Despite the fundamental pathophysiological importance of b-catenin in tumor progression, the mechanism underlying its final transcriptional output has been partially elucidated. Here, we report that b-arrestin-1 (b-arr1) is an epigenetic regulator of endothelin (ET)-1-induced b-catenin signaling in epithelial ovarian cancer (EOC). In response to ET A receptor (ET A R) activation by ET-1, b-arr1 increases its nuclear translocation and direct binding to b-catenin. This in turn enhanced b-catenin nuclear accumulation and transcriptional activity, which was prevented by expressing a mutant b-arr1 incapable of nuclear distribution. b-arr1-b-catenin interaction controls b-catenin target gene expressions, such as ET-1, Axin 2, Matrix metalloproteinase 2, and Cyclin D1, by promoting histone deacetylase 1 (HDAC1) dissociation and the recruitment of p300 acetyltransferase on these promoter genes, resulting in enhanced H3 and H4 histone acetylation, and gene transcription, required for cell migration, invasion and epithelial-to-mesenchymal transition. These effects are abrogated by b-arr1 silencing or by mutant b-arr1, as well as by b-catenin or p300 silencing, confirming that nuclear b-arr1 forms a functional complex capable of regulating epigenetic changes in b-catenindriven invasive behavior. In a murine orthotopic model of metastatic human EOC, silencing of b-arr1 or mutant b-arr1 expression, as well as ET A R blockade, inhibits metastasis. In human EOC tissues, b-arr1-b-catenin nuclear complexes are selectively enriched at b-catenin target gene promoters, correlating with tumor grade, confirming a direct in vivo b-arr1-b-catenin association at specific set of genes involved in EOC progression. Collectively, our study provides insights into how a b-arr1-mediated epigenetic mechanism controls b-catenin activity, unraveling new components required for its nuclear function in promoting metastasis.Oncogene ( 1,2 b-arrs, consisting of b-arr1 and b-arr2, are involved in G-protein-coupled receptor signaling, and guide the receptor signals also in malignant cells. [3][4][5][6][7][8][9][10][11][12] A global proteomics analysis of b-arr1-interacting proteins demonstrated that b-arr1 interactions take place not only in the cytoplasm, but also in the nucleus, 13 suggesting that b-arr1 has a role in transcriptional regulation in different human cells.14-16 Previous studies indicate that under d-or k-opioid receptor stimulations, nuclear b-arr1 specifically accumulates at p27 and c-fos promoters through scaffolding cyclic adenosine monophosphate responseelement (CRE) binding protein and histone acetyltransferase p300, thus enhancing local histone acetylation and gene transcription.