Effects of Thyroxine (T4), 3,5,3'-triiodo-L-thyronine (T3) and their Metabolites on Osteoblast Differentiation

Studies involving human genetic mutations and mutant mouse models have provided irrevocable evidence for a key role for thyroid hormones (THs) in the regulation of skeletal growth. While T3 binds to TH receptors with higher affinity than T4, T4 occupied TH receptors have also been reported in the nucleus under euthyroid conditions raising the possibility that T4 bound nuclear receptors may be biologically relevant in thyroid syndromes with elevated free T4 and reduced T3 levels. We, therefore, evaluated the direct effects of T4, T3, and their metabolites (rT3 and T2) in stimulating osteoblast differentiation using MC3T3-E1 preosteoblasts which do not produce detectable levels of deiodinases. Under serum-free conditions, a 24-h treatment of MC3T3-E1 cells with THs and their metabolites caused a dose-dependent increase in the expression of osteoblast differentiation markers, osterix, and osteocalcin. Circulating concentrations of T3 (similar to 1 ng/ml) and T4 (similar to 30 ng/ml) showed similar potency in stimulating osteoblast differentiation marker expression, while rT3 and T2 were less potent than T3 and T4. Moreover, T3 and T4 treatments elevated the IGF-1 mRNA level suggesting the involvement of IGF-1 signaling in the TH regulation of osteoblast differentiation. We conclude that an elevated T4 level in the absence of T3 may exert stimulatory effects on osteoblast differentiation. The establishment of cell-specific effects of T4 on osteoblasts may provide a strategy to generate T4 mimics that exert skeletal specific effects without the confounding T3 effects on other tissues.

涉及人类基因突变和突变小鼠模型的研究为甲状腺激素（THs）在骨骼生长调节中的关键作用提供了确凿的证据。虽然T3与甲状腺激素受体的亲和力高于T4，但在甲状腺功能正常的情况下，细胞核中也有被T4占据的甲状腺激素受体的报道，这增加了一种可能性，即与T4结合的核受体在游离T4升高且T3水平降低的甲状腺综合征中可能具有生物学相关性。因此，我们使用不产生可检测水平的脱碘酶的MC3T3 - E1前成骨细胞评估了T4、T3及其代谢产物（rT3和T2）在刺激成骨细胞分化方面的直接作用。在无血清条件下，用甲状腺激素及其代谢产物对MC3T3 - E1细胞处理24小时，导致成骨细胞分化标志物（osterix和骨钙素）的表达呈剂量依赖性增加。循环中T3（约1纳克/毫升）和T4（约30纳克/毫升）在刺激成骨细胞分化标志物表达方面显示出相似的效力，而rT3和T2的效力低于T3和T4。此外，T3和T4处理提高了IGF - 1 mRNA水平，表明IGF - 1信号通路参与了甲状腺激素对成骨细胞分化的调节。我们得出结论，在缺乏T3的情况下，升高的T4水平可能对成骨细胞分化产生刺激作用。确定T4对成骨细胞的细胞特异性作用可能提供一种策略，用于开发T4模拟物，使其在不对其他组织产生T3的混杂影响的情况下发挥对骨骼的特异性作用。