Effect of alcohol on adipose tissue: a review on ethanol mediated adipose tissue injury

Abstract

Background: Alcohol consumption has been in existence in the world for many centuries and it is the major cause of death and injury worldwide. Alcoholic liver disease (ALD) is caused due to excess and chronic alcohol intake. Studies across the globe have identified several pathways leading to ALD. Adipose tissue which has been considered as an energy storage organ is also found to play a major role in ALD progression by secreting hormones and cytokines known as adipokines or adipocytokines. Ethanol affects the metabolic and innate immune activities of adipose tissue contributing to alcohol-induced injury of the tissues.

Objective: We aimed at 1) summarizing the metabolism and progression of ALD 2) summarizing about the structure and effect of ethanol induced oxidative stress on adipose tissue 3) reviewing the available data on the effect of ethanol on adipose tissue mass and adipokine secretion in both rodent models and alcoholic patients.

Methods: The article is summarized based on the original literature and reviews in studying the effect of ethanol on adipose tissue.

Results: Studies on alcoholic patients and rodent models has shown that chronic ethanol consumption reduces adipose tissue mass and causes CYP2E1 mediated oxidative stress and inflammation of adipose tissue. Further hyperlipolysis is observed in adipose tissue that leads to excess fatty acid release that gets transported and deposited in the liver resulting in hepatic steatosis.

Conclusion: Studies show that adipose tissue plays a major role in the progression of ALD. So understanding of the mechanisms linking ethanol induced adipose tissue injury with ALD progression would help us in identifying potential therapeutic targets.

Keywords:

• adipokines
• alcohol
• alcoholic liver disease
• CYP2E1
• oxidative stress
• white adipose tissue

Abbreviations

ALD	=	Alcoholic liver disease
CYP2E1	=	cytochrome P450 2E1
WAT	=	white adipose tissue
BAT	=	brown adipose tissue
ROS	=	reactive oxygen species

Introduction

Alcohol is a psychoactive substance and its consumption may lead to alcoholism or alcohol abuse. Alcohol uptake at harmful levels may cause morbidity and mortality together with socio-economic burden. The pattern and the amount of alcohol consumed determine the extent of alcohol related harm. It is among the world's top 5 risk factors causing injury, disease, disability, or death.^1,2 It is the cause for more than 200 disease and injury conditions and is associated with a danger of developing alcohol dependence, liver damage, and cancers.³ According to 2014 WHO global status report on alcohol and health, alcohol consumption is responsible for 5.9% of all global deaths and 5.1% of DALYs (disability-adjusted life years).

Alcohol Metabolism

Alcohol is primarily metabolized in the body via diverse pathways by the catalytic activity of 3 different enzymes–alcohol dehydrogenase, cytochrome P450 2E1 (CYP2E1), and catalase. Chronic ethanol exposure induces CYP2E1 expression. Although the expression of CYP2E1 is greatest in the liver, it is also expressed in adipose tissue of rats after chronic ethanol consumption.⁴

Asians are more susceptible to negative effects of alcohol and alcohol related cancers due to genetic differences in alcohol dehydrogenase and aldehyde dehydrogenase enzymes.⁵⁷

Alcohol disease progression

Alcohol consumption affects multiple signaling pathways and it is the cause of a multifactorial disease known as alcoholic liver disease. The disease progression is characterized by pathologic conditions like steatosis (fatty liver), steatohepatitis, fibrosis, end stage liver disease known as cirrhosis, and finally hepatocellular carcinoma.⁸ Alcohol consumption affects multiple hepatic lipid metabolic pathways like de novo lipogenesis, oxidation of fatty acids, uptake and export of lipids.^9,10 However, studies show that alcohol mediated effect on hepatic lipid metabolism is also due to certain extra hepatic factors known as adipokines^11,12 secreted by the adipose tissue.

Adipose tissue histology, locationand function

Based on function and histology, adipose tissue is subdivided into white adipose tissue (WAT) and brown adipose tissue (BAT). BAT plays a role in thermogenesis and body temperature regulation while WAT acts as an energy storage and endocrine organ. In mammals WAT is the major energy storage organ and it provides both thermal and mechanical insulation to the body. The location of WAT is not confined but it is distributed as physically unconnected individual pads throughout the organism. Based on the location WAT can be classified as epididymal, mesenteric, subcutaneous, retroperitoneal, and pericardial fat pads.^13,14

Adipose tissue is diverse in its cellular composition comprising of mature adipocytes, pre-adipocytes, macrophages, fibroblasts, and endothelial cells. Mature adipocytes can amass excess energy sources with the help of glucose transporters and lipoprotein lipase enzymes in the form of triglycerides inside the cell without compromising their cellular function.¹⁵

WAT has 3 main functions: 1) energy storage 2) triglyceride hydrolysis to free fatty acids for supporting tissue energy needs and 3) adipokine release.¹⁶

WAT under positive energy balance conditions stores excess energy in the form of triglycerides whereas under negative energy balance conditions it provides energy for other organs by releasing free fatty acids. Fat storage function disorder of WAT leads to excess influx of fatty acid into the liver leading to steatosis. So, healthy adipose tissue is required to maintain lipid homeostasis throughout the body at the adipose tissue-liver axis.^17-19

Alcohol, adipose tissue, CYP2E1 and innate immunity

Ethanol consumption leads to the development of oxidative stress in a number of tissues²⁰ together with adipose tissue. Activation of CYP2E1 mediated ethanol breakdown results in oxidative stress and endoplasmic reticulum (ER) stress leading to adipokine dysregulation and subsequent progression of ALD. Oxidative stress indicators like 4-hydroxynonenal are detected in adipose tissue after chronic ethanol feeding.^21,22 Increased ethanol-induced CYP2E1 expression is found to be critical to the development of adipose tissue inflammation via activating redox-sensitive transcription factors that cause increased ROS production. Alternatively, increased CYP2E1 expression and activity also leads to C1q dependent complement system activation and apoptosis mediated by Bid resulting in an indirect mechanism of CYP2E1 mediated inflammatory responses.²³ Also levels of inflammatory cytokine, macrophage migration inhibitory factor (MIF) are found to be increased in alcoholic hepatitis and cirrhosis patients. Although the exact mechanism of MIF increase is not known it is known that CYP2E1 activation is partially responsible for the increased MIF expression.^24-26 A summary of the effects of alcohol consumption on adipose tissue mass and inflammatory mediators is presented in Figure 1 and Table 1.

Table 1. The effect of alcohol consumption in rodents and humans with respect to adipokine secretion and adipose tissue mass

Animal Model	Effects caused by alcohol consumption	Reference
Rodent	↓ in adiponectin and cartonectin levels	^15,16,32,75,77,90,92,93
	↑ in leptin, resistin, IL-6, visfatin, chemerin, TNF-α, MCP-1, RBP-4	^32,35,39,42,71
	↓ in adipose tissue mass and adipocyte size	^38,40
	↑ in triglyceride degradation	^38
	Insulin resistance	^39
Human	↑ in adiponectin, leptin, resistin, grehlin, TNF-α, IL-6	^14,18,29,33,36,37,62,65,71,88
	↓ in acylation stimulating protein	^36
	↓ in fat mass and ↑ in hepatic fat levels	^2,3,48
	↑ in VAT and ↓ in SAT	^44,57,83
	Glucose intolerance	^22,55,84

Figure 1. Alcohol consumption causes dysregulated adipokine secretion and macrophage infiltration into adipose tissue resulting in liver steatosis and inflammation.

Endocrine Function of Adipose Tissue

Knowledge about the endocrine function of adipose tissue has emerged after the discovery of leptin expression and production by adipocytes.²⁷ Studies following leptin discovery have shown that adipose tissue secretes a wide variety of peptide molecules (both hormones and cytokines) known as adipokines or adipocytokines. Adipokines exert their activity on metabolism, immune function, as well as neuroendocrine pathways regulating feeding behaviors. All the cell types contribute to the secretory function of WAT. Both physiological environment and adipose tissue expansion status regulate adipokine production ^27,28

The extent of metabolic activity of adipose tissue fat pads varies with the location and certain fat depots secrete specific adipokines more actively than others. Non-adipose tissues also secrete certain adipokines. For example, resistin is found in mouse brain and pituitary,²⁹ and in humans resistin mRNA is also observed in placenta³⁰ and monocytes.^31,32

Effect of alcohol on WAT

Alcohol consumption causes alcoholic fatty liver disease. In human and animal models, alcohol intake causes susceptibility toward non-alcoholic fatty liver disease.³³ Studies in both humans and animals have shown that alcohol consumption affects both adipose tissue mass and adipokine secretion. Insulin resistance, increased macrophage infiltration, inflammatory cytokine,³⁴ reduced ability of insulin to stimulate glucose uptake and inhibit lipolysis leading to hepatic steatosis^21,4 are observed after chronic ethanol feeding.

Studies on rodents

Reduction in adipose tissue mass^21,35 and increase in fatty acid uptake by hepatocytes is observed in alcohol exposed rodents.^36,37 Ethanol consumption in rats apart from decreasing glucose uptake in rat adipocytes,³⁸ it also increases the rate of degradation of triglycerides in adipose tissue thus resulting in increased free fatty acid circulation.²¹ Studies on C57BL/6N mice showed that lipid homeostasis at the adipose tissue-liver axis is disturbed on chronic ethanol consumption and this demonstrates that WAT lipolysis is stimulated on alcohol consumption resulting in an excess fatty acid release that get transported to the liver and become deposited as triglycerides.³⁹ Anatomically, VAT is connected to hepatic system via the mesenteric and portal veins. Thus chronic alcohol consumption may cause simultaneous accumulation of free fatty acid in liver cells and mesenteric fat tissues. Adipocyte size is also found to be reduced in chronic alcohol exposed mice.⁴⁰

Adipose triglyeride lipase and hormone sensitive lipase is found to be activated in association with a significant increase in the release of fatty acid from adipose tissue explants in mice fed with alcohol. Lipolysis in WAT is primarily due to hormone sensitive lipase (HSL) enzyme.⁴¹ Up on lipolytic hormones stimulation, HSL is activated via phosphorylation by cAMP/PKA pathway.⁴² The enzyme is inactivated by protein phosphatase 2A (PP2A) mediated dephosphorylation.⁴³ Methyaltion of PP2A by leucine carboxyl methyltransferase 1 (LCMT1) enhances its activity.⁴⁴

Studies on mice chronically fed with alcohol containing diet showed hypomethyaltion of PP2A enzyme as evident by reduced SAM/SAH ratio resulting in uninhibited HSL activation leading to increased lipolysis and release of free fatty acids from adipose tissue.⁴⁵

Alcohol exposure up regulates fatty acid transport proteins, causes accumulation of lipids in the liver, induces insulin intolerance, inactivates adipose protein phosphatase 1, and also upregulates phosphatase and tensin homolog (PTEN) and suppressor of cytokine signaling 3(SOCS3) proteins in mice. WAT dysfunction can directly impact lipid homeostasis of liver by reverse triglyceride transport. Hyperlipolysis is found to be the major functional defect of WAT after chronic ethanol exposure. Studies suggest that alcohol induced adipose tissue hyperlipolysis and insulin resistance are due to inactivation of adipose protein phosphatase 1 and upregulation of negative regulators of insulin signaling like PTEN and SOCS3 there by dysregulating or inhibiting insulin signal transduction pathway. Study by Zhong et al showed a significant loss in WAT in alcohol fed mouse.⁴⁰

Studies on alcoholics

Clinical studies in alcoholics have shown an association of lower fat mass with higher liver fat.^46,47 Lower body mass index (BMI) and fat mass (FM) and higher hepatic fat levels are observed in alcoholic patients.^46-48 Alcohol intake enhances cortisol secretion that results in change of fat distribution pattern, together with an increase in abdominal and hepatic fat deposition⁴⁹ and SAT lipolysis.⁵⁰ Studies show that alcohol consumption is associated with decreased accumulation of SAT and increased accumulation of VAT. Impaired function of adipocytes or surpass in the storage capacity of SAT results in accumulation of fat outside SAT thus resulting in buildup of VAT.⁵¹ VAT accumulation results in glucose intolerance, onset of diabetes, and an increase in apolipoproteins.^52-54 The Framingham Offspring Study reported that intake of large amounts of alcohol is associated with decrease of SAT in women and increase of VAT in men.⁵⁵

Consumption of alcohol more than 14 standard drinks is shown to be related to an increased risk of metabolic syndrome. Further no protective effect on adipose tissue accumulation is shown with low to moderate alcohol intake. Alcohol energy content of 7.1g/kcal is relatively high⁵⁶ and it increases appetite and promotes energy intake.⁵⁷ A study on the dietary intake of energy from food and alcohol in Koreans showed that with higher consumption of alcohol there was an increase in total energy intake and further it was observed that there was an increase in VAT accumulation with either decrease or no change in SAT accumulation.⁵⁸

Alcohol and adipokines

Chronic consumption of alcohol disrupts gut permeability leading to the diffusion of bacterial endotoxin (LPS) into the portal blood circulation.^59-61 This result in TLR₄ signaling activation mediated via MyD88 dependent and independent pathways resulting in an increased production of inflammatory cytokines.^62,63

Alcohol consumption is known to disrupt adipokine release from adipose tissue,^11,64 and promote infiltration of macrophages and this leads to adipose tissue phenotype alteration accredited to alcohol metabolism induced oxidative stress.⁶⁵ Qin and colleagues study results provided evidence for an increased adipose tissue inflammation due to alcohol binging prior to burn injury.⁶⁶ These observations have important inferences as studies show that increases in pro-inflammatory cytokines not only alter the functional integrity of adipose tissue but also dysregulate metabolism of adipose tissue.⁶⁷

Studies have shown that consumption of alcohol at moderate levels improves human health, especially cardiovascular related morbidities.⁶⁸ In vitro and in vivo experimental data demonstrate that alcohol acts as anti-inflammatory agent in presence of stimuli like bacterial lipopolysaccharides that up regulate inflammatory cytokines expression.⁶⁹ However, in the absence of inflammatory stimuli alcohol acts as a pro-inflammatory agent and upregulates the production of inflammatory cytokines and enzymes like IL-6, TNF-α, iNOS, and COX-2.^70,71

Studies on rodents

Adiponectin and leptin are the key adipokines that modulate hepatic lipid homeostasis toward lowering liver lipid content. Reduced adipose tissue weight, serum leptin and adiponectin concentrations together with the development of fatty liver are observed in chronic alcohol exposed rodents.^{21,35,36,40,72-78} Adiponectin signaling via adiponectin receptor activates adenosine monophosphate-activated protein kinase (AMPK) pathway that stimulates fatty acid oxidation and decreases hepatic lipid influx and de novo lipogenesis thereby regulating hepatic lipid content.^11,12,16,79 Chronic consumption of alcohol results in impaired metabolism of methionine leading to hyperhomocysteinemia and decreased ratio of SAM/SAH in adipose tissue. This contributes to alcohol induced reduction in adiponectin secretion from adipocytes.⁷⁵ Exogenous adiponectin administration or endogenous adiponectin production stimulation or Rosiglitazone, a PPAR-γ agonist that mainly targets adipocytes has been shown to attenuate alcohol induced fatty liver in mice^{72,73,76,80,81} as adiponectin has anti-inflammatory effects that are mediated via a heme-oxygenase-1 dependent pathway that attenuates TLR₄ signaling mediated by MyD88 dependent and independent pathways.^62,63 Leptin signaling via leptin receptor b activates AMPK and signal transducer Stat3 pathways.^16,79 Decreased serum leptin levels are observed on either chronic alcohol or acute alcohol consumption. Studies show that leptin deficiency is associated with WAT mass reduction and exogenous leptin administration in leptin deficient mice restored alcohol induced hypoleptinemia and hepatic steatosis.^82-85 Studies show that during hyperinsulinemic-euglycemic clamp subcutaneous, epididymal, and omental adipose tissue glucose uptake is decreased after chronic ethanol feeding. Studies on male Wistar rats showed macrophage infiltration in epididymal adipose tissue, alteration in mRNA expression levels of adipocytokines like TNFα, IL-6, MCP-1, adiponectin, and RBP4 on chronic ethanol feeding.³⁴

Intragastric feeding of male Wistar rats for 22 weeks showed a decrease in the production of cartonectin and adiponectin in VAT and an increase in the production of leptin, visfatin, chemerin,⁸⁶ and resistin in both sera and VAT in a dose dependent manner.⁸⁷

Concentration dependent up regulation of inflammatory genes IL-6 and TNF-α genes and inflammatory enzymes COX-2 and iNOS is observed in 3T3L1 pre-adipocytes⁸⁸ treated with ethanol.

Studies on alcoholic patients

Higher levels of adiponectin (Acrp30) and resistin are found in patients with ALD when compared to control subjects. The increase in levels of resistin is due to inflammation in alcoholic patients while the increase in adiponectin levels suggests a protective and anti-inflammatory role of adiponectin.⁸⁹ This may be explained by Behre's hypothesis which states that increased adiponectin blood levels have a protective role and also help the body to adapt during fasting. In ALD progression malnutrition is frequently observed and this might be the reason for increased plasma levels of Acrp30 in alcoholic patients when compared to controls.⁹⁰

Studies show that moderate alcohol consumption increased adiponectin and grehlin levels while acylation-stimulating protein concentrations are decreased and these concentrations improve insulin sensitivity.⁹¹ Recent data shows that high levels of TNFα in the SAT are observed in patients with alcoholic hepatitis and a positive correlation is observed between the liver IL6 and liver histological lesions and SAT TNFα levels.⁹² Osteopontin expression levels from adipose tissue also correlate with fibrosis of liver in alcohol patients.⁹³ These data suggest the existence of similar pro-inflammatory mechanisms in adipose tissue and liver. Alcohol consumption leads to pro inflammatory cytokine expression from SAT. One week after alcohol withdrawal adipose tissue macrophages polarize into a M2 phenotype and alleviation in SAT inflammation is observed in patients with ALD.⁹⁴

Serum concentrations of leptin, an adipokine with pro-inflammatory and pro-fibrotic properties, are high in alcoholic patients irrespective of the presence or absence of cirrhosis.⁶⁴

Studies on patients with ALD established a correlation between TNF-α and IL-10 produced by adipose tissue with histological lesions in the liver and this suggests a relationship between adipose tissue inflammatory process and ALD progression in the liver.⁹²

Studies on alcoholic patients also established a correlation between the osteopontin expression in adipose tissue and hepatic fibrosis emphasizing the concept of adipose tissue inflammation in ALD.⁹³

In vitro studies on human adipose tissue fragments showed a significant anti-inflammatory effect of ethanol in a dose and time dependent manner.⁹⁵

Conclusion

Alcohol induced effects on tissue are multifactorial and involves crosstalk among various organs and tissues. Studies show that adipose tissue plays a major role in alcohol induced liver disease progression. These effects of ethanol on the adipose tissue innate immunity and metabolic activities lead to ethanol induced tissue injury⁹⁶ and they also influence alcohol consumption regulating behaviors.^97,98 Research till date suggests that progression of ALD can be reduced by improving the function of adipose tissue. Though much progress has been made in understanding the role of adipose tissue in alcohol related tissue injury, the exact underlying molecular mechanisms linking adipose tissue damage and liver disease progression due to alcohol consumption still needs to be elucidated. As studies show that adipokines play a key role in alcoholic liver disease progression, effective drug leads targeting adipokine signaling pathways need to be developed and tested for ethanol induced liver disease amelioration.

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Funding

This work was supported by grants from DST (Department of Science and Technology), CSIR (Council of Scientific and Industrial Research) and by UGC (University Grant Commission) for providing fellowship to Venkata Harini Kema.