Endocrine Abstracts

T3, GH and IGF-1 are essential for skeletal development. GH acts directly on growth plate chondrocytes and also via a local paracrine pathway involving IGF-1, but it is unclear whether T3 stimulates bone formation directly or indirectly. To address this, we determined expression of GH-receptor, IGF-1 and IGF-1 receptor (IGF-1R) by in situ hybridisation and activation of downstream STAT5 and AKT signalling by immunohistochemistry in growth plates from four T3-receptor (TR) mutant mouse strains. Knockout mice that lack TR alpha (TRa0/0) or beta (TRb-/-), and mice with a dominant-negative mutation (PV) targeted to TR alpha (TRa1PV) or beta (TRbPV) were examined. TRa0/0 and TRa1PV mice were euthyroid and displayed growth retardation with delayed ossification, indicating skeletal hypothyroidism. In contrast, TRb-/- and TRbPV had advanced bone formation and skeletal thyrotoxicosis. This phenotype was more severe in TRbPV mice, which have 10-15 fold increased thyroid hormone concentrations, than TRb-/- animals in which circulating hormones were increased 4-fold. GH-receptor expression was markedly reduced in TRa0/0 and TRa1PV growth plates, but was increased in TRb-/- and TRbPV mice, compared to wild-type littermates at 2, 3 and 4 weeks of age. These findings correlated with reduced phospho-STAT5 expression in TRa0/0 and TRa1PV mice, and increased expression in TRbPV mice. Phospho-STAT5 was unchanged in TRb-/- mice, in which systemic hyperthyroidism was less severe. IGF-1R expression was also markedly reduced in TRa0/0 and TRa1PV mice, but increased in TRb-/- and TRbPV animals. These findings correlated with reduced phospho-AKT in TRa0/0 and TRa1PV mice and increased expression in TRbPV mice, but again no change was evident in TRb-/- mice. IGF-1 expression was unaffected in all mutants. Thus, genetic manipulation of skeletal thyroid status interferes with growth plate GH/IGF-1 signalling. These data identify a novel developmental pathway in bone where GH and IGF-1 receptors are downstream of TR alpha.