Increased expression of the transforming growth factor β (TGFβ) gene following polyamine depletion in IEC-6 cell line

Polyamine depletion and the peptide TGFβ inhibit cell proliferation. The current study tests the hypothesis that TGFβ gene expression is altered by polyamine depletion resulting in the growth inhibition of intestinal epithelial cells IEC-6 cells, derived from rat small intestinal crypt cells, were grown in Dulbecco's minimal essential medium in the presence or absence of DFMO, a specific inhibitor of polyamine biosynthesis. Exposure of the IEC-6 cells to DFMO for 6 and 12 days not only depleted intracellular polyamines but also strikingly increased the mRNA levels of TGFβ. Increased expression of TGFβ mRNA in DFMO-treated cells occurred as cell proliferation ceased. To determined if increased expression of TGFβ mRNA was caused by the stimulation of transcription or posttranscription, the synthesis and half-life of the mRNA were measured. Depletion of intracellular polyamines by DFMO had no effect on the rate of TGFβ gene transcription as measured by nuclear run-on assay. The half-life of mRNA for TGFβ in normal cells was ∼2 h and increased to more than 20 h in cells treated with DFMO for 6 or 12 days. In the presence of DFMO, administration of exogenous polyamine reversed the effect of DFMO on the stability of TGFβ mRNA in IEC-6 cells These results indicate that 1) depletion of intracellular polyamines is associated with both the increased expression of TGFβ gene and growth inhibition, and 2) increased TGFβ mRNA levels in polyamine-deficient cells result from a delay in the rate of degradation rather than from the activation of the gene transcription.