Apoptotic pathways activated by histone deacetylase inhibitors: implications for the drug-resistant phenotype

Changes in chromatin accessibility and transcriptional landscape induced by HDAC inhibitors in TP53 mutated patient-derived colon cancer organoids

Here we present the generation and characterization of patient-derived organoids (PDOs) from colorectal cancer patients. PDOs derived from two patients with TP53 mutations were tested with two different HDAC inhibitors (SAHA and NKL54). Cell death induction, transcriptome, and chromatin accessibility changes were analyzed. HDACIs promote the upregulation of low expressed genes and the downregulation of highly expressed genes. A similar differential effect is observed at the level of chromatin accessibility. Only SAHA is a potent inducer of cell death, which is characterized by the upregulation of BH3-only genes BIK and BMF. Up-regulation of BIK is associated with increased accessibility in an intronic region that has enhancer properties. SAHA, but not NKL54, also causes downregulation of BCL2L1 and decreases chromatin accessibility in three distinct regions of the BCL2L1 locus. Both inhibitors upregulate the expression of innate immunity genes and members of the MHC family. In summary, our exploratory study indicates a mechanism of action for SAHA and demonstrate the low efficacy of NKL54 as a single agent for apoptosis induction, using two PDOs. These observations need to be validated in a larger cohort of PDOs.

Effects of 4-hexylresorcinol administration on the submandibular glands in a growing rat model

Background

4-Hexylresorcinol (4HR) is a food additive and class I histone deacetylase inhibitor. In this study, we examined the effects of 4HR administration on the submandibular gland in a growing rat model.

Methods

Four-week-old rats were used in this study. The experimental group (nine males and eight females) received 12.8 mg/kg of 4HR weekly for 12 weeks. Ten rats (five males and five females) were used as controls. The submandibular glands of rats were collected 12 weeks after the first administration of 4HR. The weight of the glands was measured. Histological analysis, immunoprecipitation-high-performance liquid chromatography (IP-HPLC), and western blotting were performed.

Results

The weights of the rat submandibular glands were higher in the experimental groups than in the control group, especially in male rats (P < 0.05). The vascular endothelial growth factor (VEGF) and testosterone in the submandibular glands were more highly expressed in 4HR-treated male rats than in untreated rats, as detected by both western blotting and immunohistochemistry. The IP-HPLC results demonstrated that the expression levels of Ki67, epidermal growth factor, and testosterone in the submandibular glands were higher in 4HR-treated male rats than in untreated rats.

Conclusions

This study demonstrated that the systemic administration of 4HR increased the weight of submandibular glands in male rats. In addition, the testosterone and VEGF expression levels in the submandibular glands increased owing to 4HR administration.

Selective class IIa HDAC inhibitors: myth or reality

The prospect of intervening, through the use of a specific molecule, with a cellular alteration responsible for a disease, is a fundamental ambition of biomedical science. Epigenetic-based therapies appear as a remarkable opportunity to impact on several disorders, including cancer. Many efforts have been made to develop small molecules acting as inhibitors of histone deacetylases (HDACs). These enzymes are key targets to reset altered genetic programs and thus to restore normal cellular activities, including drug responsiveness. Several classes of HDAC inhibitors (HDACis) have been generated, characterized and, in certain cases, approved for the use in clinic. A new frontier is the generation of subtype-specific inhibitors, to increase selectivity and to manage general toxicity. Here we will discuss about a set of molecules, which can interfere with the activity of a specific subclass of HDACs: the class IIa.

Histone deacetylase inhibitors disrupt the mitotic spindle assembly checkpoint by targeting histone and nonhistone proteins

Histone deacetylase inhibitors exhibit pleiotropic effects on cell functions, both in vivo and in vitro. One of the more dramatic effects of these drugs is their ability to disrupt normal mitotic division, which is a significant contributor to the anticancer properties of these drugs. The most important feature of the disrupted mitosis is that drug treatment overcomes the mitotic spindle assembly checkpoint and drives mitotic slippage, but in a manner that triggers apoptosis. The mechanism by which histone deacetylase inhibitors affect mitosis is now becoming clearer through the identification of a number of chromatin and nonchromatin protein targets that are critical to the regulation of normal mitotic progression and cell division. These proteins are directly regulated by acetylation and deacetylation, or in some cases indirectly through the acetylation of essential partner proteins. There appears to be little contribution from deacetylase inhibitor-induced transcriptional changes to the mitotic effects of these drugs. The overall mitotic phenotype of drug treatment appears to be the sum of these disrupted mechanisms.

Trichostatin A reduced phospholipase C gamma-1 transcript and protein contents in MCF-7 breast cancer cells

It has recently been demonstrated that phospholipase C gamma-1 (PLCγ1) activation may contribute to breast carcinoma cell motility and their metastasis. Employing MCF-7 breast cancer cells, we showed the effect of trichostatin A (TSA) on the cellular contents of the PLCγ1 molecule. Using reverse transcription, real-time quantitative PCR and western blot analysis, we demonstrated that TSA reduced the PLCγ1 transcript and protein levels in MCF-7 cells. We also found that TSA decreased the half-life of the PLCγ1 transcript from approximately 7hours to 5hours. Moreover, we observed that protein synthesis appears to be essential in the TSA reduction of PLCγ1 mRNA stability. Since PLCγ1 activation is considered a key factor in the initiation of events that increase malignant cell motility, our observations may support the validity of TSA in anticancer studies.

Sensitization of tumor cells by targeting histone deacetylases

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

Upregulated histone deacetylase 1 expression in pancreatic ductal adenocarcinoma and specific siRNA inhibits the growth of cancer cells

OBJECTIVES

So far, there are no investigations about the role of histone deacetylase 1 (HDAC1) in tumorigenesis of pancreatic ductal adenocarcinoma. This study was designed to elucidate the roles and mechanisms of HDAC1 in tumorigenesis of pancreatic ductal adenocarcinoma.

METHODS

Real-time reverse transcription-polymerase chain reaction and immunohistochemistry techniques were adopted to detect the expression of HDAC1 in human pancreatic ductal adenocarcinoma tissues and paired paracancerous tissues. The roles of HDAC1 in human pancreatic cell line PaTu8988 were investigated using siRNA.

RESULTS

Histone deacetylase 1 mRNA in pancreatic cancer tissues were significantly higher than in paracancerous tissues (P < 0.05). Immunohistochemistry showed that the indices of HDAC1 in pancreatic cancer tissues and paracancerous tissues were 56.4% (SD, 23.1%) and 6.7% (SD, 5.0%), respectively (P < 0.001). Knockdown of HDAC1 can generate a remarkable defect in proliferation and also can significantly induce apoptosis and S-phase arrest in PaTu8988 cells (P < 0.05). The Bcl-2 mRNA expression was significantly downregulated, whereas the p21 and Bax mRNA expression were significantly upregulated.

CONCLUSIONS

The HDAC1 overexpression might play an important role in tumorigenesis of pancreatic cancer. Our data support the development of selective inhibitors targeting HDAC1 for the treatment of pancreatic ductal adenocarcinoma. Histone deacetylase 1 could be a new gene therapy target in pancreatic ductal adenocarcinoma.

Histone H1 subtype preferences of DFF40 and possible nuclear localization of DFF40/45 in normal and trichostatin A-treated NB4 leukemic cells

A major hallmark of the terminal stages of apoptosis is the internucleosomal DNA fragmentation. The endonuclease responsible for this type of DNA degradation is the DNA fragmentation factor (DFF). DFF is a complex of the endonuclease DFF40 and its chaperone/inhibitor, DFF45. In vitro work has shown that histone H1 and HMGB1/2 recruit/target DFF40 to the internucleosomal linker regions of chromatin and that histone H1 directly interacts with DFF40 conferring DNA binding ability and enhancing its nuclease activity. The histone H1 family is comprised of many subtypes, which recent work has shown may have distinct roles in chromatin function. Thus we studied the binding association of DFF40 with specific H1 subtypes and whether these binding associations are altered after the induction of apoptosis in an in vivo cellular context. The apoptotic agent used in this study is the histone deacetylase inhibitor, trichostatin A (TSA). We separated the insoluble chromatin-enriched fraction from the soluble nuclear fraction of the NB4 leukemic cell line. Using MNase digestion, we provide evidence which strongly suggests that the heterodimer, DFF40-DFF45, is localized to the chromatin fraction under apoptotic as well as non-apoptotic conditions. Moreover, we present results that show that DFF40 interacts with the all H1 subtypes used in this study, but preferentially interacts with specific H1 subtypes after the induction of apoptosis by TSA. These results illustrate for the first time the association of DFF40 with individual H1 subtypes, under a specific apoptotic stimulus in an in vivo cellular context.

Rational combinations using HDAC inhibitors

In addition to well-characterized genetic abnormalities that lead to cancer onset and progression, it is now recognized that alterations to the epigenome may also play a significant role in oncogenesis. As a result, epigenetic-modulating agents such as histone deacetylase inhibitors (HDACi) have attracted enormous attention as anticancer drugs. In numerous in vitro and preclinical settings, these compounds have shown their vast potential as single agent anticancer therapies, but unfortunately equivalent responses have not always been observed in patients. Given the pleiotropic effects HDACi have on malignant cells, their true therapeutic potential most likely lies in combination with other anticancer drugs. In this review we will focus on the anticancer effects of HDACi when combined with other cancer therapeutics with an emphasis on those combinations based on a strong molecular rationale.

Histone deacetylase inhibition in the treatment of heart disease

Recent work has demonstrated the importance of chromatin remodeling, especially histone acetylation, in the control of gene expression in the heart. Studies in preclinical models suggest that inhibition of histone deacetylase (HDAC) activity - using compounds that show promise in ongoing oncology trials - blunts pathologic growth of cardiac myocytes. Indeed, small-molecule inhibitors of HDACs are members of an evolving class of pharmacologic agents in development for the treatment of several diseases. If proved effective in the treatment of heart disease, HDAC inhibitors could have a significant impact on public health, as cardiovascular disease remains the leading cause of death in the US. This paper reviews understanding of the mechanisms of action of HDAC inhibitors in the heart and summarizes emerging data regarding their effects on disease-related cardiac remodeling and function.

PP2A regulates HDAC4 nuclear import

Different signal-regulated serine/threonine kinases phosphorylate class II histone deacetylases (HDACs) to promote nuclear export, cytosolic accumulation, and activation of gene transcription. However, little is known about mechanisms operating in the opposite direction, which, possibly through phosphatases, should promote class II HDACs nuclear entry and subsequent gene repression. Here we show that HDAC4 forms a complex with the PP2A holoenzyme C alpha, A alpha, B/PR55 alpha. In vitro and in vivo binding studies demonstrate that the N-terminus of HDAC4 interacts with the catalytic subunit of PP2A. HDAC4 is dephosphorylated by PP2A and experiments using okadaic acid or RNA interference have revealed that PP2A controls HDAC4 nuclear import. Moreover, we identified serine 298 as a putative phosphorylation site important for HDAC4 nuclear import. The HDAC4 mutant mimicking phosphorylation of serine 298 is defective in nuclear import. Mutation of serine 298 to alanine partially rescues the defect in HDAC4 nuclear import observed in cells with down-regulated PP2A. These observations suggest that PP2A, via the dephosphorylation of multiple serines including the 14-3-3 binding sites and serine 298, controls HDAC4 nuclear import.

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.

Combinatorial action of the HDAC inhibitor trichostatin A and etoposide induces caspase-mediated AIF-dependent apoptotic cell death in non-small cell lung carcinoma cells

Commonly used regimens in cancer therapy rely on the induction of apoptotic cell death, and drug resistance can be attributed, at least in part, to a disabled apoptotic program. Non-small cell lung carcinomas (NSCLC), exhibit an intrinsic resistance to chemotherapy. Here, we show that co-treatment with etoposide (VP16) and the pan-histone deacetylase (HDAC) inhibitor trichostatin A (TSA), but not valproic acid (VPA), induced apoptotic cell death in drug-resistant NSCLC cells. Co-treatment, but not single treatment, with VP16 and TSA induced apoptosis in a caspase-dependent manner accompanied by a crucial decrease in Bcl-xL expression allowing Bax activation and subsequent initiation of the apoptosis inducing factor (AIF)-dependent death pathway. Importantly, AIF proved to be required for the effects of TSA/VP16 as RNA knockdown of AIF resulted in a complete abolishment of TSA/VP16-induced apoptotic cell death in drug-resistant NSCLC cells. Our results thus provide evidence for the requirement of both caspase-dependent and caspase-independent apoptotic pathways in TSA/VP16-mediated death of drug-resistant NSCLC cells, and extend previous suggestions that HDAC inhibitors in combination with conventional chemotherapeutic drugs could be valuable in the treatment of NSCLC cancer and other malignancies in which Bcl-xL is overexpressed.

Histone deacetylase inhibitor NVP-LAQ824 sensitizes human nonsmall cell lung cancer to the cytotoxic effects of ionizing radiation

Stage III nonsmall cell lung cancer is primarily treated with combined chemotherapy and radiation therapy. Relapses for progression of disease within irradiated sites remains a primary pattern of failure. To evaluate the interaction between histone deacetylase inhibitors and irradiation in nonsmall cell lung cancer, we studied NVP-LAQ824 in mouse models of human lung cancer. Colony formation assays were performed to determine whether LAQ824 sensitized nonsmall cell lung cancer to the cytotoxic effects of ionizing radiation. LAQ824 reduced clonogenic survival of the H23 and H460 cell lines five-fold compared with controls and four-fold compared with either agent alone (P<0.001). Western blot analysis of caspase cleavage, microscopic analysis of nuclei and Annexin-fluorescein isothiocyanate/propidium iodide flow cytometry assays showed that LAQ824 enhanced radiation-induced apoptosis and attenuated mitosis (P<0.001). Immunostaining for gamma-H2AX nuclear foci was performed to determine the effect of LAQ824 on radiation-induced DNA double-strand breaks. Combined modality treatment delayed the resolution of gamma-H2AX foci with over 30% of cells staining positive 6 h after treatment versus approximately 5 and 3% in cells treated with LAQ824 or radiation alone (P<0.001). Additionally, an in-vivo xenograft model was utilized to study the effects of fractioned irradiation and LAQ824 on tumor growth. Fractioned irradiation and LAQ824 delayed tumor growth by 19 days versus 7 and 4 days for treatment with LAQ824 and radiation alone. This study shows the effectiveness of histone deacetylase inhibitors to enhance the cytotoxic effects of radiation by attenuating DNA repair and inducing apoptosis in human nonsmall cell lung cancer.

Dephosphorylation and caspase processing generate distinct nuclear pools of histone deacetylase 4

From the nucleus, histone deacetylase 4 (HDAC4) regulates a variety of cellular processes, including growth, differentiation, and survival, by orchestrating transcriptional changes. Extracellular signals control its repressive influence mostly through regulating its nuclear-cytoplasmic shuttling. In particular, specific posttranslational modifications such as phosphorylation and caspase-mediated proteolytic processing operate on HDAC4 to promote its nuclear accumulation or export. To understand the signaling properties of this deacetylase, we investigated its cell death-promoting activity and the transcriptional repression potential of different mutants that accumulate in the nucleus. Here we show that, compared to that of other nuclear forms of HDAC4, a caspase-generated nuclear fragment exhibits a stronger cell death-promoting activity coupled with increased repressive effect on Runx2- or SRF-dependent transcription. However, this mutant displays reduced repressive action on MEF2C-driven transcription. Photobleaching experiments and quantitative analysis of the raw data, based on a two-binding-state compartmental model, demonstrate the existence of two nuclear pools of HDAC4 with different chromatin-binding properties. The caspase-generated fragment is weakly bound to chromatin, whereas an HDAC4 mutant defective in 14-3-3 binding or the wild-type HDAC5 protein forms a more stable complex. The tightly bound species show an impaired ability to induce cell death and repress Runx2- or SRF-dependent transcription less efficiently. We propose that, through specific posttranslation modifications, extracellular signals control two distinct nuclear pools of HDAC4 to differentially dictate cell death and differentiation. These two nuclear pools of HDAC4 are characterized by different repression potentials and divergent dynamics of chromatin interaction.

The differentiation-associated linker histone, H1.0, during the in vitro aging and senescence of human diploid fibroblasts

There are numerous similarities between aging/senescence and differentiation. One key similarity is that in both biological processes chromatin remodeling events occur. It is now known that during both processes there is a reorganization of eu- and heterochromatic domains and an increase in heterochromatin, known as heterochromatinization. Previous work of more than two decades has shown that the replacement H1 linker histone subtype, H1.0, accumulates during terminal differentiation in numerous cell/tissue systems. However, work with this differentiation-associated H1 subtype in aging cell systems has only recently been accomplished. In this article, we outline the cumulative results from our investigations of H1.0 protein and mRNA levels in the in vitro aging cell system of human diploid fibroblasts (HDFs) and discuss the potential rationale of why this particular subtype was found to accumulate during both these processes.

Role for histone deacetylase 1 in human tumor cell proliferation

Posttranslational modifications of core histones are central to the regulation of gene expression. Histone deacetylases (HDACs) repress transcription by deacetylating histones, and class I HDACs have a crucial role in mouse, Xenopus laevis, zebra fish, and Caenorhabditis elegans development. The role of individual class I HDACs in tumor cell proliferation was investigated using RNA interference-mediated protein knockdown. We show here that in the absence of HDAC1 cells can arrest either at the G(1) phase of the cell cycle or at the G(2)/M transition, resulting in the loss of mitotic cells, cell growth inhibition, and an increase in the percentage of apoptotic cells. On the contrary, HDAC2 knockdown showed no effect on cell proliferation unless we concurrently knocked down HDAC1. Using gene expression profiling analysis, we found that inactivation of HDAC1 affected the transcription of specific target genes involved in proliferation and apoptosis. Furthermore, HDAC2 downregulation did not cause significant changes compared to control cells, while inactivation of HDAC1, HDAC1 plus HDAC2, or HDAC3 resulted in more distinct clusters. Loss of these HDACs might impair cell cycle progression by affecting not only the transcription of specific target genes but also other biological processes. Our data support the idea that a drug targeting specific HDACs could be highly beneficial in the treatment of cancer.

Impaired activity of volume-sensitive Cl− channel is involved in cisplatin resistance of cancer cells

The platinum-based drug cisplatin is a widely used anticancer drug which acts by causing the induction of apoptosis. However, resistance to the drug is a major problem. In this study we show that the KCP-4 human epidermoid cancer cell line, which serves as a model of acquired resistance to cisplatin, has virtually no volume-sensitive, outwardly rectifying (VSOR) chloride channel activity. The VSOR chloride channel's molecular identity has not yet been determined, and semi-quantitative RT-PCR experiments in this study suggested that the channel corresponds to none of three candidate genes. However, because it is known that the channel current plays an essential role in apoptosis, we hypothesized that lack of the current contributes to cisplatin resistance in these cells and that its restoration would reduce resistance. To test this hypothesis, we attempted to restore VSOR chloride current in KCP-4 cells. It was found that treatment with trichostatin A (TSA), a histone deacetylase inhibitor, caused VSOR chloride channel function to be partially restored. Treatment of the cells with both TSA and cisplatin resulted in an increase in caspase-3 activity at 24 h and a decrease in cell viability at 48 h. These effects were blocked by simultaneous treatment of the cells with a VSOR chloride channel blocker. These results indicate that restoration of the channel's functional expression by TSA treatment leads to a decrease in the cisplatin resistance of KCP-4 cells. We thus conclude that impaired activity of the VSOR chloride channel is involved in the cisplatin resistance of KCP-4 cancer cells.

The histone deacetylase inhibitor, suberoylanilide hydroxamic acid, overcomes resistance of human breast cancer cells to Apo2L/TRAIL

While the apoptosis-inducing ligand Apo2L/TRAIL is a promising new agent for the treatment of cancer, the sensitivity of cancer cells for induction of apoptosis by Apo2L/TRAIL varies considerably. Identification of agents that can be used in combination with Apo2L/TRAIL to enhance apoptosis in breast cancer cells would increase the potential utility of this agent as a breast cancer therapeutic. Here, we show that the histone deacetylase inhibitor, suberoylanilide hydroxamic acid (SAHA), can sensitize Apo2L/TRAIL-resistant breast cancer cells to Apo2L/TRAIL-induced apoptosis. Importantly, neither Apo2L/TRAIL alone, nor in combination with SAHA, affected the viability of normal human cells in culture. Apo2L/TRAIL-resistant MDA-MB-231 breast cancer cells, generated by long-term culture in the continuous presence of Apo2L/TRAIL, were resensitized to Apo2L/TRAIL-induced apoptosis by SAHA. The sensitization of these cells by SAHA was accompanied by activation of caspase 8, caspase 9 and caspase 3 and was concomitant with Bid and PARP cleavage. The expression of the proapoptotic protein, Bax, increased significantly with SAHA treatment and high levels of Bax were maintained in the combined treatment with Apo2L/TRAIL. Treatment with SAHA increased cell surface expression of DR5 but not DR4. Interestingly, SAHA treatment also resulted in a significant increase in cell surface expression of DcR1. Taken together, our findings indicate that the use of these 2 agents in combination may be effective for the treatment of breast cancer.

Involvement of P-glycoprotein and MRP1 in resistance to cyclic tetrapeptide subfamily of histone deacetylase inhibitors in the drug-resistant osteosarcoma and Ewing's sarcoma cells

Despite recent improvements in multimodal therapies for osteosarcoma (OS) and Ewing's family of tumors (EFTs), the prognosis of relapsed cases remains very poor because of the resistance to chemotherapy. Histone deacetylase inhibitors (HDACIs), including members of the cyclic tetrapeptide family such as FK228 and apicidin, are novel antitumor agents that can induce cell cycle arrest and apoptosis in various cancer cells. HDACIs also exhibit potent antitumor effects on OS and EFTs. However, to date there have been no studies to our knowledge reporting the effects of HDACIs on drug-resistant OS and EFTs. Here, we demonstrated that FK228 and apicidin exhibited strong resistance in doxorubicin-resistant clones of OS and EFTs expressing P-glycoprotein (P-gp) and multidrug resistance-associated protein 1 (MRP1) and that P-gp and MRP1 might play a crucial role in the resistance mechanism to FK228 and apicidin. A P-gp inhibitor (verapamil) and an MRP1 inhibitor (MK571) could independently reverse the resistance to FK228 and apicidin in the drug-resistant clones. Moreover, the combination of verapamil and MK571 could enhance HDACI-induced cell number reduction in drug-resistant clones to a similar extent as that in their parental clones. Although these findings suggest the difficulty in treating drug-resistant tumors expressing P-gp and/or MRP1 with these HDACIs, the combination of P-gp and MRP1 inhibitors might reverse the resistance to the HDACIs in the treatment of those tumors. Because HDACIs are potent and promising antitumor drugs and seem to be close to clinical use, it is necessary to pay attention to the resistance mechanisms against HDACIs.

The role of caspases in cell death and differentiation

The complexity, redundancy and interdependence of the biological systems involved in tumour response to different treatments hamper progress towards developing specific and effective therapies. In addition, the many and even contradictory roles played by certain key proteins can significantly amend our view on tumourigenesis. The role of caspases in the modulation of cell death and differentiation is a prominent example of such a complexity. Here we focus on the role of caspases in apoptotic cell death, mainly in haematological malignancies, tumourigenesis, sepsis, T-cell proliferation and cell differentiation.

Histone Deacetylase Inhibitors: Emerging Anticancer Therapeutic Agents?

Histone deacetylase inhibitors are novel anticancer agents in clinical development that target the family of histone deacetylase (HDAC) enzymes responsible for deacetylating core nucleosomal histones and other proteins. The precise mechanisms resulting in the antiproliferative biologic effects of these agents are not yet known, but there are several proposed mechanistic models, including transcriptional and nontranscriptional processes. Clinical experience with these agents indicates that they are generally well tolerated, and anticancer activity has been observed in early clinical trials in several tumor types including non-small-cell lung cancer. The development of these agents continues, with an emphasis on the discovery of HDAC isoform-selective compounds. Successful future development relies on clearer understanding of the dominant mechanisms involved in the observed antiproliferative effects.

Genome-wide screening for target regions of histone deacetylases in cardiomyocytes

The acetylation status of core histones in cardiomyocytes has been linked to the development of cardiac hypertrophy and heart failure. Little is known, however, of the genes affected by abnormal histone acetylation in such pathological conditions. We recently developed a genome-wide screening method, differential chromatin scanning (DCS), to isolate genomic fragments associated with histones subject to differential acetylation. We have now applied DCS to H9C2 rat embryonic cardiomyocytes incubated with or without trichostatin A (TSA), a specific inhibitor of histone deacetylase (HDAC) activity. About 200 genomic fragments were readily isolated by DCS on the basis of the preferential acetylation of associated histones in TSA-treated cells. Quantitation of the amount of DNA in chromatin immunoprecipitates prepared with antibodies to acetylated histone H3 revealed that 37 of 38 randomly chosen DCS clones were preferentially precipitated from the TSA-treated cells, thus verifying the high fidelity of DCS. Epigenetic regulation of DCS clones was further confirmed in cells treated with sodium butyrate, another HDAC inhibitor, as well as in cardiac myocytes isolated from neonatal rats. The mRNA level of 9 (39%) of 23 genes corresponding to DCS clones changed in parallel with the level of histone acetylation in H9C2 cells. Furthermore, a physiological hypertrophic stimulus, cardiotrophin-1, affected the acetylation level of histones associated with genomic regions corresponding to certain DCS clones. Our data thus establish a genome-wide profile of HDAC targets in cardiomyocytes, which should provide a basis for further investigations into the role of epigenetic modification in cardiac disorders.

Clinical development of histone deacetylase inhibitors as anticancer agents

Acetylation is a key posttranslational modification of many proteins responsible for regulating critical intracellular pathways. Although histones are the most thoroughly studied of acetylated protein substrates, histone acetyltransferases (HATs) and deacetylases (HDACs) are also responsible for modifying the activity of diverse types of nonhistone proteins, including transcription factors and signal transduction mediators. HDACs have emerged as uncredentialed molecular targets for the development of enzymatic inhibitors to treat human cancer, and six structurally distinct drug classes have been identified with in vivo bioavailability and intracellular capability to inhibit many of the known mammalian members representing the two general types of NAD+-independent yeast HDACs, Rpd3 (HDACs 1, 2, 3, 8) and Hda1 (HDACs 4, 5, 6, 7, 9a, 9b, 10). Initial clinical trials indicate that HDAC inhibitors from several different structural classes are very well tolerated and exhibit clinical activity against a variety of human malignancies; however, the molecular basis for their anticancer selectivity remains largely unknown. HDAC inhibitors have also shown preclinical promise when combined with other therapeutic agents, and innovative drug delivery strategies, including liposome encapsulation, may further enhance their clinical development and anticancer potential. An improved understanding of the mechanistic role of specific HDACs in human tumorigenesis, as well as the identification of more specific HDAC inhibitors, will likely accelerate the clinical development and broaden the future scope and utility of HDAC inhibitors for cancer treatment.

Enhancement of therapeutic potential of TRAIL by cancer chemotherapy and irradiation: mechanisms and clinical implications

Activation of cell surface death receptors by their cognate ligands triggers apoptosis. Several human death receptors (Fas, TNF-R1, TRAMP, DR4, DR5, DR6, EDA-R and NGF-R) have been identified. The most promising cytokine for anticancer therapy is TRAIL/APO-2L, which induces apoptosis in cancer cells by binding to death receptors TRAIL-R1/DR4 and TRAIL-R2/DR5. The cytotoxic activity of TRAIL is relatively selective to cancer cells compared to normal cells. Signaling by TRAIL and its receptors is tightly regulated process essential for key physiological functions in a variety of organs, as well as the maintenance of immune homeostasis. Despite early promising results, recent studies have identified several TRAIL-resistant cancer cells of various origins. Based on molecular analysis of death-receptor signaling pathways several new approaches have been developed to increase the efficacy of TRAIL. Resistance of cancer cells to TRAIL appears to occur through the modulation of various molecular targets. They may include differential expression of death receptors, constitutively active Akt and NFkappaB, overexpression of cFLIP and IAPs, mutations in Bax and Bak genes, and defects in the release of mitochondrial proteins in resistant cells. Conventional chemotherapeutic and chemopreventive drugs, and irradiation can sensitize TRAIL-resistant cells to undergo apoptosis. Thus, these agents enhance the therapeutic potential of TRAIL in TRAIL-sensitive cells and sensitize TRAIL-resistant cells. TRAIL and TRAIL-receptor antibodies may prove to be useful for cancer therapy, either alone or in association with conventional approaches such as chemotherapy or radiation therapy. This review discusses intracellular mechanisms of TRAIL resistance and various approaches that can be taken to sensitize TRAIL-resistant cancer cells.

Synergistic killing of human leukemia cells by antioxidants and trichostatin A

PURPOSE

Antioxidants and trichostatin A (TSA) are promising anticancer drugs, and are capable of enhancing the neoplastic toxicity of other chemicals that exert anticancer activity via different mechanisms. Since antioxidants and TSA (the specific inhibitor of histone deacetylase) are believed to combat cancer via different mechanisms, we sought to determine whether combining them would improve their anticancer activity in human leukemia cells (HL-60).

MATERIALS AND METHODS

HL-60 cells were treated with antioxidants (ascorbic acid, AA and N-acetyl-cysteine, NAC), TSA or their combination, and cell proliferation arrest, lactate dehydrogenase (LDH) release and cell viability were measured as indicators of cell damage. Accumulation of reactive oxygen species (ROS) and the acetylation of histones were also measured.

RESULTS

The cytotoxicity of AA, NAC and TSA increased in a time- and dose-dependent manner. AA (1, 2 and 4 mM) and NAC (0.2, 0.5 and 1 mM) were able to diminish ROS generation but showed no influence on histone acetylation in HL-60 cells. In contrast, TSA (20, 50, 100 and 200 nM) did not inhibit ROS generation but significantly increased histone acetylation, indicating a possible role for both scavenging ROS and increasing histone acetylation in the induction of cell death in HL-60 cells. This conclusion was further confirmed by the finding that the combination of antioxidant and TSA not only diminished ROS generation, but also increased histone acetylation, and hence showed greater cytotoxicity in HL-60 cells than either component alone.

CONCLUSIONS

Our findings show that combining antioxidants and TSA can enhance their neoplastic toxicity at least in human leukemia HL-60 cells, providing a new approach to the design of chemotherapy strategies and the development of anticancer drugs.

Subcellular Localization Determines the Protective Effects of Activated ERK2 against Distinct Apoptogenic Stimuli in Myeloid Leukemia Cells

ERKs, mitogen-activated protein kinases, are well characterized as key mediators in the conveyance of signals that promote cell survival in cells of hemopoietic origin, a key factor in the upbringing of leukemogenesis. It is also well known that ERKs phosphorylate a wide array of substrates distributed throughout distinct cellular locations such as the nucleus, cytoplasm, and cell periphery, but the relative contribution of these compartmentalized signal components to the overall survival signal generated by activation of ERKs has yet to be established. To this end, we have utilized constitutively activated forms of ERK2, whose expression is restricted to the nucleus or to the cytoplasm, to investigate the consequences of compartmentalized activation of ERK in the survival of chronic myelogenous leukemia cells subjected to distinct apoptogenic stimuli. We show that cytoplasmic ERK2 activity protected against apoptosis caused by prolonged serum starvation, whereas ERK2 activation restricted to the nucleus antagonized apoptosis induced by the Bcr-Abl inhibitor STI571. On the other hand, neither cytoplasmic nor nuclear ERK2 activities were effective in counteracting apoptosis induced by UV light. These results demonstrate that the protective effects of ERK2 against defined apoptogenic stimuli are strictly dependent on the cellular localization where ERK activation takes place. Furthermore, we present evidence suggesting that the complex I kappa B-NF kappa B participates on ERK2-mediated survival mechanisms, in a fashion dependent on the cellular location where ERK2 is active and on the causative apoptogenic stimulus.

DNA and the chromosome - varied targets for chemotherapy

The nucleus of the cell serves to maintain, regulate, and replicate the critical genetic information encoded by the genome. Genomic DNA is highly associated with proteins that enable simple nuclear structures such as nucleosomes to form higher-order organisation such as chromatin fibres. The temporal association of regulatory proteins with DNA creates a dynamic environment capable of quickly responding to cellular requirements and distress. The response is often mediated through alterations in the chromatin structure, resulting in changed accessibility of specific DNA sequences that are then recognized by specific proteins. Anti-cancer drugs that target cellular DNA have been used clinically for over four decades, but it is only recently that nuclease specific drugs have been developed to not only target the DNA but also other components of the nuclear structure and its regulation. In this review, we discuss some of the new drugs aimed at primary DNA sequences, DNA secondary structures, and associated proteins, keeping in mind that these agents are not only important from a clinical perspective but also as tools for understanding the nuclear environment in normal and cancer cells.