Exploiting endocytic factors as druggable targets to enhance sodium iodide symporter activity with clinical implications for radioiodide therapy

Objective: Suboptimal radioiodide (RAI) treatment is frequently associated with diminished targeting and retention of the sodium iodide symporter (NIS) at the plasma membrane (PM). The mechanisms which govern the endocytosis of NIS away from the PM are however ill‐defined and may have therapeutic potential. We recently demonstrated that NIS internalisation was modulated by the interaction of a C‐terminal diacidic motif with the heterotetramer Adaptor Protein 2 (AP2) – a key regulatory factor in endocytosis. Here, we determined whether NIS endocytosis represents a druggable process to enhance RAI uptake.

Methods: PM localisation of NIS was quantified via NanoBRET and cell surface biotinylation assays (CSBA). RAI (¹²⁵I) uptake assays were used to monitor NIS function in vitro. Intravenous technetium‐m pertechnetate (^99mTc) uptake was used to evaluate NIS function in wild‐type BALB/c mice.

Results: The drug chloroquine (CQ) rapidly increased ¹²⁵I uptake in TPC1‐NIS (2.54‐fold; P  < 0.0001) and 8505C‐NIS (1.93‐fold; P  < 0.05) cells peaking after 8 hr, which was abrogated by co‐treatment with the endocytosis inhibitor Dynasore. Subsequent CSBA confirmed elevated levels of cell‐surface NIS in CQ‐treated thyroid cancer cells. This finding was supported in live CQ‐treated cells via KRAS‐NanoBRET assays, where CQ gave a strong BRET signal similar to Dynasore, suggesting that NIS was retained at the PM. To challenge this, we ablated PICALM, an endocytic factor known to recruit AP2/clathrin to the PM which prevented significant induction of RAI uptake by CQ. In vivo, CQ treatment of BALB/c mice significantly enhanced thyroidal ^99mTc‐uptake in combination with the HDACi SAHA (52.7%; n = 10; P = 0.0003), as well as increasing thyroidal expression of NIS (2.2‐fold; P  < 0.0001), TSHR (1.9‐fold; P = 0.001) and PAX8 mRNA (1.6‐fold; P = 0.003).

Conclusion: Our findings suggest that CQ interferes with the PICALM/AP2/clathrin machinery which controls NIS endocytosis, identifying it as an FDA‐approved pharmaceutical agent which alters NIS endocytosis, with translatable potential to improve radioiodide therapy.