New insights into the role of iron in the development of nonalcoholic fatty liver disease

High omega arachidonic acid/docosahexaenoic acid ratio induces mitochondrial dysfunction and altered lipid metabolism in human hepatoma cells

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is a common cause of liver disease worldwide and is a growing epidemic. A high ratio of omega-6 fatty acids to omega-3 fatty acids in the diet has been implicated in the development of NAFLD. However, the inflicted cellular pathology remains unknown. A high ratio may promote lipogenic pathways and contribute to reactive oxygen species (ROS)-mediated damage, perhaps leading to mitochondrial dysfunction. Therefore, these parameters were investigated to understand their contribution to NAFLD development.

AIM

To examine the effect of increasing ratios of omega-6:3 fatty acids on mitochondrial function and lipid metabolism mediators.

METHODS

HepG2-derived VL-17A cells were treated with normal (1:1, 4:1) and high (15:1, 25:1) ratios of omega-6: omega-3 fatty acids [arachidonic acid (AA): docosahexaenoic acid (DHA)] at various time points. Mitochondrial activity and function were examined via MTT assay and Seahorse XF24 analyzer, respectively. Triglyceride accumulation was determined by using EnzyChrom™ and levels of ROS were measured by fluorescence intensity. Protein expression of the mediators of lipogenic, lipolytic and endocannabinoid pathways was assessed by Western blotting.

RESULTS

High AA:DHA ratio decreased mitochondrial activity (P < 0.01; up to 80%) and promoted intracellular triglyceride accumulation (P < 0.05; 40%-70%). Mechanistically, it altered the mediators of lipid metabolism; increased the expression of stearoyl-CoA desaturase (P < 0.05; 22%-35%), decreased the expression of peroxisome proliferator-activated receptor-alpha (P < 0.05; 30%-40%) and increased the expression of cannabinoid receptor 1 (P < 0.05; 31%). Furthermore, the high ratio increased ROS production (P < 0.01; 74%-115%) and reduced mitochondrial respiratory functions such as basal and maximal respiration, ATP production, spare respiratory capacity and proton leak (P < 0.01; 35%-68%).

CONCLUSION

High AA:DHA ratio induced triglyceride accumulation, increased oxidative stress and disrupted mitochondrial functions. Stimulation of lipogenic and steroidal transcription factors may partly mediate these effects and contribute to NAFLD development.

Control of oxidative stress in hepatocellular carcinoma: Helpful or harmful?

Oxidative stress is becoming recognized as a key factor in the progression of chronic liver disease (CLD) and hepatocarcinogenesis. The metabolically important liver is a major reservoir of mitochondria that serve as sources of reactive oxygen species, which are apparently responsible for the initiation of necroinflammation. As a result, CLD could be a major inducer of oxidative stress. Chronic hepatitis C is a powerful generator of oxidative stress, causing a high rate of hepatocarcinogenesis among patients with cirrhosis. Non-alcoholic steatohepatitis is also associated with oxidative stress although its hepatocarcinogenic potential is lower than that of chronic hepatitis C. Analyses of serum markers and histological findings have shown that hepatocellular carcinoma correlates with oxidative stress and experimental data indicate that oxidative stress increases the likelihood of developing hepatocarcinogenesis. However, the results of antioxidant therapy have not been favorable. Physiological oxidative stress is a necessary biological response, and thus adequate control of oxidative stress and a balance between oxidative and anti-oxidative responses is important. Several agents including metformin and L-carnitine can reportedly control mechanistic oxidative stress. This study reviews the importance of oxidative stress in hepatocarcinogenesis and of control strategies for the optimal survival of patients with CLD and hepatocellular carcinoma.

Interactions between hepatic iron and lipid metabolism with possible relevance to steatohepatitis

The liver is an important site for iron and lipid metabolism and the main site for the interactions between these two metabolic pathways. Although conflicting results have been obtained, most studies support the hypothesis that iron plays a role in hepatic lipogenesis. Iron is an integral part of some enzymes and transporters involved in lipid metabolism and, as such, may exert a direct effect on hepatic lipid load, intrahepatic metabolic pathways and hepatic lipid secretion. On the other hand, iron in its ferrous form may indirectly affect lipid metabolism through its ability to induce oxidative stress and inflammation, a hypothesis which is currently the focus of much research in the field of non-alcoholic fatty liver disease/non-alcoholic steatohepatitis (NAFLD/NASH). The present review will first discuss how iron might directly interact with the metabolism of hepatic lipids and then consider a new perspective on the way in which iron may have a role in the two hit hypothesis for the progression of NAFLD via ferroportin and the iron regulatory molecule hepcidin. The review concludes that iron has important interactions with lipid metabolism in the liver that can impact on the development of NAFLD/NASH. More defined studies are required to improve our understanding of these effects.

Hepatic iron stores are increased as assessed by magnetic resonance imaging in a Chinese population with altered glucose homeostasis

BACKGROUND

Emerging scientific evidence has disclosed a correlation between iron metabolism and type 2 diabetes (T2D).

OBJECTIVE

The objective of this study was to test the hypothesis that body iron stores are higher in a Chinese population with altered glucose homeostasis.

DESIGN

Serum iron, ferritin, and soluble transferrin receptor concentrations were measured in 298 subjects, including 70 subjects with normal glucose tolerance (NGT group), 60 subjects with prediabetes (prediabetes group), and 168 subjects with T2D (T2D group). Hepatic iron stores in 88 subjects were assessed by using a magnetic resonance imaging (MRI) T2* gradient-recalled-echo technique. A general linear model ANOVA was performed for comparisons between groups after adjustment for age and BMI. Stepwise multiple linear regression analysis was used to identify factors associated with the MRI-estimated hepatic iron concentration (M-HIC).

RESULTS

Mean (±SD) M-HIC and R2* values in the prediabetes and T2D groups were significantly higher than in the NGT group (M-HIC: 40.6 ± 8.6 and 39.3 ± 10.7 μmol/g compared with 27.8 ± 9.1 μmol/g; R2* values: 47.9 ± 11.9 and 47.3 ± 11.5 s(-1) compared with 34.9 ± 7.0 s(-1); all P < 0.01). No significant difference was shown in M-HIC and R2* values between prediabetes and T2D groups. The M-HIC independently contributed to 43.3% of the glycated hemoglobin variance after adjustment for main clinical indexes (P < 0.001). The proportions of subjects with mild hepatic iron overload in the NGT, prediabetes, and T2D groups were 12.5%, 70.6%, and 63.6%, respectively.

CONCLUSIONS

To our knowledge, our findings provide novel evidence to support the hypothesis of a mild iron overload in patients with prediabetes and T2D. A cohort study concerned with the effect of the attenuation of excess iron on glucose metabolism in a prediabetic population is warranted.

Non-alcoholic fatty liver disease: is iron relevant?

Non-alcoholic fatty liver disease (NAFLD) is a common and ubiquitous disorder (Bedogni et al. in Hepatology 42:44-52, 2005; Bellentani et al. in Ann Intern Med 132:112-117, 2000) which in a proportion of subjects leads to non-alcoholic steatohepatitis (NASH), advanced liver disease and hepatocellular carcinoma. Although the factors responsible for progression of disease are still uncertain, there is evidence that insulin resistance (IR) is a key operative mechanism (Angulo et al. in Hepatology 30:1356-1362, 1999) and that two stages are involved. The first is the accumulation of triglycerides in hepatocytes followed by a "second hit" which promotes cellular oxidative stress. Several factors may be responsible for the induction of oxidative stress but hepatic iron has been implicated in various studies. The topic is controversial, however, with early studies showing an association between hepatic iron (with or without hemochromatosis gene mutations) and the progression to hepatic fibrosis. Subsequent studies, however, could not confirm an association between the presence of hepatic iron and any of the histological determinants of NAFLD or NASH. Recent studies have reactivated interest in this subject firstly, with the demonstration that hepatic iron loading increases liver cholesterol synthesis with increased lipid deposition in the liver increasing the cellular lipid burden and secondly, a large clinical study has concluded that hepatocellular iron deposition is associated with an increased risk of hepatic fibrosis, thus, strongly supporting the original observation made over a decade ago. An improvement in insulin sensitivity has been demonstrated following phlebotomy therapy but a suitably powered controlled clinical trial is required before this treatment can be implemented.