Differential effects of histone deacetylase inhibitors in tumor and normal cells-what is the toxicological relevance?

Revisiting Histone Deacetylases in Human Tumorigenesis: The Paradigm of Urothelial Bladder Cancer

Urinary bladder cancer is a common malignancy, being characterized by substantial patient mortality and management cost. Its high somatic-mutation frequency and molecular heterogeneity usually renders tumors refractory to the applied regimens. Hitherto, methotrexate-vinblastine-adriamycin-cisplatin and gemcitabine-cisplatin represent the backbone of systemic chemotherapy. However, despite the initial chemosensitivity, the majority of treated patients will eventually develop chemoresistance, which severely reduces their survival expectancy. Since chromatin regulation genes are more frequently mutated in muscle-invasive bladder cancer, as compared to other epithelial tumors, targeted therapies against chromatin aberrations in chemoresistant clones may prove beneficial for the disease. “Acetyl-chromatin” homeostasis is regulated by the opposing functions of histone acetyltransferases (HATs) and histone deacetylases (HDACs). The HDAC/SIRT (super-)family contains 18 members, which are divided in five classes, with each family member being differentially expressed in normal urinary bladder tissues. Since a strong association between irregular HDAC expression/activity and tumorigenesis has been previously demonstrated, we herein attempt to review the accumulated published evidences that implicate HDACs/SIRTs as critical regulators in urothelial bladder cancer. Moreover, the most extensively investigated HDAC inhibitors (HDACis) are also analyzed, and the respective clinical trials are also described. Interestingly, it seems that HDACis should be preferably used in drug-combination therapeutic schemes, including radiation.

Effect of Genistein in Comparison with Trichostatin A on Reactivation of DNMTs Genes in Hepatocellular Carcinoma

Background and Aims: DNA methylation and histone modification are epigenetic modifications essential for normal function of mammalian cells. The processes are mediated by biochemical interactions between DNA methyltransferases (DNMTs) and histone deacetylases. Promoter hypermethylation and deacetylation of tumor suppressor genes play major roles in cancer induction, through transcriptional silencing of these genes. DNA hypermethylation is carried out by a family of DNMTs including DNMT1, DNMT3a and DNMT3b. In hepatocellular carcinoma, a significant positive correlation between over-expression of these genes and cancer induction has been reported. The DNA demethylating agent genistein (GE) has been demonstrated to reduce different cancers. Previously, we reported that GE can induce apoptosis and inhibit proliferation in hepatocellular carcinoma PLC/PRF5 and HepG2 cell lines. Besides, histone deacetylase inhibitors, such as trichostatin A (TSA), were successfully used to inhibit cancer cell growth. The present study was designed to assess the effect of GE in comparison with TSA on DNMT1, DNMT3a and DNMT3b gene expression, cell growth inhibition and apoptosis induction in the HepG2 cell line. Methods: Cells were seeded and treated with various doses of GE and TSA. The MTT assay, flow cytometry assay, and real-time RT-PCR were used to determine viability, apoptosis, and DNMT1, DNMT3a and DNMT3b gene expression respectively. Results: Both agents inhibited cell growth, induced apoptosis and reactivated DNMT1, DNMT3a and DNMT3b gene expression. Furthermore, TSA demonstrated a significantly greater apoptotic effect than the other agent, whereas GE improved gene expression more significantly than TSA. Conclusions: Our findings suggest that GE and TSA can significantly inhibit cell growth, induce apoptosis and restore DNMT1, DNMT3a and DNMT3b gene reactivation.

Systems biology applications to study mechanisms of human immunodeficiency virus latency and reactivation

Eradication of human immunodeficiency virus (HIV) in infected individuals is currently not possible because of the presence of the persistent cellular reservoir of latent infection. The identification of HIV latency biomarkers and a better understanding of the molecular mechanisms contributing to regulation of HIV expression might provide essential tools to eliminate these latently infected cells. This review aims at summarizing gene expression profiling and systems biology applications to studies of HIV latency and eradication. Studies comparing gene expression in latently infected and uninfected cells identify candidate latency biomarkers and novel mechanisms of latency control. Studies that profiled gene expression changes induced by existing latency reversing agents (LRAs) highlight uniting themes driving HIV reactivation and novel mechanisms that contribute to regulation of HIV expression by different LRAs. Among the reviewed gene expression studies, the common approaches included identification of differentially expressed genes and gene functional category assessment. Integration of transcriptomic data with other biological data types is presently scarce, and the field would benefit from increased adoption of these methods in future studies. In addition, designing prospective studies that use the same methods of data acquisition and statistical analyses will facilitate a more reliable identification of latency biomarkers using different model systems and the comparison of the effects of different LRAs on host factors with a role in HIV reactivation. The results from such studies would have the potential to significantly impact the process by which candidate drugs are selected and combined for future evaluations and advancement to clinical trials.

Models and methods for in vitro testing of hepatic gap junctional communication

Inherent to their pivotal roles in controlling all aspects of the liver cell life cycle, hepatocellular gap junctions are frequently disrupted upon impairment of the homeostatic balance, as occurs during liver toxicity. Hepatic gap junctions, which are mainly built up by connexin32, are specifically targeted by tumor promoters and epigenetic carcinogens. This renders inhibition of gap junction functionality a suitable indicator for the in vitro detection of nongenotoxic hepatocarcinogenicity. The establishment of a reliable liver gap junction inhibition assay for routine in vitro testing purposes requires a cellular system in which gap junctions are expressed at an in vivo-like level as well as an appropriate technique to probe gap junction activity. Both these models and methods are discussed in the current paper, thereby focusing on connexin32-based gap junctions.

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

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

Survival of primary, but not of cancer cells after combined Plk1-HDAC inhibition

In the current study we examined the combination of SAHA and SBE13 in cancer and non-cancer cells. HeLa cells displayed a synergistically reduced cell proliferation, which was much weaker in hTERT-RPE1 or NIH-3T3 cells. Cell cycle distribution differed in HeLa, hTERT-RPE1 and NIH-3T3 cells. SAHA-treated HeLa cells showed slightly increasing cell numbers in G2/M phase, but after combination with SBE13 strongly elevated cell numbers in G2/M and S phase, accompanied by decreasing G0/G1 percentages. hTERT-RPE1 and NIH-3T3 cells showed strongly enriched cell numbers in G0/G1 phase. Western blot and quantitative real time analyses revealed reduced Plk1 mRNA and protein in all cells. p21 protein was strongly induced in cancer, but not in non-cancer cells, corresponding to a different localization in immunofluorescence studies. Additionally, these revealed an abundantly present pRb protein in HeLa cells after any treatment but almost completely vanished pRb staining in treated hTERT-RPE1 cells. These differences could be approved in Western blots against Parp and Caspase 3, which were activated in HeLa, but not in hTERT-RPE1 cells. Thus, we observed for the first time a differential effect of cancer versus non-cancer cells after treatment with SAHA and SBE13, which might be due to the dual role of p21.

Mechanisms of Histone Deacetylase Inhibitor-Regulated Gene Expression in Cancer Cells

SIGNIFICANCE

Class I and II histone deacetylase inhibitors (HDACis) are approved for the treatment of cutaneous T-cell lymphoma and are undergoing clinical trials as single agents, and in combination, for other hematological and solid tumors. Understanding their mechanisms of action is essential for their more effective clinical use, and broadening their clinical potential.

RECENT ADVANCES

HDACi induce extensive transcriptional changes in tumor cells by activating and repressing similar numbers of genes. These transcriptional changes mediate, at least in part, HDACi-mediated growth inhibition, apoptosis, and differentiation. Here, we highlight two fundamental mechanisms by which HDACi regulate gene expression—histone and transcription factor acetylation. We also review the transcriptional responses invoked by HDACi, and compare these effects within and across tumor types.

CRITICAL ISSUES

The mechanistic basis for how HDACi activate, and in particular repress gene expression, is not well understood. In addition, whether subsets of genes are reproducibly regulated by these agents both within and across tumor types has not been systematically addressed. A detailed understanding of the transcriptional changes elicited by HDACi in various tumor types, and the mechanistic basis for these effects, may provide insights into the specificity of these drugs for transformed cells and specific tumor types.

FUTURE DIRECTIONS

Understanding the mechanisms by which HDACi regulate gene expression and an appreciation of their transcriptional targets could facilitate the ongoing clinical development of these emerging therapeutics. In particular, this knowledge could inform the design of rational drug combinations involving HDACi, and facilitate the identification of mechanism-based biomarkers of response.

Anti-Cancer Effect of IN-2001 in T47D Human Breast Cancer

Histone deacetylases (HDACs) are enzymes involved in the remodelling of chromatin, and have a key role in the epigenetic regulation of gene expression. Histone deacetylase (HDAC) inhibitors are emerging as an exciting new class of potential anti-cancer agents. In recent years, a number of structurally diverse HDAC inhibitors have been identified and these HDAC inhibitors induce growth arrest, differentiation and/or apoptosis of cancer cells in vitro and in vivo. However, the underlying molecular mechanisms remain unclear. This study aimed at investigating the anti-tumor activity of various HDAC inhibitors, IN-2001, using T47D human breast cancer cells. Moreover, the possible mechanism by which HDAC inhibitors exhibit anti-tumor activity was also explored. In estrogen receptor positive T47D cells, IN-2001, HDAC inhibitor showed anti-proliferative effects in dose-and time-dependent manner. In T47D human breast cancer cells showed anti-tumor activity of IN-2001 and the growth inhibitory effects of IN-2001 were related to the cell cycle arrest and induction of apoptosis. Flow cytometry studies revealed that IN-2001 showed accumulation of cells at G2/M phase. At the same time, IN-2001 treatment time-dependently increased sub-G1 population, representing apoptotic cells. IN-2001-mediated cell cycle arrest was associated with induction of cdk inhibitor expression. In T47D cells, IN-2001 as well as other HDAC inhibitors treatment significantly increased p21(WAF1) and p27(KIP1) expression. In addition, thymidylate synthase, an essential enzyme for DNA replication and repair, was down-regulated by IN-2001 and other HDAC inhibitors in the T47D human breast cancer cells. In summary, IN-2001 with a higher potency than other HDAC inhibitors induced growth inhibition, cell cycle arrest, and eventual apoptosis in human breast cancer possibly through modulation of cell cycle and apoptosis regulatory proteins, such as cdk inhibitors, cyclins, and thymidylate synthase.

Regulation of connexin- and pannexin-based channels by post-translational modifications

Connexin (Cx) and pannexin (Panx) proteins form large conductance channels, which function as regulators of communication between neighbouring cells via gap junctions and/or hemichannels. Intercellular communication is essential to coordinate cellular responses in tissues and organs, thereby fulfilling an essential role in the spreading of signalling, survival and death processes. The functional properties of gap junctions and hemichannels are modulated by different physiological and pathophysiological stimuli. At the molecular level, Cxs and Panxs function as multi-protein channel complexes, regulating their channel localisation and activity. In addition to this, gap junctional channels and hemichannels are modulated by different post-translational modifications (PTMs), including phosphorylation, glycosylation, proteolysis, N-acetylation, S-nitrosylation, ubiquitination, lipidation, hydroxylation, methylation and deamidation. These PTMs influence almost all aspects of communicating junctional channels in normal cell biology and pathophysiology. In this review, we will provide a systematic overview of PTMs of communicating junction proteins and discuss their effects on Cx and Panx-channel activity and localisation.

A histone deacetylase inhibitor, largazole, decreases liver fibrosis and angiogenesis by inhibiting transforming growth factor-β and vascular endothelial growth factor signalling

BACKGROUND & AIMS

Largazole is a novel histone deacetylase (HDAC) inhibitor. This study investigated the effects of largazole against liver fibrosis.

METHODS

The in vitro effects of largazole were examined using hepatic stellate cells (HSCs). In vivo effects of largazole were studied using a mouse liver fibrotic model induced by CCl4 .

RESULTS

Largazole augmented acetylation of histone H3 (H3) and histone H4 (H4) in HSCs. It directly inhibited the activation of HSCs owing to HDAC inhibitory activity as the antifibrotic effect of largazole was significantly decreased in cells with HDAC1, HDAC2 and HDAC3 knockdown. Largazole also induced apoptosis of HSCs. Largazole not only inhibited the expression of TGFβR2, but also reduced phosphorylation of Smad2 and Akt induced by TGF-β1. Largazole also inhibited the expression of vascular endothelial growth factor (VEGF) and its receptor. VEGF-induced proliferation of HSCs and activation of Akt and p38MAPK were also suppressed by largazole. In vivo, largazole reduced the expression of collagen I, α-smooth muscle actin and tissue inhibitor of metalloproteinase-1 in CCl4 -induced fibrosis, and these antifibrotic effects were associated with increased acetylation of H3 and H4. Largazole also induced HSCs to undergo apoptosis in vivo, which was correlated with downregulation of bcl-2 and bcl-xL. Furthermore, largazole inhibited angiogenesis in vivo as evidenced by reduced expression of CD34, VEGF and VEGFR. In addition to its antifibrotic activity, the drug reduced inflammatory activity in CCl4 -induced liver fibrosis.

CONCLUSIONS

Our findings revealed a novel role of largazole in the treatment of liver fibrosis. Through multiple mechanisms, largazole could be a potentially effective antifibrotic agent.

Expression of the coxsackie adenovirus receptor in neuroendocrine lung cancers and its implications for oncolytic adenoviral infection

Coxsackie adenovirus receptor (CAR) is the primary receptor to which oncolytic adenoviruses have to bind for internalization and viral replication. A total of 171 neuroendocrine lung tumors in form of multitissue arrays have been analyzed resulting in a positivity of 112 cases (65.5%). Immunostaining correlated statistically significant with histopathology and development of recurrence. The subtype small cell lung cancer (SCLC) showed the highest CAR expression (77.6%), moreover the CAR level was correlated to the disease-free survival. Further, high CAR expression level in SCLC cell lines was found in vitro and in vivo when cell lines had been transplanted into immunodeficient mice. A correlation between CAR expression in the primary tumors and metastases development in the tumor model underlined the clinical relevance. Cell lines with high CAR level showed a high infectivity when infected with a replication-deficient adenovirus. Low levels of CAR expression in SCLC could be upregulated with Trichostatin A, a histone deacetylase inhibitor. As a result of the unaltered poor prognosis of SCLC and its high CAR expression it seems to be the perfect candidate for oncolytic therapy. With our clinically relevant tumor model, we show that xenograft experiments are warrant to test the efficiency of oncolytic adenoviral therapy.

Role of connexin-related signalling in hepatic homeostasis and its relevance for liver-based in vitro modelling

Direct intercellular communication mediated by gap junctions constitutes a major regulatory platform in the control of hepatic homeostasis. Hepatocellular gap junctions are composed of two hemichannels of adjacent cells which are built up by connexin proteins,

in casu Cx32. Mathieu Vinken, Pofessor at the Department of Toxicology of the Free University Brussels-Belgium, was one of the first investigators to demonstrate that hepatic connexin expression is controlled by epigenetic mechanisms. In particular, he found that inhibitors of histone deacetylase enzymes enhance Cx32 production and gap junction activity in cultures of primary hepatocytes, a finding that is of importance for liver-based in vitro modelling. Professor Dr. Mathieu Vinken’s recent work is focussed on the elucidation of the role of connexin proteins and their channels in the hepatocyte life cycle. Specific attention is paid to apoptosis in this context, whereby it has been found that Cx32 hemichannels control the termination of induced cell death in cultures of primary hepatocytes. Overall, Professor Dr. Mathieu Vinken’s research can be considered as an important contribution to the field of hepatic connexin physiology.

Romidepsin (Istodax, NSC 630176, FR901228, FK228, depsipeptide): a natural product recently approved for cutaneous T-cell lymphoma

Romidepsin (Istodax), a selective inhibitor of histone deacetylases (HDACs), was approved for the treatment of cutaneous T-cell lymphoma in November 2009 by the US Food and Drug Administration. This unique natural product was discovered from cultures of Chromobacterium violaceum, a Gram-negative bacterium isolated from a Japanese soil sample. This bicyclic compound acts as a prodrug, its disulfide bridge being reduced by glutathione on uptake into the cell, allowing the free thiol groups to interact with Zn ions in the active site of class I and II HDAC enzymes. Due to the synthetic complexity of the compound, as well as the low yield from the producing organism, analogs are sought to create synthetically accessible alternatives. As a T-cell lymphoma drug, romidepsin offers a valuable new treatment for diseases with few effective therapies.

Adamantanyl-histone deacetylase inhibitor H6CAHA exhibits favorable pharmacokinetics and augments prostate cancer radiation sensitivity

PURPOSE

To evaluate pharmacological properties of H6CAHA, an adamantyl-hydroxamate histone deacetylase inhibitor, and to investigate its effect on prostate cancer cells following exposure to γ-radiation in vitro and in vivo.

METHODS AND MATERIALS

H6CAHA was assessed for in vitro solubility, lipophilicity and growth inhibition, and in vivo plasma pharmacokinetics. The effect of H6CAHA on radiation clonogenic survival and DNA damage repair was evaluated in human prostate cancer (PC3, DU145, LNCaP) and nonmalignant control epithelial (RWPE1 and 267B1) cell lines. The effect of this agent on the growth of prostate cancer xenografts was also assessed in mice.

RESULTS

H6CAHA demonstrated good solubility and permeability profiles and preferentially inhibited the growth of prostate cancer cells over nonmalignant cells. Plasma pharmacokinetics revealed that the area under the curve of H6CAHA was 8.08 ± 0.91 μM × h, and its half-life was 11.17 ± 0.87 h. Radiation clonogenic assays revealed that H6CAHA decreased the survival of prostate cancer cells at the dose that exerted limited effect on normal cells. Concomitantly, delayed DNA damage repair following combination treatment was evident in cancer cells, indicated by the prolonged appearance of γH2AX and Rad51 foci and suppression of DNA damage repair genes (ATM, BRCA1, and BRCA2). Combined modality of H6CAHA (daily intraperitoneal injections for 10 days) with γ-radiation (10 × 2 Gy) completely blocked the growth of PC3 tumor xenografts (p < 0.001) over 60 days.

CONCLUSION

These results support the potential therapeutic value of H6CAHA in combination with radiation and support the rationale for further clinical investigation.

The histone deacetylase inhibitor trichostatin A synergistically resensitizes a cisplatin resistant human bladder cancer cell line

PURPOSE

Cisplatin is the mainstay of treatment for advanced bladder cancer. However, intrinsic or acquired resistance to cisplatin is common, which severely limits its therapeutic potential. We determined the synergistic antitumor effect of cisplatin and the histone deacetylase inhibitor trichostatin A in cisplatin resistant human bladder cancer cells.

MATERIALS AND METHODS

The cisplatin resistant human bladder cancer cell line T24R2 was exposed to cisplatin and/or trichostatin A. Tumor cell proliferation was examined by cell counting kit assay. Synergism between 2 drugs was examined by the combination index. Changes in cell cycle and apoptosis were determined by flow cytometry. We analyzed the expression of caspase-3, 8 and 9, poly(adenosine diphosphate-ribose) polymerase, p21WAF1/CIP1, cyclin A, B1 and D1, Cdc2c, p-Cdc2c, Cdc25c, p-Cdc25c, cytochrome c, p-Akt, t-Akt, Bcl-2, Bax, Bad, vascular endothelial growth factor and fetal liver kinase-1 by Western blot and colorimetric assay.

RESULTS

Based on the combination index and isobole analysis of the Cell Counting Kit-8 assay we observed a strong synergistic antitumor effect between cisplatin and trichostatin A, allowing a 3.5 and 4.9-fold dose reduction in cisplatin and trichostatin A, respectively, while achieving an estimated 90% kill of T24R2 cells. The underlying mechanism could be synergistic cell cycle arrest, induction of caspase mediated apoptosis or up-regulated expression of pro-apoptotic Bad and Bax.

CONCLUSIONS

Results indicate that trichostatin A may synergistically enhance the antitumor effect of cisplatin and resensitize cisplatin resistant bladder cancer cells. These findings suggest the potential use of histone deacetylase inhibitor as a combination agent to enhance the antitumor effect of cisplatin in patients with advanced bladder cancer.

Histone deacetylase inhibitor: antineoplastic agent and radiation modulator

Inhibitors of histone deacetylases (HDACs) have emerged as a new class of anticancer agents based on their actions in cancer cell growth and cell cycle arrest, terminal differentiation, and apoptosis. Previously, we rationally designed and developed a new class of hydroxamide- and mercaptoacetamide-bearing HDAC inhibitors. A subset of these inhibitors exhibited chemo-radiation sensitizing properties in various human cancer cells. Furthermore, some HDAC inhibitors protected normal cells from radiation-induced damage and extended the survival of mice following total body exposure to lethal dose radiation. Pathological analyses revealed that intestinal and bone marrow cellularities recovered significantly from radiation-induced damage by structural compartments restoration, suggesting the mechanism of action of these HDAC inhibitors. These findings support the hypothesis that epigenetic regulation may play a crucial role in the functional recovery of normal tissues from radiation injuries.

The effect of combined treatment with cisplatin and histone deacetylase inhibitors on HeLa cells

OBJECTIVE

To investigate the combined effects of cisplatin and the histone deacetylase (HDAC) inhibitors suberoylanilide hydroxamic acid (SAHA) or sirtinol on HeLa cells and assess the mechanism underlying HDAC inhibitor-cisplatin synergy.

METHODS

The antineoplastic actions of cisplatin, SAHA and sirtinol, alone and in combination, were evaluated using the tetrazolium dye-based MTT cell proliferation assay, DAPI nuclear staining and cytotoxicity analysis.

RESULTS

Exposure to cisplatin, SAHA or sirtinol alone induced a dose-dependent reduction in HeLa cell viability. Combined treatment with cisplatin and SAHA or sirtinol was significantly more cytotoxic than cisplatin alone. Individually, cisplatin, SAHA and sirtinol activated caspase-3 and induced apoptosis, but the effects of combined treatment were greater. Importantly, both HDAC inhibitors dose-dependently inhibited the expression of the antiapoptotic proteins Bcl-2 and x-linked inhibitor of apoptosis protein (XIAP).

CONCLUSION

The combination of cisplatin and SAHA or sirtinol had synergistic effect on the HeLa cell viability. This potentiation of cisplatin activity was associated with HDAC inhibitor-mediated down-regulation of Bcl-2 and XIAP. These may result from the relaxation of chromatin by these HDAC inhibitors that increase cisplatin sensitivity by enhancing the accessibility of DNA to cisplatin and transcriptional regulators.

Targeting the epigenome: effects of epigenetic treatment strategies on genomic stability in healthy human cells

Epigenetic treatment concepts have long been ascribed as being tumour-selective. Over the last decade, it has become evident that epigenetic mechanisms are essential for a wide range of intracellular functions in healthy cells as well. Evaluation of possible side-effects and their underlying mechanisms in healthy human cells is necessary in order to improve not only patient safety, but also to support future drug development. Since epigenetic regulation directly interacts with genomic and chromosomal packaging density, increasing genomic instability may be a result subsequent to drug-induced epigenetic modifications. This review highlights past and current research efforts on the influence of epigenetic modification on genomic stability in healthy human cells.

Novel histone deacetylase inhibitors in clinical trials as anti-cancer agents

Histone deacetylases (HDACs) can regulate expression of tumor suppressor genes and activities of transcriptional factors involved in both cancer initiation and progression through alteration of either DNA or the structural components of chromatin. Recently, the role of gene repression through modulation such as acetylation in cancer patients has been clinically validated with several inhibitors of HDACs. One of the HDAC inhibitors, vorinostat, has been approved by FDA for treating cutaneous T-cell lymphoma (CTCL) for patients with progressive, persistent, or recurrent disease on or following two systemic therapies. Other inhibitors, for example, FK228, PXD101, PCI-24781, ITF2357, MGCD0103, MS-275, valproic acid and LBH589 have also demonstrated therapeutic potential as monotherapy or combination with other anti-tumor drugs in CTCL and other malignancies. At least 80 clinical trials are underway, testing more than eleven different HDAC inhibitory agents including both hematological and solid malignancies. This review focuses on recent development in clinical trials testing HDAC inhibitors as anti-tumor agents.

Effects of histone deacetylase inhibitors on rat mesangial cells

Glomerular mesangial cells (MCs) proliferate and produce extracellular matrix proteins in many progressive renal diseases. Recently, histone deacetylase inhibitors (HDIs) were shown to have antiproliferative and antifibrogenic effects in some in vitro and in vivo models. Using the [(3)H]-thymidine incorporation test, we have found that the HDI trichostatin A (TSA) effectively inhibits MC growth at nontoxic nanomolar concentrations. Similarly, the HDI valproic acid also inhibited MCs proliferation. Cell-cycle analysis indicated an arrest in G(0)/G(1) phase in response to TSA, which was accompanied by elevation in synthesis of the cyclin-dependent kinase inhibitors (CDKIs) p21/Waf1 and p27/Kip1. TSA treatment suppressed alpha-smooth muscle actin, transforming growth factor-beta1, and collagen protein synthesis by MCs and induced myofibroblast-like appearance of proliferating MCs. In the in vivo model of the anti-Thy1.1-induced glomerulonephritis, TSA and valproic acid treatments significantly suppressed proteinuria. Collectively, these data suggest a therapeutic potential for HDIs in the treatment of mesangial proliferative diseases and glomerulosclerosis.

Identification of LIV1, a putative zinc transporter gene responsible for HDACi-induced apoptosis, using a functional gene screen approach

Histone deacetylase inhibitors (HDACi) show promise as a novel class of antitumoral agents and have shown the ability to induce apoptosis of tumor cells. To gain a better understanding of the action of HDACi, we conducted a functional gene screen approach named suppression of mortality by antisense rescue technique to identify the key genes responsible for the tumor-selective killing trichostatin A. Over 20 genes associated with HDACi-induced mortality were identified. One of the confirmed positive hits is LIV1, a putative zinc transporter. LIV1 is significantly induced by treatment with HDACi in a number of tumor cells, but not in normal cells. Knockdown of LIV1 suppressed apoptosis induced by HDACi in tumor cells. Although HDACi induced a slight increase in the free intracellular zinc concentration, knockdown of LIV1 significantly enhanced the intracellular zinc level, which was associated with resistance to apoptosis. On the other hand, pretreatment of the cells with a specific zinc chelator TPEN reversed the apoptosis resistance conferred by knockdown of LIV1. However, the biological effects of TPEN were abolished by addition of physiologic concentrations of zinc. Taken together, the present study identifies LIV1 as a critical mediator responsible for HDACi-induced apoptosis. The effect of LIV1 is, at least in part, mediated by affecting intracellular zinc homeostasis, which may be related to alteration of the catalytic activity of the Caspase 3 and expression of some BCL-2 family genes. As such, these findings highlight a novel mechanism underlying the action of HDACi that could be potentially useful in the clinical setting.

In vitro differentiation of embryonic and adult stem cells into hepatocytes: state of the art

Stem cells are a unique source of self-renewing cells within the human body. Before the end of the last millennium, adult stem cells, in contrast to their embryonic counterparts, were considered to be lineage-restricted cells or incapable of crossing lineage boundaries. However, the unique breakthrough of muscle and liver regeneration by adult bone marrow stem cells at the end of the 1990s ended this long-standing paradigm. Since then, the number of articles reporting the existence of multipotent stem cells in skin, neuronal tissue, adipose tissue, and bone marrow has escalated, giving rise, both in vivo and in vitro, to cell types other than their tissue of origin. The phenomenon of fate reprogrammation and phenotypic diversification remains, though, an enigmatic and rare process. Understanding how to control both proliferation and differentiation of stem cells and their progeny is a challenge in many fields, going from preclinical drug discovery and development to clinical therapy. In this review, we focus on current strategies to differentiate embryonic, mesenchymal(-like), and liver stem/progenitor cells into hepatocytes in vitro. Special attention is paid to intracellular and extracellular signaling, genetic modification, and cell-cell and cell-matrix interactions. In addition, some recommendations are proposed to standardize, optimize, and enrich the in vitro production of hepatocyte-like cells out of stem/progenitor cells.

Role of epigenetics in liver-specific gene transcription, hepatocyte differentiation and stem cell reprogrammation

Controlling both growth and differentiation of stem cells and their differentiated somatic progeny is a challenge in numerous fields, from preclinical drug development to clinical therapy. Recently, new insights into the underlying molecular mechanisms have unveiled key regulatory roles of epigenetic marks driving cellular pluripotency, differentiation and self-renewal/proliferation. Indeed, the transcription of genes, governing cell-fate decisions during development and maintenance of a cell's differentiated status in adult life, critically depends on the chromatin accessibility of transcription factors to genomic regulatory and coding regions. In this review, we discuss the epigenetic control of (liver-specific) gene-transcription and the intricate interplay between chromatin modulation, including histone (de)acetylation and DNA (de)methylation, and liver-enriched transcription factors. Special attention is paid to their role in directing hepatic differentiation of primary hepatocytes and stem cells in vitro.

Histone deacetylase 7, a potential target for the antifibrotic treatment of systemic sclerosis

OBJECTIVE

We have recently shown a significant reduction in cytokine-induced transcription of type I collagen and fibronectin in systemic sclerosis (SSc) skin fibroblasts upon treatment with trichostatin A (TSA). Moreover, in a mouse model of fibrosis, TSA prevented the dermal accumulation of extracellular matrix. The purpose of this study was to analyze the silencing of histone deacetylase 7 (HDAC-7) as a possible mechanism by which TSA exerts its antifibrotic function.

METHODS

Skin fibroblasts from patients with SSc were treated with TSA and/or transforming growth factor beta. Expression of HDACs 1-11, extracellular matrix proteins, connective tissue growth factor (CTGF), and intercellular adhesion molecule 1 (ICAM-1) was analyzed by real-time polymerase chain reaction, Western blotting, and the Sircol collagen assay. HDAC-7 was silenced using small interfering RNA.

RESULTS

SSc fibroblasts did not show a specific pattern of expression of HDACs. TSA significantly inhibited the expression of HDAC-7, whereas HDAC-3 was up-regulated. Silencing of HDAC-7 decreased the constitutive and cytokine-induced production of type I and type III collagen, but not fibronectin, as TSA had done. Most interestingly, TSA induced the expression of CTGF and ICAM-1, while silencing of HDAC-7 had no effect on their expression.

CONCLUSION

Silencing of HDAC-7 appears to be not only as effective as TSA, but also a more specific target for the treatment of SSc, because it does not up-regulate the expression of profibrotic molecules such as ICAM-1 and CTGF. This observation may lead to the development of more specific and less toxic targeted therapies for SSc.

Epigenetic therapy using the histone deacetylase inhibitor for increasing therapeutic gain in oral cancer: prevention of radiation-induced oral mucositis and inhibition of chemical-induced oral carcinogenesis

In addition to genetic changes, epigenetic aberrations also play important roles in radiation- and chemical-induced disorders and carcinogenesis. The present study investigated whether epigenetic therapy with a histone deacetylase (HDAC) inhibitor has dual benefits for radiation-induced oral mucositis and chemical-induced oral carcinogenesis, which should be treated at the same time. The HDAC inhibitor phenylbutyrate was first tested to determine if it influences DNA damage repair and survival in irradiated normal cells in vitro by investigating the patterns and dynamics of phospho-gammaH2AX foci, Rad51 foci and phospho-gammaH2AX/Rad51 colocalization and using the comet and clonogenic assays. Oral mucositis or carcinogenesis was induced in hamsters using radiation or 7,12-dimethylbenz[a]anthracene (DMBA) irritation to the cheek pouch. The ability of phenylbutyrate formed in proper carriers to prevent radiation-induced oral mucositis and inhibit chemical-induced oral carcinogenesis was assessed. The treated or untreated irradiated or DMBA-irritated oral tissues or mucosal epithelia were subjected to the studies of histology, immunohistochemistry, gene expression, comet assay, HDAC activity or oxidative stress. We found that phenylbutyrate promoted DNA repair and survival in normal cells after radiation. Compared with blank or vehicle-treated hamsters, the irradiated mucosa treated with phenylbutyrate had significantly lower oxidative stress and tumor necrosis factor-alpha expression and less severe oral mucositis of a shorter duration. A reduction of the oral tumor incidence, burden and progression by phenylbutyrate correlated with the suppression of oncomiRs and Rad51 overexpression, the upregulation of differentiation markers and the decrease of intracellular HDAC activity and oxidative stress during DMBA-induced oral carcinogenesis. Thus, epigenetic therapy using the HDAC inhibitor as an adjuvant to radiotherapy for chemical-induced oral cancer may provide a promising strategy combining the prevention of radiation-induced oral mucositis and the inhibition of oral carcinogenesis.

Apoptotic insults to human HepG2 cells induced by S-(+)-ketamine occurs through activation of a Bax-mitochondria-caspase protease pathway

BACKGROUND

Ketamine is widely used as an i.v. anaesthetic agent and as a drug of abuse. Hepatocytes contribute to the metabolism of endogenous and exogenous substances. This study evaluated the toxic effects of S-(+)-ketamine and possible mechanisms using human hepatoma HepG2 cells as the experimental model.

METHODS

HepG2 cells were exposed to S-(+)-ketamine. Cell viability and the release of lactate dehydrogenase (LDH) and gamma-glutamyl transpeptidase (GPT) were measured to determine the toxicity of S-(+)-ketamine to HepG2 cells. Cell morphology, DNA fragmentation, and apoptotic cells were analysed to evaluate the mechanism of S-(+)-ketamine-induced cell death. Amounts of Bax, an apoptotic protein, and cytochrome c in the cytoplasm or mitochondria were quantified by immunoblotting. Cellular adenosine triphosphate levels were analysed using a bioluminescence assay. Caspases-3, -9, and -6 were measured fluorometrically.

RESULTS

Exposure of HepG2 cells to S-(+)-ketamine increased the release of LDH and GPT, but decreased cell viability (all P<0.01). S-(+)-Ketamine time-dependently caused shrinkage of HepG2 cells. Exposure to S-(+)-ketamine led to significant DNA fragmentation and cell apoptosis (P=0.003 and 0.002). S-(+)-Ketamine increased translocation of Bax from the cytoplasm to mitochondria, but decreased the mitochondrial membrane potential and cellular adenosine triphosphate levels (all P<0.01). Sequentially, cytosolic cytochrome c levels and activities of caspases-9, -3, and -6 were augmented after S-(+)-ketamine administration (all P<0.001). Z-VEID-FMK, an inhibitor of caspase-6, alleviated the S-(+)-ketamine-induced augmentation of caspase-6 activity, DNA fragmentation, and cell apoptosis (all P<0.001).

CONCLUSIONS

This study shows that S-(+)-ketamine can induce apoptotic insults to human HepG2 cells via a Bax-mitochondria-caspase protease pathway. Thus, we suggest that S-(+)-ketamine at a clinically relevant or an abused concentration may induce liver dysfunction possibly due to its toxicity to hepatocytes.

Evaluation of the effects of histone deacetylase inhibitors on cells from canine cancer cell lines

OBJECTIVE

To determine whether exposure of canine cancer cells to histone deacetylase (HDAC) inhibitors S(+)-N-hydroxy-4-(3-methyl-2-phenyl-butyrylamino)benzamide (OSU-HDAC42) or suberoylanilide hydroxamic acid (SAHA) results in increased histone acetylation and decreased cell viability and whether any changes in viability involve induction of apoptosis or alterations in progression of the cell cycle.

SAMPLE POPULATION

9 canine cancer cell lines.

PROCEDURES

Cells from 9 canine cancer cell lines were treated with dimethyl sulfoxide vehicle, OSU-HDAC42, or SAHA, then assays of cell viability were performed. Histone acetylation was assessed by use of western blot analysis. Apoptosis was assessed via ELISA to detect fragmentation of cytoplasmic nucleosomal DNA and western blot analysis to detect cleavage of caspase 3. Cell cycle analysis was performed by use of propidium iodide staining and flow cytometry. RESULTS-Concentrations of OSU-HDAC42 and SAHA required to achieve 50% inhibition of cell viability (IC(50)) were reached in cells of 6 and 4 canine cancer cell lines, respectively, and ranged from approximately 0.4 to 1.3 microM for OSU-HDAC42 and 0.6 to 4.8 microM for SAHA. Cells from T-cell lymphoma, mast cell tumor, osteosarcoma, and histiocytic sarcoma lines were most sensitive to HDAC inhibition, with IC(50)s of < 1 microM for OSU-HDAC42 and < 5 microM for SAHA. Induction of apoptosis was indicated via cleavage of caspase 3 and increases in cytoplasmic nucleosomes and the subG(1) cell population.

CONCLUSIONS AND CLINICAL RELEVANCE

Micromolar concentrations of HDAC inhibitors OSU-HDAC42 and SAHA induced histone acetylation, cytotoxicity, and apoptosis in canine cancer cells. In general, OSU-HDAC42 was more potent than SAHA.

Differential effects of hydroxamate histone deacetylase inhibitors on cellular functionality and gap junctions in primary cultures of mitogen-stimulated hepatocytes

Histone deacetylase (HDAC)-inhibitors are well known to induce proliferative blocks and concomitant differentiation boosts in a plethora of tumor cells. Despite their promising potential as clinical therapeutics, however, the biological outcome of HDAC-inhibitors in non-tumorous cells has been poorly documented. We previously reported that the HDAC-inhibitor trichostatin A (TSA) and its metabolically more stable structural analogue 5-(4-dimethylaminobenzoyl)-aminovaleric acid hydroxamide (4-Me2N-BAVAH) cause cell cycle arrests in primary cultures of mitogen-stimulated hepatocytes. The present study was set up to explore whether this proliferative block in non-tumorous cells is also associated with inducing effects on the differentiated hepatocellular phenotype, a scenario that is usually observed in tumorous cells. In particular, the molecular actions of TSA and 4-Me2N-BAVAH on hepatic functionality and gap junctions, gatekeepers of liver homeostasis, in primary cultures of mitogen-stimulated hepatocytes are investigated. Both HDAC-inhibitors were found to promote albumin secretion and CYP1A1 gene transcription and functionality, whereas CYP2B1 gene transcription and activity were only slightly enhanced. The protein production of the gap junction component Cx26 was downregulated, whereas Cx32 expression was upregulated in response to HDAC-inhibition. Furthermore, TSA increased protein levels of the non-specific hepatocellular Cx43, whereas 4-Me2N-BAVAH rather diminished its expression. These data provide new insight into the biological impact of HDAC-inhibitors on the homeostatic balance in hepatocytes, being major executors of xenobiotic biotransformation and primary targets of drug-induced toxicity.

Histone deacetylase inhibitors: mechanisms and clinical significance in cancer: HDAC inhibitor-induced apoptosis

Epigenic modifications, mainly DNA methylation and acetylation, are recognized as the main mechanisms contributing to the malignant phenotype. Acetylation and deacetylation are catalyzed by specific enzymes, histone acetyltransferases (HATs) and histone deacetylases (HDACs), respectively. While histones represent a primary target for the physiological function of HDACs, the antitumor effect of HDAC inhibitors might also be attributed to transcription-independent mechanisms by modulating the acetylation status of a series of non-histone proteins. HDAC inhibitors may act through the transcriptional reactivation of dormant tumor suppressor genes. They also modulate expression of several other genes related to cell cycle, apoptosis, and angiogenesis. Several HDAC inhibitors are currently in clinical trials both for solid and hematologic malignancies. Thus, HDAC inhibitors, in combination with DNA-demethylating agents, chemopreventive, or classical chemotherapeutic drugs, could be promising candidates for cancer therapy. Here, we review the molecular mechanisms and therapeutic potential of HDAC inhibitors for the treatment of cancer.

Epigenetic regulation of established human type 1 versus type 2 cytokine responses

BACKGROUND

Multiple biologic factors influence maintenance of immunologic responsiveness. Here, we studied whether epigenetics has a regulatory function in maintaining pre-established T(H)1-like and T(H)2-like immunity in human beings.

OBJECTIVE

We focused on delineating the role of endogenous histone deacetylase (HDAC) activity in regulating cytokine recall responses.

METHODS

Using RT-PCR and ELISA, the effect of increasing cellular acetylation on T(H)1/T(H)2 cytokine expression was systematically examined in 58 children by inhibiting HDAC activity with trichostatin A.

RESULTS

Phytohemagglutinin activation selectively stimulates antigen-experienced CD45RO+ T cells, eliciting recall cytokine responses. Trichostatin A reduced HDAC activity by approximately 1/3. The resulting cellular hyperacetylation led to increased T(H)2-associated (IL-13, 139%; IL-5, 168%; P < .0001) and reduced T(H)1-associated recall responses (IFN-gamma, 76%; CXCL10, 47%; P < .0001). IL-2 and IL-10 production were reduced 25% to 55% (P < .0001). These alterations in T(H)2-associated and T(H)1-associated recall responses were associated with increased expression of Gata-3 and sphingosine kinase 1, a T(H)1-negative regulator, independent of T-bet expression. Overall, inhibition of endogenous HDAC activity shifted T(H)1:T(H)2 ratios by 3-fold to 8-fold (P < or = .0001), skewing recall responses toward a more T(H)2-like phenotype, independent of the stimulus used.

CONCLUSION

Endogenous HDAC activity plays a crucial role in maintaining the balance of pre-established T(H)1-like and T(H)2-like responses, inhibiting excessive T(H)2 immunity.

Inhibition of NF-kappaB activation by the histone deacetylase inhibitor 4-Me2N-BAVAH induces an early G1 cell cycle arrest in primary hepatocytes

OBJECTIVE

Benzoylaminoalkanohydroxamic acids, including 5-(4-dimethylaminobenzoyl)aminovaleric acid hydroxamide (4-Me(2)N-BAVAH), are structural analogues of Trichostatin A, a naturally occurring histone deacetylase inhibitor (HDACi). 4-Me(2)N-BAVAH has been shown to induce histone hyperacetylation and to inhibit proliferation in Friend erythroleukaemia cells in vitro. However, the molecular mechanisms have remained unidentified.

MATERIALS AND METHODS

In this study, we evaluated the effects of 4-Me(2)N-BAVAH on proliferation in non-malignant cells, namely epidermal growth factor-stimulated primary rat hepatocytes.

RESULTS AND CONCLUSION

We have found that 4-Me(2)N-BAVAH inhibits HDAC activity at non-cytotoxic concentrations and prevents cells from responding to the mitogenic stimuli of epidermal growth factor. This results in an early G(1) cell cycle arrest that is independent of p21 activity, but instead can be attributed to inhibition of cyclin D1 transcription through a mechanism involving inhibition of nuclear factor-kappaB activation. In addition, 4-Me(2)N-BAVAH delays the onset of spontaneous apoptosis in primary rat hepatocyte cultures as evidenced by down-regulation of the pro-apoptotic proteins Bid and Bax, and inhibition of caspase-3 activation.

Targeting glycosylation pathways and the cell cycle: sugar-dependent activity of butyrate-carbohydrate cancer prodrugs

Short-chain fatty acid (SCFA)-carbohydrate hybrid molecules that target both histone deacetylation and glycosylation pathways to achieve sugar-dependent activity against cancer cells are described in this article. Specifically, n-butyrate esters of N-acetyl-D-mannosamine (But4ManNAc, 1) induced apoptosis, whereas corresponding N-acetyl-D-glucosamine (But4GlcNAc, 2), D-mannose (But5Man, 3), or glycerol (tributryin, 4) derivatives only provided transient cell cycle arrest. Western blots, reporter gene assays, and cell cycle analysis established that n-butyrate, when delivered to cells via any carbohydrate scaffold, functioned as a histone deacetylase inhibitor (HDACi), upregulated p21WAF1/Cip1 expression, and inhibited proliferation. However, only 1, a compound that primed sialic acid biosynthesis and modulated the expression of a different set of genes compared to 3, ultimately killed the cells. These results demonstrate that the biological activity of butyrate can be tuned by sugars to improve its anticancer properties.

The novel histone deacetylase inhibitor 4-Me2N-BAVAH differentially affects cell junctions between primary hepatocytes

Histone deacetylase (HDAC) inhibitors show great pharmaceutical potential, particularly in relation to cancer. However, very little is known about their biological outcome on hepatocytes, the major executors of xenobiotic biotransformation in the organism. The current study was set up to investigate the effects of the newly synthesized HDAC inhibitor 5-(4-dimethylaminobenzoyl)-aminovaleric acid hydroxamate (4-Me(2)N-BAVAH) on hepatocyte gap junctions and adherens junctions, being main guardians of liver homeostasis. For that purpose, freshly isolated rat hepatocytes were cultivated for 7 days either in the absence or presence of 50 microM 4-Me(2)N-BAVAH. Gap junction activity became promoted upon exposure to 4-Me(2)N-BAVAH, which was associated with elevated Cx32 protein levels. By contrast, both Cx26 and Cx43 protein levels were negatively affected. The modifications in connexin protein content were not reflected at the transcriptional level. Finally, neither the expressions nor the cellular localizations of the adherens junction building stones E-cadherin, beta-catenin and gamma-catenin were altered by 4-Me(2)N-BAVAH, a finding that is in contrast to what is commonly observed in tumor cells following exposure to HDAC inhibitors.

The histone deacetylase inhibitor suberoylanilide hydroxamic acid induces growth inhibition and enhances gemcitabine-induced cell death in pancreatic cancer

PURPOSE

Pancreatic cancer is an aggressive human malignancy that is generally refractory to chemotherapy. Histone deacetylase inhibitors are novel agents that modulate cell growth and survival. In this study, we sought to determine whether a relatively new histone deacetylase inhibitor, suberoylanilide hydroxamic acid (SAHA), inhibits pancreatic cancer cell growth.

EXPERIMENTAL DESIGN

The effects of SAHA on the growth of three pancreatic cancer cell lines (BxPC3, COLO-357, and PANC-1) were examined with respect to cell cycle progression, p21 induction and localization, and interactions with the nucleoside analogue gemcitabine.

RESULTS

SAHA induced a G(1) cell cycle arrest in BxPC-3 cells and COLO-357 cells but not in PANC-1 cells. This arrest was dependent, in part, on induction of p21 by SAHA, as p21 was not induced in PANC-1 cells, and knockdown of p21 using small interfering RNA oligonucleotides nearly completely suppressed the effects of SAHA on cell cycle arrest in COLO-357 and partly attenuated the effects of SAHA in BxPC-3. COLO-357 and BxPC-3 cells, but not PANC-1 cells, were also sensitive to gemcitabine. In the gemcitabine-resistant PANC-1 cells, a 48-h cotreatment with SAHA rendered the cells sensitive to the inhibitory and proapoptotic effects of gemcitabine. An additive effect on growth inhibition by SAHA and gemcitabine was observed in COLO-357 and BxPC-3 cells. Moreover, analysis of p21 distribution in COLO-357 cells revealed that SAHA induced the cytoplasmic localization of both p21 and phospho-p21.

CONCLUSIONS

These data indicate that SAHA exerts proapoptotic effects in pancreatic cancer cells, in part, by up-regulating p21 and sequestering it in the cytoplasm, raising the possibility that SAHA may have therapeutic potential in the treatment of pancreatic cancer.

Chromatin remodeling agent trichostatin A: a key-factor in the hepatic differentiation of human mesenchymal stem cells derived of adult bone marrow

Background

The capability of human mesenchymal stem cells (hMSC) derived of adult bone marrow to undergo in vitro hepatic differentiation was investigated.

Results

Exposure of hMSC to a cocktail of hepatogenic factors [(fibroblast growth factor-4 (FGF-4), hepatocyte growth factor (HGF), insulin-transferrin-sodium-selenite (ITS) and dexamethasone)] failed to induce hepatic differentiation. Sequential exposure to these factors (FGF-4, followed by HGF, followed by HGF+ITS+dexamethasone), however, resembling the order of secretion during liver embryogenesis, induced both glycogen-storage and cytokeratin (CK)18 expression. Additional exposure of the cells to trichostatin A (TSA) considerably improved endodermal differentiation, as evidenced by acquisition of an epithelial morphology, chronological expression of hepatic proteins, including hepatocyte-nuclear factor (HNF)-3β, alpha-fetoprotein (AFP), CK18, albumin (ALB), HNF1α, multidrug resistance-associated protein (MRP)2 and CCAAT-enhancer binding protein (C/EBP)α, and functional maturation, i.e. upregulated ALB secretion, urea production and inducible cytochrome P450 (CYP)-dependent activity.

Conclusion

hMSC are able to undergo mesenchymal-to-epithelial transition. TSA is hereby essential to promote differentiation of hMSC towards functional hepatocyte-like cells.

Differential role of epigenetic modulators in malignant and normal stem cells: a novel tool in preclinical in vitro toxicology and clinical therapy

Adult stem cells are primitive cells that undergo asymmetric division, thereby giving rise to one clonogenic, self-renewing cell and one cell able to undergo multipotent differentiation. Disturbance of this controlled process by epigenetic alterations, including imbalance of histone acetylation/histone deacetylation and DNA methylation/demethylation, may result in uncontrolled growth, formation of self-renewing malignant stem cells and eventually cancer. In view of this notion, several epigenetic modulators, in particular those with histone deacetylase inhibiting activity, are currently being tested in phase I and II clinical trials for their promising chemotherapeutic properties in cancer therapy. As chromatin modulation is also involved in regulation of differentiation, normal development, embryonic and adult stem cell functions and maintenance of their plasticity during embryonic organogenesis, the question can be raised whether predestined cell fate can be modified through epigenetic interference. And if so, could this strategy enforce adult stem cells to differentiate into different types of functional cells? In particular, functional hepatocytes seem important for preclinical toxicity screening of candidate drugs. This paper reviews the potential use and relevance of epigenetic modifiers, including inhibitors of histone deacetylases and DNA methyltransferases (1) to change cell fate and 'trans'differentiate normal adult stem cells into hepatocyte-like cells and (2) to cure disorders, caused by uncontrolled growth of malignant stem cells.

Assessment of developmental toxicity of vorinostat, a histone deacetylase inhibitor, in Sprague-Dawley rats and Dutch Belted rabbits

BACKGROUND

The developmental toxicity potential of vorinostat (suberoylanilide hydroxamic acid [SAHA], ZOLINZA), a potent inhibitor of histone deacetylase (HDAC), was assessed in Sprague-Dawley rats and Dutch Belted rabbits. HDAC inhibitors have been shown to mediate the regulation of gene expression, induce cell growth, cell differentiation, and apoptosis of tumor cells. Range-finding studies established oral dose levels of 5, 15, or 50 mg/kg/day and 20, 50, or 150 mg/kg/day in rats and rabbits, respectively.

METHODS

Animals were dosed on Gestation Days 6-20 or 7-20, respectively, with litter/fetal parameters evaluated on GD 21 and 28, respectively. Separate studies evaluated toxicokinetic parameters at the mid- and high-dose levels.

RESULTS

There was no maternal toxicity observed at the highest dose levels; however, hematology and serum biochemistry changes were characterized in the range-finding studies. Vorinostat did not induce morphological malformations in either rat or rabbit fetuses. In rats, drug-related developmental toxicity was observed only in the high-dose group and consisted of markedly decreased fetal weight and increases in fetuses with a limited number of skeletal variations. In rabbits, drug-related developmental toxicity was also observed only in the high-dose group and consisted of slightly decreased fetal weight and increases in fetuses with a short 13th rib and incomplete ossification of metacarpals. Maternal exposures to vorinostat based on AUC and Cmax values were comparable at the high-dose levels of both species. Rabbits tolerated higher dosages probably due to more extensive metabolism. Maternal concentrations of vorinostat were approximately 1,000-fold above the known in vitro HDAC inhibitory concentration.

CONCLUSIONS

Review of previous work with valproic acid, another HDAC inhibitor, suggest that the developmental toxicity profiles of these 2 compounds are not the result of HDAC inhibition but involve other mechanisms.

Hepatocyte cell lines: their use, scope and limitations in drug metabolism studies

Gaining knowledge on the metabolism of a drug, the enzymes involved and its inhibition or induction potential is a necessary step in pharmaceutical development of new compounds. Primary human hepatocytes are considered a cellular model of reference, as they express the majority of drug-metabolising enzymes, respond to enzyme inducers and are capable of generating in vitro a metabolic profile similar to what is found in vivo. However, hepatocytes show phenotypic instability and have a restricted accessibility. Different alternatives have been explored in the past recent years to overcome the limitations of primary hepatocytes. These include immortalisation of adult or fetal human hepatic cells by means of transforming tumour virus genes, oncogenes, conditionally immortalised hepatocytes, and cell fusion. New strategies are currently being used to upregulate the expression of drug-metabolising enzymes in cell lines or to derive hepatocytes from progenitor cells. This paper reviews the features of liver-derived cell lines, their suitability for drug metabolism studies as well as the state-of-the-art of the strategies pursued in order to generate metabolically competent hepatic cell lines.

Involvement of cell junctions in hepatocyte culture functionality

In liver, like in other multicellular systems, the establishment of cellular contacts is a prerequisite for normal functioning. In particular, well-defined cell junctions between hepatocytes, including adherens junctions, desmosomes, tight junctions, and gap junctions, are known to play key roles in the performance of liver-specific functionality. In a first part of this review article, we summarize the current knowledge concerning cell junctions and their roles in hepatic (patho)physiology. In a second part, we discuss their relevance in liver-based in vitro modeling, thereby highlighting the use of primary hepatocyte cultures as suitable in vitro models for preclinical pharmaco-toxicological testing. We further describe the actual strategies to regain and maintain cell junctions in these in vitro systems over the long-term.

Trichostatin a enhances gap junctional intercellular communication in primary cultures of adult rat hepatocytes

The effects of histone deacetylase inhibitor Trichostatin A (TSA) on connexin (Cx) expression and gap junctional intercellular communication (GJIC) were investigated in primary cultures of adult rat hepatocytes. GJIC was monitored by using the scrape-loading/dye transfer method. Immunoblotting and immunocytochemistry were used to investigate Cx protein levels and localization. Cx gene expression was studied by means of quantitative reverse transcriptase-polymerase chain reaction. TSA increased Cx32 protein levels and affected negatively the Cx26 protein levels. The latter was preferentially located in the cytosol of cultured cells. TSA also promoted the appearance of Cx43 in the nuclear compartment of primary cultured hepatocytes. Overall, this resulted in enhanced GJIC activity. It is important to note that the time of onset of TSA treatment was crucial for the extent of its outcome and that the effects of TSA on Cx protein levels occurred independently of transcriptional changes. TSA differentially affects Cx proteins in primary rat hepatocyte cultures, suggesting distinct regulation and/or distinct roles of the different Cx species in the control of hepatic homeostasis. TSA enhances GJIC between primary cultured rat hepatocytes, an interesting finding supporting its use to further optimize liver-based in vitro models for pharmacotoxicological purposes.

Suberoylanilide Hydroxamic Acid Enhances Gap Junctional Intercellular Communication via Acetylation of Histone Containing Connexin 43 Gene Locus

A histone deacetylase (HDAC) inhibitor, suberoylanilide hydroxamic acid (SAHA), induces apoptosis in neoplastic cells, but its effect on gap junctional intercellular communication in relation to apoptosis was unclear. Therefore, we carried out a comparative study of the effects of two HDAC inhibitors, SAHA and trichostatin-A, on gap junctional intercellular communication in nonmalignant human peritoneal mesothelial cells (HPMC) and tumorigenic ras oncogene-transformed rat liver epithelial cells (WB-ras) that showed a significantly lower level of gap junctional intercellular communication than did HPMC. Gap junctional intercellular communication was assessed by recovery rate of fluorescence recovery after photobleaching. Treatment of HPMC with SAHA at nanomolar concentrations caused a dose-dependent increase of recovery rate without inducing apoptosis. This effect was accompanied by enhanced connexin 43 (Cx43) mRNA and protein expression and increased presence of Cx43 protein on cell membrane. Trichostatin-A induced apoptosis in HPMC but was less potent than SAHA in enhancing the recovery rate. In contrast, treatment of WB-ras cells with SAHA or trichostatin-A induced apoptosis at low concentrations, in spite of smaller increases in recovery rate, Cx43 mRNA, and protein than in HPMC. Chromatin immunoprecipitation analysis revealed that SAHA enhanced acetylated histones H3 and H4 in the chromatin fragments associated with Cx43 gene in HPMC. These results indicate that SAHA at low concentrations selectively up-regulates Cx43 expression in normal human cells without induction of apoptosis, as a result of histone acetylation in selective chromatin fragments, in contrast to the apoptotic effect observed in tumorigenic WB-ras cells. These results support a cancer therapeutic and preventive role for specific HDAC inhibitors.