High fat plus high cholesterol diet lead to hepatic steatosis in zebrafish larvae: a novel model for screening anti-hepatic steatosis drugs

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of human liver dysfunction and possesses an enormous risk to human health [1]. It is estimated that about 20 %-30 % of the general population suffer from simple steatosis and the prevalence of non-alcoholic steatohepatitis (NASH) is 3 %-10 % [2]. Blocking hepatic steatosis is crucial for preventing the development of NAFLD. Although there have been improvements in the progression of hepatic steatosis to more advanced liver inflammation owing to the changes of lifestyle and the use of supplements [3, 4], the pathogenesis of NAFLD remains unclear, and therapeutic drugs are still restricted.

Rodents are the traditional model organisms used to reveal mechanisms underlying NAFLD, as well as to test the therapeutic potency of agents. There are many studies that have established NAFLD models in rodents through hepatotoxic drugs and dietary treatments, as well as genetic modification [5]. However, rodents are unfit for high-throughput drug screening because these animals produce small litters, mature slowly and incur high financial costs in terms of considerable daily expenditure and raising space owing to their relatively large body size. Furthermore, rodents must be sacrificed and their internal organs should be isolated in order to examine their liver for abnormalities. This requirement does not allow for continuous and real-time monitoring of liver condition in large samples [6]. Current situation highlights new approaches to complement rodent-based researches. Nowadays, zebrafish (Danio rerio) with its unique advantages in physiology, genetics and genomics, have come to be recognized as an attractive model organism for studying human disorders and screening drug candidates on a large scale [6‐8]. Specific to the study of NAFLD, the livers of larvae are matured by 4 days post fertilization (dpf) and possess the main types of liver cells that are anatomically and functionally analogous to those of mammals [9, 10]. Moreover, lipid metabolism in zebrafish, which have been reported as absorption occurs in the intestine tract with the help of bile produced by the liver, cholesterol transport is mediated by lipoproteins, and triglyceride is stored in intramuscular adipocyte, subcutaneous, and visceral, are similar to humans [11, 12]. Additionally, lipid accumulation can be visualized directly in the livers of transparent larvae and chemical agents can be simply added to fish water [13‐15]. These advantages make them a powerful system for studying lipid-related liver disease and assessing the effects of drugs across hundreds of fishes via a relatively simple protocol in vivo.

Zebrafish models of NAFLD are well established by mutagenesis or transgenic technology, chemical and dietary treatment in recent years [6, 8]. Studies have shown that zebrafish develop hepatic steatosis in response to methionine metabolism disorder [16], endoplasmic reticulum stress [17, 18] and aberrant β-hydroxybutyrate transport [19]. Hepatotoxic small molecules, such as thioacetamide, tunicamycin and γ-hexachlorocyclohexane cause an increase of hepatic lipid content [20‐23]. Dietary-induced models of NAFLD in adult fish have involved forced overfeeding or high-fat diet [24‐26]. Based on previous reports, there has been a lack of knowledge about diet-induced steatosis in larval zebrafish. Recently, Sapp et al. have reported that zebrafish larvae treated with fructose exhibited steatosis and inflammation, indicating that zebrafish larvae is a promising model organism to study diet-induced steatosis [27]. Along with an increase use of fructose, a growing consumption of high fat and cholesterol-enriched food, have been implicated in the rise of obesity and the prevalence of NAFLD [28, 29]. Furthermore, various evidences indicate that dietary cholesterol plays a critical role in the development of steatosis and the progression to steatohepatitis [29‐31]. Herein, we fed larvae a high fat diet alone or combined with high cholesterol diet to induce steatosis and determined whether the hepatic manifestations in these fishes were similar to those of humans. In addition, ezetimibe and simvastatin were used to confirm the reliability of HFC diet-induced hepatic steatosis model in larvae. Our results indicate that the zebrafish larvae hepatic steatosis model developed in this study is convenient and low-cost for rapid assessment of anti-steatosis agents.

Here we have demonstrated that HF and HFC diets treatment of zebrafish larvae induced a significant phenotype similar to human steatosis, characterized by excessive lipid droplets deposit in the liver and genes expression changes involved in lipid metabolism and ER stress. Moreover, the incidence and degree of steatosis were more severe in HFC diet-fed larvae compared with the HF diet-fed larvae, suggesting that cholesterol promotes the hepatic lipid accumulation. We then confirmed that steatosis could be ameliorated by ezetimibe and simvastatin treatment, supporting the use of HFC model for testing anti-steatosis drugs. This model will be a low-cost and effective system for studying mechanisms of the pathological processes in early steatosis, as well as for testing novel lipid-regulating agents.

The advantages of zebrafish as a model organism are versatile. Symptomatic fish are generated quickly in large numbers in a single experiment, which provides distinct benefits over rodent models [6, 8]. Previous studies have proved their utility in modeling NAFLD induced by all kinds of mechanisms, but there has been a lack of a tractable model to study diet-induced steatosis in larval zebrafish [16‐26]. Coincidentally, Sapp et al. have recently established a fructose-induced NASH model in 7 dpf larvae, our HFC model of steatosis therefore would be an indispensable complement for fructose model to understand diet-induced steatosis in larval zebrafish [27]. In contrast, our HFC model of steatosis induction was superior and the background steatosis was inferior to fructose model. This might because the food intake was strictly restricted every day and the raising period was relatively longer. Moreover, we observed that adding cholesterol to HF diet promotes the hepatic lipid accumulation by whole-mount oil red O staining. Consistent with the staining results, hepatic FC and TC levels in HFC group were much higher than the control and HF groups. These findings, along with previous studies, demonstrates that dietary cholesterol is a critical factor in the development of steatosis and FC accumulated in the liver is related to the severity of steatosis or liver injury in larvae as well as in mammals [29‐31].

Regarding to the gene changes of zebrafish during HF and HFC diets feeding periods, we found that HF and HFC diet-fed zebrafish possessed higher mRNA levels of genes involved in lipid oxidation (pparab, cpt1a and acox3) and lower mRNA levels of genes involved in lipid synthesis (acaca, fasn, srebf2, hmgcs1 and hmgcra) than the control diet-fed ones, suggesting that excessive fat and cholesterol in diet stimulated the hepatic catabolic pathway and inhibited the anabolic pathway. Furthermore, ER stress pathway which is known to incur steatosis, promote the progression of NASH, and activate the inflammatory signaling was also affected [36, 37]. Here, we found that the expression levels of atf6, hspa5 and hsp90b1 were elevated in the livers of HF and HFC diets-fed larvae compared to the control, indicating that HF and HFC diets lead to the dysfunction of ER. Although 10 days of HF and HFC diets exposure did not lead to a significant elevate in liver inflammation, the expression levels of tnfa, il1b and il6 were remarkable increased in the livers of HF and HFC diets-fed larvae after 20 days of feeding. These data suggested that HF and HFC diets lead to the disorder of lipid metabolism and dysfunction of ER within a relatively short period of time. Moreover, HF and HFC diets-fed larvae have a trend toward the progression of NASH after longer duration of feeding.

The ability to conduct high-throughput drug screening is an outstanding advantage of larvae model for NAFLD studies. Unlike adult fish, lipids can be visualized directly in transparent larvae and juveniles using whole-mount oil red O staining method [13, 38‐41]. This simple method provides a high-throughput means to detect the incidence of steatosis in larvae, enabling us to examine steatosis across a large number of fish. Here, we chose two drugs, ezetimibe and simvastatin, to demonstrate the validity of our model to screen for anti-steatosis drugs. Accumulating evidences indicate that ezetimibe and simvastatin can improve steatosis and NASH in fish models and mammals [42‐44]. In our HFC model, we also found that these two drugs induced a significant decrease lipid accumulation in the liver, which implies the potential of this model to test novel lipid-regulating drugs.

Here, we established a larval zebrafish model of steatosis within a relatively short period maintained on diets. Our HFC diet-fed steatosis model has several advantages. First, diet induced-steatosis most closely resembles the human pathology of NAFLD. HFC diet is not a special diet and easily imitates human dietary habits. Second, zebrafish models of NAFLD in previous studies mostly focused on the stage of early larvae and adult zebrafish, there is lack of late larvae and early juveniles models of NAFLD. In consideration of this, HFC model was useful for providing more information about diet-induced steatosis in the period of late larvae and early juveniles. Third, this model has great potential for exploring the pathogenesis of early steatosis and conducting high-throughput drug screening. Nevertheless, this model has some disadvantages. For example, female fish that rarely mated with males are apt to have offspring with poor quality, which may cause high mortality. Furthermore, multiple serum biochemical parameters are hard to detect because larval zebrafish are too small to draw blood. Additionally, it’s a time-consuming work to dissect the livers from the small fish.

In conclusion, we established a novel larval zebrafish model of hepatic steatosis using HFC diet treatment. This model has great potential for exploring the relationships between diet and host factors that contribute to the pathogenesis of hepatic steatosis and the ability to conduct high-throughput drug screening is crucial for the development of new interventions or therapies. Our model will provide a powerful new tool in the search for new drugs to prevent and treat NAFLD.