The sweet side of AMPK signaling: regulation of GFAT1

Mechanism of fatty acid transposase (CD36) promoting fat accumulation in mule ducks

Mule ducks accumulate a large amount of fat in their livers when fed high-energy feed, which is predominantly used for producing fatty livers. Nevertheless, there is limited research on the molecular mechanisms underlying the formation of fatty liver in mule ducks. Fatty acid translocase (CD36) is a sensor for fatty acids and lipid metabolism regulator, which may play a crucial role in the accumulation of fat in the liver of mule ducks. In this study, Overexpression and CD36 gene interference for 24 h was followed by induction of liver cells with 400 µmol/L palmitic acid (PA) for 24 h. The results demonstrated that CD36 overexpression increased hepatic triglyceride content, lipid droplet deposition, oxidative stress, and cell apoptosis. However, interference with CD36 had the opposite effect. CD36 overexpression suppressed the expression of AMPK and CPT-1A genes but enhanced the expression of ACC1 and LKB1 genes, with interference yielding contrasting results. Additionally, the expression of CD36 inhibited the AMPK pathway, reduced AMPK phosphorylation, downregulated AMPK protein expression, and upregulated SREBP1 protein expression. This promoted palmitic acid-induced hepatocyte fat accumulation. In summary, CD36 promotes palmitic acid-induced fat accumulation in primary mule duck liver cells through the AMPK signaling pathway.

Fetal Reprogramming of Nutrient Surplus Signaling, O-GlcNAcylation, and the Evolution of CKD

ABSTRACT

Fetal kidney development is characterized by increased uptake of glucose, ATP production by glycolysis, and upregulation of mammalian target of rapamycin (mTOR) and hypoxia-inducible factor-1 alpha (HIF-1 α ), which (acting in concert) promote nephrogenesis in a hypoxic low-tubular-workload environment. By contrast, the healthy adult kidney is characterized by upregulation of sirtuin-1 and adenosine monophosphate-activated protein kinase, which enhances ATP production through fatty acid oxidation to fulfill the needs of a normoxic high-tubular-workload environment. During stress or injury, the kidney reverts to a fetal signaling program, which is adaptive in the short term, but is deleterious if sustained for prolonged periods when both oxygen tension and tubular workload are heightened. Prolonged increases in glucose uptake in glomerular and proximal tubular cells lead to enhanced flux through the hexosamine biosynthesis pathway; its end product-uridine diphosphate N -acetylglucosamine-drives the rapid and reversible O-GlcNAcylation of thousands of intracellular proteins, typically those that are not membrane-bound or secreted. Both O-GlcNAcylation and phosphorylation act at serine/threonine residues, but whereas phosphorylation is regulated by hundreds of specific kinases and phosphatases, O-GlcNAcylation is regulated only by O-GlcNAc transferase and O-GlcNAcase, which adds or removes N-acetylglucosamine, respectively, from target proteins. Diabetic and nondiabetic CKD is characterized by fetal reprogramming (with upregulation of mTOR and HIF-1 α ) and increased O-GlcNAcylation, both experimentally and clinically. Augmentation of O-GlcNAcylation in the adult kidney enhances oxidative stress, cell cycle entry, apoptosis, and activation of proinflammatory and profibrotic pathways, and it inhibits megalin-mediated albumin endocytosis in glomerular mesangial and proximal tubular cells-effects that can be aggravated and attenuated by augmentation and muting of O-GlcNAcylation, respectively. In addition, drugs with known nephroprotective effects-angiotensin receptor blockers, mineralocorticoid receptor antagonists, and sodium-glucose cotransporter 2 inhibitors-are accompanied by diminished O-GlcNAcylation in the kidney, although the role of such suppression in mediating their benefits has not been explored. The available evidence supports further work on the role of uridine diphosphate N -acetylglucosamine as a critical nutrient surplus sensor (acting in concert with upregulated mTOR and HIF-1 α signaling) in the development of diabetic and nondiabetic CKD.

Foetal recapitulation of nutrient surplus signalling by O-GlcNAcylation and the failing heart

The development of the foetal heart is driven by increased glucose uptake and activation of mammalian target of rapamycin (mTOR) and hypoxia-inducible factor-1α (HIF-1α), which drives glycolysis. In contrast, the healthy adult heart is governed by sirtuin-1 (SIRT1) and adenosine monophosphate-activated protein kinase (AMPK), which promote fatty-acid oxidation and the substantial mitochondrial ATP production required for survival in a high-workload normoxic environment. During cardiac injury, the heart recapitulates the foetal signalling programme, which (although adaptive in the short term) is highly deleterious if sustained for long periods of time. Prolonged increases in glucose uptake in cardiomyocytes under stress leads to increased flux through the hexosamine biosynthesis pathway; its endproduct - uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) - functions as a critical nutrient surplus sensor. UDP-GlcNAc drives the post-translational protein modification known as O-GlcNAcylation, which rapidly and reversibly modifies thousands of intracellular proteins. Both O-GlcNAcylation and phosphorylation act at serine/threonine residues, but whereas phosphorylation is regulated by hundreds of specific kinases and phosphatases, O-GlcNAcylation is regulated by only two enzymes, O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA), which adds or removes GlcNAc (N-acetylglucosamine), respectively, from target proteins. Recapitulation of foetal programming in heart failure (regardless of diabetes) is accompanied by marked increases in O-GlcNAcylation, both experimentally and clinically. Heightened O-GlcNAcylation in the heart leads to impaired calcium kinetics and contractile derangements, arrhythmias related to activation of voltage-gated sodium channels and Ca2+ /calmodulin-dependent protein kinase II, mitochondrial dysfunction, and maladaptive hypertrophy, microvascular dysfunction, fibrosis and cardiomyopathy. These deleterious effects can be prevented by suppression of O-GlcNAcylation, which can be achieved experimentally by upregulation of AMPK and SIRT1 or by pharmacological inhibition of OGT or stimulation of OGA. The effects of sodium-glucose cotransporter 2 (SGLT2) inhibitors on the heart are accompanied by reduced O-GlcNAcylation, and their cytoprotective effects are reportedly abrogated if their action to suppress O-GlcNAcylation is blocked. Such an action may represent one of the many mechanisms by which enhanced AMPK and SIRT1 signalling following SGLT2 inhibition leads to cardiovascular benefits. These observations, taken collectively, suggest that UDP-GlcNAc functions as a critical nutrient surplus sensor (which acting in concert with mTOR and HIF-1α) can promote the development of cardiomyopathy.

Multitranscript analysis reveals an effect of 2-deoxy-d-glucose on gene expression linked to unfolded protein response and integrated stress response in primary human monocytes and monocyte-derived macrophages

BACKGROUND

Glycolytic inhibitor 2-deoxy-d-glucose (2-DG) binds to hexokinase in a non-competitive manner and phosphoglucose isomerase in a competitive manner, blocking the initial steps of the glycolytic pathway. Although 2-DG stimulates endoplasmic reticulum (ER) stress, activating the unfolded protein response to restore protein homeostasis, it is unclear which ER stress-related genes are modulated in response to 2-DG treatment in human primary cells. Here, we aimed to determine whether the treatment of monocytes and monocyte-derived macrophages (MDMs) with 2-DG leads to a transcriptional profile specific to ER stress.

METHODS

We performed bioinformatics analysis to identify differentially expressed genes (DEGs) in previously reported RNA-seq datasets of 2-DG treated cells. RT-qPCR was performed to verify the sequencing data on cultured MDMs.

RESULTS

A total of 95 common DEGs were found by transcriptional analysis of monocytes and MDMs treated with 2-DG. Among these, 74 were up-regulated and 21 were down-regulated. Multitranscript analysis showed that DEGs are linked to integrated stress response (GRP78/BiP, PERK, ATF4, CHOP, GADD34, IRE1α, XBP1, SESN2, ASNS, PHGDH), hexosamine biosynthetic pathway (GFAT1, GNA1, PGM3, UAP1), and mannose metabolism (GMPPA and GMPPB).

CONCLUSIONS

Results reveal that 2-DG triggers a gene expression program that might be involved in restoring protein homeostasis in primary cells.

GENERAL SIGNIFICANCE

2-DG is known to inhibit glycolysis and induce ER stress; however, its effect on gene expression in primary cells is not well understood. This work shows that 2-DG is a stress inducer shifting the metabolic state of monocytes and macrophages.

Disorders of cancer metabolism: The therapeutic potential of cannabinoids

Abnormal energy metabolism, as one of the important hallmarks of cancer, was induced by multiple carcinogenic factors and tumor-specific microenvironments. It comprises aerobic glycolysis, de novo lipid biosynthesis, and glutamine-dependent anaplerosis. Considering that metabolic reprogramming provides various nutrients for tumor survival and development, it has been considered a potential target for cancer therapy. Cannabinoids have been shown to exhibit a variety of anticancer activities by unclear mechanisms. This paper first reviews the recent progress of related signaling pathways (reactive oxygen species (ROS), AMP-activated protein kinase (AMPK), mitogen-activated protein kinases (MAPK), phosphoinositide 3-kinase (PI3K), hypoxia-inducible factor-1alpha (HIF-1α), and p53) mediating the reprogramming of cancer metabolism (including glucose metabolism, lipid metabolism, and amino acid metabolism). Then we comprehensively explore the latest discoveries and possible mechanisms of the anticancer effects of cannabinoids through the regulation of the above-mentioned related signaling pathways, to provide new targets and insights for cancer prevention and treatment.

AMPK is required for recovery from metabolic stress induced by ultrasound microbubble treatment

Ultrasound-stimulated microbubble (USMB) treatment is a promising strategy for cancer therapy. USMB promotes drug delivery by sonoporation and enhanced endocytosis, and also impairs cell viability. However, USMB elicits heterogeneous effects on cell viability, with apparently minimal effects on a subset of cells. This suggests that mechanisms of adaptation following USMB allow some cells to survive and/or proliferate. Herein, we used several triple negative breast cancer cells to identify the molecular mechanisms of adaptation to USMB-induced stress. We found that USMB alters steady-state levels of amino acids, glycolytic intermediates, and citric acid cycle intermediates, suggesting that USMB imposes metabolic stress on cells. USMB treatment acutely reduces ATP levels and stimulates the phosphorylation and activation of AMP-activated protein kinase (AMPK). AMPK is required to restore ATP levels and support cell proliferation post-USMB treatment. These results suggest that AMPK and metabolic perturbations are likely determinants of the antineoplastic efficacy of USMB treatment.

The role and potential mechanism of O-Glycosylation in gastrointestinal tumors

Glycosylation is a critical post-translational modification (PTM) that affects the function of proteins and regulates cell signaling, thereby regulating various biological processes. Protein oxygen-N-acetylglucosamine (O-GlcNAc) glycosylation modifications are glycochemical modifications that occur within cells in the signal transduction and are frequently found in the cytoplasm and nucleus. Due to the rapid and reversible addition and removal, O-GlcNAc modifications are able to reversibly compete with certain phosphorylation modifications, immediately regulate the activity of proteins, and participate in kinds of cellular metabolic and signal transduction pathways, playing a pivotal role in the regulation of tumors, diabetes, and other diseases. This article provided a brief overview of O-GlcNAc glycosylation modification, introduced its role in altering the progression and immune response regulation of gastrointestinal tumors, and discussed its potential use as a marker of tumor neogenesis.

The interplay among oxidative stress, brain insulin resistance and AMPK dysfunction contribute to neurodegeneration in type 2 diabetes and Alzheimer disease

Alzheimer's disease (AD) is the most common form of dementia in the elderly followed by vascular dementia. In addition to clinically diagnosed dementia, cognitive dysfunction has been reported in diabetic patients. Recent studies are now beginning to recognize type 2 diabetes mellitus (T2DM), characterized by chronic hyperglycemia and insulin resistance, as a risk factor for AD and other cognitive disorders. While studies on insulin action have remained traditionally in the domain of peripheral tissues, the detrimental effects of insulin resistance in the central nervous system on cognitive dysfunction are increasingly being reported in recent clinical and preclinical studies. Brain functions require continuous supply of glucose and oxygen and a tight regulation of metabolic processes. Loss of this metabolic regulation has been proposed to be a contributor to memory dysfunction associated with neurodegeneration. Within the above scenario, this review will focus on the interplay among oxidative stress (OS), insulin resistance and AMPK dysfunctions in the brain by highlighting how these neurotoxic events contribute to neurodegeneration. We provide an overview on the detrimental effects of OS on proteins regulating insulin signaling and how these alterations impact cell metabolic dysfunctions through AMPK dysregulation. Such processes, we assert, are critically involved in the molecular pathways that underlie AD.

High-Fat Diet Leads to Reduced Protein O-GlcNAcylation and Mitochondrial Defects Promoting the Development of Alzheimer's Disease Signatures

The disturbance of protein O-GlcNAcylation is emerging as a possible link between altered brain metabolism and the progression of neurodegeneration. As observed in brains with Alzheimer's disease (AD), flaws of the cerebral glucose uptake translate into reduced protein O-GlcNAcylation, which promote the formation of pathological hallmarks. A high-fat diet (HFD) is known to foster metabolic dysregulation and insulin resistance in the brain and such effects have been associated with the reduction of cognitive performances. Remarkably, a significant role in HFD-related cognitive decline might be played by aberrant protein O-GlcNAcylation by triggering the development of AD signature and mitochondrial impairment. Our data support the impairment of total protein O-GlcNAcylation profile both in the brain of mice subjected to a 6-week high-fat-diet (HFD) and in our in vitro transposition on SH-SY5Y cells. The reduction of protein O-GlcNAcylation was associated with the development of insulin resistance, induced by overfeeding (i.e., defective insulin signaling and reduced mitochondrial activity), which promoted the dysregulation of the hexosamine biosynthetic pathway (HBP) flux, through the AMPK-driven reduction of GFAT1 activation. Further, we observed that a HFD induced the selective impairment of O-GlcNAcylated-tau and of O-GlcNAcylated-Complex I subunit NDUFB8, thus resulting in tau toxicity and reduced respiratory chain functionality respectively, highlighting the involvement of this posttranslational modification in the neurodegenerative process.

Molecular genetic study on GATA5 gene promoter in acute myocardial infarction

Background

Acute myocardial infarction (AMI) is a severe type of coronary artery disease, caused by coronary occlusion and followed by cardiac ischaemia. GATA binding protein 5 (GATA5) is an important member of GATA family and plays an important role in vascular inflammation, endothelial function, oxidative stress and cell metabolism. Previous studies have shown that the DNA sequence variants (DSVs) in GATA4 and GATA6 promoter can increase susceptibility to AMI. In this study, we explored the relationship between GATA5 promoter and AMI for the first time, hoping to provide a new genetic basis for understanding the pathogenesis of AMI.

Methods

GATA5 promoter was sequenced in 683 individuals (332 AMI patients and 351 controls). The transcriptional activity of the GATA5 promoter with or without DSVs in HEK-293 cells, H9c2 cells and primary neonatal rat cardiomyocytes were examined by Promega Dual-Luciferase® Reporter Assay system. Electrophoretic mobility shift assay (EMSA) was performed to explore whether the DSVs interfered with the binding of transcription factors (TFs).

Results

Nine mutations have been found in GATA5 promoter, eight of them evidently altered the transcriptional activity of the GATA5 promoter, five of them disrupted the binding of TFs (such as farnesoid X receptor). Furthermore, haplotype AT (across rs80197101 and rs77067995) is a dangerous haplotype of AMI. Genotype GA and allele A of rs80197101 and genotype CT and allele T of rs77067995 are the risk factors of AMI.

Conclusions

DSVs in GATA5 promoter can increase susceptibility to AMI. But the mechanism remains to be verified in vivo.

Signaling Pathways Involved in Nutrient Sensing Control in Cancer Stem Cells: An Overview

Cancer cells characteristically have a high proliferation rate. Because tumor growth depends on energy-consuming anabolic processes, including biosynthesis of protein, lipid, and nucleotides, many tumor-associated conditions, including intermittent oxygen deficiency due to insufficient vascularization, oxidative stress, and nutrient deprivation, results from fast growth. To cope with these environmental stressors, cancer cells, including cancer stem cells, must adapt their metabolism to maintain cellular homeostasis. It is well- known that cancer stem cells (CSC) reprogram their metabolism to adapt to live in hypoxic niches. They usually change from oxidative phosphorylation to increased aerobic glycolysis even in the presence of oxygen. However, as opposed to most differentiated cancer cells relying on glycolysis, CSCs can be highly glycolytic or oxidative phosphorylation-dependent, displaying high metabolic plasticity. Although the influence of the metabolic and nutrient-sensing pathways on the maintenance of stemness has been recognized, the molecular mechanisms that link these pathways to stemness are not well known. Here in this review, we describe the most relevant signaling pathways involved in nutrient sensing and cancer cell survival. Among them, Adenosine monophosphate (AMP)-activated protein kinase (AMPK) pathway, mTOR pathway, and Hexosamine Biosynthetic Pathway (HBP) are critical sensors of cellular energy and nutrient status in cancer cells and interact in complex and dynamic ways.

The Dysregulation of OGT/OGA Cycle Mediates Tau and APP Neuropathology in Down Syndrome

Protein O-GlcNAcylation is a nutrient-related post-translational modification that, since its discovery some 30 years ago, has been associated with the development of neurodegenerative diseases. As reported in Alzheimer's disease (AD), flaws in the cerebral glucose uptake translate into reduced hexosamine biosynthetic pathway flux and subsequently lead to aberrant protein O-GlcNAcylation. Notably, the reduction of O-GlcNAcylated proteins involves also tau and APP, thus promoting their aberrant phosphorylation in AD brain and the onset of AD pathological markers. Down syndrome (DS) individuals are characterized by the early development of AD by the age of 60 and, although the two conditions present the same pathological hallmarks and share the alteration of many molecular mechanisms driving brain degeneration, no evidence has been sought on the implication of O-GlcNAcylation in DS pathology. Our study aimed to unravel for the first time the role of protein O-GlcNacylation in DS brain alterations positing the attention of potential trisomy-related mechanisms triggering the aberrant regulation of OGT/OGA cycle. We demonstrate the disruption of O-GlcNAcylation homeostasis, as an effect of altered OGT and OGA regulatory mechanism, and confirm the relevance of O-GlcNAcylation in the appearance of AD hallmarks in the brain of a murine model of DS. Furthermore, we provide evidence for the neuroprotective effects of brain-targeted OGA inhibition. Indeed, the rescue of OGA activity was able to restore protein O-GlcNAcylation, and reduce AD-related hallmarks and decreased protein nitration, possibly as effect of induced autophagy.

GFAT1/HBP/O-GlcNAcylation Axis Regulates β-Catenin Activity to Promote Pancreatic Cancer Aggressiveness

Reprogrammed glucose and glutamine metabolism are essential for tumor initiation and development. As a branch of glucose and metabolism, the hexosamine biosynthesis pathway (HBP) generates uridine diphosphate N-acetylglucosamine (UDP-GlcNAc) and contributes to the O-GlcNAcylation process. However, the spectrum of HBP-dependent tumors and the mechanisms by which the HBP promotes tumor aggressiveness remain areas of active investigation. In this study, we analyzed the activity of the HBP and its prognostic value across 33 types of human cancers. Increased HBP activity was observed in pancreatic ductal adenocarcinoma (PDAC), and higher HBP activity predicted a poor prognosis in PDAC patients. Genetic silencing or pharmacological inhibition of the first and rate-limiting enzyme of the HBP, glutamine:fructose-6-phosphate amidotransferase 1 (GFAT1), inhibited PDAC cell proliferation, invasive capacity, and triggered cell apoptosis. Notably, these effects can be restored by addition of UDP-GlcNAc. Moreover, similar antitumor effects were noticed by pharmacological inhibition of GFAT1 with 6-diazo-5-oxo-l-norleucine (DON) or Azaserine. PDAC is maintained by oncogenic Wnt/β-catenin transcriptional activity. Our data showed that GFAT1 can regulate β-catenin expression via modulation of the O-GlcNAcylation process. TOP/FOP-Flash and real-time qPCR analysis showed that GFAT1 knockdown inhibited β-catenin activity and the transcription of its downstream target genes CCND1 and MYC. Ectopic expression of a stabilized form of β-catenin restored the suppressive roles of GFAT1 knockdown on PDAC cell proliferation and invasion. Collectively, our findings indicate that higher GFAT1/HBP/O-GlcNAcylation exhibits tumor-promoting roles by maintaining β-catenin activity in PDAC.

AMP-Activated Protein Kinase and O-GlcNAcylation, Two Partners Tightly Connected to Regulate Key Cellular Processes

The AMP-activated protein kinase (AMPK) is an important cellular energy sensor. Its activation under energetic stress is known to activate energy-producing pathways and to inactivate energy-consuming pathways, promoting ATP preservation and cell survival. AMPK has been shown to play protective role in many pathophysiological processes including cardiovascular diseases, diabetes, and cancer. Its action is multi-faceted and comprises short-term regulation of enzymes by direct phosphorylation as well as long-term adaptation via control of transcription factors and cellular events such as autophagy. During the last decade, several studies underline the particular importance of the interaction between AMPK and the post-translational modification called O-GlcNAcylation. O-GlcNAcylation means the O-linked attachment of a single N-acetylglucosamine moiety on serine or threonine residues. O-GlcNAcylation plays a role in multiple physiological cellular processes but is also associated with the development of various diseases. The first goal of the present review is to present the tight molecular relationship between AMPK and enzymes regulating O-GlcNAcylation. We then draw the attention of the reader on the putative importance of this interaction in different pathophysiological events.