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.

Citation Format: Alice Fletcher, Martin L. Read, Vikki L. Poole, Vicki E. Smith, Christopher J. McCabe. 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 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 920.