BCL11A-dependent recruitment of SIRT1 to a promoter template in mammalian cells results in histone deacetylation and transcriptional repression

BCL11A Expression in Non-Small Cell Lung Cancers

B-cell leukemia/lymphoma 11A (BCL11A) may be one of the potential biomarkers of non-small cell lung cancer (NSCLC). However, its role in the development of this cancer has not yet been precisely established. The aim of this study was to investigate the expression of BCL11A at the mRNA and protein levels in NSCLC cases and non-malignant lung tissue (NMLT) and to determine the relationship between BCL11A expression and the clinicopathological factors and Ki-67, Slug, Snail and Twist. The localization and the level of BCL11A protein were examined using immunohistochemistry (IHC) on 259 cases of NSCLC, and 116 NMLT samples were prepared as tissue microarrays and using immunofluorescence (IF) in the following cell lines: NCI-H1703, A549 and IMR-90. The mRNA expression of BCL11A was determined using real-time PCR in 33 NSCLC cases, 10 NMLT samples and the cell lines. BCL11A protein expression was significantly higher in NSCLC cases compared to NMLT. Nuclear expression was found in lung squamous cell carcinoma (SCC) cells, while cytoplasmic expression was demonstrated in adenocarcinoma (AC) cells. Nuclear expression of BCL11A decreased with increasing malignancy grade and correlated positively with Ki-67 and Slug and Twist expression. The opposite relationships were found for the cytoplasmic expression of BCL11A. Nuclear expression of BCL11A in NSCLC cells may affect tumor cell proliferation and change their phenotype, thus promoting tumor progression.

KLF14 regulates the growth of hepatocellular carcinoma cells via its modulation of iron homeostasis through the repression of iron-responsive element-binding protein 2

BACKGROUND

Hepatocellular carcinoma (HCC) is a multifactor-driven malignant tumor with rapid progression, which causes the difficulty to substantially improve the prognosis of HCC. Limited understanding of the mechanisms in HCC impedes the development of efficacious therapies. Despite Krüpple-Like factors (KLFs) were reported to be participated in HCC pathogenesis, the function of KLF14 in HCC remains largely unexplored.

METHODS

We generated KLF14 overexpressed and silenced liver cancer cells, and nude mouse xenograft models for the in vitro and in vivo study. Luciferase reporter assay, ChIP-qPCR, Co-IP, immunofluorescence were performed for mechanism research. The expression of KLF14 in HCC samples was analyzed by quantitative RT-PCR, Western blotting, and immunohistochemistry (IHC) analysis.

RESULTS

KLF14 was significantly downregulated in human HCC tissues, which was highly correlated with poor prognosis. Inhibition of KLF14 promoted liver cancer cells proliferation and overexpression of KLF14 suppressed cells growth. KLF14 exerts its anti-tumor function by inhibiting Iron-responsive element-binding protein 2 (IRP2), which then causes transferrin receptor-1(TfR1) downregulation and ferritin upregulation on the basis of IRP-IREs system. This then leading to cellular iron deficiency and HCC cells growth suppression in vitro and in vivo. Interestingly, KLF14 suppressed the transcription of IRP2 via recruiting SIRT1 to reduce the histone acetylation of the IRP2 promoter, resulting in iron depletion and cell growth suppression. More important, we found fluphenazine is an activator of KLF14, inhibiting HCC cells growth through inducing iron deficiency.

CONCLUSION

KLF14 acts as a tumor suppressor which inhibits the proliferation of HCC cells by modulating cellular iron metabolism via the repression of IRP2. We identified Fluphenazine, as an activator of KLF14, could be a potential compound for HCC therapy. Our findings therefore provide an innovative insight into the pathogenesis of HCC and a promising therapeutic target.

Transcription factors Bcl11a and Bcl11b are required for the production and differentiation of cortical projection neurons

The generation and differentiation of cortical projection neurons are extensively regulated by interactive programs of transcriptional factors. Here, we report the cooperative functions of transcription factors Bcl11a and Bcl11b in regulating the development of cortical projection neurons. Among the cells derived from the cortical neural stem cells, Bcl11a is expressed in the progenitors and the projection neurons, while Bcl11b expression is restricted to the projection neurons. Using conditional knockout mice, we show that deficiency of Bcl11a leads to reduced proliferation and precocious differentiation of cortical progenitor cells, which is exacerbated when Bcl11b is simultaneously deleted. Besides defective neuronal production, the differentiation of cortical projection neurons is blocked in the absence of both Bcl11a and Bcl11b: Expression of both pan-cortical and subtype-specific genes is reduced or absent; axonal projections to the thalamus, hindbrain, spinal cord, and contralateral cortical hemisphere are reduced or absent. Furthermore, neurogenesis-to-gliogenesis switch is accelerated in the Bcl11a-CKO and Bcl11a/b-DCKO mice. Bcl11a likely regulates neurogenesis through repressing the Nr2f1 expression. These results demonstrate that Bcl11a and Bcl11b jointly play critical roles in the generation and differentiation of cortical projection neurons and in controlling the timing of neurogenesis-to-gliogenesis switch.

Network Analysis Identifies Regulators of Basal-Like Breast Cancer Reprogramming and Endocrine Therapy Vulnerability

Basal-like breast cancer is the most aggressive breast cancer subtype with the worst prognosis. Despite its high recurrence rate, chemotherapy is the only treatment for basal-like breast cancer, which lacks expression of hormone receptors. In contrast, luminal A tumors express ERα and can undergo endocrine therapy for treatment. Previous studies have tried to develop effective treatments for basal-like patients using various therapeutics but failed due to the complex and dynamic nature of the disease. In this study, we performed a transcriptomic analysis of patients with breast cancer to construct a simplified but essential molecular regulatory network model. Network control analysis identified potential targets and elucidated the underlying mechanisms of reprogramming basal-like cancer cells into luminal A cells. Inhibition of BCL11A and HDAC1/2 effectively drove basal-like cells to transition to luminal A cells and increased ERα expression, leading to increased tamoxifen sensitivity. High expression of BCL11A and HDAC1/2 correlated with poor prognosis in patients with breast cancer. These findings identify mechanisms regulating breast cancer phenotypes and suggest the potential to reprogram basal-like breast cancer cells to enhance their targetability. SIGNIFICANCE: A network model enables investigation of mechanisms regulating the basal-to-luminal transition in breast cancer, identifying BCL11A and HDAC1/2 as optimal targets that can induce basal-like breast cancer reprogramming and endocrine therapy sensitivity.

CTIP2 and lipid metabolism: regulation in skin development and associated diseases

INTRODUCTION

COUP-TF INTERACTING PROTEIN 2 (CTIP2) is a crucial transcription factor exhibiting its control through coupled modulation of epigenetic modification and transcriptional regulation of key genes related to skin, immune, and nervous system development. Previous studies have validated the essential role of CTIP2 in skin development and maintenance, propagating its effects in epidermal permeability barrier (EPB) homeostasis, wound healing, inflammatory diseases, and epithelial cancers. Lipid metabolism dysregulation, on the other hand, has also established its independent emerging role over the years in normal skin development and various skin-associated ailments. This review focuses on the relatively unexplored connections between CTIP2-mediated control of lipid metabolism and alteration of EPB homeostasis, delayed wound healing, inflammatory diseases exacerbation, and cancer promotion and progression.

AREAS COVERED

Here we have discussed the intricate interplay of various endogenous lipids and lipoproteins accompanying skin development and associated disease processes and the possible link to CTIP2-mediated regulation of lipid metabolism.

EXPERT OPINION

Establishing the link between CTIP2 and lipid metabolism alterations in the context of skin morphogenesis and diverse types of skin diseases including cancer can help us identify novel targets for effective therapeutic intervention.

Synthetic modified Fezf2 mRNA (modRNA) with concurrent small molecule SIRT1 inhibition enhances refinement of cortical subcerebral/corticospinal neuron identity from mouse embryonic stem cells

During late embryonic development of the cerebral cortex, the major class of cortical output neurons termed subcerebral projection neurons (SCPN; including the predominant population of corticospinal neurons, CSN) and the class of interhemispheric callosal projection neurons (CPN) initially express overlapping molecular controls that later undergo subtype-specific refinements. Such molecular refinements are largely absent in heterogeneous, maturation-stalled, neocortical-like neurons (termed "cortical" here) spontaneously generated by established embryonic stem cell (ES) and induced pluripotent stem cell (iPSC) differentiation. Building on recently identified central molecular controls over SCPN development, we used a combination of synthetic modified mRNA (modRNA) for Fezf2, the central transcription factor controlling SCPN specification, and small molecule screening to investigate whether distinct chromatin modifiers might complement Fezf2 functions to promote SCPN-specific differentiation by mouse ES (mES)-derived cortical-like neurons. We find that the inhibition of a specific histone deacetylase, Sirtuin 1 (SIRT1), enhances refinement of SCPN subtype molecular identity by both mES-derived cortical-like neurons and primary dissociated E12.5 mouse cortical neurons. In vivo, we identify that SIRT1 is specifically expressed by CPN, but not SCPN, during late embryonic and postnatal differentiation. Together, these data indicate that SIRT1 has neuronal subtype-specific expression in the mouse cortex in vivo, and that its inhibition enhances subtype-specific differentiation of highly clinically relevant SCPN / CSN cortical neurons in vitro.

Epigenetic Dynamics in the Function of T-Lineage Regulatory Factor Bcl11b

The transcription factor Bcl11b is critically required to support the development of diverse cell types, including T lymphocytes, type 2 innate lymphoid cells, neurons, craniofacial mesenchyme and keratinocytes. Although in T cell development its onset of expression is tightly linked to T-lymphoid lineage commitment, the Bcl11b protein in fact regulates substantially different sets of genes in different lymphocyte populations, playing strongly context-dependent roles. Somewhat unusually for lineage-defining transcription factors with site-specific DNA binding activity, much of the reported chromatin binding of Bcl11b appears to be indirect, or guided in large part by interactions with other transcription factors. We describe evidence suggesting that a further way in which Bcl11b exerts such distinct stage-dependent functions is by nucleating changes in regional suites of epigenetic modifications through recruitment of multiple families of chromatin-modifying enzyme complexes. Herein we explore what is - and what remains to be - understood of the roles of Bcl11b, its cofactors, and how it modifies the epigenetic state of the cell to enforce its diverse set of context-specific transcriptional and developmental programs.

Bcl11 Transcription Factors Regulate Cortical Development and Function

Transcription factors regulate multiple processes during brain development and in the adult brain, from brain patterning to differentiation and maturation of highly specialized neurons as well as establishing and maintaining the functional neuronal connectivity. The members of the zinc-finger transcription factor family Bcl11 are mainly expressed in the hematopoietic and central nervous systems regulating the expression of numerous genes involved in a wide range of pathways. In the brain Bcl11 proteins are required to regulate progenitor cell proliferation as well as differentiation, migration, and functional integration of neural cells. Mutations of the human Bcl11 genes lead to anomalies in multiple systems including neurodevelopmental impairments like intellectual disabilities and autism spectrum disorders.

The Regulatory Role of NAD in Human and Animal Cells

Nicotinamide adenine dinucleotide (NAD) and its phosphorylated form NADP are the major coenzymes in the redox reactions of various essential metabolic pathways. NAD+ also serves as a substrate for several families of regulatory proteins, such as protein deacetylases (sirtuins), ADP-ribosyltransferases, and poly(ADP-ribose) polymerases, that control vital cell processes including gene expression, DNA repair, apoptosis, mitochondrial biogenesis, unfolded protein response, and many others. NAD+ is also a precursor for calcium-mobilizing secondary messengers. Proper regulation of these NAD-dependent metabolic and signaling pathways depends on how efficiently cells can maintain their NAD levels. Generally, mammalian cells regulate their NAD supply through biosynthesis from the precursors delivered with the diet: nicotinamide and nicotinic acid (vitamin B3), as well as nicotinamide riboside and nicotinic acid riboside. Administration of NAD precursors has been demonstrated to restore NAD levels in tissues (i.e., to produce beneficial therapeutic effects) in preclinical models of various diseases, such as neurodegenerative disorders, obesity, diabetes, and metabolic syndrome.

A cellular perspective of adipogenesis transcriptional regulation

Adipose cells store lipids in the cytoplasm and signal systemically through secretion of adipokines and other molecules that regulate body energy metabolism. Differentiation of fat cells and its regulation has been the focus of extensive research since the early 1970s. In this review, we had attempted to examine the research bearing on the control of adipose cell differentiation, some of it dating back to the early days when Howard Green and his group described the preadipocyte cell lines 3T3-L1 and 3T3-F442A during 1974-1975. We also concentrated our attention on research published during the last few years, emphasizing data described on transcription factors that regulate adipose differentiation, outside of those that were reported earlier as part of the canonical adipogenic transcriptional cascade, which has been the subject of ample reviews by several groups of researchers. We focused on the studies carried out with the two preadipocyte cell culture models, the 3T3-L1 and 3T3-F442A cells that have provided essential data on adipose biology.

Direct Promoter Repression by BCL11A Controls the Fetal to Adult Hemoglobin Switch

Fetal hemoglobin (HbF, α₂γ₂) level is genetically controlled and modifies severity of adult hemoglobin (HbA, α₂β₂) disorders, sickle cell disease, and β-thalassemia. Common genetic variation affects expression of BCL11A, a regulator of HbF silencing. To uncover how BCL11A supports the developmental switch from γ- to β- globin, we use a functional assay and protein binding microarray to establish a requirement for a zinc-finger cluster in BCL11A in repression and identify a preferred DNA recognition sequence. This motif appears in embryonic and fetal-expressed globin promoters and is duplicated in γ-globin promoters. The more distal of the duplicated motifs is mutated in individuals with hereditary persistence of HbF. Using the CUT&RUN approach to map protein binding sites in erythroid cells, we demonstrate BCL11A occupancy preferentially at the distal motif, which can be disrupted by editing the promoter. Our findings reveal that direct γ-globin gene promoter repression by BCL11A underlies hemoglobin switching.

SIRT1 activates the expression of fetal hemoglobin genes

High fetal hemoglobin (HbF, α₂ γ₂ ) levels ameliorate the clinical manifestations of sickle cell disease and β thalassemia. The mechanisms that repress HbF expression and silence γ-globin genes in adults are incompletely characterized and only a single HbF inducer, hydroxyurea, is approved for treatment, and only in patients with sickle cell disease. We identified SIRT1, a protein deacetylase, as a new inducer of γ-globin. SIRT1 knockdown decreased, while SIRT1 ectopic expression upregulated γ-globin gene (HBG) expression in primary human erythroid cells and in K562 cells. The small molecule SIRT1 activators SRT2104 and SRT1720 enhanced HBG expression in cord blood human erythroblasts and reactivated silenced HBG in adult human erythroblasts. Furthermore, SIRT1 binds in the β-globin gene cluster locus control region (LCR) and HBG promoters, promotes the looping of the LCR to HBG promoter, and increases the binding of RNA polymerase II and H4K16Ac in the HBG promoter. SIRT1 suppressed the expression of the HBG suppressors BCL11A, KLF1, HDAC1 and HDAC2. Lastly, SIRT1 did not change the proliferation of human erythroid progenitor cells or the expression of differentiation marker CD235a. These data suggest that SIRT1 activates HBG expression through facilitating LCR looping to the HBG promoter, inhibiting the expression of transcriptional suppressors of HBG, and indirectly increasing histone acetylation in the HBG promoter. SIRT1 is a potential therapeutic target for γ-globin gene induction, and small molecule SIRT1 activators might serve as a lead compound for the development of new HbF inducers.

BCL11A expression in acute myeloid leukemia

BACKGROUND

BCL11A encodes a C2H2 type zinc-finger protein. During normal haematopoietic cell differentiation BCL11A expression is down-regulated. Data in mice suggest up-regulation of BCL11A is involved in the pathogenesis of myeloid leukaemias. BCL11A expression in persons with acute myeloid leukaemia (AML) is not systematically studied.

OBJECTIVE

Interrogate associations between BCL11A expression at diagnosis and clinical and laboratory valuables and outcomes in newly-diagnosed persons with AML.

METHODS

We determined BCL11A mRNA levels in bone marrow and blood mononuclear cells in 292 consecutive newly-diagnosed subjects with AML by reverse transcript and real-time polymerase chain reaction. Data were compared to mRNA levels in bone marrow cells of normals.

RESULTS

Subjects with BCL11A transcript levels at diagnosis exceeding the median value of 2.434 (±3.423 SD; 25th-75th inter-quartile range, 1.33-4.29) had higher WBC levels, a greater proportion of bone marrow myeloblasts, were more likely to be FAB M0 subtype, less likely to be FAB M3 subtype, more likely to be in the intermediate cytogenetic risk cohort, less likely to have a complex karyotype and more likely to have DNMT3A(R882) and FLT3-ITD mutations than subjects with transcript levels below the median value. In 89 subjects receiving conventional induction chemotherapy the complete remission rate was 54% (95% confidence interval [CI]; 33, 75%) in the lower BCL11A cohort and 65% (45, 85%; P=0.26) in the higher BCL11A cohort. 3 year survival was 33% (2, 65%) in the lower BCL11A cohort and 15% (0, 39%; P=0.35) in the high BCL11A cohort.

CONCLUSION

BCL11A transcript levels at diagnosis was significantly associated with several clinical and laboratory variables. There were also non-significant associations with complete remission rate and survival. These data suggest a possible role for BCL11A expression in AML biology.

The Specification of Cortical Subcerebral Projection Neurons Depends on the Direct Repression of TBR1 by CTIP1/BCL11a

The acquisition of distinct neuronal fates is fundamental for the function of the cerebral cortex. We find that the development of subcerebral projections from layer 5 neurons in the mouse neocortex depends on the high levels of expression of the transcription factor CTIP1; CTIP1 is coexpressed with CTIP2 in neurons that project to subcerebral targets and with SATB2 in those that project to the contralateral cortex. CTIP1 directly represses Tbr1 in layer 5, which appears as a critical step for the acquisition of the subcerebral fate. In contrast, lower levels of CTIP1 in layer 6 are required for TBR1 expression, which directs the corticothalamic fate. CTIP1 does not appear to play a critical role in the acquisition of the callosal projection fate in layer 5. These findings unravel a key step in the acquisition of cell fate for closely related corticofugal neurons and indicate that differential dosages of transcriptions factors are critical to specify different neuronal identities.

Calorie Restriction Prevents Metabolic Aging Caused by Abnormal SIRT1 Function in Adipose Tissues

Adipose tissue is a pivotal organ determining longevity, due largely to its role in maintaining whole-body energy homeostasis and insulin sensitivity. SIRT1 is a NAD-dependent protein deacetylase possessing antiaging activities in a wide range of organisms. The current study demonstrates that mice with adipose tissue-selective overexpression of hSIRT1(H363Y), a dominant-negative mutant that disrupts endogenous SIRT1 activity, show accelerated development of metabolic aging. These mice, referred to as Adipo-H363Y, exhibit hyperglycemia, dyslipidemia, ectopic lipid deposition, insulin resistance, and glucose intolerance at a much younger age than their wild-type littermates. The metabolic defects of Adipo-H363Y are associated with abnormal epigenetic modifications and chromatin remodeling in their adipose tissues, as a result of excess accumulation of biotin, which inhibits endogenous SIRT1 activity, leading to increased inflammation, cellularity, and collagen deposition. The enzyme acetyl-CoA carboxylase 2 plays an important role in biotin accumulation within adipose tissues of Adipo-H363Y. Calorie restriction prevents biotin accumulation, abolishes abnormal histone biotinylation, and completely restores the metabolic and adipose functions of Adipo-H363Y. The effects are mimicked by short-term restriction of biotin intake, an approach potentially translatable to humans for maintaining the epigenetic and chromatin remodeling capacity of adipose tissues and preventing aging-associated metabolic disorders.

BCL11A is a triple-negative breast cancer gene with critical functions in stem and progenitor cells

Triple-negative breast cancer (TNBC) has poor prognostic outcome compared with other types of breast cancer. The molecular and cellular mechanisms underlying TNBC pathology are not fully understood. Here, we report that the transcription factor BCL11A is overexpressed in TNBC including basal-like breast cancer (BLBC) and that its genomic locus is amplified in up to 38% of BLBC tumours. Exogenous BCL11A overexpression promotes tumour formation, whereas its knockdown in TNBC cell lines suppresses their tumourigenic potential in xenograft models. In the DMBA-induced tumour model, Bcl11a deletion substantially decreases tumour formation, even in p53-null cells and inactivation of Bcl11a in established tumours causes their regression. At the cellular level, Bcl11a deletion causes a reduction in the number of mammary epithelial stem and progenitor cells. Thus, BCL11A has an important role in TNBC and normal mammary epithelial cells. This study highlights the importance of further investigation of BCL11A in TNBC-targeted therapies.

The SIRT1 modulators AROS and DBC1 regulate HSF1 activity and the heat shock response

The heat shock response, the cellular response to protein damaging stress, is critical in maintaining proteostasis. The heat shock response is regulated by the transcription factor HSF1, which is activated upon heat shock and other stresses to induce the expression of molecular chaperones. SIRT1 has previously been shown to activate HSF1 by deacetylating it, leading to increased DNA binding ability. We have investigated how the heat shock response may be controlled by factors influencing SIRT1 activity. We found that heat shock results in an increase in the cellular NAD⁺/NADH ratio and an increase in recruitment of SIRT1 to the hsp70 promoter. Furthermore, we found that the SIRT1 modulators AROS and DBC1 have an impact on hsp70 transcription, HSF1 acetylation status, and HSF1 recruitment to the hsp70 promoter. Therefore, AROS and DBC1 are now two new targets available for therapeutic regulation of the heat shock response.

Using mouse models to study function of transcriptional factors in T cell development

Laboratory mice have widely been used as tools for basic biological research and models for studying human diseases. With the advances of genetic engineering and conditional knockout (CKO) mice, we now understand hematopoiesis is a dynamic stepwise process starting from hematopoietic stem cells (HSCs) which are responsible for replenishing all blood cells. Transcriptional factors play important role in hematopoiesis. In this review we compile several studies on using genetic modified mice and humanized mice to study function of transcriptional factors in lymphopoiesis, including T lymphocyte and Natural killer (NK) cell development. Finally, we focused on the key transcriptional factor Bcl11b and its function in regulating T cell specification and commitment.

NF-Y recruits both transcription activator and repressor to modulate tissue- and developmental stage-specific expression of human γ-globin gene

The human embryonic, fetal and adult β-like globin genes provide a paradigm for tissue- and developmental stage-specific gene regulation. The fetal γ-globin gene is expressed in fetal erythroid cells but is repressed in adult erythroid cells. The molecular mechanism underlying this transcriptional switch during erythroid development is not completely understood. Here, we used a combination of in vitro and in vivo assays to dissect the molecular assemblies of the active and the repressed proximal γ-globin promoter complexes in K562 human erythroleukemia cell line and primary human fetal and adult erythroid cells. We found that the proximal γ-globin promoter complex is assembled by a developmentally regulated, general transcription activator NF-Y bound strongly at the tandem CCAAT motifs near the TATA box. NF-Y recruits to neighboring DNA motifs the developmentally regulated, erythroid transcription activator GATA-2 and general repressor BCL11A, which in turn recruit erythroid repressor GATA-1 and general repressor COUP-TFII to form respectively the NF-Y/GATA-2 transcription activator hub and the BCL11A/COUP-TFII/GATA-1 transcription repressor hub. Both the activator and the repressor hubs are present in both the active and the repressed γ-globin promoter complexes in fetal and adult erythroid cells. Through changes in their levels and respective interactions with the co-activators and co-repressors during erythroid development, the activator and the repressor hubs modulate erythroid- and developmental stage-specific transcription of γ-globin gene.

Role of the transcription factor Bcl11b in development and lymphomagenesis

Bcl11b is a lineage-specific transcription factor expressed in various cell types and its expression is important for development of T cells, neurons and others. On the other hand, Bcl11b is a haploinsufficient tumor suppressor and loss of a Bcl11b allele provides susceptibility to mouse thymic lymphoma and human T-cell acute lymphoblastic leukemia. Although there are many transcription factors affecting both cell differentiation and cancer development, Bcl11b has several unique properties. This review describes phenotypes given by loss of Bcl11b and roles of Bcl11b in cell proliferation, differentiation and apoptosis, taking tissue development and lymphomagenesis into consideration.

Sirtuin 1 in immune regulation and autoimmunity

The NAD-dependent histone deacetylase sirtuin (Sirt)1 is implicated in a wide variety of physiological processes, ranging from tumorigenesis to mitochondrial biogenesis to neuronal development. Recent studies indicate that Sirt1 is a critical regulator of both the innate and adaptive immune response in mice and its altered functions are likely involved in autoimmune diseases. Small molecules that modulate Sirt1 functions are potential therapeutic reagents for autoimmune inflammatory diseases. In this review, we highlight the functions of Sirt1 in the immune system focusing on the underlying molecular mechanisms, and the potential of Sirt1 as a therapeutic target for autoimmune diseases.

Mammalian Sirt1: insights on its biological functions

Sirt1 (member of the sirtuin family) is a nicotinamide adenosine dinucleotide (NAD)-dependent deacetylase that removes acetyl groups from various proteins. Sirt1 performs a wide variety of functions in biological systems. The current review focuses on the biological functions of Sirt1 in obesity-associated metabolic diseases, cancer, adipose tissue, aging, cellular senescence, cardiac aging and stress, prion-mediated neurodegeneration, inflammatory signaling in response to environmental stress, development and placental cell survival.

NAD: a master regulator of transcription

Cellular processes such as proliferation, differentiation and death are intrinsically dependent upon the redox status of a cell. Among other indicators of redox flux, cellular NAD(H) levels play a predominant role in transcriptional reprogramming. In addition to this, normal physiological functions of a cell are regulated in response to perturbations in NAD(H) levels (for example, due to alterations in diet/metabolism) to maintain homeostatic conditions. Cells achieve this homeostasis by reprogramming various components that include changes in chromatin structure and function (transcription). The interdependence of changes in gene expression and NAD(H) is evolutionarily conserved and is considered crucial for the survival of a species (by affecting reproductive capacity and longevity). Proteins that bind and/or use NAD(H) as a co-substrate (such as, CtBP and PARPs/Sirtuins respectively) are known to induce changes in chromatin structure and transcriptional profiles. In fact, their ability to sense perturbations in NAD(H) levels has been implicated in their roles in development, stress responses, metabolic homeostasis, reproduction and aging or age-related diseases. It is also becoming increasingly clear that both the levels/activities of these proteins and the availability of NAD(H) are equally important. Here we discuss the pivotal role of NAD(H) in controlling the functions of some of these proteins, the functional interplay between them and physiological implications during calorie restriction, energy homeostasis, circadian rhythm and aging.

SIRT1-dependent regulation of chromatin and transcription: linking NAD(+) metabolism and signaling to the control of cellular functions

Sirtuins comprise a family of NAD(+)-dependent protein deacetylases and ADP-ribosyltransferases. Mammalian SIRT1 - a homolog of yeast Sir2, the prototypical member of the sirtuin family - is an important regulator of metabolism, cell differentiation and senescence, stress response, and cancer. As an NAD(+)-dependent enzyme, SIRT1 regulates gene expression programs in response to cellular metabolic status, thereby coordinating metabolic adaptation of the whole organism. Several important mechanisms have emerged for SIRT1-dependent regulation of transcription. First, SIRT1 can modulate chromatin function through direct deacetylation of histones as well as by promoting alterations in the methylation of histones and DNA, leading to the repression of transcription. The latter is accomplished through the recruitment of other nuclear enzymes to chromatin for histone methylation and DNA CpG methylation, suggesting a broader role of SIRT1 in epigenetic regulation. Second, SIRT1 can interact and deacetylate a broad range of transcription factors and coregulators, thereby regulating target gene expression both positively and negatively. Cellular energy state, specifically NAD(+) metabolism, plays a major role in the regulation of SIRT1 activity. Recent studies on the NAD(+) biosynthetic enzymes in the salvage pathway, nicotinamide phosphoribosyltransferase (NAMPT) and nicotinamide mononucleotide adenylyltransferase 1 (NMNAT-1), have revealed important functions for these enzymes in SIRT1-dependent transcription regulation. The collective molecular actions of SIRT1 control specific patterns of gene expression that modulate a wide variety of physiological outcomes.

Bcl11A/CTIP1 regulates expression of DCC and MAP1b in control of axon branching and dendrite outgrowth

The extension of axon branches is important for target innervation but how axon branching is regulated is currently not well understood. Here, we report that Bcl11A/CTIP1/Evi9, a zinc finger transcription factor, downregulates axon branching. Knockdown of Bcl11A induced axon branching and multi-axon formation, as well as dendrite outgrowth. Due to alternative splicing, a single Bcl11A gene encodes two protein products, Bcl11A-L and -S. Bcl11A-L was found to be the main Bcl11A player in regulation of neurite arborization; Bcl11A-S is an antagonist of Bcl11A-L. Time-lapse study further suggests that Bcl11A-L knockdown enhances axon dynamics and increases the duration of axon outgrowth. Finally, the expression of DCC and MAP1b, two molecules involved in direction and branching of axon outgrowth, is controlled by Bcl11A-L. DCC overexpression rescues the phenotype induced by Bcl11A-L knockdown. In conclusion, this report provides the first evidence that Bcl11A is important for neurite arborization.

CTIP2 expression in human head and neck squamous cell carcinoma is linked to poorly differentiated tumor status

Background

We have demonstrated earlier that CTIP2 is highly expressed in mouse skin during embryogenesis and in adulthood. CTIP2 mutant mice die at birth with epidermal differentiation defects and a compromised epidermal permeability barrier suggesting its role in skin development and/or homeostasis. CTIP2 has also been suggested to function as tumor suppressor in cells, and several reports have described a link between chromosomal rearrangements of CTIP2 and human T cell acute lymphoblast leukemia (T-ALL). The aim of the present study was to look into the pattern of CTIP2 expression in Head and Neck Squamous Cell Carcinoma (HNSCC).

Methodology/Principal Findings

In the present study, we analyzed CTIP2 expression in human HNSCC cell lines by western blotting, in paraffin embedded archival specimens by immunohistochemistry (IHC), and in cDNA samples of human HNSCC by qRT-PCR. Elevated levels of CTIP2 protein was detected in several HNSCC cell lines. CTIP2 staining was mainly detected in the basal layer of the head and neck normal epithelium. CTIP2 expression was found to be significantly elevated in HNSCC (p<0.01), and increase in CTIP2 expression was associated with poorly differentiated tumor status. Nuclear co-localization of CTIP2 protein and cancer stem cell (CSC) marker BMI1 was observed in most, if not all of the cells expressing BMI1 in moderately and poorly differentiated tumors.

Conclusions/Significance

We report for the first time expression of transcriptional regulator CTIP2 in normal human head and neck epithelia. A statistically significant increase in the expression of CTIP2 was detected in the poorly differentiated samples of the human head and neck tumors. Actual CTIP2, rather than the long form of CTIP2 (CTIP2_L) was found to be more relevant to the differentiation state of the tumors. Results demonstrated existence of distinct subsets of cancer cells, which express CTIP2 and underscores the use of CTIP2 and BMI1 co-labeling to distinguish tumor initiating cells or cancer stem cells (CSCs) from surrounding cancer cells.

Dual role of COUP-TF-interacting protein 2 in epidermal homeostasis and permeability barrier formation

COUP-TF-interacting protein 2 (CTIP2; also known as Bcl11b) is a transcription factor that plays key roles in the development of the central nervous and immune systems. CTIP2 is also highly expressed in the developing epidermis, and at lower levels in the dermis and in adult skin. Analyses of mice harboring a germline deletion of CTIP2 revealed that the protein plays critical roles in skin during development, particularly in keratinocyte proliferation and late differentiation events, as well as in the development of the epidermal permeability barrier. At the core of all of these actions is a relatively large network of genes, described herein, that is regulated directly or indirectly by CTIP2. The analysis of conditionally null mice, in which expression of CTIP2 was ablated specifically in epidermal keratinocytes, suggests that CTIP2 functions in both cell and non-cell autonomous contexts to exert regulatory influence over multiple phases of skin development, including barrier establishment. Considered together, our results suggest that CTIP2 functions as a top-level regulator of skin morphogenesis.

A retroviral mutagenesis screen reveals strong cooperation between Bcl11a overexpression and loss of the Nf1 tumor suppressor gene

NF1 inactivation occurs in specific human cancers, including juvenile myelomonocytic leukemia, an aggressive myeloproliferative disorder of childhood. However, evidence suggests that Nf1 loss alone does not cause leukemia. We therefore hypothesized that inactivation of the Nf1 tumor suppressor gene requires cooperating mutations to cause acute leukemia. To search for candidate genes that cooperate with Nf1 deficiency in leukemogenesis, we performed a forward genetic screen using retroviral insertion mutagenesis in Nf1 mutant mice. We identified 43 common proviral insertion sites that contain candidate genes involved in leukemogenesis. One of these genes, Bcl11a, confers a growth advantage in cultured Nf1 mutant hematopoietic cells and causes early onset of leukemia of either myeloid or lymphoid lineage in mice when expressed in Nf1-deficient bone marrow. Bcl11a-expressing cells display compromised p21(Cip1) induction, suggesting that Bcl11a's oncogenic effects are mediated, in part, through suppression of p21(Cip1). Importantly, Bcl11a is expressed in human chronic myelomonocytic leukemia and juvenile myelomonocytic leukemia samples. A subset of AML patients, who had poor outcomes, of 16 clusters, displayed high levels of BCL11A in leukemic cells. These findings suggest that deregulated Bcl11a cooperates with Nf1 in leukemogenesis, and a therapeutic strategy targeting the BCL11A pathway may prove beneficial in the treatment of leukemia.

Therapeutic application of histone deacetylase inhibitors for central nervous system disorders

Histone deacetylases (HDACs)--enzymes that affect the acetylation status of histones and other important cellular proteins--have been recognized as potentially useful therapeutic targets for a broad range of human disorders. Pharmacological manipulations using small-molecule HDAC inhibitors--which may restore transcriptional balance to neurons, modulate cytoskeletal function, affect immune responses and enhance protein degradation pathways--have been beneficial in various experimental models of brain diseases. Although mounting data predict a therapeutic benefit for HDAC-based therapy, drug discovery and development of clinical candidates face significant challenges. Here, we summarize the current state of development of HDAC therapeutics and their application for the treatment of human brain disorders such as Rubinstein-Taybi syndrome, Rett syndrome, Friedreich's ataxia, Huntington's disease and multiple sclerosis.

BCL11A is a major HbF quantitative trait locus in three different populations with beta-hemoglobinopathies

Increased HbF levels or F-cell (HbF containing erythrocyte) numbers can ameliorate the disease severity of beta-thalassemia major and sickle cell anemia. Recent genome-wide association studies reported that single nucleotide polymorphisms (SNPs) in BCL11A gene on chromosome 2p16.1 were correlated with F-cells among healthy northern Europeans, and HbF among Sardinians with beta-thalassemias. In this study, we showed that SNPs in BCL11A were associated with F-cell numbers in Chinese with beta-thalassemia trait, and with HbF levels in Thais with either beta-thalassemia or HbE trait and in African Americans with sickle cell anemia. Taken together, the data suggest that the functional motifs responsible for modulating F-cells and HbF levels reside within a 3 kb region in the second intron of BCL11A.

Transcription Regulation by Class III Histone Deacetylases (HDACs)-Sirtuins

Sirtuins are NAD(+)-dependent histone deacetylases (Class III HDACs). Recently, Sirtuins have been shown to play important roles, both direct and indirect, in transcriptional regulation. This transcriptional control, through incorporation of Sirtuins into transcription complexes and deacetylation of histones locally at gene promoters, or direct interaction with specific transcription factors, is central to the participation of Sirtuins in multiple diverse processes, including aging, apoptosis, hormone responses, stress tolerance, differentiation, metabolism and development. Here we review the contribution of the Sirtuin family, at multiple molecular levels, to transcriptional regulation.

Genome-wide association study shows BCL11A associated with persistent fetal hemoglobin and amelioration of the phenotype of beta-thalassemia

beta-Thalassemia and sickle cell disease both display a great deal of phenotypic heterogeneity, despite being generally thought of as simple Mendelian diseases. The reasons for this are not well understood, although the level of fetal hemoglobin (HbF) is one well characterized ameliorating factor in both of these conditions. To better understand the genetic basis of this heterogeneity, we carried out genome-wide scans with 362,129 common SNPs on 4,305 Sardinians to look for genetic linkage and association with HbF levels, as well as other red blood cell-related traits. Among major variants affecting HbF levels, SNP rs11886868 in the BCL11A gene was strongly associated with this trait (P < 10(-35)). The C allele frequency was significantly higher in Sardinian individuals with elevated HbF levels, detected by screening for beta-thalassemia, and patients with attenuated forms of beta-thalassemia vs. those with thalassemia major. We also show that the same BCL11A variant is strongly associated with HbF levels in a large cohort of sickle cell patients. These results indicate that BCL11A variants, by modulating HbF levels, act as an important ameliorating factor of the beta-thalassemia phenotype, and it is likely they could help ameliorate other hemoglobin disorders. We expect our findings will help to characterize the molecular mechanisms of fetal globin regulation and could eventually contribute to the development of new therapeutic approaches for beta-thalassemia and sickle cell anemia.

A QTL influencing F cell production maps to a gene encoding a zinc-finger protein on chromosome 2p15

F cells measure the presence of fetal hemoglobin, a heritable quantitative trait in adults that accounts for substantial phenotypic diversity of sickle cell disease and beta thalassemia. We applied a genome-wide association mapping strategy to individuals with contrasting extreme trait values and mapped a new F cell quantitative trait locus to BCL11A, which encodes a zinc-finger protein, on chromosome 2p15. The 2p15 BCL11A quantitative trait locus accounts for 15.1% of the trait variance.

Expression of zinc finger transcription factor Bcl11A/Evi9/CTIP1 in rat brain

Bcl11A/Evi9/CTIP1, a Kruppel-like zinc finger gene, plays an important role in B-cell development. In addition to expression in B lymphocytes, Bcl11A/Evi9/CTIP1 is also highly expressed in the brain, although its function there is still unclear. In the present study, regional and subcellular distributions of Bcl11A/Evi9/CTIP1 in rat brain were investigated by immunostaining and biochemical fractionation. Using antibodies recognizing the first 18 amino acid residues of Bcl11A/Evi9/CTIP1, the distribution of 2 isoforms of Bcl11A/Evi9/CTIP1 gene products, Bcl11A-L/Evi9a and Bcl11A-S/Evi9c, was examined. In rat brain, both Bcl11A-L/Evi9a and Bcl11A-S/Evi9c were expressed, although the amount of Bcl11A-S/Evi9c protein was higher. Bcl11A-S/Evi9c was widely expressed in different regions of the rat brain. In contrast, Bcl11A-L/Evi9a was more restricted, being expressed in the cerebral cortex, hippocampus, and olfactory bulb. At the subcellular level, biochemical fractionation and confocal analysis of adult rat brain revealed that, in addition to being in the nuclei of neurons, fractions of Bcl11A-L/Evi9a and Bcl11A-S/Evi9c could be found in extranuclear locations. Double staining with the synaptic marker synaptophysin indicated a synaptic distribution of Bcl11A/Evi9/CTIP1. Postsynaptic density was also biochemically purified and subjected to immunoblotting using Bcl11A/Evi9/CTIP1 antibodies. The results showed that Bcl11A-L/Evi9a was enriched in the PSD I and PSD II fractions. In contrast, only a trace amount of Bcl11A-S was detected in PSD fractions. Our study also indicated that a fraction of Bcl11A/Evi9/CTIP1 was present in the cytoplasm, even at synapses. To regulate gene expression in the nuclei, nuclear translocation of Bcl11A/Evi9/CTIP1 may be one of the mechanisms controlling neuronal Bcl11A/Evi9/CTIP1 function.

Sirtuins in mammals: insights into their biological function

Sirtuins are a conserved family of proteins found in all domains of life. The first known sirtuin, Sir2 (silent information regulator 2) of Saccharomyces cerevisiae, from which the family derives its name, regulates ribosomal DNA recombination, gene silencing, DNA repair, chromosomal stability and longevity. Sir2 homologues also modulate lifespan in worms and flies, and may underlie the beneficial effects of caloric restriction, the only regimen that slows aging and extends lifespan of most classes of organism, including mammals. Sirtuins have gained considerable attention for their impact on mammalian physiology, since they may provide novel targets for treating diseases associated with aging and perhaps extend human lifespan. In this review we describe our current understanding of the biological function of the seven mammalian sirtuins, SIRT1-7, and we will also discuss their potential as mediators of caloric restriction and as pharmacological targets to delay and treat human age-related diseases.

Sirtuins in mammals: insights into their biological function

Sirtuins are a conserved family of proteins found in all domains of life. The first known sirtuin, Sir2 (silent information regulator 2) of Saccharomyces cerevisiae, from which the family derives its name, regulates ribosomal DNA recombination, gene silencing, DNA repair, chromosomal stability and longevity. Sir2 homologues also modulate lifespan in worms and flies, and may underlie the beneficial effects of caloric restriction, the only regimen that slows aging and extends lifespan of most classes of organism, including mammals. Sirtuins have gained considerable attention for their impact on mammalian physiology, since they may provide novel targets for treating diseases associated with aging and perhaps extend human lifespan. In this review we describe our current understanding of the biological function of the seven mammalian sirtuins, SIRT1-7, and we will also discuss their potential as mediators of caloric restriction and as pharmacological targets to delay and treat human age-related diseases.

Inhibition of BCL11B expression leads to apoptosis of malignant but not normal mature T cells

The B-cell chronic lymphocytic leukemia (CLL)/lymphoma 11B gene (BCL11B) encodes a Krüppel-like zinc-finger protein, which plays a crucial role in thymopoiesis and has been associated with hematopoietic malignancies. It was hypothesized that BCL11B may act as a tumor-suppressor gene, but its precise function has not yet been elucidated. Here, we demonstrate that the survival of human T-cell leukemia and lymphoma cell lines is critically dependent on Bcl11b. Suppression of Bcl11b by RNA interference selectively induced apoptosis in transformed T cells whereas normal mature T cells remained unaffected. The apoptosis was effected by simultaneous activation of death receptor-mediated and intrinsic apoptotic pathways, most likely as a result of tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) upregulation and suppression of the Bcl-xL antiapoptotic protein. Our data indicate an antiapoptotic function of Bcl11b. The resistance of normal mature T lymphocytes to Bcl11b suppression-induced apoptosis and restricted expression pattern make it an attractive therapeutic target in T-cell malignancies.

Modeling a whole organ using proteomics: the avian bursa of Fabricius

While advances in proteomics have improved proteome coverage and enhanced biological modeling, modeling function in multicellular organisms requires understanding how cells interact. Here we used the chicken bursa of Fabricius, a common experimental system for B cell function, to model organ function from proteomics data. The bursa has two major functional cell types: B cells and the supporting stromal cells. We used differential detergent fractionation-multidimensional protein identification technology (DDF-MudPIT) to identify 5198 proteins from all cellular compartments. Of these, 1753 were B cell specific, 1972 were stroma specific and 1473 were shared between the two. By modeling programmed cell death (PCD), cell differentiation and proliferation, and transcriptional activation, we have improved functional annotation of chicken proteins and placed chicken-specific death receptors into the PCD process using phylogenetics. We have identified 114 transcription factors (TFs); 42 of the bursal B cell TFs have not been reported before in any B cells. We have also improved the structural annotation of a newly sequenced genome by confirming the in vivo expression of 4006 "predicted", and 6623 ab initio, ORFs. Finally, we have developed a novel method for facilitating structural annotation, "expressed peptide sequence tags" (ePSTs) and demonstrate its utility by identifying 521 potential novel proteins from the chicken "unassigned chromosome".

Resveratrol—A boon for treating Alzheimer's disease?

Resveratrol, a red wine polyphenol, is known to protect against cardiovascular diseases and cancers, as well as to promote antiaging effects in numerous organisms. It also modulates pathomechanisms of debilitating neurological disorders, such as strokes, ischemia, and Huntington's disease. The role of resveratrol in Alzheimer's disease is still unclear, although some recent studies on red wine bioactive compounds suggest that resveratrol modulates multiple mechanisms of Alzheimer's disease pathology. Emerging literature indicates that mechanisms of aging and Alzheimer's disease are intricately linked and that these mechanisms can be modulated by both calorie restriction regimens and calorie restriction mimetics, the prime mediator of which is the SIRT1 protein, a human homologue of yeast silent information regulator (Sir)-2, and a member of NAD+-dependent histone deacetylases. Calorie restriction regimens and calorie restriction-mimetics trigger sirtuins in a wide variety of organisms, ranging from bacteria to mouse. In a mouse model of Huntington's disease, resveratrol-induced SIRT1 was found to protect neurons against ployQ toxicity and in Wallerian degeneration slow mice, resveratrol was found to protect the degeneration of neurons from axotomy, suggesting that resveratrol may possess therapeutic value to neuronal degeneration. This paper mainly focuses on the role of resveratrol in modulating AD pathomechanisms.

CTIP2 associates with the NuRD complex on the promoter of p57KIP2, a newly identified CTIP2 target gene

Chicken ovalbumin upstream promoter transcription factor (COUP-TF)-interacting protein 2 (CTIP2), also known as Bcl11b, is a transcriptional repressor that functions by direct, sequence-specific DNA binding activity or by recruitment to the promoter template by interaction with COUP-TF family members. CTIP2 is essential for both T cell development and axonal projections of corticospinal motor neurons in the central nervous system. However, little is known regarding the molecular mechanism(s) by which CTIP2 contributes to either process. CTIP2 complexes that were isolated from SK-N-MC neuroblastoma cells were found to harbor substantial histone deacetylase activity, which was likely conferred by the nucleosome remodeling and deacetylation (NuRD) complex. CTIP2 was found to associate with the NuRD complex through direct interaction with both RbAp46 and RbAp48, and components of the NuRD complex were found to be recruited to an artificial promoter template in a CTIP2-dependent manner in transfected cells. Finally, the NuRD complex and CTIP2 were found to co-occupy the promoter template of p57KIP2, a gene encoding a cyclin-dependent kinase inhibitor, and identified herein as a novel transcriptional target of CTIP2 in SK-N-MC cells. Therefore, it seems likely that the NuRD complex may be involved in transcriptional repression of CTIP2 target genes and contribute to the function(s) of CTIP2 within a neuronal context.

BCL11B participates in the activation of IL2 gene expression in CD4+ T lymphocytes

BCL11A and BCL11B are transcriptional regulators important for lymphopoiesis and previously associated with hematopoietic malignancies. Ablation of the mouse Bcl11b locus results in failure to generate double-positive thymocytes, implicating a critical role of Bcl11b in T-cell development. However, BCL11B is also expressed in CD4+ T lymphocytes, both in resting and activated states. Here we show both in transformed and primary CD4+ T cells that BCL11B participates in the control of the interleukin-2 (IL2) gene expression following activation through T-cell receptor (TCR). BCL11B augments expression from the IL2 promoter through direct binding to the US1 site. In addition, BCL11B associates with the p300 coactivator in CD4+ T cells activated through TCR, which may account for its transcriptional activation function. These results provide the first evidence that BCL11B, originally described as a transcriptional repressor, activates transcription of a target gene in the context of T-cell activation.

Inhibition of SIRT1 reactivates silenced cancer genes without loss of promoter DNA hypermethylation

The class III histone deactylase (HDAC), SIRT1, has cancer relevance because it regulates lifespan in multiple organisms, down-regulates p53 function through deacetylation, and is linked to polycomb gene silencing in Drosophila. However, it has not been reported to mediate heterochromatin formation or heritable silencing for endogenous mammalian genes. Herein, we show that SIRT1 localizes to promoters of several aberrantly silenced tumor suppressor genes (TSGs) in which 5′ CpG islands are densely hypermethylated, but not to these same promoters in cell lines in which the promoters are not hypermethylated and the genes are expressed. Heretofore, only type I and II HDACs, through deactylation of lysines 9 and 14 of histone H3 (H3-K9 and H3-K14, respectively), had been tied to the above TSG silencing. However, inhibition of these enzymes alone fails to re-activate the genes unless DNA methylation is first inhibited. In contrast, inhibition of SIRT1 by pharmacologic, dominant negative, and siRNA (small interfering RNA)–mediated inhibition in breast and colon cancer cells causes increased H4-K16 and H3-K9 acetylation at endogenous promoters and gene re-expression despite full retention of promoter DNA hypermethylation. Furthermore, SIRT1 inhibition affects key phenotypic aspects of cancer cells. We thus have identified a new component of epigenetic TSG silencing that may potentially link some epigenetic changes associated with aging with those found in cancer, and provide new directions for therapeutically targeting these important genes for re-expression.

The propensity for cancer to arise and progress is influenced not only by gene mutations (genetic abnormalities), but also by defects in gene expression programs that are inherited from one dividing cell to another. This change in the inheritance of gene expression patterns not associated with changes in the primary DNA sequence is referred to as an epigenetic abnormality. In virtually every form of cancer, tumor suppressor genes (TSGs) and candidate TSGs are epigenetically altered such that the ability of these genes to become activated and lead to production of the corresponding proteins is lost. This so-called gene “silencing” is often linked with abnormal accumulation of methyl groups to DNA (DNA methylation) in a region of the gene that controls its expression. The SIRT1 protein is an enzyme that can remove acetyl groups attached to specific amino acids in a number of different protein targets and thereby regulate gene silencing in yeast. However, in mammalian cells this has not been demonstrated. Here, the authors show SIRT1 is involved in epigenetic silencing of DNA-hypermethylated TSGs in cancer cells. Inhibition of SIRT1 by multiple approaches leads to TSG re-expression and a block in tumor-causing networks of cell signaling that are activated by loss of the TSGs in a wide range of cancers. This finding has important ramifications for the biology of cancer in terms of what maintains abnormal gene silencing. Furthermore, the authors propose that their observations may have potential clinical relevance in suggesting new means for restoring expression of abnormally silenced genes in cancer.

[Chromosome arm 17p13.3: could HIC1 be the one ?]

Loss of heterozygosity (LOH) of the short arm of chromosome 17 (17p) is one of the most frequent genetic alterations in human cancers. Most often, allelic losses coincide with p53 mutations at 17p13.1. However, in many types of solid tumors including sporadic breast cancers, ovarian cancers, medulloblastomas and small cell lung carcinomas, frequent LOH or DNA methylation changes occur in a more telomeric region at 17p13.3, in absence of any p53 genetic alterations. These results suggest that one or more tumor suppressor genes located at 17p13.3 could be involved in tumorigenesis. In addition, the 17p13.3 region has also been implicated in the Miller-Dieker syndrome (MDS), a severe form of lissencephaly accompanied by developmental anomalies caused by heterozygous gene deletions. Analyses of deletion mapping and CpG island methylation patterns have resulted in the identification of two tumor suppressor genes at 17p13.3, HIC1 (hypermethylated in cancer 1) and OVCA1 (ovarian cancer gene 1). HIC1 is a tumor suppressor gene that encodes a transcriptional repressor with five Krüppel-like C2H2 zinc finger motifs and a N-terminal BTB/POZ domain. Clues to the tumor suppressor function of HIC1 have come from the study of heterozygous Hic1+/- mice, which develop spontaneous malignant tumors of different types. Generation of double heterozygous knockout mice Hic1+/- p53+/- provides strong evidence that epigenetically silenced genes such as HIC1 can significantly influence tumorigenesis driven by mutations of classic tumor suppressor genes. This functional cooperation between HIC1 and p53 is interesting and recently, its has been demonstrated that HIC1 was involved in a certain feedback regulation for p53 in tumor suppression through the histone deacetylase SIRT1. However, despite the fact that epigenetic oncogenesis is one of the most vibrant areas of biologic research, the determinants between genetic versus epigenetic routes of tumor suppressor gene inactivation remain elusive.