Histone deacetylase inhibitor-mediated radiosensitization of human cancer cells: class differences and the potential influence of p53

Quisinostat is a brain-penetrant radiosensitizer in glioblastoma

Histone deacetylase (HDAC) inhibitors have garnered considerable interest for the treatment of adult and pediatric malignant brain tumors. However, owing to their broad-spectrum nature and inability to effectively penetrate the blood-brain barrier, HDAC inhibitors have failed to provide substantial clinical benefit to patients with glioblastoma (GBM) to date. Moreover, global inhibition of HDACs results in widespread toxicity, highlighting the need for selective isoform targeting. Although no isoform-specific HDAC inhibitors are currently available, the second-generation hydroxamic acid-based HDAC inhibitor quisinostat possesses subnanomolar specificity for class I HDAC isoforms, particularly HDAC1 and HDAC2. It has been shown that HDAC1 is the essential HDAC in GBM. This study analyzed the neuropharmacokinetic, pharmacodynamic, and radiation-sensitizing properties of quisinostat in preclinical models of GBM. It was found that quisinostat is a well-tolerated and brain-penetrant molecule that extended survival when administered in combination with radiation in vivo. The pharmacokinetic-pharmacodynamic-efficacy relationship was established by correlating free drug concentrations and evidence of target modulation in the brain with survival benefit. Together, these data provide a strong rationale for clinical development of quisinostat as a radiosensitizer for the treatment of GBM.

Targeting the p53 signaling pathway in cancers: Molecular mechanisms and clinical studies

Tumor suppressor p53 can transcriptionally activate downstream genes in response to stress, and then regulate the cell cycle, DNA repair, metabolism, angiogenesis, apoptosis, and other biological responses. p53 has seven functional domains and 12 splice isoforms, and different domains and subtypes play different roles. The activation and inactivation of p53 are finely regulated and are associated with phosphorylation/acetylation modification and ubiquitination modification, respectively. Abnormal activation of p53 is closely related to the occurrence and development of cancer. While targeted therapy of the p53 signaling pathway is still in its early stages and only a few drugs or treatments have entered clinical trials, the development of new drugs and ongoing clinical trials are expected to lead to the widespread use of p53 signaling-targeted therapy in cancer treatment in the future. TRIAP1 is a novel p53 downstream inhibitor of apoptosis. TRIAP1 is the homolog of yeast mitochondrial intermembrane protein MDM35, which can play a tumor-promoting role by blocking the mitochondria-dependent apoptosis pathway. This work provides a systematic overview of recent basic research and clinical progress in the p53 signaling pathway and proposes that TRIAP1 is an important therapeutic target downstream of p53 signaling.

Targeted Cancer Therapy Based on Acetylation and Deacetylation of Key Proteins Involved in Double-Strand Break Repair

DNA double-strand breaks (DSBs) play an important role in promoting genomic instability and cell death. The precise repair of DSBs is essential for maintaining genome integrity during cancer progression, and inducing genomic instability or blocking DNA repair is an important mechanism through which chemo/radiotherapies exert killing effects on cancer cells. The two main pathways that facilitate the repair of DSBs in cancer cells are homologous recombination (HR) and non-homologous end-joining (NHEJ). Accumulating data suggest that the acetylation and deacetylation of DSB repair proteins regulate the initiation and progression of the cellular response to DNA DSBs, which may further affect the chemosensitivity or radiosensitivity of cancer cells. Here, we focus on the role of acetylation/deacetylation in the regulation of ataxia-telangiectasia mutated, Rad51, and 53BP1 in the HR pathway, as well as the relevant roles of PARP1 and Ku70 in NHEJ. Notably, several histone deacetylase (HDAC) inhibitors targeting HR or NHEJ have been demonstrated to enhance chemo/radiosensitivity in preclinical studies. This review highlights the essential role of acetylation/deacetylation in the regulation of DSB repair proteins, suggesting that HDAC inhibitors targeting the HR or NHEJ pathways that downregulate DNA DSB repair genes may be worthwhile cancer therapeutic agents.

DNMT3A and DNMT3B Targeting as an Effective Radiosensitizing Strategy in Embryonal Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is the most common soft tissue sarcoma in childhood. Recently, we demonstrated the overexpression of both DNA methyltransferase 3A (DNMT3A) and 3B (DNMT3B) in RMS tumour biopsies and cell lines compared to normal skeletal muscle. Radiotherapy may often fail due to the abnormal expression of some molecules able to drive resistance mechanisms. The aim of this study was to analyse the involvement of DNMT3A and DNMT3B in radioresistance in RMS. RNA interference experiments against DNMT3A/3B were performed in embryonal RMS cells, upon ionizing radiation (IR) exposure and the effects of the combined treatment on RMS cells were analysed. DNMT3A and DNMT3B knocking down increased the sensitivity of RMS cells to IR, as indicated by the drastic decrease of colony formation ability. Interestingly, DNMT3A/3B act in two different ways: DNMT3A silencing triggers the cellular senescence program by up-regulating p16 and p21, whilst DNMT3B depletion induces significant DNA damage and impairs the DNA repair machinery (ATM, DNA-PKcs and Rad51 reduction). Our findings demonstrate for the first time that DNMT3A and DNMT3B overexpression may contribute to radiotherapy failure, and their inhibition might be a promising radiosensitizing strategy, mainly in the treatment of patients with metastatic or recurrent RMS tumours.

Low Dose of Trichostatin A Improves Radiation Resistance by Activating Akt/Nrf2-Dependent Antioxidation Pathway in Cancer Cells

Numerous studies have shown that histone deacetylase inhibitors (HDACis) improve cellular acetylation while also enhancing the radiation sensitivity. In this work, however, we confirmed that low-dose trichostatin A (TSA) as a typical HDACi could reduce rather than increase the radiosensitivity of cancer cells, while the cellular acetylation was also increased with TSA-induced epigenetic modification. The surviving fraction of HeLa/HepG2 cells pretreated with 25 nM TSA for 24 h was higher at 1 Gy/2 Gy of γ-ray radiation than that of the cells with the same radiation dose but without TSA pretreatment. To understand the underlying mechanism, we investigated the effect of low-dose TSA on HO-1, SOD and CAT induction and activating Akt together with its downstream Nrf2 signaling pathway. Our results indicated that TSA activated HO-1, SOD and CAT expression by increasing the phosphorylation level of Nrf2 in an Akt-dependent manner. In addition, we also observed that the 25-nM-TSA-pretreated group showed a significant increase in the antioxidant capacity in terms of SOD and CAT activities. Therefore, our results suggest that low-dose TSA can activate the Akt/Nrf2 pathway and upregulate expression of HO-1, SOD and CAT to stimulate the cellular defense mechanism. This work demonstrates that low-dose TSA treatment may activate the adaptation mechanism against the oxidative stress induced by ionizing radiation, and application of HDACi treatment should be undertaken with caution to avoid its possible radioresistance in radiotherapy.

Targeting histone deacetylase 8 as a therapeutic approach to cancer and neurodegenerative diseases

Histone deacetylase 8 (HDAC8), a unique class I zinc-dependent HDAC, is an emerging target in cancer and other diseases. Its substrate repertoire extends beyond histones to many nonhistone proteins. Besides being a deacetylase, HDAC8 also mediates signaling via scaffolding functions. Aberrant expression or deregulated interactions with transcription factors are critical in HDAC8-dependent cancers. Many potent HDAC8-selective inhibitors with cellular activity and anticancer effects have been reported. We present HDAC8 as a druggable target and discuss inhibitors of different chemical scaffolds with cellular effects. Furthermore, we review HDAC8 activators that revert activity of mutant enzymes. Isotype-selective HDAC8 targeting in patients with HDAC8-relevant cancers is challenging, however, is promising to avoid adverse side effects as observed with pan-HDAC inhibitors.

Isotype-Specific Inhibition of Histone Deacetylases: Identification of Optimal Targets for Radiosensitization

Purpose

Histone deacetylase (HDAC) inhibitors radiosensitize tumor cells. To elucidate mechanisms underlying radiosensitization by HDAC inhibition, understanding of differential contributions of HDAC isotypes is needed. The aim of this study was to investigate involvement of known HDAC isotypes in modulation of cellular radiosensitivity.

Materials and Methods

Because pharmacologic HDAC inhibitors lack isotype-specificity, RNA interference against 11 HDAC isotypes was used to inhibit HDAC in an isotype-specific manner. Radiation cell survival was evaluated using a clonogenic assay in SQ20B cells transfected with small interfering RNA specifically targeting HDAC isotypes. Immunocytochemistry was performed for detection of γH2AX foci. Protein expression was measured using Western blotting.

Results

Among 11 HDAC isotypes tested, specific inhibition of 7 isotypes (HDAC1, HDAC3, HDAC4, HDAC6, HDAC7, HDAC10, and HDAC11) enhanced radiation lethality in SQ20B cells. Radiosensitization by inhibition of these HDAC isotypes was accompanied by delay of DNA double strand break repair. Radiosensitivity of SQ20B cells was not altered by selective inhibition of the remaining four isotypes (HDAC2, HDAC5, HDAC8, and HDAC9). Inhibition of HDAC isotypes resulted in downregulation of various proteins involved in pro-survival and DNA damage repair pathways.

Conclusion

Isotype-specificity exists in HDAC inhibition-induced radiosensitization. Different HDAC isotypes are differentially involved in modulation of cellular radiosensitivity.

Radiosensitization with combined use of olaparib and PI-103 in triple-negative breast cancer

BACKGROUND

Triple-negative breast cancer (TNBC) shows aggressive clinical behavior, but the treatment options are limited due to lack of a specific target. TNBC shares many clinical and pathological similarities with BRCA-deficient breast cancer, for which poly(ADP-ribose) polymerase (PARP) inhibitor is effective, but PARP inhibitor alone failed to show clinical effects in patients with sporadic TNBC. Radiation induces DNA double-strand breaks, and the phosphoinositide 3-kinase (PI3K) signaling pathway has been known to regulate steady-state levels of homologous recombination. A recent preclinical study showed that PI3K inhibition impairs BRCA1/2 expression and sensitizes BRCA-proficient TNBC to PARP inhibition. Therefore, we assessed the radiosensitizing effect, and the underlying mechanism of combination treatment with PARP inhibitor olaparib and PI3K inhibitor PI-103 in BRCA-proficient TNBC cells.

METHODS

MDA-MB-435S cells were divided into four treatment groups, irradiation (IR) alone, olaparib plus IR, PI-103 plus IR, and olaparib plus PI-103 plus IR. Cells were exposed to the drugs for 2 hours prior to irradiation, and the cell survival curve was obtained using a clonogenic assay. Western blotting and immunofluorescent detection of γH2AX foci were performed. Xenograft and bioluminescence imaging were carried out to assess in vivo radiosensitivity.

RESULTS

Combined use of olaparib and PI-103 enhanced radiation-induced death of MDA-MB-435S (sensitizer enhancement ratio[SER]0.05,1.7) and MDA-MB-231-BR (SER0.05,2.1) cells and significantly reduced tumor volume in a xenograft models (P < 0.001). Treatment with PI-103 showed persistent γH2AX foci, indicating delayed repair of DNA strand breaks. PI-103 alone increased levels of poly(ADP-ribose) and phosphorylated extracellular signal-regulated kinase, and downregulated BRCA1.

CONCLUSIONS

Combined use of olaparib and PI-103 enhanced radiation-induced cell death in BRCA-proficient MDA-MB-435S and MDA-MB-231-BR cells and xenografts. TNBC patients have high incidences of locoregional relapse and distant metastasis, and radiation therapy targets both locoregional control and treatment of distant recurrences such as brain metastasis or other oligometastasis. Targeting of the PI3K signaling pathway combined with PARP inhibition maybe a feasible approach to enhance effects of radiation in BRCA-proficient TNBC.

Experimental study on the regulation of erlotinib-induced radiosensitization with an anti-c-MET monoclonal antibody

PURPOSE

Erlotinib is a novel therapeutic agent for cancer treatment. This study was performed to investigate the role of c-MET-PI3K-AKT pathway in the regulation of erlotinib-induced radiosensitization.

METHODS

A973 lung adenocarcinoma cells treated with 6 Gy of radiation were incubated in the presence of erlotinib. The apoptotic rate after 24 hours, the colony-formating rate after 14 days, and changes in the c-MET expression levels after 14 days of irradiation were examined. Surviving fractions in different treatment groups (blank control, radiation alone, erlotinib alone, anti-c-MET monoclonal antibody alone, combined erlotinib and radiation, and combined erlotinib and radiation with anti-c-MET monoclonal antibody groups) were determined, the survival curves were plotted, and the sensitizer enhancement ratio was calculated using colony formation assays. Expressions of c-MET, p-c-MET, PI3K, AKT, and p-AKT in cells in different treatment groups were examined by Western blot analysis.

RESULTS

The apoptotic rate in the combined erlotinib and radiation group was higher than those in single treatment groups; however, the colony-forming rate remained approximately 2.04 ± 1.02%. The expression of c-MET in colony-forming cells in the combined group significantly increased, and the blockade of c-MET activity significantly enhanced the radiosensitizing effect of erlotinib. The expression of c-Met, p-c-MET, PI3K, AKT, and p-AKT among colony-forming cells significantly decreased upon the inhibition of c-MET.

CONCLUSIONS

Upregulated activity of the c-MET-PI3K-AKT pathway was found to be important for cell survival under combined the treatment with erlotinib and radiation. The blockade of the c-MET-PI3K-AKT signaling pathway enhanced the radiosensitizing effect of erlotinib.

Histone deacetylase inhibitors in glioblastoma: pre-clinical and clinical experience

Epigenetic mechanisms are increasingly recognized as a major factor contributing to pathogenesis of cancer including glioblastoma, the most common and most malignant primary brain tumour in adults. Enzymatic modifications of histone proteins regulating gene expression are being exploited for therapeutic drug targeting. Over the last decade, numerous studies have shown promising results with histone deacetylase (HDAC) inhibitors in various malignancies. This article provides a brief overview of mechanism of anti-cancer effect and pharmacology of HDAC inhibitors and summarizes results from pre-clinical and clinical studies in glioblastoma. It analyses experience with HDAC inhibitors as single agents as well as in combination with targeted agents, cytotoxic chemotherapy and radiotherapy. Hallmark features of glioblastoma, such as uncontrolled cellular proliferation, invasion, angiogenesis and resistance to apoptosis, have been shown to be targeted by HDAC inhibitors in experiments with glioblastoma cell lines. Vorinostat is the most advanced HDAC inhibitor that entered clinical trials in glioblastoma, showing activity in recurrent disease. Multiple phase II trials with vorinostat in combination with targeted agents, temozolomide and radiotherapy are currently recruiting. While the results from pre-clinical studies are encouraging, early clinical trials showed only modest benefit and the value of HDAC inhibitors for clinical practice will need to be confirmed in larger prospective trials. Further research in epigenetic mechanisms driving glioblastoma pathogenesis and identification of molecular subtypes of glioblastoma is needed. This will hopefully lead to better selection of patients who will benefit from treatment with HDAC inhibitors.

Chromatin structure and radiation-induced DNA damage: from structural biology to radiobiology

Genomic DNA in eukaryotic cells is basically divided into chromosomes, each consisting of a single huge nucleosomal fiber. It is now clear that chromatin structure and dynamics play a critical role in all processes involved in DNA metabolism, e.g. replication, transcription, repair and recombination. Radiation is a useful tool to study the biological effects of chromatin alterations. Conversely, radiotherapy and radiodiagnosis raise questions about the influence of chromatin integrity on clinical features and secondary effects. This review focuses on the link between DNA damage and chromatin structure at different scales, showing how a comprehensive multiscale vision is required to understand better the effect of radiations on DNA. Clinical aspects related to high- and low-dose of radiation and chromosomal instability will be discussed. At the same time, we will show that the analysis of the radiation-induced DNA damage distribution provides good insight on chromatin structure. Hence, we argue that chromatin "structuralists" and radiobiological "clinicians" would each benefit from more collaboration with the other. We hope that this focused review will help in this regard.

Sequence-Dependent Radiosensitization of Histone Deacetylase Inhibitors Trichostatin A and SK-7041

PURPOSE

This preclinical study is to determine whether the capacity of histone deacetylase (HDAC) inhibitors to enhance radiation response depends on temporal sequences of HDAC inhibition and irradiation.

MATERIALS AND METHODS

The effects of HDAC inhibitors trichostatin A (TSA) and SK-7041 on radiosensitivity in human lung cancer cells were examined using a clonogenic assay, exposing cells to HDAC inhibitors in various sequences of HDAC inhibition and radiation. We performed Western blot of acetylated histone H3 and flow cytometry to analyze cell cycle phase distribution.

RESULTS

TSA and SK-7041 augmented radiation cell lethality in an exposure time-dependent manner when delivered before irradiation. The impact of TSA and SK-7041 on radiosensitivity rapidly diminished when HDAC inhibition was delayed after irradiation. Radiation induced the acetylation of histone H3 in cells exposed to TSA, while irradiation alone had no effect on the expression of acetylated histone H3 in TSA-naïve cells. Preirradiation exposure to TSA abrogated radiation-induced G2/M-phase arrest. When delivered after irradiation, TSA had no effect on the peak of radiation-induced G2/M-phase arrest.

CONCLUSION

TSA and SK-7041 enhances radiosensitivity only when delivered before irradiation. Unless proven otherwise, it seems prudent to apply scheduling including preirradiation HDAC inhibition so that maximal radiosensitization is obtained.

p53-dependent and p53-independent anticancer effects of different histone deacetylase inhibitors

Background:

Histone deacetylase inhibitors (HDACi) are promising antineoplastic agents, but their precise mechanisms of actions are not well understood. In particular, the relevance of p53 for HDACi-induced effects has not been fully elucidated. We investigated the anticancer effects of four structurally distinct HDACi, vorinostat, entinostat, apicidin and valproic acid, using isogenic HCT-116 colon cancer cell lines differing in p53 status.

Methods:

Effects were assessed by MTT assay, flow-cytometric analyses of propidium iodide uptake, mitochondrial depolarisation and cell-cycle distribution, as well as by gene expression profiling.

Results:

Vorinostat was equally effective in p53 wild-type and null cells, whereas entinostat was less effective in p53 null cells. Histone deacetylase inhibitors treatment suppressed the expression of MDM2 and increased the abundance of p53. Combination treatments showed that vorinostat enhanced the cytotoxic activity of TRAIL and bortezomib, independent of the cellular p53 status. Investigations into the effects of an inhibitor of the sirtuin class of HDAC, tenovin-1, revealed that tenovin-1-mediated cell death hinged on p53.

Conclusion:

These results demonstrate that vorinostat activates p53, but does not require p53 for inducing its anticancer action. Yet they also demonstrate that entinostat-induced cytotoxic effects partially depend on p53, indicating that different HDACi have a different requirement for p53.

The histone deacetylase inhibitor SAHA acts in synergism with fenretinide and doxorubicin to control growth of rhabdoid tumor cells

BACKGROUND

Rhabdoid tumors are highly aggressive malignancies affecting infants and very young children. In many instances these tumors are resistant to conventional type chemotherapy necessitating alternative approaches.

METHODS

Proliferation assays (MTT), apoptosis (propidium iodide/annexin V) and cell cycle analysis (DAPI), RNA expression microarrays and western blots were used to identify synergism of the HDAC (histone deacetylase) inhibitor SAHA with fenretinide, tamoxifen and doxorubicin in rhabdoidtumor cell lines.

RESULTS

HDAC1 and HDAC2 are overexpressed in primary rhabdoid tumors and rhabdoid tumor cell lines. Targeting HDACs in rhabdoid tumors induces cell cycle arrest and apoptosis. On the other hand HDAC inhibition induces deregulated gene programs (MYCC-, RB program and the stem cell program) in rhabdoid tumors. These programs are in general associated with cell cycle progression. Targeting these activated pro-proliferative genes by combined approaches of HDAC-inhibitors plus fenretinide, which inhibits cyclinD1, exhibit strong synergistic effects on induction of apoptosis. Furthermore, HDAC inhibition sensitizes rhabdoid tumor cell lines to cell death induced by chemotherapy.

CONCLUSION

Our data demonstrate that HDAC inhibitor treatment in combination with fenretinide or conventional chemotherapy is a promising tool for the treatment of chemoresistant rhabdoid tumors.

The different radiosensitivity when combining erlotinib with radiation at different administration schedules might be related to activity variations in c-MET-PI3K-AKT signal transduction

OBJECTIVES

The aim of this paper was to investigate the efficacy and activity variation associated with phosphoinositide 3-kinase (PI3K) signal transduction when combining erlotinib with radiation, using different administration schedules.

MATERIALS AND METHODS

Erlotinib was delivered to A973 cancer cells in the following three ways: (1) irradiation after administration, (2) irradiation upon administration, and, (3) irradiation before administration. The cell-survival rates were detected using colony-forming assays, while cell apoptosis was detected with flow cytometry. The expression levels of C-MET, p-C-MET, AKT, and p-AKT were determined via Western blotting analysis, under 6 Gy irradiation with/ without erlotinib.

RESULTS

The sensitizer enhancement ratios (SERs) of erlotinib irradiation after administration, irradiation upon administration, and irradiation before administration groups were 2.19, 1.53, and 1.38, respectively. A higher apoptosis rate was observed when irradiation was delivered after erlotinib. In addition, changes in cell apoptosis were found to be related to concurrent changes in C-MET, p-C-MET, AKT, and p-AKT expression. Protein expression increased in the combination groups, with trends showing a negative relationship with cell apoptosis.

CONCLUSION

The radiosensitive effect of erlotinib varied because of the different administration schedules; this variation may be related to PI3K signal transduction and its associated regulating effect.

DNMT (DNA methyltransferase) inhibitors radiosensitize human cancer cells by suppressing DNA repair activity

BACKGROUND

Histone modifications and DNA methylation are two major factors in epigenetic phenomenon. Unlike the histone deacetylase inhibitors, which are known to exert radiosensitizing effects, there have only been a few studies thus far concerning the role of DNA methyltransferase (DNMT) inhibitors as radiosensitizers. The principal objective of this study was to evaluate the effects of DNMT inhibitors on the radiosensitivity of human cancer cell lines, and to elucidate the mechanisms relevant to that process.

METHODS

A549 (lung cancer) and U373MG (glioblastoma) cells were exposed to radiation with or without six DNMT inhibitors (5-azacytidine, 5-aza-2'-deoxycytidine, zebularine, hydralazine, epigallocatechin gallate, and psammaplin A) for 18 hours prior to radiation, after which cell survival was evaluated via clonogenic assays. Cell cycle and apoptosis were analyzed via flow cytometry. Expressions of DNMT1, 3A/3B, and cleaved caspase-3 were detected via Western blotting. Expression of γH2AX, a marker of radiation-induced DNA double-strand break, was examined by immunocytochemistry.

RESULTS

Pretreatment with psammaplin A, 5-aza-2'-deoxycytidine, and zebularine radiosensitized both A549 and U373MG cells. Pretreatment with psammaplin A increased the sub-G1 fraction of A549 cells, as compared to cells exposed to radiation alone. Prolongation of γH2AX expression was observed in the cells treated with DNMT inhibitors prior to radiation as compared with those treated by radiation alone.

CONCLUSIONS

Psammaplin A, 5-aza-2'-deoxycytidine, and zebularine induce radiosensitivity in both A549 and U373MG cell lines, and suggest that this effect might be associated with the inhibition of DNA repair.

Exploring inhibitor release pathways in histone deacetylases using random acceleration molecular dynamics simulations

Molecular channel exploration perseveres to be the prominent solution for eliciting structure and accessibility of active site and other internal spaces of macromolecules. The volume and silhouette characterization of these channels provides answers for the issues of substrate access and ligand swapping between the obscured active site and the exterior of the protein. Histone deacetylases (HDACs) are metal-dependent enzymes that are involved in the cell growth, cell cycle regulation, and progression, and their deregulations have been linked with different types of cancers. Hence HDACs, especially the class I family, are widely recognized as the important cancer targets, and the characterizations of their structures and functions have been of special interest in cancer drug discovery. The class I HDACs are known to possess two different protein channels, an 11 Å and a 14 Å (named channels A and B1, respectively), of which the former is a ligand or substrate occupying tunnel that leads to the buried active site zinc ion and the latter is speculated to be involved in product release. In this work, we have carried out random acceleration molecular dynamics (RAMD) simulations coupled with the classical molecular dynamics to explore the release of the ligand, N-(2-aminophenyl) benzamide (LLX) from the active sites of the recently solved X-ray crystal structure of HDAC2 and the computationally modeled HDAC1 proteins. The RAMD simulations identified significant structural and dynamic features of the HDAC channels, especially the key 'gate-keeping' amino acid residues that control these channels and the ligand release events. Further, this study identified a novel and unique channel B2, a subchannel from channel B1, in the HDAC1 protein structure. The roles of water molecules in the LLX release from the HDAC1 and HDAC2 enzymes are also discussed. Such structural and dynamic properties of the HDAC protein channels that govern the ligand escape reactions will provide further mechanistic insights into the HDAC enzymes, which, in the long run, have a potential to bring new ideas for developing more promising HDAC inhibitors as well as extend our atomic level understandings on their mechanisms of action.

Histone deacetylase inhibitors sensitize human non-small cell lung cancer cells to ionizing radiation through acetyl p53-mediated c-myc down-regulation

INTRODUCTION

Histone deacetylase inhibitors (HDACIs) induce growth arrest and apoptosis in cancer cells. In addition to their intrinsic anticancer properties, HDACIs modulate cellular responses to ionizing radiation (IR). We examined the molecular mechanism(s) associated with the radiosensitizing effects of HDACIs in human lung cancer cells.

METHODS

Lung cancer cells were pretreated with the appropriate concentrations of suberoylanilide hydroxamic acid or trichostatin A. After 2 hours, cells were irradiated with various doses of γ-IR, and then we performed 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, fluorescence-activated cell sorting analysis, clonogenic assay, and Western blotting to detect cell viability or apoptosis and changes of specific proteins expression levels.

RESULTS

In this study, we showed that HDACIs (including suberoylanilide hydroxamic acid and trichostatin A) and IR synergistically trigger cell death in human non-small cell lung cancer cells. Cell viability and clonogenic survival were markedly decreased in cultures cotreated with HDACIs and IR. Interestingly, p53 acetylation at lysine 382 was significantly increased, and c-myc expression simultaneously down-regulated in cotreated cells. Radiosensitization by HDACIs was inhibited on transfection with small interfering RNA against p53 and c-myc overexpression, supporting the involvement of p53 and c-myc in this process. Furthermore, c-myc down-regulation and apoptotic cell death coinduced by IR and HDACI were suppressed in cells transfected with mutant K382R p53 and C135Y p53 displaying loss of acetylation at lysine 382 and DNA-binding activity, respectively.

CONCLUSIONS

Our results collectively demonstrate that the degree of radiosensitization by HDACIs is influenced by acetyl p53-mediated c-myc down-regulation.

Sensitization of tumor cells by targeting histone deacetylases

Epigenetic mechanisms may contribute to drug resistance by interfering with tumor growth regulatory pathways and pro-apoptotic programs. Since gene expression is regulated by acetylation status of histones, a large variety of histone deacetylase (HDAC) inhibitors have been studied as antitumor agents. On the basis of their pro-apoptotic activity, HDAC inhibitors have been combined with conventional antitumor agents or novel target-specific agents to increase susceptibility to apoptosis and drug sensitivity of cancer cells. Several combination strategies including HDAC inhibitors have been explored in preclinical studies. Promising therapeutic effects have been reported in combination with DNA damaging agents, taxanes, targeted agents, death receptor agonists and hormonal therapies. Some histone deacetylases, such as HDAC6, can also modulate the function of non-histone proteins involved in critical regulatory processes which may be relevant as therapeutic targets. Given the pleiotropic effects of most of the available inhibitors, the mechanisms of the sensitization are not completely elucidated. A better understanding of the involved mechanisms will provide a rational basis to improve the therapeutic outcome of the available antitumor agents.

Valproic acid sensitizes human glioma cells for temozolomide and γ-radiation

Temozolomide (TMZ) is given in addition to radiotherapy in glioma patients, but its interaction with the commonly prescribed antiepileptic drug valproic acid (VPA) is largely unknown. Induction of DNA demethylation by VPA could potentially induce expression of the O(6)-methylguanine-DNA-methyltransferase (MGMT) protein, causing resistance to TMZ and thereby antagonizing its effect. Therefore, this study investigates the interaction between VPA, TMZ, and γ-radiation. Two glioma cell lines were used that differ in TMZ sensitivity caused by the absence (D384) or presence (T98) of the MGMT protein. VPA was administered before (24/48 h) or after (24 h) single doses of γ-radiation; or, after 24 h, VPA treatment was accompanied by a single dose of TMZ for another 24 h. For trimodal treatment the combination of VPA and TMZ was followed by single doses of γ-radiation. In both cell lines VPA caused enhancement of the radiation response after preincubation (DMF(0.2) 1.4 and 1.5) but not after postirradiation (DMF(0.2) 1.1 and 1.0). The combination of VPA and TMZ caused enhanced cytotoxicity (DMF(0.2) 1.7) in both the TMZ-sensitive cell line (D384) and the TMZ-resistant cell line (T98). The combination of VPA and TMZ caused a significant radiation enhancement (DMF(0.2) 1.9 and 1.6) that was slightly more effective than that of VPA alone. VPA does not antagonize the cytotoxic effects of TMZ. Preincubation with VPA enhances the effect of both γ-radiation and TMZ, in both a TMZ-sensitive and a TMZ-resistant human glioma cell line. VPA combined with TMZ may lead to further enhancement of the radiation response.

γ-radiation induces cellular sensitivity and aberrant methylation in human tumor cell lines

PURPOSE

Ionizing radiation induces cellular damage through both direct and indirect mechanisms, which may include effects from epigenetic changes. The purpose of this study was to determine the effect of ionizing radiation on DNA methylation patterns that may be associated with altered gene expression.

MATERIALS AND METHODS

Sixteen human tumor cell lines originating from various cancers were initially tested for radiation sensitivity by irradiating them with γ-radiation in vitro and subsequently, radiation sensitive and resistant cell lines were treated with different doses of a demethylating agent, 5-Aza-2'-Deoxycytidine (5-aza-dC) and a chromatin modifier, Trichostatin-A (TSA). Survival of these cell lines was measured using 3-(4, 5-Dimethylthiazol- 2-yl)-2, 5-diphenyltetrazolium (MTT) and clonogenic assays. The effect of radiation on global DNA methylation was measured using reverse phase high performance liquid chromatography (RP-HPLC). The transcription response of methylated gene promoters, from cyclin-dependent kinase inhibitor 2A (p16(INK4a)) and ataxia telangiectasia mutated (ATM) genes, to radiation was measured using a luciferase reporter assay.

RESULTS

γ-radiation resistant (SiHa and MDAMB453) and sensitive (SaOS2 and WM115) tumor cell lines were examined for the relationship between radiation sensitivity and DNA methylation. Treatment of cells with 5-aza-dC and TSA prior to irradiation enhanced DNA strand breaks, G2/M phase arrest, apoptosis and cell death. Exposure to γ-radiation led to global demethylation in a time-dependent manner in tumor cells in relation to resistance and sensitivity to radiation with concomitant activation of p16(INK4a) and ATM gene promoters.

CONCLUSION

These results provide important information on alterations in DNA methylation as one of the determinants of radiation effects, which may be associated with altered gene expression. Our results may help in delineating the mechanisms of radiation resistance in tumor cells, which can influence diagnosis, prognosis and eventually therapy for human cancers.

Therapeutic strategies to enhance the anticancer efficacy of histone deacetylase inhibitors

Histone acetylation is a posttranslational modification that plays a role in regulating gene expression. More recently, other nonhistone proteins have been identified to be acetylated which can regulate their function, stability, localization, or interaction with other molecules. Modulating acetylation with histone deacetylase inhibitors (HDACi) has been validated to have anticancer effects in preclinical and clinical cancer models. This has led to development and approval of the first HDACi, vorinostat, for the treatment of cutaneous T cell lymphoma. However, to date, targeting acetylation with HDACi as a monotherapy has shown modest activity against other cancers. To improve their efficacy, HDACi have been paired with other antitumor agents. Here, we discuss several combination therapies, highlighting various epigenetic drugs, ROS-generating agents, proteasome inhibitors, and DNA-damaging compounds that together may provide a therapeutic advantage over single-agent strategies.

Low-dose valproic acid enhances radiosensitivity of prostate cancer through acetylated p53-dependent modulation of mitochondrial membrane potential and apoptosis

Histone deacetylase inhibitors (HDI) have shown promise as candidate radiosensitizers for many types of cancers, including prostate cancer. However, the mechanisms of action are not well understood. In this study, we show in prostate cancer cells that valproic acid (VPA) at low concentrations has minimal cytotoxic effects yet can significantly increase radiation-induced apoptosis. VPA seems to stabilize a specific acetyl modification (lysine 120) of the p53 tumor suppressor protein, resulting in an increase in its proapoptotic function at the mitochondrial membrane. These effects of VPA are independent of any action of the p53 protein as a transcription factor in the nucleus, since these effects were also observed in native and engineered prostate cancer cells containing mutant forms of p53 protein having no transcription factor activity. Transcription levels of p53-related or Bcl-2 family member proapoptotic proteins were not affected by VPA exposure. The results of this study suggest that, in addition to nuclear-based pathways previously reported, HDIs may also result in radiosensitization at lower concentrations via a specific p53 acetylation and its mitochondrial-based pathway(s).

Rational therapeutic combinations with histone deacetylase inhibitors for the treatment of cancer

Histone deacetylases (HDACs) regulate the acetylation of a variety of histone and nonhistone proteins, controlling the transcription and regulation of genes involved in cell cycle control, proliferation, survival, DNA repair and differentiation. Unsurprisingly, HDAC expression is frequently altered in hematologic and solid tumor malignancies. Two HDAC inhibitors (vorinostat and romidepsin) have been approved by the US FDA for the treatment of cutaneous T-cell lymphoma. As single agents, treatment with HDAC inhibitors has demonstrated limited clinical benefit for patients with solid tumors, prompting the investigation of novel treatment combinations with other cancer therapeutics. In this article, the rationales and clinical progress of several combinations with HDAC inhibitors are presented, including DNA-damaging chemotherapeutic agents, radiotherapy, hormonal therapies, DNA methyltransferase inhibitors and various small-molecule inhibitors. The future application of HDAC inhibitors as a treatment for cancer is discussed, examining current hurdles to overcome before realizing the potential of this new approach.

Induction of cell cycle arrest and DNA damage by the HDAC inhibitor panobinostat (LBH589) and the lipid peroxidation end product 4-hydroxynonenal in prostate cancer cells

Histone deacetylase inhibitors (HDACIs) are promising antineoplastic agents for the treatment of cancer. Here we report that the lipid peroxidation end product 4-hydroxynonenal (HNE) significantly potentiates the anti-tumor effects of the HDAC inhibitor panobinostat (LBH589) in the PC3 prostate cancer cell model. Panobinostat and HNE inhibited proliferation of PC3 cells and the combination of the two agents resulted in a significant combined effect. Cell cycle analysis revealed that both single agents and, to a greater extent, their combined treatment induced G2/M arrest, but cell death occurred in the combined treatment only. Furthermore, HNE and, to a greater extent, the combined treatment induced dephosphorylation of Cdc2 leading to progression into mitosis as confirmed by α-tubulin/DAPI staining and phospho-histone H3 (Ser10) analysis. To evaluate possible induction of DNA damage we utilized the marker phosphorylated histone H2A.X. Results showed that the combination of panobinostat and HNE induced significant DNA damage concomitant with the mitotic arrest. Then, by using androgen receptor (AR)-expressing PC3 cells we observed that the responsiveness to HNE and panobinostat was independent of the expression of functional AR. Taken together, our data suggest that HNE potentiates the antitumoral effect of the HDACI panobinostat in prostate cancer cells.

Novel histone deacetylase inhibitor CG200745 induces clonogenic cell death by modulating acetylation of p53 in cancer cells

Histone deacetylase (HDAC) plays an important role in cancer onset and progression. Therefore, inhibition of HDAC offers potential as an effective cancer treatment regimen. CG200745, (E)-N(1)-(3-(dimethylamino)propyl)-N(8)-hydroxy-2-((naphthalene-1-loxy)methyl)oct-2-enediamide, is a novel HDAC inhibitor presently undergoing a phase I clinical trial. Enhancement of p53 acetylation by HDAC inhibitors induces cell cycle arrest, differentiation, and apoptosis in cancer cells. The purpose of the present study was to investigate the role of p53 acetylation in the cancer cell death caused by CG200745. CG200745-induced clonogenic cell death was 2-fold greater in RKO cells expressing wild-type p53 than in p53-deficient RC10.1 cells. CG200745 treatment was also cytotoxic to PC-3 human prostate cancer cells, which express wild-type p53. CG200745 increased acetylation of p53 lysine residues K320, K373, and K382. CG200745 induced the accumulation of p53, promoted p53-dependent transactivation, and enhanced the expression of MDM2 and p21(Waf1/Cip1) proteins, which are encoded by p53 target genes. An examination of CG200745 effects on p53 acetylation using cells transfected with various p53 mutants showed that cells expressing p53 K382R mutants were significantly resistant to CG200745-induced clonogenic cell death compared with wild-type p53 cells. Moreover, p53 transactivation in response to CG200745 was suppressed in all cells carrying mutant forms of p53, especially K382R. Taken together, these results suggest that acetylation of p53 at K382 plays an important role in CG200745-induced p53 transactivation and clonogenic cell death.

Histone deacetylase inhibition attenuates cell growth with associated telomerase inhibition in high-grade childhood brain tumor cells

Aberrant epigenetic regulation of gene expression contributes to tumor initiation and progression. Studies from a plethora of hematologic and solid tumors support the use of histone deacetylase inhibitors (HDACi) as potent anticancer agents. However, the mechanism of HDACi action with respect to the temporal order of induced cellular events is unclear. The present study investigates the anticancer effects of the HDACi trichostatin A in high-grade childhood brain tumor cells. Acute exposure to trichostatin A resulted in marked inhibition of cell proliferation, an increase in the proportion of G(2)-M cells, activation of H2A.X, and subsequent induction of apoptosis in the majority of cell lines. These phenotypic effects were associated with abrogation of telomerase activity and human telomerase reverse transcriptase downregulation in the majority of cell lines. In contrast, no cytotoxicity was observed in primary ependymal cells with respect to cilia function. Thus, inhibition of histone deacetylases leads to antiproliferative and proapoptotic effects in childhood brain tumor cells, likely to involve altered chromatin regulation at the human telomerase reverse transcriptase promoter.

Targeting epidermal growth factor receptor-associated signaling pathways in non-small cell lung cancer cells: implication in radiation response

Several studies have shown solid evidence for the potential value of targeting epidermal growth factor receptor (EGFR) signaling to enhance the antitumor activity of radiation. However, therapeutic resistance has emerged as an important clinical issue. Here, we investigated whether strategies for targeting EGFR-associated downstream signaling would radiosensitize a panel of non-small cell lung cancer cell lines. Inhibition of K-RAS using RNA interference attenuated downstream signaling and increased radiosensitivity of A549 and H460 cells, whereas inhibition of EGFR did not. A549 cells harboring a K-RAS mutation at codon V12 were radiosensitized by small interfering RNA (siRNA) targeting this codon. H460 cells having mutation at codon V61 was radiosensitized by siRNA targeting of this mutation. K-RAS siRNA did not radiosensitize H1299 cells possessing wild-type K-RAS. Inhibition of the phosphoinositide 3-kinase (PI3K)-AKT-mammalian target of rapamycin pathway led to significant radiosensitization of the two cell lines, whereas selective inhibition of extracellular signal-regulated kinase signaling did not. Inhibitors targeting the PI3K-AKT-mTOR pathway also abrogated G(2) arrest following irradiation and induced gammaH2AX foci formation. A dual inhibitor of class I PI3K and mammalian target of rapamycin effectively increased the radiosensitivity of A549 and H460 cells. Inhibition of PI3K-AKT signaling was associated with the downregulation of DNA-PKs. Although apoptosis was the primary mode of cell death when cells were pretreated with LY294002 or AKT inhibitor VIII, cells pretreated with rapamycin or PI-103 showed mixed modes of cell death, including apoptosis and autophagy. Our results suggest possible mechanisms for counteracting EGFR prosurvival signaling implicated in radioresistance and offer an alternative strategy for overcoming resistance to EGFR inhibitors used in combination with irradiation.

Replication independent DNA double-strand break retention may prevent genomic instability

Background

Global hypomethylation and genomic instability are cardinal features of cancers. Recently, we established a method for the detection of DNA methylation levels at sites close to endogenous DNA double strand breaks (EDSBs), and found that those sites have a higher level of methylation than the rest of the genome. Interestingly, the most significant differences between EDSBs and genomes were observed when cells were cultured in the absence of serum. DNA methylation levels on each genomic location are different. Therefore, there are more replication-independent EDSBs (RIND-EDSBs) located in methylated genomic regions. Moreover, methylated and unmethylated RIND-EDSBs are differentially processed. Euchromatins respond rapidly to DSBs induced by irradiation with the phosphorylation of H2AX, γ-H2AX, and these initiate the DSB repair process. During G0, most DSBs are repaired by non-homologous end-joining repair (NHEJ), mediated by at least two distinct pathways; the Ku-mediated and the ataxia telangiectasia-mutated (ATM)-mediated. The ATM-mediated pathway is more precise. Here we explored how cells process methylated RIND-EDSBs and if RIND-EDSBs play a role in global hypomethylation-induced genomic instability.

Results

We observed a significant number of methylated RIND-EDSBs that are retained within deacetylated chromatin and free from an immediate cellular response to DSBs, the γ-H2AX. When cells were treated with tricostatin A (TSA) and the histones became hyperacetylated, the amount of γ-H2AX-bound DNA increased and the retained RIND-EDSBs were rapidly repaired. When NHEJ was simultaneously inhibited in TSA-treated cells, more EDSBs were detected. Without TSA, a sporadic increase in unmethylated RIND-EDSBs could be observed when Ku-mediated NHEJ was inhibited. Finally, a remarkable increase in RIND-EDSB methylation levels was observed when cells were depleted of ATM, but not of Ku86 and RAD51.

Conclusions

Methylated RIND-EDSBs are retained in non-acetylated heterochromatin because there is a prolonged time lag between RIND-EDSB production and repair. The rapid cellular responses to DSBs may be blocked by compact heterochromatin structure which then allows these breaks to be repaired by a more precise ATM-dependent pathway. In contrast, Ku-mediated NHEJ can repair euchromatin-associated EDSBs. Consequently, spontaneous mutations in hypomethylated genome are produced at faster rates because unmethylated EDSBs are unable to avoid the more error-prone NHEJ mechanisms.

Sensitization of ovarian carcinoma cells to the atypical retinoid ST1926 by the histone deacetylase inhibitor, RC307: enhanced DNA damage response

The synthetic atypical retinoids containing an adamantyl group exhibit antiproliferative or proapoptotic activities. Apoptosis induction is a dose-dependent effect independent of retinoid receptors. We have reported that induction of apoptosis by the atypical retinoid, ST1926, is associated with early manifestations of genotoxic stress. Indeed, in this study performed in ovarian carcinoma cells, we show that exposure to ST1926 resulted in an increase of early markers of DNA damage, including ATM and H2AX phosphorylation. In addition, we found that a novel histone deacetylase (HDAC) inhibitor (RC307) was able to enhance sensitivity of ovarian carcinoma cells to ST1926. Under conditions where single-agent treatment caused only antiproliferative effects, the combination of the atypical retinoid and HDAC inhibitor resulted in marked apoptotic cell death with a more rapid onset in wild-type p53 ovarian carcinoma cells. The sensitization to ST1926-induced apoptosis was associated with an enhanced DNA damage response, because a prolonged expression of DNA damage markers (e.g., H2AX, p53 and RPA-2 phosphorylation) and a marked activation of DNA damage checkpoint kinases (in particular, phosphorylation of Chk1) were observed indicating an accumulation of DNA damage by the ST1926/HDAC inhibitor combination. The study provides additional support to the role of DNA damage as a primary event leading to the activation of apoptosis in ovarian carcinoma cells by adamantyl retinoids and documents the potential therapeutic efficacy of the combination of ST1926 and HDAC inhibitors of the novel series.

Lin28-let7 modulates radiosensitivity of human cancer cells with activation of K-Ras

PURPOSE

To evaluate the potential of targeting Lin28-let7 microRNA regulatory network for overcoming the radioresistance of cancer cells having activated K-Ras signaling.

METHODS AND MATERIALS

A549 lung carcinoma cells and ASPC1 pancreatic cancer cells possessing K-RAS mutation were transfected with pre-let7a microRNA or Lin28 siRNA, respectively. Clonogenic assay, quantitative reverse transcription polymerase chain reaction, and Western analysis were performed. The effects of Lin28 on SQ20B cells having wild-type K-RAS, and a normal fibroblast were also assessed.

RESULTS

The overexpression of let-7a decreased expression of K-Ras and radiosensitized A549 cells. Inhibition of Lin28, a repressor of let-7, attenuated K-Ras expression and radiosensitized A549 and ASPC1 cells. Neither SQ20B cells expressing wild-type K-RAS nor HDF, the normal human fibroblasts, were radiosensitized by this approach.

CONCLUSIONS

The Lin28-let7 regulatory network may be a potentially useful therapeutic target for overcoming the radioresistance of human cancers having activated K-Ras signaling.

HDAC inhibitor-mediated radiosensitization in human carcinoma cells: a general phenomenon?

Histone deacetylase inhibitors (HDIs) have attracted considerable attention for anticancer therapy strategy, including radiosensitization. Regarding a potential application of HDI as a radiosensitizer in the treatment of solid tumors, an important question is whether treatment efficacy would be influenced by intrinsic differences between cancer cells, such as different histologic origin and status of ATM or p53. First we have observed the in vitro radiosensitization by Trichostatin A (TSA) on the broad spectrum of human tumor cell lines having different histologic origin such as HCT116 adenocarcinoma of colon, A549 adenocarcinoma of lung, HN-3 squamous cell carcinoma of head/neck, and HeLa squamous carcinoma of uterine cervix, using clonogenic assay. Next, we have systematically assessed the radiosensitization on the cell lines having different ATM or p53 status. We found that pretreatment of HDI consistently resulted in radiosensitization of all cell lines tested, though the sensitizer enhancement ratio of individual cell lines was variable. We also observed that TSA-mediated radiosensitization was clearly influenced by p53 and ATM status of cells tested. The data presented here indicate that HDI enhances the radiation induced cell killing in the various cancer cells having intrinsic differences and may serve as a general strategy for enhancing tumor cell radiosensitivity. These results have potential implications for the clinical utility of HDI in increasing the anticancer efficacy of radiation.

Radiosensitising agents for the radiotherapy of cancer: novel molecularly targeted approaches

BACKGROUND

The efficacy of radiotherapy (RT) for cancer treatment is limited by normal tissue toxicity and by the intrinsic or acquired radioresistance of many tumours. Therefore, continuing efforts are conducted to identify radiosensitising agents that preferentially sensitise tumour cells to the cytotoxic action of RT. Recent progresses in molecular oncology have uncovered an array of novel targets, which may be exploited for RT enhancement.

OBJECTIVE

To survey the patent literature of the past 4 years pertaining to the development of molecularly targeted agents as potential tumour radiosensitisers.

METHODS

Patents were searched with a set of relevant keywords using several search engines. A Medline search on the same topics was performed in parallel.

RESULTS/CONCLUSION

A total of 48 patents/applications were selected. These concerned agents target molecular components of pathways involved in DNA damage repair, cell growth and survival signalling, apoptosis modulation and tumour angiogenesis. Current trials of some of these agents may reveal their value as clinical radiosensitisers.

Effects of sodium butyrate on cell death induced by photodynamic therapy in U373-MG and D54-MG astrocytoma cell lines

The damage induced by end products of photodynamic therapy (PDT) in astrocytoma tumors leads to cytotoxicity and cell death. Chromatin modifiers such as sodium butyrate (NaB) induce several genes involved in apoptosis, among others. The PDT improvement was evaluated by the measurement of its effectiveness in the treatment of U373-MG and D54-MG astrocytoma cell lines exposed to NaB. Cells exposed to 80 microg mL(-1) of delta-aminolevulinic acid (ALA) as precursor of endogenous photosensitizer (PS), protoporphyrin IX (PpIX), induced 16.67% and 28.9% of mortality in U373-MG and D54-MG, respectively. The mortality increased to 70.62% and 96.7%, respectively, when U373-MG and D54-MG cells were exposed for 24 h to 8 mm NaB prior to ALA-induction. In this condition, re-expression of some genes related to apoptosis in U373-MG, and differentiation in D54-MG were induced. PpIX accumulation was higher than ALA-induction and the acetylation of histone H4 induced by NaB was verified by immunocytochemistry in both cells. It can be concluded that modified chromatin and genes induced by NaB increment the cellular death induced by PDT in astrocytoma cells using PpIX as endogenous PS.

HDACi--targets beyond chromatin

Histone deacetylases (HDACs) play an important role in gene regulation. Inhibitors of HDACs (HDACi) are novel anti-cancer drugs, which induce histone (hyper-) acetylation and counteract aberrant gene repression. On the other hand, HDACi treatment can also result in decreased gene expression, and targeting HDACs affects more than chromatin. Recently, HDACi were shown to evoke non-histone protein acetylation, which can alter signaling networks relevant for tumorgenesis. Furthermore, HDACi can promote the degradation of (proto-) oncoproteins. Here, we summarize these findings and discuss how these substances could be beneficial for the treatment and prevention of human ailments, such as cancer and unbalanced immune functions.

Splitomicin suppresses human platelet aggregation via inhibition of cyclic AMP phosphodiesterase and intracellular Ca++ release

Splitomicin is derived from beta-naphthol and is an inhibitor of Silent Information Regulator 2 (SIR2). Its naphthoic moiety might be responsible for its inhibitory effects on platelets. The major goal of our study was to examine possible mechanisms of action of splitomicin on platelet aggregation in order to promote development of a novel anti-platelet aggregation therapy for cardiovascular and cerebrovascular diseases. To study the inhibitory effects of splitomicin on platelet aggregation, we used washed human platelets, and monitored platelet aggregation and ATP release induced by thrombin (0.1 U/ml), collagen (2 microg/ml), arachidonic acid (AA) (0.5 mM), U46619 (2 microM) or ADP (10 microM). Splitomicin inhibited platelet aggregation induced by thrombin, collagen, AA and U46619 with a concentration dependent manner. Splitomicin increased cAMP and this effect was enhanced when splitomicin (150 microM) was combined with PGE1 (0.5 microM). It did not further increase cAMP when combined with IBMX. This data indicated that splitomicin increases cAMP by inhibiting activity of phosphodiestease. In addition, splitomicin (300 microM) attenuated intracellular Ca(++) mobilization, and production of thromboxane B2 (TXB2) in platelets that was induced by thrombin, collagen, AA or U46619. The inhibitory mechanism of splitomicin on platelet aggregation may increase cyclic AMP levels via inhibition of cyclic AMP phosphodiesterase activity and subsequent inhibition of intracellular Ca(++) mobilization, TXB2 formation and ATP release.

Specific activity of class II histone deacetylases in human breast cancer cells

Although numerous studies have underlined the role of histone deacetylases (HDAC) in breast physiology and tumorigenesis, little is known on the particular contribution of the various classes of HDACs in these processes. Using estrogen receptor-alpha (ERalpha)-positive MCF-7 breast cancer cells, the effects of MC1575 and MC1568, two novel class II-specific HDAC inhibitors, were analyzed on cell proliferation, apoptosis, and estrogen signaling. The specificity of these HDAC inhibitors was validated by measuring histone and alpha-tubulin acetylation and by the specific in vitro inhibition of recombinant HDAC4 using histone and nonhistone substrates, contrasting with the lack of inhibition of class I HDACs. In addition, MC1575 did not inhibit class I HDAC gene expression, thus confirming the specific targeting of class II enzymes. Similar to trichostatin A (TSA), MC1575 displayed a dose-dependent antiproliferative effect and induced cell cycle arrest although this blockade occurred at a different level than TSA. Moreover, and in contrast to TSA, MC1575 had no effect on MCF-7 cells apoptosis. Interestingly, MC1575 was able to increase p21(waf1/CIP1) mRNA levels but did not regulate the expression of other genes such as cyclin D1, p27, p14(ARF), Bcl2, Baxalpha, Trail-R1, and Trail-R2. Finally, MC1575 strongly induced ERbeta gene expression but did not decrease ERalpha expression, nor did it switch hydroxytamoxifen to an agonist activity. Altogether, these data suggest that the class II HDAC subfamily may exert specific roles in breast cancer progression and estrogen dependence.

Postradiation sensitization of the histone deacetylase inhibitor valproic acid

PURPOSE

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

EXPERIMENTAL DESIGN

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

RESULTS

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

CONCLUSIONS

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