2', 3'-Dideoxycytidine represses thymidine kinases 1 and 2 expression in T-lymphoid cells

Differential gene expression in human hepatocyte cell lines exposed to the antiretroviral agent zidovudine

Zidovudine (3'-azido-3'-deoxythymidine; AZT) is the most widely used nucleoside reverse transcriptase inhibitor for the treatment of AIDS patients and prevention of mother-to-child transmission of HIV-1. Previously, we demonstrated that AZT had significantly greater growth inhibitory effects upon the human liver carcinoma cell line HepG2 as compared to the immortalized human liver cell line THLE2. We have now used gene expression profiling to determine the molecular pathways associated with toxicity in both cell lines. HepG2 cells were incubated with 0, 2, 20, or 100 μM AZT for 2 weeks; THLE2 cells were treated with 0, 50, 500, or 2,500 μM AZT, concentrations that were equi-toxic to those used in the HepG2 cells. After the treatment, total RNA was isolated and subjected to microarray analysis. Global analysis of gene expression, with a false discovery rate ≤0.01 and a fold change ≥1.5, indicated that 6- to 70-fold more genes were differentially expressed in a significant concentration-dependent manner in HepG2 cells when compared to THLE2 cells. Comparative analysis indicated that 7 % of these genes were common to both cell lines. Among the common differentially expressed genes, 70 % changed in the same direction, most of which were associated with cell death and survival, cell cycle, cell growth and proliferation, and DNA replication, recombination, and repair. As determined by the uptake of [methyl-(3)H]AZT, the intracellular levels of total AZT were approximately twofold higher in THLE2 cells than in HepG2 cells. The expression of thymidine kinase 1 (TK1) and UDP-glucuronosyltransferase 2B7 (UGT2B7) genes that regulate the metabolic activation and deactivation of AZT, respectively, was increased in HepG2 cells but decreased in THLE2 cells after treatment with AZT. This differential response in AZT metabolism was confirmed by real-time PCR, western blotting, and/or enzymatic assays. These data indicate that molecular pathways involved with cell death and survival, cell cycle, cell growth and proliferation, and DNA replication, recombination, and repair are involved in the toxicities associated with AZT in both human cell lines, and that the difference in expression of TK1 and UGT2B7 in response to AZT treatment in HepG2 cells and THLE2 cells might explain why HepG2 cells are more sensitive than THLE2 cells to the toxicity of AZT.

Long-term AZT exposure alters the metabolic capacity of cultured human lymphoblastoid cells

The antiretroviral efficacy of 3′-azido-3′-deoxythymidine (AZT) is dependent upon intracellular mono-, di-, and triphosphorylation and incorporation into DNA in place of thymidine. Thymidine kinase 1 (TK-1) catalyzes the first step of this pathway. MOLT-3, human lymphoblastoid cells, were exposed to AZT continuously for 14 passages (P₁–P₁₄) and cultured for an additional 14 passages (P₁₅–P₂₈) without AZT. Progressive and irreversible depletion of the enzymatically active form of the TK-1 24-kDa monomer with loss of active protein was demonstrated during P₁–P₅ of AZT exposure. From P₁₅ to P₂₈, both the 24- and the 48-kDa forms of TK-1 were undetectable and a tetrameric 96-kDa form was present. AZT-DNA incorporation was observed with values of 150, 133, and 108 molecules of AZT/10⁶ nucleotides at the 10μM plasma-equivalent AZT dose at P₁, P₅, and P₁₄, respectively. An exposure-related increase in the frequency of micronuclei (MN) was observed in cells exposed to either 10 or 800μM AZT during P₁–P₁₄. Analysis of the cell cycle profile revealed an accumulation of S-phase cells and a decrease in G₁-phase cells during exposure to 800μM AZT for 14 passages. When MOLT-3 cells were grown in AZT-free media (P₁₅–P₂₉), there was a reduction in AZT-DNA incorporation and MN formation; however, TK-1 depletion and the persistence of S-phase delay were unchanged. These data suggest that in addition to known mutagenic mechanisms, cells may become resistant to AZT partially through inactivation of TK-1 and through modulation of cell cycle components.

Expression profiling of nucleotide metabolism-related genes in human breast cancer cells after treatment with 5-fluorouracil

5-Fluorouracil (5-FU) is one of the most widely used drugs for treatment of cancers, including breast cancer that exhibits its anticancer activity by inhibiting DNA synthesis and also incorporated into DNA and RNA. The objective of this investigation was to find out the total nucleotide metabolism genes regulated by 5-FU in breast cancer cell line. The breast cancer cell line MCF-7 was treated with the drug 5-FU. To analyze the expression of genes, we have conducted the experiment using 1.7 k and 19k human microarray slide and confirmed the expression of genes by semiquantitative reverse transcription-polymerase chain reaction. The expression of 44 genes involved in the nucleotide metabolism pathway was quantified. Of these 44 genes analyzed, transcription of 6 genes were upregulated and 9 genes were downregulated. Earlier studies revealed that the transcription of genes for key enzymes like thymidylate synthase, thymidine kinase, and dihydropyrimidine dehydrogenase are regulated by 5-FU. This study identified some novel genes like thioredoxin reductase, ectonucleotide triphosphate dephosphorylase, and CTP synthase are regulated by 5-FU. The data also reveal large-scale perturbation in transcription of genes not involved directly in the known mechanism of action of 5-FU.

Radiopharmaceuticals in preclinical and clinical development for monitoring of therapy with PET

This review article discusses PET agents, other than (18)F-FDG, with the potential to monitor the response to therapy before, during, or after therapeutic intervention. This review deals primarily with non-(18)F-FDG PET tracers that are in the final stages of preclinical development or in the early stages of clinical application for monitoring the therapeutic response. Four sections related to the nature of the tracers are included: radiotracers of DNA synthesis, such as the 2 most promising agents, the thymidine analogs 3'-(18)F-fluoro-3'-deoxythymidine and (18)F-1-(2'-deoxy-2'-fluoro-beta-d-arabinofuranosyl)thymine; agents for PET imaging of hypoxia within tumors, such as (60/62/64)Cu-labeled diacetyl-bis(N(4)-methylthiosemicarbazone) and (18)F-fluoromisonidazole; amino acids for PET imaging, including the most popular such agent, l-[methyl-(11)C]methionine; and agents for the imaging of tumor expression of androgen and estrogen receptors, such as 16beta-(18)F-fluoro-5alpha-dihydrotestosterone and 16alpha-(18)F-fluoro-17beta-estradiol, respectively.

Increased mitochondrial DNA copy-number in CEM cells resistant to delayed toxicity of 2',3'-dideoxycytidine

The nucleoside analog 2',3'-dideoxycytidine (ddC) has been used for treatment of human immunodeficiency virus (HIV) infections. ddC causes delayed toxicity when cells are exposed to the drug at low concentration for prolonged periods of time. The delayed toxicity is due to inhibition of mitochondrial DNA (mtDNA) replication, which results in mtDNA depletion and mitochondrial dysfunction. In the present study we have cultured CEM T-lymphoblast cells in the presence of low concentrations of ddC to generate two cell lines resistant to the delayed toxicity of the drug. Both cell lines were resistant to mtDNA depletion by ddC. The mechanism of ddC resistance was investigated and we showed that the resistant cells had decreased mRNA expression of the nucleoside kinases deoxycytidine kinase and thymidine kinase 2. We also studied the mitochondrial DNA in the cells and showed that the ddC-resistant cells had structurally intact mtDNA but 1.5-2-fold increased mtDNA copy-number as well as increased levels of the mitochondrial transcription factor A (Tfam). Our study suggests that cells may increase their level of mtDNA to counteract mtDNA depletion induced by ddC, while keeping pronounced antiviral activity of the drug.

Mechanisms of genotoxicity of nucleoside reverse transcriptase inhibitors

Nucleoside analogs were first approved by the U.S. Food and Drug Administration for use against HIV-AIDS in 1987. Since then, these agents, now commonly referred to as nucleoside reverse transcriptase inhibitors (NRTIs), have become essential components of the Highly Active Antiretroviral Therapy (HAART) drug combinations used for treatment of Human Immunodeficiency Virus-1 (HIV-1) infections. Their antiretroviral activity is likely two-fold: incorporation of the drug into viral DNA and inhibition of the viral reverse transcriptase. However, incorporation of the drug into host nuclear and mitochondrial DNA may be largely responsible for dose-limiting toxicities. Azidothymidine (AZT, 3'-azido-3'-deoxythymidine, zidovudine), the first NRTI approved for the therapy of HIV-1, is incorporated into DNA, causes mutations in the hypoxanthine-guanine phosphoribosyl-transferase (HPRT) and thymidine kinase (TK) genes, and induces micronuclei, chromosomal aberrations, sister chromatid exchange, shortened telomeres, and other genotoxic effects in cultured cells. Genomic instability would be predicted as a consequence of these events. Metabolic pathways that result in the phosphorylation of AZT play a crucial role in AZT-DNA incorporation, and may be altered after prolonged treatment. For example, thymidine kinase 1, the enzyme responsible for AZT mono-phosphorylation, is down-regulated during long-term exposure and appears to be associated with AZT-induced replication inhibition and the accumulation of cells in S-phase. Detailed information on the mechanisms underlying NRTI-associated antiretroviral efficacy, toxicity, and metabolic resistance were not available when AZT was first approved for use as an antiretroviral agent. Current insights, based on 15 years of research, may lead to intervention strategies to attenuate toxicity without altering drug efficacy.

The effects of prolonged treatment with zidovudine, lamivudine, and abacavir on a T-lymphoblastoid cell line

A human T-lymphoblastoid cell line that is resistant to the antiviral activity of zidovudine (ZDV) and moderately resistant to lamivudine (3TC) has been obtained as a result of prolonged treatment with a combination of three nucleoside analogues (NA), ZDV, 3TC, and abacavir (ABV). These cells, called CEM(ZLA), are fully sensitive to ABV. The cellular resistance of the CEM(ZLA) cells to ZDV correlates with significant reductions in thymidine kinase (TK) activity and in the amount of intracellular TK protein. Interestingly, the reduction in TK activity led to impairment of the ability of CEM(ZLA) to accumulate the triphosphate metabolite of ZDV. However, the moderately 3TC-resistant phenotype of CEM(ZLA) cannot be ascribed to a similar reduction in deoxycytidine kinase activity. Compared to the parental CEM cells, CEM(ZLA) cells express a high level of multidrug resistance protein 4 (MRP4), which could reduce the intracellular concentration of 3TC. This study shows that the exposure of cells to a combination of NAs is capable of simultaneously affecting more than one target site to confer resistance and that NAs display differing abilities to select cellular resistance mechanisms.

Effects of benzo(a)pyrene on protein expression in Jurkat T-cells

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental pollutants of air, water and soil, and are produced by the incomplete combustion of organic materials. The International Agency for Research on Cancer has characterized PAHs as carcinogens. In this study, we investigated the effects of benzo(a)pyrene (B(a)P), which is the most carcinogenic member of the PAHs, on Jurkat cell protein by proteomic analysis. Jurkat cells were treated with various concentrations of B(a)P (0, 2.5, 5, 10, 20 or 40 microM) for 24 or 48 h and 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium and lactate dehydrogenase assays were carried out to determine cytotoxicity and a Comet assay was used to determinate genotoxicity. The cytotoxicity assays showed that 2.5 microM of B(a)P was the maximal concentration that did not cause any toxicity, but nevertheless, at this level B(a)P produced significant DNA damage in Jurkat cells at 48 h. Proteomic analysis using three different pI ranges and large two-dimensional gel electrophoresis showed 3427 protein spots. A total of 46 (13 up- and 33 down-regulated) proteins were identified as biomarkers of B(a)P and showed dose-dependent expressions in Jurkat T-cell line exposed to B(a)P. Of these, 27 protein spots were identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry. Two functionally differentiated protein groups were found. The protein group involving apoptosis and tumor suppression were found to be up-regulated, and B(a)P down-regulated enzyme was involved in energy metabolism, DNA synthesis and in cell structure and motility.