Molecular and biologic characterization and drug sensitivity of pan-histone deacetylase inhibitor-resistant acute myeloid leukemia cells

Definitions, Biology, and Current Therapeutic Landscape of Myelodysplastic/Myeloproliferative Neoplasms

Myelodysplastic/myeloproliferative neoplasms (MDS/MPN) are hematological disorders characterized by both proliferative and dysplastic features. According to the 2022 International Consensus Classification (ICC), MDS/MPN consists of clonal monocytosis of undetermined significance (CMUS), chronic myelomonocytic leukemia (CMML), atypical chronic myeloid leukemia (aCML), MDS/MPN with SF3B1 mutation (MDS/MPN-T-SF3B1), MDS/MPN with ring sideroblasts and thrombocytosis not otherwise specified (MDS/MPN-RS-T-NOS), and MDS/MPN-NOS. These disorders exhibit a diverse range of genetic alterations involving various transcription factors (e.g., RUNX1), signaling molecules (e.g., NRAS, JAK2), splicing factors (e.g., SF3B, SRSF2), and epigenetic regulators (e.g., TET2, ASXL1, DNMT3A), as well as specific cytogenetic abnormalities (e.g., 8 trisomies, 7 deletions/monosomies). Clinical studies exploring therapeutic options for higher-risk MDS/MPN overlap syndromes mostly involve hypomethylating agents, but other treatments such as lenalidomide and targeted agents such as JAK inhibitors and inhibitors targeting PARP, histone deacetylases, and the Ras pathway are under investigation. While these treatment modalities can provide partial disease control, allogeneic bone marrow transplantation (allo-BMT) is the only potentially curative option for patients. Important prognostic factors correlating with outcomes after allo-BMT include comorbidities, splenomegaly, karyotype alterations, and the bone marrow blasts percentage at the time of transplantation. Future research is imperative to optimizing therapeutic strategies and enhancing patient outcomes in MDS/MPN neoplasms. In this review, we summarize MDS/MPN diagnostic criteria, biology, and current and future treatment options, including bone marrow transplantation.

Lysine Acetylation, Cancer Hallmarks and Emerging Onco-Therapeutic Opportunities

Simple Summary

Several histone deacetylase inhibitors have been approved by FDA for cancer treatment. Intensive efforts have been devoted to enhancing its anti-cancer efficacy by combining it with various other agents. Yet, no guideline is available to assist in the choice of candidate drugs for combination towards optimal solutions for different clinical problems. Thus, it is imperative to characterize the primary cancer hallmarks that lysine acetylation is associated with and gain knowledge on the key cancer features that each combinatorial onco-therapeutic modality targets to aid in the combinatorial onco-therapeutic design. Cold atmospheric plasma represents an emerging anti-cancer modality via manipulating cellular redox level and has been demonstrated to selectively target several cancer hallmarks. This review aims to delineate the intrinsic connections between lysine acetylation and cancer properties, and forecast opportunities histone deacetylase inhibitors may have when combined with cold atmospheric plasma as novel precision onco-therapies.

Abstract

Acetylation, a reversible epigenetic process, is implicated in many critical cellular regulatory systems including transcriptional regulation, protein structure, activity, stability, and localization. Lysine acetylation is the most prevalent and intensively investigated among the diverse acetylation forms. Owing to the intrinsic connections of acetylation with cell metabolism, acetylation has been associated with metabolic disorders including cancers. Yet, relatively little has been reported on the features of acetylation against the cancer hallmarks, even though this knowledge may help identify appropriate therapeutic strategies or combinatorial modalities for the effective treatment and resolution of malignancies. By examining the available data related to the efficacy of lysine acetylation against tumor cells and elaborating the primary cancer hallmarks and the associated mechanisms to target the specific hallmarks, this review identifies the intrinsic connections between lysine acetylation and cancer hallmarks and proposes novel modalities that can be combined with HDAC inhibitors for cancer treatment with higher efficacy and minimum adverse effects.

Characterization of the MYB-inhibitory potential of the Pan-HDAC inhibitor LAQ824

A large body of work has shown that MYB acts as a master transcription regulator in hematopoietic cells and has pinpointed MYB as a potential drug target for acute myeloid leukemia (AML). Here, we have examined the MYB-inhibitory potential of the HDAC inhibitor LAQ824, which was identified in a screen for novel MYB inhibitors. We show that nanomolar concentrations of LAQ824 and the related HDAC inhibitors vorinostat and panobinostat interfere with MYB function in two ways, by inducing its degradation and inhibiting its activity. Reporter assays show that the inhibition of MYB activity by LAQ824 involves the MYB transactivation domain and the cooperation of MYB with co-activator p300, a key MYB interaction partner and driver of MYB activity. In AML cells, LAQ824-induced degradation of MYB is accompanied by expression of myeloid differentiation markers and apoptotic and necrotic cell death. The ability of LAQ824 to inhibit MYB activity is supported by the observation that down-regulation of direct MYB target genes MYC and GFI1 occurs without apparent decrease of MYB expression already after 2 h of treatment with LAQ824. Furthermore, ectopic expression of an activated version of MYB In HL60 cells counteracts the induction of myeloid differentiation by LAQ824. Overall, our data identify LAQ824 and related HDAC inhibitors as potent MYB-inhibitory agents that exert dual effects on MYB expression and activity in AML cells.

Histone deacetylases: A novel class of therapeutic targets for pancreatic cancer

Pancreatic cancer is the seventh leading cause of cancer death worldwide, with a low 5-year survival rate. Novel agents are urgently necessary to treat the main pathological type, known as pancreatic ductal carcinoma (PDAC). The dysregulation of histone deacetylases (HDACs) has been identified in association with PDAC, which can be more easily targeted by small molecular inhibitors than gene mutations and may represent a therapeutic breakthrough for PDAC. However, the contributions of HDACs to PDAC remain controversial, and pharmacokinetic challenges have limited the application of HDAC inhibitors (HDACis) in PDAC. This review summarizes the mechanisms associated with success and failure of HDACis in PDAC and discusses the recent progress made in HDACi development and application, such as combination therapies designed to enhance efficacy. More precise strategies involving HDACis might eventually improve the outcomes of PDAC treatment.

Entamoeba histolytica Adaption to Auranofin: A Phenotypic and Multi-Omics Characterization

Auranofin (AF), an antirheumatic agent, targets mammalian thioredoxin reductase (TrxR), an important enzyme controlling redox homeostasis. AF is also highly effective against a diversity of pathogenic bacteria and protozoan parasites. Here, we report on the resistance of the parasite Entamoeba histolytica to 2 µM of AF that was acquired by gradual exposure of the parasite to an increasing amount of the drug. AF-adapted E. histolytica trophozoites (AFAT) have impaired growth and cytopathic activity, and are more sensitive to oxidative stress (OS), nitrosative stress (NS), and metronidazole (MNZ) than wild type (WT) trophozoites. Integrated transcriptomics and redoxomics analyses showed that many upregulated genes in AFAT, including genes encoding for dehydrogenase and cytoskeletal proteins, have their product oxidized in wild type trophozoites exposed to AF (acute AF trophozoites) but not in AFAT. We also showed that the level of reactive oxygen species (ROS) and oxidized proteins (OXs) in AFAT is lower than that in acute AF trophozoites. Overexpression of E. histolytica TrxR (EhTrxR) did not protect the parasite against AF, which suggests that EhTrxR is not central to the mechanism of adaptation to AF.

A novel dual HDAC and HSP90 inhibitor, MPT0G449, downregulates oncogenic pathways in human acute leukemia in vitro and in vivo

Acute leukemia is a highly heterogeneous disease; therefore, combination therapy is commonly used for patient treatment. Drug–drug interaction is a major concern of combined therapy; hence, dual/multi-target inhibitors have become a dominant approach for cancer drug development. HDACs and HSP90 are involved in the activation of various oncogenic signaling pathways, including PI3K/AKT/mTOR, JAK/STAT, and RAF/MEK/ERK, which are also highly enriched in acute leukemia gene expression profiles. Therefore, we suggest that dual HDAC and HSP90 inhibitors could represent a novel therapeutic approach for acute leukemia. MPT0G449 is a dual effect inhibitor, and it showed cytotoxic effectiveness in acute leukemia cells. Molecular docking analysis indicated that MPT0G449 possessed dual HDAC and HSP90 inhibitory abilities. Furthermore, MPT0G449 induced G₂ arrest and caspase-mediated cell apoptosis in acute leukemia cells. The oncogenic signaling molecules AKT, mTOR, STAT3, STAT5, MEK, and ERK were significantly downregulated after MPT0G449 treatment in HL-60 and MOLT-4 cells. In vivo xenograft models confirmed the antitumor activity and showed the upregulation of acetyl-histone H3 and HSP70, biomarkers of pan-HDAC and HSP90 inhibition, with MPT0G449 treatment. These findings suggest that the dual inhibition of HDAC and HSP90 can suppress the expression of oncogenic pathways in acute leukemia, and MPT0G449 represents a novel therapeutic for anticancer treatment.

HDN-1 induces cell differentiation toward apoptosis in promyelocytic leukemia cells depending on its selective effect on client proteins of Hsp90

Heat Shock Protein 90 (Hsp90) is frequently upregulated in many cancers, and its inhibition simultaneously blocks multiple signaling pathways, resulting in cell differentiation or apoptosis. However, the complexity of Hsp90 in differentiation and its relation with apoptosis have remained unsettled. In this study, we demonstrated that HDN-1, a C-terminal inhibitor of Hsp90, induced the differentiation of HL-60 cells toward apoptosis. HDN-1 induced the differentiation of cells containing mutant AML1-ETO into mature granulocytes, which was related to its selective effect on client proteins of Hsp90. HDN-1 destabilized AML1-ETO and preserved C/EBPβ at the same time, thereby induced a total increase in C/EBPβ levels because of AML1-ETO negative regulation to C/EBPβ expression. Neither HDN-1 nor 17-AAG (an N-terminal inhibitor of Hsp90) led to the differentiation of NB4 cells because mutant PML-RARα was not affected as a client protein of Hsp90; thus, no additional expression of C/EBPβ was induced. 17-AAG did not affect the differentiation of HL-60 cells due to decreased AML1-ETO and C/EBPβ levels. These results indicate that HDN-1 drives cell differentiation toward apoptosis depending on its selective influence on client proteins of Hsp90, establishing the relationship between differentiation and apoptosis and uncovering the mechanism of HDN-1 in promyelocytic leukemia cell differentiation. Moreover, HDN-1 is very promising for the development of anticancer agents with the induction of differentiation.

Resistance to histone deacetylase inhibitors confers hypersensitivity to oncolytic reovirus therapy

Despite the promising antilymphoma activity of histone deacetylase (HDAC) inhibitors as a drug class, resistance is a significant clinical issue. Elucidating the molecular mechanisms driving HDAC inhibitor resistance and/or the specific targets that are altered in drug-resistant cells may facilitate the development of strategies that overcome drug resistance and are more effective for refractory patients. We generated novel T-cell lymphoma (TCL) cell line models of acquired resistance to the HDAC inhibitor belinostat to identify potential effective therapies. Belinostat-resistant cells displayed significant cross-resistance to other HDAC inhibitors including romidepsin, panobinostat, and vorinostat. Consistent with a lack of sensitivity to HDAC inhibitors, the resistant cells failed to induce increased acetylated histones. Drug-resistant cells featured significantly decreased expression of the key antiviral mediators IRF1 and STAT1. On the basis of these findings, we investigated the efficacy of the clinical formulation of reovirus (Reolysin) in parental and drug-resistant models. Our investigation revealed that HDAC inhibitor-resistant cells displayed enhanced vulnerability to reovirus replication and cell death in both in vitro and in vivo models compared with their parental counterparts. Importantly, Reolysin also significantly increased the antilymphoma activity of belinostat in HDAC inhibitor-resistant cells. Our data demonstrate that Reolysin alone or in combination with belinostat is a novel therapeutic strategy to treat TCL patients who develop resistance to HDAC inhibitors.

Post-translational modifications of Hsp90 and translating the chaperone code

Cells have a remarkable ability to synthesize large amounts of protein in a very short period of time. Under these conditions, many hydrophobic surfaces on proteins may be transiently exposed, and the likelihood of deleterious interactions is quite high. To counter this threat to cell viability, molecular chaperones have evolved to help nascent polypeptides fold correctly and multimeric protein complexes assemble productively, while minimizing the danger of protein aggregation. Heat shock protein 90 (Hsp90) is an evolutionarily conserved molecular chaperone that is involved in the stability and activation of at least 300 proteins, also known as clients, under normal cellular conditions. The Hsp90 clients participate in the full breadth of cellular processes, including cell growth and cell cycle control, signal transduction, DNA repair, transcription, and many others. Hsp90 chaperone function is coupled to its ability to bind and hydrolyze ATP, which is tightly regulated both by co-chaperone proteins and post-translational modifications (PTMs). Many reported PTMs of Hsp90 alter chaperone function and consequently affect myriad cellular processes. Here, we review the contributions of PTMs, such as phosphorylation, acetylation, SUMOylation, methylation, O-GlcNAcylation, ubiquitination, and others, toward regulation of Hsp90 function. We also discuss how the Hsp90 modification state affects cellular sensitivity to Hsp90-targeted therapeutics that specifically bind and inhibit its chaperone activity. The ultimate challenge is to decipher the comprehensive and combinatorial array of PTMs that modulate Hsp90 chaperone function, a phenomenon termed the "chaperone code."

Understanding Failure and Improving Treatment Using HDAC Inhibitors for Prostate Cancer

Novel treatment regimens are required for castration-resistant prostate cancers (CRPCs) that become unresponsive to standard treatments, such as docetaxel and enzalutamide. Histone deacetylase (HDAC) inhibitors showed promising results in hematological malignancies, but they failed in solid tumors such as prostate cancer, despite the overexpression of HDACs in CRPC. Four HDAC inhibitors, vorinostat, pracinostat, panobinostat and romidepsin, underwent phase II clinical trials for prostate cancers; however, phase III trials were not recommended due to a majority of patients exhibiting either toxicity or disease progression. In this review, the pharmacodynamic reasons for the failure of HDAC inhibitors were assessed and placed in the context of the advancements in the understanding of CRPCs, HDACs and resistance mechanisms. The review focuses on three themes: evolution of androgen receptor-negative prostate cancers, development of resistance mechanisms and differential effects of HDACs. In conclusion, advancements can be made in this field by characterizing HDACs in prostate tumors more extensively, as this will allow more specific drugs catering to the specific HDAC subtypes to be designed.

Ligand-independent activation of the P2X7 receptor by Hsp90 inhibition stimulates motor neuron apoptosis

Activation of the extracellular ATP ionotropic receptor P2X7 stimulates motor neuron apoptosis, whereas its inhibition in cell and animal models of amyotrophic lateral sclerosis can be protective. These observations suggest that P2X7 receptor activation is relevant to motor neuron disease and that it could be targeted for therapeutic development. Heat shock protein 90 (Hsp90) is an integral regulatory component of the P2X7 receptor complex, antagonizing ligand-induced receptor activation. Here, we show that the repressive activity of Hsp90 on P2X7 receptor activation in primary motor neurons is highly sensitive to inhibition. Primary motor neurons in culture are 100-fold more sensitive to Hsp90 inhibition by geldanamycin than other neuronal populations. Pharmacological inhibition and down-regulation of the P2X7 receptor prevented motor neuron apoptosis triggered by Hsp90 inhibition, which occurred in the absence of extracellular ATP. These observations suggest that inhibition of a seemingly motor neuron specific pool of Hsp90 leads to ligand independent activation of P2X7 receptor and motor neuron death. Downstream of Hsp90 inhibition, P2X7 receptor activated the phosphatase and tensin homolog (TPEN), which in turn suppressed the pro-survival phosphatidyl inositol 3 kinase (PI3K)/Akt pathway, leading to Fas-dependent motor neuron apoptosis. Conditions altering the interaction between P2X7 receptor and Hsp90, such as recruitment of Hsp90 to other subcellular compartments under stress conditions, or nitration following oxidative stress can induce motor neuron death. These findings may have broad implications in neurodegenerative disorders, including amyotrophic lateral sclerosis, in which activation of P2X7 receptor may be involved in both autonomous and non-autonomous motor neurons death.

HSP90 Molecular Chaperones, Metabolic Rewiring, and Epigenetics: Impact on Tumor Progression and Perspective for Anticancer Therapy

Heat shock protein 90 (HSP90) molecular chaperones are a family of ubiquitous proteins participating in several cellular functions through the regulation of folding and/or assembly of large multiprotein complexes and client proteins. Thus, HSP90s chaperones are, directly or indirectly, master regulators of a variety of cellular processes, such as adaptation to stress, cell proliferation, motility, angiogenesis, and signal transduction. In recent years, it has been proposed that HSP90s play a crucial role in carcinogenesis as regulators of genotype-to-phenotype interplay. Indeed, HSP90 chaperones control metabolic rewiring, a hallmark of cancer cells, and influence the transcription of several of the key-genes responsible for tumorigenesis and cancer progression, through either direct binding to chromatin or through the quality control of transcription factors and epigenetic effectors. In this review, we will revise evidence suggesting how this interplay between epigenetics and metabolism may affect oncogenesis. We will examine the effect of metabolic rewiring on the accumulation of specific metabolites, and the changes in the availability of epigenetic co-factors and how this process can be controlled by HSP90 molecular chaperones. Understanding deeply the relationship between epigenetic and metabolism could disclose novel therapeutic scenarios that may lead to improvements in cancer treatment.

Inhibition of TFEB oligomerization by co-treatment of melatonin with vorinostat promotes the therapeutic sensitivity in glioblastoma and glioma stem cells

Glioblastoma (GBM) is the most aggressive malignant glioma and most lethal form of human brain cancer (Clin J Oncol Nurs. 2016;20:S2). GBM is also one of the most expensive and difficult cancers to treat by the surgical resection, local radiotherapy, and temozolomide (TMZ) and still remains an incurable disease. Oncomine platform analysis and Gene Expression Profiling Interactive Analysis (GEPIA) show that the expression of transcription factor EB (TFEB) was significantly increased in GBMs and in GBM patients above stage IV. TFEB requires the oligomerization and localization to regulate transcription in the nucleus. Also, the expression and oligomerization of TFEB proteins contribute to the resistance of GBM cells to conventional chemotherapeutic agents such as TMZ. Thus, we investigated whether the combination of vorinostat and melatonin could overcome the effects of TFEB and induce apoptosis in GBM cells and glioma cancer stem cells (GSCs). The downregulation of TFEB and oligomerization by vorinostat and melatonin increased the expression of apoptosis-related genes and activated the apoptotic cell death process. Significantly, the inhibition of TFEB expression dramatically decreased GSC tumor-sphere formation and size. The inhibitory effect of co-treatment resulted in decreased proliferation of GSCs and induced the expression of cleaved PARP and p-γH2AX. Taken together, our results definitely demonstrate that TFEB expression contributes to enhanced resistance of GBMs to chemotherapy and that vorinostat- and melatonin-activated apoptosis signaling in GBM cells by inhibiting TFEB expression and oligomerization, suggesting that co-treatment of vorinostat and melatonin may be an effective therapeutic strategy for human brain cancers.

The Therapeutic Strategy of HDAC6 Inhibitors in Lymphoproliferative Disease

Histone deacetylases (HDACs) are master regulators of chromatin remodeling, acting as epigenetic regulators of gene expression. In the last decade, inhibition of HDACs has become a target for specific epigenetic modifications related to cancer development. Overexpression of HDAC has been observed in several hematologic malignancies. Therefore, the observation that HDACs might play a role in various hematologic malignancies has brought to the development of HDAC inhibitors as potential antitumor agents. Recently, the class IIb, HDAC6, has emerged as one potential selective HDACi. This isoenzyme represents an important pharmacological target for selective inhibition. Its selectivity may reduce the toxicity related to the off-target effects of pan-HDAC inhibitors. HDAC6 has also been studied in cancer especially for its ability to coordinate a variety of cellular processes that are important for cancer pathogenesis. HDAC6 has been reported to be overexpressed in lymphoid cells and its inhibition has demonstrated activity in preclinical and clinical study of lymphoproliferative disease. Various studies of HDAC6 inhibitors alone and in combination with other agents provide strong scientific rationale for the evaluation of these new agents in the clinical setting of hematological malignancies. In this review, we describe the HDACs, their inhibitors, and the recent advances of HDAC6 inhibitors, their mechanisms of action and role in lymphoproliferative disorders.

Molecular, biological characterization and drug sensitivity of chidamide-resistant non-small cell lung cancer cells

Chidamide, a histone deacetylase (HDAC) inhibitor, has been applied in clinical trials for various types of hematological and solid tumors. Although acquired resistance is common in chemotherapy, the mechanism of resistance to chidamide is poorly characterized. The goal of the present study was to explore, in detail, the mechanism for the induced resistance to chidamide, and investigate a potential cross-resistance to other chemotherapeutic drugs. A549 cells were exposed to gradually increasing chidamide concentrations to establish a chidamide-resistant non-small cell lung cancer cell line (A549-CHI-R). The IC₅₀ for chidamide, the proliferation inhibition rate, the total HDAC activity and the HDAC protein level were determined by an MTT assay, colony formation, a fluorometric HDAC activity assay and western blotting, respectively. Overexpression of the HDAC1 gene and HDAC1 gene-knockdown were achieved via plasmid transfection. A549-CHI-R cells demonstrated increased resistance to chidamide (8.6-fold). HDAC1 protein degradation was inhibited and HDAC activity was significantly higher in the A549-CHI-R cells relative to the parental A549 cells. A549-CHI-R cells demonstrated cross-resistance to paclitaxel, vinorelbine and gemcitabine, but not to cisplatin (CDDP) or 5-fluorouracil (5-FU). These results indicated that HDAC1 may be associated with resistance to chidamide, and HDAC1 may therefore be a predictive marker for chidamide sensitivity in cancer. In addition, A549-CHI-R cells remained sensitive to 5-FU and CDDP, indicating a potential strategy for cancer therapy.

Panobinostat for the treatment of acute myelogenous leukemia

INTRODUCTION

Therapeutic strategies in patients with acute myeloid leukemia (AML) have not changed significantly over the last decades. Appropriate strategies are ultimately driven by the assessment of patients' fitness to define suitability for intensive induction chemotherapy, which produces high initial remission rates but, increased likelihood of relapse. Old/unfit AML patients still represent an urgent and unmet therapeutic need. Epigenetic deregulation represents a strategic characteristic of AML pathophysiology whereby aberrant gene transcription provides an advantage to leukemic cell survival. Efforts to re-establish impaired epigenetic regulation include hypomethylating agents and histone deacetylase inhibitors (HDACi).

AREAS COVERED

The review discusses the underlying mechanisms leading to disruption of lysine acetyltransferases (KAT or HAT)/deacetylase (KDAC or HDAC) balance and the rationale for using the HDACi panobinostat (LBH-589) in AML.

EXPERT OPINION

Although panobinostat has produced significant results in myeloma, its efficacy remains limited in AML. Panobinostat exerts pleiotropic activity and lack of specificity, which likely contributes to its inadequate safety in elderly AML patients. Phase I-II trials, utilizing panobinostat associated with well-known chemotherapeutic agents are ongoing and combinations with other druggable targets may likely be evaluated in future trials. The clinical use of this HDACi in AML the near future does not appearing promising.

A novel approach to detect resistance mechanisms reveals FGR as a factor mediating HDAC inhibitor SAHA resistance in B-cell lymphoma

Histone deacetylase (HDAC) inhibitors such as suberoylanilide hydroxamic acid (SAHA) are not commonly used in clinical practice for treatment of B-cell lymphomas, although a subset of patients with refractory or relapsed B-cell lymphoma achieved partial or complete remissions. Therefore, the purpose of this study was to identify molecular features that predict the response of B-cell lymphomas to SAHA treatment. We designed an integrative approach combining drug efficacy testing with exome and captured target analysis (DETECT). In this study, we tested SAHA sensitivity in 26 B-cell lymphoma cell lines and determined SAHA-interacting proteins in SAHA resistant and sensitive cell lines employing a SAHA capture compound (CC) and mass spectrometry (CCMS). In addition, we performed exome mutation analysis. Candidate validation was done by expression analysis and knock-out experiments. An integrated network analysis revealed that the Src tyrosine kinase Gardner-Rasheed feline sarcoma viral (v-fgr) oncogene homolog (FGR) is associated with SAHA resistance. FGR was specifically captured by the SAHA-CC in resistant cells. In line with this observation, we found that FGR expression was significantly higher in SAHA resistant cell lines. As functional proof, CRISPR/Cas9 mediated FGR knock-out in resistant cells increased SAHA sensitivity. In silico analysis of B-cell lymphoma samples (n = 1200) showed a wide range of FGR expression indicating that FGR expression might help to stratify patients, which clinically benefit from SAHA therapy. In conclusion, our comprehensive analysis of SAHA-interacting proteins highlights FGR as a factor involved in SAHA resistance in B-cell lymphoma.

Hsp70 protects from stroke in atrial fibrillation patients by preventing thrombosis without increased bleeding risk

AIMS

Atrial fibrillation (AF) is a major risk factor for cardio-embolic stroke. Anticoagulant drugs are effective in preventing AF-related stroke. However, the high frequency of anticoagulant-associated major bleeding is a major concern. This study sought to identify new targets to develop safer antithrombotic therapies.

METHODS AND RESULTS

Here, microarray analysis in peripheral blood cells in eight patients with AF and stroke and eight AF subjects without stroke brought to light a stroke-related gene expression pattern. HSPA1B, which encodes for heat-shock protein 70 kDa (Hsp70), was the most differentially expressed gene. This gene was down-regulated in stroke subjects, a finding confirmed further in an independent AF cohort of 200 individuals. Hsp70 knock-out mice subjected to different thrombotic challenges developed thrombosis significantly earlier than their wild-type (WT) counterparts. Remarkably, the tail bleeding time was unchanged. Accordingly, both TRC051384 and tubastatin A, i.e. two Hsp70 inducers via different pathways, delayed thrombus formation in WT mice, the tail bleeding time still being unaltered. Most interestingly, Hsp70 inducers did not increase the bleeding risk even when aspirin was concomitantly administered. Hsp70 induction was associated with an increased vascular thrombomodulin expression and higher circulating levels of activated protein C upon thrombotic stimulus.

CONCLUSIONS

Hsp70 induction is a novel approach to delay thrombus formation with minimal bleeding risk, and is especially promising for treating AF patients and in other situations where there is also a major bleeding hazard.

Melanoma and the Unfolded Protein Response

The UPR (unfolded protein response) has been identified as a key factor in the progression and metastasis of cancers, notably melanoma. Several mediators of the UPR are upregulated in cancers, e.g., high levels of GRP78 (glucose-regulator protein 78 kDa) correlate with progression and poor outcome in melanoma patients. The proliferative burden of cancer induces stress and activates several cellular stress responses. The UPR is a tightly orchestrated stress response that is activated upon the accumulation of unfolded proteins within the ER (endoplasmic reticulum). The UPR is designed to mediate two conflicting outcomtes, recovery and apoptosis. As a result, the UPR initiates a widespread signaling cascade to return the cell to homeostasis and failing to achieve cellular recovery, initiates UPR-induced apoptosis. There is evidence that ER stress and subsequently the UPR promote tumourigenesis and metastasis. The complete role of the UPR has yet to be defined. Understanding how the UPR allows for adaption to stress and thereby assists in cancer progression is important in defining an archetype of melanoma pathology. In addition, elucidation of the mechanisms of the UPR may lead to development of effective treatments of metastatic melanoma.

SUMOylation-disrupting WAS mutation converts WASp from a transcriptional activator to a repressor of NF-κB response genes in T cells

In Wiskott-Aldrich syndrome (WAS), immunodeficiency and autoimmunity often comanifest, yet how WAS mutations misregulate chromatin-signaling in Thelper (TH) cells favoring development of auto-inflammation over protective immunity is unclear. Previously, we identified an essential promoter-specific, coactivator role of nuclear-WASp in TH1 gene transcription. Here we identify small ubiquitin-related modifier (SUMO)ylation as a novel posttranslational modification of WASp, impairment of which converts nuclear-WASp from a transcriptional coactivator to a corepressor of nuclear factor (NF)-κB response genes in human (TH)1-differentiating cells. V75M, one of many disease-causing mutations occurring in SUMO*motif (72-ψψψψKDxxxxSY-83) of WASp, compromises WASp-SUMOylation, associates with COMMD1 to attenuate NF-κB signaling, and recruits histone deacetylases-6 (HDAC6) to p300-marked promoters of NF-κB response genes that pattern immunity but not inflammation. Consequently, proteins mediating adaptive immunity (IFNG, STAT1, TLR1) are deficient, whereas those mediating auto-inflammation (GM-CSF, TNFAIP2, IL-1β) are paradoxically increased in TH1 cells expressing SUMOylation-deficient WASp. Moreover, SUMOylation-deficient WASp favors ectopic development of the TH17-like phenotype (↑IL17A, IL21, IL22, IL23R, RORC, and CSF2) under TH1-skewing conditions, suggesting a role for WASp in modulating TH1/TH17 plasticity. Notably, pan-histone deacetylase inhibitors lift promoter-specific repression imposed by SUMOylation-deficient WASp and restore misregulated gene expression. Our findings uncovering a SUMOylation-based mechanism controlling WASp's dichotomous roles in transcription may have implications for personalized therapy for patients carrying mutations that perturb WASp-SUMOylation.

Romidepsin for the treatment of non-Hodgkin's lymphoma

INTRODUCTION

Patients with relapsed or refractory lymphoma remain a population with unmet medical needs. Histone deacetylase inhibitors (HDACIs) represent a novel class of anticancer drugs currently in development in several malignancies. Inhibition of HDACs leads to acetylation of histone and non-histone proteins, which in turn results in epigenetic modification of gene expression that leads to a plethora of effects, such as cell cycle arrest, apoptosis and inhibition of angiogenesis. Romidepsin is a novel HDACI that has demonstrated preclinical and clinical activity.

AREAS COVERED

This review discusses the different HDACs and epigenetic regulation with a particular focus on the preclinical and clinical development of romidepsin in lymphoma. The review of romidepsin includes: the mechanism of action, its synergistic interaction with novel agents, pivotal clinical trials that lead to its US FDA approval in cutaneous T-cell lymphoma and peripheral T-cell lymphoma as well as active combinations currently in clinical trials.

EXPERT OPINION

Romidepsin is a potent HDACI with clinical activity in T-cell lymphoma where novel agents and combinations are desperately needed. A deeper understanding of the molecular characteristics of this class of agents will allow the design of more potent drugs with improved toxicity profiles and future rational combinations that will expand the indication and benefit from these novel agents.

Hsp90 inhibitor as a sensitizer of cancer cells to different therapies (review)

Hsp90 is a molecular chaperone that maintains the structural and functional integrity of various client proteins involved in signaling and many other functions of cancer cells. The natural inhibitors, ansamycins influence the Hsp90 chaperone function by preventing its binding to client proteins and resulting in their proteasomal degradation. N- and C-terminal inhibitors of Hsp90 and their analogues are widely tested as potential anticancer agents in vitro, in vivo as well as in clinical trials. It seems that Hsp90 competitive inhibitors target different tumor types at nanomolar concentrations and might have therapeutic benefit. On the contrary, some Hsp90 inhibitors increased toxicity and resistance of cancer cells induced by heat shock response, and through the interaction of survival signals, that occured as side effects of treatments, could be very effectively limited via combination of therapies. The aim of our review was to collect the data from experimental and clinical trials where Hsp90 inhibitor was combined with other therapies in order to prevent resistance as well as to potentiate the cytotoxic and/or antiproliferative effects.

HSP90 inhibitor NVP-AUY922 enhances TRAIL-induced apoptosis by suppressing the JAK2-STAT3-Mcl-1 signal transduction pathway in colorectal cancer cells

TRAIL has been shown to induce apoptosis in cancer cells, but in some cases, certain cancer cells are resistant to this ligand. In this study, we explored the ability of representative HSP90 (heat shock protein 90) inhibitor NVP-AUY922 to overcome TRAIL resistance by increasing apoptosis in colorectal cancer (CRC) cells. The combination of TRAIL and NVP-AUY922 induced synergistic cytotoxicity and apoptosis, which was mediated through an increase in caspase activation. The treatment of NVP-AUY922 dephosphorylated JAK2 and STAT3 and decreased Mcl-1, which resulted in facilitating cytochrome c release. NVP-AUY922-mediated inhibition of JAK2/STAT3 signaling and down-regulation of their target gene, Mcl-1, occurred in a dose and time-dependent manner. Knock down of Mcl-1, STAT3 inhibitor or JAK2 inhibitor synergistically enhanced TRAIL-induced apoptosis. Taken together, our results suggest the involvement of the JAK2-STAT3-Mcl-1 signal transduction pathway in response to NVP-AUY922 treatment, which may play a key role in NVP-AUY922-mediated sensitization to TRAIL. By contrast, the effect of the combination treatments in non-transformed colon cells was minimal. We provide a clinical rationale that combining HSP90 inhibitor with TRAIL enhances therapeutic efficacy without increasing normal tissue toxicity in CRC patients.

BET protein antagonist JQ1 is synergistically lethal with FLT3 tyrosine kinase inhibitor (TKI) and overcomes resistance to FLT3-TKI in AML cells expressing FLT-ITD

Recently, treatment with bromodomain and extraterminal protein antagonist (BA) such as JQ1 has been shown to inhibit growth and induce apoptosis of human acute myelogenous leukemia (AML) cells, including those expressing FLT3-ITD. Here, we demonstrate that cotreatment with JQ1 and the FLT3 tyrosine kinase inhibitor (TKI) ponatinib or AC220 synergistically induce apoptosis of cultured and primary CD34(+) human AML blast progenitor cells (BPC) expressing FLT3-ITD. Concomitantly, as compared with each agent alone, cotreatment with JQ1 and the FLT3-TKI caused greater attenuation of c-MYC, BCL2, and CDK4/6. Simultaneously, cotreatment with JQ1 and the FLT3-TKI increased the levels of p21, BIM, and cleaved PARP, as well as mediated marked attenuation of p-STAT5, p-AKT, and p-ERK1/2 levels in AML BPCs. Conversely, cotreatment with JQ1 and FLT3-TKI was significantly less active against CD34(+) normal bone marrow progenitor cells. Knockdown of BRD4 by short hairpin RNA also sensitized AML cells to FLT3-TKI. JQ1 treatment induced apoptosis of mouse Ba/F3 cells ectopically expressing FLT3-ITD with or without FLT3-TKI-resistant mutations F691L and D835V. Compared with the parental human AML FLT3-ITD-expressing MOLM13, MOLM13-TKIR cells resistant to AC220 were markedly more sensitive to JQ1-induced apoptosis. Furthermore, cotreatment with JQ1 and the pan-histone deacetylase inhibitor (HDI) panobinostat synergistically induced apoptosis of FLT3-TKI-resistant MOLM13-TKIR and MV4-11-TKIR cells. Collectively, these findings support the rationale for determining the in vivo activity of combined therapy with BA and FLT3-TKI against human AML cells expressing FLT3-ITD or with BA and HDI against AML cells resistant to FLT3-TKI.

The HDAC inhibitor LBH589 induces ERK-dependent prometaphase arrest in prostate cancer via HDAC6 inactivation and down-regulation

Histone deacetylase inhibitors (HDACIs) have potent anti-cancer activity in a variety of cancer models. Understanding the molecular mechanisms involved in the therapeutic responsiveness of HDACI is needed before its clinical application. This study aimed to determine if a potent HDACI, LBH589 (Panobinostat), had differential therapeutic responsiveness towards LNCaP and PC-3 prostate cancer (PCa) cells. The former showed prometaphase arrest with subsequent apoptosis upon LBH589 treatment, while the latter was less sensitive and had late G2 arrest. The LBH589 treatment down-regulated HDAC6 and sustained ERK activation, and contributed to prometaphase arrest. Mechanistically, LBH589 inhibited HDAC6 activity, caused its dissociation from protein phosphatase PP1α, and increased 14-3-3ζ acetylation. Acetylated 14-3-3ζ released its mask effect on serine 259 of c-Raf and serine 216 of Cdc25C subsequent to de-phosphorylation by PP1α, which contributed to ERK activation. Enhanced ERK activity by LBH589 further down-regulated HDAC6 protein levels and sustained ERK activation by free-forward regulation. The sustained Cdc25C and ERK activation resulted in early M-phase (prometaphase) arrest and subsequent apoptosis in the most sensitive LNCaP cells but not in PC-3 cells. This study provides pre-clinical evidence that HDAC6 may serve as a sensitive therapeutic target in the treatment of prostate cancer with HDACI LBH589 for clinical translation. This study also posits a novel mechanism of HDAC6 participation in regulating the c-Raf-PP1-ERK signaling pathway and contributing to M phase cell-cycle transition.

STAT5 acetylation: Mechanisms and consequences for immunological control and leukemogenesis

The cytokine-inducible transcription factors signal transducer and activator of transcription 5A and 5B (STAT5A and STAT5B) are important for the proper development of multicellular eukaryotes. Disturbed signaling cascades evoking uncontrolled expression of STAT5 target genes are associated with cancer and immunological failure. Here, we summarize how STAT5 acetylation is integrated into posttranslational modification networks within cells. Moreover, we focus on how inhibitors of deacetylases and tyrosine kinases can correct leukemogenic signaling nodes involving STAT5. Such small molecules can be exploited in the fight against neoplastic diseases and immunological disorders.

Vorinostat-induced autophagy switches from a death-promoting to a cytoprotective signal to drive acquired resistance

Histone deacetylase inhibitors (HDACi) have shown promising activity against hematological malignancies in clinical trials and have led to the approval of vorinostat for the treatment of cutaneous T-cell lymphoma. However, de novo or acquired resistance to HDACi therapy is inevitable, and their molecular mechanisms are still unclear. To gain insight into HDACi resistance, we developed vorinostat-resistant clones from the hematological cell lines U937 and SUDHL6. Although cross-resistant to some but not all HDACi, the resistant cell lines exhibit dramatically increased sensitivity toward chloroquine, an inhibitor of autophagy. Consistent with this, resistant cells growing in vorinostat show increased autophagy. Inhibition of autophagy in vorinostat-resistant U937 cells by knockdown of Beclin-1 or Lamp-2 (lysosome-associated membrane protein 2) restores sensitivity to vorinostat. Interestingly, autophagy is also activated in parental U937 cells by de novo treatment with vorinostat. However, in contrast to the resistant cells, inhibition of autophagy decreases sensitivity to vorinostat. These results indicate that autophagy can switch from a proapoptotic signal to a prosurvival function driving acquired resistance. Moreover, inducers of autophagy (such as mammalian target of rapamycin inhibitors) synergize with vorinostat to induce cell death in parental cells, whereas the resistant cells remain insensitive. These data highlight the complexity of the design of combination strategies using modulators of autophagy and HDACi for the treatment of hematological malignancies.

Bone Marrow and Peripheral Blood AML Cells Are Highly Sensitive to CNDAC, the Active Form of Sapacitabine

Achieving improvements in survival and reducing relapse remains a challenge in acute myelogenous leukemia (AML) patients. This study evaluated the in vitro efficacy of the active form of novel agent sapacitabine, CNDAC, compared to current chemotherapeutic drugs Ara-C and mitoxantrone using two AML cell lines, HL-60 (promyelocytic) and THP-1 (monocytic), as well as bone marrow (BM) and peripheral blood (PB) cells collected from AML patients. Cell lines were exposed to compound for 3-6 days and primary cells for 4 days. The viability of primary cells was additionally evaluated 3, 7, and 31 days after removal of tested compound to determine the durability of the response. Our studies indicate that CNDAC and mitoxantrone have a greater impact on viability than ara-C in primary AML cells and AML cell lines. CNDAC is more effective at reducing viability and inducing apoptosis than ara-C at equivalent concentrations in the THP-1 cell line, which is defined as displaying resistance to ara-C. As sapacitabine has shown in vivo activity at clinically achievable doses, future studies are warranted to assess the potential for combining it with ara-C and/or mitoxantrone, with an emphasis on cells and patients insensitive to ara-C treatment.

JAK2 the future: therapeutic strategies for JAK-dependent malignancies

The Janus kinase (JAK) proteins are a family of intracellular nonreceptor tyrosine kinases involved in cytokine signaling via the JAK-STAT (signal transducers and activators of transcription) pathway. Genetic studies have identified somatic JAK2(V617F) mutations and other mutant alleles that activate JAK-STAT signaling in most patients with myeloproliferative neoplasms (MPNs). As a result, JAK inhibitors have been developed to treat various malignancies and have been shown to be efficacious in both preclinical and clinical settings. However, available ATP-competitive JAK (type I) inhibitors are associated with dose-dependent toxicities, and do not yet reduce disease burden in MPN patients. Recent studies suggest that genetic and epigenetic mechanisms can cause insensitivity to type I JAK inhibitors. Novel therapies include the development of type II JAK inhibitors and the use of alternative strategies to abrogate JAK-STAT signaling, perhaps with histone deacetylase (HDAC) and heat shock protein 90 (HSP90) inhibitors. These innovative therapies may translate to treatment of other diseases that are dependent on JAK signaling, including B-precursor acute lymphoblastic leukemia (B-ALL).

Influence of natural and synthetic histone deacetylase inhibitors on chromatin

SIGNIFICANCE

Histone deacetylase inhibitors (HDACIs) have emerged as a new class of anticancer therapeutics. The hydroxamic acid, suberoylanilide hydroxamic acid (Vorinostat, Zolinza™), and the cyclic peptide, depsipeptide (Romidepsin, Istodax™), were approved by the U.S. Food and Drug Administration (FDA) for the treatment of cutaneous T-cell lymphoma in 2006 and 2009, respectively. At least 15 HDACIs are currently undergoing clinical trials either alone or in combination with other therapeutic modalities for the treatment of numerous hematological and solid malignancies.

RECENT ADVANCES

The potential utility of HDACIs has been extended to nononcologic applications, including autoimmune disorders, inflammation, diseases of the central nervous system, and malaria.

CRITICAL ISSUES

Given the promise of HDACIs, there is growing interest in the potential of dietary compounds that possess HDAC inhibition activity. This review is focused on the identification of and recent findings with HDACIs from dietary, medicinal plant, and microbial sources. We discuss the mechanisms of action and clinical potential of natural HDACIs.

FUTURE DIRECTIONS

Apart from identification of further HDACI compounds from dietary sources, further research will be aimed at understanding the effects on gene regulation on lifetime exposure to these compounds. Another important issue that requires clarification.

Current limitations and future opportunities for epigenetic therapies

This article reviews progress in epigenetic therapies that hope to improve the treatment of cancer. Tumors show widespread, aberrant epigenetic changes, leading to changes in the expression of genes involved in all the hallmarks of cancer. These epigenetic changes can potentially be reversed using small-molecule inhibitors of enzymes involved in maintenance of the epigenetic state. DNA-demethylating agents and histone deacetylase inhibitors have shown anti-tumor activity against certain hematological malignancies; however, their activity in solid tumors remains more uncertain. Major challenges remain in delivery of epigenetic therapy, maintenance of a pharmacodynamic response and achievement of a therapeutic index. We believe histone lysine methyl transferases are a highly promising epigenetic target, which has yet to be clinically exploited. Crystallographic studies on histone lysine methyl transferases provide insights into their mechanism and specificity crucial for the design and development of small-molecule inhibitors.

Characterization of cells resistant to the potent histone deacetylase inhibitor spiruchostatin B (SP-B) and effect of overexpressed p21waf1/cip1 on the SP-B resistance or susceptibility of human leukemia cells

We previously showed that the B cell leukemia cell line NALM-6 had the highest susceptibility among a number of leukemia cell lines to spiruchostatin B (SP-B), a potent histone deacetylase (HDAC) inhibitor. We also showed that SP-B-induced cytotoxicity depended on induction of apoptosis that was mediated by p21^waf1/cip1 expression. In the present study, we generated and characterized a stable, SP-B-resistant NALM-6 cell line (NALM-6/SP-B) by continuous exposure to SP-B, starting with a low SP-B concentration. NALM-6/SP-B cells were also more resistant to FK228, which has a similar chemical structure to SP-B, and were slightly more resistant to the P-gp substrates doxorubicin and vincristine than parental cells, but displayed similar susceptibility to other HDAC inhibitors and to paclitaxel as the parental cells. There was little change in the basal mRNA expression of HDAC1, p53, Bax, Bcl-2, Fas, caspase-3, c-Myc and MDR1 in NALM-6/SP-B compared to parental cells, but the mRNA expression of p21^waf1/cip1 was decreased. The introduction of an exogenous p21^waf1/cip1 expression vector restored SP-B induction of NALM-6/SP-B cell apoptosis. Moreover, overexpressed p21^waf1/cip1 enhanced SP-B induction of the apoptosis of the human erythroleukemia leukemia cell line K562 which is less susceptible to SP-B than NALM-6 cells. These results suggest that downregulation of p21^waf1/cip1, which is a characteristic feature of NALM-6/SP-B cells, was important for their resistance to SP-B, and that this SP-B resistance could be overcome by the introduction of exogenous p21^waf1/cip1. Furthermore, introduction of p21^waf1/cip1 to other leukemia cells such as K562 may enhance their susceptibility to SP-B. This is the first report of the characterization of SP-B-resistant cells and of the effect of overexpressed p21^waf1/cip1 on the resistance or susceptibility of human leukemia cells to SP-B.

Mechanisms of resistance to histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors are a new class of anticancer agents. HDAC inhibitors induce acetylation of histones and nonhistone proteins which are involved in regulation of gene expression and in various cellular pathways including cell growth arrest, differentiation, DNA damage and repair, redox signaling, and apoptosis (Marks, 2010). The U.S. Food and Drug Administration has approved two HDAC inhibitors, vorinostat and romidepsin, for the treatment of cutaneous T-cell lymphoma (Duvic & Vu, 2007; Grant et al., 2010; Marks & Breslow, 2007). Over 20 chemically different HDAC inhibitors are in clinical trials for hematological malignancies and solid tumors. This review considers the mechanisms of resistance to HDAC inhibitors that have been identified which account for the selective effects of these agents in inducing cancer but not normal cell death. These mechanisms, such as functioning Chk1, high levels of thioredoxin, or the prosurvival BCL-2, may also contribute to resistance of cancer cells to HDAC inhibitors.

Histone deacetylase inhibitors: emerging mechanisms of resistance

The histone deacetylase inhibitors (HDIs) have shown promise in the treatment of a number of hematologic malignancies, leading to the approval of vorinostat and romidepsin for the treatment of cutaneous T-cell lymphoma and romidepsin for the treatment of peripheral T-cell lymphoma by the U.S. Food and Drug Administration. Despite these promising results, clinical trials with the HDIs in solid tumors have not met with success. Examining mechanisms of resistance to HDIs may lead to strategies that increase their therapeutic potential in solid tumors. However, relatively few examples of drug-selected cell lines exist, and mechanisms of resistance have not been studied in depth. Very few clinical translational studies have evaluated resistance mechanisms. In the current review, we summarize many of the purported mechanisms of action of the HDIs in clinical trials and examine some of the emerging resistance mechanisms.

A phase 2 study of vorinostat in acute myeloid leukemia

BACKGROUND

This two-stage, multi-institutional, randomized phase 2 trial assessed the toxicity and response rate associated with two treatment schedules of the histone deacetylase inhibitor, vorinostat (suberoylanilide hydroxamic acid; SAHA) in patients with relapsed acute myeloid leukemia and in selected untreated patients with high-risk acute myeloid leukemia.

DESIGN AND METHODS

Patients with relapsed or untreated acute myeloid leukemia who were not candidates for chemotherapy entered one of the two treatment arms. In both arms a total dose of 8400 mg of vorinostat was delivered in each 21-day cycle of treatment: in arm A the dose regimen was 400 mg daily whereas in arm B the dose regimen was 200 mg three times daily for 14 days followed by 1 week rest.

RESULTS

Data from all 37 patients were used for the analyses. In arm A (n=15), the confirmed complete remission rate was 0% (95% CI, 0% to 23%); this arm was closed at the planned interim analysis. In arm B (n=22), the confirmed complete remission rate was 4.5% (1 response; 95% CI, 0.4% to 24%), with a duration of response exceeding 398 days. The median time to treatment failure in arm A was 42 days (95% CI, 26 to 57); although a minimum of four cycles of treatment were planned, 11 patients (79%) received no more than two cycles. The median time to treatment failure in arm B was 46 days (95% CI, 20 to 71); 13 patients (59%) received no more than two cycles of treatment.

CONCLUSIONS

Vorinostat monotherapy demonstrated minimal activity in this group of patients with acute myeloid leukemia. Therapy was discontinued in many patients before the planned four cycles had been administered, either because of failure of vorinostat to control the leukocyte count or patients' and physicians' preference. Future studies of vorinostat in acute myeloid leukemia should focus on combinations with other drugs with which it might interact pharmacodynamically. ClinicalTrials.gov Identifier: NCT00305773.

Increased gene expression of histone deacetylases in patients with Philadelphia-negative chronic myeloproliferative neoplasms

Myeloproliferation, myeloaccumulation (decreased apoptosis), inflammation, bone marrow fibrosis and angiogenesis are cardinal features of the Philadelphia-negative chronic myeloproliferative neoplasms: essential thrombocythemia (ET), polycythemia vera (PV) and primary myelofibrosis (PMF). Histone deacetylases (HDACs) have a critical role in modulating gene expression and, accordingly, in the control of cell pathobiology and cancer development. HDAC inhibition has been shown to inhibit tumor growth (impaired myeloproliferation), to modulate the balance between pro- and antiapoptotic proteins in favor of apoptosis (enhanced apoptosis) and also to inhibit angiogenesis. Recently, enhanced HDAC enzyme activity has been found in CD34+cells from patients with PMF, enzyme activity levels highly exceeding those recorded in other chronic myeloproliferative neoplasms (CMPNs). The raised levels correlated to the degree of splenomegaly, suggesting that HDAC might be recruited as ET or PV progresses into myelofibrosis or PMF progresses into a more advanced stage. Accordingly, HDAC inhibition is an obvious novel therapeutic approach in these neoplasms. Using global gene expression profiling of whole blood from patients with CMPNs, we have found a pronounced deregulation of HDAC genes, involving significant up-regulation of the HDAC genes 9 and 11, with the highest expression levels being found in patients with ET (HDAC9 and 11), PMF (HDAC9) and CMPNs (both HDAC9 and HDAC11). Furthermore, we have identified that the HDAC6 gene is progressively expressed in patients with ET, PV and PMF, reflecting a steady accumulation of abnormally expressed HDAC6 during disease evolution. Our results lend further support to HDACs as important epigenetic targets in the future treatment of patients with CMPNs. Since the highest expression levels of HDAC genes were recorded in ET, in PMF and in the entire CMPN group, their down-regulation by HDAC inhibitors might be associated with decreased disease activity, including reduction of splenomegaly.

Targeting apoptosis proteins in hematological malignancies

The apoptotic machinery plays a key role in hematopoietic cell homeostasis. Terminally differentiated cells are eliminated, at least in part, by apoptosis, whereas part of the apoptotic machinery, including one or several caspases, is required to go through very specific steps of the differentiation pathways. A number of hematological diseases involve a deregulation of this machinery, which in most cases is a decrease in cell sensitivity to pro-apoptotic signals through over-expression of anti-apoptotic molecules. In some situations however, e.g. in the erythroid lineage of low grade myelodysplastic syndromes, cell sensitivity to apoptosis is increased in a death receptor-dependent manner and cell death pathways are inhibited only when these diseases progress into high grade and acute leukemia. Therapeutic strategies targeting the apoptotic machinery specifically block cell death inhibitors that are over-expressed in transformed cells, mainly Bcl-2-related proteins and Inhibitor of Apoptosis Proteins (IAPs). Another strategy is the activation of the extrinsic pathway to apoptosis, mainly through the death receptor agonist Tumor necrosis factor-Related Apoptosis Inducing Ligand (TRAIL) or agonistic antibodies targeting TRAIL receptors. The use of inhibitors of death receptors could make sense when these receptors are involved in excessive cell death or activation of survival pathways. Most of the drugs targeting apoptotic pathways introduced in clinics have demonstrated their tolerability. Their efficacy, either alone or in combination with other drugs such as demethylating agents and histone deacetylase inhibitors, is currently tested in both myeloid and lymphoid hematological diseases.

Myelodysplastic syndrome and histone deacetylase inhibitors: "to be or not to be acetylated"?

Myelodysplastic syndrome (MDS) represents a heterogeneous group of diseases with clonal proliferation, bone marrow failure and increasing risk of transformation into an acute myeloid leukaemia. Structured guidelines are developed for selective therapy based on prognostic subgroups, age, and performance status. Although many driving forces of disease phenotype and biology are described, the complete and possibly interacting pathogenetic pathways still remain unclear. Epigenetic investigations of cancer and haematologic diseases like MDS give new insights into the pathogenesis of this complex disease. Modifications of DNA or histones via methylation or acetylation lead to gene silencing and altered physiology relevant for MDS. First clinical trials give evidence that patients with MDS could benefit from epigenetic treatment with, for example, DNA methyl transferase inhibitors (DNMTi) or histone deacetylase inhibitors (HDACi). Nevertheless, many issues of HDACi remain incompletely understood and pose clinical and translational challenges. In this paper, major aspects of MDS, MDS-associated epigenetics and the potential use of HDACi are discussed.

Autophagy inhibition enhances vorinostat-induced apoptosis via ubiquitinated protein accumulation

Autophagy is an evolutionarily conserved cell survival pathway that enables cells to recoup ATP and other critical biosynthetic molecules during nutrient deprivation or exposure to hypoxia, which are hallmarks of the tumour microenvironment. Autophagy has been implicated as a potential mechanism of resistance to anticancer agents as it can promote cell survival in the face of stress induced by chemotherapeutic agents by breaking down cellular components to generate alternative sources of energy. Disruption of autophagy with chloroquine (CQ) induces the accumulation of ubiquitin-conjugated proteins in a manner similar to the proteasome inhibitor bortezomib (BZ). However, CQ-induced protein accumulation occurs at a slower rate and is localized to lysosomes in contrast to BZ, which stimulates rapid buildup of ubiquitinated proteins and aggresome formation in the cytosol. The histone deacetylase (HDAC) inhibitor vorinostat (VOR) blocked BZ-induced aggresome formation, but promoted CQ-mediated ubiquitinated protein accumulation. Disruption of autophagy with CQ strongly enhanced VOR-mediated apoptosis in colon cancer cells. Accordingly, knockdown of the essential autophagy gene Atg7 also sensitized cells to VOR-induced apoptosis. Knockdown of HDAC6 greatly enhanced BZ-induced apoptosis, but only marginally sensitized cells to CQ. Subsequent studies determined that the CQ/VOR combination promoted a large increase in superoxide generation that was required for ubiquitinated protein accumulation and cell death. Finally, treatment with the CQ/VOR combination significantly reduced tumour burden and induced apoptosis in a colon cancer xenograft model. Collectively, our results establish that inhibition of autophagy with CQ induces ubiquitinated protein accumulation and VOR potentiates CQ-mediated aggregate formation, superoxide generation and apoptosis.

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.

Vorinostat in acute myeloid leukemia and myelodysplastic syndromes

INTRODUCTION

the results of conventional treatment for acute myeloid leukemia (AML) and myelodysplastic syndromes (MDS) remain poor and innovative strategies are warranted. With this aim, epigenetic modulation became a promising approach over the last years. Vorinostat is an epigenetic targeted drug belonging to the histone deacetylase (HDAC) inhibitors family. It is the second-generation HDAC inhibitor that has been more extensively studied in AML and MDS.

AREAS COVERED

in this review, we will present the rationale for its use in AML and MDS, describe the drug pharmacologic properties and review the current data available from clinical trials. The data presented here will allow the reader to overview the common mechanisms of action of this class of compounds in AML and MDS, as well as to better understand the biological specificities of vorinostat, and its current and future clinical development in the field.

EXPERT OPINION

vorinostat has shown an acceptable toxicity profile with mainly gastrointestinal and constitutional side effects. Efficacy as a single agent is limited in that group of patients, but promising results are currently observed with combinations of vorinostat and either conventional chemotherapy or investigational agents, including demethylating agents.

Overcoming resistance to histone deacetylase inhibitors in human leukemia with the redox modulating compound β-phenylethyl isothiocyanate

Mechanisms of action and resistance of histone deacetylase inhibitors (HDACIs) are not well understood. A gene expression analysis performed in a phase 1 trial of vorinostat in leukemia indicated that overexpression of genes involved in antioxidant defense was associated with clinical resistance. We hypothesized that nonepigenetic mechanisms may be involved in resistance to HDACI therapy in leukemia. Here we confirmed up-regulation of a series of antioxidants in a pan-HDACI-resistant leukemia cell line HL60/LR. Vorinostat induced reactive oxygen species (ROS) through nicotinamide adenine dinucleotide phosphate oxidase in leukemia cells. An increase in ROS resulted in translocation of nuclear factor E2-related factor 2 from cytosol to nucleus, leading to up-regulation of antioxidant genes, including a majority of glutathione-associated enzymes as a cellular protective mechanism. Addition of β-phenylethyl isothiocyanate, a natural compound capable of depleting cellular glutathione, significantly enhanced the cytotoxicity of vorinostat in leukemia cell lines and primary leukemia cells by inhibiting the cytoprotective antioxidant response. These results suggest that ROS plays an important role in action of vorinostat and that combination with a redox-modulating compound increases sensitivity to HDACIs and also overcomes vorinostat resistance. Such a combination strategy may be an effective therapeutic regimen and have potential clinical application in leukemia.

Role of CAAT/enhancer binding protein homologous protein in panobinostat-mediated potentiation of bortezomib-induced lethal endoplasmic reticulum stress in mantle cell lymphoma cells

PURPOSE

Bortezomib induces unfolded protein response (UPR) and endoplasmic reticulum stress, as well as exhibits clinical activity in patients with relapsed and refractory mantle cell lymphoma (MCL). Here, we determined the molecular basis of the improved in vitro and in vivo activity of the combination of the pan-histone deacetylase inhibitor panobinostat and bortezomib against human, cultured, and primary MCL cells.

EXPERIMENTAL DESIGN

Immunoblot analyses, reverse transcription-PCR, and immunofluorescent and electron microscopy were used to determine the effects of panobinostat on bortezomib-induced aggresome formation and endoplasmic reticulum stress in MCL cells.

RESULTS

Treatment with panobinostat induced heat shock protein 90 acetylation; depleted the levels of heat shock protein 90 client proteins, cyclin-dependent kinase 4, c-RAF, and AKT; and abrogated bortezomib-induced aggresome formation in MCL cells. Panobinostat also induced lethal UPR, associated with induction of CAAT/enhancer binding protein homologous protein (CHOP). Conversely, knockdown of CHOP attenuated panobinostat-induced cell death of MCL cells. Compared with each agent alone, cotreatment with panobinostat increased bortezomib-induced expression of CHOP and NOXA, as well as increased bortezomib-induced UPR and apoptosis of cultured and primary MCL cells. Cotreatment with panobinostat also increased bortezomib-mediated in vivo tumor growth inhibition and improved survival of mice bearing human Z138C MCL cell xenograft.

CONCLUSION

These findings suggest that increased UPR and induction of CHOP are involved in enhanced anti-MCL activity of the combination of panobinostat and bortezomib.