G protein-coupled estrogen receptor 1 as a novel regulator of blood pressure

Abstract

The mechanisms underlying hypertension are multifaceted and incompletely understood. New evidence suggests that G protein-coupled estrogen receptor 1 (GPER1) mediates protective actions within the cardiovascular and renal systems. This mini-review focuses on recent advancements in our understanding of the vascular, renal, and cardiac GPER1-mediated mechanisms that influence blood pressure regulation. We emphasize clinical and basic evidence that suggests GPER1 as a novel target to aid therapeutic strategies for hypertension. Furthermore, we discuss current controversies and challenges facing GPER1-related research.

INTRODUCTION

Hypertension is a serious public health problem that impacts one-third of Americans (88). Referred to as the “silent killer” because it does not cause overt early symptoms, hypertension is associated with multiple comorbidities and contributes to 10% of deaths worldwide (16). Despite the availability of multiple antihypertensive regimens, almost half of individuals diagnosed with hypertension in the United States do not have their blood pressure under control (90), highlighting the need to identify new therapeutic targets for the management of high blood pressure.

Accumulating evidence indicates that estrogen confers protection from hypertension in premenopausal women (89). Estrogen binds two classical nuclear receptors, estrogen receptor (ER)-α and ER-β, as well as G protein-coupled estrogen receptor 1 (GPER1), which initiates a rapid signaling response (73, 79). GPER1, previously known as GPR30, is a seven-transmembrane domain receptor (93). Initial research showed that GPER1 activation results in transactivation of the epidermal growth factor receptor and downstream signaling to mitogen-activated protein kinases (24). Subsequent studies showed that GPER1 activation can also stimulate cAMP production (25, 79), intracellular Ca²⁺ mobilization (7, 73), and phosphatidylinositol 3,4,5-trisphosphate synthesis (73). In addition to mediating these rapid signaling events, GPER1 has been shown to indirectly regulate transcriptional activity (48, 57, 71).

GPER1 signaling regulates physiological functions in multiple organ systems, including the cardiovascular and renal systems (93). Importantly, evidence points to multiple protective effects of GPER1 activation within the cardiovascular and renal systems, which together suggest a potential role for GPER1 signaling in the maintenance of blood pressure.

Here, we provide a brief overview of the physiological actions of GPER1, with emphasis on the contribution of vascular, renal, and cardiac GPER1 in the maintenance of blood pressure. We also highlight the potential of therapeutically targeting GPER1 signaling for the management of hypertension. Finally, we discuss the current controversies and challenges facing the field of GPER1-related research.

GPER1 IN CARDIOVASCULAR AND RENAL HEALTH

Estradiol has been implicated in sex-specific discrepancies in hypertension (5, 64). However, the mechanisms underlying the protective effects of estradiol within the cardiovascular and renal systems are complicated and not yet fully defined. Indeed, the protective effects mediated by ER-α and ER-β have been studied (17, 80, 92), yet cardioprotective and renoprotective estrogenic actions are evident in the absence of these classical ERs (37, 40, 82), suggesting a role for nonclassical estrogen receptors, such as GPER1, in these protective effects.

GPER1 is ubiquitously expressed throughout the cardiovascular and renal systems (38). The identification of G1 as a highly selective synthetic GPER1 agonist allowed studies that have vastly improved our understanding of GPER1 function (70). Of note, G1 displays no binding affinity for ER-α or ER-β (1, 6) and no activity in GPER1 knockout mice (30, 56, 83). Intravenous infusion of G1 results in an acute decline in blood pressure in normotensive male Sprague-Dawley rats (30). Moreover, long-term treatment with G1 systemically elicits an antihypertensive effect in ovariectomized mRen2.Lewis (50) or Sprague-Dawley rats (27). Importantly, evidence from human studies further suggests a role for GPER1 dysfunction in hypertension (11, 21, 44, 55, 78).

Blood Vessels

GPER1 is widely expressed throughout the vascular system, including vascular endothelial cells and smooth muscle cells (8, 29, 49, 50, 52, 63). Extensive evidence shows that GPER1 plays an important role in regulating the vascular tone in different vascular beds. For example, studies have demonstrated that activation of GPER1 evokes endothelium-dependent or nitric oxide-dependent vasodilatation in male and female animals (8, 26, 32, 52, 67, 81). Conversely, pharmacological blockade of GPER1 results in vasoconstriction of aortas from female mRen2.Lewis rats (53), suggesting that endogenous activation of GPER1 by circulating estrogen facilitates vasodilatation under basal conditions. GPER1 activation in male mice ameliorates the vasoconstrictor response to other vasoactive agents, including adipose-derived contracting factor (62) and endothelium-dependent contracting factors such as prostanoids (59) and endothelin-1 (60). Consistently, vasodilator responses to G1 are also evident in male and female human vascular beds (4).

Although evidence indicates that GPER1 modulates vascular tone and vascular reactivity in both sexes, sex differences in GPER1 expression and/or vascular responses have been documented. For example, in mRen2.Lewis rats, GPER1 protein abundance is higher in mesenteric arteries of female rats than in those of male rats (51). In contrast, Peixoto and colleagues (68) showed that, in Wistar rats, GPER1 protein abundance is greater in mesenteric arteries of male rats. Interestingly, despite the sex difference in GPER1 expression, G1-induced vasodilation in mesenteric arteries did not differ by sex in these rats (68). Moreover, G1-induced vasodilation is more pronounced in human resistance arteries isolated from postmenopausal women than in those from age-matched men (4). These findings indicate further studies are needed to clarify male-female differences in the localization, expression, and function of GPER1 under physiological and pathophysiological conditions.

The mechanisms by which GPER1 activation exerts vasoprotective effects under pathophysiological conditions are likely multifactorial and only partially understood as well. Liu et al. (54) showed that systemic GPER1 activation attenuates salt-induced aortic remodeling in female mRen2.Lewis rats. It has also been shown that treatment of isolated carotid and cerebral arteries with G1 reduces NADPH-stimulated superoxide production in male and female Sprague-Dawley rats (8), suggesting that GPER1 exerts an antioxidant effect. In female mice, GPER1 deletion enhances atherosclerosis and increases total and low-density lipoprotein-cholesterol levels (63), suggesting an atheroprotective effect of GPER1 that is related to adipogenesis. Moreover, GPER1 has been reported to regulate proinflammatory proteins in human umbilical vein endothelial cells (12), pointing to GPER1 as a modulator of vascular inflammation. However, the exact signaling cascades by which GPER1 mitigates inflammation, reactive oxygen species production, and atherosclerosis have not been fully defined.

The Kidney

Expression of GPER1 has been demonstrated in renal tubular and epithelial cells (15, 53). Specifically, GPER1 has been detected in the cortical epithelia (15), brush border of proximal tubules (53), distal convoluted tubule, loop of Henle, proximal convoluted tubule (15), mesangial cells (46), renal interlobular artery smooth muscle and endothelial cells (13), inner medullary collecting duct cells (14), and smooth muscle cells of the renal pelvic region (33).

Activation of GPER1 systemically mitigates salt-induced renal injury and proteinuria in female mRen2.Lewis rats in a blood pressure-independent manner (53). In addition, systemic GPER1 activation protects against renal ischemic injury in ovariectomized Sprague-Dawley rats (13). Furthermore, infusion of G1 to the renal medulla evokes a natriuretic response in female, but not male, Sprague-Dawley rats (27), possibly due to the markedly higher GPER1 mRNA expression and protein abundance detected in renal outer medullary tissues of female rats (27). Similarly, Hutson et al. (38) showed greater expression of renal GPER1 mRNA expression in female versus male Sprague-Dawley rats. More investigation is needed to delineate the specific role of renal GPER1 in regulating blood pressure and whether sex plays a role in this context.

The renoprotective actions of GPER1 include increasing estimated glomerular filtration rate (53) and megalin expression (53) and reducing oxidative stress (42, 53), renal hypertrophy (53), and glomerular permeability (36). GPER1 also regulates mesangial cell migration and extracellular matrix production (46). In addition, GPER1 activation inhibits podocyte apoptosis by reducing the production of reactive oxygen species (72). Similarly, treatment with G1 augments superoxide dismutase activity and decreases malondialdehyde levels in renal proximal tubular epithelial cells treated with methotrexate (42), further highlighting the antioxidant potential of GPER1 activation.

Consistent with the established vasodilatory responses to GPER1 activation in multiple extrarenal vascular beds, systemic G1 improves nitric oxide-dependent vasodilatory function in the renal interlobular artery of ovariectomized Sprague-Dawley rats (13). In addition, G1 promotes vasodilation in isolated perfused male or female rat kidneys preconstricted with phenylephrine (43). However, under basal renal perfusion pressure, G1 induces a vasoconstrictor effect in the isolated perfused kidneys of male or female rats (43). Further studies are necessary to understand the contrasting response to GPER1 activation in the renal vascular bed under different perfusion pressures.

The Heart

In the myocardium, GPER1 is expressed in cardiac myocytes (20, 85), fibroblasts (87), and mast cells (91). Multiple lines of evidence demonstrate that, in female subjects, GPER1 exerts cardioprotective effects against various stressors, including high-salt diet (39), myocardial ischemia-reperfusion injury (20, 45, 66), ovariectomy, and aging (3, 84). In ovariectomized rats, systemic GPER1 activation restores cardiac structure and function through antiproliferative actions in cardiac fibroblasts (87) and mast cells (91). Systemic treatment of female mRen2.Lewis rats with G1 attenuates high salt-induced cardiomyocyte hypertrophy and wall thickening in a blood pressure-independent manner (39). Systemic activation of GPER1 also reverses age- and ovariectomy-induced loss of myocardial relaxation in female Fischer344 × Brown Norway rats by decreasing interstitial fibrosis and increasing Ca²⁺-ATPase expression in the sarcoplasmic reticulum (3). Furthermore, cardiomyocyte-specific deletion of GPER1 increases cardiac oxidative stress in female mice (86).

In male rats, systemic GPER1 activation mitigates doxorubicin-induced cardiotoxicity, as revealed by reduced levels of oxidative stress (18). Moreover, cardiomyocyte-specific deletion of GPER1 results in left ventricular dysfunction and increases wall thickness in male and female mice (85). Treatment of isolated Sprague-Dawley rat hearts with G1 also improves functional recovery and reduces infarct size after ischemia-reperfusion injury in a sex-independent manner (20). Furthermore, no sex differences have been detected in GPER1 protein abundance or mRNA expression in total rat heart homogenates (20, 38). Similarly, GPER1 protein abundance does not differ by sex in the coronary arteries of Wistar rats, yet G1-induced vasodilation of the coronary bed is more pronounced in female rats than in male rats (19). Collectively, these findings suggest GPER1 as a therapeutic target for coronary heart disease (31). A thorough understanding of the mechanisms underlying the cardioprotective actions of GPER1 in both sexes will advance this and other therapeutic developments for heart diseases.

Overall, the expression of GPER1 within the cardiovascular and renal systems and the regulation of blood pressure by GPER1, as shown using pharmacological and genetic approaches, suggest a role for this receptor in the pathophysiology of cardiovascular and kidney disease.

AN EVOLVING ROLE FOR GPER1 AS A THERAPEUTIC TARGET FOR WOMEN WITH HYPERTENSION

In the United States, the prevalence of hypertension was 13.0%, 49.9%, and 73.9% among adult women aged 18–39, 40–59, and ≥60 yr old, respectively, in 2017–2018 (65a). Notably, compared with age-matched men (65a), premenopausal women are largely protected from hypertension, but consistent with the age-related increase in the prevalence of hypertension among women in the United States, this protective effect appears to diminish in women after menopause (41, 47). In addition to the studies discussed above that pointed to a role for GPER1 in regulating blood pressure, a recent study demonstrated that postmenopausal women who are hypertensive have lower serum GPER1 levels than those who are normotensive (55), establishing a clear association between serum GPER1 level and hypertension in women after menopause (55). Moreover, Feldman et al. (21) used a genetic variant approach to determine the impact of a common missense single nucleotide variant of GPER1, GPER P16L, on the risk of hypertension in humans. This study revealed that women, but not men, carrying this GPER1 hypovariant have higher blood pressure than counterparts without the variant (21), supporting a central role for GPER1 in regulating blood pressure in women. In addition, Hussain et al. (35) demonstrated that women carrying this GPER1 genetic variant have increased levels of plasma low-density lipoprotein and total cholesterol. This effect is also sex specific, as it was not evident in men (35). Of note, the GPER1 gene has been mapped to chromosome 7p22, a genetic locus that has been associated with hypertension in humans (11, 44, 78).

In addition, animal studies have shown that GPER1 protects the cardiovascular and renal systems in models of surgical menopause or aging (50, 58). Thus, the emerging clinical and preclinical findings support the need for further research on GPER1 as a potential therapeutic target for controlling blood pressure in postmenopausal women.

CONTROVERSIES AND CHALLENGES IN THE FIELD OF GPER1 RESEARCH

Aldosterone-GPER1 Interaction

In addition to mediating estrogen-induced rapid signaling events, GPER1 appears to be involved in aldosterone-induced rapid signaling events. Antagonism of mineralocorticoid receptor (MR) or GPER1 inhibits aldosterone-induced signaling in the rat aorta (29). Similarly, it has been shown that GPER1 mediates aldosterone-induced signaling in mouse mesenteric arteries (22) and breast cancer cells (74). However, studies have demonstrated that aldosterone does not bind directly to GPER1 in breast cancer cells or whole kidney tissues (15, 74). Rather, it appears that aldosterone triggers an interaction between MR and GPER1 (74). Additional mechanistic study is necessary to determine whether the MR-GPER1 interaction is evident in a tissue- or cell-specific manner in male and female subjects. Of note, it has been recently demonstrated that GPER1 activation elicits a potent aldosterone secretagogue effect in human adrenocortical cells (10, 75). Overall, the cross-talk between GPER1 and MR may be a novel mechanism by which GPER1 regulates the renin-angiotensin aldosterone system and, consequently, blood pressure.

GPER1 Subcellular Localization

Cell membrane localization of GPER1 has been reported (23, 79). However, GPER1 localization has been detected in the endoplasmic reticulum and Golgi apparatus as well (73, 76), although this localization may be related to GPER1 protein synthesis. This controversy over the subcellular localization of GPER1 might be related to the lack of specificity provided by commercially available anti-GPER1 antibodies. The development of antibodies that specifically recognize GPER1 will be crucial for accurately detecting GPER1 protein in tissues.

GPER1 in Cycling Female Subjects

Animal studies have demonstrated that GPER1 expression level in the brain (77), kidney (15), and pituitary gland (9) varies throughout the estrus cycle. Notably, the subcellular localization of renal GPER1 also fluctuates with the estrus cycle (15). Similarly, human studies have revealed that endometrial expression of GPER1 in women is regulated by the menstrual cycle (34, 69). The observation that the estrus/menstrual cycle influences GPER1 expression level and/or localization adds another layer of complexity to studying GPER1 function in female subjects.

Sex and Age Differences in GPER1 Antihypertensive Actions

G1-mediated activation of GPER1 does not alter blood pressure chronically in male or ovary-intact female mRen2.Lewis rats (50). However, chronic treatment with G1 decreases blood pressure in ovariectomized mRen2.Lewis and Sprague-Dawley rats (27, 50), suggesting that the antihypertensive potential of GPER1 may be more robust after loss of ovarian function. Additionally, genetic deletion of GPER1 results in increased blood pressure in aged, but not young, female mice (58, 65), indicating that age critically affects GPER1-mediated blood pressure regulation in female subjects. Whether age impacts blood pressure regulation by GPER1 in male subjects is not clear yet.

CONCLUSIONS

To date, studies have identified considerable evidence of a role for GPER1 in regulating blood pressure. However, important questions remain to be answered. In particular, the sex differences in GPER1 expression and function must be clarified. Moreover, in light of the relatively high abundance of GPER1 in the brain, the possibility that GPER1 within the central nervous system regulates blood pressure should also be examined. Finally, identifying the upstream regulators and downstream signaling cascades for GPER1 within the cardiovascular and renal systems will advance our understanding of nonclassical estrogenic signaling and its clinical relevance in cardiovascular and renal disease.

GRANTS

This work was supported by National Institute of Diabetes and Digestive and Kidney Diseases Grant K99DK119413 (to E.Y.G.).

DISCLOSURES

No conflicts of interest, financial or otherwise, is declared by the author.

AUTHOR CONTRIBUTIONS

E.Y.G. interpreted results of experiments; E.Y.G. prepared figures; E.Y.G. drafted manuscript; E.Y.G. edited and revised manuscript; E.Y.G. approved final version of manuscript.