Transcriptional control of human cytochrome P1-450 gene expression by 2,3,7,8-tetrachlorodibenzo-p-dioxin in human tissue culture cell lines.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces aryl hydrocarbon hydroxylase (AHH) activity and human cytochrome P1-450 mRNA in the human breast carcinoma MCF-7 and hepatoblastoma HepG2 tissue culture cell lines. Although AHH activities induced by 100 nM TCDD are comparable in these cell lines, the EC50 values for TCDD differ: EC50 approximately equal to 1 nM for HepG2; EC50 greater than 20 nM for MCF-7. In order to determine the mechanism responsible for this difference in EC50, we have examined putative regulatory factors such as the intracellular TCDD receptor as well as the kinetics of mRNA transcription and accumulation in these cells. TCDD increases transcription of hP(1)450 mRNA in both MCF-7 and HepG2 cells; however, MCF-7 cells require higher concentrations of TCDD to produce transcriptional activation comparable to that observed for HepG2 cells. These data indicate that the difference in EC50 is determined at an early step in the induction of hP(1)450 mRNA. With the use of a sensitive assay based on high-performance anion-exchange liquid chromatography, an intracellular protein which binds TCDD with high affinity was detected in HepG2 cytosolic fractions but not in MCF-7 cells. Thus, the difference in EC50 for TCDD can be correlated to the differences in TCDD binding. We postulate that MCF-7 cells contain a "defective" receptor with decreased affinity for TCDD.