An Ets site in the whey acidic protein gene promoter mediates transcriptional activation in the mammary gland of pregnant mice but is dispensable during lactation.

The whey acidic protein (WAP) gene is specifically expressed in mammary tissue, and its transcription is induced several thousand-fold during pregnancy and remains high throughout lactation. A purine-rich sequence (PRS) located around -110 of the WAP gene promoter is conserved between mice, rats, and rabbits, suggesting that it features a regulatory element. This PRS contains an invariant GGAA/T core motif characteristic of the binding site for Ets transcription factors. Electromobility shift assays demonstrate that Ets1 binds specifically to the PRS. Experiments in transgenic mice further demonstrate that this PRS/Ets site plays a critical role in the activation of WAP transgenes during pregnancy, but that its presence is not required for high expression throughout lactation. Transgenes with an intact PRS/Ets site are expressed at high levels at day 13 of pregnancy, with little further increase during late pregnancy and lactation. In contrast, WAP transgenes with a mutation in the PRS/Ets site, which abrogates the binding of Ets1, are not expressed at midpregnancy, but their transcriptional activity is not affected during lactation. These results demonstrate that Ets-signaling pathways can function as stage-specific transcriptional activators of milk protein genes in the developing mammary gland. In addition, this work extends earlier findings that gene activation during pregnancy and lactation is mediated, in part, by different mechanisms.