Depsipeptide-resistant KU812 cells show reversible P-glycoprotein expression, hyper-acetylated histones, and modulated gene expression profile

The Methyltransferases METTL7A and METTL7B Confer Resistance to Thiol-Based Histone Deacetylase Inhibitors

Histone deacetylase inhibitors (HDACi) are part of a growing class of epigenetic therapies used for the treatment of cancer. Although HDACis are effective in the treatment of T-cell lymphomas, treatment of solid tumors with this class of drugs has not been successful. Overexpression of the multidrug resistance protein P-glycoprotein (P-gp), encoded by ABCB1, is known to confer resistance to the HDACi romidepsin in vitro, yet increased ABCB1 expression has not been associated with resistance in patients, suggesting that other mechanisms of resistance arise in the clinic. To identify alternative mechanisms of resistance to romidepsin, we selected MCF-7 breast cancer cells with romidepsin in the presence of the P-gp inhibitor verapamil to reduce the likelihood of P-gp-mediated resistance. The resulting cell line, MCF-7 DpVp300, does not express P-gp and was found to be selectively resistant to romidepsin but not to other HDACis such as belinostat, panobinostat, or vorinostat. RNA-sequencing analysis revealed upregulation of the mRNA coding for the putative methyltransferase, METTL7A, whose paralog, METTL7B, was previously shown to methylate thiol groups on hydrogen sulfide and captopril. As romidepsin has a thiol as the zinc-binding moiety, we hypothesized that METTL7A could inactivate romidepsin and other thiol-based HDACis via methylation of the thiol group. We demonstrate that expression of METTL7A or METTL7B confers resistance to thiol-based HDACis and that both enzymes are capable of methylating thiol-containing HDACis. We thus propose that METTL7A and METTL7B confer resistance to thiol-based HDACis by methylating and inactivating the zinc-binding thiol.

Transcriptome Profiling Analysis Identifies LCP1 as a Contributor for Chidamide Resistance in Gastric Cancer

BACKGROUND

Gastric cancer (GC) remains a significant health problem and carries with it substantial morbidity and mortality. Chidamide is a novel and orally administered histone deacetylase (HDAC) inhibitor and has been demonstrated its anti-tumor efficacy on different kinds of hematological and solid tumors. However, the underlying mechanism of chidamide resistance is still poorly characterized.

METHODS

We established chidamide resistant GC cell lines, AGS ChiR and MGC803 ChiR and investigated the toxicologic effects through cell survival, colony formation and flow cytometry assays in vitro, and a subcutaneous xenograft model in vivo. RNA-sequence was then performed to screen chidamide resistance-associated genes between AGS and AGS ChiR cells. The role of Lymphocyte cytosolic protein 1 (LCP1) in chidamide resistance was explored by gain- and loss-of-function analyses.

RESULTS

We found that chidamide significantly inhibited cell proliferation and induced the apoptosis in a concentration-dependent manner in wild-type GC cell lines as compared to chidamide resistant cell lines. The transcriptomic profiling, quantitative RT-PCR, and western blot data revealed that LCP1 was upregulated in AGS ChiR cells compared with parental cells. Overexpression of LCP1 conferred and knockdown of LCP1 attenuated the chidamide resistance of GC cells. Epigenetic derepression of LCP1 by chidamide may be a possible reason for the contribution of LCP1 to chidamide resistance.

CONCLUSIONS

These findings illustrated that LCP1 may play a chidamide resistance role in GC, suggesting that LCP1 could be a potential target for the therapy of GC combined with chidamide.

Dual Inhibition of Histone Deacetylases and the Mechanistic Target of Rapamycin Promotes Apoptosis in Cell Line Models of Uveal Melanoma

Purpose

Over 90% of uveal melanomas harbor pathogenic variants of the GNAQ or GNA11 genes that activate survival pathways. As previous studies found that Ras-mutated cell lines were vulnerable to a combination of survival pathway inhibitors and the histone-deacetylase inhibitor romidepsin, we investigated whether this combination would be effective in models of uveal melanoma.

Methods

A small-scale screen of inhibitors of bromodomain-containing protein 4 (BRD4; OTX-015), extracellular signal-related kinase (ERK; ulixertinib), mechanistic target of rapamycin (mTOR; AZD-8055), or phosphoinositide 3-kinase (PI3K; GDC-0941) combined with a clinically relevant administration of romidepsin was performed on a panel of uveal melanoma cell lines (92.1, Mel202, MP38, and MP41) and apoptosis was quantified by flow cytometry after 48 hours. RNA sequencing analysis was performed on Mel202 cells treated with romidepsin alone, AZD-8055 alone, or the combination, and protein changes were validated by immunoblot.

Results

AZD-8055 with romidepsin was the most effective combination in inducing apoptosis in the cell lines. Increased caspase-3 and PARP cleavage were noted in the cell lines when they were treated with romidepsin and mTOR inhibitors. RNA sequencing analysis of Mel202 cells revealed that apoptosis was the most affected pathway in the romidepsin/AZD-8055-treated cells. Increases in pro-apoptotic BCL2L11 and decreases in anti-apoptotic BIRC5 and BCL2L1 transcripts noted in the sequencing analysis were confirmed at the protein level in Mel202 cells.

Conclusions

Our data suggest that romidepsin in combination with mTOR inhibition could be an effective treatment strategy against uveal melanoma due in part to changes in apoptotic proteins.

Emerging roles of dehydrogenase/reductase member 2 (DHRS2) in the pathology of disease

Dehydrogenase/reductase member 2 (DHRS2) belongs to the short-chain dehydrogenase/reductase (SDR) family. It was initially isolated from the nuclear extract of hepatocellular carcinoma HepG2 cells and was identified as a specific cell cycle regulator. DHRS2 is a reduced nicotinamide adenine dinucleotide phosphate (NADPH)-dependent carbonyl reductase and catalyzes the reduction of dicarbonyl compounds. It is also functionally active in lipid metabolism and acts as a metabolic enzyme of hormones. Recent studies have shown that DHRS2 reprograms lipid metabolism and redox homeostasis to regulate proliferation, migration, invasion, and drug resistance of cancer cells. Here, we describe the structure, organelle localization and function of DHRS2, and also highlight its roles in the pathologic progression of diseases.

The Selective Class IIa Histone Deacetylase Inhibitor TMP195 Resensitizes ABCB1- and ABCG2-Overexpressing Multidrug-Resistant Cancer Cells to Cytotoxic Anticancer Drugs

Multidrug resistance caused by the overexpression of the ATP-binding cassette (ABC) proteins in cancer cells remains one of the most difficult challenges faced by drug developers and clinical scientists. The emergence of multidrug-resistant cancers has driven efforts from researchers to develop innovative strategies to improve therapeutic outcomes. Based on the drug repurposing approach, we discovered an additional action of TMP195, a potent and selective inhibitor of class IIa histone deacetylase. We reveal that in vitro TMP195 treatment significantly enhances drug-induced apoptosis and sensitizes multidrug-resistant cancer cells overexpressing ABCB1 or ABCG2 to anticancer drugs. We demonstrate that TMP195 inhibits the drug transport function, but not the protein expression of ABCB1 and ABCG2. The interaction between TMP195 with these transporters was supported by the TMP195-stimulated ATPase activity of ABCB1 and ABCG2, and by in silico docking analysis of TMP195 binding to the substrate-binding pocket of these transporters. Furthermore, we did not find clear evidence of TMP195 resistance conferred by ABCB1 or ABCG2, suggesting that these transporters are unlikely to play a significant role in the development of resistance to TMP195 in cancer patients.

Dehydrogenase/reductase SDR family member 2 silencing sensitizes an oxaliplatin‑resistant cell line to oxaliplatin by inhibiting excision repair cross‑complementing group 1 protein expression

Oxaliplatin (Oxa)-based chemotherapy is widely used as the first-line treatment for colorectal cancer (CRC). However, Oxa-resistance is common for many postoperative CRC patients. To explore drug resistance in CRC, an Oxa-resistant cell line, HCT116/Oxa, was established from parental HCT116 cells. These Oxa-resistant cells exhibited characteristics of epithelial-mesenchymal transition (EMT) and a higher migratory capacity than parental cells. Protein profiles of HCT116/Oxa and HCT116 cells were compared using a tandem mass tag-based quantitative proteomics technique. The protein dehydrogenase/reductase SDR family member 2 (DHRS2) was revealed to be highly expressed in HCT116/Oxa cells. Silencing of DHRS2 in HCT116/Oxa cells effectively restored Oxa-sensitivity by suppressing the expression of excision repair cross-complementing group 1 protein via a p53-dependent pathway, and reversed the EMT phenotype. Overall, the suppression of DHRS2 expression may be a promising strategy for the prevention of Oxa-resistance in CRC.

HOXA13 contributes to gastric carcinogenesis through DHRS2 interacting with MDM2 and confers 5-FU resistance by a p53-dependent pathway

5-FU-based chemotherapy is recently most recommended as the first-line treatment for gastric cancer (GC). However, 5-FU resistance is common for many postoperative GC patients. Homeobox A13 (HOXA13) is a member of homeobox genes highly expressed in many human tumors. Its potential roles and mechanisms of resistance to 5-FU in GC are poorly understood. In this study, we discovered that HOXA13 played an oncogenic role in vivo and in vitro. The patients with HOXA13 overexpression were closely related with poor prognosis and more prone to be resistant to 5-FU. Moreover, dehydrogenase/reductase 2 (DHRS2) was identified as a downstream gene of HOXA13. HOXA13 played a role of carcinogenesis through directly down-regulating DHRS2 to increase MDM2. Furthermore, HOXA13 conferred 5-FU resistance through MRP1 by a p53-dependent pathway. Therefore, HOXA13 might serve as a potential signature that recognized patients who were insensitive to 5-FU, and timely recommended them to other chemotherapy regimens.

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

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

HDAC inhibitor-induced drug resistance involving ATP-binding cassette transporters (Review)

Histone deacetylase (HDAC) inhibitors are becoming a novel and promising class of antineoplastic agents that have been used for cancer therapy in the clinic. Two HDAC inhibitors, vorinostat and romidepsin, have been approved by the Food and Drug Administration to treat T-cell lymphoma. Nevertheless, similar to common anticancer drugs, HDAC inhibitors have been found to induce multidrug resistance (MDR), which is an obstacle for the success of chemotherapy. The most common cause of MDR is considered to be the increased expression of adenosine triphosphate binding cassette (ABC) transporters. Numerous studies have identified that the upregulation of ABC transporters is often observed following treatment with HDAC inhibitors, particularly the increased expression of P-glycoprotein, which leads to drug efflux, reduces intracellular drug concentration and induces MDR. The present review summarizes the key ABC transporters involved in MDR following various HDAC inhibitor treatments in a range of cancer cell lines and also explored the potential mechanisms that result in MDR, including the effect of nuclear receptors, which are the upstream regulatory factors of ABC transporters.

MAPK pathway activation leads to Bim loss and histone deacetylase inhibitor resistance: rationale to combine romidepsin with an MEK inhibitor

To identify molecular determinants of histone deacetylase inhibitor (HDI) resistance, we selected HuT78 cutaneous T-cell lymphoma (CTCL) cells with romidepsin in the presence of P-glycoprotein inhibitors to prevent transporter upregulation. Resistant sublines were 250- to 385-fold resistant to romidepsin and were resistant to apoptosis induced by apicidin, entinostat, panobinostat, belinostat, and vorinostat. A custom TaqMan array identified increased insulin receptor (INSR) gene expression; immunoblot analysis confirmed increased protein expression and a four- to eightfold increase in mitogen-activated protein kinase (MAPK) kinase (MEK) phosphorylation in resistant cells compared with parental cells. Resistant cells were exquisitely sensitive to MEK inhibitors, and apoptosis correlated with restoration of proapoptotic Bim. Romidepsin combined with MEK inhibitors yielded greater apoptosis in cells expressing mutant KRAS compared with romidepsin treatment alone. Gene expression analysis of samples obtained from patients with CTCL enrolled on the NCI1312 phase 2 study of romidepsin in T-cell lymphoma suggested perturbation of the MAPK pathway by romidepsin. Immunohistochemical analysis of Bim expression demonstrated decreased expression in some skin biopsies at disease progression. These findings implicate increased activation of MEK and decreased Bim expression as a resistance mechanism to HDIs, supporting combination of romidepsin with MEK inhibitors in clinical trials.

Toxicological and metabolic considerations for histone deacetylase inhibitors

INTRODUCTION

Vorinostat and romidepsin were the first histone deacetylase (HDAC) inhibitors (HDi) that fulfilled the preclinical promise of anticancer potential in clinical trials. Nevertheless, they merely opened a new chapter in the history of cancer therapy. Demonstration of their antitumor activity was a straightforward task in in vitro setting. Proving their efficacy in vivo was much more difficult, since the effects of an administrated drug strongly depend on its absorption, distribution, metabolism and excretion.

AREAS COVERED

This article summarizes clinical data on the pharmacokinetic properties of HDi that are currently at more advanced stages of clinical development. Specific attention is paid to the metabolic pathways. Moreover, a comprehensive overview of HDi-related adverse effects is given.

EXPERT OPINION

At this moment, HDi form one of the most interesting classes of therapeutics, yet their efficacy and safety profiles could still be improved by i) designing better formulations, ii) more extensive characterization of their disposition at the preclinical stage, iii) targeting of individual disease-related deacetylase isoforms and/or their complexes, iv) selecting a target patient population with the highest probability of response based on molecular signatures.

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

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

Histone deacetylase inhibition restores cisplatin sensitivity of Hodgkin's lymphoma cells

Increasing evidence suggest that epigenetic mechanisms (e.g. histone modification by histone deacetylases) play a major role in the pathogenesis of Hodgkin's lymphoma (HL). We treated HL cell lines with the histone deacetylase inhibitor vorinostat and investigated the gene expression profile of these cells by using DNA microarrays. Vorinostat inhibited cell proliferation and induced chances in the gene expression profile of HL cells, including down regulation of interleukin-26 and CD30. Vorinostat also increased sensitivity for cisplatin. Our data suggest that the combination of vorinostat and chemotherapy might be an interesting option for patients with chemoresistant HL.

Mechanisms of resistance to histone deacetylase inhibitors

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

HDAC inhibitors in parasitic diseases

Parasitic diseases cause significant global morbidity and mortality, particularly in underdeveloped regions of the world. Malaria alone causes ~800000 deaths each year, with children and pregnant women being at highest risk. There is no licensed vaccine available for any human parasitic disease and drug resistance is compromising the efficacy of many available anti-parasitic drugs. This is driving drug discovery research on new agents with novel modes of action. Histone deacetylase (HDAC) inhibitors are being investigated as drugs for a range of diseases, including cancers and infectious diseases such as HIV/AIDS, and several parasitic diseases. This review focuses on the current state of knowledge of HDAC inhibitors targeted to the major human parasitic diseases malaria, schistosomiasis, trypanosomiasis, toxoplasmosis and leishmaniasis. Insights are provided into the unique challenges that will need to be considered if HDAC inhibitors are to be progressed towards clinical development as potential new anti-parasitic drugs.

Histone deacetylase inhibitors: emerging mechanisms of resistance

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

Activation of STAT5 confers imatinib resistance on leukemic cells through the transcription of TERT and MDR1

We used two imatinib resistant cell lines, K562-ADM cells, which over-express P-glycoprotein (a product of the ABCB1 gene, more commonly known as MDR1), and K562-hTERT cells, which over-express the telomerase reverse transcriptase (TERT), as models to show that the acquisition of multidrug resistance in CML is associated with the enhanced phosphorylation of signal transducer and activator of transcription 5 (STAT5). The induction of P-glycoprotein expression that occurred in response to adriamycin treatment was accompanied by increased phosphorylation of BCR-ABL and STAT5, as well as increased telomerase protein expression. Intriguingly, a ChIP assay using an anti-STAT5 antibody revealed direct binding of STAT5 to the promoter regions of both the human TERT gene and the MDR1 gene in K562-ADM cells. Conversely, silencing of endogenous STAT5 expression by siRNA significantly reduced both the expression of P-glycoprotein and telomerase activity and resulted in the recovery of the imatinib sensitivity of K562-ADM cells. These findings indicate a critical role for STAT5 in the induction of P-glycoprotein and in the modulation of telomerase activity in drug-resistant CML cells. Furthermore, primary leukemic cells obtained from patients in blast crisis showed increased levels of phospho-STAT5, P-glycoprotein and telomerase. In contrast, none of these proteins were detectable in the cells obtained from patients in the chronic phase. Together, these findings indicate a novel mechanism that contributes toward multidrug resistance involving STAT5 as a sensor for cytotoxic drugs in CML patients.

Recent advances in histone deacetylase targeted cancer therapy

Epigenetic regulators such as histone acetyltransferases (HATs) and histone deacetylases (HDACs) are known to play an important role in gene expression. Of these enzymes, HDACs have been shown to be commonly associated with many types of cancers and to affect cancer development. Consequently, HDACs have been considered as promising targets for cancer therapy. In addition, the inhibition of HDACs by histone deacetylase inhibitors (HDACIs) shifts the balance between the deacetylating activity of HDACs and the acetylating activity of HATs in the regulation of gene expression. Therefore, HDACIs are an exciting new addition in cancer therapies. Numerous HDACIs have been identified and some have recently been used in clinical trials for cancer treatment, although the mechanisms underlying the anticancer effects of HDACIs remain unclear. In this review, we examine the most recent developments in HDACIs and various aspects of HDAC-targeted cancer treatment.

Laboratory correlates for a phase II trial of romidepsin in cutaneous and peripheral T-cell lymphoma

Romidepsin has shown promise in the treatment of T-cell lymphomas, and so we evaluated molecular endpoints gathered from 61 patients enrolled on a phase II trial of romidepsin in cutaneous and peripheral T-cell lymphoma at the National Institutes of Health. The endpoints included histone H3 acetylation and ABCB1 gene expression in peripheral blood mononuclear cells (PBMCs); ABCB1 gene expression in tumour biopsy samples; and blood fetal haemoglobin levels (HbF), all of which were increased following romidepsin treatment. The fold increase in histone acetylation in PBMCs at 24 h was weakly to moderately well correlated with the pharmacokinetic parameters C(max) and area under the curve (AUC)(last) (rho = 0.37, P = 0.03 and rho = 0.36, P = 0.03 respectively) and inversely associated with clearance (rho = -0.44; P = 0.03). Histone acetylation in PBMCs at 24 h was associated with response (P = 0.026) as was the increase in fetal haemoglobin (P = 0.014); this latter association may be due to the longer on-study duration for patients with disease response. Together, these results suggest that pharmacokinetics may be an important determinant of response to histone deacetylase inhibitors (HDIs) - the association with histone acetylation in PBMCs at 24 h is consistent with a hypothesis that potent HDIs are needed for a critical threshold of drug exposure and durable activity.

Gene expression-signature of belinostat in cell lines is specific for histone deacetylase inhibitor treatment, with a corresponding signature in xenografts

Belinostat is a hydroxamate-type histone deactylase inhibitor (HDACi), which has recently entered phase I and II clinical trials. Microarray-based analysis of belinostat-treated cell lines showed an impact on genes associated with the G2/M phase of the cell cycle and downregulation of the aurora kinase pathway. Expression of 25 dysregulated genes was measured in eight differentially sensitive cell lines using a novel high-throughput assay that combines multiplex reverse transcriptase-PCR and fluorescence capillary electrophoresis. Sensitivity to belinostat and the magnitude of changes in overall gene modulation were significantly correlated. A belinostat-gene profile was specific for HDACi in three cell lines when compared with equipotent concentrations of four mechanistically different chemotherapeutic agents: 5-fluorouracil, cisplatin, paclitaxel, and thiotepa. Belinostat- and trichostatin A (HDACi)-induced gene responses were highly correlated with each other, but not with the limited changes in response to the other non-HDACi agents. Moreover, belinostat treatment of mice bearing human xenografts showed that the preponderance of selected genes were also modulated in vivo, more extensively in a drug-sensitive tumor than a more resistant model. We have demonstrated a gene signature that is selectively regulated by HDACi when compared with other clinical agents allowing us to distinguish HDACi responses from those related to other mechanisms.

Segregation of megakaryocytic or erythroid cells from a megakaryocytic leukemia cell line (JAS-R) by adhesion during culture

Adhesion is one of the important biologic characteristics of leukemic cells. We previously reported a new megakaryocytic-erythroid cell line, JAS-R. In this study, JAS-R cells were segregated into two types by the differences of attachment to culture dishes. One type (designated as JAS-RAD cells) adhered to the substratum of the culture dishes, while the other (JAS-REN cells) grew as a single-cell suspension. Adhesion of JAS-RAD was inhibited by treatment with RGDS oligopeptide. Flow cytometric analysis revealed that JAS-RAD cells had high expression of CD41a and CD61 versus low CD235a expression, and JAS-REN showed low expression of CD41a, and CD61, and high CD235a. The two phenotypes were reciprocally exchangeable by selecting adherent or suspended cells from each type of culture. Microarray analysis and RT-PCR revealed that JAS-RAD cells expressed four major alpha-granule genes and JAS-REN cells expressed beta-globin. Interestingly, erythropoietin was only secreted by JAS-RAD cells. With regard to transcription factors, it was shown that GFI1, FLI1 and RUNX1 were strongly expressed in JAS-RAD cells while GATA1, FOG1 and NFE2 were equally expressed by both types. These findings indicate that adhesion via integrins is related to the phenotypic shift of JAS-R cells between megakaryocytic and erythroid lineages.

The histone deacetylase inhibitor trichostatin A downregulates human MDR1 (ABCB1) gene expression by a transcription-dependent mechanism in a drug-resistant small cell lung carcinoma cell line model

Tumour drug-resistant ABCB1 gene expression is regulated at the chromatin level through epigenetic mechanisms. We examined the effects of the histone deacetylase inhibitor trichostatin A (TSA) on ABCB1 gene expression in small cell lung carcinoma (SCLC) drug-sensitive (H69WT) or etoposide-resistant (H69VP) cells. We found that TSA induced an increase in ABCB1 expression in drug-sensitive cells, but strongly decreased it in drug-resistant cells. These up- and downregulations occurred at the transcriptional level. Protein synthesis inhibition reduced these modulations, but did not completely suppress them. Differential temporal patterns of histone acetylation were observed at the ABCB1 promoter: increase in H4 acetylation in both cell lines, but different H3 acetylation with a progressive increase in H69WT cells but a transient one in H69VP cells. ABCB1 regulations were not related with the methylation status of the promoter -50GC, -110GC, and Inr sites, and did not result in further changes to these methylation profiles. Trichostatin A treatment did not modify MBD1 binding to the ABCB1 promoter and similarly increased PCAF binding in both H69 cell lines. Our results suggest that in H69 drug-resistant SCLC cell line TSA induces downregulation of ABCB1 expression through a transcriptional mechanism, independently of promoter methylation, and MBD1 or PCAF recruitment.

Will broad-spectrum histone deacetylase inhibitors be superseded by more specific compounds?

Histone deacetylase (HDAC) inhibitors can induce differentiation, cell cycle and growth arrest or in certain cases apoptosis in cancer cells. In a remarkably short period of time, especially considering that their mechanism of action remains largely undefined, HDAC inhibitors have realized both success and failure as therapeutics for cancer in clinical trials. Notably, the pleiotropic HDAC inhibitors, suberoylanilide hydroxamic acid (SAHA) and depsipeptide, have shown efficacy in a wide range of cancers, in particular for cutaneous T-cell lymphoma (CTCL), and are progressing in phase II clinical studies. However, evidence is accumulating that specific HDAC enzymes are important with respect to clinical efficacy, calling the usefulness of the classical inhibitors into question. Class I enzymes are being heralded as the most clinically relevant, however, this is still controversial and much of the information is in the private domain. Nevertheless, the potential to alter the expression of a more focused, disease-related subset of genes and to limit adverse effects has prompted the development of isoform-specific HDAC inhibitors. Here, we consider the growing view that broad-spectrum HDAC inhibitors may be superseded by more specific compounds.

The antiepileptic and anticancer agent, valproic acid, induces P-glycoprotein in human tumour cell lines and in rat liver

BACKGROUND AND PURPOSE

The antiepileptic drug valproic acid, a histone deacetylase (HDAC) inhibitor, is currently being tested as an anticancer agent. However, HDAC inhibitors may interact with anticancer drugs through induction of P-glycoprotein (P-gp, MDR1) expression. In this study we assessed whether valproic acid induces P-gp function in tumour cells. We also investigated effects of valproic acid on the mRNA for P-gp and the cytochrome P450, CYP3A, in rat livers.

EXPERIMENTAL APPROACH

Effects of valproic acid on P-gp were assessed in three tumour cell lines, SW620, KG1a and H4IIE. Accumulation of acetylated histone H3 in rats' livers treated for two or seven days with valproic acid was evaluated using a specific antibody. Hepatic expression of the P-gp genes, mdr1a, mdr1b and mdr2, was determined by real-time polymerase chain reaction. The effects of valproic acid on CYP3A were assessed by Northern blot analysis and CYP3A activity assays.

KEY RESULTS

Valproic acid (0.5-2.0 mM) induced P-gp expression and function up to 4-fold in vitro. The effect of a series of valproic acid derivatives on P-gp expression in SW620 and KG1a cells correlated with their HDAC inhibition potencies. Treatment of rats with 1 mmol kg(-1) valproic acid for two and seven days increased hepatic histone acetylation (1.3- and 3.5-fold, respectively) and the expression of mdr1a and mdr2 (2.2-4.1-fold). Valpromide (0.5-2.0 mM) did not increase histone acetylation or P-gp expression in rat livers, but induced CYP3A expression.

CONCLUSIONS

Valproic acid increased P-gp expression and function in human tumour cell lines and in rat liver. The clinical significance of this increase merits further investigation.

Defining the role of gene regulation in resistance to HDAC inhibitors—Mechanisms beyond P-glycoprotein

The manuscript by Yamada et al. "Depsipeptide-resistant KU812 cells show reversible P-glycoprotein expression, hyperacetylated histones, and modulated gene expression profile," describes not only development of the major resistance mechanism, P-glycoprotein expression, but how histone deacetylase inhibitors can modulate the gene expression profile to promote survival and resistance to depsipeptide.