Effects of triiodothyronine on DNA synthesis and differentiation markers of normal human osteoblast-like cells in vitro

To study the direct effects of thyroid hormones on human osteoblasts we examined the effects of triiodothyronine (T3) on proliferation and differentiation of human osteoblast-like (hOB) cells in vitro. T3 increased 3H-thymidine incorporation in DNA of hOB cells (p < 0.05, n = 10). Half maximal effects obtained at a T3 concentration of 1-10 nM which lies within the physiological concentration of the hormone. In addition, T3 increased alkaline phosphatase production (p < 0.05, n = 13) and inhibited procollagen type I carboxyterminal propeptide (PICP) production (p < 0.05, n = 13). T3 interaction with 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) was also studied. 1,25-(OH)2D3 (10(-9)M) alone doubled AP production and induced osteocalcin expression by hOB cells. Concurrent addition of T3 and 1,25-(OH)2D3 did not further increase production of AP, PICP or osteocalcin by hOB cells. In conclusion, T3 exerts significant effects on osteoblast proliferation and differentiation, suggesting that human osteoblasts are targets for thyroid hormones.