2.3. Diet-based animal models
The animals are fed on specific modified diets for a particular duration
which results in cirrhosis (Table 3). The strain of rodents plays a
vital role in inducing disease. It has been observed that C57BL/6 mice
establish hepatic cirrhosis rapidly with the use of a modified diet
(76). These diets are as follows:
2.3.1. Methionine and choline-deficient diet: Methionine is an
amino acid that plays important in wound healing and prevents liver
damage. Choline is a nutrient essential for metabolism, brain function
and helps to treat liver ailments. This diet comprises a sucrose content
of up to 40%, fat content of 10%, and is deficient in methionine and
choline. After regular administration of this diet for 8-10 weeks, mice
and rats develop liver cirrhosis (77, 78). This diet results in
increased fatty acid uptake by adipose tissue, which results in
increased synthesis of triglycerides. An imbalance between the
antioxidant enzymes and free radical production along with the induction
of inflammatory response contributes to the development of liver
cirrhosis (79).
2.3.2. Liebere De Carli liquid diet: It is also called chronic
ethanol feeding and is the first method that involves drinking water
incorporated with ethanol. The liquid diet containing ethanol is given
as a substitute for water and food. Its constituents include 36%
calories of ethanol with a diet concentration of 5% (w/v),
carbohydrates 11%, fat 35%, and proteins 18%. The daily ethanol
intake by the animal is 14-25g/kg of body weight (80, 81). After taking
this diet for 4-12 weeks, mice and rats develop liver cirrhosis. Ethanol
oxidizes via the alcohol dehydrogenase pathway, which affects lipid,
carbohydrate, protein, and purine metabolism. Cell membrane damage
occurs as a result of lipid peroxidation and reactive oxygen species
production by the action of cytochrome P-450 (P-4501IEI) causing liver
fibrosis (82).
Bin Gao et al developed a slight modification in Liebere De Carli liquid
diet containing 5%v/v ethanol continuously for the 10 days, followed by
a single dose of ethanol (5mg/kg) on the 11th day and
euthanized after nine hours for observation. This experiment can be
extended up to 8 weeks for the examination of steatosis, liver cell
damage, liver neutrophil migration, and liver function parameters. Thus
this model could be used for the evaluation of alcohol-induced liver
injury in individuals (83).
2.3.3. High-fat diet: Fats comprise of fatty acid and tri
esters of glycerol. These are required for cell growth and provide
energy to the body. In this model, the rodents are fed on a high-fat
diet. The diet comprises different compositions of nutrients, such as
11% carbohydrates, 71% fat and 18% proteins. The animals fed on this
diet develop liver fibrosis within 2-3 weeks in rats and 9 weeks in mice
(84, 85). High-fat diet generates aberrant mitochondria and fatty acid
influx resulting in the development of insulin resistance and obesity.
The beta-oxidation of fatty acids leads to the secretion of very
low-density lipoproteins along with free radicals causing liver
cirrhosis (86).
The experimental model involves non-alcoholic fatty liver disease
(NAFLD) that involves a diet rich in fats without any nutritional
deficiency. The diet is composed of 45-74% of fat in total diet intake
for the study of hepatic steatosis (87). The C57BL/6 mice are the
specialized rodents that are prone to metabolic syndromes i.e. diabetes
mellitus. 0.1% cholesterol diet along with fructose-rich diet results
in obesity and NAFLD progression in 52 weeks in these animals (88). The
physiological, histological, and cellular signaling pathways can be
easily understood by this experimental model. There is a limitation in
the availability of knockout mice (88).
2.3.4. High fructose diet: In this model, the rodents feed on a
high fructose diet leading to development of hepatic fibrosis. At first,
10% fructose is incorporated in the drinking water. The animals are
allowed to feed on a high content of fructose (up to 70%) and develop
liver cirrhosis within 8 weeks. This method can be used in both mice and
rats. Fructose, after metabolism, induces synthesis of fatty acids in
the liver. As a result, accumulation of lipids takes place in the liver
which leads to cytokine release along with enhanced nodule formation in
hepatocytes inducing liver cirrhosis (89-91).
2.3.5. Ethanol Ad libitum feeding: It is one of the oldest
methods to establish cirrhosis in rodents. It includes incorporation of
alcohol in drinking water but rodents are permitted to feed freely with
a chow diet as per standards. The animal fed on ad libitum diet water
with 20% (v/v) of ethanol for the duration of 8 weeks in mice, and with
40% (v/v) of ethanol for the duration of up to 29 weeks in rats induces
liver cirrhosis.
Experimental alcohol administration in a single bottle, multiple alcohol
containing bottles and drinking in dark models were the established
models for the evaluation of hepatoprotective drugs. The earliest method
of alcohol drinking constituted 5%-20% (v/v) of ethanol supplemented
in place of water for 8-10 weeks. In the multiple bottle model, two
bottles were incorporated, one of distilled water and the other of
5%-40% ethanol for 29 weeks. Hepatotoxicity is reported when rodents
show steatosis, fibrosis and hepatic injury upon acute inflammation and
for histological findings include development of focal necrosis and
perivenular fibrosis in chronic inflammation (92).
Drinking in the dark model involves 20% alcohol consumption for 2-4
hours in mice quarantined singly. This model is compatible with
achieving high blood alcohol concentration i.e. >1.0mg/ml
and is used for mimicking prenatal human binge drinking (93). Despite
the diversity in alcohol drinking models, several hepatic stressors such
as diethyl nitrosamine was used to for progression of hepatic necrosis
to hepatocellular carcinoma (94).
Excessive ethanol drinking raises the expression and activity of
cytochrome P450 (CYP2E1), which enhances acetaldehyde synthesis by
forming reactive oxygen species (ROS). Due to stimulation of
interleukins, there is initiation of the inflammatory response and
oxidant scavenging capacity is reduced resulting in hepatic fibrosis
(95-97).