Histone deacetylase inhibitors: apoptotic effects and clinical implications (Review)

It has been shown that epigenetic modifications play an important role in tumorigenesis. Thus, affecting epigenetic tumorigenic alterations can represent a promising strategy for anticancer targeted therapy. Among the key chromatin modifying enzymes which influence gene expression, histone acetyltransferases (HATs) and histone deacetylases (HDACs) have recently attracted interest because of their impact on tumor development and progression. Increased expression of HDACs and disrupted activities of HATs have been found in several tumor types, with a consequent hypoacetylated state of chromatin that can be strictly correlated with low expression of either tumor suppressor or pro-apoptotic genes. Histone deacetylase inhibitors (HDACIs) represent a new and promising class of antitumor drugs that influence gene expression by enhancing acetylation of histones in specific chromatin domains. HDACIs have been shown to exert potent anticancer activities inducing cell cycle arrest and apoptosis. Notably, a high efficacy of these drugs has been selectively revealed in malignant cells rather than in normal cells. Moreover, the therapeutic potential of these agents is also supported by the evidence that HDACIs downregulate genes involved in tumor progression, invasion and angiogenesis. Several HDACIs are currently under clinical investigation, including vorinostat (SAHA), romidepsin (depsipeptide, FK-228), LAQ824/LBH589 and belinostat (PXD101), compounds that have shown therapeutic potential in many types of malignancies including solid tumors. Based on the ability of HDACIs to regulate many signaling pathways, co-treatment of these compounds with molecular targeted drugs is a promising strategy against many types of tumors.     

Histone deacetylase inhibitors induce cell death in supratentorial primitive neuroectodermal tumor cells

Histone deacetylase inhibitors (HDIs) are a promising new class of antineoplastic agents with the capacity to induce differentiation and/or apoptosis of cancer cells. The objective of this study was to evaluate the activity of HDIs against supratentorial primitive neuroectodermal tumor (sPNET) cells. We show that the HDIs, suberoylanilide hydroxamic acid, sodium butyrate, and trichostatin A, induced cell death, and activated caspase-3 and -9 in a sPNET cell line, PFSK. The poly-caspase inhibitor z-VAD-fmk partially prevented the action of HDIs, as judged by determining the mitochondrial membrane potential and by quantifying internucleosomal DNA fragmentation. In conclusion, the HDIs explored possess potent activity against sPNET cells, suggesting that HDIs may be effective in the treatment of sPNET.     

p53-dependent and p53-independent anticancer effects of different histone deacetylase inhibitors

Background:

Histone deacetylase inhibitors (HDACi) are promising antineoplastic agents, but their precise mechanisms of actions are not well understood. In particular, the relevance of p53 for HDACi-induced effects has not been fully elucidated. We investigated the anticancer effects of four structurally distinct HDACi, vorinostat, entinostat, apicidin and valproic acid, using isogenic HCT-116 colon cancer cell lines differing in p53 status.

Methods:

Effects were assessed by MTT assay, flow-cytometric analyses of propidium iodide uptake, mitochondrial depolarisation and cell-cycle distribution, as well as by gene expression profiling.

Results:

Vorinostat was equally effective in p53 wild-type and null cells, whereas entinostat was less effective in p53 null cells. Histone deacetylase inhibitors treatment suppressed the expression of MDM2 and increased the abundance of p53. Combination treatments showed that vorinostat enhanced the cytotoxic activity of TRAIL and bortezomib, independent of the cellular p53 status. Investigations into the effects of an inhibitor of the sirtuin class of HDAC, tenovin-1, revealed that tenovin-1-mediated cell death hinged on p53.

Conclusion:

These results demonstrate that vorinostat activates p53, but does not require p53 for inducing its anticancer action. Yet they also demonstrate that entinostat-induced cytotoxic effects partially depend on p53, indicating that different HDACi have a different requirement for p53.     

Histone deacetylase inhibitors prime medulloblastoma cells for chemotherapy-induced apoptosis by enhancing p53-dependent Bax activation

Despite aggressive therapies, the prognosis of children with high-risk medulloblastoma is still poor, thus underscoring the need to develop novel treatment strategies. Here, we report that histone deacetylase inhibitors (HDACI), that is, MS-275, valproic acid or SAHA, provide a novel strategy for sensitization of medulloblastoma to DNA-damaging drugs such as Doxorubicin, VP16 and Cisplatin by promoting p53-dependent, mitochondrial apoptosis. Mechanistic studies reveal that single-agent treatment with MS-275 causes acetylation of the non-histone protein Ku70, an event reported to release Bax from Ku70, whereas DNA-damaging drugs trigger p53 acetylation and accumulation. Combined treatment with MS-275 and Doxorubicin or VP16 cooperates to promote binding of p53 to Bax and p53-dependent Bax activation, resulting in enhanced loss of mitochondrial membrane potential, cytochrome c release and caspase-dependent apoptosis. Overexpression of Bcl-2 almost completely abolishes the MS-275-mediated chemosensitization, underlining the importance of the mitochondrial pathway for inducing apoptosis. Also, MS-275 cooperates with chemotherapeutics to inhibit long-term clonogenic survival. Most importantly, MS-275 increases chemotherapeutic drug-induced apoptosis in primary medulloblastoma samples, and cooperates with Doxorubicin to suppress medulloblastoma growth in an in vivo model, which underscores the clinical relevance of the findings. Thus, HDACI such as MS-275 present a promising approach for chemosensitization of medulloblastoma by enhancing mitochondrial apoptosis in a p53-dependent manner. These findings have important clinical implications for the design of experimental treatment protocols for medulloblastoma.     

Histone deacetylase inhibitors VPA and TSA induce apoptosis and autophagy in pancreatic cancer cells

Purpose

Histone deacetylase inhibitors (HDACi) are anti-neoplastic agents that are known to affect the growth of different cancer types, but their underlying mechanisms are still incompletely understood. Here, we compared the effects of two HDACi, i.e., Trichostatin A (TSA) and Valproic Acid (VPA), on the induction of cell death and autophagy in pancreatic cancer-derived cells that exhibit a high metastatic capacity and carry KRAS/p53 double mutations.

Methods

Cell viability and proliferation tests were carried out using Trypan blue dye exclusion, MTT and BrdU assays. FACS analyses were carried out to assess cell cycle progression, apoptosis, reactive oxygen species (ROS) production and mitochondrial depolarization, while Western blot and immunoprecipitation analyses were employed to detect proteins involved in apoptosis and autophagy.

Results

We found that both VPA and TSA can induce apoptosis in Panc1 and PaCa44 pancreatic cancer-derived cells by triggering mitochondrial membrane depolarization, Cytochrome c release and Caspase 3 activation, although VPA was more effective than TSA, especially in Panc1 cells. As underlying molecular events, we found that ERK1/2 was de-phosphorylated and that the c-Myc and mutant p53 protein levels were reduced after VPA and, to a lesser extent, after TSA treatment. Up-regulation of p21 and Puma was also observed, concomitantly with mutant p53 degradation. In addition, we found that in both cell lines VPA increased the pro-apoptotic Bim level, reduced the anti-apoptotic Mcl-1 level and increased ROS production and autophagy, while TSA was able to induce these effects only in PaCA44 cells.

Conclusions

From our results we conclude that both VPA and TSA can induce pancreatic cancer cell apoptosis and autophagy. VPA appears have a stronger and broader cytotoxic effect than TSA and, thus, may represent a better choice for anti-pancreatic cancer therapy.

     

Histone deacetylase 2 modulates p53 transcriptional activities through regulation of p53-DNA binding activity

Histone deacetylase (HDAC) inhibitors are emerging as promising cancer therapeutics. HDAC inhibitors have been found to induce cellular activities that are strikingly similar to p53-mediated responses to genotoxic stress. For example, HDAC inhibitors induce cell cycle arrest, apoptosis, and cellular senescence. Because at least 11 HDACs are affected by the current HDAC inhibitors, the HDAC critical for tumor cell survival and proliferation remains unknown. Thus, we sought to characterize the distinct roles of HDACs in the p53 pathway. Through the use of stable MCF7 cell lines which inducibly express short hairpin RNA targeting HDAC2, we found that HDAC2 plays important roles in the p53 pathway. Specifically, we found that knockdown of HDAC2 inhibited cellular proliferation in a dose-dependent manner which was also partly p53-dependent. Furthermore, knockdown of HDAC2 induced cellular senescence. Importantly, we found that knockdown of HDAC2 enhanced p53-dependent trans-repression and trans-activation of a subset of target genes. We found that the enhancement was due to increased p53-DNA binding activity but not alterations in p53 stability or posttranslational modification(s). Thus, for the first time, our data suggest that HDAC inhibitors function through the p53 pathway, at least in part, by activating p53-DNA binding activity.     

Histone deacetylase inhibitors induce growth suppression and cell death in human rhabdomyosarcoma in vitro.

PURPOSE: A group of histone deacetylase inhibitors has been shown to be effective in suppressing the growth of a variety of transformed cell lines in vitro and in vivo. The effects of two of these agents, suberoylanilide hydroxamic acid (SAHA) and suberoyl-3-aminopyridineamide hydroxamic acid (pyroxamide), were investigated for their growth-suppressive effects on rhabdomyosarcoma (RMS) cells. EXPERIMENTAL DESIGN AND RESULTS: Dose-response experiments of two RMS cell lines, RD (embryonal) and RH30B (alveolar), were performed with SAHA (0.25-3.0 micro M) and pyroxamide (1.25-20.0 micro M). Both agents caused a dose-dependent decrease in viable cell number and an increase in percentage of dead cells over time. Exposure of the RMS cells to SAHA and pyroxamide resulted in an accumulation of acetylated histones with increasing doses by Western blot analysis. Additionally, there was an induction of p21/WAF1 at 15 and 24 h when the cells were cultured with SAHA (2.0 micro M) or pyroxamide (20.0 micro M), concentrations that were tested because they successfully induced inhibition of cell growth and initiated cell death in both RMS cell lines. An increase in nuclei with hypodiploid or sub-G(1) fraction was found by flow cytometry with increasing doses of both SAHA (0.25-3.0 micro M) and pyroxamide (1.25-20.0 micro M) over time. This finding is consistent with DNA fragmentation and cell death by apoptosis. CONCLUSIONS: SAHA and pyroxamide induce growth suppression and cell death in human RMS in vitro. Accumulation of acetylated histones and induction of p21/WAF1 expression are observed in cells exposed to either agent.     

Histone deacetylase inhibitors require caspase activity to induce apoptosis in lung and prostate carcinoma cells

Histone deacetylase inhibitors (HDIs) are a promising new class of antineoplastic agents with the capacity to induce growth arrest and/or apoptosis of cancer cells. However, their precise mechanism of action is uncertain; particularly, the role of caspases in the apoptotic response to HDIs is controversial. Here, we show that the HDIs explored, suberoylanilide hydroxamic acid, sodium butyrate and trichostatin A, activated caspase-3 in A549 and PC-3 carcinoma cells. Additionally, the poly-caspase inhibitor z-VAD-fmk prevented HDI-induced apoptosis, as judged by determining mitochondrial membrane potential and by quantifying internucleosomal DNA fragmentation. Importantly, z-VAD-fmk also significantly inhibited HDI-elicited cell death, as assessed by measuring propidium iodide uptake. As an accessory finding, with the inhibition of caspases, a HDI-induced G2-M arrest became evident. Taken together, these results provide evidence that HDIs require activated caspases to induce apoptosis of carcinoma cells.     

Histone deacetylase inhibitors and aspirin interact synergistically to induce cell death in ovarian cancer cells

Histone deacetylase inhibitors (HDIs) as well as non-steroidal anti-inflammatory drugs including aspirin show promise as antineoplastic agents. The treatment with both HDIs and aspirin can result in hyperacetylation of proteins. In this study, we investigated whether HDIs and aspirin interacted in inducing anticancer activity and histone acetylation. We found that the HDIs, suberoylanilide hydroxamic acid and sodium butyrate, and aspirin cooperated to induce cell death in the ovarian cancer cell line, A2780. The effect was synergistic, as evidenced by CI-isobologram analysis. However, aspirin had no effect on histone acetylation, neither in the absence nor presence of HDIs. To gain insight into the mechanism underlying the synergistic action of HDIs and aspirin, we employed the deacetylated metabolite of aspirin, salicylic acid, and the cyclooxygenase-1- and -2-selective inhibitors, SC-560 and NS-398, respectively. We found that HDIs and salicylic acid interacted synergistically, albeit less efficiently than HDIs and aspirin, to induce cancer cell death, suggesting that the acetyl and the salicyl moiety contributed to the cooperative interaction of aspirin with HDIs. SC-560 and NS-398 had little effect both when applied alone or in conjunction with HDIs, indicating that the combinatorial effect of HDIs and aspirin was not the result of cyclo-oxygenase inhibition. In conclusion, our study demonstrates that HDIs and aspirin synergize to induce cancer cell death and, thus, provides a rationale for a more in-depth exploration into the potential of combining HDIs and aspirin as a strategy for anticancer therapy.     

Histone deacetylase inhibitors sensitize tumour cells for cytotoxic effects of natural killer cells

Histone deacetylase inhibitors (HDIs) are emerging as potent anti-tumour agents which induce cell cycle arrest, differentiation and/or apoptosis in many tumour cells. Furthermore, they render tumour cells more sensitive to other therapeutic regimens like ionizing radiation, chemotherapy and recombinant tumour necrosis factor-related apoptosis-inducing ligand (TRAIL). Here, we show that the HDIs suberoylanilide hydroxamic acid (SAHA; vorinostat), sodium butyrate (NaB) and MS-275 sensitized DAOY and PC3 tumour cells for the cytotoxic effects of IL-2-activated PBMCs. In (51)Cr-release assays, blockade of the activating NK receptors DNAM-1, NKG2D and the NCRs completely abrogated tumour cell lysis, revealing that NK cells were the main effector cells involved. HDIs increased the tumour surface expression of ligands for the activating NK receptors NKG2D and DNAM-1 thereby facilitating tumour cell recognition by NK cells. These results suggest that the combination of HDIs and immunotherapy may be an effective strategy for anti-cancer therapy.     

Killing effects of different physical factors on extracorporeal HepG2 human hepatoma cells

Objective: To determine the killing effects on extracorporeal HepG2 cells under different temperatures, pressures of permeability and lengths of treatment time. Method: According to different temperatures, pressures of permeability and lengths of treating time, extracorporeal HepG2 cells of human hepatoma cell-line were grouped to 80 groups. Cell index (CI) as the measurement of killing effect were calculated by monotetrazolium (MTT) methods, i.e., CI =1- (the OD value in treated group - the OD value in blank control group) / (mean of untreated control group – mean of blank control group). According to the factorial design, data were fed into SPSS 10.0 and analyzed by three-way ANOVA (analysis of variance). Result: Temperature, pressure of permeability and length of treating time all had effects on the CI (cell index) level. Length of treating time was the most influential factor of the three. Additionally, any two of them all had statistically significant interactive effects on the CI level. When treated for 5-30 min, destilled water at 46℃ stably generated the highest CI. Conclusion: The “46℃-destilled water-60 min” was considered as the optimal combination of conditions which lead to highest CI. We suggest exerting celiac lavage for 15 min with stilled water at 40℃-43℃ in surgical practice as a hyperthermia treatment to achieve ideal killing effects on free cancer cells, which is feasible, practical, and clinically effective.     

Regulation of p53-, Bcl-2- and caspase-dependent signaling pathway in xanthorrhizol-induced apoptosis of HepG2 hepatoma cells

Xanthorrhizol is a sesquiterpenoid compound extracted from the rhizome of Curcuma xanthorrhiza. This study investigated the antiproliferative effect and the mechanism of action of xanthorrhizol on human hepatoma cells, HepG2, and the mode of cell death. An antiproliferative assay using methylene blue staining revealed that xanthorrhizol inhibited the proliferation of the HepG2 cells with a 50% inhibition of cell growth (IC50) value of 4.17 +/- 0.053 microg/ml. The antiproliferative activity of xanthorrhizol was due to apoptosis induced in the HepG2 cells and not necrosis, which was confirmed by the Tdt-mediated dUTP nick end labeling (TUNEL) assay. The xanthorrhizol-treated HepG2 cells showed typical apoptotic morphology such as DNA fragmentation, cell shrinkage and elongated lamellipodia. The apoptosis mediated by xanthorrhizol in the HepG2 cells was associated with the activation of tumor suppressor p53 and down-regulation of antiapoptotic Bcl-2 protein expression, but not Bax. The levels of Bcl-2 protein expression decreased 24-h after treatment with xanthorrhizol and remained lower than controls throughout the experiment, resulting in a shift in the Bax to Bcl-2 ratio thus favouring apoptosis. The processing of the initiator procaspase-9 was detected. Caspase-3 was also found to be activated, but not caspase-7. Xanthorrhizol exerts antiproliferative effects on HepG2 cells by inducing apoptosis via the mitochondrial pathway.     

Potent inhibitors of cyclin-dependent kinase 2 induce nuclear accumulation of wild-type p53 and nucleolar fragmentation in human untransformed and tumor-derived cells

The cdk2 gene has been identified as a human cdc2/CDC28-related gene that encodes a protein kinase essential for the G1/S transition in mammalian cells, but not for the G2/M transition, which requires Cdk1, another p34cdc2/CDC28 homolog. Novel potential functions of Cdk2 have been uncovered by using two potent and specific inhibitors of its kinase activity, roscovitine and olomoucine, on human wt p53-expresser untransformed and tumor-derived cells. At concentrations equal or superior to respectively 30- and 20-fold their in vitro IC50 values for cyclin B/Cdk1, cyclin A/Cdk2 and cyclin E/Cdk2, the Cdk inhibitors precipitately induce a dramatic nuclear accumulation of wt p53 and a delocalization of nucleolin from the nucleolus in all interphase cells, whatever their cell cycle status, acting in this way like the DNA-damaging drug, mitomycin C (7 microg/ml). These early events are soon followed by a nucleolar fragmentation in both normal and tumor cells in the presence of the Cdk inhibitors but not in the presence of the DNA-damaging drug. Yet, treatment with either type of compounds eventually triggers rapidly the death of the tumor cells and, much more slowly, that of the normal cells. The Cdk inhibitors, however, stimulate cell death from any stage of the cell cycle, whereas the DNA-damaging drug kills more efficaciously S phase cells. These observations provide a hint that the Cdk2 kinase might be involved in controlling the nuclear levels of the tumor suppressor wt p53 protein and in maintaining the nucleolar integrity and function, linking in this way the cell division cycle machinery to survival functions and overall cell metabolism via the control of nucleocytoplasmic transport and of ribosome production.     

Asiatic acid,a triterpene,induces apoptosis through intracellular Ca2+ release and enhanced expression of p53 in HepG2 human hepatoma cells

Asiatic acid (AA), a triterpene, decreased viability and induced apoptosis of HepG2 human hepatoma cells in a dose-dependent manner. AA also markedly increased intracellular Ca(2+) level, which was blocked by TMB-8 and dantrolene, intracellular Ca(2+) release blockers, but not by EGTA, an extracellular Ca(2+) chelator. Moreover, AA-induced apoptosis was significantly suppressed by treatment with TMB-8 and dantrolene, suggesting that intracellular Ca(2+) release may play an essential role in the AA-induced apoptosis. In addition, AA profoundly increased protein level of p53, which was also inhibited by BAPTA/AM, an intracellular Ca(2+) chelator, TMB-8 and dantrolene. Treatment with A23187, a Ca(2+) ionophore, or thapsigargin, a Ca(2+)-ATPase inhibitor, alone enhanced p53 nuclear accumulation, indicating that p53 accumulation is dependent on intracellular Ca(2+) increase. Furthermore, the viability of Hep3B, p53-null cells, was much higher than that of HepG2, p53-wild type cells, when treated with AA. Taken together, these results suggest that AA induced apoptosis through increased intracellular Ca(2+), which, in turn, enhanced p53 expression in HepG2 cells. These results further suggest that AA may be a valuable agent for the therapeutic intervention of human hepatomas.