Characteristics of different cytoplasmic and nuclear estrogen receptors appearing with continuous hormonal exposure

Non-nuclear estrogen receptor beta and alpha in the hippocampus of male and female rats

Estrogens play important roles in the brain, acting through two receptor types, ERalpha and ERbeta, both recognized as transcription factors. In this study, we investigated the ERbeta mRNA and protein expression in the male and female rat brain, focusing on the hippocampus, and comparing with well-known ERalpha expression patterns. Extranuclear ERbeta localization, as shown by light microscopic immunocytochemistry and tissue fractionation experiments, was noted in the hippocampus, whereas nuclear ERbeta was present in the amygdala. Despite these marked differences in subcellular localizations, similar expression levels of ERbeta proteins as well as the profile of ERbeta mRNA isoforms were observed in the two brain structures. ERalpha was localized to the nucleus more so than ERbeta, yet not without an extranuclear component. Our results suggest that cytoplasmic estrogen receptors may play an important role in hippocampal physiology.

Discrepancies of the biochemical and immunohistochemical estrogen receptor assays in breast cancer

We examined the estrogen receptor (ER) content of 124 primary breast cancers by hormone binding and immunohistochemical (ER-ICA) assays. Both assays were in agreement in 110 tumors (89%; P less than .0001); 68 tumors were positive and 42 were negative. In 14 cases (11%), the assays yielded discordant results. Three tumors showed hormone binding in the absence of immunohistochemically detectable ER; the false positive hormone binding resulted from the presence of normal epithelium adjacent to ER-ICA negative malignant cells. Eleven tumors failed to show hormone binding but were ER-ICA positive. Four of these were from premenopausal patients whose circulating endogenous estrogen may occupy the receptor, giving rise to false negative hormone binding assays. In four cases, the discrepancy of negative hormone binding assay and positive ER-ICA assay was attributed to scant tumor cells in the tissue sample. The remaining three discrepancies could not be resolved with certainty, but possibly resulted from alteration of the hormone binding site with preservation of the immunoreactive epitope on the ER molecule. These results indicate that the ER-ICA assay is more accurate than the hormone binding assay in identifying the presence of ER in cancer cells. The heterogeneous immunostaining of ER in tumor sections, which may reflect mosaicism of tumor cells, rate of cell proliferation, or phase of cell cycle, remains unexplained.

Progesterone-modulation of estrogen action: rapid down regulation of nuclear acceptor sites for the estrogen receptor

Our previous studies demonstrated that progesterone down regulates the occupied form of nuclear estrogen receptor (Re). Using the density shift method, we discovered that progestins stimulate the turnover of nuclear Re within 3 h of treatment, and Re synthesis is suppressed subsequently. Thus, the primary site of progestin action in down-regulating Re is the stimulation of nuclear Re turnover followed by the inhibition of Re replenishment. A major breakthrough in our understanding of how progestin controls Re turnover was made by studying nuclear acceptor sites for Re that were found to decrease markedly within 2 h of progestin treatment. These and other results indicate that progestin induces a factor called the Re regulatory factor (ReRF) which acts to block nuclear Re acceptor sites, and this in turn decreases nuclear Re retention on chromatin acceptor sites, leading to an enhanced turnover (or processing) of nuclear Re.

Analysis of monomeric-dimeric states of the estrogen receptor with monoclonal antiestrophilins

We studied the antibody-combining properties of 3 forms of the estrogen receptor found in buffers of high ionic strength. Shifts to a faster sedimenting peak on sucrose gradients or a faster eluting peak on a gel filtration column with antibody addition allowed us to determine whether a given form contained one, two or more antibody-binding sites. The monomeric cytosolic estrogen receptor, ERC, contained one antibody binding site for each of 2 monoclonal antiestrophilins (H222 and H165, provided by Abbott Laboratories). Both the heat-transformed cytosolic estrogen receptor, ERC*, and a major fraction of the estrogen receptor extracted from nuclei, ERN, contained two sites for H165, but only one for H222. A minor fraction of ERN had only one site for each antibody. The kinetics of transformation of ERC to a species with two H165 binding sites were appropriate to a dimerization of ERC*. Addition of H222, but not H165, before the onset of the heat-induced transformation blocked the formation of ERC to ERC. These data suggest that ERC* and a major form of ERN are comprised of two immunologically similar subunits identical to ERC. Also, the antigenic determinant for H222, but not H165, appears to be located close to the dimerization domain. The minor form of ERN appears to contain an altered or dissimilar subunit.

Non-stoichiometric nuclear-cytoplasmic redistribution of estrogen receptor in adult rat uterus, following estradiol injection

In immature and ovariectomized rats acutely injected with estradiol (E2), accumulation of estradiol receptor complexes (E2R) from the uterine cytosol to the nucleus has been shown to be quantitative by numerous investigators. In the present study, translocation of E2R from the cytosol to the nuclear fraction in adult and ovariectomized estrogen prestimulated rats was analyzed. Twenty micrograms of E2, dissolved in saline containing 10% ethanol and 1 g% bovine serum albumin (B.S.A.) were injected intraperitoneally to the animals and 2 h later E2R in the cytosol and crude nuclear fractions were assayed by exchange techniques. Unlike a 91% recovery of the depleted cytosol E2R in the nuclear fraction of ovariectomized rats, only 39.2 and 27.5% were recovered in the adult and ovariectomized estrogen prestimulated rat uterus respectively. Moreover, depending on the temperature and duration of nuclear suspension incubation, from 18 up to 80% of the recovered nuclear E2R were solubilized in the incubation medium and nuclear post-incubation washes and could be measured by hydroxylapatite treatment (HAP). Saturation assays showed a plateau from 12 nM E2 3H onwards up to 80 nM. The Kd values computed for the receptors in the nucleus and HAP in all the three groups were of the order of 2 X 10(-9) M. In conclusion, after E2 administration to adult or ovariectomized estrogen prestimulated rats, a stoichiometric recovery of the depleted cytosol E2R in the nuclear fraction was not observed, even when leakage of nuclear receptor into the medium in course of exchange was taken into account. Chronic estrogenization appeared to modify the dynamics of uterine receptor.

Cryptic estrogen binding protein complicates analysis of estrogen receptor distribution

The response of rat uterine estrogen receptor sub-species to injection of 5 micrograms estradiol has been investigated in intact and 4-weeks' ovariectomized adult animals. Determinations of occupied and unoccupied receptor subcellular fluctuations reveal significant differences not detectable under standard assays which measure only total nuclear and unoccupied cytosolic receptors. Both animal models manifest a high level of unoccupied nuclear receptors which are inaccessible to estrogen. In contrast to the intact animal, uteri from castrate animals have a high level of occupied receptors in the cytosol, which remains high following estrogen exposure. Receptor processing occurs in the castrate, but not the intact, animal. The results demonstrate that traditional assays are complicated by the presence and simultaneous measurement of non-responsive receptor species which quantitatively differ widely among animal models and will give rise to an erroneous interpretation of the pattern of estrogen-induced turnover of its receptor.

A novel nuclear form of estradiol receptor in MCF-7 human breast cancer cells

Nuclear estrogen receptor from MCF-7 cells undergoes a time-dependent, hormone-inducible transformation to a form that is less extractable from nuclei and less exchangeable with ligand. This receptor-modifying, intranuclear event is independent of receptor loss (processing) and appears associated with hormone responsiveness (progesterone-receptor induction) in these cells. The magnitude of receptor loss, however, is variable and apparently not a prerequisite for hormone action to induce progesterone receptor.