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

A bstract 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 are not well characterized. We observed that MCF-7 cells treated acutely with 17β-estradiol (E2) retain a substantial amount of the hormone even upon 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. While circulating E2 is sequestered by sex hormone binding globulin (SHBG), the potential mechanisms of intracellular E2 retention are poorly understood. We found that a mislocalization of a steroid-binding GRAM-domain containing protein, ASTER-B, to the nucleus, which is observed in a subset of breast cancer patients, 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. S ignificance This study demonstrates how E2 can be accumulated and retained intracellularly to drive a pro-proliferative gene expression program in ER+ breast cancer cells. Mechanistically, intracellular E2 retention is mediated in part by mislocalized nuclear ASTER-B, which binds estradiol to support the functions of ER, including E2-mediated gene expression