Expression of multiple thyroid hormone receptor isoforms in rat femoral and vertebral bone and in bone marrow osteogenic cultures

Thyroid hormones influence both bone formation and bone resorption. Clinica l data and animal studies provide evidence of skeletal site heterogeneity ( hip vs. spine) of bone responses to thyroid hormones. In vitro studies also demonstrate direct effects of thyroid hormones on cells of the osteoblast lineage. Transcriptional regulation by thyroid hormone is mediated by ligan d-dependent transcription factors called thyroid hormone receptors (TRs). T wo genes, c-ErbA alpha and c-ErbA beta, generate at least four TR isoforms in the rat: TR alpha(1), c-erbA alpha(2), TR beta(1), and TR beta(2). Altho ugh functional TRs have been identified in cells of the osteoblast lineage, it is still not known if TR isoform expression in bone differs depending u pon which skeletal site is examined. We have used ribonuclease protection a ssay and Northern blot analysis to simultaneously examine the expression of TR isoform mRNAs in adult rat femoral and vertebral bone. TR alpha(1), c-e rbA alpha(2), and TR beta(1) are expressed in both femur and vertebra whole bone. Bone marrow cells from both skeletal sites were also cultured under conditions whereby the osteoprogenitors differentiated into osteoblasts and formed a mineralized extracellular matrix. TR alpha(1), c-erbA alpha(2), a nd TR beta(1) mRNAs are each expressed in both femoral and vertebral osteob last cultures. The presence of TR alpha(1), c-erbA alpha(2), and beta(1) pr oteins was confirmed by Western analysis of nuclear protein extracts from f emoral and vertebral cell cultures. These results indicate that the three p redominant TR isoforms are highly expressed in bone and osteoblasts from fe murs and vertebrae. Whether there are distinct mechanisms of thyroid hormon e action mediated by TR alpha(1), c-erbA alpha(2), and TR beta(1) at these separate skeletal sites remain to be shown, (C) 1999 Wiley-Liss, Inc.