THE REGULATORY ROLE OF THYROID-HORMONES IN BONE CELL-GROWTH AND DIFFERENTIATION

The clinical effects of thyroid hormones on bone in hypo- and hyperthy roidism are well known but their fundamental role in the regulation of bone remodeling is still poorly understood. In this review the curren t literature is summarized and experimental data from our laboratory a re presented. The direct stimulation of bone resorption by thyroid hor mones in organ culture, which in part is mediated by prostaglandins an d TGF-beta, and the effect of different agents thereon are reviewed. M ore recent data concerning thyroid hormone action in the osteoblastic cell line MC3T3E1, are summarized. From their effect on proliferation and alkaline phosphatase activity, we conclude that thyroid hormones a ccelerate osteoblastic differentiation. The regulation of the transcri ptional expression of certain genes by nuclear T3 receptors and their effect on osteoblastic target genes like IGF-I are reviewed. In additi on a novel role of triiodothyronine as inhibitor of growth factor indu ced transcriptional expression of regulatory genes (c-fos, c-jun) is s uggested.