Combination therapy with the histone deacetylase inhibitor LBH589 and radiation is an effective regimen for prostate cancer cells

A Bifunctional PARP-HDAC Inhibitor with Activity in Ewing Sarcoma

PURPOSE

Histone deacetylase (HDAC) inhibition has been shown to induce pharmacologic "BRCAness" in cancer cells with proficient DNA repair activity. This provides a rationale for exploring combination treatments with HDAC and PARP inhibition in cancer types that are insensitive to single-agent PARP inhibitors (PARPi). Here, we report the concept and characterization of a novel bifunctional PARPi (kt-3283) with dual activity toward PARP1/2 and HDAC enzymes in Ewing sarcoma cells.

EXPERIMENTAL DESIGN

Inhibition of PARP1/2 and HDAC was measured using PARP1/2, HDAC activity, and PAR formation assays. Cytotoxicity was assessed by IncuCyte live cell imaging, CellTiter-Glo, and spheroid assays. Cell-cycle profiles were determined using propidium iodide staining and flow cytometry. DNA damage was examined by γH2AX expression and comet assay. Inhibition of metastatic potential by kt-3283 was evaluated via ex vivo pulmonary metastasis assay (PuMA).

RESULTS

Compared with FDA-approved PARP (olaparib) and HDAC (vorinostat) inhibitors, kt-3283 displayed enhanced cytotoxicity in Ewing sarcoma models. The kt-3283-induced cytotoxicity was associated with strong S and G2-M cell-cycle arrest in nanomolar concentration range and elevated DNA damage as assessed by γH2AX tracking and comet assays. In three-dimensional spheroid models of Ewing sarcoma, kt-3283 showed efficacy in lower concentrations than olaparib and vorinostat, and kt-3283 inhibited colonization of Ewing sarcoma cells in the ex vivo PuMA model.

CONCLUSIONS

Our data demonstrate the preclinical justification for studying the benefit of dual PARP and HDAC inhibition in the treatment of Ewing sarcoma in a clinical trial and provides proof-of-concept for a bifunctional single-molecule therapeutic strategy.

Molecular Targeted Therapies in Metastatic Prostate Cancer: Recent Advances and Future Challenges

Prostate cancer is the most frequent malignant tumor in men, and, despite the great improvements in survival in patients with localized cancer, the prognosis for metastatic disease remains poor. Novel molecular targeted therapies, which block specific molecules or signaling pathways in tumor cells or in their microenvironment, have shown encouraging results in metastatic castration-resistant prostate cancer. Among these therapeutic approaches, prostate-specific membrane antigen-targeted radionuclide therapies and DNA repair inhibitors represent the most promising ones, with some therapeutic protocols already approved by the FDA, whereas therapies targeting tumor neovascularization and immune checkpoint inhibitors have not yet demonstrated clear clinical benefits. In this review, the most relevant studies and clinical trials on this topic are illustrated and discussed, together with future research directions and challenges.

The Combination of Panobinostat and Melphalan for the Treatment of Patients with Multiple Myeloma

Histone deacetylase inhibitors show synergy with several genotoxic drugs. Herein, we investigated the biological impact of the combined treatment of panobinostat and melphalan in multiple myeloma (MM). DNA damage response (DDR) parameters and the expression of DDR-associated genes were analyzed in bone marrow plasma cells (BMPCs) and peripheral blood mononuclear cells (PBMCs) from 26 newly diagnosed MM patients. PBMCs from 25 healthy controls (HC) were examined in parallel. Compared with the ex vivo melphalan-only treatment, combined treatment with panobinostat and melphalan significantly reduced the efficiency of nucleotide excision repair (NER) and double-strand-break repair (DSB/R), enhanced the accumulation of DNA lesions (monoadducts and DSBs), and increased the apoptosis rate only in patients’ BMPCs (all p < 0.001); marginal changes were observed in PBMCs from the same patients or HC. Accordingly, panobinostat pre-treatment decreased the expression levels of critical NER (DDB2, XPC) and DSB/R (MRE11A, PRKDC/DNAPKc, RAD50, XRCC6/Ku70) genes only in patients’ BMPCs; no significant changes were observed in PBMCs from patients or HC. Together, our findings demonstrate that panobinostat significantly increased the melphalan sensitivity of malignant BMPCs without increasing the melphalan sensitivity of PBMCs from the same patients, thus paving the way for combination therapies in MM with improved anti-myeloma efficacy and lower side effects.

From Therapy Resistance to Targeted Therapies in Prostate Cancer

Prostate cancer (PCa) is the second most common malignancy among men worldwide. Although early-stage disease is curable, advanced stage PCa is mostly incurable and eventually becomes resistant to standard therapeutic options. Different genetic and epigenetic alterations are associated with the development of therapy resistant PCa, with specific players being particularly involved in this process. Therefore, identification and targeting of these molecules with selective inhibitors might result in anti-tumoral effects. Herein, we describe the mechanisms underlying therapy resistance in PCa, focusing on the most relevant molecules, aiming to enlighten the current state of targeted therapies in PCa. We suggest that selective drug targeting, either alone or in combination with standard treatment options, might improve therapeutic sensitivity of resistant PCa. Moreover, an individualized analysis of tumor biology in each PCa patient might improve treatment selection and therapeutic response, enabling better disease management.

Pharmacologic Induction of BRCAness in BRCA-Proficient Cancers: Expanding PARP Inhibitor Use

The poly(ADP-ribose) polymerase (PARP) family of proteins has been implicated in numerous cellular processes, including DNA repair, translation, transcription, telomere maintenance, and chromatin remodeling. Best characterized is PARP1, which plays a central role in the repair of single strand DNA damage, thus prompting the development of small molecule PARP inhibitors (PARPi) with the intent of potentiating the genotoxic effects of DNA damaging agents such as chemo- and radiotherapy. However, preclinical studies rapidly uncovered tumor-specific cytotoxicity of PARPi in a subset of cancers carrying mutations in the BReast CAncer 1 and 2 genes (BRCA1/2), which are defective in the homologous recombination (HR) DNA repair pathway, and several PARPi are now FDA-approved for single agent treatment in BRCA-mutated tumors. This phenomenon, termed synthetic lethality, has now been demonstrated in tumors harboring a number of repair gene mutations that produce a BRCA-like impairment of HR (also known as a 'BRCAness' phenotype). However, BRCA mutations or BRCAness is present in only a small subset of cancers, limiting PARPi therapeutic utility. Fortunately, it is now increasingly recognized that many small molecule agents, targeting a variety of molecular pathways, can induce therapeutic BRCAness as a downstream effect of activity. This review will discuss the potential for targeting a broad range of molecular pathways to therapeutically induce BRCAness and PARPi synthetic lethality.

Histone Deacetylases and Their Potential as Targets to Enhance Tumour Radiosensitisation

In mammalian cells, genomic DNA is packaged with histone proteins and condensed into chromatin. To gain access to the DNA, chromatin remodelling is required that is enhanced through histone post-translational modifications, which subsequently stimulate processes including DNA repair and transcription. Histone acetylation is one of the most well understood modifications and is controlled by histone acetyltransferases (HATs) and histone deacetylases (HDACs). These enzymes play critical roles in normal cellular functioning, and the dysregulation of HDAC expression in particular has been linked with the development of a number of different cancer types. Conversely, tumour cell killing following radiotherapy is triggered through DNA damage and HDACs can help co-ordinate the cellular DNA damage response which promotes radioresistance. Consequently, HDAC inhibitors have been investigated as potential radiosensitizers in vitro and in vivo to improve the efficacy or radiotherapy in specific tumour types. In this review, we provide an up-to-date summary of HDACs and their cellular functions, including in DNA damage repair. We also review evidence demonstrating that HDAC inhibitors can effectively enhance tumour radiosensitisation, and which therefore show potential for translation into the clinic for cancer patient benefit.

Screening and Validation of Molecular Targeted Radiosensitizers

The development of molecular targeted drugs with radiation and chemotherapy are critically important for improving the outcomes of patients with hard-to-treat, potentially curable cancers. However, too many preclinical studies have not translated into successful radiation oncology trials. Major contributing factors to this insufficiency include poor reproducibility of preclinical data, inadequate preclinical modeling of inter-tumoral genomic heterogeneity that influences treatment sensitivity in the clinic, and a reliance on tumor growth delay instead of local control (TCD50) endpoints. There exists an urgent need to overcome these barriers to facilitate successful clinical translation of targeted radiosensitizers. To this end, we have employed 3D cell culture assays to better model tumor behavior in vivo. Examples of successful prediction of in vivo effects with these 3D assays include radiosensitization of head and neck cancers by inhibiting epidermal growth factor receptor or focal adhesion kinase signaling, and radioresistance associated with oncogenic mutation of KRAS. To address the issue of tumor heterogeneity we leveraged institutional resources that allow high-throughput 3D screening of radiation combinations with small molecule inhibitors across genomically characterized cell lines from lung, head and neck, and pancreatic cancers. This high-throughput screen is expected to uncover genomic biomarkers that will inform the successful clinical translation of targeted agents from the NCI CTEP portfolio and other sources. Screening "hits" need to be subjected to refinement studies that include clonogenic assays, addition of disease-specific chemotherapeutics, target/biomarker validation, and integration of patient-derived tumor models. The chemoradiosensitizing activities of the most promising drugs should be confirmed in TCD50 assays in xenograft models with/without relevant biomarker and utilizing clinically relevant radiation fractionation. We predict that appropriately validated and biomarker-directed targeted therapies will have a higher likelihood than past efforts to be successfully incorporated into the standard management of hard-to-treat tumors.

Hydralazine and Panobinostat Attenuate Malignant Properties of Prostate Cancer Cell Lines

Among the well-established alterations contributing to prostate cancer (PCa) pathogenesis, epigenetics is an important player in its development and aggressive disease state. Moreover, since no curative therapies are available for advanced stage disease, there is an urgent need for novel therapeutic strategies targeting this subset of patients. Thus, we aimed to evaluate the combined antineoplastic effects of DNA methylation inhibitor hydralazine and histone deacetylase inhibitors panobinostat and valproic acid in several prostate cell lines. The effect of these drugs was assessed in four PCa (LNCaP, 22Rv1, DU145 and PC-3) cell lines, as well as in non-malignant epithelial (RWPE-1) and stromal (WPMY-1) cell lines, using several assays to evaluate cell viability, apoptosis, proliferation, DNA damage and clonogenic potential. We found that exposure to each epidrug separately reduced viability of all PCa cells in a dose-dependent manner and that combined treatments led to synergic growth inhibitory effects, impacting also on colony formation, invasion, apoptotic and proliferation rates. Interestingly, antitumoral effects of combined treatment were particularly expressive in DU145 cells. We concluded that hydralazine and panobinostat attenuate malignant properties of PCa cells, constituting a potential therapeutic tool to counteract PCa progression.

Downregulation of Mcl-1 by Panobinostat Potentiates Proton Beam Therapy in Hepatocellular Carcinoma Cells

Epigenetic modulation by histone deacetylase (HDAC) inhibitors is an attractive anti-cancer strategy for diverse hematological and solid cancers. Herein, we explored the relative effectiveness of the pan-HDAC inhibitor panobinostat in combination with proton over X-ray irradiation in HCC cells. Clonogenic survival assays revealed that radiosensitization of Huh7 and Hep3B cells by panobinostat was more evident when combined with protons than X-rays. Panobinostat increased G2/M arrest and production of intracellular reactive oxygen species, which was further enhanced by proton irradiation. Immunofluorescence staining of γH2AX showed that panobinostat enhanced proton-induced DNA damage. Panobinostat dose-dependently decreased expression of an anti-apoptotic protein, Mcl-1, concomitant with increasing acetylation of histone H4. The combination of panobinostat with proton irradiation enhanced apoptotic cell death to a greater extent than that with X-ray irradiation. Depletion of Mcl-1 by RNA interference enhanced proton-induced apoptosis and proton radiosensitization, suggesting a potential role of Mcl-1 in determining proton sensitivity. Together, our findings suggest that panobinostat may be a promising combination agent for proton beam therapy in HCC treatment.

CD44 collaborates with ERBB2 mediate radiation resistance via p38 phosphorylation and DNA homologous recombination pathway in prostate cancer

CD44, a glycoprotein, has been reported to have relationship with resistance to radiation in prostate cancer (Cap) cells. However, its molecular mechanism remains unknown. In this study, we demonstrated that inhibited CD44 enhanced the radiosentivity in Cap cells. It has been hypothesized that CD44 combine with ERBB2 and activate downstream phosphated protein to mediate DNA damage repair. Therefore, we conducted a detailed analysis of effects of radiation by clonogenic assay and immunofluorescence stain for p-H2AX foci. The downstream of CD44/ERBB2 and DNA damage repair proteins was detected by western blot. The results reveal that CD44 interacted with ERBB2, the downstream of CD44/ERBB2 was p-p38 when Cap cells were irradiated. Among the pathways, homologous recombination (HR) related proteins Mre11 and Rad50 were involved in CD44/ERBB2/p-p38 mediated radioresistance in Cap. In conclusion, CD44 could stabilize ERBB2 and co-activate p-p38 expression then promote the DNA damage repair by HR pathway, which finally contribute to the radioresistance of CaP.

Combination Therapy With Histone Deacetylase Inhibitors (HDACi) for the Treatment of Cancer: Achieving the Full Therapeutic Potential of HDACi

Genetic and epigenetic changes in DNA are involved in cancer development and tumor progression. Histone deacetylases (HDACs) are key regulators of gene expression that act as transcriptional repressors by removing acetyl groups from histones. HDACs are dysregulated in many cancers, making them a therapeutic target for the treatment of cancer. Histone deacetylase inhibitors (HDACi), a novel class of small-molecular therapeutics, are now approved by the Food and Drug Administration as anticancer agents. While they have shown great promise, resistance to HDACi is often observed and furthermore, HDACi have shown limited success in treating solid tumors. The combination of HDACi with standard chemotherapeutic drugs has demonstrated promising anticancer effects in both preclinical and clinical studies. In this review, we summarize the research thus far on HDACi in combination therapy, with other anticancer agents and their translation into preclinical and clinical studies. We additionally highlight the side effects associated with HDACi in cancer therapy and discuss potential biomarkers to either select or predict a patient's response to these agents, in order to limit the off-target toxicity associated with HDACi.

Proteomic identification of the lactate dehydrogenase A in a radioresistant prostate cancer xenograft mouse model for improving radiotherapy

Radioresistance is a major challenge for prostate cancer (CaP) metastasis and recurrence after radiotherapy. This study aimed to identify potential protein markers and signaling pathways associated with radioresistance using a PC-3 radioresistant (RR) subcutaneous xenograft mouse model and verify the radiosensitization effect from a selected potential candidate. PC-3RR and PC-3 xenograft tumors were established and differential protein expression profiles from two groups of xenografts were analyzed using liquid chromatography tandem-mass spectrometry. One selected glycolysis marker, lactate dehydrogenase A (LDHA) was validated, and further investigated for its role in CaP radioresistance. We found that 378 proteins and 51 pathways were significantly differentially expressed between PC-3RR and PC-3 xenograft tumors, and that the glycolysis pathway is closely linked with CaP radioresistance. In addition, we also demonstrated that knock down of LDHA with siRNA or inhibition of LDHA activity with a LDHA specific inhibitor (FX-11), could sensitize PC-3RR cells to radiotherapy with reduced epithelial-mesenchymal transition, hypoxia, DNA repair ability and autophagy, as well as increased DNA double strand breaks and apoptosis. In summary, we identified a list of potential RR protein markers and important signaling pathways from a PC-3RR xenograft mouse model, and demonstrate that targeting LDHA combined with radiotherapy could increase radiosensitivity in RR CaP cells, suggesting that LDHA is an ideal therapeutic target to develop combination therapy for overcoming CaP radioresistance.

Targeting histone deacetylase 4/ubiquitin-conjugating enzyme 9 impairs DNA repair for radiosensitization of hepatocellular carcinoma cells in mice

Several strategies to improve the efficacy of radiation therapy against hepatocellular carcinoma (HCC) have been investigated. One approach is to develop radiosensitizing compounds. Because histone deacetylase 4 (HDAC4) is highly expressed in liver cancer and known to regulate oncogenesis through chromatin structure remodeling and controlling protein access to DNA, we postulated that HDAC4 inhibition might enhance radiation's effect on HCC cells. HCC cell lines (Huh7 and PLC5) and an ectopic xenograft were pretreated with HDAC inhibitor or short hairpin RNA to knock down expression of HDAC4 and then irradiated (2.5-10.0 Gy). We evaluated cell survival by a clonogenic assay; apoptosis by Annexin V immunofluorescence; γH2AX, Rad51, and HDAC4 by immunofluorescence staining; HDAC4, Rad51, and ubiquitin-conjugating enzyme 9 (Ubc9) in HCC cell nuclei by cell fractionation and confocal microscopy; physical interaction between HDAC4/Rad51/Ubc9 by immunoprecipitation; and the downstream targets of HDAC4 knockdown by immunoblotting. Both HDAC4 knockdown and HDAC inhibitor enhanced radiation-induced cell death and reduced homologous recombination repair of DNA double-strand breaks and protein kinase B activation, leading to increased apoptosis. HDAC4 knockdown with or without an HDAC inhibitor significantly delayed tumor growth in a radiation-treated xenograft model. Radiation stimulated nuclear translocation of Rad51 in an HDAC4-dependent manner and the binding of Ubc9 directly to HDAC4, which led to Ubc9 acetylation. Moreover, these effects were accompanied by HDAC4/Ubc9/Rad51 complex dissociation through inhibiting nuclear translocation. Conclusion: HDAC4 signaling blockade enhances radiation-induced lethality in HCC cells and xenografts. These findings raise the possibility that HDAC4/Ubc9/Rad51 complex in DNA repair may be a target for radiosensitization of HCC. (Hepatology 2018;67:586-599).

Synergistic antitumor interaction between valproic acid, capecitabine and radiotherapy in colorectal cancer: critical role of p53

Background

Recurrence with distant metastases has become the predominant pattern of failure in locally advanced rectal cancer (LARC), thus the integration of new antineoplastic agents into preoperative fluoropyrimidine-based chemo-radiotherapy represents a clinical challenge to implement an intensified therapeutic strategy.

The present study examined the combination of the histone deacetylase inhibitor (HDACi) valproic acid (VPA) with fluoropyrimidine-based chemo-radiotherapy on colorectal cancer (CRC) cells.

Methods

HCT-116 (p53-wild type), HCT-116 p53^−/− (p53-null), SW620 and HT29 (p53-mutant) CRC cell lines were used to assess the antitumor interaction between VPA and capecitabine metabolite 5′-deoxy-5-fluorouridine (5′-DFUR) in combination with radiotherapy and to evaluate the role of p53 in the combination treatment. Effects on proliferation, clonogenicity and apoptosis were evaluated, along with γH2AX foci formation as an indicator for DNA damage.

Results

Combined treatment with equipotent doses of VPA and 5′-DFUR resulted in synergistic effects in CRC lines expressing p53 (wild-type or mutant). In HCT-116 p53^−/− cells we observed antagonist effects. Radiotherapy further potentiated the antiproliferative, pro-apoptotic and DNA damage effects induced by 5′-DFUR/VPA combination in p53 expressing cells.

Conclusions

These results highlighted the role of VPA as valuable candidate to be added to preoperative chemo-radiotherapy in LARC. On these bases we launched the ongoing phase I/II study of VPA and short-course radiotherapy plus capecitabine as preoperative treatment in low-moderate risk rectal cancer (V-shoRT-R3).

Electronic supplementary material

The online version of this article (10.1186/s13046-017-0647-5) contains supplementary material, which is available to authorized users.

Concomitant Use of Panobinostat and Reirradiation in Progressive DIPG: Report of 2 Cases

Diffuse intrinsic pontine glioma (DIPG) remains a devastating disease. Panobinostat has been shown to have therapeutic efficacy both in vitro and in DIPG orthotopic xenograft models; however, clinical data in patients with DIPG are lacking. We present 2 cases of DIPG, who were treated with panobinostat at 22 to 25 mg/m/dose, 3 times weekly for 2 weeks in 3-week cycles and concomitant reirradiation after disease progression. Two episodes of asymptomatic thrombocytopenia were observed in 1 patient. Hyperacetylation of histone H4 of peripheral blood mononuclear cells was evident following treatment. In our experience, panobinostat administered with reirradiation was well tolerated at a relatively higher dose than that used in adult studies.

Combination treatment with naftopidil increases the efficacy of radiotherapy in PC-3 human prostate cancer cells

PURPOSE

Clinically, radiotherapy (RT) often leads to the development of prostate cancer (PCa) resistance because of protective responses in cancer cells. One of the mechanisms includes the upregulation of RT-induced antioxidant enzymes. Thus, combination therapy with RT and certain pharmaceutical drugs targeting antioxidant enzymes may be ideal for increasing the efficacy of RT with minimum side effects. Naftopidil is a subtype-selective α_1D-adrenoceptor antagonist used for the treatment of benign prostatic hyperplasia (BPH). In our drug repositioning study, naftopidil showed not only unique growth-inhibitory effects but also AKT phosphorylation-inhibitory effects in PC-3 human PCa cells. Here, we examined the efficacy of additive naftopidil treatment in combination with RT in PC-3 cells.

METHODS

The effects of combination therapy with RT plus naftopidil were analyzed using an animal model of PC-3 xenografts in BALB/c nude mice. The expression of the antioxidant enzyme manganese superoxide dismutase (MnSOD) was evaluated by western blotting.

RESULTS

Combination therapy with RT plus naftopidil induced a more efficacious delay in PC-3 xenograft tumor growth as compared with monotherapy with naftopidil or RT. In PC-3 tumors, combination therapy with RT plus naftopidil suppressed the upregulation of RT-induced MnSOD expression. In vitro, neither AKT inhibitor IV nor naftopidil directly altered MnSOD expression. Upregulation of RT-induced MnSOD expression was markedly suppressed by combination treatment with RT plus AKT inhibitor IV or naftopidil.

CONCLUSIONS

These results suggest that additive naftopidil treatment in combination with RT may increase the efficacy of RT for the treatment of PCa.

Epigenetic modulators as therapeutic targets in prostate cancer

Prostate cancer is one of the most common non-cutaneous malignancies among men worldwide. Epigenetic aberrations, including changes in DNA methylation patterns and/or histone modifications, are key drivers of prostate carcinogenesis. These epigenetic defects might be due to deregulated function and/or expression of the epigenetic machinery, affecting the expression of several important genes. Remarkably, epigenetic modifications are reversible and numerous compounds that target the epigenetic enzymes and regulatory proteins were reported to be effective in cancer growth control. In fact, some of these drugs are already being tested in clinical trials. This review discusses the most important epigenetic alterations in prostate cancer, highlighting the role of epigenetic modulating compounds in pre-clinical and clinical trials as potential therapeutic agents for prostate cancer management.

Histone deacetylase inhibitors stimulate the susceptibility of A549 cells to a plasma-activated medium treatment

The number of potential applications of non-thermal atmospheric pressure plasma (NTAPP) discharges in medicine, particularly in cancer therapy, has increased in recent years. NTAPP has been shown to affect cells not only by direct irradiation, but also by an indirect treatment with previously prepared plasma-activated medium (PAM). Histone deacetylase (HDAC) inhibitors have the potential to enhance susceptibility to anticancer drugs and radiation. The aim of the present study was to demonstrate the advantage of the combined application of PAM and HDAC inhibitors on A549 cancer cell survival and elucidate the underlying mechanisms. Cell death with DNA breaks in the nucleus was greater using combined regimens of PAM and HDAC inhibitors such as trichostatin A (TSA) and valproic acid (VPA) than a single PAM treatment and was accompanied by the activation of poly (ADP-ribose) polymerase-1 (PARP-1), depletion of ATP, and elevations in intracellular calcium levels. Moreover, the expression of Rad 51, a DNA repair factor in homologous recombination pathways, was significantly suppressed by the treatment with HDAC inhibitors. These results demonstrate that HDAC inhibitors may synergistically induce the sensitivity of cancer cells to PAM components.

The histone deacetylase inhibitor PCI-24781 as a putative radiosensitizer in pediatric glioblastoma cell lines

BACKGROUND

Glioblastoma (GBM) is considered to be one of the most aggressive tumors of the central nervous system (CNS). Even with the use of modern treatment protocols, the prognosis remains reserved, with children with GBM having a mean survival of 12-15 months.

METHODS

In the present study we investigated the potential radiosensitizing effect of PCI-24781, a potent pan-histone deacetylase inhibitor (HDACi), on the SF188 and KNS42 cell lines of pediatric GBM. Cell proliferation rates, clonogenicity and apoptosis were compared in the presence and absence of treatment with PCI-24781. We also compared the clonogenicity rates of the irradiated SF188 and KNS42 cell lines with or without previous treatment with PCI-24781 at the doses of 0.25-16 μM. In addition, we investigated the effects of PCI-24781 on the expression of some of the main proteins responsible for the repair of double-strand DNA breaks caused by irradiation.

RESULTS

The inhibitor blocked cell proliferation, induced death by apoptosis and reduced the colony forming capacity of the cell lines, both of them showing a significant decrease of colony formation at all irradiation doses. The expression of the Rad51 protein, important for the homologous recombination (HR) repair pathway, and of the DNA-PKcs, Ku70 and Ku86 proteins, important for the non-homologous end joining (NHEJ) repair pathway, was more reduced when the irradiated cell line was previously treated with PCI-24781 than when it was treated exclusively with radiotherapy.

CONCLUSIONS

These findings demonstrate that HDACi PCI-24781 has a radiosensitizing profile that compromises the repair of double-strand DNA breaks in cells of pediatric GBM treated with radiotherapy.

Cellular Pathways in Response to Ionizing Radiation and Their Targetability for Tumor Radiosensitization

During the last few decades, improvements in the planning and application of radiotherapy in combination with surgery and chemotherapy resulted in increased survival rates of tumor patients. However, the success of radiotherapy is impaired by two reasons: firstly, the radioresistance of tumor cells and, secondly, the radiation-induced damage of normal tissue cells located in the field of ionizing radiation. These limitations demand the development of drugs for either radiosensitization of tumor cells or radioprotection of normal tissue cells. In order to identify potential targets, a detailed understanding of the cellular pathways involved in radiation response is an absolute requirement. This review describes the most important pathways of radioresponse and several key target proteins for radiosensitization.

Synergistic suppressive effect of PARP-1 inhibitor PJ34 and HDAC inhibitor SAHA on proliferation of liver cancer cells

Poly (ADP-ribose) polymerase-1 (PARP-1) inhibitors and histone deacetylase (HDAC) inhibitors have recently emerged as promising anticancer drugs. The aim of this study was to investigate the effect of combination treatment with the PARP inhibitor PJ34 and HDAC inhibitor SAHA on the proliferation of liver cancer cells. Cell proliferation and apoptosis were assessed in three human liver cancer cell lines (HepG2, Hep3B and HCC-LM3) treated with PJ34 (8 μmol/L) and SAHA (1 μmol/L), alone or combined, by Cell Counting Kit-8 assay and flow cytometry, respectively. The nude mice bearing subcutaneous HepG2 tumors were administered different groups of drugs (10 mg/kg PJ34, 25 mg/kg SAHA, 10 mg/kg PJ34+25 mg/kg SAHA), and the inhibition rates of tumor growth were compared between groups. The results showed that combined use of PJ34 and SAHA could synergistically inhibit the proliferation of liver cancer cell lines HepG2, Hep3B and HCC-LM3. The apoptosis rate of HepG2 cells treated with PJ34+SAHA was significantly higher than that of HepG2 cells treated with PJ34 or SAHA alone (P<0.05). In vivo, the tumor inhibition rates were 53.5%, 61.4% and 82.6% in PJ34, SAHA and PJ34+SAHA groups, respectively. The combined use of PJ34 and SAHA could significantly inhibit the xenograft tumor growth when compared with use of PJ34 or SAHA alone (P<0.05). It was led to conclude that PJ34 and SAHA can synergistically suppress the proliferation of liver cancer cells.

Molecular Mechanism of the Cell Death Induced by the Histone Deacetylase Pan Inhibitor LBH589 (Panobinostat) in Wilms Tumor Cells

Background

Wilms tumor (WT) is an embryonic kidney cancer, for which histone acetylation might be a therapeutic target. LBH589, a novel targeted agent, suppresses histone deacetylases in many tumors. This study investigated the antitumor activity of LBH589 in SK-NEP-1 and G401 cells.

Methods

SK-NEP-1 and G401 cell growth was assessed by CCK-8 and in nude mice experiments. Annexin V/propidium iodide staining followed by flow cytometry detected apoptosis in cell culture. Gene expressions of LBH589-treated tumor cells were analyzed using an Arraystar Human LncRNA Array. The Multi Experiment View cluster software analyzed the expression data. Differentially expressed genes from the cluster analyses were imported into the Ingenuity Pathway Analysis tool.

Results

LBH589 inhibited cell proliferation of SK-NEP-1 and G401 cells in a dose-dependent manner. Annexin V, TUNEL and Hochest 33342 staining analysis showed that LBH589-treated cells showed more apoptotic features compared with the control. LBH589 treatment inhibited the growth of SK-NEP-1 xenograft tumors in nude mice. Arraystar Human LncRNA Array analysis of genes and lncRNAs regulated by LBH589 identified 6653 mRNAs and 8135 lncRNAs in LBH589-treated SK-NEP-1 cells. The most enriched gene ontology terms were those involved in nucleosome assembly. KEGG pathway analysis identified cell cycle proteins, including CCNA2, CCNB2, CCND1, CCND2, CDK4, CDKN1B and HDAC2, etc. Ingenuity Pathway Analysis identified important upstream molecules: HIST2H3C, HIST1H4A, HIST1A, HIST1C, HIST1D, histone H1, histone H3, RPRM, HSP70 and MYC.

Conclusions

LBH589 treatment caused apoptosis and inhibition of cell proliferation of SK-NEP-1and G401 cells. LBH589 had a significant effect and few side effects on SK-NEP-1 xenograft tumors. Expression profiling, and GO, KEGG and IPA analyses identified new targets and a new “network” of genes responding to LBH589 treatment in SK-NEP-1 cells. RPRM, HSP70 and MYC may be important regulators during LBH589 treatment. Our results provide new clues to the proapoptotic mechanism of LBH589.

Adapting a drug screening platform to discover associations of molecular targeted radiosensitizers with genomic biomarkers

Large collections of annotated cancer cell lines are powerful tools for precisely matching targeted drugs with genomic alterations that can be tested as biomarkers in the clinic. Whether these screening platforms, which utilize short-term cell survival to assess drug responses, can be applied to precision radiation medicine is not established. To this end, 32 cancer cell lines were screened using 18 targeted therapeutic agents with known or putative radiosensitizing properties (227 combinations). The cell number remaining after drug exposure with or without radiation was assessed by nonclonogenic assays. We derived short-term radiosensitization factors (SRF2Gy) and calculated clonogenic survival assay-based dose enhancement factors (DEFSF0.1). Radiosensitization was characterized by SRF2Gy values of mostly ∼1.05 to 1.2 and significantly correlated with drug-induced changes in apoptosis and senescence frequencies. SRF2Gy was significantly correlated with DEFSF0.1, with a respective sensitivity and specificity of 91.7% and 81.5% for a 3-day endpoint, and 82.8% and 84.2% for a robotic 5-day assay. KRAS mutations (codons 12/13) were found to be a biomarker of radiosensitization by midostaurin in lung cancer, which was pronounced under conditions that enriched for stem cell-like cells. In conclusion, although short-term proliferation/survival assays cannot replace the gold-standard clonogenic survival assay for measuring cellular radiosensitivity, they capture with high accuracy the relative change in radiosensitivity that is caused by a radiosensitzing targeted agent.

IMPLICATIONS

This study supports a paradigm shift regarding the utility of short-term assays for precision radiation medicine, which should facilitate the identification of genomic biomarkers to guide the testing of novel drug/radiation combinations.

Histone deacetylase inhibitor sodium butyrate suppresses DNA double strand break repair induced by etoposide more effectively in MCF-7 cells than in HEK293 cells

Background

Histone deacetylase inhibitors (HDACi’s) are emerging as promising anticancer drugs alone or in combination with chemotherapy or radiotherapy agents. Previous research suggests that HDACi’s have a high degree of selectivity for killing cancer cells, but little is known regarding the impact of different cellular contexts on HDACi treatment. It is likely that the molecular mechanisms of HDACi’s involve processes that depend on the chromatin template, such as DNA damage and repair. We sought to establish the connection between the HDACi sodium butyrate and DNA double-strand break (DSB) damage in human breast cancer MCF-7 and non-cancerous human embryonic kidney293 (HEK293) cells.

Results

Sodium butyrate inhibited the proliferation of both HEK293 and MCF-7 cells in a dose- and time- dependent manner, but the effects on MCF-7 cells were more obvious. This differential effect on cell growth was not explained by differences in cell cycle arrest, as sodium butyrate caused an arrest in G₁/G₂ phase and a decrease in S phase for both cell lines. At high doses of sodium butyrate or in combination with etoposide, MCF-7 cells formed fewer colonies than HEK293 cells. Furthermore, sodium butyrate enhanced the formation of etoposide-induced γ-H2AX foci to a greater extent in MCF-7 than in HEK293 cells. The two cells also displayed differential patterns in the nuclear expression of DNA DSB repair proteins, which could, in part, explain the cytotoxic effects of sodium butyrate.

Conclusions

These studies suggest that sodium butyrate treatment leads to a different degree of chromatin relaxation in HEK293 and cancerous MCF-7 cells, which results in differential sensitivity to the toxic effects of etoposide in controlling damaged DNA repair.

PI3K/Akt/mTOR pathway inhibitors enhance radiosensitivity in radioresistant prostate cancer cells through inducing apoptosis, reducing autophagy, suppressing NHEJ and HR repair pathways

The PI3K/Akt/mTOR pathway has a central role in cancer metastasis and radiotherapy. To develop effective therapeutics to improve radiosensitivity, understanding the possible pathways of radioresistance involved and the effects of a combination of the PI3K/Akt/mTOR inhibitors with radiotherapy on prostate cancer (CaP) radioresistant cells is needed. We found that compared with parent CaP cells, CaP-radioresistant cells demonstrated G0/G1 and S phase arrest, activation of cell cycle check point, autophagy and DNA repair pathway proteins, and inactivation of apoptotic proteins. We also demonstrated that compared with combination of single PI3K or mTOR inhibitors (BKM120 or Rapamycin) and radiation, low-dose of dual PI3K/mTOR inhibitors (BEZ235 or PI103) combined with radiation greatly improved treatment efficacy by repressing colony formation, inducing more apoptosis, leading to the arrest of the G2/M phase, increased double-strand break levels and less inactivation of cell cycle check point, autophagy and non-homologous end joining (NHEJ)/homologous recombination (HR) repair pathway proteins in CaP-radioresistant cells. This study describes the possible pathways associated with CaP radioresistance and demonstrates the putative mechanisms of the radiosensitization effect in CaP-resistant cells in the combination treatment. The findings from this study suggest that the combination of dual PI3K/Akt/mTOR inhibitors (BEZ235 or PI103) with radiotherapy is a promising modality for the treatment of CaP to overcome radioresistance.