Abstract LB-327: Identification of novel sodium iodide symporter (NIS) interactors which modulate iodide uptake

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Abstract Ablative radioiodide therapy is an effective treatment for thyroid cancer, but around a quarter of patients do not accumulate therapeutically-sufficient doses of radioiodide due to decreased expression and/or plasma membrane (PM) localization of the sodium iodide symporter (NIS). Putative use of radioiodide therapy in breast cancer is hampered by low levels of NIS membrane localisation. Currently, the regulation of NIS trafficking is ill-defined. Mass spectrometry was performed on proteins co-immunoprecipitating with lentivirally expressed NIS in whole cell and plasma membrane extracts. NIS activity was significantly altered by ADP-ribosylation factor 4 (ARF4) and valosin containing protein (VCP) in TPC1 thyroid and MDA-MB-231 breast cancer cells lentivirally-expressing NIS. ARF4 downregulation markedly decreased radioiodide uptake, and VCP downregulation increased radioiodide uptake in thyroid and breast cells. Transient overexpression of these genes significantly reversed siRNA effects on NIS function. Co-immunoprecipitation assays confirmed NIS interacts with ARF4 and VCP in vitro, and proximity ligation assays revealed the subcellular sites of interaction. Critically, human primary thyroid culture experiments confirmed that ARF4 and VCP regulate endogenous NIS function in vivo. In addressing the mechanisms of NIS trafficking we identified a C-terminal VXPX motif of NIS which is required for Arf4 regulation, whilst TIRF microscopy revealed coincident Arf4 and NIS localization to shared PM vesicles. Pharmacological inhibitor studies identified that the VCP inhibitors Eeyarastatin-1 and NMS-873 overcome VCP inhibition of NIS function. VCP, which is central to ER-associated Degradation (ERAD), selectively targeted a dimeric form of NIS, which we now identify through co-immunoprecipitation, 3D modelling and Förster resonance energy transfer (FRET) assays. TCGA data analysis of 58 matched papillary thyroid cancers revealed that Arf4 was significantly repressed and VCP highly upregulated in thyroid cancer, providing a putative explanation for repressed NIS function. Our data suggest a possible model for NIS trafficking: NIS dimerises in the ER, a process monitored by VCP and the ERAD pathway, with correctly processed and folded NIS being trafficked to the PM via C-terminal interaction with Arf4. We thus now identify two new potential therapeutic targets for enhancing radioiodide uptake in patients with radioiodide-refractory thyroid cancer. Citation Format: Alice Fletcher, Rebecca Thompson, Martin Read, Andrew Turnell, Vicki Smith, Chris J. McCabe. Identification of novel sodium iodide symporter (NIS) interactors which modulate iodide uptake [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr LB-327.