The Molecular Mechanisms of Radiation-Induced Apoptosis in Different pH Environments

초록

We have investigated the molecular mechanism underlying the radiation-induced apoptosis in different pH environment using RKO.C human colorectal cancer cells carrying wild-type p53 and RKOmp53 cells carrying mutant p53 and RC10.1 cells, a subline of RKO.C cells transfected with E6. An irradiation with 2-20 Gy of γ-ray induced apoptosis in these cells and the radiation-induced apoptosis occurred after the cells are released from G2/M arrest. The radiation-induced G2/M arrest was longer in the RKOmp53 cekks and RC10.1 cells than that in the RKO.C cells. Consequently the rate of apperance of radiation-induced apoptosis in the RKOmp53 cells and RC10.1 cells was much slower than that in the RKO.C cells. In RKO.C cells, a pH 6.6 environment markedly prolonged G2/M arrest and thus delayed the radiation-induced apoptosis. The effect of acicic pH on the radiation-induced cell cycle progression and development of apoptosis in the RKOmp53 cells and RC10.1 cells arreared to be less than that in the RKO.C cells. The radiation-induced increase in p53 and p21 was less in RC10.1 cells than that in other cells. The radiation-induced increase in p53 and p21 level was significantly less in pH 6.6 environment than in pH 7.5 environment in RKO.C, RKOmp53, and RC10.1 cells. The level of Bax increased while that of Bcl-2 unchanged in both pHs after radiation in RKO.C cells whereas neither Bax nor Bcl-2 changed in RKOmp53 and RC10.1 cells after irradiation. The radiation induced cleavage of PARP(poly(ADP-ribose)polymerase)was greater in RKO.C cells than in other cells and lower pH environment suppressed the radiation-induced PARP cleavage. Taking these together, we concluded that:(1)Radiation-induced apoptosis occurs after the cells pass through the G2/M checkpoint, even though apoptotic signals such as an increase in p53 and Bax takes places soon after radiation, (2) an acidic environment prolongs radiation-induced G2/M arrest and expression of pro-apoptotic signal such as p53 and Bax re