106b cluster, or miR-93 alone, increases fat mass and, subsequently, insulin resistance. Mechanistically, we discovered an intricate interplay between enhanced adipocyte precursor turnover and increased adipogenesis. First, miR-93 controls Tbx3, thereby limiting self-renewal in early adipocyte precursors. Second, miR-93 inhibits the metabolic target Sirt7, which we identified as a major driver of in vivo adipogenesis via induction of differentiation and maturation of early adipocyte precursors. Using mouse parabiosis, obesity in mir-25-93-106b(-/-) mice could be rescued by restoring levels of circulating miRNA and subsequent inhibition of Tbx3 and Sirt7. Downregulation of miR-93 also occurred in obese ob/ob mice, and this phenocopy of mir-25-93-106b(-/-) was partially reversible with injection of miR-93 mimics. Our data establish miR-93 as a negative regulator of adipogenesis and a potential therapeutic option for obesity and the metabolic syndrome.

The leukocyte activation marker CD69 is a novel regulator of the immune response, modulating the production of cytokines including transforming growth factor-␤ (TGF-␤). We have generated an antimurine CD69 monoclonal antibody (mAb), CD69.2.2, which down-regulates CD69 expression in vivo but does not deplete CD69-expressing cells. Therapeutic administration of CD69.2.2 to wild-type mice induces significant natural killer (NK) cell-dependent antitumor responses to major histocompatibility complex (MHC) class I low RMA-S lymphomas and to RM-1 prostatic carcinoma lung metastases. These in vivo antitumor responses are comparable to those seen in CD69 ؊/؊ mice. Enhanced host NK cytotoxic activity correlates with a reduction in NK-cell TGF-␤ production and is independent of tumor priming. In vitro studies demonstrate the novel ability of anti-CD69 mAbs to activate resting NK cells in an Fc receptor-independent manner, resulting in a substantial increase in both NK-cell cytolytic activity and interferon ␥ (IFN␥) production. Modulation of the innate immune system with monoclonal antibodies to host CD69 thus provides a novel means to antagonize tumor growth and metastasis. ( IntroductionCD69 is expressed in all leukocytes during activation, 1 with rapid kinetic onset (2-4 hours) in lymphoid cells. 2 The CD69 gene is located within the natural killer (NK) complex controlling NK-cell activity. 1,2 Both the genetic and the biochemical characteristics of the mouse CD69 molecule are quite similar to its human homolog. 1,[3][4][5] Our recent studies show that the CD69 molecule is in fact a negative regulator of the immune response in part through production of transforming growth factor-␤ (TGF-␤). CD69 deficiency promotes both autoimmune and antitumor responses. 6,7 The in vivo antitumor response was evaluated by analyzing the susceptibility of CD69 Ϫ/Ϫ mice to tumors. 7 CD69 Ϫ/Ϫ mice showed an enhanced NK-mediated antitumor response that led to greater protection and rejection of major histocompatibility complex (MHC) class I low tumor cells compared with wild-type (wt) mice. This potent antitumor response was associated with altered cytokine patterns, reduced TGF-␤ production, and an increase in local lymphocyte responses. Consistent with these data, the CD69 Ϫ/Ϫ mice showed a greater incidence and severity of collagen-induced arthritis, again correlated with reduced TGF-␤ production. 6 In addition, CD69 engagement promoted active TGF-␤ production in murine and human leukocytes in vitro, further evidence for the role of the CD69 molecule as an inhibitor of immune responses. TGF-␤1 is known to regulate both the production of immune mediators and lymphocyte differentiation. [8][9][10][11][12] Generally, TGF-␤1 down-regulates immunoresponse and plays a major role in tumor responses, [13][14][15] both in animal models and in human tumor cells in vitro. 16,17 Significant progress is now being made in augmenting specific immune effector functions in experimental tumor therapy, particularly using monoclonal antibodies (mAbs). [18][...