Transcriptional response of BALB/c mouse thyroids following in vivo astatine-211 exposure reveals distinct gene expression profiles

Abstract

BackgroundAstatine-211 (211At) is an alpha particle emitting halogen with almost optimal linear energy transfer for creating DNA double-strand breaks and is thus proposed for radionuclide therapy when bound to tumor-seeking agents. Unbound 211At accumulates in the thyroid gland, and the concept of basal radiation-induced biological effects in the thyroid tissue is, to a high degree, unknown and is most valuable.MethodsFemale BALB/c nude mice were intravenously injected with 0.064 to 42 kBq of 211At, resulting in absorbed doses of 0.05 to 32 Gy in the thyroid gland. Thyroids were removed 24 h after injection; total RNA was extracted from pooled thyroids and processed in triplicate using Illumina MouseRef-8 Whole-Genome Expression Beadchips.ResultsThyroids exposed to 211At revealed distinctive gene expression profiles compared to non-irradiated controls. A larger number of genes were affected at low absorbed doses (0.05 and 0.5 Gy) compared to intermediate (1.4 Gy) and higher absorbed doses (11 and 32 Gy). The proportion of dose-specific genes increased with decreased absorbed dose. Additionally, 1.4 Gy often exerted opposite regulation on gene expression compared to the other absorbed doses. Using Gene Ontology data, an immunological effect was detected at 0.05 and 11 Gy. Effects on cellular response to external stress and cell cycle regulation and proliferation were detected at 1.4 and 11 Gy.ConclusionsConclusively, the cellular response to ionizing radiation is complex and differs with absorbed dose. The response acquired at high absorbed doses cannot be extrapolated down to low absorbed doses or vice versa. We also demonstrated that the thyroid - already at absorbed doses similar to those obtained in radionuclide therapy - responds with expression of a high number of genes. Due to the increased heterogeneous irradiation at low absorbed doses, we suggest that this response partly originates from non-irradiated cells in the tissue, i.e., bystander cells.

Comparative Analysis of Transcriptional Gene Regulation Indicates Similar Physiologic Response in Mouse Tissues at Low Absorbed Doses from Intravenously Administered211At

“…This hypothesis is supported by previously presented findings: the biological process of thyroid hormone generation was affected at 1.4 and 11 Gy absorbed dose (17). Moreover, Ttr (encoding for the carrier of the thyroid hormone thyroxine) was downregulated at all absorbed doses, and Slc5a8 (Solute carrier family 5, member 8) was downregulated at 1.4 Gy but upregulated at all other absorbed doses (17). Potentially, there could also be additional radiation-induced signaling factors originating from the affected thyroid on ionizing radiation exposure, which could mediate a nontargeted or systemic bystander response in vivo.…”

“…A thyroid-induced response is likely based on thyroid hormones, which would be dysregulated because of thyroid damage at sufficiently high absorbed doses from 211 At. This hypothesis is supported by previously presented findings: the biological process of thyroid hormone generation was affected at 1.4 and 11 Gy absorbed dose (17). Moreover, Ttr (encoding for the carrier of the thyroid hormone thyroxine) was downregulated at all absorbed doses, and Slc5a8 (Solute carrier family 5, member 8) was downregulated at 1.4 Gy but upregulated at all other absorbed doses (17).…”

“…In thyroid tissues, which were taken from the same mice as the tissues investigated in this study, a similar threshold phenomenon was also reported between 0.64 and 1.8 kBq of 211 At; however, this threshold corresponded to much higher absorbed doses (i.e., 50 and 500 mGy, respectively) (17). On the basis of the similar response in the thyroid (17) and the tissues investigated in this study, despite large differences in absorbed dose, it is hypothesized that this threshold phenomenon seen in tissues other than the thyroid is likely due to a thyroid-based systemic regulation rather than a tissue-based direct response to radiation exposure. Theoretically, 3 basic stages would be distinguishable for the tissues in dependence of increasing 211 At activity: first, a dominance of thyroid-induced response; second, a concomitance of thyroid-induced and radiation-induced response; and third, a dominance of radiation-induced response.…”

“…a mean absorbed dose of 1.2 Gy would correspond to a statistical mean value of 1 a-particle track per thyroid cell (28). Accordingly, absorbed doses of 50 and 500 mGy at 0.064 and 0.64 kBq of 211 At, respectively, would result in around 4% and 40% of single-hit cells in thyroid tissue (17). Because the lungs, liver, spleen, and kidney tissues received much lower absorbed doses at these activities, the fraction of cells traversed by a-particles would be considerably lower than the nonhit population.…”

“…A previous study has demonstrated distinct transcriptional gene expression profiles in the thyroid of female BALB/c nude mice at 24 h after the intravenous administration of 211 At and showed dose-dependent gene regulation responses (17). Gene regulation responses in vivo are a complex topic, because the responses are affected by signals on the organizational level of cells, tissues, organs, and the organism as a whole.…”