Effect of iodide on nicotinamide adenine dinucleotide phosphate oxidase activity and Duox2 protein expression in isolated porcine thyroid follicles

Identification of a novel partner of duox: EFP1, a thioredoxin-related protein

H(2)O(2) is a crucial substrate of thyroproxidase (TPO) to iodinate thyroglobulin and synthesize thyroid hormones in thyroid. ThOX proteins (thyroid oxidase) also called Duox are believed to be responsible for H(2)O(2) generation. Duoxs expressed in transfected cells do not generate an active system, nor permit their membrane localization suggesting that other proteins are required to fulfill these functions. In this study, we demonstrate interactions of Duoxs with TPO and with p22(phox) without any effect on Duox activity. By yeast two-hybrid method using EF-hand fragment of dog Duox1 as the bait we have isolated EFP1 (EF-hand binding protein 1), one partner of Duoxs that belongs to the thioredoxin-related protein family. EFP1 shares moderate similarities with other members of thioredoxin-related proteins, but the characteristic active site and the folding structures are well conserved. EFP1 can be co-immunoprecipitated with Duoxs in transfected COS cells as well as in primary cultured human thyrocytes. It interacts also with TPO but not thyroglobulin. Immunofluorescence studies show that EFP1 and Duox proteins are co-localized inside the transfected cells, suggesting that EFP1 is not sufficient to induce either the expression of Duox at the plasma membrane or to permit H(2)O(2) production. EFP1 and Duox mRNA share similar distribution in nine different tissues. These results suggest that EFP1 could be one of the partners in the assembly of the multiprotein complex constituting the thyroid H(2)O(2) generating system but is certainly not sufficient to permit H(2)O(2) generation.

[Enzymes involved in thyroid iodide organification]

Thyroid hormone biosynthesis depends on iodide uptake and its incorporation into the acceptor protein thyroglobulin (Tg), a high molecular weight protein secreted into the follicular lumen. The sodium-iodide symporter (NIS) is responsible for thyroid iodide uptake, the first step in thyroid hormonogenesis. Iodide is subsequently transported through the cellular membrane by pendrin (PDS) and then incorporated into Tg. Iodide oxidation and organification occur mainly in the thyrocyte apical surface and these reactions are catalyzed by thyroperoxidase (TPO) in the presence of hydrogen peroxide. Thus, thyroid iodide organification depends on TPO activity, which is modulated by the concentration of substrates (thyroglobulin and iodide) and cofactor (hydrogen peroxide). Hydrogen peroxide generation is catalyzed by the thyroid NADPH oxidase (ThOx), which is present in the apical pole of thyrocytes, is stimulated by thyrotropin and is inhibited by iodide. Hydrogen peroxide generation is the limiting step in thyroid hormone biosynthesis under iodine sufficiency conditions.

Targeting of the dual oxidase 2 N-terminal region to the plasma membrane

Dual oxidase 2 (Duox2) is a cell surface glycoprotein that probably provides thyroperoxidase with the H2O2 required to catalyze thyroid hormone synthesis. No functional H2O2-generating system has yet been obtained after transfecting Duox2 into non-thyroid cell lines, because it is retained in the endoplasmic reticulum (ER). We investigated the level of maturation of various Duox2 truncated proteins in an attempt to identify the region of Duox2 responsible for its remaining in the ER. Duox2-Q686X mutant, corresponding to the N-terminal ectodomain including the first putative transmembrane domain, was expressed in different cell lines. Carbohydrate content analysis revealed that complex type-specific Golgi apparatus (GA) oligosaccharides were present on pig Duox2-Q686X, whereas human truncated Duox2 carried only high mannose-type sugar chains characteristic of the ER. Further characterization using surface biotinylation and flow cytometry assays indicated that pig Duox2-Q686X was present at the plasma membrane, whereas human Duox2-Q686X remained inside the cell. The replacement of the last 90 residues of the human Duox2-Q686X with the pig equivalent region allowed the chimerical peptide to reach the Golgi apparatus. Pig mutants containing the complete first intracellular loop with or without the second transmembrane domain accumulated in the ER. These findings show that 1) the human Duox2-Q686X region encompassing residues 596-685 prevents mutant exportation from the ER and 2) there is a pig Duox2 retention domain in the first intracellular loop. In addition, missense mutations of four cysteines (Cys-351, -370, -568, or -582) completely inhibited the emergence of pig Duox2-Q686X from the ER compartment, indicating their importance in Duox2 maturation.