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

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