Estrogen receptor-mediated cross-talk with growth factor signaling pathways

Estrogen (E 2) palys critical roles in the development of tumors in female reproductive organs. Development of most breast cancers is dependent on E 2 in most cases. Most E 2 actions are considered to be exerted through two subtypes of Estrogen receptors (ERs), ER α and ERβ. ERs belong to the nuclear receptor superfamily, and act as ligand-inducible transcription factors to activate transcription of a particular set of the target genes. Ligand-bound ER recruits at least two distinct classes of coactivator complexes. In estrogen-dependent breast cancer, growth factors are shown to often act synergisticaly with E 2, and the breast cancer often become resistant to treatment of estogen antagonists. However, the molecular basis of this coupled regulation of growth factor- and ER-mediated signaling and hormone-resistance are largely unknown. We have previously shown that MAP (mitogen-activated protein) kinase (MAPK) activated by growth factors phosphorylates and potentiates the N-terminal transactivation function (AF-1), indicating a possible molecular mechanism of a novel cross-talk between two signalings (Kato et al, 1995). Furthermore, we have identified a coactivator that specifically interacts with ER α AF-1 (Endoh et al, 1999). In this review, this cross-talk is discussed in terms of the transactivation function of ERs and their coactivators.