Expression of the TGF-beta receptor gene and sensitivity to growth inhibition following polyamine depletion

Our previous studies have shown that inhibition of polyamine biosynthesis i ncreases the sensitivity of intestinal epithelial cells to growth inhibitio n induced by exogenous transforming growth factor-beta (TGF-beta). This stu dy went further to determine whether expression of the TGF-beta receptor ge nes is involved in this process. Studies were conducted in the IEC-6 cell l ine, derived from rat small intestinal crypt cells. Administration of alpha -difluoromethylornithine (DFMO), a specific inhibitor of ornithine decarbox ylase (the rate-limiting enzyme for polyamine synthesis), for 4 and 6 days depleted cellular polyamines putrescine, spermidine, and spermine in IEC-6 cells. Polyamine depletion by DFMO increased levels of the TGF-beta type I receptor (TGF-beta RI) mRNA and protein but had no effect on the TGF-beta t ype II receptor expression. The induced TGF-beta RI expression after polyam ine depletion was associated with an increased sensitivity to growth inhibi tion induced by exogenous TGF-beta but not by somatostatin. Extracellular m atrix laminin inhibited IEC-6 cell growth without affecting the TGF-beta re ceptor expression. Laminin consistently failed to induce the sensitivity of TGF-beta-mediated growth inhibition. In addition, decreasing TGF-beta RI e xpression by treatment with retinoic acid not only decreased TGF-beta-media ted growth inhibition in normal cells but also prevented the increased sens itivity to exogenous TGF-beta in polyamine-deficient cells. These results i ndicate that 1) depletion of cellular polyamines by DFMO increases expressi on of the TGF-beta RI gene and 2) increased TGF-beta RI expression plays an important role in the process through which polyamine depletion sensitizes intestinal epithelial cells to growth inhibition induced by TGF-beta.