Abstract
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. J. Cell. Biochem. 82:467–479, 2001. © 2001 Wiley‐Liss, Inc.