Local impact of thyroid hormone inactivation

Deiodination is a critical process by which the minimally active thyroxine (T4) molecule is converted into the favorite ligand for thyroid hormone (TH) receptors, triiodothyronine (T3). The iodothyronine deiodinases type 1, 2, and 3 (D1, D2, and D3) constitute a potent mechanism of TH activation (D1 and D2) or inactivation (D3), which functions by tissue specifically regulating TH bioavailability. D2 and D3 are widely expressed and in a dynamically and tightly coordinated fashion, thereby allowing cells to customize their own TH activity. D3, the major T3 and T4 inactivating deiodinase, catalyzes their conversion to 3,3′-diiodothyronine and to reverse T3 respectively. According to common wisdom, D3 plays a major role in lowering serum TH concentrations during development, as supported by the much wider D3 tissue expression in the embryo structures than in the adult tissues. However, several recent studies show that D3 is reexpressed in adult life in various pathophysiological contexts, which strengthens the concept that cell-specific TH inactivation is a critical mediator in cellular TH metabolism. This review focuses on the progress made in understanding the physiological function and significance of D3. It summarizes the intriguing evidence that D3 plays a pivotal role in defining local TH concentration in the developing fetus and in several conditions in adult life.

The Thyroid Hormone-Inactivating Deiodinase Functions as a Homodimer

The type 3 deiodinase (D3) inactivates thyroid hormone action by catalyzing tissue-specific inner ring deiodination, predominantly during embryonic development. D3 has gained much attention as a player in the euthyroid sick syndrome, given its robust reactivation during injury and/or illness. Whereas much of the structure biology of the deiodinases is derived from studies with D2, a dimeric endoplasmic reticulum obligatory activating deiodinase, little is known about the holostructure of the plasma membrane resident D3, the deiodinase capable of thyroid hormone inactivation. Here we used fluorescence resonance energy transfer in live cells to demonstrate that D3 exists as homodimer. While D3 homodimerized in its native state, minor heterodimerization was also observed between D3:D1 and D3:D2 in intact cells, the significance of which remains elusive. Incubation with 0.5–1.2 m urea resulted in loss of D3 homodimerization as assessed by bioluminescence resonance energy transfer and a proportional loss of enzyme activity, to a maximum of approximately 50%. Protein modeling using a D2-based scaffold identified potential dimerization surfaces in the transmembrane and globular domains. Truncation of the transmembrane domain (ΔD3) abrogated dimerization and deiodinase activity except when coexpressed with full-length catalytically inactive deiodinase, thus assembled as ΔD3:D3 dimer; thus the D3 globular domain also exhibits dimerization surfaces. In conclusion, the inactivating deiodinase D3 exists as homo- or heterodimer in living intact cells, a feature that is critical for their catalytic activities.