SCLEROTOME-RELATED HELIX-LOOP-HELIX TYPE TRANSCRIPTION FACTOR (SCLERAXIS) MESSENGER-RNA IS EXPRESSED IN OSTEOBLASTS AND ITS LEVEL IS ENHANCED BY TYPE-BETA TRANSFORMING GROWTH-FACTOR

Scleraxis is a recently identified transcription factor with a basic h elix-loop-helix motif, which is expressed in sclerotome during embryon ic development. We have examined the expression of scleraxis mRNA in r at 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-beta transforming growth factor (TGF beta) treatment. The TGF beta effect was observed i n 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 TGF beta effect was blocked by cycloheximide, while no effect on scleraxis mRNA stability was observed. TGF beta tre atment enhanced scleraxis-E box (Scx-E) binding activity in the nuclea r extracts of ROS17/2.8 cells. Furthermore, TGF beta enhanced transcri ptional activity of the CAT constructs which contain the Scx-E box seq uence. TGF beta treatment also enhanced scleraxis gene expression in o steoblast-enriched cells derived from primary rat calvaria. These find ings 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 TGF beta.