Resistance to transforming growth factor-beta occurs in the presence of normal Smad activation.
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.
Berger
David H
DH
Department of Surgery, The Baylor College of Medicine, Houston, Tex, USA.
Feng
Xin-Hua
XH
Yao
Jun
J
Saha
Debabrata
D
Beauchamp
R Daniel
RD
Lin
Xia
X
D000818:Animals; D002455:Cell Division / Q000187:drug effects / Q000502:physiology; D002478:Cells, Cultured; D004268:DNA-Binding Proteins / Q000378:metabolism*; D015536:Down-Regulation / Q000187:drug effects; D004351:Drug Resistance; D015870:Gene Expression / Q000502:physiology; D007413:Intestinal Mucosa / Q000166:cytology / Q000378:metabolism*; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology; D051899:Smad2 Protein; D051901:Smad4 Protein; D051905:Smad6 Protein; D015534:Trans-Activators / Q000378:metabolism*; D016212:Transforming Growth Factor beta / Q000494:pharmacology*; D053773:Transforming Growth Factor beta1
D004268:DNA-Binding Proteins; D016271:Proto-Oncogene Proteins c-myc; D051899:Smad2 Protein; C497784:Smad2 protein, rat; D051901:Smad4 Protein; C490811:Smad4 protein, rat; D051905:Smad6 Protein; C497804:Smad6 protein, rat; C508960:TGFB1 protein, human; C508962:Tgfb1 protein, rat; D015534:Trans-Activators; D016212:Transforming Growth Factor beta; D053773:Transforming Growth Factor beta1