Proton Beam Induces P53-mediated Cell Cycle Arrest in HepG2 Hepatocellular Carcinoma Cells

A proton beam (PB) can be used to treat various cancers. In the present study, we evaluated the effect of PB on HepG2 hepatocellular carcinoma cells. The results showed that P53 phosphorylation at Ser15 was significantly induced by PB irradiation in a dose-dependent manner. Furthermore, dose-dependent increases of P21 expression and H2A.X phosphorylation by PB were observed at 24 h and 48 h after irradiation. In contrast, PB did not alter the expressions of Bcl-xL, AIF, or caspase-3 or PARP cleavage. In addition, PB suppressed ERK phosphorylation, enhanced p38 phosphorylation at 24 h and 48 h after irradiation, and induced G2/M phase cell cycle arrest. Moreover, although no significant decrease in HepG2 cell viability was observed within 72 h after irradiation of 0.5 to 16 Gy, colony-forming assay results showed that PB induced cell death at 14 days after irradiation. In summary, this investigation demonstrates that PB irradiation induces early-stage G2/M cell cycle arrest, which is mediated by the induction of P53-linked P21 expression via regulation of the ERK and P38 signaling pathways. In conclusion, although PB irradiation was observed to produce late cell death, a PB-sensitizing strategy is needed because PB-induced early-stage G2/M cell cycle arrest may enable the development of resistance to PB irradiation.