Uridine prevents the glucose deprivation-induced death of immunostimulated astrocytes via the action of uridine phosphorylase

Nucleosides are overlooked fuels in central carbon metabolism

From our daily nutrition and synthesis within cells, nucleosides enter the bloodstream and circulate throughout the body and tissues. Nucleosides and nucleotides are classically viewed as precursors of nucleic acids, but recently they have emerged as a novel energy source for central carbon metabolism. Through catabolism by nucleoside phosphorylases, the ribose sugar group is released and can provide substrates for lower steps in glycolysis. In environments with limited glucose, such as at sites of infection or in the tumor microenvironment (TME), cells can use, and may even require, this alternative energy source. Here, we discuss the implications of these new findings in health and disease and speculate on the potential new roles of nucleosides and nucleic acids in energy metabolism.

Metabolites that feed upper glycolytic branches support glucose independent human cytomegalovirus replication

The broad tissue distribution and cell tropism of human cytomegalovirus indicates that the virus successfully replicates in tissues with various nutrient environments. HCMV requires and reprograms central carbon metabolism for viral replication. However, many studies focus on reprogramming of metabolism in high nutrient conditions that do not recapitulate physiological nutrient environments in the body. In this study, we investigate how HCMV successfully replicates when nutrients are suboptimal. We limited glucose following HCMV infection to determine how glucose supports virus replication and how nutrients potentially present in the physiological environment contribute to successful glucose independent HCMV replication. Glucose is required for HCMV viral genome synthesis, viral protein production and glycosylation, and virus production. However, supplement of glucose-free cultures with uridine, ribose, or UDP-GlcNAc-metabolites that support upper glycolytic branches-resulted in partially restored viral genome synthesis and subsequent partial restoration of viral protein levels. Low levels of virus production were also restored. Supplementing lower glycolysis in glucose-free cultures using pyruvate had no effect on virus replication. These results indicate nutrients that support upper glycolytic branches like the pentose phosphate pathway and hexosamine pathway can compensate for glucose during HCMV replication to support low levels of virus production. More broadly, our findings suggest that HCMV could successfully replicate in diverse metabolic niches, including those in the body with low levels of glucose, through alternative nutrient usage.

Uridine-derived ribose fuels glucose-restricted pancreatic cancer

Pancreatic ductal adenocarcinoma (PDA) is a lethal disease notoriously resistant to therapy1,2. This is mediated in part by a complex tumour microenvironment3, low vascularity4, and metabolic aberrations5,6. Although altered metabolism drives tumour progression, the spectrum of metabolites used as nutrients by PDA remains largely unknown. Here we identified uridine as a fuel for PDA in glucose-deprived conditions by assessing how more than 175 metabolites impacted metabolic activity in 21 pancreatic cell lines under nutrient restriction. Uridine utilization strongly correlated with the expression of uridine phosphorylase 1 (UPP1), which we demonstrate liberates uridine-derived ribose to fuel central carbon metabolism and thereby support redox balance, survival and proliferation in glucose-restricted PDA cells. In PDA, UPP1 is regulated by KRAS-MAPK signalling and is augmented by nutrient restriction. Consistently, tumours expressed high UPP1 compared with non-tumoural tissues, and UPP1 expression correlated with poor survival in cohorts of patients with PDA. Uridine is available in the tumour microenvironment, and we demonstrated that uridine-derived ribose is actively catabolized in tumours. Finally, UPP1 deletion restricted the ability of PDA cells to use uridine and blunted tumour growth in immunocompetent mouse models. Our data identify uridine utilization as an important compensatory metabolic process in nutrient-deprived PDA cells, suggesting a novel metabolic axis for PDA therapy.

Salvage of ribose from uridine or RNA supports glycolysis in nutrient-limited conditions

Glucose is vital for life, serving as both a source of energy and carbon building block for growth. When glucose is limiting, alternative nutrients must be harnessed. To identify mechanisms by which cells can tolerate complete loss of glucose, we performed nutrient-sensitized genome-wide genetic screens and a PRISM growth assay across 482 cancer cell lines. We report that catabolism of uridine from the medium enables the growth of cells in the complete absence of glucose. While previous studies have shown that uridine can be salvaged to support pyrimidine synthesis in the setting of mitochondrial oxidative phosphorylation deficiency1, our work demonstrates that the ribose moiety of uridine or RNA can be salvaged to fulfil energy requirements via a pathway based on: (1) the phosphorylytic cleavage of uridine by uridine phosphorylase UPP1/UPP2 into uracil and ribose-1-phosphate (R1P), (2) the conversion of uridine-derived R1P into fructose-6-P and glyceraldehyde-3-P by the non-oxidative branch of the pentose phosphate pathway and (3) their glycolytic utilization to fuel ATP production, biosynthesis and gluconeogenesis. Capacity for glycolysis from uridine-derived ribose appears widespread, and we confirm its activity in cancer lineages, primary macrophages and mice in vivo. An interesting property of this pathway is that R1P enters downstream of the initial, highly regulated steps of glucose transport and upper glycolysis. We anticipate that 'uridine bypass' of upper glycolysis could be important in the context of disease and even exploited for therapeutic purposes.

Nutritional Niches of Cancer Therapy-Induced Senescent Cells

Therapy-induced senescence (TIS) is a state of stable proliferative arrest of both normal and neoplastic cells that is triggered by exposure to anticancer treatments. TIS cells acquire a senescence-associated secretory phenotype (SASP), which is pro-inflammatory and actively promotes tumor relapse and adverse side-effects in patients. Here, we hypothesized that TIS cells adapt their scavenging and catabolic ability to overcome the nutritional constraints in their microenvironmental niches. We used a panel of mechanistically-diverse TIS triggers (i.e., bleomycin, doxorubicin, alisertib, and palbociclib) and Biolog Phenotype MicroArrays to identify (among 190 different carbon and nitrogen sources) candidate metabolites that support the survival of TIS cells in limiting nutrient conditions. We provide evidence of distinguishable TIS-associated nutrient consumption profiles involving a core set of shared (e.g., glutamine) and unique (e.g., glucose-1-phosphate, inosine, and uridine) nutritional sources after diverse senescence-inducing interventions. We also observed a trend for an inverse correlation between the intensity of the pro-inflammatory SASP provoked by different TIS agents and diversity of compensatory nutritional niches utilizable by senescent cells. These findings support the detailed exploration of the nutritional niche as a new metabolic dimension to understand and target TIS in cancer.

The Multifaceted Roles of Adipose Tissue-Therapeutic Targets for Diabetes and Beyond: The 2015 Banting Lecture

The Banting Medal for Scientific Achievement is the highest scientific award of the American Diabetes Association (ADA). Given in memory of Sir Frederick Banting, one of the key investigators in the discovery of insulin, the Banting Medal is awarded annually for scientific excellence, recognizing significant long-term contributions to the understanding, treatment, or prevention of diabetes. Philipp E. Scherer, PhD, of the Touchstone Diabetes Center, The University of Texas Southwestern Medical Center, Dallas, TX, received the prestigious award at the ADA's 75th Scientific Sessions, 5-9 June 2015, in Boston, MA. He presented the Banting Lecture, "The Multifaceted Roles of Adipose Tissue-Therapeutic Targets for Diabetes and Beyond," on Sunday, 7 June 2015.A number of different cell types contribute to the cellular architecture of adipose tissue. Although the adipocyte is functionally making important contributions to systemic metabolic homeostatis, several additional cell types contribute a supportive role to bestow maximal flexibility on the tissue with respect to many biosynthetic and catabolic processes, depending on the metabolic state. These cells include vascular endothelial cells, a host of immune cells, and adipocyte precursor cells and fibroblasts. Combined, these cell types give rise to a tissue with remarkable flexibility with respect to expansion and contraction, while optimizing the ability of the tissue to act as an endocrine organ through the release of many protein factors, critically influencing systemic lipid homeostasis and biochemically contributing many metabolites. Using an example from each of these categories-adiponectin as a key adipokine, sphingolipids as critical mediators of insulin sensitivity, and uridine as an important metabolite contributed by the adipocyte to the systemic pool-I will discuss the emerging genesis of the adipocyte over the past 20 years from metabolic bystander to key driver of metabolic flexibility.

Neuroprotective effects of uridine in a rat model of neonatal hypoxic-ischemic encephalopathy

Neonatal hypoxic-ischemic encephalopathy (HIE) is a major cause of neurological disability requiring newer therapeutic strategies. Uridine is the principal circulating pyrimidine in humans and a substrate for nucleotides and membrane phospholipids. The objective of this study was to investigate the effects of uridine in a neonatal rat model of HIE. Rat pups subjected to hypoxic-ischemic insult on postnatal day 7 were injected intraperitoneally with either saline or uridine (100, 300 or 500mg/kg) for three consecutive days and brains were collected for evaluation of brain infarct volume and apoptosis. Compared with Control group, uridine at 300 and 500mg/kg doses significantly reduced percent infarct volume, TUNEL(+) cell ratio and active Caspase-3 immunoreactivity in the cortex, as well as in CA1 and CA3 regions of the hippocampus. Uridine (300 and 500mg/kg) also decreased active Caspase-3 expression in the ipsilateral hemisphere. These data indicate that uridine dose-dependently reduces brain injury in a rat model of neonatal HIE by decreasing apoptosis.

A novel structural mechanism for redox regulation of uridine phosphorylase 2 activity

Uridine phosphorylase (UPP) catalyzes the reversible conversion of uridine to uracil and ribose-1-phosphate and plays an important pharmacological role in activating fluoropyrimidine nucleoside chemotherapeutic agents such as 5-fluorouracil and capecitabine. Most vertebrate animals, including humans, possess two homologs of this enzyme (UPP1 & UPP2), of which UPP1 has been more thoroughly studied and is better characterized. Here, we report two crystallographic structures of human UPP2 (hUPP2) in distinctly active and inactive conformations. These structures reveal that a conditional intramolecular disulfide bridge can form within the protein that dislocates a critical phosphate-coordinating arginine residue (R100) away from the active site, disabling the enzyme. In vitro activity measurements on both recombinant hUPP2 and native mouse UPP2 confirm the redox sensitivity of this enzyme, in contrast to UPP1. Sequence analysis shows that this feature is conserved among UPP2 homologs and lacking in all UPP1 proteins due to the absence of a necessary cysteine residue. The state of the disulfide bridge has further structural consequences for one face of the enzyme that suggest UPP2 may have additional functions in sensing and initiating cellular responses to oxidative stress. The molecular details surrounding these dynamic aspects of hUPP2 structure and regulation provide new insights as to how novel inhibitors of this protein may be developed with improved specificity and affinity. As uridine is emerging as a promising protective compound in neuro-degenerative diseases, including Alzheimer's and Parkinson's, understanding the regulatory mechanisms underlying UPP control of uridine concentration is key to improving clinical outcomes in these illnesses.

Active site conformational dynamics in human uridine phosphorylase 1

Uridine phosphorylase (UPP) is a central enzyme in the pyrimidine salvage pathway, catalyzing the reversible phosphorolysis of uridine to uracil and ribose-1-phosphate. Human UPP activity has been a focus of cancer research due to its role in activating fluoropyrimidine nucleoside chemotherapeutic agents such as 5-fluorouracil (5-FU) and capecitabine. Additionally, specific molecular inhibitors of this enzyme have been found to raise endogenous uridine concentrations, which can produce a cytoprotective effect on normal tissues exposed to these drugs. Here we report the structure of hUPP1 bound to 5-FU at 2.3 A resolution. Analysis of this structure reveals new insights as to the conformational motions the enzyme undergoes in the course of substrate binding and catalysis. The dimeric enzyme is capable of a large hinge motion between its two domains, facilitating ligand exchange and explaining observed cooperativity between the two active sites in binding phosphate-bearing substrates. Further, a loop toward the back end of the uracil binding pocket is shown to flexibly adjust to the varying chemistry of different compounds through an "induced-fit" association mechanism that was not observed in earlier hUPP1 structures. The details surrounding these dynamic aspects of hUPP1 structure and function provide unexplored avenues to develop novel inhibitors of this protein with improved specificity and increased affinity. Given the recent emergence of new roles for uridine as a neuron protective compound in ischemia and degenerative diseases, such as Alzheimer's and Parkinson's, inhibitors of hUPP1 with greater efficacy, which are able to boost cellular uridine levels without adverse side-effects, may have a wide range of therapeutic applications.

Cellular processes of v-Src transformation revealed by gene profiling of primary cells--implications for human cancer

Background

Cell transformation by the Src tyrosine kinase is characterized by extensive changes in gene expression. In this study, we took advantage of several strains of the Rous sarcoma virus (RSV) to characterize the patterns of v-Src-dependent gene expression in two different primary cell types, namely chicken embryo fibroblasts (CEF) and chicken neuroretinal (CNR) cells. We identified a common set of v-Src regulated genes and assessed if their expression is associated with disease-free survival using several independent human tumor data sets.

Methods

CEF and CNR cells were infected with transforming, non-transforming, and temperature sensitive mutants of RSV to identify the patterns of gene expression in response to v-Src-transformation. Microarray analysis was used to measure changes in gene expression and to define a common set of v-Src regulated genes (CSR genes) in CEF and CNR cells. A clustering enrichment regime using the CSR genes and two independent breast tumor data-sets was used to identify a 42-gene aggressive tumor gene signature. The aggressive gene signature was tested for its prognostic value by conducting survival analyses on six additional tumor data sets.

Results

The analysis of CEF and CNR cells revealed that cell transformation by v-Src alters the expression of 6% of the protein coding genes of the genome. A common set of 175 v-Src regulated genes (CSR genes) was regulated in both CEF and CNR cells. Within the CSR gene set, a group of 42 v-Src inducible genes was associated with reduced disease- and metastasis-free survival in several independent patient cohorts with breast or lung cancer. Gene classes represented within this group include DNA replication, cell cycle, the DNA damage and stress responses, and blood vessel morphogenesis.

Conclusion

By studying the v-Src-dependent changes in gene expression in two types of primary cells, we identified a set of 42 inducible genes associated with poor prognosis in breast and lung cancer. The identification of these genes provides a set of biomarkers of aggressive tumor behavior and a framework for the study of cancer cells characterized by elevated Src kinase activity.

Neuroprotective effects of salidroside in the PC12 cell model exposed to hypoglycemia and serum limitation

The hypoglycemia and serum limitation-induced cell death in cultured PC12 cells represents a useful in vitro model for the study of brain ischemia and neurodegenerative disorders. Salidroside is a phenylpropanoid glycoside isolated from Rhodiola rosea L., a traditional Chinese medicinal plant, and has displayed a broad spectrum of pharmacological properties. In this study, MTT assay, Hoechst 33342 staining, and flow cytometry with annexin V/PI staining collectively showed that pretreatment with salidroside attenuated, in a dose-dependent manner, cell viability loss, and apoptotic cell death in cultured PC12 cells induced by hypoglycemia and serum limitation. RT-PCR, Western blot analysis, and enzymatic colorimetric assay indicated the changes in expression levels of Bcl-2, Bax, and caspase3 in PC12 cells on exposure to hypoglycemia and serum limitation with and without salidroside pretreatment, respectively. Rhodamine 123 staining and flow cytometry with 2',7'-Dichlorofluorescin diacetate staining revealed the changes in the mitochondrial membrane potential and radical oxygen species (ROS) production in PC12 cells on exposure to hypoglycemia and serum limitation with and without salidroside pretreatment, respectively. The experimental results suggest that salidroside protects the PC12 cells against hypoglycemia and serum limitation-induced cytotoxicity possibly by the way of the modulation of apoptosis-related gene expression, the restoration of the mitochondrial membrane potential, and the inhibition of the intracellular ROS production. Our findings might raise a possibility of potential therapeutic applications of salidroside for preventing and treating cerebral ischemic and neurodegenerative diseases.

Purine and pyrimidine nucleosides preserve human astrocytoma cell adenylate energy charge under ischemic conditions

The brain depends on both glycolysis and mitochondrial oxidative phosphorylation for maintenance of ATP pools. Astrocytes play an integral role in brain functions providing trophic supports and energy substrates for neurons. In this paper, we report that human astrocytoma cells (ADF) undergoing ischemic conditions may use both purine and pyrimidine nucleosides as energy source to slow down cellular damage. The cells are subjected to metabolic stress conditions by exclusion of glucose and incubation with oligomycin (an inhibitor of oxidative phosphorylation). This treatment brings about a depletion of the ATP pool, with a concomitant increase in the AMP levels, which results in a significant decrease of the adenylate energy charge. The presence of purine nucleosides in the culture medium preserves the adenylate energy charge, and improves cell viability. Besides purine nucleosides, also pyrimidine nucleosides, such as uridine and, to a lesser extent, cytidine, are able to preserve the ATP pool. The determination of lactate in the incubation medium indicates that nucleosides can preserve the ATP pool through anaerobic glycolysis, thus pointing to a relevant role of the phosphorolytic cleavage of the N-glycosidic bond of nucleosides which generates, without energy expense, the phosphorylated pentose, which through the pentose phosphate pathway and glycolysis can be converted to energetic intermediates also in the absence of oxygen. In fact, ADF cells possess both purine nucleoside phosphorylase and uridine phosphorylase activities.