The Estrogen-responsive B Box Protein Is a Novel Regulator of the Retinoid Signal *

Abstract Retinoic acid (RA) induces growth arrest, cell death, and differentiation in many human cancer cells in vitro and has entered routine clinical use for the treatment of several human cancer types. One mechanism by which cancer cells evade retinoid-induced effects is through repression of retinoic acid receptor β (RARβ) gene transcription. The RA response element β (βRARE) is the essential DNA sequence required for retinoid-induced RARβ transcription. Here we show that the estrogen-responsive B box protein (EBBP), a member of the RING-B box-coiled-coil protein family, is a βRARE-binding protein. EBBP undergoes serine threonine phosphorylation and enhanced protein stability after RA treatment. Following RA treatment, we also observed increased nuclear EBBP levels in aggregates with the promyelocytic leukemia protein at promyelocytic leukemia nuclear bodies. EBBP enhanced RA-responsive RARβ transcription in RA-sensitive and -resistant cancer cells, which were resistant to both a histone deacetylase inhibitor and a demethylating agent. EBBP-specific small interfering RNA reduced basal and RA-induced RARβ expression. EBBP increased βRARE-transactivating function through its coiled-coil domain. Taken together, our work suggests that EBBP may have a pivotal role in the retinoid anti-cancer signal.