Flow Cytometric Analysis of Nucleoside Transporters Activity in Chemoresistant Prostate Cancer Model

Summary

Background:
Nucleoside analogues represent a relevant class of antimetabolites used for therapy of various types of cancer. However, their effectivity is limited by drug resistance. The nucleoside transport capability of tumour cells is considered to be a determinant of the clinical outcome of treatment regimens using antimetabolites. Due to hydrophilic properties of antimetabolites, their transport across the plasma membrane is mediated by two families of transmembrane proteins, the SLC28 family of cation-linked concentrative nucleoside transporters (hCNTs) and SLC29 family of energy-independent equilibrative nucleoside transporters (hENTs). Loss of functional nucleoside transporters has been associated with reduced efficacy of antimetabolites and their derivatives and treatment failure in diverse malignancies including solid tumours, such as pancreatic adenocarcinoma.

Material and Methods:
The effectivity and kinetics of antimetabolite uptake were analysed using control and docetaxel-resistant PC3 cells. For this purpose, fluorescent nucleoside analogue probe uridine-furane and inhibitor of nucleoside transporters, S-(4-nitrobenzyl) -6-thioinosine were exploited. Combination of flow cytometry, confocal microscopy and real-time quantitative polymerase chain reaction methodology were used for the analysis.

Results:
Here we utilized flow cytometric assay for analysis of nucleoside transporters activity employing fluorescent nucleoside analogue, uridine-furane. We have determined the long-time kinetics of uridine-furane incorporation and quantified its levels in the parental prostate cancer cell line PC3 and its chemoresistant derivative. Finally, we have shown an association between the activity and mRNA expression of nucleoside transporters and sensitivity to various nucleoside analogues.

Conclusion:
Fluorescent techniques can serve as an effective tool for the detection of nucleoside transporter activity which has the potential for application in clinical oncology.

Key words:
nucleoside transporter proteins – drug resistance – prostatic neoplasm – chemotherapy

Introduction

Nucleoside analogues represent an important class of antimetabolites, used for cancer therapy either as single agents or in combination regimens [1,2]. The therapeutic effectivity of these drugs in tumour cells is severely limited by different mechanisms of resistance. Nucleoside transporters (NTs) are important determinants for salvage of preformed nucleosides and mediated uptake of antimetabolite nucleoside drugs into target cells [3].

The transport of nucleosides across the plasma membrane is mediated by two families of transmembrane proteins, the SLC28 family of cation-linked concentrative NTs (hCNTs) and SLC29 family of energy-independent equilibrative NTs (hENTs) [4]. With the exception of the nucleoside uptake, NTs play also a significant role in the effectivity of many chemotherapeutic drugs. The most prominent NT, hENT1, was shown to be crucial for the uptake of gemcitabine, standard frontline chemotherapy for pancreatic cancer [5]. Patients with a detectable expression of SLC29A1 had significantly higher overall median survival compared to those lacking SLC29A1 expression [6]. Moreover, hENT1-dependent nucleoside uptake was associated with resistance to nucleoside analogue Ara-C (Cytarabine) in (AML) patients with acute myeloid leukaemia [7,8]. More recently, down-regulation of SLC29A1 gene was associated with decreased drug sensitivity, higher proliferation rate, invasion and induction of epithelial-to-mesenchymal transition [9]. Based on these findings, the expression profile of hENT1 and other nucleoside transporters can be considered as a prognostic factor for chemosensitivity to antimetabolites. The analysis of NTs expression and localization using antibody-based approaches including flow cytometry and microscopy is an important parameter, but not relevant for the determination of its activity. Until now, there has not been identified any effective approach for the detection of NTs activity in the clinical routine. The most frequently used experimental approach is exploiting radioactively-labeled nucleoside analogues or another nucleoside transporter ligands [10]. Despite the sensitivity and specificity of radioactive assays, radioactively-labelled probes remain one of the major obstacles of the analysis. Moreover, it also does not allow single cells analysis and real-time monitoring of intracellular trafficking of nucleosides. Fluorescent nucleoside analogues or reporters are suitable for real-time measurement of NTs activity at single cell level using both fluorescence microscopy and flow cytometry [11,12].

Here we utilized the assay for the functional analysis of nucleoside transporters using flow cytometry and confocal microscopy exploiting fluorescent nucleoside analogue probe uridine-furane and inhibitor of equilibrative nucleoside transporter proteins, S-(4-nitrobenzyl) -6-thioinosine (NBTI) [12] in the experimental model of chemoresistant prostate cancer cells [13].

Material and Methods

Docetaxel-resistant PC3 prostate cancer cell line was derived as previously reported [13] and kindly provided by prof. Zoran Culig (Medical University, Innsbruck, Austria). Docetaxel resistance was maintained by a continuous supply of docetaxel (Cell Signaling Technology, Ins., 9886, USA) in the final concentration 12.5 nM. Cell lines were maintained at 37 °C (5% CO₂) in RPMI 1640 (ThermoFisher Scientific, 72400-021, USA) media supplemented with 10% fetal bovine serum and 100 U/mL penicillin/streptomycin. Uridine-furane (UF) was prepared in six steps from uridine by slightly modified published procedure [12]. The compound was purified by flash column chromatography on silica gel (eluent – dichloromethane/methanol 3: 2) followed by trituration with diethyl ether. The ¹H and ¹³C nuclear magnetic resonance (NMR) spectra of the prepared material matched those published. The kinetics and activity of uridine-furane incorporation were determined by flow cytometry analysis (BD FACSAria II SORP, Beckton Dickinson) and confocal microscopy (Leica TCS SP5 X) using UV laser. Acquired data were evaluated within FlowJo software TreeStar, USA). Dead cells were excluded from analysis based on their positivity to propidium iodide stain. Cell aggregates and debris were excluded based on a dual-parameter dot plot in which the pulse ratio (signal height/y-axis vs. signal area/x-axis) was displayed. The relative mRNA levels of nucleoside transporters were assessed by quantitative real-time polymerase chain reaction (qRT-PCR) (LightCycler 480, Roche). Dose-response analysis of tested drugs was performed as described previously [14].

Results and discussion

Kinetics of uridine-furane incorporation

To explore the impact of nucleoside transporter activity we employed the prostate cancer cell line PC3 and its docetaxel-resistant deriative [13]. Using in vitro live cell microscopic analysis, we investigated the incorporation potential of fluorescent nucleoside analogue UF as a promising probe for the analysis of nucleoside transporter activity. Microscopic data nicely reflected the formation of UF foci in a early stage of exposure located mostly in the plasmatic membrane with the subsequent signal extension to the nucleus (Fig. 1A). Moreover, the small molecule inhibitor NBTI was fully capable to avoid the incorporation of UF in both models, confirming the role of hENT’s in the transportation of nucleoside analogues into the cells (Fig. 1B). Fluorescence increase and UF incorporation were detected continuously throughout the whole measurement. Obtained data are in the favour of previously published study introducing uridine-furane as useful fluorescent nucleoside derivative for the analysis of nucleoside analogue uptake and nucleoside transporters function [12].

Docetaxel-resistant model possess the lower expression of nucleoside transporters

Our next aim was to determine the expression profile of nucleoside transporter families SLC29 and SLC28. We hypothesized that docetaxel-resistant cells possess decreased gene levels of both concentrative and equlibrative nucleoside transporters. We have employed qRT-PCR and identified three nucleoside transporters, hENT1, hENT3 and hCNT2 down-regulated in the docetaxel-resistant model compared to controls (Fig. 2). These findings are consistent with other study suggesting an association between decreased levels of NTs and resistance to various chemotherapy drugs [15].

Flow cytometric analysis of nucleoside transporter activity

As expression levels are not the relevant parameter for the evaluation of the nucleoside transporter function we employed flow cytometric analysis of UF incorporation via NTs. We have utilized an approach for the determination of nucleoside transporter activity exploiting NTs inhibition using NBTI, reflecting the phenotype of drug uptake suppression. Preliminary analysis of the kinetics showed more robust incorporation of UF into the control cells compared to the docetaxel-resistant derivatives, disabled by NBTI pre-treatment in both models (Fig 3A). Although endpoint data showed higher incorporation in docetaxel-resistant cells based on fluorescence intensity, normalization and the ratio between the mean of fluorescence intensity of UF alone and in combination with NBTI revealed higher NTs activity in the control cells (Fig. 3B). Finally, we also performed dose-response analysis of cytotoxicity for three clinically used nucleoside analogues – gemcitabine, cytarabine and fludarabine. Determined IC50 values were then associated it with the activity of NTs. The results showed that decreased sensitivity of docetaxel-resistant PC3 cells to all three nucleoside analogues is related to the decreased activity of NTs (Fig. 3C). This observation is in agreement with the recent study demonstrating hENT1 expression as a marker of better prognostic for a patient with pancreatic cancer treated with adjuvant gemcitabine chemotherapy [16]. Collectively, the results showed a decrease in the NTs activity in docetaxel-resistant model, predicting worse response to the treatment by various nucleoside analogues. These findings correspond with the studies considering decreased expression and function of hENT as factors causatively responsible for the acquisition of Ara-C or trifluorothymidine resistance, resp. [15,17].

Conclusions

Our experimental results suggest that analysis of nucleoside transporters activity, particularly hENT1, might serve as a reliable parameter for the prediction of treatment response in chemoresistant prostate cancer. Fluorescent techniques are useful tools for analysis of NTs activity with the potential for utilization in clinical practice.

Acknowledgement

Authors would like to thank Iva Liskova, Martina Urbankova and Katerina Svobodova for technical assistance; prof. Zoran Culig for providing PC3 AG and PC3 DR cell lines.

The authors declare they have no potential conflicts of interest concerning drugs, products, or services used in the study.

The Editorial Board declares that the manuscript met the ICMJE recommendation for biomedical papers.

This work was supported by Ministry of Health of the Czech Republic, grant nr. 15-33999A, all rights reserved (to K. Souček, K. Paruch and M. Svoboda). 

Submitted: 10. 4. 2018

Accepted: 19. 4. 2018

Mgr. Karel Soucek, Ph.D.

Department of Cytokinetics Institute of Biophysics Academy of Sciences of the Czech Republic, v. v. i.

Kralovopolska 135 612 65 Brno Czech Republic

e-mail: ksoucek@ibp.cz