Nonalcoholic fatty liver disease (NAFLD) is becoming the most common chronic liver disease in the western hemisphere affecting 1 in 3 adults and 1 in 10 children1. Today, liver biopsies are the only gold standard for precise diagnosis of NAFLD2. Because of this, there are tremendous efforts on the research and clinical side to introduce newer, less invasive methodologies which will aid in diagnosing and monitoring NAFLD and its progression to nonalcoholic steatohepatitis (NASH). The following sheds light on the biology of NAFLD and NASH, and introduces emerging biomarker research in NAFLD and NASH that may prove to be more effective, less invasive, and more affordable methods of NAFLD diagnosis and monitoring.
The liver helps the body maintain homeostasis by playing an essential role in lipid (fat) metabolism. Depending on the body’s needs, the liver either synthesizes (lipogenesis) or stores fatty acids. The hormones insulin and glucagon are involved in regulating these processes. When insulin and glucagon regulation is disrupted, there is an excessive buildup of lipids which can lead to fatty liver disease3. This process is commonly referred to as steatosis. Alcohol consumption has been shown to be one direct link to the disruption of fat homeostasis in the liver, but it is not the only cause. When damaging fat accumulation in the liver occurs from other causes unrelated to alcohol consumption, it is known as NAFLD3.
NAFLD progresses through several stages. Research has shown that the rate of progression through each stage is highly variable and dependent on risk factors such as genetics, diabetes, obesity, microbiome, and age4. NAFLD starts with steatosis which has the potential to lead to liver failure (cirrhosis) or liver cancer4.
Fat accumulation in the liver leads to inflammation which damages the liver cells. Damaged liver cells are noticeably different from normal liver cells because they become enlarged, which is known as ballooning5. This damage marks the progression of NAFLD to nonalcoholic steatohepatitis (NASH)5.
Liver cells can go into regeneration overdrive in an attempt to overcompensate for inflammation. The regeneration response can cause a buildup of extracellular matrix proteins such as collagen, causing scarring and eventually fibrosis6. As this scarring continues to build up, the liver becomes stiff and less responsive to proper blood flow. When blood flow becomes restricted, the result is cirrhosis7.
Both invasive and noninvasive methods are currently used to detect and track NAFLD and NASH. Liver biopsies are used to measure how much ballooning, inflammation, and scarring has occurred in the liver. Different scoring scales and algorithms are used alongside liver biopsies to help classify the stages of NAFLD. Noninvasive imaging techniques have improved over time and may be used to help determine the progression of steatosis, fibrosis, and liver stiffness5.
Recently, researchers began focusing on multiple types of biomarkers for detecting, classifying, and tracking NAFLD and NASH including hormones, pro-inflammatory cytokines and proteins, adipokines, and carrier proteins.
ADP has an anti-steatotic effect on liver cells because it has been shown to increase free fatty acid (FFA) oxidation, decrease gluconeogenesis, decrease FFA influx, and de novo lipogenesis8. Additionally, ADP has been found to protect liver cells from apoptosis and exert anti-inflammatory and anti-fibrotic actions8. Evidence demonstrates that levels of this hormone are lower in NAFLD and NASH individuals as compared to levels in controls or those with simple steatosis8.
Some studies have found that TNF-α is elevated in the liver and in serum of those with NASH9. Research has indicated that increased levels of TNF-α can interfere with insulin signaling in the liver and are involved in the development of both insulin resistance and NAFLD10. The TNF-α signaling cascade is a possible therapeutic target for NAFLD and NASH.
Research has shown that leptin may protect against hepatic steatosis in the initial stages of the disease, but may also act as an inflammatory and fibrogenic factor as the disease progresses8,11. Leptin levels have been shown to be higher in NAFLD and NASH individuals as compared to levels in controls or those in with simple steatosis8.
Recent studies indicate that CRP levels are much higher in individuals with NAFLD, and can be a strong predictor of NAFLD12, even in obese individuals13. Research also suggests that FFA build up in both adipose tissue and the liver can cause an increase in CRP levels leading to NAFLD13.
Studies have shown that serum IL-6 levels are higher in NAFLD patients than in controls10,14, and are positively correlated with severity of hepatocyte inflammation, stage of fibrosis, and systemic insulin resistance in patients with NASH2,15.
Research in NASH mice models has shown how oxLDL can build up in liver Kupffer cells (macrophages), causing the formation of foam cells, similar to foamy macrophages seen in atherosclerosis. Studies on humans have indicated that oxLDL is a potential biomarker for NAFLD progression to NASH and may be a therapeutic target16,17,18.
Data from mouse models has indicated that the pro-inflammatory cytokine interleukin-17 (IL-17) is involved in the progression of NAFLD to NASH. In mice, a high fat diet leads to increased levels of IL-17, resulting in the production of IL-6 and TNF- α by liver macrophages19.
With the prevalence of diabetes and obesity continuing to grow around the world, experts project that instances of NAFLD and NASH will also continue to rise. It is clear that alternative screening methods need to be researched and approved in order to provide additional tools to diagnose and monitor the progression of NAFLD. Emerging biomarker research in NAFLD and NASH will help pave the way for alternative noninvasive methodologies and potential therapies.
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