Role of cell cycle in mediating sensitivity to radiotherapy

p53-dependent radiobiological responses to internalised indium-111 in human cells

INTRODUCTION

The p53 tumour suppressor protein plays a pivotal role in the response of mammalian cells to DNA damage. It regulates cell cycle progression, apoptosis and DNA repair mechanisms and is therefore likely to influence response to targeted radionuclide therapy. This study investigated the role of p53 in the cellular response to the Auger-emitting radionuclide indium-111.

METHODS

Two stable clones of a HT1080 fibrosarcoma cell line, differing only in p53 status due to RNAi-mediated knockdown of p53 expression, were incubated for 1 h with [¹¹¹In]-oxinate (0-10 MBq/ml). Radiopharmaceutical uptake into HT1080 cells was measured in situ using a non-contact phosphorimager method. Cellular sensitivity and DNA damage were measured by, respectively, clonogenic survival analysis and the single cell gel electrophoresis (Comet) assay.

RESULTS

Mean uptake of [¹¹¹In]-oxinate in HT1080 cells was unaffected by p53 status, reaching a maximum of 9Bq/cell. [¹¹¹In]-oxinate-induced cytotoxicity was also identical in both clones, as measured by IC50 (0.68 MBq/ml). However the formation of DNA damage, measured immediately after treatment with [¹¹¹In]-oxinate, was found to be up to 2.5-fold higher in the p53-deficient HT1080 clone.

CONCLUSIONS

The increased DNA damage induced in p53-deficient HT1080 cells suggests an early deficiency in the repair of DNA damage during the treatment period. However, the similarity in cellular sensitivity, irrespective of p53 status, suggests that reduced p53 leads to a concomitant reduction in p53-dependent cytotoxicity despite the persistence of DNA damage. The results may provide insight into how tumours that differ in p53 status respond to therapeutic radionuclides.

c-Met inhibitor SU11274 enhances the response of the prostate cancer cell line DU145 to ionizing radiation

Hormone-refractory prostate cancer shows substantial resistance to most conventional therapies including radiotherapy, constitutes a key impediment to curing patients with the disease. c-Met overexpression plays a key role in prostate cancer tumorigenesis and disease progression. Here, we demonstrate that c-Met inhibition by SU11274 could significantly suppress cell survival and proliferation as well as enhance the radiosensitivity of DU145 cells. The underlying mechanisms of the effects of SU11274 on DU145 cells may include the inhibition of c-Met signaling, depolarization of the mitochondrial membrane potential, impairment of DNA repair function, abrogation of cell cycle arrest, and enhancement of cell death. Our study is the first to show the effectiveness of combining c-Met inhibition with ionizing radiation to cure hormone-refractory prostate cancer.

Curcumin improves the antitumor effect of X-ray irradiation by blocking the NF-κB pathway: an in-vitro study of lymphoma

Curcumin, a phenolic compound from the rhizomes of Curcuma longa, inhibits the growth of a variety of malignant cell types including lymphoma cells. We investigated the role of curcumin in modulating the response of Burkitt's lymphoma cells to ionizing radiation (IR) in vitro and explored the mechanisms that mediated this effect. We treated three Burkitt's lymphoma cell lines with vehicle, curcumin, IR, and curcumin in combination with IR. Cell viability, apoptosis, and cell cycle distribution were determined to ascertain the radiosensitization effect of curcumin. Nuclear factor-kappa B (NF-κB) activation was assessed by nuclear translocation of p65. Apoptosis-related proteins were monitored by western blot assay and real-time RT-PCR. Pretreatment of curcumin sensitized lymphoma cells to IR-induced apoptosis and increased G2/M phase arrest in the cell cycle distribution. Accordingly, the antiapoptotic Bcl-xL protein, cell cycle modulating protein CDC2, and cyclin B1 were downregulated by the curcumin treatment. IR activated NF-κB as evidenced by an increased nuclear p65 translocation and cytoplasmic IκBα expression. However, pretreatment with curcumin significantly decreased the nuclear translocation of p65 and cytoplasmic IκBα degradation. Survivin and hexokinase II, downstream effectors of NF-κB that mediate the antiapoptotic effect of NF-κB, were suppressed by the pretreatment of curcumin. These observations suggest that the activated NF-κB pathway plays a prosurvival role in Burkitt's lymphoma in response to IR. Curcumin blocks this pathway and has therapeutic potential for improving the antitumor effects of radiotherapy.

Bcl-xL protein: predictor of complete tumor response in patients with oral cancer treated with curative radiotherapy

BACKGROUND

We earlier observed altered expression of p53 and Bcl-xL in oral cancer cell lines/tissues and wanted to evaluate these proteins for prediction of radiotherapy response and outcome.

METHODS

Thirty-nine paraffin-embedded, pretreatment oral cancer biopsies were analyzed for protein expression using immunohistochemistry and correlated with tumor response to radiotherapy and disease-free survival (DFS).

RESULT

High p53 (p = .040) was observed in female versus male patients. Increased p53 intensity (p = .063) was observed in tobacco habitués (chewers ± smokers) versus patients with no habits. In univariate analysis, nodal positivity (p = .044) and favorable/complete tumor response (p = .002) exhibited a significant correlation with DFS, whereas tumor response emerged as an independent predictor of DFS in multivariate analysis. Significantly high Bcl-xL (p = .048) was observed in the unfavorable versus favorable responders.

CONCLUSION

Our study suggests that Bcl-xL expression along with clinical parameters may be useful for identifying patients with oral cancer likely to draw maximum benefit from curative radiotherapy.

Curcumin-cyclodextrin complexes potentiate gemcitabine effects in an orthotopic mouse model of lung cancer

Background:

Overall clinical outcome for advanced lung cancer remains very disappointing despite recent advances in treatment. Curcumin has been reported as potentially active against cancer.

Methods:

Owing to poor curcumin solubility, we have used cyclodextrins (CD) as an excipient allowing a considerable increase of aqueous solubility and bioavailability of curcumin. The effects of solubilised curcumin have been evaluated in cell cultures as well as in an in vivo orthotopic lung tumour mouse model.

Results:

Cell proliferation was reduced while apoptosis rates were increased when lung epithelial tumour cells were cultured in the presence of curcumin–CD complexes. For in vivo experiments, cells were grafted into lungs of C57Bl/6 mice treated by an oral administration of a non-soluble form of curcumin, CDs alone or curcumin–CD complexes, combined or not with gemcitabine. The size of orthotopically implanted lung tumours was reduced upon curcumin complex administration as compared with treatments with placebo or non-solubilised curcumin. Moreover, curcumin potentiated the gemcitabine-mediated antitumour effects.

Conclusion:

Our data demonstrate that curcumin, when given orally in a CD-solubilised form, reduces lung tumour size in vivo. In vitro experiments show impaired tumour cell proliferation and increased cell apoptosis. Moreover, our data underline a potential additive effect of curcumin with gemcitabine thus providing an efficient therapeutic option for antilung cancer therapy.

5-aza-2'-Deoxycytidine enhances the radiosensitivity of breast cancer cells

PURPOSE

To investigate the effect of the DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine (DAC), on radiosensitivity in breast cancer cells.

MATERIALS AND METHODS

Two breast cancer cell lines, MDA-MB-231 and MDA-MB-435, were evaluated. The methylation status and the mRNA expression of three genes (ER, PR, and HIC-1) that were frequently hypermethylated in these cell lines were determined as a function of DAC exposure. 3-(4,5)-dimethylthiahiazo-(-z-y1)-3,5-di-phenytetrazoliumromide (MTT) absorbance and a colony-forming assay were used to estimate cell viability and radiosensitivity. Using cell-cycle analysis, γ-histone H2A (γ-H2AX) formation assays and methylation-specific polymerase chain reaction (MSP) analysis of three genes correlated with radiosensitivity (BRCA1, 14-3-3σ, and E-cadherin), the mechanism of DAC enhancement of radiosensitivity was explored.

RESULTS

DAC induced the demethylation and reactivation of silent genes in both cell lines. The combination of DAC and irradiation induced growth suppression in vitro. DAC, 5 μM, enhanced sensitivity to ionizing radiation. DAC followed by irradiation correlated with G2/M arrest and the retardation of repair of radiation-induced double strand breaks. DAC reversed the methylation status of genes connected with radiosensitization. The different radiosensitization effects observed with different breast cancers cells may correlate with the primary methylation status of radiosensitizing genes.

CONCLUSIONS

Treatment strategies that include DAC present promising options for sensitizing breast cancer cells to irradiation.

Physical basis and biological mechanisms of gold nanoparticle radiosensitization

The unique properties of nanomaterials, in particular gold nanoparticles (GNPs) have applications for a wide range of biomedical applications. GNPs have been proposed as novel radiosensitizing agents due to their strong photoelectric absorption coefficient. Experimental evidence supporting the application of GNPs as radiosensitizing agents has been provided from extensive in vitro investigation and a relatively limited number of in vivo studies. Whilst these studies provide experimental evidence for the use of GNPs in combination with ionising radiation, there is an apparent disparity between the observed experimental findings and the level of radiosensitization predicted by mass energy absorption and GNP concentration. This review summarises experimental findings and attempts to highlight potential underlying biological mechanisms of response in GNP radiosensitization.

Radiosensitization effects of berberine on human breast cancer cells

Berberine, an isoquinoline derivative alkaloid, has recently been shown to have antitumor activity. The present study aimed to investigate the effects of the concomitant administration of berberine and radiation on breast cancer. The effects of berberine on the radiosensitivity of MCF-7 and MDA-MB-468 cells were evaluated by using cell clonogenic assays. Cells pre-treated with berberine or dimethyl sulfoxide (DMSO) for 24 h were irradiated using a Faxitron Cabinet X-ray System to deliver the indicated doses (0, 1, 2, 3 and 4 Gy). Changes in cell cycle distribution were determined by flow cytometry. γ-H2AX foci were detected by immunofluorescence staining. The levels of Ku70, Ku86 and RAD51 proteins were evaluated by western blot analysis. We observed that berberine increased the MCF-7 and MDA-MB-468 cell radiosensitivity with cell clonogenic assays. the radiation-induced G2/M cell cycle delay was reduced in the MCF-7 cells pre-teated with berberine. Berberine pre-treatment prolonged the persistence of DNA double-strand breaks in the MCF-7 cell line. In comparison with the control cells, the protein levels of RAD51 were decreased in the MCF-7 and MDA-MB-468 cells treated with berberine, and in the cells pre-treated with 15 µM berberine for 24 h, the level of RAD51 protein decreased significantly at the indicated time-points (0, 2, 6 and 24 h) following X-ray exposure. In conclusion, berberine sensitizes human breast cancer cells to ionizing radiation by inducing cell cycle arrest and the downregulation of the homologous recombination repair protein, RAD51. Berberine may be a promising radiosensitizer for the treatment of breast cancer.

Homologous recombination mediates S-phase-dependent radioresistance in cells deficient in DNA polymerase eta

DNA polymerase eta (pol η) is the only DNA polymerase causally linked to carcinogenesis in humans. Inherited deficiency of pol η in the variant form of xeroderma pigmentosum (XPV) predisposes to UV-light-induced skin cancer. Pol η-deficient cells demonstrate increased sensitivity to cisplatin and oxaliplatin chemotherapy. We have found that XP30R0 fibroblasts derived from a patient with XPV are more resistant to cell kill by ionising radiation (IR) than the same cells complemented with wild-type pol η. This phenomenon has been confirmed in Burkitt's lymphoma cells, which either expressed wild-type pol η or harboured a pol η deletion. Pol η deficiency was associated with accumulation of cells in S-phase, which persisted after IR. Cells deficient in pol η demonstrated increased homologous recombination (HR)-directed repair of double strand breaks created by IR. Depletion of the HR protein, X-ray repair cross-complementing protein 3 (XRCC3), abrogated the radioresistance observed in pol η-deficient cells as compared with pol η-complemented cells. These findings suggest that HR mediates S-phase-dependent radioresistance associated with pol η deficiency. We propose that pol η protein levels in tumours may potentially be used to identify patients who require treatment with chemo-radiotherapy rather than radiotherapy alone for adequate tumour control.

Ionizing radiation induces mitochondrial reactive oxygen species production accompanied by upregulation of mitochondrial electron transport chain function and mitochondrial content under control of the cell cycle checkpoint

Whereas ionizing radiation (Ir) instantaneously causes the formation of water radiolysis products that contain some reactive oxygen species (ROS), ROS are also suggested to be released from biological sources in irradiated cells. It is now becoming clear that these ROS generated secondarily after Ir have a variety of biological roles. Although mitochondria are assumed to be responsible for this Ir-induced ROS production, it remains to be elucidated how Ir triggers it. Therefore, we conducted this study to decipher the mechanism of Ir-induced mitochondrial ROS production. In human lung carcinoma A549 cells, Ir (10 Gy of X-rays) induced a time-dependent increase in the mitochondrial ROS level. Ir also increased mitochondrial membrane potential, mitochondrial respiration, and mitochondrial ATP production, suggesting upregulation of the mitochondrial electron transport chain (ETC) function after Ir. Although we found that Ir slightly enhanced mitochondrial ETC complex II activity, the complex II inhibitor 3-nitropropionic acid failed to reduce Ir-induced mitochondrial ROS production. Meanwhile, we observed that the mitochondrial mass and mitochondrial DNA level were upregulated after Ir, indicating that Ir increased the mitochondrial content of the cell. Because irradiated cells are known to undergo cell cycle arrest under control of the checkpoint mechanisms, we examined the relationships between cell cycle and mitochondrial content and cellular oxidative stress level. We found that the cells in the G2/M phase had a higher mitochondrial content and cellular oxidative stress level than cells in the G1 or S phase, regardless of whether the cells were irradiated. We also found that Ir-induced accumulation of the cells in the G2/M phase led to an increase in cells with a high mitochondrial content and cellular oxidative stress level. This suggested that Ir upregulated mitochondrial ETC function and mitochondrial content, resulting in mitochondrial ROS production, and that Ir-induced G2/M arrest contributed to the increase in the mitochondrial ROS level by accumulating cells in the G2/M phase.

Effect of irradiation on cell transcriptome and proteome of rat submandibular salivary glands

Salivary glands (SGs) are irreversibly damaged by irradiation (IR) treatment in head and neck cancer patients. Here, we used an animal irradiation model to investigate and define the molecular mechanisms affecting SGs following IR, focusing on saliva proteome and global transcription profile of submandibular salivary gland (SSG) tissue.We show that saliva secretion was gradually reduced to 50% of its initial level 12 weeks post-IR. Saliva protein composition was further examined by proteomic analysis following mass spectrometry (MS) analysis that revealed proteins with reduced expression originating from SSGs and proteins with increased expression derived from the serum, both indicating salivary tissue damage. To examine alterations in mRNA expression levels microarray analysis was performed. We found significant alterations in 95 genes, including cell-cycle arrest genes, SG functional genes and a DNA repair gene.Tissue damage was seen by confocal immunofluorescence of α-amylase and c-Kit that showed an increase and decrease, respectively, in protein expression. This was coherent with real-time PCR results.This data indicates that IR damages the SSG cells' ability to produce and secrete saliva and proteins, and maintain the physiological barrier between serum and saliva. The damage does not heal due to cell-cycle arrest, which prevents tissue regeneration. Taken together, our results reveal a new insight into IR pathobiology.

A bioinformatics filtering strategy for identifying radiation response biomarker candidates

The number of biomarker candidates is often much larger than the number of clinical patient data points available, which motivates the use of a rational candidate variable filtering methodology. The goal of this paper is to apply such a bioinformatics filtering process to isolate a modest number (<10) of key interacting genes and their associated single nucleotide polymorphisms involved in radiation response, and to ultimately serve as a basis for using clinical datasets to identify new biomarkers. In step 1, we surveyed the literature on genetic and protein correlates to radiation response, in vivo or in vitro, across cellular, animal, and human studies. In step 2, we analyzed two publicly available microarray datasets and identified genes in which mRNA expression changed in response to radiation. Combining results from Step 1 and Step 2, we identified 20 genes that were common to all three sources. As a final step, a curated database of protein interactions was used to generate the most statistically reliable protein interaction network among any subset of the 20 genes resulting from Steps 1 and 2, resulting in identification of a small, tightly interacting network with 7 out of 20 input genes. We further ranked the genes in terms of likely importance, based on their location within the network using a graph-based scoring function. The resulting core interacting network provides an attractive set of genes likely to be important to radiation response.

Microtubule stabilising agents and ionising radiation: multiple exploitable mechanisms for combined treatment

Combined radiochemotherapy treatment modalities are in use for many indications and therefore of high interest. Even though a combined modality in clinical use is often driven by pragmatic aspects, mechanistic preclinical-based concepts of interaction are of importance in order to translate and implement an optimal combination and scheduling of two modalities into the clinics. The use of microtubule stabilising agents is a promising strategy for anti-cancer therapy as a part of combined treatment modality with ionising radiation. Traditionally, microtubule targeting agents are classified as cytotoxic chemotherapeutics and are mostly used in a maximally tolerated dose regimen. Apart from direct cytotoxicity and similar to mechanisms of molecular targeting agents, microtubule stabilising agents interfere with multiple cellular processes, which can be exploited as part of combined treatment modalities. Recent preclinical investigations on the combination of ionising radiation and microtubule stabilising agents reveal new mechanistic interactions on the cellular and tumour level and elucidate the supra-additive tumour response observed particularly in vivo. The major focus on the mechanism of interaction was primarily based on radiosensitisation due to cell cycle arrest in the most radiosensitive G2/M-phase of the cell cycle. However, other mechanisms of interaction such as reoxygenation and direct as well as indirect endothelial damage have also been identified. In this review we summarise and allocate additive and synergistic effects induced by the combined treatment of clinically relevant microtubule stabilising agents and ionising radiation along a described radiobiological framework encompassing distinct mechanisms relevant for exploiting the combination of drugs and ionising radiation.

EGFR inhibition in glioma cells modulates Rho signaling to inhibit cell motility and invasion and cooperates with temozolomide to reduce cell growth

Enforced EGFR activation upon gene amplification and/or mutation is a common hallmark of malignant glioma. Small molecule EGFR tyrosine kinase inhibitors, such as erlotinib (Tarceva), have shown some activity in a subset of glioma patients in recent trials, although the reported data on the cellular basis of glioma cell responsiveness to these compounds have been contradictory. Here we have used a panel of human glioma cell lines, including cells with amplified or mutant EGFR, to further characterize the cellular effects of EGFR inhibition with erlotinib. Dose-response and cellular growth assays indicate that erlotinib reduces cell proliferation in all tested cell lines without inducing cytotoxic effects. Flow cytometric analyses confirm that EGFR inhibition does not induce apoptosis in glioma cells, leading to cell cycle arrest in G₁. Interestingly, erlotinib also prevents spontaneous multicellular tumour spheroid growth in U87MG cells and cooperates with sub-optimal doses of temozolomide (TMZ) to reduce multicellular tumour spheroid growth. This cooperation appears to be schedule-dependent, since pre-treatment with erlotinib protects against TMZ-induced cytotoxicity whereas concomitant treatment results in a cooperative effect. Cell cycle arrest in erlotinib-treated cells is associated with an inhibition of ERK and Akt signaling, resulting in cyclin D1 downregulation, an increase in p27^kip1 levels and pRB hypophosphorylation. Interestingly, EGFR inhibition also perturbs Rho GTPase signaling and cellular morphology, leading to Rho/ROCK-dependent formation of actin stress fibres and the inhibition of glioma cell motility and invasion.

Cucurbitacin B Causes Increased Radiation Sensitivity of Human Breast Cancer Cells via G2/M Cell Cycle Arrest

Purpose. To explore the effects of cucurbitacin B on the radiation survival of human breast cancer cells and to elucidate the cellular mechanism of radiosensitization if any. Materials and Methods. Human breast carcinoma cell lines were treated with cucurbitacin B before irradiation with 0–10 Gy of ¹³⁷Cs gamma rays. The effect of cucurbitacin B on cell-survival following irradiation was evaluated by colony-forming assay. Cell cycle distributions were investigated using flow cytometry. Real-time PCR and western blots were performed to investigate the expression of cell cycle checkpoints. Results. Cucurbitacin B inhibited breast cancer cell proliferation in a dose-dependent manner. Only MDA-MB-231 and MCF7:5C cells but not SKBR-3 cells were radiosensitized by cucurbitacin B. Flow cytometric analysis for DNA content indicated that cucurbitacin B resulted in G2/M arrest in MDA-MB-231 and MCF7:5C but not SKBR-3 cells. Moreover, Real-time PCR and western blot analysis demonstrated upregulated p21 expression before irradiation, a likely cause of the cell cycle arrest. Conclusion. Taken together, these findings suggest that cucurbitacin B causes radiosensitization of some breast cancer cells, and that cucurbitacin B induced G2/M arrest is an important mechanism. Therefore, combinations of cucurbitacin B with radiotherapy may be appropriate for experimental breast cancer treatment.

Pharmacological concentrations of ascorbate radiosensitize glioblastoma multiforme primary cells by increasing oxidative DNA damage and inhibiting G2/M arrest

Glioblastoma multiforme (GBM) has a very poor prognosis because of its chemo- and radiation therapy resistance. Here we investigated the ability of pharmacological concentrations of ascorbate to radiosensitize primary cells isolated from six GBM patients, mouse astrocytoma cells, and mouse astrocytes. We measured cell viability by trypan blue exclusion, generation of double-stranded DNA breaks by H2AX phosphorylation using fluorescently labeled antibodies and FACS analysis, apoptosis by annexin V/propidium iodide staining, inhibition of autophagy by 3-methyladenine, and cell cycle progression by propidium iodide staining of permeabilized cells. We showed that 5 mM ascorbate in combination with 6 Gy radiation killed more GBM primary cells by generating significantly more double-stranded breaks than either treatment alone (p<0.05). Combined treatment affected viability and double-stranded break generation in normal astrocytes to a much smaller extent. Radiation, but not 5 mM ascorbate, caused G2/M arrest in GBM cells and ascorbate prevented radiation-induced G2/M arrest in combined treatment. Cell death in response to 5 mM ascorbate or combination treatment was not mediated by apoptosis or autophagy. In conclusion, pharmacological concentrations of ascorbate radiosensitize GBM primary cells to a much greater extent than astrocytes; this large therapeutic ratio may be of clinical significance in radiation-resistant cancers.

The radiation dose-regulated AND gate genetic circuit, a novel targeted and real-time monitoring strategy for cancer gene therapy

The AND gate functions such that when all inputs are activated the downstream gene will be transcribed and it is off otherwise. To accomplish optimal and targeted gene therapy in solid tumor patients, we have constructed an AND gate genetic circuit and investigated whether it could be activated by low-dose radiation in vitro and in vivo. The enhancement green fluorescent protein (EGFP) expression in different tumor cells transfected with control vector plxsn-EGFP confirmed that 2 Gy of radiation and 1% O(2) for 3 h could activate our AND gate. Besides, the obvious different levels of EGFP expression between 2 and 6 Gy of radiation demonstrated that the AND gate could be regulated by radiation doses. Additionally, through EGFP expression and the codistribution of p53 and HIF-1α in xenografts, we illustrated the targeted activation property of the AND gate and real-time monitoring to hypoxic districts in vivo. Moreover, significant growth inhibition and cell cycle arrest in vitro and apoptosis-inducing effects in vitro and in vivo proved that the AND gate induced ideal antitumor effects. In conclusion, the radiation dose-regulated AND gate genetic circuit could not only effectively monitor the therapeutic process in real-time but also induce ideal antitumor efficacy, and can be further exploited for personal therapy in clinical tumor patients.

Differential response of three cell types to dual stress of nitric oxide and radiation

The perception of toxicity to nitric oxide (NO) and irradiation (IR) by three different cell types has been studied. The three cell types are the macrophage like RAW264.7 cells, EL4 lymphoma cells, and splenocytes, which represent the different components of a tumor. These three cell types respond differently to NO donors (SNP and SNAP) and radiation treatment. The macrophages were found to be most radio-resistant and insensitive to NO donors. The innate resistance of the macrophages was not due to its antioxidant defense system since there was no significant activation of the enzymes (superoxide dismutases, catalase, and glutathione peroxidase) in RAW264.7 cells after NO donor and irradiation. But the cell cycle arrest of the three cell types was different from each other. The EL4 cells were found to arrest in the G2/M phase while the macrophages were found arrested in the G1 phase of the cell cycle. Such specific killing of the tumor cell in response to NO donor while sparing the macrophages can be of immense importance to radiotherapy.

Targeting a cornerstone of radiation resistance: cancer stem cell

In radiation oncology, cancer stem cells (CSCs) have become an important research field. In fact, it appears that most cancer types contain populations of cells that exhibit stem-cell properties. CSCs have the ability to renew indefinitely, which can drive tumor development and metastatic invasion. As those cells are classically resistant to conventional chemotherapy and to radiation therapy, they may contribute to treatment failure and relapse. Over past decades, preclinical research has highlighted that variations in the CSCs content within tumor could affect their radiocurability by interfering with mechanisms of DNA repair, redistribution in the cell cycle, tumor cells repopulation, and hypoxia. It is now possible to isolate particular cells expressing specific surface markers and thus better investigating CSCs pathways. Numerous inhibitory agents targeting these specific signaling pathways, such as Notch and Wnt/B-catenin, are currently evaluated in early clinical trials. By targeting CSCs, tumor radioresistance could be potentially overcome to improve outcome for patients with solid malignancies. Radiation therapy using ion particles (proton and carbon) may be also more effective than classic photon on CSCs. This review presents the major pathophysiological mechanisms involved in CSCs radioresistance and recent developments for targeted strategies.

Oxaliplatin as a radiosensitiser for upper and lower gastrointestinal tract malignancies: what have we learned from a decade of translational research?

Some of the greatest advances in the treatment of solid malignancies have resulted from the combination of chemotherapy and radiotherapy treatments. This article comprehensively reviews the current clinical evidence for oxaliplatin-based chemo-radiotherapy that may improve local control and survival. In order to understand how clinical studies should be designed, the pre-clinical evidence for the use of oxaliplatin chemotherapy as a radiosensitising agent is appraised. Particular focus is placed on oxaliplatin's biological mechanisms of action, including cell cycle effects, the formation of DNA adducts and interstrand cross-links and the role of DNA repair proteins. At a clinical level, there is currently no evidence to suggest that oxaliplatin provides an additional benefit to concurrent chemo-radiation regimes that utilise fluoropyrimidines; we evaluate the reasons for this observation, the limitations of clinical trial design and the opportunities that currently exist to design clinical trials which are underpinned by an understanding of the basic biology.

Suberoylanilide hydroxamic acid affects γH2AX expression in osteosarcoma, atypical teratoid rhabdoid tumor and normal tissue cell lines after irradiation

PURPOSE

Osteosarcoma and atypical teratoid rhabdoid tumors are tumor entities with varying response to common standard therapy protocols. Histone acetylation affects chromatin structure and gene expression which are considered to influence radiation sensitivity. The aim of this study was to investigate the effect of the combination therapy with the histone deacetylase inhibitor suberoylanilide hydroxamic acid (SAHA) and irradiation on atypical teratoid rhabdoid tumors and osteosarcoma compared to normal tissue cell lines.

METHODS

Clonogenic assay was used to determine cell survival. DNA double-strand breaks (DSB) were examined by pulsed-field electrophoresis (PFGE) as well as by γH2AX immunostaining involving flow cytometry, fluorescence microscopy, and immunoblot analysis.

RESULTS

SAHA lead to an increased radiosensitivity in tumor but not in normal tissue cell lines. γH2AX expression as an indicator for DSB was significantly increased when SAHA was applied 24 h before irradiation to the sarcoma cell cultures. In contrast, γH2AX expression in the normal tissue cell lines was significantly reduced when irradiation was combined with SAHA. Analysis of initial DNA fragmentation and fragment rejoining by PFGE, however, did not reveal differences in response to the SAHA pretreatment for either cell type.

CONCLUSION

SAHA increases radiosensitivity in tumor but not normal tissue cell lines. The increased H2AX phosphorylation status of the SAHA-treated tumor cells post irradiation likely reflects its delayed dephosphorylation within the DNA damage signal decay rather than chromatin acetylation-dependent differences in the overall efficacy of DSB induction and rejoining. The results support the hypothesis that combining SAHA with irradiation may provide a promising strategy in the treatment of solid tumors.

Prognostic role of sensitive-to-apoptosis gene expression in rectal cancer

AIM: To investigate the association between prognosis of rectal cancer treated with chemoradiotherapy (CRT) and expression of sensitive-to-apoptosis (SAG), B-cell lymphoma-extra large (Bcl-X_L) and Bcl-2 homologous antagonist/killer (Bak).

METHODS: Real-time quantitative polymerase chain reaction was used to determine the expression of proteins of interest, namely SAG, Bcl-X_L, Bak and β-actin, in rectal carcinoma patients who had a follow-up period of 3 years after CRT. Biopsy specimens were excised from the rectal tumor preceding CRT.

RESULTS: SAG, Bcl-X_L and Bak proteins showed significant correlations with each other. In multivariate analysis, patients with high vs low SAG expression showed a statistically significant difference in 2-year survival rates: 56% vs 73%, respectively (P = 0.056). On the other hand, there were no significant correlations between the expression levels of all three genes and metastatic rates or tumor responses to CRT. Mean overall survival in the patients with elevated SAG expression was 27.1 mo ± 3.9 mo [95% confidence interval (CI): 19.3-34.9], and in patients with reduced expression, it was 32.1 mo ± 2.5 mo (95% CI: 27.3-36.9). The corresponding values for Bcl-X_L were 28.0 mo ± 4.1 mo (95% CI: 19.9-36.1) and 31.7 mo ± 2.9 mo (95% CI: 26.0-37.5), and those for Bak were 29.8 mo ± 3.7 mo (95% CI: 22.5-37.2) and 30.6 mo ± 2.4 mo (95% CI: 25.5-35.0), respectively.

CONCLUSION: Two-year survival rates significantly correlated with low SAG expression, and SAG may be a candidate gene for good prognosis, independent of therapeutic response of different individuals.

Near-infrared exposure changes cellular responses to ionizing radiation

Near infrared (NIR) and X-rays are radiations from different sides of the wavelength spectrum but both are used during medical treatments, as they have severe impacts on cellular processes, including metabolism, gene expression, proliferation and survival. However, both radiations differ strictly in their consequences for exposed patients: NIR effects are generally supposed to be positive, mostly ascribed to a stimulation of metabolism, whereas X-ray leads to genetic instability, an increase of reactive oxygen species (ROS) and DNA damages and finally to cellular death by apoptosis in tumor cells. Since genomic stability after X-irradiation depends on the mitochondrial metabolism, which is well known to be regulated by NIR, we analyzed the impact of NIR on cellular responses of fibroblasts, retinal progenitor cells and keratinocytes to X-radiation. Our data show that previous exposure to naturally occurring doses of nonthermal NIR combined with clinically relevant X-ray doses leads to (1) increased genomic instability, indicated by elevated ratios of mitotic catastrophes, (2) increased ROS, (3) higher amounts of X-irradiated cells entering S-phase and (4) impaired DNA double-strand break repair. Taken together, our data show tremendous effects of NIR on cellular responses to X-rays, probably affecting the results of radiotherapy after NIR exposure during cancer treatment.

CXCL10 enhances radiotherapy effects in HeLa cells through cell cycle redistribution

Radiotherapy is a crucial treatment for cervical cancer, the second most common type of cancer in women worldwide. In this study, we investigated the effects of CXC chemokine ligand 10 (CXCL10) gene therapy combined with radiotherapy on cervical cancer using HeLa cells. TUNEL assay revealed that the apoptotic rate in the combined treatment of CXCL10 gene therapy and radiotherapy was greatly increased compared with that of CXCL10 or radiotherapy alone. Flow cytometry showed that CXCL10 overexpression in HeLa cells resulted in a prolonged G1 phase and shortened S phase at 72 h post-transfection. Western blot analysis revealed that p27(Kip1) was up-regulated in CXCL10-treated HeLa cells; however, cyclin E was down-regulated. These results indicate that the combination of CXCL10 gene therapy and radiotherapy is an effective strategy for the growth suppression of HeLa cells, and that CXCL10 enhances the radiotherapy effects through cell cycle redistribution. Our data provide new insight into the treatment of cervical carcinoma, involving an effective combination of gene therapy and radiotherapy against tumors.

Head and neck inflammatory myofibroblastic tumor (IMT): evaluation of clinicopathologic and prognostic features

Owing to rarity and awareness deficiency towards inflammatory myofibroblastic tumor (IMT), we sought to review on its clinicopathological features; arising awareness to achieve early diagnosis; exploring prognostic factors and then establishing a treatment protocol. Retrospective study was performed on patients with histological proven IMT between January 2003 and December 2010. Their demographic data, clinical and histological presentations were recorded. Overall survival (OS) and progression-free-survival (PFS) were estimated via Kaplan-Meier method. Cox regression model was applied to determine the significant of prognostic factors. Logistic regression model was established to predict the probability of relapse. A total of 28 patients. Five-year PFS was 65%. Surgical margins primarily and independently determined the survival, followed by size, pseudocapsule of the lesion, intra-lesional necrosis and lastly Ki-67 and ALK overexpression. Logistic model in prediction of relapse was established, with the formula as probability of relapse = 1/(1 + e(-z)) where e = exponential function, z = constant value (3.9) + B*margin + B*size + B*immunohistochemical expression + B*pseudocapsule + B*intra-lesional necrosis. Immunohistochemical overexpression was significant if Ki-67 was strongly expressed with a conditioned ALK overexpression simultaneously. Staining intensity must be at least moderate for those ALK nuclear staining was less than 25%. Weak ALK staining intensity is only significant if nuclear staining was more than 25%. Diagnosis of IMT is achieved via exclusion. Radical resection and obtaining negative margins remains the mainstay of treatment. Both high and moderate-risk groups required post-operative radiotherapy. In low-risk group, post-operative radiotherapy was recommended if the lesion is larger than 5 cm in diameter with a conditioned ALK & Ki-67 overexpression.

A mitochondria-targeted inhibitor of cytochrome c peroxidase mitigates radiation-induced death

The risk of radionuclide release in terrorist acts or exposure of healthy tissue during radiotherapy demand potent radioprotectants/radiomitigators. Ionizing radiation induces cell death by initiating the selective peroxidation of cardiolipin in mitochondria by the peroxidase activity of its complex with cytochrome c leading to release of haemoprotein into the cytosol and commitment to the apoptotic program. Here we design and synthesize mitochondria-targeted triphenylphosphonium-conjugated imidazole-substituted oleic and stearic acids that blocked peroxidase activity of cytochrome c/cardiolipin complex by specifically binding to its haem-iron. We show that both compounds inhibit pro-apoptotic oxidative events, suppress cyt c release, prevent cell death, and protect mice against lethal doses of irradiation. Significant radioprotective/radiomitigative effects of imidazole-substituted oleic acid are observed after pretreatment of mice from 1 h before through 24 h after the irradiation.

Combination of suberoylanilide hydroxamic acid with heavy ion therapy shows promising effects in infantile sarcoma cell lines

Introduction

The pan-HDAC inhibitor (HDACI) suberoylanilide hydroxamic acid (SAHA) has previously shown to be a radio-sensitizer to conventional photon radiotherapy (XRT) in pediatric sarcoma cell lines. Here, we investigate its effect on the response of two sarcoma cell lines and a normal tissue cell line to heavy ion irradiation (HIT).

Materials and methods

Clonogenic assays after different doses of heavy ions were performed. DNA damage and repair were evaluated by measuring γH2AX via flow-cytometry. Apoptosis and cell cycle analysis were also measured via flow cytometry. Protein expression of repair proteins, p53 and p21 were measured using immunoblot analysis. Changes of nuclear architecture after treatment with SAHA and HIT were observed in one of the sarcoma cell lines via light microscopy after staining towards chromatin and γH2AX.

Results

Corresponding with previously reported photon data, SAHA lead to an increase of sensitivity to heavy ions along with an increase of DSB and apoptosis in the two sarcoma cell lines. In contrast, in the osteoblast cell line (hFOB 1.19), the combination of SAHA and HIT showed a significant radio-protective effect. Laser scanning microscopy revealed no significant morphologic changes after HIT compared to the combined treatment with SAHA. Immunoblot analysis revealed no significant up or down regulation of p53. However, p21 was significantly increased by SAHA and combination treatment as compared to HIT only in the two sarcoma cell lines - again in contrast to the osteoblast cell line. Changes in the repair kinetics of DSB p53-independent apoptosis with p21 involvement may be part of the underlying mechanisms for radio-sensitization by SAHA.

Conclusion

Our in vitro data suggest an increase of the therapeutic ratio by the combination of SAHA with HIT in infantile sarcoma cell lines.

Soy isoflavones sensitize cancer cells to radiotherapy

Soy isoflavones are dietary compounds isolated from soybeans, which are safe for human use and have mild anti-cancer properties. Soy isoflavones inhibit the activity of transcription factors and genes essential for tumor cell proliferation, invasion, and neovascularization, and it appears that soy isoflavones may enhance the effectiveness of conventional therapies against cancer. Soy isoflavones could be an effective complementary treatment given that they inhibit the survival signaling pathways of various cancer cells through altered activation of APE1/Ref-1, NF-κB, and HIF-1α, which are genes essential for tumor cell survival, tumor growth, and angiogenesis, thus making such cells more sensitive to radiotherapy. Studies in which soy isoflavones were given in conjunction with radiotherapy to prostate cancer patients suggest that soy isoflavones might also mitigate the adverse effects of radiation on normal tissues, probably by acting as antioxidants. These observations open new avenues for exploiting soy isoflavones as supplements to conventional therapies.

Nitric oxide radical-induced radioadaptation and radiosensitization are G2/M phase-dependent

The aim of this study was to examine biological effects of nitric oxide (NO) on radiosensitivity and chromosome aberrations in different phases of the cell cycle in human cancer cells with a wild-type p53 (wtp53) genotype. H1299/wtp53 cells were pre-treated with isosorbide dinitrate (ISDN) at different concentrations or pre-irradiated with a low dose of X-rays, and then exposed to a high dose of X-rays. Cell synchronization was achieved with serum starvation. Cellular radiosensitivity, cell cycle distributions, and chromosome aberrations were assayed with colony-forming assays, flow cytometry and chromosome banding techniques, respectively. After treatment with ISDN at a low concentration or after an exposure to 0.02 Gy of X-rays, radioresistance and a reduction in the number of chromosome aberrations were observed mainly 17.5 h after plating mitotic cells. This radioadaptation effect was observed during a clearly shortened G(2)/M phase and a slightly prolonged S phase. In contrast, in the presence of a high concentration of ISDN, radiosensitization and the enhancement of chromosome aberrations were detected principally 17.5 h after plating mitotic cells, and this radiosensitization was observed during a significantly prolonged G(2)/M phase and a slightly shortened S phase. A range of concentrations of NO induced opposing effects on radiosensitivity and chromosome aberrations in human non-small cell lung cancer cells bearing wtp53 gene status, and these different effects produced by NO depended on the cell cycle phase.

The microtubule stabilizer patupilone (epothilone B) is a potent radiosensitizer in medulloblastoma cells

Concurrent radiochemotherapy for medulloblastoma includes the microtubule disrupting agent vincristine; however, vincristine alone or as part of a combined treatment regimen is highly toxic. A major goal is therefore to replace vincristine with novel potent chemotherapeutic agents-in particular, with microtubule stabilizing and destabilizing compounds-with a larger therapeutic window. Here, we investigated the antiproliferative, cytotoxic and radiosensitizing effect of patupilone (epothilone B [EPO906]), a novel, non-taxane-related and nonneurotoxic microtubule-stabilizing agent in human medulloblastoma cell lines. The antiproliferative and cytotoxic effects of patupilone alone and in combination with ionizing radiation was determined in the 3 representative human medulloblastoma cell lines D341Med, D425Med, and DAOY. Patupilone alone effectively reduced the proliferative activity and clonogenicity of all medulloblastoma cell lines tested at picomolar concentrations (50-200 pM) and resulted in an at least additive anticlonogenic effect in combination with clinically relevant doses of ionizing radiation (2 or 5 Gy). Cell-cycle analysis revealed a sequential G2-M arrest and sub-G1 accumulation in a dose- and treatment-dependent manner after exposure to patupilone. In tumor xenografts derived from D425Med cells, a minimal treatment regimen with patupilone and fractionated irradiation (1 × 2 mg/kg plus 3 × 3 Gy) resulted in an extended tumor growth delay for the 2 single treatment modalities alone and a supra-additive treatment response for the combined treatment modality, with complete tumor regressions. These results demonstrate the potent efficacy of patupilone against medulloblastoma cell lines and indicate that patupilone represents a promising candidate to replace vincristine as part of a combined treatment strategy with ionizing radiation.

Arsenic trioxide and radiation enhance apoptotic effects in HL-60 cells through increased ROS generation and regulation of JNK and p38 MAPK signaling pathways

The induction of apoptotic cell death is a significant mechanism of tumor cells under the influence of radio-/chemotherapy, and resistance to these treatments has been linked to some cancer cell lines with a low propensity for apoptosis. The present study aimed to investigate the enhanced effects and mechanisms in apoptosis and the cycle distribution of HL-60 cells, a human leukemia cell line lacking a functional p53 protein, after combination treatment with arsenic trioxide (ATO) and irradiation (IR). Our results indicated that combined treatment led to increased cytotoxicity and apoptotic cell death in HL-60 cells, which was correlated with the activation of cdc-2 and increased expression of cyclin B, the induction of intracellular reactive oxygen species (ROS) generation, the loss of mitochondria membrane potential, and the activation of caspase-3. The combined treatment of HL-60 cells pre-treated with Z-VAD or NAC resulted in a significant reduction in apoptotic cells. In addition, activation of JNK and p38 MAPK may be involved in combined treatment-mediated apoptosis. The data suggest that a combination of IR and ATO could be a potential therapeutic strategy against p53-deficient leukemia cells.

MicroRNA-221 and -222 regulate radiation sensitivity by targeting the PTEN pathway

PURPOSE

MicroRNAs (miRNAs) are noncoding RNAs inhibiting expression of numerous target genes by posttranscriptional regulation. miRNA-221 and miRNA-222 (miRNA-221/-222) expression is elevated in radioresistant tumor cell lines; however, it is not known whether and how miRNAs control cellular responses to ionizing irradiation.

METHODS AND MATERIALS

We used bioinformatic analyses, luciferase reporter assay, and genetic knockdown and biochemical assays to characterize the regulation pathways of miRNA-221/-222 in response to radiation treatment.

RESULTS

We identified the PTEN gene as a target of miRNA-221/-222. Furthermore, we found that knocking down miRNA-221/-222 by antisense oligonucleotides upregulated PTEN expression. Upregulated PTEN expression suppressed AKT activity and increased radiation-induced apoptosis, resulting in enhancement of radiosensitivity in tumor cells.

CONCLUSIONS

miRNA-221/-222 control radiation sensitivity by regulating the PTEN/AKT pathway and can be explored as novel targets for radiosensitization.

Costunolide causes mitotic arrest and enhances radiosensitivity in human hepatocellular carcinoma cells

PURPOSE

This work aimed to investigate the effect of costunolide, a sesquiterpene lactone isolated from Michelia compressa, on cell cycle distribution and radiosensitivity of human hepatocellular carcinoma (HCC) cells.

METHODS

The assessment used in this study included: cell viability assay, cell cycle analysis by DNA histogram, expression of phosphorylated histone H3 (Ser 10) by flow cytometer, mitotic index by Liu's stain and morphological observation, mitotic spindle alignment by immunofluorescence of alpha-tubulin, expression of cell cycle-related proteins by Western blotting, and radiation survival by clonogenic assay.

RESULTS

Our results show that costunolide reduced the viability of HA22T/VGH cells. It caused a rapid G2/M arrest at 4 hours shown by DNA histogram. The increase in phosphorylated histone H3 (Ser 10)-positive cells and mitotic index indicates costunolide-treated cells are arrested at mitosis, not G2, phase. Immunofluorescence of alpha-tubulin for spindle formation further demonstrated these cells are halted at metaphase. Costunolide up-regulated the expression of phosphorylated Chk2 (Thr 68), phosphorylated Cdc25c (Ser 216), phosphorylated Cdk1 (Tyr 15) and cyclin B1 in HA22T/VGH cells. At optimal condition causing mitotic arrest, costunolide sensitized HA22T/VGH HCC cells to ionizing radiation with sensitizer enhancement ratio up to 1.9.

CONCLUSIONS

Costunolide could reduce the viability and arrest cell cycling at mitosis in hepatoma cells. Logical exploration of this mitosis-arresting activity for cancer therapeutics shows costunolide enhanced the killing effect of radiotherapy against human HCC cells.

Proton MR spectroscopy of neural stem cells: does the proton-NMR peak at 1.28 ppm function as a biomarker for cell type or state?

Recently, a peak at 1.28 ppm in proton magnetic resonance spectroscopy ((1)H-MRS) of neural stem cells (NSCs) was introduced as a noninterventional biomarker for neurogenesis in vivo. This would be an urgently needed requisite for translational studies in humans regarding the beneficial role of adult neurogenesis for the structural and functional integrity of the brain. However, many concerns have risen about the validity of the proposed signal as a specific marker for NSCs. The peak has also been related to cell-type-independent phenomena such as apoptosis or necrosis. Thus, we compared the 1.28-ppm peak in various immature stem cell populations, including embryonic stem cells, mouse embryonic fibroblasts, embryonic stem cell- and induced pluripotent stem cell-derived NSCs, ex vivo isolated embryonic NSCs, as well as mature and tumor cell types from different germ layers. To correlate the integral peak intensity with cell death, we induced both apoptosis with camptothecin and necrosis with sodium azide. A peak at 1.28 ppm was found in most cell types, and in most, but not all, NSCH cultures, demonstrating no specificity for NSCs. The intensities of the 1.28-ppm resonance significantly correlated with the rate of apoptosis, but not with the rate of necrosis, cell cycle phase distribution, cell size, or type. Multiple regression analysis displayed a significant predictive value of the peak intensity for apoptosis only. In this context, its specificity for apoptosis as a major selection process during neurogenesis may suggest this resonance as an indirect marker for neurogenesis in vivo.

Down-regulation of microRNA 106b is involved in p21-mediated cell cycle arrest in response to radiation in prostate cancer cells

BACKGROUND

microRNAs (miRNAs) are endogenous short non-coding RNAs, and play a pivotal role in regulating of a variety of cellular processes, including proliferation and apoptosis, both of which are cellular responses to radiation treatment. The purpose of this study is to identify candidate miRNAs whose levels are altered in response to radiation in prostate cancer cells and to investigate the molecular pathway of such miRNAs in the regulation of radiation-induced cellular response.

METHODS

Using a miRNA microarray assay, we screened 132 cancerous miRNAs in LNCaP cells in response to radiation treatment. The function of one candidate miRNA was investigated for checkpoint protein expression, cell cycle arrest, cell proliferation, and cell survival in cells transfected with precursor or antisense miRNA.

RESULTS

In response to radiation, multiple miRNAs, including mi-106b, showed altered expression. Cells transfected with precursor miR-106b were able to suppress radiation-induced p21 activation. Functionally, exogenous addition of precursor miR-106b overrode the G2/M arrest in response to radiation and resulted in a transient diminishment of radiation-induced growth inhibition.

CONCLUSION

We have shown a novel role of miR-106b, in the setting of radiation treatment, in regulating the p21-activated cell cycle arrest. Our finding that miR-106b is able to override radiation-induced cell cycle arrest and cell growth inhibition points to a potential therapeutic target in certain prostate cancer cells whose radiation resistance is likely due to consistently elevated level of miR-106b.

Activation of p53 by nutlin-3a induces apoptosis and cellular senescence in human glioblastoma multiforme

Glioblastoma multiforme (GBM) is the most common and aggressive primary brain tumor in adults. Despite concerted efforts to improve current therapies and develop novel clinical approaches, patient survival remains poor. As such, increasing attention has focused on developing new therapeutic strategies that specifically target the apoptotic pathway in order to improve treatment responses. Recently, nutlins, small-molecule antagonists of MDM2, have been developed to inhibit p53-MDM2 interaction and activate p53 signaling in cancer cells. Glioma cell lines and primary cultured glioblastoma cells were treated with nutlin-3a. Nutlin-3a induced p53-dependent G1- and G2-M cell cycle arrest and apoptosis in glioma cell lines with normal TP53 status. In addition, nutlin-arrested glioma cells show morphological features of senescence and persistent induction of p21 protein. Furthermore, senescence induced by nutlin-3a might be depending on mTOR pathway activity. In wild-type TP53 primary cultured cells, exposure to nutlin-3a resulted in variable degrees of apoptosis as well as cellular features of senescence. Nutlin-3a-induced apoptosis and senescence were firmly dependent on the presence of functional p53, as revealed by the fact that glioblastoma cells with knockdown p53 with specific siRNA, or cells with mutated or functionally impaired p53 pathway, were completely insensitive to the drug. Finally, we also found that nutlin-3a increased response of glioma cells to radiation therapy. The results provide a basis for the rational use of MDM2 antagonists as a novel treatment option for glioblastoma patients.

Silencing Prx1 and/or Prx5 sensitizes human esophageal cancer cells to ionizing radiation and increases apoptosis via intracellular ROS accumulation

AIM

To investigate whether down-regulation of peroxiredoxin 1 (Prx1) and/or peroxiredoxin 5 (Prx5) sensitizes human esophageal cancer cells to ionizing radiation (IR).

METHODS

Human esophageal carcinoma cell lines Eca-109 and TE-1 were used. Prx mRNA expression profiles in Eca-109 and TE-1 cells were determined using RT-PCR. Two highly expressed isoforms of Prxs, Prx1 and Prx5, were silenced by RNA interference (RNAi). Following IR, intracellular reactive oxygen species (ROS) and apoptosis were measured using flow cytometry, the activities of catalase, superoxide dismutase and glutathione peroxidase were measured, and the radiosensitizing effect of RNAi was observed. Tumor xenograft model was also used to examine the radiosensitizing effect of RNAi in vivo.

RESULTS

Down-regulation of Prx1 and/or Prx5 by RNAi does not alter the activities of catalase, superoxide dismutase and glutathione peroxidase, but made human tumor cells more sensitive to IR-induced apoptosis both in vitro and in vivo. When the two isoforms were decreased simultaneously, intracellular ROS and apoptosis significantly increased after IR.

CONCLUSION

Silencing Prx1 and/or Prx5 by RNAi sensitizes human Eca-109 and TE-1 cells to IR, and the intracellular ROS accumulation may contribute to the radiosensitizing effect of the RNAi.