Obesity is markedly linked to an increased risk of all-cause mortality and its prevalence
has risen to unacceptable levels in the developed world.
1,2
Obesity and insulin resistance compose the core of most cases of metabolic syndrome
(MS), which is a group of conditions and traits associated with an increased risk
of cardiovascular disease and diabetes (approximately 2-fold and 5-fold, respectively).
3
Non-alcoholic fatty liver disease (NAFLD) is the hepatic manifestation of MS and has
gained attention not only for obesity-related disorders but also for the progression
to non-alcoholic steatohepatitis (NASH), liver cirrhosis, and hepatocellular carcinoma.
Similar to the other metabolic components involved in MS, NAFLD treatment is based
on lifestyle changes that are difficult to achieve in clinical trials, making preclinical
studies an excellent option to increase the knowledge about the disease and to test
the interventions proposed.
However, in vitro models have severe limitations to evaluate the hepatic and extrahepatic
findings of human NASH due to its multifactorial etiology.
4
As a result, data obtained from animals are largely assessed, with a growing interest
in the development of mice models. Even if many of these models do not develop steatohepatitis
as the strict definition applied to human liver tissue, they are still an outstanding
source of knowledge about NAFLD and NASH.
Rodent models need to mimic NAFLD regarding their development by diet-induced obesity,
which is deemed the most common risk factor for NAFLD in humans.
5
Above all, the diet given to the mice should resemble human diets regarding macronutrient
composition, not depending on toxins and leading to obesity, insulin resistance and
systemic inflammation.
6
Some of the most used mice models to simulate NAFLD are the methionine and choline-deficient
diet and the choline-deficient L-amino-defined diet, but both have been criticized
because the animals suffer weight loss and do not develop insulin resistance.
7
In addition, the high-cholesterol diet promotes only slight increases in liver weight,
triglyceride levels and serum liver enzymes, at the expense of an unusual amount of
dietary cholesterol (1%) compared to human diets.
8
The Cholesterol and Cholate (CC) diet also require a high amount of cholesterol (1.25%).
It induces inflammation, hepatocellular ballooning, steatosis and fibrosis over 6-24
weeks.
4
Furthermore, additions of 60% fat can shorten the development of NASH to 12 weeks.
9
The model also causes lipid peroxidation, serum lipids increase and oxidative stress.
However, mice submitted to this diet lose weight, have low plasma triglyceride levels
and do not develop insulin resistance.
10
The high-fat diet (71% fat, 18% proteins and 11% carbohydrates) is another interesting
option to simulate NASH, developing insulin resistance and panlobular steatosis, but
it is highly dependent on the animal strain and the diet composition.
4
The high-fructose diet promotes hepatic inflammation and oxidative stress but does
not induce the hepatic findings of NASH when administered ad libitum. Therefore, it
would be expected that a high fat, fructose and cholesterol diet would be an excellent
option to induce NASH by combining all the main components of each models mentioned
above. Nevertheless, some mice models fed with these components do not develop advanced
liver fibrosis.
4
Other models bring some advantages, but they are more expensive and involve more technical
issues.
An additional option to the rodent models of NAFLD is now presented by Muniz et al.,
in which mice were fed with a high-fat diet composed by lard (20%), cholesterol (1%)
and cholate (0.1%). The regimen given to the mice is similar to the CC diet, leading
to dyslipidemia and severe liver damage as seen in the human NASH.
11
The animals not only gained weight but their liver weight was marked increased as
well, reaching 5% of the total body weight. The authors postulated that the use of
cholate could have accelerated the metabolic disturbances induced by the diet. Of
note, the total fat composed only 44% of the dietary energy content, similar to the
amount consumed by obese people (43-55%). Also, the authors obtained significant results
after 6 weeks, whereas similar studies needed 9-12 weeks to achieve their aims.
The histological analysis presented in the article showed pronounced steatosis and
marked inflammation. New studies should be developed in order to evaluate if these
findings would lead to liver fibrosis and other consequences of human NASH. For the
moment, the model presented by Muniz et al.,
11
is a fast and low-cost option to induce NASH in mice by using a diet that resembles
the ones consumed by obese people. Hopefully, this model can bring new information
on NAFLD, NASH and MS, increasing the current knowledge about their pathophysiology
and allowing evaluating new treatments against these metabolic disorders.