Introduction

Hyperlipidic–hypercaloric diets are now frequently consumed in modern societies, coupled with low levels of physical activity. These factors contribute importantly to the development of pathological conditions, which include obesity, hypertension, insulin resistance, dyslipidemia, and liver diseases. The liver is a central player in the physiological regulation of whole-body energy homeostasis as well as in the pathogenesis of the epidemiologically relevant metabolic disorders, such as obesity and diabetes. In particular, chronic dietary overload with fructose and saturated fatty acids, typical of western societies, will enhance accumulation of lipid metabolites and oxidative stress in liver [1–3]. The ectopic fat accumulation in liver is tightly associated with the development of insulin resistance [4, 5]. In fact, hepatic accumulation of diacylglycerol (DAG) and ceramide, as well as DAG-induced activation of protein kinase C, can impair insulin signaling, most notably at the level of the insulin receptor substrates [6–9].

The rate at which fat accumulates in tissues is determined by several factors, such as the rate of lipid uptake from the circulation and the utilization of lipids within the tissues [10]. Mitochondria are the main cellular sites devoted to fatty acid oxidation. For these reasons, a role for mitochondrial dysfunction in the onset of insulin resistance has been proposed and a number of studies have dealt with possible alteration in mitochondrial function in obesity and diabetes, both in humans and animal models, using different experimental paradigms and with different approaches. In particular, it should be taken into account that the mitochondrial oxidation of metabolic fuels depends on organelle number, organelle activity, and energetic efficiency of the mitochondrial machinery in synthesizing ATP from the oxidation of fuels. Therefore, the main goal of this review was to collect and analyze the available data on liver mitochondria and diet-induced obesity with the focus on all the three above parameters. Search in PubMed of relevant articles from 2003 to 2014 was conducted, by using query “liver mitochondria and obesity” “hepatic mitochondria and obesity” “liver mitochondria and high fat diet” and “hepatic mitochondria and high fat diet” with the inclusion of related articles by the same groups. A summary of the obtained results is presented in Tables 1 and 2.

Table 1 Summary of data on mitochondrial function and insulin resistance/high-fat diet
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Table 2 Summary of diet-induced changes in liver mitochondrial compartment, oxidative stress, and insulin sensitivity
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Mitochondrial functionality and insulin resistance

Mitochondria are key organelles in energy metabolism, especially in tissues with high metabolic activity, such as liver. Therefore, mitochondrial dysfunction could play an important role in the pathogenesis of the metabolic disorders.

Mitochondrial production of reactive oxygen species (ROS) is frequently reported to be increased in the physiopathology of insulin resistance and could cause damage to cellular macromolecules, thus causing damage to cellular structures, including mitochondria. Mitochondrial impairment would lead to a decreased oxidative capacity, thus favoring intracellular lipid storage that is considered a key player in the development of insulin resistance.

An alternative pathway leading to hepatic insulin resistance could involve an elevated β-oxidation [11–14], that is viewed as an adaptive mechanism, but provides large amounts of reduced equivalents (NADH, H+ and FADH2) and electrons to the respiratory chain regardless of the ATP demand. Thus, oxidative phosphorylation would be unbalanced, promoting successively increasing ROS production, mitochondrial and cellular damages, and reduction of insulin signal transduction.

Several works, by using different physiological approaches, have investigated the relationship between onset of insulin resistance and mitochondrial functioning in liver, that exerts a deep impact on glucose homeostasis. These studies have included measurements of mitochondrial mass and function and are based on assessment of mitochondrial membrane potential, proton leak kinetics, mitochondrial content by ultrastructural observations, citrate synthase activity, ratio of mitochondrial relative to nuclear DNA, polarographic determination of oxygen consumption rates, enzyme activities of mitochondrial respiratory complexes I–V, markers of oxidative stress such as lipid peroxidation products, and antioxidant capacity such as superoxide dismutase specific activity [15–22]. The results that have been obtained report either decreased, unchanged or even increased hepatic mitochondrial function and oxidative phosphorylation capacity in insulin resistant states.

Mitochondrial functionality and high-fat diet

In rats fed a high-fat diet, a loss of cristae in hepatic mitochondria has been shown, as well as swollen mitochondria and decreased matrix density [23, 24]. When considering mitochondrial respiration and ATP production, the activity assessed in isolated liver mitochondria from rats exposed to a high-fat diet has been found either decreased [19], unchanged [25] or even increased [26] when compared with controls. Several discrepancies exist regarding possible high-fat diet-induced changes in mitochondrial respiratory chain activity and respiration. Cytochrome oxidase activity has been found increased [27], unchanged [25, 28] or reduced [18, 29]. State 3 oxygen consumption in isolated liver mitochondria with different substrates was either reported as unchanged [25, 28, 30–32], increased [33, 34] or significantly reduced [18, 29, 35–37].

Mitochondria generate most of the energy used by cells, and the efficiency with which ATP is synthesized by mitochondrial oxidative phosphorylation is dependent on mitochondrial coupling. Excess in energy intake and/or high degree of mitochondrial coupling cause an increase in proton motive force to a maximum, with respiratory complexes that become highly reduced and may release electrons directly to oxygen resulting in a higher ROS production, thus altering cell functioning and leading to several pathologies. Therefore, it appears important to study the link between mitochondria and insulin resistance with a focus on the degree of mitochondrial coupling. Again, when examining the results obtained in literature, the efficiency of oxidative phosphorylation and the mitochondrial membrane potential were either increased [28] or unchanged [19, 25, 33, 38].

To gain more insight into this link, we have used a rat model that displays several correlates human obesity [39]. In these high-fat fed rats, the combination of slight increases in metabolisable energy intake and slight decreases in energy expenditure resulted in positive energy balance that cumulated over the 7-week study period to result in marked increases in body energy gain and lipid gain; the latter resulting in part from an increase in metabolic efficiency [39]. Other metabolic characteristics that resembles human obesity in the above rats fed a high-fat diet for 7 weeks are insulin resistance and hepatic steatosis [39], a common complication of diet-induced obesity [40]. Since mitochondria are the major cellular site involved in fatty acid metabolism and the main source of ROS, they could play a key role in ectopic fat storage and related complications. An increase in mitochondrial protein mass together with a significant decrease in State 3 respiratory capacities were found in rats fed high-fat diet [39]. These modifications of mitochondrial compartment are similar to those found in response to aging [41] and oxidative stress [42]. The results strongly suggest that high-fat feeding causes an early onset of mitochondrial decay in adult rats. An additional mechanism that can regulate mitochondrial energy production is the degree of coupling of oxidative phosphorylation, which in turn depends on mitochondrial inner membrane permeability to protons (proton leak). The significant decrease in proton leak exhibited by mitochondria from rats fed a high-fat diet suggests an increase in mitochondrial coupling in this condition. When mitochondrial coupling is higher, less substrates need to be burned to obtain a given amount of ATP, with a following decrease in liver ability to fatty acids delivered from the blood. In addition, in coupled mitochondria, an increase in the production of ROS by the respiratory chain could take place, since one of the postulated roles for mitochondrial proton leak is to maintain membrane potential below the critical threshold for ROS production [43]. The results showed an increased oxidative damage in rats fed a high-fat diet, with no compensatory increase in antioxidant by SOD activity [39], a condition that contributes to the development of insulin resistance and hepatic disease [44].

Similar alterations in hepatic mitochondrial protein mass, capacity and degree of oxidative stress were found when rats fed a high-fat diet for only 2 weeks [45, 46], at a time point when no alteration of insulin signaling could be detected [45, 46]. Accordingly, in hyperphagic obese OLETF rats it has been found that hepatic mitochondrial dysfunction precedes the development of insulin resistance and hepatic steatosis [47]. Taken together, these results suggest that the above modifications of mitochondrial compartment in liver precede and can contribute to the subsequent development of insulin resistance.

Mitochondrial functionality and fructose-rich diet

Diet-induced obesity and insulin resistance can also be elicited by fructose-rich diets. In fact, dietary fructose intake has risen considerably in the last decades due to increase in the consumption of pre-packaged foods, soft drinks and juice beverages containing sucrose or high-fructose corn syrup [48–50]. In addition, the 25 % increase in fructose consumption over the past 30 years coincides closely with the increase in the prevalence of obesity [48–50] and in the risk of diabetes, cardiometabolic disease and gout, as well as with lipid disturbances [51]. In humans it is difficult to assess the contribution of fructose intake alone to the development of the above metabolic disorders, since, in everyday life, additional factors are involved, such as hypercaloric diet rich in saturated fat and low physical activity. For these reasons, animal models could help to shed light on the role of dietary fructose on excessive lipid depots and correlated metabolic diseases. Following long-term intake of a fructose-rich low fat diet in adult, sedentary rats we have found several metabolic derangements typical of human obesity, such as increased body lipids [52], ectopic lipid deposition and altered insulin sensitivity [53]. In addition, a stimulation of whole-body and hepatic net de novo lipogenesis contributes to excess lipid accumulation in our fructose-fed rats. This latter result fits well with literature data showing an increased de novo lipogenesis in humans after long-term fructose feeding [54, 55] as well as with several animal studies reporting that hepatic de novo lipogenesis is stimulated by fructose intake, alone [56, 57] or in combination with high dietary fat [46]. On the other hand, the effect of fat feeding on hepatic de novo lipogenesis is less clear, since this metabolic pathway has been found reduced [56–60] or even increased [61–63].

Using our animal model to study the metabolic effects of fructose, we have also found alteration in hepatic mitochondrial energetics [64]. Respiratory capacities evaluated in isolated liver mitochondria were found unchanged, while ATP needed for biosynthetic pathways is obtained at a lower cost, since hepatic mitochondria display increased degree of coupling and are less responsive to the uncoupling effect of fatty acids. Higher coupling efficiency implies lower fuel burning that could partly explain the higher body lipids found in fructose-fed rats. Another unwanted consequence of the increased degree of coupling is higher ROS production, and in fact, hepatic mitochondria showed signs of oxidative damage, both in the lipid and in the protein component, together with decreased activity of SOD, one of the enzymatic component of the antioxidant system. After short-term feeding with a fructose-rich diet, at variance with the results obtained with high-fat diet [45, 46], other groups have found reduced insulin sensitivity [65], together with increased oxidative stress in liver [66], in face of unchanged mitochondrial capacity [67]. Interestingly, higher hepatic mitochondrial efficiency and oxidative damage have also been found in rats fed high-fat diet [39], indicating similar effects of fructose-rich or high-fat diet, while the two diets exhibit a different effect on mitochondrial oxidative capacity (Table 2).

Conclusions

It is clear that hepatic mitochondrial function is impaired by high-fat or high-fructose feeding. Since mitochondria are the main producers of both cellular energy and free radicals, dysfunctional mitochondria can play an important role in the development of insulin resistance and ectopic fat storage in the liver, thus supporting the emerging idea that mitochondrial dysfunction could be closely related to the development of metabolic diseases, such as obesity, type 2 diabetes mellitus and non-alcoholic steatohepatitis.