Growth inhibitory signalling by TGFbeta is blocked in Ras-transformed intestinal epithelial cells at a post-receptor locus.

The transforming growth factor beta (TGFbeta) family of growth regulatory peptides plays an important role in the regulation of gastrointestinal epithelial cell homeostasis. Loss of growth inhibitory signalling by TGFbeta is common in the context of Ras-transformation and it has been hypothesized that loss of TGFbeta receptor II (TGFbetaRII) expression accounts for the emergence of TGFbeta resistance. Here we examine the functional significance of reduced TGFbetaRII expression in intestinal epithelial cells transformed by oncogenic Ras. TGFbeta-induced signalling events downstream of TGFbetaRII were examined in Ras-transformed RIE-1 cells (RIE-Ras) and compared to the parental RIE-1 line. RIE-Ras cells were resistant to growth inhibition by TGFbeta. Neither overexpression of TGFbetaRII in RIE-Ras cells nor expression of constitutively active TGFbetaRI restored sensitivity to TGFbeta. TGFbeta-mediated phosphorylation of Smad2 occurred in TGFbeta-resistant RIE-Ras cells, as well as other TGFbeta-resistant cells lines (HT-29, SW620) expressing low levels of TGFbetaRII. Nuclear translocation of Smad2 and Smad4 occurred equally in RIE-Ras and parental RIE cells. The activity of TIEG2, a TGFbeta-inducible SP1-like transcription factor, was reduced in RIE-Ras cells, implying that resistance in Ras-transformed RIE cells occurs by a transcriptional mechanism.