Current Status in Testing for Nonalcoholic Fatty Liver Disease (NAFLD) and Nonalcoholic Steatohepatitis (NASH)

Abstract

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease in Western countries with almost 25% affected adults worldwide. The growing public health burden is getting evident when considering that NAFLD-related liver transplantations are predicted to almost double within the next 20 years. Typically, hepatic alterations start with simple steatosis, which easily progresses to more advanced stages such as nonalcoholic steatohepatitis (NASH), fibrosis and cirrhosis. This course of disease finally leads to end-stage liver disease such as hepatocellular carcinoma, which is associated with increased morbidity and mortality. Although clinical trials show promising results, there is actually no pharmacological agent approved to treat NASH. Another important problem associated with NASH is that presently the liver biopsy is still the gold standard in diagnosis and for disease staging and grading. Because of its invasiveness, this technique is not well accepted by patients and the method is prone to sampling error. Therefore, an urgent need exists to find reliable, accurate and noninvasive biomarkers discriminating between different disease stages or to develop innovative imaging techniques to quantify steatosis.

Keywords: algorithms; biomarkers; fibrosis; grading; imaging; nonalcoholic steatohepatitis; scores; staging.

Figure 1

Risk factors, symptoms, diagnosis, and treatment of nonalcoholic liver disease. Predisposition and disease progression in NAFLD/NASH is influenced by comorbidities [6], genetic determinants [7], and environmental factors including drugs and toxins [8]. The range of resulting symptoms and complication of NAFLD/NASH can vary from very mild to life-threating. Diagnosis and monitoring of disease can be done by blood tests [9], liver function tests [10] and imaging [11]. However, a liver biopsy should be performed based on an individualized decision and is still the gold standard in scoring and grading of steatosis, inflammation, and fibrosis [12]. Although an ultimate therapy is still missing, beneficial effects on NAFLD/NASH progression are energy restriction, lifestyle changes, improved diets, and elevated physical activity [13]. In addition, in biopsy-proven NASH and fibrosis, medication with antiglycemic drugs, insulin sensitizers, synbiotics, or compounds interfering with fat metabolism or preventing oxidative stress have favorable effects on disease outcome [13,14]. Surgical procedures, including bariatric surgery, to treat obesity and liver transplant after liver failure are extreme forms in NAFLD/NASH treatment [15].

Figure 2

Established compounds and drug candidates evaluated for the treatment of NAFLD/NASH in clinical trial phase III. Pioglitazone (PPARγ agonist), vitamin E (antioxidant), pentoxifylline (anti-tumor-necrosis factor-α (TNF-α) agent), Obeticholic acid (farnesoid X receptor antagonist), cenicriviroc (CCR2/CCR5 inhibitor), elafibranor (PPARα/δ agonist), and selonsertib (ASK1 inhibitor) have different molecular targets. The different biological activities of the drugs point to the complexity of NAFLD/NASH, having a large variety of potential therapeutic drug targets. The depicted structures were generated with the open source molecule viewer Jmol using data deposited in the PubChem compound database with CIDs: 4829, 14985, 4740, 447715, 11285792, 9864881, and 71245288). For more details about the biological activity of each drug refer to [32].

Figure 3

Significance of laboratory parameters in the diagnosis NAFLD and NASH. In the scheme, the biological source and alteration of individual biochemical markers during progression of NAFLD/NASH are indicated. High caloric intake and elevated quantities of fat result in hepatic steatosis, triggering steatohepatitis. The inflammatory response is triggered by infiltrating immune cells and liver-resident Kupffer cells releasing a plenitude of inflammatory triggers. As a consequence, the expression of acute phase response proteins (α₂ macroglobulin (α₂M) and ferritin) is increased in hepatocytes. In addition, the increase of cholesterol and triglycerides provokes cellular fat accumulation, damage, and cellular leakage of hepatocytes. This is indicated by elevated quantities of aspartate aminotransferase (AST), alanine aminotransferase (ALT), bilirubin, and γ-glutamyltransferase (γ-GT). Subsequently, the overall capacity of these cells to synthesize typical liver proteins (haptoglobuin, thrombopoietin) decreases. Lower quantities of thrombopoietin results in reduced formation of platelets within the bones. Dysfunction of synovial lining cells (reduced capacity to degrade hyaluronic acid (HA)) and ongoing fibrogenesis lead to elevated levels of HA. In addition, the transdifferentiation of hepatic stellate cells (HSC) to myofibroblast (MFB) is associated with the occurrence of typical biomarkers (tissue inhibitor of matrix metalloproteinase-1 (TIMP-1), amino-terminal propeptide of type III procollagen (PIIINP)), which correlate to extracellular matrix formation and/or turnover. The metabolic syndrome associated with NAFLD/NASH results in higher quantities of fasting insulin and basal glucose triggering the non-enzymatic formation of glycated hemoglobin (HbA1c). All these parameters are diagnostically relevant in the diagnosis or scoring of NAFLD/NASH and are the basis of various blood biomarker panels to identify inflammatory liver disease.

Figure 4

Gene alterations in NASH. Gene alterations/modifications associated with the pathogenesis of NASH affect the GCKR, TM6SF2, PNPLA3, and MBOAT7 genes. The chromosomal location of respective genes is depicted in the ideogram. Gene annotations were done with the Genome Decoration Page (https://www.ncbi.nlm.nih.gov/genome/tools/gdp) and genomic coordinates deposited in the Online Mendelian Inheritance in Man (OMIM) database under accession no. 609567 (PNPLA3, 22q13.31), 606563 (TM6SF2, 19p13.11), 606048 (MBOAT7, 19q13.42), and 600842 (GCKR, 2p23.3).