Concentrative Nucleoside Transporter, CNT, Results in Selective Toxicity of Toyocamycin against Candida albicans

Toyocamycin (TM) is an adenosine-analog antibiotic isolated from Streptomyces toyocaensis. It inhibits Candida albicans, several plant fungal pathogens, and human cells, but many fungi, including Saccharomyces cerevisiae, are much less susceptible to TM. Aiming to clarify why TM and its analogs tubercidin and 5-iodotubercidin are active against C. albicans but not S. cerevisiae, this study focused on the absence of purine nucleoside transport activity from S. cerevisiae. When the concentrative nucleoside transporter (CNT) of C. albicans was expressed in S. cerevisiae, the recombinant strain became sensitive to TM and its analogs. The expression of C. albicans purine nucleoside permease in S. cerevisiae did not result in sensitivity to TM. Clustered regularly interspaced short palindromic repeat-mediated disruption of CNT was performed in C. albicans. The CNTΔ strain of C. albicans became insensitive to TM and its analogs. These data suggest that the toxicity of TM and its analogs toward C. albicans results from their transport via CNT. Interestingly, S. cerevisiae also became sensitive to TM and its analogs if human CNT3 was introduced into cells. These findings enhance our understanding of the mechanisms of action of adenosine analogs toward Candida pathogens and human cells. IMPORTANCE We investigated the mechanism of toxicity of TM and its analogs to C. albicans. Inspired by the effect of the copresence of TM and purine nucleosides on cell growth of C. albicans, we investigated the involvement of CNT in the toxicity mechanism by expressing CNT of C. albicans (CaCNT) in S. cerevisiae and deleting CaCNT in C. albicans. Our examinations clearly demonstrated that CaCNT is responsible for the toxicity of TM to C. albicans. S. cerevisiae expressing the human ortholog of CaCNT also became sensitive to TM and its analogs, and the order of effects of the TM analogs was a little different between CaCNT- and hCNT3-expressing S. cerevisiae. These findings are beneficial for an understanding of the mechanisms of action of adenosine analogs toward Candida pathogens and human cells and also the development of new antifungal drugs.

Repeat-swap homology modeling of secondary active transporters: updated protocol and prediction of elevator-type mechanisms

Secondary active transporters are critical for neurotransmitter clearance and recycling during synaptic transmission and uptake of nutrients. These proteins mediate the movement of solutes against their concentration gradients, by using the energy released in the movement of ions down pre-existing concentration gradients. To achieve this, transporters conform to the so-called alternating-access hypothesis, whereby the protein adopts at least two conformations in which the substrate binding sites are exposed to one or other side of the membrane, but not both simultaneously. Structures of a bacterial homolog of neuronal glutamate transporters, Glt_Ph, in several different conformational states have revealed that the protein structure is asymmetric in the outward- and inward-open states, and that the conformational change connecting them involves a elevator-like movement of a substrate binding domain across the membrane. The structural asymmetry is created by inverted-topology repeats, i.e., structural repeats with similar overall folds whose transmembrane topologies are related to each other by two-fold pseudo-symmetry around an axis parallel to the membrane plane. Inverted repeats have been found in around three-quarters of secondary transporter folds. Moreover, the (a)symmetry of these systems has been successfully used as a bioinformatic tool, called “repeat-swap modeling” to predict structural models of a transporter in one conformation using the known structure of the transporter in the complementary conformation as a template. Here, we describe an updated repeat-swap homology modeling protocol, and calibrate the accuracy of the method using Glt_Ph, for which both inward- and outward-facing conformations are known. We then apply this repeat-swap homology modeling procedure to a concentrative nucleoside transporter, VcCNT, which has a three-dimensional arrangement related to that of Glt_Ph. The repeat-swapped model of VcCNT predicts that nucleoside transport also occurs via an elevator-like mechanism.

Attenuation of nucleoside and anti-cancer nucleoside analog drug uptake in prostate cancer cells by Cimicifuga racemosa extract BNO-1055

This study aimed to investigate the mechanisms underlying the anti-proliferative effects of the ethanolic Cimicifuga racemosa extract BNO-1055 on prostate cells and evaluate its therapeutic potential. BNO-1055 dose-dependently attenuated cellular uptake and incorporation of thymidine and BrdU and significantly inhibited cell growth after long-time exposure. Similar results were obtained using saponin-enriched sub-fractions of BNO-1055. These inhibitory effects of BNO-1055 could be mimicked using pharmacological inhibitors and isoform-specific siRNAs targeting the equilibrative nucleoside transporters ENT1 and ENT2. Moreover, BNO-1055 attenuated the uptake of clinically relevant nucleoside analogs, e.g. the anti-cancer drugs gemcitabine and fludarabine. Consistent with inhibition of the salvage nucleoside uptake pathway BNO-1055 potentiated the cytotoxicity of the de novo nucleotide synthesis inhibitor 5-FU without significantly altering its uptake. Collectively, these data show for the first time that the anti-proliferative effects of BNO-1055 result from hindered nucleoside uptake due to impaired ENT activity and demonstrate the potential therapeutic use of BNO-1055 for modulation of nucleoside transport.