p53-dependent thermal enhancement of cellular sensitivity in human squamous cell carcinomas in relation to LET

To investigate the dependence on p53 gene status of the thermal enhancement of cellular sensitivity against different levels of linear energy transfer (LET) from X-rays or carbon-ion (C-) beams.

Two kinds of human squamous cell carcinoma cell lines were used with an identical genotype except for the p53 gene. SAS/mp53 cells were established by transfection with mutated p53 (mp53) gene to SAS cells having functional wild-type p53 (wtp53). As the control, a neo vector was transfected to the SAS cells (SAS/neo cells). Both cells were exposed to X-rays or accelerated C-beams (30-150 KeV microm(-1)) followed by heating at 44 degrees C. Cellular sensitivity was determined by colony-forming activity. Induction of apoptosis was analysed by Hoechst 33342 staining of apoptotic bodies and agarose-gel electrophoresis for the formation of DNA ladders.

It was found that (1) there was no significant difference in cellular sensitivity between SAS/neo and SAS/mp53 cells to LET radiation of >30 KeV microm(-1), although the radiosensitivity of SAS/neo cells to X-rays was higher (1.2-fold) than that of SAS/mp53 cells; (2) there was an interactive thermal enhancement of radiosensitivity below an LET of 70 KeV microm(-1) in SAS/neo cells, although only additive thermal enhancement was observed in SAS/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 dose-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 sensitivity to X-rays was due to induction of p53-dependent apoptosis.

The findings suggest that thermal enhancement of radiosensitivity may result from p53-dependent apoptosis induced by inhibition of p53-dependent 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.