Overexpression of a nuclear protein, TIEG, mimics transforming growth factor-β action in human osteoblast cells

Although transforming growth factor-beta (TGF-beta) is a growth factor with many known regulatory activities in many different cell types, its intracellular signaling pathway is still not fully understood. A TGF-beta-inducible early gene (TIEG) was discovered and shown by this laboratory to be a 3-zinc finger transcription factor family member; its expression is rapidly induced in cells treated with TGF-beta. To ascertain whether TIEG plays a major role in the TGF-beta pathway, human osteosarcoma MG-63 cells were stably transfected either with an expression vector containing a TIEG cDNA or with the vector alone. Clones that contain only the vector express normal levels of TIEG mRNA and protein and display the same patterns of gene expression and levels of cell proliferation as the nontransfected, non-TGF-beta-treated parental cells. However, transfected cells that overexpress TIEG mRNA and protein (TIEG-6 and TIEG-7) display changes that mimic those of MG-63 cells treated with TGF-beta, i.e. increased alkaline phosphatase activity, decreased levels of osteocalcin mRNA and protein, and decreased cell proliferation. The degree of these changes correlated with the level of TIEG expressed in the cell lines. TGF-beta treatment of the overexpressed cells showed no added effects. These findings and other published reports support a primary role of TIEG as a transcription factor in the TGF-beta signaling pathway.