Sclerotome-related helix-loop-helix type transcription factor (scleraxis) mRNA is expressed in osteoblasts and its level is enhanced by type-β transforming growth factor

Scleraxis is a recently identified transcription factor with a basic helix-loop-helix motif, which is expressed in sclerotome during embryonic development. We have examined the expression of scleraxis mRNA in rat osteoblastic cells and found that the scleraxis gene was expressed as a 1.2 kb mRNA species in osteoblastic osteosarcoma ROS17/2.8 cells. The scleraxis mRNA expression was enhanced by type-β transforming growth factor (TGFβ) treatment. The TGFβ effect was observed in a dose-dependent manner starting at 0.2 ng/ml and saturating at 2 ng/ml. The effect was time-dependent and was first observed within 12 h and peaked at 24 h. The TGFβ effect was blocked by cycloheximide, while no effect on scleraxis mRNA stability was observed. TGFβ treatment enhanced scleraxis-E box (Scx-E) binding activity in the nuclear extracts of ROS17/2.8 cells. Furthermore, TGFβ enhanced transcriptional activity of the CAT constructs which contain the Scx-E box sequence. TGFβ treatment also enhanced scleraxis gene expression in osteoblast-enriched cells derived from primary rat calvaria. These findings indicated for the first time that the novel helix-loop-helix type transcription factor (scleraxis) mRNA is expressed in osteoblasts and its expression is regulated by TGFβ.