Metabolic regulation of gene transcription in mammals

Modulation of the replication of positive-sense RNA viruses by the natural plant metabolite xanthohumol and its derivatives

The COVID-19 pandemic has highlighted the importance of identifying new potent antiviral agents. Nutrients as well as plant-derived substances are promising candidates because they are usually well tolerated by the human body and readily available in nature, and consequently mostly cheap to produce. A variety of antiviral effects have recently been described for the hop chalcone xanthohumol (XN), and to a lesser extent for its derivatives, making these hop compounds particularly attractive for further investigation. Noteworthy, mounting evidence indicated that XN can suppress a wide range of viruses belonging to several virus families, all of which share a common reproductive cycle. As a result, the purpose of this review is to summarize the most recent research on the antiviral properties of XN and its derivatives, with a particular emphasis on the positive-sense RNA viruses human hepatitis C virus (HCV), porcine reproductive and respiratory syndrome virus (PRRSV), and severe acute respiratory syndrome corona virus (SARS-CoV-2).

Lipid Metabolic Alterations in KRAS Mutant Tumors: Unmasking New Vulnerabilities for Cancer Therapy

KRAS is one of the most commonly mutated genes, an event that leads to development of highly aggressive and resistant to any type of available therapy tumors. Mutated KRAS drives a complex network of lipid metabolic rearrangements to support the adaptation of cancer cells to harsh environmental conditions and ensure their survival. Because there has been only a little success in the continuous efforts of effectively targeting KRAS-driven tumors, it is of outmost importance to delineate the exact mechanisms of how they get rewired, leading to this distinctive phenotype. Therefore, the aim of this review is to summarize the available data acquired over the last years with regard to the lipid metabolic regulation of KRAS-driven tumors and elucidate their specific characteristics in an attempt to unravel novel therapeutic targets.

Comprehensive Metabolic Profiling of MYC-Amplified Medulloblastoma Tumors Reveals Key Dependencies on Amino Acid, Tricarboxylic Acid and Hexosamine Pathways

Simple Summary

The oncogene MYC alters cellular metabolism. Medulloblastoma is the most common malignant pediatric brain tumor. MYC-amplified medulloblastoma has a poor prognosis, and the metabolism of MYC-amplified medulloblastoma is poorly understood. We performed comprehensive metabolic profiling of MYC-amplified medulloblastoma and found increased reliance on potentially targetable pathways. We also found that the metabolism of MYC-amplified cell lines differed from orthotopic brain tumors in vitro and in flank tumors, suggesting that analyses conducted in vitro or in flank tumors may miss key vulnerabilities.

Abstract

Reprograming of cellular metabolism is a hallmark of cancer. Altering metabolism allows cancer cells to overcome unfavorable microenvironment conditions and to proliferate and invade. Medulloblastoma is the most common malignant brain tumor of children. Genomic amplification of MYC defines a subset of poor-prognosis medulloblastoma. We performed comprehensive metabolic studies of human MYC-amplified medulloblastoma by comparing the metabolic profiles of tumor cells in three different conditions—in vitro, in flank xenografts and in orthotopic xenografts in the cerebellum. Principal component analysis showed that the metabolic profiles of brain and flank high-MYC medulloblastoma tumors clustered closely together and separated away from normal brain and in vitro MYC-amplified cells. Compared to normal brain, MYC-amplified medulloblastoma orthotopic xenograft tumors showed upregulation of the TCA cycle as well as the synthesis of nucleotides, hexosamines, amino acids and glutathione. There was significantly higher glucose uptake and usage in orthotopic xenograft tumors compared to flank xenograft tumors and cells in culture. In orthotopic tumors, glucose was the main carbon source for the de novo synthesis of glutamate, glutamine and glutathione through the TCA cycle. In vivo, the glutaminase II pathway was the main pathway utilizing glutamine. Glutathione was the most abundant upregulated metabolite in orthotopic tumors compared to normal brain. Glutamine-derived glutathione was synthesized through the glutamine transaminase K (GTK) enzyme in vivo. In conclusion, high MYC medulloblastoma cells have different metabolic profiles in vitro compared to in vivo, and key vulnerabilities may be missed by not performing in vivo metabolic analyses.

High OGT activity is essential for MYC-driven proliferation of prostate cancer cells

O-GlcNAc transferase (OGT) is overexpressed in aggressive prostate cancer. OGT modifies intra-cellular proteins via single sugar conjugation (O-GlcNAcylation) to alter their activity. We recently discovered the first fast-acting OGT inhibitor OSMI-2. Here, we probe the stability and function of the chromatin O-GlcNAc and identify transcription factors that coordinate with OGT to promote proliferation of prostate cancer cells.

Methods: Chromatin immunoprecipitation (ChIP) coupled to sequencing (seq), formaldehyde-assisted isolation of regulatory elements, RNA-seq and reverse-phase protein arrays (RPPA) were used to study the importance of OGT for chromatin structure and transcription. Mass spectrometry, western blot, RT-qPCR, cell cycle analysis and viability assays were used to establish the role of OGT for MYC-related processes. Prostate cancer patient data profiled for both mRNA and protein levels were used to validate findings.

Results: We show for the first time that OGT inhibition leads to a rapid loss of O-GlcNAc chromatin mark. O-GlcNAc ChIP-seq regions overlap with super-enhancers (SE) and MYC binding sites. OGT inhibition leads to down-regulation of SE-dependent genes. We establish the first O-GlcNAc chromatin consensus motif, which we use as a bait for mass spectrometry. By combining the proteomic data from oligonucleotide enrichment with O-GlcNAc and MYC ChIP-mass spectrometry, we identify host cell factor 1 (HCF-1) as an interaction partner of MYC. Inhibition of OGT disrupts this interaction and compromises MYC's ability to confer androgen-independent proliferation to prostate cancer cells. We show that OGT is required for MYC-mediated stabilization of mitotic proteins, including Cyclin B1, and/or the increased translation of their coding transcripts. This implies that increased expression of mRNA is not always required to achieve increased protein expression and confer aggressive phenotype. Indeed, high expression of Cyclin B1 protein has strong predictive value in prostate cancer patients (p=0.000014) while mRNA does not.

Conclusions: OGT promotes SE-dependent gene expression. OGT activity is required for the interaction between MYC and HCF-1 and expression of MYC-regulated mitotic proteins. These features render OGT essential for the androgen-independent, MYC-driven proliferation of prostate cancer cells. Androgen-independency is the major mechanism of prostate cancer progression, and our study identifies OGT as an essential mediator in this process.

Tanycyte Gene Expression Dynamics in the Regulation of Energy Homeostasis

Animal survival relies on a constant balance between energy supply and energy expenditure, which is controlled by several neuroendocrine functions that integrate metabolic information and adapt the response of the organism to physiological demands. Polarized ependymoglial cells lining the floor of the third ventricle and sending a single process within metabolic hypothalamic parenchyma, tanycytes are henceforth described as key components of the hypothalamic neural network controlling energy balance. Their strategic position and peculiar properties convey them diverse physiological functions ranging from blood/brain traffic controllers, metabolic modulators, and neural stem/progenitor cells. At the molecular level, these functions rely on an accurate regulation of gene expression. Indeed, tanycytes are characterized by their own molecular signature which is mostly associated to their diverse physiological functions, and the detection of variations in nutrient/hormone levels leads to an adequate modulation of genetic profile in order to ensure energy homeostasis. The aim of this review is to summarize recent knowledge on the nutritional control of tanycyte gene expression.

Comparison of the Biochemical Composition and Nutritional Quality Between Diploid and Triploid Hong Kong Oysters, Crassostrea hongkongensis

This study is the first systematic comparison of the biochemical composition and nutritional quality between diploid and triploid Hong Kong oysters, Crassostrea hongkongensis. Results showed that in the reproductive season, the glycogen content in five tissues (gill, mantle, adductor muscle, labial palps and gonad) was significantly higher (P < 0.05) in triploids than in diploids, with odds ratios (ORs) of 96.26, 60.17, 72.59, 53.56, and 128.52%, respectively. In the non-reproductive phase, significant differences in glycogen content (P < 0.05) between diploid and triploid oysters existed only in gill and gonad. In both diploid and triploid Hong Kong oysters, quantitative real-time PCR analysis of the glycogen synthesis gene (ChGS) and glycogen phosphorylase gene (ChGP) showed that the gene expression patterns matched the pattern of variation in glycogen content. Moreover, in both the reproductive and the non-reproductive phases, triploid Hong Kong oysters had a well balance of essential amino acids and were thus a well source of high-quality protein. Surprisingly, in both phases, significantly higher (P < 0.05) percentages of four essential fatty acids (α-linolenic acid, linoleic acid, eicosapentaenoic acid, and docosahexaenoic acid) were observed in triploids than in diploids. Additionally, the ratio of n-3/n-6 polyunsaturated fatty acids (PUFAs) was much higher in triploids than that in diploids. Variations in Biochemical composition were consistent with the relative expression of the citrate synthase gene (ChCS) and the α-ketoglutarate dehydrogenase gene (ChKD), which are key enzyme genes of the tricarboxylic acid cycle. Overall, the triploid Hong Kong oyster has a better nutritional value and taste than the diploid in terms of glycogen content, protein quality and fatty acid content.

New Strategies and Challenges in Lung Proteomics and Metabolomics. An Official American Thoracic Society Workshop Report

This document presents the proceedings from the workshop entitled, "New Strategies and Challenges in Lung Proteomics and Metabolomics" held February 4th-5th, 2016, in Denver, Colorado. It was sponsored by the National Heart Lung Blood Institute, the American Thoracic Society, the Colorado Biological Mass Spectrometry Society, and National Jewish Health. The goal of this workshop was to convene, for the first time, relevant experts in lung proteomics and metabolomics to discuss and overcome specific challenges in these fields that are unique to the lung. The main objectives of this workshop were to identify, review, and/or understand: (1) emerging technologies in metabolomics and proteomics as applied to the study of the lung; (2) the unique composition and challenges of lung-specific biological specimens for metabolomic and proteomic analysis; (3) the diverse informatics approaches and databases unique to metabolomics and proteomics, with special emphasis on the lung; (4) integrative platforms across genetic and genomic databases that can be applied to lung-related metabolomic and proteomic studies; and (5) the clinical applications of proteomics and metabolomics. The major findings and conclusions of this workshop are summarized at the end of the report, and outline the progress and challenges that face these rapidly advancing fields.

Beneficiary effect of Commiphora mukul ethanolic extract against high fructose diet induced abnormalities in carbohydrate and lipid metabolism in wistar rats

The present study was proposed to elucidate the effect of Commiphora mukul gum resin elthanolic extract treatment on alterations in carbohydrate and lipid metabolisms in rats fed with high-fructose diet. Male Wistar rats were divided into four groups: two of these groups (group C and C+CM) were fed with standard pellet diet and the other two groups (group F and F+CM) were fed with high fructose (66 %) diet. C. mukul suspension in 5% Tween-80 in distilled water (200 mg/kg body weight/day) was administered orally to group C+CM and group F+CM. At the end of 60-day experimental period, biochemical parameters related to carbohydrate and lipid metabolisms were assayed. C. mukul treatment completely prevented the fructose-induced increased body weight, hyperglycemia, and hypertriglyceridemia. Hyperinsulinemia and insulin resistance observed in group F decreased significantly with C. mukul treatment in group F+CM. The alterations observed in the activities of enzymes of carbohydrate and lipid metabolisms and contents of hepatic tissue lipids in group F rats were significantly restored to near normal values by C. mukul treatment in group F+CM. In conclusion, our study demonstrated that C. mukul treatment is effective in preventing fructose-induced insulin resistance and hypertriglyceridemia while attenuating the fructose induced alterations in carbohydrate and lipid metabolisms by the extract which was further supported by histopathological results from liver samples which showed regeneration of the hepatocytes. This study suggests that the plant can be used as an adjuvant for the prevention and/or management of insulin resistance and disorders related to it.

Increased mitochondrial arginine metabolism supports bioenergetics in asthma

High levels of arginine metabolizing enzymes, including inducible nitric oxide synthase (iNOS) and arginase (ARG), are typical in asthmatic airway epithelium; however, little is known about the metabolic effects of enhanced arginine flux in asthma. Here, we demonstrated that increased metabolism sustains arginine availability in asthmatic airway epithelium with consequences for bioenergetics and inflammation. Expression of iNOS, ARG2, arginine synthetic enzymes, and mitochondrial respiratory complexes III and IV was elevated in asthmatic lung samples compared with healthy controls. ARG2 overexpression in a human bronchial epithelial cell line accelerated oxidative bioenergetic pathways and suppressed hypoxia-inducible factors (HIFs) and phosphorylation of the signal transducer for atopic Th2 inflammation STAT6 (pSTAT6), both of which are implicated in asthma etiology. Arg2-deficient mice had lower mitochondrial membrane potential and greater HIF-2α than WT animals. In an allergen-induced asthma model, mice lacking Arg2 had greater Th2 inflammation than WT mice, as indicated by higher levels of pSTAT6, IL-13, IL-17, eotaxin, and eosinophils and more mucus metaplasia. Bone marrow transplants from Arg2-deficient mice did not affect airway inflammation in recipient mice, supporting resident lung cells as the drivers of elevated Th2 inflammation. These data demonstrate that arginine flux preserves cellular respiration and suppresses pathological signaling events that promote inflammation in asthma.

Differential regulation of human Eag1 channel expression by serum and epidermal growth factor in lung and breast cancer cells

Oncogenic ether à-go-go-1 (Eag1) potassium channels are overexpressed in most primary human solid tumors. Low oxygen and nutrient/growth factor concentrations play critical roles in tumorigenesis. However, the mechanisms by which tumor cells survive and proliferate under growth factor-depleted conditions remain elusive. Here, we investigated whether serum-deprived conditions and epidermal growth factor (EGF) regulate Eag1 expression in human lung and breast cancer cells. The human cancer cell lines A549 and MCF-7 (from the lungs and breast, respectively) were obtained from the American Type Culture Collection and cultured following the manufacturer’s recommendations. Eag1 gene and protein expression were studied by real-time PCR and immunocytochemistry, respectively. Cell proliferation was evaluated using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, and ERK1/2 phosphorylation was investigated by Western blot. Serum-deprived conditions increased Eag1 mRNA and protein expression in both cell lines. This Eag1 upregulation was prevented by EGF and the ERK1/2 inhibitor U0126 in only lung cancer cells; vascular endothelial growth factor did not prevent Eag1 upregulation. Our results suggest that Eag1 may act as a survival and mitogenic factor under low-serum and nutrient conditions and may be a clinical target during the early stages of tumor development.

Non-classical gluconeogenesis-dependent glucose metabolism in Rhipicephalus microplus embryonic cell line BME26

In this work we evaluated several genes involved in gluconeogenesis, glycolysis and glycogen metabolism, the major pathways for carbohydrate catabolism and anabolism, in the BME26 Rhipicephalus microplus embryonic cell line. Genetic and catalytic control of the genes and enzymes associated with these pathways are modulated by alterations in energy resource availability (primarily glucose). BME26 cells in media were investigated using three different glucose concentrations, and changes in the transcription levels of target genes in response to carbohydrate utilization were assessed. The results indicate that several genes, such as glycogen synthase (GS), glycogen synthase kinase 3 (GSK3), phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6 phosphatase (GP) displayed mutual regulation in response to glucose treatment. Surprisingly, the transcription of gluconeogenic enzymes was found to increase alongside that of glycolytic enzymes, especially pyruvate kinase, with high glucose treatment. In addition, RNAi data from this study revealed that the transcription of gluconeogenic genes in BME26 cells is controlled by GSK-3. Collectively, these results improve our understanding of how glucose metabolism is regulated at the genetic level in tick cells.

Dissecting the global variation of gene expression for the functional interpretation of transcriptome data

To perform their biological functions, individual genes exhibit varying ranges of expression levels. Thus, considering the intrinsic variability of gene expression can improve geneset-based functional analyses which are typically used to interpret transcriptome data. Through the extensive quantitative analysis of the expressional variability of individual genes using large collections of transcriptome and proteome data, we found the existence of the intrinsic variability of gene expression at the transcriptional level. Interestingly, genes under post-translational regulation were not sensitively regulated at the transcriptional level. Because genes have intrinsically different levels of regulation at the transcription and translation stages, the functional geneset-based interpretation of transcriptome data should only include genes that are significantly varied at the transcriptional level. Thus, by removing genes with low transcriptional variation from the DNA microarray data, we showed that geneset enrichment analysis could provide improved resolution in prioritizing target functional pathways in several different experimental datasets.

Effect of the combined probiotics with aflatoxin B₁-degrading enzyme on aflatoxin detoxification, broiler production performance and hepatic enzyme gene expression

In order to degrade aflatoxin B₁ (AFB₁), AFB₁-degrading microbes (probiotics) such as Lactobacillus casei, Bacillus subtilis and Pichia anomala, and the AFB₁-degrading enzyme from Aspergillus oryzae were selected and combined to make feed additive. Seventy-five 43-day-old male Arbor Acres broilers were randomly divided into 5 groups, 15 broilers for each group. The broilers were given with 5 kinds of diets such as the basal diet, 400 μg/kg AFB₁ supplement without feed additive, and 200, 400, 800 μg/kg AFB₁ supplement with 0.15% feed additive. The feeding experimental period was 30 d, which was used to determine production performance of broilers. In addition, serum, liver and chest muscle were selected for measuring AFB₁ residues, gene expressions, microscopic and antioxidant analyses. The results showed that adding 0.15% feed additive in broiler diets could significantly relieve the negative effect of AFB₁ on chicken's production performance and nutrient metabolic rates (P<0.05). It could also improve AFB₁ metabolism, hepatic cell structure, antioxidant activity, and many hepatic enzyme gene expressions involved in oxidoreductase, apoptosis, cell growth, immune system and metabolic process (P<0.05). It could be concluded that the feed additive was able to degrade AFB₁ and improve animal production.

Impacts of CA9 gene polymorphisms and environmental factors on oral-cancer susceptibility and clinicopathologic characteristics in Taiwan

Background

In Taiwan, oral cancer has causally been associated with environmental carcinogens. Carbonic anhydrase 9 (CA9) is reportedly overexpressed in several types of carcinomas and is generally considered a marker of malignancy. The current study explored the combined effect of CA9 gene polymorphisms and exposure to environmental carcinogens on the susceptibility of developing oral squamous cell carcinoma (OSCC) and the clinicopathological characteristics of the tumors.

Methodology and Principal Findings

Four single-nucleotide polymorphisms (SNPs) of the CA9 gene from 462 patients with oral cancer and 519 non-cancer controls were analyzed by a real-time polymerase chain reaction (PCR). While the studied SNPs (CA9 rs2071676, rs3829078, rs1048638 and +376 Del) were not associated with susceptibility to oral cancer, the GAA haplotype of 3 CA9 SNPs (rs2071676, rs3829078, and rs1048638) was related to a higher risk of oral cancer. Moreover, the four CA9 SNPs combined with betel quid chewing and/or tobacco consumption could robustly elevate susceptibility to oral cancer. Finally, patients with oral cancer who had at least one G allele of CA9 rs2071676 were at higher risk for developing lymph-node metastasis (p = 0.022), compared to those patients homozygous for AA.

Conclusions

Our results suggest that the haplotype of rs2071676, rs3829078, and rs1048638 combined has potential predictive significance in oral carcinogenesis. Gene-environment interactions of CA9 polymorphisms, smoking, and betel-quid chewing might alter oral cancer susceptibility and metastasis.

Strategic role for mitochondria in Alzheimer's disease and cancer

SIGNIFICANCE

Detailed knowledge about cell death and cell survival mechanisms and how these pathways are impaired in neurodegenerative disorders and cancer forms the basis for future drug development for these diseases that affect millions of people around the world.

RECENT ADVANCES

In neurodegenerative disorders such as Alzheimer's disease (AD), cell death pathways are inappropriately activated, resulting in neuronal cell death. In contrast, cancer cells develop resistance to apoptosis by regulating anti-apoptotic proteins signaling via mitochondria. Mounting evidence shows that mitochondrial function is central in both cancer and AD. Cancer cells typically shut down oxidative phosphorylation (OXPHOS) in mitochondria and switch to glycolysis for ATP production, making them resistant to hypoxia. In AD, for example, amyloid-β peptide (Aβ) and reactive oxygen species impair mitochondrial function. Neurons therefore also switch to glycolysis to maintain ATP production and to produce molecules involved in antioxidant metabolism in an attempt to survive.

CRITICAL ISSUES

One critical difference between cancer cells and neurons is that cancer cells can survive without OXPHOS, while neurons are dependent on OXPHOS for long-term survival.

FUTURE DIRECTIONS

This review will focus on these abnormalities of mitochondrial function shared in AD and cancer and discuss the potential mechanisms underlying links that may be key steps in the development of therapeutic strategies.

Short-term serum deprivation confers sensitivity to taxanes in platinum-resistant human ovarian cancer cells

BACKGROUND

Based on the evidences showing that serum deprivation provokes apoptosis in a variety of cells, we have investigated the effect of serum deprivation on drug sensitivity.

METHODS

After human ovarian cancer cells were preincubated in 0.5 % serum containing medium for 12 hours, cellular drug sensitivities were determined by colony-forming assay.

RESULTS

Serum deprivation treatment resulted in significant increase in paclitaxel sensitivity by factors of mean ± SD, 148.6 ± 28.1 and 10.1 ± 1.0 (n = 3; P < 0.001) fold in platinum-resistant C13 and CP70 cells, respectively. Similarly, serum deprivation induced significant docetaxel sensitivity in these cell lines. However, no enhancement effect of serum deprivation was observed in platinum-sensitive 2008 and A2780 cells. Serum deprivation did not have any effect on the sensitivities to cisplatin, vincristin, and doxorubicin in all of these cells. More than 7-fold increase of apoptotic cells were observed in C13 or CP70 cells when they were treated by serum deprivation followed by paclitaxel compared with the treatment of either serum deprivation or paclitaxel alone. Confocal laser microscopy using rhodamine 123 and flow cytometric analysis with 3,3'-dihexyloxacarbocyanine iodide revealed that serum deprivation decreased mitochondrial membrane potential in C13 or CP70 cells, whereas no change was observed in 2008 and A2780 cells. This indicates that serum deprivation induced depolarization specifically in platinum-resistant cells. Electron microscopy revealed that serum deprivation caused regeneration of mitochondrial matrix structure in C13 or CP70 cells where mitochondria were usually destructed and disappeared.

DISCUSSIONS

These results indicate that serum deprivation confers taxane hypersensitivity specifically in platinum-resistant cells by recovering their impaired mitochondrial functions. The evidence might be clinically beneficial for the development of new chemotherapeutic technology, particularly for the patients with platinum-resistant ovarian cancer.

Molecular mechanisms involved in the adaptation to amino acid limitation in mammals

In mammals, metabolic adaptations are required to cope with episodes of protein deprivation and malnutrition. Consequently, mammals have to adjust physiological functions involved in the adaptation to amino acid availability. Part of this regulation involves the modulation of the expression of numerous genes. In particular, it has been shown that amino acids by themselves can modify the expression of target genes. This review describes the regulation of amino acids homeostasis and the their role as signal molecules. The recent advances in the understanding of the molecular mechanisms involved in the control of mammalian gene expression in response to amino acid limitation will be described.

Ethanol enhances hepatitis C virus replication through lipid metabolism and elevated NADH/NAD+

Ethanol has been suggested to elevate HCV titer in patients and to increase HCV RNA in replicon cells, suggesting that HCV replication is increased in the presence and absence of the complete viral replication cycle, but the mechanisms remain unclear. In this study, we use Huh7 human hepatoma cells that naturally express comparable levels of CYP2E1 as human liver to demonstrate that ethanol, at subtoxic and physiologically relevant concentrations, enhances complete HCV replication. The viral RNA genome replication is affected for both genotypes 2a and 1b. Acetaldehyde, a major product of ethanol metabolism, likewise enhances HCV replication at physiological concentrations. The potentiation of HCV replication by ethanol is suppressed by inhibiting CYP2E1 or aldehyde dehydrogenase and requires an elevated NADH/NAD(+) ratio. In addition, acetate, isopropyl alcohol, and concentrations of acetone that occur in diabetics enhance HCV replication with corresponding increases in the NADH/NAD(+). Furthermore, inhibiting the host mevalonate pathway with lovastatin or fluvastatin and fatty acid synthesis with 5-(tetradecyloxy)-2-furoic acid or cerulenin significantly attenuates the enhancement of HCV replication by ethanol, acetaldehyde, acetone, as well as acetate, whereas inhibiting beta-oxidation with beta-mercaptopropionic acid increases HCV replication. Ethanol, acetaldehyde, acetone, and acetate increase the total intracellular cholesterol content, which is attenuated with lovastatin. In contrast, both endogenous and exogenous ROS suppress the replication of HCV genotype 2a, as previously shown with genotype 1b.

CONCLUSION

Therefore, lipid metabolism and alteration of cellular NADH/NAD(+) ratio are likely to play a critical role in the potentiation of HCV replication by ethanol rather than oxidative stress.

Plant-derived sucrose is a key element in the symbiotic association between Trichoderma virens and maize plants

Fungal species belonging to the genus Trichoderma colonize the rhizosphere of many plants, resulting in beneficial effects such as increased resistance to pathogens and greater yield and productivity. However, the molecular mechanisms that govern the recognition and association between Trichoderma and their hosts are still largely unknown. In this report, we demonstrate that plant-derived sucrose (Suc) is an important resource provided to Trichoderma cells and is also associated with the control of root colonization. We describe the identification and characterization of an intracellular invertase from Trichoderma virens (TvInv) important for the mechanisms that control the symbiotic association and fungal growth in the presence of Suc. Gene expression studies revealed that the hydrolysis of plant-derived Suc in T. virens is necessary for the up-regulation of Sm1, the Trichoderma-secreted elicitor that systemically activates the defense mechanisms in leaves. We determined that as a result of colonization of maize (Zea mays) roots by T. virens, photosynthetic rate increases in leaves and the functional expression of tvinv is crucial for such effect. In agreement, the steady-state levels of mRNA for Rubisco small subunit and the oxygen-evolving enhancer 3-1 were increased in leaves of plants colonized by wild-type T. virens. We conclude that during the symbiosis, the sucrolytic activity in the fungal cells affects the sink activity of roots, directing carbon partitioning toward roots and increasing the rate of photosynthesis in leaves. A discussion of the role of Suc in controlling the fungal proliferation on roots and its pivotal role in the coordination of plant-microbe associations is provided.

Multifunctional roles of enolase in Alzheimer's disease brain: beyond altered glucose metabolism

Enolase enzymes are abundantly expressed, cytosolic carbon-oxygen lyases known for their role in glucose metabolism. Recently, enolase has been shown to possess a variety of different regulatory functions, beyond glycolysis and gluconeogenesis, associated with hypoxia, ischemia, and Alzheimer's disease (AD). AD is an age-associated neurodegenerative disorder characterized pathologically by elevated oxidative stress and subsequent damage to proteins, lipids, and nucleic acids, appearance of neurofibrillary tangles and senile plaques, and loss of synapse and neuronal cells. It is unclear if development of a hypometabolic environment is a consequence of or contributes to AD pathology, as there is not only a significant decline in brain glucose levels in AD, but also there is an increase in proteomics identified oxidatively modified glycolytic enzymes that are rendered inactive, including enolase. Previously, our laboratory identified alpha-enolase as one the most frequently up-regulated and oxidatively modified proteins in amnestic mild cognitive impairment (MCI), early-onset AD, and AD. However, the glycolytic conversion of 2-phosphoglycerate to phosphoenolpyruvate catalyzed by enolase does not directly produce ATP or NADH; therefore it is surprising that, among all glycolytic enzymes, alpha-enolase was one of only two glycolytic enzymes consistently up-regulated from MCI to AD. These findings suggest enolase is involved with more than glucose metabolism in AD brain, but may possess other functions, normally necessary to preserve brain function. This review examines potential altered function(s) of brain enolase in MCI, early-onset AD, and AD, alterations that may contribute to the biochemical, pathological, clinical characteristics, and progression of this dementing disorder.

Insulin signaling and the general amino acid control response. Two distinct pathways to amino acid synthesis and uptake

ATF4 is a transcription factor that induces a genetic program for amino acid synthesis and amino acid uptake. Previous work demonstrated that ATF4 expression is increased either by insulin or by the general amino acid control (GAAC) response, an evolutionarily ancient pathway that is activated when eukaryotic cells are deprived of amino acids. It is not known whether insulin and the GAAC pathway increase ATF4 expression by the same or different mechanisms. In these studies, we demonstrate that insulin-mediated ATF4 expression occurs as part of a coordinated anabolic program that does not require an essential component of the GAAC pathway, the protein kinase GCN2. Moreover, insulin and the GAAC pathway have an additive effect on expression of ATF4 and downstream mRNAs for amino acid synthesis and uptake. These data suggest that the GAAC pathway may facilitate insulin-mediated anabolism when exogenous amino acids are limiting. We conclude that insulin signaling and the GAAC response comprise two distinct yet complimentary pathways to ATF4 expression, allowing anabolism to be finely tuned to amino acid availability.

Detailed molecular analysis of the induction of the L-PK gene by glucose

Glucose has powerful effects on gene expression and participates in the fasted-to-fed transition of the liver. However, the molecular mechanism of glucose-regulated gene expression has not been completely described. In the present study, we performed a detailed analysis of the molecular events of the insulin-independent glucose response of the liver-type pyruvate kinase (L-PK) gene. L-PK mRNA was increased by glucose at the transcriptional level as determined by real-time RT-PCR, mRNA stability measurements, and nuclear run-on assays. LY294002 and LY303511 inhibited the glucose response of the L-PK gene at the transcriptional level. Histones H3 and H4 associated with the L-PK gene promoter were hyperacetylated and HNF4alpha was constitutively bound in low and high glucose. Treatment with 20mM glucose increased recruitment of ChREBP, additional HNF4alpha, and RNA polymerase II. Glucose-stimulated the phosphorylation of the C-terminal domain of RNA polymerase II, with increased Ser5 phosphorylation near the transcription start site and increased Ser2 phosphorylation near the termination signal. LY294002 and LY303511 blocked the recruitment of RNA polymerase II to the L-PK gene, reducing the rate of transcription. The results of these studies demonstrate fundamental details of the molecular mechanism of glucose activated gene expression.

Identification and function of phosphorylation in the glucose-regulated transcription factor ChREBP

In the liver, induction of genes encoding enzymes involved in de novo lipogenesis occurs in response to increased glucose metabolism. ChREBP (carbohydrate-response-element-binding protein) is a basic helix-loop-helix/leucine zipper transcription factor that regulates expression of these genes. To evaluate the potential role of ChREBP phosphorylation in its regulation, we used MS to identify modified residues. In the present paper, we report the detection of multiple phosphorylation sites of ChREBP expressed in hepatocytes, several of which are only observed under high-glucose conditions. Mutation of each of these serine/threonine residues of ChREBP did not alter its ability to respond to glucose. However, mutation of five N-terminal phosphoacceptor sites resulted in a major decrease in activity under high-glucose conditions. These phosphorylated residues are located within a region of ChREBP (amino acids 1-197) that is critical for glucose regulation. Mutation of Ser(56) within this region to an aspartate residue resulted in increased nuclear accumulation and activity under high-glucose conditions. Together, these data suggest that ChREBP activity is regulated by complex multisite phosphorylation patterns involving its N-terminal regulatory region.

c-Myc and ChREBP regulate glucose-mediated expression of the L-type pyruvate kinase gene in INS-1-derived 832/13 cells

Increased glucose flux generates metabolic signals that control transcriptional programs through poorly understood mechanisms. Previously, we demonstrated a necessity in hepatocytes for c-Myc in the regulation of a prototypical glucose-responsive gene, L-type pyruvate kinase (L-PK) (Collier JJ, Doan TT, Daniels MC, Schurr JR, Kolls JK, Scott DK. J Biol Chem 278: 6588-6595, 2003). Pancreatic beta-cells have many features in common with hepatocytes with respect to glucose-regulated gene expression, and in the present study we determined whether c-Myc was required for the L-PK glucose response in insulin-secreting (INS-1)-derived 832/13 cells. Glucose increased c-Myc abundance and association with its heterodimer partner, Max. Manipulations that prevented the formation of a functional c-Myc/Max heterodimer reduced the expression of the L-PK gene. In addition, glucose augmented the binding of carbohydrate response element binding protein (ChREBP), c-Myc, and Max to the promoter of the L-PK gene in situ. The transactivation of ChREBP, but not of c-Myc, was dependent on high glucose concentrations in the contexts of either the L-PK promoter or a heterologous promoter. The glucose-mediated transactivation of ChREBP was independent of mutations that alter phosphorylation sites thought to regulate the cellular location of ChREBP. We conclude that maximal glucose-induced expression of the L-PK gene in INS-1-derived 832/13 cells involves increased c-Myc abundance, recruitment of c-Myc, Max, and ChREBP to the promoter, and a glucose-stimulated increase in ChREBP transactivation.

Regulation of rat hepatic L-pyruvate kinase promoter composition and activity by glucose, n-3 polyunsaturated fatty acids, and peroxisome proliferator-activated receptor-alpha agonist

Carbohydrate regulatory element-binding protein (ChREBP), MAX-like factor X (MLX), and hepatic nuclear factor-4alpha (HNF-4alpha) are key transcription factors involved in the glucose-mediated induction of hepatic L-type pyruvate kinase (L-PK) gene transcription. n-3 polyunsaturated fatty acids (PUFA) and WY14643 (peroxisome proliferator-activated receptor alpha (PPARalpha) agonist) interfere with glucose-stimulated L-PK gene transcription in vivo and in rat primary hepatocytes. Feeding rats a diet containing n-3 PUFA or WY14643 suppressed hepatic mRNA(L-PK) but did not suppress hepatic ChREBP or HNF-4alpha nuclear abundance. Hepatic MLX nuclear abundance, however, was suppressed by n-3 PUFA but not WY14643. In rat primary hepatocytes, glucose-stimulated accumulation of mRNA(LPK) and L-PK promoter activity correlated with increased ChREBP nuclear abundance. This treatment also increased L-PK promoter occupancy by RNA polymerase II (RNA pol II), acetylated histone H3 (Ac-H3), and acetylated histone H4 (Ac-H4) but did not significantly impact L-PK promoter occupancy by ChREBP or HNF-4alpha. Inhibition of L-PK promoter activity by n-3 PUFA correlated with suppressed RNA pol II, Ac-H3, and Ac-H4 occupancy on the L-PK promoter. Although n-3 PUFA transiently suppressed ChREBP and MLX nuclear abundance, this treatment did not impact ChREBP-LPK promoter interaction. HNF4alpha-LPK promoter interaction was transiently suppressed by n-3 PUFA. Inhibition of L-PK promoter activity by WY14643 correlated with a transient decline in ChREBP nuclear abundance and decreased Ac-H4 interaction with the L-PK promoter. WY14643, however, had no impact on MLX nuclear abundance or HNF4alpha-LPK promoter interaction. Although overexpressed ChREBP or HNF-4alpha did not relieve n-3 PUFA suppression of L-PK gene expression, overexpressed MLX fully abrogated n-3 PUFA suppression of L-PK promoter activity and mRNA(L-PK) abundance. Overexpressed ChREBP, but not MLX, relieved the WY14643 inhibition of L-PK. In conclusion, n-3 PUFA and WY14643/PPARalpha target different transcription factors to control L-PK gene transcription. MLX, the heterodimer partner for ChREBP, has emerged as a novel target for n-3 PUFA regulation.

Insulin regulation of glucokinase gene expression: evidence against a role for sterol regulatory element binding protein 1 in primary hepatocytes

Liver key genes for carbohydrate and lipid homeostasis are regulated by insulin and glucose. The sterol regulatory-element binding protein-1c (SREBP-1c) has emerged as a mediator of insulin effects on gene transcription, particularly on glucokinase (GK). In this paper, we show that despite stimulation of GK promoter transcription by overexpression of mature SREBP-1c, insulin induced GK transcription at least 4h ahead of accumulation of mature SREBP-1c in the nucleus. Importantly, the knockdown of SREBP-1 mRNA using a RNA-interference technique reduced the level of nuclear SREBP-1 protein, diminished fatty acid synthase mRNA level, but did not affect GK and L-pyruvate kinase mRNA levels. We concluded that SREBP-1 is unlikely to be the mediator of the early insulin effect on GK gene transcription.

Molecular cloning and seasonal expression of oyster glycogen phosphorylase and glycogen synthase genes

To investigate the control at the mRNA level of glycogen metabolism in the cupped oyster Crassostrea gigas, we report in the present paper the cloning and characterization of glycogen phosphorylase and synthase cDNAs (Cg-GPH and Cg-GYS, respectively, transcripts of main enzymes for glycogen use and storage), and their first expression profiles depending on oyster tissues and seasons. A strong expression of both genes was observed in the labial palps and the gonad in accordance with specific cells located in both tissues and ability to store glucose. Cg-GPH expression was also found mainly in muscle suggesting ability to use glycogen as readily available glucose to supply its activity. For seasonal examinations, expression of Cg-GYS and Cg-GPH genes appeared to be regulated according to variation in glycogen content. Relative levels of Cg-GYS transcripts appeared highest in October corresponding to glycogen storage and resting period. Relative levels of Cg-GPH transcripts were highest in May corresponding to mobilization of glycogen needed for germ cell maturation. Expression of both genes would likely be driven by the oyster's reproductive cycle, reflecting the central role of glycogen in energy storage and gametogenic development in C. gigas. Both genes are useful molecular markers in the regulation of glycogen metabolism and reproduction in C. gigas but enzymatic regulation of glycogen phosphorylase and synthase remains to be elucidated.

A modest glucokinase overexpression in the liver promotes fed expression levels of glycolytic and lipogenic enzyme genes in the fasted state without altering SREBP-1c expression

Hepatic genes crucial for carbohydrate and lipid homeostasis are regulated by insulin and glucose metabolism. However, the relative contributions of insulin and glucose to the regulation of metabolic gene expression are poorly defined in vivo. To address this issue, adenovirus-mediated hepatic overexpression of glucokinase was used to determine the effects of increased hepatic glucose metabolism on gene expression in fasted or ad libitum fed rats. In the fasted state, a 3 fold glucokinase overexpression was sufficient to mimic feeding-induced increases in pyruvate kinase and acetyl CoA carboxylase mRNA levels, demonstrating a primary role for glucose metabolism in the regulation of these genes in vivo. Conversely, glucokinase overexpression was unable to mimic feeding-induced alterations of fatty acid synthase, glucose-6-phosphate dehydrogenase, carnitine palmitoyl transferase I or PEPCK mRNAs, indicating insulin as the primary regulator of these genes. Interestingly, glucose-6-phosphatase mRNA was increased by glucokinase overexpression in both the fasted and fed states, providing evidence, under these conditions, for the dominance of glucose over insulin signaling for this gene in vivo. Importantly, glucokinase overexpression did not alter sterol regulatory element binding protein 1-c mRNA levels in vivo and glucose signaling did not alter the expression of this gene in primary hepatocytes. We conclude that a modest hepatic overexpression of glucokinase is sufficient to alter expression of metabolic genes without changing the expression of SREBP-1c.

Direct inhibition of the ubiquitin–proteasome pathway by ester bond-containing green tea polyphenols is associated with increased expression of sterol regulatory element-binding protein 2 and LDL receptor

Green tea has been shown to lower plasma cholesterol, associated with up-regulation of the low-density lipoprotein receptor (LDLR) although the responsible molecular mechanism is unknown. Previously, we reported that ester bond-containing green tea polyphenols (GTPs), such as (-)-epigallocatechin-3-gallate [(-)-EGCG], potently inhibit the tumor cellular proteasome activity, which may contribute to the cancer-preventative effect of green tea. In the current study, we hypothesize that the proteasome is a heart disease-associated molecular target of GTPs. We have shown that ester bond-containing GTPs, including (-)-EGCG, potently inhibit the proteasomal activity in intact hepatocellular carcinoma HepG2 and cervical carcinoma HeLa cells, as evident by accumulation of ubiquitinated proteins and three natural proteasome targets (p27, IkappaB-alpha and Bax). (-)-EGCG selectively inhibits the chymotrypsin-like, but not trypsin-like, activity of the proteasome. Associated with proteasome inhibition by ester bond-containing GTPs, there was a significant, time- and concentration-dependent increase in levels of the cleaved, activated, but not the precursor, form of sterol regulatory element-binding protein 2 (SREBP-2), an essential factor for LDLR transcription. Subsequently, LDL receptor expression was increased dramatically in HepG2 and HeLa cells treated with (-)-EGCG. Our results suggest that ester bond-containing GTPs inhibit ubiquitin/proteasome-mediated degradation of the active SREBP-2, resulting in up-regulation of LDLR. This identified molecular mechanism may be related to the previously reported cholesterol-lowering and heart disease-preventative effects of green tea.

Deficiency of carbohydrate response element-binding protein (ChREBP) reduces lipogenesis as well as glycolysis

The liver provides for long-term energy needs of the body by converting excess carbohydrate into fat for storage. Insulin is one factor that promotes hepatic lipogenesis, but there is increasing evidence that glucose also contributes to the coordinated regulation of carbohydrate and fat metabolism in liver by mechanisms that are independent of insulin. In this study, we show that the transcription factor, carbohydrate response element-binding protein (ChREBP), is required both for basal and carbohydrate-induced expression of several liver enzymes essential for coordinated control of glucose metabolism, fatty acid, and the synthesis of fatty acids and triglycerides in vivo.

Insulin-independent induction of sterol regulatory element-binding protein-1c expression in the livers of streptozotocin-treated mice

Insulin and glucose together have been previously shown to regulate hepatic sterol regulatory element-binding protein (SREBP)-1c expression. We sought to explore the nutritional regulation of lipogenesis through SREBP-1c induction in a setting where effects of sugars versus insulin could be distinguished. To do so, mice were insulin depleted by streptozotocin (STZ) administration and subjected to a fasting-refeeding protocol with glucose, fructose, or sucrose. Unexpectedly, the insulin-depleted mice exhibited a marked induction of SREBP-1c on all sugars, and this increase in SREBP-1c was even more dramatic than in the non-STZ-administered controls. The time course of changes in SREBP-1 induction varied depending on the type of sugars in both control and STZ-administered mice. Glucose refeeding gave a peak of SREBP-1c induction, whereas fructose refeeding caused slow and gradual increments, and sucrose refeeding fell between these two responses. Expression of various lipogenic enzymes were also gradually increased over time, irrespective of the types of sugars, with greater intensities in STZ-administered than in nontreated mice. In contrast, induction of hepatic glucokinase and suppression of phoshoenolpyruvate carboxykinase were insulin dependent in an early refed state. These data clearly demonstrate that nutritional regulation of SREBP-1c and lipogenic genes may be completely independent of insulin as long as sufficient carbohydrates are available.

A low protein diet alters gene expression in rat pancreatic islets

Insulin secretion is regulated mainly by circulating nutrients, particularly glucose, and is also modulated by hormonal and neuronal inputs. Nutritional alterations during fetal and early postnatal periods, induced by either low protein or energy-restricted diets, produce beta-cell dysfunction. As a consequence, insulin secretion in response to different secretagogues is reduced, as is the number of beta-cells and the size and vascularization of islets. In this study, we used a cDNA macroarray technique and RT-PCR to assess the pattern of gene expression in pancreatic islets from rats fed isocaloric low (6 g/100 g, LP) and normal (17 g/100 g, NP) protein diets, after weaning. Thirty-two genes related to metabolism, neurotransmitter receptors, protein trafficking and targeting, intracellular kinase network members and hormones had altered expression (up- or down-regulated). RT-PCR confirmed the macroarray results for five selected genes, i.e., clusterin, secretogranin II precursor, eukaryotic translation initiation factor 2, phospholipase A(2) and glucose transporter. Thus, cDNA macroarray analysis revealed significant changes in the gene expression pattern in rats fed a low protein diet after weaning. The range of proteins affected indicated that numerous mechanisms are involved in the intracellular alterations in the endocrine pancreas, including impaired glucose-induced insulin secretion.

Mlx is the functional heteromeric partner of the carbohydrate response element-binding protein in glucose regulation of lipogenic enzyme genes

The expression of genes encoding enzymes involved in de novo triglyceride synthesis (lipogenesis) is transcriptionally induced in the liver in response to increased glucose metabolism. The carbohydrate response element-binding protein (ChREBP) is a newly identified basic helix-loop-helix/leucine zipper transcription factor proposed to regulate the expression of the glucose-responsive gene pyruvate kinase. This gene contains a carbohydrate response element (ChoRE) consisting of two E box motifs separated by 5 bp that is necessary and sufficient for glucose regulation. We demonstrate that overexpression of ChREBP in primary rat hepatocytes activates other ChoRE-containing promoters in a manner consistent with their ability to respond to glucose. In vitro binding of ChREBP to ChoRE sequences was not detected. Because E box-binding proteins function as obligate dimers, we performed a yeast two-hybrid screen of a mouse liver cDNA library to identify potential heteromeric partners. Mlx (Max-like protein X) was selected as the only basic helix-loop-helix/leucine zipper interaction partner in this screen. When a plasmid expressing either Mlx or ChREBP was cotransfected with a ChoRE-containing reporter plasmid into human embryonic kidney 293 cells, no increase in promoter activity was observed. However, the expression of both proteins dramatically enhanced promoter activity. This activation was observed with reporters containing ChoREs from several different lipogenic enzyme genes. In contrast, reporters containing non-glucose-responsive E box elements were not activated by ChREBP-Mlx expression. In vitro binding of ChREBP to ChoRE-containing oligonucleotides was observed only in the presence of Mlx. ChREBP-Mlx binding discriminated between E box sites that are glucose-responsive and those that are not. We conclude that Mlx is a functional heteromeric partner of ChREBP in regulating the expression of glucose-responsive genes.

Regulation of protein synthesis by branched-chain amino acids in vivo

Recent advances in the understanding of mRNA translation have facilitated molecular studies on the regulation of protein synthesis by nutrients and the interplay between nutrients and hormonal signals. Numerous reports have established that, in skeletal muscle, the branched-chain amino acids (BCAAs) have the unique ability to initiate signal transduction pathways that modulate translation initiation. Of the BCAAs, leucine is the most potent. Oral administration of leucine to food-deprived rats enhances muscle protein synthesis, in part, through activation of the mRNA binding step of translation initiation. Interestingly, leucine signaling in skeletal muscle differs from that in liver, suggesting that the responses may be tissue specific. The purpose of this paper was to briefly review the current knowledge of how BCAAs act as regulators of protein synthesis in physiologically important tissues, with particular focus on the mechanisms by which BCAAs regulate translation initiation.

Amino acids as regulators of gene expression: molecular mechanisms

Regulation of gene expression by nutrients in mammals is an important mechanism allowing them to adapt their physiological functions according to the supply of nutrient in the diet. It has been shown recently that amino acids are able to regulate by themselves the expression of numerous genes. CHOP, asparagine synthetase, and IGFBP-1 regulation following AA starvation will be described in this review with special interest in the molecular mechanisms involved.

Regulation of glucose-6-phosphatase gene expression in cultured hepatocytes and H4IIE cells by short-chain fatty acids: role of hepatic nuclear factor-4alpha

Mechanisms underlying dietary nutrient regulation of glucose-6-phosphatase (Glc-6-Pase) gene expression are not well understood. Here we investigated the effects of short-chain fatty acids on the expression of this gene in primary cultures of rat hepatocytes and H4IIE hepatoma cells. Propionate, butyrate, valerate, and caproate induced severalfold increases in the expression of Glc-6-Pase mRNA. In reporter gene assays, propionate, valerate, caproate, and also octanoate increased Glc-6-Pase promoter activity by 6-16-fold. Butyrate, by itself, had little or no effect on promoter activity, but it induced a robust increase (45-fold) in promoter activity in cells co-transfected with a plasmid expressing the transcription factor HNF-4alpha (alpha isoforms of hepatic nuclear factor 4). HNF-4alpha also enhanced promoter activity induced by other short-chain fatty acids. A dominant negative form of HNF-4alpha abrogated the fatty acid-induced promoter activity, a finding that accentuates a role for HNF-4alpha in the transcription process studied here. In cells transfected with HNF-4alpha, short-chain fatty acids and trichostatin A, an inhibitor of histone deacetylase, synergistically enhanced promoter activity, suggesting that hyperacetylation of histones is an important component of the transactivation of the Glc-6-Pase gene promoter by HNF-4alpha. Region-751/-466 of this promoter contains seven putative HNF-4alpha-binding motifs. Binding of HNF-4alpha to this region was confirmed by electrophoretic mobility shift and chromatin immunoprecipitation assays, indicating that HNF-4alpha is recruited to the Glc-6-Pase gene promoter during short-chain fatty acid-induced transcription from this promoter.

Recent advances in the understanding of amino acid regulation of gene expression

In mammals, the impact of nutrients on gene expression has become an important area of research. Because amino acids have multiple and important functions, their homeostasis has to be finely maintained. However, amino acidemia can be affected by certain nutritional conditions or various forms of stress. Consequently, mammals must adjust several of the physiological functions involved in the adaptation to amino acid availability by regulating expression of numerous genes. It has been shown that amino acids alone can modify the expression of target genes. However, understanding of amino acid-dependent control of gene expression has just started to emerge. This review focuses on recent advances in the understanding of mechanisms involved in the amino acid control of gene expression.

Xylulose 5-phosphate mediates glucose-induced lipogenesis by xylulose 5-phosphate-activated protein phosphatase in rat liver

Carbohydrate-responsive element binding protein (ChREBP) is a transcription factor that activates lipogenic genes in liver in response to excess carbohydrate in the diet. ChREBP is regulated in a reciprocal manner by glucose and cAMP. cAMP-dependent protein kinase (protein kinase A) phosphorylates two physiologically important sites in ChREBP, Ser-196, which is located near nuclear localization signal sequence (NLS), and Thr-666, within the basic helix-loop-helix (bHLH) site, resulting in inactivation of nuclear translocation of ChREBP and of the DNA-binding activity, respectively. We demonstrate here that crude cytosolic extracts from livers of rats fed a high carbohydrate diet contained protein phosphatase (PPase) activity that dephosphorylated a peptide containing Ser-196, whereas a PPase in the nuclear extract catalyzed dephosphorylation of Ser-568 and Thr-666. All these PPases are activated specifically by xylulose 5-phosphate (Xu5P), but not by other sugar phosphates. Furthermore, addition of Xu5P elevated PPase activity to the level observed in extracts of fed liver cells. These partially purified PPases were characterized as PP2A-AB delta C by immunoblotting with specific antibodies. These results suggest that (ia) Xu5P-dependent PPase is responsible for activation of transcription of the L-type pyruvate kinase gene and lipogenic enzyme genes, and (ii) Xu5P is the glucose signaling compound. Thus, we propose that the same Xu5P-activated PPase controls both acute and long-term regulation of glucose metabolism and fat synthesis.

c-Myc is required for the glucose-mediated induction of metabolic enzyme genes

Glucose exerts powerful effects on hepatocyte gene transcription by mechanisms that are incompletely understood. c-Myc regulates hepatic glucose metabolism by increasing glycolytic enzyme gene transcription while concomitantly decreasing gluconeogenic and ketogenic enzyme gene expression. However, the molecular mechanisms by which c-Myc exerts these effects is not known. In this study, the glucose-mediated induction of L-type pyruvate kinase and glucose-6-phosphatase mRNA levels was diminished by maneuvers involving recombinant adenoviral vectors that interfere with (i) c-Myc protein levels by antisense expression or (ii) c-Myc function through a dominant-negative Max protein. These results were obtained using both HL1C rat hepatoma cells and primary rat hepatocytes. Furthermore, a decrease in c-Myc abundance reduced glucose production in HL1C cells, presumably by decreasing glucose-6-phosphatase activity. The repression of hormone-activated phosphoenolpyruvate carboxykinase gene transcription by glucose was not affected by a reduction in c-Myc levels. The basal mRNA levels for L-pyruvate kinase and glucose-6-phosphatase were not altered to any significant degree by adenoviral treatment. Furthermore, adenoviral overexpression of the c-Myc protein induced glucose-6-phosphatase mRNA in the absence of glucose stimulation. We conclude that multiple mechanisms exist to communicate the glucose-derived signal and that c-Myc has a key role in the hepatic glucose signaling pathway.

Signaling cross-talk between hypoxia and glucose via hypoxia-inducible factor 1 and glucose response elements

The substrates oxygen and glucose are important for the appropriate regulation of metabolism, angiogenesis, tumorigenesis and embryonic development. The knowledge about an interaction between these two signals is limited. We demonstrated that the regulation of glucagon receptor, insulin receptor and L-type pyruvate kinase (L-PK) gene expression in liver is dependent upon a cross-talk between oxygen and glucose. The periportal to perivenous drop in O2 tension was proposed to be an endocrine key regulator for the zonated gene expression in liver. In primary rat hepatocyte cultures, the expression of the glucagon receptor and the L-PK mRNA was maximally induced by glucose under arterial pO2 whereas the insulin receptor was maximally induced under perivenous pO2. It was demonstrated for the L-PK gene that the modulation by O2 of the glucose-dependent induction occured at the glucose-responsive element (Glc(PK)RE) in the L-PK gene promoter. The reduction of the glucose-dependent induction of the L-PK gene expression under venous pO2 appeared to be mediated via an interference between hypoxia-inducible factor 1 (HIF-1) and the glucose-responsive transcription factors at the Glc(PK)RE. The glucose response element (GlcRE) also functioned as a hypoxia response element and, vice versa, a hypoxia-responsive element was functioning as a GlcRE. Thus, our findings implicate that the cross-talk between oxygen and glucose might have a fundamental role in the regulation of several physiological and pathophysiological processes.

Characterization of a carbohydrate response element regulating the gene for human galactose-1-phosphate uridyltransferase

Human galactose-1-phosphate uridyltransferase (hGALT) is a central enzyme in the conserved pathway by which galactose is converted to energy, UDP-galactose and UDP-glucose. A natural mutation that deleted -119 to -116 bp (delGTCA) of the promoter decreased hGALT mRNA and enzyme activity and prompted analysis of hGALT gene regulation. Regulatory domains were identified by inspection and confirmed in a reporter system. Previous studies by others were confirmed that HepG2 cells grown in D-glucose increased hGALT enzyme activity and mRNA by 30%. We extended these observations by sequencing the promoter region and identifying a potential carbohydrate response element (ChoRE). The response to glucose rose to 190% when a plasmid construct containing a luciferase reporter and only the -165 bp region as a promoter was transfected into HepG2 and NIH:OVCAR-3. By contrast, fibroblasts transfected with the identical construct failed to respond to glucose. Within the -165 bp region there were two enhancer (E-box) motifs that encompassed the delGTCA mutation. The deletion diminished the positive regulatory response, but an additional GTCA repeat unexpectedly increased the response. Using this postulated ChoRE as a probe in electrophoretic mobility shift assays, multiple nuclear proteins bound and one was identified as a member of the basic/helix-loop-helix/leucine zipper enhancer-binding (b/HLH/LZ) family. Increased binding of proteins correlated with increased hGALT expression when the spacing between E-box motifs was enlarged but the carbohydrate response was dampened. When the 3(')E-box was mutated, b/HLH/LZ binding and gene expression were abolished. We conclude that the hGALT promoter region contains a ChoRE in which the spacing between and the sequence of its E-box motifs are critical. One nuclear protein of the b/HLH/LZ family is necessary, but not sufficient for the carbohydrate response.

The role of SREBP-1c in nutritional regulation of lipogenic enzyme gene expression

A high carbohydrate diet up-regulates the transcription of enzymes of triglyceride biosynthesis (lipogenesis) in the mammalian liver. This treatment stimulates hepatic insulin signaling, leading to transcription of sterol regulatory element-binding protein-1c (SREBP-1c). SREBP-1c has been implicated as a major factor that up-regulates lipogenic genes in response to carbohydrate feeding. However, we presented evidence for another factor, carbohydrate response factor, which is also involved in this response, and we proposed a model wherein SREBP-1c and carbohydrate response factor are independent transcription factors that act in response to insulin and glucose, respectively. In this study, we examined the contribution of SREBP-1c to the expression of lipogenic genes in glucose- and insulin-treated primary rat hepatocytes using an inducible adenovirus system. We found that SREBP-1c overexpression leads to a modest induction of fatty acid synthase, S(14), and acetyl-CoA carboxylase mRNAs to 20% (fatty acid synthase), 10% (S(14)), and 5% (acetyl-CoA carboxylase) of the induction seen by high glucose and insulin treatment. Restoring insulin to cells overexpressing SREBP-1c did not further increase these mRNA levels. In contrast, adenovirus-expressed SREBP-1c did not induce pyruvate kinase mRNA, suggesting that induction of this gene is SREBP-1c-independent. SREBP-1c does indeed play a role in the induction of lipogenic enzyme genes in response to insulin treatment, but it is not sufficient for the induction seen when hepatocytes are treated with insulin and high glucose.

Carbohydrate responsive element-binding protein (ChREBP): a key regulator of glucose metabolism and fat storage

Feeding a high carbohydrate diet induces transcription of more than 15 genes involved in the metabolic conversion of glucose to fat. A new transcription factor binding to a glucose response element of the pyruvate kinase and lipogenesis enzyme genes was discovered recently. This factor, termed carbohydrate responsive element-binding protein (ChREBP), is activated in response to high glucose and up-regulates these genes. Cyclic AMP and a high fat diet inhibit ChREBP and slow down glucose utilization. ChREBP is able to control transcription of lipogenic enzyme genes in response to nutritional and hormonal inputs, and may play an important role in disease states such as diabetes, obesity, and hypertension.

The role of C/EBP in nutrient and hormonal regulation of gene expression

C/EBPs are a family of transcription factors that play important roles in energy metabolism. Although initially thought to be constitutive regulators of transcription, an increasing amount of evidence indicates that their transactivating capacity within the cell can be modulated by nutrients and hormones. There are several mechanisms whereby this occurs. First, hormones/nutrients are known to directly alter the expression of C/EBPs. Second, hormones/nutrients may cause an alteration in the phosphorylation state of C/EBPs, which can affect their DNA-binding activity or transactivating capacity. Third, C/EBPs can function as accessory factors on gene promoters within a hormone response unit, interacting with other transcription factors to enhance the degree of responsiveness to specific hormones. Given their role in regulating genes involved in a wide variety of metabolic events, advancing our understanding of the molecular mechanism of action of C/EBPs will undoubtedly further our appreciation for the role these transcription factors play in both health and disease.