Radiobiological effects of the thyroid gland - transcriptomic and proteomic responses to 131l and 211At exposure

Abstract

Radionuclides are widely used in medicine. 131I is one of the most employed radionuclides and is administered to patients either bound to tumor targeting molecules or as halide to target the thyroid or thyroid cancer. 211At is proposed for radionuclide therapy and preclinical and clinical research on 211At-labeled tumor targeting molecules is on-going. The thyroid gland accumulates both 131I and 211At as halides and is an organ at risk. Additionally, 131I exposure of thyroid may occur from radioactive fallout from e.g. nuclear accidents. There is a lack of knowledge of molecular mechanisms in thyroid cells after 131I or 211At exposure. The overall aim of this work was to examine the transcriptomic and proteomic effects of 131I and 211At exposure on normal thyroid tissue in vivo. The influence of absorbed dose, dose-rate, time after administration, and radiation quality on gene expression regulation was studied. Another aim was to identify radiation-responsive genes in thyroid. Mice and rats were i.v. injected with 0.064-42 kBq 211At or 9-4700 kBq 131I. Resulting absorbed dose to thyroid from 211At and 131I exposures were 0.023-32 and 0.0058-34 Gy, respectively. Transcriptomic and proteomic responses in thyroid and plasma were measured 1-168 h after administration using RNA microarray and liquid chromatography mass spectrometry, respectively. Fold-change and adjusted p-value cut-offs of 1.5 and 0.01 were used to determine statistically significantly regulated transcripts. Pathway analyses were performed using Gene Ontology and the Ingenuity Pathway Analysis tool (p-value < 0.05). Plasma T4 and TSH levels were measured in rats using ELISA. The transcriptional response in thyroid tissue after 131I and 211At exposure varied with absorbed dose, dose-rate, time after administration, and radiation quality. In mice, 27 recurrently regulated genes were identified after 131I or 211At exposure and genes with similar function shared similar transcriptional regulation patterns. Additionally, regulation of several kallikrein genes was identified in mouse thyroid tissue after 131I or 211At administration. In rats, 2 recurrently genes were identified: Dbp and Slc47a2. Different biological functions were affected in response to different exposure conditions. For example, effects on immune response were found at 1, 6, and 168, but not 24 h after 1.7 kBq 211At administration in mice. An impact on rat thyroid function with regulation of 13 genes crucial for thyroid hormone synthesis was identified. The proteomic response to 32 Gy suggests hypoxia in thyroid and decreased thyroid function. Profound effects on gene expression regulation with distinct differences in response to different exposures were identified in mouse and rat thyroid tissue following 131I or 211At exposure. The transcriptional response likely depends to a varying degree on absorbed dose, dose-rate, time after administration, and radiation quality. Recurrently regulated genes were identified, and the biomarker applicability of these genes should be further assessed.