Endocrine Abstracts

Although thyroid hormone (T3) and the T3-receptor (TR) alpha gene are essential for skeletal development, T3-signalling pathways in bone have not been characterised. We identified fibroblast growth factor receptor-1 (FGFR1) as a novel T3-inducible gene in osteoblasts by subtraction hybridisation. FGFR1 mRNA was induced 2-3 fold in osteosarcoma cells and primary cultured osteoblasts treated with T3 (0.1-1000nM, 0-48h) and FGFR1 protein was stimulated 2-4 fold over the same period. T3 did not increase FGFR1 gene transcription in nuclear-run on experiments. However, induction of FGFR1 was independent of mRNA half-life and abolished by actinomycin D and cycloheximide, indicating that T3-stimulation of FGFR1 involves transcriptional activation of an intermediary T3-regulated protein. FGF (0.5-5ng/ml, 5-30min) caused a dose-dependent activation of the mitogen-activated protein kinase cascade in ROS 17/2.8 cells and primary osteoblasts. Pre-treatment with T3 (100nM, 6h) induced a more rapid response to FGF that was increased in magnitude by 2-3 fold. These effects were abolished in the presence of either of the specific FGFR1 inhibitors PD166866 and PD161570. Pre-treatment with T3 for 30 minutes failed to enhance FGF signalling, suggesting that T3 acts via its nuclear TR to enhance FGFR1 activity rather than by a possible non-genomic pathway. In situ hybridisation analysis of TR alpha knockout mice revealed that FGFR1 mRNA expression was markedly reduced in osteoblasts, osteocytes and growth plate chondrocytes compared to wild-type littermate controls. TR alpha null mice display skeletal growth retardation with delayed ossification and mineralisation, indicating that FGFR1 lies downstream of T3 in the signalling pathway required for bone development. Activating mutations of FGFR1 cause Pfeiffer's and Crouzon craniosynostosis syndromes in man. Thus, craniosynostosis in juvenile thyrotoxicosis, and other effects of T3 on growth and bone remodelling, may be mediated via FGF signalling pathways.