Non-alcoholic fatty liver disease (NAFLD) has recently become the most common cause of chronic liver disease in children and adults in the United States (Della Corte et al., 2012). The disease was first discovered by Ludwig et al. in a clinical study of twenty people with liver inflammation and liver cell death (1980). Upon liver biopsy, Ludwig found that the liver inflammation and cell death was caused by fat, not alcohol. Studies suggest that NAFLD now affects 30% of the U.S. adult population, 8% of children under the age of 20, and 70-80% of the obese population (Roth et al., 2012). NAFLD-related cirrhosis is also the cause of 10% of all liver transplantations in the United States. The rise of NAFLD in developed nations has paralleled the rise in obesity over the past several decades. Recent statistics demonstrate that there are now 30% more obese people in the world than those that are malnourished, which was not the case only two decades ago (Lustig, 2013). It is critical that researchers better understand the mechanisms by which NAFLD arises and progresses, since the number of people affected will only continue to rise as obesity continues to advance through the developed world.
The Progression of NAFLD: From Healthy Liver to Cirrhotic Liver
The term non-alcoholic fatty liver disease represents a wide spectrum of the disease. The first form that the disease takes is known as steatosis. Steatosis is characterized by an accumulation of fat in hepatocytes in the form of lipid droplets. When triglyceride levels exceed 55mg per gram of liver (the 95th percentile for lean individuals) or lipid droplets are present in more than 5% of hepatocytes, the liver is said to be steatotic (Cohen et al., 2011). In most cases, steatosis will not progress any further, but in approximately 10% of cases steatosis will progress to non-alcoholic steatohepatitis (Gentile et al., 2010). NASH is marked by hepatocyte cell death, inflammation, and fibrosis (Cohen et al., 2011). Anywhere from 10-29% of individuals with NASH develop cirrhosis which is characterized by irreversible scarring of the liver (Cohen et al., 2011). At this point, NASH-induced cirrhosis leads to hepatocellular carcinoma in 4 to 27% of individuals and severely cirrhotic livers need to be replaced by liver transplantation (Cohen et al., 2011).
NAFLD Symptoms and Diagnosis
The majority of individuals with NAFLD are asymptomatic and in these cases, the disease is initially identified accidentally by abnormal liver-test or ultrasound results. In a minority of NAFLD cases, patients will present with pain in the upper right quadrant of their abdomen and fatigue. Upon palpation of the upper right quadrant, physicians typically find hepatomegaly, a swollen liver, in patients with NAFLD (Angulo 2002). Moderately elevated levels of aspartate aminotransferase (AST) and/or alanine aminotransferase (ALT) are indicative of NAFLD, but do not confirm the diagnosis (Angulo 2002). Additionally, imaging studies, while helpful in displaying the presence of fat accumulation in the liver, are unable to determine the severity of liver damage. Therefore, liver biopsy, although invasive, is the only diagnostic tool capable of confirming NAFLD and identifying its severity (Angulo 2002).
Running Like a Well-Oiled Machine: The Liver in a Healthy State
The liver’s role in filtering toxins from the blood is widely known, however, the liver also plays a critical role in the flux of free fatty acids and transport of triglycerides. After digestion in the mouth and stomach, dietary fats are absorbed in the small intestine after emulsification and packaged into chylomicrons, lipoproteins which transport triglycerides. From this point, chylomicrons are transported to endothelial cells where the triglycerides they contain are hydrolyzed before entering the blood stream. The hydrolysis of triglycerides results in free fatty acids which are primarily taken up by adipocytes or muscle tissue from the blood stream and then repackaged into triglycerides in adipocytes for storage.
Upon stimulation by insulin, adipocyte triglyceride hydrolase catalyzes the hydrolysis of triglycerides (Cohen et al., 2011). The free fatty acids (FFAs) are then transported to the liver bound to albumin. In the liver, FFAs can be oxidized in the mitochondria, reesterified to triglycerides and stored in the form of lipid droplets, or transported out of the liver in the form of very-low-density lipoproteins (Cohen et al., 2011). Free fatty acids can also be directly taken up by the liver out of the bloodstream and will undergo the same processes already outlined.
Other important players in the disease-state of NAFLD are carbohydrates. In a healthy human body, glucose stimulates the release of insulin from the pancreas. Insulin then stimulates a transcription factor known as SREBP-1c (sterol response element binding protein) through a signaling cascade. SREBP-1c is the primary isoform in humans and is highly expressed in the liver (Eberlé et al., 2004). SREBP-1c activates the expression of genes involved in fatty acid biosynthetic pathways like ATP citrate lyase, acetyl-CoA carboxylase, fatty acid synthase, and glycerol-3-phosphate acyltransferase (Horton et al., 2002). SREBP-1c also activates three genes responsible for generating NADPH, which is critical for fatty acid biosynthesis (Horton et al., 2002). Glucose also activates the transcription factor, carbohydrate responsive element-binding protein (ChREBP), which then enhances the expression of liver-type pyruvate kinase, leading to more acetyl-coA, which means more substrate for fatty acid biosynthesis.
The healthy state of the liver requires a balance between free fatty acids and triglycerides entering and triglycerides leaving in the form of VLDL. Although implicated in the disease state, the uptake of free fatty acids and triglycerides in the liver as well as the upregulation of genes involved in the biosynthesis of fatty acids are not the sole cause of NAFLD. These enzymes and transcription factors only support the progression of the disease when overstimulated by external factors or when the genes that they regulate are mutated.
Advance to The Molecular Basis of NAFLD to learn how these enzymes and molecules are altered in the disease state of NAFLD!