Identification of novel sodium iodide symporter (NIS) interactors which modulate iodide uptake

By exploiting the canonical function of the sodium iodide symporter (NIS) ablative radioiodine therapy is an effective treatment for thyroid cancer. However, a subset of patients are unable to accumulate sufficient radioiodine for effective treatment due to the dysregulation of NIS, which can occur through decreased expression and/or reduced plasma membrane localisation. Although NIS localisation at the plasma membrane is critical for radioiodine uptake the mechanism of NIS trafficking remains ill-defined. The importance of understanding protein interactomes for cellular trafficking is well-documented. We previously identified the first proven modulator of NIS localisation, pituitary tumor-transforming gene (PTTG) binding factor (PBF). In order to expand the NIS interactome, and subsequently unravel the mechanism of NIS trafficking, we have now used mass spectrometry to identify proteins that bind to NIS specifically at the plasma membrane. Using plasma membrane extracts from a cell line stably-expressing NIS, we have identified a number of novel NIS-interactors. To examine the biological impact on NIS-mediated radioiodine uptake, siRNA knockdown of the six highest ranked NIS-interactors was followed by functional screening. NIS activity was significantly altered by three proteins; ADP-ribosylation factor 4 (ARF4), Rab18 and valosin containing protein (VCP). Each of these proteins have roles in protein trafficking and/or endocytosis. siRNA knockdown of ARF4 decreased NIS-mediated radioiodine uptake by 31%, whereas Rab18 and VCP downregulation increased NIS-mediated radioiodine uptake by 58% and 68% respectively (P<0.01). Critically, through co-immunoprecipitation we have confirmed that both ARF4 and VCP bind NIS in vitro (n=3). Taken together these data highlight a number of novel NIS-interactors that alter NIS-mediated radioiodine uptake. Our studies may thus aid the understanding of NIS plasma membrane localisation, and also provide therapeutic targets which could be manipulated to increase radioiodine uptake in those patients who are radioiodine-refractory.