GPER

Discovery

The classical estrogen receptors first characterized in 1958 are water-soluble proteins located in the interior of cells that are activated by estrogenenic hormones such as estradiol and several of its metabolites such as estrone or estriol. These proteins belong to the nuclear hormone receptor class of transcription factors that regulate gene transcription. Since it takes time for genes to be transcribed into RNA and translated into protein, the effects of estrogens binding to these classical estrogen receptors is delayed. However, estrogens are also known to have effects that are too fast to be caused by regulation of gene transcription. In 2005, it was discovered that a member of the G protein-coupled receptor (GPCR) family, GPR30 also binds with high affinity of estradiol and is responsible in part for the rapid non-genomic actions of estradiol. Based on its ability to be bind estradiol, GPR30 was renamed as G protein-coupled estrogen receptor (GPER). Unlike other members of the GPCR family, which reside in the outer membrane of cells, GPER is localized to the endoplasmic reticulum.

Ligands

GPER binds estradiol though not other endogenous estrogens, such as estrone or estriol, nor for other endogenous steroids, including progesterone, testosterone, and cortisol. Although potentially involved in signaling by aldosterone, GPER does not show any detectable binding towards aldosterone. Niacin and nicotinamide bind to the receptor in vitro with very low affinity.

CCL18 has been identified as an endogenous antagonist of the GPER.

Function

This protein is a member of the rhodopsin-like family of G protein-coupled receptors and is a multi-pass membrane protein that localizes to the endoplasmic reticulum. The protein binds estradiol, resulting in intracellular calcium mobilization and synthesis of phosphatidylinositol (3,4,5)-trisphosphate in the nucleus. This protein therefore plays a role in the rapid nongenomic signaling events widely observed following stimulation of cells and tissues with estradiol. The distribution of GPER is well established in the rodent, with high expression observed in the hypothalamus, pituitary gland, adrenal medulla, kidney medulla and developing follicles of the ovary.

Reproductive tissue

GPER is expressed in the breasts, and activation by estradiol produces cell proliferation in both normal and malignant breast epithelial tissue. However, GPER knockout mice show no overt mammary phenotype, unlike ERα knockout mice, but similarly to ERβ knockout mice. This indicates that although GPER and ERβ play a modulatory role in breast development, ERα is the main receptor responsible for estrogen-mediated breast tissue growth. GPER is expressed in germ cells and has been found to be essential for male fertility, specifically, in spermatogenesis. GPER has been found to modulate gonadotropin-releasing hormone (GnRH) secretion in the hypothalamic-pituitary-gonadal (HPG) axis.

Cardiovascular effects

GPER is expressed in the blood vessel endothelium and is responsible for vasodilation and as a result, blood pressure lowering effects of estrogen. GPER also regulates components of the renin–angiotensin system, which also controls blood pressure, and is required for superoxide-mediated cardiovascular function and aging.

Central nervous system activity

GPER and ERα, but not ERβ, have been found to mediate the antidepressant-like effects of estradiol. Contrarily, activation of GPER has been found to be anxiogenic in mice, while activation of ERβ has been found to be anxiolytic. There is a high expression of GPER, as well as ERβ, in oxytocin neurons in various parts of the hypothalamus, including the paraventricular nucleus and the supraoptic nucleus. It is speculated that activation of GPER may be the mechanism by which estradiol mediates rapid effects on the oxytocin system, for instance, rapidly increasing oxytocin receptor expression. Estradiol has also been found to increase oxytocin levels and release in the medial preoptic area and medial basal hypothalamus, actions that may be mediated by activation of GPER and/or ERβ. Estradiol, as well as tamoxifen and fulvestrant, have been found to rapidly induce lordosis through activation of the GPER in female rats.

Metabolic roles

Female GPER knockout mice display hyperglycemia and impaired glucose tolerance, reduced body growth, and increased blood pressure. Male GPER knockout mice are observed to have increased growth, body fat, insulin resistance and glucose intolerance, dyslipidemia, increased osteoblast function (mineralization), resulting in higher bone mineral density and trabecular bone volume, and persistent growth plate activity resulting in longer bones.

Clinical significance

GPER plays a role in breast cancer progression and tamoxifen resistance. GPER has also been proposed as a biomarker in triple-negative breast cancer.