TG-interacting factor (TGIF) is a transcriptional co-repressor that directly associates with Smad (Sma- and Mad-related protein) proteins and inhibits Smad-mediated transcriptional activation. By using Affymetrix (Santa Clara, CA, U.S.A.) oligonucleotide microarray analysis, we found that TGIF mRNA level was elevated by transforming-growth-factor-β (TGF-β) treatment in a human T-cell line, HuT78. Subsequent reverse-transcription PCR assays indicated that TGF-β1 and activin were able to induce a rapid and transient increase in the level of TGIF in both HuT78 and HepG2 hepatoma cells. To analyse whether or not the regulation of TGIF mRNA occurs at the transcriptional level, a 2.4kb human TGIF promoter was isolated. A primer extension assay was performed to localize the putative transcription initiation site of the promoter. When transiently expressed in HepG2 cells, this promoter was stimulated by TGF-β1 and activin treatment in a time-dependent manner. A series of deletion mutants of the TGIF promoter were also generated to further characterize the TGF-β responsive region of the promoter. In addition, expression of TGIF was able to cause a dose-dependent inhibition of TGF-β and activin signalling. Taken together, these experiments indicated that TGIF is a novel transcriptional target of TGF-β and activin signalling and is likely involved in a negative feedback loop to desensitize TGF-β/activin action.
Abbreviations used: (CA-) ALK, (constitutively active) activin receptor-like kinase; FBS, fetal-bovine serum; G3PDH, glyceraldehyde-3-phosphate dehydrogenase; HEK-293, human embryonic kidney 293 cells; HPE, holoprosencephaly; NIH, National Institutes of Health; RT-PCR, reverse-transcription PCR; SBE, Smad-binding element; Smad, Sma- and Mad-related protein; TGF-β, transforming growth factor-β; TGIF, TG-interacting factor.