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Role of farnesoid X receptor in hepatic steatosis in nonalcoholic fatty liver disease
Biomedicine & Pharmacotherapy 121 (2020) 109609
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Review
Role of farnesoid X receptor in hepatic steatosis in nonalcoholic fatty liver disease
T
Yingfei Xia, Hongshan Lib,* a b
Medical School of Ningbo University, Ningbo, Zhejiang, 315211, China Liver Disease Department, Hwa Mei Hospital, University of Chinese Academy of Sciences, Ningbo, Zhejiang 315010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Farnesoid X receptor Hepatic steatosis Nonalcoholic fatty liver disease Mechanism
With the increased incidence of obesity, nonalcoholic fatty liver disease (NAFLD) has become a major global health concern. The pathogenesis of NAFLD has not yet been fully elucidated, and as few efficient pharmaceutical treatments are available for the condition, economic and medical burdens are heavy. Hepatic steatosis, as a precursor of NAFLD, plays a vital role in the pathological process of NAFLD. Hepatic steatosis is a consequence of lipid acquisition (i.e. free fatty acid uptake and de novo lipogenesis) exceeding lipid disposal (i.e. fatty acid oxidation and export as very-low-density lipoproteins). Therefore, restoring lipid homeostasis in the liver is an important therapeutic strategy of NAFLD. Farnesoid X receptor (FXR) is a major member of the ligandactivated nuclear receptor superfamily. Previous reviews have shown that FXR is a multipurpose receptor that plays an important role in regulating bile acid homeostasis, glucose and lipid metabolism, intestinal bacterial growth, and hepatic regeneration. This review focuses on the role of FXR in individual pathways that contribute to hepatic steatosis; it further demonstrates the molecular function of FXR in the pathogenesis of NAFLD.
1. Introduction Nonalcoholic fatty liver disease (NAFLD) is characterized by the accumulation of triglycerides (TGs) within hepatocytes (more than 5%), after excluding other causes of fat depots (e.g. excessive alcohol consumption, use of medications, chronic liver disease) [1,2]. NAFLD encompasses a broad clinical spectrum, ranging from nonalcoholic fatty liver (NAFL) to the more severe form, nonalcoholic steatohepatitis (NASH), which can progress to advanced fibrosis that may ultimately lead to cirrhosis or hepatocellular carcinoma (HCC) [2]. With the increasing incidence of obesity, the prevalence of NAFLD is rapidly rising.
Currently, the global prevalence of NAFLD is approximately 25.24%, and the pooled prevalence of NAFLD in Asia and Europe is estimated to be about 27.37% and 23.71%, respectively [3]. Indeed, NAFLD has become the most common chronic liver disease in Western countries, leading to heavy economic and medical burdens [4]. NAFLD is a complex multifaceted disease commonly associated with metabolic comorbidities such as type 2 diabetes, dyslipidemia, obesity, and hyperlipidemia [5]. Although the pathogenesis of NAFLD has not yet been fully elucidated, hepatic steatosis, as a precursor, is important in the pathogenesis of NAFLD [6]. Hepatic steatosis results from an imbalance between lipid acquisition and lipid removal, which is
Abbreviations: NAFLD, nonalcoholic fatty liver disease; FXR, farnesoid X receptor; BA, bile acid; TG, triglyceride; NAFL, nonalcoholic fatty liver; NASH, nonalcoholic steatohepatitis; HCC, hepatocellular carcinoma; FFA, free fatty acid; FAO, fatty acid oxidation; VLDL, very-low-density lipoprotein; OCA, obeticholic acid; FA, fatty acid; FATP, fatty acid-transport protein; CD36, fatty acid translocase; PPAR, peroxisome proliferator-activated receptor; FABP, fatty acid-binding protein; SHP, small heterodimer partner; LXR, liver X receptor; HNF4α, hepatocyte nuclear factor 4 alpha; ACC, acetyl-CoA carboxylase; FAS, fatty acid synthase; SREBP, sterol regulatory element-binding protein; ChREBP, carbohydrate response element-binding protein; SCD1, stearoyl-CoA desaturase 1; INSIG2, insulin-induced gene 2 protein; SCAP, SREBP cleavage-activating protein; LPK, liver-type pyruvate kinase; ChoRE, carbohydrate-response element; CDCA, chenodeoxycholic acid; CA, cholic acid; ACS, acyl-CoA synthetase; SMRT, silencing mediator of retinoic acid and thyroid hormone receptor; FGF, fibroblast growth factor; FGFR, fibroblast growth factor receptor; CYP7A1, cholesterol 7 alpha-hydroxylase; STAT, signal transducer and activator of transcription; PGC, peroxisome proliferator-activated receptor gamma coactivator; T-β-MCA, tauro-β-muricholic acid; Gly-MCA, glycine-β-muricholic acid; ROS, reactive oxygen species; VLCFA, very long-chain fatty acid; ACOX, fatty acyl-CoA oxidase; CPT, carnitine palmitoyl transferase; LCAD, long chain acyl-CoA dehydrogenase; MCAD, medium chain acyl-CoA dehydrogenase; RXR, retinoid X receptor; PPRE, peroxisome proliferator response element; FXRE, farnesoid X-activated receptor elements; Ces1, carboxylesterase 1; PDK-4, pyruvate dehydrogenase kinase isozyme 4; MTTP, microsomal triglyceride transfer protein; ApoB, apolipoprotein B; ANGPTL3, angiopoietin-like protein 3; LPL, lipoprotein lipase; ApoC, apolipoprotein C; PLA2G12B, phospholipase A2 G12B; VLDLR, very-low-density lipoprotein receptor ⁎ Corresponding author. E-mail address: [email protected] (H. Li). https://doi.org/10.1016/j.biopha.2019.109609 Received 8 August 2019; Received in revised form 23 October 2019; Accepted 25 October 2019 0753-3322/ © 2019 The Author(s). Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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Fig. 1. FXR-mediated lipid metabolism in the liver. Intestinal and hepatic FXR represses lipogenesis and promotes FA oxidation. Hepatic FXR also reduces FA uptake and VLDL secretion. (a) FXR activation leads to the reduction of CD36, and thus, reduces FA uptake. (b) FXR represses lipogenesis via the FXR-SHP-SREBP-1c pathway. (c) FXR increases FA oxidation by stimulating the expressions of PPARα and CPT1. (d) FXR inhibits VLDL secretion via HNF4α-mediated expression of MTTP and ApoB. FA, fatty acid; FXR, farnesoid X receptor; CD36, fatty acid translocase; TG, triglyceride; ChoRE, carbohydrate-response element; SHP, small heterodimer partner; LPK, liver-type pyruvate kinase; LXR, liver X receptor; SREBP, sterol regulatory element binding protein; FAS, fatty acid synthase; ACC, acetyl-CoA carboxylase; SCD1, stearoyl-CoA desaturase 1; FGF, fibroblast growth factor; FGFR, fibroblast growth factor receptor; PPAR, peroxisome proliferator-activated receptor; Ces1, carboxylesterase 1; PDK-4, pyruvate dehydrogenase kinase isozyme 4; CPT, carnitine palmitoyl transferase; FAO, fatty acid oxidation; ApoC, apolipoprotein C; ANGPTL3, angiopoietin-like protein 3; VLDLR, very-low-density lipoprotein receptor; HNF4α, hepatocyte nuclear factor 4 alpha; MTTP, microsomal triglyceride transfer protein; VLDL, very-low-density lipoprotein; LPL, lipoprotein lipase.
constitute three membrane proteins, which are associated with protein trafficking and lipid droplet formation [17]. Caveolin-1 is enhanced in the liver of animal models with NAFLD [18], whereas whole body deletion of caveolin-1 reduces hepatic lipid levels [19]. Following uptake, FAs within hydrophobic proteins combine with fatty acid-binding protein (FABP) [13]. The activation of FXR definitely induces the expression of small heterodimer partner (SHP) [12]. SHP is an atypical nuclear receptor that lacks a DNA-binding domain and exerts a repressive effect via dimerization with several nuclear receptors, such as liver X receptor (LXR), hepatocyte nuclear factor 4α (HNF4α), and PPARs [20–23]. In NAFLD, overexpression of CD36 increases hepatic uptake of FAs [24], whereas knockdown of CD36 decreases lipid accumulation in both dietinduced and genetic steatosis [25], suggesting that CD36 is critical in the development of hepatic steatosis. Treatment of diet-induced obese mice with FXR agonist GW4064 resulted in reversal of hepatic steatosis and reduction in plasma lipid level through considerable reduction in the expression of CD36 at both protein and mRNA level [26]. In addition, GW4064 inhibited PPARγ in wild-type mice but not in SHP-deficient mice, suggesting that SHP is required for the repression of PPARγ promoter activity by FXR [27]. In HNF4α-deficient mice, PPARγ mRNA levels reduced dramatically. PPARγ promoter activity was induced by overexpression of HNF4α through transfection assays, and this induction was inhibited by overexpression of SHP, indicating that SHP suppresses PPARγ through HNF4α [27]. As previously mentioned, CD36 is regulated by PPARγ. Hence, we hypothesize that FXR inhibits the expression of CD36 via SHP-mediated reduced activity of PPARγ, which subsequently reduces FA uptake and hepatic steatosis.
regulated by four primary mechanisms: free fatty acid (FFA) uptake, de novo lipogenesis, fatty acid oxidation (FAO), and export from hepatocytes as very-low-density lipoproteins (VLDLs) [7]. Disturbance in one or more of these pathways may result in the retention of fat in the liver, leading to NAFLD [7]. FXR belongs to the nuclear receptor superfamily, and is mostly expressed in the liver, kidney, intestinal villi, and adrenal cortex [8]. Obeticholic acid (OCA), an agonist of FXR, improves histological hepatic steatosis, inflammation, and fibrosis in patients with NASH [9]; this shows the importance of FXR in the treatment of NAFLD. In recent years, this receptor has been demonstrated to be critical in modulating various metabolic processes in the liver. Several previous reviews have illustrated that the activation of FXR by endogenous (e.g. bile acid [BA]) or synthetic (e.g. OCA, GW4064, WAY-362450) ligands improves BA homeostasis, glucose metabolism, lipid accumulation, inflammation, and fibrogenesis in NAFLD [8–12]. However, no review has systematically revealed the interaction of FXR with lipid accumulation in NAFLD. In this review, we clearly demonstrate the role of FXR in the progress of hepatic steatosis; this may assist in the search for new pharmaceutical treatments for NAFLD (Fig. 1).
2. Hepatic lipid uptake and FXR The uptake of fatty acids (FAs) by the liver depends mainly on specific fatty acid transporters located in the plasma membrane [13]. Fatty acid transport protein (FATP), cluster of differentiation 36 (CD36), and caveolins are the primary proteins involved in the transport of FAs [6]. Among the six FATP isoforms in mammals, FATP2 and FATP5 are highly expressed in the liver [13]. Knockout of FATP2 or FATP5 in mice decreases the uptake of FAs and reverses hepatic steatosis [14,15]. The transmembrane protein CD36 accelerates the transport of long-chain fatty acids and is mediated by the peroxisome proliferator-activated receptor (PPAR)-γ [16]. Under normal conditions, CD36 is not highly expressed in the liver; however, it is overexpressed in diet-induced obesity [6]. In patients with NAFLD, expression of CD36 is positively correlated with triglyceride (TG) concentration in the liver [6]. Caveolins
3. De novo lipogenesis and FXR De novo lipogenesis is an integrated process that utilizes acetyl-CoA (derived from glycolysis) to synthesize saturated FA. Acetyl-CoA is converted to malonyl-CoA by acetyl-CoA carboxylase (ACC), and malonyl-CoA is then converted to palmitate by fatty acid synthase (FAS) [7]. The de novo-synthesized FA may then undergo desaturation and 2
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induction was significantly repressed upon activation of FXR [33]. FXR represses glucose-induced expression of LPK through a transrepressive mechanism involving the release of the transcription factor ChREBP and the recruitment of the transcriptional corepressor silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) to the ChoRE of the LPK promoter [33], thereby decreasing the production of acetyl-CoA for de novo lipogenesis and diminishing hepatic steatosis. FXR also negatively regulates the expression of other ChREBP target genes, likely by molecular mechanisms similar to those involved in the repression of the LPK gene, at least for FAS [33]. Activation of FXR in the ileal enterocytes via intestinal BA also has important implications for lipid hemostasis. Intestinal FXR induces the production of fibroblast growth factor 19 (FGF-19, FGF-15 in mice) [39], which binds to FGFR4/β-klotho complex within hepatocytes and could inhibit BA synthesis in the liver through repressing cholesterol 7 alpha-hydroxylase (CYP7A1) gene transcription [40]. Transgenic expression of human FGF-19 in obese mice leads to increased energy expenditure and decreased adiposity [41]. In the liver, FGF-19 inhibits insulin-induced stimulation of FA synthesis by reducing the activity of SREBP-1c, along with increased activity of signal transducer and activator of transcription 3 (STAT3, an inhibitor of SREBP-1c expression), and decreased expression of PPARγ coactivator-1β (PGC-1β, an activator of SREBP-1c activity) [42]. In addition, FGF-19 upregulates the expression of SHP, which suppresses the expression of lipogenic enzymes through an SREBP-1c-independent mechanism [42]. A previous study by Zhang et al. reported that PPARγ coactivator 1α (PGC-1α) increases FXR transcriptional activity during fasting [43]. Fasting induces the expression of PGC-1α and FXR in the liver and leads to decreased plasma TG concentrations in wild-type mice, but not in FXR-deficient mice, supporting the theory that FXR limits lipogenesis under conditions of nutrient deprivation [10,43]. Interestingly, some studies indicated that inhibition of FXR signaling in the intestine prevents NAFLD. Mice with diet-induced obesity treated with antibiotics or tempol showed decreased FA biosynthesis in the liver depending on modulation of the gut microbiota and BA metabolism, enriched levels of the FXR antagonist tauro-β-muricholic acid (T-β-MCA), and antagonized FXR signaling in the intestine [44]. However, another report indicated that T-β-MCA was rapidly hydrolyzed by gut bacterial bile salt hydrolase and failed to reach concentrations that inhibit gut-specific FXR signaling in vivo [45]. As a derivative of T-β-MCA, glycine-β-muricholic acid (Gly-MCA) proved to be an intestine-selective FXR inhibitor [45]. Treatment with Gly-MCA reduced obesity and hepatic steatosis in obese mice owing to reduced biosynthesis of intestinal-derived ceramides, which directly compromised the thermogenic function of beige fat [45]. This finding is consistent with the observation that ceramide treatment reverses the beneficial effects of Gly-MCA in mice with diet-induced obesity [45].
esterification to form TG, which is eventually stored as TG vacuoles or exported as VLDL particles [7]. De novo lipogenesis is regulated by multiple transcription factors, among which sterol regulatory elementbinding protein 1c (SREBP-1c) and carbohydrate response elementbinding protein (ChREBP) are the primary regulators [6]. SREBP-1c is the predominant isoform of SREBPs, and it is activated by insulin and liver X receptor α (LXRα) [28]. In transfected hepatocytes, SREBP-1c augments the expression of enzymes involved in lipogenesis, such as FAS, ACC, and stearoyl-CoA desaturase 1(SCD1), whereas SREBP-1c-deficient mice show decreased hepatic lipid levels, further substantiating the lipogenic action of SREBP-1c [28–30]. Insulin activates SREBP-1c indirectly via inhibition of the interaction between insulin-induced gene 2 protein (INSIG2) and SREBP cleavage-activating protein (SCAP) under feeding conditions. LXRα directly activates SREBP-1c via LXR binding sites in the SREBP-1c gene promoter [31]. In addition, LXRα promotes lipogenesis directly through inducing the expressions of FAS, ACC, and SCD1 [28]. ChREBP regulates glucose-induced lipogenesis [32]. Under highglucose conditions, activated ChREBP promotes the expression of its target genes, such as the glycolytic liver-type pyruvate kinase (LPK), FAS, and ACC1 (an enzyme involved in the production of malonyl-CoA) via acceleration of the combination of Max-like protein (Mlx) with carbohydrate-response element (ChoRE) to form two Ebox-like motifs, which are present in the promoter of its target genes [33]. Moreover, fasting hormones, such as glucagon, suppress the expression of ChREBP [34]. In obese and insulin-resistant ob/ob mice, the expressions of both SREBP-1c and ChREBP are markedly increased in the liver, and a decrease in either factor helps alleviate hepatic steatosis, indicating the importance of these transcription factors in de novo lipogenesis and accumulation of TGs [35]. Diabetic KK-Ay and C57BL/6 J mice fed a dietary supplement of cholic acid (CA) showed reduced expression of genes encoding enzymes involved in the biosynthesis of FAs and TGs, such as acyl-CoA synthetase (ACS), malic enzyme, and SCD1 [36], and FXR-deficient mice exhibited a conspicuous induction of lipogenic genes, such as FAS, SREBP-1c, and SCD1 [37], suggesting that FXR is critical in downregulating lipogenesis. In patients with hypotriglyceridemia and gallstones, serum TG levels decreased on treatment with chenodeoxycholic acid (CDCA) [10]. Indeed, FXR has a role in the amelioration of blood TG levels, partly through the inhibition of FA and TG biosynthesis [10]. In mouse primary hepatocytes in vitro, CDCA lowers the expression of endogenous SREBP-1c and its target genes [36]. In vivo, the expression of endogenous SREBP-1c was induced by LXR, and this expression was negatively correlated with the amount of CDCA [36]. These data show that the expression of endogenous SREBP-1c is markedly modulated by BAs. The activation of FXR is known to induce the expression of SHP. In wild-type mice, treatment with CA and GW4064 considerably reduced serum TG levels and genes involved in lipogenesis; however, the reduction was not observed in SHP-null mice [36]. Meanwhile, in SHP-deficient mice, suppression of SREBP-1c and its target genes by CA or GW4064 was not observed [38]. These experiments demonstrate that FXR inhibits de novo lipogenesis through an FXR–SHP–SREBP-1c pathway. In wild-type mice, treatment with CA lowers serum TG levels and the expression of SREBP-1c [36]. This decrease in serum TG levels and SREBP-1c expression was abolished in LXR-deficient mice [36]. These results show that LXR is critical in the reduction of SHP-mediated TGs in vivo. In brief, activation of FXR induces the expression of SHP, which then suppresses the activation of LXR and LXR-induced SREBP-1c, thus inhibiting the expression of lipogenic enzymes [10]. Further, in fasting wild-type mice, the hepatic levels of LPK mRNA were considerably increased by high-carbohydrate refeeding, whereas FXR synthetic agonist INT-747 markedly blunted this induction [33]. In immortalized human hepatocytes and HepaRG cells, the expression of LPK gene was increased via incubation with high glucose concentrations, whereas this
4. Fatty acid oxidation and FXR In mammalian cells, FAO plays a central role in energy generation, especially in cases of low circulating glucose concentrations [7,46]. Three cellular organelles mediate FAO—mitochondria and peroxisomes contribute to FA β-oxidation, and cytochromes participate in FA ωoxidation [47,48]. Although NAFLD is characterized by lipid overload and defective β-oxidation owing to dysfunctional mitochondria, defects in ω-oxidation could also contribute to the disease [46,47]. These processes generate oxidative stress, reactive oxygen species (ROS), and toxic dicarboxylic acids, which potentially aggravate inflammation and disease progression [47]. Peroxisomal β-oxidation metabolize very long-chain fatty acids (VLCFAs), dicarboxylic acid, and certain branched-chain FAs [49,50]. In two studies, patients with NAFLD showed elevated mRNA levels of fatty acyl-CoA oxidase (ACOX) and branched chain acyl-CoA oxidase (two peroxisomal enzymes involved in FAO), indicating that elevated peroxisomal FAO may be a complemental response attempting to remit 3
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which is the only way to reduce lipid content in the liver other than FAO [65]. VLDL particles comprise hydrophobic core lipids containing TGs and cholesterol esters, which are covered with hydrophilic phospholipids, and apolipoproteins [66]. The process of assembly of VLDL particles initially occurs in the endoplasmic reticulum and is catalyzed by microsomal triglyceride transfer protein (MTTP), an enzyme that facilitates the lipidation of apolipoprotein B 100 (ApoB 100) with TG [7,67]. Nascent VLDL particles are transported into Golgi apparatus; during this process, nascent particles that are further lipidated ultimately form the mature VLDL particles [61]. Indeed, hepatic steatosis is common in patients with mutations in ApoB100 (hypobetalipoproteinemia) or in MTTP (abetalipoproteinemia) [6,7,61], emphasizing the importance of these proteins in hepatic secretion of VLDL and lipid homeostasis. Insulin is crucial in the regulation of the assembly and secretion of hepatic VLDL; it decreases lipid export by inducing post-translational degradation of ApoB100 and inhibiting MTTP synthesis at the transcription level [61]. In patients with NAFLD, selective hepatic insulin resistance allows insulin to stimulate de novo lipogenesis without inhibiting the production of VLDL [7], resulting in the synthesis of TGs exceeding its secretion, leading to increased TG concentration in the liver. Consequently, patients with NAFLD exhibit hypertriglyceridemia and hepatic steatosis. Prolonged exposure to FAs promotes endoplasmic reticulum stress in the liver; this inhibits the secretion of ApoB100, thereby decreasing the secretion VLDL and worsening hepatic steatosis [61]. The decreased serum TG concentration partially results from FXRmediated TG clearance [10]. FXR promotes TG clearance through mediating the transcription of regulators (e.g. ApoC-II, ApoC-III, and ANGPTL3) of lipoprotein lipase (LPL), which hydrolyzes TG from VLDL particles and chylomicrons, thereby lowering serum TG levels [36]. ApoC-II, an activator of LPL, has been reported as an FXR target gene [68]. ApoC-II mRNA levels were increased both in HepG2 cells cultured with CDCA and wild-type mice treated with 1% CA diet, and this induction was blunted in FXR-deficient mice [68]. FXR heterodimerizes with RXR to form FXR/RXR heterodimer and binds to FXRE, which contains distal enhancer elements located upstream of the ApoC-II promotor; subsequently, it upregulates the expression of ApoC-II gene [68]. Regulation of ApoC-III (an inhibitor of LPL) by FXR is supported by decreased expression of ApoC-III upon treatment with taurocholic acid in mice and upon treatment with CDCA in human primary hepatocytes and HepG2 cells, but not in FXR-deficient mice [69]. Activated FXR causes the FXR/RXR heterodimer to bind to FXRE upstream of the ApoC-III promoter, which is occupied by HNF4α, a strong regulator of ApoC-III, in the absence of FXR agonists [69]. As a negative regulation, the expression of ApoC-III gene is downregulated [69]. Further, treatment with CA reduces the expression of ANGPTL3 through an SHPdependent mechanism [36]. ANGPTL3 can inactivate LPL, thereby increasing serum TG levels [36]. Taken together, FXR improves TG clearance by inducing the hepatic expression of ApoC-II and repressing the expression of ApoC-III and ANGPTL3 [36,68,69]. VLDL secretion is also mediated by FXR. Treatment with CDCA reduced the production of VLDLs in fructose-feeding hamsters and in patients with hypertriglyceridemia [10]. When HepG2 cells were administered CDCA, mRNA levels of MTTP and ApoB decreased [70]. FXR inhibits VLDL secretion through SHP-mediated repression of HNF4α, which can induce the expression of MTTP and ApoB [70]. Liu et al. (2016) reported that FXR decreases VLDL secretion partly through reduction of phospholipase A2G12B (PLA2G12B) expression [71]. They speculated that FXR-induced SHP expression accounts for the repression of PLA2G12B; nevertheless, whether decreased VLDL secretion by FXR activation is definitely regulated by PLA2G12B has not yet been proven. Additionally, FXR activation induces the expressions of syndecan-1 and VLDL receptor, which accounts for enhanced clearance of remnant particles and TG-rich lipoproteins, respectively
the progression of steatosis in NAFLD [30,51]. However, the peroxisomes also generate ROS, which may induce oxidative stress such as ωoxidation in the cytochromes [52]. Mitochondrial β-oxidation is the primary pathway of FAO, which involves catabolization of short-, medium-, and long-chain FAs derived from diet [46]. The entry of activated FAs into mitochondria depends on the presence of carnitine palmitoyl transferase 1 (CPT1) at the mitochondrial outer membrane, which catalyzes the formation of acylcarnitine from acyl-CoA [6]. Mitochondria are a dominant site for generation of ROS; typically, consumption of only about 1%–2% oxygen in the mitochondria result in the generation of ROS [53]. Excessive production of ROS, partially from peroxisomal and cytochromemediated FAO, leads to lipid peroxidation of the mitochondrial membranes, which, in turn, impairs mitochondrial function [53,54]. Thus, dysfunctional mitochondria not only decrease the catabolism of LCFACoA, which can cause lipotoxicity, but also generate more ROS, leading to a vicious circle [54]. Activation of FXR in the liver also drives FAO. Wild-type C57BL/6 J mice treated with CA showed increased expression of FAO genes in medium-chain acyl-CoA dehydrogenase (MCAD) and long-chain acylCoA dehydrogenase (LCAD) enzymes [36]. These changes in gene expressions are probably due to FXR-mediated induction of PPARα expression [55]. PPARα is a transcription factor activated by FA, and it regulates all three FAO systems. PPARα forms a heterodimer with retinoid X receptor (RXR), which binds to the peroxisome proliferator response element (PPRE), a specific DNA sequence present on its target gene promoter [56]. These transcriptional complexes upregulate the expression of genes related to FAO, such as MCAD in mitochondrial βoxidation, ACOX1, and enoyl-CoA hydratase in peroxisomal β-oxidation, and CYP4A1 and CYP4A3 in cytochrome-mediated ω-oxidation [57]. PPARα-deficient ob/ob mice show severe hepatic steatosis [57], further supporting the importance of PPARα in hepatic lipid homeostasis. However, the expression of PPARα in the liver is negatively correlated with the severity of steatosis [58]. Incubation of HepG2 cells and human primary hepatocytes with CDCA and GW4064 resulted in upregulation of the expression of human PPARα gene [55], whereas in FXR-deficient ob/ob mice, the expression of PPARα was strongly downregulated [59]. Indeed, FXR induces PPARα expression by binding to the functional farnesoid X-activated receptor element (FXRE), αFXRE, within the human PPARα promoter [55]. Further, when HepG2 cells were treated with CDCA, the expression of PPARα target gene CPT1 (which facilitates the entry of FAs into the mitochondria) was also enhanced [55]. As previously described, intestinal FXR induces the expression of FGF-19. FGF19-transgenic mice showed decreased expression of ACC2, which negatively regulated FAO through malonyl-CoA-mediated inhibition of CPT1 [41,42]. FXR activated by BA or GW4064 elevated the expression and secretion of FGF-21 [60]. FGF-21 stimulated FAO, ketogenesis, and gluconeogenesis during fasting by binding to the FGFR1c/b-klotho complex in the liver [40,61]. Moreover, FXR also enhanced FGF-21 expression through FGF-19 [60]. Increased hepatic FGF-21 expression results in reduced TG levels [59]. Interestingly, a previous study reported that overexpression of hepatic carboxylesterase 1 (Ces1) induced by FXR ligands is essential for improved lipid homeostasis [62]. The authors indicated that FXR directly induces Ces1 expression via binding to FXRE upstream of Ces1 promotor; overexpressed Ces1 increases the release of FA, consequently activating PPARα, and then stimulates FAO. Additionally, FXR ligands augment the expression of PPARα and that of its target gene pyruvate dehydrogenase kinase isozyme 4 (PDK-4), which may promote FAO, consistent with concomitantly decreased plasma TG levels [63]. 5. Lipid export and FXR TGs are exported from the hepatocytes in the form of VLDLs [64], 4
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[59].
metabolism, angiogenesis, and behavior, Sci. Signal. 2 (2009) re3. [17] M.A. Fernandez, C. Albor, M. Ingelmo-Torres, S.J. Nixon, C. Ferguson, T. Kurzchalia, et al., Caveolin-1 is essential for liver regeneration, Science 313 (2006) 1628–1632. [18] Y. Qiu, S. Liu, H.T. Chen, C.H. Yu, X.D. Teng, H.T. Yao, et al., Upregulation of caveolin-1 and SR-B1 in mice with non-alcoholic fatty liver disease, HBPD INT 12 (2013) 630–636. [19] I.W. Asterholm, D.I. Mundy, J. Weng, R.G. Anderson, P.E. Scherer, Altered mitochondrial function and metabolic inflexibility associated with loss of caveolin-1, Cell Metab. 15 (2012) 171–185. [20] W. Seol, H.S. Choi, D.D. Moore, An orphan nuclear hormone receptor that lacks a DNA binding domain and heterodimerizes with other receptors, Science 272 (1996) 1336–1339. [21] J.H. Lee, H. Kim, S.J. Park, J.H. Woo, E.H. Joe, I. Jou, Small heterodimer partner SHP mediates liver X receptor (LXR)-dependent suppression of inflammatory signaling by promoting LXR SUMOylation specifically in astrocytes, Sci. Signal. 9 (2016) ra78. [22] T.T. Lu, M. Makishima, J.J. Repa, K. Schoonjans, T.A. Kerr, J. Auwerx, et al., Molecular basis for feedback regulation of bile acid synthesis by nuclear receptors, Mol. Cell 6 (2000) 507–515. [23] Y.S. Kim, C.Y. Han, S.W. Kim, J.H. Kim, S.K. Lee, D.J. Jung, et al., The orphan nuclear receptor small heterodimer partner as a novel coregulator of nuclear factorkappa b in oxidized low density lipoprotein-treated macrophage cell line RAW 264.7, J. Biol. Chem. 276 (2001) 33736–33740. [24] D.P. Koonen, R.L. Jacobs, M. Febbraio, M.E. Young, C.L. Soltys, H. Ong, et al., Increased hepatic CD36 expression contributes to dyslipidemia associated with dietinduced obesity, Diabetes 56 (2007) 2863–2871. [25] C.G. Wilson, J.L. Tran, D.M. Erion, N.B. Vera, M. Febbraio, E.J. Weiss, Hepatocytespecific disruption of CD36 attenuates fatty liver and improves insulin sensitivity in HFD-Fed mice, Endocrinology 157 (2016) 570–585. [26] Y. Ma, Y. Huang, L. Yan, M. Gao, D. Liu, Synthetic FXR agonist GW4064 prevents diet-induced hepatic steatosis and insulin resistance, Pharm. Res. 30 (2013) 1447–1457. [27] K. Jadhav, Y. Xu, Y. Xu, Y. Li, J. Xu, Y. Zhu, et al., Reversal of metabolic disorders by pharmacological activation of bile acid receptors TGR5 and FXR, Mol. Metab. 9 (2018) 131–140. [28] S.B. Joseph, B.A. Laffitte, P.H. Patel, M.A. Watson, K.E. Matsukuma, R. Walczak, et al., Direct and indirect mechanisms for regulation of fatty acid synthase gene expression by liver X receptors, J. Biol. Chem. 277 (2002) 11019–11025. [29] D. Eberle, B. Hegarty, P. Bossard, P. Ferre, F. Foufelle, SREBP transcription factors: master regulators of lipid homeostasis, Biochimie 86 (2004) 839–848. [30] M. Kohjima, M. Enjoji, N. Higuchi, M. Kato, K. Kotoh, T. Yoshimoto, et al., Reevaluation of fatty acid metabolism-related gene expression in nonalcoholic fatty liver disease, Int. J. Mol. Med. 20 (2007) 351–358. [31] T. Yoshikawa, H. Shimano, M. Amemiya-Kudo, N. Yahagi, A.H. Hasty, T. Matsuzaka, et al., Identification of liver X receptor-retinoid X receptor as an activator of the sterol regulatory element-binding protein 1c gene promoter, Mol. Cell. Biol. 21 (2001) 2991–3000. [32] K. Iizuka, R.K. Bruick, G. Liang, J.D. Horton, K. Uyeda, Deficiency of carbohydrate response element-binding protein (ChREBP) reduces lipogenesis as well as glycolysis, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 7281–7286. [33] S. Caron, C. Huaman Samanez, H. Dehondt, M. Ploton, O. Briand, F. Lien, et al., Farnesoid X receptor inhibits the transcriptional activity of carbohydrate response element binding protein in human hepatocytes, Mol. Cell. Biol. 33 (2013) 2202–2211. [34] G. Filhoulaud, S. Guilmeau, R. Dentin, J. Girard, C. Postic, Novel insights into ChREBP regulation and function, Trends Endocrinol. Metab. 24 (2013) 257–268. [35] R. Dentin, F. Benhamed, I. Hainault, V. Fauveau, F. Foufelle, J.R.B. Dyck, et al., Liver-specific inhibition of ChREBP improves hepatic steatosis and insulin resistance in ob/ob mice, Diabetes 55 (2006) 2159–2170. [36] M. Watanabe, S.M. Houten, L. Wang, A. Moschetta, D.J. Mangelsdorf, R.A. Heyman, et al., Bile acids lower triglyceride levels via a pathway involving FXR, SHP, and SREBP-1c, J. Clin. Invest. 113 (2004) 1408–1418. [37] K. Ma, P.K. Saha, L. Chan, D.D. Moore, Farnesoid X receptor is essential for normal glucose homeostasis, J. Clin. Invest. 116 (2006) 1102–1109. [38] J.R. Crouse 3rd, Hypertriglyceridemia: a contraindication to the use of bile acid binding resins, Am. J. Med. 83 (1987) 243–248. [39] S. Kir, Y. Zhang, R.D. Gerard, S.A. Kliewer, D.J. Mangelsdorf, Nuclear receptors HNF4α and LRH-1 cooperate in RegulatingCyp7a1 in vivo, J. Biol. Chem. 287 (2012) 41334–41341. [40] C. Cicione, C. Degirolamo, A. Moschetta, Emerging role of fibroblast growth factors 15/19 and 21 as metabolic integrators in the liver, Hepatology 56 (2012) 2404–2411. [41] E. Tomlinson, L. Fu, L. John, B. Hultgren, X. Huang, M. Renz, et al., Transgenic mice expressing human fibroblast growth factor-19 display increased metabolic rate and decreased adiposity, Endocrinology 143 (2002) 1741–1747. [42] S. Bhatnagar, H.A. Damron, F.B. Hillgartner, Fibroblast growth factor-19, a novel factor that inhibits hepatic fatty acid synthesis, J. Biol. Chem. 284 (2009) 10023–10033. [43] Y. Zhang, L.W. Castellani, C.J. Sinal, F.J. Gonzalez, P.A. Edwards, Peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha) regulates triglyceride metabolism by activation of the nuclear receptor FXR, Genes Dev. 18 (2004) 157–169. [44] C. Jiang, C. Xie, F. Li, L. Zhang, R.G. Nichols, K.W. Krausz, et al., Intestinal farnesoid X receptor signaling promotes nonalcoholic fatty liver disease, J. Clin. Invest. 125 (2015) 386–402.
6. Conclusions In conclusion, FXR regulates multiple pathways involved in hepatic steatosis. FXR induces the expression of SHP, which represses lipid synthesis via the inhibition of LXR-mediated SREBP-1c and decreases VLDL secretion via the suppression of HNF4α-mediated MTTP and ApoB. FXR also stimulates the expression of PPARα, which can increase FAO through several mechanisms. Moreover, FXR promotes TG clearance via overexpression of LPL by inducing the expression of ApoC-II, and inhibiting the expressions of ApoC-III and ANGPTL3. In addition, activation of intestinal-specific FXR by BA could induce the expression of FGF19, which decreases lipogenesis via a SREBP-1c-dependent mechanism and increases FAO via the inhibition of ACC2. FXR activation also induces the expression of FGF21, which is involved in FAO during fasting. However, the mechanism by which FXR inhibits CD36 expression is still unclear. Whether PDK-4 (a target gene of PPARα) facilitates FAO is uncertain. Therefore, further research is warranted to unravel the exact role of FXR in hepatic steatosis, which may contribute to the development of better pharmaceutical treatment for NAFLD in the future. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgment This study was supported by the National Natural Science Foundation of China (grant number 81873109). References [1] G.B. Goh, A.J. McCullough, Natural history of nonalcoholic fatty liver disease, Dig. Dis. Sci. 61 (2016) 1226–1233. [2] B.J. Perumpail, M.A. Khan, E.R. Yoo, G. Cholankeril, D. Kim, A. Ahmed, Clinical epidemiology and disease burden of nonalcoholic fatty liver disease, World J. Gastroenterol. 23 (2017) 8263–8276. [3] Z.M. Younossi, A.B. Koenig, D. Abdelatif, Y. Fazel, L. Henry, M. Wymer, Global epidemiology of nonalcoholic fatty liver disease-meta-analytic assessment of prevalence, incidence, and outcomes, Hepatology 64 (2016) 73–84. [4] Z.M. Younossi, D. Blissett, R. Blissett, L. Henry, M. Stepanova, Y. Younossi, et al., The economic and clinical burden of nonalcoholic fatty liver disease in the United States and Europe, Hepatology 64 (2016) 1577–1586. [5] C.I. Andronescu, M.R. Purcarea, P.A. Babes, Nonalcoholic fatty liver disease: epidemiology, pathogenesis and therapeutic implications, J. Med. Life 11 (2018) 20–23. [6] S.H. Koo, Nonalcoholic fatty liver disease: molecular mechanisms for the hepatic steatosis, Clin. Mol. Hepatol. 19 (2013) 210–215. [7] D.H. Ipsen, J. Lykkesfeldt, P. Tveden-Nyborg, Molecular mechanisms of hepatic lipid accumulation in non-alcoholic fatty liver disease, Cell. Mol. Life Sci. 75 (2018) 3313–3327. [8] P. Lefebvre, B. Cariou, F. Lien, F. Kuipers, B. Staels, Role of bile acids and bile acid receptors in metabolic regulation, Physiol. Rev. 89 (2009) 147–191. [9] B.A. Neuschwander Tetri, R. Loomba, A.J. Sanyal, J.E. Lavine, M.L. Van Natta, M.F. Abdelmalek, et al., Farnesoid X nuclear receptor ligand obeticholic acid for non-cirrhotic, non-alcoholic steatohepatitis (FLINT): a multicentre, randomised, placebo-controlled trial, Lancet 385 (2015) 956–965. [10] V. Massafra, S.W.C. van Mil, Farnesoid X receptor: a "homeostat" for hepatic nutrient metabolism, Biochim. Biophys. Acta Mol. Basis Dis. 1864 (2018) 45–59. [11] J.P. Arab, S.J. Karpen, P.A. Dawson, M. Arrese, M. Trauner, Bile acids and nonalcoholic fatty liver disease: molecular insights and therapeutic perspectives, Hepatology 65 (2017) 350–362. [12] F.Y. Lee, H. Lee, M.L. Hubbert, P.A. Edwards, Y. Zhang, FXR, a multipurpose nuclear receptor, Trends Biochem. Sci. 31 (2006) 572–580. [13] D.G. Mashek, Hepatic fatty acid trafficking: multiple forks in the road1-3, Adv. Nutr. 4 (2013) 697–710. [14] A. Falcon, H. Doege, A. Fluitt, B. Tsang, N. Watson, M.A. Kay, et al., FATP2 is a hepatic fatty acid transporter and peroxisomal very long-chain acyl-CoA synthetase, Am. J. Physiol. Endocrinol. Metab. 299 (2010) E384–393. [15] H. Doege, R.A. Baillie, A.M. Ortegon, B. Tsang, Q. Wu, S. Punreddy, et al., Targeted deletion of FATP5 reveals multiple functions in liver metabolism: alterations in hepatic lipid homeostasis, Gastroenterology 130 (2006) 1245–1258. [16] R.L. Silverstein, M. Febbraio, CD36, a scavenger receptor involved in immunity,
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Y. Xi and H. Li
[59] Y. Li, K. Jadhav, Y. Zhang, Bile acid receptors in non-alcoholic fatty liver disease, Biochem. Pharmacol. 86 (2013) 1517–1524. [60] H.A. Cyphert, X.M. Ge, A.B. Kohan, L.M. Salati, Y.Q. Zhang, H.F. Bradley, Activation of the farnesoid X receptor induces hepatic expression and secretion of fibroblast growth factor 21, J. Biol. Chem. 287 (2012) 25123. [61] Y. Kawano, D.E. Cohen, Mechanisms of hepatic triglyceride accumulation in nonalcoholic fatty liver disease, J. Gastroenterol. 48 (2013) 434–441. [62] J. Xu, Y. Li, W.D. Chen, Y. Xu, L. Yin, X. Ge, et al., Hepatic carboxylesterase 1 is essential for both normal and farnesoid X receptor-controlled lipid homeostasis, Hepatology 59 (2014) 1761–1771. [63] R.S. Savkur, K.S. Bramlett, L.F. Michael, T.P. Burris, Regulation of pyruvate dehydrogenase kinase expression by the farnesoid X receptor, Biochem. Biophys. Res. Commun. 329 (2005) 391–396. [64] M.M. Hussain, J. Shi, P. Dreizen, Microsomal triglyceride transfer protein and its role in apoB-lipoprotein assembly, J. Lipid Res. 44 (2003) 22–32. [65] R.J. Perry, V.T. Samuel, K.F. Petersen, G.I. Shulman, The role of hepatic lipids in hepatic insulin resistance and type 2 diabetes, Nature 510 (2014) 84–91. [66] E. Fabbrini, B.S. Mohammed, F. Magkos, K.M. Korenblat, B.W. Patterson, S. Klein, Alterations in adipose tissue and hepatic lipid kinetics in obese men and women with nonalcoholic fatty liver disease, Gastroenterology 134 (2008) 424–431. [67] I.V.A. Pereira, J.T. Stefano, C.P.M.S. Oliveira, Microsomal triglyceride transfer protein and nonalcoholic fatty liver disease, Expert Rev. Gastroenterol. Hepatol. 5 (2011) 245–251. [68] H.R. Kast, C.M. Nguyen, C.J. Sinal, S.A. Jones, B.A. Laffitte, K. Reue, et al., Farnesoid X-activated receptor induces apolipoprotein C-II transcription: a molecular mechanism linking plasma triglyceride levels to bile acids, Mol. Endocrinol. 15 (2001) 1720–1728. [69] T. Claudel, Y. Inoue, O. Barbier, D. Duran-Sandoval, V. Kosykh, J. Fruchart, et al., Farnesoid X receptor agonists suppress hepatic apolipoprotein CIII expression, Gastroenterology. 125 (2003) 544–555. [70] H. Hirokane, M. Nakahara, S. Tachibana, M. Shimizu, R. Sato, Bile acid reduces the secretion of very low density lipoprotein by repressing microsomal triglyceride transfer protein gene expression mediated by hepatocyte nuclear factor-4, J. Biol. Chem. 279 (2004) 45685–45692. [71] Q. Liu, M. Yang, X. Fu, R. Liu, C. Sun, H. Pan, et al., Activation of farnesoid X receptor promotes triglycerides lowering by suppressing phospholipase A2 G12B expression, Mol. Cell. Endocrinol. 436 (2016) 93–101.
[45] C. Jiang, C. Xie, Y. Lv, J. Li, K.W. Krausz, J. Shi, et al., Intestine-selective farnesoid X receptor inhibition improves obesity-related metabolic dysfunction, Nat. Commun. 6 (2015) 10166. [46] P. Misra, J.K. Reddy, Peroxisome proliferator-activated receptor-alpha activation and excess energy burning in hepatocarcinogenesis, Biochimie 98 (2014) 63–74. [47] M.S. Rao, J.K. Reddy, Peroxisomal beta-oxidation and steatohepatitis, Semin. Liver Dis. 21 (2001) 043–056. [48] K. Begriche, J. Massart, M.A. Robin, F. Bonnet, B. Fromenty, Mitochondrial adaptations and dysfunctions in nonalcoholic fatty liver disease, Hepatology 58 (2013) 1497–1507. [49] P. Misra, N. Viswakarma, J.K. Reddy, Peroxisome proliferator-activated receptor-α signaling in hepatocarcinogenesis, Subcell. Biochem. 69 (2013) 77–99. [50] J.K. Reddy, T. Hashimoto, Peroxisomal beta-oxidation and peroxisome proliferatoractivated receptor alpha: an adaptive metabolic system, Annu. Rev. Nutr. 21 (2001) 193–230. [51] M. Nakamuta, M. Kohjima, N. Higuchi, M. Kato, K. Kotoh, T. Yoshimoto, et al., The significance of differences in fatty acid metabolism between obese and non-obese patients with non-alcoholic fatty liver disease, Int. J. Mol. Med. 22 (2008) 663–667. [52] G.H. Koek, P.R. Liedorp, A. Bast, The role of oxidative stress in non-alcoholic steatohepatitis, Clin. Chim. Acta 412 (2011) 1297–1305. [53] A. Boveris, B. Chance, The mitochondrial generation of hydrogen peroxide. General properties and effect of hyperbaric oxygen, Biochem. J. 134 (1973) 707–716. [54] F. Nassir, J.A. Ibdah, Role of mitochondria in nonalcoholic fatty liver disease, Int. J. Mol. Sci. 15 (2014) 8713–8742. [55] I. Pineda Torra, T. Claudel, C. Duval, V. Kosykh, J.C. Fruchart, B. Staels, Bile acids induce the expression of the human peroxisome proliferator-activated receptor alpha gene via activation of the farnesoid X receptor, Mol. Endocrinol. 17 (2003) 259–272. [56] S.A. Kliewer, K. Umesono, D.J. Noonan, R.A. Heyman, R.M. Evans, Convergence of 9-cis retinoic acid and peroxisome proliferator signalling pathways through heterodimer formation of their receptors, Nature 358 (1992) 771–774. [57] Q. Gao, Y. Jia, G. Yang, X. Zhang, P.C. Boddu, B. Petersen, et al., PPARalpha-deficient ob/ob obese mice become more obese and manifest severe hepatic steatosis due to decreased fatty acid oxidation, Am. J. Pathol. 185 (2015) 1396–1408. [58] S. Francque, A. Verrijken, S. Caron, J. Prawitt, R. Paumelle, B. Derudas, et al., PPARalpha gene expression correlates with severity and histological treatment response in patients with non-alcoholic steatohepatitis, J. Hepatol. 63 (2015) 164–173.
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Review
Role of farnesoid X receptor in hepatic steatosis in nonalcoholic fatty liver disease
T
Yingfei Xia, Hongshan Lib,* a b
Medical School of Ningbo University, Ningbo, Zhejiang, 315211, China Liver Disease Department, Hwa Mei Hospital, University of Chinese Academy of Sciences, Ningbo, Zhejiang 315010, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Farnesoid X receptor Hepatic steatosis Nonalcoholic fatty liver disease Mechanism
With the increased incidence of obesity, nonalcoholic fatty liver disease (NAFLD) has become a major global health concern. The pathogenesis of NAFLD has not yet been fully elucidated, and as few efficient pharmaceutical treatments are available for the condition, economic and medical burdens are heavy. Hepatic steatosis, as a precursor of NAFLD, plays a vital role in the pathological process of NAFLD. Hepatic steatosis is a consequence of lipid acquisition (i.e. free fatty acid uptake and de novo lipogenesis) exceeding lipid disposal (i.e. fatty acid oxidation and export as very-low-density lipoproteins). Therefore, restoring lipid homeostasis in the liver is an important therapeutic strategy of NAFLD. Farnesoid X receptor (FXR) is a major member of the ligandactivated nuclear receptor superfamily. Previous reviews have shown that FXR is a multipurpose receptor that plays an important role in regulating bile acid homeostasis, glucose and lipid metabolism, intestinal bacterial growth, and hepatic regeneration. This review focuses on the role of FXR in individual pathways that contribute to hepatic steatosis; it further demonstrates the molecular function of FXR in the pathogenesis of NAFLD.
1. Introduction Nonalcoholic fatty liver disease (NAFLD) is characterized by the accumulation of triglycerides (TGs) within hepatocytes (more than 5%), after excluding other causes of fat depots (e.g. excessive alcohol consumption, use of medications, chronic liver disease) [1,2]. NAFLD encompasses a broad clinical spectrum, ranging from nonalcoholic fatty liver (NAFL) to the more severe form, nonalcoholic steatohepatitis (NASH), which can progress to advanced fibrosis that may ultimately lead to cirrhosis or hepatocellular carcinoma (HCC) [2]. With the increasing incidence of obesity, the prevalence of NAFLD is rapidly rising.
Currently, the global prevalence of NAFLD is approximately 25.24%, and the pooled prevalence of NAFLD in Asia and Europe is estimated to be about 27.37% and 23.71%, respectively [3]. Indeed, NAFLD has become the most common chronic liver disease in Western countries, leading to heavy economic and medical burdens [4]. NAFLD is a complex multifaceted disease commonly associated with metabolic comorbidities such as type 2 diabetes, dyslipidemia, obesity, and hyperlipidemia [5]. Although the pathogenesis of NAFLD has not yet been fully elucidated, hepatic steatosis, as a precursor, is important in the pathogenesis of NAFLD [6]. Hepatic steatosis results from an imbalance between lipid acquisition and lipid removal, which is
Abbreviations: NAFLD, nonalcoholic fatty liver disease; FXR, farnesoid X receptor; BA, bile acid; TG, triglyceride; NAFL, nonalcoholic fatty liver; NASH, nonalcoholic steatohepatitis; HCC, hepatocellular carcinoma; FFA, free fatty acid; FAO, fatty acid oxidation; VLDL, very-low-density lipoprotein; OCA, obeticholic acid; FA, fatty acid; FATP, fatty acid-transport protein; CD36, fatty acid translocase; PPAR, peroxisome proliferator-activated receptor; FABP, fatty acid-binding protein; SHP, small heterodimer partner; LXR, liver X receptor; HNF4α, hepatocyte nuclear factor 4 alpha; ACC, acetyl-CoA carboxylase; FAS, fatty acid synthase; SREBP, sterol regulatory element-binding protein; ChREBP, carbohydrate response element-binding protein; SCD1, stearoyl-CoA desaturase 1; INSIG2, insulin-induced gene 2 protein; SCAP, SREBP cleavage-activating protein; LPK, liver-type pyruvate kinase; ChoRE, carbohydrate-response element; CDCA, chenodeoxycholic acid; CA, cholic acid; ACS, acyl-CoA synthetase; SMRT, silencing mediator of retinoic acid and thyroid hormone receptor; FGF, fibroblast growth factor; FGFR, fibroblast growth factor receptor; CYP7A1, cholesterol 7 alpha-hydroxylase; STAT, signal transducer and activator of transcription; PGC, peroxisome proliferator-activated receptor gamma coactivator; T-β-MCA, tauro-β-muricholic acid; Gly-MCA, glycine-β-muricholic acid; ROS, reactive oxygen species; VLCFA, very long-chain fatty acid; ACOX, fatty acyl-CoA oxidase; CPT, carnitine palmitoyl transferase; LCAD, long chain acyl-CoA dehydrogenase; MCAD, medium chain acyl-CoA dehydrogenase; RXR, retinoid X receptor; PPRE, peroxisome proliferator response element; FXRE, farnesoid X-activated receptor elements; Ces1, carboxylesterase 1; PDK-4, pyruvate dehydrogenase kinase isozyme 4; MTTP, microsomal triglyceride transfer protein; ApoB, apolipoprotein B; ANGPTL3, angiopoietin-like protein 3; LPL, lipoprotein lipase; ApoC, apolipoprotein C; PLA2G12B, phospholipase A2 G12B; VLDLR, very-low-density lipoprotein receptor ⁎ Corresponding author. E-mail address: [email protected] (H. Li). https://doi.org/10.1016/j.biopha.2019.109609 Received 8 August 2019; Received in revised form 23 October 2019; Accepted 25 October 2019 0753-3322/ © 2019 The Author(s). Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
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Fig. 1. FXR-mediated lipid metabolism in the liver. Intestinal and hepatic FXR represses lipogenesis and promotes FA oxidation. Hepatic FXR also reduces FA uptake and VLDL secretion. (a) FXR activation leads to the reduction of CD36, and thus, reduces FA uptake. (b) FXR represses lipogenesis via the FXR-SHP-SREBP-1c pathway. (c) FXR increases FA oxidation by stimulating the expressions of PPARα and CPT1. (d) FXR inhibits VLDL secretion via HNF4α-mediated expression of MTTP and ApoB. FA, fatty acid; FXR, farnesoid X receptor; CD36, fatty acid translocase; TG, triglyceride; ChoRE, carbohydrate-response element; SHP, small heterodimer partner; LPK, liver-type pyruvate kinase; LXR, liver X receptor; SREBP, sterol regulatory element binding protein; FAS, fatty acid synthase; ACC, acetyl-CoA carboxylase; SCD1, stearoyl-CoA desaturase 1; FGF, fibroblast growth factor; FGFR, fibroblast growth factor receptor; PPAR, peroxisome proliferator-activated receptor; Ces1, carboxylesterase 1; PDK-4, pyruvate dehydrogenase kinase isozyme 4; CPT, carnitine palmitoyl transferase; FAO, fatty acid oxidation; ApoC, apolipoprotein C; ANGPTL3, angiopoietin-like protein 3; VLDLR, very-low-density lipoprotein receptor; HNF4α, hepatocyte nuclear factor 4 alpha; MTTP, microsomal triglyceride transfer protein; VLDL, very-low-density lipoprotein; LPL, lipoprotein lipase.
constitute three membrane proteins, which are associated with protein trafficking and lipid droplet formation [17]. Caveolin-1 is enhanced in the liver of animal models with NAFLD [18], whereas whole body deletion of caveolin-1 reduces hepatic lipid levels [19]. Following uptake, FAs within hydrophobic proteins combine with fatty acid-binding protein (FABP) [13]. The activation of FXR definitely induces the expression of small heterodimer partner (SHP) [12]. SHP is an atypical nuclear receptor that lacks a DNA-binding domain and exerts a repressive effect via dimerization with several nuclear receptors, such as liver X receptor (LXR), hepatocyte nuclear factor 4α (HNF4α), and PPARs [20–23]. In NAFLD, overexpression of CD36 increases hepatic uptake of FAs [24], whereas knockdown of CD36 decreases lipid accumulation in both dietinduced and genetic steatosis [25], suggesting that CD36 is critical in the development of hepatic steatosis. Treatment of diet-induced obese mice with FXR agonist GW4064 resulted in reversal of hepatic steatosis and reduction in plasma lipid level through considerable reduction in the expression of CD36 at both protein and mRNA level [26]. In addition, GW4064 inhibited PPARγ in wild-type mice but not in SHP-deficient mice, suggesting that SHP is required for the repression of PPARγ promoter activity by FXR [27]. In HNF4α-deficient mice, PPARγ mRNA levels reduced dramatically. PPARγ promoter activity was induced by overexpression of HNF4α through transfection assays, and this induction was inhibited by overexpression of SHP, indicating that SHP suppresses PPARγ through HNF4α [27]. As previously mentioned, CD36 is regulated by PPARγ. Hence, we hypothesize that FXR inhibits the expression of CD36 via SHP-mediated reduced activity of PPARγ, which subsequently reduces FA uptake and hepatic steatosis.
regulated by four primary mechanisms: free fatty acid (FFA) uptake, de novo lipogenesis, fatty acid oxidation (FAO), and export from hepatocytes as very-low-density lipoproteins (VLDLs) [7]. Disturbance in one or more of these pathways may result in the retention of fat in the liver, leading to NAFLD [7]. FXR belongs to the nuclear receptor superfamily, and is mostly expressed in the liver, kidney, intestinal villi, and adrenal cortex [8]. Obeticholic acid (OCA), an agonist of FXR, improves histological hepatic steatosis, inflammation, and fibrosis in patients with NASH [9]; this shows the importance of FXR in the treatment of NAFLD. In recent years, this receptor has been demonstrated to be critical in modulating various metabolic processes in the liver. Several previous reviews have illustrated that the activation of FXR by endogenous (e.g. bile acid [BA]) or synthetic (e.g. OCA, GW4064, WAY-362450) ligands improves BA homeostasis, glucose metabolism, lipid accumulation, inflammation, and fibrogenesis in NAFLD [8–12]. However, no review has systematically revealed the interaction of FXR with lipid accumulation in NAFLD. In this review, we clearly demonstrate the role of FXR in the progress of hepatic steatosis; this may assist in the search for new pharmaceutical treatments for NAFLD (Fig. 1).
2. Hepatic lipid uptake and FXR The uptake of fatty acids (FAs) by the liver depends mainly on specific fatty acid transporters located in the plasma membrane [13]. Fatty acid transport protein (FATP), cluster of differentiation 36 (CD36), and caveolins are the primary proteins involved in the transport of FAs [6]. Among the six FATP isoforms in mammals, FATP2 and FATP5 are highly expressed in the liver [13]. Knockout of FATP2 or FATP5 in mice decreases the uptake of FAs and reverses hepatic steatosis [14,15]. The transmembrane protein CD36 accelerates the transport of long-chain fatty acids and is mediated by the peroxisome proliferator-activated receptor (PPAR)-γ [16]. Under normal conditions, CD36 is not highly expressed in the liver; however, it is overexpressed in diet-induced obesity [6]. In patients with NAFLD, expression of CD36 is positively correlated with triglyceride (TG) concentration in the liver [6]. Caveolins
3. De novo lipogenesis and FXR De novo lipogenesis is an integrated process that utilizes acetyl-CoA (derived from glycolysis) to synthesize saturated FA. Acetyl-CoA is converted to malonyl-CoA by acetyl-CoA carboxylase (ACC), and malonyl-CoA is then converted to palmitate by fatty acid synthase (FAS) [7]. The de novo-synthesized FA may then undergo desaturation and 2
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induction was significantly repressed upon activation of FXR [33]. FXR represses glucose-induced expression of LPK through a transrepressive mechanism involving the release of the transcription factor ChREBP and the recruitment of the transcriptional corepressor silencing mediator of retinoic acid and thyroid hormone receptor (SMRT) to the ChoRE of the LPK promoter [33], thereby decreasing the production of acetyl-CoA for de novo lipogenesis and diminishing hepatic steatosis. FXR also negatively regulates the expression of other ChREBP target genes, likely by molecular mechanisms similar to those involved in the repression of the LPK gene, at least for FAS [33]. Activation of FXR in the ileal enterocytes via intestinal BA also has important implications for lipid hemostasis. Intestinal FXR induces the production of fibroblast growth factor 19 (FGF-19, FGF-15 in mice) [39], which binds to FGFR4/β-klotho complex within hepatocytes and could inhibit BA synthesis in the liver through repressing cholesterol 7 alpha-hydroxylase (CYP7A1) gene transcription [40]. Transgenic expression of human FGF-19 in obese mice leads to increased energy expenditure and decreased adiposity [41]. In the liver, FGF-19 inhibits insulin-induced stimulation of FA synthesis by reducing the activity of SREBP-1c, along with increased activity of signal transducer and activator of transcription 3 (STAT3, an inhibitor of SREBP-1c expression), and decreased expression of PPARγ coactivator-1β (PGC-1β, an activator of SREBP-1c activity) [42]. In addition, FGF-19 upregulates the expression of SHP, which suppresses the expression of lipogenic enzymes through an SREBP-1c-independent mechanism [42]. A previous study by Zhang et al. reported that PPARγ coactivator 1α (PGC-1α) increases FXR transcriptional activity during fasting [43]. Fasting induces the expression of PGC-1α and FXR in the liver and leads to decreased plasma TG concentrations in wild-type mice, but not in FXR-deficient mice, supporting the theory that FXR limits lipogenesis under conditions of nutrient deprivation [10,43]. Interestingly, some studies indicated that inhibition of FXR signaling in the intestine prevents NAFLD. Mice with diet-induced obesity treated with antibiotics or tempol showed decreased FA biosynthesis in the liver depending on modulation of the gut microbiota and BA metabolism, enriched levels of the FXR antagonist tauro-β-muricholic acid (T-β-MCA), and antagonized FXR signaling in the intestine [44]. However, another report indicated that T-β-MCA was rapidly hydrolyzed by gut bacterial bile salt hydrolase and failed to reach concentrations that inhibit gut-specific FXR signaling in vivo [45]. As a derivative of T-β-MCA, glycine-β-muricholic acid (Gly-MCA) proved to be an intestine-selective FXR inhibitor [45]. Treatment with Gly-MCA reduced obesity and hepatic steatosis in obese mice owing to reduced biosynthesis of intestinal-derived ceramides, which directly compromised the thermogenic function of beige fat [45]. This finding is consistent with the observation that ceramide treatment reverses the beneficial effects of Gly-MCA in mice with diet-induced obesity [45].
esterification to form TG, which is eventually stored as TG vacuoles or exported as VLDL particles [7]. De novo lipogenesis is regulated by multiple transcription factors, among which sterol regulatory elementbinding protein 1c (SREBP-1c) and carbohydrate response elementbinding protein (ChREBP) are the primary regulators [6]. SREBP-1c is the predominant isoform of SREBPs, and it is activated by insulin and liver X receptor α (LXRα) [28]. In transfected hepatocytes, SREBP-1c augments the expression of enzymes involved in lipogenesis, such as FAS, ACC, and stearoyl-CoA desaturase 1(SCD1), whereas SREBP-1c-deficient mice show decreased hepatic lipid levels, further substantiating the lipogenic action of SREBP-1c [28–30]. Insulin activates SREBP-1c indirectly via inhibition of the interaction between insulin-induced gene 2 protein (INSIG2) and SREBP cleavage-activating protein (SCAP) under feeding conditions. LXRα directly activates SREBP-1c via LXR binding sites in the SREBP-1c gene promoter [31]. In addition, LXRα promotes lipogenesis directly through inducing the expressions of FAS, ACC, and SCD1 [28]. ChREBP regulates glucose-induced lipogenesis [32]. Under highglucose conditions, activated ChREBP promotes the expression of its target genes, such as the glycolytic liver-type pyruvate kinase (LPK), FAS, and ACC1 (an enzyme involved in the production of malonyl-CoA) via acceleration of the combination of Max-like protein (Mlx) with carbohydrate-response element (ChoRE) to form two Ebox-like motifs, which are present in the promoter of its target genes [33]. Moreover, fasting hormones, such as glucagon, suppress the expression of ChREBP [34]. In obese and insulin-resistant ob/ob mice, the expressions of both SREBP-1c and ChREBP are markedly increased in the liver, and a decrease in either factor helps alleviate hepatic steatosis, indicating the importance of these transcription factors in de novo lipogenesis and accumulation of TGs [35]. Diabetic KK-Ay and C57BL/6 J mice fed a dietary supplement of cholic acid (CA) showed reduced expression of genes encoding enzymes involved in the biosynthesis of FAs and TGs, such as acyl-CoA synthetase (ACS), malic enzyme, and SCD1 [36], and FXR-deficient mice exhibited a conspicuous induction of lipogenic genes, such as FAS, SREBP-1c, and SCD1 [37], suggesting that FXR is critical in downregulating lipogenesis. In patients with hypotriglyceridemia and gallstones, serum TG levels decreased on treatment with chenodeoxycholic acid (CDCA) [10]. Indeed, FXR has a role in the amelioration of blood TG levels, partly through the inhibition of FA and TG biosynthesis [10]. In mouse primary hepatocytes in vitro, CDCA lowers the expression of endogenous SREBP-1c and its target genes [36]. In vivo, the expression of endogenous SREBP-1c was induced by LXR, and this expression was negatively correlated with the amount of CDCA [36]. These data show that the expression of endogenous SREBP-1c is markedly modulated by BAs. The activation of FXR is known to induce the expression of SHP. In wild-type mice, treatment with CA and GW4064 considerably reduced serum TG levels and genes involved in lipogenesis; however, the reduction was not observed in SHP-null mice [36]. Meanwhile, in SHP-deficient mice, suppression of SREBP-1c and its target genes by CA or GW4064 was not observed [38]. These experiments demonstrate that FXR inhibits de novo lipogenesis through an FXR–SHP–SREBP-1c pathway. In wild-type mice, treatment with CA lowers serum TG levels and the expression of SREBP-1c [36]. This decrease in serum TG levels and SREBP-1c expression was abolished in LXR-deficient mice [36]. These results show that LXR is critical in the reduction of SHP-mediated TGs in vivo. In brief, activation of FXR induces the expression of SHP, which then suppresses the activation of LXR and LXR-induced SREBP-1c, thus inhibiting the expression of lipogenic enzymes [10]. Further, in fasting wild-type mice, the hepatic levels of LPK mRNA were considerably increased by high-carbohydrate refeeding, whereas FXR synthetic agonist INT-747 markedly blunted this induction [33]. In immortalized human hepatocytes and HepaRG cells, the expression of LPK gene was increased via incubation with high glucose concentrations, whereas this
4. Fatty acid oxidation and FXR In mammalian cells, FAO plays a central role in energy generation, especially in cases of low circulating glucose concentrations [7,46]. Three cellular organelles mediate FAO—mitochondria and peroxisomes contribute to FA β-oxidation, and cytochromes participate in FA ωoxidation [47,48]. Although NAFLD is characterized by lipid overload and defective β-oxidation owing to dysfunctional mitochondria, defects in ω-oxidation could also contribute to the disease [46,47]. These processes generate oxidative stress, reactive oxygen species (ROS), and toxic dicarboxylic acids, which potentially aggravate inflammation and disease progression [47]. Peroxisomal β-oxidation metabolize very long-chain fatty acids (VLCFAs), dicarboxylic acid, and certain branched-chain FAs [49,50]. In two studies, patients with NAFLD showed elevated mRNA levels of fatty acyl-CoA oxidase (ACOX) and branched chain acyl-CoA oxidase (two peroxisomal enzymes involved in FAO), indicating that elevated peroxisomal FAO may be a complemental response attempting to remit 3
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which is the only way to reduce lipid content in the liver other than FAO [65]. VLDL particles comprise hydrophobic core lipids containing TGs and cholesterol esters, which are covered with hydrophilic phospholipids, and apolipoproteins [66]. The process of assembly of VLDL particles initially occurs in the endoplasmic reticulum and is catalyzed by microsomal triglyceride transfer protein (MTTP), an enzyme that facilitates the lipidation of apolipoprotein B 100 (ApoB 100) with TG [7,67]. Nascent VLDL particles are transported into Golgi apparatus; during this process, nascent particles that are further lipidated ultimately form the mature VLDL particles [61]. Indeed, hepatic steatosis is common in patients with mutations in ApoB100 (hypobetalipoproteinemia) or in MTTP (abetalipoproteinemia) [6,7,61], emphasizing the importance of these proteins in hepatic secretion of VLDL and lipid homeostasis. Insulin is crucial in the regulation of the assembly and secretion of hepatic VLDL; it decreases lipid export by inducing post-translational degradation of ApoB100 and inhibiting MTTP synthesis at the transcription level [61]. In patients with NAFLD, selective hepatic insulin resistance allows insulin to stimulate de novo lipogenesis without inhibiting the production of VLDL [7], resulting in the synthesis of TGs exceeding its secretion, leading to increased TG concentration in the liver. Consequently, patients with NAFLD exhibit hypertriglyceridemia and hepatic steatosis. Prolonged exposure to FAs promotes endoplasmic reticulum stress in the liver; this inhibits the secretion of ApoB100, thereby decreasing the secretion VLDL and worsening hepatic steatosis [61]. The decreased serum TG concentration partially results from FXRmediated TG clearance [10]. FXR promotes TG clearance through mediating the transcription of regulators (e.g. ApoC-II, ApoC-III, and ANGPTL3) of lipoprotein lipase (LPL), which hydrolyzes TG from VLDL particles and chylomicrons, thereby lowering serum TG levels [36]. ApoC-II, an activator of LPL, has been reported as an FXR target gene [68]. ApoC-II mRNA levels were increased both in HepG2 cells cultured with CDCA and wild-type mice treated with 1% CA diet, and this induction was blunted in FXR-deficient mice [68]. FXR heterodimerizes with RXR to form FXR/RXR heterodimer and binds to FXRE, which contains distal enhancer elements located upstream of the ApoC-II promotor; subsequently, it upregulates the expression of ApoC-II gene [68]. Regulation of ApoC-III (an inhibitor of LPL) by FXR is supported by decreased expression of ApoC-III upon treatment with taurocholic acid in mice and upon treatment with CDCA in human primary hepatocytes and HepG2 cells, but not in FXR-deficient mice [69]. Activated FXR causes the FXR/RXR heterodimer to bind to FXRE upstream of the ApoC-III promoter, which is occupied by HNF4α, a strong regulator of ApoC-III, in the absence of FXR agonists [69]. As a negative regulation, the expression of ApoC-III gene is downregulated [69]. Further, treatment with CA reduces the expression of ANGPTL3 through an SHPdependent mechanism [36]. ANGPTL3 can inactivate LPL, thereby increasing serum TG levels [36]. Taken together, FXR improves TG clearance by inducing the hepatic expression of ApoC-II and repressing the expression of ApoC-III and ANGPTL3 [36,68,69]. VLDL secretion is also mediated by FXR. Treatment with CDCA reduced the production of VLDLs in fructose-feeding hamsters and in patients with hypertriglyceridemia [10]. When HepG2 cells were administered CDCA, mRNA levels of MTTP and ApoB decreased [70]. FXR inhibits VLDL secretion through SHP-mediated repression of HNF4α, which can induce the expression of MTTP and ApoB [70]. Liu et al. (2016) reported that FXR decreases VLDL secretion partly through reduction of phospholipase A2G12B (PLA2G12B) expression [71]. They speculated that FXR-induced SHP expression accounts for the repression of PLA2G12B; nevertheless, whether decreased VLDL secretion by FXR activation is definitely regulated by PLA2G12B has not yet been proven. Additionally, FXR activation induces the expressions of syndecan-1 and VLDL receptor, which accounts for enhanced clearance of remnant particles and TG-rich lipoproteins, respectively
the progression of steatosis in NAFLD [30,51]. However, the peroxisomes also generate ROS, which may induce oxidative stress such as ωoxidation in the cytochromes [52]. Mitochondrial β-oxidation is the primary pathway of FAO, which involves catabolization of short-, medium-, and long-chain FAs derived from diet [46]. The entry of activated FAs into mitochondria depends on the presence of carnitine palmitoyl transferase 1 (CPT1) at the mitochondrial outer membrane, which catalyzes the formation of acylcarnitine from acyl-CoA [6]. Mitochondria are a dominant site for generation of ROS; typically, consumption of only about 1%–2% oxygen in the mitochondria result in the generation of ROS [53]. Excessive production of ROS, partially from peroxisomal and cytochromemediated FAO, leads to lipid peroxidation of the mitochondrial membranes, which, in turn, impairs mitochondrial function [53,54]. Thus, dysfunctional mitochondria not only decrease the catabolism of LCFACoA, which can cause lipotoxicity, but also generate more ROS, leading to a vicious circle [54]. Activation of FXR in the liver also drives FAO. Wild-type C57BL/6 J mice treated with CA showed increased expression of FAO genes in medium-chain acyl-CoA dehydrogenase (MCAD) and long-chain acylCoA dehydrogenase (LCAD) enzymes [36]. These changes in gene expressions are probably due to FXR-mediated induction of PPARα expression [55]. PPARα is a transcription factor activated by FA, and it regulates all three FAO systems. PPARα forms a heterodimer with retinoid X receptor (RXR), which binds to the peroxisome proliferator response element (PPRE), a specific DNA sequence present on its target gene promoter [56]. These transcriptional complexes upregulate the expression of genes related to FAO, such as MCAD in mitochondrial βoxidation, ACOX1, and enoyl-CoA hydratase in peroxisomal β-oxidation, and CYP4A1 and CYP4A3 in cytochrome-mediated ω-oxidation [57]. PPARα-deficient ob/ob mice show severe hepatic steatosis [57], further supporting the importance of PPARα in hepatic lipid homeostasis. However, the expression of PPARα in the liver is negatively correlated with the severity of steatosis [58]. Incubation of HepG2 cells and human primary hepatocytes with CDCA and GW4064 resulted in upregulation of the expression of human PPARα gene [55], whereas in FXR-deficient ob/ob mice, the expression of PPARα was strongly downregulated [59]. Indeed, FXR induces PPARα expression by binding to the functional farnesoid X-activated receptor element (FXRE), αFXRE, within the human PPARα promoter [55]. Further, when HepG2 cells were treated with CDCA, the expression of PPARα target gene CPT1 (which facilitates the entry of FAs into the mitochondria) was also enhanced [55]. As previously described, intestinal FXR induces the expression of FGF-19. FGF19-transgenic mice showed decreased expression of ACC2, which negatively regulated FAO through malonyl-CoA-mediated inhibition of CPT1 [41,42]. FXR activated by BA or GW4064 elevated the expression and secretion of FGF-21 [60]. FGF-21 stimulated FAO, ketogenesis, and gluconeogenesis during fasting by binding to the FGFR1c/b-klotho complex in the liver [40,61]. Moreover, FXR also enhanced FGF-21 expression through FGF-19 [60]. Increased hepatic FGF-21 expression results in reduced TG levels [59]. Interestingly, a previous study reported that overexpression of hepatic carboxylesterase 1 (Ces1) induced by FXR ligands is essential for improved lipid homeostasis [62]. The authors indicated that FXR directly induces Ces1 expression via binding to FXRE upstream of Ces1 promotor; overexpressed Ces1 increases the release of FA, consequently activating PPARα, and then stimulates FAO. Additionally, FXR ligands augment the expression of PPARα and that of its target gene pyruvate dehydrogenase kinase isozyme 4 (PDK-4), which may promote FAO, consistent with concomitantly decreased plasma TG levels [63]. 5. Lipid export and FXR TGs are exported from the hepatocytes in the form of VLDLs [64], 4
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[59].
metabolism, angiogenesis, and behavior, Sci. Signal. 2 (2009) re3. [17] M.A. Fernandez, C. Albor, M. Ingelmo-Torres, S.J. Nixon, C. Ferguson, T. Kurzchalia, et al., Caveolin-1 is essential for liver regeneration, Science 313 (2006) 1628–1632. [18] Y. Qiu, S. Liu, H.T. Chen, C.H. Yu, X.D. Teng, H.T. Yao, et al., Upregulation of caveolin-1 and SR-B1 in mice with non-alcoholic fatty liver disease, HBPD INT 12 (2013) 630–636. [19] I.W. Asterholm, D.I. Mundy, J. Weng, R.G. Anderson, P.E. Scherer, Altered mitochondrial function and metabolic inflexibility associated with loss of caveolin-1, Cell Metab. 15 (2012) 171–185. [20] W. Seol, H.S. Choi, D.D. Moore, An orphan nuclear hormone receptor that lacks a DNA binding domain and heterodimerizes with other receptors, Science 272 (1996) 1336–1339. [21] J.H. Lee, H. Kim, S.J. Park, J.H. Woo, E.H. Joe, I. Jou, Small heterodimer partner SHP mediates liver X receptor (LXR)-dependent suppression of inflammatory signaling by promoting LXR SUMOylation specifically in astrocytes, Sci. Signal. 9 (2016) ra78. [22] T.T. Lu, M. Makishima, J.J. Repa, K. Schoonjans, T.A. Kerr, J. Auwerx, et al., Molecular basis for feedback regulation of bile acid synthesis by nuclear receptors, Mol. Cell 6 (2000) 507–515. [23] Y.S. Kim, C.Y. Han, S.W. Kim, J.H. Kim, S.K. Lee, D.J. Jung, et al., The orphan nuclear receptor small heterodimer partner as a novel coregulator of nuclear factorkappa b in oxidized low density lipoprotein-treated macrophage cell line RAW 264.7, J. Biol. Chem. 276 (2001) 33736–33740. [24] D.P. Koonen, R.L. Jacobs, M. Febbraio, M.E. Young, C.L. Soltys, H. Ong, et al., Increased hepatic CD36 expression contributes to dyslipidemia associated with dietinduced obesity, Diabetes 56 (2007) 2863–2871. [25] C.G. Wilson, J.L. Tran, D.M. Erion, N.B. Vera, M. Febbraio, E.J. Weiss, Hepatocytespecific disruption of CD36 attenuates fatty liver and improves insulin sensitivity in HFD-Fed mice, Endocrinology 157 (2016) 570–585. [26] Y. Ma, Y. Huang, L. Yan, M. Gao, D. Liu, Synthetic FXR agonist GW4064 prevents diet-induced hepatic steatosis and insulin resistance, Pharm. Res. 30 (2013) 1447–1457. [27] K. Jadhav, Y. Xu, Y. Xu, Y. Li, J. Xu, Y. Zhu, et al., Reversal of metabolic disorders by pharmacological activation of bile acid receptors TGR5 and FXR, Mol. Metab. 9 (2018) 131–140. [28] S.B. Joseph, B.A. Laffitte, P.H. Patel, M.A. Watson, K.E. Matsukuma, R. Walczak, et al., Direct and indirect mechanisms for regulation of fatty acid synthase gene expression by liver X receptors, J. Biol. Chem. 277 (2002) 11019–11025. [29] D. Eberle, B. Hegarty, P. Bossard, P. Ferre, F. Foufelle, SREBP transcription factors: master regulators of lipid homeostasis, Biochimie 86 (2004) 839–848. [30] M. Kohjima, M. Enjoji, N. Higuchi, M. Kato, K. Kotoh, T. Yoshimoto, et al., Reevaluation of fatty acid metabolism-related gene expression in nonalcoholic fatty liver disease, Int. J. Mol. Med. 20 (2007) 351–358. [31] T. Yoshikawa, H. Shimano, M. Amemiya-Kudo, N. Yahagi, A.H. Hasty, T. Matsuzaka, et al., Identification of liver X receptor-retinoid X receptor as an activator of the sterol regulatory element-binding protein 1c gene promoter, Mol. Cell. Biol. 21 (2001) 2991–3000. [32] K. Iizuka, R.K. Bruick, G. Liang, J.D. Horton, K. Uyeda, Deficiency of carbohydrate response element-binding protein (ChREBP) reduces lipogenesis as well as glycolysis, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 7281–7286. [33] S. Caron, C. Huaman Samanez, H. Dehondt, M. Ploton, O. Briand, F. Lien, et al., Farnesoid X receptor inhibits the transcriptional activity of carbohydrate response element binding protein in human hepatocytes, Mol. Cell. Biol. 33 (2013) 2202–2211. [34] G. Filhoulaud, S. Guilmeau, R. Dentin, J. Girard, C. Postic, Novel insights into ChREBP regulation and function, Trends Endocrinol. Metab. 24 (2013) 257–268. [35] R. Dentin, F. Benhamed, I. Hainault, V. Fauveau, F. Foufelle, J.R.B. Dyck, et al., Liver-specific inhibition of ChREBP improves hepatic steatosis and insulin resistance in ob/ob mice, Diabetes 55 (2006) 2159–2170. [36] M. Watanabe, S.M. Houten, L. Wang, A. Moschetta, D.J. Mangelsdorf, R.A. Heyman, et al., Bile acids lower triglyceride levels via a pathway involving FXR, SHP, and SREBP-1c, J. Clin. Invest. 113 (2004) 1408–1418. [37] K. Ma, P.K. Saha, L. Chan, D.D. Moore, Farnesoid X receptor is essential for normal glucose homeostasis, J. Clin. Invest. 116 (2006) 1102–1109. [38] J.R. Crouse 3rd, Hypertriglyceridemia: a contraindication to the use of bile acid binding resins, Am. J. Med. 83 (1987) 243–248. [39] S. Kir, Y. Zhang, R.D. Gerard, S.A. Kliewer, D.J. Mangelsdorf, Nuclear receptors HNF4α and LRH-1 cooperate in RegulatingCyp7a1 in vivo, J. Biol. Chem. 287 (2012) 41334–41341. [40] C. Cicione, C. Degirolamo, A. Moschetta, Emerging role of fibroblast growth factors 15/19 and 21 as metabolic integrators in the liver, Hepatology 56 (2012) 2404–2411. [41] E. Tomlinson, L. Fu, L. John, B. Hultgren, X. Huang, M. Renz, et al., Transgenic mice expressing human fibroblast growth factor-19 display increased metabolic rate and decreased adiposity, Endocrinology 143 (2002) 1741–1747. [42] S. Bhatnagar, H.A. Damron, F.B. Hillgartner, Fibroblast growth factor-19, a novel factor that inhibits hepatic fatty acid synthesis, J. Biol. Chem. 284 (2009) 10023–10033. [43] Y. Zhang, L.W. Castellani, C.J. Sinal, F.J. Gonzalez, P.A. Edwards, Peroxisome proliferator-activated receptor-gamma coactivator 1alpha (PGC-1alpha) regulates triglyceride metabolism by activation of the nuclear receptor FXR, Genes Dev. 18 (2004) 157–169. [44] C. Jiang, C. Xie, F. Li, L. Zhang, R.G. Nichols, K.W. Krausz, et al., Intestinal farnesoid X receptor signaling promotes nonalcoholic fatty liver disease, J. Clin. Invest. 125 (2015) 386–402.
6. Conclusions In conclusion, FXR regulates multiple pathways involved in hepatic steatosis. FXR induces the expression of SHP, which represses lipid synthesis via the inhibition of LXR-mediated SREBP-1c and decreases VLDL secretion via the suppression of HNF4α-mediated MTTP and ApoB. FXR also stimulates the expression of PPARα, which can increase FAO through several mechanisms. Moreover, FXR promotes TG clearance via overexpression of LPL by inducing the expression of ApoC-II, and inhibiting the expressions of ApoC-III and ANGPTL3. In addition, activation of intestinal-specific FXR by BA could induce the expression of FGF19, which decreases lipogenesis via a SREBP-1c-dependent mechanism and increases FAO via the inhibition of ACC2. FXR activation also induces the expression of FGF21, which is involved in FAO during fasting. However, the mechanism by which FXR inhibits CD36 expression is still unclear. Whether PDK-4 (a target gene of PPARα) facilitates FAO is uncertain. Therefore, further research is warranted to unravel the exact role of FXR in hepatic steatosis, which may contribute to the development of better pharmaceutical treatment for NAFLD in the future. Declaration of Competing Interest The authors declare that they have no conflict of interest. Acknowledgment This study was supported by the National Natural Science Foundation of China (grant number 81873109). References [1] G.B. Goh, A.J. McCullough, Natural history of nonalcoholic fatty liver disease, Dig. Dis. Sci. 61 (2016) 1226–1233. [2] B.J. Perumpail, M.A. Khan, E.R. Yoo, G. Cholankeril, D. Kim, A. Ahmed, Clinical epidemiology and disease burden of nonalcoholic fatty liver disease, World J. Gastroenterol. 23 (2017) 8263–8276. [3] Z.M. Younossi, A.B. Koenig, D. Abdelatif, Y. Fazel, L. Henry, M. Wymer, Global epidemiology of nonalcoholic fatty liver disease-meta-analytic assessment of prevalence, incidence, and outcomes, Hepatology 64 (2016) 73–84. [4] Z.M. Younossi, D. Blissett, R. Blissett, L. Henry, M. Stepanova, Y. Younossi, et al., The economic and clinical burden of nonalcoholic fatty liver disease in the United States and Europe, Hepatology 64 (2016) 1577–1586. [5] C.I. Andronescu, M.R. Purcarea, P.A. Babes, Nonalcoholic fatty liver disease: epidemiology, pathogenesis and therapeutic implications, J. Med. Life 11 (2018) 20–23. [6] S.H. Koo, Nonalcoholic fatty liver disease: molecular mechanisms for the hepatic steatosis, Clin. Mol. Hepatol. 19 (2013) 210–215. [7] D.H. Ipsen, J. Lykkesfeldt, P. Tveden-Nyborg, Molecular mechanisms of hepatic lipid accumulation in non-alcoholic fatty liver disease, Cell. Mol. Life Sci. 75 (2018) 3313–3327. [8] P. Lefebvre, B. Cariou, F. Lien, F. Kuipers, B. Staels, Role of bile acids and bile acid receptors in metabolic regulation, Physiol. Rev. 89 (2009) 147–191. [9] B.A. Neuschwander Tetri, R. Loomba, A.J. Sanyal, J.E. Lavine, M.L. Van Natta, M.F. Abdelmalek, et al., Farnesoid X nuclear receptor ligand obeticholic acid for non-cirrhotic, non-alcoholic steatohepatitis (FLINT): a multicentre, randomised, placebo-controlled trial, Lancet 385 (2015) 956–965. [10] V. Massafra, S.W.C. van Mil, Farnesoid X receptor: a "homeostat" for hepatic nutrient metabolism, Biochim. Biophys. Acta Mol. Basis Dis. 1864 (2018) 45–59. [11] J.P. Arab, S.J. Karpen, P.A. Dawson, M. Arrese, M. Trauner, Bile acids and nonalcoholic fatty liver disease: molecular insights and therapeutic perspectives, Hepatology 65 (2017) 350–362. [12] F.Y. Lee, H. Lee, M.L. Hubbert, P.A. Edwards, Y. Zhang, FXR, a multipurpose nuclear receptor, Trends Biochem. Sci. 31 (2006) 572–580. [13] D.G. Mashek, Hepatic fatty acid trafficking: multiple forks in the road1-3, Adv. Nutr. 4 (2013) 697–710. [14] A. Falcon, H. Doege, A. Fluitt, B. Tsang, N. Watson, M.A. Kay, et al., FATP2 is a hepatic fatty acid transporter and peroxisomal very long-chain acyl-CoA synthetase, Am. J. Physiol. Endocrinol. Metab. 299 (2010) E384–393. [15] H. Doege, R.A. Baillie, A.M. Ortegon, B. Tsang, Q. Wu, S. Punreddy, et al., Targeted deletion of FATP5 reveals multiple functions in liver metabolism: alterations in hepatic lipid homeostasis, Gastroenterology 130 (2006) 1245–1258. [16] R.L. Silverstein, M. Febbraio, CD36, a scavenger receptor involved in immunity,
5
Biomedicine & Pharmacotherapy 121 (2020) 109609
Y. Xi and H. Li
[59] Y. Li, K. Jadhav, Y. Zhang, Bile acid receptors in non-alcoholic fatty liver disease, Biochem. Pharmacol. 86 (2013) 1517–1524. [60] H.A. Cyphert, X.M. Ge, A.B. Kohan, L.M. Salati, Y.Q. Zhang, H.F. Bradley, Activation of the farnesoid X receptor induces hepatic expression and secretion of fibroblast growth factor 21, J. Biol. Chem. 287 (2012) 25123. [61] Y. Kawano, D.E. Cohen, Mechanisms of hepatic triglyceride accumulation in nonalcoholic fatty liver disease, J. Gastroenterol. 48 (2013) 434–441. [62] J. Xu, Y. Li, W.D. Chen, Y. Xu, L. Yin, X. Ge, et al., Hepatic carboxylesterase 1 is essential for both normal and farnesoid X receptor-controlled lipid homeostasis, Hepatology 59 (2014) 1761–1771. [63] R.S. Savkur, K.S. Bramlett, L.F. Michael, T.P. Burris, Regulation of pyruvate dehydrogenase kinase expression by the farnesoid X receptor, Biochem. Biophys. Res. Commun. 329 (2005) 391–396. [64] M.M. Hussain, J. Shi, P. Dreizen, Microsomal triglyceride transfer protein and its role in apoB-lipoprotein assembly, J. Lipid Res. 44 (2003) 22–32. [65] R.J. Perry, V.T. Samuel, K.F. Petersen, G.I. Shulman, The role of hepatic lipids in hepatic insulin resistance and type 2 diabetes, Nature 510 (2014) 84–91. [66] E. Fabbrini, B.S. Mohammed, F. Magkos, K.M. Korenblat, B.W. Patterson, S. Klein, Alterations in adipose tissue and hepatic lipid kinetics in obese men and women with nonalcoholic fatty liver disease, Gastroenterology 134 (2008) 424–431. [67] I.V.A. Pereira, J.T. Stefano, C.P.M.S. Oliveira, Microsomal triglyceride transfer protein and nonalcoholic fatty liver disease, Expert Rev. Gastroenterol. Hepatol. 5 (2011) 245–251. [68] H.R. Kast, C.M. Nguyen, C.J. Sinal, S.A. Jones, B.A. Laffitte, K. Reue, et al., Farnesoid X-activated receptor induces apolipoprotein C-II transcription: a molecular mechanism linking plasma triglyceride levels to bile acids, Mol. Endocrinol. 15 (2001) 1720–1728. [69] T. Claudel, Y. Inoue, O. Barbier, D. Duran-Sandoval, V. Kosykh, J. Fruchart, et al., Farnesoid X receptor agonists suppress hepatic apolipoprotein CIII expression, Gastroenterology. 125 (2003) 544–555. [70] H. Hirokane, M. Nakahara, S. Tachibana, M. Shimizu, R. Sato, Bile acid reduces the secretion of very low density lipoprotein by repressing microsomal triglyceride transfer protein gene expression mediated by hepatocyte nuclear factor-4, J. Biol. Chem. 279 (2004) 45685–45692. [71] Q. Liu, M. Yang, X. Fu, R. Liu, C. Sun, H. Pan, et al., Activation of farnesoid X receptor promotes triglycerides lowering by suppressing phospholipase A2 G12B expression, Mol. Cell. Endocrinol. 436 (2016) 93–101.
[45] C. Jiang, C. Xie, Y. Lv, J. Li, K.W. Krausz, J. Shi, et al., Intestine-selective farnesoid X receptor inhibition improves obesity-related metabolic dysfunction, Nat. Commun. 6 (2015) 10166. [46] P. Misra, J.K. Reddy, Peroxisome proliferator-activated receptor-alpha activation and excess energy burning in hepatocarcinogenesis, Biochimie 98 (2014) 63–74. [47] M.S. Rao, J.K. Reddy, Peroxisomal beta-oxidation and steatohepatitis, Semin. Liver Dis. 21 (2001) 043–056. [48] K. Begriche, J. Massart, M.A. Robin, F. Bonnet, B. Fromenty, Mitochondrial adaptations and dysfunctions in nonalcoholic fatty liver disease, Hepatology 58 (2013) 1497–1507. [49] P. Misra, N. Viswakarma, J.K. Reddy, Peroxisome proliferator-activated receptor-α signaling in hepatocarcinogenesis, Subcell. Biochem. 69 (2013) 77–99. [50] J.K. Reddy, T. Hashimoto, Peroxisomal beta-oxidation and peroxisome proliferatoractivated receptor alpha: an adaptive metabolic system, Annu. Rev. Nutr. 21 (2001) 193–230. [51] M. Nakamuta, M. Kohjima, N. Higuchi, M. Kato, K. Kotoh, T. Yoshimoto, et al., The significance of differences in fatty acid metabolism between obese and non-obese patients with non-alcoholic fatty liver disease, Int. J. Mol. Med. 22 (2008) 663–667. [52] G.H. Koek, P.R. Liedorp, A. Bast, The role of oxidative stress in non-alcoholic steatohepatitis, Clin. Chim. Acta 412 (2011) 1297–1305. [53] A. Boveris, B. Chance, The mitochondrial generation of hydrogen peroxide. General properties and effect of hyperbaric oxygen, Biochem. J. 134 (1973) 707–716. [54] F. Nassir, J.A. Ibdah, Role of mitochondria in nonalcoholic fatty liver disease, Int. J. Mol. Sci. 15 (2014) 8713–8742. [55] I. Pineda Torra, T. Claudel, C. Duval, V. Kosykh, J.C. Fruchart, B. Staels, Bile acids induce the expression of the human peroxisome proliferator-activated receptor alpha gene via activation of the farnesoid X receptor, Mol. Endocrinol. 17 (2003) 259–272. [56] S.A. Kliewer, K. Umesono, D.J. Noonan, R.A. Heyman, R.M. Evans, Convergence of 9-cis retinoic acid and peroxisome proliferator signalling pathways through heterodimer formation of their receptors, Nature 358 (1992) 771–774. [57] Q. Gao, Y. Jia, G. Yang, X. Zhang, P.C. Boddu, B. Petersen, et al., PPARalpha-deficient ob/ob obese mice become more obese and manifest severe hepatic steatosis due to decreased fatty acid oxidation, Am. J. Pathol. 185 (2015) 1396–1408. [58] S. Francque, A. Verrijken, S. Caron, J. Prawitt, R. Paumelle, B. Derudas, et al., PPARalpha gene expression correlates with severity and histological treatment response in patients with non-alcoholic steatohepatitis, J. Hepatol. 63 (2015) 164–173.
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