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

Our previous studies have shown that inhibition of polyamine biosynthesis increases the sensitivity of intestinal epithelial cells to growth inhibition induced by exogenous transforming growth factor-β (TGF-β). This study went further to determine whether expression of the TGF-β receptor genes is involved in this process. Studies were conducted in the IEC-6 cell line, derived from rat small intestinal crypt cells. Administration of α-difluoromethylornithine (DFMO), a specific inhibitor of ornithine decarboxylase (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-β type I receptor (TGF-βRI) mRNA and protein but had no effect on the TGF-β type II receptor expression. The induced TGF-βRI expression after polyamine depletion was associated with an increased sensitivity to growth inhibition induced by exogenous TGF-β but not by somatostatin. Extracellular matrix laminin inhibited IEC-6 cell growth without affecting the TGF-β receptor expression. Laminin consistently failed to induce the sensitivity of TGF-β-mediated growth inhibition. In addition, decreasing TGF-βRI expression by treatment with retinoic acid not only decreased TGF-β-mediated growth inhibition in normal cells but also prevented the increased sensitivity to exogenous TGF-β in polyamine-deficient cells. These results indicate that 1) depletion of cellular polyamines by DFMO increases expression of the TGF-βRI gene and 2) increased TGF-βRI expression plays an important role in the process through which polyamine depletion sensitizes intestinal epithelial cells to growth inhibition induced by TGF-β.