Endocrine Abstracts

PTTG and PBF as targets for augmenting radioiodine uptake in thyroid cancer

V Smith

University of Birmingham, Birmingham, UK.

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Radioiodine ablation of differentiated thyroid cancers and their metastases utilises the ability of the thyroid to accumulate iodide. Treatment is therefore dependent on the expression and function of the sodium iodide symporter (NIS), which transports iodide into thyroid follicular cells for thyroid hormone biosynthesis. However, in thyroid cancers, NIS expression levels are variable and the avidity of the thyroid for radioiodine can be reduced by transcriptional and post-translational effects on NIS expression. In a subset of thyroid cancers and metastases, radioiodide uptake is insufficient for effective therapy and results in a poor prognosis. Until recently, the mechanisms by which NIS activity is diminished in thyroid cancer have been poorly understood. We have now elucidated mechanisms by which pituitary tumor transforming gene (PTTG) and its interacting partner, PTTG binding factor (PBF), repress iodide uptake through the down-regulation of NIS function.

PTTG and PBF are proto-oncogenes over-expressed in thyroid tumours. Our group has shown that both PTTG and PBF are able to repress NIS mRNA expression and iodide uptake in primary human thyroid cultures. Subsequently, promoter studies in both primary human thyroid cultures and FRTL-5 rat thyroid cells determined that this was a promoter-specific effect. Both PTTG and PBF significantly repressed the activity of the human NIS upstream enhancer (hNUE) located ~9 kb from the gene. More specifically, a region in which a PAX8 and putative USF1 consensus sequence overlap was critical to this repression. Hence, PTTG and PBF inhibit transcriptional regulation of NIS via the hNUE, potentially by interfering in the intrinsic binding of the transcription factors PAX8 or USF1.

More recently we have elucidated a mechanism by which PBF directly modulates NIS activity in vitro. In COS-7 and FRTL-5 subcellular localisation studies, PBF and NIS demonstrated co-localisation within intracellular vesicles. A physical interaction between PBF and NIS proteins was identified in GST pull-down and co-immunoprecipitation experiments. Transient PBF over-expression was associated with an increase in NIS staining within intracellular vesicles and a significant reduction in plasma membrane-associated NIS, an observation quantified through cell surface biotinylation assays. To assess the functional consequence of the NIS-PBF interaction, iodide uptake studies were performed in FRTL-5 cells. PBF significantly repressed iodide uptake whereas three deletion mutants of PBF, which do not co-localise with NIS within intracellular vesicles, were unable to inhibit NIS activity.

In summary, we have demonstrated two discrete mechanisms by which PTTG and PBF can down-regulate iodide uptake. Both proto-oncogenes can transcriptionally repress NIS expression via its upstream enhancer. Additionally, we have elucidated an entirely novel mechanism by which PBF binds NIS and alters its subcellular localisation, thereby regulating its ability to uptake iodide. We have now developed transgenic mouse models with thyroid-specific PTTG and PBF overexpression, allowing us to test these observations in mouse models of thyroid cancer. Overall, we intend to enhance ablative treatment for radioiodine-refractory thyroid cancers and metastases, and thereby improve the otherwise poor prognosis for these tumours.

Note: The above abstract does not appear in the print version of the abstract book.

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Endocrine Abstracts (2010) 21 YEP1