Abstract 2049: The role of epigenetic mark profile, cell cycle alteration and DNA repair in resistance of glioblastoma cells to photodynamic therapy
Authors: Ghahe Somayeh Shahmoradi , Kopania Karolina , Ciuba Agata , Foksinski Marek , Oliński Ryszard , Tudek Barbara 
Published: July 2017

Abstract
Glioblastoma is the most aggressive primary brain tumor. Treatment regimens for glioblastoma tumors, such as surgery, radiotherapy, and chemotherapy, are very invasive and can only prolong the median patient life to several months. It was shown that 5-aminolevulinic acid (5-ALA)-based photodynamic therapy (PDT) is a promising and less aggressive adjuvant modality for diagnosis and treatment of glioblastoma.
Limiting point of PDT outcome is appearing of cells with intrinsic or acquired PDT resistance that finally results in repopulating the tumor and short-term survival of glioma patients. Therefore, our study was designed to determine therapy resistance markers in PDT resistant glioblastoma cell line. Potential markers subsequently can be used for sensitization of glioblastoma to PDT.
Glioblastoma (U-87) cell line resistant to PDT (U-87R) was isolated from parental, sensitive line (U-87P) by applying several cycles of ALA-PDT, and further growing of surviving cells. U-87R cells subsequently were characterized from different aspects such as epigenetic markers, cell cycle events, oxidative stress and DNA repair capacity.
Assessment of nucleotide modifications and epigenetic marks in PDT resistant glioblastoma and its parental cell line showed significant higher level of two epigenetic marks including 5-methylcytosine (5-mC) and 5-hydroxymethylcytosine (5-hmC) in U-87R compare to U-87P, before and after PDT.
Doubling time of U-87R cells was considerably longer than that of U-87P. Cell cycle analysis showed longer G1 phase in U-87R cells compare to U-87P. Moreover, accumulation in G2 phase following PDT was observed earlier in resistant cells than in parental line. That suggests the role of early activation of DNA damage response in resistant cells. Resistant glioblastoma cells were then sensitized to PDT by applying inhibitor of one of the main kinases of DNA damage response, ATM kinase. In comparison with parental cells, U-87R cells also showed higher activity of some DNA base excision repair (BER) enzymes including glycosylases and AP-endonuclease1 (APE1).
Studies of the whole protein profile of PDT resistant and parental cells demonstrated that the level of superoxide dismutase (SOD) was considerably higher in U-87R than in U-87P. That data was then confirmed by detecting lower oxidative stress in U-87R following PDT.
PDT resistant glioblastoma cells as a model of resistant cells in glioblastoma tumor population demonstrated the significant role of epigenetic mark profile, cell cycle alteration, higher DNA repair capacity, and antioxidant defense in conferring resistance to photodynamic therapy.
This work was supported by the Polish National Science Centre (NCN) grants: DEC-2012/07/B/NZ1/00008, UMO-2014/15/B/NZ5/01444, UMO-2014/13/N/NZ3/00863
Note: This abstract was not presented at the meeting.
Citation Format: Somayeh Shahmoradi Ghahe, Karolina Kopania, Agata Ciuba, Marek Foksinski, Ryszard Oliński, Barbara Tudek. The role of epigenetic mark profile, cell cycle alteration and DNA repair in resistance of glioblastoma cells to photodynamic therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2049. doi:10.1158/1538-7445.AM2017-2049