As mentioned previously, the only definitive method of staging NAFLD is a liver biopsy. This presents a significant complication to the study of human fatty liver disease at several levels, including identifying patients for study, obtaining tissue samples, and tracking disease progression. Further, human DME expression and activity can vary widely between patients due to a number of variables (e.g., genetic polymorphism, diet, xenobiotic exposure, age, and so on) (Guengerich, 2006
; Gomez-Lechon et al., 2009
). As a result of these limitations, the majority of human NAFLD studies employ one of two tissue sources: cadaveric organs and bariatric surgery patients. Postmortem tissue samples provide sufficient material for analysis of mRNA and protein expression, as well as activity, though a detailed medical history of the patient is often lacking. The patient population undergoing bariatric surgery (i.e., morbidly obese patients), have a much higher rate of both NAFLD and NASH (Lazo and Clark, 2008
). The routine intraoperative liver biopsies performed during bariatric surgery allow a detailed determination of NASH status in these patients (Tanaka et al., 2006
; Dolce et al., 2009
Because of the difficulty in identifying properly diagnosed NAFLD patients, as well as the ethical and practical issues with obtaining liver samples for analysis, the majority of studies on NAFLD gene alterations have employed animal models. Although some of these models are of limited accuracy in relation to either histological outcome or disease context (reviewed by Larter and Yeh, 2008
; Anstee and Goldin, 2006
), a detailed presentation of the various benefits and faults of each is not the focus of this review. However, a brief description of the models used in the studies described below may prove beneficial to properly compare results between models and species. Though NAFLD studies using nonrodent species have been described (Leclercq et al., 1998
), the vast majority of research has been performed in rat and mouse models.
These rodent models are genetic, dietary, or a combination of the two. In both rats and mice, the most common genetic models involve dysregulation of leptin signaling, leading to hyperphagia and obesity. Obese Zucker (fa/fa
) rats and db/db
mice are deficient in the leptin receptor, whereas ob/ob
mice are deficient in leptin itself. Though these models exhibit insulin resistance and obesity, their liver pathology rarely progresses beyond steatosis to NASH without a second insult (Larter and Yeh, 2008
). Because of this, drug-metabolism data presented from these genetic models can be assumed to correlate with a human diagnosis of simple steatosis.
To stimulate the progression to NASH, a dietary model is often used. The most well-characterized model of rodent NASH is a methionine-choline–deficient (MCD) diet, which rapidly induces steatohepatitis with a liver histopathology closely recapitulating the human condition. However, in contrast to human NASH, animals fed the MCD diet experience significant weight loss, hypoin-sulinemia, and are insulin sensitive. Despite these shortcomings, in our experience, the MCD diet accurately models the expression changes of a majority of ADME, genes.
An alternate dietary model, the high-fat diet, has been used to more closely recapitulate the modern “Western-diet.” However, rodents adapt to high-fat feeding and may take several months to progress to NASH. Additionally, the composition of the experimental chow may vary a great deal in a number of constituents, with varying experimental results. In the absence of a stated hepatic histological staging, it is difficult to properly assign these models to steatosis or NASH (Larter and Yeh, 2008
). Additional animal models employed include dietary orotic acid and forced intragastric feeding, which result in steatosis and steatohepatitis, respectively (Zhang et al., 2007
; Deng et al., 2005
As mentioned above, each of the animal models currently in use have limitations that may complicate the extrapolation of disease results to human patients. It is important to also note that in certain cases, the mechanisms of enzyme regulation may differ significantly between species. Despite these shortcomings, animal models provide a valuable tool to investigate the effects of fatty liver.