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Bile acids alter the subcellular localization of CNT2 (concentrative nucleoside cotransporter) and increase CNT2-related transport activity in liver parenchymal cells.

ABSTRACT: CNT2 (concentrative nucleoside cotransporter) is a plasma membrane high-affinity Na+-coupled adenosine transporter, also localized in intracellular structures. This transporter protein may play additional roles other than nucleoside salvage, since it has recently been shown to be under purinergic control via K(ATP) channels, by a mechanism that does not seem to involve changes in its subcellular localization. In an attempt to identify the agents that promote CNT2 trafficking, bile acids were found to increase CNT2-related transport activity in a K(ATP) channel-independent manner in both Fao hepatoma and rat liver parenchymal cells. A maximum effect was recorded after treatment with hydrophilic anions such as TCA (taurocholate). However, this effect did not involve changes in the amount of CNT2 protein, it was instead associated with a subcellular redistribution of CNT2, resulting in an accumulation of the transporter at the plasma membrane. This was deduced from subcellular fractionation studies, biotinylation of plasma membrane proteins and subsequent CNT2 detection in streptavidin precipitates and in vivo confocal microscopic analysis of the distribution of a YFP (yellow fluorescent protein)-CNT2 construct. The induction of CNT2 translocation, triggered by TCA, was inhibited by wortmannin, dibutyryl-AMPc, PD98059 and colchicine, thus suggesting the involvement of the PI3K/ERK (phosphoinositide 3-kinase/extracellular-signal related kinase) pathway in microtubule-dependent activation of recombinant CNT2. These are novel effects of bile-acid physiology and provide the first evidence for short-term regulation of CNT2 translocation into and from the plasma membrane.

SUBMITTER: Fernandez-Veledo S

PROVIDER: S-EPMC1422760 | biostudies-other | 2006 Apr

REPOSITORIES: biostudies-other

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Project description:Transporters are specialized integral membrane proteins, which mediate the passage of virtually all molecules through cell membranes. They are expressed in a broad range of human and animal tissues and play important roles in both normal and disease states. For these reasons, they are evaluated when developing and testing drugs. Two major families of drug transporters, the adenosine 5'-triphosphate-binding cassette and solute carrier transporters (SLC), have critical roles in the absorption, distribution, metabolism, and elimination of drugs. The SLC family contains known nucleoside transporters and therefore are important when nucleoside analogs are used as drugs to prevent or treat viral infections. In this study, we wanted to determine if it was possible to locate one member of the SLC family, the human concentrative nucleoside transporter 3 (CNT3) in human vaginal epithelial cells. The CNT3 protein has important roles in drug delivery, subsequent drug tissue distribution, and, hence, efficacy. Vaginal epithelial cells, taken from two human volunteers (one Caucasian and one African American), were labeled for light and electron microscopy, with a commercial antibody to a cytoplasmic domain of CNT3, the protein product of the SLC28A3 gene. Fluorescent secondary antibodies or protein A-gold were used to detect antibody binding. By electron microscopy, gold particle binding was quantified to determine labeling specificity. By light microscopy, positive labeling with anti-CNT3 antibodies was detected on human vaginal epithelial cells, but specificity to any intracellular structure was not easily determined, most likely a result of specimen preparation. Electron microscopy revealed that the CNT3 transporter protein was present predominantly on microvilli located on one side of some human vaginal epithelial cells. Quantification confirmed specific anti-CNT3 labeling over human vaginal epithelial cell microvilli. The CNT3 protein, present in the microvilli of human vaginal epithelial cells, may have a role in redistributing nucleoside homologues delivered to the vaginal tract. Transporter proteins such as CNT3 could shuttle nucleosides and their analogs through the vaginal epithelium to immune cells located in lower cell layers. Outer layers of cells, which are eventually shed from the epithelium, may remove accumulated nucleoside drug analogs from the vaginal tract.

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Bile acids alter the subcellular localization of CNT2 (concentrative nucleoside cotransporter) and increase CNT2-related transport activity in liver parenchymal cells.

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