Two- and Three-Dimensional Models for Risk Assessment of Radiation-Enhanced Colorectal Tumorigenesis

Abstract

Astronauts may be at an increased risk for developing colorectal cancer after a prolonged interplanetary mission given the potential for greater carcinogenic effects of radiation to the colon. In addition, with an increase in age, there is a greater incidence of premalignant colon adenomas with age. In the present study, we have compared the effects of radiation on human colon epithelial cells in two-dimensional (2D) monolayer culture, in three-dimensional (3D) culture, and in intact human colon tissue biopsies. Immortalized colon epithelial cells were irradiated at the NASA Space Radiation Laboratory (NSRL) with either 1 Gy 1 GeV/nucleon 56 Fe particles or 1 Gy 1 GeV/nucleon protons and were stained at various times to assess DNA damage and repair responses. The results show more persisting damage at 24 h with iron-particle radiation compared to protons. Similar results were seen in 3D colon epithelial cell cultures in which 56 Fe-particle-irradiated specimens show more persisting damage at 24 h than those irradiated with low-LET γ rays. We compared these results to those obtained from human colon tissue biopsies irradiated with 1 Gy γ rays or 1 Gy 1 GeV 56 Fe particles. Observations of radiation-induced DNA damage and repair in γ-irradiated specimens revealed more pronounced early DNA damage responses in the epithelial cell compartment compared to the stromal cell compartment. After low-LET irradiation, the damage foci mostly disappeared at 24 h. Antibodies to more than one type of DNA repair factor display this pattern of DNA damage, and staining of nonirradiated cells with non-phosphorylated DNA-PKcs shows a predominance of epithelial staining over stromal cells. Biopsy specimens irradiated with high-LET radiations also show a pattern of predominance of the DNA damage response in the highly proliferative epithelial cell compartment. Persistent unrepaired DNA damage in colon epithelial cells and the differing repair responses between the epithelial and mesenchymal compartments in tissues may enhance tumorigenesis by both stem cell transformation and alterations in the radiation-induced permissive tissue microenvironment that may potentiate cancer progression.

Induction and repair of DNA double-strand breaks assessed by gamma-H2AX foci after irradiation with pulsed or continuous proton beams

“…Our data support the idea that the amount of DNA damage inflicted is not decidedly affected by the ultrahigh dose rate of a pulsed delivery mode when compared to irradiation times in the millisecond range. Results of the present experiments with longer repair time showed no differences for pulsed or continuous protons after 24-h repair what is well line with our previous experiments (Schmid et al 2010a) and with the experiments of Roig et al (2009) in logarithmically growing cells and in 3D colon epithelial cell 24 h after irradiation with protons.…”

Report of the AAPM TG‐256 on the relative biological effectiveness of proton beams in radiation therapy

“…Compared to 2D monolayers, 3D cultures allow development of cell morphology, cell–cell and cell–extracellular matrix interactions and differentiation more characteristic of in vivo . Published data comparing responses of normal cells irradiated with photons in 2D and 3D do not all agree; some report that cells in 3D show increased survival and DNA repair, while others indicate no differences in responses . Most of these studies have used detection of DNA repair protein‐related foci as the damage endpoint, so the relationship to clonogenic survival is not clear.…”

“…133 Published data comparing responses of normal cells irradiated with photons in 2D and 3D do not all agree; some report that cells in 3D show increased survival and DNA repair, 135 while others indicate no differences in responses. 136,137 Most of these studies have used detection of DNA repair proteinrelated foci as the damage endpoint, so the relationship to clonogenic survival is not clear. No data with clinical energy protons appear to have been published, so there is no indication of whether RBE values would change in 3D cultures compared to 2D.…”

Mechanism of cluster DNA damage repair in response to high-atomic number and energy particles radiation

“…Although these systems have provided useful and powerful information, monolayer cell cultures do not recapitulate the three-dimensional structural organization or functional differentiation of the cells in viv o. Recently, radiation-induced cellular responses in three-dimensional (3D) cell culture models have been reported [128, 129]. Though cells grown in 3D culture recapitulate some of the in vivo tissue architecture, they do not truly reflect the in vivo tissue physiology.…”