CHARACTERIZATION OF THYROID-HORMONE (T-3) RECEPTORS IN 3 OSTEOSARCOMA CELL-LINES OF DISTINCT OSTEOBLAST PHENOTYPE - INTERACTIONS AMONG T-3, VITAMIN-D-3, AND RETINOID SIGNALING

T-3 is required for normal skeletal development, but its cellular targets in bone are unknown. T-3 regulates target gene transcription via a specific nuclear receptor (T(3)R), which can heterodimerize with 9-cis-retinoic acid, 1 alpha,25-dihydroxyvitamin D-3, or retinoic acid receptors to modify T-3 responsiveness. Serum-free cultures were developed to investigate hormone interactions in three osteosarcoma cell lines, ROS25/1, UMR106, and ROS17/2.8, that express fibroblast-like, preosteoblast, and mature osteoblast phenotypes. ROS25/1 expressed T(3)R alpha 1, but only low levels of T(3)R beta 1, whereas UMR106 and ROS17/2.8 cells expressed both receptor proteins. All cells expressed c-erb-A alpha 2 protein and equal levels of 1 alpha,25-dihydroxyvitamin D-3 receptor, 9-cis-retinoic acid receptor, and retinoic acid receptor messenger RNAs. Endogenous T(3)R activity and the effects of D-3 and 9-cis-RA on T-3 responsiveness were determined in transfections using reporter genes containing T-3 response elements from rat malic enzyme or alpha-myosin heavy chain genes. Cell-specific T-3 responses were associated with differing patterns of T(3)R gene expression and stages of osteoblast phenotype expression. A change in T(3)R beta 1 gene expression during osteoblast phenotype differentiation may modify T-3 action in developing bone.