Functional production of mammalian concentrative nucleoside transporters in Saccharomyces cerevisiae.

The transport of nucleosides and nucleobases in the yeast Saccharomyces cerevisiae is reviewed and the use of this organism to study recombinant mammalian concentrative nucleoside transport (CNT) proteins is described. A selection strategy based on the ability of an expressed nucleoside transporter cDNA to mediate thymidine uptake by yeast under a selective condition that depletes endogenous thymidylate was used to assess the transport capacity of heterologous transporter proteins. The pyrimidine-nucleoside selective concentrative transporters from human (hCNT1) and rat (rCNT1) complemented the imposed thymidylate depletion in S. cerevisiae, as did N-terminally truncated versions of hCNT1 and rCNT1 lacking up to 31 amino acids. Transporter-mediated rescue of S. cerevisiae by both nucleoside transporters was inhibited by cytidine, uridine and adenosine, but not by guanosine or inosine. This work represents the development of a new model system for the functional production of recombinant nucleoside transporters of the CNT family of membrane proteins.

Distinct regional distribution of human equilibrative nucleoside transporter proteins 1 and 2 (hENT1 and hENT2) in the central nervous system.

Nucleoside transport processes play an important role in human cells in salvage of nucleosides used in the biosynthesis of nucleic acids and in regulating endogenous adenosine concentrations in the human central nervous system (CNS). By altering the levels of adenosine available to interact with cell-surface receptors, nucleoside transporters have profound effects on the ability of adenosine to modulate neurotransmission, vascular tone and other physiological events. Although the human equilibrative nucleoside transporters 1 and 2 (hENT1 and hENT2) are believed to play a crucial role in modulating brain function, their distribution within the major divisions of the human CNS is not known. In this work, antibodies specific for hENT1 and hENT2 were produced against fragments of the transporter proteins and used for immunoblot analysis of enriched membrane fractions prepared from several regions of the human brain. While hENT1 was most prevalent in the frontal and parietal lobes of the cerebral cortex, thalamus, midbrain and basal ganglia, hENT2 was concentrated in the cerebellum and brainstem regions, particularly the pons. The apparent reciprocal distribution of hENT1 and hENT2 in human brain suggests that these nucleoside transporter proteins are produced in distinct regions of the CNS where they function in nucleoside salvage and/or regulation of exogenous adenosine. Within the brain regions that were investigated, the pattern of hENT1 distribution correlated well with adenosine A(1) receptor abundance. The regional co-localization of hENT1 and A(1) receptor protein suggests an important role of hENT1-mediated transport process in the control of neuromodulatory actions mediated by adenosine A(1) receptors in human brain.

Nucleoside transporter proteins of Saccharomyces cerevisiae. Demonstration of a transporter (FUI1) with high uridine selectivity in plasma membranes and a transporter (FUN26) with broad nucleoside selectivity in intracellular membranes.

FUI1 and function unknown now 26 (FUN26) are proteins of uncertain function with sequence similarities to members of the uracil/allantoin permease and equilibrative nucleoside transporter families of transporter proteins, respectively. [(3)H]Uridine influx was eliminated by disruption of the gene encoding FUI1 (fui1) and restored by expression of FUI1 cDNA, whereas influx in transport-competent and fui1-negative yeast were unaffected, respectively, by disruption of the FUN26 gene or overexpression of FUN26 cDNA. FUI1 transported uridine with high affinity (K(m), 22 +/- 3 micrometer) and was unaffected or inhibited only partially by high concentrations (1 mm) of a variety of ribo- and deoxyribonucleosides or nucleobases. When FUN26 cDNA was expressed in oocytes of Xenopus laevis, inward fluxes of [(3)H]uridine, [(3)H]adenosine, and [(3)H]cytidine were stimulated, and uridine influx was independent of pH and not inhibited by dilazep, dipyridamole, or nitrobenzylmercaptopurine ribonucleoside. Fractionation of yeast membranes containing immunotagged recombinant FUN26 (shown to be functional in oocytes) demonstrated that the protein was primarily in intracellular membranes. These results indicated that FUI1 has high selectivity for uracil-containing ribonucleosides and imports uridine across cell-surface membranes, whereas FUN26 has broad nucleoside selectivity and most likely functions to transport nucleosides across intracellular membranes.

Nucleoside transporters of mammalian cells.

In this review, we have summarized recent advances in our understanding of the biology of nucleoside transport arising from new insights provided by the isolation and functional expression of cDNAs encoding the major nucleoside transporters of mammalian cells. Nucleoside transporters are required for permeation of nucleosides across biological membranes and are present in the plasma membranes of most cell types. There is growing evidence that functional nucleoside transporters are required for translocation of nucleosides between intracellular compartments and thus are also present in organellar membranes. Functional studies during the 1980s established that nucleoside transport in mammalian cells occurs by two mechanistically distinct processes, facilitated diffusion and Na(+)-nucleoside cotransport. The determination of the primary amino acid sequences of the equilibrative and concentrative transporters of human and rat cells has provided a structural basis for the functional differences among the different transporter subtypes. Although nucleoside transporter proteins were first purified from human erythrocytes a decade ago, the low abundance of nucleoside transporter proteins in membranes of mammalian cells has hindered analysis of relationships between transporter structure and function. The molecular cloning of cDNAs encoding nucleoside transporters and the development of heterologous expression systems for production of recombinant nucleoside transporters, when combined with recombinant DNA technologies, provide powerful tools for characterization of functional domains within transporter proteins that are involved in nucleoside recognition and translocation. As relationships between molecular structure and function are determined, it should be possible to develop new approaches for optimizing the transportability of nucleoside drugs into diseased tissues, for development of new transport inhibitors, including reagents that are targeted to the concentrative transporters, and, eventually, for manipulation of transporter function through an understanding of the regulation of transport activity.

Interleukin 6: a fibroblast-derived growth inhibitor of human melanoma cells from early but not advanced stages of tumor progression.

Recently we reported that human dermal fibroblasts, or conditioned media obtained from such cells, affect the growth of human melanoma cells as a direct function of tumor progression: melanoma cells obtained from early-stage (metastatically incompetent) primary lesions were growth inhibited, whereas cells obtained from more advanced (metastatically competent) primary lesions, or metastases, were growth stimulated. Ion-exchange and gel-filtration chromatography of fibroblast conditioned medium revealed the inhibitor to be a protein of molecular mass between 20 and 30 kDa and distinct from the stimulator. This is the approximate molecular mass of interleukin 6 (IL-6), a ubiquitous multifunctional cytokine known to affect in particular many kinds of hemopoietic and lymphoid cells. Since this cytokine is known to be made by fibroblasts, we attempted to determine if the human fibroblast-derived growth inhibitor (hFDGI) was identical to IL-6. Neutralizing antibodies specific for IL-6 completely eliminated the inhibitory activity of hFDGI. Moreover, exposure to human recombinant IL-6 was found to inhibit the growth of early-stage melanoma cells obtained from radial growth phase (RGP) or early vertical growth phase (VGP) primary lesions in three of four cases. In contrast, melanoma cells from a number of more advanced VGP primary lesions, or from distant metastases, were completely resistant to this IL-6-mediated growth inhibition. Acquisition of an "IL-6-resistant" phenotype by metastatically competent melanoma cell variants may provide such cells with a proliferative advantage within the dermal mesenchyme (a hallmark of melanoma cells that are malignant), helping them eventually to dominate advanced primary lesions and to establish secondary growths elsewhere.