Fructose consumption: potential mechanisms for its effects to increase visceral adiposity and induce dyslipidemia and insulin resistance

PURPOSE OF REVIEW: Based on interim results from an ongoing study, we have reported that consumption of a high-fructose diet, but not a high-glucose diet, promotes the development of three of the pathological characteristics associated with metabolic syndrome: visceral adiposity, dyslipidemia, and insulin resistance. From these results and a review of the current literature, we present two potential sequences of events by which fructose consumption may contribute to metabolic syndrome. RECENT FINDINGS: The earliest metabolic perturbation resulting from fructose consumption is postprandial hypertriglyceridemia, which may increase visceral adipose deposition. Visceral adiposity contributes to hepatic triglyceride accumulation, novel protein kinase C activation, and hepatic insulin resistance by increasing the portal delivery of free fatty acids to the liver. With insulin resistance, VLDL production is upregulated and this, along with systemic free fatty acids, increase lipid delivery to muscle. It is also possible that fructose initiates hepatic insulin resistance independently of visceral adiposity and free fatty acid delivery. By providing substrate for hepatic lipogenesis, fructose may result in a direct lipid overload that leads to triglyceride accumulation, novel protein kinase C activation, and hepatic insulin resistance. SUMMARY: Our investigation and future studies of the effects of fructose consumption may help to clarify the sequence of events leading to development of metabolic syndrome.     

PKM2, cancer metabolism,and the road ahead

A major metabolic aberration associated with cancer is a change in glucose metabolism. Isoform selection of the glycolytic enzyme pyruvate kinase has been implicated in the metabolic phenotype of cancer cells, and specific pyruvate kinase isoforms have been suggested to support divergent energetic and biosynthetic requirements of cells in tumors and normal tissues. PKM2 isoform expression has been closely linked to embryogenesis, tissue repair, and cancer. In contrast, forced expression of the PKM1 isoform has been associated with reduced tumor cell proliferation. Here, we discuss the role that PKM2 plays in cells and provide a historical perspective for how the study of PKM2 has contributed to understanding cancer metabolism. We also review recent studies that raise important questions with regard to the role of PKM2 in both normal and cancer cell metabolism.     

Fructose-mediated stress signaling in the liver: implications for hepatic insulin resistance

Organisms reprogram metabolic pathways to adapt to changes in nutrient availability. This requires that nutrient-based stimuli are sensed, signals are transmitted, and highly specific responses are engaged. We propose that in the liver, the mitogen-activated protein kinase, c-jun N-terminal kinase (JNK), links excessive nutrient metabolism with impaired insulin regulation of glucose production. The liver, by virtue of its anatomic position and selective regulatory features, buffers and is highly responsive to changes in nutrient delivery. In particular, sugars such as sucrose and fructose uniquely regulate and are selectively metabolized by the liver. We propose that when hepatic fructose uptake exceeds requirements for glycogen and energy (hepatic sugar excess), the JNK-signaling pathway is engaged as part of the adaptive response.     

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.     

Potent Therapeutic Activity Against Peritoneal Dissemination and Malignant Ascites by the Novel Anti-Folate Receptor Alpha Antibody KHK2805

Many ovarian cancer patients often show peritoneal metastasis with malignant ascites. However, unmet medical needs remain regarding controlling these symptoms after tumors become resistant to chemotherapies. We developed KHK2805, a novel anti-folate receptor α (FOLR1) humanized antibody with enhanced antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). The primary aim of the present study was to evaluate whether the anti-tumor activity of KHK2805 was sufficient for therapeutic application against peritoneal dissemination and malignant ascites of platinum-resistant ovarian cancer in preclinical models. Here, both the ADCC and CDC of KHK2805 were evaluated in ovarian cancer cell lines and patient-derived samples. The anti-tumor activity of KHK2805 was evaluated in a SCID mouse model of platinum-resistant peritoneal dissemination. As results, KHK2805 showed specific binding to FOLR1 with high affinity at a novel epitope. KHK2805 exerted potent ADCC and CDC against ovarian cancer cell lines. Furthermore, primary platinum-resistant malignant ascites cells were susceptible to autologous ADCC with KHK2805. Patient-derived sera and malignant ascites induced CDC of KHK2805. KHK2805 significantly reduced the total tumor burden and amount of ascites in SCID mice with peritoneal dissemination and significantly prolonged their survival. In addition, the parental rat antibody strongly stained serous and clear cell-type ovarian tumors by immunohistochemistry. Overall, KHK2805 showed cytotoxicity against both ovarian cancer cell lines and patient-derived cells. These translational study findings suggest that KHK2805 may be promising as a novel therapeutic agent for platinum-resistant ovarian cancer with peritoneal dissemination and malignant ascites.     

Fatty acid mobilization and their use in adipose tissue

An excess of fat mass excess predisposes to multiple complications such as type 2 diabetes, cardiovascular diseases or cancer. A dysregulation of lipid metabolism contributes to the development of obesity and the metabolic syndrome. Recent data on lipid mobilization in adipose tissue have revealed a complex pathway involving a human specific hormonal control of lipolysis via the natriuretic peptides and a new triglyceride lipase, ATGL. Activation of fatty acid reesterification and oxidation can lead to an increase in fatty acid utilization. Targeting these key steps of lipid metabolism (adipose tissue lipolysis and fatty acid oxidation) constitutes a potential strategy for the treatment of obesity and associated metabolic disorders.     

Regulation and function of mitophagy in development and cancer

Beyond its role in recycling intracellular components nonselectively to sustain survival in response to metabolic stresses, autophagy can also selectively degrade specific cargoes such as damaged or dysfunctional organelles to maintain cellular homeostasis. Mitochondria, known as the power plant of cells, are the critical and dynamic organelles playing a fundamental role in cellular metabolism. Mitophagy, the selective autophagic elimination of mitochondria, has been identified both in yeast and in mammalian cells. Moreover, defects in mitophagy may contribute to a variety of human disorders such as neurodegeneration and myopathies. However, the role of mitophagy in development and cancer remains largely unclear. In this review, we summarize our current knowledge of the regulation and function of mitophagy in development and cancer.     

Comparative metabolic flux analysis of lysine-producing Corynebacterium glutamicum cultured on glucose or fructose

A comprehensive approach to (13)C tracer studies, labeling measurements by gas chromatography-mass spectrometry, metabolite balancing, and isotopomer modeling, was applied for comparative metabolic network analysis of lysine-producing Corynebacterium glutamicum on glucose or fructose. Significantly reduced yields of lysine and biomass and enhanced formation of dihydroxyacetone, glycerol, and lactate in comparison to those for glucose resulted on fructose. Metabolic flux analysis revealed drastic differences in intracellular flux depending on the carbon source applied. On fructose, flux through the pentose phosphate pathway (PPP) was only 14.4% of the total substrate uptake flux and therefore markedly decreased compared to that for glucose (62.0%). This result is due mainly to (i) the predominance of phosphoenolpyruvate-dependent phosphotransferase systems for fructose uptake (PTS(Fructose)) (92.3%), resulting in a major entry of fructose via fructose 1,6-bisphosphate, and (ii) the inactivity of fructose 1,6-bisphosphatase (0.0%). The uptake of fructose during flux via PTS(Mannose) was only 7.7%. In glucose-grown cells, the flux through pyruvate dehydrogenase (70.9%) was much less than that in fructose-grown cells (95.2%). Accordingly, flux through the tricarboxylic acid cycle was decreased on glucose. Normalized to that for glucose uptake, the supply of NADPH during flux was only 112.4% on fructose compared to 176.9% on glucose, which might explain the substantially lower lysine yield of C. glutamicum on fructose. Balancing NADPH levels even revealed an apparent deficiency of NADPH on fructose, which is probably overcome by in vivo activity of malic enzyme. Based on these results, potential targets could be identified for optimization of lysine production by C. glutamicum on fructose, involving (i) modification of flux through the two PTS for fructose uptake, (ii) amplification of fructose 1,6-bisphosphatase to increase flux through the PPP, and (iii) knockout of a not-yet-annotated gene encoding dihydroxyacetone phosphatase or kinase activity to suppress overflow metabolism. Statistical evaluation revealed high precision of the estimates of flux, so the observed differences for metabolic flux are clearly substrate specific.     

Glucose Transporter 8 (GLUT8) Mediates Fructose-induced de Novo Lipogenesis and Macrosteatosis

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease in the world, and it is thought to be the hepatic manifestation of the metabolic syndrome. Excess dietary fructose causes both metabolic syndrome and NAFLD in rodents and humans, but the pathogenic mechanisms of fructose-induced metabolic syndrome and NAFLD are poorly understood. GLUT8 (Slc2A8) is a facilitative glucose and fructose transporter that is highly expressed in liver, heart, and other oxidative tissues. We previously demonstrated that female mice lacking GLUT8 exhibit impaired first-pass hepatic fructose metabolism, suggesting that fructose transport into the hepatocyte, the primary site of fructose metabolism, is in part mediated by GLUT8. Here, we tested the hypothesis that GLUT8 is required for hepatocyte fructose uptake and for the development of fructose-induced NAFLD. We demonstrate that GLUT8 is a cell surface-localized transporter and that GLUT8 overexpression or GLUT8 shRNA-mediated gene silencing significantly induces and blocks radiolabeled fructose uptake in cultured hepatocytes. We further show diminished fructose uptake and de novo lipogenesis in fructose-challenged GLUT8-deficient hepatocytes. Finally, livers from long term high-fructose diet-fed GLUT8-deficient mice exhibited attenuated fructose-induced hepatic triglyceride and cholesterol accumulation without changes in hepatocyte insulin-stimulated Akt phosphorylation. GLUT8 is thus essential for hepatocyte fructose transport and fructose-induced macrosteatosis. Fructose delivery across the hepatocyte membrane is thus a proximal, modifiable disease mechanism that may be exploited to prevent NAFLD.     

Effects of Excess Energy Intake on Glucose and Lipid Metabolism in C57BL/6 Mice

Excess energy intake correlates with the development of metabolic disorders. However, different energy-dense foods have different effects on metabolism. To compare the effects of a high-fat diet, a high-fructose diet and a combination high-fat/high-fructose diet on glucose and lipid metabolism, male C57BL/6 mice were fed with one of four different diets for 3 months: standard chow; standard diet and access to fructose water; a high fat diet; and a high fat diet with fructose water. After 3 months of feeding, the high-fat and the combined high-fat/high-fructose groups showed significantly increased body weights, accompanied by hyperglycemia and insulin resistance; however, the high-fructose group was not different from the control group. All three energy-dense groups showed significantly higher visceral fat weights, total cholesterol concentrations, and low-density lipoprotein cholesterol concentrations compared with the control group. Assays of basal metabolism showed that the respiratory quotient of the high-fat, the high-fructose, and the high-fat/high-fructose groups decreased compared with the control group. The present study confirmed the deleterious effect of high energy diets on body weight and metabolism, but suggested that the energy efficiency of the high-fructose diet was much lower than that of the high-fat diet. In addition, fructose supplementation did not worsen the detrimental effects of high-fat feeding alone on metabolism in C57BL/6 mice.     

Inhibition of Nicotinamide Phosphoribosyltransferase (NAMPT), an Enzyme Essential for NAD+ Biosynthesis,Leads to Altered Carbohydrate Metabolism in Cancer Cells

Nicotinamide phosphoribosyltransferase (NAMPT) has been extensively studied due to its essential role in NAD⁺ biosynthesis in cancer cells and the prospect of developing novel therapeutics. To understand how NAMPT regulates cellular metabolism, we have shown that the treatment with FK866, a specific NAMPT inhibitor, leads to attenuation of glycolysis by blocking the glyceraldehyde 3-phosphate dehydrogenase step (Tan, B., Young, D. A., Lu, Z. H., Wang, T., Meier, T. I., Shepard, R. L., Roth, K., Zhai, Y., Huss, K., Kuo, M. S., Gillig, J., Parthasarathy, S., Burkholder, T. P., Smith, M. C., Geeganage, S., and Zhao, G. (2013) Pharmacological inhibition of nicotinamide phosphoribosyltransferase (NAMPT), an enzyme essential for NAD⁺ biosynthesis, in human cancer cells: metabolic basis and potential clinical implications. J. Biol. Chem. 288, 3500–3511). Due to technical limitations, we failed to separate isotopomers of phosphorylated sugars. In this study, we developed an enabling LC-MS methodology. Using this, we confirmed the previous findings and also showed that NAMPT inhibition led to accumulation of fructose 1-phosphate and sedoheptulose 1-phosphate but not glucose 6-phosphate, fructose 6-phosphate, and sedoheptulose 7-phosphate as previously thought. To investigate the metabolic basis of the metabolite formation, we carried out biochemical and cellular studies and established the following. First, glucose-labeling studies indicated that fructose 1-phosphate was derived from dihydroxyacetone phosphate and glyceraldehyde, and sedoheptulose 1-phosphate was derived from dihydroxyacetone phosphate and erythrose via an aldolase reaction. Second, biochemical studies showed that aldolase indeed catalyzed these reactions. Third, glyceraldehyde- and erythrose-labeling studies showed increased incorporation of corresponding labels into fructose 1-phosphate and sedoheptulose 1-phosphate in FK866-treated cells. Fourth, NAMPT inhibition led to increased glyceraldehyde and erythrose levels in the cell. Finally, glucose-labeling studies showed accumulated fructose 1,6-bisphosphate in FK866-treated cells mainly derived from dihydroxyacetone phosphate and glyceraldehyde 3-phosphate. Taken together, this study shows that NAMPT inhibition leads to attenuation of glycolysis, resulting in further perturbation of carbohydrate metabolism in cancer cells. The potential clinical implications of these findings are also discussed.     

Comparative Metabolic Flux Analysis of Lysine-Producing Corynebacterium glutamicum Cultured on Glucose or Fructose

A comprehensive approach to ¹³C tracer studies, labeling measurements by gas chromatography-mass spectrometry, metabolite balancing, and isotopomer modeling, was applied for comparative metabolic network analysis of lysine-producing Corynebacterium glutamicum on glucose or fructose. Significantly reduced yields of lysine and biomass and enhanced formation of dihydroxyacetone, glycerol, and lactate in comparison to those for glucose resulted on fructose. Metabolic flux analysis revealed drastic differences in intracellular flux depending on the carbon source applied. On fructose, flux through the pentose phosphate pathway (PPP) was only 14.4% of the total substrate uptake flux and therefore markedly decreased compared to that for glucose (62.0%). This result is due mainly to (i) the predominance of phosphoenolpyruvate-dependent phosphotransferase systems for fructose uptake (PTS_Fructose) (92.3%), resulting in a major entry of fructose via fructose 1,6-bisphosphate, and (ii) the inactivity of fructose 1,6-bisphosphatase (0.0%). The uptake of fructose during flux via PTS_Mannose was only 7.7%. In glucose-grown cells, the flux through pyruvate dehydrogenase (70.9%) was much less than that in fructose-grown cells (95.2%). Accordingly, flux through the tricarboxylic acid cycle was decreased on glucose. Normalized to that for glucose uptake, the supply of NADPH during flux was only 112.4% on fructose compared to 176.9% on glucose, which might explain the substantially lower lysine yield of C. glutamicum on fructose. Balancing NADPH levels even revealed an apparent deficiency of NADPH on fructose, which is probably overcome by in vivo activity of malic enzyme. Based on these results, potential targets could be identified for optimization of lysine production by C. glutamicum on fructose, involving (i) modification of flux through the two PTS for fructose uptake, (ii) amplification of fructose 1,6-bisphosphatase to increase flux through the PPP, and (iii) knockout of a not-yet-annotated gene encoding dihydroxyacetone phosphatase or kinase activity to suppress overflow metabolism. Statistical evaluation revealed high precision of the estimates of flux, so the observed differences for metabolic flux are clearly substrate specific.     

mTOR signaling in disease

The target of rapamycin (TOR) is a highly conserved serine/threonine kinase and a central controller of cell growth, metabolism and aging. Mammalian TOR (mTOR) is activated in response to nutrients, growth factors and cellular energy. Dysregulated mTOR signaling has been implicated in major disease. Here we review recent findings on the role of mTOR in cancer, metabolic disorders, neurological diseases, and inflammation.     

The fat side of prostate cancer

Prostate cancer (PCa) metabolism appears to be unique in comparison with other types of solid cancers. Normal prostate cells mainly rely on glucose oxidation to provide precursors for the synthesis and secretion of citrate, resulting in an incomplete Krebs cycle and minimal oxidative phosphorylation for energy production. In contrast, during transformation, PCa cells no longer secrete citrate and they reactivate the Krebs cycle as energy source. Moreover, primary PCas do not show increased aerobic glycolysis and therefore they are not efficiently detectable with ¹⁸F-FDG-PET. However, increased de novo lipid synthesis, strictly intertwined with deregulation in classical oncogenes and oncosuppressors, is an early event of the disease. Up-regulation and increased activity of lipogenic enzymes (including fatty acid synthase and choline kinase) occurs throughout PCa carcinogenesis and correlates with worse prognosis and poor survival. Thus, lipid precursors such as acetate and choline have been successfully used as alternative tracers for PET imaging. Lipid synthesis intermediates and FA catabolism also emerged as important players in PCa maintenance. Finally, epidemiologic studies suggested that systemic metabolic disorders including obesity, metabolic syndrome, and diabetes as well as hypercaloric and fat-rich diets might increase the risk of PCa. However, how metabolic disorders contribute to PCa development and whether dietary lipids and de novo lipids synthesized intra-tumor are differentially metabolized still remains unclear. In this review, we examine the switch in lipid metabolism supporting the development and progression of PCa and we discuss how we can exploit its lipogenic nature for therapeutic and diagnostic purposes. This article is part of a Special Issue entitled Lipid Metabolism in Cancer.     

Targeting cellular metabolism to improve cancer therapeutics

The metabolic properties of cancer cells diverge significantly from those of normal cells. Energy production in cancer cells is abnormally dependent on aerobic glycolysis. In addition to the dependency on glycolysis, cancer cells have other atypical metabolic characteristics such as increased fatty acid synthesis and increased rates of glutamine metabolism. Emerging evidence shows that many features characteristic to cancer cells, such as dysregulated Warburg-like glucose metabolism, fatty acid synthesis and glutaminolysis are linked to therapeutic resistance in cancer treatment. Therefore, targeting cellular metabolism may improve the response to cancer therapeutics and the combination of chemotherapeutic drugs with cellular metabolism inhibitors may represent a promising strategy to overcome drug resistance in cancer therapy. Recently, several review articles have summarized the anticancer targets in the metabolic pathways and metabolic inhibitor-induced cell death pathways, however, the dysregulated metabolism in therapeutic resistance, which is a highly clinical relevant area in cancer metabolism research, has not been specifically addressed. From this unique angle, this review article will discuss the relationship between dysregulated cellular metabolism and cancer drug resistance and how targeting of metabolic enzymes, such as glucose transporters, hexokinase, pyruvate kinase M2, lactate dehydrogenase A, pyruvate dehydrogenase kinase, fatty acid synthase and glutaminase can enhance the efficacy of common therapeutic agents or overcome resistance to chemotherapy or radiotherapy.     

腺苷一磷酸激活蛋白激酶激活剂研究进展

腺苷一磷酸激活蛋白激酶（AMPK）是调控能量代谢的重要激酶,在代谢障碍、心血管疾病及肿瘤等疾病的病理进程中都有重要的调节作用。对AMPK 的结构及其生理调节作用进行介绍,并重点综述AMPK 间接激活剂和直接激活剂的研究进展,旨在为AMPK 激活剂的深入开发提供参考。     

Development of protein kinase activators: AMPK as a target in metabolic disorders and cancer

AMP-activated protein kinase (AMPK) is a cellular energy sensor activated by metabolic stresses that either inhibit ATP synthesis or accelerate ATP consumption. Activation of AMPK in response to an increase in the cellular AMP:ATP ratio results in inhibition of ATP-consuming processes such as gluconeogenesis and fatty acid synthesis, while stimulating ATP-generating processes, including fatty acid oxidation. These alterations in lipid and glucose metabolism would be expected to ameliorate the pathogenesis of obesity, type 2 diabetes and other metabolic disorders. Recently, AMPK has also been identified as a potential target for cancer prevention and/or treatment. Cell growth and proliferation are energetically demanding, and AMPK may act as an “energy checkpoint” that permits growth and proliferation only when energy reserves are sufficient. Thus, activators of AMPK could have potential as novel therapeutics both for metabolic disorders and for cancer, which together constitute two of the most prevalent groups of diseases worldwide.     

5′-AMP-activated Protein Kinase (AMPK) Supports the Growth of Aggressive Experimental Human Breast Cancer Tumors

Rapid tumor growth can establish metabolically stressed microenvironments that activate 5′-AMP-activated protein kinase (AMPK), a ubiquitous regulator of ATP homeostasis. Previously, we investigated the importance of AMPK for the growth of experimental tumors prepared from HRAS-transformed mouse embryo fibroblasts and for primary brain tumor development in a rat model of neurocarcinogenesis. Here, we used triple-negative human breast cancer cells in which AMPK activity had been knocked down to investigate the contribution of AMPK to experimental tumor growth and core glucose metabolism. We found that AMPK supports the growth of fast-growing orthotopic tumors prepared from MDA-MB-231 and DU4475 breast cancer cells but had no effect on the proliferation or survival of these cells in culture. We used in vitro and in vivo metabolic profiling with [¹³C]glucose tracers to investigate the contribution of AMPK to core glucose metabolism in MDA-MB-231 cells, which have a Warburg metabolic phenotype; these experiments indicated that AMPK supports tumor glucose metabolism in part through positive regulation of glycolysis and the nonoxidative pentose phosphate cycle. We also found that AMPK activity in the MDA-MB-231 tumors could systemically perturb glucose homeostasis in sensitive normal tissues (liver and pancreas). Overall, our findings suggest that the contribution of AMPK to the growth of aggressive experimental tumors has a critical microenvironmental component that involves specific regulation of core glucose metabolism.