Ras-Mediated Suppression of TGFβRII Expression in Intestinal Epithelial Cells Involves Raf-Independent Signaling

Abstract

Ras-transformed intestinal epithelial cells are resistant to the growth inhibitory actions of TGFβ and have a marked decrease in expression of the TGFβ type II receptor (TGFβRII). Rat intestinal epithelial cells (RIE) were stably transfected with activated Ras, Sos and Raf constructs and tested for expression of TGFβRII and sensitivity to growth inhibition by TGFβ. The parental RIE line and the RIE-Raf cells were nontransformed in morphology and were sensitive to TGFβ (70–90% inhibited). In contrast, the RIE-Ras and RIE-Sos lines were transformed, resistant to TGFβ and expressed 5- to 10-fold decreased levels of the TGFβRII mRNA and protein. Cyclin D1 protein expression was repressed by TGFβ treatment in parental RIE and RIE-Raf cells, whereas levels of cyclin D1 in RIERas and RIE-Sos cells remained unchanged. Treatment of RIE-Ras cells with 25 μM farnesyl transferase inhibitor, FTI L739,749, for 48 hours restored expression of TGFβRII to levels equivalent to control cells. In addition, treatment of RIE-Ras cells for 48 hours with PD-98059, a specific MAPKK inhibitor, also increased expression of TGF,3RII to control levels. Collectively these results suggest that downregulation of TGFβRII and loss of sensitivity to growth inhibition by TGFβ in Ras-transformed intestinal epithelial cells is not mediated exclusively by the conventional Ras/Raf/ MAPKK/MAPK pathway. However, activation of MAPK, perhaps by an alternate Ras effector pathway, appears to be necessary for Ras-mediated downregulation of TGFβRII.