Absorption of dietary iodide, presumably in the small intestine, is the first step in iodide (I(-)) utilization. From the bloodstream, I(-) is actively taken up via the Na(+)/I(-) symporter (NIS) in the thyroid for thyroid hormone biosynthesis and in such other tissues as lactating breast, which supplies I(-) to the newborn in the milk. The molecular basis for intestinal I(-) absorption is unknown. We sought to determine whether I(-) is actively accumulated by enterocytes and, if so, whether this process is mediated by NIS and regulated by I(-) itself. NIS expression was localized exclusively at the apical surface of rat and mouse enterocytes. In vivo intestine-to-blood transport of pertechnetate, a NIS substrate, was sensitive to the NIS inhibitor perchlorate. Brush border membrane vesicles accumulated I(-) in a sodium-dependent, perchlorate-sensitive manner with kinetic parameters similar to those of thyroid cells. NIS was expressed in intestinal epithelial cell line 6, and I(-) uptake in these cells was also kinetically similar to that in thyrocytes. I(-) downregulated NIS protein expression and its own NIS-mediated transport both in vitro and in vivo. We conclude that NIS is functionally expressed on the apical surface of enterocytes, where it mediates active I(-) accumulation. Therefore, NIS is a significant and possibly central component of the I(-) absorption system in the small intestine, a system of key importance for thyroid hormone biosynthesis and thus systemic intermediary metabolism.

I − uptake in the thyroid, the first step in thyroid hormone biosynthesis, is mediated by the Na þ ∕I − symporter (NIS) with an electrogenic 2Na þ : 1I − stoichiometry. We have obtained mechanistic information on NIS by characterizing the congenital I − transport defect-causing NIS mutant G93R. This mutant is targeted to the plasma membrane but is inactive. Substitutions at position 93 show that the longer the side chain of the neutral residue at this position, the higher the K m for the anion substrates. Unlike WT NIS, which mediates symport of Na þ and the environmental pollutant perchlorate electroneutrally, G93T/N/Q/E/D NIS, strikingly, do it electrogenically with a 2∶1 stoichiometry. Furthermore, G93E/Q NIS discriminate between anion substrates, a discovery with potential clinical relevance. A 3D homology model of NIS based on the structure of the bacterial Na þ ∕galactose transporter identifies G93 as a critical player in the mechanism of the transporter: the changes from an outwardly to an inwardly open conformation during the transport cycle use G93 as a pivot.iodide transport defect | homology modeling | radioiodide therapy | sodium solute cotransporter family

Despite considerable progress in our understanding of the interplay between immune and endocrine systems, the role of thyroid hormones and their receptors in the control of adaptive immunity is still uncertain. Here, we investigated the role of thyroid hormone receptor (TR) ␤ 1 signaling in modulating dendritic cell (DC) physiology and the intracellular mechanisms underlying these immunoregulatory effects. Exposure of DCs to triiodothyronine (T 3 ) resulted in a rapid and sustained increase in Akt phosphorylation independently of phosphatidylinositol 3-kinase activation, which was essential for supporting T 3 -induced DC maturation and interleukin (IL)-12 production. This effect was dependent on intact TR␤ 1 signaling as small interfering RNA-mediated silencing of TR␤ 1 expression prevented T 3 -induced DC maturation and IL-12 secretion as well as Akt activation and IB-⑀ degradation. In turn, T 3 up-regulated TR␤ 1 expression through mechanisms involving NF-B, suggesting an autocrine regulatory loop to control hormone-dependent TR␤ 1 signaling. These findings were confirmed by chromatin immunoprecipitation analysis, which disclosed a new functional NF-B consensus site in the promoter region of the TRB1 gene. Thus, a T 3 -induced NF-B-dependent mechanism controls TR␤ 1 expression, which in turn signals DCs to promote maturation and function via an Akt-dependent but PI3K-independent pathway. These results underscore a novel unrecognized target that regulates DC maturation and function with critical implications in immunopathology at the crossroads of the immune-endocrine circuits.The endocrine and immune systems are interconnected via a bidirectional network in which hormones affect immune function, and, in turn, immune responses are reflected in neuroendocrine changes. This bidirectional communication is possible as both systems share common ligands (hormones and cytokines) and their specific receptors (1). Thyroid hormones (TH) 5 play critical roles in differentiation, growth, and metabolism. The classic genomic actions of TH are mediated by nuclear TH receptors (TR) that act mainly as hormone-inducible transcription factors. Several TR␣ and TR␤ isoforms are encoded by the TRA and TRB genes, respectively. The TR␣ 1 , TR␣ 2 , TR␤ 1 , and TR␤ 3 isoforms are widely expressed, whereas TR␤ 2 is predominantly restricted to the hypothalamus-pituitary axis (2). Recent emerging evidence has also characterized the interactions of TR with co-repressor proteins, namely the nuclear co-repressor and the silencing mediator of retinoid and TH receptors. These effects involve histone deacetylase activity that mediates TR silencing in the absence of triiodothyronine (T 3 ) and several co-activator proteins that exhibit histone acetylase activity in the presence of this hormone (2). However, the notion of classical or genomic mechanisms as unique actions mediated by TRs has been challenged in the past decade by descriptions of TH actions that involve extranuclear (nongenomic) effects in a variety of cell types. These TH-dependent ...

Iodide (I(-)) is an irreplaceable constituent of thyroid hormones and an important regulator of thyroid function, because high concentrations of I(-) down-regulate sodium/iodide symporter (NIS) expression and function. In thyrocytes, activation of phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) cascade also inhibits NIS expression and function. Because I(-) excess and PI3K/Akt signaling pathway induce similar inhibitory effects on NIS expression, we aimed to study whether the PI3K/Akt cascade mediates the acute and rapid inhibitory effect of I(-) excess on NIS expression/activity. Here, we reported that the treatment of PCCl3 cells with I(-) excess increased Akt phosphorylation under normal or TSH/insulin-starving conditions. I(-) stimulated Akt phosphorylation in a PI3K-dependent manner, because the use of PI3K inhibitors (wortmannin or 2-(4-Morpholinyl)-8-phenyl-4H-1-benzopyran-4-one) abrogated the induction of I(-) effect. Moreover, I(-) inhibitory effect on NIS expression and function were abolished when the cells were previously treated with specific inhibitors of PI3K or Akt (Akt1/2 kinase inhibitor). Importantly, we also found that the effect of I(-) on NIS expression involved the generation of reactive oxygen species (ROS). Using the fluorogenic probes dihydroethidium and mitochondrial superoxide indicator (MitoSOX Red), we observed that I(-) excess increased ROS production in thyrocytes and determined that mitochondria were the source of anion superoxide. Furthermore, the ROS scavengers N-acetyl cysteine and 2-phenyl-1,2-benzisoselenazol-3-(2H)-one blocked the effect of I(-) on Akt phosphorylation. Overall, our data demonstrated the involvement of the PI3K/Akt signaling pathway as a novel mediator of the I(-)-induced thyroid autoregulation, linking the role of thyroid oxidative state to the Wolff-Chaikoff effect.