Gut–liver on a chip toward an in vitro model of hepatic steatosis

Nonalcoholic fatty liver disease is a common cause of chronic liver disease in the general population. Nonalcoholic steatohepatitis (NASH), the aggressive form of nonalcoholic fatty liver disease, is associated with an increased risk of liver-related mortality and cardiovascular disease. At present, a liver biopsy is the only generally acceptable method for the diagnosis of NASH and assessment of its progression toward cirrhosis. Although several treatments have shown evidence of efficacy in clinical trials of varying design, there are no approved treatments for NASH, and published trials are often too divergent to allow meaningful comparisons. There is thus a lack of established noninvasive, point-of-care diagnostics and approved treatment on one hand and a substantial population burden of disease on the other. These provide the rationale for developing consensus on key endpoints and clinical trial design for NASH. This article summarizes the consensus arrived at a meeting of the American Association for the Study of Liver Diseases on the key endpoints and specific trial design issues that are germane for development of diagnostic biomarkers and treatment trials for NASH. Copyright © 2011 American Association for the Study of Liver Diseases.

Production of short-chain fatty acids (SCFA) in the intestinal lumen may play an important role in the maintenance of the intestinal barrier. However, overproduction/accumulation of SCFA in the bowel may be toxic to the intestinal mucosa and has been hypothesized to play a role in the pathogenesis of neonatal necrotizing enterocolitis (NEC). By using a Caco-2 cell monolayer model of intestinal barrier, we report here that the effect of butyrate on the intestinal barrier is paradoxical. Butyrate at a low concentration (2 mM) promotes intestinal barrier function as measured by a significant increase in transepithelial electrical resistance (TER) and a significant decrease in inulin permeability. Butyrate at a high concentration (8 mM) reduces TER and increases inulin permeability significantly. Butyrate induces apoptosis and reduces the number of viable Caco-2 cells in a dose-dependent manner. Intestinal barrier function impairment induced by high concentrations of butyrate is most likely related to butyrate-induced cytotoxicity due to apoptosis. We conclude that the effect of butyrate on the intestinal barrier is paradoxical; i.e. whereas low concentrations of butyrate may be beneficial in promoting intestinal barrier function, excessive butyrate may induce severe intestinal epithelial cell apoptosis and disrupt intestinal barrier.

The present study was designed to define an experimental model of hepatocellular steatosis with a fat overaccumulation profile in which the metabolic and cytotoxic/apoptotic effects could be separated. This was accomplished by defining the experimental conditions of lipid exposure that lead to significant intracellular fat accumulation in the absence of overt cytotoxicity, therefore allowing to differentiate between cytotoxic and apoptotic effects. Palmitic (C16:0) and oleic (C18:1) acids are the most abundant fatty acids (FFAs) in liver triglycerides in both normal subjects and patients with nonalcoholic fatty liver disease (NAFLD). Therefore, human hepatocytes and HepG2 cells were incubated with a mixture of different proportions of saturated (palmitate) and unsaturated (oleate) FFAs to induce fat-overloading. Similar intracellular levels of lipid accumulation as in the human steatotic liver were achieved. Individual FFAs have a distinct inherent toxic potential. Fat accumulation, cytotoxicity and apoptosis in cells exposed to the FFA mixtures were investigated. The FFA mixture containing a low proportion of palmitic acid (oleate/palmitate, 2:1 ratio) is associated with minor toxic and apoptotic effects, thus representing a cellular model of steatosis that mimics benign chronic steatosis. On the other hand, a high proportion of palmitic acid (oleate/palmitate, 0:3 ratio) might represent a cellular model of steatosis in which saturated FFAs promote an acute harmful effect of fat overaccumulation in the liver. These hepatic cellular models are apparently suitable to experimentally investigate the impact of fat overaccumulation in the liver excluding other factors that could influence hepatocyte behaviour.