Transforming growth factor β suppresses peroxisome proliferator-activated receptor γ expression via both SMAD binding...

Area of Science:

• Molecular Biology
• Cell Signaling
• Fibrosis Research

Background:

• Transforming growth factor β (TGF-β) is crucial for wound healing but drives pathological fibrosis when dysregulated.
• Peroxisome proliferator-activated receptor γ (PPARγ) acts as an anti-fibrotic agent, and its expression is repressed in fibrosis via unknown SMAD-signaling pathways.
• TGF-β and PPARγ reciprocally regulate each other's expression.

Purpose of the Study:

• To elucidate the novel SMAD-signaling mechanisms by which TGF-β represses PPARγ expression.
• To identify the specific molecular interactions and regulatory elements involved in TGF-β-mediated PPARγ repression.

Main Methods:

• Investigated the interaction of SMAD3 with SMAD4, E2F4, and p107 in TGF-β-treated cells.
• Utilized chromatin immunoprecipitation assays to identify SMAD complex binding sites on the PPARG promoter.
• Performed reporter assays to assess the transcriptional activity of the PPARG promoter with identified regulatory elements.

Main Results:

• TGF-β induces the formation of a SMAD3-SMAD4-E2F4-p107 complex that translocates to the nucleus.
• This complex binds to both a novel TGF-β inhibitory element (TIE) and canonical SMAD-binding elements (SBEs) in the PPARG promoter.
• Both TIE and SBEs independently mediate partial repression of PPARG transcription, demonstrating their cooperative function within the same promoter.
• TGF-β-treated fibroblasts exhibited SMAD complexes with dual activity: repressing PPARG and activating other SMAD target genes.

Conclusions:

• Novel SMAD-signaling mechanisms underlying TGF-β-induced PPARγ repression have been detailed.
• The findings reveal a previously unrecognized TIE and SBE interaction within the PPARG promoter.
• The discovery of dual SMAD complex activity in fibroblasts provides new insights into TGF-β's role in fibrosis.