AP-2 and moesin regulate the internalisation of the sodium-iodide symporter and affect I¹²⁵ Uptake in thyroid cancer cells

Dysregulation of sodium-iodide symporter (NIS) function is common in differentiated thyroid cancer, resulting in sub-optimal radioiodide therapy and poorer clinical outcome. Recent developments in identifying proteins that regulate the function of the sodium iodide symporter have highlighted two proteins involved in internalisation of NIS from the plasma membrane: AP-2 and moesin. Clathrin-mediated endocytosis (CME) of NIS is facilitated through the adaptor protein 2 (AP2) complex which selectively sorts membrane proteins for recycling or fusion with early endosomes. Moesin has an established role in regulating CME by bridging integral membrane proteins with actin filaments to facilitate the cytoskeletal rearrangements necessary for internalising proteins. Our studies have shown that AP2°1 gene knockdown effectively inhibits CME causing NIS retention at the plasma membrane and a significant increase in ¹²⁵I uptake (3.4-fold; P < 0.001). Blocking dynamin-mediated scission via the GTPase inhibitor Dynasore (100uM) resulted in a significant increase in ¹²⁵I uptake (2.34 to 2.89- fold; P < 0.001) in TPC1-NIS and 8505C-NIS cells and significantly increased NIS protein expression. Additionally, we investigated the ability of moesin to interact with NIS, and assessed potential functional consequences. NanoBiT protein-protein interaction assays confirmed a stringent interaction between moesin and NIS in HeLa cells (P < 0.05 compared to controls. Critically, a functional role for moesin in regulating NIS was indicated as depletion of moesin significantly increased I¹²⁵ uptake in NIS-expressing thyroid cell lines (TPC-1-NIS cells: 2.01-fold, P < 0.05; 8505C-NIS cells: 1.74-fold, P < 0.001). These studies have further outlined the processes that regulate the internalisation of NIS which can be dysregulated in differentiated thyroid cancer and may contribute to a radioiodide refractory tumour phenotype.