Endocrine Abstracts

The thyroid hormones, L-thyroxine (T4) and 3,5,3'-triiodothyronine (T3), are essential for skeletal development and regulate bone mineralisation in the adult. T4 is a pro-hormone that is converted to active T3 by the type I and II 5'-iodothyronine deiodinase enzymes (D1 and D2). T4 and T3 are substrates for the type III 5-deiodinase enzyme (D3) which catalyses the irreversible production of inactive metabolites. The deiodinases are expressed widely in T3-target tissues and their levels of activity determine T3 availability to the T3 receptor, thereby controlling hormone responsiveness. To investigate whether this system is important in endochondral bone formation, we examined whether ATDC5 cells, which undergo hypertrophic chondrocyte differentiation in vitro in response to T3, respond to T4 (100nM) in the absence or presence of propylthiouracil (PTU), a specific D1 inhibitor. T4, like T3, promoted chondrocyte differentiation by increasing secretion of an alcian blue positive matrix and inducing alkaline phosphatase activity earlier than in controls. The T4 response was not inhibited by PTU, suggesting that D1 is not expressed in differentiating chondrocytes and that D2 is responsible for the generation of T3 to facilitate hormone responsiveness. To investigate this hypothesis, we determined which enzymes were expressed in ATDC5 cells and in primary rat tibial growth plate chondrocytes by RT-PCR. D2 and D3 mRNAs were expressed during ATDC5 cell differentiation over a 21 day culture period and were also expressed in primary chondrocytes. In contrast, and correlating with the lack of PTU effect on T4-responsiveness, D1 was not expressed in ATDC5 cells or primary chondrocytes. These data indicate that the D2 and D3 enzymes, which are the most efficient deiodinases (Km in nM range), determine ligand supply to growth plate chondrocytes and may play a key role in the local fine-tuning of thyroid hormone actions on growth.