A hormone pulse induces transient changes in the subcellular distribution and leads to a lysosomal accumulation of the estradiol receptor α in target tissues

An intrauterine pulse-stimulation with estradiol induced changes in the subcellular localization of estrogen receptor α in porcine endometrium, as detected with F(ab′) fragments of various anti-receptor antibodies covalently linked to nanogold. The low-sterically hindered immunoreagents – recognizing different epitopes within the hormone binding domain – allowed for an efficient immunolabeling of estradiol receptor α, detecting it both in the cytoplasm and the nucleus of non-stimulated epithelium cells. In the cytoplasm, the receptor often seemed to be associated with actin filaments and the endoplasmatic reticulum. After the stimulation with estradiol, a predominantly nuclear localization and a labeling of nucleoli was observed. Our immunoelectron microscopy study demonstrates a localization of the receptor in cytoplasmic organelles that increased after the hormone pulse. These organelles exhibited the morphological properties of lysosomes and relocated to the perinuclear area. In analogous cytoplasmic organelles, the presence of cathepsin D was detected via indirect immunogold labeling, justifying their classification as lysosomes. Quantitative examinations revealed that not only the number of lysosomes in the proximity of the nucleus but also their immunostaining for estradiol receptor α increased significantly after the hormone pulse. Thus, estradiol induces both the rapid shift of receptor into the nucleus, a slower perinuclear accumulation of lysosomes and an increase of lysosomal ERα-immunoreactivity. These results suggest a role for lysosomes in the degradation of receptor shuttling out of the nucleus. This could serve as termination of the estradiol receptor α-dependent activation of target cells. This hypothesis is strengthened by the fact that the receptor content in uterine tissue declined drastically few hours after the hormone pulse.