All-trans-retinoic acid promotes trafficking of human concentrative nucleoside transporter-3 (hCNT3) to the plasma membrane by a TGF-beta1-mediated mechanism

Nucleoside transporters and immunosuppressive adenosine signaling in the tumor microenvironment: Potential therapeutic opportunities

Adenosine compartmentalization has a profound impact on immune cell function by regulating adenosine localization and, therefore, extracellular signaling capabilities, which suppresses immune cell function in the tumor microenvironment. Nucleoside transporters, responsible for the translocation and cellular compartmentalization of hydrophilic adenosine, represent an understudied yet crucial component of adenosine disposition in the tumor microenvironment. In this review article, we will summarize what is known regarding nucleoside transporter's function within the purinome in relation to currently devised points of intervention (i.e., ectonucleotidases, adenosine receptors) for cancer immunotherapy, alterations in nucleoside transporter expression reported in cancer, and potential avenues for targeting of nucleoside transporters for the desired modulation of adenosine compartmentalization and action. Further, we put forward that nucleoside transporters are an unexplored therapeutic opportunity, and modulation of nucleoside transport processes could attenuate the pathogenic buildup of immunosuppressive adenosine in solid tumors, particularly those enriched with nucleoside transport proteins.

The Antagonist of Retinoic Acid Receptor α, ER-50891 Antagonizes the Inhibitive Effect of All-Trans Retinoic Acid and Rescues Bone Morphogenetic Protein 2-Induced Osteoblastogenic Differentiation

Background

Hypervitaminosis A, alcoholism or medical treatment for acute promyelocytic leukaemia may cause unphysiologically high accumulation of all-trans retinoic acid (ATRA), which could inhibit osteoblastogenesis, thereby triggering osteoporosis. We have shown that bone morphogenetic protein-2 (BMP-2) can only partially antagonize the inhibitive effects of ATRA. In this study, we hypothesized that antagonists of retinoic acid receptors (RARs) could further antagonize the inhibitive effect of ATRA and rescue BMP2-induced osteoblastogenesis.

Materials and Methods

We first screened the dose-dependent effects of the specific antagonists of RAR α, β and γ and transforming growth factor-beta receptor (ER-50891, LE-135, MM11253, and SB-43142, respectively) on ATRA-induced inhibition of the total cell metabolic activity and proliferation of preosteoblasts. We selected ER-50891 and tested its effects on osteoblastogenesis with the presence or absence of 1 μM ATRA and/or 200 ng/mL BMP-2. We measured the following parameters: Alkaline phosphatase activity (ALP), osteocalcin (OCN) expression and extracellular matrix mineralization as well as the level of phosphorylated Smad1/5.

Results

ER-50891 but not LE-135, MM11253, or SB-431542 significantly antagonized the inhibition of ATRA and enhanced the total cell metabolic activity and proliferation of preosteoblasts. Dose-dependent assays show ER-50891 could also rescue ATRA inhibited OCN expression and mineralization with or without the induction of BMP. ER-50891 also suppressed the ALP activity that was synergistically enhanced by BMP and ATRA. Neither ATRA, nor ER-50891 or their combination significantly affected the level of BMP-induced phosphorylated Smad1/5.

Conclusion

The antagonist of RARα, ER-50891 could significantly attenuate ATRA’s inhibitive effects on BMP 2-induced osteoblastogenesis.

Association of SLC28A3 Gene Expression and CYP2B6*6 Allele with the Response to Fludarabine Plus Cyclophosphamide in Chronic Lymphocytic Leukemia Patients

Fludarabine plus cyclophosphamide (FC) chemotherapy is the basis of treatment protocols used in management of chronic lymphocytic leukemia (CLL). In some patients, response to therapy may be affected by aberrant function of genes involved in pharmacokinetics and pharmacodynamics of the drugs. The aim of this research was to assess the impact of pharmacogenetic variability, namely expression of SLC28A3 gene and the presence of CYP2B6*6 variant allele, on the FC treatment efficacy. Forty-four CLL patients with functional TP53 gene at the time of FC initiation were enrolled in this study. CYP2B6 genotyping was performed by polymerase chain reaction and direct sequencing. SLC28A3 expression was measured by quantitative reverse-transcriptase polymerase chain reaction. Significantly higher pretreatment levels of SLC28A3 mRNA were detected in patients who failed to respond to FC in comparison to patients who achieved complete and partial response (p = 0.01). SLC28A3 high-expressing cases were almost ten times more likely not to respond to FC than low-expressing cases (OR = 9.8; p = 0.046). However, association of SLC28A3 expression with progression-free survival (PFS) and overall survival (OS) was not observed. CYP2B6*6 allele, detected in 24 patients (54.6%), exerted no association with the attainment of response to FC, as well as with PFS and OS. The results of this study demonstrate that SLC28A3 expression is a significant predictor of FC efficacy in CLL patients with intact TP53. Elevated SLC28A3 mRNA levels are associated with inferior short-term response to FC, suggesting that, if validated on larger cohorts, SLC28A3 expression may become a biomarker useful for pretreatment stratification of patients.

Expression of Concentrative Nucleoside Transporters ( SLC28A) in the Human Placenta: Effects of Gestation Age and Prototype Differentiation-Affecting Agents

Equilibrative ( SLC29A) and concentrative ( SLC28A) nucleoside transporters contribute to proper placental development and mediate uptake of nucleosides/nucleoside-derived drugs. We analyzed placental expression of SLC28A mRNA during gestation. Moreover, we studied in choriocarcinoma-derived BeWo cells whether SLC29A and SLC28A mRNA levels can be modulated by activity of adenylyl cyclase, retinoic acid receptor activation, CpG islands methylation, or histone acetylation, using forskolin, all- trans-retinoic acid, 5-azacytidine, and sodium butyrate/sodium valproate, respectively. We found that expression of SLC28A1, SLC28A2, and SLC28A3 increases during gestation and reveals considerable interindividual variability. SLC28A2 was shown to be a dominant subtype in the first-trimester and term human placenta, while SLC28A1 exhibited negligible expression in the term placenta only. In BeWo cells, we detected mRNA of SLC28A2 and SLC28A3. Levels of the latter were affected by 5-azacytidine and all- trans-retinoic acid, while the former was modulated by sodium valproate (but not sodium butyrate), all- trans-retinoic acid, 5-azacytidine, and forskolin that caused 25-fold increase in SLC28A2 mRNA; we documented by analysis of syncytin-1 that the observed changes in SLC28A expression do not correlate with the morphological differentiation state of BeWo cells. Upregulated SLC28A2 mRNA was reflected in elevated uptake of [³H]-adenosine, high-affinity substrate of concentrative nucleoside transporter 2. Using KT-5720 and inhibitors of phosphodiesterases, we subsequently confirmed importance of cAMP/protein kinase A pathway in SLC28A2 regulation. On the other hand, SLC29A genes exhibited constitutive expression and none of the tested compounds increased SLC28A1 expression to detectable levels. In conclusion, we provide the first evidence that methylation status and activation of retinoic acid receptor affect placental SLC28A2 and SLC28A3 transcription and substrates of concentrative nucleoside transporter 2 might be taken up in higher extent in placentas with overactivated cAMP/protein kinase A pathway and likely in the term placenta.

FLT3 is implicated in cytarabine transport by human equilibrative nucleoside transporter 1 in pediatric acute leukemia

FLT3 abnormalities are negative prognostic markers in acute leukemia. Infant leukemias are a subgroup with frequent MLL (KMT2A) rearrangements, FLT3 overexpression and high sensitivity to cytarabine, but dismal prognosis. Cytarabine is transported into cells by Human Equilibrative Nucleoside Transporter-1 (hENT1, SLC29A1), but the mechanisms that regulate hENT1 in acute leukemia have been scarcely studied.We explored the expression and functional link between FLT3 and main cytarabine transporters in 50 pediatric patients diagnosed with acute lymphoblastic leukemia and MLL rearrangement (ALL-MLL+) and other subtypes of leukemia, and in leukemia cell lines.A significant positive correlation was found between FLT3 and hENT1 expression in patients. Cytarabine uptake into cells was mediated mainly by hENT1, hENT2 and hCNT1. hENT1-mediated uptake of cytarabine was transiently abolished by the FLT3 inhibitor PKC412, and this effect was associated with decreased hENT1 mRNA and protein levels. Noticeably, the cytotoxicity of cytarabine was lower when cells were first exposed to FLT3 inhibitors (PKC412 or AC220), probably due to decreased hENT1 activity, but we observed a higher cytotoxic effect if FLT3 inhibitors were administered after cytarabine.FLT3 regulates hENT1 activity and thereby affects cytarabine cytotoxicity. The sequence of administration of cytarabine and FLT3 inhibitors is important to maintain their efficacy.

All-Trans Retinoic Acid Induces TGF-β2 in Intestinal Epithelial Cells via RhoA- and p38α MAPK-Mediated Activation of the Transcription Factor ATF2

Objective

We have shown previously that preterm infants are at risk of necrotizing enterocolitis (NEC), an inflammatory bowel necrosis typically seen in infants born prior to 32 weeks’ gestation, because of the developmental deficiency of transforming growth factor (TGF)-β₂ in the intestine. The present study was designed to investigate all-trans retinoic acid (atRA) as an inducer of TGF-β₂ in intestinal epithelial cells (IECs) and to elucidate the involved signaling mechanisms.

Methods

AtRA effects on intestinal epithelium were investigated using IEC6 cells. TGF-β₂ expression was measured using reverse transcriptase-quantitative polymerase chain reaction (RT-qPCR) and Western blots. Signaling pathways were investigated using Western blots, transiently-transfected/transduced cells, kinase arrays, chromatin immunoprecipitation, and selective small molecule inhibitors.

Results

AtRA-treatment of IEC6 cells selectively increased TGF-β₂ mRNA and protein expression in a time- and dose-dependent fashion, and increased the activity of the TGF-β₂ promoter. AtRA effects were mediated via RhoA GTPase, Rho-associated, coiled-coil-containing protein kinase 1 (ROCK1), p38α MAPK, and activating transcription factor (ATF)-2. AtRA increased phospho-ATF2 binding to the TGF-β₂ promoter and increased histone H2B acetylation in the TGF-β₂ nucleosome, which is typically associated with transcriptional activation.

Conclusions

AtRA induces TGF-β₂ expression in IECs via RhoA- and p38α MAPK-mediated activation of the transcription factor ATF2. Further studies are needed to investigate the role of atRA as a protective/therapeutic agent in gut mucosal inflammation.

Fluorescent nucleoside derivatives as a tool for the detection of concentrative nucleoside transporter activity using confocal microscopy and flow cytometry

The abundance and function of transporter proteins at the plasma membrane are likely to be crucial in drug responsiveness. Functional detection of human concentrative nucleoside transporters (hCNTs) is of interest for predicting drug sensitivity because of their ability to transport most nucleoside-derived drugs. In the present study, two fluorescent nucleoside analogues, uridine-furan and etheno-cytidine, were evaluated as tools to study in vivo nucleoside transporter-related functions. These two molecules showed high affinity interactions with hCNT1 and hCNT3 and were shown to be substrates of both transporters. Both fluorescence microscopy and flow cytometry experiments showed that uridine-furan uptake was better suited for distinguishing cells that express hCNT1 or hCNT3. These data highlight the usefulness of fluorescent nucleoside derivatives, as long as they fulfill the requirements of confocal microscopy and flow cytometry, for in vivo analysis of hCNT-related function.

Nucleoside transporters and human organic cation transporter 1 determine the cellular handling of DNA-methyltransferase inhibitors

BACKGROUND AND PURPOSE

Inhibitors of DNA methyltransferases (DNMTs), such as azacytidine, decitabine and zebularine, are used for the epigenetic treatment of cancer. Their action may depend upon their translocation across the plasma membrane. The aim of this study was to identify transporter proteins contributing to DNMT inhibitor action.

EXPERIMENTAL APPROACH

Drug interactions with selected hCNT and hENT proteins were studied in transiently transfected HeLa and MDCK cells. Interaction with human organic cation transporters (hOCTs) was assessed in transiently transfected HeLa cells and Xenopus laevis oocytes.

KEY RESULTS

Zebularine uptake was mediated by hCNT1, hCNT3 and hENT2. Decitabine interacted with but was not translocated by any nucleoside transporter (NT) type. hCNT expression at the apical domain of MDCK cells promoted net vectorial flux of zebularine. Neither hOCT1 nor hOCT2 transported decitabine, but both were involved in the efflux of zebularine, suggesting these proteins act as efflux transporters. hOCT1 polymorphic variants, known to alter function, decreased zebularine efflux.

CONCLUSIONS AND IMPLICATIONS

This study highlights the influence of human NTs and hOCTs on the pharmacokinetics and pharmacodynamics of selected DNMT inhibitors. As hOCTs may also behave as efflux transporters, they could contribute either to chemoresistance or to chemosensitivity, depending upon the nature of the drug or combination of drugs being used in cancer therapy.

Adenosine transporters and receptors: key elements for retinal function and neuroprotection

Adenosine is an important neuroactive substance in the central nervous system, including in the retina where subclasses of adenosine receptors and transporters are expressed since early stages of development. Here, we review some evidence showing that adenosine plays important functions in the mature as well as in the developing tissue. Adenosine transporters are divided into equilibrative and concentrative, and the major transporter subtype present in the retina is the ENT1. This transporter is responsible for a bidirectional transport of adenosine and the uptake or release of this nucleoside appears to be regulated by different signaling pathways that are also controlled by activation of adenosine receptors. Adenosine receptors are also key players in retina physiology regulating a variety of functions in the mature and developing tissue. Regulation of excitatory neurotransmitter release and neuroprotection are the main functions played be adenosine in the mature tissue, while regulation of cell survival and neurogenesis are some of the functions played by adenosine in developing retina. Since adenosine is neuroprotective against excitotoxic and metabolic dysfunctions observed in neurological and ocular diseases, the search for adenosine-related drugs regulating adenosine transporters and receptors can be important for advancement of therapeutic strategies against these diseases.

Ribavirin uptake into human hepatocyte HHL5 cells is enhanced by interferon-α via up-regulation of the human concentrative nucleoside transporter (hCNT2)

Ribavirin is a broad spectrum antiviral that increases the response rate in chronic hepatitis C patients when administered in combination with IFNα. Ribavirin is a purine nucleoside derivative, transported into hepatocytes by nucleoside transporters. hCNT2 is the best candidate to mediate ribavirin uptake into hepatocytes due to its high-affinity for purines and its capacity to concentrate its substrates intracellularly. The aim of this study was to determine whether hCNT2 function is under IFNα modulation. IFNα treatment of the nontransformed human hepatocyte-derived cell line HHL5 induced a rapid and transient increase in hCNT2 activity after cytokine addition. hCNT2 activity up-regulation was associated with increased ribavirin accumulation into cells. This increase was consistent with the translocation of hCNT2-containing vesicles to the plasma membrane via a mechanism requiring ERK 1/2 and ROCK activation and cytoskeleton integrity. Longer treatments with IFNα induced transcriptional activation of the hCNT2-encoding gene (SLC28A2), resulting in a sustained increase in hCNT2-related activity. These observations are proof of concept for at least one of the putative mechanisms underlying the synergistic responses induced by combination therapy with IFNα and ribavirin.

Retinoid and TGF-β families: crosstalk in development, neoplasia, immunity, and tissue repair

Transforming growth factor-β (TGF-β) isoforms are profibrotic cytokines, par excellence, and have complex multifunctional effects on many systems, depending on the biologic setting. Retinoids are vitamin A derivatives that also have diverse effects in development, physiology, and disease. The interactions between these classes of molecules are, not surprisingly, highly complex and are dependent on the tissue, cellular, and molecular settings.

Translocation of nucleoside analogs across the plasma membrane in hematologic malignancies

Nucleoside analogs are currently used in the treatment of various hematologic malignancies due to their ability to induce apoptosis of lymphoid cells. For nucleoside-derived drugs to exert their action, they must enter cells via nucleoside transporters from two gene families, SLC28 and SLC29 (CNT and ENT, respectively). Once inside the cell, these drugs must be phosphorylated to their active forms. In contrast, some members of the ATP-binding cassette (ABC) protein family have been identified as responsible for the efflux of the phosphorylated forms of these nucleoside-derived drugs. Here, we review the main nucleoside analogs used in hematologic malignancies and focus especially on those that are currently used in chronic lymphocytic leukemia (CLL). Moreover, we discuss the pharmacological profile of the nucleoside transporters, which determines the bioavailability of and cell sensitivity to these nucleoside-derived drugs. We also discuss the expression of nucleoside transporters and their activities in CLL as well as the possibility of modulating these transporter activities as a means of modulating intracellular drug availability and, consequently, responsiveness to therapy.

New role of the human equilibrative nucleoside transporter 1 (hENT1) in epithelial-to-mesenchymal transition in renal tubular cells

Epithelial-to-mesenchymal transition (EMT) is an important pro-fibrotic event in which tubular epithelial cells are transformed into myofibroblasts. Nucleoside transporters (NT) are regulated by many factors and processes, some of which are involved in fibrosis, such as cytokines, inflammation, and proliferation. Equilibrative nucleoside transporter 1 (ENT1) has been proved to be the most widely expressed adenosine transporter. In that sense, ENT1 may be a key player in cell damage signaling. Here we analyze the role of human ENT1 (hENT1) in the EMT process in proximal tubular cells. Addition of the main inducer of EMT, the transforming growth factor-β1, to HK-2 cells increased hENT1 mRNA and protein level expression. ENT1-mediated adenosine uptake was also enhanced. When cells were incubated with dipyridamole to evaluate the potential contribution of ENT1 to EMT by blocking its transport activity, EMT was induced. Moreover, the knock down of hENT1 with siRNA induced EMT and collagen production in HK-2 cells. Kidneys isolated from ENT1 knockout mice showed higher levels of interstitial collagen and α-SMA positive cells than wild-type mice. Our results point to a new potential role of hENT1 as a modulator of EMT in proximal tubular cells. In this sense, hENT1 could be involved in renal protection processes, and the loss or reduced expression of hENT1 would lead to an increased vulnerability of cells to the onset and/or progression of renal fibrosis.

Enhancement of fludarabine sensitivity by all-trans-retinoic acid in chronic lymphocytic leukemia cells

BACKGROUND

A subset of patients with fludarabine-resistant chronic lymphocytic leukemia has previously been shown to express elevated intracellular levels of the concentrative high-affinity fludarabine transporter hCNT3, without any detectable related activity. We have recently shown that all-trans-retinoic acid is capable of inducing hCNT3 trafficking to plasma membrane in the MEC1 cell line. We, therefore, evaluated the effect of all-trans-retinoic acid on hCNT3 in primary chronic lymphocytic leukemia cells as a suitable mechanism to improve fludarabine-based therapy of chronic lymphocytic leukemia.

DESIGN AND METHODS

Cells from 23 chronic lymphocytic leukemia patients wild-type for P53 were analyzed for ex vivo sensitivity to fludarabine. hCNT3 activity in chronic lymphocytic leukemia cell samples was evaluated by measuring the uptake of [8-(3)H]-fludarabine. The amounts of transforming growth factor-β1 and hCNT3 messenger RNA were analyzed by real-time polymerase chain reaction. The effect of all-trans-retinoic acid on hCNT3 subcellular localization was analyzed by confocal microscopy and its effect on fludarabine-induced apoptosis was evaluated by flow cytometry analysis using annexin V staining.

RESULTS

Chronic lymphocytic leukemia cases showing higher ex vivo basal sensitivity to fludarabine also had a greater basal hCNT3-associated fludarabine uptake capacity compared to the subset of patients showing ex vivo resistance to the drug. hCNT3 transporter activity in chronic lymphocytic leukemia cells from the latter patients was either negligible or absent. Treatment of the fludarabine-resistant subset of chronic lymphocytic leukemia cells with all-trans-retinoic acid induced increased fludarabine transport via hCNT3 which was associated with a significant increase in fludarabine sensitivity.

CONCLUSIONS

Improvement of ex vivo fludarabine sensitivity in chronic lymphocytic leukemia cells is associated with increased hCNT3 activity after all-trans-retinoic acid treatment.

Differential adenosine uptake in mixed neuronal/glial or purified glial cultures of avian retinal cells: modulation by adenosine metabolism and the ERK cascade

Adenosine is an important modulator of neuronal survival and differentiation in the CNS. Our previous work showed that nucleoside transporters (NTs) are present in cultures of chick retinal cells, but little is known about the mechanisms regulating adenosine transport in these cultures. Our aim in the present work was to study the participation of the adenosine metabolism as well as the ERK pathway on adenosine uptake in different types of retinal cultures (mixed and purified glial cultures). Kinetic analysis in both cultures revealed that the uptake reached equilibrium after 30 min and presented two components. Incubation of cultures with S-(p-nitrobenzyl)-6-thioinosine (NBTI) or dipyridamole, different inhibitors of equilibrative nucleoside transporters (ENTs), produced a significant and concentration-dependent uptake reduction in both cultures. However, while dipyridamole presented similar maximal inhibitory effects in both cultures (although in different concentrations), the inhibition by NBTI was smaller in glial cultures than in mixed cultures, suggesting the presence of different transporters. Moreover, pre-incubation of [(3)H]-adenosine with adenosine deaminase (ADA) or adenosine kinase (ADK) inhibition with iodotubercidin promoted significant uptake inhibition in both cultures, indicating that the uptake is predominantly for adenosine and not inosine, and that taken up adenosine is preferentially directed to the synthesis of adenine nucleotides. In both cultures, the MEK inhibitors PD98059 or UO126, but not the inactive analog U0124, induced a significant and concentration-dependent uptake decrease. We have not observed any change in adenosine metabolism induced by MEK inhibitors, suggesting that this pathway is mediating a direct effect on NTs. Our results show the expression of different NTs in retinal cells in culture and that the activity of these transporters can be regulated by the ERK pathway or metabolic enzymes such as ADK which are then potential targets for regulation of Ado levels in normal or pathological conditions.

Different N-terminal motifs determine plasma membrane targeting of the human concentrative nucleoside transporter 3 in polarized and nonpolarized cells

Human concentrative nucleoside transporter 3 (hCNT3) is a broad-selectivity, high-affinity protein implicated in the uptake of most nucleoside-derived anticancer and antiviral drugs. Regulated trafficking of hCNT3 has been recently postulated as a suitable way to improve nucleoside-based therapies. Moreover, the recent identification of a putative novel hCNT3-type transporter lacking the first 69 amino acids and retained at the endoplasmic reticulum anticipated that the N terminus of hCNT3 contains critical motifs implicated in trafficking. In the current study, we have addressed this issue by using deletions and site-directed mutagenesis and plasma membrane expression and nucleoside uptake kinetic analysis. Data reveal that 1) a segment between amino acids 50 and 62 contains plasma membrane-sorting determinants in nonpolarized cells; 2) in particular, the Val(57)-Thr(58)-Val(59) tripeptide seems to be the core of the export signal, whereas acidic motifs upstream and downstream of it seem to be important for the kinetics of the process; and 3) in polarized epithelia, the β-turn-forming motif (17)VGFQ(20) is necessary for proper apical expression of the protein.