Activation of deoxycytidine kinase during inhibition of DNA synthesis by 2-chloro-2'-deoxyadenosine (Cladribine) in human lymphocytes

Doxorubicin effects on leukemia and breast cancer cells in culture on the TK1 protein levels using AroCell TK 210 ELISA: a tool for drug development

In this study changes in thymidine kinase 1 (TK1) levels after 24 hours of Doxorubicin (Dox) exposure of CEM and MDA MB-231 cells were determined using the commercially available AroCell TK 210 ELISA test. In cell extracts, TK1 levels increased twofold with 1 µM Dox in both cell lines, while the TK1 levels in the culture media increased with 5 and 10 µM of Dox only in case of CEM cells. In conclusion, this study reveals that the TK 210 ELISA can measure changes in intra- and extracellular TK1 levels apparently related to the mechanism of cytotoxicity of anti-cancer agents.

Cladribine and Fludarabine Nucleotides Induce Distinct Hexamers Defining a Common Mode of Reversible RNR Inhibition

The enzyme ribonucleotide reductase (RNR) is a major target of anticancer drugs. Until recently, suicide inactivation in which synthetic substrate analogs (nucleoside diphosphates) irreversibly inactivate the RNR-α2β2 heterodimeric complex was the only clinically proven inhibition pathway. For instance, this mechanism is deployed by the multifactorial anticancer agent gemcitabine diphosphate. Recently reversible targeting of RNR-α-alone coupled with ligand-induced RNR-α-persistent hexamerization has emerged to be of clinical significance. To date, clofarabine nucleotides are the only known example of this mechanism. Herein, chemoenzymatic syntheses of the active forms of two other drugs, phosphorylated cladribine (ClA) and fludarabine (FlU), allow us to establish that reversible inhibition is common to numerous drugs in clinical use. Enzyme inhibition and fluorescence anisotropy assays show that the di- and triphosphates of the two nucleosides function as reversible (i.e., nonmechanism-based) inhibitors of RNR and interact with the catalytic (C site) and the allosteric activity (A site) sites of RNR-α, respectively. Gel filtration, protease digestion, and FRET assays demonstrate that inhibition is coupled with formation of conformationally diverse hexamers. Studies in 293T cells capable of selectively inducing either wild-type or oligomerization-defective mutant RNR-α overexpression delineate the central role of RNR-α oligomerization in drug activity, and highlight a potential resistance mechanism to these drugs. These data set the stage for new interventions targeting RNR oligomeric regulation.

DrugGenEx-Net: a novel computational platform for systems pharmacology and gene expression-based drug repurposing

Background

The targeting of disease-related proteins is important for drug discovery, and yet target-based discovery has not been fruitful. Contextualizing overall biological processes is critical to formulating successful drug-disease hypotheses. Network pharmacology helps to overcome target-based bottlenecks through systems biology analytics, such as protein-protein interaction (PPI) networks and pathway regulation.

Results

We present a systems polypharmacology platform entitled DrugGenEx-Net (DGE-NET). DGE-NET predicts empirical drug-target (DT) interactions, integrates interaction pairs into a multi-tiered network analysis, and ultimately predicts disease-specific drug polypharmacology through systems-based gene expression analysis. Incorporation of established biological network annotations for protein target-disease, −signaling pathway, −molecular function, and protein-protein interactions enhances predicted DT effects on disease pathophysiology. Over 50 drug-disease and 100 drug-pathway predictions are validated. For example, the predicted systems pharmacology of the cholesterol-lowering agent ezetimibe corroborates its potential carcinogenicity.

When disease-specific gene expression analysis is integrated, DGE-NET prioritizes known therapeutics/experimental drugs as well as their contra-indications. Proof-of-concept is established for immune-related rheumatoid arthritis and inflammatory bowel disease, as well as neuro-degenerative Alzheimer’s and Parkinson’s diseases.

Conclusions

DGE-NET is a novel computational method that predicting drug therapeutic and counter-therapeutic indications by uniquely integrating systems pharmacology with gene expression analysis. DGE-NET correctly predicts various drug-disease indications by linking the biological activity of drugs and diseases at multiple tiers of biological action, and is therefore a useful approach to identifying drug candidates for re-purposing.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-016-1065-y) contains supplementary material, which is available to authorized users.

The apoptotic effects of toosendanin are partially mediated by activation of deoxycytidine kinase in HL-60 cells

Triterpenoid toosendanin (TSN) exhibits potent cytotoxic activity through inducing apoptosis in a variety of cancer cell lines. However, the target and mechanism of the apoptotic effects by TSN remain unknown. In this study, we captured a specific binding protein of TSN in HL-60 cells by serial affinity chromatography and further identified it as deoxycytidine kinase (dCK). Combination of direct activation of dCK and inhibition of TSN-induced apoptosis by a dCK inhibitor confirmed that dCK is a target for TSN partially responsible for the apoptosis in HL-60 cells. Moreover, the activation of dCK by TSN was a result of conformational change, rather than auto-phosphorylation. Our results further imply that, in addition to the dATP increase by dCK activation in tumor cells, dCK may also involve in the apoptotic regulation.

Phosphorylation of deoxycytidine kinase on Ser-74: impact on kinetic properties and nucleoside analog activation in cancer cells

Deoxycytidine kinase (dCK) (EC 2.7.1.74) is a key enzyme in the activation of several therapeutic nucleoside analogs (NA). Its activity can be increased in vivo by Ser-74 phosphorylation, a property that could be used for enhancing NA activation and clinical efficacy. In line with this, studies with recombinant dCK showed that mimicking Ser-74 phosphorylation by a S74E mutation increases its activity toward pyrimidine analogs. However, purine analogs had not been investigated. Here, we show that the S74E mutation increased the k(cat) for cladribine (CdA) by 8- or 3-fold, depending on whether the phosphoryl donor was ATP or UTP, for clofarabine (CAFdA) by about 2-fold with both ATP and UTP, and for fludarabine (F-Ara-A) by 2-fold, but only with UTP. However, the catalytic efficiencies (k(cat)/Km) were not, or slightly, increased. The S74E mutation also sensitized dCK to feed-back inhibition by dCTP, regardless of the phosphoryl donor. Importantly, we did not observe an increase of endogenous dCK activity toward purine analogs after in vivo-induced increase of Ser-74 phosphorylation. Accordingly, treatment of CLL cells with aphidicolin, which enhances dCK activity through Ser-74 phosphorylation, did not modify the conversion of CdA or F-Ara-A into their active triphosphate form. Nevertheless, the same treatment enhanced activation of gemcitabine (dFdC) into dFdCTP in CLL as well as in HCT-116 cells and produced synergistic cytotoxicity. We conclude that increasing phosphorylation of dCK on Ser-74 might constitute a valuable strategy to enhance the clinical efficacy of some NA, like dFdC, but not of CdA or F-Ara-A.

Influence of phosphorylation of THR-3, SER-11, and SER-15 on deoxycytidine kinase activity and stability

Deoxycytidine kinase (dCK) is a key enzyme in the salvage of deoxyribonucleosides and in the activation of several anticancer and antiviral nucleoside analogues. We have recently shown that dCK is a phosphoprotein. Four in vivo phosphorylation sites were identified: Thr-3, Ser-11, Ser-15, and Ser-74. Site-directed mutagenesis demonstrated that phosphorylation of Ser-74, the major phosphorylated residue, strongly influences dCK activity in eucaryotic cells. Here, we show that phosphorylation of the three other sites, located in the N-terminal extremity of the protein, does not significantly modify dCK activity, but phosphorylation of Thr-3 could promote dCK stability.

Casein kinase 1delta activates human recombinant deoxycytidine kinase by Ser-74 phosphorylation, but is not involved in the in vivo regulation of its activity

Deoxycytidine kinase (dCK) is a key enzyme in the salvage of deoxynucleosides and in the activation of several anticancer and antiviral nucleoside analogues. We recently showed that dCK was activated in vivo by phosphorylation of Ser-74. However, the protein kinase responsible was not identified. Ser-74 is located downstream a Glu-rich region, presenting similarity with the consensus phosphorylation motif of casein kinase 1 (CKI), and particularly of CKI delta. We showed that recombinant CKI delta phosphorylated several residues of bacterially overexpressed dCK: Ser-74, but also Ser-11, Ser-15, and Thr-72. Phosphorylation of dCK by CKI delta correlated with increased activity reaching at least 4-fold. Site-directed mutagenesis demonstrated that only Ser-74 phosphorylation was involved in dCK activation by CKI delta, strengthening the key role of this residue in the control of dCK activity. However, neither CKI delta inhibitors nor CKI delta siRNA-mediated knock-down modified Ser-74 phosphorylation or dCK activity in cultured cells. Moreover, these approaches did not prevent dCK activation induced by treatments enhancing Ser-74 phosphorylation. Taken together, the data preclude a role of CKI delta in the regulation of dCK activity in vivo. Nevertheless, phosphorylation of dCK by CKI delta could be a useful tool for elucidating the influence of Ser-74 phosphorylation on the structure-activity relationships in the enzyme.

Positive regulation of deoxycytidine kinase activity by phosphorylation of Ser-74 in B-cell chronic lymphocytic leukaemia lymphocytes

Deoxycytidine kinase (dCK) activates several antileukaemic nucleoside analogues. We have recently reported that the activity of dCK, overexpressed in HEK 293T cells, correlates with its phosphorylation level on Ser-74. Here, we show that dCK from B-cell chronic lymphocytic leukaemia (B-CLL) lymphocytes can be detected by an anti-phospho-Ser-74 antibody and that interindividual variability in dCK activity is related to its phosphorylation level on Ser-74. Moreover, pharmacological intervention modified Ser-74 phosphorylation, in close parallel with changes in dCK activity. These results suggest that activation of dCK via phosphorylation of Ser-74 might constitute a new therapeutic strategy to enhance activation and efficacy of nucleoside analogues.

The pattern of deoxycytidine- and deoxyguanosine kinase activity in relation to messenger RNA expression in blood cells from untreated patients with B-cell chronic lymphocytic leukemia

Deoxycytidine kinase (dCK) and deoxyguanosine kinase (dGK) catalyze the first step in the intracellular cascade of fludarabine (2-fluoroadenine-beta-D-arabinofuranoside) and cladribine (2-chlorodeoxyadenosine) phosphorylation, which leads to activation of these prodrugs, commonly used for treatment of chronic lymphocytic leukemia (CLL). Thus, resistance to nucleoside analogues may primarily be due to low levels of deoxynucleoside kinase activity. The purpose of this study was to investigate the activity profiles of dCK and dGK and characterize the possible relationship between the levels of dCK enzymatic activities and mRNA levels in B-CLL cells from untreated patient samples in an attempt to determine the best approach for predicting sensitivity to nucleoside analogues and thereby optimizing treatment of CLL. For this purpose, dCK and dGK analyses were done in blood cells from 59 untreated symptomatic patients with CLL. The dGK activity towards 2-chlorodeoxyadenosine was significantly lower than of dCK (median 73 pmol/mg protein/min (85-121, 95% CI) versus 353 pmol/mg protein/min (331-421)). The median dCK mRNA level was 0.107 (0.096-0.120, 95% CI). There was a lack of correlation between the activities of dCK and dGK, which indicates that these proteins are regulated independently. We also found that the dCK and dGK activity measurement towards their endogenous substrates were comparable to the nucleoside analogues tested. Such variations in enzyme activities and mRNA levels may well explain differences in clinical responses to treatment. There was no correlation between the levels of dCK mRNAs and enzymatic activities using a quantitative real-time PCR procedure. Sequencing of dCK mRNA did not reveal alternate splicing or mutations in the coding region. The relation between activity and mRNA levels was studied by short interfering RNA (siRNA) method, which showed that in the siRNA treated cells the down-regulation of dCK expression, and activity followed each other. However, in control cells the mRNA levels remained stable but the protein activity markedly decreased. These data demonstrate that the dCK activity is not reflected by dCK mRNA expression that indicates a post-translational mechanism(s).

Identification of in vivo phosphorylation sites on human deoxycytidine kinase. Role of Ser-74 in the control of enzyme activity

Deoxycytidine kinase (dCK) catalyzes the rate-limiting step of the deoxyribonucleoside salvage pathway in mammalian cells and plays a key role in the activation of numerous nucleoside analogues used in anti-cancer and antiviral chemotherapy. Although compelling evidence indicated that dCK activity might be regulated by phosphorylation/dephosphorylation, direct demonstration was lacking. Here we showed that dCK overexpressed in HEK 293T cells was labeled after incubating the cells with [32P]orthophosphate. Sorbitol, which was reported to decrease dCK activity, also decreased the labeling of dCK. These results indicated that dCK may exist as a phosphoprotein in vivo and that its activity can be correlated with its phosphorylation level. After purification of 32P-labeled dCK, digestion by trypsin, and analysis of the radioactive peptides by tandem mass spectrometry, the following four in vivo phosphorylation sites were identified: Thr-3, Ser-11, Ser-15, and Ser-74, the latter being the major phosphorylation site. Site-directed mutagenesis and use of an anti-phospho-Ser-74 antibody demonstrated that Ser-74 phosphorylation was crucial for dCK activity in HEK 293T cells, whereas phosphorylation of other identified sites did not seem essential. Phosphorylation of Ser-74 was also detected on endogenous dCK in leukemic cells, in which the Ser-74 phosphorylation state was increased by agents that enhanced dCK activity. Our study provided direct evidence that dCK activity can be controlled by phosphorylation in intact cells and highlights the importance of Ser-74 for dCK activity.

Quantitation of synergism of arabinosylcytosine and cladribine against the growth of arabinosylcytosine-resistant human lymphoid cells

This report presents a quantitative analysis of the synergistic interaction of arabinosylcytosine (araC) and cladribine (CdA) in human H9-lymphoid cell lines sensitive and resistant to araC (H9-araC cells). H9-araC cells obtained by cultivation of H9 cells in the presence of 0.5 microM arabinosylcytosine (araC) had lower deoxycytidine kinase (dCK) than the parental cell line. The IC50 values of araC and CdA calculated by using median-effect analysis and CalcuSyn software were: 0.55 microM and 1.16 microM for CdA and 0.0058 microM and 3.5 microM for araC in H9 and H9-araC cells, respectively. These values were reduced to 0.10 microM and 0.38 microM for CdA and to 0.004 microM and to 0.77 microM for araC when the drugs were used in combination. Computerized simulation of dose reduction index (DRI) indicated that at 50-99% growth inhibition levels, the doses of araC could be reduced by 2.0 to 11.9-fold and 2.9 to 5.3-fold and the doses of CdA by 5.9 and 183.7-fold and 3.1 to 164.8-fold in H9 and H9-araC cells, respectively, when the drugs are used in combination. Assessment by combination index (CI) analysis showed that the combination exhibited moderate to strong synergistic lympho-cytotoxic effects. CdA metabolic studies (influx and activation) in the presence of deoxyadenosine, deoxycytidine, or araC suggested that CdA enters cells by a deoxyadenosine-inhibitable transport system, which is different than that of araC and deoxycytidine transport system. Thus, in addition to the known mechanisms, other mechanisms might be involved in the metabolism of CdA. The demonstration that araC and CdA combinations exert synergistic cytotoxicity even in the resistant cells raises hope that such a combination may be useful in tumors that were found resistant to these drugs.

Cyclopentenyl cytosine-induced activation of deoxycytidine kinase increases gemcitabine anabolism and cytotoxicity in neuroblastoma

The effect of the CTP synthetase inhibitor cyclopentenyl cytosine (CPEC) on the metabolism and cytotoxicity of 2',2'-difluorodeoxycytidine (dFdC, gemcitabine) and the expression and activity of deoxycytidine kinase (dCK) was studied in human neuroblastoma cell lines. The cytotoxicity of dFdC and CPEC was studied in a panel of MYCN-amplified and MYCN-single-copy neuroblastoma cell lines using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide-assays. dFdC-metabolism was studied in SK-N-BE(2)c cells using [3H]-radiolabeled dFdC. dCK activity and expression were measured using enzyme assays, immunoblot and quantitative PCR, respectively. Both MYCN-amplified and MYCN-single-copy neuroblastoma cell lines were highly sensitive to dFdC, with concentration of the drug resulting in 50% effect when compared to untreated controls (ED50) values in the nanomolar range after a 3-h exposure to dFdC. There was no correlation of the observed ED50 with the dCK activity. Treatment with dFdC induced cell death in MYCN-amplified cells whereas MYCN-single-copy-cell lines underwent neuronal differentiation. Pre-incubation with CPEC significantly increased dFdC-cytotoxicity from 1.3 to 5.3-fold in 13 out of 15 cell lines. [3H]dFdC-anabolism increased 6-44 fold in SK-N-BE(2)c cells after incubation with CPEC and was paralleled by a significant increase in expression and activity of dCK. In conclusion, the combination of dFdC and CPEC is highly toxic to neuroblastoma in vitro.

Deoxy-adenosine-monophosphate (dAMP) di-n-butylester induces apoptosis by increasing the dATP level in HL-60 cells

Deoxy-ATP is a potent inducer of apoptosis. We intended to synthesize a lipophilic dAMP derivative which, according to our working hypothesis penetrates into the cell, is converted to dAMP by intracellular esterases and to dATP by nucleotide kinases. We synthesized dAMP-di-n-butylester (DAB) and tested it. We found that it fulfills the above-described expectations. DAB treatment decreases the viability of HL-60 cells, increases the dATP concentration and induces apoptogenic cytochrome c release from mitochondria with concomitant elevation of caspase-9 activity. Our results indicate that use of dAMP derivatives with masked phosphate may be a feasible approach for pharmacological elevation of intracellular dATP and induction of apoptosis.

Activation of deoxycytidine kinase by deoxyadenosine: implications in deoxyadenosine-mediated cytotoxicity

The inborn deficiency of adenosine deaminase is characterised by accumulation of excess amounts of cytotoxic deoxyadenine nucleotides in lymphocytes. Formation of dATP requires phosphorylation of deoxyadenosine by deoxycytidine kinase (dCK), the main nucleoside salvage enzyme in lymphoid cells. Activation of dCK by a number of genotoxic agents including 2-chlorodeoxyadenosine, a deamination-resistant deoxyadenosine analogue, was found previously. Here, we show that deoxyadenosine itself is also a potent activator of dCK if its deamination was prevented by the adenosine deaminase inhibitor deoxycoformycin. In contrast, deoxycytidine was found to prevent stimulation of dCK by various drugs. The activated form of dCK was more resistant to tryptic digestion, indicating that dCK undergoes a substrate-independent conformational change upon activation. Elevated dCK activities were accompanied by decreased pyrimidine nucleotide levels whereas cytotoxic dATP pools were selectively enhanced. dCK activity was found to be downregulated by growth factor and MAP kinase signalling, providing a potential tool to slow the rate of dATP accumulation in adenosine deaminase deficiency.

New evidences for a regulation of deoxycytidine kinase activity by reversible phosphorylation

Recent studies indicate that deoxycytidine kinase (dCK), which activates various nucleoside analogues used in antileukemic therapy, can be regulated by post-translational modification, most probably through reversible phosphorylation. To further unravel its regulation, dCK was overexpressed in HEK-293 cells as a His-tag fusion protein. Western blot analysis showed that purified overexpressed dCK appears as doublet protein bands. The slower band disappeared after treatment with protein phosphatase lambda (PP lambda) in parallel with a decrease of dCK activity, providing additional arguments in favor of both phosphorylated and unphosphorylated forms of dCK.

Stimulation of deoxycytidine kinase results in prolonged maintenance of the enzyme activity

A number of genotoxic and antiproliferative agents such as 2-chlorodeoxyadenosine (Cladribine; CdA) and aphidicolin (APC) have been shown to stimulate the activity of deoxycytidine kinase, the main deoxynucleoside salvage enzyme in lymphocytes. Here we show that enzyme activation could be prevented by treating cells with the membrane-permeant calcium chelator BAPTA-AM. Long-term incubations demonstrated that CdA and APC not only stimulated but also sustained deoxycytidine kinase activity in the cellular context, as compared to the control and BAPTA-AM treated enzyme activities.

Selective increase of dATP pools upon activation of deoxycytidine kinase in lymphocytes: implications in apoptosis

Stimulation of the activity of deoxycytidine kinase (dCK), the principal deoxynucleoside salvage enzyme, has been recently considered as a protective cellular response to a wide range of agents interfering with DNA repair and apoptosis. In light of this, the potential contribution of dCK activation to apoptosis induction--presumably by supplying dATP or its analogues for the apoptosome formation--deserves consideration. Two-hour exposure of human tonsillar lymphocytes to 2-chloro-deoxyadenosine (CdA) led to a two-fold activation of dCK. This activation process was inhibited by pifithrin-alpha, a potent inhibitor of p53. When the dNTP pools were determined, both deoxypyrimidine triphosphate and dGTP pools were reduced after the treatments, while dATP levels elevated by 62%, 77% and 50% in the CdA, aphidicolin and etoposide-treated cells, respectively. We assume that dCK activation elicited by cellular damage might be a proapoptotic factor in terms of generating dATP well before the release of cytochrome c and deoxyguanosine kinase from mitochondria.

Activation of deoxycytidine kinase by protein kinase inhibitors and okadaic acid in leukemic cells

Deoxycytidine kinase (dCK) is a key enzyme in the deoxynucleoside salvage pathway and in the activation of numerous nucleoside analogues used in cancer and antiviral chemotherapy. Recent studies indicate that dCK activity might be regulated through reversible phosphorylation. Here, we report the effects of a large panel of protein kinase inhibitors on dCK activity in the B-leukemia cell line EHEB, both in basal conditions and in the presence of the nucleoside analogue 2-chloro-2'-deoxyadenosine (CdA) which induces activation of dCK. Except staurosporine and H-7 that significantly reduced the activation of dCK by CdA, no specific protein kinase inhibitor diminished basal dCK activity or its activation by CdA. In contrast, genistein, a general protein tyrosine kinase inhibitor, and AG-490, an inhibitor of JAK2 and JAK3, increased basal dCK activity more than two-fold. Two specific inhibitors of the MAPK/ERK pathway, PD-98059 and U-0126, also enhanced dCK activity. These data suggest that the JAK/MAPK pathway could be involved in the regulation of dCK. Moreover, we show that the activity of dCK, raised by CdA, can return to its initial level by treatment with protein phosphatase-2A (PP2A). Accordingly, dCK activity in intact cells increased upon incubation with okadaic acid (OA) at concentrations that should inhibit PP2A, but not protein phosphatase-1. Activation of dCK by protein kinase inhibitors and OA was also observed in CCRF-CEM cells and in chronic lymphocytic leukemia B-lymphocytes, suggesting a general mechanism of post-translational regulation of dCK, which could be exploited to enhance the activation of antileukemic nucleoside analogues.

Pharmacological basis for cladribine resistance

The inherent or acquired resistance of leukemic cells to cytostatic agents is a major clinical challenge. The purpose of this review was to elucidate and analyse the available data concerning mechanisms of resistance of cladribine with emphasis on recent advances in the characterization of activating and inactivating enzymes in the induction of resistance to cladribine. All available in vitro and clinical data on cladribine was undertaken. Cladribine, unlike many other drugs, is toxic to both dividing and indolent lymphoid malignancies. Cladribine is a prodrug and must be phosphorylated intracellularly to cladribine-monophosphate (MP) by the nuclear/cystosol enzyme deoxycytidine kinase (dCK) and the mitochondrial enzyme deoxyguanosine kinase. The cytotoxicity mainly depends on the accumulation of cladribine-triphosphates (TP) after phosphorylation of cladribine-MP by nucleoside monophosphate kinase and nucleoside diphosphate kinase. 5'-Nucleotidase (5'-NT) dephosphorylates cladribine-MP and the accumulation of cladribine-TP depends on the ratio of dCK and 5'-NT in the cells. The mechanisms underlying cladribine resistance are multifactorial, e.g. decreased nucleoside transport, decreased activity or deficiency of dCK, altered intracellular pools of competing nucleotides, altered regulation of ribonucleotide reductase and increased drug inactivation by 5'-NT. Finally, cladribine resistance may be a consequence of a defective induction of apoptosis. In spite of the fact that more than one mechanism can contribute to a cladribine resistance phenotype, a reduction in dCK activity is probably the major determinant of cladribine resistance. Insight into the mechanism of action and resistance to cladribine is crucial for its optimal use as well as for the development of newer analogues.

Determination of the deoxycytidine kinase activity in cell homogenates with a non-radiochemical assay using reversed-phase high performance liquid chromatography

A non-radioactive procedure to measure the deoxycytidine kinase (dCK) activity in crude cell free homogenates was developed. 2-Chlorodeoxyadenosine (CdA) was used as the substrate for dCK and was separated from its product 2-chlorodeoxyadenosine-5'-monophosphate (CdAMP) by reversed-phase HPLC. A complete separation of CdA and its metabolites was achieved in 30 min. The minimum amount of CdAMP that could be detected was 1 pmol. The assay was linear with reaction times up to at least 3h. With respect to the protein concentration, the reaction was linear with protein concentrations up to 760 microg/ml in the assay. An amount of 8 x 10(3) cells was already sufficient to determine the specific dCK activity in SK-N-BE(2)c cells. CdA was not only converted to CdAMP but also to 2-chloroadenine and, surprisingly, also to 2-chlorodeoxyinosine, in MOLT-3 cells. The deamination of CdA was completely inhibited by deoxycoformycin, which clearly demonstrates that CdA is a substrate for adenosine deaminase.

Resistance to gemcitabine in a human follicular lymphoma cell line is due to partial deletion of the deoxycytidine kinase gene

Background

Gemcitabine is an analogue of deoxycytidine with activity against several solid tumors. In order to elucidate the mechanisms by which tumor cells become resistant to gemcitabine, we developed the resistant subline RL-G from the human follicular lymphoma cell line RL-7 by prolonged exposure of parental cells to increasing concentrations of gemcitabine.

Results

In vitro, the IC₅₀ increased from 0.015 μM in parental RL-7 cells to 25 μM in the resistant variant, RL-G. Xenografts of both cell lines developed in nude mice were treated with repeated injections of gemcitabine. Under conditions of gemcitabine treatment which totally inhibited the development of RL-7 tumors, RL-G derived tumors grew similarly to those of untreated animals, demonstrating the in vivo resistance of RL-G cells to gemcitabine. HPLC experiments showed that RL-G cells accumulated and incorporated less gemcitabine metabolites into DNA and RNA than RL-7 cells. Gemcitabine induced an S-phase arrest in RL-7 cells but not in RL-G cells. Exposure to gemcitabine induced a higher degree of apoptosis in RL-7 than in RL-G cells, with poly-(ADP-ribose) polymerase cleavage in RL-7 cells. No modifications of Bcl-2 nor of Bax expression were observed in RL-7 or RL-G cells exposed to gemcitabine. These alterations were associated with the absence of the deoxycytidine kinase mRNA expression observed by quantitative RT-PCR in RL-G cells. PCR amplification of désoxycytidine kinase gene exons showed a partial deletion of the dCK gene in RL-G cells.

Conclusions

These results suggest that partial deletion of the dCK gene observed after selection in the presence of gemcitabine is involved with resistance to this agent both in vitro and in vivo.

2-Chloro-2'-deoxyadenosine synergistically enhances azidothymidine cytotoxicity in azidothymidine resistant T-lymphoid cells

This report presents quantitative analysis of the synergistic interaction of azidothymidine (AZT) and cladribine (CdA) in human H9-lymphoid cell lines sensitive and resistant to AZT (H9-araC cells). H9-araC cells obtained by cultivation of H9 cells in the presence of 0.5 microM arabinosyl-cytosine (araC) had lower deoxycytidine kinase and thymidine kinase (TK) activities and expressed cross-resistance to araC and AZT. The IC(50) values of AZT and CdA were calculated by using median-effect analysis and CalcuSyn software. The IC(50) values were 0.44 and 0.82 microM for CdA and 67.8 and 30,310 microM for AZT in H9 and H9-araC cells, respectively. However, when the drugs were used in combination the IC(50) values of CdA and AZT were reduced to 0.12 and 15.5 microM in H9 cells and to 0.19 and 24.9 microM in H9-araC cells, respectively. Calculation of dose reduction index (DRI) indicated that at 50-90% growth inhibition level, the combination of the drugs caused 3.6-5.8- and 4.1-11.5-fold reduction in the dose of CdA and 4.4-37.6- and > 1000-fold reduction in the dose of AZT in H9 and H9-araC cells, respectively. The combination index (CI) values simulated from these data suggested synergistic to very strong synergistic lymphocytotoxic effects of AZT combined with CdA. These findings suggest the potential usefulness of a double-targeted approach for designing efficacious therapeutics for the kinase deficient drug resistant tumors.

Activation of deoxycytidine kinase in lymphocytes is calcium dependent and involves a conformational change detectable by native immunostaining

Deoxycytidine kinase (dCK), the principal deoxynucleoside salvage enzyme, plays a seminal role in the bioactivation of a wide array of cytotoxic nucleoside analogues. Recently, activation of dCK has been considered as a protective cellular response to a number of DNA-damaging agents in lymphocytes. Regarding the molecular mechanism of the enzyme activation, a post-translational modification by protein phosphorylation has been suggested. Here we provide evidence that both the activation process and the maintenance of the activated state require free cytosolic calcium. BAPTA-AM, a cell-permeable calcium chelator selectively inhibited the activation of dCK in a time- and concentration-dependent manner while extracellular calcium depletion had no effect. On the other hand, elevation of cytoplasmic calcium levels by thapsigargin did not potentiate the enzyme, referring to the permissive function of calcium in the activation process. Denaturing Western blots of extracts from lymphocytes incubated with 2-chlorodeoxyadenosine, aphidicolin and/or BAPTA-AM clearly demonstrated that dCK protein levels were unchanged during these treatments. However, a striking correlation was found between enzyme activity and the intensity of dCK-specific signals in native Western blots. Extracts from CdA-treated cells were much better recognized by the antibody raised against the C-terminal peptide of dCK than the BAPTA-AM-treated samples. These results indicate that the calcium-dependent activation of dCK is accompanied by a conformational change that renders the C-terminal epitope more accessible to the antibody.

Increased sensitivity to gemcitabine of P-glycoprotein and multidrug resistance-associated protein-overexpressing human cancer cell lines

Gemcitabine (2',2'-difluorodeoxycytidine) is a deoxycytidine analogue that is activated by deoxycytidine kinase (dCK) to its monophosphate and subsequently to its triphosphate dFdCTP, which is incorporated into both RNA and DNA, leading to DNA damage. Multidrug resistance (MDR) is characterised by an overexpression of the membrane efflux pumps P-glycoprotein (P-gP) or multidrug resistance-associated protein (MRP). Gemcitabine was tested against human melanoma, non-small-cell lung cancer, small-cell lung cancer, epidermoid carcinoma and ovarian cancer cells with an MDR phenotype as a result of selection by drug exposure or by transfection with the mdr1 gene. These cell lines were nine- to 72-fold more sensitive to gemcitabine than their parental cell lines. The doxorubicin-resistant cells 2R120 (MRP1) and 2R160 (P-gP) were nine- and 28-fold more sensitive to gemcitabine than their parental SW1573 cells, respectively (P<0.01), which was completely reverted by 25 micro M verapamil. In 2R120 and 2R160 cells, dCK activities were seven- and four-fold higher than in SW1573, respectively, which was associated with an increased dCK mRNA and dCK protein. Inactivation by deoxycytidine deaminase was 2.9- and 2.2-fold decreased in 2R120 and 2R160, respectively. dFdCTP accumulation was similar in SW1573 and its MDR variants after 24 h exposure to 0.1 micro M gemcitabine, but dFdCTP was retained longer in 2R120 (P<0.001) and 2R160 (P<0.003) cells. 2R120 and 2R160 cells also incorporated four- and six-fold more [(3)H]gemcitabine into DNA (P<0.05), respectively. P-glycoprotein and MRP1 overexpression possibly caused a cellular stress resulting in increased gemcitabine metabolism and sensitivity, while reversal of collateral gemcitabine sensitivity by verapamil also suggests a direct relation between the presence of membrane efflux pumps and gemcitabine sensitivity.

Activation of deoxycytidine kinase by gamma-irradiation and inactivation by hyperosmotic shock in human lymphocytes

Deoxycytidine kinase (dCK) is a key enzyme in the intracellular metabolism of deoxynucleosides and their analogues, phosphorylating a wide range of drugs used in the chemotherapy of leukaemia and solid tumours. Previously, we found that activity of dCK can be enhanced by incubating primary cultures of lymphocytes with substrate analogues of the enzyme, as well as with various genotoxic agents. Here we present evidence that exposure of human lymphocytes to 0.5-2 Gy dosage of gamma-radiation as well as incubation of cells with calyculin A, a potent inhibitor of protein phosphatase 1 and 2A, both elevate dCK activity without changing the level of dCK protein. When cells were gamma-irradiated in the presence of calyculin A, a more pronounced activation of dCK was observed. In contrast, both basal and stimulated dCK activities were reduced by hyperosmotic treatment of the cells. DNA repair determined by the Comet assay and by thymidine incorporation was induced by irradiation. Complete repair of gamma-irradiated DNA was detected within 1 hr following the irradiation along with dCK activation, but the rate of repair was not accelerated by calyculin A. These data provide evidence for the activation of dCK upon DNA damage and repair that seems to be mediated by phosphorylation of the enzyme, suggesting the role of dCK in DNA repair processes.

Selective activation of deoxycytidine kinase by thymidine-5'-thiosulphate and release by deoxycytidine in human lymphocytes

Deoxycytidine kinase (dCK) catalyses the rate-limiting step of the salvage of three natural deoxyribonucleosides as well as several therapeutic nucleoside analogues, which in turn can enhance its enzymatic activity [Biochem Pharmacol 56 (1998) 1175], improving the efficacy of the cytostatic therapy. Here, we measured the effect of the 5'-thiosulphate (5'-TS) derivatives of four deoxyribonucleosides (deoxyadenosine, deoxycytidine (dCyd), azidothymidine, thymidine) and two ribonucleosides (ribopurine, ribouridine (Urd)) on the activity of the two main salvage deoxynucleoside kinases, and on the salvage of dCyd and deoxythymidine (dThd). It turned out that only 2'-deoxythymidine-5'-thiosulphate (dThd-5'-TS) can potentiate the dCK activity, without influencing the thymidine kinase isoenzymes during short-time treatments of human peripheral blood and tonsillar lymphocytes. The enhancement of dCK activity by dThd-5'-TS can be reversed by dCyd, but dThd had no effect on the enzyme activation in cells. Neither dThd-5'-TS nor Urd-5'-TS had any effect on the dCK and thymidine kinase activities tested in cell-free extracts. The stimulation of dCK activity in cells was accompanied by an imbalance in the dThd and dCyd metabolism. The incorporation of 3H-dThd into DNA was suppressed by 90% in cells by dThd-5'-TS, while Urd-5'-TS only slightly influenced the same process. The 3H-dCyd incorporation into DNA was inhibited only to 50% of the control, while the 3H-dCyd labelling of the nucleotide fraction was enlarged in dThd-5'-TS-treated cells, as a consequence of the increased dCK activity. We suggest that the enhancement of dCK activity is a compensatory mechanism in cells that might be induced by different "inhibitors" of DNA synthesis leading to damage of DNA. The increased dCK activity is able to supply the repair of DNA with dNTPs in quiescent cells; this suggestion seems to be supported by the counteracting effect of extracellular dCyd, too.

Activation of deoxycytidine kinase by UV-C-irradiation in chronic lymphocytic leukemia B-lymphocytes

Deoxycytidine kinase (dCK), a key enzyme of the deoxynucleoside salvage pathway, might have a preponderant role in DNA synthesis in resting chronic lymphocytic leukemia B-lymphocytes. In these cells, two important enzymes in deoxynucleoside triphosphate production, ribonucleotide reductase and thymidine kinase (TK), both cell-cycle regulated, are indeed very weakly expressed. This study investigated the regulation of dCK activity in response to UV-C light, a condition which causes DNA lesions and DNA repair synthesis. We observed that activity of dCK in B-CLL cells was upregulated up to 3-fold, 30 min after irradiation with 30 J/m(2) UV-C, whereas TK activity was unchanged. Activation of dCK by UV-C light was caused neither by a change in concentration of a low molecular weight metabolite nor by an increase in the amount of dCK protein. Activation of dCK by UV-C was mimicked by H(2)O(2), markedly counteracted by N-acetylcysteine, a general antioxidant, and completely abolished by the growth factor receptor inhibitor suramin. Taken together, these results indicate that dCK activity is upregulated by UV-C light through a postranslational modification that may be initiated at the cell surface through oxidative mechanisms. Suramin also suppressed the increase in DNA repair synthesis elicited by UV-C irradiation, suggesting that upregulation of dCK activity could contribute to the normal completion of DNA repair synthesis elicited by UV light.

Nucleoside analogues: mechanisms of drug resistance and reversal strategies

Nucleoside analogues (NA) are essential components of AML induction therapy (cytosine arabinoside), effective treatments of lymphoproliferative disorders (fludarabine, cladribine) and are also used in the treatment of some solid tumors (gemcitabine). These important compounds share some general common characteristics, namely in terms of requiring transport by specific membrane transporters, metabolism and interaction with intracellular targets. However, these compounds differ in regard to the types of transporters that most efficiently transport a given compound, and their preferential interaction with certain targets which may explain why some compounds are more effective against rapidly proliferating tumors and others on neoplasia with a more protracted evolution. In this review, we analyze the available data concerning mechanisms of action of and resistance to NA, with particular emphasis on recent advances in the characterization of nucleoside transporters and on the potential role of activating or inactivating enzymes in the induction of clinical resistance to these compounds. We performed an extensive search of published in vitro and clinical data in which the levels of expression of nucleoside-activating or inactivating enzymes have been correlated with tumor response or patient outcome. Strategies aiming to increase the intracellular concentrations of active compounds are presented.

Collateral sensitivity to gemcitabine (2',2'-difluorodeoxycytidine) and cytosine arabinoside of daunorubicin- and VM-26-resistant variants of human small cell lung cancer cell lines

Multidrug resistance (MDR), characterized by a cross-resistance to many natural toxin-related compounds, may be caused either by overexpression of a drug efflux pump such as P-glycoprotein, (P-gP), multidrug resistance proteins MRP1-3, or BCRP/MXR or, in the case of DNA topoisomerase II active drugs, by a decrease in the enzymatic activity of the target molecule termed altered topoisomerase MDR (at-MDR). However, human small cell lung carcinoma (SCLC) cell lines showed a collateral sensitivity to 2',2'-difluorodeoxycytidine (gemcitabine, dFdC) and 1-beta-D-arabinofuranosylcytosine (ara-C). H69/DAU, a daunorubicin (DAU)-resistant variant of H69 with a P-gP overexpression, and NYH/VM, a VM-26 (teniposide)-resistant variant of NYH with an at-MDR, were both 2-fold more sensitive to gemcitabine and 7- and 2-fold more sensitive to ara-C, respectively. MDR variants had a 4.3- and 2.0-fold increased activity of deoxycytidine kinase (dCK), respectively. dCK catalyzes the first rate-limiting activation step of both gemcitabine and ara-C. In addition, deoxycytidine deaminase, responsible for inactivation of dFdC and ara-C, was 9.0-fold lower in H69/DAU cells. The level of thymidine kinase 2, a mitochondrial enzyme that can also phosphorylate deoxycytidine and gemcitabine, was not significantly different between the variants. These differences most likely caused an increased accumulation of the active metabolites (dFdCTP, 2.1- and 1.6-fold in NYH/VM and H69/DAU cells, respectively) and of ara-CTP (1.3-fold in NYH/VM cells). Ara-CTP accumulation was not detectable in either H69 variant. The pools of all ribonucleoside and deoxyribonucleoside triphosphates were at least 3- to 4-fold higher in the NYH variants compared to the H69 variants; for dCTP and dGTP this difference was even larger. The higher ribonucleotide pools might explain the >10-fold higher accumulation of dFdCTP in NYH compared to H69 variants. Since dCTP is low, H69 cells might not need a high ara-CTP accumulation to inhibit DNA polymerase. This might be related to the lack of ara-CTP in H69 variants. In addition, the increased CTP, ATP, and UTP pools in the MDR variants might explain the increased ara-CTP and dFdCTP accumulation. In conclusion, the MDR variants of the human SCLC cell lines were collaterally sensitive due to an increased dCK activity, and consequently an increased ara-CTP and dFdCTP accumulation.

Activation of deoxycytidine kinase by inhibition of DNA synthesis in human lymphocytes

Deoxycytidine kinase (dCK, EC.2.7.1.74) is a key enzyme in the intracellular metabolism of 2-chlorodeoxyadenosine, 1-beta-D-arabinofuranosylcytosine, difluorodeoxycytidine, and other drugs used in chemotherapy of different leukaemias and solid tumours. Recently, stimulation of dCK activity was shown by these analogues and by other genotoxic agents such as etoposide and NaF, all of which cause severe inhibition of DNA synthesis in cell cultures. Here we describe that direct inhibition of DNA polymerases by aphidicolin stimulated dCK activity in normal lymphocytes and acute myeloid leukaemic cells, as well as in HL 60 promyelocytic cell cultures. Increased dCK activity was not due to new protein synthesis under our conditions, as measured by immunoblotting. Partial purification by diethylaminoethyl-Sephadex chromatography revealed that the activated form of dCK survived purification procedure. Moreover, it was possible to inactivate purified dCK preparations by recombinant protein phosphatase with Ser/Thr/Tyr dephosphorylating activity. These data suggest that the activation of dCK may be due to phosphorylation, and that deoxynucleoside salvage is promoted during inhibition of DNA synthesis in human lymphocytes.

Sequence dependent effect of paclitaxel on gemcitabine metabolism in relation to cell cycle and cytotoxicity in non-small-cell lung cancer cell lines

Gemcitabine and paclitaxel are active agents in the treatment of non-small-cell lung cancer (NSCLC). To optimize treatment drug combinations, simultaneously and 4 and 24 h intervals, were studied using DNA flow cytometry and multiple drug effect analysis in the NSCLC cell lines H460, H322 and Lewis Lung. All combinations resulted in comparable cytotoxicity, varying from additivity to antagonism (combination index: 1.0-2.6). Gemcitabine caused a S (48%) and G1 (64%) arrest at IC-50 and 10 x IC-50 concentrations, respectively. Paclitaxel induced G2/M arrest (70%) which was maximal within 24 h at 10 x IC-50. Simultaneous treatment increased S-phase arrest, while at the 24 h interval after 72 h the first drug seemed to dominate the effect. Apoptosis was more pronounced when paclitaxel preceded gemcitabine (20% for both intervals) as compared to the reverse sequence (8%, P = 0.173 for the 4 h and 12%, P = 0.051 for the 24 h time interval). In H460 cells, paclitaxel increased 2-fold the accumulation of dFdCTP, the active metabolite of gemcitabine, in contrast to H322 cells. Paclitaxel did not affect deoxycytidine kinase levels, but ribonucleotide levels increased possibly explaining the increase in dFdCTP. Paclitaxel did not affect gemcitabine incorporation into DNA, but seemed to increase incorporation into RNA. Gemcitabine almost completely inhibited DNA synthesis in both cell lines (70-89%), while paclitaxel had a minor effect and did not increase that of gemcitabine. In conclusion, various gemcitabine-paclitaxel combinations did not show sequence dependent cytotoxic effects; all combinations were not more than additive. However, since paclitaxel increased dFdCTP accumulation, gemcitabine incorporation into RNA and the apoptotic index, the administration of paclitaxel prior to gemcitabine might be favourable as compared to reversed sequences.

Treatment of normal and malignant cells with nucleoside analogues and etoposide enhances deoxycytidine kinase activity

Deoxycytidine kinase (dCK), one of the rate-limiting enzymes in the intracellular metabolism of many antileukaemic drugs, was shown to be stimulated after treatment of human tonsillar lymphocytes by 2-chloro-2'-deoxyadenosine (cladribine, CdA) (Sasvári-Székely, et al., Biochem Pharmacol 1998, 56, 1175-1179). Here we present a comparative study of different normal and malignant cells in respect to the activation of dCK by CdA. G-phase lymphocytes showed a higher sensitivity for dCK stimulation than S-phase cells. Normal and leukaemic peripheral blood mononuclear cells, as well as the promyelocytic cell line HL60 responded to CdA treatment by a 2-5-fold increase in activity of dCK. However, no significant stimulation was detected either in CCRF-CEM T-lymphoblastoid cells, or in K562 myeloid cells. Thymidine kinase (TK) activity was not stimulated in any cases. Treatment of these cells with several other analogues beside CdA, such as 2-chloro-2'-arabino-fluoro-2'-deoxyadenosine (CAFdA), 2-fluoro-1-beta-D-arabinosyladenine (Fludarabine, FaraA) and 1-beta-D-arabinosylcytosine (cytarabine, araC) gave similar results to CdA treatment. Enhancement of dCK activity could also be achieved with the topoisomerase II inhibitor, etoposide. In contrast, 2-chloro-riboadenosine (CrA) had no effect on the dCK at concentrations of 10 microM or less, while dCyd and 5-aza-dCyd caused slight inhibition. These results indicate that treatment of cells with several inhibitors of DNA synthesis potentiates the dCK activity. The drugs widely differ in their stimulatory effect on dCK, and there are also 'responsive' and 'non-responsive' cells with respect to dCK activation. Thus, enhancement of the dCK activity by specific drugs in 'responsive' cells might give a rationale for combination chemotherapy.