INTRODUCTION — Nonalcoholic fatty liver disease (NAFLD) is a clinico-histopathologic entity with histologic features that resemble alcohol-induced liver injury, but by definition, it occurs in patients with little or no history of alcohol consumption. It encompasses a histologic spectrum that ranges from fat accumulation in hepatocytes without concomitant inflammation or fibrosis (simple hepatic steatosis) to hepatic steatosis with a necroinflammatory component (steatohepatitis) that may or may not have associated fibrosis. The latter condition, referred to as nonalcoholic steatohepatitis (NASH), may progress to cirrhosis in up to 20 percent of patients [1]. NASH is now recognized to be a leading cause of cryptogenic cirrhosis [2].
The pathogenesis of NAFLD has not been fully elucidated. The most widely supported theory implicates insulin resistance as the key mechanism leading to hepatic steatosis, and perhaps also to steatohepatitis. Others have proposed that a "second hit," or additional oxidative injury, is required to manifest the necroinflammatory component of steatohepatitis. Hepatic iron, gut hormones, antioxidant deficiencies, and intestinal bacteria have all been implicated in the pathogenesis of NAFLD.
This topic review will focus on the pathogenesis of NAFLD. An approach to such patients is presented separately. (See "Epidemiology, clinical features, and diagnosis of nonalcoholic fatty liver disease in adults" and "Management of nonalcoholic fatty liver disease in adults".)
GENETICS
Risk factors — NAFLD is a complex disorder with both environmental and genetic contributions. Twin studies have demonstrated a strong hereditary component (approximately 50 percent) to both hepatic fat content and hepatic fibrosis [3]. At least four genetic variants in four different genes responsible for encoding hepatic lipid metabolism regulatory proteins are associated with the development and progression of NAFLD [4,5]. Furthermore, genetic polymorphisms involved in insulin signaling are associated with hepatic fibrosis [6]. Interestingly, successful therapeutic intervention with bariatric surgery demonstrated partial reversibility of methylation of insulin signaling genes, a process that has been shown to have some role in the development of NAFLD [7,8].
Protective factors — Genome-wide association studies have identified loss-of-function genetic variants with protective effects against liver diseases including NASH and cirrhosis [9,10]. In a large exome sequencing study that included clinical data (eg, liver enzymes, liver histology), individuals with a loss-of-function mutation in one of two copies of the CIDEB gene had lower risk of cirrhosis from any cause (odds ratio [OR] per allele 0.50, 95% CI 0.36-0.70) and lower risk of nonalcoholic liver disease (OR per allele, 0.64, 95% CI 0.52-0.80) [10]. (See "Epidemiology, clinical features, and diagnosis of nonalcoholic fatty liver disease in adults", section on 'NAFLD activity score'.)
The CIDEB gene codes for a lipid droplet protein related to fat storage within liver cells [11,12]. Thus, a variant that codes for an inactive protein may prevent fat accumulation within liver cells.
CAUSES OF LIPID ACCUMULATION — Hepatic steatosis is a manifestation of excessive accumulation of toxic lipids in the liver, including triglycerides, free fatty acids (FFA), ceramides, and free cholesterol [13]. This can occur from the excessive importation of FFA from adipose tissue, from diminished hepatic export of FFA (secondary to reduced synthesis or secretion of very low-density lipoprotein [VLDL]), or from impaired beta-oxidation of FFA. The major sources of triglycerides are from fatty acids stored in adipose tissue and fatty acids newly made within the liver through de novo lipogenesis [14].
●Excessive importation of FFA can result from either increased delivery of triglycerides to the liver (as seen with obesity and rapid weight loss), or from excessive conversion of carbohydrates and proteins to triglycerides (eg, secondary to overfeeding or use of total parenteral nutrition).
●Impaired VLDL synthesis and secretion can result from abetalipoproteinemia, protein malnutrition, or choline deficiency. Patients with nonalcoholic steatohepatitis (NASH) may have a defect in postprandial Apo B secretion, leading to triglyceride accumulation [15]. In addition, a defect in the lipidation of Apo B, caused by an inhibition of microsomal triglyceride transfer protein (MTP), may be a key mechanism in drug-induced NAFLD, such as seen with amiodarone and tetracycline [16]. Depletion of the orphan receptor small heterodimer partner (SHP) results in increased VLDL secretion, elevated MTP levels, and increased insulin sensitivity, whereas induction of SHP results in the rapid accumulation of hepatocyte lipids [17]. Impaired VLDL synthesis and secretion were also more apparent in patients with NASH compared with patients with hepatic steatosis [18]. This may have resulted in the induction of lipid oxidation and oxidative hepatocyte damage.
●Treatment of hypertriglyceridemia with eicosapentaenoic acid (EPA) reduces steatosis, oxidative stress, inflammation, and progression of fibrosis in a NASH animal model [19].
●In the 96-week Pioglitazone versus Vitamin E versus Placebo for the Treatment of Nondiabetic Patients with Nonalcoholic Steatohepatitis (PIVENS) trial, patients who had resolution of NASH seen on biopsy also had significant decreases in triglyceride levels (-21.1 mg/dL versus -2.3 mg/dL) [20].
●Impaired beta-oxidation of FFA to adenosine triphosphate (ATP) may be seen with vitamin B5 (pantothenic acid) deficiency, excessive alcohol consumption, or coenzyme A deficiency (as can occur with valproic acid or chronic aspirin use). Activation of peroxisome proliferator-activated receptor alpha appears to have a central role in stimulating beta-oxidation and disposing hepatic fatty acids in NASH [21]. The ability to recover from hepatic ATP depletion is severely impaired in patients with obesity-related NASH [22]. Compromised hepatic ATP homeostasis may predispose to injury from other insults. Adiponectin, a fat derived hormone, appears to have a pivotal role in improving fatty acid oxidation and decreasing fatty acid synthesis [23]. Administration of adiponectin improved hepatomegaly, steatosis, and alanine aminotransferase (ALT) levels in obese, leptin-deficient mice. Also implicated in the steatosis pathway is the cannabinoid receptor type 1 (CB1). Administration of a CB1 receptor antagonist rapidly abolished hepatic steatosis, improved aminotransferase levels, reduced the levels of proinflammatory cytokines, and increased adiponectin levels in leptin-deficient mice [24].
●Triglyceride synthesis and oxidation appear to be regulated, at least in part, by the enzyme acyl-CoA:diacylglycerol acyltransferase 1 (DGAT1). DGAT1-deficient hepatocytes were protected from hepatic steatosis by reducing synthesis and increasing oxidation of fatty acids in an animal knockout model. DGAT1 activity was necessary for the manifestation of hepatic steatosis [25].
●Micro-RNA (MiRNA) also has a role in hepatocellular lipid metabolism and immunity. These are small noncoding RNAs that post-translationally modulate gene function and are involved in cellular processes, including cellular proliferation, inflammation, and apoptosis [26]. Alterations in MiRNA activity result in hepatocellular injury, apoptosis, and portal fibrosis [27]. In NAFLD, specific MiRNA appear to regulate genes involved in fatty acid biosynthesis. Antagonism of specific MiRNA leads to decreased fatty acid synthesis and increased hepatic fatty acid oxidation [28].
●Ceramides, a family of lipid molecules, are involved in inflammation and cell toxicity via interaction with tumor necrosis factor alpha. Inhibition of ceramide synthesis has been shown to diminish steatosis, cell injury, and insulin sensitivity in animal models of NAFLD [29].
●Data suggest that inhibitors of lipogenesis may have a role in treating NASH. In a trial including 99 patients with NASH, use of a fatty acid synthase inhibitor (50 mg daily) resulted in greater reduction in liver fat compared with placebo after 12 weeks (-28.1 versus 4.5 percent) [30]. Fatty acid synthase inhibitor therapy also improved metabolic, pro-inflammatory, and fibrosis markers.
INSULIN RESISTANCE — Insulin resistance has a key role in the development of hepatic steatosis and, potentially, steatohepatitis [31-37]. Obesity and type 2 diabetes, conditions associated with peripheral insulin resistance, are frequently observed in patients with NAFLD. Insulin resistance has also been observed in patients with nonalcoholic steatohepatitis (NASH) who are not overweight and those who have normal glucose tolerance [33,38-40]. Despite the strong association, not all patients with NASH exhibit insulin resistance. This suggests that NASH may be a heterogeneous syndrome with more than one cause.
The genetic basis for insulin resistance associated with NASH remains unclear. One report found an association with certain polymorphisms in the gene encoding for apolipoprotein C3 [41], while another study demonstrated that interleukin-6 (IL-6) polymorphisms are associated with NAFLD and markers of insulin resistance and inflammation [42]. A third report found polymorphisms in a gene encoding for protein expressed in adipose tissue (adiponutrin) and involved in triglyceride metabolism [43]. Certain variants of the gene were strongly associated with the histologic severity of NAFLD. In addition, alterations in the transcriptional activity of the peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PPARGC1A) promoter correlated with the insulin resistance phenotype and the presence of NAFLD [44]. In both adults and children, a single nucleotide polymorphism in the peroxisome proliferator-activated receptor gamma coactivator 1-alpha gene (PPARGC1A) has been associated with an increased risk for developing NAFLD [45,46]. A clinical trial supports the receptor's pathophysiological role. Patients who took elafibranor (an agonist of peroxisome proliferator-activated receptors alpha and delta) 120 mg daily for 52 weeks had resolution of NASH with improvements in fibrosis, liver enzymes, glucose and lipid profiles, and systemic inflammatory markers more often than those who received placebo [47]. Similarly, treatment with saroglitazar, a dual PPAR alpha/gamma agonist, resulted in marked improvements in hepatocyte ballooning and steatosis [48].
Increases in visceral adipose tissue and intrahepatic fat correlate with increased gluconeogenesis, increased free fatty acid (FFA) levels, and insulin resistance [49]. Visceral fat has also been associated with liver inflammation and fibrosis in patients with NASH independently of insulin resistance, an effect possibly mediated by IL-6 (a proinflammatory cytokine) [50]. Increased expression of hepatic IL-6 correlated with insulin resistance in another report [51]. Several other cytokines and adipokines involved in insulin receptor signaling appear to be altered in omental adipose tissue of NASH patients [52]. The activation of tumor necrosis factor alpha-converting enzyme correlated with levels of insulin resistance, macrovesicular steatosis, and balloon degeneration in an animal model [53].
Further supporting the role of insulin resistance are the observations from pilot studies, which have demonstrated beneficial effects of glucose-sensitizing medications in patients with NAFLD. Treatment with rosiglitazone reduced the expression of hepatic acute phase reactants (C-reactive protein and serum amyloid A), suggesting that improvements in insulin sensitivity correspond with a reduction in inflammation [54]. Similarly, pioglitazone therapy led to significant improvements in steatosis and necroinflammation in patients with NASH and impaired glucose tolerance, correlating with decreases in adipose tissue insulin resistance [55]. In other reports, patients with NAFLD and glucose intolerance were significantly more insulin resistant than glucose intolerant patients without fatty liver [56,57]. (See "Management of nonalcoholic fatty liver disease in adults", section on 'Patients with NASH and diabetes'.)
Resistance to the action of insulin results in important changes in lipid metabolism. These include enhanced peripheral lipolysis, increased triglyceride synthesis [58], and increased hepatic uptake of fatty acids. Each of these may contribute to the accumulation of hepatocellular triglyceride [32], which in turn results in a preferential shift from carbohydrate to FFA beta-oxidation, an occurrence that has been demonstrated in patients with insulin resistance [32,59]. Significantly increased FFA levels have been observed in patients with NAFLD and type 2 diabetes mellitus, compared with type 2 diabetics without NAFLD [57]. (See "Pathogenesis of type 2 diabetes mellitus", section on 'Role of diet, obesity, and inflammation'.)
The molecular pathways leading to insulin resistance are complex and have not been completely elucidated. Several molecules appear to be involved in interfering with the actions of insulin on a cellular level.
Lipophilic bile acids have been demonstrated to promote insulin sensitivity and decrease hepatic gluconeogenesis and triglyceridemia via binding to the farnesoid X nuclear receptor [60]. A randomized trial with 141 patients demonstrated that 45 percent of the patients who received obeticholic acid (a potent farnesoid X nuclear receptor activator) had a two point or greater improvement in NAFLD activity score on liver biopsy after 72 weeks of treatment compared with 21 percent of patients in the placebo group, supporting the receptor's role in NAFLD [61].
HEPATOCELLULAR INJURY — FFAs are inducers of several cytochrome p-450 microsomal lipoxygenases, capable of producing hepatotoxic free oxygen radical species [62]. Furthermore, the shift to FFA beta-oxidation in the setting of preexisting defects in mitochondrial oxidative phosphorylation may result in increased free radical formation, hepatocellular injury, and fibrosis [32]. Electron microscopy of hepatocytes from patients with NAFLD demonstrated that significant mitochondrial structural abnormalities were present in patients with nonalcoholic steatohepatitis (NASH), but not those with simple hepatic steatosis [32]. These investigators hypothesized that, in the absence of these mitochondrial defects, peripheral insulin resistance will lead only to the development of simple fatty liver. Consistent with this theory is the observation that several genes important for mitochondrial function were significantly underexpressed in NASH patients, suggesting that there is a transcriptional or pretranscriptional basis for impaired mitochondrial function [63]. On the other hand, it is possible that mitochondrial structural abnormalities may simply be a consequence of increased lipid peroxidation since lipid peroxidation products alter both mitochondrial DNA and mitochondrial respiration [64,65].
Others have suggested that the development of hepatocellular injury requires the presence of both insulin resistance and a second defect that results in the accumulation of damaging free oxygen radical species. Several potential oxidative stressors have been proposed to result in necroinflammation.
The activation of nuclear factor kappa-beta and increased cytokine production appear to mediate the hepatocyte inflammatory process. Numerous proinflammatory cytokines and inflammatory mediators have been identified as having a role in hepatocyte inflammation and injury, including the activation of tumor necrosis factor-alpha, the complement system [66], plasma myeloperoxidase [67], natural killer cells [68], among others. Estrogens may protect against fibrogenesis in NAFLD patients, as men and postmenopausal women have been found to have a high risk for more severe fibrosis compared with premenopausal women [69]. Furthermore, the induction of the Hedgehog ligand pathway appears to correlate with the severity of portal inflammation in both adult and pediatric patients with NAFLD [70,71]. Evidence also points to a pathogenic role of caspase-2, a protease involved in cellular apoptosis, whose expression strongly correlates with liver disease severity in patients with NAFLD [72]. In animal models, caspace-2 deficiency protected the liver from steatohepatitis [73].
ANTIOXIDANTS — Lipid peroxidation and free oxygen radical species can deplete antioxidant enzymes such as glutathione, vitamin E, beta-carotene, and vitamin C, thus rendering the liver susceptible to oxidative injury [74,75]. Serum levels of xanthine oxidase, a generator of reactive oxygen species, are higher in patients with nonalcoholic steatohepatitis (NASH) compared with controls, whereas levels of multiple antioxidant enzymes are lower [76]. In addition, the induction of heme oxygenase-1, an antioxidant defense enzyme, interrupted the progression of steatohepatitis by inducing an antioxidant pathway and suppressing proinflammatory cytokines [77]. A correlation between disease severity and increased expression of oxidative scavenger receptors has been described [78]. Serotonin has been implicated as a source of reactive oxygen species in NASH. Increased catabolism of serotonin resulted in increased levels of reactive oxidative species and necroinflammation in an animal NASH model [79].
There is indirect evidence supporting the role of antioxidants in preventing oxidative liver injury. Vitamin E therapy normalized serum aminotransferase elevations in children with fatty liver disease [80]. In another report, a six-month course of combination therapy with vitamin E and vitamin C resulted in significant histologic improvement, both with respect to inflammation and fibrosis scores [81]. In a third report, dietary intake of antioxidant vitamins was significantly lower in NASH patients, compared with age and body mass index-matched controls [15].
The beneficial effect of antioxidant therapy may be mediated by regulatory T cells, which are depleted in steatotic mice [82]. Antioxidant therapy resulted in reduced regulatory T-cell apoptosis and decreased hepatic inflammation.
IRON — Increased hepatic iron may also have a role in the development of nonalcoholic steatohepatitis (NASH).
●Insulin resistance is associated with increased hepatic iron levels [83], and improved glycemic control is associated with improvements in serum ferritin and hepatic iron concentrations [84].
●The prevalence of heterozygosity for the hemochromatosis gene mutation (HFE) may be increased in patients with NASH associated with increased hepatic iron concentrations and alanine aminotransferase (ALT) levels [85]. In one report, there was a significantly increased prevalence of HFE mutations and iron overload in patients with primary hypertriglyceridemia, a condition that is associated with NAFLD [86]. However, another report found no association between the presence of an HFE mutation or a specific HFE genotype with the severity of hepatic fibrosis in patients with NAFLD [87].
●Increased parenchymal hepatic iron concentration in NASH appears to correlate with the severity of fibrosis [87,88]. A study of 840 patients with NAFLD demonstrated that the pattern of iron staining correlated with the severity of histologic injury [89]. Among the 35 percent of patients with NAFLD who had stainable hepatic iron, those with a reticuloendothelial system (RES) cell pattern of iron staining were more likely to have portal inflammation, hepatocellular ballooning, definite steatohepatitis, and fibrosis. Patients with RES iron were also significantly more likely to have advanced fibrosis.
The specific mechanism by which hepatic iron may contribute to necroinflammation is unknown, but may be related to the generation of free oxygen radical species that occurs in the process of reduction of Fe 3+ to Fe 2+ [90]. In one study, iron, even at normal levels, was an important factor in determining sensitivity to insulin [56]. In addition, compared with glucose intolerant patients without fatty liver disease, glucose intolerant patients with fatty liver disease were 2.5 times more hyperinsulinemic at baseline. In those with fatty liver disease, both hyperinsulinemia and aminotransferase elevations were exceptionally responsive to iron depletion, even though all of the patients had normal iron indices. Interestingly, the glucose intolerant patients without fatty liver disease did not demonstrate significant improvements in fasting insulin levels after phlebotomy.
Still unexplained is the observation that homozygosity for the HFE does not appear to confer an increased risk for the development of NAFLD [91]. Furthermore, the clinical significance of iron overload in NASH with respect to clinically relevant endpoints is unclear. In an unselected cohort of 65 patients with NASH, iron accumulation was not associated with increased overall mortality, liver-related mortality, or development of cirrhosis [92]. Additionally, a prospective study did not demonstrate an improvement in hepatic steatosis (based on magnetic resonance imaging), ALT levels, or insulin sensitivity indices among patients who underwent six months of phlebotomy [93].
LEPTIN — Leptin is a peptide produced primarily in adipose tissue. An absence of leptin is associated with massive obesity in mice (ob/ob) and in humans. (See "Physiology of leptin".)
Leptin may contribute to the development of fibrosis in nonalcoholic steatohepatitis (NASH). Leptin induces dephosphorylation of insulin-receptor substrate 1, rendering hepatocytes more insulin-resistant [94]. Blood leptin levels correlate with the degree of fibrosis in patients with chronic hepatitis C [95], and leptin-deficient obese mice that are exposed to a methionine-choline-deficient diet, a necroinflammatory insult, do not develop hepatic fibrosis. Administration of leptin into the central nervous system of mice with fatty liver corrected insulin resistance and fatty liver, while peripheral administration did not [96]. This suggests resistance to leptin in the central nervous system, rather than the liver, may be important in the pathogenesis of NASH. On the other hand, no relationship between leptin levels and fibrosis stage (after adjusting for potential confounders) was found in a study of 88 patients with NAFLD [97].
ADIPONECTIN — Adiponectin is a hormone secreted exclusively by adipose tissue that produces beneficial effects on lipid metabolism, enhancing both lipid clearance from plasma and beta oxidation of fatty acids in muscle [23]. It also has direct antiinflammatory effects, suppressing tumor necrosis factor-alpha production in the liver [23]. In one report, low serum adiponectin levels correlated with the presence of NAFLD, hepatic fibrosis, and the severity of the metabolic syndrome [98]. Furthermore, adiponectin expression was markedly reduced in adipose tissue from ob/ob (leptin-deficient) mice [23]. A study in nonalcoholic obese ob/ob mice demonstrated significant improvements in hepatic steatosis, hepatomegaly, and aminotransferase elevations following administration of adiponectin [23]. Reduced circulating levels of adiponectin correlate with the severity of liver histology in nonalcoholic steatohepatitis (NASH) [99]. Adiponectin appears to have a role in modulating insulin sensitivity. In one report, plasma adiponectin levels were significantly associated with hepatic insulin sensitivity [100]. Furthermore, administration of pioglitazone increased adiponectin levels, which correlated with improvements in hepatic steatosis, necroinflammation, and fibrosis [100,101].
RESISTIN — Resistin is an adipose-derived protein that may have an important physiological role in the development of insulin resistance. Overexpression of resistin in a mouse model led to glucose intolerance, hyperinsulinemia, and impaired suppression of free fatty acid (FFA) levels [102]. In addition, administering antisense oligonucleotides against resistin mRNA completely reversed the marked increase in resistin levels and severe insulin resistance that developed in mice fed high-fat diets [103].
INCRETINS — Incretins, such as glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1), are gut-derived hormones that potentiate insulin secretion after meal ingestion and play an important role in glucose homeostasis [13]. GLP-1 receptor agonists have been shown to improve glucose and lipid metabolism, reduce hepatic fat content, and improve liver enzymes [104,105]. The use of GLP-1 receptor agonists for patients with diabetes and nonalcoholic steatohepatitis (NASH) is discussed separately. (See "Management of nonalcoholic fatty liver disease in adults", section on 'Potential pharmacologic therapies'.)
INTESTINAL MICROBES — Intestinal microbes have been implicated as a potential source of hepatotoxic oxidative injury, and changes in the microbiome have been demonstrated to play a role in lipotoxicity and pathogenesis of NAFLD. In one report, small intestinal bacterial overgrowth was observed significantly more often in patients with nonalcoholic steatohepatitis (NASH) compared with controls [106]. Another report found increased intestinal permeability in patients with NAFLD, possibly related to small intestinal bacterial overgrowth [107].
Studies suggest that the specific composition of gut microbiota may play a role in both the inflammatory and fibrosis responses in patients with NAFLD [108]. Among 57 patients with biopsy-proven NAFLD, those with Bacteroides genus counts in the second and third tertile had a twofold increase in NASH compared with those with lower Bacteroides counts who were found to also have an abundance of Prevotella bacteria. With respect to fibrosis stage, those with Ruminococcus counts in the third tertile were found to have a twofold increase in stage 2 or greater fibrosis compared with those with lower levels of Ruminococcus. Specific gut microbiome signatures were linked to the severity of NAFLD and degree of fibrosis in additional studies [109,110]. A specific microbial metabolite, 3-(4-hydroxyphenyl) lactate, correlated significantly with hepatic fibrosis and specific bacterial species (Firmicutes, Bacteroidetes, and Proteobacteria) [109]. Alterations in macronutrient composition appear to affect the intestinal flora composition, as has been shown in diets high in saturated fat and fructose [111]. The resulting dysbiosis leads to a cascade of events including increased intestinal barrier permeability, bacterial translocation, and activation of hepatic receptor-induced inflammation.
One proposed mechanism pertains to the production of endogenous alcohol and acetaldehyde [112]. Colonic bacteria and yeast possess an enormous metabolic capacity for generating both ethanol and acetaldehyde, and can oxidize ethanol to high levels of acetaldehyde, even at low ethanol concentrations. Acetaldehyde is easily absorbed into the portal blood stream and can initiate histologic changes similar to those seen in NAFLD [113]. High concentrations of endogenous alcohol production have been found in humans [114] and animals with intestinal blind loops [115]. Increased breath alcohol levels have been described in patients with Candida albicans overgrowth given a carbohydrate load and in females with obesity [116,117].
It has also been demonstrated that patients with hepatic steatosis have higher plasma levels of an intestinal microbe metabolite, N,N,N-trimethyl-5-aminovaleric acid (TMAVA), which has been shown to reduce carnitine synthesis and hepatic fatty acid oxidation, thus promoting hepatic steatosis [118].
Intestinal bacteria may also contribute to hepatic injury by means of endotoxin production. Rats injected with lipopolysaccharide develop steatohepatitis, while antitumor necrosis factor antibodies can improve steatosis [119,120]. Increased intestinal permeability and increased portal endotoxemia have been demonstrated in genetically obese mice, which are consistent with this theory [121,122]. Significant increases in markers of intestinal permeability and a higher prevalence of small intestinal bacterial overgrowth have been demonstrated in humans with NAFLD [107]. These changes correlated with the severity of hepatic steatosis.
Other possible mechanisms by which intestinal bacteria may contribute to hepatocellular injury include deconjugation of bile salts and inactivation of hepatic lipotropes, such as choline. Further supporting a pathogenic role for intestinal bacteria is the observation that the administration of antibiotics, such as polymyxin B, improved steatosis grades in both rats and humans on total parenteral nutrition, and in alcohol-exposed rats [123-125]. In addition, metronidazole administration improved hepatic steatosis following intestinal bypass surgery [126]. Some probiotic species are associated with increased levels of glucagon-like peptide-1 (GLP-1) and improvement in fatty liver in children [127]. Furthermore, the administration of probiotics to mice with NAFLD led to improvements in steatosis, hepatomegaly, and nuclear factor kappa-beta activity after four weeks of therapy [128].
Exercise may also impact NAFLD risk through modification of the intestinal biome. Moderate intensity exercise training reversed gut dysbiosis in a small trial that used ribosomal RNA analysis and metatranscriptomics to analyze microbial and functional diversity [129].
BILE ACIDS — Bile acids are cholesterol-derived carboxylic acids, synthesized in the liver, that facilitate absorption of lipids in the small intestine. Through binding to the farnesoid X receptor, bile acids play a role in lipid and glucose metabolism [130], and also in preventing intestinal bacterial proliferation [131]. In an interim analysis of a large cohort of patients with nonalcoholic steatohepatitis (NASH), obeticholic acid (25 mg daily) resulted in greater improvement in hepatic fibrosis compared with placebo (23 versus 12 percent) [132]. (See "Overview of the management of primary biliary cholangitis", section on 'Subsequent therapy'.)
OTHER FACTORS — Obstructive sleep apnea has been proposed to have a role in inducing inflammation in NAFLD [133]. Compared with controls, mice on a high-fat, high-cholesterol diet exposed to six months of chronic intermittent hypoxia exhibited histologic signs of liver injury, including lobular inflammation and fibrosis. Also noted were significant increases in hepatic lipid peroxidation and levels of proinflammatory cytokines. An observational study in patients undergoing gastric bypass surgery found that patients with higher oxygen desaturation index scores (number of drops in oxygen saturation of 3 percent per hour) had more severe histopathologic changes on liver biopsy than patients with lower oxygen desaturation index scores [134]. This observation was confirmed in a study of 362 patients with obesity who underwent bariatric surgery [135]. In this cohort, the severity of obstructive sleep apnea, as determined by the apnea-hypopnea index, correlated with the histologic severity of NAFLD.
Dietary cholesterol may also be an independent factor in the development of hepatic inflammation [136]. Hyperlipidemic mice fed high-fat diets with cholesterol-developed steatosis with severe inflammation, compared with normolipidemic control animals that developed only steatosis. Hepatic inflammation was linked to increased plasma very low-density lipoprotein (VLDL) cholesterol levels. Omitting cholesterol and lowering VLDL levels prevented hepatic inflammation. On the cellular level, activation of the nuclear receptor constitutive androstane receptor (CAR) is important in the development of lipid peroxidation and steatohepatitis. In a dietary model of nonalcoholic steatohepatitis (NASH), activation of the CAR receptor resulted in hepatic inflammation and fibrosis, in contrast with increased steatosis alone in CAR-negative animals [137].
Fibroblast growth factor 21 (FGF21) has been demonstrated to be an important modulator of metabolism, and it is has been shown to stimulate oxidation of fatty acids and production of ketone bodies, and to inhibit lipogenesis [138]. In a trial of 80 patients with biopsy-confirmed NASH, daily treatment with a pegylated FGF21 analogue resulted in a greater reduction in hepatic fat compared with placebo after eight weeks (-7 versus -1 percent) [139].
Thyroid hormone receptor-beta (a target of lipid metabolism) may also have a role in NAFLD pathogenesis. In a trial including 125 patients with biopsy-confirmed NASH, treatment with MGL-3196 (Resmetirom, a liver-directed selective thyroid hormone receptor-beta agonist) resulted in a greater reduction in hepatic fat compared with placebo after 12 weeks (-33 versus -10 percent) and at week 36 (-37 versus -9 percent) [140]. In addition, studies suggest that higher thyroid-stimulating hormone (TSH) levels, even within the reference range, were associated with the development of NAFLD, independent of other metabolic factors [141].
Sex hormone-related factors appear to impact NAFLD risk. In a large cohort study, later age at menarche was associated with a lower risk for developing NAFLD [142]. However, compared with natural menopause, oophorectomy was associated with an increased risk of NAFLD. Postmenopausal exogenous hormone use was also associated with increased risk for NAFLD.
FIBROSIS — Perisinusoidal (zone 3) fibrosis in patients with nonalcoholic steatohepatitis (NASH) is primarily a consequence of the chronic inflammatory process with activation of lobular stellate cells. Portal fibrosis is commonly a feature of progressive disease. It stems from the activation of a secondary replicative pathway involving hepatic progenitor cells [143]. Hepatic progenitor cells appear to proliferate in the setting of primary replicative senescence from chronic hepatocyte injury. A ductular reaction ensues, leading to periportal fibrogenesis. Increases in the ductular reaction correlated with the grade of NASH activity, the degree of fibrosis, and the extent of primary hepatocyte replicative arrest, which in turn correlated with insulin resistance [143].
Data have suggested that activated platelets play a role in hepatic fibrosis via stimulation of hepatic stellate cells and enhancing expression of proinflammatory prostaglandins. Regular aspirin use has been associated with lower risk of progression to advanced fibrosis. (See "Management of nonalcoholic fatty liver disease in adults", section on 'Potential pharmacologic therapies'.)
SUMMARY AND RECOMMENDATIONS
●Background – Nonalcoholic fatty liver disease (NAFLD) is a spectrum of disorders that range from simple hepatic steatosis without significant inflammation or fibrosis to nonalcoholic steatohepatitis (NASH) with varying degrees of inflammation and fibrosis. (See 'Introduction' above.)
●Insulin resistance – Strong epidemiological, biochemical, and therapeutic evidence supports the premise that the primary pathophysiological derangement in most patients with NAFLD is insulin resistance. Insulin resistance leads to increased lipolysis, triglyceride synthesis, increased hepatic uptake of free fatty acids (FFA), and accumulation of hepatic triglyceride. (See 'Insulin resistance' above.)
Several fat-derived hormones, such as adiponectin, leptin, and resistin, are important regulators of hepatic insulin sensitivity. At the cellular level, these effects appear to be modulated through altered activation of numerous receptors, membrane glycoproteins, and cytokines. (See 'Adiponectin' above and 'Leptin' above and 'Resistin' above.)
●Hepatic inflammation – Factors that determine the presence and extent of necroinflammation are not yet well understood. Several possible mechanisms have been theorized, including host factors, such as defects in mitochondrial structure and function, impaired free oxygen radical scavenging, increased hepatic iron, and hepatotoxic byproducts of intestinal bacteria. (See 'Antioxidants' above and 'Iron' above and 'Intestinal microbes' above.)
●Hepatic fibrogenesis – The factors involved in hepatic fibrogenesis are slowly becoming understood. Activation of both lobular stellate cells and hepatic progenitor cells has been observed in NAFLD. (See 'Fibrosis' above.)
