Nuclear vs translocating steroid receptor models and the excluded middle

Differential cellular localization of estrogen receptor alpha in uterine and mammary cells

The classical alpha isoform of the estrogen receptor (ER) has been reported to localize almost exclusively in the nucleus. However, studies on non-genomic steroid effects have also suggested the existence of ERs residing at the cell surface. In this work, we present immunological data supporting extra-nuclear ERalpha localization in uterine (SHM) and mammary (MCF-7) cell lines. Immunocytological studies performed on SHM cells revealed that native ERs mainly localize as a perinuclear cytoplasmic ring. The receptors were rapidly translocated to the nucleus by 17beta-estradiol. In addition to nuclear ERs, a peripheral reservoir of ERalpha immunoreactivity, most probably associated with the plasma membrane, was detected in MCF-7 cells. These results were confirmed by the detection of membrane estrogen binding sites using fluorescent estrogen-BSA derivatives and ligand binding assays to intact cells, where [3H]-estradiol could be partly displaced by impeded estrogen conjugates. Partial inhibition of radioligand binding by an antibody against the steroid binding domain of the ERalpha suggests that the isoform faces the extracellular media in MCF-7 cells. Moreover, ERalpha-like proteins ( approximately 67 kDa) were found to be associated in isolated membrane subfractions from the cells. However, immunocytology of COS-1 (ER-negative) and SHM cells transfected with the complete cDNA coding for the cloned ERalpha and beta isoforms showed exclusive nuclear localization of the overexpressed ERs. The non-classical distribution of native ERalpha-like proteins in each cell line, suggests an alternative mode of ERalpha cellular localization/function. Cell type-dependent processing may account for the differential localization shown by native and expressed receptors in the systems considered.

Role of molecular chaperones in subnuclear trafficking of glucocorticoid receptors

The delivery of activated steroid receptors to high-affinity genomic sites must be efficient enough to account for the rapidity and selectivity of many transcriptional responses to steroid hormones. Thus, the signal transduction capacity of steroid hormone receptors will be influenced by the efficiency of receptor trafficking both between different subcellular compartments (that is, the cytoplasm and nucleus) and within a specific compartment (that is, the nucleus). Molecular chaperones, such as heat shock proteins, have long been recognized to play important roles in the management of protein folding in both stressed and nonstressed cells. In recent years, the participation of these proteins in various signal transduction pathways (for example, steroid hormone responses) has also been recognized. In this review, recent results that implicate a role for distinct heat shock proteins in subnuclear trafficking of glucocorticoid receptors are discussed. These studies also highlight the importance of mobilizing the cellular chaperone machinery for managing steroid receptor folding within the nucleus.

Expression and subcellular distribution of the beta-isoform of the human glucocorticoid receptor

Alternative splicing of the human glucocorticoid receptor (hGR) primary transcript produces two highly homologous protein isoforms, termed hGR alpha and hGRbeta, that differ at their carboxy-termini. In contrast to the well characterized hGR alpha isoform, which modulates gene expression in a hormone-dependent fashion, the biological significance of hGRbeta has only recently begun to emerge. We and others have shown that the hGRbeta messenger RNA transcript is widely expressed in human tissues and that the hGRbeta protein functions as a dominant negative inhibitor of hGR alpha in transfected cells. Unfortunately, these initial studies did not determine whether the hGRbeta protein was made in vivo. Such analyses are hindered because available anti-hGR antibodies cannot discriminate between the similarly sized hGR alpha and hGRbeta proteins. Therefore, to investigate the expression of the hGRbeta protein, we have produced an antipeptide, hGRbeta-specific antibody termed BShGR. This antibody was made against the unique 15-amino acid peptide at the carboxy-terminus of hGRbeta and recognizes both the native and denatured conformations of hGRbeta, but does not cross-react with hGR alpha. Using BShGR on Western blots and in immunoprecipitation experiments, we detected the hGRbeta protein in a variety of human cell lines and tissues. Immunocytochemistry was then performed with BShGR on HeLa S3 and CEM-C7 cells and on tissue sections prepared from lung, thymus, and liver to assess the cellular and subcellular distribution of hGRbeta. In all immunopositive cells, hGRbeta was found in the nucleus independent of glucocorticoid treatment. Within tissues, the hGRbeta protein was expressed most abundantly in the epithelial cells lining the terminal bronchiole of the lung, forming the outer layer of Hassall's corpuscle in the thymus, and lining the bile duct in the liver. As a potential in vivo inhibitor of hGR alpha activity, expression of hGRbeta may be an important factor regulating target cell responsiveness to glucocorticoids.

17 beta-Estradiol potentiates kainate-induced currents via activation of the cAMP cascade

Evidence for nongenomic actions of steroids is now coming from a variety of fields of steroid research. Mechanisms of steroid action are being studied with regard to the membrane receptors and the activation of second messengers. The present study investigated the mechanism for the rapid effect of estrogen on acutely dissociated hippocampal CA1 neurons by using the whole-cell, voltage-clamp recording. Under the perforated patch configuration, 17 beta-estradiol potentiated kainate-induced currents in 38% of tested neurons. The potentiation was stereospecific, rapid in onset, and reversible after the removal of the steroid. Dose-response curves show that the potentiation by 17 beta-estradiol was evident at a concentration as low as 10 nM and saturated at 10 microM. 17 beta-Estradiol did not affect the kinetics (i.e., affinity and cooperativity) and reversal potential of kainate-induced currents. This suggests that the potentiation did not result from direct interaction with kainate receptors nor the activation of ion channels other than kainate receptor-channels. The potentiation by 17 beta-estradiol was similar to the enhancement of kainate-induced currents evoked by 8-bromo-cAMP, and was modulated by an inhibitor of phosphodiesterase (IBMX). The estrogen potentiation was blocked by a specific blocker of PKA (Rp-cAMPS). Under standard recording configuration, the effect was significantly affected by intracellular perfusing with GDP-beta-S or GTP-gamma-S. The data suggest that the potentiation of kainate-induced currents by 17-beta-estradiol was likely a G-protein(s) coupled, cAMP-dependent phosphorylation event. By involvement of this non-genomic mechanism, estrogen may play a role in the modulation of excitatory synaptic transmission in the hippocampus.

Membrane estrogen receptor-enriched GH(3)/B6 cells have an enhanced non-genomic response to estrogen

We immunoselected GH(3)/B6 cells for a membrane estrogen receptor (mER) using antibodies generated against the rat intracellular ER (iER). Immunocytochemistry with anti-ER antibodies revealed bright fluorescence distributed in patches over the surface of mER-enriched cells, while cells immuno-depleted for mER showed only low-level membrane immunofluorescence. Quantitation via digital image analysis confirmed that immunoenriched populations show increases in both stained cell numbers and intensity of staining. Short-term culturing with serum reversibly decreased the intensity of immunostaining in mER-enriched cells to immuno-depleted cell levels. The mER-enriched populations initially contained ∼85% immunopositive cells in defined medium, but when cultured continuously with serum gradually decline to ∼22% immunopositive cells by 10 weeks. Cells enriched for mER showed a significant increase in rapid (after 2 or 5 min) prolactin release when treated with 17β-estradiol, while mER-depleted cells lacked this response. Immunoprecipitabie membrane proteins isolated from mER-enriched cells were 60,000, 74,000 and ∼ 200,000 MW, compared to an iER size of 67,000. Therefore, the presence and level of an mER that is antigenically related to iER is correlated with the ability of GH(3)/B6 cells to mediate a rapid action of estrogen.