Endocrine Abstracts

Thyroid hormone (T3) is an important regulator of skeletal development and bone turnover. Thyrotoxicosis is associated with accelerated growth, advanced bone age and osteoporosis. T3-action in bone is mediated mainly by T3-receptors (TRs). TRβ^PV mice harbour a dominant-negative mutation in the TRβ gene that impairs negative feedback control of the hypothalamic-pituitary-thyroid axis resulting in elevated thyroid hormone and TSH concentrations. TRβ^PV mice consequently exhibit advanced endochondral and intramembranous ossification with persistent short stature that results from skeletal thyrotoxicosis. To investigate mechanisms underlying advanced bone formation in TRβ^PV mice, primary osteoblast RNA was obtained from wild-type and homozygous TRβ^PV/PV littermate mice and expression profiles of 24,000 genes were determined in cDNA microarray studies. Of a subset of 192 differentially expressed genes, 85 were up-regulated in TRβ^PV/PV mice whereas 107 were down-regulated. Functional analysis software was used to cluster differentially expressed genes into groups with similar molecular functions. Up-regulation of cyclin D1 and frizzled-7 in TRβ^PV/PV osteoblasts implicated altered Wnt/β-catenin signalling in the TRβ^PV phenotype. Gain-of-function mutations of the Wnt-activated LRP5/frizzled co-receptor cause high bone density, whereas loss-of-function mutations cause osteoporosis. Thus, real-time PCR array analysis was employed to characterise expression profiles of the 84 genes involved in mediating Wnt-activated signal transduction pathways. These studies revealed generalised down-regulation of the Wnt pathway in TRβ^PV/PV osteoblasts, and the findings were validated by independent quantitative RT-PCR studies in primary osteoblasts. In further experiments, murine osteoblastic MC3T3 cells were transfected with a β-catenin responsive T-cell factor reporter gene. T3-treatment resulted in reduced β-catenin expression in Western blots and reduced reporter gene activity, indicating that T3 inhibits β-catenin signal transduction in osteoblasts. These data suggest that reduced Wnt/β-catenin signalling contributes to the advanced bone formation observed in TRβ^PV mice and may be implicated in osteoporosis associated with thyrotoxicosis.