Sequential valproic acid/all-trans retinoic acid treatment reprograms differentiation in refractory and high-risk acute myeloid leukemia

Cross-species functional genomic analysis identifies resistance genes of the histone deacetylase inhibitor valproic acid

The mechanisms of successful epigenetic reprogramming in cancer are not well characterized as they involve coordinated removal of repressive marks and deposition of activating marks by a large number of histone and DNA modification enzymes. Here, we have used a cross-species functional genomic approach to identify conserved genetic interactions to improve therapeutic effect of the histone deacetylase inhibitor (HDACi) valproic acid, which increases survival in more than 20% of patients with advanced acute myeloid leukemia (AML). Using a bidirectional synthetic lethality screen revealing genes that increased or decreased VPA sensitivity in C. elegans, we identified novel conserved sensitizers and synthetic lethal interactors of VPA. One sensitizer identified as a conserved determinant of therapeutic success of HDACi was UTX (KDM6A), which demonstrates a functional relationship between protein acetylation and lysine-specific methylation. The synthetic lethal screen identified resistance programs that compensated for the HDACi-induced global hyper-acetylation, and confirmed MAPKAPK2, HSP90AA1, HSP90AB1 and ACTB as conserved hubs in a resistance program for HDACi that are drugable in human AML cell lines. Hence, these resistance hubs represent promising novel targets for refinement of combinatorial epigenetic anti-cancer therapy.

Valproic acid triggers erythro/megakaryocyte lineage decision through induction of GFI1B and MLLT3 expression

Histone deacetylase inhibitors represent a family of targeted anticancer compounds that are widely used against hematological malignancies. So far little is known about their effects on normal myelopoiesis. Therefore, in order to investigate the effect of histone deacetylase inhibitors on the myeloid commitment of hematopoietic stem/progenitor cells, we treated CD34(+) cells with valproic acid (VPA). Our results demonstrate that VPA treatment induces H4 histone acetylation and hampers cell cycle progression in CD34(+) cells sustaining high levels of CD34 protein expression. In addition, our data show that VPA treatment promotes erythrocyte and megakaryocyte differentiation. In fact, we demonstrate that VPA treatment is able to induce the expression of growth factor-independent protein 1B (GFI1B) and of mixed-lineage leukemia translocated to chromosome 3 protein (MLLT3), which are crucial regulators of erythrocyte and megakaryocyte differentiation, and that the up-regulation of these genes is mediated by the histone hyperacetylation at their promoter sites. Finally, we show that GFI1B inhibition impairs erythroid and megakaryocyte differentiation induced by VPA, while MLLT3 silencing inhibits megakaryocyte commitment only. As a whole, our data suggest that VPA sustains the expression of stemness-related markers in hematopoietic stem/progenitor cells and is able to interfere with hematopoietic lineage commitment by enhancing erythrocyte and megakaryocyte differentiation and by inhibiting the granulocyte and mono-macrophage maturation.

Effects of PPARγ Ligands on Leukemia

Peroxisome proliferator-activated receptors (PPARs) and retinoic acid receptors (RARs), members of the nuclear receptor superfamily, are transcription factors that regulate a variety of important cellular functions. PPARs form heterodimers retinoid X receptor (RXR), an obligate heterodimeric partner for other nuclear receptors. Several novel links between retinoid metabolism and PPAR responses have been identified, and activation of PPAR/RXR expression has been shown to increase response to retinoids. PPARγ has emerged as a key regulator of cell growth and survival, whose activity is modulated by a number of synthetic and natural ligands. While clinical trials in cancer patients with thiazolidinediones (TZD) have been disappointing, novel structurally different PPARγ ligands, including triterpenoids, have entered clinical arena as therapeutic agents for epithelial and hematopoietic malignancies. Here we shall review the antitumor advances of PPARγ, alone and in combination with RARα ligands in control of cell proliferation, differentiation, and apoptosis and their potential therapeutic applications in hematological malignancies.

Cancer risk in people with epilepsy using valproate-sodium

OBJECTIVES

Based on reports of antitumour properties of sodium-valproate, we hypothesised that valproate has a cancer-protective effect in people with epilepsy. We aimed to determine cancer risk in people with epilepsy using sodium-valproate.

MATERIALS AND METHODS

Continuous data for 2997 people with epilepsy who had been prescribed valproate for at least two years, and for 11,988 unexposed people were provided by the UK General Practice Research Database. Hazard ratios (HRs) for all cancers and individual cancers between the exposed and unexposed groups, with smoking and alcohol consumption and age as covariates, were calculated using the Cox proportional hazards method.

RESULTS

Exposure to valproate had no influence on the incidence of the composite of all cancers [HR: 1.19, 95% CI: 0.97-1.47, P = 0.10]; there was, however, a significant excess of colon cancers [HR: 3.95, 95% CI: 1.97-7.92, P = 0.001] and a trend towards an excess of prostate neoplasms [HR: 2.15, 95% CI: 0.92-5.02, P = 0.08] and in addition, a trend towards reduced incidence of breast cancer [HR: 0.40, 95% CI: 0.14-1.30, P = 0.08] in the exposed group.

CONCLUSIONS

The lack of an inverse association between valproate use and hazard ratios for all cancers and several individual cancer sites does not lend support for a cancer-protective role for valproate.

Valproic acid triggers differentiation and apoptosis in AML1/ETO-positive leukemic cells specifically

Valproic acid (VPA) has extensive effects on leukemic blasts through its inhibition of histone deacetylases. The main goal of this study was to identify the subgroup of patients who may benefit most from VPA treatment. We examined the significance of t(8;21) chromosomal aberration for VPA treatment response among acute myeloid leukemia (AML) patients by direct comparison of AML1/ETO-negative vs. positive leukemic cell-lines as well as bone marrow blasts from AML patients. In t(8;21) AML, leukemogenesis is supposed to be induced via aberrant recruitment of histone deacetylases. AML cell lines of different genotypes (Kasumi-1, Kasumi-6, MV4;11, K562) and diagnostic bone marrow samples from patients were treated with VPA. VPA induced apoptosis in AML1/ETO-positive and MLL-AF4-positive cells in a dose-dependent manner. Differentiation, as indicated by changes in immunophenotype, was observed only in AML1/ETO-positive cells. VPA increased the expression of AML1 target genes - PU.1, C/EBPa, BPI and IGFBP7 only in AML1/ETO-positive cells. This AML1/ETO-specific effect was confirmed also using patient blasts isolated at the time of diagnosis. AML1/ETO-positive leukemia shows specific mechanism of VPA residing from differentiation followed by apoptosis that is accompanied by an increase in the expression of repressed AML1 target genes. Our data suggest that AML1/ETO-positive patients might derive the greatest benefit from VPA treatment.

Valproic acid combined with cytosine arabinoside in elderly patients with acute myeloid leukemia has in vitro but limited clinical activity

Elderly patients with acute myeloid leukemia (AML) have a poor prognosis. The authors examined the in vitro and clinical activity of the histone deacetylase inhibitor valproic acid (VA) combined with cytosine arabinoside (AraC) in elderly patients with AML unsuited to intensive therapy. For the in vitro studies, primary AML cells from 11 patients were treated with AraC and VA and analyzed for apoptosis, cytostatic effects, differentiation and acetyl histone H3 induction. VA (alone and with AraC) enhanced apoptosis and induced acetyl histone H3. VA inhibited cell proliferation. For the clinical trial, 15 patients were treated with VA and subcutaneous AraC and assessed for toxicity and response. No complete or partial remissions were achieved. In conclusion, VA has in vitro activity against AML and has additional activity with AraC. However, in this study, this combination demonstrated limited clinical activity in elderly patients with AML.

Critical Role of c-Myc in Acute Myeloid Leukemia Involving Direct Regulation of miR-26a and Histone Methyltransferase EZH2

Increased expression or aberrant activation of c-Myc plays an important role in leukemogenesis. Here, we show that in acute myeloid leukemia (AML), c-Myc directly controls the expression of EZH2, a component of the Polycomb repressive complex 2, and miR-26a. miR-26a is downregulated in primary blasts from AML patients and, during myeloid differentiation of AML cells, is induced together with a decrease in c-Myc and Ezh2 levels. Previously, EZH2 was shown to be regulated by miR-26a at the translational levels in lymphomas. However, we demonstrate that in AML, the variation of EZH2 mainly depends on c-Myc transcriptional control. We also show that enforced expression of miR-26a in AML cells is able to inhibit cell cycle progression by downregulating cyclin E2 expression. In addition, increased levels of miR-26a potentiate the antiproliferative effects of 1,25-dihydroxyvitamin D(3) (VitD) and stimulate myeloid differentiation. Our results identify new molecular targets of c-Myc in AML and highlight miR-26a attractiveness as a therapeutic target in leukemia.

A multicenter phase II trial of decitabine as first-line treatment for older patients with acute myeloid leukemia judged unfit for induction chemotherapy

BACKGROUND

The treatment of acute myeloid leukemia of older, medically non-fit patients still poses a highly unmet clinical need, and only few large, prospective studies have been performed in this setting. Given the established activity of hypomethylating agents such as 5-aza-2'-deoxycytidine (decitabine) in myelodysplastic syndromes and acute myeloid leukemia with 20-30% bone marrow blasts, we investigated whether this drug is also active in patients with more than 30% blasts.

DESIGN AND METHODS

To evaluate the efficacy and toxicity of decitabine in patients over 60 years old with untreated acute myeloid leukemia ineligible for induction chemotherapy, 227 patients (median age, 72 years), many with comorbidities, adverse cytogenetics and/or preceding myelodysplastic syndrome were treated with this hypomethylating agent. During the initial decitabine treatment (135 mg/m(2) total dose infused intravenously over 72 hours every 6 weeks), a median of two cycles was administered (range, 1-4). All-trans retinoic acid was administered to 100 patients during course 2. Fifty-two patients who completed four cycles of treatment subsequently received a median of five maintenance courses (range, 1-19) with a lower dose of decitabine (20 mg/m(2)) infused over 1 hour on 3 consecutive days every 4-6 weeks.

RESULTS

The complete and partial remission rate was 26%, 95% CI (20%, 32%), and an antileukemic effect was noted in 26% of patients. Response rates did not differ between patients with or without adverse cytogenetics; patients with monosomal karyotypes also responded. The median overall survival from the start of decitabine treatment was 5.5 months (range, 0-57.5+) and the 1-year survival rate was 28%, 95%CI (22%,34%). Toxicities were predominantly hematologic.

CONCLUSIONS

Decitabine is well tolerated by older, medically non-fit patients with acute myeloid leukemia; myelosuppression is the major toxicity. The response rate and overall survival were not adversely influenced by poor-risk cytogenetics or myelodysplastic syndrome. Because of these encouraging results, randomized studies evaluating single-agent decitabine versus conventional treatment are warranted. The study is registered with the German Clinical Trials Registry, number DRKS00000069.

Differential expression of neuronal dopamine and serotonin transporters DAT and SERT in megakaryocytes and platelets generated from human MEG-01 megakaryoblasts

In the central nervous system, serotonergic and dopaminergic signaling is terminated by the activity of specialized transporter proteins for serotonin (SERT) and dopamine (DAT). These transporter proteins are found both on the cell surface and in intracellular transport vesicles. Trafficking between these subcellular domains regulates the efficiency of removal of extracellular neurotransmitters and hence the efficacy of neuronal signaling. Therefore, it is of high interest to gain more insight into the regulatory mechanisms of the human DAT and SERT cell surface expression in their natural surroundings, i.e., in human cells. Because it is not possible to cultivate human neuronal cells expressing these transporter proteins, there is a need to find other human cells expressing these neuronal proteins. Here, we have investigated the expression of human SERT and DAT on developing megakaryocytes and platelet-like particles derived from the megakaryocyte progenitor cell line MEG-01 upon differentiation by valproic acid (VPA) and all-trans retinoic acid (ATRA). Our results show that MEG-01 cells express SERT and DAT and that VPA and ATRA induce a significant increase of transporter expression on developing megakaryocytes and platelets. As compared to ATRA, VPA more efficiently induced SERT expression but not DAT expression. Comparable to naïve platelets and neurons, SERT was localized to both the cell surface and intracellular compartments. Hence, VPA and ATRA-treated MEG-01 cells provide a model well-suited to studying neuronal monoamine transporter expression, not only during transcription and translation but also with respect to protein trafficking to and from the cell surface.

Interpreting clinical assays for histone deacetylase inhibitors

As opposed to genetics, dealing with gene expressions by direct DNA sequence modifications, the term epigenetics applies to all the external influences that target the chromatin structure of cells with impact on gene expression unrelated to the sequence coding of DNA itself. In normal cells, epigenetics modulates gene expression through all development steps. When "imprinted" early by the environment, epigenetic changes influence the organism at an early stage and can be transmitted to the progeny. Together with DNA sequence alterations, DNA aberrant cytosine methylation and microRNA deregulation, epigenetic modifications participate in the malignant transformation of cells. Their reversible nature has led to the emergence of the promising field of epigenetic therapy. The efforts made to inhibit in particular the epigenetic enzyme family called histone deacetylases (HDACs) are described. HDAC inhibitors (HDACi) have been proposed as a viable clinical therapeutic approach for the treatment of leukemia and solid tumors, but also to a lesser degree for noncancerous diseases. Three epigenetic drugs are already arriving at the patient's bedside, and more than 100 clinical assays for HDACi are registered on the National Cancer Institute website. They explore the eventual additive benefits of combined therapies. In the context of the pleiotropic effects of HDAC isoforms, more specific HDACi and more informative screening tests are being developed for the benefit of the patients.

Retinoids, retinoic acid receptors, and cancer

Retinoids (i.e., vitamin A, all-trans retinoic acid, and related signaling molecules) induce the differentiation of various types of stem cells. Nuclear retinoic acid receptors mediate most but not all of the effects of retinoids. Retinoid signaling is often compromised early in carcinogenesis, which suggests that a reduction in retinoid signaling may be required for tumor development. Retinoids interact with other signaling pathways, including estrogen signaling in breast cancer. Retinoids are used to treat cancer, in part because of their ability to induce differentiation and arrest proliferation. Delivery of retinoids to patients is challenging because of the rapid metabolism of some retinoids and because epigenetic changes can render cells retinoid resistant. Successful cancer therapy with retinoids is likely to require combination therapy with drugs that regulate the epigenome, such as DNA methyltransferase and histone deacetylase inhibitors, as well as classical chemotherapeutic agents. Thus, retinoid research benefits both cancer prevention and cancer treatment.

Valproic acid: growth inhibition of head and neck cancer by induction of terminal differentiation and senescence

BACKGROUND

There are limited studies on the effects of drugs that modulate epigenetic regulation for head and neck squamous cell carcinoma (HNSCC). This study determined the effect of valproic acid (VPA) on HNSCC.

METHODS

Growth inhibition effects of VPA alone or in combination with 5-aza-2'deoxycytidine (5-aza-dC) or all-trans retinoic acid (ATRA) was evaluated with MTT and clonogenic assays on 5 HNSCC cell lines. The mechanism of growth inhibition was investigated by looking at markers of terminal differentiation and senescence.

RESULTS

Growth inhibition profiles of HNSCC cell lines varied in response to VPA. Inhibition of clonogenic survival in response to VPA was associated with an upregulation of p21, expression of terminal differentiation markers, and cellular senescence. Notably, a combination treatment of 5-Aza-dC-VPA-ATRA enhanced growth inhibition in cells resistant to VPA.

CONCLUSION

VPA is a potent inhibitor of proliferation in some HNSCC cell lines, and may be used to treat HNSCC.

Histone deacetylase inhibitors for the treatment of myelodysplastic syndrome and acute myeloid leukemia

Epigenetic changes have been identified in recent years as important factors in the pathogenesis of myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Histone deacetylase inhibitors (HDACIs) regulate the acetylation of histones as well as other non-histone protein targets. Treatment with HDACIs results in chromatin remodeling that permits re-expression of silenced tumor suppressor genes in cancer cells, which, in turn, can potentially result in cellular differentiation, inhibition of proliferation and/or apoptosis. Several classes of HDACIs are currently under development for the treatment of patients with MDS and AML. Although modest clinical activity has been reported with the use of HDACIs as single-agent therapy, marked responses have been observed in selected subsets of patients. More importantly, HDACIs appear to be synergistic in vitro and improve response rates in vivo when combined with other agents, such as hypomethylating agents. Furthermore, HDACIs are also being investigated in combination with non-epigenetic therapies. This article synthesizes the most recent results reported with HDACIs in clinical trials conducted in patients with MDS and other myeloid malignancies.

The DAC system and associations with acute leukemias and myelodysplastic syndromes

Imbalances of histone acetyltransferase (HAT) and deacetylase activity (DAC) that result in deregulated gene expression are commonly observed in leukemias. These alterations provide the basis for novel therapeutic approaches that target the epigenetic mechanisms implicated in leukemogenesis. As the acetylation status of histones has been linked to transcriptional regulation of genes involved particularly in differentiation and apoptosis, DAC inhibitors (DACi) have attracted considerable attention for treatment of hematologic malignancies. DACi encompass a structurally diverse family of compounds that are being explored as single agents as well as in combination with chemotherapeutic drugs, small molecule inhibitors of signaling pathways and hypomethylating agents. While DACi have shown clear evidence of activity in acute myeloid leukemia, myelodysplastic syndromes and lymphoid malignancies, their precise role in treatment of these different entities remain to be elucidated. Successful development of these compounds as elements of novel targeted treatment strategies for leukemia will require that clinical studies be performed in conjunction with translational research including efforts to identify predictive biomarkers.

Mechanisms of synergistic antileukemic interactions between valproic acid and cytarabine in pediatric acute myeloid leukemia

PURPOSE

To determine the possibility of synergistic antileukemic activity and the underlying molecular mechanisms associated with cytarabine combined with valproic acid (VPA; a histone deacetylase inhibitor and a Food and Drug Administration-licensed drug for treating both children and adults with epilepsy) in pediatric acute myeloid leukemia (AML).

EXPERIMENTAL DESIGN

The type and extent of antileukemic interactions between cytarabine and VPA in clinically relevant pediatric AML cell lines and diagnostic blasts from children with AML were determined by MTT assays and standard isobologram analyses. The effects of cytarabine and VPA on apoptosis and cell cycle distributions were determined by flow cytometry analysis and caspase enzymatic assays. The effects of the two agents on DNA damage and Bcl-2 family proteins were determined by Western blotting.

RESULTS

We showed synergistic antileukemic activities between cytarabine and VPA in four pediatric AML cell lines and nine diagnostic AML blast samples. t(8;21) AML blasts were significantly more sensitive to VPA and showed far greater sensitivities to combined cytarabine and VPA than non-t(8;21) AML cases. Cytarabine and VPA cooperatively induced DNA double-strand breaks, reflected in induction of γH2AX and apoptosis, accompanied by activation of caspase-9 and caspase-3. Further, VPA induced Bim expression and short hairpin RNA knockdown of Bim resulted in significantly decreased apoptosis induced by cytarabine and by cytarabine plus VPA.

CONCLUSIONS

Our results establish global synergistic antileukemic activity of combined VPA and cytarabine in pediatric AML and provide compelling evidence to support the use of VPA in the treatment of children with this deadly disease.

[Histone deacetylase inhibitors: highlight on epigenetic regulation]

HDAC, by modifiing relations between DNA and histones, are major proteins of the epigenetic regulation. They play part in the signal transduction and in many cellular processes: cell cycle control, apoptosis, protein degradation, angiogenesis, invasion and cell motility. In several models of cancer HDAC inhibitors (HDACIs) are able to up regulate tumor suppressing gene (p53, p21, pRB...) and to down regulate oncogenes (SRC, HIF-Ialpha,HER2...). Many inhibitors are currently in clinical development and promising results have been reported in cutaneous T cell lymphoma, Hodgkin's disease and non-hodgkin lymphoma. Combination with chemotherapy and molecular targeted agents seem to be effective in myeloma, lung cancer and myeloïd neoplasms. In this review, we focus on recent biologic and clinical data that highlitght the anti-neoplastic role of HDACIs.

Phase I trial of ATRA-IV and Depakote in patients with advanced solid tumor malignancies

Retinoic acid derivatives have shown their greatest benefit in acute promyelocytic leukemia, but have also demonstrated pre-clinical anti-cancer effects in some solid tumors. Histone deacetylase inhibitors, by upregulating gene expression, are able to limit cancer cell proliferation and induce apoptosis. The combination of all-trans retinoic acid (ATRA) and the histone deacetylase inhibitor valproic acid has been previously studied in hematologic malignancies. We conducted a phase I two-step dose escalation trial of the liposomal ATRA analog ATRA-IV and divalproex sodium (Depakote) in nine patients with advanced solid tumors refractory to prior therapy. Side effects attributed to therapy had a severity <or=grade 2 and included skin toxicity and thrombocytopenia. The best disease response seen was disease stabilization in one patient. Expression of cellular retinoic acid binding protein-2 in peripheral blood mononuclear cells was detected as a marker of drug effect. The maximum tolerated dose (MTD) of both drugs in combination could not be established due to early closure of the trial resulting from a halt in the commercial availability of ATRA-IV.

Differential radiosensitizing effect of valproic acid in differentiation versus self-renewal promoting culture conditions

PURPOSE

It has been shown that valproic acid (VA) enhances the proliferation and self-renewal of normal hematopoietic stem cells and that breast cancer stem/progenitor cells can be resistant to radiation. From these data, we hypothesized that VA would fail to radiosensitize breast cancer stem/progenitor cells grown to three-dimensional (3D) mammospheres.

METHODS AND MATERIALS

We used the MCF7 breast cancer cell line grown under stem cell-promoting culture conditions (3D mammosphere) and standard nonstem cell monolayer culture conditions (two-dimensional) to examine the effect of pretreatment with VA on radiation sensitivity in clonogenic survival assays and on the expression of embryonic stem cell transcription factors.

RESULTS

3D-cultured MCF-7 cells expressed higher levels of Oct4, Nanog, and Sox2. The 3D passage enriched self-renewal and increased radioresistance in the 3D mammosphere formation assays. VA radiosensitized adherent cells but radioprotected 3D cells in single-fraction clonogenic assays. Moreover, fractionated radiation sensitized VA-treated adherent MCF7 cells but did not have a significant effect on VA-treated single cells grown to mammospheres.

CONCLUSION

We have concluded that VA might preferentially radiosensitize differentiated cells compared with those expressing stem cell surrogates and that stem cell-promoting culture is a useful tool for in vitro evaluation of novel cancer therapeutic agents and radiosensitizers.

Histone deacetylase inhibition modulates cell fate decisions during myeloid differentiation

BACKGROUND

The clinical use of chromatin-modulating drugs, such as histone deacetylase inhibitors, for the treatment of bone marrow failure and hematopoietic malignancies has increased dramatically over the last few years. Nonetheless, little is currently known concerning their effects on myelopoiesis.

DESIGN AND METHODS

We utilized an ex vivo differentiation system in which umbilical cord blood-derived CD34(+) cells were treated with trichostatin A, sodium butyrate and valproic acid to evaluate the effect of histone deacetylase inhibitor treatment on myeloid lineage development, colony-forming potential, proliferation, and terminal neutrophil differentiation.

RESULTS

Trichostatin A treatment modestly reduced progenitor proliferation, while sodium butyrate and valproic acid resulted in concentration-dependent effects on proliferation and apoptosis. Addition of valproic acid uniquely stimulated CD34(+) proliferation. Sodium butyrate treatment inhibited terminal neutrophil differentiation both quantitatively and qualitatively. Addition of 100 microM valproic acid resulted in increased numbers of mature neutrophils with a block in differentiation at increasing concentrations. Sodium butyrate and valproic acid treatment resulted in increased acetylation of histones 3 and 4 while trichostatin A, sodium butyrate and valproic acid had differential effects on the acetylation of non-histone proteins.

CONCLUSIONS

Individual histone deacetylase inhibitors had specific effects on cell fate decisions during myeloid development. These data provide novel insights into the effects of histone deacetylase inhibitors on the regulation of normal hematopoiesis, which is of importance when considering utilizing these compounds for the treatment of myeloid malignancies and bone marrow failure syndromes.

Histone modification therapy of cancer

The state of modification of histone tails plays an important role in defining the accessibility of DNA for the transcription machinery and other regulatory factors. It has been extensively demonstrated that the posttranslational modifications of the histone tails, as well as modifications within the nucleosome domain, regulate the level of chromatin condensation and are therefore important in regulating gene expression and other nuclear events. Together with DNA methylation, they constitute the most relevant level of epigenetic regulation of cell functions. Histone modifications are carried out by a multipart network of macromolecular complexes endowed with enzymatic, regulatory, and recognition domains. Not surprisingly, epigenetic alterations caused by aberrant activity of these enzymes are linked to the establishment and maintenance of the cancer phenotype and, importantly, are potentially reversible, since they do not involve genetic mutations in the underlying DNA sequence. Histone modification therapy of cancer is based on the generation of drugs able to interfere with the activity of enzymes involved in histone modifications: new drugs have recently been approved for use in cancer patients, clinically validating this strategy. Unfortunately, however, clinical responses are not always consistent and do not parallel closely the results observed in preclinical models. Here, we present a brief overview of the deregulation of chromatin-associated enzymatic activities in cancer cells and of the main results achieved by histone modification therapeutic approaches.

Leukaemogenesis: more than mutant genes

Acute leukaemias are characterized by recurring chromosomal aberrations and gene mutations that are crucial to disease pathogenesis. It is now evident that epigenetic modifications, including DNA methylation and histone modifications, substantially contribute to the phenotype of leukaemia cells. An additional layer of epigenetic complexity is the pathogenetic role of microRNAs in leukaemias, and their key role in the transcriptional regulation of tumour suppressor genes and oncogenes. The genetic heterogeneity of acute leukaemias poses therapeutic challenges, but pharmacological agents that target components of the epigenetic machinery are promising as a component of the therapeutic arsenal for this group of diseases.

Valproic acid increases CXCR4 expression in hematopoietic stem/progenitor cells by chromatin remodeling

A major limitation of cord blood (CB) hematopoietic stem/progenitor cell (HSPC) transplantation in adult patients is the low cell dose available, which is associated with delayed or failed engraftment. This has prompted intensive research to develop novel strategies to improve HSPC engraftment and reconstitution. The chemokine receptor CXCR4 and its ligand stromal cell-derived factor (SDF)-1alpha play a crucial role in the homing and repopulation capacity of HSPCs. We hypothesized that in HSPCs the CXCR4 receptor is regulated through chromatin remodeling by histone deacetylase inhibitors (HDIs) such as valproic acid (VPA). Using CB CD34(+) cells and the models of immature hematopoietic cells expressing CD34 antigen, namely the leukemic cell lines KG-1a and KG-1, we found that VPA increases surface and mRNA CXCR4 levels in these cells, thereby enhancing their migration toward an SDF-1alpha gradient. We also found that modulation of CXCR4 gene transcription by VPA correlates with the acetylation status of histone H4 in CB CD34(+) and KG-1 cells. Hence we suggest that in CB transplantation priming of HSPCs with VPA could improve homing and engraftment.

Differentiation therapy of leukemia: 3 decades of development

A characteristic feature of leukemia cells is a blockade of differentiation at a distinct stage in cellular maturation. In the 1970s and 1980s, studies demonstrating the capabilities of certain chemicals to induce differentiation of hematopoietic cell lines fostered the concept of treating leukemia by forcing malignant cells to undergo terminal differentiation instead of killing them through cytotoxicity. The first promising reports on this notion prompted a review article on this subject by us 25 years ago. In this review, we revisit this interesting field of study and report the progress achieved in the course of nearly 3 decades. The best proof of principle for differentiation therapy has been the treatment of acute promyelocytic leukemia with all-trans retinoic acid. Attempts to emulate this success with other nuclear hormone ligands such as vitamin D compounds and PPARgamma agonists or different classes of substances such as hematopoietic cytokines or compounds affecting the epigenetic landscape have not been successful on a broad scale. However, a multitude of studies demonstrating partial progress and improvements and, finally, the new powerful possibilities of forward and reverse engineering of differentiation pathways by manipulation of transcription factors support the continued enthusiasm for differentiation therapy of leukemia in the future.

Histone deacetylase inhibitors as a new weapon in the arsenal of differentiation therapies of cancer

Absent or altered differentiation is one of the major features of cancer cells. Histone deacetylases (HDACs) play a central role in the epigenetic regulation of gene expression. Aberrant activity of HDACs has been documented in several types of cancers, leading to the development of HDAC inhibitors (HDACi) as anti-tumor drugs. In vitro and in vivo experimental evidences show that HDACi are able to resume the process of maturation in undifferentiated cancer cells, justifying their introduction as differentiating agents in several clinical trials. Modulation of cell fate by HDACi is observed at several levels, including the stem cell compartment: HDACi can act both on cancer stem cells, and with the rest of the tumor cell mass, leading to complex biological outputs. As a note of caution, when used as single agent, HDACi show only a moderate and limited biological response, which is augmented in combinatorial therapies with drugs designed against other epigenetic targets. The optimal employment of these molecules may be therefore in combination with other epigenetic drugs acting against the set of enzymes responsible for the set-up and maintenance of epigenetic information.

Anti-leukemic activity of valproic acid and imatinib mesylate on human Ph+ ALL and CML cells in vitro

The armamentarium of anti-leukemic drugs has increased substantially since anti-leukemic activities were recently found for a variety of non-classical cytostatic drugs, among them the histone deacetylase (HDAC) inhibitor valproic acid (VPA). This study investigated the effect of VPA on proliferation and apoptosis of human Philadelphia chromosome-positive (Ph+) acute lymphatic (ALL) and chronic myeloid leukemia (CML) cells and on colony formation of human chronic-phase CML progenitor cells. Strong anti-proliferative and pro-apoptotic effects of VPA were observed on human ALL and CML cell lines at concentrations achievable in vivo. These effects were most pronounced in ALL cell lines as well as in primary ALL cells. Notably, VPA revealed enhanced activity with imatinib mesylate, nilotinib, the farnesyl transferase inhibitor SCH66336, interferon-alpha and cytosine arabinoside. VPA inhibited the growth of colony-forming cells from 12 Ph+ chronic-phase CML patients but also of those from normal healthy controls in a dose-dependent fashion. HDAC-inhibiting activity of VPA was confirmed on ALL and CML cells. In conclusion, VPA, whether alone or in combination with other non-classical anti-leukemic compounds, exerts significant anti-leukemic effects on human ALL and CML cells.

Epigenetic therapy of leukemia: An update

Carcinogenesis is classically thought to result from genetic alterations in DNA sequence such as deletions, mutations, or chromosomal translocations. These in turn may lead to the activation of oncogenes, inactivation of tumor suppressor genes or formation of chimeric oncoproteins. Epigenetics, in contrast, refers to a number of biochemical modifications of chromatin, either to DNA directly or to its associated protein complexes that affect gene expression without altering the primary sequence of DNA [Robertson KD, Wolffe AP. DNA methylation in health and disease. Nat Rev Genet 2000;1:11-9; Jones PA, Baylin SB. The epigenomics of cancer. Cell. 2007;128:683-92]. A fundamental difference between genetic and epigenetic alterations is the irreversible nature of genetic lesions whereas epigenetic ones are potentially reversible, allowing for therapeutic intervention. In the last decade, it has become apparent that epigenetic changes play an important role in cancer, particularly in leukemia. Significant advances have been made in the elucidation of these processes as well as in translating this knowledge to the clinic, as in the development of new prognostic biomarkers or targeted therapies. In this review, we will focus on recent advances in epigenetic therapy in leukemia.

Elevated FOSB-expression; a potential marker of valproate sensitivity in AML

Histone deacetylase inhibitors (HDIs) are emerging as valuable new agents in the treatment of acute myeloid leukaemia (AML). However, since response rates to these agents alone are low, we sought to identify markers associated with responsiveness. In a trial of 20 patients treated with the HDI sodium valproate (VPA) in combination with all trans retinoic acid and theophylline, three patients responded clinically with one complete remission (CR) and two partial remissions. The in vivo response of the CR patient was mirrored by high in vitro sensitivity of their blasts to VPA, indicating that similar factors determine both in vivo and in vitro sensitivity. Microarray analysis of the primary AMLs and a panel of haemato-lymphoid cell lines, with a similar range of VPA sensitivities as the primary leukaemic blasts, identified elevated FOSB-expression as a potential marker of VPA sensitivity. Quantitative polymerase chain reaction confirmed overexpression of FOSB in the CR patient blasts compared to patients failing to achieve CR, and in a subset of a larger panel of AML samples. Overexpression of FOSB in K562 myeloid cells significantly increased in vitro sensitivity to VPA. Thus, we propose that FOSB is a novel, potential marker of VPA sensitivity in AML.

Emerging treatment strategies for acute myeloid leukemia (AML) in the elderly

Acute myeloid leukemia (AML) is more prevalent in older adults, with an incidence in the United States of 17.6 per 100,000 for those 65 years of age, compared with an incidence of 1.8 per 100,000 for those <65 years of age. While there have been improvements in survival during the last decade for younger patients, prognosis in elderly patients is still poor; approximately 50% achieve complete responses, but many of them relapse. With increasing age, more patients are suboptimal candidates for standard induction chemotherapy due to poor performance status, pre-existing myelodysplasia, unfavorable cytogenetics, treatment-related AML, multidrug resistance protein expression, and CD34 positivity, which are often characteristic of this patient population. In addition, the presence of comorbid conditions make many treatment options less tolerable for elderly patients. Several investigators have described subgroups showing no benefit after intensive treatment approaches in recent years. However, several novel agents have been developed to treat elderly AML patients. These include new chemotherapeutic agents, such as nucleoside analogs, as well as targeted therapies like farnesyltransferase inhibitors, tyrosine kinase inhibitors, epigenetic drugs, and antibodies. On the other hand new insights into the biology of the disease lead to a better understanding of its heterogeneity. Thus, with a variety of novel substances at hand it is increasingly important to introduce a risk-adapted approach for the optimal management of patients. This review will identify subgroups not likely to benefit from intensive chemotherapy and highlight the efficacy and tolerability of new agents in the treatment of AML.

Epigenetic reprogramming of breast cancer cells by valproic acid occurs regardless of estrogen receptor status

Estrogen receptors (ERs) are a recognized prognostic factor and therapeutic target in breast cancer. The loss of ER expression relates to poor prognosis, poor clinical outcome and impairs the use of anti-estrogenic treatment. Histone deacetylase inhibitors are candidate drugs for cancer therapy. Among them, valproic acid (VPA) is a long used and safe anti-epileptic drug. We studied the biological consequences of the chromatin remodeling action of VPA in a normal human mammary epithelial cell line and in ERalpha-positive and ERalpha-negative breast cancer cell lines. In these cells and regardless of their ER status, VPA-induced cell differentiation, as shown by increased milk lipids production, decreased expression of the CD44 antigen and growth arrest in the G(0)-G(1) phase of the cell cycle. These effects were accompanied by decreased Rb phosphorylation, hyperacetylation of the p21(WAF1/CIP1) gene promoter and increased p21 protein expression. Only in breast cancer cells, cyclin B1 expression was decreased and the cells accumulated also in G(2). ERalpha expression decreased in ERalpha-positive, increased in ERalpha-negative and was unchanged in normal mammary epithelial cells, as did the expression of progesterone receptor, a physiological ERalpha target. VPA decreased the expression of the invasiveness marker pS2 in ERalpha-positive breast cancer cells, but did not cause its re-expression in ERalpha-negative cells. Overall, these data suggest that in both ERalpha-positive and -negative malignant mammary epithelial cells VPA reprograms the cells to a more differentiated and "physiologic" phenotype that may improve the sensitivity to endocrine therapy and/or chemotherapy in breast cancer patients.

Photocarcinogenesis of topical tazarotene and isotretinoin alone and in combination with valproic acid in hairless mice

Retinoids and the histone deacetylase inhibitor valproic acid have shown anticancer properties, but the photocarcinogenic or photoprotective effect is unclear. Therefore, we investigated whether a topical formulation of valproic acid is photocarcinogenic or photoprotective in hairless female C3.Cg/TifBomTac immunocompetent mice exposed to simulated solar radiation (SSR) and whether valproic acid changes the effect of the retinoids: tazarotene and isotretinoin. The products were applied on the dorsal skin of 400 mice (five times weekly) followed by SSR (three times weekly) 3-4 h after the application. This was performed during 12 months or until death. Tumors appeared sooner in groups treated with tazarotene and isotretinoin compared with that of the group treated with valproic acid and the control group. The present study shows that valproic acid alone is not photocarcinogenic or photoprotective in hairless mice. When valproic acid is combined with tazarotene or isotretinoin, it does not change their photocarcinogenicity significantly.

Epigenetic plasticity of chromatin in embryonic and hematopoietic stem/progenitor cells: therapeutic potential of cell reprogramming

During embryonic development and adult life, the plasticity and reversibility of modifications that affect the chromatin structure is important in the expression of genes involved in cell fate decisions and the maintenance of cell-differentiated state. Epigenetic changes in DNA and chromatin, which must occur to allow the accessibility of transcriptional factors at specific DNA-binding sites, are regarded as emerging major players for embryonic and hematopoietic stem cell (HSC) development and lineage differentiation. Epigenetic deregulation of gene expression, whether it be in conjunction with chromosomal alterations and gene mutations or not, is a newly recognized mechanism that leads to several diseases, including leukemia. The reversibility of epigenetic modifications makes DNA and chromatin changes attractive targets for therapeutic intervention. Here we review some of the epigenetic mechanisms that regulate gene expression in pluripotent embryonic and multipotent HSCs but may be deregulated in leukemia, and the clinical approaches designed to target the chromatin structure in leukemic cells.

Current status of epigenetic treatment in myelodysplastic syndromes

Epigenetic deregulation plays an important role in cancer development. The great interest in epigenetics in hematology and oncology results from the fact that epigenetic, in contrast to genetic, alterations are, in principle, amenable to pharmacological reversal. Epigenetically active drugs currently within clinical trials include histone deacetylase inhibitors (HDACi) and DNA methyltransferase (DNMT) inhibitors. The first treatment approved by the Food and Drug Administration for the treatment of myelodysplastic syndromes (MDS) was the DNMT-inhibitor 5-azacytidine. Currently, two out of three drugs FDA approved for MDS therapy, 5-azacytidine and 5-aza-2'-deoxycytidine, are epigenetically active drugs. Recent clinical trials investigate new dosing schedules, routes of administration, and combination regimens. Several structurally distinct HDACi have been developed. Available data is mostly restricted to phase I trials. The largest experience in MDS and acute myeloid leukemia exists with the anticonvulsant valproic acid. This review summarizes the existing clinical experience on HDACi and DNMT inhibitors.

Histone hyperacetylation occurs on promoters of lytic cycle regulatory genes in Epstein-Barr virus-infected cell lines which are refractory to disruption of latency by histone deacetylase inhibitors

Activation of the Epstein-Barr virus (EBV) lytic cycle is mediated through the combined actions of ZEBRA and Rta, the products of the viral BZLF1 and BRLF1 genes. During latency, these two genes are tightly repressed. Histone deacetylase inhibitors (HDACi) can activate viral lytic gene expression. Therefore, a widely held hypothesis is that Zp and Rp, the promoters for BZLF1 and BRLF1, are repressed by chromatin and that hyperacetylation of histone tails, by allowing the access of positively acting factors, leads to transcription of BZLF1 and BRLF1. To investigate this hypothesis, we used chromatin immunoprecipitation (ChIP) to examine the acetylation and phosphorylation states of histones H3 and H4 on Zp and Rp in three cell lines, Raji, B95-8, and HH514-16, which differ in their response to EBV lytic induction by HDACi. We studied the effects of three HDACi, sodium butyrate (NaB), trichostatin A (TSA), and valproic acid (VPA). We also examined the effects of tetradecanoyl phorbol acetate (TPA) and 5-aza-2'-deoxycytidine, a DNA methyltransferase inhibitor, on histone modification. In Raji cells, TPA and NaB act synergistically to activate the EBV lytic cycle and promote an increase in histone H3 and H4 acetylation and phosphorylation at Zp and Rp. Surprisingly, however, when Raji cells were treated with NaB or TSA, neither of which is sufficient to activate the lytic cycle, an increase of comparable magnitude of hyperacetylated and phosphorylated histone H3 at Zp and Rp was observed. In B95-8 cells, NaB inhibited lytic induction by TPA, yet NaB promoted hyperacetylation of H3 and H4. In HH514-16 cells, NaB and TSA strongly activated the EBV lytic cycle and caused hyperacetylation of histone H3 on Zp and Rp. However, when HH514-16 cells were treated with VPA, lytic cycle mRNAs or proteins were not induced, although histone H3 was hyperacetylated as measured by immunoblotting or by ChIP on Zp and Rp. Taken together, our data suggest that open chromatin at EBV BZLF1 and BRLF1 promoters is not sufficient to activate EBV lytic cycle gene expression.

Epigenetic treatment of myelodysplastic syndromes and acute myeloid leukemias

Epigenetic mechanisms affecting chromatin structure contribute to regulate gene expression and assure the inheritance of information, which are essential for the proper expression of key regulatory genes in healthy cells, tissues and organs. In the medical field, an increasing body of evidence indicates that altered gene expression or de-regulated gene function lead to disease. Cancer cells also suffer a profound change in the genomic methylation patterns and chromatin status. Aberrant DNA methylation patterns, changes in chromatin structure and in gene expression are common in all kind of tumor types. However, studies on leukemias have provided paradigmatic examples for the functional implications of the epigenetic alterations in cancer development and progression as well as their relevance for therapeutical targeting.

NKG2D ligand expression in AML increases in response to HDAC inhibitor valproic acid and contributes to allorecognition by NK-cell lines with single KIR-HLA class I specificities

This study exploited alloreactivity of natural killer (NK) cells for augmenting the recognition of human acute myeloid leukemia (AML). To circumvent the inhibitory effect of killer immunoglobulin receptor (KIR) signaling, we generated NK-cell lines with single KIR specificities for major human leukocyte antigen (HLA) class I allotypes. We demonstrated efficient cytolysis of KIR-HLA class I-mismatched primary AML blasts even at low effector-to-target ratios. To define the impact of tumor-associated activating NKG2D-ligands (NKG2D-L), 66 AML patients at diagnosis were analyzed. NKG2D-L were selectively expressed on monoblastic cells in AML M4 and M5 yet absent or weakly expressed on myeloblastic cells in all AML subtypes. Paucity of cell-surface NKG2D-L was not the result of shedding because levels of soluble ULBP1 ligand measured in AML plasma were in the normal range. Notably, purified NKG2D-L(+) monoblastic cells were more susceptible to NK-mediated killing than NKG2D-L(-) myeloblastic cells. Accordingly, induction of cell-surface NKG2D-L by treatment with the histone deacetylase inhibitor, valproic acid, rendered cells more sensitive to NK cytolysis. These data suggest that adoptive transfer of selected populations of alloreactive HLA class I-mismatched NK cells in combination with pharmacologic induction of NKG2D-L merits clinical evaluation as novel approaches to immunotherapy of human AML.

Valproic acid for the treatment of myeloid malignancies

Valproic acid (VPA) has been used as an anticonvulsant for decades. Recently, it was demonstrated that VPA also acts as a histone deacetylase inhibitor and induces differentiation and apoptosis in a variety of malignant cells in vitro. The effect of VPA on tumor cells differs according to cell type, degree of differentiation, and underlying genetic alterations. Clinical trials with VPA have focused on acute myeloid leukemia and the myelodysplastic syndromes. When it was used as monotherapy or in combination with all-trans retinoic acid, which synergizes in vitro, VPA achieved hematologic improvement in a subset of patients. Similar to other inhibitors of histone deacetylases, complete or partial remissions rarely were observed. In this report, the authors reviewed the in vitro and in vivo data obtained with VPA, and they considered possible combination regimens aimed at improving therapeutic efficacy.

Downregulation of c-Myc is critical for valproic acid-induced growth arrest and myeloid differentiation of acute myeloid leukemia

Valproic acid (VPA), an agent used for neurological disorders, has been shown to be a novel class of histone deacetylase inhibitor (HDACI), able to induce apoptosis and myeloid differentiation of acute myeloid leukemia (AML). In this study, we examined the underlying mechanisms in VPA-mediated activities in AML cells. VPA not only inhibited the growth of HL-60, U937 and NB4 cells by causing cell-cycle arrest at G(0)/G(1) phase and apoptosis, but also induced morphologic and phenotypic changes. VPA markedly increased p21WAF1, and downregulated c-Myc expression at transcriptional levels. Ectopic expression of wildtype c-Myc and T58A mutant significantly inhibited VPA-mediated growth inhibition. As with results from cell line studies, VPA also downregulated c-Myc levels, and induced apoptosis and myeloid differentiation of primary AML cells, leading to decreased colony-forming ability. Given the role of c-Myc in leukemogenesis, our study suggests that VPA might be a potential therapeutic agent for AML.

Diverse actions of retinoid receptors in cancer prevention and treatment

Retinoids (retinol [vitamin A] and its biologically active metabolites) are essential signaling molecules that control various developmental pathways and influence the proliferation and differentiation of a variety of cell types. The physiological actions of retinoids are mediated primarily by the retinoic acid receptors alpha, beta, and gamma (RARs) and rexinoid receptors alpha, beta, and gamma. Although mutations in RARalpha, via the PML-RARalpha fusion proteins, result in acute promyelocytic leukemia, RARs have generally not been reported to be mutated or part of fusion proteins in carcinomas. However, the retinoid signaling pathway is often compromised in carcinomas. Altered retinol metabolism, including low levels of lecithin:retinol acyl trasferase and retinaldehyde dehydrogenase 2, and higher levels of CYP26A1, has been observed in various tumors. RARbeta(2) expression is also reduced or is absent in many types of cancer. A greater understanding of the molecular mechanisms by which retinoids induce cell differentiation, and in particular stem cell differentiation, is required in order to solve the issue of retinoid resistance in tumors, and thereby to utilize RA and synthetic retinoids more effectively in combination therapies for human cancer.

Safety and clinical activity of the combination of 5-azacytidine, valproic acid, and all-trans retinoic acid in acute myeloid leukemia and myelodysplastic syndrome

The combination of a DNA hypomethylating agent with a histone deacetylase inhibitor has synergistic antileukemia activity and may restore sensitivity to all-trans retinoic acid (ATRA). We conducted a phase 1/2 study of the combination of 5-azacitidine (5-AZA), valproic acid (VPA), and ATRA in patients with acute myeloid leukemia or high-risk myelodysplastic syndrome. 5-AZA was administered subcutaneously at a fixed dose of 75 mg/m(2) daily for 7 days. VPA was dose-escalated and given orally daily for 7 days concomitantly with 5-AZA. ATRA was given at 45 mg/m(2) orally daily for 5 days, starting on day 3. A total of 53 patients were treated. Their median age was 69 years (range, 5-84 years). The maximum tolerated dose of VPA in this combination was 50 mg/kg daily for 7 days. Dose-limiting toxicity was reversible neurotoxicity. The overall response rate was 42%. In previously untreated older patients, the response rate was 52%. Median number of courses to response was 1 (range, 1-3 courses). Median remission duration was 26 weeks, and median survival has not been reached. A significant decrease in global DNA methylation and induction of histone acetylation were achieved. VPA blood levels were higher in responders (P < .005). In conclusion, the combination studied is safe and has significant clinical activity. This clinical trial was registered at www.clinicaltrials.gov as no. NCT00326170.

Histone deacetylase inhibitors--turning epigenic mechanisms of gene regulation into tools of therapeutic intervention in malignant and other diseases

Histone deacetylase inhibitors reside among the most promising targeted anticancer agents that are potent inducers of growth arrest, differentiation, and/or apoptotic cell death of transformed cells. In October 2006, the US Food and Drug Administration approved the first drug of this new class, vorinostat (1, Zolinza, Merck). Several histone deacetylase (HDAC) inhibitors more are in clinical trials. HDAC inhibitors have shown significant activity against a variety of hematological and solid tumors at doses that are well tolerated by patients, both in monotherapy as well as in combination therapy with other drugs. This paper reviews the most recent developments in HDAC inhibitor design, particularly in the context of anticancer therapy, and other possible pharmaceutical applications.