Enhancement ofin vitro andin vivo tumor cell radiosensitivity by valproic acid

Cell type-specific anti-cancer properties of valproic acid: independent effects on HDAC activity and Erk1/2 phosphorylation

Background

The anti-epileptic drug valproic acid (VPA) has attracted attention as an anti-cancer agent.

Methods

The present study investigated effects of VPA exposure on histone deacetylase (HDAC) inhibition, cell growth, cell speed, and the degree of Erk1/2 phosphorylation in 10 cell lines (BT4C, BT4Cn, U87MG, N2a, PC12-E2, CSML0, CSML100, HeLa, L929, Swiss 3T3).

Results

VPA induced significant histone deacetylase (HDAC) inhibition in most of the cell lines, but the degree of inhibition was highly cell type-specific. Moreover, cell growth, motility and the degree of Erk1/2 phosphorylation were inhibited, activated, or unaffected by VPA in a cell type-specific manner. Importantly, no relationship was found between the effects of VPA on HDAC inhibition and changes in the degree of Erk1/2 phosphorylation, cell growth, or motility. In contrast, VPA-induced modulation of the MAPK pathway downstream of Ras but upstream of MEK (i.e., at the level of Raf) was important for changes in cell speed.

Conclusions

These results suggest that VPA can modulate the degree of Erk1/2 phosphorylation in a manner unrelated to HDAC inhibition and emphasize that changes in the degree of Erk1/2 phosphorylation are also important for the anti-cancer properties of VPA.

Histone deacetylase inhibition attenuates cell growth with associated telomerase inhibition in high-grade childhood brain tumor cells

Aberrant epigenetic regulation of gene expression contributes to tumor initiation and progression. Studies from a plethora of hematologic and solid tumors support the use of histone deacetylase inhibitors (HDACi) as potent anticancer agents. However, the mechanism of HDACi action with respect to the temporal order of induced cellular events is unclear. The present study investigates the anticancer effects of the HDACi trichostatin A in high-grade childhood brain tumor cells. Acute exposure to trichostatin A resulted in marked inhibition of cell proliferation, an increase in the proportion of G(2)-M cells, activation of H2A.X, and subsequent induction of apoptosis in the majority of cell lines. These phenotypic effects were associated with abrogation of telomerase activity and human telomerase reverse transcriptase downregulation in the majority of cell lines. In contrast, no cytotoxicity was observed in primary ependymal cells with respect to cilia function. Thus, inhibition of histone deacetylases leads to antiproliferative and proapoptotic effects in childhood brain tumor cells, likely to involve altered chromatin regulation at the human telomerase reverse transcriptase promoter.

A combined pretreatment of 1,25-dihydroxyvitamin D3 and sodium valproate enhances the damaging effect of ionizing radiation on prostate cancer cells

Radiotherapy is one of the curative treatment options for prostate cancer (PCa). However, effective doses of ionizing radiation (IR) have a high risk of side effects. To increase sensitivity of PCa to IR we pretreated human androgen-refractory DU145 PCa cells with a combination of sodium valproate (VPA), a well-tolerated drug with histone deacetylases inhibiting activity, and 1,25-dihydroxyvitamin D3, 1,25(OH)2D3, the active metabolite of vitamin D, a well known anticancer agent. The results show that irradiation (4Gy) of DU145 PCa cells pretreated with a combination of 1 mM VPA and 100 nM 1,25(OH)2D3 efficiently suppressed (87.9%) PCa cell proliferation. IR after combined pretreatment resulted in increased DNA double-strand breaks expressed as levels of phosphorylated histone H2A.X, compared with non-treated cells the increase was 58.1% in pretreated cells and 11.8% in non-pretreated cells (p<0.002). Combined pretreatment enhanced IR-induced activation of DNA damage checkpoint kinase Chk2, 39.0% in pretreated cells compared to 23.8% in non-pretreated cells (p<0.05). These molecular changes led to DNA replication blockade, S-phase cell-cycle arrest and enhanced apoptosis. Cumulatively, the results indicate that combined pretreatment with VPA and 1,25(OH)2D3 followed by IR is a highly effective treatment for human PCa cells. This observation may have important implications for reducing doses of radiation administered to cancer patients thus limiting the severity of side effects.

Enhancement of radiation response in osteosarcoma and rhabdomyosarcoma cell lines by histone deacetylase inhibition

PURPOSE

Histone deacetylase inhibitors (HDACIs) can enhance the sensitivity of cells to photon radiation treatment (XRT) by altering numerous molecular pathways. We investigated the effect of pan-HDACIs such as suberoylanilide hydroxamic acid (SAHA) on radiation response in two osteosarcoma (OS) and two rhabdomyosarcoma (RMS) cell lines.

METHODS AND MATERIALS

Clonogenic survival, cell cycle analysis, and apoptosis were examined in OS (KHOS-24OS, SAOS2) and RMS (A-204, RD) cell lines treated with HDACI and HDACI plus XRT, respectively. Protein expression was investigated via immunoblot analysis, and cell cycle analysis and measurement of apoptosis were performed using flow cytometry.

RESULTS

SAHA induced an inhibition of cell proliferation and clonogenic survival in OS and RMS cell lines and led to a significant radiosensitization of all tumor cell lines. Other HDACI such as M344 and valproate showed similar effects as investigated in one OS cell line. Furthermore, SAHA significantly increased radiation-induced apoptosis in the OS cell lines, whereas in the RMS cell lines radiation-induced apoptosis was insignificant with and without SAHA. In all investigated sarcoma cell lines, SAHA attenuated radiation-induced DNA repair protein expression (Rad51, Ku80).

CONCLUSION

Our results show that HDACIs enhance radiation action in OS and RMS cell lines. Inhibition of DNA repair, as well as increased apoptosis induction after exposure to HDACIs, can be mechanisms of radiosensitization by HDACIs.

Differential radiosensitizing effect of valproic acid in differentiation versus self-renewal promoting culture conditions

PURPOSE

It has been shown that valproic acid (VA) enhances the proliferation and self-renewal of normal hematopoietic stem cells and that breast cancer stem/progenitor cells can be resistant to radiation. From these data, we hypothesized that VA would fail to radiosensitize breast cancer stem/progenitor cells grown to three-dimensional (3D) mammospheres.

METHODS AND MATERIALS

We used the MCF7 breast cancer cell line grown under stem cell-promoting culture conditions (3D mammosphere) and standard nonstem cell monolayer culture conditions (two-dimensional) to examine the effect of pretreatment with VA on radiation sensitivity in clonogenic survival assays and on the expression of embryonic stem cell transcription factors.

RESULTS

3D-cultured MCF-7 cells expressed higher levels of Oct4, Nanog, and Sox2. The 3D passage enriched self-renewal and increased radioresistance in the 3D mammosphere formation assays. VA radiosensitized adherent cells but radioprotected 3D cells in single-fraction clonogenic assays. Moreover, fractionated radiation sensitized VA-treated adherent MCF7 cells but did not have a significant effect on VA-treated single cells grown to mammospheres.

CONCLUSION

We have concluded that VA might preferentially radiosensitize differentiated cells compared with those expressing stem cell surrogates and that stem cell-promoting culture is a useful tool for in vitro evaluation of novel cancer therapeutic agents and radiosensitizers.

Epigenetics of neurological cancers

Epigenetic mechanisms involving DNA methylation, histone modifications and noncoding RNAs regulate and maintain gene-expression states. Similar to genetic mutations, alterations in epigenetic regulation can lead to uncontrolled cell division, tumor initiation and growth, invasiveness and metastasis. Research in brain cancer, particularly gliomas, has uncovered global and gene-specific DNA hypomethylation, local DNA hypermethylation of gene promoters and the de-regulation of microRNA expression. Understanding epigenetic dysregulation in brain cancers has provided new tools for prognostication, as well as suggesting new approaches to therapy. There is significant interest in new sequencing-based technologies that map genetic and epigenetic alterations comprehensively and at high resolution. These methods are being applied to brain tumors, and will better define the contribution of epigenetic defects to tumorigenesis.

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

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

Inhibition of type I histone deacetylase increases resistance of checkpoint-deficient cells to genotoxic agents through mitotic delay

Histone deacetylase (HDAC) inhibitors potently inhibit tumor growth and are currently being evaluated for their efficacy as chemosensitizers and radiosensitizers. This efficacy is likely to be limited by the fact that HDAC inhibitors also induce cell cycle arrest. Deletion of the class I HDAC Rpd3 has been shown to specifically suppress the sensitivity of Saccharomyces cerevisiae DNA damage checkpoint mutants to UV and hydroxyurea. We show that in the fission yeast Schizosaccharomyces pombe, inhibition of the homologous class I HDAC specifically suppresses the DNA damage sensitivity of checkpoint mutants. Importantly, the prototype HDAC inhibitor Trichostatin A also suppressed the sensitivity of DNA damage checkpoint but not of DNA repair mutants to UV and HU. TSA suppressed DNA damage activity independently of the mitogen-activated protein kinase-dependent and spindle checkpoint pathways. We show that TSA delays progression into mitosis and propose that this is the main mechanism for suppression of the DNA damage sensitivity of S. pombe checkpoint mutants, partially compensating for the loss of the G(2) checkpoint pathway. Our studies also show that the ability of HDAC inhibitors to suppress DNA damage sensitivity is not species specific. Class I HDACs are the major target of HDAC inhibitors and cancer cells are often defective in checkpoint activation. Effective use of these agents as chemosensitizers and radiosensitizers may require specific treatment schedules that circumvent their inhibition of cell cycle progression.

Vorinostat enhances the radiosensitivity of a breast cancer brain metastatic cell line grown in vitro and as intracranial xenografts

Vorinostat (suberoylanilide hydroxamic acid), a histone deacetylase inhibitor, is currently undergoing clinical evaluation as therapy for cancer. We investigated the effects of vorinostat on tumor cell radiosensitivity in a breast cancer brain metastasis model using MDA-MB-231-BR cells. In vitro radiosensitivity was evaluated using clonogenic assay. Cell cycle distribution and apoptosis was measured using flow cytometry. DNA damage and repair was evaluated using gammaH2AX. Mitotic catastrophe was measured by immunostaining. Growth delay and intracranial xenograft models were used to evaluate the in vivo tumor radiosensitivity. Cells exposed to vorinostat for 16 hours before and maintained in the medium after irradiation had an increase in radiosensitivity with a dose enhancement factor of 1.57. gammaH2AX, as an indicator of double-strand breaks, had significantly more foci per cell in the vorinostat plus irradiation group. Mitotic catastrophe, measured at 72 hours, was significantly increased in cells receiving vorinostat plus irradiation. Irradiation of s.c. MDA-MB-231-BR tumors in mice treated with vorinostat resulted in an increase in radiation-induced tumor growth delay. Most importantly, animals with intracranial tumor implants lived the longest after combination treatment. These results indicate that vorinostat enhances tumor cell radiosensitivity in vitro and in vivo. There was a greater than additive improvement in survival in our intracranial model. Combining vorinostat with radiation may be a potential treatment option for patients with breast cancer who develop brain metastases.

Radiosensitization by SAHA in experimental colorectal carcinoma models-in vivo effects and relevance of histone acetylation status

PURPOSE

Histone deacetylase inhibitors are being evaluated as antitumor agents in ongoing clinical trials, and promising preclinical results, combined with favorable toxicity profiles, have rendered the drugs as interesting candidates for combination with other treatment modalities, such as radiotherapy. The aim of the present study was to evaluate the radiosensitizing properties of suberoylanilide hydroxamic acid (SAHA) and the possible requirement of histone hyperacetylation at radiation exposure.

METHODS AND MATERIALS

Radiosensitization by SAHA was assessed in a colorectal carcinoma cell line and in two colorectal xenograft models by analysis of clonogenic survival and tumor growth delay, respectively. Histone acetylation status at radiation exposure was evaluated by Western blot.

RESULTS

In vitro, radiosensitization was demonstrated when cells were preincubated with SAHA, and, in the xenografts, tumor growth was delayed when the mice were treated with fractionated radiation combined with daily SAHA injections compared with radiation alone. Surprisingly, the SAHA-dependent growth delay was still present when radiation was delivered at restored baseline acetylation levels compared with maximal histone hyperacetylation.

CONCLUSION

SAHA was an effective radiosensitizer in experimental colorectal carcinoma models, suggesting that histone deacetylase inhibition might constitute a valuable supplement to current multimodal treatment strategies in rectal cancer. The presence of histone hyperacetylation at radiation was not required to obtain an increased radiation response, questioning the validity of using histone hyperacetylation as a molecular marker for radiosensitivity.

Enhancing the apoptotic and therapeutic effects of HDAC inhibitors

Histone deacetylase inhibitors (HDACi) are anti-cancer drugs that have moved rapidly through clinical development and in 2006 vorinostat (SAHA, Zolinza) was given FDA approval for the treatment of cutaneous T cell lymphoma. Class I, II and IV HDACs that are targets for these compounds deacetylate histone proteins, resulting in chromatin remodelling and altered gene transcription. In addition, numerous non-histone proteins are modified by acetylation and the inhibition of HDAC activity can therefore affect various molecular processes. This broad effect on protein function may account for the pleiotropic anti-tumor responses elicited by HDACi that include induction of tumor cell apoptosis, cell cycle arrest, differentiation and senescence, modulation of immune responses and altered angiogenesis. The ability of HDACi to selectively induce tumor cells to undergo apoptosis is important for the therapeutic efficacy observed in pre-clinical models. Moreover, HDACi can augment the apoptotic effects of other anti-cancer agents that have diverse molecular targets. While HDACi are promising anti-cancer drugs, particularly given the scope to combine HDACi with other agents, identifying the key molecular events that determine the biological response of cells to HDACi treatment remains a challenge. Herein we focus on HDACi-induced apoptosis and discuss the various proteins and pathways that are affected by HDACi to mediate this programmed cell death response. In addition, we highlight the ability of HDACi to synergise with other anti-cancer agents to potently kill tumor cells and discuss the possible molecular processes that underpin the combination effect.

HDACi--targets beyond chromatin

Histone deacetylases (HDACs) play an important role in gene regulation. Inhibitors of HDACs (HDACi) are novel anti-cancer drugs, which induce histone (hyper-) acetylation and counteract aberrant gene repression. On the other hand, HDACi treatment can also result in decreased gene expression, and targeting HDACs affects more than chromatin. Recently, HDACi were shown to evoke non-histone protein acetylation, which can alter signaling networks relevant for tumorgenesis. Furthermore, HDACi can promote the degradation of (proto-) oncoproteins. Here, we summarize these findings and discuss how these substances could be beneficial for the treatment and prevention of human ailments, such as cancer and unbalanced immune functions.

Targeted therapy in the treatment of malignant gliomas

Malignant gliomas are invasive tumors with the potential to progress through current available therapies. These tumors are characterized by a number of abnormalities in molecular signaling that play roles in tumorigenesis, spread, and survival. These pathways are being actively investigated in both the pre-clinical and clinical settings as potential targets in the treatment of malignant gliomas. We will review many of the therapies that target the cancer cell, including the epidermal growth factor receptor, mammalian target of rapamycin, histone deacetylase, and farnesyl transferase. In addition, we will discuss strategies that target the extracellular matrix in which these cells reside as well as angiogenesis, a process emerging as central to tumor development and growth. Finally, we will briefly touch on the role of neural stem cells as both potential targets as well as delivery vectors for other therapies. Interdependence between these varied pathways, both in maintaining health and in causing disease, is clear. Thus, attempts to easily classify some targeted therapies are problematic.

Brain tumors and epilepsy: pathophysiology of peritumoral changes

Epilepsy commonly develops among patients with brain tumors, frequently even as the presenting symptom, and such patients consequently experience substantial morbidity from both the seizures and the underlying disease. At clinical presentation, these seizures are most commonly focal with secondary generalization and conventional medical management is often met with less efficacy. The molecular pathophysiology of these seizures is being elucidated with findings that both the tumoral and peritumoral microenvironments may exhibit epileptogenic phenotypes owing to disordered neuronal connectivity and regulation, impaired glial cell function, and the presence of altered vascular supply and permeability. Neoplastic tissue can itself be the initiation site of seizure activity, particularly for tumors arising from neuronal cell lines, such as gangliogliomas or dysembryoblastic neuroepithelial tumors. Conversely, a growing intracranial lesion can both structurally and functionally alter the surrounding brain tissue with edema, vascular insufficiency, inflammation, and release of metabolically active molecules, hence also promoting seizure activity. The involved mechanisms are certain to be multifactorial and depend on specific tumor histology, integrity of the blood brain barrier, and characteristics of the peritumoral environment. Understanding these changes that underlie tumor-related epilepsy may have roles in both optimal medical management for the seizure symptom and optimal surgical objective and management of the underlying disease.

Reactivation of death receptor 4 (DR4) expression sensitizes medulloblastoma cell lines to TRAIL

OBJECT

Apoptosis, a key cellular response to therapeutic agents is often inactivated in tumor cells. In this study, we evaluated the expression of the tumor necrosis family of death receptors, DR4 and DR5, in medulloblastoma tumor samples and cell lines to determine if epigenetic modulation of gene expression could sensitize tumor cell lines to TRAIL-mediated apoptosis.

METHODS

Human medulloblastoma samples and cell lines were analyzed for DR4 and DR5 expression by quantitative PCR and immunofluorescence assays. Cell lines with downregulated expression of one or both genes were treated with the histone deacetylase inhibitor, MS-275, and the expression of DR4 and DR5 measured by quantitative PCR, Western blotting, flow cytometry and chromatin immunoprecipitation assays. Induction of apoptosis in the presence of MS-275 was evaluated by TUNEL assay and its ability to augment TRAIL-mediated cytotoxicity was determined by MTT assays, Western blotting and flow cytometry.

RESULTS

Compared to normal cerebellum, DR4, but not DR5 expression was consistently downregulated in medulloblastoma tumor samples and in Daoy and D283 cell lines. Interestingly, MS-275 decreased cell growth and induced apoptosis in Daoy and D283 cells. In Daoy cells, this coincided with increased histone H3 and H4 acetylation at the DR4 promoter and enhanced DR4 gene and protein expression as well as elevated Caspase-8 activity. The involvement of DR4 in the cellular response to MS-275 was further confirmed by the observation that knockdown of DR4 and FADD abrogated apoptosis. Further, addition of TRAIL to MS-275 treated cells resulted in an enhancement of apoptosis, suggesting that the upregulated death receptors were functional.

CONCLUSION

Our study provides an understanding of the role of DR4 in apoptosis of medulloblastoma cell lines and suggests a potential contribution of aberrant histone deacetylation to the resistance of medulloblastoma cells to therapeutic death.

Histone acetylation resulting in resistance to methotrexate in choroid plexus cells

Choroid plexus carcinomas are rare tumors that typically occur in young children. Prognosis is poor, and very little information is available to optimize treatment protocols. We used a cell culture model to evaluate whether combining chemotherapeutic agents such as methotrexate with histone deacetylase inhibitors (HDACI) such as valproic acid and MS-275 could improve efficacy. Valproic acid increased the cytotoxicity of radiation and of all the chemotherapeutic agents in Z310 and SV11 mouse choroid plexus cell lines, with the exception of methotrexate. Both HDACIs made choroid plexus cells resistant to this folate antagonist. Searching for a molecular explanation, we found that thymidylate synthase was up regulated when the cells were incubated with HDACI. We also confirmed this finding in human choroid plexus carcinoma cells. Methotrexate should not be combined with HDACI in the treatment of choroid plexus carcinoma.

Postradiation sensitization of the histone deacetylase inhibitor valproic acid

PURPOSE

Preclinical studies evaluating histone deacetylase (HDAC) inhibitor-induced radiosensitization have largely focused on the preirradiation setting based on the assumption that enhanced radiosensitivity was mediated by changes in gene expression. Our previous investigations identified maximal radiosensitization when cells were exposed to HDAC inhibitors in both the preradiation and postradiation setting. We now expand on these studies to determine whether postirradiation exposure alone affects radiosensitivity.

EXPERIMENTAL DESIGN

The effects of the HDAC inhibitor valproic acid (VA) on postirradiation sensitivity in human glioma cell lines were evaluated using a clonogenic assay, exposing cells to VA up to 24 h after irradiation. DNA damage repair was evaluated using gammaH2AX and 53BP1 foci and cell cycle phase distribution was analyzed by flow cytometry. Western blot of acetylated gammaH2AX was done following histone extraction on AUT gels.

RESULTS

VA enhanced radiosensitivity when delivered up to 24 h after irradiation. Cells accumulated in G(2)-M following irradiation, although they returned to baseline at 24 h, mitigating the role of cell cycle redistribution in postirradiation sensitization by VA. At 12 h after irradiation, significant gammaH2AX and 53BP1 foci dispersal was shown in the control, although cells exposed to VA after irradiation maintained foci expression. VA alone had no effect on the acetylation or phosphorylation of H2AX, although it did acetylate radiation-induced gammaH2AX.

CONCLUSIONS

These results indicate that VA enhances radiosensitivity at times up to 24 h after irradiation, which has direct clinical application.

Differential expression of 12 histone deacetylase (HDAC) genes in astrocytomas and normal brain tissue: class II and IV are hypoexpressed in glioblastomas

BACKGROUND

Glioblastoma is the most lethal primary malignant brain tumor. Although considerable progress has been made in the treatment of this aggressive tumor, the clinical outcome for patients remains poor. Histone deacetylases (HDACs) are recognized as promising targets for cancer treatment. In the past several years, HDAC inhibitors (HDACis) have been used as radiosensitizers in glioblastoma treatment. However, no study has demonstrated the status of global HDAC expression in gliomas and its possible correlation to the use of HDACis. The purpose of this study was to evaluate and compare mRNA and protein levels of class I, II and IV of HDACs in low grade and high grade astrocytomas and normal brain tissue and to correlate the findings with the malignancy in astrocytomas.

METHODS

Forty-three microdissected patient tumor samples were evaluated. The histopathologic diagnoses were 20 low-grade gliomas (13 grade I and 7 grade II) and 23 high-grade gliomas (5 grade III and 18 glioblastomas). Eleven normal cerebral tissue samples were also analyzed (54 total samples analyzed). mRNA expression of class I, II, and IV HDACs was studied by quantitative real-time polymerase chain reaction and normalized to the housekeeping gene beta-glucuronidase. Protein levels were evaluated by western blotting.

RESULTS

We found that mRNA levels of class II and IV HDACs were downregulated in glioblastomas compared to low-grade astrocytomas and normal brain tissue (7 in 8 genes, p < 0.05). The protein levels of class II HDAC9 were also lower in high-grade astrocytomas than in low-grade astrocytomas and normal brain tissue. Additionally, we found that histone H3 (but not histone H4) was more acetylated in glioblastomas than normal brain tissue.

CONCLUSION

Our study establishes a negative correlation between HDAC gene expression and the glioma grade suggesting that class II and IV HDACs might play an important role in glioma malignancy. Evaluation of histone acetylation levels showed that histone H3 is more acetylated in glioblastomas than normal brain tissue confirming the downregulation of HDAC mRNA in glioblastomas.

Valproic acid was well tolerated in heavily pretreated pediatric patients with high-grade glioma

Valproic acid (VPA) inhibits histone deacetylase and has been reported to induce apoptosis in glioma. We report 44 heavily pretreated pediatric patients with high-grade glioma or diffuse intrinsic pontine glioma who received VPA as oral continues maintenance treatment with individual dose adaptation. The tumor status when starting the drug was: no measurable disease in 12, measurable but stable disease in 12, and measurable progressive disease in 22 patients. Average trough blood levels of VPA were 99 mg/l. The most frequent complaint was somnolence (three patients), but no severe toxicity was reported. One relapse patient responded, early progression of disease was observed in three frontline patients and in six relapsed patients. Median overall survival duration for all patients was 1.33 years, with large differences between first-line (5-year overall survival, 44%) and relapse therapy (5-year overall survival, 14%). This shows that valproate is safe in this patient population. The moderate tumor efficacy encourages studying the drug further as an element of multi-agent protocols.

Histone deacetylase inhibitors and genomic instability

Histone deacetylase inhibitors (HDACIs) are a promising new class of anticancer drugs. However, their mechanism of action has not been fully elucidated. Most studies have investigated the effect of HDACIs on the regulation of gene transcription. HDAC inhibition also leads to genomic instability by a variety of mechanisms. This phenomenon, which has been largely overlooked, may contribute to the cytotoxic effects of these drugs. Indeed, HDACIs sensitize DNA to exogenous genotoxic damage and induce the generation of reactive oxygen species. Moreover, HDACIs target mitosis resulting in chromosome segregation defects. Here, we review the effects of HDACI treatment on DNA damage and repair, and chromosome segregation control.

Valproic acid as a therapeutic agent for head and neck squamous cell carcinomas

PURPOSES

Here we investigate if valproic acid (VA) can enhance the efficacy of commonly used therapies for head and neck squamous cell carcinomas (HNSCC) and the molecular mechanisms that may be related to its anticancer effects.

METHODS

Proliferation and viability of distinct cell types subjected to VA treatment alone or in combination regimens were measured through BrdU incorporation and LDH release, respectively. Molecular markers compatible with histone deacetylase inhibitory activity of VA were assessed through western blots assays in lysates obtained from cultured cells and tumour biopsies.

RESULTS

Treatment of all cell types with VA resulted in a dose-dependent increase in histone H3 acetylation and p21 expression, as well as dose-dependent cytostasis. In contrast, the cytotoxic response to VA was variable and did not correlate with cytostasis, histone acetylation or p21 induction. The variability in response to VA was also observed in tumour biopsy samples collected from patients prior to and following a 1 week oral course of VA. In addition, we found that a combination of a clinically achievable concentration of VA plus cisplatin caused a threefold to sevenfold increase in cisplatin cytotoxicity in vitro.

CONCLUSIONS

VA acts as a histone deacetylase inhibitor (HDI) in SCC cells and normal human keratinocytes (HKs), potentiates the cytotoxic effect of cisplatin in SCC cell lines and decreases the viability of SCC cells as opposed to HKs. Taken together, the results provide initial evidence that VA might be a valuable drug in the development of better therapeutic regimens for HNSCC.

Functions of normal and malignant prostatic stem/progenitor cells in tissue regeneration and cancer progression and novel targeting therapies

This review summarizes the recent advancements that have improved our understanding of the functions of prostatic stem/progenitor cells in maintaining homeostasis of the prostate gland. We also describe the oncogenic events that may contribute to their malignant transformation into prostatic cancer stem/progenitor cells during cancer initiation and progression to metastatic disease stages. The molecular mechanisms that may contribute to the intrinsic or the acquisition of a resistant phenotype by the prostatic cancer stem/progenitor cells and their differentiated progenies with a luminal phenotype to the current therapies and disease relapse are also reviewed. The emphasis is on the critical functions of distinct tumorigenic signaling cascades induced through the epidermal growth factor system, hedgehog, Wnt/beta-catenin, and/or stromal cell-derived factor-1/CXC chemokine receptor-4 pathways as well as the deregulated apoptotic signaling elements and ATP-binding cassette multidrug transporter. Of particular therapeutic interest, we also discuss the potential beneficial effects associated with the targeting of these signaling elements to overcome the resistance to current treatments and prostate cancer recurrence. The combined targeted strategies toward distinct oncogenic signaling cascades in prostatic cancer stem/progenitor cells and their progenies as well as their local microenvironment, which could improve the efficacy of current clinical chemotherapeutic treatments against incurable, androgen-independent, and metastatic prostate cancers, are also described.

Histone deacetylase inhibitors enhance the chemosensitivity of tumor cells with cross-resistance to a wide range of DNA-damaging drugs

Although DNA-damaging agents are among the most effective anticancer drugs in clinical use, their overall effectiveness is limited by the development of cross-resistance to these drugs. Given that histone deacetylase (HDAC) inhibitors increase the acetylation of core histones, resulting in an open chromatin configuration that is more accessible to DNA-targeting agents, we examined whether HDAC inhibitors might enhance the cytotoxicity of DNA-damaging drugs in six human ovarian tumor cell lines that exhibit different cisplatin sensitivities. Low concentrations of HDAC inhibitors, which alone exhibited little cytotoxicity, markedly enhanced the induction of apoptotic cell death not only by cisplatin but also by a wide variety of DNA-targeting anticancer drugs in these tumor cell lines, irrespective of their sensitivities to the respective drugs. In contrast, HDAC inhibitors did not increase the cytotoxicity of metabolic antagonists or microtubule-targeting agents. HDAC inhibitors potentiated both the phosphorylation of histone H2AX on serine-139 (a marker of DNA double-strand breaks) as well as the accumulation of reactive oxygen species induced by DNA-damaging agents in tumor cells. The enhanced generation of reactive oxygen species appeared to be responsible for the enhanced apoptotic cell death induced by the combination of these drugs. These results indicate that the combination of an HDAC inhibitor with a wide variety of DNA-damaging agents is a promising chemotherapeutic strategy for the eradication of tumor cells, regardless of whether the cells are sensitive or resistant to the DNA-damaging anticancer drugs.

Radiosensitization by the histone deacetylase inhibitor PCI-24781

PURPOSE

PCI-24781 is a novel broad spectrum histone deacetylase inhibitor that is currently in phase I clinical trials. The ability of PCI-24781 to act as a radiation sensitizer and the mechanisms of radiosensitization were examined.

EXPERIMENTAL DESIGN

Exponentially growing human SiHa cervical and WiDr colon carcinoma cells were exposed to 0.1 to 10 micromol/L PCI-24781 in vitro for 2 to 20 h before irradiation and 0 to 4 h after irradiation. Single cells and sorted populations were analyzed for histone acetylation, H2AX phosphorylation, cell cycle distribution, apoptotic fraction, and clonogenic survival.

RESULTS

PCI-24781 treatment for 4 h increased histone H3 acetylation and produced a modest increase in gammaH2AX but negligible cell killing or radiosensitization. Treatment for 24 h resulted in up to 80% cell kill and depletion of cells in S phase. Toxicity reached maximum levels at a drug concentration of approximately 1 micromol/L, and cells in G(1) phase at the end of treatment were preferentially spared. A similar dose-modifying factor (DMF(0.1) = 1.5) was observed for SiHa cells exposed for 24 h at 0.1 to 3 micromol/L, and more radioresistant WiDr cells showed less sensitization (DMF(0.1) = 1.2). Limited radiosensitization and less killing were observed in noncycling human fibroblasts. Cell sorting experiments confirmed that depletion of S-phase cells was not a major mechanism of radiosensitization and that inner noncycling cells of SiHa spheroids could be sensitized by nontoxic doses. PCI-24781 pretreatment increased the fraction of cells with gammaH2AX foci 24 h after irradiation but did not affect the initial rate of loss of radiation-induced gammaH2AX or the rate of rejoining of DNA double-strand breaks.

CONCLUSIONS

PCI-24781 shows promise as a radiosensitizing agent that may compromise the accuracy of repair of radiation damage.

Medical management of brain tumor patients

Brain tumors can present challenging medical problems. Seizures, peritumoral edema, venous thromboembolism, fatigue, and cognitive dysfunction can complicate the treatment of patients who have primary or metastatic brain tumors. Effective medical management results in decreased morbidity and mortality and improved quality of life for affected patients.

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.

HDAC inhibitor PCI-24781 decreases RAD51 expression and inhibits homologous recombination

Histone deacetylase (HDAC) inhibitors such as the phenyl hydroxamic acid PCI-24781 have emerged recently as a class of therapeutic agents for the treatment of cancer. Recent data showing synergy of HDAC inhibitors with ionizing radiation and other DNA-damaging agents have suggested that HDAC inhibitors may act, in part, by inhibiting DNA repair. Here we present evidence that HDAC enzymes are important for homologous recombinational repair of DNA double-strand breaks. Combination studies of PCI-24781 with the poly(ADP-ribose) polymerase inhibitor PJ34, an agent thought to produce lesions repaired by homologous recombination (HR), resulted in a synergistic effect on apoptosis. Immunofluorescence analysis demonstrated that HDAC inhibition caused a complete inhibition of subnuclear repair foci in response to ionizing radiation. Mechanistic investigations revealed that inhibition of HDAC enzymes by PCI-24781 led to a significant reduction in the transcription of genes specifically associated with HR, including RAD51. RAD51 protein levels were significantly decreased after 24 h of drug exposure both in vitro and in vivo. Consistent with inhibition of HR, treatment with PCI-24781 resulted in a decreased ability to perform homology directed repair of I-SceI-induced chromosome breaks in transfected CHO cells. In addition, an enhancement of cell killing was observed in Ku mutant cells lacking functional nonhomologous end joining compared with WT cells. Together these results demonstrate that HDAC enzymes are critically important to enable functional HR by controlling the expression of HR-related genes and promoting the proper assembly of HR-directed subnuclear foci.

Phenylbutyrate sensitizes human glioblastoma cells lacking wild-type p53 function to ionizing radiation

PURPOSE

Histone deacetylase (HDAC) inhibitors induce growth arrest, differentiation, and apoptosis in cancer cells. Phenylbutyrate (PB) is a HDAC inhibitor used clinically for treatment of urea cycle disorders. Because of its low cytotoxicity, cerebrospinal fluid penetration, and high oral bioavailability, we investigated PB as a potential radiation sensitizer in human glioblastoma cell lines.

METHODS AND MATERIALS

Four glioblastoma cell lines were selected for this study. Phenylbutyrate was used at a concentration of 2 mM, which is achievable in humans. Western blots were used to assess levels of acetylated histone H3 in tumor cells after treatment with PB. Flow cytometry was used for cell cycle analysis. Clonogenic assays were performed to assess the effect of PB on radiation sensitivity. We used shRNA against p53 to study the role of p53 in radiosensitization.

RESULTS

Treatment with PB alone resulted in hyperacetylation of histones, confirmed by Western blot analysis. The PB alone resulted in cytostatic effects in three cell lines. There was no evidence of G(1) arrest, increase in sub-G(1) fraction or p21 protein induction. Clonogenic assays showed radiosensitization in two lines harboring p53 mutations, with enhancement ratios (+/- SE) of 1.5 (+/- 0.2) and 1.3 (+/- 0.1), respectively. There was no radiopotentiating effect in two cell lines with wild-type p53, but knockdown of wild-type p53 resulted in radiosensitization by PB.

CONCLUSIONS

Phenylbutyrate can produce p21-independent cytostasis, and enhances radiation sensitivity in p53 mutant human glioblastoma cells in vitro. This suggests the potential application of combined PB and radiotherapy in glioblastoma harboring mutant p53.

Inhibition of Histone Deacetylation: A Strategy for Tumor Radiosensitization

Recently, strategies to enhance tumor radiosensitivity have begun to focus on targeting the molecules and processes that regulate cellular radioresponse. A molecular target that has begun to receive considerable attention is histone acetylation. Histone acetylation is determined by the dynamic interaction of two families of enzymes: histone acetylases and histone deacetylases (HDACs). Histone acetylation plays a role in regulating chromatin structure and gene expression—two parameters that have long been considered determinants of radioresponse. As a means of modifying histone acetylation status, considerable effort has been put into the development of inhibitors of HDAC activity. This has led to the generation of a relatively large number of structurally diverse compounds that can inhibit HDAC activity resulting in histone hyperacetylation. Many of the newer HDAC inhibitor compounds have been designed with better bioavailability or pharmacology than the first-generation compounds. Whereas a number of these second-generation HDAC inhibitors have antitumor activity in preclinical cancer models when delivered as single agents, early clinical data demonstrate only cytostasis when used as monotherapy. However, recent preclinical studies have indicated that HDAC inhibitors from structurally diverse classes can enhance both the in vitro and in vivo radiosensitivity of human tumor cell lines generated from a spectrum of solid tumors. HDAC inhibitors are in clinical trials as single modalities, in combination with chemotherapeutic agents, and recently, in combination with radiotherapy.

Histone deacetylase inhibitor and demethylating agent chromatin compaction and the radiation response by cancer cells

It now appears that epigenetics plays a central role in transformation, both in vitro and in vivo. The expression and regulation of DNA methylation and the subsequent chromatin structure are significantly altered in tumor cells, suggesting a direct role in the process of in vivo cellular transformation. If epigenetics and posttranslational modifications of histones play a role in transformation, then it seems logical that the genes regulating chromatin compaction may also be molecular targets and markers in profiling tumor cell resistance. Local remodeling of chromatin is a key step in the regulation of gene expression, and altering the expression of these genes might also favorably alter how tumor cells respond to anticancer agents. Several new agents that alter chromatin compaction, either methyltransferase or histone deacetylases inhibitors, are progressing through clinical trials and have shown promising preclinical interactions when combined with radiation. In this review, we discuss the potential for histone deacetylases inhibitors as radiosensitizing agents.

Histone deacetylase inhibitors: biology and mechanism of action

Histone deacetylases (HDACs) and histone acetyltransferases are enzymes that regulate chromatin structure and function through the removal and addition, respectively, of the acetyl group from the lysine residues of core nucleosomal histones. This posttranslational modification of histones is an important process in the regulation of gene expression. Aberrant expression and recruitment and disrupted activities of HDACs and histone acetyltransferases have been found in malignant tissues, implicating their involvement in cancer. HDAC inhibitors (HDACIs) function through diverse mechanisms, including the promotion of cell cycle arrest and apoptosis and the inhibition of angiogenesis. Malignant cells appear more sensitive to the proapoptotic effects of HDACIs, underscoring the therapeutic potential of these agents. Multiple HDACIs are currently under investigation in clinical trials, including vorinostat (suberoylanilide hydroxamic acid), which was recently approved by the U.S. Food and Drug Administration for the treatment of cutaneous manifestations of cutaneous T-cell lymphoma in patients with progressive, persistent, or recurrent disease on or after 2 systemic therapies.

Homologous recombination and prostate cancer: a model for novel DNA repair targets and therapies

Using elegant targeting techniques such as IMRT, radiation oncology has improved the therapeutic ratio of prostate cancer radiotherapy through increased physical precision (e.g. increased local control through dose-escalation without increased normal tissue toxicity). The therapeutic ratio might be further improved by the addition of "biologic precision and escalation" pertaining to the use of molecular inhibitors of DNA damage sensing and repair. Indeed, proteins involved in the ATM-p53 damage signaling axis and the homologous (HR) and non-homologous end-joining (NHEJ) pathways of DNA double-strand break (DNA-dsb) rejoining pathways may be attractive candidates to elucidate cancer risk, prognosis, prediction of response and to develop sensitizers towards oxic and hypoxic prostate tumor cells. This review highlights DNA-dsb in prostate cancer research in terms of novel molecular inhibitors, the role of the microenvironment in DNA-dsb repair and potential DNA-dsb biomarkers for clinical trials.

Histone deacetylase inhibitors in cancer therapy

Histones are a family of nuclear proteins that interact with DNA, resulting in DNA being wrapped around a core of histone octamer within the nucleosome. Acetylation/deacetylation of histones is an important mechanism that regulates gene expression and chromatin remodeling. Histone deacetylase (HDAC) inhibitors are a new class of chemotherapeutic drugs that regulate gene expression by enhancing the acetylation of histones, and thus inducing chromatin relaxation and altering gene expression. HDAC inhibitors have been shown in preclinical studies to have potent anticancer activities. A range of structurally diverse HDAC inhibitors have been purified as natural products or synthetically produced. Due to the promising preclinical activity of these agents, numerous clinical trials have been initiated. In this review, the results of published data of single agent and combination trials of these drugs are reviewed, with a focus on dosing, scheduling and toxicity. Although still early in drug development, there is a picture that is starting to develop as to the common toxicities and which tumors seem to be the most susceptible to this class of drugs.

The regulation of cyclin D1 degradation: roles in cancer development and the potential for therapeutic invention

Cyclin D1 is an important regulator of cell cycle progression and can function as a transcriptionl co-regulator. The overexpression of cyclin D1 has been linked to the development and progression of cancer. Deregulated cyclin D1 degradation appears to be responsible for the increased levels of cyclin D1 in several cancers. Recent findings have identified novel mechanisms involved in the regulation of cyclin D1 stability. A number of therapeutic agents have been shown to induce cyclin D1 degradation. The therapeutic ablation of cyclin D1 may be useful for the prevention and treatment of cancer. In this review, current knowledge on the regulation of cyclin D1 degradation is discussed. Novel insights into cyclin D1 degradation are also discussed in the context of ablative therapy. A number of unresolved questions regarding the regulation of cellular cyclin D1 levels are also addressed.

In vivo efficacy of a novel histone deacetylase inhibitor in combination with radiation for the treatment of gliomas

Histone modification has emerged as a promising approach to cancer therapy. We explored the in vivo efficacy of a butyric acid derivative, pivaloyloxymethyl butyrate (AN-9), for the treatment of gliomas. Relative to control and single-modality treatments, the combination of AN-9 and radiation significantly inhibited tumor growth and prolonged time to failure in mice bearing glioma xenografts. The enhanced response to radiation was accompanied by inhibition of cellular proliferation and by increased phosphorylation of H2AX, implicating DNA double-strand breaks in the antineoplastic effects of AN-9 and radiation. The data suggest that AN-9 in combination with radiation may be an effective therapy for malignant gliomas.

Disparity of histone deacetylase inhibition on repair of radiation-induced DNA damage on euchromatin and constitutive heterochromatin compartments

Epigenetic regulation of chromatin structure is central to the process of DNA repair. A well-characterized epigenetic feature is the dynamic phosphorylation of the histone H2AX (gammaH2AX) and mobilization of double strand break (DSB) recognition and repair factors to the site. How chromatin structure is altered in response to DNA damage and how such alterations influence DSB repair mechanisms are currently relevant issues. Despite the clear link between histone deacetylases (HDACs) and radiosensitivity, how histone hyperacetylation influence DSB repair remains poorly understood. We have determined the structure of chromatin is a major factor determining radiosensitivity and repair in human cells. Trichostatin A (TSA) enhances radiosensitivity with dose modification factors of 1.2 and 1.9 at 0.2 and 1 microM, respectively. Cells treated with TSA causing hyperacetylation and remodelling on euchromatic alleles coexist with gammaH2AX accumulation in radiosensitized cells. Formation of gammaH2AX on heterochromatin was significantly reduced even when cells were treated with TSA, suggesting that chromatin structure and histone hyperacetylation are pronounced features of radiation sensitivity and repair in euchromatic regions.

The corepressor silencing mediator for retinoid and thyroid hormone receptor facilitates cellular recovery from DNA double-strand breaks

Cells are frequently challenged by DNA double-strand breaks (DSB) that threaten their normal function and survival. In mammalian cells, the repair of DSBs is predominantly mediated by the DNA-dependent protein kinase (DNA-PK) complex. We unexpectedly found that the corepressor silencing mediator for retinoid and thyroid hormone receptor (SMRT) associates with the DNA-PK repair complex. The SMRT/histone deacetylase 3 complex is required for the transcriptional repressive property of the Ku70 subunit of the repair complex. Moreover, SMRT, but not the related Nuclear Receptor Corepressor, is required for cellular recovery from DNA DSBs induced by ionizing radiation or DNA damage-inducing drugs. Thus, the corepressor SMRT plays a novel and critical role in the cellular response to DSBs.

Anticancer activities of histone deacetylase inhibitors

Histone deacetylases (HDACs) are enzymes involved in the remodelling of chromatin, and have a key role in the epigenetic regulation of gene expression. In addition, the activity of non-histone proteins can be regulated through HDAC-mediated hypo-acetylation. In recent years, inhibition of HDACs has emerged as a potential strategy to reverse aberrant epigenetic changes associated with cancer, and several classes of HDAC inhibitors have been found to have potent and specific anticancer activities in preclinical studies. However, such studies have also indicated that the effects of HDAC inhibitors could be considerably broader and more complicated than originally understood. Here we summarize recent advances in the understanding of the molecular events that underlie the anticancer effects of HDAC inhibitors, and discuss how such information could be used in optimizing the development and application of these agents in the clinic, either as monotherapies or in combination with other anticancer drugs.

Radiosensitization of colorectal carcinoma cell lines by histone deacetylase inhibition

Background

The tumor response to preoperative radiotherapy of locally advanced rectal cancer varies greatly, warranting the use of experimental models to assay the efficacy of molecular targeting agents in rectal cancer radiosensitization. Histone deacetylase (HDAC) inhibitors, agents that cause hyperacetylation of histone proteins and thereby remodeling of chromatin structure, may override cell cycle checkpoint responses to DNA damage and amplify radiation-induced tumor cell death.

Methods

Human colorectal carcinoma cell lines were exposed to ionizing radiation and HDAC inhibitors, and cell cycle profiles and regulatory factors, as well as clonogenicity, were analyzed.

Results

In addition to G₂/M phase arrest following irradiation, the cell lines displayed cell cycle responses typical for either intact or defective p53 function (the presence or absence, respectively, of radiation-induced expression of the cell cycle inhibitor p21 and subsequent accumulation of G₁ phase cells). In contrast, histone acetylation was associated with complete depletion of the G₁ population of cells with functional p53 but accumulation of both G₁ and G₂/M populations of cells with defective p53. The cellular phenotypes upon HDAC inhibition were consistent with the observed repression of Polo-like kinase-1, a regulatory G₂/M phase kinase. Following pre-treatment with HDAC inhibitors currently undergoing clinical investigation, the inhibitory effect of ionizing radiation on clonogenicity was significantly amplified.

Conclusion

In these experimental models, HDAC inhibition sensitized the tumor cells to ionizing radiation, which is in accordance with the concept of increased probability of tumor cell death when chromatin structure is modified.

Vorinostat, a histone deacetylase inhibitor, enhances the response of human tumor cells to ionizing radiation through prolongation of gamma-H2AX foci

Vorinostat (suberoylanilide hydroxamic acid) is the prototype of a family of hybrid polar compounds that can induce growth arrest in transformed cells and shows promise for the treatment of cancer. Vorinostat specifically binds to and inhibits the activity of histone deacetylases resulting in acetylation of nucleosomal histones and an activation of gene transcription. Because histone deacetylases modulate chromatin structure and gene expression, both of which can influence radioresponse, this study was designed to examine the capacity of Vorinostat to influence radiation response in human tumor cells and investigate the mechanism underlying these interactions. Vorinostat induced hyperacetylation of histone H4 in a dose-dependent manner. We tested its ability to radiosensitize three human tumor cell lines (A375, MeWo, and A549) using clonogenic cell survival assays. Clonogenic cell survival assay showed that Vorinostat significantly radiosensitized all three tumor cell lines, substantially reducing the surviving fraction at 2 Gy. We examined potential mechanisms that may contribute to the enhanced radiation response induced by Vorinostat. Vorinostat and radiation alone did not induce apoptosis in the melanoma cell line. However, enhanced apoptosis was observed when cells were exposed to both Vorinostat and radiation, suggesting that Vorinostat renders tumor cells more susceptible to radiation-induced apoptosis. Results from DNA damage repair analysis in cultured A375 cells showed that Vorinostat had a strong inhibitory effect on the nonhomologous end joining pathway after radiation. A detailed examination of the involvement of the DNA repair pathway following Vorinostat treatment showed that Vorinostat reduced the expression of the repair-related genes Ku70, Ku80, and Rad50 in A375 cells as detected by Western blot analysis. We also examined gamma-H2AX phosphorylation as a predictive marker of radiotherapy response to Vorinostat and observed that the combination of Vorinostat and radiation caused a prolongation of expression of DNA repair proteins such as gamma-H2AX. Overall, we conclude that Vorinostat enhances tumor radioresponse by multiple mechanisms that may involve antiproliferative growth inhibition and effects on DNA repair after exposure to radiation.

Medical management of patients with brain tumors

The most common medical problems in brain tumor patients include the management of seizures, peritumoral edema, medication side effects, venous thromboembolism (VTE), fatigue and cognitive dysfunction. Despite their importance, there are relatively few studies specifically addressing these issues. There is increasing evidence that brain tumor patients who have not had a seizure do not benefit from prophylactic antiepileptic medications. Patients on corticosteroids are at greater risk of Pneumocystis jerovecii pneumonia and may benefit from prophylactic therapy. There is also growing evidence suggesting that anticoagulation may be more effective than inferior vena cava IVC) filtration devices for treating VTE in brain tumor patients and the risk of hemorrhage with anticoagulation is relatively small. Low-molecular weight heparin may be more effective than coumadin. Medications such as modafinil and methylphenidate have assumed an increasing role in the treatment of fatigue, while donepezil and memantine may be helpful with memory loss.

Histone deacetylase inhibitor-mediated radiosensitization of human cancer cells: class differences and the potential influence of p53

Histone deacetylase inhibitors (HDI) are emerging as potentially useful components of the anticancer armamentarium and as useful tools to dissect mechanistic pathways. HDIs that globally inhibit histone deacetylases (HDAC) have radiosensitizing effects, but the relative contribution of specific HDAC classes remains unclear. Newly characterized HDIs are now available that preferentially inhibit specific HDAC classes, including SK7041 (inhibits class I HDACs) and splitomicin (inhibits class III HDACs). We investigated in human cancer cells the relative radiosensitizations that result from blocking specific HDAC classes. We found that trichostatin A (TSA; inhibitor of both class I and II HDACs) was the most effective radiosensitizer, followed by the class I inhibitor SK7041, whereas splitomicin (inhibitor of class III) had least effect. Interestingly, radiosensitization by TSA in cell lines expressing p53 was more pronounced than in isogenic lines lacking p53. Radiosensitization of cells expressing p53 by TSA was reduced by pifithrin-alpha, a small-molecule inhibitor of p53. In contrast, the radiosensitization by TSA of cells expressing low levels of p53 was enhanced by transfection of wild-type p53-expressing vector or pretreatment with leptomycin B, an inhibitor of nuclear export that increased intracellular levels of p53. These effects on radiosensitization were respectively muted or not seen in cells treated with SK7041 or splitomicin. To our knowledge, this may be among the first systematic investigations of the comparative anticancer effects of inhibiting specific classes of HDACs, with results suggesting differences in the degrees of radiosensitization, which in some cell lines may be influenced by p53 expression.

Butyric acid prodrugs are histone deacetylase inhibitors that show antineoplastic activity and radiosensitizing capacity in the treatment of malignant gliomas

Histone modification has emerged as a promising approach to cancer therapy. We explored the efficacy of a novel class of histone deacetylase inhibitors in the treatment of malignant gliomas. Treatment of glioma cell lines with two butyric acid derivatives, pivaloylomethyl butyrate (AN-9) and butyroyloxymethyl butyrate (AN-1), induced hyperacetylation, increased p21(Cip1) expression, inhibited proliferation, and enhanced apoptosis. Histone deacetylase inhibitor-induced apoptosis was mediated primarily by caspase-8. Treatment of cells with AN-1 or AN-9 for 24 hours before exposure to gamma-irradiation potentiated further caspase-8 activity and resultant apoptosis. Clonogenic survival curves revealed marked reductions in cell renewal capacity of U251 MG cells exposed to combinations of AN-1 and radiation. Preliminary in vivo experiments using human glioma cell lines grown as xenografts in mouse flanks suggest in vivo efficacy of AN-9. The data suggest that novel butyric acid prodrugs provide a promising treatment strategy for malignant gliomas as single agents and in combination with radiation therapy.

Modulation of cellular radiation responses by histone deacetylase inhibitors

Histone deacetylase (HDAC) inhibitors are emerging as a new class of targeted cancer chemotherapeutics. Several HDAC inhibitors are currently in clinical trials and promising anticancer effects at well-tolerated doses have been observed for both hematologic and solid cancers. HDAC inhibitors have been shown to induce cell-cycle and growth arrest, differentiation and in certain cases apoptosis in cell cultures and in vivo. However, it is known that these compounds induce varying responses in different cells and biological settings, and identifying their precise mechanisms of action is an area of great interest. Important findings are continually expanding our understanding of the cellular effects of HDAC inhibitors and recent studies will be briefly outlined in this review. In addition to their intrinsic anticancer properties, numerous studies have demonstrated that HDAC inhibitors can modulate cellular responses to other cytotoxic modalities including ionizing radiation, ultraviolet radiation and chemotherapeutic drugs. Hence, there is a growing interest in potential clinical use of HDAC inhibitors in combination with conventional cancer therapies. In this review, the interaction of HDAC inhibitors with other anticancer agents is discussed. The focus of the article is on the different mechanisms by which HDAC inhibitors enhance the sensitivity of cells to the effects of ionizing radiation.