Thyroid hormone receptor-β1 signaling is critically involved in regulating secondary ossification via promoting transcription of the Ihh gene in the epiphysis

Thyroid hormone (TH) action is mediated through two nuclear TH receptors, THR alpha and THR beta. Although the role of THR alpha is well established in bone, less is known about the relevance of THR beta-mediated signaling in bone development. On ther basis of our recent finding that TH signaling is essential for initiation and formation of secondary ossification center, we evaluated the role of THRs in mediating TH effects on epiphysial bone formation. Two-day treatment of TH-deficient Tshr(-/-) mice with TH increased THR beta 1 mRNA level 3.4-fold at day 7 but had no effect on THR alpha 1 mRNA level at the proximal tibia epiphysis. Treatment of serum-free cultures of tibias from 3-day-old mice with T-3 increased THR beta 1 expression 2.1- and 13-fold, respectively, at 24 and 72 h. Ten-day treatment of Tshr(-/-) newborns (days 5-14) with THR beta 1 agonist GC1 at 0.2 or 2.0 mu g/day increased BV/TV at day 21 by 225 and 263%, respectively, compared with vehicle treatment. Two-day treatment with GC1 (0.2 mu g/day) increased expression levels of Indian hedgehog (Ihh) 100-fold, osterix 15-fold, and osteocalcin 59-fold compared with vehicle at day 7 in the proximal tibia epiphysis. Gel mobility shift assay demonstrated that a putative TH response element in the distal promoter of mouse Ihh gene interacted with THR beta 1. GC1 treatment (1 nM) increased Ihh distal promoter activity 20-fold after 48 h in chondroctyes. Our data suggest a novel role for THR beta 1 in secondary ossification at the epiphysis that involves transcriptional upregulation of Ihh gene.