ESTRO 2023 - Abstract Book

S273

Sunday 14 May 2023

ESTRO 2023

tumors became reduced. Analyses of CAF-markers in human specimens are being conducted and final outcomes, including prognostic and predictive impact, will be presented at the meeting. Conclusions Radiation induces important phenotypic changes in CAFs, however irradiation do not seem to alter substantially the CAF- mediated pro-tumorigenic or radioprotective functions on tumor cells. CAFs abundance and phenotype remain largely unchanged upon external beam radiotherapy of subcutaneously grown tumors. The ultimate role of CAFs on tumor responses to radiotherapy warrant further investigations.

SP-0351 Novel mechanisms and biomarkers associated to therapy-induced senescent cells TBC

SP-0352 Targeting radiation-induced senescence to augment glioblastoma therapy S. Burma 1 1 University of Texas Health Science Center at San Antonio, Neurosurgery, San Antonio, USA

Abstract Text Glioblastomas (GBM) are routinely treated with high doses of ionizing radiation (IR), yet these tumors inevitably recur, and the recurrent tumors are highly therapy resistant. Understanding mechanisms driving tumor recurrence after radiotherapy is an unmet need in the field of GBM therapy. During GBM therapy, both the tumor and the normal brain tissue surrounding the tumor are irradiated with up to 60 Gy of IR. IR is a potent inducer of senescence, and senescent stromal cells are known to promote the growth of neighboring tumor cells by secreting proteases, cytokines and growth factors that create a senescence-associated secretory phenotype (SASP). We have reported recently that IR-induced senescence of astrocytes in the brain leads to the secretion of SASP factors that promote the growth and invasiveness of tumor cells in mouse models of GBM. Briefly, we found that cranial irradiation of C57BL/6J mice triggered p21-dependent senescence of astrocytes. Senescent astrocytes showed upregulation of several SASP genes including HGF. HGF secreted by senescent astrocytes activated the receptor tyrosine kinase Met in orthotopically implanted glioma cells thereby promoting their growth and invasiveness. Elimination of senescent astrocytes using the senolytic ABT-263 (Navitoclax) thwarted tumor growth, underscoring the usefulness of senolytics for improving GBM therapy. Following up on this study, we next irradiated a panel of GBM cell lines and found that a significant fraction of these cells senesced within 10 days, as evaluated by analyzing senescence markers like SA-beta-galactosidase staining, induction of p21 and loss of nuclear Lamin B1. Senescent glioma cells exhibited upregulation of a number of bona fide SASP genes in a NF-kB-dependent manner. The interleukin IL6, which is an activator of the JAK-STAT pathway, was most prominently upregulated in these cells. Conditioned media from senescent GBM cells activated the JAK-STAT pathway in non-senescent GBM cells and promoted tumor cell proliferation and radiation resistance. These effects could be neutralized by antibodies targeting IL6 or its receptor IL6R, as well as by JAK 1/2 inhibitors. Interestingly, conditioned media from senescent GBM cells could also activate the NF-kB pathway in non-senescent cells resulting in induction of SASP genes in the recipient cells. These results indicate that senescent GBM cells can activate pro-tumorigenic pathways in their non-senescent counterparts and SASP can spread from senescent to non-senescent cells, thereby promoting tumor recurrence. Bioinformatic analyses of the transcriptomic profiles of naïve and irradiated GBM cell lines and the TCGA database revealed that the inhibitor of apoptosis protein cIAP2 is a critical survival factor for senescent glioma cells. We find that targeting cIAP2 using Smac mimetics triggers cell death specifically in senescent GBM cells with minimal toxicity towards normal brain cells such as astrocytes. Upregulation of cIAP2 in irradiated tumor cells was also seen in PDX models of GBM. Using these PDX models, we are currently validating novel senolytic-based therapeutic approaches to prevent tumor recurrence after radiotherapy. In sum, our findings illustrate how senescence in both stromal and tumor cells promote GBM recurrence via different mechanisms and underscore the potential utility of adjuvant senolytic therapy for blunting GBM recurrence after radiotherapy. SP-0353 Effects of irradiation on Head and Neck Squamous Cell Carcinoma derived CAFs M. Ansems 1 , R. Peters 2 , V. Mekers 2 , A. Beerkens 2 , J. Honings 3 , W. Weijs 4 , G. Adema 2 , J. Bussink 5 , J. Kaanders 5 1 RIMLS, Radiation Oncology, Nijmegen, The Netherlands; 2 Radboud Institute for Molecular Life Sciences, Radiation Oncology, Nijmegen, The Netherlands; 3 Radboud Institute for Health Sciences, Otorhinolaryngology-Head and Neck Surgery, Nijmegen, The Netherlands; 4 Radboud Institute for Health Sciences, Oral and Maxillofacial Surgery, Nijmegen, The Netherlands; 5 Radboud Institute for Health Sciences, Radboudumc, Radiation Oncology, nijmegen, The Netherlands Abstract Text Purpose: Head and Neck Squamous Cell Carcinoma (HNSCC) is the sixth most common cancer worldwide, resulting in more than 450.000 deaths a year. Half of the patients with advanced HNSCC die within five years. Most treatments aim to kill cancer cells, but this is often insufficient to cure advanced HNSCC. Besides cancer cells, also cancer-associated fibroblasts (CAFs) make up a large part of the tumour. Their presence, however, is often ignored. Currently, the majority of HNSCC patients will be treated with radiotherapy. Despite the abundance of CAFs, the effect of radiotherapy on their function in HNSCC is largely unknown. To increase the success rate of treatment strategies in HNSCC, more in-depth knowledge regarding CAFs and the effect of radiotherapy on their role is essential. Materials and methods: CAFs were isolated from HNSCC patients and exposed to ionizing radiation ex vivo . The effect of different doses of radiation on CAFs was determined by microscopic analyses of cell growth, cell size, DNA damage and quantification of senescence. Results: Our data shows that human HNSCC patient-derived CAFs are widely affected by radiotherapy; they dose- dependently decrease cell growth (Fig1A), increase their cell size (Fig.B,C) and have permanent DNA damage as measured by the presence of 53BP1 (Fig1D,E,F). As these effects are often associated with senescence, we established -and confirmed- radiation-induced senescence by staining the irradiated CAFs for β -galactosidase (Fig1G,H), the most commonly used marker to identify cellular senescence. Conclusion: Our data show that radiotherapy modulates the phenotype of HNSCC patient-derived CAFs. Radiation of CAFs affects their morphology and induces senescence. Current research focuses on the effect of radiotherapy on the function of CAFs in vivo and their effect to modulate the anti-tumour immune response.