ESTRO 36 Abstract Book

S542 ESTRO 36 _______________________________________________________________________________________________

rhabdospheres

derived

from

the

embryonal

rhabdomyosarcoma cell lines. Material and Methods

Rhabdospheres enriched in cancer stem like cells were obtained growing ERMS cells in non adherent condition in stem cell medium. Stem cell markers were evaluated by FACS analysis and immunoblotting. ERK1/2, myogenic markers, proteins of DNA repair and bone marrow X-linked kinase (BMX) expression were evaluated by immunoblotting analysis. Radiation was delivered using an x-6 MV photon linear accelerator. Xenografts were obtained in NOD/SCID mice by subcutaneously injection of rhabdosphere cells or cells pretreated with U0126 in stem cell medium. Results MEK/ERK inhibitor U0126 dramatically prevented rhabdosphere formation and down-regulated stem cell markers CD133, CXCR4 and Nanog expression, but enhanced ALDH, MAPK phospho-active p38 and differentiative myogenic markers. By contrast, MAPK p38 inhibition accelerated rhabdosphere formation and enhanced phospho-active ERK1/2 and Nanog expression. ERMS cells, chronically treated with U0126 and then xeno- transplanted in NOD/SCID mice, delayed tumor development and reduced tumor mass when compared with tumor induced by rhabdosphere cells. U0126 intraperitoneal administration to mice bearing rhabdosphere-derived tumors inhibited tumor growth . The MEK/ERK pathway role in rhabdosphere radiosensitivity was investigated in vitro. Disassembly of rhabdospheres was induced by both radiation or U0126, and further enhanced by combined treatment. In U0126- treated rhabdospheres, the expression of the stem cell markers CD133 and CXCR4 decreased and dropped even more markedly following combined treatment. The expression of BMX, a negative regulator of apoptosis, also decreased following combined treatment, which suggests an increase in radiosensitivity of rhabdosphere cells. Conclusion Our results indicate that the MEK/ERK pathway plays a prominent role in maintaining the stem-like phenotype of ERMS cells, their survival and their innate radioresistance. Thus, therapeutic strategies that target cancer stem cells, which are resistant to traditional cancer therapies, may benefit from MEK/ERK inhibition combined with traditional radiotherapy, thereby providing a promising therapy for embryonal rhabdomyosarcoma. PO-0981 Disturbance of redox status enhances radiosensitivity of hepatocellular carcinoma H. Zhang 1 , C. Sun 1 1 Institute of Modern Physics- Chinese Academy of Sciences, Department of Heavy Ion Radiation Biology and Medicine, Lanzhou, China Purpose or Objective High constitutive expression of Nrf2 has been found in many types of cancers, and this high level of Nrf2 also favors resistance to drugs and radiation. Here we investigate how isoliquiritigenin (ISL), a natural antioxidant, inhibits the Nrf2-dependent antioxidant pathway and enhances the radiosensitivity of HepG2 cells Treatment of HepG2 cells with ISL for 6 h, Keap1 and ubiquitination of Nrf2 were measured by RT-PCR and Western blot. Pretreatment with ISL for 6 h followed by X- ray irradiation, confocal microscopy was used to visualize Nrf2 translocation to the nucleus and γ-H2AX foci. To investigate the radiosensitization effect of ISL, apoptosis, clonogenic potential and HepG2 xenografts were examined. Results Treatment of HepG2 cells with ISL for 6 h selectively enhanced transcription and expression of Keap1. Keap1 and HepG2 xenografts. Material and Methods

Figure 1. Day fourteen clonogenic cell-survival curves for MRT and CRT. Data are presented as mean ± SEM, n = 3, *P < 0.05, **P < 0.01.

Conclusion This is the first study to compare the response of DIPG cell lines to MRT and CRT. Although MRT caused more DNA damage that was detrimental to the cell cycle compared to CRT, the JHH-1 cell demonstrated radio-resistance regardless of the radiation modality used. The findings of this study support the use of MRT as a potential alternative to CRT for patients with radiosensitive tumours and also contribute to our understanding of the differential response of cancer cells to MRT and CRT. PO-0980 MEK/ERK pathway sustains radioresistance of embryonal rhabdomyosarcoma stem-like cell population. F. Marampon 1 , G. Gravina 1 , C. Festuccia 1 , C. Ciccarelli 1 , F. De Felice 2 , D. Musio 2 , V. Tombolini 2 1 University of L'Aquila, Department of Biotechnological

and Applied Clinical Sciences, L'Aquila, Italy 2 University of Rome "Sapienza", Department of Radiotherapy, Rome, Italy

Purpose or Objective The identification of signaling pathways that affect the cancer stem-like phenotype may provide insights into therapeutic targets for combating embryonal rhabdomyosarcoma. The aim of this study was to investigate the role of the MEK/ERK pathway in controlling the cancer stem-like phenotype using a model of