Ghrelin regulates adipose tissue metabolism: Role in hepatic steatosis
Introduction
Excessive alcohol use is a serious problem in US and worldwide. Among individuals with alcohol use disorder, 90% of the people develop hepatic steatosis [1,2] which is characterized by accumulation of lipids in the hepatocytes [3]. Although steatosis is often benign and reversible, it is widely believed to be the precursor to fibrosis, cirrhosis and cancer and therefore is a prime target for therapeutic intervention [4]. Central among the many mechanisms proposed to play a role in the development of alcoholic steatosis is dysregulation of adipose tissue lipid metabolism. Particularly, it is the enhanced adipose lipolysis-derived free fatty acids and their delivery to the liver that contributes to the development of alcoholic steatosis [5,6].
Studies have demonstrated that the adipose-liver axis is modulated by hormones and cytokines, particularly adipokines [7,8]. The hormones, insulin and adiponectin play a major role in adipose-liver cross-talk [9,10]. Insulin profoundly affects both carbohydrate and lipid metabolism in both liver and adipose tissue. In the liver, insulin stimulates the export of fat as very-low density lipoproteins (VLDL) that are secreted into the circulation and taken up by various organs, including adipocytes to provide free fatty acids for their use [11,12]. Adipocytes use the free fatty acids to synthesize triglycerides (TG) which are stored in lipid droplets for energy needs. Insulin inhibits lipolysis in adipocytes thus allowing for increased lipid storage in these cells [13,14]. Adiponectin has both autocrine and paracrine functions. In adipocytes, adiponectin helps adipocytes expansion by improving insulin sensitivity for saving lipids in the form of lipid droplets to avoid lipotoxicity in peripheral organs [15,16]. In the liver, adiponectin reduces lipogenesis and promotes lipid oxidation [8,17,18].
In addition to peptide hormones, adipose-derived cytokines and chemokines play a major role in adipose-liver interplay, specifically progression of steatosis to advanced stage liver disease. Importantly, tumor-necrosis factor α (TNFα), interleukin-6 (IL-6) and a macrophage chemoattractant, CCL2 (chemokine [C–C motif] ligand 2), all play central roles in insulin sensitivity and lipid metabolism by paracrine and endocrine functions [[19], [20], [21], [22], [23]].
We and others have reported that alcohol administration decreases the circulating levels of both insulin [5,[24], [25], [26]] and adiponectin [[27], [28], [29], [30], [31], [32]] in humans and animal models while increasing circulating ghrelin levels [26,33,34]. Further, it has also been reported that alcohol administration increases TNF-α, IL-6 and CCL2 levels in adipose tissues [28,35].
In investigating further, we recently reported that the decrease in circulating insulin observed upon alcohol administration was due to increased serum ghrelin levels which impairs insulin secretion from pancreatic β-cells [25,26]. Adipose tissue expresses the ghrelin receptors, GHS-R1a [36,37] and previous studies have reported that ghrelin inhibits the differentiation of 3T3-L1 preadipocytes to mature adipocytes [38]. Based on these considerations, we hypothesized that the alcohol-induced ghrelin increase is responsible for the adipose lipid dysregulation observed during the development of alcoholic steatosis. Thus, the aim of this preliminary study was to investigate the direct role of ghrelin on adipocyte lipid breakdown and adipokine secretion which together can have a significant effect in the development of alcoholic fatty liver disease.
Section snippets
3T3-L1 preadipocyte cell Culture and differentiation
Mouse 3T3-L1 fibroblasts were purchased from ATCC. Cells were grown in DMEM media containing 10% FBS and 1% penicillin-streptomycin. At 70% confluence, cells were induced to differentiate by adding methylisobutylxanthine, dexamethasone and insulin to the cultured media as described previously [39]. After three days of treatment with differentiation media, cells were cultured in insulin supplemented media for 6 days. After that cells were maintained in FBS containing DMEM media until sufficient
Ethanol metabolizing enzyme expression
Differentiated 3T3-L1 adipocytes express both ADH (alcohol dehydrogenase) and CYP2E1 (cytochrome P450 2E1), the two major enzymes that metabolize ethanol (Fig. 1). The expression of both these enzymes was increased by ethanol exposure. These cells also express ghrelin receptor (GHS-R) (Fig. 1) as has been shown previously [43]. Collectively, these data indicate that 3T3-L1 adipocytes are an appropriate in vitro model to examine the effects of ghrelin and ethanol.
Ghrelin significantly inhibits adipose differentiation
Peroxisome
Discussion
Fatty liver, characterized by an accumulation of lipids in hepatocytes, is one of the earliest changes in the pathogenesis of both alcoholic fatty liver disease. Once the liver becomes steatotic, it is more prone to inflammatory mediators leading to progression to hepatitis, fibrosis and eventually cirrhosis and HCC [4]. It has been reported that adipose tissue plays a central role in development and progression of fatty liver disease. Impaired adipose metabolism, particularly lipid metabolism,
Grant
This research was supported by K01 AA024254 (KR) from the National Institute on Alcohol Abuse and Alcoholism of the National Institutes of Health and the Merit Review grant BX004053 (KKK) from the U.S. Department of Veterans Affairs.
Author contribution
KR was responsible for the conception and design of the study, and analyzed the data; KR, JLK, KLK and LH performed the experiments; KR, CAC and KKK interpreted the results of experiments, analyzed the data, and co-wrote the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
We thank Dr. Vasilis Vasiliou (Yale School of Public Health/Yale School of Medicine, New Haven, CT) for giving the opportunity to share our findings at the “Alcohol and Cancer 2019” meeting.
References (55)
Alcoholic fatty liver: its pathogenesis and mechanism of progression to inflammation and fibrosis
Alcohol
(2004)- et al.
Chronic ethanol and triglyceride turnover in white adipose tissue in rats: inhibition of the anti-lipolytic action of insulin after chronic ethanol contributes to increased triglyceride degradation
J. Biol. Chem.
(2007) Obesity and free fatty acids
Endocrinol Metab. Clin. N. Am.
(2008)- et al.
Adiponectin promotes adipocyte differentiation, insulin sensitivity, and lipid accumulation
J. Lipid Res.
(2005) - et al.
Transmembrane tumor necrosis factor (TNF)-alpha inhibits adipocyte differentiation by selectively activating TNF receptor 1
J. Biol. Chem.
(1999) - et al.
Tumor necrosis factor (TNF)-alpha inhibits insulin signaling through stimulation of the p55 TNF receptor and activation of sphingomyelinase
J. Biol. Chem.
(1996) - et al.
Chronic ethanol consumption inhibits glucokinase transcriptional activity by Atf3 and triggers metabolic syndrome in vivo
J. Biol. Chem.
(2014) - et al.
Influence of daily alcohol consumption on serum adiponectin levels in men
Metabolism
(2013) - et al.
Protocol for effective differentiation of 3T3-L1 cells to adipocytes
Anal. Biochem.
(2012) - et al.
A simple method for the isolation and purification of total lipides from animal tissues
J. Biol. Chem.
(1957)
A simple, rapid, and sensitive DNA assay procedure
Anal. Biochem.
PPARgamma: an essential regulator of adipogenesis and modulator of fat cell function
Cell
PPARgamma in adipocyte differentiation and metabolism--novel insights from genome-wide studies
FEBS Lett.
Chronic alcohol exposure stimulates adipose tissue lipolysis in mice: role of reverse triglyceride transport in the pathogenesis of alcoholic steatosis
Am. J. Pathol.
Harmful effect of adipose tissue on liver lesions in patients with alcoholic liver disease
J. Hepatol.
Recent trends in the epidemiology of alcoholic liver disease
Clin. Liver Dis.
Mortality due to cirrhosis and liver cancer in the United States, 1999-2016: observational study
BMJ
Alcoholic liver disease: pathologic, pathogenetic and clinical aspects
Alcohol Clin. Exp. Res.
Chronic alcohol exposure disturbs lipid homeostasis at the adipose tissue-liver axis in mice: analysis of triacylglycerols using high-resolution mass spectrometry in combination with in vivo metabolite deuterium labeling
PloS One
Adiponectin: more than just another fat cell hormone?
Diabetes Care
Adipose tissue in obesity-related inflammation and insulin resistance: cells, cytokines, and chemokines
ISRN Inflamm.
Alcohol, adipose tissue and lipid dysregulation
Biomolecules
Hepatic insulin signaling regulates VLDL secretion and atherogenesis in mice
J. Clin. Invest.
Insulin regulates hepatic triglyceride secretion and lipid content via signaling in the brain
Diabetes
Carbohydrate metabolism in non-insulin-dependent diabetes mellitus
N. Engl. J. Med.
Insulin signalling and the regulation of glucose and lipid metabolism
Nature
Adiponectin promotes preadipocyte differentiation via the PPARgamma pathway
Mol. Med. Rep.
Cited by (0)
- 1
These authors contributed equally and are co-senior/last authors.