Regulation of the Activity of AIB1 an Estrogen Receptor Coactivator by Growth Factor Signals

摘要

The AIB1 protein is a member of a coactivator family that potentiates the transcriptional activity of the nuclear hormone receptors. AIB1, is amplified in certain breast and ovarian cancers and has been suggested that AIB1 plays a causative role in breast cancer development. Our lab recently identified AIB1 as a target of the MAPK signaling pathway (Font de Mora and Brown, Mol Cell Biol 20:5041, 2000). Based on these findings, we propose that the phosphorylation of AIB1 by MAPK may represent part of the molecular mechanism that integrates signals from steroid hormones and growth factors. In order to identify the sites of AIB1 that are phosphorylated by MAPK, seven potential phosphorylation sites were targeted for point mutations and or deletions. Previously we showed by an In vitro phosphorylation assay by (Erk2) that three out of the seven putative phosphorylation sites tested were most likely to be the specific MAPK phosphorylation sites. In order to investigate the physiological relevance of the AIB1 phosphorylation sites, we subcloned all mutants into GAL4DBD vector. We found that those mutants that showed low phosphorytation in vitro, gave the lowest transactivation levels when tested in vivo. In addition we were able to differentiate the sub-cellular localization of AIB1 after phosphorylation by performing transient transfection in COS cells using a constitutively active MAPK vector and a kinase inhibitor vector and further on performing immunofluorescence.