A class of hybrid polar inducers of transformed cell differentiation inhibits histone deacetylases

Differentiation therapy of human cancer: basic science and clinical applications

Current cancer therapies are highly toxic and often nonspecific. A potentially less toxic approach to treating this prevalent disease employs agents that modify cancer cell differentiation, termed 'differentiation therapy.' This approach is based on the tacit assumption that many neoplastic cell types exhibit reversible defects in differentiation, which upon appropriate treatment, results in tumor reprogramming and a concomitant loss in proliferative capacity and induction of terminal differentiation or apoptosis (programmed cell death). Laboratory studies that focus on elucidating mechanisms of action are demonstrating the effectiveness of 'differentiation therapy,' which is now beginning to show translational promise in the clinical setting.

Genes, chromatin, and breast cancer: an epigenetic tale

The production of heritable changes in gene expression is the driving force in the development and progression of breast cancer. Such changes can result from mutations or from epigenetic events such as hypermethylation of DNA and hypoacetylation of histones. Histone acetylation and DNA methylation are major determinants of chromatin structure, and chromatin structure is a primary regulator of gene transcription. Cancer cells frequently contain both mutated genes and genes with altered expression due to one or more epigenetic mechanisms. This review describes the epigenetic changes that disrupt normal chromatin architecture and modify the expression of key genes in breast cancer cells. The structural integrity of the latter genes is usually intact, but their expression has been substantially altered due to methylation in their promoter region or deacetylation of histones that interact with their promoter region or both mechanisms. Genes affected by epigenetic changes in breast cancers include HoxA5, p21WAF, gelsolin, BRCA1, BRCA2, E-cadherin, steroid hormone receptors, and retinoic acid receptor II. Because these epigenetic modifications are usually reversible by treatment with certain drugs, they represent vulnerabilities in the cancer cell that can be exploited as novel targets for new prevention and therapeutic strategies.

Transcriptional activation of p21(WAF1/CIP1) by apicidin, a novel histone deacetylase inhibitor

Apicidin [cyclo(N-O-methyl-L-tryptophanyl-L-isoleucinyl-D-pipecolinyl-l-2-amino-8-oxodecanoyl)], a novel histone deacetylase inhibitor, has been identified as an antiprotozoal and antiproliferative agent. In this study, we show apicidin induces transcriptional activation of p21(WAF1/CIP1) (p21) in human prostate carcinoma cells. Apicidin induces expression of p21 protein and mRNA and activation of p21 promoter-luciferase reporter constructs. Apicidin causes an accumulation of acetylated histones H3 and H4 in total cellular chromatin. Chromatin immunoprecipitation shows p21 promoter DNA is associated with hyperacetylated histones H3 and H4 after treatment with apicidin. Therefore, the data here demonstrate that apicidin activates p21 transcription associated with the acetylation of histones H3 and H4.

Induction of differentiation and apoptosis- a possible strategy in the treatment of adult acute myelogenous leukemia

A differentiation block with accumulation of immature myeloid cells characterizes acute myelogenous leukemia (AML). However, native AML cells often show some morphological signs of differentiation that allow a classification into different subsets, and further differentiation may be induced by exposure to various soluble mediators, e.g., all trans-retinoic acid (ATRA) and several cytokines. Combination therapy with ATRA and chemotherapy should now be regarded as the standard treatment for the acute promyelocytic leukemia variant of AML. Several agents can induce leukemic cell differentiation for other AML subtypes, although these effects differ between patients. Differentiation may then be associated with induction of apoptosis, and differentiation-inducing therapy may therefore become useful in combination with intensive chemotherapy to increase the susceptibility of AML blasts to drug-induced apoptosis. However, it should be emphasized that differentiation and apoptosis can occur as separate events with different regulation in AML cells, and future studies in AML should therefore focus on: A) the identification of new agents with more predictable effects on differentiation and apoptosis; B) the use of clinical and laboratory parameters to define new subsets of AML patients in which differentiation/apoptosis induction has a predictable and beneficial effect, and C) further characterization of how AML blast sensitivity to drug-induced apoptosis is modulated by differentiation induction.

Trichostatin A reverses skewed expression of CD154, interleukin-10, and interferon-gamma gene and protein expression in lupus T cells

In systemic lupus erythematosus (SLE), T helper cells exhibit increased and prolonged expression of cell-surface CD40 ligand (CD154), spontaneously overproduce interleukin-10 (IL-10), but underproduce interferon-gamma (IFN-gamma). We tested the hypothesis that the imbalance of these gene products reflects skewed expression of CD154, IL-10, and IFN-gamma genes. Here, we demonstrate that the histone deacetylase inhibitor, trichostatin A, significantly down-regulated CD154 and IL-10 and up-regulated IFN-gamma gene expression in SLE T cells. This reversal corrected the aberrant expression of these gene products, thereby enhancing IFN-gamma production and inhibiting IL-10 and CD154 expression. That trichostatin A can simultaneously reverse the skewed expression of multiple genes implicated in the immunopathogenesis of SLE suggests that this pharmacologic agent may be a candidate for the treatment of this autoimmune disease.

Broad spectrum antiprotozoal agents that inhibit histone deacetylase: structure-activity relationships of apicidin. Part 1

Apicidin, a natural product recently isolated at Merck, inhibits both mammalian and protozoan histone deacetylases (HDACs). The conversion of apicidin, a nanomolar inhibitor of HDACs, into a series of side-chain analogues that display picomolar enzyme affinity is described within this structure-activity study.

Potent histone deacetylase inhibitors built from trichostatin A and cyclic tetrapeptide antibiotics including trapoxin

Trichostatin A (TSA) and trapoxin (TPX) are potent inhibitors of histone deacetylases (HDACs). TSA is proposed to block the catalytic reaction by chelating a zinc ion in the active-site pocket through its hydroxamic acid group. On the other hand, the epoxyketone is suggested to be the functional group of TPX capable of alkylating the enzyme. We synthesized a novel TPX analogue containing a hydroxamic acid instead of the epoxyketone. The hybrid compound cyclic hydroxamic acid-containing peptide (CHAP) 1 inhibited HDAC1 at low nanomolar concentrations. The HDAC1 inhibition by CHAP1 was reversible as it was by TSA, in contrast to the irreversible inhibition by TPX. CHAP with an aliphatic chain length of five, which corresponded to that of acetylated lysine, was stronger than those with other lengths. These results suggest that TPX is a substrate mimic and that the replacement of the epoxyketone with the hydroxamic acid converted TPX to an inhibitor chelating the zinc like TSA. Interestingly, HDAC6, but not HDAC1 or HDAC4, was resistant to TPX and CHAP1, whereas TSA inhibited these HDACs to a similar extent. HDAC6 inhibition by TPX at a high concentration was reversible, probably because HDAC6 is not alkylated by TPX. We further synthesized the counterparts of all known naturally occurring cyclic tetrapeptides containing the epoxyketone. HDAC1 was highly sensitive to all these CHAPs much more than HDAC6, indicating that the structure of the cyclic tetrapeptide framework affects the target enzyme specificity. These results suggest that CHAP is a unique lead to develop isoform-specific HDAC inhibitors.

Potent histone deacetylase inhibitors built from trichostatin A and cyclic tetrapeptide antibiotics including trapoxin

Trichostatin A (TSA) and trapoxin (TPX) are potent inhibitors of histone deacetylases (HDACs). TSA is proposed to block the catalytic reaction by chelating a zinc ion in the active-site pocket through its hydroxamic acid group. On the other hand, the epoxyketone is suggested to be the functional group of TPX capable of alkylating the enzyme. We synthesized a novel TPX analogue containing a hydroxamic acid instead of the epoxyketone. The hybrid compound cyclic hydroxamic acid-containing peptide (CHAP) 1 inhibited HDAC1 at low nanomolar concentrations. The HDAC1 inhibition by CHAP1 was reversible as it was by TSA, in contrast to the irreversible inhibition by TPX. CHAP with an aliphatic chain length of five, which corresponded to that of acetylated lysine, was stronger than those with other lengths. These results suggest that TPX is a substrate mimic and that the replacement of the epoxyketone with the hydroxamic acid converted TPX to an inhibitor chelating the zinc like TSA. Interestingly, HDAC6, but not HDAC1 or HDAC4, was resistant to TPX and CHAP1, whereas TSA inhibited these HDACs to a similar extent. HDAC6 inhibition by TPX at a high concentration was reversible, probably because HDAC6 is not alkylated by TPX. We further synthesized the counterparts of all known naturally occurring cyclic tetrapeptides containing the epoxyketone. HDAC1 was highly sensitive to all these CHAPs much more than HDAC6, indicating that the structure of the cyclic tetrapeptide framework affects the target enzyme specificity. These results suggest that CHAP is a unique lead to develop isoform-specific HDAC inhibitors.

Mechanisms of suberoylanilide hydroxamic acid inhibition of mammary cell growth

The mechanism of suberoylanilide hydroxamic acid in cell growth inhibition involved induction of pRb-2/p130 interaction and nuclear translocation with E2F-4, followed by significant repression in E2F-1 and PCNA nuclear levels, which led to inhibition in DNA synthesis in mammary epithelial cell lines.

Expression and Up-regulation of alternatively spliced transcripts of melastatin, a melanoma metastasis-related gene, in human melanoma cells

Loss of expression of a novel suppressor of metastasis, melastatin (MLSN1), has recently been reported to correlate with metastatic potential of melanoma cells. Using differential display analysis, we identified MLSN1 among genes overexpressed in pigmented metastatic human melanoma cells treated with the differentiation inducer hexamethylene bisacetamide (HMBA). In this study, we show that multiple short transcripts of MLSN1 are present in melanocytes and pigmented metastatic melanoma cell lines while the full-length 5. 4-kb mRNA is detectable only in melanocytes. Treatment of pigmented melanoma cells with the differentiation-inducing agent, HMBA, results in up-regulation of the 5.4-kb MLSN1 mRNA as well as short RNAs. Analysis of a panel of nonpigmented primary and metastatic melanoma cell lines showed weak expression of a 1.8-kb mRNA in a few melanoma cell lines. Northern blot and RT-PCR analyses with DNA probes and oligonucleotide primers that correspond to distinct regions of full-length MLSN1 mRNA indicated that the short transcripts contained sequences corresponding primarily to either 5'- or 3'-end of the 5.4-kb mRNA. HMBA appears to up-regulate MLSN1 transcripts derived mainly from the 5'-end. Modulators of cAMP and protein kinase C pathways had no significant effect on MLSN1 expression. Our data show that multiple MLSN1 transcripts, both constitutively expressed and inducible, are present in cultured pigmented melanoma cells, and suggest that MLSN1 expression can be regulated at the level of both transcription and mRNA processing.

Lovastatin induces apoptosis in a primitive neuroectodermal tumor cell line in association with RB down-regulation and loss of the G1 checkpoint

To develop a new approach to the treatment of primitive neuroectodermal tumors we evaluated the effect of the HMG-CoA reductase inhibitor lovastatin on the Ewing's sarcoma cell line CHP-100. Lovastatin induced neural morphology and markers including neuron-specific enolase and neurofilament protein. The acquisition of neural morphology required new mRNA synthesis, and cDNA microarray analysis confirmed that lovastatin altered the program of gene expression. After morphologic differentiation the cells underwent rapidly progressive apoptosis. In normal development of neuronal progenitors, differentiation signals trigger p21WAF1 accumulation, RB hypophosphorylation, enhanced RB-E2F-1 association, and G1 arrest, and these events have been shown to protect from apoptosis. In contrast, in the Ewing's sarcoma cells lovastatin triggered differentiation without causing cell cycle arrest: p21WAF1 was not induced, RB remained hyperphosphorylated, and RB protein expression and RB-E2F-1 association were markedly downregulated, suggesting that loss of an RB-regulated G1 checkpoint promoted apoptosis. Consistent with this hypothesis, adenoviral p21WAF1 decreased DNA synthesis and partially protected from lovastatin-induced cytotoxicity. The data demonstrate a new model for examining the genetic regulation of cell fate in a neural progenitor tumor and suggest a new approach to the treatment of this neoplasm.

Modifying histones to tame cancer: clinical development of sodium phenylbutyrate and other histone deacetylase inhibitors

Compounds that inhibit histone deacetylase may enable the re-expression of silenced regulatory genes in neoplastic cells, reversing the malignant phenotype. Although several molecules that inhibit histone deacetylase are undergoing preclinical development, butyric acid derivatives have undergone clinical investigation for several years, initially for non-malignant indications and more recently for the treatment of cancer. Of the butyric acid derivatives, sodium phenylbutyrate has undergone the most extensive systematic investigation. Administration of phenylbutyrate by iv. and oral routes is well-tolerated clinically at concentrations which effect acetylation of histones in vitro. Higher doses lead to reversible CNS depression. The studies presented to date have been Phase I studies and do not enable assessment of efficacy. However, current development of phenylbutyrate is proceeding in combination with other agents based on rational biologically-based in vitro studies. The parallel development of combination therapy including phenylbutyrate and early clinical development of other, more potent histone deacetylase inhibitors will hopefully lead to feasible, clinically tolerable strategies for altering the malignant phenotype of cancer cells.

Histone deacetylase inhibitors and retinoic acids inhibit growth of human neuroblastoma in vitro

BACKGROUND

Neuroblastoma is a common childhood cancer with a poor overall prognosis. Retinoic acids (RAs) have been studied as a potential therapy, showing promise in recurrent disease. The histone deacetylase inhibitor (HDACI) M-carboxycinnamic acid bishydroxamide (CBHA) is another potential therapy, which we recently described. Combinations of RAs and HDACIs currently under investigation display synergy in certain neoplasms. In this study, we evaluate the effect of combinations of RAs and HDACIs on human neuroblastoma cells.

PROCEDURE

Established cell lines were cultured in increasing concentrations of HDACIs, RAs, and combinations thereof. Following exposure, viable cell number was quantified by trypan blue dye exclusion on a hemacytometer. Cell cycle analysis was performed by propidium iodide staining and FACS.

RESULTS

All assayed HDACIs and RAs decreased viable cell number. Lower concentrations of each agent were effective when the two were combined. The primary reason for decreased cell number appears to be apoptosis following HDACI exposure and G1 arrest following RA exposure. Both effects are seen with cotreatment. Caspase inhibition abrogates the apoptotic response.

CONCLUSIONS

CBHA causes apoptosis of human neuroblastoma in vitro, an effect that can add to the effects of RA. HDACIs and RAs inhibit neuroblastoma in significantly lower concentrations when used together than when used individually. Combination therapy may improve the ultimate efficacy while reducing the side effects of these agents in clinical use.

Activation of the p21WAF1/CIP1 promoter independent of p53 by the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) through the Sp1 sites

Suberoylanilide hydroxamic acid (SAHA) is a novel histone deacetylase inhibitor with high potency in inducing differentiation of cultured murine erythroleukemia cells. We have recently demonstrated that SAHA induces cell cycle arrest and apoptosis in human breast cancer cells, accompanied by up-regulation of the cyclin-dependent kinase inhibitor, p21WAF1/CIP1, via a p53-independent mechanism. In this study, we used p21 gene expression as a model system to elucidate the molecular mechanism(s) underlying SAHA-mediated gene activation. Treatment of human breast cancer cell line MCF7 cells with SAHA induced p21 mRNA as a consequence of an immediate-early gene activation. Moreover, SAHA activated the p21 promoter primarily through two Spl sites located at -82 and -69 relative to the transcription start site. Furthermore, Sp1 and Sp3 proteins were the major factors binding to the Spl site of the p21 promoter. However, SAHA did not alter their DNA binding activities, suggesting that SAHA mediates p21 promoter activity by a mechanism other than altering the DNA binding activities of Sp1 and Sp3. Further studies using the GAL4 luciferase assay system demonstrated that both GAL4-Sp1 and GAL4-Sp3 fusion proteins supported SAHA-mediated gene activation from a promoter driven by five GAL4 DNA binding sites, and that GAL4-Sp3 fusion protein was suppressive in the absence of SAHA treatment. Collectively, our results suggest that SAHA activates the p21 promoter through the Spl sites, and that both Spl and Sp3 proteins can mediate SAHA-induced gene activation.

Rapid induction of histone hyperacetylation and cellular differentiation in human breast tumor cell lines following degradation of histone deacetylase-1

Quinidine inhibits proliferation and promotes cellular differentiation in human breast tumor epithelial cells. Previously we showed quinidine arrested MCF-7 cells in G(1) phase of the cell cycle and led to a G(1) to G(0) transition followed by apoptotic cell death. The present experiments demonstrated that MCF-7, MCF-7ras, T47D, MDA-MB-231, and MDA-MB-435 cells transiently differentiate before undergoing apoptosis in response to quinidine. The cells accumulated lipid droplets, and the cytokeratin 18 cytoskeleton was reorganized. Hyperacetylated histone H4 appeared within 2 h of the addition of quinidine to the medium, and levels were maximal by 24 h. Quinidine-treated MCF-7 cells showed elevated p21(WAF1), hypophosphorylation and suppression of retinoblastoma protein, and down-regulation of cyclin D1, similar to the cell cycle response observed with cells induced to differentiate by histone deacetylase inhibitors, trichostatin A, and trapoxin. Quinidine did not show evidence for direct inhibition of histone deacetylase enzymatic activity in vitro. HDAC1 was undetectable in MCF-7 cells 30 min after addition of quinidine to the growth medium. The proteasome inhibitors MG-132 and lactacystin completely protected HDAC1 from the action of quinidine. We conclude that quinidine is a breast tumor cell differentiating agent that causes the loss of HDAC1 via a proteasomal sensitive mechanism.

Aminohexanoic hydroxamate is a potent inducer of the differentiation of mouse neuroblastoma cells

Deoxyhypusine synthase is the key enzyme for modifying a lysine residue to hypusine in the cellular protein eukaryotic initiation factor 5A (eIF-5A). Deletion of the deoxyhypusine synthase or the eIF-5A gene in yeast produces lethal phenotype. Inhibition of deoxyhypusine synthase by 1-guanidino-7-aminoheptane (GC7) suppresses tumor cell growth. Hypusine formation represents one of the most specific polyamine-dependent biochemical reactions. In view of the importance of polyamines in growth regulation and cancer biology, deoxyhypusine synthase has been considered to be a good target for chemotherapeutic drug design. Using GC7 as a prototype we have synthesized and tested three classes of diamine analogs, namely, guanidino-, pyrimidino-, and hydroxamate derivatives, as potential inhibitors for deoxyhypusine synthase. Our study shows that (i) among all the compounds tested, GC7 remained to be the most potent inhibitor for deoxyhypusine synthase; (ii) N,N'-bispyrimidino-1, 9-diaminononane, although a poor inhibitor of deoxyhypusine synthase, was a potent growth inhibitor; and (iii) one of the hydroxamate derivatives, 6-aminohexanoic hydroxamate (HC6), prominently induced the differentiation of mouse neuroblastoma cells at sub-millimolar concentrations. Interestingly, other hydroxamates with different chain length were not nearly as effective as HC6 in inducing neuroblastoma cell differentiation. The effect of HC6 was also unique in that it could induce neurite outgrowth and the expression of neuron-specific genes such as synapsin I and MAP-2 in neuroblastoma cells in the absence of other promoting agents such as cAMP. The effect of HC6 on neuroblastoma cell differentiation was comparable with, or better than that of N(6),O(2)'-dibutyryl cAMP (Bt(2)cAMP), a standard reagent commonly used for inducing the differentiation of mouse and human neuroblastoma cells in culture.

P21-dependent g(1)arrest with downregulation of cyclin D1 and upregulation of cyclin E by the histone deacetylase inhibitor FR901228

Depsipeptide, FR901228, a novel cyclic peptide inhibitor of histone deacetylase with a unique cytotoxicity profile is currently in phase I clinical trials. Here we demonstrate that, in addition to G2/M arrest, FR901228 causes G1 arrest with Rb hypophosphorylation. In vitro kinase assays demonstrated no direct inhibition of CDK activity, however, an inhibition was observed in CDKs extracted from cells exposed to FR901228. Cyclin D1 protein disappeared between 6 and 12 hours after treatment with FR901228, whereas cyclin E was upregulated. While it did not induce wt p53, FR901228 did induce p21(WAF1/CIP1)in a p53-independent manner. Cell clones lacking p21 were not arrested in G1 phase, but continued DNA synthesis and were arrested in G2/M phase following FR901228 treatment. Finally, FR901228 blunted ERK-2/MAPK activation by EGF whereas early signal transduction events remained intact since overall cellular tyrosine phosphorylation after EGF stimulation was unaffected. Thus, FR901228, while not directly inhibiting kinase activity, causes cyclin D1 downregulation and a p53-independent p21 induction, leading to inhibition of CDK and dephosphorylation of Rb resulting in growth arrest in the early G1 phase. In contrast to the G1 arrest, the G2/M arrest is p21-independent, but is associated with significant cytotoxicity.

Histone deacetylase inhibitor selectively induces p21WAF1 expression and gene-associated histone acetylation

Histone deacetylases (HDACs) catalyze the removal of acetyl groups on the amino-terminal lysine residues of core nucleosomal histones. This activity is associated generally with transcriptional repression. We have reported previously that inhibition of HDAC activity by hydroxamic acid-based hybrid polar compounds, such as suberoylanilide hydroxamic acid (SAHA), induces differentiation and/or apoptosis of transformed cells in vitro and inhibits tumor growth in vivo. SAHA is a potentially new therapeutic approach to cancer treatment and is in Phase I clinical trials. In several tumor cell lines examined, HDAC inhibitors alter the expression of less than 1% of expressed genes, including the cell cycle kinase inhibitor p21(WAF1). In T24 bladder carcinoma cells, SAHA induces up to a 9-fold increase in p21(WAF1) mRNA and protein, which is, at least in part, because of an increase in the rate of transcription of the gene. SAHA causes an accumulation of acetylated histones H3 and H4 in total cellular chromatin by 2 h, which is maintained through 24 h of culture. An increase in the accumulation of acetylated H3 and H4 was detected throughout the p21(WAF1) promoter and the structural gene after culture with SAHA. The level of histone acetylation did not change in chromatin associated with the actin and p27 genes, and their mRNA expression was not altered during culture of T24 cells with SAHA. Thus, the present findings indicate that the induction of p21(WAF1) by SAHA is regulated, at least in part, by the degree of acetylation of the gene-associated histones and that this induced increase in acetylation is gene selective.

Histone deacetylase inhibitors: inducers of differentiation or apoptosis of transformed cells

Histone deacetylase (HDAC) inhibitors have been shown to be potent inducers of growth arrest, differentiation, and/or apoptotic cell death of transformed cells in vitro and in vivo. One class of HDAC inhibitors, hydroxamic acid-based hybrid polar compounds (HPCs), induce differentiation at micromolar or lower concentrations. Studies (x-ray crystallographic) showed that the catalytic site of HDAC has a tubular structure with a zinc atom at its base and that these HDAC inhibitors, such as suberoylanilide hydroxamic acid and trichostatin A, fit into this structure with the hydroxamic moiety of the inhibitor binding to the zinc. HDAC inhibitors cause acetylated histones to accumulate in both tumor and normal tissues, and this accumulation can be used as a marker of the biologic activity of the HDAC inhibitors. Hydroxamic acid-based HPCs act selectively to inhibit tumor cell growth at levels that have little or no toxicity for normal cells. These compounds also act selectively on gene expression, altering the expression of only about 2% of the genes expressed in cultured tumor cells. In general, chromatin fractions enriched in actively transcribed genes are also enriched in highly acetylated core histones, whereas silent genes are associated with nucleosomes with a low level of acetylation. However, HDACs can also acetylate proteins other than histones in nucleosomes. The role that these other targets play in the induction of cell growth arrest, differentiation, and/or apoptotic cell death has not been determined. Our working hypothesis is that inhibition of HDAC activity leads to the modulation of expression of a specific set of genes that, in turn, result in growth arrest, differentiation, and/or apoptotic cell death. The hydroxamic acid-based HPCs are potentially effective agents for cancer therapy and, possibly, cancer chemoprevention.

Cyclin-dependent kinase inhibitors: novel anticancer agents

In current models of cell cycle control, the transition between different cell cycle states is regulated at checkpoints. Transition through the cell-cycle is induced by a family of protein kinase holoenzymes, the cyclin-dependent kinases (CDKs) and their heterodimeric cyclin partner. Orderly progression through the cell-cycle involves co-ordinated activation of the CDKs, which in the presence of an associated CDK-activating kinase, phosphorylate target substrates including members of the 'pocket protein' family. This family includes the product of the retinoblastoma susceptibility gene (the pRb protein) and the related p107 and p130 proteins. Activity of these holoenzymes is regulated by post-translational modification. Phosphorylation of inhibitory sites on a conserved threonine residue within the activation segment is regulated by CDK7/cyclin H, referred to as CDK-activating kinase [1]. In addition, the cdc25 phosphatases activate the CDKs by dephosphorylating their inhibitory tyrosine and threonine phosphorylated residues [2,3]. Among the many roles for endogenous inhibitors (CDKIs), including members of the p21(CIP1/Waf1) family and the p16 family, one role is to regulate cyclin activity. Cellular neoplastic transformation is accompanied by loss of regulation of cell cycle checkpoints in conjunction with aberrant expression of CDKs and/or cyclins and the loss or mutation of the negative regulators (the CDKIs or the pocket protein pRb). One strategy to inhibit malignant cellular proliferation involves inhibiting CDK activity or enhancing function of the CDKI. Novel inhibitors of CDKs showing promise in the clinic include flavopiridol and UCN-01, which show early evidence of human tolerability in clinical trials. This review examines pertinent advances in the field of CDK inhibitors.

Histone deacetylases, transcriptional control, and cancer

A key event in the regulation of eukaryotic gene expression is the posttranslational modification of nucleosomal histones, which converts regions of chromosomes into transcriptionally active or inactive chromatin. The most well studied posttranslational modification of histones is the acetylation of epsilon-amino groups on conserved lysine residues in the histones' amino-terminal tail domains. Significant advances have been made in the past few years toward the identification of histone acetyltransferases and histone deacetylases. Currently, there are over a dozen cloned histone acetyltransferases and at least eight cloned human histone deacetylases. Interestingly, many histone deacetylases can function as transcriptional corepressors and, often, they are present in multi-subunit complexes. More intriguing, at least some histone deacetylases are associated with chromatin-remodeling machines. In addition, several studies have pointed to the possible involvement of histone deacetylases in human cancer. The availability of the cloned histone deacetylase genes has provided swift progress in the understanding of the mechanisms of deacetylases, their role in transcription, and their possible role in health and disease.

Characterization of two putative histone deacetylase genes from Aspergillus nidulans

In eukaryotic organisms, acetylation of core histones plays a key role in the regulation of transcription. Multiple histone acetyltransferases (HATs) and histone deacetylases (HDACs) maintain a dynamic equilibrium of histone acetylation. The latter form a highly conserved protein family in many eukaryotic species. In this paper, we report the cloning and sequencing of two putative histone deacetylase genes (rpdA, hosA) of Aspergillus nidulans, which are the first to be analyzed from filamentous fungi. Hybridization with a chromosome-specific cosmid library of A. nidulans allowed the localization of rpdA to chromosome III and hosA to chromosome II, respectively. PCR analyses and Southern hybridization experiments revealed that no further members of the RPD3 family are present in the genome of the fungus. Although sequence alignment displays significant amino acid similarity to other eukaryotic RPD3-type deacetylases, the deduced RPDA sequence reveals an unusual 200-amino acid extension at the C-terminus. Expression of both genes was determined by RNA blot analysis. Treatment of the cells with trichostatin A (TSA), a potent inhibitor of HDACs, was found to stimulate expression of rpdA of A. nidulans.

Histone deacetylase inhibitors trigger a G2 checkpoint in normal cells that is defective in tumor cells

Important aspects of cell cycle regulation are the checkpoints, which respond to a variety of cellular stresses to inhibit cell cycle progression and act as protective mechanisms to ensure genomic integrity. An increasing number of tumor suppressors are being demonstrated to have roles in checkpoint mechanisms, implying that checkpoint dysfunction is likely to be a common feature of cancers. Here we report that histone deacetylase inhibitors, in particular azelaic bishydroxamic acid, triggers a G2 phase cell cycle checkpoint response in normal human cells, and this checkpoint is defective in a range of tumor cell lines. Loss of this G2 checkpoint results in the tumor cells undergoing an aberrant mitosis resulting in fractured multinuclei and micronuclei and eventually cell death. This histone deacetylase inhibitor-sensitive checkpoint appears to be distinct from G2/M checkpoints activated by genotoxins and microtubule poisons and may be the human homologue of a yeast G2 checkpoint, which responds to aberrant histone acetylation states. Azelaic bishydroxamic acid may represent a new class of anticancer drugs with selective toxicity based on its ability to target a dysfunctional checkpoint mechanism in tumor cells.

Cloning and characterization of a novel human class I histone deacetylase that functions as a transcription repressor

Histone acetylation alters chromatin state by modifying lysines on histone and plays an important role in modulating gene transcription. A dynamic balance of histone acetylation/deacetylation is maintained by histone acetyltransferases and histone deacetylases. Emerging evidence suggests that a family of histone deacetylases may exist to regulate diverse cellular functions, including chromatin structure, gene expression, cell cycle progression, and oncogenesis. We describe here a novel human histone deacetylase, named HDAC8, cloned from human kidney. HDAC8 encodes 377 amino acid residues and shares extensive homology to several known HDACs, in particular a histone deacetylase from Arabidopsis thaliana. Northern blot analyses revealed that HDAC8 expression pattern for HDAC8 is distinct from that for HDAC1 and HDAC3, and expression of HDAC8 mRNA occurs in multiple organs including heart, lung, kidney, and pancreas. HDAC8 mRNA was also observed in several cell lines derived from cancerous tissues. When expressed in HEK293 cells, HDAC8 exhibited deacetylase activity toward acetylated histone, indicating that this protein is a bona fide histone deacetylase. Its histone deacetylase activity was inhibited by trichostatin and other known histone deacetylase inhibitors. Furthermore, active recombinant HDAC8 was expressed and purified from Escherichia coli. When ectopically expressed in cells, HDAC8 was found to be localized to the nucleus. Co-transfection experiments demonstrated that expression of HDAC8 repressed a viral SV40 early promoter activity. These results indicate that HDAC8 is a novel member of the histone deacetylase family, which may play a role in the development of a broad range of tissues and potentially in the etiology of cancer.

Anti-malarial effect of histone deacetylation inhibitors and mammalian tumour cytodifferentiating agents

The histones of Plasmodium falciparum represent a potential new target for anti-malarial compounds. A naturally occurring compound, apicidin, has recently been shown to inhibit the in vitro growth of P. falciparum. Apicidin was shown to hyperacetylate histones, suggesting that its mode of action is through histone deacetylase inhibition. We have tested the ability of known histone deacetylase inhibitors, mammalian tumour suppressor compounds, and cytodifferentiating agents to inhibit the in vitro growth of a drug sensitive and resistant strain of P. falciparum. Seven of the tested compounds had microM IC50 values, and trichostatin A, a histone deacetylation inhibitor and cytodifferentiating agent, was active at low nM concentrations. One compound, suberic acid bisdimethylamide, which selectively arrests tumour cells as opposed to normal mammalian cells, had an in vivo cytostatic effect against the acute murine malaria Plasmodium berghei, and one round of treatment with the compound failed to select for resistant mutations. These results suggest a promising role for histone deacetylase inhibitors and cytodifferentiating agents as antimalarial drug candidates.

Differentiation therapy in acute myelogenous leukemia (non-APL)

Successful treatment of acute promyelocytic leukemia (APL) has identified several novel approaches to induce leukemic cell differentiation and selective apoptosis by overcoming the site-specific transcriptional repression by dominant fusion leukemogenic proteins characteristic of APL and other forms of acute myelogenous leukemia (AML). These therapeutic approaches include the use of site-specific ligands, receptors and cytokines, disruption of dominant fusion leukemogenic proteins, chromatin remodeling and combining the above with cytotoxic chemotherapy. With the exception of cytotoxic chemotherapy, the above therapeutic strategies do not significantly affect normal hematopoiesis and their combinations have been shown to be synergistic in inducing myeloid differentiation and apoptosis in several AML cell lines and in patients with APL. These approaches are, in general, non-cross resistant and should be well tolerated particularly in elderly patients with AML. Clinical studies which include biologic end points for differentiation induction, histone acetylation and selective apoptosis are presently in development to evaluate these strategies in the treatment of AML.

mSin3A regulates murine erythroleukemia cell differentiation through association with the TAL1 (or SCL) transcription factor

Activation of the TAL1 (or SCL) gene is the most frequent gain-of-function mutation in T-cell acute lymphoblastic leukemia (T-ALL). TAL1 belongs to the basic helix-loop-helix (HLH) family of transcription factors that bind as heterodimers with the E2A and HEB/HTF4 gene products to a nucleotide sequence motif termed the E-box. Reported to act both as an activator and as a repressor of transcription, the mechanisms underlying TAL1-regulated gene expression are poorly understood. We report here that the corepressor mSin3A is associated with TAL1 in murine erythroleukemia (MEL) and human T-ALL cells. Interaction mapping showed that the basic-HLH domain of TAL1 was both necessary and sufficient for TAL1-mSin3A interaction. TAL1 was found, in addition, to interact with the histone deacetylase HDAC1 in vitro and in vivo, and a specific histone deacetylase inhibitor, trichostatin A (TSA), relieved TAL1-mediated repression of an E-box-containing promoter and a GAL4 reporter linked to a thymidine kinase minimal promoter. Further, TAL1 association with mSin3A and HDAC1 declined during dimethyl sulfoxide-induced differentiation of MEL cells in parallel with a decrease in mSin3A abundance. Finally, TSA had a synergistic effect with enforced TAL1 expression in stimulating MEL cells to differentiate, while constitutive expression of mSin3A inhibited MEL cell differentiation. These results demonstrate that a corepressor complex containing mSin3A and HDAC1 interacts with TAL1 and restricts its function in erythroid differentiation. This also has implications for this transcription factor's actions in leukemogenesis.

Arsenic induces apoptosis of multidrug-resistant human myeloid leukemia cells that express Bcr-Abl or overexpress MDR, MRP, Bcl-2, or Bcl-xL

We investigated the in vitro growth inhibitory and apoptotic effects of clinically achievable concentrations of As2O3 (0.5 to 2.0 μmol/L) against human myeloid leukemia cells known to be resistant to a number of apoptotic stimuli. These included chronic myelocytic leukemia (CML) blast crisis K562 and HL-60/Bcr-Abl cells, which contain p210 and p185 Bcr-Abl, respectively, and HL-60 cell types that overexpress Bcl-2 (HL-60/Bcl-2), Bcl-xL(HL-60/Bcl-xL), MDR (HL-60/VCR), or MRP (HL-60/AR) protein. The growth-inhibitory IC50 values for As2O3 treatment for 7 days against all these cell types ranged from 0.8 to 1.5 μmol/L. Exposure to 2 μmol/L As2O3 for 7 days induced apoptosis of all cell types, including HL-60/Bcr-Abl and K562 cells. This was associated with the cytosolic accumulation of cyt c and preapoptotic mitochondrial events, such as the loss of inner membrane potential (▵Ψm) and the increase in reactive oxygen species (ROS). Treatment with As2O3 (2 μmol/L) generated the activities of caspases, which produced the cleavage of the BH3 domain containing proapoptotic Bid protein and poly (ADP-ribose) polymerase. Significantly, As2O3-induced apoptosis of HL-60/Bcr-Abl and K562 cells was associated with a decline in Bcr-Abl protein levels, without any significant alterations in the levels of Bcl-xL, Bax, Apaf-1, Fas, and FasL. Although As2O3 treatment caused a marked increase in the expression of the myeloid differentiation marker CD11b, it did not affect Hb levels in HL-60/Bcr-Abl, K562, or HL-60/neo cells. However, in these cells, As2O3 potently induced hyper-acetylation of the histones H3 and H4. These findings characterize As2O3 as a growth inhibiting and apoptosis-inducing agent against a variety of myeloid leukemia cells resistant to multiple apoptotic stimuli.

Identification of a transcriptional repressor related to the noncatalytic domain of histone deacetylases 4 and 5

Histone deacetylases (HDACs) are involved in regulating transcription by modifying the core histones of the nucleosome. To date, six HDACs have been identified in mammalian cells: the yeast RPD3 homologs HDAC1, 2, and 3 and the yeast HDA1 homologs HDAC4, 5, and 6. HDAC4 and HDAC5 contain a noncatalytic N-terminal domain. Herein, we report the identification of a protein HDRP (HDAC-related protein) that shares 50% identity in deduced amino acid sequence to the noncatalytic N-terminal domain of HDAC4 and 5. The steady-state levels of HDRP mRNA are high in human brain, heart, and skeletal muscle and low in the several other tissues. HDRP has an apparent molecular mass of approximately 75 kDa. HDRP does not possess intrinsic HDAC activity but forms complexes with both HDAC1 and HDAC3. HDRP represses both basal and activated transcription in transient transfection assays when tethered to DNA as a Gal4-fusion protein. HDAC inhibitors do not reverse transcriptional repression mediated by Gal4-HDRP. Thus, HDRP is a transcriptional repressor and can repress transcription in the presence of HDAC inhibitors.

Induction of apoptosis in U937 human leukemia cells by suberoylanilide hydroxamic acid (SAHA) proceeds through pathways that are regulated by Bcl-2/Bcl-XL, c-Jun, and p21CIP1, but independent of p53

Determinants of differentiation and apoptosis in myelomonocytic leukemia cells (U937) exposed to the novel hybrid polar compound SAHA (suberoylanilide hydroxamic acid) have been examined. In contrast to hexamethylenbisacetamide (HMBA), SAHA-related maturation was limited and accompanied by marked cytoxicity. SAHA-mediated apoptosis occurred within the G0G1 and S phase populations, and was associated with decreased mitochondrial membrane potential, caspase-3 activation, PARP degradation, hypophosphorylation/cleavage of pRB, and down-regulation of c-Myc, c-Myb, and B-Myb. Enforced expression of Bcl-2 or Bcl-XL inhibited SAHA-induced apoptosis, but only modestly potentiated differentiation. While SAHA induced the cyclin-dependent kinase inhibitor p21CIP1, antisense ablation of this CDKI increased, rather than decreased, SAHA-related lethality. In contrast, conditional expression of wild-type p53 failed to modify SAHA actions, but markedly potentiated HMBA-induced apoptosis. Finally, SAHA modestly increased expression/activation of the stress-activated protein kinase (SAPK/JNK); moreover, SAHA-related lethality was partially attenuated by a dominant-negative c-Jun mutant protein (TAM67). SAHA did not stimulate mitogen-activated protein kinase (MAPK), nor was lethality diminished by the specific MEK/MAPK inhibitor PD98059. These findings indicate that SAHA potently induces apoptosis in human leukemia cells via a pathway that is p53-independent but at least partially regulated by Bcl-2/Bcl-XL, p21CIP1, and the c-Jun/AP-1 signaling cascade.

Amide analogues of trichostatin A as inhibitors of histone deacetylase and inducers of terminal cell differentiation

Inhibitors of histone deacetylase (HD) bear great potential as new drugs due to their ability to modulate transcription and to induce apoptosis or differentiation in cancer cells. We have described previously analogues of the complex natural HD inhibitors trapoxin B and trichostatin A with activities in the submicromolar range. Here we report structure-activity relationship analyses of further analogues of trichostatin A with respect to in vitro inhibition of maize HD-2 and their ability to induce terminal cell differentiation in Friend leukemic cells. This is the first report that shows the correlation between HD inhibitory activity and action on cancer cells on a larger series of similar compounds. Only the compounds that inhibit HD induce differentiation and/or exert antiproliferative activities in cell culture. Our studies support the use of in vitro systems as screening tools and provide structure-activity relationships that merit further investigation of this interesting target.

Histone deacetylase inhibitors: novel anticancer agents

Previous studies have established that the regulation of gene expression is dependent upon the nucleosomal integrity of nuclear DNA. To a large extent, this integrity is dictated by the acetylation status of the core histone particles. The acetylation of histones is, in turn, controlled by the combined activity of specific acetylases and deacetylases. Moreover, disruption of histone acetylases and deacetylases has been linked to a wide variety of human cancers. For this reason, the recent availability of potent and specific histone deacetylase inhibitors has provoked a great deal of interest amongst cancer biologists, oncologists and pharmacologists. Within the past 2- 3 years, several novel histone deacetylase inhibitors have been reported, many of which have already been tested in vivo in mouse models of cancer. In this review we focus on the rationale behind the use of histone deacetylase inhibitors as anticancer agents. Moreover, we review some of the recent findings pertaining to the use of these compounds as anticancer agents.

In vivo analysis of the molecular pathogenesis of acute promyelocytic leukemia in the mouse and its therapeutic implications

Acute promyelocytic leukemia (APL) is characterized by the expansion of malignant myeloid cells blocked at the promyelocytic stage of hemopoietic development, and is associated with reciprocal chromosomal translocations always involving the retinoic acid receptor alpha (RARalpha) gene on chromosome 17. As a consequence of the translocation RARalpha variably fuses to the PML, PLZF, NPM and NUMA genes (X genes), leading to the generation of RARalpha-X and X-RARalpha fusion genes. The aberrant chimeric proteins encoded by these genes may exert a crucial role in leukemogenesis. Retinoic acid (RA), a metabolite of vitamin A, can overcome the block of maturation at the promyelocytic stage and induce the malignant cells to terminally mature into granulocytes resulting in complete albeit transient disease remission. APL has become, for this reason, the paradigm for 'cancer differentiation therapy'. Furthermore, APL associated with translocation between the RARalpha and the PLZF genes (PLZF-RARalpha) shows a distinctly worse prognosis with poor response to chemotherapy and little or no response to treatment with RA, thus defining a new APL syndrome. Here we will focus our attention on the recent progresses made in defining the molecular mechanisms underlying the pathogenesis of this paradigmatic disease in vivo in the mouse. We will review the critical contribution of mouse modeling in unraveling the transcriptional basis for the differential response to RA in APL. We will also discuss how this new understanding has allowed to propose, develop and test in these murine leukemia models as well as in human APL patients novel therapeutic strategies.

Structures of a histone deacetylase homologue bound to the TSA and SAHA inhibitors

Histone deacetylases (HDACs) mediate changes in nucleosome conformation and are important in the regulation of gene expression. HDACs are involved in cell-cycle progression and differentiation, and their deregulation is associated with several cancers. HDAC inhibitors, such as trichostatin A (TSA) and suberoylanilide hydroxamic acid (SAHA), have anti-tumour effects, as they can inhibit cell growth, induce terminal differentiation and prevent the formation of tumours in mice models, and they are effective in the treatment of promyelocytic leukemia. Here we describe the structure of the histone deacetylase catalytic core, as revealed by the crystal structure of a homologue from the hyperthermophilic bacterium Aquifex aeolicus, that shares 35.2% identity with human HDAC1 over 375 residues, deacetylates histones in vitro and is inhibited by TSA and SAHA. The deacetylase, deacetylase-TSA and deacetylase-SAHA structures reveal an active site consisting of a tubular pocket, a zinc-binding site and two Asp-His charge-relay systems, and establish the mechanism of HDAC inhibition. The residues that make up the active site and contact the inhibitors are conserved across the HDAC family. These structures also suggest a mechanism for the deacetylation reaction and provide a framework for the further development of HDAC inhibitors as antitumour agents.

Organization of chromatin in cancer cells: role of signalling pathways

The role of mechanical and chemical signalling pathways in the organization and function of chromatin is the subject of this review. The mechanical signalling pathway consists of the tissue matrix system that links together the three-dimensional skeletal networks, the extracellular matrix, cytoskeleton, and nuclear matrix. Intermediate filament proteins are associated with nuclear DNA, suggesting that intermediate filaments may have a role in the organization of chromatin. In human hormone-dependent breast cancer cells, the interaction between cytokeratins and chromatin is regulated by estrogens. Transcription factors, histone acetyltransferases, and histone deacetylases, which are associated with the nuclear matrix, are components of the mechanical signalling pathway. Recently, we reported that nuclear matrix-bound human and chicken histone deacetylase 1 is associated with nuclear DNA in situ, suggesting that histone deacetylase has a role in the organization of nuclear DNA. Chemical signalling pathways such as the Ras/mitogen-activated protein kinase (Ras/MAPK) pathway stimulate the activity of kinases that modify transcription factors, nonhistone chromosomal proteins, and histones. The levels of phosphorylated histones are increased in mouse fibroblasts transformed with oncogenes, the products of which stimulate the Ras/MAPK pathway. Histone phosphorylation may lead to decondensation of chromatin, resulting in aberrant gene expression.Key words: histone acetylation, histone phosphorylation, nuclear matrix, cytoskeleton, histone deacetylase, cancer.

Cloning and expression of a murine histone deacetylase 3 (mHdac3) cDNA and mapping to a region of conserved synteny between murine chromosome 18 and human chromosome 5

Histone deacetylases (HDACs) are enzymes that play a pivotal role in transcription, differentiation, and cell cycle progression. We previously cloned human HDAC3 cDNA and showed that its transfection into THP-1 cells results in G2/M cell cycle accumulation. Using bioinformatic screening and PCR, we have now cloned the murine Hdac3 cDNA, which encodes a 428-amino-acid protein with near complete identity to its human ortholog. To establish a link to a potential disease locus, we performed PCR-based chromosomal mapping for the mHdac3 gene and chromosomal fluorescence in situ hybridization (FISH) for the human gene. mHdac3 localizes to chromosome 18 and human HDAC3 gene localizes to a syntenic region in chromosome 5 at band q31.3-q32 telomeric to the cytokine gene cluster. Transfection of mHdac3 into HeLa cells led to accumulation in G2/M. Our results suggest a cell cycle function for murine Hdac3, reflecting the complex regulatory roles of this gene family.

Cloning of the cDNA encoding phenylalanyl tRNA synthetase regulatory alpha-subunit-like protein whose expression is down-regulated during differentiation

Hybrid polar compounds (HPCs), such as suberoylanilide hydroxamic acid (SAHA), induce differentiation of transformed cells. Differential display of RNA was used to identify genes whose expression is changed during SAHA-induced differentiation of murine erythroleukemia (MEL) cells. One such cDNA was identified whose mRNA level decreased by 50% after 8h of SAHA treatment as determined by Northern blot analysis. The full-length cDNA (1944bp in length) was cloned by sequencing of an EST clone and rapid amplification of 5' cDNA ends (5'-RACE). The predicted amino acid sequence is 589 amino acids and shares 45% identity with the yeast cytoplasmic phenylalanyl tRNA synthetase (PheRS) regulatory alpha-subunit. Human EST clones which share over 90% identity of predicted amino acid sequence with this cDNA map to chromosome 2 near the paired box homeotic gene 3 (PAX3) locus, a region syngenic to mouse chromosome 1. This is the first report of the cloning of the full-length cDNA for the murine PheRS regulatory alpha-subunit-like protein. The level of PheRS alpha-subunit-like mRNA is regulated during differentiation but not during cell cycle progression.

Anti-tumour activity in vitro and in vivo of selective differentiating agents containing hydroxamate

A series of hydroxamates, which are not metalloprotease inhibitors, have been found to be selectively toxic to a range of transformed and human tumour cells without killing normal cells (fibroblasts, melanocytes) at the same concentrations. Within 24 h of treatment, drug action is characterized by morphological reversion of tumour cells to a more normal phenotype (dendritic morphology), and rapid and reversible acetylation of histone H4 in both tumour and normal cells. Two hydroxamates inhibited growth of xenografts of human melanoma cells in nude mice; resistance did not develop in vivo or in vitro. A third hydroxamate, trichostatin A, was active in vitro but became inactivated and had no anti-tumour activity in vivo. Development of dendritic morphology was found to be dependent upon phosphatase activity, RNA and protein synthesis. Proliferating hybrid clones of sensitive and resistant cells remained sensitive to ABHA, indicating a dominant-negative mechanism of sensitivity. Histone H4 hyperacetylation suggests that these agents act at the chromatin level. This work may lead to new drugs that are potent, and selective anti-tumour agents with low toxicity to normal cells.

Photoaffinity labeling and mass spectrometry identify ribosomal protein S3 as a potential target for hybrid polar cytodifferentiation agents

The ability of a novel class of hybrid polar compounds (HPCs) to induce differentiation and consequent cessation of proliferation of transformed cells has led to their development as potential chemotherapeutic agents in the treatment of cancer. Suberoylanilide hydroxamic acid (SAHA) is a prototype of a family of hydroxamic acid based compounds (SAHA-like HPCs) that can, at micromolar concentrations, induce a variety of transformed cell lines to differentiate. The mechanism of action of the HPCs is not entirely understood. Searching for a cellular target of the SAHA-like HPCs, we synthesized a photoaffinity labeling reagent structurally based on SAHA, and probed for SAHA-binding proteins in murine erythroleukemia (MEL) cells. Photoaffinity labeling in cell free extracts identified a 32-kDa protein (p32) that was specifically labeled by the photoaffinity reagent. Cell fractionation assays localized p32 to the P100 fraction. p32 was partially purified and identified by mass spectrometry as the 40 S ribosomal protein S3. Expression of epitope-tagged S3 in bacterial lysates followed by photoaffinity labeling confirmed its specific labeling. Identification of a cytodifferentiation agent target may shed light on the mechanism by which the SAHA-like HPCs exert their antitumor effects.

A histone deacetylase inhibitor, trichostatin A, suppresses myofibroblastic differentiation of rat hepatic stellate cells in primary culture

Hepatic stellate cells are the major cellular sources of extracellular matrix in chronic liver diseases leading to fibrosis. We explored the antifibrogenic effect of two histone deacetylase inhibitors, sodium butyrate and trichostatin A (TSA), on this cell type in vitro. Primary hepatic stellate cells as well as culture activated cells were exposed to butyrate (0.01-1 mmol/L) or TSA (1-100 nmol/L); their effect on collagen types I and III and smooth muscle alpha-actin was examined by quantitative immunoprecipitation and by Northern analysis. Their antiproliferative effect was examined by 3H-thymidine incorporation and cell counting. Hyperacetylation of histones was demonstrated by acid urea/Triton-X-100 (AUT) polyacrylamide gel electrophoresis. Possible cytotoxic effects were judged on stellate cells by evaluating de novo total protein synthesis, and on hepatocytes by measuring lactate dehydrogenase (LDH) leakage, albumin secretion, and epoxide hydrolase and ethoxycoumarin O-deethylase activity. TSA at 100 nmol/L and butyrate at 1 mmol/L retarded the morphological changes characteristic for activation of primary stellate cells. TSA at 100 nmol/L inhibited synthesis of collagen types I and III and smooth muscle alpha-actin by 62%, 70%, and 88%. Butyrate at 1 mmol/L showed a modest inhibitory effect on collagen type III and smooth muscle alpha-actin, but had no effect on collagen type I. Northern analysis suggested that these inhibitory effects on collagen type III and smooth muscle alpha-actin were transcriptional, while the effect on collagen type I was largely posttranscriptional. At 100 nmol/L, TSA strongly suppressed proliferation of primary hepatic stellate cells. Inhibition of activation of stellate cells was preceded by hyperacetylation of histone H4. When tested on cells at day 14 in culture, butyrate had no inhibitory effects on the synthesis of collagens or smooth muscle alpha-actin. One hundred or 10 nmol/L TSA modestly inhibited the synthesis of collagens type I (-24%,-22%) and III (-34%,-22%), and smooth muscle alpha-actin (-27%,-12%). We conclude that TSA inhibits transdifferentiation of stellate cells into myofibroblasts by interfering with the level of acetylation of histone H4.