Pharmacological enhancement of radioiodine uptake through Src kinase inhibition

In thyroid cancer, a reduction in sodium iodide symporter (NIS) expression at the basolateral plasma membrane (PM) of thyrocytes decreases the efficacy of radioiodine imaging, ablative therapy and treatment of metastases. NIS overexpression in breast cancer has resulted in radioiodine being widely proposed as a novel therapeutic strategy. However, uptake is insufficient for tumour destruction. Augmenting NIS PM localisation represents an important therapeutic strategy for increasing radioiodine delivery in both tumour types. We previously described a mechanism by which NIS is internalised by pituitary tumor-transforming gene-binding factor (PBF) in thyroid cells, significantly reducing radioiodine uptake. PBF phosphorylation at Y174 by Src kinase mediates NIS repression, which can be rescued by the Src family kinase (SFK) inhibitor PP1. We have now replicated these findings in breast cancer cells, further elucidated the mechanism of repression and identified a more potent inhibitor of PBF-pY174. In MCF-7 and MDA-MB-231 breast cancer cells PBF significantly repressed radioiodine uptake and this was reversible with PP1 treatment. Mutation of a predicted Src consensus sequence (EEN170-172AAA) abrogated pY174 and radioiodine uptake repression. PBF-pY174 was most potently inhibited by the SFK inhibitor dasatinib, which restored PBF-mediated radioiodine uptake. In the presence of dasatinib-resistant Src (T341I), dasatinib no longer rescued PBF repression of NIS, indicating that Src specifically mediates PBF phosphorylation. A post-translational modification of Src, myristoylation, inhibits Src plasma membrane localisation. Utilising a new high affinity inhibitor of myristoylation, N-myristoyltransferase inhibitor 3 (NMTi3), radioiodine uptake in MDA-MB-231 cells lentivirally expressing NIS was significantly increased. Interestingly, combined dasatinib and NMTi3 treatment synergistically induced endogenous radioiodine uptake in MCF-7 cells (P<0.01; N=3). Taken together, these data suggest that Src inhibition can effectively enhance radioiodine uptake in multiple tumour types, with implications for improving outcomes in thyroid cancer and making radioiodine a potentially viable new strategy for breast cancer treatment.