Ligand-Independent Antiapoptotic Function of Estrogen Receptor-β in Lung Cancer Cells

Recent studies have demonstrated the presence of estrogen receptor (ER)beta in the mitochondria in various cell types and tissues, but the exact function of this localization remains unclear. In this study, we have examined the function of mitochondrial ER beta in non-small-cell lung cancer (NSCLC) cells. Down-regulation of ER beta by short hairpin RNA constructs sensitized NSCLC cells to various apoptosis-inducing agents such as cisplatin, taxol, and etoposide. The increased growth inhibition and induction of apoptosis in ER beta-knockdown cells was observed irrespective of estrogen treatment, suggesting a ligand-independent role of ER beta in regulating the intrinsic apoptotic pathway. Further, ER beta from the mitochondrial fraction physically interacted with the proapoptotic protein Bad, in a ligand-independent manner. Glutathione-S-transferase pull-down assays and molecular modeling studies revealed that the DNA-binding domain and hinge region of ER beta, and the BH3 domain of Bad were involved in these interactions. Further investigations revealed that ER beta inhibited Bad function by disrupting Bad-Bcl-X-L and Bad-Bcl-2 interactions. Reintroduction of ER beta in the mitochondria of ER beta knockdown cells reversed their sensitivity to cisplatin. Overall, our results demonstrate a ligand-independent role of ER beta in regulating apoptosis, revealing a novel function for ER beta in the mitochondria. (Molecular Endocrinology 24: 1737-1747, 2010)