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

Primary hepatocytes and their cultures for the testing of drug-induced liver injury

Drug-induced liver injury is a major reason for discontinuation of drug development and withdrawal of drugs from the market. Intensive efforts in the last decades have focused on the establishment and finetuning of liver-based in vitro models for reliable prediction of hepatotoxicity triggered by drug candidates. Of those, primary hepatocytes and their cultures still are considered the gold standard, as they provide an acceptable reflection of the hepatic in vivo situation. Nevertheless, these in vitro systems cope with gradual deterioration of the differentiated morphological and functional phenotype. The present paper gives an overview of traditional and more recently introduced strategies to counteract this dedifferentiation process in an attempt to set up culture models that can be used for long-term testing purposes. The relevance and applicability of such optimized cultures of primary hepatocytes for the testing of drug-induced cholestatic liver injury is demonstrated.

Epigenetic Modifications as Antidedifferentiation Strategy for Primary Hepatocytes in Culture

A well-known problem of cultured primary hepatocytes is their rapid dedifferentiation. During the last years, several strategies to counteract this phenomenon have been developed, of which changing the in vitro environment is the most popular one. However, mimicking the in vivo setting in vitro by adding soluble media additives or the restoration of both cell-cell and cell-extracellular matrix contacts is not sufficient and only delays the dedifferentiation process instead of counteracting it. In this chapter, new strategies to prevent the deterioration of the liver-specific phenotype of primary hepatocytes in culture by targeting the (epi)genetic mechanisms that drive hepatocellular gene expression are described.

A Phase I Protocol of Hydralazine and Valproic Acid in Advanced, Previously Treated Solid Cancers

Smokers experience aberrant gene promoter methylation in their bronchial cells, which may predispose to the development of neoplasia. Hydralazine is a DNA demethylating agent, and valproic acid is a histone deacetylase inhibitor, and both have modest but synergistic anticancer activity in vitro. We conducted a phase I trial combining valproic acid and hydralazine to determine the maximally tolerated dose (MTD) of hydralazine in combination with a therapeutic dose of valproic acid in patients with advanced, unresectable, and previously treated solid cancers. Twenty females and nine males were enrolled, with a median age of 57 years and a median ECOG performance status of 0. Grade 1 lymphopenia and fatigue were the most common adverse effects. Three subjects withdrew for treatment-related toxicities occurring after the DLT observation period, including testicular edema, rash, and an increase in serum lipase accompanied by hyponatremia in one subject each. A true MTD of hydralazine in combination with therapeutic doses of valproic acid was not reached in this trial, and the planned upper limit of hydralazine investigated in this combination was 400 mg/day without grade 3 or 4 toxicities. A median number of two treatment cycles were delivered. One partial response by Response Evaluation Criteria In Solid Tumors criteria was observed, and five subjects experienced stable disease for 3 to 6 months. The combination of hydralazine and valproic acid is simple, nontoxic, and might be appropriate for chemoprevention or combination with other cancer treatments. This trial supports further investigation of epigenetic modification as a new therapeutic strategy.

Transcriptional machinery of TNF-α-inducible YTH domain containing 2 (YTHDC2) gene

We previously demonstrated that a cellular factor, cyclosporin A (CsA) associated helicase-like protein (CAHL) that is identical to YTH domain containing 2 (YTHDC2), forms trimer complex with cyclophilin B and NS5B of hepatitis C virus (HCV) and facilitates HCV genome replication. Gene expression of YTHDC2 was shown in tumor cell lines and tumor necrosis factor (TNF)-α-treated hepatocytes, but not in untreated. However, the function of YTHDC2 in the tumor cells and the mechanism by which the YTHDC2 gene is transcribed in these cells is largely unknown. We first evaluated that the role of YTHDC2 in the proliferation of hepatocellular carcinoma (HCC) cell line Huh7 using RNA interference and found that YTHDC2-downregulated Huh7 were significantly decreased cell growth as compared to control. We next demonstrated that the cAMP response element (CRE) site in the promoter region of the YTHDC2 gene is critical for YTHDC2 transcription. To further investigate the transcription factors bound to the CRE site, we performed chromatin immunoprecipitation assays. Our findings demonstrate that c-Jun and ATF-2 bind to the CRE site in Huh7, and that TNF-α induces the biological activity of these transcription factors in hepatocytes as well as Huh7. Moreover, treatment with the HDAC inhibitor, trichostatin A (TSA), reduces YTHDC2 expression in Huh7 and in TNF-α-stimulated hepatocytes. Collectively, these data show that YTHDC2 plays an important role in tumor cells growth and activation/recruitment of c-Jun and ATF-2 to the YTHDC2 promoter is necessary for the transcription of YTHDC2, and that HDAC activity is required for the efficient expression of YTHDC2 in both of hepatocyte and HCC cells.

Primary hepatocytes and their cultures in liver apoptosis research

Apoptosis not only plays a key role in physiological demise of defunct hepatocytes, but is also associated with a plethora of acute and chronic liver diseases as well as with hepatotoxicity. The present paper focuses on the modelling of this mode of programmed cell death in primary hepatocyte cultures. Particular attention is paid to the activation of spontaneous apoptosis during the isolation of hepatocytes from the liver, its progressive manifestation upon the subsequent establishment of cell cultures and simultaneously to strategies to counteract this deleterious process. In addition, currently applied approaches to experimentally induce controlled apoptosis in this in vitro setting for mechanistic research purposes and thereby its detection using relevant biomarkers are reviewed.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Primary hepatocyte cultures for pharmaco-toxicological studies: at the busy crossroad of various anti-dedifferentiation strategies

Continuously increasing understanding of the molecular triggers responsible for the onset of diseases, paralleled by an equally dynamic evolution of chemical synthesis and screening methods, offers an abundance of pharmacological agents with a potential to become new successful drugs. However, before patients can benefit of newly developed pharmaceuticals, stringent safety filters need to be applied to weed out unfavourable drug candidates. Cost effectiveness and the need to identify compound liabilities, without exposing humans to unnecessary risks, has stimulated the shift of the safety studies to the earliest stages of drug discovery and development. In this regard, in vivo relevant organotypic in vitro models have high potential to revolutionize the preclinical safety testing. They can enable automation of the process, to match the requirements of high-throughput screening approaches, while satisfying ethical considerations. Cultures of primary hepatocytes became already an inherent part of the preclinical pharmaco-toxicological testing battery, yet their routine use, particularly for long-term assays, is limited by the progressive deterioration of liver-specific features. The availability of suitable hepatic and other organ-specific in vitro models is, however, of paramount importance in the light of changing European legal regulations in the field of chemical compounds of different origin, which gradually restrict the use of animal studies for safety assessment, as currently witnessed in cosmetic industry. Fortunately, research groups worldwide spare no effort to establish hepatic in vitro systems. In the present review, both classical and innovative methodologies to stabilize the in vivo-like hepatocyte phenotype in culture of primary hepatocytes are presented and discussed.

EFFECTS OF FLUID SHEAR STRESS AND CIGLITAZONE ON OSTEOBLASTS

Long-term use of thiazolidinedione (TZD) antidiabetic agents in patients with type 2 diabetes mellitus has been shown to increase the incidence of osteoporosis. Mechanical loading can enhance bone mass by promoting bone formation and suppressing bone resorption, which may be beneficial to patients with TZD-induced osteoporosis. In this study, we examined the cooperative effect of fluid shear stress (FSS) and ciglitazone (CIG), a type of TZD, on osteoblasts. The proliferation, osteoblast differentiation-related mRNA expression and translocation of nuclear factor κB (NFκB) of osteoblasts were assessed. The results show that CIG significantly decreased the proliferation of osteoblasts, inhibited the translocation of NFκB to the nucleus and reduced the mRNA expression of COX-2, IGF, Runx2 and OCN. At the same time, CIG also increased the mRNA expression of PPARγ. Conversely, FSS significantly increased the proliferation of osteoblasts, promoted the translocation of NFκB to the nucleus and increased the mRNA expression of COX-2, IGF, Runx2 and OCN but decreased the mRNA expression of PPARγ. When FSS and CIG were combined, FSS counteracted the effects of CIG on osteoblasts. Taken together, the current results suggest that FSS is able to arrest the effects of CIG on the proliferation and differentiation of osteoblasts.

Natural chalcones as dual inhibitors of HDACs and NF-κB

Histone deacetylase enzymes (HDACs) are emerging as a promising biological target for cancer and inflammation. Using a fluorescence assay, we tested the in vitro HDAC inhibitory activity of twenty-one natural chalcones, a widespread group of natural products with well-known anti-inflammatory and antitumor effects. Since HDACs regulate the expression of the transcription factor NF-κB, we also evaluated the inhibitory potential of the compounds on NF-κB activation. Only four chalcones, isoliquiritigenin (no. 10), butein (no. 12), homobutein (no. 15) and the glycoside marein (no. 21) showed HDAC inhibitory activity with IC₅₀ values of 60–190 μM, whereas a number of compounds inhibited TNFα-induced NF-κB activation with IC₅₀ values in the range of 8–41 μM. Interestingly, three chalcones (nos. 10, 12 and 15) inhibited both TNFα-induced NF-κB activity and total HDAC activity of classes I, II and IV. Molecular modeling and docking studies were performed to shed light into dual activity and to draw structure-activity relationships among chalcones (nos. 1–21). To the best of our knowledge this is the first study that provides evidence for HDACs as potential drug targets for natural chalcones. The dual inhibitory potential of the selected chalcones on NF-κB and HDACs was investigated for the first time. This study demonstrates that chalcones can serve as lead compounds in the development of dual inhibitors against both targets in the treatment of inflammation and cancer.

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.

Effect of Trichostatin A on miRNA expression in cultures of primary rat hepatocytes

In the present study, the effect of Trichostatin A (TSA), a histone deacetylase inhibitor, was investigated on the microRNA (miR, miRNA) expression profile in cultured primary rat hepatocytes by means of microarray analysis. Simultaneously, albumin secretory capacity and morphological features of the hepatocytes were evaluated throughout the culture time. In total, 25 out of 348 miRNAs were found to be differentially expressed between freshly isolated hepatocytes and 7-day cultured cells. Nineteen of these miRNAs were connected with 'general metabolism'. miR-21 and miR-126 were shown to be the most up and down regulated miRs upon cultivation and could be linked to the proliferative response triggered in the hepatocytes upon their isolation from the liver. miR-379 and miR-143, on the other hand, were found to be the most up and down regulated miRs upon TSA treatment. Together with the higher expression of miR-122 observed in TSA-treated versus non-treated cultures, we hypothesize that the changes observed for miR-122, miR-143 and miR-379 could be related to the inhibitory effects of TSA on hepatocellular proliferation.

Characterization of spontaneous cell death in monolayer cultures of primary hepatocytes

Monolayer cultures of primary hepatocytes, isolated from freshly removed livers, represent widely used in vitro tools in the area of liver physiology and pathology, pharmacology and toxicology. However, a major shortcoming of these systems is that they cope with dedifferentiation, which is accompanied by spontaneous cell death. The goal of the present study was to elucidate the mechanisms that drive the process of self-generated cell demise in primary hepatocyte cultures. For this purpose, isolated rat hepatocytes were cultivated under conventional conditions, and the occurrence of apoptosis and necrosis was monitored during 4 days by performing a set of acknowledged cell death assays. These included examination of cell morphology by light microscopy, quantification of apoptotic and necrotic cell populations by Hoechst 33342 and propidium iodide in situ staining, assessment of apoptotic and necrotic activities by measuring caspase 3-like activity and extracellular leakage of lactate dehydrogenase, and studying the expression of apoptosis regulators through immunoblot analysis. In essence, two cell death peaks were observed, namely shortly after cell seeding and in the final stages of the cultivation period, both involving apoptotic and necrotic actions. The outcome of this study not only sheds new light onto the molecular processes that underlie spontaneous cell death in primary hepatocyte cultures, but also opens perspectives for the establishment of strategies to increase cell survival in these popular in vitro systems.

Preservation of hepatocellular functionality in cultures of primary rat hepatocytes upon exposure to 4-Me2N-BAVAH, a hydroxamate-based HDAC-inhibitor

Great efforts are being put in the development/optimization of reliable and highly predictive models for high-throughput screening of efficacy and toxicity of promising drug candidates. The use of primary hepatocyte cultures, however, is still limited by the occurrence of phenotypic alterations, including loss of xenobiotic biotransformation capacity. In the present study, the differentiation-stabilizing effect of a new histone deacetylase inhibitor 5-(4-dimethylaminobenzoyl)-aminovaleric acid hydroxamide (4-Me(2)N-BAVAH), a structural Trichostatin A (TSA)-analogue with a more favourable pharmaco-toxicological profile, was studied at a genome-wide scale by means of microarray analysis. Several genes coding for xenobiotic biotransformation enzymes were found to be positively regulated upon exposure to 4-Me(2)N-BAVAH. For CYP1A1/2B1/3A2, these observations were confirmed by qRT-PCR and immunoblot analysis. In addition, significantly higher 7-ethoxyresorufin-O-deethylase and 7-pentoxyresorufin-O-dealkylase activity levels were measured. These effects were accompanied by an increased expression of CCAAT/enhancer binding protein alpha and hepatic nuclear factor (HNF)4α, but not of HNF1α. Finally, 4-Me(2)N-BAVAH was found to induce histone H3 acetylation at the proximal promoter of the albumin, CYP1A1 and CYP2B1 genes, suggesting that chromatin remodelling is directly involved in the transcriptional regulation of these genes. In conclusion, histone deacetylase inhibitors prove to be efficient agents for better maintaining a differentiated hepatic phenotype in rat hepatocyte cultures.

Histone deacetylase inhibition and the regulation of cell growth with particular reference to liver pathobiology

The transcriptional activity of genes largely depends on the accessibility of specific chromatin regions to transcriptional regulators. This process is controlled by diverse post-transcriptional modifications of the histone amino termini of which reversible acetylation plays a vital role. Histone acetyltransferases (HATs) are responsible for the addition of acetyl groups and histone deacetylases (HDACs) catalyse the reverse reaction. In general, though not exclusively, histone acetylation is associated with a positive regulation of transcription, whereas histone deacetylation is correlated with transcriptional silencing. The elucidation of unequivocal links between aberrant action of HDACs and tumorigenesis lies at the base of key scientific importance of these enzymes. In particular, the potential benefit of HDAC inhibition has been confirmed in various tumour cell lines, demonstrating antiproliferative, differentiating and pro-apoptotic effects. Consequently, the dynamic quest for HDAC inhibitors (HDIs) as a new class of anticancer drugs was set off, resulting in a number of compounds that are currently evaluated in clinical trials. Ironically, the knowledge with respect to the expression pattern and function of individual HDAC isoenzymes remains largely elusive. In the present review, we provide an update of the current knowledge on the involvement of HDACs in the regulation of fundamental cellular processes in the liver, being the main site for drug metabolism within the body. Focus lies on the involvement of HDACs in the regulation of growth of normal and transformed hepatocytes and the transdifferentiation process of stellate cells. Furthermore, extrapolation of our present knowledge on HDAC functionality towards innovative treatment of malignant and non-malignant, hyperproliferative and inflammatory disorders is discussed.

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.

Effects and mechanisms of silibinin on human hepatocellular carcinoma xenografts in nude mice

AIM: To investigate the in vivo effects and mechanisms of silibinin on the growth of hepatocellular carcinoma (HCC) xenografts in nude mice.

METHODS: Nude mice bearing HuH7 xenografts were used to assess the anti-HCC effects and mechanisms of silibinin.

RESULTS: Silibinin resulted in a potent dose-dependent reduction of HuH7 xenografts in association with a significant decrease in Ki-67 and α-fetoprotein production, nuclear NF-κB content, polo-like kinase 1, Rb phosphorylation, and E2F1/DP1 complex, but increased p27/CDK4 complex and checkpoint kinase 1 expression, suggesting that the in vivo effects of silibinin are mediated by inhibiting G1-S transition of the cell cycle. Silibinin-induced apoptosis of HuH7 xenografts was associated with inhibited survivin phosphorylation. Silibinin-reduced growth of HuH7 xenografts was associated with decreased p-ERK, increased PTEN expression and the activity of silibinin was correlated with decreased p-Akt production, indicating involvement of PTEN/PI₃K/Akt and ERK pathways in its in vivo anti-HCC effects. Silibinin-reduced growth of HuH7 xenografts was also associated with a significant increase in AC-H3 and AC-H4 expression and the production of superoxide dismutase (SOD)-1.

CONCLUSION: Silibinin reduces HCC xenograft growth through the inhibition of cell proliferation, cell cycle progression and PTEN/P-Akt and ERK signaling, inducing cell apoptosis, and increasing histone acetylation and SOD-1 expression.

NF-kappaB promotes iNOS and VEGF expression in salivary gland adenoid cystic carcinoma cells and enhances endothelial cell motility in vitro

Tumour cell-derived angiogenic-related factors and endothelial cell mobility are essential for neovascularization and haematogenous metastasis of adenoid cystic carcinoma (ACC) of the human salivary glands. Our previous study demonstrated that vascular endothelial growth factor (VEGF), inducible nitric oxide synthase (iNOS), and nuclear factor kappaB (NF-kappaB) staining correlated with ACC microvessel density. However, there still remains a lack of direct evidence to clarify the function of liposaccharide-induced NF-kappaB activity in ACC angiogenesis.

OBJECTIVES

This study aimed to demonstrate the function of liposaccharide-induced NF-kappaB signalling pathway (which would be relevant to angiogenesis) of ACC cell lines.

MATERIALS AND METHODS

A PCMV-IkappaBalphaM vector transfection assay was performed to inhibit NF-kappaB activity. Constitutive and liposaccharide-induced NF-kappaB activity, along with VEGF and iNOS expression, was detected by electrophoretic mobility shift assays, reverse transcription-polymerase chain reaction and Western blot assay. Tumour cell-derived NO(2) (-) was evaluated by a nitrite determination assay, and endothelial cell mobility was investigated by endothelial cell proliferation, migration and tube formation assays.

RESULTS

We demonstrated that regulation of VEGF and iNOS expression utilized the NF-kappaB signalling pathway in ACC cell lines. In the constitutive and liposaccharide-induced condition, NF-kappaB activity was blocked by phosphorylation-defective IkappaBalpha. Accordingly, mRNA and protein expression of VEGF and iNOS were significantly decreased, with a great drop in endogenous NO(2) (-). Furthermore, supernatant from ACC-2/IkBaM and ACC-M/IkBaM cells resulted in much lower endothelial cell mobility than that from ACC-2 and ACC-M cells.

CONCLUSIONS

Our results suggest that NF-kappaB promotes iNOS and VEGF expression in salivary gland ACC cells and enhances endothelial cell motility in vitro.

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.