Resistance to transforming growth factor-beta occurs in the presence of normal Smad activation

Surgery. 2002 Aug;132(2):310-6. doi: 10.1067/msy.2002.126097.

Abstract

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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Animals
  • Cell Division / drug effects
  • Cell Division / physiology
  • Cells, Cultured
  • DNA-Binding Proteins / metabolism*
  • Down-Regulation / drug effects
  • Drug Resistance
  • Gene Expression / physiology
  • Intestinal Mucosa / cytology
  • Intestinal Mucosa / metabolism*
  • Proto-Oncogene Proteins c-myc / genetics
  • Proto-Oncogene Proteins c-myc / metabolism
  • Rats
  • Signal Transduction / drug effects
  • Signal Transduction / physiology
  • Smad2 Protein
  • Smad4 Protein
  • Smad6 Protein
  • Trans-Activators / metabolism*
  • Transforming Growth Factor beta / pharmacology*
  • Transforming Growth Factor beta1

Substances

  • DNA-Binding Proteins
  • Proto-Oncogene Proteins c-myc
  • Smad2 Protein
  • Smad2 protein, rat
  • Smad4 Protein
  • Smad4 protein, rat
  • Smad6 Protein
  • Smad6 protein, rat
  • TGFB1 protein, human
  • Tgfb1 protein, rat
  • Trans-Activators
  • Transforming Growth Factor beta
  • Transforming Growth Factor beta1