Death Receptor Pathway Activation and Increase of ROS Production by the Triple Epigenetic Inhibitor UVI5008

Synergistic Antitumoral Effect of Epigenetic Inhibitors and Gemcitabine in Pancreatic Cancer Cells

Epigenetic modifications could drive some of the molecular events implicated in proliferation, drug resistance and metastasis of pancreatic ductal adenocarcinoma (PDAC). Thus, epigenetic enzyme inhibitors could be the key to revert those events and transform PDAC into a drug-sensitive tumor. We performed a systematic study with five different epigenetic enzyme inhibitors (1, UVI5008, MS275, psammaplin A, and BIX01294) targeting either Histone Deacetylase (HDAC) 1 or 1/4, DNA methyltransferase 3a (DNMT3a), Euchromatic histone lysine methyltransferase 2 (EHMT2), or Sirtuin 1 (SIRT1), as well as one drug that restores the p53 function (P53R3), in three different human PDAC cell lines (SKPC-1, MIA PaCa-2, and BxPC-3) using 2D and 3D cell cultures. The synergistic effect of these antitumoral drugs with gemcitabine was tested and the most efficient combinations were characterized by RNA-seq. The inhibition of HDAC1/4 (MS275), HDAC1/4/SIRT1/DNMT3a (UVI5008) or EHMT2 (BIX01294) induced a significant reduction on the cell viability, even in gemcitabine-resistance cells. The combination of UVI5008 or MS275 with gemcitabine induced a synergistic effect at low concentration and the RNA-Seq analysis revealed some synergy candidate genes as potential biomarkers. Reverting aberrant epigenetic modifications in combination with gemcitabine offers an alternative treatment for PDAC patients, with an important reduction of the therapeutic dose.

Marine-Derived Secondary Metabolites as Promising Epigenetic Bio-Compounds for Anticancer Therapy

Sessile organisms such as seaweeds, corals, and sponges continuously adapt to both abiotic and biotic components of the ecosystem. This extremely complex and dynamic process often results in different forms of competition to ensure the maintenance of an ecological niche suitable for survival. A high percentage of marine species have evolved to synthesize biologically active molecules, termed secondary metabolites, as a defense mechanism against the external environment. These natural products and their derivatives may play modulatory roles in the epigenome and in disease-associated epigenetic machinery. Epigenetic modifications also represent a form of adaptation to the environment and confer a competitive advantage to marine species by mediating the production of complex chemical molecules with potential clinical implications. Bioactive compounds are able to interfere with epigenetic targets by regulating key transcriptional factors involved in the hallmarks of cancer through orchestrated molecular mechanisms, which also establish signaling interactions of the tumor microenvironment crucial to cancer phenotypes. In this review, we discuss the current understanding of secondary metabolites derived from marine organisms and their synthetic derivatives as epigenetic modulators, highlighting advantages and limitations, as well as potential strategies to improve cancer treatment.

Adiponectin ameliorates lung injury induced by intermittent hypoxia through inhibition of ROS-associated pulmonary cell apoptosis

PURPOSE

Obstructive sleep apnea hypopnea syndrome has been reported to be associated with pulmonary hypertension (PH). Adiponectin (Ad) has many protective roles in the human body, including its function as an anti-inflammatory and an anti-oxidant, as well as its role in preventing insulin resistance and atherosclerosis. This study aimed to investigate the molecular mechanism of chronic intermittent hypoxia (CIH)-induced pulmonary injury and the protective role of Ad in experimental rats.

METHODS

Thirty male Sprague-Dawley rats were randomly divided into three groups with 10 rats in each group: normal control (NC) group, CIH group, and CIH + Ad group. Rats in the NC group were kept breathing room air for 12 weeks. Rats in the CIH group were intermittently exposed to a hypoxic environment for 8 h/day for 12 weeks. Rats in the CIH + Ad group received 10 μg Ad twice weekly via intravenous injection. After 12 weeks of CIH exposure, we detected the pulmonary function, pulmonary artery pressure, lung histology, pulmonary cell apoptosis, pulmonary artery endothelial cell apoptosis, mitochondrial membrane potential (MMP), and reactive oxygen species (ROS) level. We also analyzed expression proteins involved in the mitochondria-, endoplasmic reticulum (ER) stress-, and Fas receptor-associated pulmonary apoptosis pathways, as well as the SIRT3/SOD2 pathway.

RESULTS

CIH exposure for 12 weeks did not lead to abnormal pulmonary function, PH, or pulmonary artery endothelial cell apoptosis. However, we observed a significant increase in the rate of pulmonary cell apoptosis, the expression of proteins involved in mitochondria-, ER stress-, and Fas receptor-associated pulmonary apoptosis pathways, and the generation of ROS in the CIH group compared with the NC group. In contrast, the MMP and protein expressions of SIRT3/SOD2 pathway were significantly decreased in the CIH group compared with the NC group. Ad supplementation in the CIH + Ad group partially improved these changes induced by CIH.

CONCLUSION

Even though CIH did not cause abnormal pulmonary function or PH, early lung injury was detected at the molecular level in rats exposed to CIH. Treatment with Ad ameliorated the pulmonary injury by activating the SIRT3/SOD2 pathway, reducing ROS generation, and inhibiting ROS-associated lung cell apoptosis.

Enzymatic and Biological Characterization of Novel Sirtuin Modulators against Cancer

Sirtuins, a family of nicotinamide adenine dinucleotide (NAD⁺)-dependent lysine deacetylases, are promising targets for anticancer treatment. Recently, we characterized a novel pan-sirtuin (SIRT) inhibitor, MC2494, displaying antiproliferative effects and able to induce death pathways in several human cancer cell lines and decrease tumor growth in vivo. Based on the chemical scaffold of MC2494, and by applying a structure–activity relationship approach, we developed a small library of derivative compounds and extensively analyzed their enzymatic action at cellular level as well as their ability to induce cell death. We also investigated the effect of MC2494 on regulation of cell cycle progression in different cancer cell lines. Our investigations indicated that chemical substitutions applied to MC2494 scaffold did not confer higher efficacy in terms of biological activity and SIRT1 inhibition, but carbethoxy-containing derivatives showed higher SIRT2 specificity. The carbethoxy derivative of MC2494 and its 2-methyl analog displayed the strongest enzymatic activity. Applied chemical modifications improved the enzymatic selectivity of these SIRT inhibitors. Additionally, the observed activity of MC2494 via cell cycle and apoptotic regulation and inhibition of cell migration supports the potential role of SIRTs as targets in tumorigenesis and makes SIRT-targeting molecules good candidates for novel pharmacological approaches in personalized medicine.

Marine-Derived Natural Lead Compound Disulfide-Linked Dimer Psammaplin A: Biological Activity and Structural Modification

Marine natural products are considered to be valuable resources that are furnished with diverse chemical structures and various bioactivities. To date, there are seven compounds derived from marine natural products which have been approved as therapeutic drugs by the U.S. Food and Drug Administration. Numerous bromotyrosine derivatives have been isolated as a type of marine natural products. Among them, psammaplin A, including the oxime groups and carbon-sulfur bonds, was the first identified symmetrical bromotyrosine-derived disulfide dimer. It has been found to have a broad bioactive spectrum, especially in terms of antimicrobial and antiproliferative activities. The highest potential indole-derived psammaplin A derivative, UVI5008, is used as an epigenetic modulator with multiple enzyme inhibitory activities. Inspired by these reasons, psammaplin A has gradually become a research focus for pharmacologists and chemists. To the best of our knowledge, there is no systematic review about the biological activity and structural modification of psammaplin A. In this review, the pharmacological effects, total synthesis, and synthesized derivatives of psammaplin A are summarized.

3-Chloro-N'-(2-hydroxybenzylidene) benzohydrazide: An LSD1-Selective Inhibitor and Iron-Chelating Agent for Anticancer Therapy

Despite the discovery and development of novel therapies, cancer is still a leading cause of death worldwide. In order to grow, tumor cells require large quantities of nutrients involved in metabolic processes, and an increase in iron levels is known to contribute to cancer proliferation. Iron plays an important role in the active site of a number of proteins involved in energy metabolism, DNA synthesis and repair, such as ribonucleotide reductase, which induce G0/S phase arrest and exert a marked antineoplastic effect, particularly in leukemia and neuroblastoma. Iron-depletion strategies using iron chelators have been shown to result in cell cycle arrest and apoptosis. Deferoxamine (DFO) was the first FDA-approved drug for the treatment of iron overload pathologies, and has also been recognized as having anticancer properties. The high cost, low permeability and short plasma half-life of DFO led to the development of other iron-chelating drugs. Pyridoxal isonicotinoyl hydrazone (PIH) and its analogs chelate cellular iron by tridentate binding, and inhibit DNA synthesis more robustly than DFO, demonstrating an effective antiproliferative activity. Here, we investigated the biological effects of a PIH derivative, 3-chloro-N′-(2-hydroxybenzylidene)benzohydrazide (CHBH), known to be a lysine-specific histone demethylase 1A inhibitor. We showed that CHBH is able to induce cell proliferation arrest in several human cancer cell lines, including lung, colon, pancreas and breast cancer, at micromolar levels. Our findings indicate that CHBH exerts a dual anticancer action by strongly impairing iron metabolism and modulating chromatin structure and function.

Epigenetic modulator UVI5008 inhibits MRSA by interfering with bacterial gyrase

The impact of multi-drug resistant bacterial strains on human health is reaching worrisome levels. Over 2 million people are infected by resistant bacteria, and more than 700,000 people die each year because of the continuous spread of resistant strains. The development of new antibiotics and the prudent use of existing ones to prolong their lifespan require a constant effort by drug industries and healthcare workers. The re-purposing of existing drugs for use as antimicrobial agents would streamline the development of new antibacterial strategies. As part of this effort, we screened a panel of drugs previously characterized to be epigenetic modulators/pro-apoptotic/differentiative drugs. We selected a few compounds that alter Gram-positive growth. Among these, UVI5008, a derivative of the natural compound psammaplin A (Psa_A), was identified. The interaction of Psa_A with the DNA gyrase enzyme has been shown, and here, we hypothesized and confirmed the gyrase-specific activity by biochemical assays. UVI5008 exhibited growth inhibition activity against Staphylococcus aureus via structural modification of the cell wall, which was observed by SEM electron microscopy. Based on our findings, we propose UVI5008 as an alternative antibacterial compound against methicillin-resistant (Met.R) S. aureus strains.

Recent advances in inhibitors of sirtuin1/2: an update and perspective

Sirtuins (SIRT) are coenzyme NAD+-dependent histone deacetylases for the transfer of modified acetyl groups. Sirtuins are widely involved in various physiological processes and therefore associated with cardiovascular disease, diabetes, Parkinson's disease, cancer and beyond. Consequently, the development of modulators for  sirtuins has considerable clinical value. To date, a variety of SIRT1/2 inhibitors have been reported and none has been approved for the market. This review summarizes the recent progress in the discovery and development of SIRT1/2 inhibitors including their inhibitory potency, structure–activity relationship and binding mode analysis as well as discusses the perspective for the future development of SIRT1/2 inhibitors.

Cytochrome P450 1B1 promotes cancer cell survival via specificity protein 1 (Sp1)-mediated suppression of death receptor 4

Cytochrome P450 1B1 (CYP1B1), a well-known oncogene, has garnered wide attention because of its tumor-specific expression pattern and actions as a carcinogenic factor. Although CYP1B1 might play a crucial role in carcinogenesis, the detailed molecular mechanisms underlying oncogenic involvement in cancer development remain unclear. The present study investigated the manner in which CYP1B1 promotes survival of various cancer cells. Treatment with 2,2',4,6'-tetramethoxystilbene (TMS), a specific CYP1B1 inhibitor, significantly inhibited cell viability in human breast cancer and leukemia cell lines, including MCF-7, MDA-MB-231, HL-60, and U937 cells. In order to characterize the cellular functions of CYP1B1 associated with cancer cell survival, the relationship between this oncogene and death receptor 4 (DR4) was determined. Following induction or inhibition of CYP1B1, mRNA and protein expression levels of DR4 were measured, and this oncogene was found to significantly repress DR4 mRNA and protein expression. Further, the suppression of DR4 by CYP1B1 was restored with 5-aza-2'-deoxycytidine (5-aza-dC), a DNA methyltransferase inhibitor, indicating that DNA methylation may be involved in CYP1B1-mediated DR4 inhibition. Methylation-specific polymerase chain reaction (PCR) in CYP1B1-overexpressed HL-60 cells revealed that this oncogene induced hypermethylation on DR4 promoter. Interestingly, data showed that DR4 suppression of CYP1B1 is mediated by the DNA-binding ability of specificity protein 1 (Sp1). These findings suggest that CYP1B1 promotes cancer cell survival through involvement of DNA methylation-mediated DR4 inhibition and that Sp1 may act as key mediator required for oncogenic action.

Time-resolved analysis of DNA-protein interactions in living cells by UV laser pulses

Interactions between DNA and proteins are mainly studied through chemical procedures involving bi-functional reagents, mostly formaldehyde. Chromatin immunoprecipitation is used to identify the binding between transcription factors (TFs) and chromatin, and to evaluate the occurrence and impact of histone/DNA modifications. The current bottleneck in probing DNA-protein interactions using these approaches is caused by the fact that chemical crosslinkers do not discriminate direct and indirect bindings or short-lived chromatin occupancy. Here, we describe a novel application of UV laser-induced (L-) crosslinking and demonstrate that a combination of chemical and L-crosslinking is able to distinguish between direct and indirect DNA-protein interactions in a small number of living cells. The spatial and temporal dynamics of TF bindings to chromatin and their role in gene expression regulation may thus be assessed. The combination of chemical and L-crosslinking offers an exciting and unprecedented tool for biomedical applications.

c-Myc Modulation and Acetylation Is a Key HDAC Inhibitor Target in Cancer

Purpose: Histone deacetylase inhibitors (HDACi) are promising anticancer drugs. Although some HDACi have entered the clinic, the mechanism(s) underlying their tumor selectivity are poorly understood.Experimental Design and Results: Using gene expression analysis, we define a core set of six genes commonly regulated in acute myeloid leukemia (AML) blasts and cell lines. MYC, the most prominently modulated, is preferentially altered in leukemia. Upon HDACi treatment, c-Myc is acetylated at lysine 323 and its expression decreases, leading to TRAIL activation and apoptosis. c-Myc binds to the TRAIL promoter on the proximal GC box through SP1 or MIZ1, impairing TRAIL activation. HDACi exposure triggers TRAIL expression, altering c-Myc-TRAIL binding. These events do not occur in normal cells. Excitingly, this inverse correlation between TRAIL and c-Myc is supported by HDACi treatment ex vivo of AML blasts and primary human breast cancer cells. The predictive value of c-Myc to HDACi responsiveness is confirmed in vivo in AML patients undergoing HDACi-based clinical trials.Conclusions: Collectively, our findings identify a key role for c-Myc in TRAIL deregulation and as a biomarker of the anticancer action of HDACi in AML. The potential improved patient stratification could pave the way toward personalized therapies. Clin Cancer Res; 23(10); 2542-55. ©2016 AACR.

Epigenetic modulators as therapeutic targets in prostate cancer

Prostate cancer is one of the most common non-cutaneous malignancies among men worldwide. Epigenetic aberrations, including changes in DNA methylation patterns and/or histone modifications, are key drivers of prostate carcinogenesis. These epigenetic defects might be due to deregulated function and/or expression of the epigenetic machinery, affecting the expression of several important genes. Remarkably, epigenetic modifications are reversible and numerous compounds that target the epigenetic enzymes and regulatory proteins were reported to be effective in cancer growth control. In fact, some of these drugs are already being tested in clinical trials. This review discusses the most important epigenetic alterations in prostate cancer, highlighting the role of epigenetic modulating compounds in pre-clinical and clinical trials as potential therapeutic agents for prostate cancer management.

How much successful are the medicinal chemists in modulation of SIRT1: A critical review

Silent information regulator two homologue one (SIRT1) is the most widely studied member of the sirtuin family related to histone deacetylases class III super-family using nicotinamide adenine dinucleotide (NAD(+)) as its cofactor. It is located in the nucleus but also modulates the targets in cytoplasm and mainly acts as transacetylase rather than deacetylase. SIRT1 specifically cleaves the nicotinamide ribosyl bond of NAD(+) and transfers the acetyl group from proteins to their co-substrate through an ADP- ribose-peptidyl imidate intermediate. It has been indicated that SIRT1 and its histone as well as non histone targets are involved in a wide range of biological courses including metabolic diseases, age related diseases, viral infection, inflammation, tumor-cell growth and metastasis. Modulation of SIRT1 expression may present a new insight in the discovery of a number of therapeutics. This review summarizes studies about SIRT1 and mainly focuses on the various modulators of SIRT1 evolved by natural as well as synthetic means.

Epigenetic-based therapy: From single- to multi-target approaches

The treatment of cancer has traditionally been based on the identification of a single molecule and/or enzymatic function (target) responsible for a particular phenotype, and therefore on the ability to stimulate, attenuate or inhibit its activity through the use of selective compounds. However, cancer is no longer considered a disease caused by a single factor, but is now recognized as a multi-factorial disorder. Genetic, epigenetic and metabolic factors all contribute to neoplasia, causing significant changes in molecular networks that govern cell growth, development, death and specialization. Consequently, many antitumor therapies are no longer directed against a single target but the biological system as a whole, in which functions determining the onset and maintenance of a physio-pathological state are modulated. The field of epi-drug discovery is currently in a transitional phase where the search for putative anticancer drugs is shifting from single-target-oriented molecules to network-active compounds and to epi-drugs used in combination with other epi-agents and with traditional chemotherapeutics. This review illustrates the pros and cons of each therapeutic option, providing examples in support of single-target and multi (network)-target epi-drug approaches.

Targeting aberrant cancer metabolism - The role of sirtuins

Cancer cells, as opposed to normal cells, generate energy by increasing aerobic glycolysis, which is a phenomenon called "the Warburg effect". An altered energy metabolism supporting continuous cell growth and proliferation was pointed to as the new "hallmark" of cancer cells. Several hypotheses have been proposed to explain the maintenance of this seemingly wasteful catabolic state. The epigenetic mechanisms which depend on the covalent modifications of both DNA and histones have recently emerged as important players in the regulation of glucose metabolism. The sirtuin family of histone deacetylases has emerged as important regulators of diverse physiological and pathological events, including cancer metabolism. Sirtuins 1-7 (SIRT1-7) belong to class III of histone deacetylase enzymes which are dependent on NAD(+) for activity. It was recently demonstrated that SIRT6 is a tumor suppressor that modulates aerobic glycolysis by repressing HIF1 transcription. Members of this family of enzymes are considered promising pharmaceutical targets for cancer treatment. This review highlights the major functions of sirtuins in relation to cancer metabolism and the possibilities of their activation and inhibition by small molecule drugs.

Antiproliferative, antibacterial and antifungal activity of the lichen Xanthoria parietina and its secondary metabolite parietin

Lichens are valuable natural resources used for centuries throughout the world as medicine, food, fodder, perfume, spices and dyes, as well as for other miscellaneous purposes. This study investigates the antiproliferative, antibacterial and antifungal activity of the acetone extract of the lichen Xanthoria parietina (Linnaeus) Theodor Fries and its major secondary metabolite, parietin. The extract and parietin were tested for antimicrobial activity against nine American Type Culture Collection standard and clinically isolated bacterial strains, and three fungal strains. Both showed strong antibacterial activity against all bacterial strains and matched clinical isolates, particularly against Staphylococcus aureus from standard and clinical sources. Among the fungi tested, Rhizoctonia solani was the most sensitive. The antiproliferative effects of the extract and parietin were also investigated in human breast cancer cells. The extract inhibited proliferation and induced apoptosis, both effects being accompanied by modulation of expression of cell cycle regulating genes such as p16, p27, cyclin D1 and cyclin A. It also mediated apoptosis by activating extrinsic and intrinsic cell death pathways, modulating Tumor Necrosis Factor-related apoptosis-inducing ligand (TRAIL) and B-cell lymphoma 2 (Bcl-2), and inducing Bcl-2-associated agonist of cell death (BAD) phosphorylation. Our results indicate that Xanthoria parietina is a major potential source of antimicrobial and anticancer substances.

Efficient synthesis and biological activity of Psammaplin A and its analogues as antitumor agents

We describe a new concise method for the synthesis of psammaplin A and its analogues, and antitumor activity of psammaplin A analogues. Psammaplin A was obtained with 41% yield in 5 steps from 3-bromo-4-hydroxybenzaldahyde and ethyl acetoacetate via Knoevenagel condensation and α-nitrosation as key steps. Twenty eight analogues of psammaplin A were prepared employing the new synthetic approach. Structure-activity relationship study against cytotoxicity reveal that the free oxime group and disulfide functional group were responsible for high cytotoxicity. Also the bromotyrosine component was relatively tolerable and hydrophobic aromatic groups preserved the cytotoxicity. The cytotoxicity of aromatic group is dependent on the size and spatial geometry. Among them, five compounds showed comparable cytotoxicity to psammaplin A. Compound 30 exhibited potential HDAC inhibitory activity and in vivo antitumor activity.

Advancements in the delivery of epigenetic drugs

INTRODUCTION

Advancements in epigenetic treatments are not only coming from new drugs, but also from modifications or encapsulation of the existing drugs into different formulations leading to greater stability and enhanced delivery to the target site. The epigenome is highly regulated and complex; therefore, it is important that off-target effects of epigenetic drugs be minimized. The step from in vitro to in vivo treatment of these drugs often requires development of a method of effective delivery for clinical translation.

AREAS COVERED

This review covers epigenetic mechanisms such as DNA methylation, chromatin remodeling and small-RNA-mediated gene regulation. There is a section in the review with examples of diseases where epigenetic alterations lead to impaired pathways, with an emphasis on cancer. Epigenetic drugs, their targets and clinical status are presented. Advantages of using a delivery method for epigenetic drugs as well as examples of current advancements and challenges are also discussed.

EXPERT OPINION

Epigenetic drugs have the potential to be very effective therapy against a number of diseases, especially cancers and neurological disorders. As with many chemotherapeutics, undesired side effects need to be minimized. Finding a suitable delivery method means reducing side effects and achieving a higher therapeutic index. Each drug may require a unique delivery method exploiting the drug's chemistry or other physical characteristic requiring interdisciplinary participation and would benefit from a better understanding of the mechanisms of action.

Synthesis of 7-alkylidene-7,12-dihydroindolo[3,2-d]benzazepine-6-(5H)-ones (7-alkylidene-paullones) by N-cyclization–oxidative Heck cascade and characterization as sirtuin modulators

A palladium-induced cascade of N-cyclization and oxidative Heck reaction of o-alkynylanilines produced 7-alkylidene-indolobenzazepinones (paullones) that have sirtuin modulation activities.

Anticancer agents targeted to sirtuins

Sirtuins are nicotinamide adenine dinucleotide⁺-dependent deacetylases of which there are seven isoforms (SIRT1–7). Sirtuin activity is linked to gene expression, lifespan extension, neurodegeneration, and age-related disorders. Numerous studies have suggested that sirtuins could be of great significance with regard to both antiaging and tumorigenesis, depending on its targets in specific signaling pathways or in specific cancers. Recent studies have identified small chemical compounds that modulate sirtuins, and these modulators have enabled a greater understanding of the biological function and molecular mechanisms of sirtuins. This review highlights the possibility of sirtuins, especially SIRT1 and SIRT2, for cancer therapy targets, and focuses on the therapeutic potential of sirtuin modulators both in cancer prevention and treatment.

Novel antiproliferative chimeric compounds with marked histone deacetylase inhibitory activity

Given our interest in finding potential antitumor agents and in view of the multifactorial mechanistic nature of cancer, in the present work, taking advantage of the multifunctional ligands approach, new chimeric molecules were designed and synthesized by combining in single chemical entities structural features of SAHA, targeting histone deacetylases (HDACs), with substituted stilbene or terphenyl derivatives previously obtained by us and endowed with antiproliferative and pro-apoptotic activity. The new chimeric derivatives were characterized with respect to their cytotoxic activity and their effects on cell cycle progression on different tumor cell lines, as well as their HDACs inhibition. Among the other, trans -6 showed the most interesting biological profile, as it exhibited a strong pro-apoptotic activity in tumor cell lines in comparison with both of its parent compounds and a marked HDAC inhibition.

Natural compounds in epigenetics: a current view

The successful treatment of many human diseases, including cancer, has come to be considered a major challenge, as patient response to therapy is difficult to predict. Recently, considerable efforts are being focused on the development of new tools to meet the growing demand for personalized medicine. With few exceptions, synthetic compounds have been unable to meet initial expectations for their clinical use. The last twenty years have been characterized by the failure of several drugs in advanced clinical development, possibly due to the insufficient understanding of molecular pathways underlying their mechanism of action. Although the biodiversity of compounds found in nature has been poorly explored until now, the field of naturally occurring drugs is rapidly expanding. Here, we review the current knowledge on the use of natural compounds with particular emphasis on those that display a chromatin remodeling effect coupled with anticancer action.

Quinoline-based p300 histone acetyltransferase inhibitors with pro-apoptotic activity in human leukemia U937 cells

Chemical manipulations performed on 2-methyl-3-carbethoxyquinoline (1), a histone acetyltransferase inhibitor previously identified by our research group and active at the sub-millimolar/millimolar level, led to compounds bearing higher alkyl groups at the C2-quinoline or additional side chains at the C6-quinoline positions. Such compounds displayed at least threefold improved inhibitory potency toward p300 protein lysine acetyltransferase activity; some of them decreased histone H3 and H4 acetylation levels in U937 cells and induced high degrees of apoptosis (three compounds >10-fold higher than compound 1) after treatment of U937 cells.

The class I-specific HDAC inhibitor MS-275 modulates the differentiation potential of mouse embryonic stem cells

Exploitation of embryonic stem cells (ESC) for therapeutic use and biomedical applications is severely hampered by the risk of teratocarcinoma formation. Here, we performed a screen of selected epi-modulating compounds and demonstrate that a transient exposure of mouse ESC to MS-275 (Entinostat), a class I histone deacetylase inhibitor (HDAC), modulates differentiation and prevents teratocarcinoma formation. Morphological and molecular data indicate that MS-275-primed ESCs are committed towards neural differentiation, which is supported by transcriptome analyses. Interestingly, in vitro withdrawal of MS-275 reverses the primed cells to the pluripotent state. In vivo, MS275-primed ES cells injected into recipient mice give only rise to benign teratomas but not teratocarcinomas with prevalence of neural-derived structures. In agreement, MS-275-primed ESC are unable to colonize blastocysts. These findings provide evidence that a transient alteration of acetylation alters the ESC fate.

Discovery of a potent small molecule SIRT1/2 inhibitor with anticancer effects

SIRT1 and SIRT2 are deacetylase enzymes that belong to the sirtuin family and are involved in tumorigenesis. In our screen for small molecules inhibiting SIRT1/2 toxoflavin was identified. Toxoflavin potently inhibited SIRT1 activity in in vitro deacetylase assay using purified SIRT1 protein. SIRT2 activity was also inhibited by toxoflavin less potently than SIRT1 in deacetylase assay in vitro. Toxoflavin exhibited growth inhibition of various cancer cell lines including A549 lung cancer cells with a GI(50) of 48 nM. Toxoflavin treatment in A549 cells increased the acetylated form of p53, which is a substrate of SIRT1. The acetylation levels of α-tubulin, a SIRT2 substrate, were also increased by toxoflavin treatment dose-dependently. Several toxoflavin derivatives were synthesized to determine the preliminary structure-activity relationship of toxoflavin. Some of the toxoflavin derivatives showed highly selective inhibition against SIRT1. In conclusion, this study presented toxoflavin as a potent SIRT1/2 inhibitor with anticancer activity.

HDAC inhibitors: roles of DNA damage and repair

Histone deacetylase inhibitors (HDACis) increase gene expression through induction of histone acetylation. However, it remains unclear whether specific gene expression changes determine the apoptotic response following HDACis administration. Herein, we discuss evidence that HDACis trigger in cancer and leukemia cells not only widespread histone acetylation but also actual increases in reactive oxygen species (ROS) and DNA damage that are further increased following treatment with DNA-damaging chemotherapies. While the origins of ROS production are not completely understood, mechanisms, including inflammation and altered antioxidant signaling, have been reported. While the generation of ROS is an explanation, at least in part, for the source of DNA damage observed with HDACi treatment, DNA damage can also be independently induced by changes in the DNA repair activity and chromatin remodeling factors. Recent development of sirtuin inhibitors (SIRTis) has shown that, similar to HDACis, these drugs induce increases in ROS and DNA damage used singly, or in combination with HDACis and other drugs. Thus, induction of apoptosis by HDACis/SIRTis may result through oxidative stress and DNA damage mechanisms in addition to direct activation of apoptosis-inducing genes. Nevertheless, while DNA damage and stress responses could be of interest as markers for clinical responses, they have yet to be validated as markers for responses to HDACi treatment in clinical trials, alone, and in combination.

Thioester derivatives of the natural product psammaplin A as potent histone deacetylase inhibitors

There has been significant interest in the bioactivity of the natural product psammaplin A, most recently as a potent and isoform selective HDAC inhibitor. Here we report our preliminary studies on thioester HDAC inhibitors derived from the active monomeric (thiol) form of psammaplin A, as a means to improve compound delivery into cells. We have discovered that such compounds exhibit both potent cytotoxicity and enzymatic inhibitory activity against recombinant HDAC1. The latter effect is surprising since previous SAR suggested that modification of the thiol functionality should detrimentally affect HDAC potency. We therefore also report our preliminary studies on the mechanism of action of this observed effect.

Epigenetic targeting therapies to overcome chemotherapy resistance

It is now well established that epigenetic aberrations occur early in malignant transformation, raising the possibility of identifying chemopreventive compounds or reliable diagnostic screening using epigenetic biomarkers. Combinatorial therapies effective for the reexpression of tumor suppressors, facilitating resensitization to conventional chemotherapies, hold great promise for the future therapy of cancer. This approach may also perturb cancer stem cells and thus represent an effective means for managing a number of solid tumors. We believe that in the near future, anticancer drug regimens will routinely include epigenetic therapies, possibly in conjunction with inhibitors of "stemness" signal pathways, to effectively reduce the devastating occurrence of cancer chemotherapy resistance.

Highly ligand efficient and selective N-2-(Thioethyl)picolinamide histone deacetylase inhibitors inspired by the natural product psammaplin A

Novel picolinamide-based histone deacetylase (HDAC) inhibitors were developed, drawing inspiration from the natural product psammaplin A. We found that the HDAC potency and isoform selectivity provided by the oxime unit of psammaplin A could be reproduced by using carefully chosen heterocyclic frameworks. The resulting (hetero)aromatic amide based compounds displayed very high potency and isoform selectivity among the HDAC family, in addition to excellent ligand efficiency relative to previously reported HDAC inhibitors. In particular, the high HDAC1 isoform selectivity provided by the chloropyridine motif represents a valuable design criterion for the development of new lead compounds and chemical probes that target HDAC1.

Histone deacetylase inhibitors: a patent review (2009 - 2011)

INTRODUCTION

Given the involvement of histone deacetylases (HDACs) in regulation of gene expression, they are believed to be 'master regulators' of many diseases. Thus, HDAC inhibitors (HDACis) are able to modulate transcriptional activity. These molecules can induce cell cycle arrest, differentiation and apoptosis of tumor cells in culture and in animal models and therefore are emerging as an exciting new class of potential anti-cancer agents for the treatment of solid and hematological malignancies.

AREAS COVERED

The aim of this review is to provide an overview of current knowledge and molecular mechanisms of HDACis, and the most recent patents existing in the field of HDACis from 2009 until 2011.

EXPERT OPINION

In recent years, an increasing number of structurally diverse HDACis have been identified. In addition, non-cancer diseases, including neurodegeneration, metabolic, inflammatory and autoimmune disorders, infectious and cardiovascular diseases have also been proposed for an HDACi treatment. The growing body of evidence of the potential benefits of disease treatment based on the use of HDACis has led to a large number of patent applications throughout the world.

Enantioselective apoptosis induction in histiocytic lymphoma cells and acute promyelocytic leukemia cells

The aim of this study was to identify valproic acid (VPA) analogs with a broad spectrum of anti-cancer activities and an increased apoptosis-inducing potential compared with the parent VPA, which is enrolled as histone deacetylase (HDAC) inhibitor in a large number of clinical trials. We identified a chiral VPA derivative, (S)-2-pentyl-4-pentynoic acid, previously characterized as HDAC inhibitor that induced massive programmed cell death in a strongly enantioselective manner in U937 histiocytic lymphoma cells and NB4 acute promyelocytic leukemia cells. By performing fluorescence-activated cell sorting and Western blotting analyses, we established that enantiomer (S)-2-pentyl-4-pentynoic acid has higher apoptosis-inducing potential than VPA itself. The optic antipode (R)-2-pentyl-4-pentynoic acid and VPA caused under the same conditions only a weak growth inhibition without inducing cell differentiation and apoptosis. (S)-2-pentyl-4-pentynoic acid is more apoptogenic than VPA and displays enantioselective anti-cancer properties that warrant further research regarding the mechanistic basis of its activity and its potential use in cancer therapy.

Molecular pathways: the complexity of the epigenome in cancer and recent clinical advances

Human cancer is causally linked to genomic and epigenomic deregulations. Epigenetic abnormalities occurring within signaling pathways regulating proliferation, migration, growth, differentiation, transcription, and death signals may be critical in the progression of malignancies. Consequently, identification of epigenetic marks and their bioimplications in tumors represents a crucial step toward defining new therapeutic strategies both in cancer treatment and prevention. Alterations of writers, readers, and erasers in cancer may affect, for example, the methylation and acetylation state of huge areas of chromatin, suggesting that epi-based treatments may require "distinct" therapeutic strategies compared with "canonical" targeted treatments. Whereas anticancer treatments targeting histone deacetylase and DNA methylation have entered the clinic, additional chromatin modification enzymes have not yet been pharmacologically targeted for clinical use in patients. Thus, a greater insight into alterations occurring on chromatin modifiers and their impact in tumorigenesis represents a crucial advancement in exploiting epigenetic targeting in cancer prevention and treatment. Here, the interplay of the best known epi-mutations and how their targeting might be optimized are addressed.

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.

HDAC modulation and cell death in the clinic

Histone acetyltransferases (HATs) and histone deacetylases (HDACs) are two opposing classes of enzymes, which finely regulate the balance of histone acetylation affecting chromatin packaging and gene expression. Imbalanced acetylation has been associated with carcinogenesis and cancer progression. In contrast to genetic mutations, epigenetic changes are potentially reversible. This implies that epigenetic alterations are amenable to pharmacological interventions. Accordingly, some epigenetic-based drugs (epidrugs) have been approved by the Food and Drug Administration (FDA) and the European Medicines Agency (EMA) for cancer treatment. Here, we focus on the biological features of HDAC inhibitors (HDACis), analyzing the mechanism(s) of action and their current use in clinical practice.

Sirtuins and disease: the road ahead

Sirtuins represent a promising new class of conserved histone deacetylases, originally identified in yeast. The activity of the sirtuin (SirT) family – made up of seven members (SirT1-7) – is NAD⁺ dependent. Sirtuins target a wide range of cellular proteins in nucleus, cytoplasm, and mitochondria for post-translational modification by acetylation (SirT1, 2, 3, and 5) or ADP-ribosylation (SirT4 and 6). Sirtuins regulate responses to stress and ensure that damaged DNA is not propagated, thus contrasting the accumulation of mutations. To date, sirtuins have emerged as potential therapeutic targets for treatment of human pathologies such as metabolic, cardiovascular and neurodegenerative diseases, and cancer. SirT1 is the founding member of this class of enzymes and is currently the best known of the group. SirT1 acts in various cellular processes, deacetylating both chromatin and non-histone proteins, and its role in cancer and aging has been extensively studied. SirT1 may play a critical role in tumor initiation and progression as well as drug resistance by blocking senescence and apoptosis, and by promoting cell growth and angiogenesis. Recently, growing interest in sirtuin modulation has led to the discovery and characterization of small molecules able to modify sirtuin activity. The present review highlights SirT mechanism(s) of action and deregulation in cancer, focusing on the therapeutic potential of SirT modulators both in cancer prevention and treatment.