Intracellular Retention of Estradiol Is Mediated by GRAM Domain-Containing Protein ASTER-B in Breast Cancer Cells.

Elevated blood levels of estrogens have been associated with poor prognosis in estrogen receptor-positive (ER+) breast cancers, but the relationship between circulating hormone levels in the blood and intracellular hormone concentrations is not well characterized. We observed that MCF-7 cells treated acutely with 17β-estradiol (E2) retain a substantial amount of the hormone even upon the removal of the hormone from the culture medium. Moreover, global patterns of E2-dependent gene expression are sustained for hours after acute E2 treatment and hormone removal. Although circulating E2 is sequestered by sex hormone binding globulin, the potential mechanisms of intracellular E2 retention are poorly understood. We found that mislocalization of a steroid-binding GRAM domain-containing protein, ASTER-B, to the nucleus, which is observed in a subset of patients with breast cancer, is associated with higher cellular E2 retention. Accumulation and retention of E2 are related to the steroidal properties of E2 and require nuclear localization and steroid binding by ASTER-B, as shown using a panel of mutant ASTER-B proteins. Finally, we observed that nuclear ASTER-B-mediated E2 retention is required for sustained hormone-induced ERα chromatin occupancy at enhancers and gene expression, as well as subsequent cell growth responses. Our results add intracellular hormone retention as a mechanism controlling E2-dependent gene expression and downstream biological outcomes. Implications: Mislocalized nuclear ASTER-B, which binds estradiol to support the functions of ER, can provide an alternate means of enhancing the biological effects of E2 in breast cancers and may be a potential therapeutic target that addresses multiple aspects of estrogen bioavailability.