ReviewEstradiol, in the CNS, targets several physiologically relevant membrane-associated proteins
Introduction
In the last 5 years a remarkable increase in new findings pointed beyond reasonable doubt that estrogens in addition to the classical genomic pathway use membrane-initiated events to activate cellular mechanisms. For example, it is quite evident that estrogens activate the MAPK (mitogen-activated protein kinase) pathway in the CNS [45], [50], [51] as well in other tissues, such as MCF-7 breast cancer cells [21], [25], [37], human bone cells [4], [8] and endothelial cells [46], [49] to mention some; however, the initial event is still unclear. Also in other cells estrogen uses another signal transduction: the activation of Ca [29] and K [57] channels. We do not know how the steroid lead to changes in the activity of these channels: it could be either directly through specific binding sites in these proteins [57] or indirectly, through changes in cAMP [7], [29]. Though the activation of the G protein-coupled signal transduction pathway was established many years ago by the pioneer work of Szego et al. [52] a renew effort in this direction indicate that estradiol at physiological concentrations indeed used this non-genomic pathway to initiate a cascade of intracellular responses. For instance, 1 nM 17β-estradiol within 5 min stimulated the phosphorylation of Erk-1 and -2 proteins in both MCF-7 cells (with ER α(+) and ER β(+) receptors) and in SKBR3 cells which lack these receptors indicating that activation of this pathway does not require the nuclear estrogen receptors (nERs). In addition, the authors of this paper [9] clearly showed that estrogen-induced activation of the Erk proteins requires cellular expression of the G protein receptor homologue, GPR30, an orphan receptor. Curiously, under the conditions of their study ICI 182,780, considered a pure anti-estrogen, was also capable to activate the SKBR3 cells. The complexity of this pathway was also apparent from their results because the GPR30-mediated phosphorylation of the Erk proteins requires EGF receptor tyrosine kinase activity and occurs through the release of surface-associated heparin-binding EGF (epidermal growth factor). These results indicate that neither of the two nuclear receptors mediates the estrogen activation of the MAPK pathway; however, others have evidence indicating the need or presence of such receptors [42], [49], [59]. Future studies will be required to clarify such contradictory data. On the other hand, earlier work indicated that GPCRs (G protein-coupled receptors) stimulated by physiological doses of estrogen led to phospholipase C activation and that it is the G β, γ subunits rather than G α, the ones involved in the effect of 17β-estradiol in osteoblast cells [19].
The demonstration of specific and high affinity binding sites in the CNS for estradiol (for review see Ref. [40]) indicates that specific proteins mediate the so-called non-genomic actions of estradiol. Though several laboratories [5], [16], [56] including us [39] have reported proteins with affinity to estradiol, in some cases different in molecular size to the nERs, none of those proteins have been isolated and identified, except for OSCP, a subunit of the ATPase/ATP synthase [63], a novel target of estrogens. Another very interesting protein is c-erB2 because it binds with high affinity to estradiol [22], and some of the actions of this steroid may be mediated by this family of receptors [34], [35].
In this report we will review the current status of our efforts to isolate the so far elusive mER. The data clearly indicate that estradiol binds to several proteins in the CNS of which two have been isolated and characterized: GAPDH (glyceraldehyde-3-phosphate dehydrogenase) and β-tubulin. The former, a classical glycolytic enzyme but with new functions during the past 20 years [2], [12], [47]. The latter, a 55K protein essential for the formation of microtubules through a dynamic process of polymerization and depolymerization [48]. The overall review points to the concept that the elusive mER is not a single cognate protein but a myriad of membrane proteins with different but crucial functions at the cellular level.
Section snippets
The estradiol-binding proteins
Our laboratory has a long-term interest in the rapid action of estradiol and progesterone. Several papers indicate that both steroids can rapidly activate cellular processes leading to release of dopamine from rat striatal fragments superfused in vitro in the case of estradiol [1] and the E-6-BSA conjugate [39], and LHRH [41] in the case of progesterone. This latter steroid also modulates amphetamine-induced dopamine release [6] and the complex, P-3-BSA stimulated LHRH release from hypothalamic
Overview
Our efforts to isolate and identify a membrane estrogen receptor (mER) from the rat brain [38] and the liver [26] so far have failed. However, in the process of such studies we have identified several physiologically relevant estrogen binding proteins: OSCP [63], GAPDH [17], tubulin [20] and an isoform of CEH [27], all of them showing high affinity for 17β-estradiol. In this review, we will discuss the significance of the binding of estradiol to GAPDH and tubulin, the trust of the present
Conclusions and speculations
It appears from the information so far accumulated that steroids hormones activate specific brain cells by means of rapid membrane initiated events as well as by slower genomic mechanisms. The rapid initiated events by steroids involve a series of different target proteins among which GAPDH and tubulin have been identified. They should be considered in explaining some of the known effect of the sex hormones, particularly in cell morphology in general, and changes in dendritic density in the
Acknowledgements
The authors wish to thank Ms. Norma Ramirez for her technical help with the binding assays and Ms. Penny Morman for preparing the manuscript. This work was in part funded by a NIH Grant to Dr. V.D. Ramirez.
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