Thyroid hormone stimulation of osteocalcin gene expression in ROS 17/2.8 cells is mediated by transcriptional and post-transcriptional mechanisms

Abstract

We investigated the mechanism of thyroid hormone regulation of osteocalcin (OC) gene expression in osteoblast-like cells (ROS 17/2.8). Treatment with triiodothyronine (T3) (10-8 M) increased OC mRNA levels by ∼3-fold after 24 h and reached a maximum, ∼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: ∼4-fold at ∼1 × 105 cells/dish and 1.5-fold at 40-60 × 105 cells/dish. The magnitude of OC mRNA induction by T3 was ∼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 μg/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.