SFEBES2022 Oral Communications Thyroid (6 abstracts)
Enhancing radioiodide uptake by addressing the mechanism of sodium/iodide symporter (NIS) endocytosis
Ling Zha 1 , Katie Brookes 1 , Caitlin Thornton 1 , Alice Fletcher 1 , Jana Kim 2 , Kavitha Sunassee 2 , Philip J Blower 2 , Hannah R Nieto 1 , Vicki E Smith 1 , Martin L Read 1 & Christopher J McCabe 1
1Institute of Metabolism and Systems Research, University of Birmingham, Birmingham, United Kingdom; 2School of Biomedical Engineering & Imaging Sciences, King’s College London, London, United Kingdom
Background: The sodium/iodide symporter (NIS) frequently shows diminished targeting to the plasma membrane (PM) in differentiated thyroid cancer, resulting in suboptimal radioiodine treatment and poor prognosis. However, the mechanisms which govern the endocytosis of NIS away from the PM – its sole site of transport activity – are ill-defined, and may be of direct therapeutic potential. We previously showed that the proto-oncogene PBF binds NIS and enhances its internalisation, hypothesising that this was via the heterotetramer Adaptor Protein 2 (AP2). We now challenge this hypothesis experimentally.
Methods: AP2 subunits α1, μ2, and σ2 were ablated via siRNA. NanoBiT assays were used to assess the stringency of protein interactions. Technetium-99m pertechnetate (99mTc) uptake was used to evaluate NIS function in vivo.
Results: Acidic dipeptides are known to bind AP2σ2. We identified a putative acidic dipeptide within the NIS C-terminus; abrogation of this (E578A/E579A) significantly increased 125I uptake and retention of NIS at the PM as determined by immunofluorescent microscopy. Exogenous AP2σ2 was confirmed to bind NIS in NanoBiT assays with mutation of σ2 (R15H) increasing NIS binding affinity. Importantly, ablation of AP2α1 and μ2 significantly increased radioiodide uptake and NIS protein expression. NanoBiT assays showed that AP2α1 and μ2 ablation also increased NIS:PBF binding, whereas σ2 did not. The drug chloroquine induced radioiodide uptake in vitro (8hr), independently of its canonical influence on autophagy, which was blocked by AP2 ablation suggesting an impact on NIS endocytosis. In vivo, chloroquine treatment of Balb/c mice significantly enhanced thyroidal uptake of 99mTc, in combination with the HDACi SAHA.
Conclusion: We propose that NIS internalisation is modulated by the interaction of a C-terminal diacidic motif with the AP2σ2, and that the endocytosis motif of PBF is critical to this. Given our mouse data, the internalisation of NIS may now be druggable in vivo.
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Endocrine Abstracts
ISSN 1470-3947 (print) | ISSN 1479-6848 (online)
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