Introduction: The Na+/I? symporter (NIS), a basolateralplasma membrane glycoprotein, mediates I? accumulation for thyroidhormonogenesis and radioiodide therapy for thyroid cancer. Differentiatedthyroid tumors often presents as cold nodules exhibiting reduced (or evenundetectable) I− accumulation. Paradoxically, most benign andmalignant cold nodules showed normal to increase intracellular NIS expression,suggesting the presence of plasma membrane transport abnormalities. A thoroughcomprehension of the mechanisms that regulate NIS expression at the plasmamembrane would have multiple implications for radioiodide therapy.
Methods: In vitro functional experiments wereperformed in non-polarized or polarized MDCK-II cells stably expressing NISvariants. Structural modellings were performed using crystal structuresdeposited in the Protein Data Bank.
Results: We generated a NIS homology model based on the crystal structure of the benzyl-hydantointransporter to characterize the location of the intracellularly facing NIS carboxy-terminus. We showed that NIS carboxy-terminusis required for the transport of the protein to the plasma membrane. Werevealed that the proximal segment of the carboxy-terminus contains anuncharacterized endoplasmic reticulum export signal, whereas the distal segmentis dispensable for plasma membrane expression. We identified a highly conservedmonoleucine-based sorting motif that determines NIS basolateral expression.Such monoleucine-based sorting motif confers dominant basolateral information. Moreover,in clathrin adaptor protein (AP)-1B-deficient cells, NIS sorting to thebasolateral plasma membrane is compromised, leading to substantial NISmissorting to the apical plasma membrane. Computer simulations providedpreliminary evidence supporting that the AP-1B hemicomplex γ-σ1 recognizes NISmonoleucine-based basolateral sorting motif, thus mediating NIS sorting to thebasolateral plasma membrane.
Conclusions: Althoughthe molecular mechanisms that determine NIS intracellular retention in thyroidcancer remain elusive, our findings uncovering NIS basolateral sorting mightopen new avenues toidentify new molecular targets to treat radioiodide-refractory thyroid cancer.
