Transforming Growth Factor-{beta}1 Induces Transforming Growth Factor-{alpha} Promoter Activity and Transforming Growth Factor-{alpha} Secretion in the Human Colon Adenocarcinoma Cell Line FET

FET cells are well differentiated human adenocarcinoma cells whose growth is partially inhibited (50–60%) by transforming growth factor-β1 (TGF-β1). In exponentially growing cultures, TGF-β1 induces the expression of transforming growth factor-α (TGF-α) by 3-fold. To determine whether this induction is the result of increased TGF-α promoter activity, FET cells were transiently transfected with a plasmid containing 2816 base pairs of the 5′-flanking region of the TGF-α gene linked to luciferase. Transfected FET cells treated with growth-inhibitory concentrations of TGF-β1 (10 ng/ml) showed up to a 10-fold increase in luciferase activity. The increase in luciferase activity was dose dependent through the normal physiological range of TGF-β1 (0.5–20 ng/ml), saturating at 10 ng/ml. This effect was also TGF-α promoter specific, inasmuch as the Rous sarcoma virus long terminal repeat used as a control remained relatively insensitive to the effects of TGF-β1. By using progressively smaller portions of the TGF-α promoter region, the TGF-β1-responsive element was mapped between base pairs -77 and -201 of the 5′-flanking region. TGF-β1 treatment also affected epidermal growth factor receptor levels. FET cells treated with TGF-β1 (10 ng/ml) for 48 h showed a 20% decrease in the number of epidermal growth factor receptors and a 2-fold increase in the number of high affinity epidermal growth factor receptors on their surface. These results indicate that TGF-β1 acts as a positive regulator of TGF-α transcription, and they suggest a possible mechanism by which these cells circumvent the growth-inhibitory effects of TGF-β1.