Nonalcoholic Fatty Liver Disease
Nonalcoholic fatty liver disease (NAFLD) is a newly emerging obesity-related disorder characterized by fatty infiltration (steatosis) of the liver in the absence of chronic alcohol consumption. In some individuals, steatosis progresses to nonalcoholic steatohepatitis (NASH), which is characterized by steatosis, inflammation and fibrosis, and end-stage liver disease (Fig. 1). NAFLD is now recognized as the most common cause of chronic liver enzyme elevations and cryptogenic cirrhosis. The prevalence of NAFLD has nearly doubled since 1980, and current estimates indicate that NAFLD may affect up to 25% of the general population and 80% of obese and type 2 diabetic individuals. NAFLD has also emerged as a common pediatric disease, afflicting 3-9% of children in the US and up to 50% of obese children.
We are investigating the hypothesis that hepatic fatty acid composition impacts disease progression in NAFLD. More specifically we propose that an increased ratio of saturated-to-unsaturated fatty acids delivered to or stored within the liver contribute to progression from simple steatosis to NASH.
There are several potential molecular mechanisms that link fatty acid composition and liver injury; and our focus is the endoplasmic reticulum (ER). The ER is one of the largest cellular organelles, its membranes representing as much as half of the total membranes in a cell. The ER lumen comprises over 10% of the cell volume and is characterized by a unique environment that includes the highest concentration of calcium within the cell and an oxidative environment to support disulfide bond formation. An essential function of the ER is the proper assembly of proteins that are ultimately destined for intracellular organelles and the cell surface. The status of protein assembly and folding is monitored and relayed to the cytosol and nucleus by the unfolded protein response (UPR; Fig. 2). 
Activation of the UPR serves to attenuate global protein synthesis and enhance the capacity for protein folding and degradation. Failure of the UPR to re-establish ER homeostasis can lead to programmed cell death. The ER membrane is characterized by a low concentration of cholesterol and a high concentration of polyunsaturated fatty acids, a lipid environment consistent with a “disordered” membrane. Cholesterol loading activates the UPR and induces apoptosis in macrophages, suggesting that the UPR senses changes to the membrane cholesterol environment. We have demonstrated that increased long-chain fatty acid delivery to hepatocytes also induces the UPR and leads to cell death in the liver and hepatocytes. These data have lead to the hypothesis that the ratio of saturated-to-unsaturated fatty acids may be an important determinant of hepatic ER homeostasis. Current studies are examining the role of the UPR in NAFLD, using cell and animal models, as well as human liver tissue.
The Effects of Nutrient Excess on the Liver
The liver is a major contributor to the disposition of internally delivered glucose, taking up 20-30% of absorbed glucose. Phosphorylation of glucose by glucokinase is a rate-determining step in hepatic glucose uptake and metabolism. Low “catalytic” doses of fructose increase glucose uptake by increasing the cytosolic availability of glucokinase. Fructose is now an abundant source of dietary carbohydrate in the United States. In contrast to glucose, phosphorylation of fructose in the liver occurs via the enzyme Fructokinase. In addition, the metabolism of fructose-1-phosphate in the liver occurs independently of phosphofructokinase, a second rate-determining step in glucose metabolism. As a result, the liver is the primary site of fructose extraction and metabolism, with extraction approaching 50-70% of fructose delivery. Therefore, increased availability of fructose will increase not only “normal” glucose flux but also fructose metabolism in the hepatocyte.
Recent evidence has suggested that ingestion of high fructose sweeteners may contribute to hepatic steatosis. We have proposed that the presence of added sucrose or fructose in the diet will provoke rates of hepatic sugar uptake that are in excess of requirements for energy and replenishment of glycogen. Sugar uptake in excess of these requirements will be diverted to glycolysis, lipogenesis and the pentose phosphate pathway. Intermediates in these pathways, in particular the pentose phosphate pathway, may serve as intrahepatic signals of sugar excess. In the short term, the hepatocyte perceives and response to this excess by activating processes that will reduce the level of these intermediates. Long-term exposure to this change in diet composition leads to accumulation of lipid and lipid precursors, other intermediates and hepatic insulin resistance. We hypothesis that both short and long-term responses involve stress activated pathways (e.g. c-Jun Terminal Kinase) and potentially activation of the UPR.
Novel Therapies for Treatment of NAFLD
The prevalence of obesity is rising around the world at alarming rates. Obesity is associated with numerous complications, including increased risks of diabetes, cardiovascular disease, certain cancers, and non-alcoholic fatty liver disease (NAFLD). The spectrum of NAFLD ranges from simple steatosis to steatohepatitis (NASH), which can progress to end-stage liver disease. NAFLD is highly prevalent in obese individuals and individuals with type 2 diabetes (50 – 75%); and obesity, diabetes, and the metabolic syndrome are risk factors for NASH and advanced liver disease. NAFLD, not only increases the likelihood of liver disease, but appears to be significantly associated with a moderately increased risk for cardiovascular disease among type 2 diabetic individuals.
Lifestyle changes such as increasing physical activity and modifying the diet are likely to remain integral to the management of obesity and obesity-related disorders, including NAFLD. However, despite the implementation of dietary and exercise intervention strategies and the presence of numerous programs focused on behavior modification and weight reduction, the prevalence of obesity and type 2 diabetes has continued to increase. In addition, populations with the most limited access to and trust in standard, western health care (e.g. American and Alaskan Indians, Hispanics, African Americans) currently experience the highest prevalence of obesity, diabetes and NAFLD. Consequently, there is a growing need to develop novel approaches towards management and prevention of obesity and obesity-related disorders in these populations.
Plant roots secrete (exude) an enormous range of large and small molecular weight compounds into the rhizosphere, the soil zone that surrounds and is influenced by the roots of plants. These exudates constitute a novel frontier in the search for complementary and alternative therapies for disease prevention and treatment. We have recently demonstrated that onion root exudates reduce fat accumulation in liver cells in vitro and liver in vivo, and enhance insulin action. Thus, the overall goal of this project is to define the biochemical basis for these observations. Over the long-term, it is our intent to develop an experimental paradigm that uses agricultural science as a tool to identify plant-based treatments for NAFLD.
Recent Reviews
- Hepatic adaptations to sucrose and fructose. Metabolism 54:1189-1201, 2005.
- Fructose-mediated stress signaling in the liver: Implications for hepatic insulin resistance. J. Nutr. Biochem. 18:1-9, 2007.
- The role of fatty acids in the development and progression of nonalcoholic fatty liver disease. J. Nutr. Biochem. 19:567-576, 2008.
- The endoplasmic reticulum as a potential therapeutic target in nonalcoholic fatty liver disease. Curr. Opin. Invest. Drugs (In Press).