The hormone combination of insulin, dexamethasone and prolactin induced accumulation of preproepidermal growth factor (EGF) mRNA in HC11 mouse mammary epithelial cells 16-24 h after the hormones were added to the cultures. Individual hormones or combinations of two of the hormones had no effect on EGF mRNA concentrations. The same hormone combination was capable of inducing expression of a reporter gene construct containing -888 to +25 bp of the EGF gene fused with luciferase. Deletions of the promoter between -888 and -271 bp had no detectable effect on basal or hormone-induced reporter gene expression. However, further deletion from -270 to -74 bp increased baseline to approximately equal hormone-induced reporter gene expression. This deletion also abolished the hormone-induced increase in reporter gene expression. Sequence analysis suggested that this region contained a binding site for Yin-Yang-1 (YY1), which was confirmed by gelshift analysis. Mutation of the YY1 binding site increased baseline reporter gene expression to the same level as induced by insulin, dexamethasone and prolactin in the wild-type promoter. These results indicate that expression of the EGF gene in mammary epithelium is repressed by the YY1 site, and that removal of repression may play a part in regulating EGF gene expression in lactating mammary tissue.

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