Ionizing radiation (IR) is a known risk factor for colorectal (CR) cancer. Considering the high spontaneous incidence of gastrointestinal (GI) cancer in the US and even higher incidence of pre-malignant lesions, such as colonic polyps, an even modest increase by radiation exposure could have a significant effect on health risk estimates for manned space flight. During space travel, astronauts are exposed to levels of radiation, which increases their risk for acute short-term deleterious effects, such as during a solar storm, and for long-term deleterious effects like cancer. Long-duration space missions, like those at the International Space Station, lunar missions, or a mission to Mars, are of sufficient duration to allow for appreciable cumulative doses of cosmic radiation that could have profound health consequences. This project is important because there are no good estimates for intestinal, particularly CR, cancer by space radiation. Considering the increased CR cancer incidence in A-bomb survivors and the novel characteristics of space radiation, cancer causation by space radiation could potentially be even greater.
The overall goal of this project was to improve NASA risk estimates for space radiation-induced intestinal tumors. Investigators employed adenomatous polyposis coli (APC) mouse models that have been successfully used to demonstrate radiation-induced intestinal cancer. The APC models have inactivation of one allele of the APC gene and generally have sporadic loss or truncation mutations of the other gene. This results in mice developing intestinal tumors, called polyps or adenomas and more rarely invasive cancers. Space and terrestrial radiation could differentially perturb other factors of molecular pathways involved in intestinal tumorigenesis and thus initiate increased intestinal tumor development and/or progression.
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Intestinal tumor frequency and grade were assessed in APC mice and results were compared to those in APC mice after exposure to a mean absorbed dose of 2 Gy from (137)Cs y rays or 1.6 Gy from 1 GeV/n iron ions. Cellular differentiation and proliferation were also assessed in the intestinal tumors of sham-irradiated and irradiated mice. In addition, investigators transfected a miR-31-5p mimic (sense) or inhibitor (antisense) into immortalized human colonic epithelial cells followed by gamma-irradiation.
Mouse Models of Colon Cancer:
The Investigators used a solar particle event (SPE) simulation at the NASA’s Space Radiation Laboratory (NSRL) to characterize the effects of low dose rate protons in vivo. Using the colorectal cancer susceptible mouse model, they studied survival and the progression of colon cancer after total body exposure to a simulated SPE with varying energies using a total dose of 2 Gy over a 2 hour period. They also exposed mice to 2 Gy of monoenergetic proton or X-ray at a dose rate of 20 cGy/min as a reference radiation exposure. The SPE simulation was more effective in inducing an increase in the number of polyps, and the percent of invasive adenocarcinomas compared to monoenergetic acute protons or X-rays. Investigators also observed a chronic/persistent increase in oxidative stress, ß-catenin/cyclin dopamine signaling, and a subset of senescence-associated inflammatory response genes as well as a decrease in a subset of circulating miRNAs that persisted 100 days after exposure to SPE. The mice exposed to 2 Gy of silicon (28Si) particles also show a similar biological effectiveness in increasing colon cancer progression. For comparison, groups of mice were fed with a diet containing the anti-oxidant/anti-inflammatory synthetic triterpenoid, CDDO-ethyl amide (CDDO-EA), for 3 days prior to SPE or 28Si exposure to determine if mice could be protected from space radiation-induced damage. Investigators found that administration of the CDDO-EA just a few days prior to SPE or 28Si irradiation reduced oxidative stress, chronic inflammatory responses, and cancer progression. These findings suggest that exposure to low dose rate SPE protons and 28Si elicit significant changes in biological effects that have functional consequences on colon cancer progression.
Human Colonic Epithelial Cells (HCECs):
To address the cellular function of miR-31-5p, investigators transfected a miR-31-5p mimic (sense) or inhibitor (antisense) into human colonic epithelial cells and colon cancer cell lines followed by gamma-irradiation. They found that a miR-31-5p mimic sensitized cells to irradiation, while a miR-31-5p inhibitor protected normal colonic epithelial cells against radiation. miR-31-5p regulates mismatch repair (MMR) gene expression, including the human mutL homolog 1 (hMLH1). The miR-31-5p inhibitor failed to modulate radiosensitivity in a hMLH1-deficient HCT116 colon cancer cell line but protected HCT116 3-6 and DLD-1 (both hMLH1-positive) colon cancer cell lines. These findings demonstrate that miR-31-5p has an important role in radiation responses through regulation of hMLH1 expression. Targeting this pathway could be a promising therapeutic strategy for future personalized anti-cancer radiotherapy.
High charge (Z) and energy (E) (HZE) particles in deep space have significantly contributed to the biological effects of space radiation, although they only account for less than 1% of the galactic cosmic rays (GCR) particle fluxes. Investigators have shown that combined radiation exposure of 2-Gy proton (1H) followed by 0.5-Gy iron (56Fe) ion particles increase transformation in human colonic epithelial cells (HCEC CT7). This study was undertaken to characterize if additional HZE ions such as oxygen (16O) and silicon (28Si) particles also result in increased cell transformation. HCEC CT7 cells irradiated with 1-Gy 16O followed 24 hours later by 1-Gy 28Si particle showed an increase in proliferation, anchorage-independent growth, migration, and invasion abilities compared to unirradiated controls. In addition, they discovered that the ß-catenin pathway was activated and that subsets of DNA repair genes were under-expressed in these transformed cells. Pretreatment with the radioprotector, Bardoxolone methyl (CDDO-Me), 18 hours before and during irradiation prevented the HZE-induced transformation. These results can be interpreted to suggest that the mixed radiation exposure of 16O followed by 28Si has carcinogenic potential. Importantly, this transformation can be protected by CDDO-Me pre-treatment.
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