p97/VCP: A novel interactor of the sodium iodide symporter, which can be pharmacologically targeted to increase radioiodine uptake in thyroid and breast cancer cells

By exploiting the canonical function of the sodium iodide symporter (NIS), ablative radioiodine therapy is an effective treatment for papillary thyroid cancer (PTC). Unfortunately, ~25% of PTC patients are unable to accumulate therapeutically sufficient radioiodine due to diminished expression and/or altered plasma membrane localization of NIS. Patients with radioiodine-refractory PTC have reduced mean survival times. Radioiodine therapy has been proposed as a viable treatment for breast cancer but is hampered by low levels of membranous NIS localization. Currently, however, the regulation of NIS membrane localization remains ill-defined. Mass spectrometry identified the protein p97/VCP as a novel NIS interactor, which was validated through co-immunoprecipitation and proximity ligation assays. p97 siRNA depletion increased NIS-mediated radioiodine uptake by 97% and 141% in the lentivirally-expressing NIS MDA-MB-231 breast and TPC1 thyroid cancer cell lines respectively (p<0.05), and significantly boosted endogenous NIS function in human primary thyrocytes (p<0.05). Conversely, p97 overexpression significantly repressed NIS function in all cell systems, accompanied by lowered membranous NIS localization (p<0.05), as quantified via cell surface biotinylation assays. We next identified five different allosteric p97 inhibitors - Eeyarestatin-1, NMS-873, Astemizole, Clotrimazole and Ebastine - which were all able to overcome p97 inhibition of NIS function in TPC1 and MDA-MB-231 cell lines lentivirally-expressing NIS, significantly increasing NIS function by at least 100%. TCGA analyses of matched PTCs revealed p97 mRNA expression is highly upregulated in PTC compared to matched normal thyroid (n=58, p<0.05), providing a putative explanation for repressed NIS function. In a wider cohort of 413 PTC patients, high tumoral p97 expression was associated with a worse disease-free survival when compared to low p97 expression (p<0.01, hazard ratio (HR): 2.924 (95% CI, 1.280-6.683)). Strikingly, in patients that did not receive radioiodine, high tumoral p97 expression had no significant effect on disease-free survival (n=136, p=NS, HR: 0.020 (95% CI, 0.000013-31.192)), whereas in patients who received radioiodine, high tumoral p97 expression resulted in a markedly worse disease-free survival (n=194, p<0.01, HR: 6.043 (95% CI, 1.749-20.879)). This is indicative that high p97 expression correlates with a worse response to radioiodine therapy. Our data therefore highlight a new pathway of NIS regulation. Critically, two of these p97 inhibitors are already FDA-approved, highlighting a novel potential therapeutic strategy for enhancing radioiodine uptake in patients with radioiodine-refractory PTC and increasing the feasibility of radioiodine therapy in breast cancer via the transient inhibition of p97 activity.