Role of Dietary Fructose and Hepatic De Novo Lipogenesis in Fatty Liver Disease

Fructose and NAFLD: The Multifaceted Aspects of Fructose Metabolism

Among various factors, such as an unhealthy diet or a sedentarity lifestyle, excessive fructose consumption is known to favor nonalcoholic fatty liver disease (NAFLD), as fructose is both a substrate and an inducer of hepatic de novo lipogenesis. The present review presents some well-established mechanisms and new clues to better understand the pathophysiology of fructose-induced NAFLD. Beyond its lipogenic effect, fructose intake is also at the onset of hepatic inflammation and cellular stress, such as oxidative and endoplasmic stress, that are key factors contributing to the progression of simple steatosis to nonalcoholic steatohepatitis (NASH). Beyond its hepatic effects, this carbohydrate may exert direct and indirect effects at the peripheral level. Excessive fructose consumption is associated, for example, with the release by the liver of several key mediators leading to alterations in the communication between the liver and the gut, muscles, and adipose tissue and to disease aggravation. These multifaceted aspects of fructose properties are in part specific to fructose, but are also shared in part with sucrose and glucose present in energy–dense beverages and foods. All these aspects must be taken into account in the development of new therapeutic strategies and thereby to better prevent NAFLD.

Non-alcoholic fatty liver disease: an emerging driving force in chronic kidney disease

Non-alcoholic fatty liver disease (NAFLD) is caused by an accumulation of fat in the liver; the condition can progress over time to increase the risk of developing cirrhosis, end-stage liver disease and hepatocellular carcinoma. The prevalence of NAFLD is increasing rapidly owing to the global epidemics of obesity and type 2 diabetes mellitus (T2DM), and NAFLD has been predicted to become the most important indication for liver transplantation over the next decade. It is now increasingly clear that NAFLD not only affects the liver but can also increase the risk of developing extra-hepatic diseases, including T2DM, cardiovascular disease and chronic kidney disease (CKD), which have a considerable impact on health-care resources. Accumulating evidence indicates that NAFLD exacerbates insulin resistance, predisposes to atherogenic dyslipidaemia and releases a variety of proinflammatory factors, prothrombotic factors and profibrogenic molecules that can promote vascular and renal damage. Furthermore, communication or 'crosstalk' between affected organs or tissues in these diseases has the potential to further harm function and worsen patient outcomes, and increasing amounts of evidence point to a strong association between NAFLD and CKD. Whether a causal relationship between NAFLD and CKD exists remains to be definitively established.

Dietary fatty acid metabolism in prediabetes

PURPOSE OF REVIEW

Experimental evidences are strong for a role of long-chain saturated fatty acids in the development of insulin resistance and type 2 diabetes. Ectopic accretion of triglycerides in lean organs is a characteristic of prediabetes and type 2 diabetes and has been linked to end-organ complications. The contribution of disordered dietary fatty acid (DFA) metabolism to lean organ overexposure and lipotoxicity is still unclear, however. DFA metabolism is very complex and very difficult to study in vivo in humans.

RECENT FINDINGS

We have recently developed a novel imaging method using PET with oral administration of 14-R,S-F-fluoro-6-thia-heptadecanoic acid (FTHA) to quantify organ-specific DFA partitioning. Our studies thus far confirmed impaired storage of DFA per volume of fat mass in abdominal adipose tissues of individuals with prediabetes. They also highlighted the increased channeling of DFA toward the heart, associated with subclinical reduction in cardiac systolic and diastolic function in individuals with prediabetes.

SUMMARY

In the present review, we summarize previous work on DFA metabolism in healthy and prediabetic states and discuss these in the light of our novel findings using PET imaging of DFA metabolism. We herein provide an integrated view of abnormal organ-specific DFA partitioning in prediabetes in humans.

Response of genes involved in lipid metabolism in rat epididymal white adipose tissue to different fasting conditions after long-term fructose consumption

There has been much concern regarding the dietary fructose contributes to the development of metabolic syndrome. High-fructose diet changes the expression of genes involved in lipid metabolism. Levels of a number of hepatic lipogenic enzymes are increased by a high-carbohydrate diet in fasted-refed model rats/mice. Both the white adipose tissue (WAT) and the liver play a key role in the maintenance of nutrient homeostasis. Here, the aim of this study was to analyze the expression of key genes related to lipid metabolism in epididymal WAT (eWAT) in response to different fasting condition after long-term chronic fructose consumption. Rats were fed standard chow supplemented with 10% w/v fructose solution for 5 weeks, and killed after chow-fasting and fructose withdrawal (fasting) or chow-fasting and continued fructose (fructose alone) for 14 h. Blood parameters and the expression of genes involved in fatty acid synthesis (ChREBP, SREBP-1c, FAS, SCD1), triglyceride biosynthesis (DGAT-1, DGAT-2) and lipid mobilization (ATGL, HSL) in eWAT were analyzed. In addition, mRNA levels of PPAR-γ, CD36 and LPL were also detected. As expected, fructose alone increased the mRNA expression of FAS, SCD1, and correspondingly decreased ATGL and HSL mRNA levels. However, ChREBP, DGAT-2, ATGL and HSL mRNA levels restored near to normal while FAS and SCD1 tend to basic level under fasting condition. The mRNA expression of SREBP-1c, PPAR-γ and LPL did not changed at any situations but CD36 mRNA decreased remarkably in fructose alone group. In conclusion, these findings demonstrate that genes involved in lipid metabolism in rat eWAT are varied in response to different fasting conditions after long-term fructose consumption.

Consumption of the total Western diet differentially affects the response to green tea in rodent models of chronic disease compared to the AIN93G diet

SCOPE

In pre-clinical studies investigating bioactive components, the efficacy of the bioactive is likely influenced by the basal diet provided to rodents. In this study, we hypothesized that a model bioactive, green tea extract (GTE), would have different effects on colon carcinogenesis, body composition, and lipid metabolism in mice fed a basal diet formulated to promote animal health and growth (AIN93G) as compared to a Western diet that emulates typical American intakes of micro- and macronutrients, the total Western diet (TWD).

METHODS AND RESULTS

Mice were fed either AIN93G or TWD, with or without GTE added to drinking water for 18 weeks. Aberrant crypt foci (ACF) in azoxymethane-initiated mice was nearly three times greater in mice fed TWD compared to AIN93G. Consumption of GTE suppressed ACF development only in mice fed the TWD. Similarly, supplementation with GTE suppressed weight gain and fasted glucose only in mice fed TWD, while GTE suppressed fat mass in mice fed either diet. Irrespective of diet, GTE supplementation increased cecum weight and decreased cecal SCFA concentration.

CONCLUSION

Collectively, these observations indicate that the TWD influences the bioactivity of GTE in rodent models of obesity, metabolism, and carcinogenesis.

Regulatory landscape of AGE-RAGE-oxidative stress axis and its modulation by PPARγ activation in high fructose diet-induced metabolic syndrome

Background

The AGE-RAGE-oxidative stress (AROS) axis is involved in the onset and progression of metabolic syndrome induced by a high-fructose diet (HFD). PPARγ activation is known to modulate metabolic syndrome; however a systems-level investigation looking at the protective effects of PPARγ activation as related to the AROS axis has not been performed.

The aim of this work is to simultaneously characterize multiple molecular parameters within the AROS axis, using samples taken from different body fluids and tissues of a rat model of HFD-induced metabolic syndrome, in the presence or absence of a PPARγ agonist, Rosiglitazone (RGZ).

Methods

Rats were fed with 60% HFD for the first half of the treatment duration (21 days) then continued with either HFD alone or HFD plus RGZ for the second half.

Results

Rats receiving HFD alone showed metabolic syndrome manifestations including hypertension, dyslipidemia, increased glucose levels and insulin resistance, as well as abnormal kidney and inflammatory parameters. Systolic blood pressure, plasma triglyceride and glucose levels, plasma creatinine, and albuminuria were significantly improved in the presence of RGZ. The following molecular parameters of the AROS axis were significantly upregulated in our rat model: carboxymethyl lysine (CML) in urine and liver; carboxyethyl lysine (CEL) in urine; advanced glycation end products (AGEs) in plasma; receptor for advanced glycation end products (RAGE) in liver and kidney; advanced oxidation protein products (AOPP) in plasma; and 4-hydroxynonenal (HNE) in plasma, liver, and kidney. Conversely, with RGZ administration, the upregulation of AOPP and AGEs in plasma, CML and CEL in urine, RAGE in liver as well as HNE in plasma and liver was significantly counteracted/prevented.

Conclusions

Our data demonstrate (i) the systems-level regulatory landscape of HFD-induced metabolic syndrome involving multiple molecular parameters, including HNE, AGEs and their receptor RAGE, and (ii) attenuation of metabolic syndrome by PPARγ modulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12986-016-0149-z) contains supplementary material, which is available to authorized users.

Diets High in Fat or Fructose Differentially Modulate Bone Health and Lipid Metabolism

Diets high in fat or carbohydrates can lead to obesity and diabetes, two interrelated conditions that have been associated with osteoporosis. Here, we contrasted the effects of a high fat (HF) versus fructose-enriched carbohydrate (CH) versus regular chow (SC) diet on bone morphology, fat content and metabolic balance in BALB/cByJ mice over a 15-week period. For 13 weeks, there were no differences in body mass between groups with small differences in the last 2 weeks. Even without the potentially confounding factor of altered body mass and levels of load bearing, HF consumption was detrimental to bone in the distal femur with lower trabecular bone volume fraction and thinner cortices than controls. These differences in bone were accompanied by twofold greater abdominal fat content and fourfold greater plasma leptin concentrations. High-fat feeding caused a decrease in de-novo lipid synthesis in the liver, kidney, white adipose and brown adipose tissue. In contrast to HF, the fructose diet did not significantly impact bone quantity or architecture. Fructose consumption also did not significantly alter leptin levels or de-novo lipid synthesis but reduced epididymal adipose tissue and increased brown adipose tissue. Cortical stiffness was lower in the CH than in HF mice. There were no differences in glucose or insulin levels between groups. Together, a diet high in fat had a negative influence on bone structure, adipose tissue deposition and lipid synthesis, changes that were largely avoided with a fructose-enriched diet.

Liver Fatty Acid Composition and Inflammation in Mice Fed with High-Carbohydrate Diet or High-Fat Diet

Both high-carbohydrate diet (HCD) and high-fat diet (HFD) modulate liver fat accumulation and inflammation, however, there is a lack of data on the potential contribution of carbohydrates and lipids separately. For this reason, the changes in liver fatty acid (FA) composition in male Swiss mice fed with HCD or HFD were compared, at the time points 0 (before starting the diets), and after 7, 14, 28 or 56 days. Activities of stearoyl-CoA desaturase-1 (SCD-1), ∆-6 desaturase (D6D), elongases and de novo lipogenesis (DNL) were estimated. Liver mRNA expression of acetyl-CoA carboxylase 1 (ACC1) was evaluated as an additional indicator of the de novo lipogenesis. Myeloperoxidase activity, nitric oxide (NO) production, and mRNA expressions of F4/80, type I collagen, interleukin (IL)-6, IL-1β, IL-10, and tumor necrosis factor-α (TNF-α) were measured as indication of the liver inflammatory state. The HCD group had more intense lipid deposition, particularly of saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs). This group also showed higher DNL, SCD-1, and D6D activities associated with increased NO concentration, as well as myeloperoxidase activity. Livers from the HFD group showed higher elongase activity, stored more polyunsaturated fatty acids (PUFAs) and had a lower omega-6/omega-3 fatty acid (n-6/n-3) ratio. In conclusion, liver lipid accumulation, fatty acids (FA) composition and inflammation were modulated by the dietary composition of lipids and carbohydrates. The HCD group had more potent lipogenic and inflammatory effects in comparison with HFD.

Long-term fructose consumption prolongs hepatic stearoyl-CoA desaturase 1 activity independent of upstream regulation in rats

Dietary fructose is considered a risk factor for metabolic disorders, such as fatty liver disease. However, the mechanism underlying the effects of fructose is not well characterized. We investigated the hepatic expression of key regulatory genes related to lipid metabolism following fructose feeding under well-defined conditions. Rats were fed standard chow supplemented with 10% w/v fructose solution for 5 weeks, and killed after chow-fasting and fructose withdrawal (fasting) or chow-fasting and continued fructose (fructose alone) for 14 h. Hepatic deposition of triglycerides was found in rats from both groups. As expected, fructose alone increased mRNA levels of lipogenesis-related genes and correspondingly decreased mRNA levels of lipid oxidative genes in the liver. Interesting, hepatic levels of stearoyl-CoA desaturase (SCD)1 mRNA remained elevated under fructose withdrawn conditions, although expression levels of other genes, including two key transcription factors (carbohydrate response element binding protein (ChREBP) and sterol regulatory element-binding protein (SREBP)-1c) fell to normal levels, indicating that long-term fructose intake increased SCD1 activity, independent of upstream regulatory genes, such as ChREBP and SREBP-1c. In conclusion, SCD1 overexpression in fatty liver disease is not affected by fasting after long-term fructose consumption in rats. Regulation of SCD1 plays an important role in fructose-induced hepatic steatosis.

Clinical assessment of hepatic de novo lipogenesis in non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) is heralded as the next big global epidemic. Hepatic de novo lipogenesis (DNL), the synthesis of new fatty acids from non-lipid sources, is thought to play a pivotal role in the development of NAFLD. While there is currently no NAFLD-specific therapeutic agent available, pharmaceutical drugs aimed at reducing hepatic fat accretion may prove to be a powerful ally in the treatment and management of this disease. With a focus on NAFLD, the present review summarizes current techniques examining DNL from a clinical perspective, and describes the merits and limitations of three commonly used assays; stable-label isotope tracer studies, fatty acid indexes and indirect calorimetry as non-invasive measures of hepatic DNL. Finally, the application of DNL assessments in the pharmacological and nutraceutical treatment of NAFLD/NASH is summarized. In a clinical research setting, measures of DNL are an important marker in the development of anti-NAFLD treatments.

The role of the gut microbiota in NAFLD

NAFLD is now the most common cause of liver disease in Western countries. This Review explores the links between NAFLD, the metabolic syndrome, dysbiosis, poor diet and gut health. Animal studies in which the gut microbiota are manipulated, and observational studies in patients with NAFLD, have provided considerable evidence that dysbiosis contributes to the pathogenesis of NAFLD. Dysbiosis increases gut permeability to bacterial products and increases hepatic exposure to injurious substances that increase hepatic inflammation and fibrosis. Dysbiosis, combined with poor diet, also changes luminal metabolism of food substrates, such as increased production of certain short-chain fatty acids and alcohol, and depletion of choline. Changes to the microbiome can also cause dysmotility, gut inflammation and other immunological changes in the gut that might contribute to liver injury. Evidence also suggests that certain food components and lifestyle factors, which are known to influence the severity of NAFLD, do so at least in part by changing the gut microbiota. Improved methods of analysis of the gut microbiome, and greater understanding of interactions between dysbiosis, diet, environmental factors and their effects on the gut-liver axis should improve the treatment of this common liver disease and its associated disorders.

New Concepts on Pathogenesis and Diagnosis of Liver Fibrosis; A Review Article

Liver fibrosis is a potentially reversible response to hepatic insults, triggered by different chronic diseases most importantly viral hepatitis, alcoholic, and nonalcoholic fatty liver disease. In the course of the chronic liver disease, hepatic fibrogenesis may develop, which is attributed to various types of cells, molecules, and pathways. Activated hepatic stellate cell (HSC), the primary source of extracellular matrix (ECM), is fundamental in pathophysiology of fibrogenesis, and thus is the most attractable target for reversing liver fibrosis. Although, liver biopsy has long been considered as the gold standard for diagnosis and staging of hepatic fibrosis, assessing progression and regression by biopsy is hampered by its limitations. We provide recent views on noninvasive approaches including serum biomarkers and radiologic techniques.