Abstract
Abstract Purpose: Radiation quality depends on loss of energy in tissues per unit distance travelled and could be low-linear energy transfer (low-LET) or high-linear energy transfer (high-LET). The low-LET radiation like gamma-rays and x-rays loose less and hence deposits less energy in tissues and are sparsely ionizing. However, the high-LET radiation like α-particle, neutron, and heavy ions loose more and so deposits more energy in tissues traversed. Epidemiological studies showed strong correlation between radiation exposure and colorectal cancer (CRC) and CRC is the second leading cause of cancer mortality in the USA. However, it is not known if activation of molecular pathways involved in CRC and consequent intestinal tumorigenesis is radiation quality dependent. Methods: Female APCMin/+ mice (6-8 weeks) were irradiated with 5 Gy of gamma radiation, a tumorigenic dose in these mice. After 90 days mice were euthanized, intestinal tumors counted and tumor samples were flash frozen. Tumor samples were subjected to histology, western blot and immunohistochemistry (IHC) to dissect the APC/β-catenin/TCF4 pathway, commonly altered in human CRC. Results of gamma radiation were compared to two qualitatively different types of radiation - high-energy proton (energy: 1 giga electron volt (GeV)) and iron (56-Fe) ion (energy: 1 GeV/nucleon). We used 4 Gy of 56-Fe and 4.7 Gy of proton, which are isotoxic to the gamma radiation dose of 5 Gy determined using a quality factor 1.25 and 1.06 respectively. Quantitation of IHC and western blot were performed using ImageJ v2.45 softwere. Results: Quantitative observations showed enhanced intestinal tumorigenesis after exposure to proton and 56-Fe radiation. Marked increases in β-catenin and TCF4 were associated with a decreased phospho-β-catenin and increased phospho-GSK3β in proton and 56-Fe mice. Increased β-catenin was also associated with higher levels of cyclin-D1 and c-myc in these samples. Furthermore, compared to gamma radiation, strikingly lower level of p53 along with higher MDM2 was observed in proton and 56-Fe samples. Interestingly, however, the decrease in p53 and increase in MDM2 was higher in 56-Fe. These observations were further supported by IHC staining of tumor sections. Statistical analyses to support these observations in proton and 56-Fe compared to ≤ ray or spontaneous tumors will be presented. Conclusions: Considering higher active β-catenin and inactivated GSK3β, we speculate that exposure to high-LET radiation caused stabilization of oncogenic β-catenin and upregulation of c-myc and cyclin-D1 leading to higher tumorigencity. Enhanced potential of high-LET radiation for carcinogenesis is further supported by downregulation of p53. In conclusion, this is the first evidence showing greater potential of high-LET radiation for intestinal carcinogenesis mediated via activation of β-catenin/TCF4 signaling. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 4422. doi:1538-7445.AM2012-4422
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