Unveiling the effect of estrogen receptors in alcoholic liver disease: A novel outlook

Abstract

Alcoholic liver disease (ALD) has a multifaceted development, progressing from alcoholic steatosis to alcoholic hepatitis and ultimately to alcoholic cirrhosis, irreversible liver damage that can even result in hepatocellular carcinoma. The prevalence of ALD is increasing globally, particularly among middle-aged adults. Gender-based studies have revealed that ALD affects more men; however, disease progression differs between men and women. Despite this, the molecular understanding of alcohol-induced liver injury among genders and its association with changes in sex hormone metabolism, particularly with estrogen and estrogen receptors (ERs) in ALD, remains poor. This review focuses on experimental and human studies describing alcohol and its association with estrogen metabolism and signaling via ERs. Chronic alcohol consumption affects the immune response, and whether estrogen has any contributory effect remains inadequately studied. This review also discusses various therapeutic approaches currently in use and future approaches that can affect the response or progression via estrogen signaling. The role of gender on alcohol consumption and its association with steroid hormones must be elucidated for a better understanding of the pathogenesis of ALD, the development of effective therapeutic approaches, and better disease management in both men and women, as ALD remains a major public health concern.

1. Introduction

Alcohol is one of the leading causes of liver diseases, and alcoholic liver disease (ALD) accounts for 40%–50% of deaths among liver diseases worldwide.1, 2, 3 Approximately 90% of chronic alcoholics develop alcoholic fatty liver, whereas nearly 10%–20% develop advanced ALD. The amount and duration of alcohol intake are the most significant indicators of ALD progression, and other influencing factors include gender, genetic variations, ethnicity, obesity, metabolic syndrome, and viral hepatitis.⁴ Altogether, the prevalence and mortality rate of ALD are increasing, which is further predicted to rise continuously.⁵ Binge drinking is more common, particularly among young adults. As per US dietary guidelines, chronic consumption of standard alcoholic drinks (i.e., 4 g, or approximately 0.6 fluid ounces, of “pure” ethanol) of >3 for men and 2 for women per day can be associated with alcohol-related risks. However, 7.6% of men and 4.0% of women attribute global deaths to alcohol consumption.⁶

Alcohol-mediated hepatotoxicity is more likely to develop in women with low doses and shorter duration of alcohol consumption, making women more vulnerable to ALD.⁷ The mechanism behind this gender disparity suggests an involvement of sex hormones in the pathogenesis of alcohol-induced liver injury.

However, many studies have strongly pointed out the involvement of sex hormones in the pathophysiology of alcohol-induced liver injury. They have shown that chronic alcohol consumption changes the hormonal levels in the blood and liver in both gender;⁸ however, the real mechanism is unknown. The liver plays a major role in this interplay, as it is the site of steroid hormone metabolism and is responsive to sex hormones.

Alcoholic men undergo phenotypic changes as they cannot maintain optimum hormone levels and often show low testosterone and high estrogen levels.⁹ However, hormonal changes in alcoholic women are not much studied. A study reported that women with chronic alcoholism may have menstrual cycle disorders, such as amenorrhea, and luteal phase dysfunction.¹⁰ Menopause also occurs earlier in these women.¹¹ Rodent model investigations have depicted that chronic alcoholism induces the reduction in estrogen levels and is responsible for the changes in ovulation cycles, which causes reduced progesterone levels.¹² Interestingly, alcohol increases estrogen levels in the blood and alters estrogen metabolism in the liver; however, very few studies have focused on progesterone. A study suggested that estrogen levels positively correlated with the amount of alcohol consumed, and these findings can be a foundation for understanding ALDs and different disease severity based on gender.¹³ Estrogen receptors (ERs) are present in the liver and modulate hepatic immune responses, which may be an important mechanism in the progression of liver diseases.¹⁴ Moreover, current therapies for ALD management are influenced by ERs, which might decide the fate of response to therapy. However, more gender-based studies are needed to establish these differences in ALD development in humans, which affects liver hormone activity and levels of hepatoprotective factors in both men and women.

In this review, we discuss a new perspective on the role of estrogen, ERs, and ALD progression in both gender and the current and future therapies for ALD in modulating estrogen metabolism.

2. Various risk factors of ALD

The quantity and duration of alcohol consumption are the main risk factors for the development of ALD. Other risk factors include gender, obesity, genetics, leaky gut, and viral hepatitis (Fig. 1).

Fig. 1.

Various risk factors of alcoholic liver disease. The quantity and duration of alcohol consumption are the main risk factors for the development of alcoholic liver disease. Other risk factors include gender, obesity, genetics, leaky gut, and viral hepatitis.

2.1. Gender

To our earlier understanding, men have a higher ALD-associated mortality rate than women; interestingly, in the past few years, mortality has been increasing more in women than in men. A study covering the period from 2009 to 2015 reported that the incidence of ALD increased by 50% in women compared with a 30% rise in men.¹⁵ However, the Meta-analysis further depicted with that similar quantities of alcohol consumed, alcoholic cirrhosis was found more in women than in men, contributing to an increase in alcohol-related mortality in women than in men.¹⁶ Severe binge drinking was observed in female experimental mice, suggesting the role of female hormones in alcohol uptake.¹⁷ In addition, it might be due to mechanisms involved in lower gastric alcohol metabolism.^18,19 Thus, alcohol-mediated hepatotoxicity is more likely to develop in women with low doses and shorter durations of alcohol consumption, making women more vulnerable to ALD than men.⁷ Liver fibrosis and cirrhosis progress rapidly in women than in men, and fibrosis persists even after alcohol cessation.²⁰ Moreover, owing to gender differences in alcohol dehydrogenase activity, increases in alcohol bioavailability in women affect their hormone activity.²¹ Many experimental studies have further strengthened the gender-based differences and effects on ER expression, liver steatosis, and plasma endotoxin levels in alcohol-fed rats.²² More gender-based studies are needed to establish these differences in ALD development in humans, affecting liver hormone activity and levels of hepatoprotective factors in both men and women (Fig. 1).

2.2. Genetic factors

ALD is a multifactorial disease. Several patients with alcohol-related liver cirrhosis have reported having a family history of alcohol-related disorders and developing ALD. Monozygotic twins have a higher chance of alcohol-related liver cirrhosis than dizygotic twins, suggesting the influence of genetics on ALD.²³ A study indicated that monozygotic twins have a higher chance of having hereditary alcohol-related liver cirrhosis (21%–67%) than dizygotic twins.²³ The effect of behavioral genetics on excessive alcohol consumption was evaluated through a Meta-analysis, which found that heredity accounts for 30%–36% of cases.²⁴ A genome-wide association study, which was conducted by the National Institute on Alcohol Abuse and Alcoholism, identified candidate genes associated with alcoholism and neurotransmission. They found that the gamma-aminobutyric acid (GABA) receptor subunit alpha-2 (GABRA2) gene, which is an inhibitor of neurotransmitter GABA, was associated with daily alcohol consumption and alcohol withdrawal symptoms. It also correlated with the degree to which alcohol ingestion among non-dependent drinkers serves as a positive reinforcement.²⁵ Other studies found many gene polymorphisms associated with ALD such as Ras protein-specific guanine nucleotide releasing factor 2, nuclear factor erythroid 2-related factor 2, alcohol dehydrogenase 1B, patatin-like phospholipase domain-containing protein 3 (PNPLA3), dopamine receptor D2, and methylenetetrahydrofolate reductase.^26,27 Alcohol-induced epigenetic changes affect the hepatocytes, immune cells, and histone modification. Alcohol upregulates histone acetyltransferase and downregulates histone deacetylase activity, which affects H3 acetylation in hepatocytes.²⁸

2.3. Obesity

The role of obesity (body mass index >30 kg/m²) is highly associated with ALD risk because of extrahepatic and intrahepatic factors.²⁹ Adipose tissues play an important role because they contain inflammatory cytokines, and fatty acid flux contributes to hepatic inflammation and steatosis in ALD,³⁰ iron overload, and other metabolic disorders.³¹

2.4. Bacterial translocation

The gut–liver axis has been studied well in patients with ALD. A leaky gut with an altered gut barrier, increased intestinal permeability, and translocation of luminal antigens were reported in both experimental animal models of ALD and patients with advanced ALD.³² An increase in the abundance of bacterial endotoxins leads to a decrease in the levels of short-chain fatty acids (SCFAs). Bacteria such as Lachnospiraceae and Ruminococcaceae are associated with alcohol injury.³³ In fact, Enterococcus faecalis (cytolysin-positive) was identified as a novel target for ALD therapy because its presence correlated with ALD severity.³⁴

2.5. Viral factors

Viruses can influence the development and progression of ALD in several ways. First, viral infections such as hepatitis B and C can cause inflammation and damage to hepatocytes, which can exacerbate alcohol-related liver damage. This can lead to more severe forms of ALD. Second, viral infections can impair the hepatocyte’s ability to metabolize alcohol, leading to the buildup of toxic metabolites that further damage the liver. This can accelerate ALD progression and increase the risk of complications such as liver failure or hepatocellular carcinoma (HCC). Moreover, viral infections can suppress the immune system and make individuals more vulnerable to other infections, increasing the risk of complications.³⁵ Interestingly, a study found that patients with chronic hepatitis C virus (HCV) infection had higher levels of ER expression in their liver tissues than healthy ones and positively correlated with fibrosis progression, which highlights the complex role of estrogen signaling in HCV-induced liver disease, which might be also associated with ALD.³⁶

3. Estrogens and ERs in the liver

Estrogen, a steroid hormone, is lipid soluble and is one of the important sex hormones of women. To date, four types of estrogens have been identified, namely, estrone (E1), 17-beta estradiol (E2), estriol (E3), and estetrol (E4). Importantly, the term estrogen refers to E2 because it is the most widespread and physiologically active in multiple tissues or organs. Even in the liver, E2 plays a significant biological function. Three receptors for estrogen are identified, namely, classical ER alpha (ERα), ER beta (ERβ), and non-classical G–protein-coupled ER 1 (GPER1), which is also known as GPR30. Also, note that E2 has a stronger affinity for all its three receptors than other estrogens. All four estrogens have the same chemical functions and structures with 18 carbon atoms. However, the expression and function of ER subtypes vary depending on each cell type and organ. ERα is mainly highly expressed in the reproductive tissues, bones, adipose tissues, kidneys, and liver. ERβ is highly expressed in the reproductive organs of men, central nervous system, heart, lung, immune system, colon, and kidney. GPR30 is expressed in skeletal muscles, neurons, immune cells, endothelia, and effector organs. Importantly, GPR30 was reported to be more expressed in lung, breast, and ovarian cancer tissues.³⁷

Estrogens bind to their receptors, exerting genomic and non-genomic effects. Regarding its genomic effects, E2 binds to intracellular ERα and ERβ, which together make the E2 receptor dimer complex, further entering the nucleus. Inside the nucleus, this complex binds to estrogen response elements, also known as activator protein-1 (AP-1) and specificity protein-1 (SP-1), which further regulate gene transcription,³⁸ autophagy, apoptosis, survival, proliferation, etc. However, under pathological disruptions, their functions may change. E2 is also responsible for non-genomic effects by binding to its three receptors rapidly and activates nuclear transcription by ion channel regulation. This process does not undergo gene transcription or regulation because it occurs instantly, i.e., within seconds to minutes, which is also known as rapid non-genomic effects.³⁹

Interestingly, estrogens are known for immune modulation. ER signaling interferes with homeostasis, which regulates progenitor cell populations, eventually affecting the number and type of immune cells. E2 is also necessary for the self-renewal capacity of hematopoietic stem cells. Estrogen signaling is also involved in the differentiation of dendritic cells from monocytes and is highly expressed in B cells. GPR30 is abundantly present in natural killer (NK) cells, B cells, T lymphocytes, and neutrophils, showing that estrogens can induce immune responses according to the ER expression.⁴⁰

4. Relationship of estrogen and ERs to ALD

Alcohol increases estrogen levels by enhancing the levels of aromatase, which in turn converts androgen to estrogen.⁴¹ An increase of approximately 15% in the levels of estrogen was observed in alcoholic postmenstrual women.⁴² Post- and premenstrual women consuming alcohol were found to have increased estrogen levels, and researchers are associating this increase with the progression of breast cancer.⁴³ Another possible mechanism behind the increase in estrogen levels due to alcohol consumption involves the effects of phytoestrogens, as many alcoholic beverages are plant-based and contain estrogen-like substances secreted by plants.⁴⁴ Another study measured estrogen and mammographic density in premenopausal women and concluded a positive relationship between alcohol consumption and estrogen levels and mammographic density.⁴⁵ Not only women but also men with alcoholism have shown increased estrogen levels and decreased testosterone levels, hence the phenomena of feminization.⁴⁶ In another study, the expression of hepatic cytosolic ERs in livers increased in patients with chronic alcoholism. The cytosolic ER content in the livers of patients with alcoholic cirrhosis who quit drinking alcohol was found to be significantly lower than observed in those with alcoholic cirrhosis who were still drinking alcohol. Cytosolic ER content comparisons were also made in alcoholic hepatitis with cirrhosis and alcoholic hepatitis without cirrhosis, and ER levels were increased in both.⁴

Sex hormone disturbances were observed in patients suffering from alcoholism; however, whether the consequences of liver dysfunction caused by ethanol or hormone dysregulation lead to liver pathogenesis is unclear.⁴⁷ These studies have suggested that estrogen levels positively correlated with the amount of alcohol consumed, and these findings can be a foundation for understanding alcohol-related liver disease and differential disease severity based on gender. High levels of estrogen have also been correlated with increased risk and severity of liver disease in mouse models.⁴⁸

Alcohol consumption leads to gastrointestinal tract disruption, which in turn leads to endotoxin/lipopolysaccharide (LPS) secretion into the portal vein. This activates Kupffer cells and hence leads to the production of pro-inflammatory cytokines and reactive oxygen species (ROS) to remove LPS.⁴⁹ On the contrary, alcohol increased estrogen in rodent models, and these mice had increased tumor necrosis factor-alpha (TNF-α) levels. This enhances the sensitivity of Kupffer cells to LPS and thus increases the levels of toxic mediators.⁵⁰ The mechanism of ALD is the positive correlation between estrogen and growth hormones.⁵¹ An increase in growth hormone increases alcohol dehydrogenase levels, which further leads to the accumulation of toxic acetaldehyde and increases ALD risk (Fig. 2).⁵²

Fig. 2.

Schematic of the proposed mechanism showing the association of alcohol and estrogen with alcoholic liver disease. (a) Alcohol consumption disrupts the gastrointestinal tract and releases LPS into the circulation and portal vein. This induces Kupffer cells and initiates pro-inflammatory cytokine response. (b) Alcohol also increases estrogen levels that in turn enhance TNF-α levels, increasing the sensitivity of Kupffer cells to endotoxins/LPS. (c) Alcohol activates NK cells and leads to the activation of HSCs. (d) Estrogen receptors on non-parenchymal cells such as hepatocytes have shown a role in inflammatory cytokines TNF-α, TGF-β, IL, PMNs, cytokines, and ROS. (e) Estrogen is directly correlated with growth hormone; an increase in growth hormone leads to an increase in the production of alcohol dehydrogenase and toxic aldehydes in the liver. (f) High estrogen levels decrease the β-oxidation of fatty acids leading to mitochondrial dysfunction, fat accumulation in the liver, and ALI. Abbreviations: ALI, alcohol-induced liver injury; GI, gastrointestinal; HSCs, hepatic stellate cells; IL, interleukin; PAMPs, pathogen-associated molecular patterns; PMNs, polymorphonuclear cells; LPS, lipopolysaccharide; NK, natural killer; ROS, reactive oxygen species; TGF-β, transforming growth factor-beta; TNF-α, tumor necrosis factor-alpha.

Women with ALD show more symptoms of acute liver failure. A study reported the effects of liver function impairment on the hepatic uptake of estrogen and systemic plasma levels of endogenous sex steroids in men with alcohol-related liver cirrhosis, i.e., serum levels of testosterone and dehydroepiandrosterone were low, whereas the levels of androstenedione, estrone, and estradiol were significantly high in men. The level of sex hormone-binding globulin was also increased. The extent of hepatic uptake of sex steroids depends partly on the degree of liver function impairment and the degree to which they are bound to sex hormone-binding globulin. A high affinity for sex hormone-binding globulin lowers the hepatic uptake influenced by liver function impairment.⁵³ Hepatic clearance accounted for only 20%–50% of the metabolic clearance of sex steroids. Production rates, peripheral metabolism, or both, rather than hepatic uptake, account for altered circulating levels of sex steroids.⁵⁴ The response of the liver to estrogen stimulation includes the maintenance of ERs and regulation of the expression and activity of several enzymes such as steroid 5 alpha-reductase, which is responsible for testosterone clearance. This enzyme was found to be increased in HCC.⁵⁵

As mentioned earlier, the feminization of men with alcoholism occurs.⁴⁶ In these men, weak androgens of adrenal origin are converted to estrone and estradiol in extrahepatic peripheral tissues. With this conversion, the concentration of E2 is increased, which leads to an increase in the E2-to-testosterone ratio.⁵⁶ A study reported the presence of high-affinity ERs in rat liver Kupffer cells, which have the same characteristics as the ERs in hepatocytes. Kupffer cells produce pro-inflammatory cytokines and play an important role in the pathogenesis of alcohol-induced liver injury. Alcoholism increases the ER content in the liver of patients with alcoholic hepatitis who are actively drinking. Chronic alcoholism in rats leads to a significant increase in the ER content of male livers, whereas female livers did not present any significant changes.⁵⁷ Women had higher apoptosis rates and reduced hepatocyte proliferation than men. Male rats were found to have less severe liver injury than female rats because of high levels of hepatocyte proliferation, which makes male livers more resistant than female livers to the toxic effects of alcohol. The response of the liver to estrogen stimulation includes the maintenance of ERs and regulation of expression and activity of several enzymes, such as steroid 5 alpha-reductase, which are responsible for testosterone clearance.⁴

Among sex hormones, progesterone, an important hormone, has not been extensively studied in relation to ALD. Progesterone is secreted by granulosa luteal cells and is categorized as a steroid hormone.⁵⁸ Higher progesterone levels were found to be associated with different liver diseases such as drug-induced liver injury, non-alcoholic fatty liver disease (NAFLD), hepatitis E virus (HEV), fibrosis, cirrhosis, and HCC.⁵⁸ Increased progesterone levels positively correlated with HEV infection, as it enhances the rate of HEV replication.⁵⁹ Moreover, progesterone was reported to be associated with increased production of cholangiocytes, which increases the number of bile ducts in the rat model.⁶⁰ Antiprogesterone is considered the treatment of bile duct proliferation and extrahepatic cholestasis.⁶⁰ In ALD mouse models, progesterone receptor membrane component 1 (pgrmc1) knockout mice exhibited poor outcomes and induced production of catalase, which increased alcohol dehydrogenase levels, leading to severe alcohol-associated liver damage.⁶¹

Studies on the role of estrogen in ALD are summarized in Table 1.62, 63, 64, 65, 66, 67, 68, 69, 70, 71

Table 1.

Studies reporting the effects of alcohol on estrogen levels in women.

Model	Study	Sample	Hormone	Levels	Reference
Human	Postmenopausal women	Plasma	Estradiol	Increased	62
Human	Healthy adult women	Plasma	Estradiol	Increased	63
Human	Postmenopausal women	Plasma	Estradiol	Increased	64
Human	Middle-aged men and postmenstrual women	Plasma	Estradiol	No effect	65
Human	Women using oral contraceptives	Plasma	Estradiol	Increased	66
Human	Premenopausal women	Serum	Estradiol	Increased	67
Mice	OVX and NOVX mice	Plasma	Estradiol	Increased	68
Human	Adolescent female	Plasma	Estradiol	Increased	69
Human	Premenopausal women	Serum and saliva	17-beta estradiol	Increased	70
Human	Premenopausal women	Serum	Estradiol	No effect	71

Abbreviations: NOVX, non-ovariectomized; OVX, ovariectomized.

5. Role of estrogen and ERs in hepatic immune modulation

ERs are localized in different organs throughout the body, and their presence in the liver renders it sensitive to estrogen. Both parenchymal and non-parenchymal hepatocytes contain ERs.⁷² ERs in the liver play a vital role in glycoprotein synthesis when estrogen is present.⁷³ Changes in the distribution and activity of these hepatic ERs are an important mechanism in many liver-associated disorders.⁶² Studies have shown significant differences in ER concentration in alcoholic and non-alcoholic steatohepatitis (NASH). Therefore, an increase in ERs may be an important mechanism in liver disease progression. ERα is the major ER that is present in the liver, and it is thought to play a major role in metabolic dysfunction in the liver.⁸

Estrogen plays a significant role in immune modulation, including liver immune responses. Liver immune cells such as Kupffer cells also contain ERs, and their signaling can regulate their functions. Estrogen regulates the expression of various cytokines and chemokines, leading to an overall immune modulatory effect. For example, estrogen can promote the production of interleukin (IL)-4, an anti-inflammatory cytokine that is important in the regulation of liver fibrosis. In addition, estrogen inhibits the production of pro-inflammatory cytokines such as TNF-α and IL-6.⁷⁴ In the liver immune response, estrogen signaling can also modulate LPS homeostasis, an important factor in maintaining immune tolerance. The liver effectively clears LPS from the circulation, preventing the induction of inflammatory responses. Estrogen also promotes LPS clearance, leading to a decreased inflammatory response (Fig. 2).⁷⁵

The effect of carbon tetrachloride (CCl₄) hepatotoxicity was also studied in a rodent model, and male rats have shown more severe outcomes than female ones. Moreover, women with ovaries showed more severe fibrosis than women without ovaries.⁷⁶ On the contrary, estrogen and E2 interventions exerted therapeutic effects on liver cirrhotic conditions and reduced portal hypertension.⁷⁷

6. Current therapies in ALD and crosstalk with ERs

To date, no effective therapy has been available for ALD. Abstinence is considered the most effective because it improves cirrhotic conditions and resolves alcoholic steatosis. Currently, the US Food and Drug Administration (FDA)-approved drugs acamprosate, disulfiram, and naltrexone for the treatment of mild-to-moderate ALD promoted the abstinence target GABA or glutamate receptors, aldehyde dehydrogenase 2, and opioid receptors, respectively.⁷⁸ Furthermore, natural or artificial steroids, particularly prednisolone, are used extensively to suppress immune or cytokine responses.⁷⁹ Other drugs that target TNF-α, such as infliximab and etanercept,^80,81 inhibited cytokines directly; however, based on the results of clinical trials, these were not recommended further. Pentoxifylline blocks TNF-α at the transcriptional level and is mainly used for corticosteroid non-responders with severe alcoholic hepatitis (SAH).⁸²

Glucocorticoids antagonize estrogens by the glucocorticoid receptor-mediated action of estrogen sulfotransferases (SULTs). Dexamethasone (DEX) decreases estrogenic activity through glucocorticoid receptors. The inhibition of an enzyme, estrogen SULT, led to the metabolic deactivation of estrogens.⁸³ Corticosteroid therapy is reported to be the only intervention for patients with SAH and a model of end-stage liver disease score >20. Corticosteroids improved the 30-day survival of patients with very severe liver disease.⁸⁴

A study interestingly reported that during the estrous cycle, corticosteroids cause the ER-dependent antagonism of glucocorticoid-induced leucine zipper (GILZ) enhancement in both male and female rodents. A potential time-dependent interplay between sex hormone and glucocorticoid signaling in vivo was assessed by comparing GILZ enhancement by methylprednisolone in the uterus, which has high ER propensity, and in the liver, which has lower ER density expression. Rodents of both gender were dosed within proestrus (which has high E2 or progesterone levels) and estrus (with low levels of female sex hormones) cycles.⁸⁵ Unbound plasma concentrations of glucocorticoid hormones and synthetic corticosteroids were rapidly distributed into intracellular spaces between tissues.⁸⁶ Systemic administration of corticosteroids decreased endogenous corticosterone levels via a negative feedback loop on the hypothalamic–pituitary–adrenal axis. The unbound fraction of endogenous and exogenous steroids interacts competitively with cytosolic glucocorticoid receptors based on relative receptor affinity. The binding of the receptor produces an activated complex that rapidly translocates into the nucleus and binds specific glucocorticoid response elements on the target DNA. This transcriptionally enhances or downregulates the turnover rates of various target genes, including GILZ. This provides in vivo support to the hypothesis of estrogen-mediated antagonism of glucocorticoid signaling (Fig. 3).⁸⁵

Fig. 3.

Corticosteroid regulation by glucocorticoid-induced leucine zipper (GILZ) gene regulation in tissues. Methylprednisolone/exogenous corticosteroid enhances GILZ mRNA transcription, and no antagonistic effect of estrogen on methylprednisolone was found because of fewer ERs present in the liver. Therefore, the negligible effect may be caused by estrogen and ER interaction on GILZ mRNA transcription. However, the ER antagonism of methylprednisolone is prominent in the uterus because of the high ER expression. Abbreviation: ER, estrogen receptor.

Some studies have contradictory results about estrogen, as some suggest it to be protective in chronic liver diseases. However, evidence also shows the role of estrogen in HCC progression.⁸⁷ Thus, more investigations are needed to understand the role of estrogen and ERs in ALD progression and in interference with current therapies based on steroid hormones.

7. Hepatic ERs: a novel therapeutic target for ALD

In the past, the liver was not considered an estrogen-dependent organ, as was the breast and uterus. However, in both men and women, the liver has been shown to have high affinity but low capacity for ERs that regulate liver function at various levels. Moreover, altered estrogen or hepatic ER levels have established a close correlation in accordance with liver diseases.³⁸

Recently, hepatic ERα has gained attention as a therapeutic target for NAFLD/NASH, as the downregulation of estrogen signaling leads to dyslipidemia or fatty liver,⁸⁸ whereas the condition can be reversed by its agonists. Meanwhile, hepatic ERα in association with the PNPLA3 p.I148 M variant accounts for the largest cohort of fatty liver conditions and stands as a common link between NAFLD and ALD.⁸⁹ The addition of the same in HepG2 cell lines abolished hepatic fat accumulation, highlighting a new therapeutic target of particular relevance. This evidence accounts for a new therapeutic target for ALD.

7.1. Hepatic ERs and future therapeutic perspective

Many therapeutic approaches have implications but are likely to have less pathophysiological significance because of confounding factors such as the inefficiency of the liver or gut-derived obstructions. Hepatic ERs could be a potential target to carry forward the investigation of new therapeutic targets against ALD based on the aforementioned facts. Some possible drug candidates attenuated the progression of ALD are shown (Fig. 4).

Fig. 4.

Overview of drug candidates that might improve alcoholic liver disease (ALD) by regulating hepatic estrogen receptors (ERs). Puerarin activates the SIRT1/AMPK pathway that inhibits SREBP1c/FASN and may mitigate alcoholic steatosis. Resveratrol triggers MAPK/NRF2 axis, which might help in improving inflammation and oxidative stress in alcoholic hepatitis. Anastrozole inhibits aromatase and might decrease the production of estradiol. Dexamethasone may cease downstream mechanisms of nuclear hepatic receptors by sulfonating estradiol in ALD. Abbreviations: AMPK, AMP-activated protein kinase; ATGL, adipose triglyceride lipase; ERK, extracellular signal-regulated kinase; FASN, fatty acid synthase; GPR30, G-protein-coupled estrogen receptor; HO1, heme oxygenase 1; MAPK, mitogen-activated protein kinase; NRF2, nuclear factor erythroid 2-related factor 2; SIRT1, silent information regulator 1; SOD2, superoxide dismutase 2; SREBP1c, sterol regulatory element-binding protein 1c; SULT1E1, estrogen sulfotransferase.

7.1.1. Puerarin

Puerarin is a major bioactive flavonoid derived from the root of Pueraria montana lobata (Wild). It is known to have pharmacological potencies, such as antioxidant and anti-inflammatory properties,⁹⁰ and helps in the regulation of lipid metabolism in mice.⁹¹ Recently, it is also suggested as the underlying molecular mechanism behind the regulation of lipolysis in HepG2 cell lines. Puerarin was found to significantly inhibit fatty acid synthase (FASN)/sterol regulatory element-binding protein 1c (SREBP1c) and activate silent information regulator 1 (SIRT1)/AMP-activated protein kinase (AMPK)-mediated calcium-dependent G-protein ER signaling.⁹² Thus, it could be a potential candidate to inhibit alcoholic steatosis.

7.1.2. Resveratrol

Resveratrol is a potent polyphenol derived from grapes and wine. It has many beneficial properties,⁹² is extensively studied as an antioxidant against cancer cells,⁹³ and has anti-obesity effects targeting the AMPK pathway.⁹⁴ It exerts a hepatoprotective effect, and its influence on hemorrhagic injury mediated by ERs has been studied.^95,96 A recent comparative analysis of resveratrol with ICI 182,780, an ER antagonist, suggested its role in improving liver function and integrity.⁹⁷ The possible mechanism behind its role has been shown by various pathways such as p38 mitogen-activated protein kinase (MAPK)/heme oxygenase 1 (HO1) and SIRT1/superoxide dismutase 2 (SOD2).⁹⁵ This opens a new area for investigating the role of resveratrol in improving ALD, which could help in improving both the liver and gut integrity.

7.1.3. SULTs

SULTs are cytosolic enzymes belonging to a family of phase II drug-metabolizing enzymes that inactivate estrogen by catalyzing sulfo-conjugation.⁹⁸ Estrogen SULT (SULT1E1 or EST) has been identified as a major isoform responsible for the sulfonation of estrogen at physiological pH with high affinity.⁹⁹ It is manifested in Sult1e1/Est null mice, and its expression was found to be diminished in breast cancer suggesting that the reactivation of endogenous expression could be a possible therapeutic target.¹⁰⁰ However, patients with ALD have increased expression of hepatic ERs and conversion of circulatory estradiol to testosterone. Thus, SULT1E1 could be beneficial in this scenario; however, the major concern remains with the investigation of endogenous SULT1E1 expression in patients with ALD.

7.1.4. DEX

DEX is a synthetic glucocorticoid that antagonizes estrogen response.¹⁰¹ A study suggested that the underlying mechanism of DEX targeting glucocorticoid receptors activate SULT1E1, which inhibits breast cancer in vivo,⁸³ represents a novel strategy of inhibiting high estrogen levels, which could be also beneficial for ALD.

7.1.5. Anastrozole

Anastrozole is a potent aromatase inhibitor that showed impeccable outcomes better than tamoxifen against breast cancer in the International Breast Cancer Intervention Study-II (IBIS-II) trial.¹⁰² Its effects were the greatest for estrogen-positive tumors with late side effects. Thus, the UK National Institute for Health and Care Excellence and the US Preventive Services Task Force,¹⁰³ have approved the use of anastrozole against breast cancer in the high-risk group of postmenopausal women. Other aromatase inhibitors such as tamoxifen have side effects such as fatty liver, fibrosis, or cirrhosis in patients with breast cancer within 2 years of consumption, which limits their use.¹⁰³ However, to minimize the conversion of testosterone to estradiol by inhibiting aromatase enzymes in alcoholic patients, anastrozole could be a better therapeutic candidate.

8. Conclusions

Despite the poor molecular understanding of the gender-based adverse outcomes in ALD, detailed translational studies are needed to investigate the estrogen- and ER-associated pathogenesis of ALD. Experimental and human studies have suggested that alcohol does affect hepatic estrogen metabolism and that estrogen signaling plays a key role in the regulation of pro-inflammatory effects and immune dysregulation in chronic alcohol consumption. Various therapeutic approaches currently in use or clinical trials can modulate early or chronic ALD by acting via estrogen signaling to antagonize actions and utilize the crosstalk with glucocorticoid receptors. Translational studies are necessary to investigate the role of estrogen signaling in ALD pathogenesis in both gender and understanding the disease progression at various stages. Overall, ALD remains a significant public health concern, and more studies are needed to improve our understanding of this disease and develop better management strategies based on gender.

Authors’ contributions

Sukriti Baweja and Rashmi Kaul contributed to conceptualization, organization and structure of the manuscript, intellectual support, and manuscript writing as well as editing. Ashmit Mittal, Swati Thangariyal, P. Debishree Subudhi, and Shivani Gautam did the literature review, interpretation and integration of data from multiple sources. All authors have approved the final manuscript.

Declaration of competing interest

The authors declare that there is no conflicts of interest.