DNA damage response signaling pathways and targets for radiotherapy sensitization in cancer

Abstract

Radiotherapy is one of the most common countermeasures for treating a wide range of tumors. However, the radioresistance of cancer cells is still a major limitation for radiotherapy applications. Efforts are continuously ongoing to explore sensitizing targets and develop radiosensitizers for improving the outcomes of radiotherapy. DNA double-strand breaks are the most lethal lesions induced by ionizing radiation and can trigger a series of cellular DNA damage responses (DDRs), including those helping cells recover from radiation injuries, such as the activation of DNA damage sensing and early transduction pathways, cell cycle arrest, and DNA repair. Obviously, these protective DDRs confer tumor radioresistance. Targeting DDR signaling pathways has become an attractive strategy for overcoming tumor radioresistance, and some important advances and breakthroughs have already been achieved in recent years. On the basis of comprehensively reviewing the DDR signal pathways, we provide an update on the novel and promising druggable targets emerging from DDR pathways that can be exploited for radiosensitization. We further discuss recent advances identified from preclinical studies, current clinical trials, and clinical application of chemical inhibitors targeting key DDR proteins, including DNA-PKcs (DNA-dependent protein kinase, catalytic subunit), ATM/ATR (ataxia-telangiectasia mutated and Rad3-related), the MRN (MRE11-RAD50-NBS1) complex, the PARP (poly[ADP-ribose] polymerase) family, MDC1, Wee1, LIG4 (ligase IV), CDK1, BRCA1 (BRCA1 C terminal), CHK1, and HIF-1 (hypoxia-inducible factor-1). Challenges for ionizing radiation-induced signal transduction and targeted therapy are also discussed based on recent achievements in the biological field of radiotherapy.

Fig. 1

DNA damage induced by ionizing radiation. The major types of DNA damage induced by IR include base and sugar damage, single-strand breaks, double-strand breaks, clustered DNA damage, and covalent intrastrand or interstrand crosslinking

Fig. 2

Structures of major DNA damage signal sensors, their main functional domains and their interactions with their partners. The data are from the RCSB database (https://www.rcsb.org/)

Fig. 3

Damage sensors and their functional complexes in response to DNA double-strand breaks. (1) Upon DSB occurrence, the core histone protein variant H2AX is instantaneously phosphorylated on its S139 position to form γH2AX foci, which can be detected at the DSB site. γH2AX provides a platform to recruit DDR proteins, such as 53BP1, MDC1, and ATM, to DSBs to initiate DDR signal transduction. (2) DNA-dependent protein kinase (DNA-PK), composed of Ku70, Ku80, and the catalytic subunit DNA-PKcs, is a classical DSB-sensing and -binding complex. DSB binding by DNA-PK protects the broken DNA end from degradation by endogenous nucleases; on the other hand, it recruits and activates the downstream components in the NHEJ pathway of DSB repair. (3) BRCA1 and BRCA2 are key proteins involved in DSB binding and initiating the HR pathway and later repair processing. BRCA2 directly recruits RAD51 to sites of DNA damage through interaction with conserved BRCT motifs to stabilize the RAD51 nucleoprotein filament on the ssDNA end of DSBs. Following end resection of the DSBs, BRCA1 activates RAD51 to promote gene conversion of homologous recombination. (4) The MRN complex (Mre11-Rad50-Nbs1) is the primary sensor of DSBs and localizes to damage sites to initiate end resection and HR processing. The MRN complex also promotes the recruitment and activation of ATM and PARP-1. PARP-1 produces poly(ADP-ribose) polymers and extends DNA damage signaling

Fig. 4

The pathways of DNA double-strand break repair. The nonhomologous end-joining (NHEJ) pathway is an error-prone repair pathway that functions through the cell cycle. The homologous recombination pathway is an error-free repair pathway that requires intact homologous DNA as a repair template and is active in the later S and G2 phases. The alternative end-joining (a-EJ) pathway, which repairs DNA double-strand breaks (DSBs), is initiated by end resection that generates 3′ single strand

Fig. 5

Functional complexes of cyclins and cyclin-dependent kinases (CDKs) and the signaling pathways involved in the regulation of cell cycle checkpoints in response to IR-induced DNA damage. CDK4/6/cyclin D promotes progression through the G1 phase. In late G1, the active CDK2/cyclin E complex promotes the G1/S transition. The CDK2/cyclin A complex promotes progression through S phase. The CDK1/cyclin A complex regulates progression through the G2 phase in preparation for mitosis. The G2/M-phase transition is initiated and promoted by the CDK1/cyclin B complex. The activity of CDK1/cyclin B is tightly maintained by the CDC25C phosphatase. Following DSB induction by IR, ATM is activated by the MRN complex, which then phosphorylates p53. Activated p53 transactivates the expression of p21^Cip1, which inhibits CDK2, consequently inducing G1/S arrest. On the other hand, ATM phosphorylates and activates CHK2, which in turn phosphorylates and inactivates CDC25C; the latter is then cytoplasmically sequestered by 14-3-3 proteins. Consequently, the inhibitory phosphorylation of CDK1 by Wee1 and Myt1 on Tyr15 and Thr14 is maintained, and G2/M arrest is induced

Fig. 6

Interregulation of the PIKK family members DNA-PKcs and ATM and their downstream substrates in the DDR pathway activated following DSB induction by IR. The dotted arrow represents the regulation at the transcription level. The solid arrow indicates the kinase activity-dependent regulation at the post-translational level