We investigated the mechanism of thyroid hormone regulation of osteocalcin (OC) gene expression in osteoblast-like cells (ROS 17/2.8). Treatment with tri-iodothyronine (T3) (10(-8) M) increased OC mRNA levels by approximately 3-fold after 24 h and reached a maximum, approximately 5.4-fold, after 48 h. The mRNA levels of other bone-specific genes, alkaline phosphatase and osteopontin, were not affected by T3 treatment. Interestingly, T3 induction of OC mRNA varied according to cell density: approximately 4-fold at approximately 1x10(5) cells/dish and 1.5-fold at 40-60x10(5) cells/dish. The magnitude of OC mRNA induction by T3 was approximately 40% lower than induction by 1,25 dihydroxyvitamin D3 (1,25D3) alone, and the combination of T3+1,25D3 did not further stimulate OC mRNA levels. T3 induction of OC mRNA was not affected by treatment with cycloheximide (10 microg/ml) for 5 h indicating that new protein synthesis is not required for the response. To study the half-life of OC mRNA, ROS 17/2.8 cells were incubated with actinomycin D. The basal half-life of OC mRNA (means+/-s.e.m.) was 6.4+/-0.2 h which was increased significantly with either T3 or 1,25D3 treatment to 10.9+/-0.6 h and 13.5+/-0.4 h respectively. T3 modestly up-regulated the rate of OC gene transcription (1.7+/-0.2-fold) as determined by run-off assay. T3 did not induce a reporter construct containing the rat OC gene (rOC) 5'-flanking region (to -1750 bp) or the previously described rOC vitamin D response element, when transfected into ROS 17/2.8 cells. In conclusion, T3 up-regulates the OC mRNA expression in ROS 17/2.8 cells in a dose-, time- and cell confluence-dependent fashion, and does so by transcriptional and post-transcriptional mechanisms. The greater T3 induction of OC expression in ROS 17/2.8 cells at low cell density is consistent with findings of thyroid hormone action on bone development.

This article has been cited by other articles:

				F. Goulart da Silva, G. Giannocco, M. F. Santos, and M. T. Nunes Thyroid Hormone Induction of Actin Polymerization in Somatotrophs of Hypothyroid Rats: Potential Repercussions in Growth Hormone Synthesis and Secretion Endocrinology, December 1, 2006; 147(12): 5777 - 5785. [Abstract] [Full Text] [PDF]

				F. R. S. Freitas, A. S. Moriscot, V. Jorgetti, A. G. Soares, M. Passarelli, T. S. Scanlan, G. A. Brent, A. C. Bianco, and C. H. A. Gouveia Spared bone mass in rats treated with thyroid hormone receptor TR{beta}-selective compound GC-1 Am J Physiol Endocrinol Metab, November 1, 2003; 285(5): E1135 - E1141. [Abstract] [Full Text] [PDF]

				D. A. Stevens, C. B. Harvey, A. J. Scott, P. J. O'Shea, J. C. Barnard, A. J. Williams, G. Brady, J. Samarut, O. Chassande, and G. R. Williams Thyroid Hormone Activates Fibroblast Growth Factor Receptor-1 in Bone Mol. Endocrinol., September 1, 2003; 17(9): 1751 - 1766. [Abstract] [Full Text] [PDF]

				P. J. O'Shea, C. B. Harvey, H. Suzuki, M. Kaneshige, K. Kaneshige, S.-Y. Cheng, and G. R. Williams A Thyrotoxic Skeletal Phenotype of Advanced Bone Formation in Mice with Resistance to Thyroid Hormone Mol. Endocrinol., July 1, 2003; 17(7): 1410 - 1424. [Abstract] [Full Text] [PDF]

				M. J. Jeon, J. A. Kim, S. H. Kwon, S. W. Kim, K. S. Park, S.-W. Park, S. Y. Kim, and C. S. Shin Activation of Peroxisome Proliferator-activated Receptor-{gamma} Inhibits the Runx2-mediated Transcription of Osteocalcin in Osteoblasts J. Biol. Chem., June 20, 2003; 278(26): 23270 - 23277. [Abstract] [Full Text] [PDF]