Mitochondrial localization of ERα and ERβ in human MCF7 cells

We observed previously that estrogen treatment increased the transcript levels of several mitochondrial DNA (mtDNA)-encoded genes for mitochondrial respiratory chain (MRC) proteins and MRC activity in rat hepatocytes and human Hep G2 cells. Others have reported detection of estrogen receptors (ER), ERα and ERβ, in mitochondria of rabbit ovarian and uterine tissue. In this study, we have extended these observations. Using cellular fractionation and Western blot with ERα- and ERβ-specific antibodies, we observed that ERα and ERβ are present in mitochondria of human MCF7 cells and that the mitochondrial ERα and ERβ account for 10 and 18%, respectively, of total cellular ERα and ERβ in 17β-estradiol (E₂)-treated MCF7 cells. We also found that E ₂ significantly enhanced the amounts of mitochondrial ERα and ERβ in a time- and concentration-dependent manner and that these effects are accompanied by a significant increase in the transcript levels of mtDNA-encoded genes, i.e., cytochrome c oxidase subunits I and II. Moreover, we demonstrated that these E₂-mediated effects were inhibited by the pure ER antagonist, ICI-182780, indicating the involvement of ERs. Using immunohistochemistry with confocal microscopy and immunogold electron microscopy, we demonstrated that ERα and ERβ are located within the MCF7 cell mitochondrial matrix. Computer analysis identified a putative internal mitochondrial targeting peptide signal within human ERβ, suggesting an inherent potential for ERβ to enter mitochondria. These findings confirm the observations of others and provide additional support for this novel localization of the ERs and for a potentially important role of the ER in the regulation of mtDNA transcription.