Ja-Eun Kim

Sirtuin 1 Modulates Cellular Responses to Hypoxia by Deacetylating Hypoxia-Inducible Factor 1α

To survive in hypoxic environments, organisms must be able to cope with redox imbalance and oxygen deficiency. The SIRT1 deacetylase and the HIF-1alpha transcription factor act as redox and oxygen sensors, respectively. Here, we found that SIRT1 binds to HIF-1alpha and deacetylates it at Lys674, which is acetylated by PCAF. By doing so, SIRT1 inactivated HIF-1alpha by blocking p300 recruitment and consequently repressed HIF-1 target genes. During hypoxia, SIRT1 was downregulated due to decreased NAD(+) levels, which allowed the acetylation and activation of HIF-1alpha. Conversely, when the redox change was attenuated by blocking glycolysis, SIRT1 was upregulated, leading to the deacetylation and inactivation of HIF-1alpha even in hypoxia. In addition, we confirmed the SIRT1-HIF-1alpha interaction in hypoxic mouse tissues and observed in vivo that SIRT1 has negative effects on tumor growth and angiogenesis. Our results suggest that crosstalk between oxygen- and redox-responsive signal transducers occurs through the SIRT1-HIF-1alpha interaction.

BACH1/FANCJ Acts with TopBP1 and Participates Early in DNA Replication Checkpoint Control

Human TopBP1 plays a critical role in the control of DNA replication checkpoint. In this study, we report a specific interaction between TopBP1 and BACH1/FANCJ, a DNA helicase involved in the repair of DNA crosslinks. The TopBP1/BACH1 interaction is mediated by the very C-terminal tandem BRCT domains of TopBP1 and S phase-specific phosphorylation of BACH1 at Thr 1133 site. Interestingly, we demonstrate that depletion of TopBP1 or BACH1 attenuates the loading of RPA on chromatin. Moreover, both TopBP1 and BACH1 are required for ATR-dependent phosphorylation events in response to replication stress. Taken together, our data suggest that BACH1 has an unexpected early role in replication checkpoint control. A specific interaction between TopBP1 and BACH1 is likely to be required for the extension of single-stranded DNA regions and RPA loading following replication stress, which is a prerequisite for the subsequent activation of replication checkpoint.

Deficiency of H3K79 Histone Methyltransferase Dot1-like Protein (DOT1L) Inhibits Cell Proliferation

Background: DOT1L is responsible for methylation of histone H3K79. Results: DOT1L deficiency leads to senescence in lung cancer cells. Conclusion: DOT1L is required for the proper proliferation of cancer cells. Significance: The inhibition of DOT1L activity might act as a barrier to tumorigenesis. Dot1-like protein (DOT1L) is an evolutionarily conserved histone methyltransferase that methylates lysine 79 of histone H3 (H3K79). Mammalian DOT1L participates in the regulation of transcription, development, erythropoiesis, differentiation, and proliferation of normal cells. However, the role of DOT1L in cancer cell proliferation has not been fully elucidated. DOT1L siRNA-transfected A549 or NCI-H1299 lung cancer cells displayed a nonproliferating multinucleated phenotype. DOT1L-deficient cells also showed abnormal mitotic spindle formation and centrosome number, suggesting that DOT1L deficiency leads to chromosomal missegregation. This chromosomal instability in DOT1L-deficient cells led to cell cycle arrest at the G1 phase and induced senescence as determined by enhanced activity of senescence-associated β-galactosidase activity. Meanwhile, overexpression of a catalytically active DOT1L, not an inactive mutant, restored DOT1L siRNA-induced phenotypes. Overall, these data imply that down-regulation of DOT1L-mediated H3K79 methylation disturbs proliferation of human cells. In addition, although H3K79 methylation is down-regulated in aged tissues, it is up-regulated in lung cancer cell lines and tumor tissues of lung cancer patients. Therefore, H3K79 methylation is a critical histone modification that regulates cell proliferation and would be a novel histone mark for aging and cancer.

DBC1 phosphorylation by ATM/ATR inhibits SIRT1 deacetylase in response to DNA damage

Human DBC1 (deleted in breast cancer-1; KIAA1967) is a nuclear protein that, in response to DNA damage, competitively inhibits the NAD(+)-dependent deacetylase SIRT1, a regulator of p53 apoptotic functions in response to genotoxic stress. DBC1 depletion in human cells increases SIRT1 activity, resulting in the deacetylation of p53 and protection from apoptosis. However, the mechanisms regulating this process have not yet been determined. Here, we report that, in human cell lines, DNA damage triggered the phosphorylation of DBC1 on Thr454 by ATM (ataxia telangiectasia-mutated) and ATR (ataxia telangiectasia and Rad3-related) kinases. Phosphorylated DBC1 bound to and inhibited SIRT1, resulting in the dissociation of the SIRT1-p53 complex and stimulating p53 acetylation and p53-dependent cell death. Indeed, DBC1-mediated genotoxicity, which was shown in knockdown experiments to be dependent on SIRT1 and p53 expression, was defective in cells expressing the phospho-mutant DBC1(T454A). This study describes the first post-translational modification of DBC1 and provides new mechanistic insight linking ATM/ATR to the DBC1-SIRT1-p53 apoptotic axis triggered by DNA damage.

Oleamide suppresses lipopolysaccharide-induced expression of iNOS and COX-2 through inhibition of NF-κB activation in BV2 murine microglial cells

Oleamide (cis-9-octadecenamide) is an endogenous sleep-inducing fatty acid amide that accumulates in the cerebrospinal fluid of the sleep-deprived animals. Microglia are the major immune cells involved in neuroinflammation causing brain damage during infection, ischemia, and neurodegenerative disease. In this study, we examined the effects of oleamide on LPS-induced production of proinflammatory mediators and the mechanisms involved in BV2 microglia. Oleamide inhibited LPS-induced production of NO and prostaglandin E2 as well as expression of iNOS and COX-2. We showed that oleamide blocked LPS-induced NF-kappaB activation and phosphorylation of inhibitor kappaB kinase (IKK). We also showed that oleamide inhibited LPS-induced phosphorylation of Akt, p38 MAPK, and ERK, activation of PI 3-kinase, and accumulation of reactive oxygen species (ROS). Finally, we showed that a specific antagonist of the CB2 receptor, AM630, blocked the inhibitory effects of oleamide on LPS-induced production of proinflammatory mediators and activation of NF-kappaB. Taken together, our results suggest that oleamide shows an anti-inflammatory effect through inhibition of NF-kappaB activation in LPS-stimulated BV2 microglia.

Balance between SIRT1 and DBC1 expression is lost in breast cancer

SIRT1 (silent mating‐type information regulation 2 homologue 1)‐mediated cellular resistance to various stresses is negatively regulated by deleted in breast cancer 1 (DBC1), which was originally reported to be deleted in breast cancer. However, the suggested functions of SIRT1 as a potential tumor promoter and of DBC1 as a potential tumor suppressor have been challenged by observations of their respective down‐ and up‐regulation in various cancers. The aim of the present study was to simultaneously evaluate the expression levels of SIRT1 and DBC1 in the normal and tumor breast tissues from 28 breast cancer patients and to determine correlations with clinicopathological variables. SIRT1 and DBC1 expression was higher in tumor tissues than in matched normal tissues at the protein level, but not at the transcriptional level. Overexpression of SIRT1 and DBC1 in tumor tissue was correlated with favorable and unfavorable clinicopathological factors, suggesting their pleiotropic functions as a potential tumor promoter and tumor suppressor in tumorigenesis. Interestingly, although the overall expression of SIRT1 and DBC1 increased in tumor breast tissues, the correlation between SIRT1 and DBC1 expression was weaker in tumor tissue than in normal tissue. This suggests that the negative regulation of SIRT1 by DBC1 may retard tumorigenesis in breast tissue. Therefore, the correlation between SIRT1 and DBC1 is a potential prognostic indicator in breast cancer. (Cancer Sci 2010)

The histone methyltransferase Dot1/DOT1L as a critical regulator of the cell cycle

Dot1/DOT1L catalyzes the methylation of histone H3 lysine 79 (H3K79), which regulates diverse cellular processes, such as development, reprogramming, differentiation, and proliferation. In regards to these processes, studies of Dot1/DOT1L-dependent H3K79 methylation have mainly focused on the transcriptional regulation of specific genes. Although the gene transcription mediated by Dot1/DOT1L during the cell cycle is not fully understood, H3K79 methylation plays a critical role in the progression of G1 phase, S phase, mitosis, and meiosis. This modification may contribute to the chromatin structure that controls gene expression, replication initiation, DNA damage response, microtubule reorganization, chromosome segregation, and heterochromatin formation. Overall, Dot1/DOT1L is required to maintain genomic and chromosomal stability. This review summarizes the several functions of Dot1/DOT1L and highlights its role in cell cycle regulation.

Characterization of the DOT1L network: implications of diverse roles for DOT1L.

Methylation of lysine 79 on histone H3 (H3K79) is mediated by a methyltransferase called Dot1-like protein (DOT1L). DOT1L is involved in the regulation of telomeric silencing, development, cell cycle checkpoint and transcription. However, the mechanisms by which DOT1L controls these unrelated and diverse functions are unknown. To gain greater insight into DOT1L-mediated functions, we have purified a DOT1L-containing complex using tandem affinity purification. Mass spectrometry of the DOT1L-containing complex revealed that AF9, ENL and NPM1 were shown to be major DOT1L-interacting proteins. To construct a plausible DOT1L-interaction web, AF9-, ENL- and NPM1-containing complexes were also purified for mass spectrometry analysis. The data showed that DOT1L might control AF9- and ENL-mediated transcription, regulate RNA processing, and function as a histone chaperone in a NPM1-dependent manner. In addition, the purification of protein complexes identified a number of novel interacting partners associated with DOT1L, AF9, ENL and NPM1. These data define a unique DOT1L network and shed light on unknown functions of the DOT1L complex.

p30 DBC is a potential regulator of tumorigenesis

Tumorigenesis is a multistep process controlled by a number of proteins involved in diverse pathways. Traditionally, proteins are either considered as oncogenes, which promote tumorigenesis or as tumor suppressors, which prevent tumorigenesis. However, recent studies revealed quite a few proteins that could function as oncogene as well as tumor suppressor. A new member of such proteins is p30 DBC (deleted in breast cancer 1, also called DBC1). p30 DBC is one of the proteins involved in tumorigenesis that does not clearly adhere to either descriptions. Several studies show that p30 DBC is involved in cell proliferation, apoptosis and histone modification, all processes important for regulating tumorigenesis. However, there are other conflicting results regarding how p30 DBC contributes to tumorigenesis. The most interesting aspect of this is that p30 DBC is a strong inhibitor of SIRT1 protein deacetylase, whose exact role in tumorigenesis is currently under debate. This review summarizes the current understandings on p30 DBC functions, with a focus on the proposed roles of p30 DBC in tumorigenesis.

AK-1, a specific SIRT2 inhibitor, induces cell cycle arrest by downregulating Snail in HCT116 human colon carcinoma cells

SIRT2, a member of the sirtuin family, is involved in the regulation of a variety of physiological functions. In addition, SIRT2 has been studied in the context of pathological conditions including neurodegenerative diseases, metabolic syndrome, and cancer. The effect of SIRT2 on cancer cell growth depends on cancer tissue type. To investigate the role of SIRT2 in colon cancer, we treated HCT116 human colon cancer cells with the SIRT2-specific inhibitor AK-1, a cell-permeable benzylsulfonamide. AK-1 treatment induced proteasomal degradation of the Snail transcription factor through inactivation of the NF-κB/CSN2 pathway. Reduction in the level of Snail resulted in upregulation of p21, a cyclin-dependent kinase inhibitor, leading to G1 arrest, slow proliferation, and slow wound-healing activity. The regulation of Snail-p21 axis by AK-1 also occurs in HT-29 colon cancer cells. Therefore, inhibition of SIRT2 using AK-1 would be a beneficial intervention in the treatment of colon cancer.