Subcellular distribution of native estrogen receptor α and β isoforms in rabbit uterus and ovary

The association of estrogen receptors with non-nuclear/cytoplasmic compartments in target tissues has been documented. However, limited information is available on the distribution of estrogen receptor isoforms, specially with regard to the newly described β isotype. The subcellular localization of estrogen receptor α and β isoforms was investigated in rabbit uterus and ovary. Native α and β subtypes were immunodetected using specific antibodies after subjecting the tissue to fractionation by differential centrifugation. The ovary expressed α and β estrogen receptors in predominant association to cytosolic components. However, in the uterus, a substantial proportion of the total estrogen binding capacity and coexpression of the two isoforms was detected in mitochondria and microsomes. The mitochondrial-enriched subfraction represented an important source of 17β-estradiol binding, where the steroid was recognized in a stereospecific and high affinity manner. The existence of mitochondrial and membrane estrogen binding sites correlated with the presence of estrogen receptor α but mainly with estrogen receptor β proteins. Using macromolecular 17β-estradiol derivatives in Ligand Blot studies, we could confirm that both α and β isoforms were expressed as the major estrogen binding proteins in the uterus, while estrogen receptor α was clearly the dominant isoform in the ovary. Other low molecular weight estrogen receptor α-like proteins were found to represent an independent subpopulation of uterine binding sites, expressed to a lesser extent. This differential cellular partitioning of estrogen receptor α and β forms may contribute to the known diversity of 17β-estradiol effects in target organs. Both estrogen receptor α and β expression levels and cellular localization patterns among tissues, add complexity to the whole estrogen signaling system, in which membrane and mitochondrial events could also be implicated.