Caspase-dependent processing activates the proapoptotic activity of deleted in breast cancer-1 during tumor necrosis factor-alpha-mediated death signaling

The splicing factor CCAR1 regulates the Fanconi anemia/BRCA pathway

The twenty-three Fanconi anemia (FA) proteins cooperate in the FA/BRCA pathway to repair DNA interstrand cross-links (ICLs). The cell division cycle and apoptosis regulator 1 (CCAR1) protein is also a regulator of ICL repair, though its possible function in the FA/BRCA pathway remains unknown. Here, we demonstrate that CCAR1 plays a unique upstream role in the FA/BRCA pathway and is required for FANCA protein expression in human cells. Interestingly, CCAR1 co-immunoprecipitates with FANCA pre-mRNA and is required for FANCA mRNA processing. Loss of CCAR1 results in retention of a poison exon in the FANCA transcript, thereby leading to reduced FANCA protein expression. A unique domain of CCAR1, the EF hand domain, is required for interaction with the U2AF heterodimer of the spliceosome and for excision of the poison exon. Taken together, CCAR1 is a splicing modulator required for normal splicing of the FANCA mRNA and other mRNAs involved in various cellular pathways.

Mechanistic insights into the dual role of CCAR2/DBC1 in cancer

Cell cycle and apoptosis regulator 2 (CCAR2), also known as deleted in breast cancer 1 (DBC1), has been recently identified as a master regulator of transcriptional processes and plays diverse roles in physiology and pathophysiology, including as a regulator of apoptosis, DNA repair, metabolism, and tumorigenesis. CCAR2 functions as a coregulator of various transcription factors and a critical regulator of numerous epigenetic modifiers. Based on its ability to stimulate apoptosis by activating and stabilizing p53, CCAR2 was initially considered to be a tumor suppressor. However, an increasing number of studies have shown that CCAR2 also functions as a tumor-promoting coregulator by activating oncogenic transcription factors and regulating the enzymatic activity of epigenetic modifiers, indicating that CCAR2 may play a dual role in cancer progression by acting as a tumor suppressor and tumor promoter. Here, we review recent progress in understanding the dual tumor-suppressing and oncogenic roles of CCAR2 in cancer. We discuss CCAR2 domain structures, its interaction partners, and the molecular mechanisms by which it regulates the activities of transcription factors and epigenetic modifiers.

Unraveling the Dichotomy of Enigmatic Serine Protease HtrA2

Mitochondrial high-temperature requirement protease A2 (HtrA2) is an integral member of the HtrA family of serine proteases that are evolutionarily conserved from prokaryotes to humans. Involvement in manifold intricate cellular networks and diverse pathophysiological functions make HtrA2 the most enigmatic moonlighting protease amongst the human HtrAs. Despite perpetuating the oligomeric architecture and overall structural fold of its homologs that comprises serine protease and regulatory PDZ domains, subtle conformational alterations and dynamic enzymatic regulation through the distinct allosteric mode of action lead to its functional diversity. This mitochondrial protease upon maturation, exposes its one-of-a-kind N-terminal tetrapeptide (AVPS) motif that binds and subsequently cleaves Inhibitor of Apoptosis Proteins (IAPs) thus promoting cell death, and posing as an important molecule for therapeutic intervention. Interestingly, unlike its other human counterparts, HtrA2 has also been implicated in maintaining the mitochondrial integrity through a bi-functional chaperone-protease activity, the on-off switch of which is yet to be identified. Furthermore, its ability to activate a wide repertoire of substrates through both its N- and C-terminal regions presumably has calibrated its association with several cellular pathways and hence diseases including neurodegenerative disorders and cancer. Therefore, the exclusive structural attributes of HtrA2 that involve multimodal activation, intermolecular PDZ-protease crosstalk, and an allosterically-modulated trimeric active-site ensemble have enabled the protease to evolve across species and partake functions that are fine-tuned for maintaining cellular homeostasis and mitochondrial proteome quality control in humans. These unique features along with its multitasking potential make HtrA2 a promising therapeutic target both in cancer and neurodegeneration.

Interaction of DBC1 with polyoma small T antigen promotes its degradation and negatively regulates tumorigenesis

Deleted in Breast Cancer 1 (DBC1) is an important metabolic sensor. Previous studies have implicated DBC1 in various cellular functions, notably cell proliferation, apoptosis, histone modification, and adipogenesis. However, current reports about the role of DBC1 in tumorigenesis are controversial and designate DBC1 alternatively as a tumor suppressor or a tumor promoter. In the present study, we report that polyoma small T antigen (PyST) associates with DBC1 in mammalian cells, and this interaction leads to the posttranslational downregulation of DBC1 protein levels. When coexpressed, DBC1 overcomes PyST-induced mitotic arrest and promotes the exit of cells from mitosis. Using both transient and stable modes of PyST expression, we also show that cellular DBC1 is subjected to degradation by LKB1, a tumor suppressor and cellular energy sensor kinase, in an AMP kinase-independent manner. Moreover, LKB1 negatively regulates the phosphorylation as well as activity of the prosurvival kinase AKT1 through DBC1 and its downstream pseudokinase substrate, Tribbles 3 (TRB3). Using both transient transfection and stable cell line approaches as well as soft agar assay, we demonstrate that DBC1 has oncogenic potential. In conclusion, our study provides insight into a novel signaling axis that connects LKB1, DBC1, TRB3, and AKT1. We propose that the LKB1–DBC1–AKT1 signaling paradigm may have an important role in the regulation of cell cycle and apoptosis and consequently tumorigenesis.

DBC1 Regulates p53 Stability via Inhibition of CBP-Dependent p53 Polyubiquitination

The control of p53 protein stability is critical to its tumor suppressor functions. The CREB binding protein (CBP) transcriptional co-activator co-operates with MDM2 to maintain normally low physiological p53 levels in cells via exclusively cytoplasmic E4 polyubiquitination activity. Using mass spectrometry to identify nuclear and cytoplasmic CBP-interacting proteins that regulate compartmentalized CBP E4 activity, we identified deleted in breast cancer 1 (DBC1) as a stoichiometric CBP-interacting protein that negatively regulates CBP-dependent p53 polyubiquitination, stabilizes p53, and augments p53-dependent apoptosis. TCGA analysis demonstrated that solid tumors often retain wild-type p53 alleles in conjunction with DBC1 loss, supporting the hypothesis that DBC1 is selected for disruption during carcinogenesis as a surrogate for p53 functional loss. Because DBC1 maintains p53 stability in the nucleus, where p53 exerts its tumor-suppressive transcriptional function, replacement of DBC1 functionality in DBC1-deleted tumors might enhance p53 function and chemosensitivity for therapeutic benefit.

Akande et al. describe DBC1 as a negative regulator of CBP p53-directed ubiquitin ligase activity that maintains nuclear p53 stability. DBC1 loss may be selected in tumors to inactivate the p53 pathway. Because expression of DBC1 enhances p53-dependent apoptosis, restoration of DBC1 in null tumors may be of therapeutic value.

Advances on the role of the deleted in breast cancer (DBC1) in cancer and autoimmune diseases

DBC1 (deleted in breast cancer 1) is a human nuclear protein that modulates the activities of various proteins. Most of the research on DBC1 has focused on metabolism and epigenetics because it is a crucial endogenic inhibitor of deacetylase Sirtuin1 (SIRT1). In this review, we have discussed and summarized the new advances in DBC1 research, mostly focusing on its structure, regulatory function, and significance in cancer and autoimmune diseases.

A novel form of Deleted in breast cancer 1 (DBC1) lacking the N-terminal domain does not bind SIRT1 and is dynamically regulated in vivo

The protein Deleted in Breast Cancer-1 is a regulator of several transcription factors and epigenetic regulators, including HDAC3, Rev-erb-alpha, PARP1 and SIRT1. It is well known that DBC1 regulates its targets, including SIRT1, by protein-protein interaction. However, little is known about how DBC1 biological activity is regulated. In this work, we show that in quiescent cells DBC1 is proteolytically cleaved, producing a protein (DN-DBC1) that misses the S1-like domain and no longer binds to SIRT1. DN-DBC1 is also found in vivo in mouse and human tissues. Interestingly, DN-DBC1 is cleared once quiescent cells re-enter to the cell cycle. Using a model of liver regeneration after partial hepatectomy, we found that DN-DBC1 is down-regulated in vivo during regeneration. In fact, WT mice show a decrease in SIRT1 activity during liver regeneration, coincidentally with DN-DBC1 downregulation and the appearance of full length DBC1. This effect on SIRT1 activity was not observed in DBC1 KO mice. Finally, we found that DBC1 KO mice have altered cell cycle progression and liver regeneration after partial hepatectomy, suggesting that DBC1/DN-DBC1 transitions play a role in normal cell cycle progression in vivo after cells leave quiescence. We propose that quiescent cells express DN-DBC1, which either replaces or coexist with the full-length protein, and that restoring of DBC1 is required for normal cell cycle progression in vitro and in vivo. Our results describe for the first time in vivo a naturally occurring form of DBC1, which does not bind SIRT1 and is dynamically regulated, thus contributing to redefine the knowledge about its function.

Mitochondrial CCAR2/DBC1 is required for cell survival against rotenone-induced mitochondrial stress

CCAR2 (cell cycle and apoptosis regulator protein 2; formerly DBC1, deleted in breast cancer 1) functions in diverse cellular processes including responses to genotoxic and metabolic stresses. However, its role in the mitochondrial stress response has not been fully elucidated. To investigate how CCAR2 regulates stress response, we purified CCAR2-containing complexes. Interestingly, the results revealed that CCAR2 localized to the mitochondria, and also bound Hsp60 (heat shock protein 60), a mitochondrial chaperone. The binding of CCAR2 to Hsp60 increased following rotenone-induced mitochondrial stress. The deficiencies in CCAR2 and Hsp60 also disrupted the mitochondrial membrane potential, thereby promoting apoptosis following mitochondrial stress. In summary, the CCAR2-Hsp60 complex promoted cell survival during mitochondrial stress-induced apoptosis. These data suggest that CCAR2 is critical for maintaining mitochondrial homeostasis in response to stress.

CARP-1/CCAR1: a biphasic regulator of cancer cell growth and apoptosis

Targeted cancer therapy using small molecule inhibitors (SMIs) has been useful in targeting the tumor cells while sparing the normal cells. Despite clinical success of many targeted therapies, their off-target effects and development of resistance are emerging as significant and challenging problems. Thus, there is an urgent need to identify targets to devise new means to treat cancers and their drug-resistant phenotypes. CARP-1/CCAR1 (Cell division cycle and apoptosis regulator 1), a peri-nuclear phospho-protein, plays a dynamic role in regulating cell growth and apoptosis by serving as a co-activator of steroid/thyroid nuclear receptors, β-catenin, Anaphase Promoting Complex/Cyclosome (APC/C) E3 ligase, and tumor suppressor p53. CARP-1/CCAR1 also regulates chemotherapy-dependent apoptosis. CARP-1/CCAR1 functional mimetics (CFMs) are a novel SMIs of CARP-1/CCAR1 interaction with APC/C. CFMs promote apoptosis in a manner independent of p53. CFMs are potent inhibitors of a variety of cancer cells including the drug (Adriamycin or Tamoxifen)-resistant breast cancer cells but not the immortalized breast epithelial cells, while a nano-lipid formulation of the lead compound CFM-4 improves its bioavailability and efficacy in vivo when administered orally. This review focuses on the background and pleiotropic roles of CARP-1/CCAR1 as well as its apoptosis signaling mechanisms in response to chemotherapy in cancer cells.

The Proteomic Profile of Deleted in Breast Cancer 1 (DBC1) Interactions Points to a Multifaceted Regulation of Gene Expression

Deleted in breast cancer 1 (DBC1) has emerged as an important regulator of multiple cellular processes, ranging from gene expression to cell cycle progression. DBC1 has been linked to tumorigenesis both as an inhibitor of histone deacetylases, HDAC3 and sirtuin 1, and as a transcriptional cofactor for nuclear hormone receptors. However, despite mounting interest in DBC1, relatively little is known about the range of its interacting partners and the scope of its functions. Here, we carried out a functional proteomics-based investigation of DBC1 interactions in two relevant cell types, T cells and kidney cells. Microscopy, molecular biology, biochemistry, and mass spectrometry studies allowed us to assess DBC1 mRNA and protein levels, localization, phosphorylation status, and protein interaction networks. The comparison of DBC1 interactions in these cell types revealed conserved regulatory roles for DBC1 in gene expression, chromatin organization and modification, and cell cycle progression. Interestingly, we observe previously unrecognized DBC1 interactions with proteins encoded by cancer-associated genes. Among these interactions are five components of the SWI/SNF complex, the most frequently mutated chromatin remodeling complex in human cancers. Additionally, we identified a DBC1 interaction with TBL1XR1, a component of the NCoR complex, which we validated by reciprocal isolation. Strikingly, we discovered that DBC1 associates with proteins that regulate the circadian cycle, including DDX5, DHX9, and SFPQ. We validated this interaction by colocalization and reciprocal isolation. Functional assessment of this association demonstrated that DBC1 protein levels are important for regulating CLOCK and BMAL1 protein oscillations in synchronized T cells. Our results suggest that DBC1 is integral to the maintenance of the circadian molecular clock. Furthermore, the identified interactions provide a valuable resource for the exploration of pathways involved in DBC1-associated tumorigenesis.

DBC1/CCAR2 is involved in the stabilization of androgen receptor and the progression of osteosarcoma

Deleted in breast cancer 1 (DBC1/CCAR2) is a protein of interest because of its diverse roles in tumorigenesis and its possible role as an androgen receptor (AR) co-activator. However, there are limited studies on the role of DBC1 in osteosarcoma. Therefore, we investigated the role of DBC1 and AR and their relationship in osteosarcoma. Immunohistochemical expression of DBC1 and AR was significantly associated with higher clinical stage and higher histologic grade, and predicted shorter survival. Especially, DBC1 expression was an independent prognostic indicator of overall survival (p = 0.005) and relapse-free survival (p = 0.004) by multivariate analysis. In osteosarcoma cell lines, U2OS and SaOS2, the knock down of DBC1 and AR with siRNA significantly reduced cellular proliferation and inhibited proliferation-related signaling. In addition, the knock down of DBC1 and AR decreased the invasion activity and inhibited invasion-related signaling of osteosarcoma cells. Interestingly, DBC1 affects the stabilization of AR protein via a mechanism involving the ubiquitination of AR. Proteosome-mediated degradation and poly-ubiquitination of AR were increased with the knock-down of DBC1. In conclusion, this study has shown that DBC1 is involved in the stabilization of AR protein and DBC1-AR pathways might be involved in the progression of osteosarcoma.

Inflammation negatively regulates FOXP3 and regulatory T-cell function via DBC1

Forkhead box P3 (FOXP3)-positive Treg cells are crucial for maintaining immune homeostasis. FOXP3 cooperates with its binding partners to elicit Treg cells' signature and function, but the molecular mechanisms underlying the modulation of the FOXP3 complex remain unclear. Here we report that Deleted in breast cancer 1 (DBC1) is a key subunit of the FOXP3 complex. We found that DBC1 interacts physically with FOXP3, and depletion of DBC1 attenuates FOXP3 degradation in inflammatory conditions. Treg cells from Dbc1-deficient mice were more resistant to inflammation-mediated abrogation of Foxp3 expression and function and delayed the onset and severity of experimental autoimmune encephalomyelitis and colitis in mice. These findings establish a previously unidentified mechanism regulating FOXP3 stability during inflammation and reveal a pathway for potential therapeutic modulation and intervention in inflammatory diseases.

DBC1 functions as a tumor suppressor by regulating p53 stability

DBC1 (deleted in breast cancer 1), also known as CCAR2 or KIAA1967, is an important negative regulator of SIRT1 and cellular stress response. Although the Dbc1 gene localizes at a region that is homozygously deleted in breast cancer, its role in tumorigenesis remains unclear. It has been suggested to be either a tumor suppressor or an oncogene. Therefore, the function of DBC1 in cancer needs to be further explored. Here, we report that Dbc1 knockout mice are tumor prone, suggesting that DBC1 functions as a tumor suppressor in vivo. Our data suggest that the increased tumor incidence in Dbc1 knockout mice is independent of Sirt1. Instead, we found that DBC1 loss results in less p53 protein in vitro and in vivo. DBC1 directly binds p53 and stabilizes it through competition with MDM2. These studies reveal that DBC1 plays an important role in tumor suppression through p53 regulation.

DBC1/CCAR2 and CCAR1 Are Largely Disordered Proteins that Have Evolved from One Common Ancestor

Deleted in breast cancer 1 (DBC1, CCAR2, KIAA1967) is a large, predominantly nuclear, multidomain protein that modulates gene expression by inhibiting several epigenetic modifiers, including the deacetylases SIRT1 and HDAC3, and the methyltransferase SUV39H1. DBC1 shares many highly conserved protein domains with its paralog cell cycle and apoptosis regulator 1 (CCAR1, CARP-1). In this study, we examined the full-length sequential and structural properties of DBC1 and CCAR1 from multiple species and correlated these properties with evolution. Our data shows that the conserved domains shared between DBC1 and CCAR1 have similar domain structures, as well as similar patterns of predicted disorder in less-conserved intrinsically disordered regions. Our analysis indicates similarities between DBC1, CCAR1, and the nematode protein lateral signaling target 3 (LST-3), suggesting that DBC1 and CCAR1 may have evolved from LST-3. Our data also suggests that DBC1 emerged later in evolution than CCAR1. DBC1 contains regions that show less conservation across species as compared to the same regions in CCAR1, suggesting a continuously evolving scenario for DBC1. Overall, this study provides insight into the structure and evolution of DBC1 and CCAR1, which may impact future studies on the biological functions of these proteins.

Nuclear cytoplasmic trafficking of proteins is a major response of human fibroblasts to oxidative stress

We have used a subcellular spatial razor approach based on LC-MS/MS-based proteomics with SILAC isotope labeling to determine changes in protein abundances in the nuclear and cytoplasmic compartments of human IMR90 fibroblasts subjected to mild oxidative stress. We show that response to mild tert-butyl hydrogen peroxide treatment includes redistribution between the nucleus and cytoplasm of numerous proteins not previously associated with oxidative stress. The 121 proteins with the most significant changes encompass proteins with known functions in a wide variety of subcellular locations and of cellular functional processes (transcription, signal transduction, autophagy, iron metabolism, TCA cycle, ATP synthesis) and are consistent with functional networks that are spatially dispersed across the cell. Both nuclear respiratory factor 2 and the proline regulatory axis appear to contribute to the cellular metabolic response. Proteins involved in iron metabolism or with iron/heme as a cofactor as well as mitochondrial proteins are prominent in the response. Evidence suggesting that nuclear import/export and vesicle-mediated protein transport contribute to the cellular response was obtained. We suggest that measurements of global changes in total cellular protein abundances need to be complemented with measurements of the dynamic subcellular spatial redistribution of proteins to obtain comprehensive pictures of cellular function.

SIRT1 expression is associated with a poor prognosis, whereas DBC1 is associated with favorable outcomes in gastric cancer

Clinical trials of histone deacetylase (HDAC) inhibitors as antitumor therapy have been conducted for gastric cancer. Expression of SIRT1, a class III HDAC, is related to poor prognosis in some malignancies. We investigated the correlation between SIRT1 expression and progression and prognosis of gastric cancers comparing with molecules linked to SIRT1 in order to better predict the efficacy of HDAC inhibitors in treating this disease. We evaluated SIRT1 expression by western blot in 51 cases and SIRT1, DBC1, acetylated H4K16 (H4K16Ac), acetylated H3K9 (H3K9Ac), and p53 by immunohistochemistry (IHC) in 557 cases of gastric cancer. Western blotting showed that SIRT1 high expression related with statistics to advanced tumor progression, positive lymphatic invasion, positive venous invasion, and advanced stage but not to poor prognosis. IHC revealed that SIRT1 high expression correlated with worse clinico-pathological prognostic factors as same as in western blotting and related poor prognosis both by univariate and multivariate analyses. By the contrast, DBC1 and H4K16Ac were related to favorable prognostic factors and linked to favorable prognosis by univariate analysis but not by multivariate analysis. H3K16Ac correlated only favorable prognostic factors. Results of p53 were very similar to those of SIRT1. We found that SIRT1 high expression closely correlates with progression and prognosis in gastric cancer patients. And it was also indicated that SIRT1 acts as an oncogene by the results of DBC1, H4K16Ac, and H3K9Ac and might be a target molecule of HDAC inhibitor treatment for gastric cancer patients.

CCAR2 deficiency augments genotoxic stress-induced apoptosis in the presence of melatonin in non-small cell lung cancer cells

Melatonin exhibits oncostatic activity in several cancers but does not lead to cytotoxicity in estrogen receptor (ER)-negative non-small cell lung cancers (NSCLCs). In an effort to overcome the melatonin resistance of these cancers, we investigated whether cell cycle and apoptosis regulator 2 (CCAR2) depletion would promote apoptosis following genotoxic stress in melatonin-resistant cancer cells. Ordinarily, the NSCLC cell lines A549 and A427 did not undergo cell death following melatonin treatment for short period. These cell lines were irradiated with UV, a source of genotoxic damage, to trigger apoptotic signaling. Treatment with melatonin prior to irradiation did not produce any significant change in apoptosis. By contrast, in CCAR2-deficient cells, melatonin treatment increased apoptosis induced by genotoxic stress; this effect was dependent on the dose of melatonin. The increase in apoptosis in CCAR2-deficient cells was not dependent on SIRT1. The results indicate that CCAR2 is critical for maintaining cell survival in the presence of melatonin under genotoxic stress. Furthermore, CCAR2 is overexpressed in NSCLC; therefore, melatonin could be used as a potential supplement to classical anticancer drugs in therapies against CCAR2-deficient cancers.

Carboxamide SIRT1 inhibitors block DBC1 binding via an acetylation-independent mechanism

SIRT1 is an NAD (+) -dependent deacetylase that counteracts multiple disease states associated with aging and may underlie some of the health benefits of calorie restriction. Understanding how SIRT1 is regulated in vivo could therefore lead to new strategies to treat age-related diseases. SIRT1 forms a stable complex with DBC1, an endogenous inhibitor. Little is known regarding the biochemical nature of SIRT1-DBC1 complex formation, how it is regulated and whether or not it is possible to block this interaction pharmacologically. In this study, we show that critical residues within the catalytic core of SIRT1 mediate binding to DBC1 via its N-terminal region, and that several carboxamide SIRT1 inhibitors, including EX-527, can completely block this interaction. We identify two acetylation sites on DBC1 that regulate its ability to bind SIRT1 and suppress its activity. Furthermore, we show that DBC1 itself is a substrate for SIRT1. Surprisingly, the effect of EX-527 on SIRT1-DBC1 binding is independent of DBC1 acetylation. Together, these data show that protein acetylation serves as an endogenous regulatory mechanism for SIRT1-DBC1 binding and illuminate a new path to developing small-molecule modulators of SIRT1.

Resveratrol delays Wallerian degeneration in a NAD(+) and DBC1 dependent manner

Axonal degeneration is a central process in the pathogenesis of several neurodegenerative diseases. Understanding the molecular mechanisms that are involved in axonal degeneration is crucial to developing new therapies against diseases involving neuronal damage. Resveratrol is a putative SIRT1 activator that has been shown to delay neurodegenerative diseases, including Amyotrophic Lateral Sclerosis, Alzheimer, and Huntington's disease. However, the effect of resveratrol on axonal degeneration is still controversial. Using an in vitro model of Wallerian degeneration based on cultures of explants of the dorsal root ganglia (DRG), we showed that resveratrol produces a delay in axonal degeneration. Furthermore, the effect of resveratrol on Wallerian degeneration was lost when SIRT1 was pharmacologically inhibited. Interestingly, we found that knocking out Deleted in Breast Cancer-1 (DBC1), an endogenous SIRT1 inhibitor, restores the neuroprotective effect of resveratrol. However, resveratrol did not have an additive protective effect in DBC1 knockout-derived DRGs, suggesting that resveratrol and DBC1 are working through the same signaling pathway. We found biochemical evidence suggesting that resveratrol protects against Wallerian degeneration by promoting the dissociation of SIRT1 and DBC1 in cultured ganglia. Finally, we demonstrated that resveratrol can delay degeneration of crushed nerves in vivo. We propose that resveratrol protects against Wallerian degeneration by activating SIRT1 through dissociation from its inhibitor DBC1.

Role of multifunctional transcription factor TFII-I and putative tumour suppressor DBC1 in cell cycle and DNA double strand damage repair

Background:

In multicellular organisms, precise control of cell cycle and the maintenance of genomic stability are crucial to prevent chromosomal alterations. The accurate function of the DNA damage pathway is maintained by DNA repair mechanisms including homologous recombination (HR). Herein, we show that both TFII-I and DBC1 mediate cellular mechanisms of cell-cycle regulation and DNA double strand damage repair.

Methods:

Regulation of cell cycle by TFII-I and DBC1 was investigated using Trypan blue dye exclusion test, luciferase assay, and flow cytometry analysis. We also analysed the role of TFII-I and DBC1 in DNA double strand damage repair after irradiation by immunofluorescence study, clonogenicity assay, and HR assay.

Results:

Flow cytometry analysis revealed a novel function that siRNA-mediated knockdown of endogenous DBC1 resulted in G2/M phase arrest. We also have shown that both endogenous TFII-I and DBC1 activate DNA repair mechanisms after irradiation because irradiation-induced foci formation of TFII-I-γH2AX was observed, and the depletion of endogenous TFII-I or DBC1 resulted in the inhibition of normal HR efficiency.

Conclusion:

These results reveal novel mechanisms by which TFII-I and DBC1 can modulate cellular fate by affecting cell-cycle control as well as HR pathway.

Multi-omic data integration links deleted in breast cancer 1 (DBC1) degradation to chromatin remodeling in inflammatory response

This study investigated the dynamics of ubiquitinated proteins after the inflammatory stimulation of RAW 264.7 macrophage-like cells with bacterial lipopolysaccharide. Ubiquitination is a common protein post-translational modification that regulates many key cellular functions. We demonstrated that levels of global ubiquitination and K48 and K63 polyubiquitin chains change after lipopolysaccharide stimulation. Quantitative proteomic analysis identified 1199 ubiquitinated proteins, 78 of which exhibited significant changes in ubiquitination levels following stimulation. Integrating the ubiquitinome data with global proteomic and transcriptomic results allowed us to identify a subset of 88 proteins that were targeted for degradation after lipopolysaccharide stimulation. Using cellular assays and Western blot analyses, we biochemically validated DBC1 (a histone deacetylase inhibitor) as a degradation substrate that is targeted via an orchestrated mechanism utilizing caspases and the proteasome. The degradation of DBC1 releases histone deacetylase activity, linking lipopolysaccharide activation to chromatin remodeling in caspase- and proteasome-mediated signaling.

Expression of SIRT1 and DBC1 in Developing and Adult Retinas

Sirtuin 1 (SIRT1) is a deacetylase that can regulate various biological processes via repression of transcription. Its activity has been linked to the differentiation of neural progenitor cells, although little is known about its function during retinal development. The study described herein was undertaken to evaluate the expression of SIRT1 and its innate inhibitor, DBC1, in retinal tissues and progenitor cells. We found both SIRT1 and DBC1 to be widely expressed in mouse and human retinas, with subtle differences in subcellular distribution of each protein. We further demonstrate that nuclear-localized SIRT1 is only seen in human-derived retinal progenitor cells and not in adult retinas, suggesting that this nuclear localization may be important in retinal development. Moreover, we observed cytoplasmic DBC1 in a subset of progenitor cells as well as in mature ganglion cells, indicating that the progenitor cell subset, which was comprised predominantly of small cells, may represent a population of ganglion cell precursors. Collectively, the data presented in this study provide support for SIRT1 and DBC1 as regulators of retinal development and normal retinal physiology.

DBC1 does not function as a negative regulator of SIRT1 in liver cancer

The putative tumor suppressor, DBC1 (deleted in breast cancer-1), was recently found to negatively regulate SIRT1 in vitro and in vivo, but the mechanism whereby DBC1 regulates SIRT1 in liver cancer remains to be elucidated. In this study, it was found that although the expression of DBC1 and SIRT1 was not aberrantly regulated in a large cohort of human hepatocellular carcinoma (HCC) patients, these proteins were highly overexpressed in a subset of HCC tissues compared with surrounding non-cancer tissues. In liver cancer, DBC1 and SIRT1 were found to be positively correlated. Inactivation of DBC1 or SIRT1 reduced SNU-182 (a liver cancer cell line) proliferation as determined by MTT viability assays. Notably, although DBC1 functions as a negative regulator of SIRT1 in A549 lung cancer cells since it suppresses the deacetylase activity of the p53 protein, it did not affect the p53 deacetylase activity of SIRT1 in SNU-182 cells. Taken together, we conclude that DBC1 is associated with SIRT1 in HCC, but that it does not inhibit SIRT1.

The c-MYC oncoprotein, the NAMPT enzyme, the SIRT1-inhibitor DBC1, and the SIRT1 deacetylase form a positive feedback loop

Silent information regulator 1 (SIRT1) represents an NAD(+)-dependent deacetylase that inhibits proapoptotic factors including p53. Here we determined whether SIRT1 is downstream of the prototypic c-MYC oncogene, which is activated in the majority of tumors. Elevated expression of c-MYC in human colorectal cancer correlated with increased SIRT1 protein levels. Activation of a conditional c-MYC allele induced increased levels of SIRT1 protein, NAD(+), and nicotinamide-phosphoribosyltransferase (NAMPT) mRNA in several cell types. This increase in SIRT1 required the induction of the NAMPT gene by c-MYC. NAMPT is the rate-limiting enzyme of the NAD(+) salvage pathway and enhances SIRT1 activity by increasing the amount of NAD(+). c-MYC also contributed to SIRT1 activation by sequestering the SIRT1 inhibitor deleted in breast cancer 1 (DBC1) from the SIRT1 protein. In primary human fibroblasts previously immortalized by introduction of c-MYC, down-regulation of SIRT1 induced senescence and apoptosis. In various cell lines inactivation of SIRT1 by RNA interference, chemical inhibitors, or ectopic DBC1 enhanced c-MYC-induced apoptosis. Furthermore, SIRT1 directly bound to and deacetylated c-MYC. Enforced SIRT1 expression increased and depletion/inhibition of SIRT1 reduced c-MYC stability. Depletion/inhibition of SIRT1 correlated with reduced lysine 63-linked polyubiquitination of c-Myc, which presumably destabilizes c-MYC by supporting degradative lysine 48-linked polyubiquitination. Moreover, SIRT1 enhanced the transcriptional activity of c-MYC. Taken together, these results show that c-MYC activates SIRT1, which in turn promotes c-MYC function. Furthermore, SIRT1 suppressed cellular senescence in cells with deregulated c-MYC expression and also inhibited c-MYC-induced apoptosis. Constitutive activation of this positive feedback loop may contribute to the development and maintenance of tumors in the context of deregulated c-MYC.

The nucleoporin Nup358/RanBP2 promotes nuclear import in a cargo- and transport receptor-specific manner

In vertebrates, the nuclear pore complex (NPC), the gate for transport of macromolecules between the nucleus and the cytoplasm, consists of approximately 30 different nucleoporins (Nups). The Nup and SUMO E3-ligase Nup358/RanBP2 are the major components of the cytoplasmic filaments of the NPC. In this study, we perform a structure-function analysis of Nup358 and describe its role in nuclear import of specific proteins. In a screen for nuclear proteins that accumulate in the cytoplasm upon Nup358 depletion, we identified proteins that were able to interact with Nup358 in a receptor-independent manner. These included the importin α/β-cargo DBC-1 (deleted in breast cancer 1) and DMAP-1 (DNA methyltransferase 1 associated protein 1). Strikingly, a short N-terminal fragment of Nup358 was sufficient to promote import of DBC-1, whereas DMAP-1 required a larger portion of Nup358 for stimulated import. Neither the interaction of RanGAP with Nup358 nor its SUMO-E3 ligase activity was required for nuclear import of all tested cargos. Together, Nup358 functions as a cargo- and receptor-specific assembly platform, increasing the efficiency of nuclear import of proteins through various mechanisms.

Antagonists of anaphase-promoting complex (APC)-2-cell cycle and apoptosis regulatory protein (CARP)-1 interaction are novel regulators of cell growth and apoptosis

CARP-1/CCAR1, a perinuclear phosphoprotein, is a regulator of cell growth and apoptosis signaling. Although CARP-1 is a regulator of chemotherapy-dependent apoptosis, it is also a part of the NF-κB proteome and a co-activator of steroid/thyroid nuclear receptors as well as β-catenin signaling. Our yeast two-hybrid screen revealed CARP-1 binding with the anaphase-promoting complex/cyclosome E3 ubiquitin ligase component APC-2 protein. CARP-1 also binds with anaphase-promoting complex/cyclosome co-activators Cdc20 and Cdh1. Following mapping of the minimal epitopes involved in CARP-1 binding with APC-2, a fluorescence polarization assay was established that indicated a dissociation constant (K(d)) of 480 nm for CARP-1/APC-2 binding. Fluorescence polarization assay-based high throughput screening of a chemical library yielded several small molecule antagonists of CARP-1/APC-2 binding, termed CARP-1 functional mimetics. CFM-4 (1(2-chlorobenzyl)-5'-phenyl-3'H-spiro[indoline-3,2'-[1,3,4]thiadiazol]-2-one), a lead compound, binds with and stimulates CARP-1 expression. CFM-4 prevents CARP-1 binding with APC-2, causes G(2)M cell cycle arrest, and induces apoptosis with an IC(50) range of 10-15 μm. Apoptosis signaling by CFM-4 involves activation of caspase-8 and -9 and caspase-mediated ubiquitin-proteasome pathway-independent loss of cyclin B1 and Cdc20 proteins. Depletion of CARP-1, however, interferes with CFM-4-dependent cell growth inhibition, activation of caspases, and apoptosis. Because CFM-4 also suppresses growth of drug-resistant human breast cancer cells without affecting the growth of human breast epithelial MCF-10A cells, elevating CARP-1 by CFM-4 and consequent apoptosis could in principle be exploited to further elucidate, and perhaps effectively target, often deregulated cell cycle pathways in pathological conditions, including cancer.

Expression of DBC1 is associated with nuclear grade and HER2 expression in breast cancer

DBC1/KIAA1967 (deleted in breast cancer 1) is a putative tumor-suppressor gene cloned from breast cancer specimens and is reported to regulate p53-dependent apoptosis through its specific inhibition of SIRT1 deacetylase. Although SIRT1 plays a pivotal role in carcinogenesis by regulating cellular proliferation, survival and death, its role in breast cancer remains controversial. Therefore, we aimed to investigate the expression status and clinicopathological significance of DBC1 and SIRT1 in breast cancer tissues. We evaluated the expression of DBC1 and SIRT1 in breast core-needle biopsy specimens from 48 primary breast cancer patients between 2005 and 2008. These patients were treated with primary systemic chemotherapy and subsequent surgical resection of the lesions. Immunohistochemical expression scores of DBC1 and SIRT1 were evaluated, and the relationship between their expression levels and clinicopathological features of breast cancer was analyzed. The expression was observed exclusively in the nuclei of normal and neoplastic ductal cells. In breast biopsy specimens, positive expression of DBC1 and SIRT1 was noted in 85 and 98% of patients, respectively. Expression of DBC1 was significantly associated with the tumor nuclear grade (P=0.019). DBC1 and SIRT1 expression was inversely correlated with HER2 expression (P=0.026 and 0.003, respectively). Lower expression of DBC1 and SIRT1 indicated a tendency for a favorable pathological response to chemotherapy, although this was not statistically significant. Our results reveal that the expression of DBC1 and SIRT1 in breast tissues is associated with tumor characteristics.

Reciprocal roles of DBC1 and SIRT1 in regulating estrogen receptor α activity and co-activator synergy

Estrogen receptor α (ERα) plays critical roles in development and progression of breast cancer. Because ERα activity is strictly dependent upon the interaction with coregulators, coregulators are also believed to contribute to breast tumorigenesis. Cell Cycle and Apoptosis Regulator 1 (CCAR1) is an important co-activator for estrogen-induced gene expression and estrogen-dependent growth of breast cancer cells. Here, we identified Deleted in Breast Cancer 1 (DBC1) as a CCAR1 binding protein. DBC1 was recently shown to function as a negative regulator of the NAD-dependent protein deacetylase SIRT1. DBC1 associates directly with ERα and cooperates synergistically with CCAR1 to enhance ERα function. DBC1 is required for estrogen-induced expression of a subset of ERα target genes as well as breast cancer cell proliferation and for estrogen-induced recruitment of ERα to the target promoters in a gene-specific manner. The mechanism of DBC1 action involves inhibition of SIRT1 interaction with ERα and of SIRT1-mediated deacetylation of ERα. SIRT1 also represses the co-activator synergy between DBC1 and CCAR1 by binding to DBC1 and disrupting its interaction with CCAR1. Our results indicate that DBC1 and SIRT1 play reciprocal roles as major regulators of ERα activity, by regulating DNA binding by ERα and by regulating co-activator synergy.

Uncoupling of bait-protein expression from the prey protein environment adds versatility for cell and tissue interaction proteomics and reveals a complex of CARP-1 and the PKA Cbeta1 subunit

An expression-uncoupled tandem affinity purification assay is introduced which differs from the standard TAP assay by uncoupling the expression of the TAP-bait protein from the target cells. Here, the TAP-tagged bait protein is expressed in Escherichia coli and purified. The two concatenated purification steps of the classical TAP are performed after addition of the purified bait to brain tissue homogenates, cell and nuclear extracts. Without prior genetic manipulation of the target, upscaling, free choice of cell compartments and avoidance of expression triggered heat shock responses could be achieved in one go. By the strategy of separating bait expression from the prey protein environment numerous established, mostly tissue-specific binding partners of the protein kinase A catalytic subunit Cbeta1 were identified, including interactions in binary, ternary and quaternary complexes. In addition, the previously unknown small molecule inhibitor-dependent interaction of Cbeta1 with the cell cycle and apoptosis regulatory protein-1 was verified. The uncoupled tandem affinity purification procedure presented here expands the application range of the in vivo TAP assay and may serve as a simple strategy for identifying cell- and tissue-specific protein complexes.

HDAC3 is negatively regulated by the nuclear protein DBC1

HDAC3 is a member of the class I histone deacetylase family that regulates gene expression by deacetylation of histones and non-histone proteins. HDAC3 activity has been shown to be modulated by interaction with the co-repressors NCoR and SMRT. Here, we present evidence that the nuclear protein DBC1 is an endogenous inhibitor of HDAC3. DBC1 has been previously identified as a regulator of some nuclear receptors, the methyltransferase SUV39H1, and the NAD-dependent deacetylase SIRT1. Furthermore, DBC1 has been shown to influence transcription regulation and apoptosis, and it may also act as a tumor suppressor. We found that DBC1 interacts and specifically inhibits the deacetylase HDAC3. This interaction depends on the N terminus of DBC1 and the C terminus of HDAC3. Expression of DBC1 not only inhibited HDAC3 activity but also altered its subcellular distribution. In addition, knockdown of endogenous DBC1 in cells and knock-out in mouse tissues increased HDAC3 deacetylase activity. Together, these results identify DBC1 as a new regulator of HDAC3 and demonstrate that DBC1 is a negative regulator of two key distinct deacetylases, SIRT1 and HDAC3. These findings may lead to a better understanding of the biological roles of DBC1 and HDAC3 in metabolic diseases and cancer.

The LIM Protein Zyxin Binds CARP-1 and Promotes Apoptosis

Zyxin is a dual-function LIM domain protein that regulates actin dynamics in response to mechanical stress and shuttles between focal adhesions and the cell nucleus. Here we show that zyxin contributes to UV-induced apoptosis. Exposure of wild-type fibroblasts to UV-C irradiation results in apoptotic cell death, whereas cells harboring a homozygous disruption of the zyxin gene display a statistically significant survival advantage. To gain insight into the molecular mechanism by which zyxin promotes apoptotic signaling, we expressed an affinity-tagged zyxin variant in zyxin-null cells and isolated zyxin-associated proteins from cell lysates under physiological conditions. A 130-kDa protein that was co-isolated with zyxin was identified by microsequence analysis as the Cell Cycle and Apoptosis Regulator Protein-1 (CARP-1). CARP-1 associates with the LIM region of zyxin. Zyxin lacking the CARP-1 binding region shows reduced proapoptotic activity in response to UV-C irradiation. We demonstrate that CARP-1 is a nuclear protein. Zyxin is modified by phosphorylation in cells exposed to UV-C irradiation, and nuclear accumulation of zyxin is induced by UV-C exposure. These findings highlight a novel mechanism for modulating the apoptotic response to UV irradiation.

Aging and disease: connections to sirtuins

The sirtuins are highly conserved NAD-dependent deacetylases that were shown to regulate lifespan in lower organisms and affect diseases of aging in mammals, such as diabetes, cancer, and inflammation. Most relevant to the amelioration of disease, the SIR2 ortholog SIRT1 has been shown to deacetylate many important transcription factors to exert an overarching influence on numerous metabolic pathways. Here we discuss several diseases of aging for which SIRT1 has been recently shown to confer protection. These findings suggest that manipulating sirtuin activity pharmacologically may be a fruitful area to improve human health.

Identification of DBC1 as a transcriptional repressor for BRCA1

BACKGROUND

DBC1/KIAA1967 (deleted in breast cancer 1) is a putative tumour-suppressor gene cloned from a heterozygously deleted region in breast cancer specimens. Caspase-dependent processing of DBC1 promotes apoptosis, and depletion of endogenous DBC1 negatively regulates p53-dependent apoptosis through its specific inhibition of SIRT1. Hereditary breast and ovarian cancer susceptibility gene product BRCA1, by binding to the promoter region of SIRT1, is a positive regulator of SIRT1 expression.

METHODS

A physical interaction between DBC1 and BRCA1 was investigated both in vivo and in vitro. To determine the pathophysiological significance of DBC1, its role as a transcriptional factor was studied.

RESULTS

We found a physical interaction between the amino terminus of DBC1 and the carboxyl terminus of BRCA1, also known as the BRCT domain. Endogenous DBC1 and BRCA1 form a complex in the nucleus of intact cells, which is exported to the cytoplasm during ultraviolet-induced apoptosis. We also showed that the expression of DBC1 represses the transcriptional activation function of BRCT by a transient expression assay. The expression of DBC1 also inhibits the transactivation of the SIRT1 promoter mediated by full-length BRCA1.

CONCLUSION

These results revealed that DBC1 may modulate the cellular functions of BRCA1 and have important implications in the understanding of carcinogenesis in breast tissue.

Repression of estrogen receptor beta function by putative tumor suppressor DBC1

It has been well established that estrogen is involved in the pathophysiology of breast cancer. Estrogen receptor (ER) alpha appears to promote the proliferation of cancer tissues, while ERbeta can protect against the mitogenic effect of estrogen in breast tissue. The expression status of ERalpha and ERbeta may greatly influence on the development, treatment, and prognosis of breast cancer. Previous studies have indicated that the deleted in breast cancer 1 (DBC1/KIAA1967) gene product has roles in regulating functions of nuclear receptors. The gene encoding DBC1 is a candidate for tumor suppressor identified by genetic search for breast cancer. Caspase-dependent processing of DBC1 promotes apoptosis, and depletion of the endogenous DBC1 negatively regulates p53-dependent apoptosis through its specific inhibition of SIRT1. In addition, DBC1 modulates ERalpha expression and promotes breast cancer cell survival by binding to ERalpha. Here we report an ERbeta-specific repressive function of DBC1. Immunoprecipitation and immunofluorescence studies show that ERbeta and DBC1 interact in a ligand-independent manner similar to ERalpha. In vitro pull-down assays revealed a direct interaction between DBC1 amino-terminus and activation function-1/2 domain of ERbeta. Although DBC1 shows no influence on the ligand-dependent transcriptional activation function of ERalpha, the expression of DBC1 negatively regulates the ligand-dependent transcriptional activation function of ERbetain vivo, and RNA interference-mediated depletion of DBC1 stimulates the transactivation function of ERbeta. These results implicate the principal role of DBC1 in regulating ERbeta-dependent gene expressions.

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.

Deleted in breast cancer 1, a novel androgen receptor (AR) coactivator that promotes AR DNA-binding activity

Androgen receptor (AR) plays a critical role in development and maintenance of male reproductive functions and the etiology of prostate cancer. As a ligand-regulated transcription factor, identification and characterization of AR coregulators are essential for understanding the molecular mechanisms underlying its diverse biological functions. Here we reported the identification of a novel AR coactivator, deleted in breast cancer 1 (DBC1), through a biochemical approach. DBC1 interacts with AR in a ligand-stimulated manner and facilitates AR transcriptional activation in transfected cells as well as in Xenopus oocytes. In in vitro gel shift experiments, recombinant DBC1 drastically enhanced AR DNA-binding activity. Expression of DBC1 also enhanced the binding of AR to chromatinized template in vivo, whereas knockdown of DBC1 impaired the binding of AR to endogenous prostate-specific antigen (PSA) gene in the prostate cancer cell line LNCaP. Thus, our data identify DBC1 as a novel AR coactivator.

A chicken ovalbumin upstream promoter transcription factor I (COUP-TFI) complex represses expression of the gene encoding tumor necrosis factor alpha-induced protein 8 (TNFAIP8)

The orphan nuclear receptor chicken ovalbumin upstream promoter transcription factor I (COUP-TFI) plays key roles in development and homeostasis. A tandem affinity purification procedure revealed that COUP-TFI associated with a number of transcriptional regulatory proteins in HeLa S3 cells, including the nuclear receptor corepressor (NCoR), TIF1beta/KAP-1, HDAC1, and the SWI/SNF family member Brahma. The proapoptotic protein DBC1 was also identified in COUP-TFI complexes. In vitro experiments revealed that COUP-TFI interacted directly with NCoR but in a manner different from that of other nuclear receptors. DBC1 stabilized the interaction between COUP-TFI and NCoR by interacting directly with both proteins. The gene encoding the anti-apoptotic protein TNFAIP8 (tumor necrosis factor alpha (TNFalpha)-induced protein 8) was identified as being repressed by COUP-TFI in a manner that required several of the component proteins of the COUP-TFI complex. Finally, our studies highlight a central role for COUP-TFI in the induction of the TNFAIP8 promoter by TNFalpha. Together, these studies identify a novel COUP-TFI complex that functions as a repressor of transcription and may play a role in the TNFalpha signaling pathways.

Inhibition of SUV39H1 methyltransferase activity by DBC1

SUV39H1 is a histone H3K9-specific methyltransferase important for heterochromatin formation, regulation of gene expression, and induction of senescence in premalignant cells. SUV39H1 forms a complex with SirT1, and its activity is stimulated by SirT1 binding. Here we present evidence that the product of the DBC1 (deleted in breast cancer 1) gene disrupts the SUV39H1-SirT1 complex. Furthermore, DBC1 binds to the SUV39H1 catalytic domain and inhibits its ability to methylate histone H3 in vitro and in vivo. Knockdown of endogenous DBC1 increased the level of cellular H3K9 methylation. As expected, DBC1 also binds to SirT1 and inhibits the deacetylase activity of SirT1. These results identify DBC1 as a novel cellular inhibitor of SUV39H1 activity. DBC1 may be an important regulator of heterochromatin formation and genomic stability by disrupting the SUV39H1-SirT1 complex and inactivating both enzymes.

Proteome-wide identification of poly(ADP-ribose) binding proteins and poly(ADP-ribose)-associated protein complexes

Poly(ADP-ribose) (pADPr) is a polymer assembled from the enzymatic polymerization of the ADP-ribosyl moiety of NAD by poly(ADP-ribose) polymerases (PARPs). The dynamic turnover of pADPr within the cell is essential for a number of cellular processes including progression through the cell cycle, DNA repair and the maintenance of genomic integrity, and apoptosis. In spite of the considerable advances in the knowledge of the physiological conditions modulated by poly(ADP-ribosyl)ation reactions, and notwithstanding the fact that pADPr can play a role of mediator in a wide spectrum of biological processes, few pADPr binding proteins have been identified so far. In this study, refined in silico prediction of pADPr binding proteins and large-scale mass spectrometry-based proteome analysis of pADPr binding proteins were used to establish a comprehensive repertoire of pADPr-associated proteins. Visualization and modeling of these pADPr-associated proteins in networks not only reflect the widespread involvement of poly(ADP-ribosyl)ation in several pathways but also identify protein targets that could shed new light on the regulatory functions of pADPr in normal physiological conditions as well as after exposure to genotoxic stimuli.

The ups and downs of SIRT1

Reversible acetylation has emerged as a key post-translational modification of proteins. Although the number of acetylated proteins is rapidly growing, the ways in which protein acetyltransferases and deacetylases connect with extracellular stimuli remain unclear. Recently, a regulatory network has emerged that controls the expression and activity of SIRT1, a mammalian class-III protein deacetylase. SIRT1 is an important regulator of metabolism, senescence, cancer and, possibly, longevity and is connected with crucial stress-responsive signal-transduction pathways. These connections provide important clues about how protein acetylation and deacetylation mediate cellular adaptations to extrinsic stress.

SIRTUIN 1: regulating the regulator

Earlier analyses on the sirtuin family of histone deacetylases and its well-known member SIRT1 had their primary focus mostly on the identification of cellular targets exploring molecular mechanisms and functional networks in the control of metabolic homeostasis, differentiation, apoptosis and cell survival. However, only little is known about the regulation of SIRT1 itself, so far. Presently, SIRT1 is gaining increasing importance in the development of innovative treatment strategies for cancer, neurodegenerative disorders and metabolic disease. Based on differences in their catalytic activities, SIRT1 and the sirtuins in general, are insensitive to the classical class I and II HDAC inhibitors which are increasingly becoming part of treatment regimens for solid tumors and hematological malignancies. In this review we outline recent research advances on the regulation of SIRT1 which may provide the basis for the development of therapeutic inhibitors with improved specificity.

Metastatic susceptibility locus, an 8p hot-spot for tumour progression disrupted in colorectal liver metastases: 13 candidate genes examined at the DNA, mRNA and protein level

Background

Mortality from colorectal cancer is mainly due to metastatic liver disease. Improved understanding of the molecular events underlying metastasis is crucial for the development of new methods for early detection and treatment of colorectal cancer. Loss of chromosome 8p is frequently seen in colorectal cancer and implicated in later stage disease and metastasis, although a single metastasis suppressor gene has yet to be identified. We therefore examined 8p for genes involved in colorectal cancer progression.

Methods

Loss of heterozygosity analyses were used to map genetic loss in colorectal liver metastases. Candidate genes in the region of loss were investigated in clinical samples from 44 patients, including 6 with matched colon normal, colon tumour and liver metastasis. We investigated gene disruption at the level of DNA, mRNA and protein using a combination of mutation, semi-quantitative real-time PCR, western blotting and immunohistochemical analyses.

Results

We mapped a 2 Mb region of 8p21-22 with loss of heterozygosity in 73% of samples; 8/11 liver metastasis samples had loss which was not present in the corresponding matched primary colon tumour. 13 candidate genes were identified for further analysis. Both up and down-regulation of 8p21-22 gene expression was associated with metastasis. ADAMDEC1 mRNA and protein expression decreased during both tumourigenesis and tumour progression. Increased STC1 and LOXL2 mRNA expression occurred during tumourigenesis. Liver metastases with low DcR1/TNFRSF10C mRNA expression were more likely to present with extrahepatic metastases (p = 0.005). A novel germline truncating mutation of DR5/TNFRSF10B was identified, and DR4/TNFRSF10A SNP rs4872077 was associated with the development of liver metastases (p = 0.02).

Conclusion

Our data confirm that genes on 8p21-22 are dysregulated during colorectal cancer progression. Interestingly, however, instead of harbouring a single candidate colorectal metastasis suppressor 8p21-22 appears to be a hot-spot for tumour progression, encoding at least 13 genes with a putative role in carcinoma development. Thus, we propose that this region of 8p comprises a metastatic susceptibility locus involved in tumour progression whose disruption increases metastatic potential.

Negative regulation of the deacetylase SIRT1 by DBC1

SIRT1 is an NAD-dependent deacetylase critically involved in stress responses, cellular metabolism and, possibly, ageing. The tumour suppressor p53 represents the first non-histone substrate functionally regulated by acetylation and deacetylation; we and others previously found that SIRT1 promotes cell survival by deacetylating p53 (refs 4-6). These results were further supported by the fact that p53 hyperacetylation and increased radiation-induced apoptosis were observed in Sirt1-deficient mice. Nevertheless, SIRT1-mediated deacetylase function is also implicated in p53-independent pathways under different cellular contexts, and its effects on transcriptional factors such as members of the FOXO family and PGC-1alpha directly modulate metabolic responses. These studies validate the importance of the deacetylase activity of SIRT1, but how SIRT1 activity is regulated in vivo is not well understood. Here we show that DBC1 (deleted in breast cancer 1) acts as a native inhibitor of SIRT1 in human cells. DBC1-mediated repression of SIRT1 leads to increasing levels of p53 acetylation and upregulation of p53-mediated function. In contrast, depletion of endogenous DBC1 by RNA interference (RNAi) stimulates SIRT1-mediated deacetylation of p53 and inhibits p53-dependent apoptosis. Notably, these effects can be reversed in cells by concomitant knockdown of endogenous SIRT1. Our study demonstrates that DBC1 promotes p53-mediated apoptosis through specific inhibition of SIRT1.

DBC1 is a negative regulator of SIRT1

The NAD-dependent protein deacetylase Sir2 (silent information regulator 2) regulates lifespan in several organisms. SIRT1, the mammalian orthologue of yeast Sir2, participates in various cellular functions and possibly tumorigenesis. Whereas the cellular functions of SIRT1 have been extensively investigated, less is known about the regulation of SIRT1 activity. Here we show that Deleted in Breast Cancer-1 (DBC1), initially cloned from a region (8p21) homozygously deleted in breast cancers, forms a stable complex with SIRT1. DBC1 directly interacts with SIRT1 and inhibits SIRT1 activity in vitro and in vivo. Downregulation of DBC1 expression potentiates SIRT1-dependent inhibition of apoptosis induced by genotoxic stress. Our results shed new light on the regulation of SIRT1 and have important implications in understanding the molecular mechanism of ageing and cancer.

Analysis of DBC1 and its homologs suggests a potential mechanism for regulation of sirtuin domain deacetylases by NAD metabolites

Deleted in Breast Cancer-1 (DBC1) and its paralog CARP-1 are large multi-domain proteins, with a nuclear or perinuclear localization, and a role in promoting apoptosis upon processing by caspases. Recent studies on human DBC1 show that it is a specific inhibitor of the sirtuin-type deacetylase, Sirt1, which deacetylates histones and p53. Using sensitive sequence profile searches and HMM-HMM comparisons we show that the central conserved globular domain present in the DBC1 and it homologs from diverse eukaryotes is a catalytically inactive version of the Nudix hydrolase (MutT) domain. Given that Nudix domains are known to bind nucleoside diphosphate sugars and NAD, we predict that this domain in DBC1 and its homologs binds NAD metabolites such as ADP-ribose. Hence, we propose that DBC1 and its homologs are likely to regulate the activity of SIRT1 or related deacetylases by sensing the soluble products or substrates of the NAD-dependent deacetylation reaction. The complex domain architectures of the members of the DBC1 family, which include fusions to the RNA-binding S1-like domain, the DNA-binding SAP domain and EF-hand domains, suggest that they are likely to function as integrators of distinct regulatory signals including chromatin protein modification, soluble compounds in NAD metabolism, apoptotic stimuli and RNA recognition.