The estrogen receptor enhances AP-1 activity by two distinct mechanisms with different requirements for receptor transactivation functions.

Estrogen receptors (ERs alpha and beta) enhance transcription in response to estrogens by binding to estrogen response elements (EREs) within target genes and utilizing transactivation functions (AF-1 and AF-2) to recruit p160 coactivator proteins. The ERs also enhance transcription in response to estrogens and antiestrogens by modulating the activity of the AP-1 protein complex. Here, we examine the role of AF-1 and AF-2 in ER action at AP-1 sites. Estrogen responses at AP-1 sites require the integrity of the ERalpha AF-1 and AF-2 activation surfaces and the complementary surfaces on the p160 coactivator GRIP1 (glucocorticoid receptor interacting protein 1), the NID/AF-1 region, and NR boxes. Thus, estrogen-liganded ERalpha utilizes the same protein-protein contacts to transactivate at EREs and AP-1 sites. In contrast, antiestrogen responses are strongly inhibited by ERalpha AF-1 and weakly inhibited by AF-2. Indeed, ERalpha truncations that lack AF-1 enhance AP-1 activity in the presence of antiestrogens, but not estrogens. This phenotype resembles ERbeta, which naturally lacks constitutive AF-1 activity. We conclude that the ERs enhance AP-1 responsive transcription by distinct mechanisms with different requirements for ER transactivation functions. We suggest that estrogen-liganded ER enhances AP-1 activity via interactions with p160s and speculate that antiestrogen-liganded ER enhances AP-1 activity via interactions with corepressors.