A. Takahashi et al., p53-dependent thermal enhancement of cellular sensitivity in human squamous cell carcinomas in relation to LET, INT J RAD B, 77(10), 2001, pp. 1043-1051
Purpose: To investigate the dependence on p53 gene status of the thermal en
hancement of cellular sensitivity against different levels of linear energy
transfer (LET) from X-rays or carbon-ion (C-) beams.
Materials and methods: Two kinds of human squamous cell carcinoma cell line
s were used with an identical genotype except for the p53 gene. SAS/mp53 ce
lls were established by transfection with mutated p53 (mp53) gene to SAS ce
lls having functional wild-type p53 (wtp53). As the control, a neo vector w
as transfected to the SAS cells (SAS/neo cells). Both cells were exposed to
X-rays or accelerated C-beams (30-150 KeV mum(-1)) followed by heating at
44 degreesC. Cellular sensitivity was determined by colony-forming activity
. Induction of apoptosis was analysed by Hoechst 33342 staining of apoptoti
c bodies and agarose-gel electrophoresis for the formation of DNA ladders.
Results: It was found that (1) there was no significant difference in cellu
lar sensitivity between SAS/neo and SAS/mp53 cells to LET radiation of >30
KeV mum(-1), although the radiosensitivity of SAS/neo cells to X-rays was h
igher (1.2-fold) than that of SAS/mp53 cells; (2) there was an interactive
thermal enhancement of radiosensitivity below an LET of 70 KeV mum(-1) in S
AS/neo cells, although only additive thermal enhancement was observed in AS
/mp53 cells through all LET levels examined; (3) low-LET radiation induced
apoptosis only in SAS/neo cells; (4) high-LET radiation at an isosurvival d
ose-induced apoptosis of SAS/neo cells at a higher frequency compared with
that with low-LET radiation; (5) high-LET radiation-induced p53-independent
apoptosis in SAS/mp53 cells; and (6) thermal enhancement of cellular sensi
tivity to X-rays was due to induction of p53-dependent apoptosis.
Conclusions: The findings suggest that thermal enhancement of radiosensitiv
ity may result from p53-dependent apoptosis induced by inhibition of p53-de
pendent cell survival system(s) through either regulation of the cell cycle
or induction of DNA repair. It is also suggested that the analysis of p53
gene status of cancer cells may predict response to combined therapies with
low-LET radiation and hyperthermia.