T(3) RECEPTOR SUPPRESSION OF SP1-DEPENDENT TRANSCRIPTION FROM THE EPIDERMAL GROWTH-FACTOR RECEPTOR PROMOTER VIA OVERLAPPING DNA-BINDING SITES

Expression of the human epidermal growth factor receptor (EGFR) gene i s inhibited by ligand-activated thyroid hormone receptor (T3R). Bindin g sites for Sp1 and for the T3R-retinoid X receptor (RXR) complex over lap in a functional core of the EGFR promoter. Sp1 inhibited binding o f the T3R complex to this 36-base pair (bp) EGFR element in vitro but did not affect binding of the T3R complex to a positive thyroid hormon e response element (TRE). In Drosophila SL2 cells, which lack Sp1 and T3R, function of the EGFR promoter was strongly dependent on Sp1. Sp1- dependent promoter function was inhibited by ligand-activated T3R but not by mutant T3R defective in DNA or T3 binding. RXR increased the ex tent of inhibition. Sp1 enhanced activity of the 36-bp element placed 5' to a minimal TATA promoter and this enhancement was also repressed by T3R. Mutations in the 36-bp element were unable to separate Sp1 and T3R functions. However, addition of a second half-site 5' to the exis ting site in an inverted repeat configuration created a positive TRE. In the absence of ligand, T3R inhibited Spl stimulation from this alte red element; addition of T3 reversed the inhibition. When a dimeric TR E is separated from Sp1-binding sites strong synergism was observed. T he nature and location of the TRE thus strongly influence biological r esponses. A TRE site in the EGFR promoter that overlaps an Sp1-binding site inhibits Sp1 function but is unable to direct positive function.