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Resistance to transforming growth factor-beta occurs in the presence of normal Smad activation
David H Berger1, Xin-Hua Feng, Jun Yao
1Department of Surgery, The Baylor College of Medicine, Houston, Tex, USA.
Background:
Resistance to the growth inhibitory actions of transforming growth factor-beta (TGF-beta) is common in human cancers. This resistance can be a result of decreased expression of TGF-beta receptors. Downregulation of c-Myc by TGF-beta is critical for TGF-beta-mediated growth inhibition. In this study we hypothesized that decreased TGF-beta receptor expression leads to reduced Smad signaling and overexpression of c-Myc in intestinal epithelial (RIE) and transformed intestinal epithelial cells (RIE-Tr) cells.
Methods:
RIE (TGF-beta-sensitive) and RIE-Tr (TGF-beta-resistant) cells were treated with and without fetal bovine serum and TGF-beta. Western blot analysis was performed to detect levels of c-Myc, Smad2, Smad4, and phosphorylated Smad2 in RIE and RIE-Tr cells. Smad complex formation was analyzed by immunoprecipitation-coupled Western blotting.
Results:
c-Myc is overexpressed in RIE-Tr cells. TGF-beta-mediated downregulation of c-Myc is abrogated in RIE-Tr cells. Smad expression and activation is normal in RIE-Tr cells. We found that Smad2, Smad4, and Smad6 expression remained constant in RIE and RIE-Tr cells with or without serum or TGF-beta treatment. In addition, TGF-beta induced similar Smad2 phosphorylation and Smad complex formation in both RIE and RIE-Tr cells.
Conclusions:
Our data demonstrate that Smad signaling is preserved in the face of decreased TGF-beta receptor levels. We also demonstrate that Smad signaling is not sufficient for TGF-beta-mediated c-Myc repression.
Insights
Transforming growth factor-beta (TGF-beta) resistance in cancer is linked to c-Myc overexpression. Despite normal Smad signaling, TGF-beta fails to repress c-Myc in resistant cells, suggesting Smad pathway sufficiency is not the sole factor.
Area of Science:
- Cell biology
- Cancer research
- Molecular signaling
Background:
- Resistance to transforming growth factor-beta (TGF-beta) is prevalent in human cancers, often due to reduced TGF-beta receptor expression.
- TGF-beta's downregulation of c-Myc is crucial for its growth inhibitory effects, and its loss contributes to cancer progression.
Purpose of the Study:
- To investigate the hypothesis that decreased TGF-beta receptor expression leads to impaired Smad signaling and elevated c-Myc levels.
- To examine the relationship between TGF-beta receptor levels, Smad signaling, and c-Myc regulation in intestinal epithelial cells.
Main Methods:
- Western blot analysis was used to quantify c-Myc, Smad2, Smad4, and phosphorylated Smad2 levels.
- Immunoprecipitation-coupled Western blotting assessed Smad complex formation.
- Treated TGF-beta-sensitive (RIE) and TGF-beta-resistant (RIE-Tr) cells with and without serum and TGF-beta.
Main Results:
- c-Myc was overexpressed in RIE-Tr cells, and TGF-beta's ability to downregulate c-Myc was lost.
- Smad2, Smad4, and Smad6 expression levels remained consistent across cell types and treatments.
- TGF-beta induced comparable Smad2 phosphorylation and Smad complex formation in both RIE and RIE-Tr cells.
Conclusions:
- Smad signaling pathways are functional and unaffected by reduced TGF-beta receptor levels.
- Normal Smad signaling is insufficient to mediate TGF-beta-induced c-Myc repression in cancer cells.