Subcellular distribution of native estrogen receptor alpha and beta isoforms in rabbit uterus and ovary

The association of estrogen receptors with non-nuclear/cytoplasmic compartm ents in target tissues has been documented. However, limited information is available on the distribution of estrogen receptor isoforms, specially wit h regard to the newly described beta isotype. The subcellular localization of estrogen receptor alpha and beta isoforms was investigated in rabbit ute rus and ovary. Native alpha and beta subtypes were immunodetected using spe cific antibodies after subjecting the tissue to fractionation by differenti al centrifugation. The ovary expressed alpha and beta estrogen receptors in predominant association to cytosolic components. However, in the uterus, a substantial proportion of the total estrogen binding capacity and coexpres sion of the two isoforrns was detected in mitochondria and microsomes. The mitochondrial-enriched subfraction represented an important source of 17 be ta -estradiol binding, where the steroid was recognized in a stereospecific and high affinity manner. The existence of mitochondrial and membrane estr ogen binding sites correlated with the presence of estrogen receptor alpha but mainly with estrogen receptor beta proteins. Using macromolecular 17 be ta -estradiol derivatives in Ligand Blot studies, we could confirm that bot h alpha and beta isoforms were expressed as the major estrogen binding prot eins in the uterus, while estrogen receptor alpha was clearly the dominant isoform in the ovary. Other low molecular weight estrogen receptor alpha -l ike proteins were found to represent an independent subpopulation of uterin e binding sites, expressed to a lesser extent. This differential cellular p artitioning of estrogen receptor alpha and beta forms may contribute to the known diversity of 17 beta -estradiol effects in target organs. Both estro gen receptor alpha and beta expression levels and cellular localization pat terns among tissues, add complexity to the whole estrogen signaling system, in which membrane and mitochondrial events could also be implicated. (C) 2 001 Wiley-Liss, Inc.