Novel Single-Pass Exchange of Circulating Uridine in Rat Liver

Gasser, Thomas; Moyer, James D.; Handschumacher, Robert E.

doi:10.1126/science.7256279

Cited by 110 publications

(66 citation statements)

References 8 publications

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“…Nevertheless the liver, a key organ in nucleoside metabolism, has been little studied until recently. Although this organ shows a high endogenous capacity for nucleoside biosynthesis, it is extremely efficient in extracting afferent nucleosides from blood [17][18][19][20], generating intracellular concentrations far beyond the equilibrium [21]. This rules out the possibility of the involvement of a single Na + -independent facilitated transport system in nucleoside transport into liver parenchymal cells.…”

Section: Introductionmentioning

confidence: 99%

Nucleoside uptake in rat liver parenchymal cells

Mercader

Gómez‐Angelats

Santo

et al. 1996

Biochemical Journal

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Rat liver parenchymal cells express Na+-dependent and Na+-independent nucleoside transport activity. The Na+-dependent component shows kinetic properties and substrate specificity similar to those reported for plasma membrane vesicles [Ruiz-Montasell, Casado, Felipe and Pastor-Anglada (1992) J. Membr. Biol. 128, 227–233]. This transport activity shows apparent Km values for uridine in the range 8–13 μM and a Vmax of 246 pmol of uridine per 3 min per 106 cells. Most nucleosides, including the analogue formycin B, cis-inhibit Na+-dependent uridine transport, although thymidine and cytidine are poor inhibitors. Inosine and adenosine inhibit Na+-dependent uridine uptake in a dose-dependent manner, reaching total inhibition. Guanosine also inhibits Na+-dependent uridine uptake, although there is some residual transport activity (35% of the control values) that is resistant to high concentrations of guanosine but may be inhibited by low concentrations of adenosine. The transport activity that is inhibited by high concentrations of thymidine is similar to the guanosine-resistant fraction. These observations are consistent with the presence of at least two Na+-dependent transport systems. Na+-dependent uridine uptake is sensitive to N-ethylmaleimide treatment, but Na+-independent transport is not. Nitrobenzylthioinosine (NBTI) stimulates Na+-dependent uridine uptake. The NBTI effect involves a change in Vmax, it is rapid, dose-dependent, does not need preincubation and can be abolished by depleting the Na+ transmembrane electrochemical gradient. Na+-independent uridine transport seems to be insensitive to NBTI. Under the same experimental conditions, NBTI effectively blocks most of the Na+-independent uridine uptake in hepatoma cells. Thus the stimulatory effect of NBTI on the concentrative nucleoside transporter of liver parenchymal cells cannot be explained by inhibition of nucleoside efflux.

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Section: Introductionmentioning

confidence: 99%

Nucleoside uptake in rat liver parenchymal cells

Mercader

Gómez‐Angelats

Santo

et al. 1996

Biochemical Journal

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“…With all of these clinical roles in mind, it is no surprise that under physiologic conditions, the uridine plasma concentration is strictly regulated at 3-5 M among different species (2,18). However, the regulatory mechanisms of this strict homeostasis of uridine and the biological consequences of its disruption remain to be elucidated.…”

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confidence: 99%

“…In the presence of ribose-1-phosphate, UPase can also catalyze the reverse reaction, forming uridine from uracil, which in turn can be salvaged into uracil nucleotides by uridine kinase (1)(2)(3)(4)(5). Clinically, this anabolic reaction represents one of the two main pathways of activation of 5-fluorouracil (5-FU) in both normal and neoplastic tissues (1,6).…”

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confidence: 99%

Abnormalities in Uridine Homeostatic Regulation and Pyrimidine Nucleotide Metabolism as a Consequence of the Deletion of the Uridine Phosphorylase Gene

Cao

Leffert

McCabe

et al. 2005

Journal of Biological Chemistry

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We report in the present study the critical role of uridine phosphorylase (UPase) in uridine homeostatic regulation and pyrimidine nucleotide metabolism, employing newly developed UPase؊/؊ mice. Our data demonstrate that the abrogation of UPase activity led to greater than a 6-fold increase in uridine concentrations in plasma, a 5-6-fold increase in lung and gut, and a 2-3-fold increase in liver and kidney, as compared with wild type mice. Urine uridine levels increased 24-fold normal in UPase؊/؊ mice. Uridine half-life and the plasma retention of pharmacological doses of uridine were significantly prolonged. Further, in these UPase؊/؊ mice, abnormal uridine metabolism led to disorders of various nucleotide metabolisms. In the liver, gut, kidney, and lung of UPase؊/؊ mice, total uridine ribonucleotide concentrations increased 2-3 times as compared with control mice. Cytidine ribonucleotides and adenosine and guanosine ribonucleotides also increased, although to a lesser extent, in these organs. Most significant deoxyribonucleotide changes were present in the gut and lung of UPase؊/؊ mice. In these tissues, dTTP concentration increased more than 4-fold normal, and dCTP, dGTP, and dATP concentrations rose 1-2 times normal. In kidney, dTTP concentration increased 2-fold normal, and dCTP and dGTP concentrations rose less than 1-fold normal. In addition, the accumulated uridine in plasma and tissues efficiently reduced 5-fluorouracil host toxicity and altered the anesthetic effect of pentobarbital. These data indicate that UPase is a critical enzyme in the regulation of uridine homeostasis and pyrimidine nucleotide metabolism, and 5-fluorouracil activity.

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“…Two cDNAs-cNT1, involved in Na ϩ -dependent pyrimidine transport (N2 type), and sodium purine nucleoside transporter (SPNT), involved in Na ϩ -dependent purine uptake (N1 type)-have recently been cloned. 24,25 Although it has long been known that nucleosides are concentrated in liver, 26 that perfused labeled uridine is efficiently extracted from the afferent blood, 27 and that parenchymal and nonparenchymal liver cells show metabolic cooperation in degrading extracellular nucleosides, 28 the mechanisms of nucleoside uptake into hepatocytes were not studied in detail until this decade. Holstege et al, 29 using the isolated perfused rat liver model, reported indirect evidence of facilitated diffusion and sodium-dependent transport of purine and pyrimidine nucleosides in rat liver.…”

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confidence: 99%

Differential expression and regulation of nucleoside transport systems in rat liver parenchymal and hepatoma cells

et al. 1998

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Primary cultures of rat-liver parenchymal cells show carrier-mediated nucleoside uptake by a mechanism that mainly involves concentrative, Na ؉ -dependent transport activity. In contrast, the hepatoma cell line FAO shows high nucleoside transport activity, although it is mostly accounted for by Na ؉ -independent transport processes. This is associated with a low amount of sodium purine nucleoside transporter (SPNT) mRNA. SPNT encodes a purinepreferring transporter expressed in liver parenchymal cells. To analyze whether SPNT expression is modulated during cell proliferation, SPNT mRNA levels were determined in the early phase of liver growth after partial hepatectomy and in synchronized FAO cells that had been induced to proliferate. SPNT mRNA amounts increased as early as 2 hours after partial hepatectomy. FAO cells induced to proliferate after serum refeeding show an increase in SPNT mRNA levels, which is followed by an increase in Na ؉ -dependent nucleoside uptake and occurs before the peak of 3 H-thymidine incorporation into DNA. FAO cells also express significant equilibrative nucleoside transport activity, which may be accounted for by the expression of the nitrobenzylthioinosine (NBTI)-sensitive and -insensitive isoforms, rat equilibrative nucleoside transporter 1 (rENT1) and rENT2, respectively. Interestingly, rENT2 mRNA levels follow a similar pattern to that described for SPNT when FAO cells are induced to proliferate, whereas rENT1 appears to be constitutively expressed. Liver parenchymal cells show low and negligible mRNA levels for rENT1 and rENT2 transporters, respectively, although most of the equilibrative transport activity found in hepatocytes is NBTI-resistant. It is concluded that: 1) SPNT expression is regulated both in vivo and in vitro in a way that appears to be dependent on cell cycle progression; 2) SPNT expression may be a feature of differentiated hepatocytes; and 3) equilibrative transporters are differentially regulated, rENT2 expression being cell cycle-dependent. This is consistent with its putative role as a growth factor-induced delayed early response gene. (HEPATOLOGY 1998;28:1504-1511.)Nucleosides and nucleoside analogues have a wide range of potent physiological and pharmacological properties. Purines, essentially adenosine, play a multifactorial role in liver physiology by modulating key metabolic pathways of the hepatocyte 1-5 and influencing, among other functions, hepatic arterial pressure-flow autoregulation, 6 vasodilation, 7 and superoxide anion generation. 8

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Novel Single-Pass Exchange of Circulating Uridine in Rat Liver

Cited by 110 publications

References 8 publications

Nucleoside uptake in rat liver parenchymal cells

Nucleoside uptake in rat liver parenchymal cells

Abnormalities in Uridine Homeostatic Regulation and Pyrimidine Nucleotide Metabolism as a Consequence of the Deletion of the Uridine Phosphorylase Gene

Differential expression and regulation of nucleoside transport systems in rat liver parenchymal and hepatoma cells

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