Estrogen Intracellular Retention is Augmented by GRAM Domain Containing Protein ASTER-B in Breast Cancer Cells

Estrogen is the primary mitogen for ER+ BC cells, but how the hormone is retained intracellularly is poorly understood. We found that the ER+ BC cell line MCF-7 retains a substantial amount of hormone even upon removal of the hormone from the culture medium. The effect of acute E2 treatment followed by washout on gene regulation was determined by RNA-seq over an E2 treatment timecourse. In agreement with intracellular hormone levels, gene exprssion was also sustained over hours after removal of the hormone. We next identified that ASTER-B, a cholesterol-binding protein, was mislocalized into the nucleus of MCF-7 cells, and was associated with retained E2. By knocking down GRAMD1B (ASTER-B), both E2 retention and its associated gene regulatory programs (ER binding and gene expression) were significantly abrogated. Taken together, this shows E2 retention is required for sustained E2 induction, and that ASTER-B supports accumulation and retention of intracellular E2.