Novel Na+-independent and adenine-specific transport system for adenine in primary cultured rat cortical neurons

Molecular Basis of Nucleobase Transport Systems in Mammals

Nucleobases are water-soluble compounds that need specific transporters to cross biological membranes. Cumulative evidence based on studies using animal tissues and cells indicates that the carrier-mediated transport systems for purine and pyrimidine nucleobases can be classified into the following two types: concentrative transport systems that mediate nucleobase transport depending on the sodium ion concentration gradient; and other systems that mediate facilitated diffusion depending on the concentration gradient of the substrate. Recently, several molecular transporters that are involved in both transport systems have been identified. The function and activity of these transporters could be of pharmacological significance considering the roles that they play not only in nucleotide synthesis and metabolism but also in the pharmacokinetics and delivery of a variety of nucleobase analogues used in anticancer and antiviral drug therapy. The present review provides an overview of the recent advances in our understanding of the molecular basis of nucleobase transport systems, focusing on the transporters that mediate purine nucleobases, and discusses the involvement of intracellular metabolism in purine nucleobase transport and chemotherapy using ganciclovir.

A study on the influence of ethanol over the primary cultured rat cortical neurons by using the scanning electron microscopy

As an inhibitor and toxic factor of central nervous system, ethanol inhibits the action of the neurons and causes various kinds of neuronal damage. However, the precise mechanisms that ethanol-induced neuronal damage in the central nervous system remain unclear. In spite of thousands of published studies, little information is available on the neurons' morphological alteration in the central nervous system. In this study, we investigated the morphological alterations of the primary cultured rat cortical neurons after they were treated by different concentrations of ethanol using the scanning electron microscopy. Our results showed that the moderate or high concentration of ethanol could lead to morphological changes of these cultured rat cortical neurons, and they were closely associated with the duration of time. Our study will provide a new base for further studies on the effects of ethanol in the central nervous system.

Nucleoside/nucleobase transporters: drug targets of the future?

BACKGROUND

Nucleoside/nucleobase transporters have been investigated since the 1960s. In particular, equilibrative nucleoside transporters were thought to be valuable drug targets, since they are involved in various kinds of viral and parasitic diseases as well as cancers.

DISCUSSION

In the postgenomic era multiple transporters, including different subtypes, have been cloned and characterized on the molecular level. In this article we summarize recent advances regarding structure, function and localization of nucleoside/nucleobase transporters as well as the pharmacological profile of selected drugs.

CONCLUSION

Knowledge of the different kinetic properties and structural features of nucleoside transporters can either be used for the rational design of therapeutics directly targeting the transporter itself or for the delivery of drugs using the transporter as a port of entry into the target cell. Equilibrative nucleoside transporters are of considerable pharmacological interest as drug targets for the development of drugs tailored to each patient's need for the treatment of cardiac disease, cancer and viral infections.

Cloning and functional expression of a novel Gi protein-coupled receptor for adenine from mouse brain

An orphan G protein-coupled receptor from the rat has recently been demonstrated to act as a transmembrane receptor for the nucleobase adenine. The receptor is possibly involved in nociception. Here we report the cloning and functional expression of an additional G(i)-coupled receptor for adenine (Genbank accession code DQ386867). mRNA for this receptor was obtained from mouse brain and the mouse neuroblastoma x rat glioma hybrid cell line NG108-15. The new mouse protein sequence shares only 76% identity with that of the rat adenine receptor, suggesting that the receptors are not species homologs but distinct receptor subtypes. In human 1321N1 astrocytoma cells stably expressing the new mouse receptor, adenine and 2-fluoroadenine inhibited the isoproterenol-induced cAMP formation with IC(50) concentrations of 8 and 15 nM, respectively. The adenosine receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 1 muM) as well as the P2 receptor antagonist suramin (300 muM) failed to change the responses to adenine. In contrast, pretreatment of cells with pertussis toxin abolished the effect of adenine. When the novel adenine receptor was expressed in Sf21 insect cells, a specific binding site for [(3)H]adenine was detected. In competition assays, the rank order of potency of selected ligands was identical to that obtained in membranes from NG108-15 cells and rat brain cortex (adenine > 2-fluoroadenine > 7-methyladenine > 1-methyladenine >> N(6)-dimethyladenine). In summary, our data show that a second mammalian DNA sequence encodes for a G(i)-coupled GPCR activated by low, nanomolar concentrations of adenine.

Characterization of guanine and guanosine transport in primary cultured rat cortical astrocytes and neurons

In this study, we examined the transport mechanisms for guanine and guanosine in rat neurons and astrocytes, and compared their characteristics. In the both types of cell, the uptake of [(3)H]guanine and [(3)H]guanosine was time-, temperature-, and concentration-dependent, and Na(+)-independent. Their uptake decreased on the addition of purine and pyrimidine nucleobases or nucleosides, and the inhibitory effect of the purine analogues was greater than that of the pyrimidine ones. In both cell types, equilibrative nucleoside transporter (ENT) 1 and ENT2 expression was confirmed at the mRNA level, and nitrobenzylmercaptopurine riboside, a representative inhibitor for ENT, decreased their uptake at concentrations of over 10 microM. Comparing uptake characteristics between the substrates, [(3)H]guanine uptake exhibited higher affinity and clearance than [(3)H]guanosine uptake in each type of cell. Although between neurons and astrocytes, there was no difference in the apparent uptake clearance for [(3)H]guanine and [(3)H]guanosine, which was calculated based upon the cellular protein content, the cellular uptake clearance was significantly greater in astrocytes than in neurons. These findings indicate that guanine and guanosine, of which the former is a preferable substrate, are taken up into both neurons and astrocytes via ENT2, and that the extracellular concentrations of guanine and guanosine are mainly regulated by astrocytes to maintain brain physiology.

Hypoxanthine uptake and release by equilibrative nucleoside transporter 2 (ENT2) of rat microvascular endothelial cells

The cardioprotective actions of adenosine are terminated by its uptake into endothelial cells with subsequent metabolism through hypoxanthine to uric acid. This process involves xanthine oxidase-mediated generation of reactive oxygen species (ROS), which have been implicated in the vascular dysfunction observed in ischemia-reperfusion injury. The equilibrative nucleoside transporter, ENT2, mediates the transfer of hypoxanthine into cells. We hypothesize that ENT2 also mediates the cellular release of hypoxanthine, which would limit the amount of intracellular hypoxanthine available for xanthine oxidase-mediated ROS production. Rat microvascular endothelial cells (MVECs) were isolated from skeletal muscle by lectin-affinity purification. The transport of [(3)H]hypoxanthine was assessed using an oil-stop method, and hypoxanthine metabolites were identified by thin-layer chromatography. MVECs accumulated hypoxanthine with a K(m) of 300 microM and a V(max) of 2.8 pmol microl(-1) s(-1). ATP-depleted cells loaded with [(3)H]hypoxanthine released the radiolabel with kinetics similar to that obtained for [(3)H]hypoxanthine influx. The uptake and release of [(3)H]hypoxanthine were both blocked by ENT2 inhibitors with similar order of potency. Thus, ENT2 mediates both the influx and efflux of hypoxanthine. Inhibition of ENT2 in MVECs might be expected to increase the amount of intracellular hypoxanthine available for metabolism by xanthine oxidase and enhance the intracellular production of ROS.