Background: Risk of colorectal cancer (CRC) after exposure to low linear energy transfer (low-LET) radiation, such as γ-rays, is highlighted by the studies in atom bomb survivors. Long duration space missions are expected to expose astronauts and future space tourists to considerable levels of cosmic radiation consisting of highly energetic charged (HZE) heavy ions such as 56-Fe, which are densely ionizing high-LET radiation. CRC risk prediction after exposure to high-LET cosmic heavy ion radiation exposure is hindered due to scarcity of in vivo data. Therefore, intestinal tumor frequency, and intestinal epithelial cell (IEC) proliferation and differentiation were studied in APCMin/+ mice. We also studied effects of 56-Fe radiation on the β-catenin pathway, which is frequently perturbed in CRC.
Methods: Female APCMin/+ mice (n=20 mice per group; 6 to 8 wks old) were exposed to whole-body γ (2 or 5 Gy) or 56-Fe (1.6 or 4 Gy; energy-1 GeV/nucleon; LET: 148 keV/μ) radiation and dose rate was 1 Gy/min. The 56-Fe radiation doses were equitoxic to respective γ radiation doses calculated using an RBE factor of 1.25. Mice were euthanized between 100 and 110 days after radiation exposure. The small intestinal tract was surgically removed, divided into duodenum, jejunum, and ilium, flushed gently with PBS, cut open longitudinally, and tumors counted. Intestinal samples of tumor free and tumor bearing areas were either fixed in 10% buffered formalin or flash frozen in liquid nitrogen.
Results: Relative to controls and γ-ray, intestinal tumor frequency was significantly higher after 56-Fe and unlike γ-ray, tumorigenesis after 56-Fe was dose dependent. Also, tumor incidence per unit of radiation (per cGy) was higher after 1.6 Gy (28 tumors/160 cGy=0.18) than after 4 Gy (52 tumors/400 cGy=0.13) suggesting that the lower dose was relatively more tumorigenic than the higher dose. We followed the 2 Gy γ and equitoxic 1.6 Gy 56-Fe samples for differential molecular pathway alterations. Staining for phospho-histone H3, indicative of proliferation, was more and alcian blue staining, indicative of differentiation, was less in 56-Fe than γ radiation. Activation of β-catenin as well as cyclin D1 was more in 56-Fe-irradiated normal-appearing and tumor tissues.
Conclusions: Our study has implications for health risk estimates of not only astronauts undertaking exploratory missions into outer space and future space tourists but also for patients exposed to heavy ion radiotherapy. We have shown for the first time that 56-Fe radiation exposure led to increased incidence of intestinal tumors in APCMin/+ mice. Importantly, molecular analysis of tumor-free and tumor-bearing areas suggests that increased intestinal tumorigenesis after 56-Fe radiation was due to reduced differentiation and increased proliferation of IEC as a result of preferentially greater activation of β-catenin pathway.
Citation Format: Kamal Datta, Shubhankar Suman, Albert J. Fornace. Heavy ion radiation exposure triggered higher intestinal tumor frequency and greater β-catenin activation than γ radiation in APCMin/+ mice. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3600. doi:10.1158/1538-7445.AM2013-3600