ESTRO 2022 - Abstract Book

S116

Abstract book

ESTRO 2022

the clinical relevance of the identified GT. To evaluate the role of GTs on PCa radiosensitization, colony formation and sphere formation assays using PC3, DU145, and LNCaP cells with siRNA-mediated knockdown of GTs has been performed. Results Depletion of our candidates radiosensitizes PCa cell lines. Besides, reduced spherogenicity of DU145 but not LNCaP cells was observed after GTs depletion. These results are consistent with our published data showing that LNCaP cells use autophagy as a prosurvival mechanism upon disruption of Gln metabolism, which may also explain the unaltered spherogenicity of LNCaP cells upon depletion of GTs (1, 2). Unlike DU145 cells, which do not express ATG5 and do not show canonical autophagy, LNCaP cells are not solely dependent on the exogenous Gln as they can use autophagy to survive (5). Additionally, we observed statistically significant increased expression of GTs in DU145 cells compared to LNCaP cells upon Gln starvation, which may recapitulate dependency of DU145 cells on Gln for survival. Conclusion Targeting SLC1A5 , SLC7A5 , and SLC38A1 might offer a promising approach to radiosensitize PCa cells. Further studies to evaluate these GTs in clinical specimens by IHC to validate them as prospective biomarkers and in animal studies by their pharmacological inhibition may elucidate their potential as a therapeutic target for PCa radiosensitization. References 1- Mukha, A. & Kahya, U. et al. Theranostics 2021

2- Mukha, A. et al. Autophagy 2021 3- Kahya, U. et al. Cancers 2021 4- Cojoc, M. et al. Cancer Res 2015 5- Peitzsch, C. Int J Radiat Biol 2014

MO-0141 Sulfasalazine radiosensitizes hypoxic colorectal cancer cells through the inhibition of xCT.

L. Kerkhove 1 , I. Dufait 2 , M. De Ridder 2 , S. de Mey 3 , F. Geirnaert 3 , K.L. Law 3 , H. Vandenplas 2 , T. Gevaert 2

1 VUB , TROP, Brussels, Belgium; 2 UZ Brussel, Radiotherapy, Brussels, Belgium; 3 VUB, TROP, Brussels, Belgium

Purpose or Objective Hypoxia is well-known characteristic of solid tumors that contributes to radioresistance. Furthermore, (hypoxic) cancer cells are characterized by a dysregulated redox homeostasis. Cells heavily rely on the antioxidant system to sustain the balance between reactive oxygen species (ROS) production and scavenging. It is hypothesized that colorectal cancers (CRC), among others, upregulate the expression of system Xc- to cope with the increased amounts of ROS. This antiporter is responsible for the uptake of cystine, the rate-limiting precursor of glutathione (GSH) and one of the main antioxidants in the cell. Anti-rheumatoid arthritis drug sulfasalazine (SSZ) is a known inhibitor of xCT (the light subunit of system Xc-) and thus increases oxidative stress. Furthermore, it has been described that xCT inhibitors can induce ferroptosis, which is being investigated in the context of ionizing radiation. Hence, we hypothesized that targeting xCT, by FDA-approved drug SSZ, is an attractive strategy to induce radiomodulatory effects in CRC by disrupting the redox homeostasis and inducing ferroptosis. Materials and Methods mRNA levels of xCT were examined in CRC using the cBioPortal tool utilizing data from the TCGA database. Human CRC cell lines DLD-1 and HCT116 were further evaluated. Levels of xCT were determined by RT-PCR and western Blot and non-toxic doses of SSZ were determined. The levels of GSH, ROS and ferroptosis were examined after SSZ treatment. xCT knock out (KO) cell lines were generated as a proof of concept. The radio-modulatory effects of SSZ were determined under normoxic and radiobiological relevant hypoxic conditions (0.1%). Results Treatment with non-toxic doses of SSZ decreased the levels of GSH significantly, both in normoxic and hypoxic conditions, with a more pronounced effect in DLD-1 cells. SSZ slightly radiosensitized human CRC cell line DLD-1 under normoxic conditions with an enhancement ratio (ER) of 1.4. Under hypoxic conditions, the radiosensitizing effect of SSZ on DLD-1 was further increased (ER: 2.0), while only a marginal effect was observed in HCT116 (ER: 1.4). ROS levels in both cell lines were significantly upregulated (up to 10-fold increase), while induction of ferroptosis was only observed in DLD-1 cells. Conclusion Very little evidence of the radiomodulatory effect of SSZ has been described before and no evidence has been generated under hypoxic conditions. Our preliminary results suggest that SSZ significantly decreased GSH and increased ROS levels within the cells and radiosensitized DLD-1 cells through induction of ferroptosis. Underlying mechanistical pathways by using xCT KO cells and the role of ferroptosis in hypoxic radioresponses are still under investigation. Elucidating the cell line specific sensitivity towards the drug and in vivo confirmation are still necessary before SSZ, alone or in combination therapies, can be used in the clinic as a radiosensitizer.

MO-0142 Lack of robustness of in vitro clonogenic assays leads to serious uncertainty in RBE determination

R. Koch 1 , E. Bahn 2,3

Made with FlippingBook Digital Publishing Software