Recent insights into non-nuclear actions of estrogen receptor alpha
Introduction
Via direct or indirect binding to regulatory elements on target genes, estrogen receptors alpha and beta (ERα, ERβ) classically function as steroid hormone nuclear receptors regulating gene transcription [1], [2]. Over the past 15–20 years it has also become apparent that ER serve functions outside the cell nucleus that entail unique post-translational modifications and protein–protein interactions of the receptor with adaptor molecules, G proteins and kinases. Upon ligand binding to non-nuclear ER, a variety of signaling events are prompted that can themselves modify cellular behavior, or that influence cellular responses to nuclear ER activation through complex non-nuclear to nuclear cross-talk events [3], [4], [5]. In this review we will focus on recent insights gained regarding non-nuclear ER actions and their physiologic and pathologic consequences. Using endothelial cells as an example of a model system, a summary of non-nuclear ER signaling mechanisms will first be provided. Related works in non-endothelial cells that have yielded complementary insights will also be discussed. This will be followed by an overview of adaptor proteins that have been implicated in non-genomic ER function. These topics set the stage for a discussion of genetic approaches that have more recently been taken to better understand the cell biology of non-nuclear ER actions. This will be followed by a summarization of recent attempts to query the biology of non-nuclear ER in vivo in a finite number of tissue contexts, and also a review of the recent discovery of a novel partnership of another nuclear receptor with ER in plasma membrane caveolae/lipid rafts. Finally, we will point out the potential future directions for research in this field by highlighting the questions that remain to be answered and the current challenges.
Section snippets
Non-nuclear ERα signaling mechanisms
The signaling events initiated by the non-nuclear subpopulation of ER have been elucidated in a variety of cell types. These include breast cancer cells, oocytes, osteoblasts, osteoclasts, vascular smooth muscle cells, endothelial cells and other specific cell types [4], [5], [6], [7]. Because of the immediate consequences on a variety of processes in the vascular wall that greatly impact vascular health and disease, the mechanisms underlying the coupling of non-nuclear ERα to endothelial NO
Non-nuclear ER adaptor proteins
Since a subpopulation of ERα is localized to endothelial cell caveolae, and the receptor has both physical and functional interaction with other signaling molecules in that specialized microdomain, adaptor proteins are likely to be important to non-nuclear actions of ERα in endothelial cells. One such adaptor molecule is the ERα binding protein striatin, which is a member of the WD-repeat protein family. In studies performed in the endothelial cell line Eahy.926, striatin and ERα were
Mutagenesis of ER to interrogate non-nuclear action
In addition to pharmacologic strategies to antagonize signaling molecules that are activated by non-nuclear ER, or the use of peptides to interfere with potentially important protein–protein interactions, mutagenesis of ER has been employed to query the biology of non-nuclear functions of the receptor. Although not found in all studies, there is evidence that ERα interacts with caveolin-1 to facilitate receptor trafficking to caveolae/lipid rafts [22], [41]. Consistent with an earlier leading
Non-nuclear ER action in vivo in vasculature
To provide selective activation of non-nuclear ER, an estrogen-dendrimer conjugate (EDC) was created in which approximately 20 ethinyl-E2 molecules are attached to a large, positively charged nondegradable poly(amido)amine (PAMAM) dendrimer via hydrolytically stable linkages, thereby excluding EDC from the nucleus. Importantly, the nature of the chemical linkage of the PAMAM to E2 is such that the affinity of EDC-bound E2 for ER is similar to that of free E2 [11], [47]. Mirroring the actions of
Non-nuclear ER action in vivo in bone
Another tissue in which non-nuclear ER actions are potentially important is in bone [49], [50]. Estrogen deficiency adversely impacts bone because it causes an increase in bone remodeling, with increased osteoclast and osteoblast number and activity, and increased bone resorption and formation that occur in an unbalanced manner [51], [52], [53]. Estrogen also has beneficial effects on bone due to its antioxidant actions [51]. To determine the role of non-nuclear ER in estrogen actions on bone,
Novel partnership of LXR with non-nuclear ER
Like ER, Liver X receptors α and β (LXRα, LXRβ) are members of the nuclear receptor superfamily, and they serve as ligand-dependent transcription factors that play key roles in lipid metabolism [55]. In a recent study seeking to determine how LXR potentially function in endothelial cells, it was found that LXR agonists stimulate endothelial cell migration via eNOS- and LXRβ-dependent processes. Possible involvement of ER in endothelial cell LXR function was first apparent in studies in which
Unanswered questions and current challenges
Although much of our current understanding of non-nuclear ER action has been derived from experiments performed in cell culture, in vivo studies entailing either selective gain-of-function or selective loss-of-function in the last 3 years have added valuable new insights into the physiologic and pathophysiologic consequences of non-nuclear ER actions in the vascular wall and in bone. However, we do not yet know how non-nuclear ER actions contribute to a variety of processes in other cell types
Acknowledgments
The authors thank their many colleagues and collaborators who have contributed to the effort to better understand non-nuclear estrogen receptor actions. This work was supported by NIH Grant HL087564, by the Crystal Charity Ball Center for Pediatric Critical Care Research and by the Associates First Capital Corporation Distinguished Chair in Pediatrics at UT Southwestern.
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2018, Vitamins and HormonesCitation Excerpt :In addition to ligand binding, SRs and NRs are regulated by covalent modification including phosphorylation (Anbalagan & Rowan, 2015; Lannigan, 2003; Rajbhandari et al., 2012), methylation (Zhang et al., 2013), and acetylation (Abdel-Hafiz & Horwitz, 2014; Kim, Woo, Chong, Homenko, & Kraus, 2006). Ligand-activated SRs and NRs interact with coregulator proteins (coactivators and corepressors) as well as chromatin-remodeling complexes to regulate target gene expression (Dasgupta, Lonard, & O'Malley, 2014; Klinge, 2000; O'Malley, 2009; York et al., 2013) In addition, a small percentage of total SRs are also associated with the plasma membrane (PM) and PM proteins leading to activation of intracellular phosphorylation cascades mediated by MAPK and PI3K as well as other rapid signaling processes (reviewed in Banerjee, Chambliss, Mineo, & Shaul, 2014; Lang, Alevizopoulos, & Stournaras, 2013; Mueck, Ruan, Seeger, Fehm, & Neubauer, 2014; Tasker, Di, & Malcher-Lopes, 2006; Watson, Jeng, & Guptarak, 2011). This is called the “nongenomic” activity of these SRs, but this is somewhat of a misnomer since activation of these PM-associated SRs leads ultimately to changes in gene expression.