The Role of Nuclear Receptors in the Pathophysiology, Natural Course, and Drug Treatment of NAFLD in Humans
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Nonalcoholic fatty liver disease (NAFLD) describes steatosis, nonalcoholic steatohepatitis with or without fibrosis, and hepatocellular carcinoma, namely the entire alcohol-like spectrum of liver disease though observed in the nonalcoholic, dysmetabolic, individual free of competing causes of liver disease. NAFLD, which is a major public health issue, exhibits intrahepatic triglyceride storage giving rise to lipotoxicity. Nuclear receptors (NRs) are transcriptional factors which, activated by ligands, are master regulators of metabolism and also have intricate connections with circadian control accounting for cyclical patterns in the metabolic fate of nutrients. Several transcription factors, such as peroxisome proliferator-activated receptors, liver X receptors, farnesoid X receptors, and their molecular cascades, finely regulate energetic fluxes and metabolic pathways. Dysregulation of such pathways is heavily implicated in those metabolic derangements characterizing insulin resistance and metabolic syndrome and in the histogenesis of progressive NAFLD forms. We review the role of selected NRs in NAFLD pathogenesis. Secondly, we analyze the role of NRs in the natural history of human NAFLD. Next, we discuss the results observed in humans following administration of drug agonists or antagonists of the NRs pathogenically involved in NAFLD. Finally, general principles of treatment and lines of research in human NAFLD are briefly examined.
KeywordsCirrhosis Farnesoid X receptor Fibrosis Hepatocellular carcinoma Liver X receptor Natural history Nonalcoholic steatohepatitis Pathogenesis Peroxisome proliferator-activated receptor Steatosis
Nonalcoholic fatty liver disease (NAFLD) designates a heterogenous set of diseases histologically mimicking alcoholic liver disease though observed in insulin-resistant nonalcoholic individuals, in the absence of competing etiologies of liver disease [1, 2]. The NAFLD spectrum and its natural history span simple steatosis through nonalcoholic steatohepatitis (NASH), fibrosis-cirrhosis, and, in a subset of cases, hepatocellular carcinoma (HCC), with or without cirrhosis [3, 4]. On the basis of the epidemic prevalence of disease and its inherent hepatic, metabolic, oncologic, and cardiovascular disease burden, NAFLD is a major public health issue posing heavy costs on health systems [1, 5, 6, 7].
From a pathophysiological point of view, intrahepatic storage of triglycerides (TGs) is an example of an adaptive process becoming maladaptive, given that ectopic fat gives rise to lipotoxicity [8, 9]. Fatty changes will primarily result from excess de novo intrahepatic lipogenesis associated with the liver being overwhelmed by an excess of steatogenic substrates in the setting of insulin resistance (IR), impaired glucose disposal/type 2 diabetes (T2D), hyperlipidemia, visceral obesity, and other features of the metabolic syndrome (MetS) . Steatosis will also derive from the failure of the liver to oxidize and export excess lipids [3, 11, 12, 13]. Therefore, NAFLD can best be conceptualized as an abnormal storage of TGs resulting from an imbalance between intrahepatic synthesis and catabolism/disposal of fatty substrates , which is inextricably linked to IR/T2D and atherogenic dyslipidemia [13, 15].
Nuclear receptors (NRs) are transcriptional factors which, activated by ligands, are master regulators of metabolism and also have intricate connections with circadian control accounting for cyclical patterns in the metabolic fate of nutrients . Several transcription factors, such as peroxisome proliferator-activated receptors (PPARs), liver X receptors (LXRs), and farnesoid X receptors (FXRs), finely regulate energetic fluxes and metabolic pathways via the molecular cascades they trigger . Dysregulation of such pathways is heavily implicated in those metabolic derangements typically belonging to the domain of IR and MetS and in the histogenesis of progressive NAFLD forms and their clinical complications in both adults and children [3, 16, 18, 19].
This article is based on previously conducted studies and does not involve any new studies of human or animal subjects performed by any of the authors.
Pathophysiology of PPARs
The PPARs are members of the NR superfamily including PPAR-α, PPAR-β/δ, and PPAR-γ, which play a key role in regulating cellular growth and differentiation, metabolism, and inflammation [20, 21, 22]. PPAR-α (NR1C1) is highly expressed in liver, kidney, and muscle, while PPAR-γ (NR1C3) is mainly present in adipose tissue and PPAR-β/δ appears to be universally expressed. These receptors are classically ligand activated, and the best characterized natural ligands are fatty acids (FA) and their derivatives. Activated PPARs form a heterodimer with retinoid X receptor (RXR) and interact with PPAR response elements in the target genes, regulating their expression. In the liver specifically, PPARs modulate a whole spectrum of physiological processes including cholesterol and bile acid (BA) homeostasis, glucolipidic metabolism, inflammatory response, regenerative mechanisms, and cell differentiation and cycle [20, 21, 22].
Role of PPAR-α in Metabolism
In hepatocytes, PPAR-α acts as a nutritional sensor, modulating the rates of FA catabolism, lipogenesis, and ketone body synthesis in response to feeding and starvation . PPAR-α controls the expression of genes regulating peroxisomal/mitochondrial β-oxidation and transport of FA in humans and mouse models. Conversely PPAR-α regulates the glycolysis-gluconeogenesis pathway in mice but not in humans . PPAR-α activation reduces plasma TG-rich lipoprotein by enhanced FA uptake, conversion into acyl-CoA derivatives, and catabolism in the β-oxidation pathways in mouse models [22, 23]. Another mechanism accounting for the PPAR-α lipid-lowering activity in humans and rodents is increased lipolysis via induction of lipoprotein lipase (LPL), which catalyses the hydrolysis of lipoprotein TGs into free FA and monoacylglycerol .
Role of PPAR-α in NAFLD
Two experimental diet-induced NAFLD mouse models have been used with different steatogenic mechanisms. High-fat diet (HFD) induces fatty liver by upregulation of genes involved in de novo lipogenesis [e.g., PPAR-γ and sterol regulatory element-binding protein-1 (SREBP1)] opposed by PPAR-α upregulation which is, however, not sufficient to efficiently catabolize the extra load of FA [24, 25, 26]. Methionine and choline deficient (MCD) diet causes fatty liver by downregulation of genes for FA esterification and very-low-density lipoprotein (VLDL) secretion without significantly affecting PPARs expression [26, 27].
With regard to NASH, the HFD model reproduces mild NASH together with typical MetS traits whereas the MCD diet efficiently induces evolution of NASH to fibrosis, histologically similar to human disease, but without the dysmetabolic phenotype [26, 28].
In the MCD diet NASH mouse model, PPAR-α-deficient mice showed histologically more severe NASH , and treatment with a potent PPAR-α agonist (Wy-14,643) reversed fibrosis and NASH in wild-type mice . Mice lacking PPAR-α expression (PPAR-α-null) fed an HFD developed increased oxidative stress, histological steatosis, hepatic inflammation, and higher NAFLD Activity Score (NAS) compared to age-matched wild-type mice fed a standard/HFD or PPAR-α-null mice fed a standard diet .
A recent study on a PPAR-α mutant mouse model lacking its DNA-binding-dependent activity on FA metabolism showed that PPAR-α agonism (by Wy-14,643) inhibited hepatic inflammatory responses and the transition from steatosis toward NASH and fibrosis via a direct anti-inflammatory effect . Another study, conducted on a NASH model in HFD-fed foz/foz obese/diabetic mice, showed that PPAR-α agonist Wy-14,643 improved metabolic indices, steatosis, and ballooning with substantially reduction of NAS and resolution of NASH according to Kleiner’s criteria , along with suppression of liver nuclear factor (NF)-kB and c-Jun N-terminal kinase activation, reduced tumor necrosis factor (TNF)-α, and monocyte chemoattractant protein (MCP)1 expression and decreased macrophage-neutrophil infiltration in the liver . However, hepatic histological inflammatory score did not improve and adipose tissue inflammation (increased inflammatory cellular infiltrate and MCP1 expression) and hypoadiponectinemia persisted, although adipose tissue TNF-α expression was reduced. These findings suggest that residual hepatic inflammatory changes result from unsuppressed adipose tissue inflammation via inflammatory chemokines, thus potentially limiting the therapeutic efficacy of PPAR-α agonists in NASH .
Moreover, a protective effect of PPAR-α expression or activation by fibrates on liver steatosis and inflammation has been shown in the apolipoprotein-E2 knock-in (APO-E2KI) mouse model mimicking human type III hyperlipoproteinemia [35, 36].
In humans, a recent paired liver biopsy study on 85 consecutive patients showed that liver PPAR-α gene expression was negatively correlated with IR, visceral adiposity, severity of steatosis, presence of NASH, ballooning, NAS, and fibrosis, and positively correlated with adiponectin. At 1 year, histological improvement was associated with an increased expression of PPAR-α and its target genes. Liver PPAR-β/δ and PPAR-γ expression did not correlate with any histological feature nor with gluco-lipidic metabolism . However, as further discussed below (see “Fibrates and Polyunstaurated FA” and “Lipid-Lowering Agents”), PPARα agonists such as fibrates are effective in reducing steatosis in rodents , but have failed to provide convincing results in humans [16, 39].
Diet affects development and progression of NAFLD via PPARs system. Dietary monounsaturated FA (MUFA), polyunsaturated FA (PUFA), and proteins can activate PPARs which stimulate lipid oxidation and reduce inflammation and IR, leading to improvement of hepatic steatosis . Moreover, these nutrients can inhibit the expression of transcription factor SREBP-1 regulating the expression of genes involved in hepatic de novo lipogenesis, and thereby reduce liver fat . For example, obese individuals are prone to the risk of developing steatosis owing to an increased SREBP-1c/PPAR-α ratio associated with n-3 long chain PUFA depletion and IR, which favors lipogenesis over FA oxidation . Consistently, in mice, the supplementation of n-3 long chain PUFA abolishes HFD-induced enhancement in hepatic SREBP-1c/PPAR-α ratios, promoting increased FA oxidation and steatosis attenuation , decreases oxidative stress, IR , and has an additive effect, with ursodeoxycholic acid (UDCA), in alleviating histological features, in HFD-induced NASH .
Finally, conflicting evidence links genetic variants of PPAR-α with susceptibility to human NAFLD [45, 46]. The results of MUFA and/or PUFA supplementation in NAFLD is discussed below (see “Fibrates and Polyunstaurated FA”).
Role of PPAR-γ in Metabolism
PPAR-γ is abundantly expressed in the adipose tissue where it enhances the activation of genes promoting adipocyte differentiation and storage of TG [14, 16, 20, 21]. Moreover, PPAR-γ is a key regulator of glucose homeostasis through its insulin-sensitizing activity by protecting non-adipose tissues against excessive fat deposition and by balancing the secretion of adipocytokines .
Role of PPAR-γ in NAFLD
PPAR-γ is generally increased in fatty liver associated with obesity both in mouse models, except for steatosis induced by MCD diet as previously detailed [26, 27], and in humans [20, 21]. In accordance with this statement, studies in mice fed an HFD have shown that hepatocyte-specific PPAR-γ-knockout protected from hepatic steatosis and PPAR-γ-knockdown by RNA interfering-adenoviral vector injection improved fatty liver [48, 49]. Although these studies would suggest a deleterious effect of PPAR-γ on NAFLD, in mouse liver the net effect of thiazolidinedione PPAR-γ agonists results in improved hepatic steatosis and protection from NASH and hepatic fibrosis via increased insulin sensitivity in adipose tissue and skeletal muscle, overcoming the direct steatogenic effects in hepatocytes [21, 50]. PPAR-γ agonism also upregulates the secretion of adiponectin and the expression of its receptors in the liver and adipose tissue, thus improving insulin sensitivity and upregulating hepatic FA oxidation . Interestingly, increased PPAR-γ expression improves fibrosis in vitro and in mice by inhibiting the activation of hepatic stellate cells [52, 53].
In humans, the C161T, Pro12Ala PPAR-γ single nucleotide polymorphism (SNP) has been specifically associated with the development and histological progression of NAFLD, although a recent meta-analysis found this association to be valid for East Asian populations, but not for European populations .
Role of PPAR-β/δ in Metabolism
PPAR-β/δ regulate hepatic glucose utilization and lipoprotein metabolism and exert an anti-inflammatory role [55, 56]. Data on PPAR-β/δ physiopathology are mainly derived from experimental studies in animal models.
A study has shown that the activation of hepatic PPAR-β/δ stimulates glucose utilization and inhibits gluconeogenesis in HFD-fed mice . PPAR-β/δ knockout mice show glucose intolerance and hypertriglyceridemia [58, 59]. Conversely, PPAR-β/δ agonists have a plasma TG-lowering effect .
Role of PPAR-β/δ in NAFLD
The effect of PPAR-β/δ on liver lipid metabolism is still controversial. Liver PPAR-β/δ upregulation through adenoviral infection has been associated with either improved (in obese db/db mice)  or increased hepatic steatosis (in HFD-fed mice liver) . However, despite increased steatosis, HFD-fed mice showed less liver damage because PPAR-β/δ activation increased the production of protective MUFA and, conversely, reduced serum concentrations of lipotoxic saturated FA .
Moreover, one study showed that PPAR-β/δ-deficient mice treated with an hepatotoxic agent developed more liver necrosis, inflammation, and enhanced expression of profibrotic genes compared to control mice, suggesting that PPAR-β/δ protect the liver from inflammation and fibrosis .
Several studies conducted in mouse models with specific agonists of PPAR-β/δ (such as GW501516, GW0742, and L-165041) reported a beneficial effect of this NR activation on fatty liver by modulation of FA metabolism (increased β-oxidation and reduced FA synthesis) and reduced IR and inflammatory activity [63, 64, 65]. Histological features of NASH also improved following treatment with PPAR-β/δ agonists [66, 67].
Two clinical trials on overweight patients have shown that PPAR-β/δ agonists (GW501516 or MBX-8025) improved metabolic features (IR, plasmatic TGs, nonesterified FA [NEFA], apolipoprotein B-100, LDL-cholesterol), liver enzymes, and liver fat content [68, 69]. However, it should be noted that the one study assessing the effect of PPAR-β/δ agonist GW501516 on steatosis was a very small pilot study .
The recently discovered dual PPAR-α/δ agonist GFT505 has shown protective effects on liver steatosis, inflammation, and fibrosis, mediated by both PPAR-α-dependent and -independent mechanisms, in murine models of NAFLD/NASH and liver fibrosis . GFT505 reduced hepatic and peripheral IR in obese humans . GFT505 will be further discussed in “Dual PPAR-α/δ Agonists” below.
Pathophysiology of FXR and its Role in NAFLD
Once deemed to be simple detergents, physicochemically facilitating the digestive processes, BA are instead master regulators of metabolic pathways . FXR, a member of the nuclear hormone receptor superfamily, together with TGR5, a G protein-coupled BA receptor (GPBAR1), acts as a BA sensor regulating their intrahepatocyte levels and mediates the signaling effects exerted by BA on gluco-lipidic metabolism [73, 74]. FXR is mainly expressed in the liver and gut. BA are the natural ligands of FXR, and chenodeoxycholic acid (CDC) shows the highest affinity . Similar to other NRs, activated FXR forms a heterodimer with RXR that binds to the promoter region of the target genes, small heterodimer partner (SHP) and fibroblast growth factor (FGF)-19, increasing their expression and promoting pathways finally leading to reduced expression of genes involved in BA synthesis (mainly CYP7A1) [74, 75]. In particular, FGF-19 is secreted from ileum into the portal circulation and acts as an enterohepatic signal to downregulate CYP7A1 via FGF-4 activation . Moreover, FXR critically regulates lipid and glucose metabolism by multiple mechanisms . Interestingly, a study conducted in 2166 German subjects reported that SNPs in the FXR-encoding gene NR1H4 were strong determinants of fasting glucose and free FA serum levels independent of unhealthy body fat accumulation . In the liver, BA downregulate steatogenesis via the FXR-SHP pathway . A study conducted in 40 biopsy-proven NAFLD cases found that FXR and SHP and BA transporters [sodium-taurocholate cotransporting polypeptide (NTCP) and bile salt export pump (BSEP)] were significantly upregulated in NASH compared to simple steatosis suggesting that FXR may play a major role in NAFLD progression .
Normal BA synthesis is essential in maintaining normal liver histology: mice with reduced BA synthesis develop overt steatosis, which is reversed either by BA feeding or administration with obeticholic acid (OCA), an FXR agonist, demonstrating that the hepatoprotection exerted by BA is indeed FXR-dependent . Consistently, the age-dependent decline in FXR activity is a major factor in the development of fatty liver observed in aging mice .
Analysis of animal models of alcoholic liver disease has shown that FXR-deficient mice are more exposed to developing steatohepatitis and fibrosis following ethanol feeding . Conversely, the FXR agonist 6ECDCA reverses steatosis and decreases the oxidative stress induced, in rodents, by feeding ethanol with protein-deficient diet .
Similarly, once challenged with steatogenic diets, hepatic FXR-deficient mice, or mice in which FXR is acetylated, develop a liver phenotype fully recapitulating human NASH and cholestasis [85, 86, 87]. However, treatments capable of disrupting the intestinal FXR/ceramide axis signaling have led to a reduced grade of steatosis .
Either direct pharmacological FXR agonism or drug interventions/surgery eventually leading to FXR upregulation have consistently been reported to improve NAFLD histology. For example, deficiency in the electroneutral Na(+)/H(+) exchanger NHE1 (Slc9a1) leads to FXR upregulation, reduced cellular stress, and preserved insulin signaling even upon HFD feeding . In agreement with these observations, treatment with hepatic FXR agonists has resulted in beneficial effects in NAFLD animal models. Administration of the dual FXR/TGR5 agonist (INT-767) for 6 weeks resulted in significantly improved histological features of NASH associated with an increase in the proportion of intrahepatic monocytes with the anti-inflammatory phenotype . Similarly, WAY-362450, a potent synthetic and orally active FXR agonist, attenuated fatty liver by acting through multiple steatogenic mechanisms . Finally, OCA, a semisynthetic BA-selective ligand for FXR, has been shown to improve liver steatosis, inflammation, and fibrosis in preclinical models and in NAFLD patients (see “Drugs Interacting with FXR” below) [91, 92]. As far as surgical experimental models are concerned, vertical sleeve gastrectomy (VSG) produces weight loss independent of SHP status in mice; SHP molecular ablation induces a pro-inflammatory milieu, which is exacerbated after VSG despite weight loss .
FXR systemic expression has led to novel therapeutic strategies targeting cholesterol and TG metabolism, fatty liver, and cholestasis [74, 93, 94]. In contrast to such a systemic therapy, however, postprandial BA release will selectively activate intestinal FXR. By mimicking this tissue-selective effect, the gut-restricted FXR agonist fexaramine significantly induces enteric FGF-15. Fexaramine reduces obesity, systemic low-grade inflammation, and hepatic IR resulting from high-calorie diet by enhancing thermogenesis and browning of white adipose tissue, without activating hepatic FXR .
Pathophysiology of LXR and NAFLD
Cholesterol and FA play a key role not only in lipid and energy metabolism but are also involved in multiple and complex biological phenomena such as the gut–liver axis , the liver–brown adipose tissue interaction , homeostasis of cell membranes, endoplasmic reticulum stress, inflammation [98, 99], atherogenesis [99, 100], T2D, obesity [101, 102], and cancer .
LXRs, which comprise LXR-α and LXR-β, belong to the nuclear hormone receptor superfamily of ligand-activated transcription factors which, in the hepatic tissue, serve as lipid sensors and participate in regulating the expression of master genes which modulate the metabolism of cholesterol and FA .
Orchestrated collaboration between LXR and SREBP-1 is a main step in the molecular cascade of events characterizing steatogenesis. Steatosis is commonly found either in the setting of NAFLD  or associated with hepatitis C virus (HCV) infection . Both these two pathogenically interconnected diseases span steatosis to inflammatory and fibrotic changes in humans [107, 108]. Consistently, inverse agonism of LXR-α and LXR-β obtained through administration of a powerful synthetic compound (SR9238) has been reported to suppress hepatic lipogenesis, inflammation, and steatosis in an experimental NAFLD model in mice  and UDCA inhibits LXR-α-mediated hepatic lipogenesis .
On these grounds, by investigating the connections of LXR-α with other intrahepatic lipid transporters and histological inflammatory and fibrotic changes in 40 NAFLD patients, a study found a positive correlation with the intrahepatic expression of ABCG5/8, CD36, and SREBP-1c; that the expression of NPC1L1 was negatively correlated with intrahepatic inflammation and LXR-α intensity and, notably, that LXR-α expression was directly correlated with the degree of steatosis, as well as with inflammatory and fibrotic changes . These data identify LXR as a potential target for NAFLD treatment.
Natural History of NAFLD at a Glance and Role of NRs
Although the basic pathological steps of NAFLD  and the natural history of disease  are closely interconnected and largely overlap one another, for the sake of clarity, they can be dissected into four distinct phases: steatosis, NASH, fibrosis-cirrhosis, and HCC.
Fatty changes affecting more than 5% of the hepatocytes, usually associated with minor ‘sterile’ inflammatory changes , make up 70–75% of all NAFLD cases and are a common finding in the general population in most countries worldwide . Risk factors for the development of nonalcoholic steatosis include age, gender, ethnicity, western-type lifestyle habits (sedentary and hypercaloric, high-fat, high simple sugar, low in fiber diet) which, against a background of genetic predisposition, lead to the development of either individual features or the full-blown MetS [3, 5]. Gain or reduction of as little as 2.7 kg affects the risk of developing steatosis or its reversal . A study conducted with proton magnetic resonance spectroscopy in 922 subjects who participated in a population screening for NAFLD suggested that metabolic factors increase the risk of developing steatosis more than genetic factors .
Does simple steatosis evolve into NASH? The consistent findings that NASH individuals have an increased mortality compared to those with steatosis had originally led to the notion of two different conditions with a low potential, if any, for steatosis to progress to NASH [11, 116]. Challenging such a view, however, several recent reports have now clearly proven the progression from steatosis to NASH in individual cases [117, 118, 119]. A recent report by Singh et al.  reconciles these seemingly opposite views. These authors, by conducting a meta-analysis of 11 studies including 150 patients with simple steatosis and 261 with NASH, all of whom had biopsy-proven disease, were able to show that the rate of progression of one stage of fibrosis takes place in over 14.3 years for individuals with simple steatosis [95% confidence interval (CI) 9.1–50.0] versus 7.1 years for those with NASH (95% CI 4.8–14.3) . In other words, it would probably take as long as 57.2 years for steatosis versus 24.4 years for NASH to progress from early, non-fibrosis disease to cirrhosis, which confirms that, although they form a disease spectrum, these two conditions follow a fairly distinct course.
Although, sometimes, simple steatosis is alluded to as ‘nonalcoholic fatty liver’ it is, nevertheless, a disease which is worth treating not only to untrigger its potential progression to NASH but also to reduce the risk of developing T2D . Further to weight loss [122, 123], various drugs appear able to promote the reversal of steatosis, notably including statins , pentoxifylline, vitamin E, thiazolidinediones, and OCA .
Role of PPARs in Hepatic Steatogenesis
PPARs play a major role in the steatogenesis and, probably, the progression of NAFLD. Steatogenic drugs such as amiodarone, valproic acid, and tetracycline affect PPAR signaling in a mouse precision-cut liver slices model . Reduced expression of PPAR-α, a master regulator of FA oxidation, and activation of the Jun amino-terminal kinase (JNK) signaling pathway promote hepatic steatosis and hypertriglyceridemia in mice  and steatogenesis in patients with type II citrullinemia . Consistently, PPAR-α activation improves hepatic IR and steatosis in high-fructose-diet-fed mice .
PPAR-γ is upregulated in the liver of obese patients [130, 131]. Conversely, dietary short-chain FA supplementation prevented and reversed HFD-induced metabolic abnormalities in mice by decreasing PPAR-γ expression and activity therefore switching metabolic pathways from the synthesis of lipids to their utilization . Interestingly, genetic PPAR-γ variants are associated with the development and histological progression of human NAFLD .
NASH, histologically defined by concurrent steatosis with inflammatory changes associated with ballooning hepatocyte degeneration, affects approximately 25–30% of those with NAFLD .
The ballooned hepatocyte has lost its normal shape as a result of a combination of cytoskeletal injury, storage of oxidized fat microvesicles, and dilated endoplasmic reticulum. Ballooning is probably the best morphological evidence of what has been alluded to as ‘multi-organelle failure’, which mirrors unbalanced oxidative stress in a lipid-rich milieu and all these events eventually conducing to activation of immunologic pathways [11, 134].
Age, gender, and genetic and metabolic factors are independently associated with NASH [135, 136, 137, 138, 139, 140, 141]. At variance with vitamin E and OCA, thiazolidinediones do not reverse hepatocyte balloning suggesting that IR plays a key role in the early phases of disease only .
Role of LXR in Hepatic Inflammation
Once activated by elevated intracellular cholesterol levels, LXRs induce the expression of genes controlling the absorption, efflux, transport, and excretion of cholesterol. Moreover, LXRs modulate immune, inflammatory, and fibrogenic responses and are thus identified as integrators of metabolic and inflammatory signaling and an ideal target for treatment strategies [143, 144, 145, 146].
Interestingly, LXR expression correlates with the degree of hepatic fat deposition, as well as with hepatic inflammation and fibrosis in human NAFLD .
Fibrosis and Cirrhosis
Fibrosis (not NASH) is the strongest predictor of hepatic mortality in NAFLD . However, NASH, together with age, gender, and genetic and endocrine-hormonal variables, is among the independent predictors of fibrosis [148, 149, 150, 151, 152, 153, 154, 155, 156, 157, 158].
Cirrhosis (usually associated with steatohepatitis and hence defined as NASH-cirrhosis) features advanced fibrosis coupled with deeply distorted (nodular) liver histological architecture .
Recent studies have found that features of the MetS (body mass index [BMI], T2D, and steatosis) together with features of cardiovascular risk (carotid plaques and intima-media thickness) are the independent correlates of advanced fibrosis/cirrhosis [160, 161, 162]. In particular, the Rotterdam study conducted with transient elastography in 3041 participants found that the combined presence of T2D and steatosis was strongly associated with liver stiffness (8.0 kPa), suggestive of clinically relevant fibrosis .
Role of FXR in Hepatic Fibrogenesis
CDC is the natural agonist of FXR, the nuclear hormone receptor which regulates gluco-lipidic metabolism, senses BA, and inhibits BA synthesis by inducing SHP gene expression . Moreover, FXR plays a major role in experimental hepatic fibrosis and in fibrosing kidney disorders observed in diabetic humans [164, 165]. Interestingly, the expression of FXR is reduced in human and mice fibrotic livers . Experimental evidence supports that an FXR-SHP regulatory cascade promotes the reversal of hepatic fibrosis, suggesting that FXR ligands might be effective antifibrotic agents . Consistently, a phase 2 trial has shown that the administration of OCA, a semisynthetic derivative of CDC, reduced surrogate markers of liver inflammation and fibrosis in patients with NAFLD and T2D . The role of OCA is further discussed in “Drugs Interacting with FXR” below.
HCC is the end stage of NAFLD as well as of other chronic liver disorders due to varying etiology. At variance with other etiologies, however, NAFLD-HCC is associated with a less striking prevalence in the male gender and may occur in the absence of cirrhosis [168, 169, 170]. Given that these NAFLD individuals escape surveillance programs, NAFLD-HCC often undergoes a diagnostic delay which curtails the chances for radical treatment and accounts for a worse prognosis compared to other etiologies of disease, such as hepatitis B virus (HBV)-HCC and HCV-HCC [171, 172].
Several risk modifiers, such as genetics (PNPLA3 polymorphisms), age, features of the MetS (BMI and T2D), dietary habits (coffee, fish and vegetables protecting from, and alcohol predisposing to developing HCC) and drugs (statins and certain oral glucose-lowering agents seem to exert a protective effect) are strongly associated with the development of NAFLD-HCC [155, 173, 174, 175, 176, 177, 178, 179]. Moreover, concurrent T2D worsens the prognosis of HCC by reducing both disease-free survival and overall survival .
Role of NRs in HCC
There is paucity of data on the role, if any, of NRs, specifically as regards the development of NAFLD-HCC. Nevertheless, data extrapolated from various etiologies support a role for vitamin D receptor (VDR), PPAR-γ, and FXR pathways [181, 182, 183, 184, 185].
NAFLD Treatment Based on NR-Targeted Drugs
On the basis of the pivotal role of NRs in hepatic metabolic pathways and on the promising results observed in animal models of NAFLD, drugs which interfere with some of these NRs are among the strongest candidates for treating human NAFLD [14, 16]. Nevertheless, findings from experimental studies have not been systematically replicated in humans and several clinical trials utilizing pharmacological manipulation of NRs have yielded conflicting results (see Table S1 in the supplementary material).
Fibrates and Polyunstaurated FA
Treatment with drugs interacting with PPAR-α, such as fibrates and PUFA, has failed to improve NAFLD histology in humans. In particular, despite certain favorable metabolic effects, fibrate monotherapy improves liver histology only to a minimal extent [186, 187, 188, 189, 190]. Similarly, PUFA supplementation may ameliorate some metabolic parameters and probably reduces liver fat content, but has no effect on liver inflammation and fibrosis [191, 192, 193, 194, 195, 196]. Paradoxically, a recent study suggested that PUFA treatment may even worsen IR and liver histology in diabetic patients with NASH . Accordingly, despite improving dyslipidemia [198, 199], this class of drugs has a quite limited scope in treating NAFLD per se .
Although extensively evaluated in human NAFLD, the thiazolidinedione PPAR-γ agonists have failed to deliver fully convincing results so far. Overall, trials on thiazolidinediones showed an improvement in hepatic and systemic IR (despite a significant weight gain), and a reduction of hepatic steatosis and necro-inflammation [200, 201, 202, 203, 204, 205, 206, 207, 208, 209, 210]. In a phase II, double-blind, placebo-controlled, 24-month study, pioglitazone significantly improved aminotransferase levels, steatosis, ballooning, and inflammation in NASH patients with impaired glucose tolerance or T2D . The seminal PIVENS study, a large multicenter, 96-week clinical trial, randomized 247 non-diabetic patients with biopsy-proven NASH to pioglitazone (30 mg/day), vitamin E (800 IU/day), or placebo for 24 months . The primary endpoint was an improvement in the composite of NAS ≥2 points with at least a 1-point improvement in hepatocellular ballooning and a 1-point improvement in either the lobular inflammation or steatosis score, and no worsening of fibrosis. This primary endpoint was achieved in 19% of subjects in the placebo group compared to 34% in the pioglitazone group (P = 0.04, not significant). Although pioglitazone did not meet the prespecified significance level for the primary outcome (prespecified α error of 0.025), it was associated with significantly higher reductions in the individual histological features of steatosis, inflammation, and hepatocellular ballooning, as well as with the resolution of steatohepatitis in a significant proportion of subjects and with improvements in IR and liver enzymes . A recent network meta-analysis by Singh et al.  confirmed that thiazolidinediones may improve steatosis, hepatocellular ballooning, and probably lobular inflammation in NASH patients. Nevertheless, it should be pointed out that in NASH, thiazolidinediones do not reverse or may even worsen mitochondrial abnormalities , and that their benefit on liver fibrosis remains unproven .
Major concerns about treatment with thiazolidinediones in NAFLD are long-term durability of their effects and safety. Indeed, on the one hand, treatment discontinuation may lead to subsequent worsening of liver histology with recurrence of steatosis and inflammation , and, on the other hand, treatment extension over time has not been associated with additional improvement of NASH histological features .
Thiazolidinediones promote pre-adipocyte differentiation into small, insulin-sensitive adipocytes and induce weight gain attributable to an increase in adipose tissue mass through a redistribution of fat from ectopic sites, such as the liver and muscle, to the more physiological reservoir, the peripheral subcutaneous adipose tissue [206, 212]. Recent data suggest that a plateau phenomenon in the improvement in liver steatosis and inflammation probably occurs under PPAR-γ agonists, once the individual subcutaneous adipose tissue expandability reaches its storage capacity [213, 214]. Moreover, thiazolidinediones are associated with long-term safety issues, such as an increased risk of congestive heart failure, bone fractures, and bladder cancer [215, 216]. For these reasons, the risk/benefit ratio of long-term treatment with PPAR-γ agonists should be carefully assessed. Even though current guidelines suggest that pioglitazone can be used to treat biopsy-proven NASH in non-diabetic patients, thiazolidinediones are not specifically licensed for the treatment of NASH implying that off-label administration poses an additional burden of responsibilities on the individual prescribing physician [2, 217].
Dual PPAR-α/δ Agonists
Dual PPAR-α/δ agonists are a novel promising class of drugs for NAFLD treatment. In phase II clinical trials, GFT505 (elafibranor) has been proven able to improve insulin sensitivity, lipid profile, and liver enzymes in patients with MetS or prediabetic abdominal obesity [70, 71]. The GOLDEN trial, a randomized, placebo-controlled, three-arm (placebo, GFT505 80 mg, and GFT505 120 mg) 1-year phase IIb study which evaluated 274 subjects with biopsy-proven NASH and a NAS ≥3, was completed in March 2015 . The primary outcome of disappearance of NASH without worsening of fibrosis was not achieved. Nevertheless, after controlling for baseline severity and center effect, patients in the 120 mg arm had a nearly twofold higher relative risk of achieving the primary endpoint compared to placebo. Of note, in the 120 patients with moderate-to-severe activity (NAS >4) from centers recruiting at least one patient for each arm, the response rate was 29% and 5% in the 120 mg and placebo arms, respectively (P = 0.01). GFT505 120 mg significantly improved ballooning, inflammation, and steatosis, compared to placebo. Moreover, resolution of steatohepatitis was associated with a significant regression of fibrosis, and with improvement in markers of hepatocellular necrosis and cardio-metabolic risk. This trial demonstrated a favorable safety profile of GFT505 . The most common adverse events were minor gastrointestinal complaints; moreover, a mild dose-dependent increase in creatinine was noted [91, 218, 219].
Drugs Interacting with FXR
BA specifically targeting FXR may be a promising approach to treat NAFLD. UDCA was the first BA proposed as a potential treatment for NASH. However, most of the clinical trials conducted with this tertiary BA failed to demonstrate any significant benefit on NASH histology [220, 221, 222, 223]. The recently discovered FXR-antagonistic properties of UDCA may account for its limited therapeutic efficiency .
Conversely, OCA, a CDC derivative, is a potent FXR agonist which improves insulin sensitivity and gluco-lipid metabolism and exerts anti-inflammatory and marked antifibrotic effects in preclinical models . A proof-of-concept, phase 2, randomized, placebo-controlled, 6-week study in T2D patients with NAFLD showed that OCA significantly increased insulin sensitivity and reduced markers of liver inflammation and fibrosis . The recently published randomized, placebo-controlled, 72-week FLINT trial compared OCA 25 mg with placebo in 282 patients with biopsy-proven NASH and NAS ≥4 without cirrhosis . The primary endpoint was a decrease of at least two points in NAS, without worsening of fibrosis. A planned interim efficacy analysis of the histological primary outcome showed a significant superiority of OCA over placebo, which supported the decision not to perform end-of-treatment biopsies in 64 subjects. OCA treatment was significantly associated with the primary outcome and with improvement in steatosis, lobular inflammation, hepatocellular ballooning, and fibrosis, but not with NASH resolution . A recent network meta-analytic review supported direct efficacy of OCA in reversing hepatocyte balloning; moreover, OCA was associated with significant efficacy in fibrosis regression on direct comparison . These positive findings, however, are counterbalanced by pruritus and increased CVR (surge in IR, total and LDL cholesterol, and a decrease in high-density lipoprotein cholesterol) .
Treatment of NASH
Here we provide an overview of how NAFLD treatment can be effected based on currently available non-pharmacological, pharmacological, endoscopic, or surgical intervention. This topic has been extensively examined by our group recently [225, 226].
In principle, NAFLD management needs to cover various aims including amelioration of MetS and its individual features and prevention of extrahepatic NAFLD features such as T2D and atherosclerosis. For example a consistent set of data suggests that NAFLD precedes the development of T2D [3, 227] and, consistently, evidence supports that the reversal of NAFLD protects from developing T2D . Similarly, a recent study suggests that the reduction in the severity of NAFLD is associated with a reduced progression in carotid intima-media thickness . Moreover, effective antifibrotic treatment may potentially halt the progression of NASH to cirrhosis and, further to standard principles of treatment of cirrhosis of any etiology , specific aims in NASH cirrhosis may include action in preventing portal vein thrombosis (PVT) and chemoprevention of HCC [229, 230].
The available weaponry spans from simple lifestyle changes to drugs (licensed for conditions other than NAFLD) to endoscopic and surgical procedures (aimed to treat obesity and indirectly improving NAFLD). However no ‘ideal’, ‘one-fits-all’ approach is available to target all the aims listed above. Rather, a highly tailored management approach should best be implemented on the basis of age, stage of liver disease, and systemic co-morbidity .
A 7–10% weight loss is associated with histologically significant rewards such as reduced liver fat content, remission of NASH, and fibrosis reduction . Moreover, quality of life is also improved in these patients as a result of weight loss . Further to total ingested calories, saturated fats, carbohydrates, and fructose-rich beverages all need to be restricted and, conversely, increases in MUFA, long-chain PUFA, and caffeine should be promoted .
There is clear meta-analytic evidence that exercising reduces liver fat content irrespective of weight loss . This is of interest given the low cost and widespread availability of this option and that most patients fail to maintain weight loss over time. Aerobic training is superior to progressive resistance training . On the basis of guidelines, cardiorespiratory exercise training should be regularly performed .
Cardiovascular disease is the leading cause of mortality in NAFLD patients and guidelines recommend the use of statins in these individuals. However, statins are underutilized in patients with NAFLD owing to implicit concerns of hepatotoxicity [238, 239].
There is no relationship linking reduction in LDL cholesterol values and raised transaminases, and hypertransaminasemia is linked to statin dosage and physicochemical properties [240, 241]. Moreover, statins improve symptom-free survival from cardiovascular disease selectively in those patients in whom coronary artery disease coexists with NAFLD . Recent evidence also suggests that statin use improves liver histology and protects from NASH, in a dose-dependent manner, both in diabetic and non-diabetic individuals [124, 243].
Although fibrates do not improve NAFLD histology, they do offer a safe and effective treatment for dyslipidemic patients with NAFLD .
NAFLD is exceedingly common among those with T2D ; conversely, a subset of NAFLD individuals have T2D at baseline and a large proportion of them will develop T2D over a 5- year follow-up [227, 244, 245]. Principles of treatment of T2D in NAFLD have recently been discussed elsewhere . Probably, the most exciting novel finding in this arena is the possibility to implement a chemopreventive strategy of HCC with antidiabetic medications such as discussed below.
In NAFLD patients, arterial hypertension is the least prevalent among the individual features of the MetS  and, nevertheless, it is a major risk factor for the progression of hepatic fibrosis in these patients , which raises the possibility that, by treating high blood pressure with sartans, hepatic fibrosis progression may be halted. Proof-of-concept experimental studies consistently support this notion [246, 247, 248, 249, 250, 251, 252, 253].
BioEnterics intragastric balloon safely induces a sustained weight loss with diet support or for preparing patients for bariatric surgery. Those individuals attaining a BMI reduction greater than 10% experience improvement in biochemical surrogate indices of NAFLD, IR, and NAS [254, 255].
Bariatric surgery in NAFLD improves transaminases, cardiometabolic risk factors (IR, glucose and lipid metabolism, hypertension), and histological endpoints (hepatic steatosis, steatohepatitis, and fibrosis); however, it is not qualified as a treatment option for NAFLD per se, but only for the accompanying morbid obesity . Roux-en-Y gastric bypass (RYGB) is more effective than other types of surgery in inducing both weight loss and NAFLD improvement. However, additional studies designed to evaluate liver-specific mortality, liver transplantation, or quality of life are eagerly awaited .
Here we discuss some selected topics of specific relevance to NASH-cirrhosis.
Prevention of Cirrhosis via Antifibrotic Agents
This aim remains largely unaddressed in clinical practice. A recent survey reporting that aspirin use seems to exert a protection from fibrosis in the general population paves the way for randomized clinical trials (RCTs) in individuals with NASH . Moderate-quality network meta-analytic evidence supports the superiority of pentoxifylline and OCA over placebo in improving fibrosis .
Prevention of Portal Vein Thrombosis
Portal vein thrombosis (PVT) is a major event dictating the natural history of cirrhosis irrespective of etiology and the prevention of PVT with enoxaparin is associated with decreased hepatic decompensation and improved survival . As regards the relationship of the etiology of cirrhosis with the risk of PVT, a nationwide US survey enrolling, overall, 33,368 patients who underwent liver transplantation reported that NASH cirrhosis was the strongest risk factor independently associated with PVT . These data attest that NASH is a prothrombotic state and suggest that PVT prevention may be particularly indicated in this population. RCTs should assess which anticoagulants should be used in preventing PVT effectively and safely in this specific patient population.
Chemoprevention of HCC
The molecular bases of this topic have extensively been examined [176, 230]. In short, on the basis of current evidence metformin [262, 263], statins [264, 265], or their combination  may potentially be useful in this setting. However, most data derive from observational studies and RCTs are warranted before chemoprevention of HCC can be licensed for clinical practice.
Our understanding of the role of NRs in NAFLD pathophysiology, natural history, and treatment is preliminary (Fig. 1) and additional studies are required. PPAR-γ agonists seem to improve steatosis and necro-inflammation in NAFLD, but thiazolidinediones are not approved for the treatment of NASH, data in diabetic patients are scarce, and long-term safety and efficacy in NASH patients have not been established . The dual PPAR-α/δ agonist elafibranor exhibits promising results and favorable safety profile, but its efficacy should be further confirmed. The FXR agonist OCA has the potential to prevent progression to cirrhosis by improving all the histological features of NASH, notably including fibrosis. However, additional research is needed to confirm this promise and address concerns about tolerability and side effects. Finally, drugs interacting with several NRs, either discussed here, such as LXR, or not discussed in the present review, such as pregnane X receptor (PXR), constitutive androstane receptor (CAR), liver receptor homolog-1 (LRH-1), estrogen receptor beta (ERβ), thyroid hormone receptor beta (TRβ), and VDR, whose importance has been demonstrated in NAFLD animal models, await urgent evaluation in humans [267, 268, 269].
We are indebted to Ms. Jacqueline Mole for English language editing. No funding was received for publication of this article. All named authors meet the International Committee of Medical Journal Editors (ICMJE) criteria for authorship for this manuscript, take responsibility for the integrity of the work as a whole, and have given final approval for the version to be published.
Stefano Ballestri, Fabio Nascimbeni, Enrica Baldelli, and Amedeo Lonardo have nothing to disclose. Dante Romagnoli serves as a consultant for AbbVie.
Compliance with Ethics Guidelines
This article is based on previously conducted studies and does not involve any new studies of human or animal subjects performed by any of the authors.
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