Differential interactions of specific nuclear factor I isoforms with the glucocorticoid receptor and STAT5 in the cooperative regulation of WAP gene transcription

The distal region (-830 to -720 bp) of the rat whey acidic protein (WAP) ge ne contains a composite response element (CoRE), which has been demonstrate d previously to confer mammary gland-specific and hormonally regulated WAP gene expression. Point mutations in the binding sites for specific transcri ption factors present within this CoRE have demonstrated the importance of both nuclear factor I (NFI) and STAT5 as well as cooperative interactions w ith the glucocorticoid receptor (GR) in the regulation of WAP gene expressi on in the mammary gland of transgenic mice. This study reports the characte rization of NFI gene expression during mammary gland development and the id entification and cloning of specific NFI isoforms (NFI-A4, NFI-B2, and NFI- X1) from the mouse mammary gland during lactation. Some but not all of thes e NFI isoforms synergistically activate WAP gene transcription in cooperati on with GR and STAT5, as determined using transient cotransfection assays i n JEG-3 cells. On both the WAP CoRE and the mouse mammary tumor virus long terminal repeat promoter, the NFI-B isoform preferentially activated gene t ranscription in cooperation with STAT5A and GR. In contrast, the NFI-A isof orm suppressed GR and STAT cooperativity at the WAP CoRE. Finally, unlike t heir interaction with the NFI consensus binding site in the adenovirus prom oter, the DNA-binding specificities of the three NFI isoforms to the palind romic NFI site in the WAP CoRE were not identical, which may partially expl ain the failure of the NFI-A isoform to cooperate with GR and STAT5A.