HDAC4, a human histone deacetylase related to yeast HDA1, is a transcriptional corepressor

Functional characterization of an amino-terminal region of HDAC4 that possesses MEF2 binding and transcriptional repressive activity

Like the full-length histone deacetylase (HDAC) 4, its amino terminus (amino acids 1-208) without the carboxyl deacetylase domain is also known to effectively bind and repress myocyte enhancer factor 2 (MEF2). Within this repressive amino terminus, we further show that a stretch of 90 amino acids (119-208) displays MEF2 binding and repressive activity. The same region is also found to associate specifically with HDAC1 which is responsible for the repressive effect. The amino terminus of HDAC4 can associate with the DNA-bound MEF2 in vitro, suggesting that it does not repress MEF2 simply by disrupting the ability of MEF2 to bind DNA. In vivo, MEF2 induces nuclear translocation of both the full-length HDAC4 and HDAC4-(1-208), whereas the nuclear HDAC4 as well as HDAC4-(1-208) in turn specifically sequesters MEF2 to distinct nuclear bodies. In addition, we show that MyoD and HDAC4 functionally antagonize each other to regulate MEF2 activity. Combined with data from others, our data suggest that the full-length HDAC4 can repress MEF2 through multiple independent repressive domains.

Calcium/calmodulin-dependent protein kinase activates serum response factor transcription activity by its dissociation from histone deacetylase, HDAC4. Implications in cardiac muscle gene regulation during hypertrophy

Serum response factor (SRF) plays a pivotal role in cardiac myocyte development, muscle gene transcription, and hypertrophy. Previously, elevation of intracellular levels of Ca2+ was shown to activate SRF function without involving the Ets family of tertiary complex factors through an unknown regulatory mechanism. Here, we tested the hypothesis that the chromatin remodeling enzymes of class II histone deacetylases (HDAC4) regulate SRF activity in a Ca2+-sensitive manner. Expression of HDAC4 profoundly repressed SRF-mediated transcription in both muscle and nonmuscle cells. Protein interaction studies demonstrated physical association of HDAC4 with SRF in living cells. The SRF/HDAC4 co-association was disrupted by treatment of cells with hypertrophic agonists such as angiotensin-II and a Ca2+ ionophore, ionomycin. Furthermore, activation of Ca2+/calmodulin-dependent protein kinase (CaMK)-IV prevented SRF/HDAC4 interaction and derepressed SRF-dependent transcription activity. The SRF.HDAC4 complex was localized to the cell nucleus, and the activated CaMK-IV disrupted HDAC4/SRF association, leading to export of HDAC4 from the nucleus and stimulation of SRF transcription activity. Thus, these results identify SRF as a functional interacting target of HDAC4 and define a novel tertiary complex factor-independent mechanism for SRF activation by Ca2+/CaMK-mediated signaling.

Roles of cardiac transcription factors in cardiac hypertrophy

Different cell types, equipped with unique structure and function, synthesize different sets of proteins on the basis of different patterns of gene expression, even though their genomes are identical. Cardiac transcription factors have been reported to control a cardiac gene program and thus to play a crucial role in transcriptional regulation during embryogenesis. Recently, postnatal roles of cardiac transcription factors have been extensively investigated. Consistent with the direct transactivation of numerous cardiac genes reactivated in response to hypertrophic stimulation, cardiac transcription factors are profoundly involved in the generation of cardiac hypertrophy or in cardioprotection from cytotoxic stress in the adult heart. In this review, the regulation of a cardiac gene program by cardiac transcription factors is summarized, with an emphasis on their potential role in the generation of cardiac hypertrophy.

Direct interaction of Ca2+/calmodulin inhibits histone deacetylase 5 repressor core binding to myocyte enhancer factor 2

Myocyte enhancer factor 2 (MEF2) proteins play a pivotal role in the differentiation of cardiac and skeletal muscle cells. MEF2 factors are regulated by histone deacetylase enzymes such as histone deacetylase 5 (HDAC5). HDAC5 in turn is responsive to Ca(2+) signaling mediated by the intracellular calcium sensor calmodulin. Here a combination of proteolytic fragmentation, matrix-assisted laser desorption ionization mass spectrometry, Edman degradation, circular dichroism, gel filtration, and surface plasmon resonance studies is utilized to define and characterize a stable core domain of HDAC5 and to examine its interactions with MEF2a and calmodulin. Results from real time binding experiments provide evidence for direct interaction of Ca(2+)/calmodulin with HDAC5 inhibiting MEF2a association with this enzyme.

A novel EID-1 family member, EID-2, associates with histone deacetylases and inhibits muscle differentiation

An EID-1 (E1A-like inhibitor of differentiation-1) inhibits differentiation by blocking the histone acetyltransferase activity of p300. Here we report a novel inhibitor of differentiation exhibiting homology to EID-1, termed EID-2 (EID-1-like inhibitor of differentiation-2). EID-2 inhibited MyoD-dependent transcription and muscle differentiation. Unlike EID-1, EID-2 did not block p300 activity. Interestingly, EID-2 associated with class I histone deacetylases (HDACs). The N-terminal portion of EID-2 was required for the binding to HDACs. This region was also involved in the transcriptional repression and nuclear localization, suggesting the importance of the involvement of HDACs in the EID-2 function. These results indicate a new family of differentiation inhibitors, although there are several differences in the biochemical mechanisms between EID-2 and EID-1.

Class II histone deacetylases: versatile regulators

Histone acetylation and deacetylation play essential roles in modifying chromatin structure and regulating gene expression in eukaryotes. Histone deacetylases (HDACs) catalyze the deacetylation of lysine residues in the histone N-terminal tails and are found in large multiprotein complexes with transcriptional co-repressors. Human HDACs are grouped into three classes based on their similarity to known yeast factors: class I HDACs are similar to the yeast transcriptional repressor yRPD3, class II HDACs to yHDA1 and class III HDACs to ySIR2. In this review, we focus on the biology of class II HDACs. These newly discovered enzymes have been implicated as global regulators of gene expression during cell differentiation and development. We discuss their emerging biological functions and the molecular mechanisms by which they are regulated.

Molecular characterization of a familial translocation implicates disruption of HDAC9 and possible position effect on TGFbeta2 in the pathogenesis of Peters' anomaly

Peters' anomaly (PA) is a congenital defect of the anterior chamber of the eye. We identified a family in which an apparently balanced chromosomal translocation t(1;7) (q41;p21) was associated with PA. Based on this observation, detailed molecular characterizations of the breakpoint regions and candidate genes were carried out. A candidate gene from each breakpoint was identified: on chromosome 7, histone deacetylase 9 (HDAC9), disrupted by the translocation breakpoint, and on chromosome 1, transforming growth factor-beta2 (TGFbeta2) located 500 kb proximal to the breakpoint. An additional lysophospholipase-like 1 gene (LYPLAL1), localized approximately 200 kb distal to the chromosome 1 breakpoint, was also identified and characterized. Although only the HDAC9 gene is disrupted by the breakpoint, we consider that TGFbeta2 represents the main candidate gene in this family, which is elicited in mice by the Tgfbeta2-null status and by the TGFbeta2-induced cataractus changes in animal models. As an alternative scenario, which is supported by the ability of class II HDACs to mediate extracellular TGF-beta stimuli to core histone deacetylation in promoter-adjacent regions, we propose the hypothesis of digenic inheritance. Inappropriate or inadequate TGFbeta2 expression, together with deficient mediation of these signals at the transcription level, due to an altered HDAC9 isoforms ratio, may also lead to the observed ocular phenotype.

Neuronal activity-dependent nucleocytoplasmic shuttling of HDAC4 and HDAC5

The class II histone deacetylases, HDAC4 and HDAC5, directly bind to and repress myogenic transcription factors of the myocyte enhancer factor-2 (MEF-2) family thereby inhibiting skeletal myogenesis. During muscle differentiation, repression of gene transcription by MEF-2/HDAC complexes is relieved due to calcium/calmodulin-dependent (CaM) kinase-induced translocation of HDAC4 and HDAC5 to the cytoplasm. MEF-2 proteins and HDACs are also highly expressed in the nervous system and have been implicated in neuronal survival and differentiation. Here we investigated the possibility that the subcellular localization of HDACs, and thus their ability to repress target genes, is controlled by synaptic activity in neurones. We found that, in cultured hippocampal neurones, the localization of HDAC4 and HDAC5 is dynamic and signal-regulated. Spontaneous electrical activity was sufficient for nuclear export of HDAC4 but not of HDAC5. HDAC5 translocation to the cytoplasm was induced following stimulation of calcium flux through synaptic NMDA receptors or L-type calcium channels; glutamate bath application (stimulating synaptic and extrasynaptic NMDA receptors) antagonized nuclear export. Activity-induced nucleocytoplasmic shuttling of both HDACs was partially blocked by the CaM kinase inhibitor KN-62 with HDAC5 nuclear export being more sensitive to CaM kinase inhibition than that of HDAC4. Thus, the subcellular localization of HDACs in neurones is specified by neuronal activity; differences in the activation thresholds for HDAC4 and HDAC5 nuclear export provides a mechanism for input-specific gene expression.

Histone deacetylase 4 interacts with 53BP1 to mediate the DNA damage response

Anumber of proteins are recruited to nuclear foci upon exposure to double-strand DNA damage, including 53BP1 and Rad51, but the precise role of these DNA damage-induced foci remain unclear. Here we show in a variety of human cell lines that histone deacetylase (HDAC) 4 is recruited to foci with kinetics similar to, and colocalizes with, 53BP1 after exposure to agents causing double-stranded DNA breaks. HDAC4 foci gradually disappeared in repair-proficient cells but persisted in repair-deficient cell lines or cells irradiated with a lethal dose, suggesting that resolution of HDAC4 foci is linked to repair. Silencing of HDAC4 via RNA interference surprisingly also decreased levels of 53BP1 protein, abrogated the DNA damage-induced G2 delay, and radiosensitized HeLa cells. Our combined results suggest that HDAC4 is a critical component of the DNA damage response pathway that acts through 53BP1 and perhaps contributes in maintaining the G2 cell cycle checkpoint.

Delayed rRNA processing results in significant ribosome biogenesis and functional defects

mof6-1 was originally isolated as a recessive mutation in Saccharomyces cerevisiae which promoted increased efficiencies of programmed -1 ribosomal frameshifting and rendered cells unable to maintain the killer virus. Here, we demonstrate that mof6-1 is a unique allele of the histone deacetylase RPD3, that the deacetylase function of Rpd3p is required for controlling wild-type levels of frameshifting and virus maintenance, and that the closest human homolog can fully complement these defects. Loss of the Rpd3p-associated histone deacetylase function, either by mutants of rpd3 or loss of the associated gene product Sin3p or Sap30p, results in a delay in rRNA processing rather than in an rRNA transcriptional defect. This results in production of ribosomes having lower affinities for aminoacyl-tRNA and diminished peptidyltransferase activities. We hypothesize that decreased rates of peptidyl transfer allow ribosomes with both A and P sites occupied by tRNAs to pause for longer periods of time at -1 frameshift signals, promoting increased programmed -1 ribosomal frameshifting efficiencies and subsequent loss of the killer virus. The frameshifting defect is accentuated when the demand for ribosomes is highest, suggesting that rRNA posttranscriptional modification is the bottleneck in ribosome biogenesis.

Histone deacetylases: unique players in shaping the epigenetic histone code

The epigenome is defined by DNA methylation patterns and the associated posttranslational modifications of histones. This histone code determines the expression status of individual genes dependent upon their localization on the chromatin. The silencing of gene expression is associated with deacetylated histones, which are often found to be associated with regions of DNA methylation as well as methylation at the lysine 4 residue of histone 3. In contrast, the activation of gene expression is associated with acetylated histones and methylation at the lysine 9 residue of histone 3. The histone deactylases play a major role in keeping the balance between the acetylated and deacetylated states of chromatin. Histone deacetylases (HDACs) are divided into three classes: class I HDACs (HDACs 1, 2, 3, and 8) are similar to the yeast RPD3 protein and localize to the nucleus; class II HDACs (HDACs 4, 5, 6, 7, 9, and 10) are homologous to the yeast HDA1 protein and are found in both the nucleus and cytoplasm; and class III HDACs form a structurally distinct class of NAD-dependent enzymes that are similar to the yeast SIR2 proteins. Since inappropriate silencing of critical genes can result in one or both hits of tumor suppressor gene (TSG) inactivation in cancer, theoretically the reactivation of affected TSGs could have an enormous therapeutic value in preventing and treating cancer. Indeed, several HDAC inhibitors are currently being developed and tested for their potency in cancer chemotherapy. Importantly, these agents are also potentially applicable to chemoprevention if their toxicity can be minimized. Despite the toxic side effects and lack of specificity of some of the inhibitors, progress is being made. With the elucidation of the structures, functions and modes of action of HDACs, finding agents that may be targeted to specific HDACs and potentially reactivate expression of only a defined set of affected genes in cancer will be more attainable.

A role for cell cycle-regulated phosphorylation in Groucho-mediated transcriptional repression

Transcriptional corepressors of the Groucho/transducin-like Enhancer of split (Gro/TLE) family are involved in a variety of cell differentiation mechanisms in both invertebrates and vertebrates. They become recruited to specific promoter regions by forming complexes with a number of different DNA-binding proteins thereby contributing to the regulation of multiple genes. To understand how the functions of Gro/TLE proteins are regulated, it was asked whether their ability to mediate transcriptional repression might be controlled by cell cycle-dependent phosphorylation events. It is shown here that activation of p34(cdc2) kinase (cdc2) with okadaic acid is correlated with hyperphosphorylation of Gro/TLEs. Moreover, pharmacological inhibition of cdc2 activity results in Gro/TLE dephosphorylation. In agreement with these findings, a purified cdc2-cyclin B complex can directly phosphorylate Gro/TLEs in vitro. Two separate Gro/TLE domains, the CcN and SP regions, contain sequences that are phosphorylated by cdc2. Deletion of these sequences is correlated with loss of Gro/TLE phosphorylation by cdc2 in vitro and okadaic acid-induced Gro/TLE hyperphosphorylation in vivo. In addition, Gro/TLEs are phosphorylated during the G(2)/M phase of the cell cycle, and this is correlated with a decreased nuclear interaction. Finally, the transcription repression ability of Gro/TLEs is enhanced by pharmacological inhibition of cdc2. Taken together, these results demonstrate that Gro/TLE proteins are phosphorylated as a function of the cell cycle and implicate phosphorylation events occurring during mitosis in the negative regulation of Gro/TLE activity.

In vivo destabilization of dynamic microtubules by HDAC6-mediated deacetylation

Trichostatin A (TSA) inhibits all histone deacetylases (HDACs) of both class I and II, whereas trapoxin (TPX) cannot inhibit HDAC6, a cytoplasmic member of class II HDACs. We took advantage of this differential sensitivity of HDAC6 to TSA and TPX to identify its substrates. Using this approach, alpha-tubulin was identified as an HDAC6 substrate. HDAC6 deacetylated alpha-tubulin both in vivo and in vitro. Our investigations suggest that HDAC6 controls the stability of a dynamic pool of microtubules. Indeed, we found that highly acetylated microtubules observed after TSA treatment exhibited delayed drug-induced depolymerization and that HDAC6 overexpression prompted their induced depolymerization. Depolymerized tubulin was rapidly deacetylated in vivo, whereas tubulin acetylation occurred only after polymerization. We therefore suggest that acetylation and deacetylation are coupled to the microtubule turnover and that HDAC6 plays a key regulatory role in the stability of the dynamic microtubules.

Transcription factors in cardiogenesis: the combinations that unlock the mysteries of the heart

Heart formation is one of the first signs of organogenesis within the developing embryo and this process is conserved from flies to man. Completing the genetic roadmap of the molecular mechanisms that control the cell specification and differentiation of cells that form the developing heart has been an exciting and fast-moving area of research in the fields of molecular and developmental biology. At the core of these studies is an interest in the transcription factors that are responsible for initiation of a pluripotent cell to become programmed to the cardiac lineage and the subsequent transcription factors that implement the instructions set up by the cells commitment decision. To gain a better understanding of these pathways, cardiac-expressed transcription factors have been identified, cloned, overexpressed, and mutated to try to determine function. Although results vary depending on the gene in question, it is clear that there is a striking evolutionary conservation of the cardiogenic program among species. As we move up the evolutionary ladder toward man, we encounter cases of functional redundancy and combinatorial interactions that reflect the complex networks of gene expression that orchestrate heart development. This review focuses on what is known about the transcription factors implicated in heart formation and the role they play in this intricate genetic program.

Coupling of DNA synthesis and histone synthesis in S phase independent of cyclin/cdk2 activity

DNA and histone synthesis are both triggered at the beginning of S phase by cyclin/cdk2 activity. Previous studies showed that inhibition of DNA synthesis with hydroxyurea or cytosine arabinoside (AraC) triggers a concerted repression of histone synthesis, indicating that sustained histone synthesis depends on continued DNA synthesis. Here we show that ectopic expression of HIRA, the likely human ortholog of two cell cycle-regulated repressors of histone gene transcription in yeast (Hir1p and Hir2p), represses transcription of histones and that this, in turn, triggers a concerted block of DNA synthesis. Thus, in mammalian cells sustained DNA synthesis and histone synthesis are mutually dependent on each other during S phase. Although cyclin/cdk2 activity drives activation of both DNA and histone synthesis at the G1/S transition of cycling cells, concerted repression of DNA or histone synthesis in response to inhibition of either one of these is not accompanied by prolonged inhibition of cyclin A/cdk2 or E/cdk2 activity. Therefore, during S phase coupling of DNA and histone synthesis occurs, at least in part, through a mechanism that is independent of cyclin/cdk2 activity. Coupling of DNA and histone synthesis in S phase presumably contributes to the prompt and orderly assembly of newly replicated DNA into chromatin.

Association of class II histone deacetylases with heterochromatin protein 1: potential role for histone methylation in control of muscle differentiation

Class II histone deacetylases (HDACs) 4, 5, 7, and 9 repress muscle differentiation through associations with the myocyte enhancer factor 2 (MEF2) transcription factor. MEF2-interacting transcription repressor (MITR) is an amino-terminal splice variant of HDAC9 that also potently inhibits MEF2 transcriptional activity despite lacking a catalytic domain. Here we report that MITR, HDAC4, and HDAC5 associate with heterochromatin protein 1 (HP1), an adaptor protein that recognizes methylated lysines within histone tails and mediates transcriptional repression by recruiting histone methyltransferase. Promyogenic signals provided by calcium/calmodulin-dependent kinase (CaMK) disrupt the interaction of MITR and HDACs with HP1. Since the histone methyl-lysine residues recognized by HP1 also serve as substrates for deacetylation by HDACs, the interaction of MITR and HDACs with HP1 provides an efficient mechanism for silencing MEF2 target genes by coupling histone deacetylation and methylation. Indeed, nucleosomal histones surrounding a MEF2-binding site in the myogenin gene promoter are highly methylated in undifferentiated myoblasts, when the gene is silent, and become acetylated during muscle differentiation, when the myogenin gene is expressed at high levels. The ability of MEF2 to recruit a histone methyltransferase to target gene promoters via HP1-MITR and HP1-HDAC interactions and of CaMK signaling to disrupt these interactions provides an efficient mechanism for signal-dependent regulation of the epigenetic events controlling muscle differentiation.

Class II histone deacetylases are directly recruited by BCL6 transcriptional repressor

BCL6 is a member of the POZ/zinc finger (POK) family involved in survival and/or differentiation of a number of cell types and in B cell lymphoma upon chromosomal alteration. Transcriptional repression by BCL6 is thought to be achieved in part by recruiting a repressor complex containing two class I histone deacetylases (HDACs). In this study we investigated whether BCL6 could also target members of class II HDACs. Our results indicate that three related class II deacetylases, HDAC4, HDAC5, and HDAC7 can associate with BCL6 in vivo and in vitro. Using electron microscopy, we found that endogenous BCL6 and class II HDACs partially co-localize in the nucleus. Overexpression experiments showed that BCL6 and HDAC4, -5, or -7 are intermingled onto common nuclear substructures and form stable complexes. A highly conserved domain in the N-terminal region of HDAC5 and HDAC7 as well as the zinc finger region of BCL6 were found necessary for the complex formation in vivo and in vitro. Moreover, our data point to the zinc finger region of BCL6 as a multifunctional domain which, beside its known capacity to bind DNA, is involved in the nuclear targeting of the protein and in the recruitment of the class II HDACs, and hence constitutes an autonomous repressor domain. Since PLZF, a BCL6 relative, could also interact with HDAC4, -5, and 7, we suggest that class II HDACs are largely involved in the control of the POK transcription factors activity.

Acetylation in hormone signaling and the cell cycle

The last decade has seen a substantial change in thinking about the role of acetylation in regulating diverse cellular processes. The correlation between histone acetylation and gene transcription has been known for many years. The cloning and biochemical characterization of the enzymes that regulate this post-translational modification has led to an understanding of the diverse role histone acetyltransferases (HATs) play in cellular function. Histone acetylases modify histones, transcription factors, co-activators, nuclear transport proteins, structural proteins and components of the cell cycle. This review focuses on the role of histone acetylases in coordinating hormone signaling and the cell cycle. Transition through the cell cycle is regulated by a family of protein kinase holoenzymes, the cyclin-dependent kinases (Cdks) and their heterodimeric cyclin partners. Recent studies have identified important cross-talk between the cell cycle regulatory apparatus and proteins regulating histone acetylation. The evidence for a dynamic interplay between components regulating the cell cycle and acetylation of target substrates provides an important new level of complexity in the mechanisms governing hormone signaling.

MOZ and MORF histone acetyltransferases interact with the Runt-domain transcription factor Runx2

The monocytic leukemia zinc finger protein MOZ and its homologue MORF have been implicated in leukemogenesis. Both MOZ and MORF are histone acetyltransferases with weak transcriptional repression domains and strong transcriptional activation domains, suggesting that they may function as transcriptional coregulators. Here we describe that MOZ and MORF both interact with Runx2 (or Cbfa1), a Runt-domain transcription factor that is known to play important roles in T cell lymphomagenesis and bone development. Through its C-terminal SM (serine- and methionine-rich) domain, MORF binds to Runx2 in vitro and in vivo. Consistent with this, the SM domain of MORF also binds to Runx1 (or AML1), a Runx2 homologue that is frequently altered by leukemia-associated chromosomal translocations. While MORF does not acetylate Runx2, its SM domain potentiates Runx2-dependent transcriptional activation. Moreover, endogenous MORF is required for transcriptional activation by Runx2. Intriguingly, Runx2 negatively regulates the transcriptional activation potential of the SM domain. Like that of MORF, the SM domain of MOZ physically and functionally interacts with Runx2. These results thus identify Runx2 as an interaction partner of MOZ and MORF and suggest that both acetyltransferases are involved in regulating transcriptional activation mediated by Runx2 and its homologues.

Identification of HDAC10, a novel class II human histone deacetylase containing a leucine-rich domain

Histone acetylation is important for regulating chromatin structure and gene expression. Three classes of mammalian histone deacetylases have been identified. Among class II, there are five known members, namely HDAC4, HDAC5, HDAC6, HDAC7 and HDAC9. Here we describe the identification and characterization of a novel class II member termed HDAC10. It is a 669 residue polypeptide with a bipartite modular structure consisting of an N-terminal Hda1p-related putative deacetylase domain and a C-terminal leucine-rich domain. HDAC10 is widely expressed in adult human tissues and cultured mammalian cells. It is enriched in the cytoplasm and this enrichment is not sensitive to leptomycin B, a specific inhibitor known to block the nuclear export of other class II members. The leucine-rich domain of HDAC10 is responsible for its cytoplasmic enrichment. Recombinant HDAC10 protein possesses histone deacetylase activity, which is sensitive to trichostatin A, a specific inhibitor for known class I and class II histone deacetylases. When tethered to a promoter, HDAC10 is able to repress transcription. Furthermore, HDAC10 interacts with HDAC3 but not with HDAC4 or HDAC6. These results indicate that HDAC10 is a novel class II histone deacetylase possessing a unique leucine-rich domain.

Isolation and characterization of mammalian HDAC10, a novel histone deacetylase

Acetylation of histone core particles plays an important role in modulating chromatin structure and gene expression. The acetylation status of the histone tails is determined by two opposing enzymatic activities, histone acetyltransferases and histone deacetylases (HDACs). Here we describe the isolation and characterization of HDAC10, a novel class II histone deacetylase. Molecular cloning and Northern blot analyses reveal that the HDAC10 transcript is widely expressed and subjected to alternative splicing. HDAC10 is both nuclear and cytoplasmic, a feature reminiscent of HDACs 4, 5, and 7. Distinct from other family members, HDAC10 harbors an amino-terminal catalytic domain and a carboxyl pseudo-repeat that shares significant homology with its catalytic domain. Mutational analysis reveals that transcriptional repression by HDAC10 requires its intrinsic histone deacetylase activity. Taken together, HDAC10 represents a distinct HDAC that may play a role in transcription regulation.

Histone deacetylase inhibitors in cancer treatment

Histone deacetylase (HDAC) inhibitors are emerging as an exciting new class of potential anticancer agents for the treatment of solid and hematological malignancies. In recent years, an increasing number of structurally diverse HDAC inhibitors have been identified that inhibit proliferation and induce differentiation and/or apoptosis of tumor cells in culture and in animal models. HDAC inhibition causes acetylated nuclear histones to accumulate in both tumor and normal tissues, providing a surrogate marker for the biological activity of HDAC inhibitors in vivo. The effects of HDAC inhibitors on gene expression are highly selective, leading to transcriptional activation of certain genes such as the cyclin-dependent kinase inhibitor p21WAF1/CIP1 but repression of others. HDAC inhibition not only results in acetylation of histones but also transcription factors such as p53, GATA-1 and estrogen receptor-alpha. The functional significance of acetylation of non-histone proteins and the precise mechanisms whereby HDAC inhibitors induce tumor cell growth arrest, differentiation and/or apoptosis are currently the focus of intensive research. Several HDAC inhibitors have shown impressive antitumor activity in vivo with remarkably little toxicity in preclinical studies and are currently in phase I clinical trial. The focus of this review is the development and clinical application of HDAC inhibitors for the treatment of cancer.

Enzymatic activity associated with class II HDACs is dependent on a multiprotein complex containing HDAC3 and SMRT/N-CoR

Histone deacetylases (HDACs) play a key role in regulating eukaryotic gene expression. The HDAC domain, homologous to the yeast repressors RPD3 and HDA1, is considered necessary and sufficient for enzymatic activity. Here, we show that the catalytic domain of HDAC4 interacts with HDAC3 via the transcriptional corepressor N-CoR/SMRT. All experimental conditions leading to the suppression of HDAC4 binding to SMRT/N-CoR and to HDAC3 result in the loss of enzymatic activity associated with HDAC4. In vitro reconstitution experiments indicate that HDAC4 and other class II HDACs are inactive in the context of the SMRT/N-CoR-HDAC3 complex and do not contribute to its enzymatic activity. These observations indicate that class II HDACs regulate transcription by bridging the enzymatically active SMRT/N-CoR-HDAC3 complex and select transcription factors independently of any intrinsic HDAC activity.

Mechanism for nucleocytoplasmic shuttling of histone deacetylase 7

Here we show that HDAC7, a member of the class II histone deacetylases, specifically targets several members of myocyte enhancer factors, MEF2A, -2C, and -2D, and inhibits their transcriptional activity. Furthermore, we demonstrate that DNA-bound MEF2C is capable of recruiting HDAC7, demonstrating that the HDAC7-dependent repression of transcription is not due to the inhibition of the MEF2 DNA binding activity. The data also suggest that the promoter bound MEF2 is potentially capable of remodeling adjacent nucleosomes via the recruitment of HDAC7. We have also observed a nucleocytoplasmic shuttling of HDAC7 and dissected the mechanism involved. In NIH3T3 cells, HDAC7 was primarily localized in the cytoplasm, essentially due to an active CRM1-dependent export of the protein from the nucleus. Interestingly, in HeLa cells, HDAC7 was predominantly nuclear. In these cells we could restore the cytoplasmic localization of HDAC7 by expressing CaMK I. This CaMK I-induced nuclear export of HDAC7 was abolished when three critical serines, Ser-178, Ser-344, and Ser-479, of HDAC7 were mutated. We show that these serines are involved in the direct interaction of HDAC7 with 14-3-3. Mutations of these serine residues weakened the association with 14-3-3 and dramatically enhanced the repression activity of HDAC7 in NIH3T3 cells, but not in HeLa cells. Data presented in this work clearly show that the signal dependent subcellular localization of HDAC7 is essential in controlling its activities. The data also show that the cellular concentration of factors such as 14-3-3, CaMK I, and other yet unknown molecules may determine the subcellular localization of an individual HDAC member in a cell type and HDAC-specific manner.

Structure of histone deacetylases: insights into substrate recognition and catalysis

Histone deacetylases catalyze the removal of the acetyl moiety from acetyl-lysine within histones to promote gene repression and silencing. These enzymes fall into distinct families based on primary sequence homology and functional properties in vivo. Recent structural studies of histone deacetylases and their homologs from bacteria have provided important insights into the mode of substrate recognition and catalysis by these enzymes.

Control of muscle development by dueling HATs and HDACs

Skeletal muscle cells have provided an especially auspicious system in which to dissect the roles of chromatin structure in the control of cell growth, differentiation, and development. The MyoD and MEF2 families of transcription factors act cooperatively to regulate the expression of skeletal muscle-specific genes. Recent studies have shown that these two classes of transcription factors associate with histone acetyltransferases and histone deacetylases to control the activation and repression, respectively, of the muscle differentiation program. Signaling systems that regulate the growth and differentiation of muscle cells act, at least in part, by regulating the intracellular localization and associations of these chromatin remodeling enzymes with myogenic transcription factors. We describe the molecules and mechanisms involved in chromatin remodeling during skeletal muscle development.

Human HDAC7 histone deacetylase activity is associated with HDAC3 in vivo

Histone deacetylases (HDACs) are part of transcriptional corepressor complexes and play key roles in regulating chromatin structure. Three different classes of human HDACs have been defined based on their homology to HDACs found in Saccharomyces cerevisiae: RPD3 (class I), HDA1 (class II), and SIR2 (class III). Here we describe the identification and functional characterization of HDAC7, a new member of the human class II HDAC family. Although HDAC7 is localized mostly to the cell nucleus, it is also found in the cytoplasm, suggesting nucleocytoplasmic shuttling. The HDAC activity of HDAC7 maps to a carboxyl-terminal domain and is dependent on the interaction with the class I HDAC, HDAC3, in the cell nucleus. Cytoplasmic HDAC7 that is not bound to HDAC3 is enzymatically inactive. We provide evidence that the transcriptional corepressors SMRT and N-CoR could serve as critical mediators of HDAC7 activity by binding class II HDACs and HDAC3 by two distinct repressor domains. Different class II HDACs reside in the cell nucleus in stable and autonomous complexes with enzymatic activity, but the enzymatic activities associated with HDAC7 and HDAC4 rely on shared cofactors, including HDAC3 and SMRT/N-CoR.

Cloning and characterization of a histone deacetylase, HDAC9

Histone deacetylase (HDAC) catalyzes the removal of the acetyl group from the lysine residues in the N-terminal tails of nucleosomal core histones. Eight human HDACs have been identified so far. Here, we report the identification of a ninth member of the HDAC family, designated HDAC9. HDAC9 is a class II HDAC and its gene resides on human chromosome 7. HDAC9 has several alternatively spliced isoforms. One of these isoforms is histone deacetylase-related protein or myocyte enhancer-binding factor 2-interacting transcriptional repressor that we and others have previously reported and which does not possess an HDAC catalytic domain. The longest of the HDAC9 isoforms contains 1,011 aa. The isoform, designated HDAC9a, is 132 aa shorter at the C terminus than HDAC9. Also, we have identified isoforms of HDAC9 that lack the nuclear localization signal. Similar to histone deacetylase-related protein, HDAC9 transcripts are expressed at high levels in brain and skeletal muscle. The ratio of HDAC9 and HDAC9a transcripts differs among the tissues examined. HDAC9 and HDAC9a contain the HDAC catalytic domain, and Flag-tagged HDAC9 and HDAC9a possess deacetylase activity. HDAC9 and HDAC9a also repress myocyte enhancer-binding factor 2-mediated transcription. In the present study, we have identified HDAC9 and a number of alternatively spliced isoforms of HDAC9 with potentially different biological activities.

Identification of a signal-responsive nuclear export sequence in class II histone deacetylases

Activation of muscle-specific genes by the MEF2 transcription factor is inhibited by class II histone deacetylases (HDACs) 4 and 5, which contain carboxy-terminal deacetylase domains and amino-terminal extensions required for association with MEF2. The inhibitory action of HDACs is overcome by myogenic signals which disrupt MEF2-HDAC interactions and stimulate nuclear export of these transcriptional repressors. Nucleocytoplasmic trafficking of HDAC5 is mediated by binding of the chaperone protein 14-3-3 to two phosphoserine residues (Ser-259 and Ser-498) in its amino-terminal extension. Here we show that HDAC4 and -5 each contain a signal-responsive nuclear export sequence (NES) at their extreme carboxy termini. The NES is conserved in another class II HDAC, HDAC7, but is absent in class I HDACs and the HDAC-related corepressor, MEF2-interacting transcription repressor. Our results suggest that this conserved NES is inactive in unphosphorylated HDAC5, which is localized to the nucleus, and that calcium-calmodulin-dependent protein kinase (CaMK)-dependent binding of 14-3-3 to phosphoserines 259 and 498 activates the NES, with consequent export of the transcriptional repressor to the cytoplasm. A single amino acid substitution in this NES is sufficient to retain HDAC5 in the nucleus in the face of CaMK signaling. These findings provide molecular insight into the mechanism by which extracellular cues alter chromatin structure to promote muscle differentiation and other MEF2-regulated processes.

Histone deacetylase 4 possesses intrinsic nuclear import and export signals

Nucleocytoplasmic trafficking of histone deacetylase 4 (HDAC4) plays an important role in regulating its function, and binding of 14-3-3 proteins is necessary for its cytoplasmic retention. Here, we report the identification of nuclear import and export sequences of HDAC4. While its N-terminal 118 residues modulate the nuclear localization, residues 244 to 279 constitute an authentic, strong nuclear localization signal. Mutational analysis of this signal revealed that three arginine-lysine clusters are necessary for its nuclear import activity. As for nuclear export, leucine-rich sequences located in the middle part of HDAC4 do not function as nuclear export signals. By contrast, a hydrophobic motif (MXXLXVXV) located at the C-terminal end serves as a nuclear export signal that is necessary for cytoplasmic retention of HDAC4. This motif is required for CRM1-mediated nuclear export of HDAC4. Furthermore, binding of 14-3-3 proteins promotes cytoplasmic localization of HDAC4 by both inhibiting its nuclear import and stimulating its nuclear export. Unlike wild-type HDAC4, a point mutant with abrogated MEF2-binding ability remains cytoplasmic upon exogenous expression of MEF2C, supporting the notion that direct MEF2 binding targets HDAC4 to the nucleus. Therefore, HDAC4 possesses intrinsic nuclear import and export signals for its dynamic nucleocytoplasmic shuttling, and association with 14-3-3 and MEF2 proteins affects such shuttling and thus directs HDAC4 to the cytoplasm and the nucleus, respectively.

Bovine herpesvirus 1 tegument protein VP22 interacts with histones, and the carboxyl terminus of VP22 is required for nuclear localization

The bovine herpesvirus 1 (BHV-1) UL49 gene encodes a viral tegument protein termed VP22. UL49 homologs are conserved among alphaherpesviruses. Interestingly, the BHV-1 VP22 deletion mutant virus is asymptomatic and avirulent in infected cattle but produces only a slight reduction in titer in vitro. Attenuation of the BHV-1 VP22 deletion mutant virus in vivo suggests that VP22 plays a functional role in BHV-1 replication. In herpes simplex virus type 1, the VP22 homolog was previously shown to interact with another tegument protein,VP16, the alpha-transinducing factor in vitro. In this report, we show that (i) the nuclear targeting of VP22 is independent of other viral factors, (ii) the carboxyl terminus of VP22 is required for its nuclear localization, (iii) VP22 associates with histones and nucleosomes, (iv) an antihistone monoclonal antibody cross-reacts with VP22, and (v) acetylation of histone H4 is decreased in VP22-expressing cells as well as virus-infected cells. Our data suggest that VP22 may have a modulatory function during BHV-1 infection.

Differential localization of HDAC4 orchestrates muscle differentiation

The class II histone deacetylases HDAC4 and HDAC5 interact specifically with the myogenic MEF2 transcription factor and repress its activity. Here we show that HDAC4 is cytoplasmic during myoblast differentiation, but relocates to the nucleus once fusion has occurred. Inappropriate nuclear entry of HDAC4 following overexpression suppresses the myogenic programme as well as MEF2-dependent transcription. Activation of the Ca(2+)/calmodulin signalling pathway via constitutively active CaMKIV prevents nuclear entry of HDAC4 and HDAC4-mediated inhibition of differentiation. Consistent with a role of phosphorylation in HDAC4 cytoplasmic localisation, HDAC4 binds to 14-3-3 proteins in a phosphorylation-dependent manner. Together these data establish a role for HDAC4 in muscle differentiation. Recently, HDAC5 has also been implicated in muscle differentiation. However, despite the functional similarities of HDAC4 and HDAC5, their intracellular localisations are opposed, suggesting a distinct role for these enzymes during muscle differentiation.

SMRTE inhibits MEF2C transcriptional activation by targeting HDAC4 and 5 to nuclear domains

The silencing mediator for retinoic acid and thyroid hormone receptors (SMRT) mediates transcriptional repression by recruiting histone deacetylases (HDACs) to the DNA-bound nuclear receptor complex. The full-length SMRT (SMRTe) contains an N-terminal sequence that is highly conserved to the nuclear receptor corepressor N-CoR. To date, little is known about the activity and function of the full-length SMRTe protein, despite extensive studies on separated receptor interaction and transcriptional repression domains. Here we show that SMRTe inhibits MEF2C transcriptional activation by targeting selective HDACs to unique subnuclear domains. Indirect immunofluorescence studies with anti-SMRTe antibody reveal discrete cytoplasmic and nuclear speckles, which contain RARalpha in an RA-sensitive manner. Formation of the SMRTe nuclear speckles results in recruitment of several class I and class II HDACs to these subnuclear domains in a process depending on HDAC enzymatic activity. Intriguingly, although HDAC4 is located primarily in the cytoplasm, coexpression of SMRTe dramatically translocates HDAC4 from the cytoplasm into the nucleus, where HDAC4 prevents MEF2C from activating muscle differentiation. SMRTe also translocates HDAC5 from diffusive nucleoplasm into discrete nuclear domains. Accordingly, SMRTe synergizes with HDAC4 and 5 to inhibit MEF2C transactivation of target promoter, suggesting that nuclear domain targeting of HDAC4/5 may be important in preventing muscle cell differentiation. These results highlight an unexpected new function of the nuclear receptor corepressor SMRTe for its role in regulating cellular trafficking of nuclear receptor and selective HDACs that may play an important role in regulation of cell growth and differentiation.

Ca(2+)/CaM-dependent kinases: from activation to function

Calmodulin (CaM) is an essential protein that serves as a ubiquitous intracellular receptor for Ca(2+). The Ca(2+)/CaM complex initiates a plethora of signaling cascades that culminate in alteration of cellular functions. Among the many Ca(2+)/CaM-binding proteins to be discovered, the multifunctional protein kinases CaMKI, II, and IV play pivotal roles. Our review focuses on this class of CaM kinases to illustrate the structural and biochemical basis for Ca(2+)/CaM interaction with and regulation of its target enzymes. Gene transcription has been chosen as the functional endpoint to illustrate the recent advances in Ca(2+)/CaM-mediated signal transduction mechanisms.

Histone hyperacetylation induced by histone deacetylase inhibitors is not sufficient to cause growth inhibition in human dermal fibroblasts

Use of specific histone deacetylase inhibitors has revealed critical roles for the histone deacetylases (HDAC) in controlling proliferation. Although many studies have correlated the function of HDAC inhibitors with the hyperacetylation of histones, few studies have specifically addressed whether the accumulation of acetylated histones, caused by HDAC inhibitor treatment, is responsible for growth inhibition. In the present study we show that HDAC inhibitors cause growth inhibition in normal and transformed keratinocytes but not in normal dermal fibroblasts. This was despite the observation that the HDAC inhibitor, suberic bishydroxamate (SBHA), caused a kinetically similar accumulation of hyperacetylated histones. This cell type-specific response to SBHA was not due to the inactivation of SBHA by fibroblasts, nor was it due to differences in the expression of specific HDAC family members. Remarkably, overexpression of HDACs 1, 4, and 6 in normal human fibroblasts resulted in cells that could be growth-inhibited by SBHA. These data suggest that, although histone acetylation is a major target for HDAC inhibitors, the accumulation of hyperacetylated histones is not sufficient to cause growth inhibition in all cell types. This suggests that growth inhibition, caused by HDAC inhibitors, may be the culmination of histone hyperacetylation acting in concert with other growth regulatory pathways.

The transcriptional corepressor MITR is a signal-responsive inhibitor of myogenesis

Activation of muscle-specific genes by members of the myocyte enhancer factor 2 (MEF2) and MyoD families of transcription factors is coupled to histone acetylation and is inhibited by class II histone deacetylases (HDACs) 4 and 5, which interact with MEF2. The ability of HDAC4 and -5 to inhibit MEF2 is blocked by phosphorylation of these HDACs at two conserved serine residues, which creates docking sites for the intracellular chaperone protein 14-3-3. When bound to 14-3-3, HDACs are released from MEF2 and transported to the cytoplasm, thereby allowing MEF2 to stimulate muscle-specific gene expression. MEF2-interacting transcription repressor (MITR) shares homology with the amino-terminal regions of HDAC4 and -5, but lacks an HDAC catalytic domain. Despite the absence of intrinsic HDAC activity, MITR acts as a potent inhibitor of MEF2-dependent transcription. Paradoxically, however, MITR has minimal inhibitory effects on the skeletal muscle differentiation program. We show that a substitution mutant of MITR containing alanine in place of two serine residues, Ser-218 and Ser-448, acts as a potent repressor of myogenesis. Our findings indicate that promyogenic signals antagonize the inhibitory action of MITR by targeting these serines for phosphorylation. Phosphorylation of Ser-218 and Ser-448 stimulates binding of 14-3-3 to MITR, disrupts MEF2:MITR interactions, and alters the nuclear distribution of MITR. These results reveal a role for MITR as a signal-dependent regulator of muscle differentiation.

Thapsigargin-induced apoptosis involves Cabin1-MEF2-mediated induction of Nur77

Thapsigargin (TG), which inhibits endoplasmic reticulum-dependent Ca(2 +)-ATPase and thereby increases cytosolic Ca(2 +), has been reported to cause apoptosis in T lymphocytes another cell types. In this study, we investigated the molecular mechanisms that are involved in the apoptosis induced by TG in T cell hybridomas. Exposure to TG results in rapid induction of the orphan steroid receptor, Nur77, accompanied by apoptosis of T cell hybridomas. The expression of Nur77 in response to TG treatment is sensitive to cyclosporin A, implicating that activation of calcineurin is necessary for Nur77 expression. The TG-induced Nur77 expression is also inhibited by overexpression of Cabin1, an endogenous inhibitor of calcineurin and a corepressor of the transcription factor MEF2, suggesting that MEF2 activation is required for Nur77 expression. These results suggest that induction of Nur77 expression and apoptosis by TG are mediated by the same signaling pathways that are involved in T cell receptor-mediated thymocyte apoptosis, including the calcineurin pathway and Cabin1-MEF2 pathway.

Class II histone deacetylases: Structure, function, and regulation

Acetylation of histones, as well as non-histone proteins, plays important roles in regulating various cellular processes. Dynamic control of protein acetylation levels in vivo occurs through the opposing actions of histone acetyltransferases and histone deacetylases (HDACs). In the past few years, distinct classes of HDACs have been identified in mammalian cells. Class I members, such as HDAC1, HDAC2, HDAC3, and HDAC8, are well-known enzymatic transcriptional corepressors homologous to yeast Rpd3. Class II members, including HDAC4, HDAC5, HDAC6, HDAC7, and HDAC9, possess domains similar to the deacetylase domain of yeast Hda1. HDAC4, HDAC5, and HDAC7 function as transcriptional corepressors that interact with the MEF2 transcription factors and the N-CoR, BCoR, and CtBP corepressors. Intriguingly, HDAC4, HDAC5, and probably HDAC7 are regulated through subcellular compartmentalization controlled by site-specific phosphorylation and binding of 14-3-3 proteins; the regulation of these HDACs is thus directly linked to cellular signaling networks. Both HDAC6 and HDAC9 possess unique structural modules, so they may have special biological functions. Comprehension of the structure, function, and regulation of class II deacetylases is important for elucidating how acetylation regulates functions of histones and other proteins in vivo.Key words: histone acetylation, protein acetylation, histone deacetylase, 14-3-3 proteins.

Mechanisms of Ca(2+)-dependent transcription

Ca(2+) has a central role in coupling synaptic activity and transcriptional responses. Recent studies have focused on Ca(2+)-dependent nuclear mechanisms that bring to the nucleosomal level cascades of events initiated in the submembranous space at the synapse. In addition, a new Ca(2+)-dependent interaction between a calcium sensor and DNA has been shown to regulate transcription directly.

The emerging role of class II histone deacetylases

Histone acetylation and deacetylation play essential roles in modifying chromatin structure and regulating gene expression in all eukaryotes. Several histone acetyltransferases have been identified that act as transcriptional coactivators. In contrast, histone deacetylases (HDACs) are part of transcriptional corepressor complexes. Based on their similarity to known yeast factors, the human HDACs are grouped into three classes. Class I HDACs are similar to the yeast transcriptional repressor yRPD3, while class II HDACs are related to yHDA1 and class III HDACs to ySIR2. In this review, we focus on the biology of class II HDACs. These newly discovered enzymes have been implicated in cell differentiation and development, and many molecular details are emerging that shed light on class II HDAC function and regulation. We discuss the biological role of these factors in the context of physiological processes.Key words: transcriptional regulation, histone deacetylases, class II HDACs, nucleocytoplasmic shuttling, MEF2.

A dynamic role for HDAC7 in MEF2-mediated muscle differentiation

The overlapping expression profile of MEF2 and the class-II histone deacetylase, HDAC7, led us to investigate the functional interaction and relationship between these regulatory proteins. HDAC7 expression inhibits the activity of MEF2 (-A, -C, and -D), and in contrast MyoD and Myogenin activities are not affected. Glutathione S-transferase pulldown and immunoprecipitation demonstrate that the repression mechanism involves direct interactions between MEF2 proteins and HDAC7 and is associated with the ability of MEF2 to interact with the N-terminal 121 amino acids of HDAC7 that encode repression domain 1. The MADS domain of MEF2 mediates the direct interaction of MEF2 with HDAC7. MEF2 inhibition by HDAC7 is dependent on the N-terminal repression domain and surprisingly does not involve the C-terminal deacetylase domain. HDAC7 interacts with CtBP and other class-I and -II HDACs suggesting that silencing of MEF2 activity involves corepressor recruitment. Furthermore, we show that induction of muscle differentiation by serum withdrawal leads to the translocation of HDAC7 from the nucleus into the cytoplasm. This work demonstrates that HDAC7 regulates the function of MEF2 proteins and suggests that this class-II HDAC regulates this important transcriptional (and pathophysiological) target in heart and muscle tissue. The nucleocytoplasmic trafficking of HDAC7 and other class-II HDACs during myogenesis provides an ideal mechanism for the regulation of HDAC targets during mammalian development and differentiation.

Homo-oligomerisation and nuclear localisation of mouse histone deacetylase 1

Reversible histone acetylation changes the chromatin structure and can modulate gene transcription. Mammalian histone deacetylase 1 (HDAC1) is a nuclear protein that belongs to a growing family of evolutionarily conserved enzymes catalysing the removal of acetyl residues from core histones and other proteins. Previously, we have identified murine HDAC1 as a growth factor-inducible protein in murine T-cells. Here, we characterise the molecular function of mouse HDAC1 in more detail. Co-immunoprecipitation experiments with epitope-tagged HDAC1 protein reveal the association with endogenous HDAC1 enzyme. We show that HDAC1 can homo-oligomerise and that this interaction is dependent on the N-terminal HDAC association domain of the protein. Furthermore, the same HDAC1 domain is also necessary for in vitro binding of HDAC2 and HDAC3, association with RbAp48 and for catalytic activity of the enzyme. A lysine-rich sequence within the carboxy terminus of HDAC1 is crucial for nuclear localisation of the enzyme. We identify a C-terminal nuclear localisation domain, which is sufficient for the transport of HDAC1 and of reporter fusion proteins into the nucleus. Alternatively, HDAC1 can be shuttled into the nucleus by association with another HDAC1 molecule via its N-terminal HDAC association domain. Our results define two domains, which are essential for the oligomerisation and nuclear localisation of mouse HDAC1.

RBP1 recruits the mSIN3-histone deacetylase complex to the pocket of retinoblastoma tumor suppressor family proteins found in limited discrete regions of the nucleus at growth arrest

Retinoblastoma (RB) tumor suppressor family pocket proteins induce cell cycle arrest by repressing transcription of E2F-regulated genes through both histone deacetylase (HDAC)-dependent and -independent mechanisms. In this study we have identified a stable complex that accounts for the recruitment of both repression activities to the pocket. One component of this complex is RBP1, a known pocket-binding protein that exhibits both HDAC-dependent and -independent repression functions. RB family proteins were shown to associate via the pocket with previously identified mSIN3-SAP30-HDAC complexes containing exclusively class I HDACs. Such enzymes do not interact directly with RB family proteins but rather utilize RBP1 to target the pocket. This mechanism was shown to account for the majority of RB-associated HDAC activity. We also show that in quiescent normal human cells this entire RBP1-mSIN3-SAP30-HDAC complex colocalizes with both RB family members and E2F4 in a limited number of discrete regions of the nucleus that in other studies have been shown to represent the initial origins of DNA replication following growth stimulation. These results suggest that RB family members, at least in part, drive exit from the cell cycle by recruitment of this HDAC complex via RBP1 to repress transcription from E2F-dependent promoters and possibly to alter chromatin structure at DNA origins.

Functional significance of histone deacetylase diversity

Nucleocytoplasmic shuttling of histone deacetylases is emerging as a major step in determining the composition, and hence the activity, of the corresponding nuclear regulatory complexes. This shuttling process is one of the distinctive characteristics of these enzymes, themselves belonging to structurally and functionally different classes. Considering the specific features of each class of deacetylases, it is possible to determine how each member can contribute to particular cellular functions.