THE NOVEL ESTROGEN-RESPONSIVE B-BOX PROTEIN (EBBP) GENE IS TAMOXIFEN REGULATED IN CELLS EXPRESSING AN ESTROGEN-RECEPTOR DNA-BINDING DOMAIN MUTANT

We have identified a 2.6-kb mRNA whose steady state levels are increas ed 2- to 4-fold by treatment of human mammary epithelial cells (HMEC) stably expressing an estrogen receptor (ER) transgene with either estr ogen (E) or the antiestrogen, 4-hydroxy-tamoxifen (HT). The cDNA corre sponding to this mRNA encodes a 564-amino acid protein, named estrogen -responsive a box protein (EBBP), that is a new member of a subfamily within the a box zinc finger protein family, which includes transcript ion factors (e.g. TIF1), tumor suppressor proteins (e.g. PML), and pro teins implicated in development (e.g. ret finger protein, XNF7). The E BBP mRNA is detectable by Northern blot analysis in most tissues, with the exception of liver and peripheral blood lymphocytes, and the gene has been mapped to human chromosome 17p11.2. In contrast to most B bo x family members, EBBP has a predominantly cytoplasmic localization. S tudies of the estrogenic regulation of EBBP expression demonstrated th at the E-dependent increase in EBBP mRNA levels in the ER-transfected HMEC is an early, ER-mediated, and cycloheximide-insensitive process. In HMEC stably transfected with an ER mutant containing a deletion in the second zinc finger of the DNA-binding domain, E and HT had differe nt effects on EBBP gene expression; EBBP regulation by E was dramatica lly reduced while the effects of HT were augmented. These data indicat e that HT can modulate EBBP mRNA expression through a mutated ER, whic h has little activity when bound by E, and suggest that different mole cular mechanisms control the E and HT responsiveness of the EBBP gene.