Mechanism of estrogen activation of c-myc oncogene expression.
The estrogen receptor complex is a known trans-acting factor that regulates transcription of specific genes through an interaction with a specific estrogen-responsive cis-acting element (ERE). In previous studies we have shown that in estrogen-responsive human breast cancer cells estrogen rapidly activates c-myc expression. This activated expression occurs through enhanced transcription and does not require the synthesis of new protein intermediates; therefore, an ERE is present in the human c-myc gene regulatory region. To localize the ERE, constructs containing varying lengths of the c-myc 5'-flanking region ranging from -2327 to +25 (relative to the P1 promoter) placed adjacent to the chloramphenicol acetyl transferase reporter gene (CAT) were prepared. They were used in transient transfection studies in MCF-7 and HeLa cells co-transfected with an estrogen receptor expression vector. These studies reveal that all constructs containing the P2 promoter region exhibited estrogen-regulated CAT expression and that a 116-bp region upstream and encompassing the P2 TATA box is necessary for this activity. Analysis of this 116-bp region failed to identify a cis-acting element with sequences resembling the consensus ERE; however, co-transfection studies with mutant estrogen receptor expression vectors showed that the DNA-binding domain of the receptor is essential for estrogen-regulated CAT gene expression. We have also observed that anti-estrogen receptor complexes can weakly trans-activate from this 116-bp region but fail to do so from the ERE-containing ApoVLDLII-CAT construct. To explain these results we propose a new mechanism of estrogen trans-activation in the c-myc gene promoter.
Dubik
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Department of Physiology, University of Manitoba, Winnipeg, Canada.
Shiu
R P
RP
D001483:Base Sequence; D015152:Blotting, Northern; D004958:Estradiol / Q000494:pharmacology*; D015972:Gene Expression Regulation, Neoplastic / Q000187:drug effects*; D016259:Genes, myc / Q000235:genetics*; D006367:HeLa Cells; D006801:Humans; D008969:Molecular Sequence Data; D010957:Plasmids / Q000235:genetics; D011960:Receptors, Estrogen / Q000235:genetics; D011993:Recombinant Fusion Proteins / Q000235:genetics; D012045:Regulatory Sequences, Nucleic Acid / Q000235:genetics*
D011960:Receptors, Estrogen; D011993:Recombinant Fusion Proteins; D004958:Estradiol