Estrogen receptor subcellular localization and cardiometabolism

Abstract

Background: In addition to their crucial role in reproduction, estrogens are key regulators of energy and glucose homeostasis and they also exert several cardiovascular protective effects. These beneficial actions are mainly mediated by estrogen receptor alpha (ERα), which is widely expressed in metabolic and vascular tissues. As a member of the nuclear receptor superfamily, ERα was primarily considered as a transcription factor that controls gene expression through the activation of its two activation functions (ERαAF-1 and ERαAF-2). However, besides these nuclear actions, a pool of ERα is localized in the vicinity of the plasma membrane, where it mediates rapid signaling effects called membrane-initiated steroid signals (MISS) that have been well described in vitro, especially in endothelial cells.

Scope of the review: This review aims to summarize our current knowledge of the mechanisms of nuclear vs membrane ERα activation that contribute to the cardiometabolic protection conferred by estrogens. Indeed, new transgenic mouse models (affecting either DNA binding, activation functions or membrane localization), together with the use of novel pharmacological tools that electively activate membrane ERα effects recently allowed to begin to unravel the different modes of ERα signaling in vivo.

Conclusion: Altogether, available data demonstrate the prominent role of ERα nuclear effects, and, more specifically, of ERαAF-2, in the preventive effects of estrogens against obesity, diabetes, and atheroma. However, membrane ERα signaling selectively mediates some of the estrogen endothelial/vascular effects (NO release, reendothelialization) and could also contribute to the regulation of energy balance, insulin sensitivity, and glucose metabolism. Such a dissection of ERα biological functions related to its subcellular localization will help to understand the mechanism of action of "old" ER modulators and to design new ones with an optimized benefit/risk profile.

Keywords: Cardiovascular system; Energy balance; Estrogen receptors; Genomic effects; Glucose homeostasis; Membrane-initiated steroid signals.

Figure 1

ERα subcellular localization allows the activation of both nuclear and membrane-initiated signaling pathways. This schematic representation first indicates the mechanistically distinct molecular pathways used by ERα to regulate gene transcription. The classical pathway (blue arrows) sequentially includes ligand (17β-estradiol, E2) activation of cytosolic ERα bound to heat shock protein 90 (Hsp90), ERα dimerization, and direct DNA binding to estrogen response elements (ERE). The tethered pathway (middle) depends on protein–protein interaction with other transcription factors following ERα ligand activation. In that case, gene regulation relies on indirect DNA binding on AP1/SP1 sites (through such interactions with other transcription factors). The membrane-initiated signal (MISS) pathway, also known as nongenomic pathway (green arrows), is mediated by a small pool of ERα localized close to the plasma membrane through ERα posttranslational modifications such as palmitoylation of Cys447 (human) or Cys551 (mouse) and direct interaction with caveolin-1. Upon E2 activation, membrane ERα interacts with protein kinases (Src and PI3K) or G-coupled protein αi (Gαi), leading to distinct signaling cascades (Akt, PKA, ERK1/2), and activation of endothelial NO synthase (eNOS). Grey arrows indicate the possible impact of the MISS pathway on nuclear/genomic pathways.

Figure 2

ERα modular structure and molecular strategy used for the generation of ERα mutant mice targeting either nuclear or MISS signaling pathways. The full length 66 kDa ERα can be subdivided into six domains (A to F), including a DNA binding domain (DBD), a ligand binding domain (LBD), and two transcriptional activation functions, respectively named AF-1 and AF-2. For a more detailed description of ERα molecular structure, please refer to the text and the following references . Below ERα linear structure are indicated the strategies that have been held (targeted amino acids for point mutations or deletions) to inactivate specific ERα functions and generate the main mouse models available to study the respective roles of nuclear ERα (ERαAF-1⁰, ERαAF-2⁰, ERα KIKO/EAAE-ERα, leading to full or partial inactivation of genomic actions) and MISS ERα (C451A-ERα).