Histone acetylation-mediated regulation of genes in leukaemic cells

Phase I pharmacokinetic and pharmacodynamic study of LAQ824, a hydroxamate histone deacetylase inhibitor with a heat shock protein-90 inhibitory profile, in patients with advanced solid tumors

PURPOSE

To determine the safety, maximum tolerated dose, and pharmacokinetic-pharmacodynamic profile of a histone deacetylase inhibitor, LAQ824, in patients with advanced malignancy.

PATIENTS AND METHODS

LAQ824 was administered i.v. as a 3-h infusion on days 1, 2, and 3 every 21 days. Western blot assays of peripheral blood mononuclear cell lysates and tumor biopsies pretherapy and posttherapy evaluated target inhibition and effects on heat shock protein-90 (HSP90) client proteins and HSP72.

RESULTS

Thirty-nine patients (22 male; median age, 53 years; median Eastern Cooperative Oncology Group performance status 1) were treated at seven dose levels (mg/m(2)): 6 (3 patients), 12 (4 patients), 24 (4 patients), 36 (4 patients), 48 (4 patients), 72 (19 patients), and 100 (1 patient). Dose-escalation used a modified continual reassessment method. Dose-limiting toxicities were transaminitis, fatigue, atrial fibrillation, raised serum creatinine, and hyperbilirubinemia. A patient with pancreatic cancer treated at 100 mg/m(2) died on course one at day 18 with grade 3 hyperbilirubinemia and neutropenia, fever, and acute renal failure. The area under the plasma concentration curve increased proportionally with increasing dose; median terminal half-life ranged from 8 to 14 hours. Peripheral blood mononuclear cell lysates showed consistent accumulation of acetylated histones posttherapy from 24 mg/m(2); higher doses resulted in increased and longer duration of pharmacodynamic effect. Changes in HSP90 client protein and HSP72 levels consistent with HSP90 inhibition were observed at higher doses. No objective response was documented; 3 patients had stable disease lasting up to 14 months. Based on these data, future efficacy trials should evaluate doses ranging from 24 to 72 mg/m(2).

CONCLUSIONS

LAQ824 was well tolerated at doses that induced accumulation of histone acetylation, with higher doses inducing changes consistent with HSP90 inhibition.

Histone deacetylase inhibitors suppress interleukin-1beta-induced nitric oxide and prostaglandin E2 production in human chondrocytes

OBJECTIVE

Overproduction of nitric oxide (NO) and prostaglandin E(2) (PGE(2)) plays an important role in the pathogenesis of osteoarthritis (OA). In the present study, we determined the effect of trichostatin A (TSA) and butyric acid (BA), two histone deacetylase (HDAC) inhibitors, on NO and PGE(2) synthesis, inducible NO synthase (iNOS) and cyclooxygenase (COX)-2 expression, and nuclear factor (NF)-kappaB DNA-binding activity, in interleukin-1beta (IL-1)-stimulated human OA chondrocytes, and on IL-1-induced proteoglycan degradation in cartilage explants.

METHODS

Chondrocytes were stimulated with IL-1 in the absence or presence of increasing concentrations of TSA or BA. The production of NO and PGE(2) was evaluated using Griess reagent and an enzyme immunoassay, respectively. The expression of iNOS and COX-2 proteins and mRNAs was evaluated using Western blotting and real-time reverse transcriptase-polymerase chain reaction (RT-PCR), respectively. Proteoglycan degradation was measured with dimethymethylene blue assay. Electrophoretic mobility shift assay (EMSA) was utilized to analyze the DNA-binding activity of NF-kappaB.

RESULTS

HDAC inhibition with TSA or BA resulted in a dose-dependent inhibition of IL-1-induced NO and PGE(2) production. IL-17- and tumor necrosis factor-alpha (TNF-alpha)-induced NO and PGE(2) production was also inhibited by TSA and BA. This inhibition correlated with the suppression of iNOS and COX-2 protein and mRNA expression. TSA and BA also prevented IL-1-induced proteoglycan release from cartilage explants. Finally, we demonstrate that the DNA-binding activity of NF-kappaB, was induced by IL-1, but was not affected by treatment with HDAC inhibitors.

CONCLUSIONS

These data indicate that HDAC inhibitors suppressed IL-1-induced NO and PGE(2) synthesis, iNOS and COX-2 expression, as well as proteoglycan degradation. The suppressive effect of HDAC inhibitors is not due to impaired DNA-binding activity of NF-kappaB. These findings also suggest that HDAC inhibitors may be of potential therapeutic value in the treatment of OA.

Postradiation sensitization of the histone deacetylase inhibitor valproic acid

PURPOSE

Preclinical studies evaluating histone deacetylase (HDAC) inhibitor-induced radiosensitization have largely focused on the preirradiation setting based on the assumption that enhanced radiosensitivity was mediated by changes in gene expression. Our previous investigations identified maximal radiosensitization when cells were exposed to HDAC inhibitors in both the preradiation and postradiation setting. We now expand on these studies to determine whether postirradiation exposure alone affects radiosensitivity.

EXPERIMENTAL DESIGN

The effects of the HDAC inhibitor valproic acid (VA) on postirradiation sensitivity in human glioma cell lines were evaluated using a clonogenic assay, exposing cells to VA up to 24 h after irradiation. DNA damage repair was evaluated using gammaH2AX and 53BP1 foci and cell cycle phase distribution was analyzed by flow cytometry. Western blot of acetylated gammaH2AX was done following histone extraction on AUT gels.

RESULTS

VA enhanced radiosensitivity when delivered up to 24 h after irradiation. Cells accumulated in G(2)-M following irradiation, although they returned to baseline at 24 h, mitigating the role of cell cycle redistribution in postirradiation sensitization by VA. At 12 h after irradiation, significant gammaH2AX and 53BP1 foci dispersal was shown in the control, although cells exposed to VA after irradiation maintained foci expression. VA alone had no effect on the acetylation or phosphorylation of H2AX, although it did acetylate radiation-induced gammaH2AX.

CONCLUSIONS

These results indicate that VA enhances radiosensitivity at times up to 24 h after irradiation, which has direct clinical application.

Histone deacetylase inhibitors: molecular mechanisms of action

This review focuses on the mechanisms of action of histone deacetylase (HDAC) inhibitors (HDACi), a group of recently discovered 'targeted' anticancer agents. There are 18 HDACs, which are generally divided into four classes, based on sequence homology to yeast counterparts. Classical HDACi such as the hydroxamic acid-based vorinostat (also known as SAHA and Zolinza) inhibits classes I, II and IV, but not the NAD+-dependent class III enzymes. In clinical trials, vorinostat has activity against hematologic and solid cancers at doses well tolerated by patients. In addition to histones, HDACs have many other protein substrates involved in regulation of gene expression, cell proliferation and cell death. Inhibition of HDACs causes accumulation of acetylated forms of these proteins, altering their function. Thus, HDACs are more properly called 'lysine deacetylases.' HDACi induces different phenotypes in various transformed cells, including growth arrest, activation of the extrinsic and/or intrinsic apoptotic pathways, autophagic cell death, reactive oxygen species (ROS)-induced cell death, mitotic cell death and senescence. In comparison, normal cells are relatively more resistant to HDACi-induced cell death. The plurality of mechanisms of HDACi-induced cell death reflects both the multiple substrates of HDACs and the heterogeneous patterns of molecular alterations present in different cancer cells.

Bcr-Abl-independent imatinib-resistant K562 cells show aberrant protein acetylation and increased sensitivity to histone deacetylase inhibitors

Bcr-Abl-independent signaling pathways are known to be involved in imatinib resistance in some patients with chronic myelogenous leukemia (CML). In this study, to find new targets for imatinib-resistant CML displaying loss of Bcr-Abl kinase target dependence, we isolated imatinib-resistant variants, K562/R1, K562/R2, and K562/R3, which showed profound declines of Bcr-Abl levels and its tyrosine kinase activity, from K562 cells. Importantly, the imatinib resistance mechanism in these variants also included aberrant acetylation of nonhistone proteins such as p53, Ku70, and Hsp90 that was due to upregulation of histone deacetylases (HDACs) and down-regulation of histone acetyltransferase (HAT). In comparison with K562 cells, the imatinib-resistant variants showed up-regulation of HDAC1, -2, and -3 (class I HDACs) and class III SIRT1 and down-regulation of CBP/p300 and PCAF with HAT activity, and thereby p53 and cytoplasmic Ku70 were aberrantly acetylated. In addition, these were associated with down-regulation of Bax and up-regulation of Bcl-2. In contrast, the class II HDAC6 level was significantly decreased, and this was accompanied by an increase of Hsp90 acetylation in the imatinib-resistant variants, which was closely associated with loss of Bcr-Abl. These results indicate that alteration of the normal balance of HATs and HDACs leads to deregulated acetylation of Hsp90, p53, and Ku70 and thereby leads to imatinib resistance, suggesting the importance of the acetylation status of apoptosis-related nonhistone proteins in Bcr-Abl-independent imatinib resistance. We also revealed that imatinib-resistant K562 cells were more sensitive to suberoylanilide hydroxamic acid, an HDAC inhibitor, than K562 cells. These findings may have implications for HDAC as a molecular target in imatinib-resistant leukemia cells.

Effects of histone deacetylase inhibitors, sodium phenyl butyrate and vitamin B3, in combination with retinoic acid on granulocytic differentiation of human promyelocytic leukemia HL-60 cells

Water-soluble vitamin B3, niacin, and its related compounds were suggested to be applicable for medical use. In this article, we examined the anti-leukemic effects of two distinct histone deacetylase (HDAC1 and Sir2) inhibitors, sodium phenyl butyrate (PB) and vitamin B3, respectively, on human promyelocytic leukemia cells HL-60, using HDACIs alone and in combination with all trans retinoic acid (RA). We demonstrated that the HDACI combinations exert different effects on cell cycle arrest and differentiation as determined by nitro blue reduction and the expression of the early myeloid differentiation marker CD11b. The most beneficial effects were found by use of 6-h pretreatment with PB and vitamin B3 before the exposition to RA alone or in combination with vitamin B3, showing significant acceleration and a high level of granulocytic differentiation. The effects were associated with a rapid histone H4 acetylation and later histone H3 modifications. Our results suggest that the use of two HDACI altogether before the induction of differentiation and acting via chromatin remodeling may be promising for the treatment of acute promyelocytic leukemia.

The cloning and characterization of the histone acetyltransferase human homolog Dmel\TIP60 in Drosophila melanogaster: Dmel\TIP60 is essential for multicellular development

Chromatin packaging directly influences gene programming as it permits only certain portions of the genome to be activated in any given developmental stage, cell, and tissue type. Histone acetyltransferases (HATs) are a key class of chromatin regulatory proteins that mediate such developmental chromatin control; however, their specific roles during multicellular development remain unclear. Here, we report the first isolation and developmental characterization of a Drosophila HAT gene (Dmel\TIP60) that is the homolog of the human HAT gene TIP60. We show that Dmel\TIP60 is differentially expressed during Drosophila development, with transcript levels significantly peaking during embryogenesis. We further demonstrate that reducing endogenous Dmel\TIP60 expression in Drosophila embryonic cells by RNAi results in cellular defects and lethality. Finally, using a GAL4-targeted RNAi system in Drosophila, we show that ubiquitous or mesoderm/muscle-specific reduction of Dmel\TIP60 expression results in lethality during fly development. Our results suggest a mechanism for HAT regulation involving developmental control of HAT expression profiles and show that Dmel\TIP60 is essential for multicellular development. Significantly, our inducible and targeted HAT knockdown system in Drosophila now provides a powerful tool for effectively studying the roles of TIP60 in specific tissues and cell types during development.

Thioredoxin in cancer--role of histone deacetylase inhibitors

Increased levels of thioredoxin (Trx) occur in a number of human cancers, which may contribute to the resistance of cancers to therapy by scavenging reactive oxygen species (ROS) which are generated by various anti-cancer agents. Many human cancers have low levels of thioredoxin-binding protein (TBP-2). TBP-2 binds to Trx and blocks its reducing activity. Histone deacetylase inhibitors (HDACi) up-regulate TBP-2 in various transformed cells, associated with a decrease in Trx levels. Up-regulation of TBP-2 and decrease of Trx may contribute to the sensitivity of many hematologic and solid tumors to anti-cancer activity of HDACi.

The Histone Deacetylase Inhibitor FK228 Distinctly Sensitizes the Human Leukemia Cells to Retinoic Acid-Induced Differentiation

FK228 (depsipeptide) is a novel histone deacetylase inhibitor (HDACI) that has shown therapeutical efficacy in clinical trials for malignant lymphoma. In this article, we examined in vitro effects of FK228 on human leukemia cell lines, NB4 and HL-60. FK228 alone (0.2-1 ng/mL) inhibited leukemia cell growth in a dose-dependent manner and induced death by apoptosis. FK228 had selective differentiating effects on two cell lines when used for 6 h before induction of granulocytic differentiation by retinoic acid (RA) or in combination with RA. These effects were accompanied by a time- and dose-dependent histone H4 hyper-acetylation or histone H3 dephosphorylation and alterations in DNA binding of NF-kappaB in association with cell death and differentiation. Pifithrin-alpha (PFT), an inhibitor of p53 transcriptional activity, protected only NB4 cells with functional p53 from FK228-induced apoptosis and did not interfere with antiproliferative activity in p53-negative HL-60 cells. In NB4 cells, PFT inhibited p53 binding to the p21 (Waf1/Cip1) promotor and induced DNA binding of NF-kappaB leading to enhanced cell survival. Thus, beneficial effects of FK228 on human promyelocytic leukemia may be exerted through the induction of differentiation or apoptosis via histone modification and selective involvement of transcription factors, such as NF-kappaB and p53.

Histone deacetylase inhibitors as therapeutics for polyglutamine disorders

During the past 5 years, gene expression studies in cell culture, animal models and in the brains of patients have shown that the perturbation of transcription frequently results in neuronal dysfunction in polyglutamine repeat diseases such as Huntington's disease. Histone deacetylases act as repressors of transcription through interactions with co-repressor complexes, which leads to chromatin remodelling. Aberrant interactions between polyglutamine proteins and regulators of transcription could be one mechanism by which transcriptional dysregulation occurs. Here, we discuss the potential therapeutic pathways through which histone deacetylase inhibitors might act to correct the aberrant transcription observed in Huntington's disease and other polyglutamine repeat diseases.

The novel histone deacetylase inhibitor BML-210 exerts growth inhibitory, proapoptotic and differentiation stimulating effects on the human leukemia cell lines

Histone deacetylase inhibitors have a potent role in the strategy for the treatment of leukemias. BML-210 (N-(2-Aminophenyl)-N' phenyloctanol diamine) is the novel histone deacetylase inhibitor, and its mechanism of action has not been characterized. In this study, we examined the in vitro effects of BML-210 on the human leukemia cell lines (NB4, HL-60, THP-1, and K562). We found that BML-210 inhibits the growth of all cell lines and promotes apoptosis in a dose- and time-dependent manner. BML-210 alone induces HL-60 and K562 cell differentiation (up to 30%) to granulocytes and erythrocytes, respectively, and in combination with differentiation agents - all-trans retinoic acid and hemin, markedly potentates it. Those treatments cause G1 arrest and histone H4 acetylation, affects transcription factor NF-kappaB and Sp1 binding activity to their consensus sequences, the p21 or the FasL promoters, and influences expression of Sp1, NF-kappaB, p21 and FasL. These findings suggest that BML-210 could be a promising antileukemic agent to induce apoptosis and to modulate differentiation through the modulation of histone acetylation and gene expression.

Loss of oncostatin M receptor beta in metastatic melanoma cells

Oncostatin M (OSM) is an interleukin-6 (IL-6) type cytokine originally described by its capacity to inhibit melanoma proliferation in vitro. Here, the mechanisms involved in resistance to growth inhibition by OSM were analysed for the first time on a large panel of metastatic melanoma cell lines. OSM resistance did not strictly correlate with IL-6, interferon-gamma or tumor necrosis factor-alpha resistance. Rather, it correlated with a specific loss of the OSM receptor-beta (OSMRbeta) subunit, in conjunction with a lower level of histone acetylation in the OSMRbeta promoter region. Treatment of various OSM-resistant melanoma cells with the histone deacetylase inhibitor Trichostatin A increased activity and histone acetylation of the OSMRbeta promoter as well as expression of OSMRbeta mRNA and protein, allowing OSM to activate the signal transducer and activator of transcription 3 (STAT3) and to inhibit proliferation. Other defects associated with OSM resistance were identified at the level of OSMRbeta transcription or protein expression, as well as downstream of or parallel to STAT3 activation. Altogether, our results suggest a role for OSM in the prevention of melanoma progression and that metastatic melanoma cells could escape this growth control by the epigenetic silencing of OSMRbeta.

Histone deacetylase inhibitors: discovery and development as anticancer agents

Histone deacetylase (HDAC) inhibitors are a new class of targeted anticancer agents. Several HDAC inhibitors are in clinical trials and have shown significant activity against a spectrum of both haematological and solid tumours at doses that are well tolerated by patients. HDACs and histone acetyltransferases can, by reversible acetylation, modify the structure and function of histones and proteins in transcription factor complexes, which are involved in the regulation of gene expression, as well as many non-histone proteins that are involved in regulating cell proliferation and cell death. HDAC inhibitors are a structurally diverse group of molecules; these agents selectively alter the expression of genes. HDAC inhibitors can induce cancer cell death, whereas normal cells are relatively resistant to HDAC inhibitor-induced cell death.

Positive and negative regulation of the innate antiviral response and beta interferon gene expression by deacetylation

Beta interferon (IFN-beta) gene expression in response to virus infection relies on the dynamic assembly of a multiprotein enhanceosome complex that is initiated by the activation of two inducible transcription factors, interferon regulatory factor 3 (IRF3) and NF-kappaB. Virus or double-stranded RNA-induced activation of IFN-beta gene expression is prevented by the addition of protein deacetylase inhibitors. The isolated IRF-responsive positive regulatory domain was found to require deacetylation for its activity, but IRF3 protein activation leading to its nuclear translocation and DNA binding was not impaired by deacetylase inhibition. In contrast, NF-kappaB activity was not affected by deacetylase inhibitors. RNA interference indicated that several deacetylase enzymes, including histone deacetylase 1 (HDAC1), HDAC8, and HDAC6, influence IFN-beta gene expression with opposing effects. While HDAC1 and HDAC8 repress IFN-beta expression, HDAC6 acts as a coactivator essential for enhancer activity. Virus replication is enhanced in HDAC6-depleted cells, demonstrating HDAC6 is an essential component of innate antiviral immunity.

Butyric acid prodrugs are histone deacetylase inhibitors that show antineoplastic activity and radiosensitizing capacity in the treatment of malignant gliomas

Histone modification has emerged as a promising approach to cancer therapy. We explored the efficacy of a novel class of histone deacetylase inhibitors in the treatment of malignant gliomas. Treatment of glioma cell lines with two butyric acid derivatives, pivaloylomethyl butyrate (AN-9) and butyroyloxymethyl butyrate (AN-1), induced hyperacetylation, increased p21(Cip1) expression, inhibited proliferation, and enhanced apoptosis. Histone deacetylase inhibitor-induced apoptosis was mediated primarily by caspase-8. Treatment of cells with AN-1 or AN-9 for 24 hours before exposure to gamma-irradiation potentiated further caspase-8 activity and resultant apoptosis. Clonogenic survival curves revealed marked reductions in cell renewal capacity of U251 MG cells exposed to combinations of AN-1 and radiation. Preliminary in vivo experiments using human glioma cell lines grown as xenografts in mouse flanks suggest in vivo efficacy of AN-9. The data suggest that novel butyric acid prodrugs provide a promising treatment strategy for malignant gliomas as single agents and in combination with radiation therapy.

Making myc

Myc regulates to some degree every major process in the cell. Proliferation, growth, differentiation, apoptosis, and metabolism are all under myc control. In turn, these processes feed back to adjust the level of c-myc expression. Although Myc is regulated at every level from RNA synthesis to protein degradation, c-myc transcription is particularly responsive to multiple diverse physiological and pathological signals. These signals are delivered to the c-myc promoter by a wide variety of transcription factors and chromatin remodeling complexes. How these diverse and sometimes disparate signals are processed to manage the output of the c-myc promoter involves chromatin, recruitment of the transcription machinery, post-initiation transcriptional regulation, and mechanisms to provide dynamic feedback. Understanding these mechanisms promises to add new dimensions to models of transcriptional control and to reveal new strategies to manipulate Myc levels.

Unexpected Roles for Deacetylation in Interferon- and Cytokine-Induced Transcription

Protein acetylation is a reversible modification that has been implicated in epigenetic regulation of gene expression. It is widely accepted that acetylation enzymes are present at transcriptionally active promoters and deacetylation enzymes associate with transcriptionally silent loci. These results notwithstanding, recent findings indicate that positive regulation of gene expression by interferons and other cytokines requires both acetylation and deacetylation.

Sensitization of osteosarcoma cells to death receptor-mediated apoptosis by HDAC inhibitors through downregulation of cellular FLIP

Fas-mediated apoptosis plays an important role in elimination of tumor cells in vivo, but some tumor-derived cells are resistant to this mechanism. Here, we show that treatment with the histone deacetylase (HDAC) inhibitor FR901228 renders Fas-resistant osteosarcoma cell lines sensitive to Fas-mediated apoptosis by downregulating expression of cellular FLIP (cellular FLICE-inhibitory protein), an inhibitor of Fas-mediated activation of caspase-8. Moreover, sensitization to Fas-mediated apoptosis was also induced in Fas-resistant osteosarcoma cells by suppressing FLIP expression using FLIP-specific RNA interference. HDAC inhibitors including FR901228 were shown to induce downregulation of cellular FLIP through inhibiting generation of FLIP mRNA, rather than stimulating degradation at either protein or mRNA level, and the inhibition was independent of de novo protein synthesis. These results clearly indicate that some tumor cells exhibit a phenotype resistant to death receptor-mediated apoptosis by expressing cellular FLIP, and that HDAC inhibitors sensitize such resistant tumor cells by directly downregulating cellular FLIP mRNA.

Histone deacetylase inhibitors: new drugs for the treatment of inflammatory diseases?

Histone deacetylase (HDAC) inhibitors induce cell cycle arrest and differentiation in cancer cells and have been in Phase I-II clinical trials for the treatment of various solid or haematological malignancies. In recent years, HDAC inhibitors have emerged as potent contenders for anti-inflammatory drugs, offering new lines of therapeutic intervention for rheumatoid arthritis or lupus erythematosus. The molecular mode of action of HDAC inhibitors is still controversial but seems to rely on reduced inflammatory mediator production, such as nitric oxide or cytokines, which implies inhibition of the transcription factor NF-kappaB. These anti-inflammatory effects will hopefully lead us to appreciate the complex anti-tumour effects of HDAC inhibitors.

Histone deacetylases as transcriptional activators? Role reversal in inducible gene regulation

Histone deacetylation enzymes have often been associated with the suppression of eukaryotic gene transcription. In contrast, recent studies of inducible gene regulation indicate that protein deacetylation can also be required as a transcriptional activation signal. The concept of protein deacetylation as a requirement for transcription activation seems to contradict earlier conclusions about the function of deacetylation in gene suppression. However, in the context of a more global interpretation, these opposing effects of deacetylation imply its dynamic role in the overall control of gene expression. The exact requirement for deacetylation differs among promoters, depending on their specific architecture and regulation scenario.

Activation of HLXB9 by juxtaposition with MYB via formation of t(6;7)(q23;q36) in an AML-M4 cell line (GDM-1)

Mutation or dysregulation of related homeobox genes occurs in leukemia. Using RT-PCR, we screened members of the EHG family of homeobox genes, comprising EN1 (at 2q14), GBX2 (at 2q36), and EN2, GBX1, and HLXB9 (at 7q36), for dysregulation in acute myeloid leukemia (AML) cell lines indicated by chromosomal breakpoints at these sites. Only one EHG-family gene was expressed, HLXB9, in cell line GDM-1 (AML-M4). Karyotypic analysis of GDM-1 revealed a unique t(6;7)(q23;q35), also present in the patient. Fluorescence in situ hybridization analysis showed chromosomal breakpoints close to the region upstream of HLXB9, at 7q36, a region rearranged in certain AML patients, and at 6q23 upstream of MYB, a gene activated in leukemia. Detailed expression analysis suggested ectopic activation of HLXB9 occurred via juxtaposition with regions upstream of MYB, which was highly expressed in GDM-1. Our data identified a cell line model for a novel leukemic translocation involving MYB with HLXB9, further implicating HLXB9 in leukemogenesis.

Clinical studies with fetal hemoglobin-enhancing agents in sickle cell disease

Fetal hemoglobin (HbF, alpha2gamma2) decreases polymerization of sickle hemoglobin, and high levels correlate with decreased morbidity and mortality in sickle cell disease (SCD). Therefore, a therapeutic goal for patients with SCD is pharmacologic reactivation of HbF. Decreased HbF production is associated with DNA methylation (by DNA methyltransferase [DNMT]) at the gamma-globin (HbF) gene promoter. The cytosine analogs 5-azacytidine and 5-aza-2'-deoxycytidine (decitabine) hypomethylate DNA by inhibiting DNMT. In early studies, 5-azacytidine produced significant HbF elevations in patients with thalassemia and SCD, but clinical development of this class of agent was halted after a poorly controlled animal study suggested that 5-azacytidine might be carcinogenic. However, the majority of preclinical studies with decitabine have suggested a chemopreventive rather than carcinogenic effect. Furthermore, decitabine, unlike 5-azacytidine, does not incorporate into RNA and is a more directed DNA-hypomethylating agent. Therefore, we have pursued studies of decitabine to pharmacologically reactivate HbF in patients with SCD. In phase I/II studies, decitabine at DNA-hypomethylating, but noncytotoxic, doses was well tolerated and effective at increasing HbF and total hemoglobin levels both in patients who had and had not responded to prior hydroxyurea therapy. In treated patients, there were marked improvements in a range of surrogate clinical endpoints measuring red blood cell adhesion, endothelial damage, and coagulation pathway activity. Pharmacologic reactivation of HbF through DNA hypomethylation holds promise as an effective disease-modifying intervention for patients with SCD. Larger studies are required to confirm the safety and effectiveness of decitabine with chronic use, and to more clearly establish its role in patients with SCD.

Epigenetic transitions: towards therapeutic targets

Therapeutic approaches aimed at developing epigenetically-effective drugs are under intense investigation. Several classes of enzymes regulating histone acetylation and DNA methylation, which are required for epigenetic transitions, offer attractive targets for therapeutic interventions. Imbalances in histone acetylation and DNA methylation may play a significant role in the development of cancer and leukaemia and may provide a mechanistic rationale for targeting epigenetic modifications. Clinical trials designed to evaluate inhibitors of DNA methylation and histone deacetylase inhibitors are showing encouraging results in cancer patients. A growing quantity of data from preclinical research supports the notion that epigenetically-effective drugs could also find an application in other therapeutic areas. A number of emerging biomarkers may prove useful for monitoring drug effects and defining molecular signatures of response, toxicity and effective dose.

TFIIH operates through an expanded proximal promoter to fine-tune c-myc expression

A continuous stream of activating and repressing signals is processed by the transcription complex paused at the promoter of the c-myc proto-oncogene. The general transcription factor IIH (TFIIH) is held at promoters prior to promoter escape and so is well situated to channel the input of activators and repressors to modulate c-myc expression. We have compared cells expressing only a mutated p89 (xeroderma pigmentosum complementation group B [XPB]), the largest TFIIH subunit, with the same cells functionally complemented with the wild-type protein (XPB/wt-p89). Here, we show structural, compositional, and functional differences in transcription complexes between XPB and XPB/wt-89 cells at the native c-myc promoter. Remarkably, although the mean levels of c-Myc are only modestly elevated in XPB compared to those in XPB/wt-p89 cells, the range of expression and the cell-to-cell variation of c-Myc are markedly increased. Our modeling indicates that the data can be explained if TFIIH integrates inputs from multiple signals, regulating transcription at multiple kinetically equivalent steps between initiation and promoter escape. This helps to suppress the intrinsic noise of transcription and to ensure the steady transcriptional output of c-myc necessary for cellular homeostasis.

Colon Epithelial Cell Differentiation Is Inhibited by Constitutive c-Myb Expression or Mutant APC Plus Activated RAS

Blocked differentiation is a hallmark of cancer cells and the restoration of differentiation programs in vivo is an actively pursued clinical aim. Understanding the key regulators of cyto-differentiation may focus therapies on molecules that reactivate this process. c-myb expression declines rapidly when human colon cancer epithelial cells are induced to differentiate with the physiologically relevant short-chain fatty acid, sodium butyrate. These cells show increased expression of alkaline phosphatase and cytokeratin 8. Similarly, murine Immorto-epithelial cells derived from wild-type colon cells also show c-myb mRNA declines when induced to differentiate with sodium butyrate. Immorto-cells harboring a single APC mutation are indistinguishable from wild-type cells with regard to differentiation, while addition of activated RAS alone markedly enhances differentiation. In marked contrast, complete differentiation arrest occurs when both APC and RAS are mutated. Expression of MybER, a 4-hydroxytamoxifen-activatable form of c-Myb, blocks differentiation in wildtype and APC mutant Immorto-cell lines as well as LIM1215 human colon carcinoma cells. These data identify two pathways of oncogenic change that lead to retarded epithelial cell differentiation, one involving the presence of a single APC mutation in conjunction with activated RAS or alternatively constitutive c-myb expression.

Potent stimulation of gene expression by histone deacetylase inhibitors on transiently transfected DNA

Transcription activity of chromatin is associated with histone acetylation which is regulated by recruitment of histone acetyltransferases and deacetylases (HDAC) to specific chromatin regions. We have tested how expression of a transfected or stably introduced gene correlates with histone acetylation. Our results demonstrate that expression of transiently transfected green fluorescence protein (GFP) genes is significantly enhanced by HDAC inhibitors. Although HDAC treatment did not induce noticeable changes in the chromatin structure of genomic DNA, chromatin immunoprecipitation showed that more transiently transfected DNA is assembled into chromatin containing acetylated histones in HDAC inhibitor treated cells when compared to untreated cells. For stably integrated GFP, the expression response to HDAC inhibitors varies between independent stable cell lines. However, there was no difference in histone acetylation associated with the integrated transgene between HDAC inhibitor responsive and non-responsive cells. Furthermore, the overall enhancement of transgene expression by HDAC inhibitors was not as pronounced as in transiently transfected cells.

Defining the Chemotherapeutic Targets of Histone Deacetylase Inhibitors

The use of many conventional chemotherapeutic drugs is often severely restricted due to dose-limiting toxicities, as these drugs target the destruction of the proliferating fraction of cells, often with little specificity for tumor cells over proliferating normal body tissue. Many newer drugs attempt to overcome this shortcoming by targeting defective gene products or cellular mechanisms that are specific to the tumor, thereby minimizing the toxicity to normal tissue. Histone deacetylase inhibitors are an example of this type of tumor-directed drug, having significant toxicity for tumors but minimal effects on normal tissue. These drugs can affect the transcriptional program by modifying chromatin structure, but it is not yet clear whether specific transcriptional changes are directly responsible for their tumor-selective toxicity. Recent evidence suggests that transcriptional changes underlie their cytostatic activity, although this is not tumor-selective and affects all proliferating cells. Here we present evidence that supports an alternative mechanism for the tumor-selective cytotoxicity of histone deacetylase inhibitors. The target is still likely to be the chromatin histones, but rather than transcriptional changes due to modification of the transcriptionally active euchromatin, we propose that hyperacetylation and disruption of the transcriptionally inactive heterochromatin, particularly the centromeric heterochromatin, and the inability of tumor cells to cell cycle arrest in response to a specific checkpoint, underlies the tumor-selective cytotoxicity of these drugs.

DNA hypo-methylating agents and sickle cell disease

Fetal haemoglobin (HbF, alpha2) decreases polymerization of sickle haemoglobin (HbS) and high levels correlate with decreased morbidity and mortality in sickle cell disease (SSD). Therefore, a therapeutic goal in SSD is the pharmacologic reactivation of HbF. Silencing of the globin (HbF) gene is associated with DNA methylation. The cytosine analogues 5-azacytidine and 5-aza-2'-deoxycytidine (decitabine) hypomethylate DNA by inhibiting DNA methyl-transferase. In clinical trials, 5-azacytidine and decitabine have demonstrated the greatest efficacy in HbF reactivation. Clinical development of these drugs has been delayed by concerns regarding the carcinogenic potential of 5-azacytidine. Furthermore, controversy regarding DNA hypomethylation versus more generic cytotoxic effects as the mechanism of action suggested that other cytotoxic/cytostatic agents might be as effective. Additional preclinical data and clinical studies of decitabine have tempered many safety concerns and have confirmed that DNA hypomethylation is the mechanism of action. Pharmacologic reactivation of HbF through DNA hypomethylation holds promise as an effective disease modifying intervention for patients with SSD. Larger studies are required to confirm safety and effectiveness with chronic use.

Histone deacetylase inhibitor NVP-LAQ824 has significant activity against myeloid leukemia cells in vitro and in vivo

NVP-LAQ824 is a novel potent hydroxamic acid-derived histone deacetylase inhibitor that induces apoptosis in nanomolar concentrations in myeloid leukemia cell lines and patient samples. Here we show the activity of NVP-LAQ824 in acute myeloid leukemia cells and BCR/ABL-expressing cells of mouse and human origin, both sensitive and resistant to imatinib mesylate (Gleevec, STI-571). Whereas imatinib inhibited overall cellular tyrosine phosphorylation in Ba/F3.p210 cells, NVP-LAQ824 did not inhibit tyrosine phosphorylation, and did not affect BCR/ABL or ABL protein expression. Neither compound was able to inhibit cellular tyrosine phosphorylation in the imatinib-resistant Ba/F3.p210-T315I cell line. These data taken together suggest that BCR/ABL kinase activity is not a direct target of NVP-LAQ824. Synergy between NVP-LAQ824 and imatinib was demonstrated against BCR/ABL-expressing K562 myeloid leukemia cell lines. In addition, we show that NVP-LAQ824 was well tolerated in vivo in a pre-clinical murine leukemia model, with antileukemia activity resulting in significant prolongation of the survival of mice when treated with NVP-LAQ824 compared to control mice. Taken together, these findings provide the framework for NVP-LAQ824 as a novel therapeutic in myeloid malignancies.

Curcumin, a novel p300/CREB-binding protein-specific inhibitor of acetyltransferase, represses the acetylation of histone/nonhistone proteins and histone acetyltransferase-dependent chromatin transcription

Acetylation of histones and non-histone proteins is an important post-translational modification involved in the regulation of gene expression in eukaryotes and all viral DNA that integrates into the human genome (e.g. the human immunodeficiency virus). Dysfunction of histone acetyltransferases (HATs) is often associated with the manifestation of several diseases. In this respect, HATs are the new potential targets for the design of therapeutics. In this study, we report that curcumin (diferuloylmethane), a major curcumanoid in the spice turmeric, is a specific inhibitor of the p300/CREB-binding protein (CBP) HAT activity but not of p300/CBP-associated factor, in vitro and in vivo. Furthermore, curcumin could also inhibit the p300-mediated acetylation of p53 in vivo. It specifically represses the p300/CBP HAT activity-dependent transcriptional activation from chromatin but not a DNA template. It is significant that curcumin could inhibit the acetylation of HIV-Tat protein in vitro by p300 as well as proliferation of the virus, as revealed by the repression in syncytia formation upon curcumin treatment in SupT1 cells. Thus, non-toxic curcumin, which targets p300/CBP, may serve as a lead compound in combinatorial HIV therapeutics.

Histone deacetylase inhibitors – a new tool to treat cancer

Transcriptional regulation in eukaryotes is a multilevel hierarchical process. It is becoming clear that higher-order chromatin structure, occurring via modifications of histones in their nucleosome structure, plays a crucial role in regulating gene expression, both in normal and pathological states. Deacetylation of histones by histone deacetylases (HDACs) modifies the chromatin from an open gene active euchromatin structure to a closed gene silenced heterochromatin structure. Several cancer promoting mutations and chromosomal translocations result in repression of transcription through abnormal recruitment and activation of HDACs, leading to neoplastic transformation. This is the rationale for the evolvement of HDAC inhibitors as a new class in cancer therapy. Trials have shown anti-proliferation effect of histone deacetylase inhibitors in cell culture, animal models and in human with both hematological and solid tumors. The exact mechanism by which histone deacetylase inhibitors exert their effect is still obscure. Reversal of the alteration in gene expression by fusion transcription factors or overexpressed repressors is just one of several possible explanations. The territory of heterochromatin in the vicinity of the nuclear periphery raised the possibility of involvement of nuclear envelope proteins in the regulation of transcription. Our laboratory is interested in the transcription repression mechanism induced by the nuclear envelope lamina associated polypeptide 2 (LAP2) family of proteins through chromatin modification. Here, we will describe the structure of the nucleosome, review regulation of gene expression by acetylation of histones and give an update on the current phase I and phase II clinical trials with histone deacetylase inhibitors.

Polyisoprenylated benzophenone, garcinol, a natural histone acetyltransferase inhibitor, represses chromatin transcription and alters global gene expression

Histone acetylation is a diagnostic feature of transcriptionally active genes. The proper recruitment and function of histone acetyltransferases (HATs) and deacetylases (HDACs) are key regulatory steps for gene expression and cell cycle. Functional defects of either of these enzymes may lead to several diseases, including cancer. HATs and HDACs thus are potential therapeutic targets. Here we report that garcinol, a polyisoprenylated benzophenone derivative from Garcinia indica fruit rind, is a potent inhibitor of histone acetyltransferases p300 (IC50 approximately 7 microm) and PCAF (IC50 approximately 5 microm) both in vitro and in vivo. The kinetic analysis shows that it is a mixed type of inhibitor with an increased affinity for PCAF compared with p300. HAT activity-dependent chromatin transcription was strongly inhibited by garcinol, whereas transcription from DNA template was not affected. Furthermore, it was found to be a potent inducer of apoptosis, and it alters (predominantly down-regulates) the global gene expression in HeLa cells.

Interferon-stimulated transcription and innate antiviral immunity require deacetylase activity and histone deacetylase 1

The use of histone deacetylase (HDAC) inhibitors has revealed an essential role for deacetylation in transcription of IFN-responsive genes. The HDAC1 protein associates with both signal transducer and activator of transcription (STAT) 1 and STAT2, and IFN-alpha stimulation induces deacetylation of histone H4. Inhibition of HDAC1 by small interfering RNA (siRNA) decreases IFN-alpha responsiveness whereas expression of HDAC1 augments the IFN-alpha response, demonstrating that HDAC1 modulates IFN-alpha-induced transcription. Importantly, the innate antiviral response is inhibited in the absence of deacetylase activity. The requirement for deacetylase is shared by IFN-gamma transcription response and may represent a general requirement for STAT-dependent gene expression.