Endogenous fructose production and metabolism in the liver contributes to the development of metabolic syndrome

Phylogenetic articulation of uric acid evolution in mammals and how it informs a therapeutic uricase

The role of uric acid during primate evolution has remained elusive ever since it was discovered over 100 years ago that humans have unusually high levels of the small molecule in our serum. It has been difficult to generate a neutral or adaptive explanation in part because the uricase enzyme evolved to become a pseudogene in apes thus masking typical signals of sequence evolution. Adding to the difficulty is a lack of clarity on the functional role of uric acid in apes. One popular hypothesis proposes that uric acid is a potent antioxidant that increased in concentration to compensate for the lack of vitamin C synthesis in primate species ∼65 million years ago (Mya). Here, we have expanded on our previous work with resurrected ancient uricase proteins to better resolve the reshaping of uricase enzymatic activity prior to ape evolution. Our results suggest that the pivotal death-knell to uricase activity occurred between 20-30 Mya despite small sequential modifications to its catalytic efficiency for the tens of millions of years since primates lost their ability to synthesize vitamin C, and thus the two appear uncorrelated. We also use this opportunity to demonstrate how molecular evolution can contribute to biomedicine by presenting ancient uricases to human immune cells that assay for innate reactivity against foreign antigens. A highly stable and highly catalytic ancient uricase is shown to elicit a lower immune response in more human haplotypes than other uricases currently in therapeutic development.

Alternative Dietary Patterns for Americans: Low-Carbohydrate Diets

The decades-long dietary experiment embodied in the Dietary Guidelines for Americans (DGA) focused on limiting fat, especially saturated fat, and higher carbohydrate intake has coincided with rapidly escalating epidemics of obesity and type 2 diabetes (T2D) that are contributing to the progression of cardiovascular disease (CVD) and other diet-related chronic diseases. Moreover, the lack of flexibility in the DGA as it pertains to low carbohydrate approaches does not align with the contemporary trend toward precision nutrition. We argue that personalizing the level of dietary carbohydrate should be a high priority based on evidence that Americans have a wide spectrum of metabolic variability in their tolerance to high carbohydrate loads. Obesity, metabolic syndrome, and T2D are conditions strongly associated with insulin resistance, a condition exacerbated by increased dietary carbohydrate and improved by restricting carbohydrate. Low-carbohydrate diets are grounded across the time-span of human evolution, have well-established biochemical principles, and are now supported by multiple clinical trials in humans that demonstrate consistent improvements in multiple established risk factors associated with insulin resistance and cardiovascular disease. The American Diabetes Association (ADA) recently recognized a low carbohydrate eating pattern as an effective approach for patients with diabetes. Despite this evidence base, low-carbohydrate diets are not reflected in the DGA. As the DGA Dietary Patterns have not been demonstrated to be universally effective in addressing the needs of many Americans and recognizing the lack of widely available treatments for obesity, metabolic syndrome, and T2D that are safe, effective, and sustainable, the argument for an alternative, low-carbohydrate Dietary Pattern is all the more compelling.

Fructose and fructose kinase in cancer and other pathologies

Fructose metabolism and fructose kinase KHK-C/A are key factors in the development of lipid oversynthesis-promoted metabolic disorders and cancer. Here, we summarize and discuss the current knowledge about the specific features of fructose metabolism and the distinct roles of KHK-C and KHK-A in metabolic liver diseases and their relevant metabolic disorders and cancer, and we highlight the specific protein kinase activity of KHK-A in tumor development. In addition, different approaches that have been used to inhibit KHK and the exploration of KHK inhibitors in clinical treatment are introduced.

Diabetes-Independent Retinal Phenotypes in an Aldose Reductase Transgenic Mouse Model

Aldose reductase (AR), the first and rate-limiting enzyme of the polyol pathway, has been implicated in the onset and development of the ocular complications of diabetes, including cataracts and retinopathy. Despite decades of research conducted to address possible mechanisms, questions still persist in understanding if or how AR contributes to imbalances leading to diabetic eye disease. To address these questions, we created a strain of transgenic mice engineered for the overexpression of human AR (AR-Tg). In the course of monitoring these animals for age-related retinal phenotypes, we observed signs of Müller cell gliosis characterized by strong immunostaining for glial fibrillary acidic protein. In addition, we observed increased staining for Iba1, consistent with an increase in the number of retinal microglia, a marker of retinal inflammation. Compared to age-matched nontransgenic controls, AR-Tg mice showed an age-dependent loss of Brn3a-positive retinal ganglion cells and an associated decrease in PERG amplitude. Both RGC-related phenotypes were rescued in animals treated with Sorbinil in drinking water. These results support the hypothesis that increased levels of AR may be a risk factor for structural and functional changes known to accompany retinopathy in humans.

Fructose and the Liver

Chronic diseases represent a major challenge in world health. Metabolic syndrome is a constellation of disturbances affecting several organs, and it has been proposed to be a liver-centered condition. Fructose overconsumption may result in insulin resistance, oxidative stress, inflammation, elevated uric acid levels, increased blood pressure, and increased triglyceride concentrations in both the blood and liver. Non-alcoholic fatty liver disease (NAFLD) is a term widely used to describe excessive fatty infiltration in the liver in the absence of alcohol, autoimmune disorders, or viral hepatitis; it is attributed to obesity, high sugar and fat consumption, and sedentarism. If untreated, NAFLD can progress to nonalcoholic steatohepatitis (NASH), characterized by inflammation and mild fibrosis in addition to fat infiltration and, eventually, advanced scar tissue deposition, cirrhosis, and finally liver cancer, which constitutes the culmination of the disease. Notably, fructose is recognized as a major mediator of NAFLD, as a significant correlation between fructose intake and the degree of inflammation and fibrosis has been found in preclinical and clinical studies. Moreover, fructose is a risk factor for liver cancer development. Interestingly, fructose induces a number of proinflammatory, fibrogenic, and oncogenic signaling pathways that explain its deleterious effects in the body, especially in the liver.

Neural mechanisms underlying the role of fructose in overfeeding

Fructose consumption has been linked with metabolic syndrome and obesity. Fructose-based sweeteners like high fructose corn syrup taste sweeter, improve food palatability, and are increasingly prevalent in our diet. The increase in fructose consumption precedes the rise in obesity and is a contributing driver to the obesity epidemic worldwide. The role of dietary fructose in obesity can be multifactorial by promoting visceral adiposity, hypertension, and insulin resistance. Interestingly, one emergent finding from human and animal studies is that dietary fructose promotes overfeeding. As the brain is a critical regulator of food intake, we reviewed the evidence that fructose can act in the brain and elucidated the major brain systems underlying fructose-induced overfeeding. We found that fructose acts on multiple interdependent brain systems to increase orexigenic drive and the incentive salience of food while decreasing the latency between food bouts and reducing cognitive control to disinhibit feeding. We concluded that the collective actions of fructose may promote feeding behavior by producing a hunger-like state in the brain.

Endogenous Fructose Metabolism Could Explain the Warburg Effect and the Protection of SGLT2 Inhibitors in Chronic Kidney Disease

Chronic low-grade inflammation underlies the pathogenesis of non-communicable diseases, including chronic kidney diseases (CKD). Inflammation is a biologically active process accompanied with biochemical changes involving energy, amino acid, lipid and nucleotides. Recently, glycolysis has been observed to be increased in several inflammatory disorders, including several types of kidney disease. However, the factors initiating glycolysis remains unclear. Added sugars containing fructose are present in nearly 70 percent of processed foods and have been implicated in the etiology of many non-communicable diseases. In the kidney, fructose is transported into the proximal tubules via several transporters to mediate pathophysiological processes. Fructose can be generated in the kidney during glucose reabsorption (such as in diabetes) as well as from intra-renal hypoxia that occurs in CKD. Fructose metabolism also provides biosynthetic precursors for inflammation by switching the intracellular metabolic profile from mitochondrial oxidative phosphorylation to glycolysis despite the availability of oxygen, which is similar to the Warburg effect in cancer. Importantly, uric acid, a byproduct of fructose metabolism, likely plays a key role in favoring glycolysis by stimulating inflammation and suppressing aconitase in the tricarboxylic acid cycle. A consequent accumulation of glycolytic intermediates connects to the production of biosynthetic precursors, proteins, lipids, and nucleic acids, to meet the increased energy demand for the local inflammation. Here, we discuss the possibility of fructose and uric acid may mediate a metabolic switch toward glycolysis in CKD. We also suggest that sodium-glucose cotransporter 2 (SGLT2) inhibitors may slow the progression of CKD by reducing intrarenal glucose, and subsequently fructose levels.

Lean NAFLD: an underrecognized and challenging disorder in medicine

Classically, Non-Alcoholic Fatty Liver Disease (NAFLD) has been thought to be driven by excessive weight gain and obesity. The overall greater awareness of this disorder has led to its recognition in patients with normal body mass index (BMI). Ongoing research has helped to better understand potential causes of Lean NAFLD, the risks for more advanced disease, and potential therapies. Here we review the recent literature on prevalence, risk factors, severity of disease, and potential therapeutic interventions.

Fructose and metabolic diseases: too much to be good

ABSTRACT

Excessive consumption of fructose, the sweetest of all naturally occurring carbohydrates, has been linked to worldwide epidemics of metabolic diseases in humans, and it is considered an independent risk factor for cardiovascular diseases. We provide an overview about the features of fructose metabolism, as well as potential mechanisms by which excessive fructose intake is associated with the pathogenesis of metabolic diseases both in humans and rodents. To accomplish this aim, we focus on illuminating the cellular and molecular mechanisms of fructose metabolism as well as its signaling effects on metabolic and cardiovascular homeostasis in health and disease, highlighting the role of carbohydrate-responsive element-binding protein in regulating fructose metabolism.

Fructose and Uric Acid as Drivers of a Hyperactive Foraging Response: A Clue to Behavioral Disorders Associated with Impulsivity or Mania?

Several behavioral disorders, including attention deficit hyperactivity disorder (ADHD), bipolar disorder, and aggressive behaviors are linked with sugar intake and obesity. The reason(s) for this association has been unclear. Here we present a hypothesis supporting a role for fructose, a component of sugar and high fructose corn syrup (HFCS), and uric acid (a fructose metabolite), in increasing the risk for these behavioral disorders. Recent studies have shown that the reason fructose intake is strongly associated with development of metabolic syndrome is that fructose intake activates an evolutionary-based survival pathway that stimulates foraging behavior and the storage of energy as fat. While modest intake may aid animals that would like to store fat as a protective response from food shortage or starvation, we propose that high intake of sugar and HFCS causes a hyperactive foraging response that stimulates craving, impulsivity, risk taking and aggression that increases the risk for ADHD, bipolar disease and aggressive behavior. High glycemic carbohydrates and salty foods may also contribute as they can be converted to fructose in the body. Some studies suggest uric acid produced during fructose metabolism may mediate some of these effects. Chronic stimulation of the pathway could lead to desensitization of hedonic responses and induce depression. In conclusion, a hyperactive foraging response driven by high glycemic carbohydrates and sugars may contribute to affective disorders.

Suppression of epithelial to mesenchymal transition markers in mouse lens by a Smad7-based recombinant protein

Cataracts, a clouding of the eye lens, are a leading cause of visual impairment and are responsible for one of the most commonly performed surgical procedures worldwide. Although generally safe and effective, cataract surgery can lead to a secondary lens abnormality due to transition of lens epithelial cells to a mesenchymal phenotype (EMT) and opacification of the posterior lens capsular bag. Occurring in up to 40% of cataract cases over time, posterior capsule opacification (PCO) introduces additional treatment costs and reduced quality of life for patients. Studies have shown that PCO pathogenesis is driven in part by TGF-β, signaling through the action of the family of Smad coactivators to effect changes in gene transcription. In the present study, we evaluated the ability of Smad-7, a well characterized inhibitor of TGF-β -mediated Smad signaling, to suppress the EMT response in lens epithelial cells associated with PCO pathogenesis. Treatment of lens epithelial cells with a cell-permeable form of Smad7 variant resulted in suppressed expression of EMT markers such as alpha smooth muscle actin and fibronectin. A single application of cell-permeable Smad7 variant in the capsular bag of a mouse cataract surgery model resulted in suppression of gene transcripts encoding alpha smooth muscle actin and fibronectin. These results point to Smad7 as a promising biotherapeutic agent for prevention or substantial reduction in the incidence of PCO following cataract surgery.

Macronutrient Determinants of Obesity, Insulin Resistance and Metabolic Health

Obesity caused by the overconsumption of calories has increased to epidemic proportions. Insulin resistance is often associated with an increased adiposity and is a precipitating factor in the development of cardiovascular disease, type 2 diabetes, and altered metabolic health. Of the various factors contributing to metabolic impairments, nutrition is the major modifiable factor that can be targeted to counter the rising prevalence of obesity and metabolic diseases. However, the macronutrient composition of a nutritionally balanced "healthy diet" are unclear, and so far, no tested dietary intervention has been successful in achieving long-term compliance and reductions in body weight and associated beneficial health outcomes. In the current review, we briefly describe the role of the three major macronutrients, carbohydrates, fats, and proteins, and their role in metabolic health, and provide mechanistic insights. We also discuss how an integrated multi-dimensional approach to nutritional science could help in reconciling apparently conflicting findings.

The Role of Fructose in Non-Alcoholic Steatohepatitis: Old Relationship and New Insights

Non-alcoholic fatty liver disease (NAFLD) represents the result of hepatic fat overload not due to alcohol consumption and potentially evolving to advanced fibrosis, cirrhosis, and hepatocellular carcinoma. Fructose is a naturally occurring simple sugar widely used in food industry linked to glucose to form sucrose, largely contained in hypercaloric food and beverages. An increasing amount of evidence in scientific literature highlighted a detrimental effect of dietary fructose consumption on metabolic disorders such as insulin resistance, obesity, hepatic steatosis, and NAFLD-related fibrosis as well. An excessive fructose consumption has been associated with NAFLD development and progression to more clinically severe phenotypes by exerting various toxic effects, including increased fatty acid production, oxidative stress, and worsening insulin resistance. Furthermore, some studies in this context demonstrated even a crucial role in liver cancer progression. Despite this compelling evidence, the molecular mechanisms by which fructose elicits those effects on liver metabolism remain unclear. Emerging data suggest that dietary fructose may directly alter the expression of genes involved in lipid metabolism, including those that increase hepatic fat accumulation or reduce hepatic fat removal. This review aimed to summarize the current understanding of fructose metabolism on NAFLD pathogenesis and progression.

Pharmacologic inhibition of ketohexokinase prevents fructose-induced metabolic dysfunction

Objective

Recent studies suggest that excess dietary fructose contributes to metabolic dysfunction by promoting insulin resistance, de novo lipogenesis (DNL), and hepatic steatosis, thereby increasing the risk of obesity, type 2 diabetes (T2D), non-alcoholic steatohepatitis (NASH), and related comorbidities. Whether this metabolic dysfunction is driven by the excess dietary calories contained in fructose or whether fructose catabolism itself is uniquely pathogenic remains controversial. We sought to test whether a small molecule inhibitor of the primary fructose metabolizing enzyme ketohexokinase (KHK) can ameliorate the metabolic effects of fructose.

Methods

The KHK inhibitor PF-06835919 was used to block fructose metabolism in primary hepatocytes and Sprague Dawley rats fed either a high-fructose diet (30% fructose kcal/g) or a diet reflecting the average macronutrient dietary content of an American diet (AD) (7.5% fructose kcal/g). The effects of fructose consumption and KHK inhibition on hepatic steatosis, insulin resistance, and hyperlipidemia were evaluated, along with the activation of DNL and the enzymes that regulate lipid synthesis. A metabolomic analysis was performed to confirm KHK inhibition and understand metabolite changes in response to fructose metabolism in vitro and in vivo. Additionally, the effects of administering a single ascending dose of PF-06835919 on fructose metabolism markers in healthy human study participants were assessed in a randomized placebo-controlled phase 1 study.

Results

Inhibition of KHK in rats prevented hyperinsulinemia and hypertriglyceridemia from fructose feeding. Supraphysiologic levels of dietary fructose were not necessary to cause metabolic dysfunction as rats fed the American diet developed hyperinsulinemia, hypertriglyceridemia, and hepatic steatosis, which were all reversed by KHK inhibition. Reversal of the metabolic effects of fructose coincided with reductions in DNL and inactivation of the lipogenic transcription factor carbohydrate response element-binding protein (ChREBP). We report that administering single oral doses of PF-06835919 was safe and well tolerated in healthy study participants and dose-dependently increased plasma fructose indicative of KHK inhibition.

Conclusions

Fructose consumption in rats promoted features of metabolic dysfunction seen in metabolic diseases such as T2D and NASH, including insulin resistance, hypertriglyceridemia, and hepatic steatosis, which were reversed by KHK inhibition.

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PF-06835919 is a potent inhibitor of fructose metabolism in rats and humans.

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Rats fed fructose at levels consistent with the typical American diet develop hyperinsulinemia, hyperlipidemia and steatosis.

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KHK inhibition reverses fructose-induced metabolic dysfunction by blocking ChREBP activation.

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Due to the global dietary prevalence of fructose, KHK inhibition is a potential pharmacotherapy for metabolic diseases.

Salt and Sugar: Two Enemies of Healthy Blood Pressure in Children

The prevalence of essential arterial hypertension in children and adolescents has grown considerably in the last few decades, making this disease a major clinical problem in the pediatric age. The pathogenesis of arterial hypertension is multifactorial, with one of the components being represented by incorrect eating habits. In particular, excessive salt and sugar intake can contribute to the onset of hypertension in children, particularly in subjects with excess weight. Babies have an innate predisposition for sweet taste, while that for salty taste manifests after a few weeks. The recent modification of dietary styles and the current very wide availability of salt and sugar has led to an exponential increase in the consumption of these two nutrients. The dietary intake of salt and sugar in children is in fact much higher than that recommended by health agencies. The purpose of this review is to explore the mechanisms via which an excessive dietary intake of salt and sugar can contribute to the onset of arterial hypertension in children and to show the most important clinical studies that demonstrate the association between these two nutrients and arterial hypertension in pediatric age. Correct eating habits are essential for the prevention and nondrug treatment of essential hypertension in children and adolescents.

Natural Aldose Reductase Inhibitor: A Potential Therapeutic Agent for Non-alcoholic Fatty Liver Disease

Aldose reductase (AR) has been reported to be involved in the development of nonalcoholic fatty liver disease (NAFLD). Hepatic AR is induced under hyperglycemia condition and converts excess glucose to lipogenic fructose, which contributes in part to the accumulation of fat in the liver cells of diabetes rodents. In addition, the hyperglycemia-induced AR or nutrition-induced AR causes suppression of the transcriptional activity of peroxisome proliferator-activated receptor (PPAR) α and reduced lipolysis in the liver, which also contribute to the development of NAFLD. Moreover, AR induction in non-alcoholic steatohepatitis (NASH) may aggravate oxidative stress and the expression of inflammatory cytokines in the liver. Here, we summarize the knowledge on AR inhibitors of plant origin and review the effect of some plant-derived AR inhibitors on NAFLD/NASH in rodents. Natural AR inhibitors may improve NAFLD at least in part through attenuating oxidative stress and inflammatory cytokine expression. Some of the natural AR inhibitors have been reported to attenuate hepatic steatosis through the regulation of PPARα-mediated fatty acid oxidation. In this review, we propose that the natural AR inhibitors are potential therapeutic agents for NAFLD.

Dietary fructose as a model to explore the influence of peripheral metabolism on brain function and plasticity

High consumption of fructose has paralleled an explosion in metabolic disorders including obesity and type 2 diabetes. Even more problematic, sustained consumption of fructose is perceived as a threat for brain function and development of neurological disorders. The action of fructose on peripheral organs is an excellent model to understand how systemic physiology impacts the brain. Given the recognized action of fructose on liver metabolism, here we discuss mechanisms by which fructose can impact the brain by interacting with liver and other organs. The interaction between peripheral and central mechanisms is a suitable target to reduce the pathophysiological consequences of neurological disorders.

The interaction between brain and liver regulates lipid metabolism in the TBI pathology

To shed light on the impact of systemic physiology on the pathology of traumatic brain injury (TBI), we examine the effects of TBI (concussive injury) and dietary fructose on critical aspects of lipid homeostasis in the brain and liver of young-adult rats. Lipids are integral components of brain structure and function, and the liver has a role on the synthesis and metabolism of lipids. Fructose is mainly metabolized in the liver with potential implications for brain function. Lipidomic analysis accompanied by unbiased sparse partial least squares discriminant analysis (sPLS-DA) identified lysophosphatidylcholine (LPC) and cholesterol ester (CE) as the top lipid families impacted by TBI and fructose in the hippocampus, and only LPC (16:0) was associated with hippocampal-dependent memory performance. Fructose and TBI elevated liver pro-inflammatory markers, interleukin-1α (IL-1α), Interferon-γ (IFN-γ) that correlated with hippocampal-dependent memory dysfunction, and monocyte chemoattractant protein-1 (MCP-1) positively correlated with LPC levels in the hippocampus. The effects of fructose were more pronounced in the liver, in agreement with the role of liver on fructose metabolism and suggest that fructose could exacerbate liver inflammation caused by TBI. The overall results indicate that TBI and fructose interact to influence systemic and central inflammation by engaging liver lipids. The impact of TBI and fructose diet on the periphery provides a therapeutic target to counteract the TBI pathogenesis.

Vasopressin mediates fructose-induced metabolic syndrome by activating the V1b receptor

Subjects with obesity frequently have elevated serum vasopressin levels, noted by measuring the stable analog, copeptin. Vasopressin acts primarily to reabsorb water via urinary concentration. However, fat is also a source of metabolic water, raising the possibility that vasopressin might have a role in fat accumulation. Fructose has also been reported to stimulate vasopressin. Here, we tested the hypothesis that fructose-induced metabolic syndrome is mediated by vasopressin. Orally administered fructose, glucose, or high-fructose corn syrup increased vasopressin (copeptin) concentrations and was mediated by fructokinase, an enzyme specific for fructose metabolism. Suppressing vasopressin with hydration both prevented and ameliorated fructose-induced metabolic syndrome. The vasopressin effects were mediated by the vasopressin 1b receptor (V1bR), as V1bR-KO mice were completely protected, whereas V1a-KO mice paradoxically showed worse metabolic syndrome. The mechanism is likely mediated in part by de novo expression of V1bR in the liver that amplifies fructokinase expression in response to fructose. Thus, our studies document a role for vasopressin in water conservation via the accumulation of fat as a source of metabolic water. Clinically, they also suggest that increased water intake may be a beneficial way to both prevent or treat metabolic syndrome.

Fructose Metabolism in Cancer

The interest in fructose metabolism is based on the observation that an increased dietary fructose consumption leads to an increased risk of obesity and metabolic syndrome. In particular, obesity is a known risk factor to develop many types of cancer and there is clinical and experimental evidence that an increased fructose intake promotes cancer growth. The precise mechanism, however, in which fructose induces tumor growth is still not fully understood. In this article, we present an overview of the metabolic pathways that utilize fructose and how fructose metabolism can sustain cancer cell proliferation. Although the degradation of fructose shares many of the enzymes and metabolic intermediates with glucose metabolism through glycolysis, glucose and fructose are metabolized differently. We describe the different metabolic fates of fructose carbons and how they are connected to lipogenesis and nucleotide synthesis. In addition, we discuss how the endogenous production of fructose from glucose via the polyol pathway can be beneficial for cancer cells.

Ketohexokinase inhibition improves NASH by reducing fructose-induced steatosis and fibrogenesis

Background & Aims

Increasing evidence highlights dietary fructose as a major driver of non-alcoholic fatty liver disease (NAFLD) pathogenesis, the majority of which is cleared on first pass through the hepatic circulation by enzymatic phosphorylation to fructose-1-phosphate via the ketohexokinase (KHK) enzyme. Without a current approved therapy, disease management emphasises lifestyle interventions, but few patients adhere to such strategies. New targeted therapies are urgently required.

Methods

We have used a unique combination of human liver specimens, a murine dietary model of NAFLD and human multicellular co-culture systems to understand the hepatocellular consequences of fructose administration. We have also performed a detailed nuclear magnetic resonance-based metabolic tracing of the fate of isotopically labelled fructose upon administration to the human liver.

Results

Expression of KHK isoforms is found in multiple human hepatic cell types, although hepatocyte expression predominates. KHK knockout mice show a reduction in serum transaminase, reduced steatosis and altered fibrogenic response on an Amylin diet. Human co-cultures exposed to fructose exhibit steatosis and activation of lipogenic and fibrogenic gene expression, which were reduced by pharmacological inhibition of KHK activity. Analysis of human livers exposed to ¹³C-labelled fructose confirmed that steatosis, and associated effects, resulted from the accumulation of lipogenic precursors (such as glycerol) and enhanced glycolytic activity. All of these were dose-dependently reduced by administration of a KHK inhibitor.

Conclusions

We have provided preclinical evidence using human livers to support the use of KHK inhibition to improve steatosis, fibrosis, and inflammation in the context of NAFLD.

Lay summary

We have used a mouse model, human cells, and liver tissue to test how exposure to fructose can cause the liver to store excess fat and become damaged and scarred. We have then inhibited a key enzyme within the liver that is responsible for fructose metabolism. Our findings show that inhibition of fructose metabolism reduces liver injury and fibrosis in mouse and human livers and thus this may represent a potential route for treating patients with fatty liver disease in the future.

Organismal Fructose Metabolism in Health and Non-Alcoholic Fatty Liver Disease

NAFLD has alarmingly increased, yet FDA-approved drugs are still lacking. An excessive intake of fructose, especially in liquid form, is a dietary risk factor of NAFLD. While fructose metabolism has been studied for decades, it is still controversial how fructose intake can cause NAFLD. It has long been believed that fructose metabolism solely happens in the liver and accordingly, numerous studies have investigated liver fructose metabolism using primary hepatocytes or liver cell lines in culture. While cultured cells are useful for studying detailed signaling pathways and metabolism in a cell-autonomous manner, it is equally important to understand fructose metabolism at the whole-body level in live organisms. In this regard, recent in vivo studies using genetically modified mice and stable isotope tracing have tremendously expanded our understanding of the complex interaction between fructose-catabolizing organs and gut microbiota. Here, we discuss how the aberrant distribution of fructose metabolism between organs and gut microbiota can contribute to NAFLD. We also address potential therapeutic interventions of fructose-elicited NAFLD.

Novel 3,4-dihydroquinolin-2(1H)-one derivatives as dual inhibitor targeting AKR1B1/ROS for treatment of diabetic complications: Design, synthesis and biological evaluation

AKR1B1 (Aldose reductase) has been used as therapeutic intervention target for treatment of diabetic complications over 50 years, and more recently for inflammation and cancer. However, most developed small molecule inhibitors have the defect of low bioactivity. To address this limitation, novel series of 3,4-dihydroquinolin-2(1H)-one derivatives as dual inhibitor targeting AKR1B1/ROS (Reactive Oxygen Species) were designed and synthesized. Most of these derivatives were found to be potent and selective against AKR1B1, and compound 8a was the most active with an IC₅₀ value of 0.035 μM. Moreover, some prepared derivatives showed strong anti-ROS activity, and among them the phenolic 3,5-dihydroxyl compound 8b was proved to be the most potent, even comparable to that of the well-known antioxidant Trolox at a concentration of 100 μM. Thus the results suggested a success in the construction of potent dual inhibitor for the therapeutic intervention target of AKR1B1/ROS.

Hyperuricemia in Kidney Disease: A Major Risk Factor for Cardiovascular Events, Vascular Calcification, and Renal Damage

Kidney disease, especially when it is associated with a reduction in estimated glomerular filtration rate, can be associated with an increase in serum urate (uric acid), suggesting that hyperuricemia in subjects with kidney disease may be a strictly secondary phenomenon. Mendelian randomization studies that evaluate genetic scores regulating serum urate also generally have not found evidence that serum urate is a causal risk factor in chronic kidney disease. Nevertheless, this is countered by a large number of epidemiologic, experimental, and clinical studies that have suggested a potentially important role for uric acid in kidney disease and cardiovascular disease. Here, we review the topic in detail. Overall, the studies strongly suggest that hyperuricemia does have an important pathogenic role that likely is driven by intracellular urate levels. An exception may be the role of extracellular uric acid in atherosclerosis and vascular calcification. One of the more striking findings on reviewing the literature is that the primary benefit of lowering serum urate in subjects with CKD is not by slowing the progression of renal disease, but rather by reducing the incidence of cardiovascular events and mortality. We recommend large-scale clinical trials to determine if there is a benefit in lowering serum urate in hyperuricemic subjects in acute and chronic kidney disease and in the reduction of cardiovascular morbidity and mortality in subjects with end-stage chronic kidney disease.

Elevated Fructose and Uric Acid Through Aldose Reductase Contribute to Experimental and Human Alcoholic Liver Disease

BACKGROUND AND AIMS

Alcohol-associated liver disease (ALD) is a common chronic liver disease worldwide with high morbidity and mortality, and no Food and Drug Administration-approved therapies. Fructose (dietary or endogenous), its metabolite uric acid, and aldose reductase (AR, the only endogenous enzyme that produces fructose) are strongly associated with the development of nonalcoholic fatty liver disease. However, the role of AR or its metabolites in ALD remains understudied and was examined using human specimens, cultured cells, and mouse model systems.

APPROACH AND RESULTS

We demonstrated in liver specimens from patients with alcoholic hepatitis, the AR up-regulation and elevated AR metabolites (sorbitol, fructose, and uric acid), which correlated significantly with (1) increased lipid peroxidation byproducts and endoplasmic reticulum (ER) stress, (2) decreased protective ER chaperones, and (3) greater cell death and liver injury. Furthermore, we established a causal role for AR in ALD by showing that the genetic deficiency of AR (knockout mice) prevented alcohol-induced increase in harmful AR metabolites, toxic aldehydes, steatosis, ER stress, apoptosis, and liver injury. Finally, we demonstrated the therapeutic potential of pharmacological AR inhibition against alcohol-induced hepatic injury in experimental ALD.

CONCLUSIONS

Our data demonstrate that hepatic AR up-regulation, and consequent elevation in fructose, sorbitol and/or uric acid, are important factors contributing to alcohol-induced steatosis, ER stress, apoptosis, and liver injury in both experimental and human ALD. Our study provides a strong rationale to evaluate AR as a potential therapeutic target and to test AR inhibitors to ameliorate alcohol-induced liver injury.

Mechanisms of Chronic Metabolic Stress in Arrhythmias

Cardiac arrhythmias are responsible for many cardiovascular disease-related deaths worldwide. While arrhythmia pathogenesis is complex, there is increasing evidence for metabolic causes. Obesity, diabetes, and chronically consuming high-fat foods significantly increase the likelihood of developing arrhythmias. Although these correlations are well established, mechanistic explanations connecting a high-fat diet (HFD) to arrhythmogenesis are incomplete, although oxidative stress appears to be critical. This review investigates the metabolic changes that occur in obesity and after HFD. Potential therapies to prevent or treat arrhythmias are discussed, including antioxidants.

Triose Kinase Controls the Lipogenic Potential of Fructose and Dietary Tolerance

The surge in fructose consumption is a major factor behind the rapid rise of nonalcoholic fatty liver disease in modern society. Through flux and genetic analyses, we demonstrate that fructose is catabolized at a much higher rate than glucose, and triose kinase (TK) couples fructolysis with lipogenesis metabolically and transcriptionally. In the absence of TK, fructose oxidation is accelerated through the activation of aldehyde dehydrogenase (ALDH) and serine biosynthesis, accompanied by increased oxidative stress and fructose aversion. TK is also required by the endogenous fructolysis pathway to drive lipogenesis and hepatic triglyceride accumulation under high-fat diet and leptin-deficient conditions. Intriguingly, a nonsynonymous TK allele (rs2260655_A) segregated during human migration out of Africa behaves as TK null for its inability to rescue fructose toxicity and increase hepatic triglyceride accumulation. Therefore, we posit TK as a metabolic switch controlling the lipogenic potential of fructose and its dietary tolerance.

Cerebral Fructose Metabolism as a Potential Mechanism Driving Alzheimer's Disease

The loss of cognitive function in Alzheimer's disease is pathologically linked with neurofibrillary tangles, amyloid deposition, and loss of neuronal communication. Cerebral insulin resistance and mitochondrial dysfunction have emerged as important contributors to pathogenesis supporting our hypothesis that cerebral fructose metabolism is a key initiating pathway for Alzheimer's disease. Fructose is unique among nutrients because it activates a survival pathway to protect animals from starvation by lowering energy in cells in association with adenosine monophosphate degradation to uric acid. The fall in energy from fructose metabolism stimulates foraging and food intake while reducing energy and oxygen needs by decreasing mitochondrial function, stimulating glycolysis, and inducing insulin resistance. When fructose metabolism is overactivated systemically, such as from excessive fructose intake, this can lead to obesity and diabetes. Herein, we present evidence that Alzheimer's disease may be driven by overactivation of cerebral fructose metabolism, in which the source of fructose is largely from endogenous production in the brain. Thus, the reduction in mitochondrial energy production is hampered by neuronal glycolysis that is inadequate, resulting in progressive loss of cerebral energy levels required for neurons to remain functional and viable. In essence, we propose that Alzheimer's disease is a modern disease driven by changes in dietary lifestyle in which fructose can disrupt cerebral metabolism and neuronal function. Inhibition of intracerebral fructose metabolism could provide a novel way to prevent and treat this disease.

Fasting Serum Fructose Levels Are Associated With Risk of Incident Type 2 Diabetes in Middle-Aged and Older Chinese Population

OBJECTIVE

We investigated the relationship between fasting serum fructose levels and the risk of incident type 2 diabetes in a prospective Chinese cohort.

RESEARCH DESIGN AND METHODS

Among 949 community-based participants aged ≥40 years without diabetes at baseline, fasting serum fructose levels were measured using liquid chromatography-tandem mass spectrometry. The participants were followed up for the occurrence of diabetes. Cox regression models were performed to analyze the effect of fasting serum fructose levels on risk of incident diabetes.

RESULTS

During a median of 3.5 years' follow-up, 179 of 949 (18.9%) participants developed type 2 diabetes. Elevated fasting serum fructose levels were associated with an increased risk of incident diabetes in a dose-response manner. After adjustment for age, sex, BMI, lipid profiles, blood pressure, liver function, smoking and drinking status, baseline glucose level, and sugar-sweetened beverage consumption, a 1-SD increased fasting fructose level was associated with a 35% (95% CI 1.08-1.67) increased risk of developing diabetes. After further adjustment for serum uric acid and estimated glomerular filtration rate, the association was partially attenuated (hazard ratio 1.33 [95% CI 1.07-1.65]). The association was similar by age, prediabetes status, BMI, and family history of diabetes but attenuated in women (P for heterogeneity = 0.037).

CONCLUSIONS

Elevated fasting serum fructose levels were independently associated with increased risk of incident type 2 diabetes in a middle-aged and older Chinese population. Our data suggest that higher fasting serum fructose levels might serve as a biomarker and/or a contributor to incident diabetes.

Sugar causes obesity and metabolic syndrome in mice independently of sweet taste

Intake of sugars, especially the fructose component, is strongly associated with the development of obesity and metabolic syndrome, but the relative role of taste versus metabolism in driving preference, intake, and metabolic outcome is not fully understood. We aimed to evaluate the preference for sweet substances and the tendency to develop metabolic syndrome in response to these sugars in mice lacking functional taste signaling [P2X2 (P2X purinoreceptor 2)/P2X3 (P2X purinoreceptor 3) double knockout mice (DKO)] and mice unable to metabolize fructose (fructokinase knockout mice). Of interest, our data indicate that despite their inability to taste sweetness, P2X2/3 DKO mice still prefer caloric sugars (including fructose and glucose) to water in long-term testing, although with diminished preference compared with control mice. Despite reduced intake of caloric sugars by P2X2/3 DKO animals, the DKO mice still show increased levels of the sugar-dependent hormone FGF21 (fibroblast growth factor 21) in plasma and liver. Despite lower sugar intake, taste-blind mice develop severe features of metabolic syndrome due to reduced sensitivity to leptin, reduced ability to mobilize and oxidize fats, and increased hepatic de novo lipogenesis. In contrast to P2X2/3 DKO and wild-type mice, fructokinase knockout mice, which cannot metabolize fructose and are protected against fructose-induced metabolic syndrome, demonstrate reduced preference and intake for all fructose-containing sugars tested but not for glucose or artificial sweeteners. Based on these observations, we conclude that sugar can induce metabolic syndrome in mice independently of its sweet properties. Furthermore, our data demonstrate that the metabolism of fructose is necessary for sugar to drive intake and preference in mice.

Uric Acid and Hypertension: An Update With Recommendations

The association between increased serum urate and hypertension has been a subject of intense controversy. Extracellular uric acid drives uric acid deposition in gout, kidney stones, and possibly vascular calcification. Mendelian randomization studies, however, indicate that serum urate is likely not the causal factor in hypertension although it does increase the risk for sudden cardiac death and diabetic vascular disease. Nevertheless, experimental evidence strongly suggests that an increase in intracellular urate is a key factor in the pathogenesis of primary hypertension. Pilot clinical trials show beneficial effect of lowering serum urate in hyperuricemic individuals who are young, hypertensive, and have preserved kidney function. Some evidence suggest that activation of the renin-angiotensin system (RAS) occurs in hyperuricemia and blocking the RAS may mimic the effects of xanthine oxidase inhibitors. A reduction in intracellular urate may be achieved by lowering serum urate concentration or by suppressing intracellular urate production with dietary measures that include reducing sugar, fructose, and salt intake. We suggest that these elements in the western diet may play a major role in the pathogenesis of primary hypertension. Studies are necessary to better define the interrelation between uric acid concentrations inside and outside the cell. In addition, large-scale clinical trials are needed to determine if extracellular and intracellular urate reduction can provide benefit hypertension and cardiometabolic disease.

Fructose contributes to the Warburg effect for cancer growth

Obesity and metabolic syndrome are strongly associated with cancer, and these disorders may share a common mechanism. Recently, fructose has emerged as a driving force to develop obesity and metabolic syndrome. Thus, we assume that fructose may be the mechanism to explain why obesity and metabolic syndrome are linked with cancer. Clinical and experimental evidence showed that fructose intake was associated with cancer growth and that fructose transporters are upregulated in various malignant tumors. Interestingly, fructose metabolism can be driven under low oxygen conditions, accelerates glucose utilization, and exhibits distinct effects as compared to glucose, including production of uric acid and lactate as major byproducts. Fructose promotes the Warburg effect to preferentially downregulate mitochondrial respiration and increases aerobic glycolysis that may aid metastases that initially have low oxygen supply. In the process, uric acid may facilitate carcinogenesis by inhibiting the TCA cycle, stimulating cell proliferation by mitochondrial ROS, and blocking fatty acid oxidation. Lactate may also contribute to cancer growth by suppressing fat oxidation and inducing oncogene expression. The ability of fructose metabolism to directly stimulate the glycolytic pathway may have been protective for animals living with limited access to oxygen, but may be deleterious toward stimulating cancer growth and metastasis for humans in modern society. Blocking fructose metabolism may be a novel approach for the prevention and treatment of cancer.

Deletion of Fructokinase in the Liver or in the Intestine Reveals Differential Effects on Sugar-Induced Metabolic Dysfunction

Intake of fructose-containing sugars is strongly associated with metabolic syndrome. Compared with other sugars, dietary fructose is uniquely metabolized by fructokinase. However, the tissue-specific role of fructokinase in sugar-induced metabolic syndrome, and the specific roles of glucose and fructose in driving it, is not fully understood. Here, we show that in mice receiving excess fructose-glucose solutions, whole-body deletion of fructokinase, and thus full blockade of fructose metabolism, is sufficient to prevent metabolic syndrome. This protection is not only due to reduced fructose metabolism, but also due to decreased sugar intake. Furthermore, by using tissue-specific fructokinase-deficient mice, we determined that while sugar intake is controlled by intestinal fructokinase activity, metabolic syndrome is driven by fructose metabolism in the liver. Our findings show a two-pronged role for fructose metabolism in sugar-induced metabolic syndrome, one arm via the intestine that mediates sugar intake and a second arm in the liver that drives metabolic dysfunction.

Cell-Type-Specific, Ketohexokinase-Dependent Induction by Fructose of Lipogenic Gene Expression in Mouse Small Intestine

BACKGROUND

High intakes of fructose are associated with metabolic diseases, including hypertriglyceridemia and intestinal tumor growth. Although small intestinal epithelia consist of many different cell types, express lipogenic genes, and convert dietary fructose to fatty acids, there is no information on the identity of the cell type(s) mediating this conversion and on the effects of fructose on lipogenic gene expression.

OBJECTIVES

We hypothesized that fructose regulates the intestinal expression of genes involved in lipid and apolipoprotein synthesis, that regulation depends on the fructose transporter solute carrier family 2 member a5 [Slc2a5 (glucose transporter 5)] and on ketohexokinase (Khk), and that regulation occurs only in enterocytes.

METHODS

We compared lipogenic gene expression among different organs from wild-type adult male C57BL mice consuming a standard vivarium nonpurified diet. We then gavaged twice daily for 2.5 d fructose or glucose solutions (15%, 0.3 mL per mouse) into wild-type, Slc2a5-knockout (KO), and Khk-KO mice with free access to the nonpurified diet and determined expression of representative lipogenic genes. Finally, from mice fed the nonpurified diet, we made organoids highly enriched in enterocyte, goblet, Paneth, or stem cells and then incubated them overnight in 10 mM fructose or glucose.

RESULTS

Most lipogenic genes were significantly expressed in the intestine relative to the kidney, liver, lung, and skeletal muscle. In vivo expression of Srebf1, Acaca, Fasn, Scd1, Dgat1, Gk, Apoa4, and Apob mRNA and of Scd1 protein increased (P < 0.05) by 3- to 20-fold in wild-type, but not in Slc2a5-KO and Khk-KO, mice gavaged with fructose. In vitro, Slc2a5- and Khk-dependent, fructose-induced increases, which ranged from 1.5- to 4-fold (P < 0.05), in mRNA concentrations of all these genes were observed only in organoids enriched in enterocytes.

CONCLUSIONS

Fructose specifically stimulates expression of mouse small intestinal genes for lipid and apolipoprotein synthesis. Secretory and stem cells seem incapable of transport- and metabolism-dependent lipogenesis, occurring only in absorptive enterocytes.

Recent advances in the pathogenesis of hereditary fructose intolerance: implications for its treatment and the understanding of fructose-induced non-alcoholic fatty liver disease

Hereditary fructose intolerance (HFI) is a rare inborn disease characterized by a deficiency in aldolase B, which catalyzes the cleavage of fructose 1,6-bisphosphate and fructose 1-phosphate (Fru 1P) to triose molecules. In patients with HFI, ingestion of fructose results in accumulation of Fru 1P and depletion of ATP, which are believed to cause symptoms, such as nausea, vomiting, hypoglycemia, and liver and kidney failure. These sequelae can be prevented by a fructose-restricted diet. Recent studies in aldolase B-deficient mice and HFI patients have provided more insight into the pathogenesis of HFI, in particular the liver phenotype. Both aldolase B-deficient mice (fed a very low fructose diet) and HFI patients (treated with a fructose-restricted diet) displayed greater intrahepatic fat content when compared to controls. The liver phenotype in aldolase B-deficient mice was prevented by reduction in intrahepatic Fru 1P concentrations by crossing these mice with mice deficient for ketohexokinase, the enzyme that catalyzes the synthesis of Fru 1P. These new findings not only provide a potential novel treatment for HFI, but lend insight into the pathogenesis of fructose-induced non-alcoholic fatty liver disease (NAFLD), which has raised to epidemic proportions in Western society. This narrative review summarizes the most recent advances in the pathogenesis of HFI and discusses the implications for the understanding and treatment of fructose-induced NAFLD.

Fructose Production and Metabolism in the Kidney

Understanding fructose metabolism might provide insights to renal pathophysiology. To support systemic glucose concentration, the proximal tubular cells reabsorb fructose as a substrate for gluconeogenesis. However, in instances when fructose intake is excessive, fructose metabolism is costly, resulting in energy depletion, uric acid generation, inflammation, and fibrosis in the kidney. A recent scientific advance is the discovery that fructose can be endogenously produced from glucose under pathologic conditions, not only in kidney diseases, but also in diabetes, in cardiac hypertrophy, and with dehydration. Why humans have such a deleterious mechanism to produce fructose is unknown, but it may relate to an evolutionary benefit in the past. In this article, we aim to illuminate the roles of fructose as it relates to gluconeogenesis and fructoneogenesis in the kidney.

NAFLD as a driver of chronic kidney disease

Non-alcoholic fatty liver disease (NAFLD) and chronic kidney disease (CKD) are worldwide public health problems, affecting up to 25-30% (NAFLD), and up to 10-15% (CKD) of the general population. Recently, it has also been established that there is a strong association between NAFLD and CKD, regardless of the presence of potential confounding diseases such as obesity, hypertension and type 2 diabetes. Since NAFLD and CKD are both common diseases that often occur alongside other metabolic conditions, such as type 2 diabetes or metabolic syndrome, elucidating the relative impact of NAFLD on the risk of incident CKD presents a substantial challenge for investigators working in this research field. A growing body of epidemiological evidence suggests that NAFLD is an independent risk factor for CKD and recent evidence also suggests that associated factors such as metabolic syndrome, dysbiosis, unhealthy diets, platelet activation and processes associated with ageing could also contribute mechanisms linking NAFLD and CKD. This narrative review provides an overview of the literature on: a) the evidence for an association and causal link between NAFLD and CKD and b) the underlying mechanisms by which NAFLD (and factors strongly linked with NAFLD) may increase the risk of developing CKD.

Fructose metabolism as a common evolutionary pathway of survival associated with climate change, food shortage and droughts

Mass extinctions occur frequently in natural history. While studies of animals that became extinct can be informative, it is the survivors that provide clues for mechanisms of adaptation when conditions are adverse. Here, we describe a survival pathway used by many species as a means for providing adequate fuel and water, while also providing protection from a decrease in oxygen availability. Fructose, whether supplied in the diet (primarily fruits and honey), or endogenously (via activation of the polyol pathway), preferentially shifts the organism towards the storing of fuel (fat, glycogen) that can be used to provide energy and water at a later date. Fructose causes sodium retention and raises blood pressure and likely helped survival in the setting of dehydration or salt deprivation. By shifting energy production from the mitochondria to glycolysis, fructose reduced oxygen demands to aid survival in situations where oxygen availability is low. The actions of fructose are driven in part by vasopressin and the generation of uric acid. Twice in history, mutations occurred during periods of mass extinction that enhanced the activity of fructose to generate fat, with the first being a mutation in vitamin C metabolism during the Cretaceous-Paleogene extinction (65 million years ago) and the second being a mutation in uricase that occurred during the Middle Miocene disruption (12-14 million years ago). Today, the excessive intake of fructose due to the availability of refined sugar and high-fructose corn syrup is driving 'burden of life style' diseases, including obesity, diabetes and high blood pressure.

Lipid and energy metabolism in Wilson disease

Copper accumulation and deficiency are reciprocally connected to lipid metabolism. In Wilson disease (WD), which is caused by a genetic loss of function of the copper-transporting P-type ATPase beta, copper accumulates mainly in the liver and lipid metabolism is dysregulated. The underlying mechanisms linking copper and lipid metabolism in WD are not clear. Copper may impair metabolic machinery by direct binding to protein and lipid structures or by generating reactive oxygen species with consequent damage to cellular organelles vital to energy metabolism. In the liver, copper overload results in mitochondrial impairment, down-regulation of lipid metabolism, and the development of steatosis with an etiology not fully elucidated. Little is known regarding the effect of copper overload on extrahepatic energy homeostasis. This review aims to discuss alterations in hepatic energy metabolism associated with WD, highlights potential mechanisms involved in the development of hepatic and systemic dysregulation of lipid metabolism, and reviews current knowledge on the effects of copper overload on extrahepatic energy metabolism.

Bile acid sequestration reverses liver injury and prevents progression of nonalcoholic steatohepatitis in Western diet-fed mice

Nonalcoholic fatty liver disease is a rapidly rising problem in the 21st century and is a leading cause of chronic liver disease that can lead to end-stage liver diseases, including cirrhosis and hepatocellular cancer. Despite this rising epidemic, no pharmacological treatment has yet been established to treat this disease. The rapidly increasing prevalence of nonalcoholic fatty liver disease and its aggressive form, nonalcoholic steatohepatitis (NASH), requires novel therapeutic approaches to prevent disease progression. Alterations in microbiome dynamics and dysbiosis play an important role in liver disease and may represent targetable pathways to treat liver disorders. Improving microbiome properties or restoring normal bile acid metabolism may prevent or slow the progression of liver diseases such as NASH. Importantly, aberrant systemic circulation of bile acids can greatly disrupt metabolic homeostasis. Bile acid sequestrants are orally administered polymers that bind bile acids in the intestine, forming nonabsorbable complexes. Bile acid sequestrants interrupt intestinal reabsorption of bile acids, decreasing their circulating levels. We determined that treatment with the bile acid sequestrant sevelamer reversed the liver injury and prevented the progression of NASH, including steatosis, inflammation, and fibrosis in a Western diet-induced NASH mouse model. Metabolomics and microbiome analysis revealed that this beneficial effect is associated with changes in the microbiota population and bile acid composition, including reversing microbiota complexity in cecum by increasing Lactobacillus and decreased Desulfovibrio The net effect of these changes was improvement in liver function and markers of liver injury and the positive effects of reversal of insulin resistance.

Fructose and hepatic insulin resistance

Excessive caloric intake in a form of high-fat diet (HFD) was long thought to be the major risk factor for development of obesity and its complications, such as fatty liver disease and insulin resistance. Recently, there has been a paradigm shift and more attention is attributed to the effects of sugar-sweetened beverages (SSBs) as one of the culprits of the obesity epidemic. In this review, we present the data invoking fructose intake with development of hepatic insulin resistance in human studies and discuss the pathways by which fructose impairs hepatic insulin action in experimental animal models. First, we described well-characterized pathways by which fructose metabolism indirectly leads to hepatic insulin resistance. These include unequivocal effects of fructose to promote de novo lipogenesis (DNL), impair fatty acid oxidation (FAO), induce endoplasmic reticulum (ER) stress and trigger hepatic inflammation. Additionally, we entertained the hypothesis that fructose can directly impede insulin signaling in the liver. This appears to be mediated by reduced insulin receptor and insulin receptor substrate 2 (IRS2) expression, increased protein-tyrosine phosphatase 1B (PTP1b) activity, whereas knockdown of ketohexokinase (KHK), the rate-limiting enzyme of fructose metabolism, increased insulin sensitivity. In summary, dietary fructose intake strongly promotes hepatic insulin resistance via complex interplay of several metabolic pathways, at least some of which are independent of increased weight gain and caloric intake. The current evidence shows that the fructose, but not glucose, component of dietary sugar drives metabolic complications and contradicts the notion that fructose is merely a source of palatable calories that leads to increased weight gain and insulin resistance.

Cytochrome P450 endoplasmic reticulum-associated degradation (ERAD): therapeutic and pathophysiological implications

The hepatic endoplasmic reticulum (ER)-anchored cytochromes P450 (P450s) are mixed-function oxidases engaged in the biotransformation of physiologically relevant endobiotics as well as of myriad xenobiotics of therapeutic and environmental relevance. P450 ER-content and hence function is regulated by their coordinated hemoprotein syntheses and proteolytic turnover. Such P450 proteolytic turnover occurs through a process known as ER-associated degradation (ERAD) that involves ubiquitin-dependent proteasomal degradation (UPD) and/or autophagic-lysosomal degradation (ALD). Herein, on the basis of available literature reports and our own recent findings of in vitro as well as in vivo experimental studies, we discuss the therapeutic and pathophysiological implications of altered P450 ERAD and its plausible clinical relevance. We specifically (i) describe the P450 ERAD-machinery and how it may be repurposed for the generation of antigenic P450 peptides involved in P450 autoantibody pathogenesis in drug-induced acute hypersensitivity reactions and liver injury, or viral hepatitis; (ii) discuss the relevance of accelerated or disrupted P450-ERAD to the pharmacological and/or toxicological effects of clinically relevant P450 drug substrates; and (iii) detail the pathophysiological consequences of disrupted P450 ERAD, contributing to non-alcoholic fatty liver disease (NAFLD)/non-alcoholic steatohepatitis (NASH) under certain synergistic cellular conditions.

Effectiveness of vitamin D supplementation in Swedish children may be negatively impacted by BMI and serum fructose

In regions where sunlight exposure is limited, dietary vitamin D intake becomes important for maintaining status. However, Swedish children have been shown to have deficient or marginal status during the winter months even if the recommended dietary intake is met. Since low vitamin D status has been associated with several disease states, this study investigated the metabolic changes associated with improved vitamin D status due to supplementation. During the 3 winter months, 5-7-year-old children (n=170) in northern (Umeå, 63° N) and southern (Malmö, 55° N) Sweden were supplemented daily with 2 (placebo), 10 or 25 μg of vitamin D. BMI-for-age z-scores (BAZ), S-25(OH)D concentrations, insulin concentrations and the serum metabolome were assessed at baseline and follow-up. S-25(OH)D concentrations increased significantly in both supplementation groups (P<.001). Only arginine and isopropanol concentrations exhibited significant associations with improvements in S-25(OH)D. Furthermore, the extent to which S-25(OH)D increased was correlated with a combination of baseline BAZ and the change in serum fructose concentrations from baseline to follow up (P=.012). In particular, the change in S-25(OH)D concentrations was negatively correlated (P=.030) with the change in fructose concentrations for subjects with BAZ ≥0 and consuming at least 20 μg vitamin D daily. These results suggest that although the metabolic changes associated with improved vitamin D status are small, the effectiveness of dietary supplementation may be influenced by serum fructose concentrations.

Daily Fructose Traces Intake and Liver Injury in Children with Hereditary Fructose Intolerance

Background: Hereditary fructose intolerance (HFI) is a rare genetic disorder of fructose metabolism due to aldolase B enzyme deficiency. Treatment consists of fructose, sorbitol, and sucrose (FSS)-free diet. We explore possible correlations between daily fructose traces intake and liver injury biomarkers on a long-term period, in a cohort of young patients affected by HFI. Methods: Patients’ clinical data and fructose daily intake were retrospectively collected. Correlations among fructose intake, serum alanine aminotransferase (ALT) level, carbohydrate-deficient transferrin (CDT) percentage, liver ultrasonography, genotype were analyzed. Results: We included 48 patients whose mean follow-up was 10.3 ± 5.6 years and fructose intake 169 ± 145.4 mg/day. Eighteen patients had persistently high ALT level, nine had abnormal CDT profile, 45 had signs of liver steatosis. Fructose intake did not correlate with ALT level nor with steatosis severity, whereas it correlated with disialotransferrin percentage (R² 0.7, p < 0.0001) and tetrasialotransferrin/disialotransferrin ratio (R² 0.5, p = 0.0001). p.A150P homozygous patients had lower ALT values at diagnosis than p.A175D variant homozygotes cases (58 ± 55 IU/L vs. 143 ± 90 IU/L, p = 0.01). Conclusion: A group of HFI patients on FSS-free diet presented persistent mild hypertransaminasemia which did not correlate with fructose intake. Genotypes may influence serum liver enzyme levels. CDT profile represents a good marker to assess FSS intake.

Antioxidant supplements as a novel mean for blocking recurrent heat stress-induced kidney damage following rehydration with fructose-containing beverages

Recently repeated heat stress and dehydration have been reported to cause oxidative stress and kidney damage that is enhanced by rehydrating with fructose solutions. We hypothesized that antioxidants might provide a novel way to prevent kidney damage. To test this hypothesis, mild heat stress was induced by exposing rats to 37 °C during 1 h in a closed chamber. The supplementation with water-soluble antioxidants (Antiox), ascorbic acid 1% plus N-acetyl cysteine 600 mg/L was done either in the 10% fructose 2 h rehydration fluid immediately after heat stress (Fructose 10% + Antiox), and/or in the tap water (Water + Antiox) for the remainder of the day, or in both fluids. After 4 weeks, control rats exposed to heat with fructose rehydration developed impaired renal function, tubular injury, intrarenal oxidative stress, a reduction in Nrf2-Keap1 antioxidant pathway, stimulation of vasopressin and the intrarenal polyol-fructokinase pathway. In contrast, dosing the antioxidants in the tap water (i.e., before the heat exposure and rehydration with fructose) preserved renal function, prevented renal tubule dysfunction and avoided the increase in systemic blood pressure. These effects were likely due to the amplification of the antioxidant defenses through increased Nrf2 nuclear translocation stimulated by the antioxidants and by the prevention of polyol fructokinase pathway overactivation. More studies to understand the mechanisms implicated in this pathology are warranted as there is recent evidence that they may be operating in humans as well.

Carbotoxicity-Noxious Effects of Carbohydrates

Modern nutrition is often characterized by the excessive intake of different types of carbohydrates ranging from digestible polysaccharides to refined sugars that collectively mediate noxious effects on human health, a phenomenon that we refer to as "carbotoxicity." Epidemiological and experimental evidence combined with clinical intervention trials underscore the negative impact of excessive carbohydrate uptake, as well as the beneficial effects of reducing carbs in the diet. We discuss the molecular, cellular, and neuroendocrine mechanisms that link exaggerated carbohydrate intake to disease and accelerated aging as we outline dietary and pharmacologic strategies to combat carbotoxicity.

Endogenous fructose production: what do we know and how relevant is it?

PURPOSE OF REVIEW

Excessive sugar and particularly fructose consumption has been proposed to be a key player in the pathogenesis of metabolic syndrome and kidney disease in humans and animal models. However, besides its dietary source, fructose can be endogenously produced in the body from glucose via the activation of the polyol pathway. In this review, we aim to describe the most recent findings and current knowledge on the potential role of endogenous fructose production and metabolism in disease.

RECENT FINDINGS

Over the recent years, the activation of the polyol pathway and endogenous fructose production has been observed in multiple tissues including the liver, renal cortex, and hypothalamic areas of the brain. The activation occurs during the development and progression of metabolic syndrome and kidney disease and results from different stimuli including osmotic effects, diabetes, and ischemia. Even though the potential toxicity of the activation of the polyol pathway can be attributed to several intermediate products, the blockade of endogenous fructose metabolism either by using fructokinase deficient mice or specific inhibitors resulted in marked amelioration of multiple metabolic diseases.

SUMMARY

New findings suggest that fructose can be produced in the body and that the blockade of tis metabolism could be clinically relevant for the prevention and treatment of metabolic syndrome and kidney disease.

Dietary Lipids Differentially Shape Nonalcoholic Steatohepatitis Progression and the Transcriptome of Kupffer Cells and Infiltrating Macrophages

A crucial component of nonalcoholic fatty liver disease (NAFLD) pathogenesis is lipid stress, which may contribute to hepatic inflammation and activation of innate immunity in the liver. However, little is known regarding how dietary lipids, including fat and cholesterol, may facilitate innate immune activation in vivo. We hypothesized that dietary fat and cholesterol drive NAFLD progression to steatohepatitis and hepatic fibrosis by altering the transcription and phenotype of hepatic macrophages. This hypothesis was tested by using RNA-sequencing methods to characterize and analyze sort-purified hepatic macrophage populations that were isolated from mice fed diets with varying amounts of fat and cholesterol. The addition of cholesterol to a high-fat diet triggered hepatic pathology reminiscent of advanced nonalcoholic steatohepatitis (NASH) in humans characterized by signs of cholesterol dysregulation, generation of oxidized low-density lipoprotein, increased recruitment of hepatic macrophages, and significant fibrosis. RNA-sequencing analyses of hepatic macrophages in this model revealed that dietary cholesterol induced a tissue repair and regeneration phenotype in Kupffer cells (KCs) and recruited infiltrating macrophages to a greater degree than fat. Furthermore, comparison of diseased KCs and infiltrating macrophages revealed that these two macrophage subsets are transcriptionally diverse. Finally, direct stimulation of murine and human macrophages with oxidized low-density lipoprotein recapitulated some of the transcriptional changes observed in the RNA-sequencing study. These findings indicate that fat and cholesterol synergize to alter macrophage phenotype, and they also challenge the dogma that KCs are purely proinflammatory in NASH. Conclusion: This comprehensive view of macrophage populations in NASH indicates mechanisms by which cholesterol contributes to NASH progression and identifies potential therapeutic targets for this common disease.