Ras-mediated suppression of TGFβRII expression in intestinal epithelial cells involves Raf-independent signaling

Ras-transformed intestinal epithelial cells are resistant to the growth inhibitory actions of TGF beta and have a marked decrease in expression of the TGF beta type II receptor (TGF beta RII). Rat intestinal epithelial cells (RIE) were stably transfected with activated Ras, Sos and Raf constructs and tested for expression of TGF beta RII and sensitivity to growth inhibition by TGF beta. The parental RIE line and the RIE-Raf cells were nontransformed in morphology and were sensitive to TGF beta (70-90% inhibited). In contrast, the RIE-Ras and RIB-sos lines were transformed, resistant to TGF beta and expressed 5- to 10-fold decreased levels of the TGF beta RII mRNA and protein. Cyclin D1 protein expression was repressed by TGF beta treatment in parental RIE and RIE-Raf cells, whereas levels of cyclin D1 in RIE-Ras and RIB-sos cells remained unchanged. Treatment of RIE-Ras cells with 25 mu M farnesyl transferase inhibitor, FTI L739,749, for 48 hours restored expression of TGF beta RII 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 beta RII to control levels. Collectively these results suggest that downregulation of TGF beta RII and loss of sensitivity to growth inhibition by TGF beta 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 TGF beta RII.