Abstract
Uptake and accumulation of iodide is the most prominent feature of the thyroid gland. The function is regulated by both positive and negative signals acting on the NIS promoter, TSH being the strongest enhancer. Recent in vitro findings indicate that 131I-induced thyroid stunning is due to a down-regulation of the sodium/iodide (NIS) symporter. Here we investigated whether this effect of ionizing radiation is related to DNA damage and can be modified by DNA repair.
Methods: Growth-arrested pig thyroid cell monolayers cultured in bicameral chambers were exposed to the DNA double-strand break inducing drug, calicheamicin (0.02 – 1 nM) with or without DNA-PK inhibitor NU7026 (1-10 ?M), for 1 h, and thereafter stimulated with TSH (1 mU/ml) for 48 h (+/- NU7026). DNA damage was verified by detection of phosphorylated H2AX -H2AX) with Western blotting and immunofluorescense. pNIS mRNA was quantified?(with real-time RT-PCR and transepithelial iodide transport was monitored with 125I-.
Results and Conclusion: Calicheamicin inhibited TSH-stimulated iodide transport with 80-90%, which correlated with a nearly 40-fold decrease of the NIS transcript levels. This response was enhanced by co-incubation with NU7026. Interestingly, NU7026 alone reduced the iodide transport by up to 50% after exposure for 48 h, suggesting that ongoing surveillance of genomic integrity supports thyroid function. All drug effects were recognised in cells that maintained TSH-stimulated electrolyte transport signified by an unchanged ability of cyclic AMP to generate a high transepithelial potential difference. Thus, down-regulation of NIS in response to DNA damage is not due to disruption of the common TSH receptor signalling pathway. Altogether, these findings indicate that the iodide-concentrating mechanism in the thyroid is immensely sensitive to genotoxic stress.