Iodothyronine deiodinase enzyme activities in bone

Abstract

Euthyroid status is essential for normal skeletal development and maintenance of the adult skeleton, but the mechanisms which control supply of thyroid hormone to bone cells are poorly understood. Thyroid hormones enter target cells via monocarboxylate transporter-8 (MCT8), which provides a functional link between thyroid hormone uptake and metabolism in the regulation of T3-action but has not been investigated in bone. Most circulating active thyroid hormone (T3) is derived from outer ring deiodination of thyroxine (T4) mediated by the type 1 deiodinase enzyme (D1). The D2 isozyme regulates intra-cellular T3 supply and determines saturation of the nuclear T3-receptor (TR), whereas a third enzyme (D3) inactivates T4 and T3 to prevent hormone availability and reduce TR-saturation. The aim of this study was to determine whether MCT8 is expressed in the skeleton and whether chondrocytes, osteoblasts and osteoclasts express functional deiodinases. Gene expression was analyzed by RT-PCR and D1, D2 and D3 function by sensitive and highly specific determination of enzyme activities. MCT8 mRNA was expressed in chondrocytes, osteoblasts and osteoclasts at all stages of cell differentiation. D1 activity was undetectable in all cell types, D2 activity was only present in mature osteoblasts whereas D3 activity was evident throughout chondrocyte, osteoblast and osteoclast differentiation in primary cell cultures. These data suggest that T3 availability especially during skeletal development may be limited by D3-mediated catabolism rather than by MCT8 mediated cellular uptake or D2-dependent T3 production.

The type 2 deiodinase Thr92Ala polymorphism is associated with increased bone turnover and decreased femoral neck bone mineral density

“…(26) This notion is supported by a recent study suggesting that D2 may not play a physiologic role in growth plate chondrocytes. (27) The observed effects of the D2 Thr92Ala polymorphism on femoral neck BMD are in line with the importance of local availability of T 3 for bone formation. D2 activity has been found on mature osteoblasts, (34) which are the primary target cells for T 3 regulatory effects on bone formation.…”

“…However, the model we used is unique in the sense that circulating T 3 levels were similar among the three D2 genotypes, allowing us to specifically study the consequences of the polymorphism for local T 3 availability in the bone microenvironment. Williams and colleagues (27) showed no D2 activity in osteoclasts. The effects of the polymorphism on the markers of bone degradation (NTX/creatinine and CTX) therefore may not be explained by direct effects on osteoclasts but are more likely to result from changes in the interaction between osteoblasts and osteoclasts, possibly by alterations in the RANK/ RANKL/OPG signaling pathway, which potentially can be modulated by local T 3 availability in the bone microenvironment.…”

“…We believe that our study provides additional information on the role of D2 in bone metabolism and the functional consequences of the D2 Thr92Ala polymorphism, supporting a role for D2 in mature bone cells. (27) …”

“…(26) This notion was supported in a recent study that demonstrated that D2 activity is restricted to mature osteoblasts, suggesting a possible role for D2 in mature osteoblast function. (27) Devising a study addressing the potential role of deiodinases, including D2, on skeletal metabolism is difficult in humans, but study of the effects of functional D2 polymorphisms on BMD and bone turnover in humans may shed light on this role.…”

American Thyroid Association Guide to Investigating Thyroid Hormone Economy and Action in Rodent and Cell Models

“…Alternative high resolution methods using confocal microscopy can be performed in specialist laboratories (725,737). Gene expression studies can be performed in paraffin-embedded decalcified sections of bone and cartilage (in situ hybridization and IHC), using RNA extracted from tissue samples (mRNA expression and profiling) and in skeletal tissue extracts (protein expression and enzyme activity assays) (726,727,(729)(730)(731)(738)(739)(740). & …”