br Conflict of interest br Author contributions br Acknowled
Conflict of interest
Introduction The repair pathway of DNA-DSBs through HR and the role of BRCA1 in repair kinetics by HR is also an important factor to unravel the radiation sensitization of novel small molecule inhibitors. The involvement of the breast cancer susceptibility gene 1 (BRCA1) in tumorigenesis and its role in facilitating DNA resection and HR during S and G2 phases of the tranylcypromine has been well documented (Yu and Chen, 2004, Huen et al., 2010). BRCA1 is involved in the activation of cell cycle check points and plays a major part in maintaining chromosomal stability. Radiation exposure phosphorylates BRCA1 at S1423 thus facilitating DSB repair by the HR pathway. Inhibition of these kinase enzymes through a targeted approach not only sensitizes cancer cells to radiation, but also results in a decrease in cancer cell multiplication rate (Kim et al., 2002, Pajonk et al., 2005). In the absence of DNA repair cells will undergo apoptosis (Martin, 2001).
Material and methods
Discussion Western blotting and flow cytometry was employed to assess the DNA-PK inhibition of LTU28 and LTU31 in combination with radiation. It has been previously reported that phosphorylation of DNA-PKcs at the Thr2609 cluster plays an important role in DSB repair and resistance to radiation (Ding et al., 2003, Reddy et al., 2004, Nagasawa et al., 2011). Another in vivo study showed autophosphorylation of Ser2056 is also a reliable indicator of DNA-PK activation and that radiation induced DNA-PKcs autophosphorylation plays a major role in DSB repair through NHEJ. (Chen et al., 2005, Chen et al., 2007). Our results showed that LTU28 + 6Gy inhibited DNA-PK phosphorylation at Ser2056 and Thr2609 in both cell lines. The action of LTU28 and LTU31 on AKT phosphorylation, one of the downstream targets of DNA-PK was also analysed. Our results showed that LTU28 + 6Gy inhibited AKT1 phosphorylation at Ser473 and Thr308 residues though not completely. Previous studies have reported the involvement of various signalling pathways other than DNA-PKcs in the activation of AKT (Brazil and Hemmings, 2001, Sarbassov et al., 2005, Guertin et al., 2006). We are hypothesizing that other modes of activation would be involved in failure to completely inhibit AKT phosphorylation by LTU28 after exposure to radiation. The effect of LTU28 and LTU31 in combination with radiation on the activation of BRCA1, one of the key enzymes that facilitates DNA-DSB repair through HR pathway was also analysed. Our results showed that LTU31 in combination with radiation completely inhibited the phosphorylation of BRCA1. The effect of a BRCA1 inhibitor that radiosensitized breast cancer cells to radiation and inhibited DSB repair through HR has been reported earlier (Pessetto, Yan, Bessho, & Natarajan, 2012). We are proposing that LTU31 when used in combination with radiation is inhibiting BRCA1 activation and thereby inhibits the HR pathway.