non - alcoholic fatty liver disease ( nafld ) is the most common chronic liver disease in north america and has emerged as a serious public health burden .
the clinical - histological phenotype of nafld extends from non - alcoholic fatty liver ( nafl ) to nash .
the estimated worldwide prevalences of nafld and nash range from 6.3%-33% and 3%-5% , respectively .
whereas nafl can progress to cirrhosis in 2% to 3% , nash has an increased risk for the progression to cirrhosis at 15% to 20% and predisposes patients to the development of hepatocellular carcinoma and increased mortality .
although extensive research in understanding the disease pathogenesis and numerous clinical trials aimed at halting disease progression have been performed in the last two decades , optimal therapy is still lacking .
the aims of this article are to provide a short overview of important mechanisms and genetic factors influencing nafld disease progression and to present strategies in the diagnostic and therapeutic management of nash .
dysfunctional adipocytes secrete cytokines and chemokines , and this perpetuating inflammatory cycle induces a state of insulin resistance , resulting in failure to suppress lipolysis in the adipocytes .
this leads to release of free fatty acids ( ffas ) to the circulation and is followed by uptake of these ffas to the liver .
a carbohydrate - rich diet and hyperinsulinemia activate carbohydrate - related element - binding protein ( chrebp ) and sterol regulatory element - binding protein-1 ( srebp-1 ) , respectively , and promote hepatic de novo lipogenesis .
the imbalance between triglyceride acquisition and removal results in excess triglycerides stored as lipid droplets .
key players include lipotoxicity , oxidative stress , endoplasmic reticulum ( er ) stress , activation of the innate immune activity , and cytokine - mediated cellular damage .
it has been demonstrated that triglyceride synthesis is not harmful but protective against fatty acid - induced lipotoxicity .
it is known that ffas can directly activate inflammatory pathways , er stress , and the innate immune system via toll - like receptors .
the presence of biologically active lipid molecules such as free cholesterol , diacylglycerol , lysophosphatidylcholine , and ceramides can affect lipogenesis , insulin signaling , and cellular injury and contribute to nafld phenotype and disease progression [ 1012 ] .
once the capacity of the liver to store triglycerides is overwhelmed , there is increased compensation by the mitochondria and peroxisomes to oxidize fatty acids .
the oxidative capacity of these organelles becomes impaired and can lead to overproduction of reactive oxygen species .
the consequent cytokine production and lipid peroxidation have the potential to induce apoptosis , inflammation , and liver fibrosis . in response to er stress , the unfolded protein response
moreover , the gut microbiome has been more recognized to play an important role in the pathogenesis of nash .
changes in the microbiota can alter intestinal permeability and promote translocation of microbes into the portal circulation .
the gut - derived microbial products , including lipopolysaccharide and bacterial dna , can enter the liver and induce inflammation by activating toll - like receptors in kupffer cells and hepatocytes .
in addition , the innate immune system is activated in the adipocytes , and the release of various adipokines ( interleukin-6 and tumor necrosis factor - alpha ) can contribute to hepatic inflammation .
combined with the host genetic background , the complex interplay of these mechanisms can produce inflammation , hepatocellular injury , and cell death and activate fibrogenesis .
the genetic susceptibility was first demonstrated by the first genome - wide association study ( gwas ) on nafld in a population of hispanic , african american , and european american individuals .
variations in palatin - like phospholipase domain - containing 3 ( pnpla3 ) were shown to influence ancestry - related and inter - individual difference in hepatic fat content and susceptibility to nafld .
specifically , the allele rs738409 encoding for the isoleucine - to - methionine variant at protein position 148 ( i148 m ) was strongly linked to hepatic fat accumulation and inflammation .
this finding was replicated in several studies across different populations and found to be associated with nafld disease severity [ 1824 ] . through a similar gwas strategy , other single - nucleotide polymorphisms ( snps )
variants near pnpla3 , neurocan ( ncan ) , and protein phosphatase 1 , regulatory ( inhibitor ) subunit 3b ( ppp1r3b ) were associated with computed tomography - measured hepatic steatosis from several large - population studies .
these snps were subsequently genotyped in biopsy - proven nafld from the nash clinical research network and showed that variants in or near ncan , glucokinase regulator ( gckr ) , lysophospholipase - like 1 ( lyplal1 ) , and pnpla3 , but not ppp1r3b , were associated with histologic lobular inflammation or fibrosis or both .
these variants have distinct effects on various metabolic pathways indicative of genetic heterogeneity in nafld pathogenesis .
more recently , kozlitina and colleagues   reported the association of a transmembrane 6 superfamily member 2 ( tm6sf2 ) variant with hepatic triglyceride content in an exome - wide association study of patients derived from the dallas heart study .
the gene is more common in individuals of european ancestry ( 7.2% ) than in african americans ( 3.4% ) or hispanics ( 4.7% ) .
the tm6sf2 variant encoding p.glu167lys correlated with elevated serum alanine aminotransferase ( alt ) and reductions in serum alkaline phosphatase and plasma levels of triglycerides and low - density lipoprotein cholesterol in three large cohorts . although the exact function of the gene is unknown , it has been proposed that the tm6sf2 acts to promote very - low - density lipoprotein secretion . in another study ,
the tm6sf2 rs58542926 was found to be associated with nash severity and advanced liver fibrosis .
whereas nafl is characterized by the presence of hepatic steatosis with no evidence of hepatocellular injury , nash is characterized histologically by the presence of ballooned hepatocytes and lobular inflammation with or without perisinusoidal fibrosis in addition to steatosis . diagnosing nash and
although liver biopsy is considered the gold standard to diagnose and stage nash , it is invasive and has several limitations , including cost and sampling error , and is accompanied by risks ( bleeding , pain , perforation , infection , and rarely death ) .
the metabolic syndrome and diabetes are associated with increased risk for nash and can be used in selecting those patients for liver biopsy .
the levels of aminotransferases are not reliable , and imaging studies have not been accurate in the diagnosis of nash and this is because the histologic features of apoptosis , ballooning , and cell injury that characterize nash are not detectable by currently available modalities . the poor performance of liver biopsy has led to significant interest in developing non - invasive biomarkers for identifying steatohepatitis or fibrosis ( or both ) in patients with nafld .
multiple biomarkers and predictive models have been suggested to predict nash but with varying degrees of success , and the majority require external validation .
circulating keratin 18 ( ck18 ) , a marker for hepatocyte apoptosis , is the most validated biomarker that is reproduced in several studies for differentiating nash from simple steatosis [ 3437 ] . in a recent meta - analysis ,
plasma ck18 levels had a sensitivity of 78% , specificity of 87% , and an area under the receiver operating curve ( auroc ) of 0.82 for identifying steatohepatitis .
though fairly specific , this biomarker lacks the sensitivity needed for it to be an adequate screening test to stage nash .
the nafld fibrosis score and enhanced liver fibrosis ( elf ) panel are non - invasive tools that facilitate the identification of the majority of advanced fibrosis patients but without the need of a liver biopsy .
the nafld fibrosis score is a scoring tool calculated by incorporating clinical parameters that include age , glycemia , body mass index , platelet count , albumin , and aspartate transaminase - to - alt ratio .
this has been validated in many studies and confirmed by a recent meta - analysis to predict advanced fibrosis with an auroc of 0.85 .
the elf combines three serum markers of matrix turnover ( timp 1 , hyaluronic acid , and p3np ) and can identify advanced fibrosis in nash patients with an auroc of 0.87 .
however , both nafld fibrosis score and elf perform poorly in early and intermediate stages . modalities that measure liver stiffness were developed to assess liver fibrosis .
transient elastography ( fibroscan ) was first evaluated in a japanese study that reported a stepwise increase in liver stiffness with increased severity of liver fibrosis .
a meta - analysis showed a pooled auroc of 0.94 with 94% sensitivity and 95% specificity for advanced fibrosis .
a major drawback of this modality is its inconsistency in obtaining measurements in obese patients . using a new xl probe
has enabled the examination in obese patients with diagnostic accuracy comparable to that of the standard probe .
more recently , studies have shown promise of acoustic radiation force impulse ( arfi ) sonoelastography for the assessment of liver stiffness [ 4446 ] . aside from having the capability to be integrated in a standard liver ultrasound , this modality can evaluate large parts of the liver in one examination and is more reliable than transient elastography in obese patients . in a study of 57 patients with nafld
, arfi showed a diagnostic accuracy similar to that of transient elastography for advanced fibrosis when compared with liver histology .
a recent meta - analysis of various chronic liver diseases confirmed this finding , although only a small proportion of patients were in the nafld group .
another imaging modality is magnetic resonance elastography , which allows evaluation of the whole liver and shows good accuracy for detecting severity of liver fibrosis .
more studies are needed to assess the role of these imaging tools in assessing changes in fibrosis in response to treatment .
meaningful therapy for nash should target reduction of mortality , development of metabolic comorbidities , liver - related and cardiovascular outcomes , and improvement of symptoms and quality of life . despite an increasing understanding of the mechanisms of nafld pathogenesis , approved therapy is lacking . lifestyle modifications ( diet and regular exercise ) remain the initial therapeutic strategy for nash .
this simple yet challenging approach has been shown to correlate with nash improvement histologically [ 4951 ] .
the majority of the patients with nash have concurrent metabolic comorbidities , and not surprisingly the leading cause of their mortality is cardiovascular disease .
the presence of multiple features of the metabolic syndrome is associated with progressive liver disease .
therefore , regardless of nash diagnosis by liver biopsy , the treatment of obesity , diabetes , dyslipidemia , and hypertension should be optimized in nafld to reduce the risks for cardiovascular disease and liver disease progression .
pharmacologic therapy can be considered only in patients with biopsy - proven nash until reliable biomarkers are available .
insulin sensitizers , angiotensin - converting enzyme inhibitors and receptor blockers , anti - oxidants , bile acids , cytokine - mediated therapy , lipid - lowering agents , and endocannabinoid antagonists have been proposed , but the majority of these agents either lack efficacy or require further studies for validation before definitive recommendations can be made .
this article will focus on pharmacologic options for which sufficient evidence is available to make a recommendation .
vitamin e is an antioxidant that suppresses lipid peroxidation , helps replete glutathione stores , decreases cytokine production , and inhibits profibrotic activity [ 5558 ] . to date , the pioglitazone versus vitamin e versus placebo for the treatment of nondiabetic patients with nonalcoholic steatohepatitis ( pivens ) trial is the largest randomized controlled trial comparing vitamin e ( 800 iu / day ) and pioglitazone ( 30 mg / day ) with placebo for 96 weeks in non - diabetic and non - cirrhotic adult patients with nash .
vitamin e was superior to placebo in improving nash histology , but no significant improvement in fibrosis was observed .
there are insufficient data to recommend vitamin e for nash patients with concomitant diabetes or cirrhosis , and long - term efficacy and safety of vitamin e remain uncertain .
it is important to note that vitamin e at high doses has been implicated in increased all - cause mortality .
other safety concerns of vitamin e include increased risk for development of hemorrhagic stroke and prostate cancer .
therefore , competing risks should be discussed with non - diabetic patients with nash before starting on therapy .
thiazolidinediones ( tzds ) are peroxisome proliferator - activated receptor - gamma agonists that ameliorate insulin resistance in adipose tissues , liver , and muscles .
tzd increases adiponectin , reduces fatty acid synthesis , and increases fatty acid oxidation by activating 5 adenosine monophosphate - activated protein kinase ( ampk ) and counteracts pro - inflammatory cytokines .
belfort and colleagues   showed that pioglitazone improved liver histology by improving steatosis , inflammation , and ballooning in patients with nash who have impaired glucose tolerance or diabetes . in the largest clinical trial ( pivens ) , although pioglitazone did not achieve the primary endpoint as assessed by the nafld activity score ( nas ) , resolution of nash was achieved in a higher percentage of non - diabetic patients receiving pioglitazone versus placebo ( 47% versus 21% , p = 0.001 ) .
a recent meta - analysis suggested that pioglitazone may improve liver histology and fibrosis in patients with nash . in terms of safety , treatment with pioglitazone may not be without adverse effects , and its long - term use has not been studied .
potential adverse effects include weight gain , higher rate of heart failure , bone loss , and rarely bladder cancer [ 6770 ] .
overall , pioglitazone is an option in patients with biopsy - proven nash , particularly for those with impaired glucose tolerance or diabetes , but this medication must be used with caution .
obeticholic acid is a synthetic fxr agonist that has been demonstrated to decrease hepatic fat and fibrosis in animal models of nafld [ 7173 ]
more recently , neuschwander - tetri and colleagues   performed a placebo - controlled , randomized clinical trial ( flint ) that assessed the efficacy of obeticholic acid ( 25 mg / day ) versus placebo for 72 weeks in patients with non - cirrhotic nash .
the primary outcome was improvement in liver histology , defined as a decrease in nas by at least 2 points without worsening of fibrosis from baseline to the end of treatment .
treatment with obeticholic acid improved all features of the nas ( steatosis , hepatocellular ballooning , and lobular inflammation ) and fibrosis .
however , complete resolution of nash was not achieved in a substantial proportion of patients .
although obeticholic acid was generally well tolerated , it was associated with frequent pruritus , increased serum cholesterol , and changes in the lipid profile .
therefore , long - term studies are needed to assess the potential risk of obeticholic acid for the development of atherosclerosis and cardiovascular disease in these patients .
apo c2 , apolipoprotein c2 ; apo c3 , apolipoprotein c3 ; fxr , farnesoid x receptor ; ldl - cholesterol , low - density lipoprotein cholesterol ; hdl - cholesterol , high - density lipoprotein ; lxr , liver x receptor .
a better understanding of nash pathogenesis may help clarify the best approach in managing patients with nash .
more studies are warranted in understanding the influence of genetics associated with disease progression and patient response to treatment .
although liver biopsy is still required , biomarkers to diagnose nash and tools to predict advanced fibrosis may be helpful , but more studies are needed to validate their use . while obeticholic acid has shown promise in nash by improving not only liver histology but also fibrosis , not achieved by either vitamin e or pioglitazone , long - term studies are needed to assess the safety of obeticholic acid and its effects on liver- and cardiovascular - related outcomes .
the multiplicity of pathways contributing to nash and fibrosis progression provides a rationale for individualized treatment strategies .
arun j. sanyal is a consulting advisor for abbot , genetech , gilead sciences , ikaria , merck , norgine , roche , salix , takeda pharmaceutical company , nimbus , nitto denko and bristol - myers squibb , receives research grants from ikaria , takeda pharmaceutical company , astellas , novartis and galectin therapeutics and receives royalties from uptodate inc .
 non - alcoholic steatohepatitis ( nash ) can lead to advanced fibrosis , hepatocellular carcinoma , and end - stage liver disease requiring liver transplantation . 
 a myriad of pathways and genetic influence contribute to nash pathogenesis and liver disease progression . diagnosing patients with nash and 
 advanced fibrosis is critical prior to treatment and prognostication . 
 there has been ongoing interest in developing non - invasive biomarkers and tools for identifying nash and advanced fibrosis . to date 
 , there has been no approved therapy for nash . 
 recently , the flint ( farnesoid x receptor [ fxr ] ligand obeticholic acid in nash treatment ) trial provided promising results of the efficacy of obeticholic acid , a farnesoid x receptor agonist , in improving histological features of nash and fibrosis . 
 long - term studies are needed to assess the safety of obeticholic acid and its effects on liver- and cardiovascular - related outcomes . 