Fam20C stimulates de novo triiodothyronine (T3) formation within thyroglobulin

CITTERIO, CINTIA E. ; TORRES, MAURICIO; ARVAN, PETER

Washington DC

Congreso; 88th Annual Meeting of the American Thyroid Association; 2018

American Thyroid Association

Introduction: De novo T3 hormonogenesis requires iodination of Tyr residues and coupling of monoiodo- and diiodo-Tyr within Thyroglobulin (TG) - a 330kDa secretory glycoprotein comprising 4 structural regions including the C terminal Cholinesterase-Like Domain (ChEL D). Hyperactivation of thyroidal TSH receptors (TSHRs) favor increased de novo T3 formation in TG which is believed secondary to other TG post-translational modifications such as its phosphorylation. Upon TSHR hyperactivation,Fam20C (a secretory serine kinase) is upregulated, accompanied by increased T3 formation. Conversely, suppressionof Fam20C decreases de novo T3 synthesis. Interestingly, a phospho-Ser present in hTG occurs at a canonicalFam20C phosphorylation site (equivalent to position S2718 in mTG).Methods / Case Presentation: We hypothesize that Fam20C-mediated S2718 phosphorylation triggers increased T3formation in mTG. To analyze the presence of phosphate in the ChEL D, we utilized calf intestinal phosphatasefollowed by immunoblotting with anti-TG Ab. To study the effects on T3 formation upon disruption of mTG-S2718 and the restitution of the mTG 2718 phospho-environment we bioengineered mTG-S2718A and mTG-S2718E,respectively. We co-expressed the TGs +/- Fam20C, +/- Fam20C inhibitor FL-1607, performed enzymatic iodinationsof the secreted TGs, and monitored de novo T3 formation by immunoblotting with mAb anti-T3 and anti-TG Ab. Results / Discussion: Dephosphorylation of the isolated ChEL D shifts its SDS-PAGE migration and, as we reported, dephosphorylation decreases de novo T3 formation in TG. Co-expression of WT mTG and Fam20C promoted T3formation in TG and this effect was blocked by Fam20C inhibitor FL-1607. Further, disruption of the established phosphorylation site mTG-S2718A blocked Fam20C-stimulated T3 formation, and this effect was reverted in the phosphomimetic mutant mTG-S2718E.Conclusions: Our data supports the hypothesis that de novo T3 formation in TG is stimulated by Fam20C-mediatedphosphorylation of mTG-S2718.Since Fam20C levels are increased in states of TSHR hyperactivation, Fam20C-induced de novo T3 formation maybe a contributor to the increased T3 found in hyperstimulated thyroid glands such as occurs in autoimmune hyperthyroidism of Graves disease.