Selective action of 3'-azido-3'-deoxythymidine 5'-triphosphate on viral reverse transcriptases and human DNA polymerases

Phytotoxicity and Other Adverse Effects on the In Vitro Shoot Cultures Caused by Virus Elimination Treatments: Reasons and Solutions

In general, in vitro virus elimination is based on the culture of isolated meristem, and in addition thermotherapy, chemotherapy, electrotherapy, and cryotherapy can also be applied. During these processes, plantlets suffer several stresses, which can result in low rate of survival, inhibited growth, incomplete development, or abnormal morphology. Even though the in vitro cultures survive the treatment, further development can be inhibited; thus, regeneration capacity of treated in vitro shoots or explants play also an important role in successful virus elimination. Sensitivity of genotypes to treatments is very different, and the rate of destruction largely depends on the physiological condition of plants as well. Exposure time of treatments affects the rate of damage in almost every therapy. Other factors such as temperature, illumination (thermotherapy), type and concentration of applied chemicals (chemo- and cryotherapy), and electric current intensity (electrotherapy) also may have a great impact on the rate of damage. However, there are several ways to decrease the harmful effect of treatments. This review summarizes the harmful effects of virus elimination treatments applied on tissue cultures reported in the literature. The aim of this review is to expound the solutions that can be used to mitigate phytotoxic and other adverse effects in practice.

Mechanistic cross-talk between DNA/RNA polymerase enzyme kinetics and nucleotide substrate availability in cells: Implications for polymerase inhibitor discovery

Enzyme kinetic analysis reveals a dynamic relationship between enzymes and their substrates. Overall enzyme activity can be controlled by both protein expression and various cellular regulatory systems. Interestingly, the availability and concentrations of intracellular substrates can constantly change, depending on conditions and cell types. Here, we review previously reported enzyme kinetic parameters of cellular and viral DNA and RNA polymerases with respect to cellular levels of their nucleotide substrates. This broad perspective exposes a remarkable co-evolution scenario of DNA polymerase enzyme kinetics with dNTP levels that can vastly change, depending on cell proliferation profiles. Similarly, RNA polymerases display much higher Km values than DNA polymerases, possibly due to millimolar range rNTP concentrations found in cells (compared with micromolar range dNTP levels). Polymerases are commonly targeted by nucleotide analog inhibitors for the treatments of various human diseases, such as cancers and viral pathogens. Because these inhibitors compete against natural cellular nucleotides, the efficacy of each inhibitor can be affected by varying cellular nucleotide levels in their target cells. Overall, both kinetic discrepancy between DNA and RNA polymerases and cellular concentration discrepancy between dNTPs and rNTPs present pharmacological and mechanistic considerations for therapeutic discovery.

Synthesis of New Indanyl Nucleoside Analogues and their Biological Evaluation on Hepatitis C Virus (HCV) Replicon

Here, we report a convenient synthetic procedure for the preparation of four novel indanyl carbanucleoside derivatives in the racemic form. The action of these compounds against hepatitis C virus was evaluated in vitro using the replicon cell line, Huh7.5 SG. Contrary to our expectations, all these compounds did not inhibit, but rather promoted HCV genotype 1b (HCVg1b) replication. Similar effects have been reported for morphine in the replicon cell lines, Huh7 and Huh8. Several biological experiments and computational studies were performed to elucidate the effect of these compounds on HCVg1b replication. Based on all the experiments performed, we propose that the increase in HCVg1b replication could be mediated, at least in part, by a similar mechanism to that of morphine on the enhancement of this replication. The presence of opioid receptors in Huh7.5 SG cells was indirectly determined for the first time in this work.

Polymerases of paramyxoviruses and pneumoviruses

The paramyxo- and pneumoviruses are members of the order Mononegavirales, a group of viruses with non-segmented, negative strand RNA genomes. The polymerases of these viruses are multi-functional complexes, capable of transcribing subgenomic capped and polyadenylated mRNAs and replicating the genome. Although there is no native structure available for any complete paramyxo- or pneumovirus polymerase, functional and structural studies of a fragment of a pneumovirus polymerase protein and mutation analyses and resistance profiling of small-molecule inhibitors have generated a wealth of mechanistic information. This review integrates these data with the structure of a related polymerase, identifying similarities, differences, gaps in knowledge, and avenues for antiviral drug development.

Physiological Mg2+ Conditions Significantly Alter the Inhibition of HIV-1 and HIV-2 Reverse Transcriptases by Nucleoside and Non-Nucleoside Inhibitors in Vitro

Reverse transcriptases (RTs) are typically assayed in vitro with 5-10 mM Mg²⁺, whereas the free Mg²⁺ concentration in cells is much lower. Artificially high Mg²⁺ concentrations used in vitro can misrepresent different properties of human immunodeficiency virus (HIV) RT, including fidelity, catalysis, pausing, and RNase H activity. Here, we analyzed nucleoside (NRTIs) and non-nucleoside RT inhibitors (NNRTIs) in primer extension assays at different concentrations of free Mg²⁺. At low concentrations of Mg²⁺, NRTIs and dideoxynucleotides (AZTTP, ddCTP, ddGTP, and 3TCTP) inhibited HIV-1 and HIV-2 RT synthesis less efficiently than they did with large amounts of Mg²⁺, whereas inhibition by the "translocation-defective RT inhibitor" EFdA (4'-ethynyl-2-fluoro-2'-deoxyadenosine) was unaffected by Mg²⁺ concentrations. Steady-state kinetic analyses revealed that the reduced level of inhibition at low Mg²⁺ concentrations resulted from a 3-9-fold (depending on the particular nucleotide and inhibitor) less efficient incorporation (based on k_cat/K_m) of these NRTIs under this condition compared to incorporation of natural dNTPs. In contrast, EFdATP was incorporated with an efficiency similar to that of its analogue dATP at low Mg²⁺ concentrations. Unlike NRTIs, NNRTIs (nevirapine, efavirenz, and rilviripine), were approximately 4-fold (based on IC₅₀ values) more effective at low than at high Mg²⁺ concentrations. Drug-resistant HIV-1 RT mutants also displayed the Mg²⁺-dependent difference in susceptibility to NRTIs and NNRTIs. In summary, analyzing the efficiency of inhibitors under more physiologically relevant low-Mg²⁺ conditions yielded results dramatically different from those from measurements using commonly employed high-Mg²⁺ in vitro conditions. These results also emphasize differences in Mg²⁺ sensitivity between the translocation inhibitor EFdATP and other NRTIs.

Incorporation of Selected Nucleoside Phosphonates and Anti-Human Immunodeficiency Virus Nucleotide Analogues into DNA by Human DNA Polymerases α, β and γ

Incorporation of selected diphosphates of nucleoside phosphonates and triphosphates of currently approved anti-human immunodeficiency virus nucleoside analogues into DNA by human DNA polymerases α, β and γ was studied. All three polymerases were able to incorporate diphosphates of 9-(2-phosphonomethoxyethyl)adenine (PMEApp), 9-(2-phosphonomethoxyethyl)guanine (PMEGpp), ( R)-9-(2-phosphonomethoxypropyl)adenine (PMPApp), ( R)-9-(2-phosphononomethoxypropyl)-2,6-diaminopurine (PMPDAPpp) and ( 2R,5R)-9-[2,5-dihydro-5-(phosphonomethoxy)-2-furanyl]adenine (D4APpp) into primer/template DNA of defined sequence. After incorporation, these nucleoside phosphonates acted as terminators of primer extension. Kinetic constants of their incorporation were determined and compared with those for incorporation of ddATP, ddCTP, (-)-2′-deoxy-3′-thiacytidine triphosphate (3TC-TP), 2′,3′-didehydro-3′-deoxythymidine triphosphate (d4T-TP) and 3′-azido-3′-deoxythymidine triphosphate (AZT-TP). Relative efficiencies of incorporation (percentage of the incorporation efficiency for the corresponding natural deoxynucleoside triphosphate) by DNA polymerase a ranged from 0.05% for 3TC-TP to 51% for PMEGpp. DNA polymerase β catalysed the incorporation with relative efficiencies ranging from 0.014% for AZT-TP to 125% for ddCTP, and efficiencies of incorporation by DNA polymerase γ varied between 0.13% for 3TC-TP and 25% for ddCTP. Generally, the lowest incorporation efficiencies with all three polymerases were found for PMPApp (0.06–1.4%) and PMPDAPpp (0.075–2.2%).

Intracellular Metabolism of 3′-azido-3′-deoxythymidine and 2′,3′-dideoxyinosine in Combination in the Absence and Presence of Ribavirin

We examined the intracellular phosphorylation of 3′-azido-3′-deoxythymidine (AZT) and 2′,3′-dideoxyinosine (ddl) and the effects on rNTP and dNTP pools when AZT and ddl were incubated separately and in combination in lymphocytes. We also compared the effect of adding ribavirin (RBV) to the two-drug combination of AZT + ddl. AZT and ddl, used singly or in combination, had no effect on the dNTP pools of CEM lymphoblastoid cells. Neither did the combination of AZT + ddl have any effect on the rNTP pools. RBV, a known inhibitor of IMP dehydrogenase, caused a decrease in GTP and an increase in dTTP whether incubated alone or with the drug combination of AZT + ddl. The addition of AZT + ddl therefore did not alter the effects of RBV upon cellular nucleotide pools. AZT was phosphorylated to a much greater extent than ddl. The activation of ddl to ddA-TP was increased 2-fold in the presence of AZT, whereas AZT phosphorylation was unchanged when combined with ddl. This increase in ddl activation may explain in part the synergistic antiviral activity of the combination of AZT + ddl. The increased activation was not due to increased phosphorylation of ddl resulting from IMP dehydrogenase inhibition. The addition of 10 μm RBV to the two-drug combination of AZT + ddl did not change the intracellular phosphorylation of AZT or ddl. The activation of ddl to ddA-TP, when combined with AZT, appeared to be maximal and could not be further increased by addition of RBV to this combination.

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.

Susceptibility of the human retrovirus XMRV to antiretroviral inhibitors

Background

XMRV (xenotropic murine leukemia virus-related virus) is the first known example of an exogenous gammaretrovirus that can infect humans. A limited number of reports suggest that XMRV is intrinsically resistant to many of the antiretroviral drugs used to treat HIV-1 infection, but is sensitive to a small subset of these inhibitors. In the present study, we used a novel marker transfer assay to directly compare the antiviral drug sensitivities of XMRV and HIV-1 under identical conditions in the same host cell type.

Results

We extend the findings of previous studies by showing that, in addition to AZT and tenofovir, XMRV and HIV-1 are equally sensitive to AZddA (3'-azido-2',3'-dideoxyadenosine), AZddG (3'-azido-2',3'-dideoxyguanosine) and adefovir. These results indicate that specific 3'-azido or acyclic nucleoside analog inhibitors of HIV-1 reverse transcriptase (RT) also block XMRV infection with comparable efficacy in vitro. Our data confirm that XMRV is highly resistant to the non-nucleoside RT inhibitors nevirapine and efavirenz and to inhibitors of HIV-1 protease. In addition, we show that the integrase inhibitors raltegravir and elvitegravir are active against XMRV, with EC₅₀ values in the nanomolar range.

Conclusions

Our analysis demonstrates that XMRV exhibits a distinct pattern of nucleoside analog susceptibility that correlates with the structure of the pseudosugar moiety and that XMRV is sensitive to a broader range of antiretroviral drugs than has previously been reported. We suggest that the divergent drug sensitivity profiles of XMRV and HIV-1 are partially explained by specific amino acid differences in their respective protease, RT and integrase sequences. Our data provide a basis for choosing specific antiretroviral drugs for clinical studies in XMRV-infected patients.

Differential inhibition of homotrimeric dUTPases by the 3'-azido derivative of dideoxy-UTP

The inhibitory effects of 3'-azido-2',3'-dideoxyuridine-5'-triphosphate in complex with the Mg2+ ion (azido-ddUTP.Mg) on the dUTPases of the human, E. coli, and equine infectious anemia virus have been compared. Azido-ddUTP is analogous to drugs used in the treatment of HIV. Here it is shown to inhibit the bacterial dUTPase in a competitive manner (Ki = 9.3 microM), but to exhibit only marginal or no binding to the human and viral dUTPases, respectively. This is the first demonstration of an inhibitor with a strong preference for binding to a bacterial dUTPase over the human enzyme. The specific binding to the E. coli dUTPase is surprising in view of the close to identical substrate pockets among the three dUTPases tested. The results are discussed with reference to the possibility of designing active site directed inhibitors that bind to the homotrimeric dUTPase of a pathogen but not to the human form.

The mitochondrial DNA polymerase in health and disease

Since mutations in mitochondrial DNA (mtDNA) have been shown to be a cause of many mitochondrial diseases as well as aging, it is important to understand the origin of these mutations and how replication proteins modulate this process. DNA polymerase gamma (pol gamma) is the polymerase that is responsible for replication and repair of mtDNA. Pol gamma has three main roles in mtDNA maintenance and mutagenesis. As the only known DNA polymerase in mitochondria, pol gamma is required for all replication and repair functions and is the main source of errors produced in human mtDNA. Pol gamma is also sensitive to a host of antiviral nucleoside analogs used to treat HIV-1 infections, which can cause an induced mitochondrial toxicity. Finally, the gene for pol gamma, POLG, is a genetic locus for several mitochondrial disease with over 150 genetic mutations currently identified.

Long-term exposure to zidovudine delays cell cycle progression, induces apoptosis, and decreases telomerase activity in human hepatocytes

Zidovudine (3'-azido-3'-deoxythymidine; AZT), which is currently used in the treatment of acquired immunodeficiency syndrome, has been shown to have anticancer properties. In the present study, we examined the mechanisms contributing to increased sensitivity of cancer cells to the growth-inhibitory effects of AZT. This was accomplished by incubating a hepatoma cell line (HepG2) and a normal liver cell line (THLE2) with AZT in continuous culture for up to 4 weeks and evaluating the number of viable and necrotic cells, the induction of apoptosis, cell cycle alterations, and telomerase activity. In HepG2 cells, AZT (2-100 microM) caused significant dose-dependent decreases in the number of viable cells at exposures > 24 h. During a 1-week recover period, there was only a slight increase in the number of viable cells treated with AZT. The decrease in viable cells was associated with an induction of apoptosis, a decrease in telomerase activity, and S and G2/M phase arrest of the cell cycle. During the recovery period, the extent of apoptosis and telomerase activity returned to control levels, whereas the disruption of cell cycle progression persisted. Western blot analysis indicated that AZT caused a decrease in checkpoint kinase 1 (Chk1) and kinase 2 (Chk2) and an increase in phosphorylated Chk1 (Ser345) and Chk2 (Thr68). Similar effects, to lesser extent, were observed in THLE2 cells given much higher concentrations of AZT (50-2500 microM). These data show that HepG2 cells are much more sensitive than THLE2 cells to AZT. They also indicate that a combination of a delay of cell cycle progression, an induction of apoptosis, and a decrease in telomerase activity is contributing to the decrease in the number of viable cells from AZT treatment, and that checkpoint enzymes Chk1 and Chk2 may play an important role in the delay of cell cycle progression.

Interference of cell cycle progression by zidovudine and lamivudine in NIH 3T3 cells

Zidovudine (3'-azido-3'-deoxythymidine; AZT) and lamivudine [(-)2',3'-dideoxy-3'-thiacytidine; 3TC] are nucleoside reverse transcriptase inhibitors used to treat and prevent human immunodeficiency virus-1 infections. In short-term incubations (<48 h), AZT, but not 3TC, has been shown to interfere with cell cycle progression. In the present study, we examined if these alterations persist during long-term incubations in which cells were exposed to AZT (0-1000 microM) or 3TC (0-500 microM) in continuous culture for up to 5 weeks. In addition, we investigated the reversibility of these effects upon removal of the drugs. Both drugs caused concentration- and time-dependent decreases in the number of viable cells, with the effect being more pronounced with AZT. There was only a slight increase in the number of viable cells treated with AZT for 5 weeks and then allowed a 1-week recovery period; cell viability in cells treated with 3TC returned to control levels during the recovery period. The decrease in viable cells was not due to apoptotic or necrotic cell death, but rather was associated with S and G2/M phase cell cycle arrest. Western blot analysis indicated that AZT treatment caused a decrease in checkpoint kinase 1 (Chk1) and checkpoint kinase 2 (Chk2) at all time points. Cyclin-dependent kinase 1 was decreased at later time points, while cyclin A was increased at early times. These data indicate that AZT and, to a lesser extent, 3TC interfere with cell growth by slowing cell cycle progression and that checkpoint proteins Chk1 and Chk2 may play an important role in this delay.

Effect of Nucleoside and Nucleotide Reverse Transcriptase Inhibitors of HIV on Endogenous Nucleotide Pools

Background

Alterations in endogenous nucleotide pools as a result of HIV therapy with nucleoside and nucleotide reverse transcriptase inhibitors (N[t]RTIs) is a proposed mechanism for therapy-related adverse events and drug interactions resulting in treatment failure. In vitro studies were performed in order to understand the effect of N(t)RTIs on endogenous nucleotide pools.

Methods

The T-cell line CEM-CCRF was treated with control antimetabolites or the N(t)RTIs abacavir, didanosine, lamivudine, tenofovir (TFV) and zidovudine (AZT), either alone or in combination. The levels of natural 2′-deoxynucleoside triphosphates (dNTP) and ribonucleoside triphophosphates were determined by liquid chromatography coupled with triple quadrupole mass spectrometry.

Results

Antimetabolites altered nucleotide pools in a manner consistent with their known mechanisms of action. AZT was the only N(t)RTI that significantly altered dNTP pools. Incubation of 10 μM AZT, either alone or in combination with other N(t)RTIs, increased 2′-deoxyadenosine triphosphate, 2′-deoxyguanosine triphosphate and thymidine triphosphate levels by up to 1.44-fold the concentrations observed in untreated cells. At higher than pharmacological concentrations of AZT, evidence for inhibition of 2′-deoxycytidylate deaminase and enzymes involved in the salvage of thymidine was also observed. Phosphorylated metabolites of TFV are known to inhibit purine nucleoside phosphorylase (PNP). However, in contrast to a potent PNP inhibitor, TFV was unable to alter intracellular dNTP pools upon addition of exogenous 2′-deoxyguanosine.

Conclusions

N(t)RTIs have the potential to alter nucleotide pools; however, at the pharmacologically relevant concentrations, tested N(t)RTI or their combinations did not have an effect on nucleotide pools with the notable exception of AZT.

Mitochondrial DNA impairment in nucleoside reverse transcriptase inhibitor-associated cardiomyopathy

Acquired immune deficiency syndrome (AIDS) is a global epidemic that continues to escalate. Recent World Health Organization estimates include over 33 million people currently diagnosed with HIV/AIDS. Another 20 million HIV-infected individuals died over the past quarter century. Antiretrovirals are effective treatments that changed the outcome of HIV infection from a fatal disease to a chronic illness. Cardiomyopathy (CM) is a bona fide component of HIV/AIDS with occurrence that is higher in HIV positive individuals. CM may result from individual or combined effects of HIV, immune reactions, or toxicities of prolonged antiretrovirals. Nucleoside reverse transcriptase inhibitors (NRTIs) are the cornerstone of antiretroviral therapy. Despite pharmacological benefits of NRTIs, NRTI side effects include increased risk for CM. Clinical observations and in vitro and in vivo studies support various mechanisms of CM. This perspective highlights some of the hypotheses and focuses on mitochondrial-associated pathways of NRTI- related CM.

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.

Administration of fludarabine-loaded autologous red blood cells in simian immunodeficiency virus-infected sooty mangabeys depletes pSTAT-1-expressing macrophages and delays the rebound of viremia after suspension of antiretroviral therapy

A major limitation of highly active antiretroviral therapy is that it fails to eradicate human immunodeficiency virus (HIV) infection due to its limited effects on viral reservoirs carrying replication-competent HIV, including monocytes/macrophages (M/M). Therefore, therapeutic approaches aimed at targeting HIV-infected M/M may prove useful in the clinical management of HIV-infected patients. In previous studies, we have shown that administration of fludarabine-loaded red blood cells (RBC) in vitro selectively induces cell death in HIV-infected M/M via a pSTAT1-dependent pathway. To determine the in vivo efficacy of this novel therapeutic strategy, we treated six naturally simian immunodeficiency virus (SIV)-infected sooty mangabeys (SMs) with either 9-[2-(R)-(phosphonomethoxy)propyl]adenine (PMPA) only, fludarabine-loaded RBC only, or PMPA in association with fludarabine-loaded RBC. The rationale of this treatment was to target infected M/M with fludarabine-loaded RBC at a time when PMPA is suppressing viral replication taking place in activated CD4+ T cells. In vivo administration of fludarabine-loaded RBC was well tolerated and did not induce any discernible side effect. Importantly, addition of fludarabine-loaded RBC to PMPA delayed the rebound of viral replication after suspension of therapy, thus suggesting a reduction in the size of SIV reservoirs. While administrations of fludarabine-loaded RBC did not induce any change in the CD4+ or CD8+ T-cell compartments, we observed, in chronically SIV-infected SMs, a selective depletion of M/M expressing pSTAT1. This study suggests that therapeutic strategies based on the administration of fludarabine-loaded RBC may be further explored as interventions aimed at reducing the size of the M/M reservoirs during chronic HIV infection.

Highly selective action of triphosphate metabolite of 4'-ethynyl D4T: a novel anti-HIV compound against HIV-1 RT

2',3'-Didehydro-3'-deoxy-4'-ethynylthymidine (4'-Ed4T), is a recently discovered nucleoside reverse transcriptase inhibitor (NRTI) showing a 5- to 10-fold greater anti-human immunodeficiency virus type 1 (HIV-1) activity and less cellular and mitochondrial toxicity than its parental compound, stavudine (D4T). It is also active against a variety of NRTI-resistant HIV-1 mutants under non-cytotoxic concentrations. In this study, the effects of 4'-Ed4TTP, which is the triphosphate metabolite of 4'-Ed4T, on HIV-1 reverse transcriptase (RT) activity were investigated. We found that 4'-Ed4TTP was a substrate of HIV-1 RT serving as a DNA chain terminator, and it inhibited the DNA polymerase activity of RT more efficiently than D4TTP. The value of Ki(4'-Ed4TTP)/Km(dTTP) is 0.15 for DNA/RNA primer/template duplex (P/T), but 0.7 for DNA/DNA P/T, suggesting 4'-Ed4TTP inhibits RT more efficiently during RNA-dependent DNA synthesis than DNA-dependent DNA synthesis. 4'-Ed4TTP was also found to inhibit the 3TC (Lamivudine)-resistant RT mutant, M184V, with 3-fold less efficiency than the wild type (wt) RT. 4'-Ed4TTP showed much less inhibitory effects toward major host DNA polymerases. Overall, our results suggest that 4'-Ed4TTP is the active form for anti-HIV-1 activity via its inhibitory effect against RT.

Why do HIV-1 and HIV-2 use different pathways to develop AZT resistance?

The human immunodeficiency virus type 1 (HIV-1) develops resistance to all available drugs, including the nucleoside analog reverse transcriptase inhibitors (NRTIs) such as AZT. ATP-mediated excision underlies the most common form of HIV-1 resistance to AZT. However, clinical data suggest that when HIV-2 is challenged with AZT, it usually accumulates resistance mutations that cause AZT resistance by reduced incorporation of AZTTP rather than selective excision of AZTMP. We compared the properties of HIV-1 and HIV-2 reverse transcriptase (RT) in vitro. Although both RTs have similar levels of polymerase activity, HIV-1 RT more readily incorporates, and is more susceptible to, inhibition by AZTTP than is HIV-2 RT. Differences in the region around the polymerase active site could explain why HIV-2 RT incorporates AZTTP less efficiently than HIV-1 RT. HIV-1 RT is markedly more efficient at carrying out the excision reaction with ATP as the pyrophosphate donor than is HIV-2 RT. This suggests that HIV-1 RT has a better nascent ATP binding site than HIV-2 RT, making it easier for HIV-1 RT to develop a more effective ATP binding site by mutation. A comparison of HIV-1 and HIV-2 RT shows that there are numerous differences in the putative ATP binding sites that could explain why HIV-1 RT binds ATP more effectively. HIV-1 RT incorporates AZTTP more efficiently than does HIV-2 RT. However, HIV-1 RT is more efficient at ATP-mediated excision of AZTMP than is HIV-2 RT. Mutations in HIV-1 RT conferring AZT resistance tend to increase the efficiency of the ATP-mediated excision pathway, while mutations in HIV-2 RT conferring AZT resistance tend to increase the level of AZTTP exclusion from the polymerase active site. Thus, each RT usually chooses the pathway best suited to extend the properties of the respective wild-type enzymes.

Ex vivo zidovudine (AZT) treatment of CD34+ bone marrow progenitors causes decreased steady state mitochondrial DNA (mtDNA) and increased lactate production

Haematopoietic suppression is one of the dose-limiting side effects of chronic zidovudine (AZT) therapy. We tested the hypothesis that AZT would reduce mitochondrial DNA (mtDNA) content in haematopoietic progenitors causing impaired haematopoiesis and mitochondrial dysfunction. We studied the effects of AZT 0-50 microM in vitro, on normal human CD34+ haematopoietic progenitor cells cultured ex vivo for up to 12 days. The mean AZT IC50 for granulocyte (phenotype CD15+/CD14-) and erythroid (phenotype glycophorin+/CD45-) cell proliferation was 2.5 microM (SD+/-0.7) and 0.023 microM (SD+/-0.005), respectively. In myeloid-rich cell cultures, the mean lactate content of the media, compared to untreated controls, increased by 86% (SD+/-23) at 10 microM AZT and in erythroid-rich cultures it increased by 134% (SD+/-24) in the presence of 0.5 microM AZT. In myeloid-rich cultures the AZT IC50 for the reduction in the mitochondrial/nuclear DNA content ratio was 5.6 microM, whereas in erythroid rich cultures this AZT IC50 was < 0.0005 microM. AZT produced concentration-dependent inhibition of CD34+ progenitor proliferation into both myeloid and erythroid lineages; erythropoiesis was more sensitive than myelopoiesis. Concurrently, AZT reduced steady state mtDNA content, while increasing lactate production. These findings support the hypothesis that mtDNA is one of the intracellular targets involved in the pathogenesis of AZT-associated bone marrow progenitor cell toxicity.

Mitochondrial toxicity of NRTI antiviral drugs: an integrated cellular perspective

Highly active antiretroviral therapy (HAART) regimes based on nucleoside reverse transcriptase inhibitors (NRTIs) have revolutionized the treatment of AIDS in recent years. Although HAART can successfully suppress viral replication in the long term, it is not without significant toxicity, which can seriously compromise treatment effectiveness. A major toxicity that has been recognized for more than a decade is NRTI-related mitochondrial toxicity, which manifests as serious side effects such as hepatic failure and lactic acidosis. However, a lack of understanding of the mechanisms underlying mitochondrial toxicity has hampered efforts to develop novel drugs with better side-effect profiles. This review characterizes the pharmacological mechanisms and pathways that are involved in mitochondrial dysfunction caused by NRTIs, and suggests opportunities for future pharmacological research.

Synthesis and QSAR study of the anticancer activity of some novel indane carbocyclic nucleosides

A set of 14 indane carbocyclic nucleosides were synthesized and experimentally assayed for their inhibitory effects in the proliferation of murine leukemia (L1210/0) and human T-lymphocyte (Molt4/C8, CEM/0) cells. The compounds have promising inhibitory activity judging from the IC(50) values obtained for all these cellular lines. Multiple linear regression analysis was then applied to build up consistent QSAR models based on quantum mechanics-derived molecular descriptors. The derived models reproduce well the experimental data of both three cells (r(2) >/=0.90), display a good predictive power and are, above all, easily interpretable. They show that frontier-orbital energies and hydrophobicity are mainly responsible for the activity of the synthesized compounds and also, suggest similar mechanisms of action. The final QSAR-models involve only two descriptors: the lowest unoccupied molecular orbital energy and the solvent accessible-hydrophobic surface area, but describe a sound correlation between predicted and experimental activity data (r(2)=0.931, r(2)=0.936 and r(2)=0.931 for the cells L1210/0, Molt4/C8 and CEM/0, respectively).

Structural determinants in human DNA polymerase gamma account for mitochondrial toxicity from nucleoside analogs

Although antiviral nucleoside analog therapy successfully delays progression of HIV infection to AIDS, these drugs cause unwelcome side-effects by inducing mitochondrial toxicity. We and others have demonstrated that the mitochondrial polymerase, DNA polymerase gamma (pol gamma), participates in mitochondrial toxicity by incorporating these chain-terminating antiviral nucleotide analogs into DNA. Here, we explore the role of three highly conserved amino acid residues in the active site of human pol gamma that modulate selection of nucleotide analogs as substrates for incorporation. Sequence alignments, crystal structures and mutagenesis studies of family A DNA polymerases led us to change Tyr951 and Tyr955 in polymerase motif B to Phe and Ala, and Glu895 in polymerase motif A was changed to Ala. The mutant polymerases were tested for their ability to incorporate natural nucleotides and the five antiviral nucleoside analogs currently approved for antiviral therapy: AZT, ddC, D4T, 3TC and carbovir. Steady-state kinetic analysis of the pol gamma derivatives with the normal and antiviral nucleotides demonstrated that Tyr951 is largely responsible for the ability of pol gamma to incorporate dideoxynucleotides and D4T-MP. Mutation of Tyr951 to Phe renders the enzyme resistant to dideoxynucleotides and D4T-TP without compromising the activity of the polymerase. Alteration of Glu895 and Tyr955 to Ala had the largest effect on overall polymerase activity with normal nucleotides, producing dramatic increases in K(m(dNTP)) and large decreases in k(cat). Mutation of Tyr955 in pol gamma causes the degenerative disease progressive external ophthalmoplegia in humans, and we show that this residue partially accounts for the ability of pol gamma to incorporate D4T-MP and carbovir. Alteration of Glu895 to Ala slightly increased discrimination against dideoxynucleotides and D4T-TP. The mechanisms by which pol gamma selects certain nucleotide analogs are discussed.

Advances in QSAR studies of HIV-1 reverse transcriptase inhibitors

A review is presented of the recent advances in quantitative structure-activity relationship (QSAR) studies of HIV-1 reverse transcriptase (RT) inhibitors. These inhibitors have been put into two classes: nucleoside RT inhibitors (NRTIs), which are 2',3'-dideoxynucleoside analogues (ddNs), and non-nucleoside RT inhibitors (NNRTIs). For NRTIs (ddNs), which act as competitive inhibitors or alternate substrates of RT and hence interact at the substrate binding site of the enzyme, QSARs have pointed out the major role of the electronic factors governing their activity. For NNRTIs, which bind to a site entirely distinct from the substrate binding site, the activity has been shown to be largely dependent upon the hydrophobic nature of the compounds or substituents. The hydrophobic nature of the active site in the receptor with which the NNRTIs interact provides relatively few possibilities for the molecules to have polar interactions or hydrogen bondings, but QSARs have indicated that NNRTIs do involve some polar interactions and hydrogen bondings with some pockets of the enzyme. QSARs also indicate the significant roles of steric interactions and conformational shape of the molecule.

Effects of specific zidovudine resistance mutations and substrate structure on nucleotide-dependent primer unblocking by human immunodeficiency virus type 1 reverse transcriptase

Nucleotide-dependent unblocking of chain-terminated DNA by human immunodeficiency virus type 1 reverse transcriptase (RT) is enhanced by the presence of mutations associated with 3'-azido-3'-deoxythymidine (AZT) resistance. The increase in unblocking activity was greater for mutant combinations associated with higher levels of in vivo AZT resistance. The difference between mutant and wild-type activity was further enhanced by introduction of a methyl group into the nucleotide substrate and was decreased for a nonaromatic substrate, suggesting that pi-pi interactions between RT and an aromatic structure may be facilitated by these mutations.

Direct analysis of mitochondrial toxicity of antiretroviral drugs

OBJECTIVES

Mitochondrial toxicity is a serious side-effect of antiretroviral drugs, especially nucleoside reverse transcriptase inhibitors (NRTI). An in vitro assay to predict mitochondrial toxicity of in-use and developmental NRTI would be invaluable. To test the ability of a cytofluorimetric technique to predict the mitochondrial-dependent pancreatic and hepatic toxicity we used didanosine (ddI) alone or in combination with hydroxyurea (HU).

METHODS

The technique is based on the ability of the lipophilic cation JC-1 to enter selectively into mitochondria and change its colour as the membrane potential changes due to toxicity. Mitochondrial toxicity by HU and ddI was evaluated in pancreatic and hepatic human cell lines. The results were expressed as mitochondrial toxicity index (MTI), ranging from 0 to 100: the negative control was 0, and 100 indicating maximal toxicity.

RESULTS

Dose-dependent pancreatic toxicity of ddI was evident after 14 days of culture (MTI 34 +/- 4 at 100 microM, 10 +/- 4 at 10 microM, 2 +/- 3 at 1 microM ddI). HU alone was not toxic (MTI 7 +/- 10 at 100 microM, 2 +/- 2 at 50 microM and 2 +/- 4 at 10 microM HU); however, HU increased the toxicity of high, but not low, concentrations of ddI. For example, the MTI of 10 microM ddI plus 50 microM HU was 54 +/- 9. Negligible mitochondrial toxicity was observed in the hepatic cell line exposed to ddI alone or in combination with HU.

CONCLUSIONS

This in vitro assay might have in vivo relevance. First, ddI-related pancreatitis is dose dependent, and is reported more frequently than hepatic failure, consistent with our in vitro results. Second, patients who developed pancreatitis during randomized, controlled trials were treated with HU in combination with 400 mg ddI once daily (high peak concentration of ddI in the blood). In contrast, no pancreatitis was observed when HU was combined with 200 mg ddI twice daily (low peak concentration of ddI). These in vivo results are consistent with our in vitro observation that HU increases pancreatic cell toxicity in the presence of high concentrations of ddI. The in vitro assay described here might be used to predict the mitochondrial toxicity of other NRTI, alone or in combination.

Metabolic complications associated with antiretroviral therapy

Mortality rates in the HIV-infected patient population have decreased with the advent of highly active antiretroviral therapy (HAART) for the treatment of AIDS. Due to the chronic nature of HAART, long-term metabolic complications are associated with therapy, such as hyperlipidemia, fat redistribution and diabetes mellitus. Currently, all of these symptoms are classified as the lipodystrophy (LD) syndrome(s). However, hyperlipidemia and fat redistribution occur independently, indicating there may be multiple syndromes associated with HAART. Although fat gain/loss and dyslipidemia occur in protease inhibitor (PI) naïve patients treated with nucleoside reverse transcriptase inhibitors (NRTIs), combination therapies (PI and NRTI) accelerate the syndrome. Recent clinical trials, cell culture and animal studies indicate that these effects are not drug class specific and select PIs, NRTIs and non-nucleoside reverse transcriptase inhibitors (NNRTIs) can be associated with metabolic complications. Moreover, the effects can vary between various members of the same class of antiretroviral agents (i.e. not all PIs cause the same adverse reactions) and may be influenced by duration of infection, genetics and environmental factors. Although HAART increases the risk of metabolic complications, this does not outweigh the benefits of survival. In this review, we summarize the latest clinical and scientific information on these metabolic complications, examine current hypotheses explaining the syndromes and comment on the existing methods available to manage these metabolic side effects.

Differential incorporation and removal of antiviral deoxynucleotides by human DNA polymerase gamma

Mitochondrial toxicity can result from antiviral nucleotide analog therapy used to control human immunodeficiency virus type 1 infection. We evaluated the ability of such analogs to inhibit DNA synthesis by the human mitochondrial DNA polymerase (pol gamma) by comparing the insertion and exonucleolytic removal of six antiviral nucleotide analogs. Apparent steady-state K(m) and k(cat) values for insertion of 2',3'-dideoxy-TTP (ddTTP), 3'-azido-TTP (AZT-TP), 2',3'-dideoxy-CTP (ddCTP), 2',3'-didehydro-TTP (D4T-TP), (-)-2',3'-dideoxy-3'-thiacytidine (3TC-TP), and carbocyclic 2',3'-didehydro-ddGTP (CBV-TP) indicated incorporation of all six analogs, albeit with varying efficiencies. Dideoxynucleotides and D4T-TP were utilized by pol gamma in vitro as efficiently as natural deoxynucleotides, whereas AZT-TP, 3TC-TP, and CBV-TP were only moderate inhibitors of DNA chain elongation. Inefficient excision of dideoxynucleotides, D4T, AZT, and CBV from DNA predicts persistence in vivo following successful incorporation. In contrast, removal of 3'-terminal 3TC residues was 50% as efficient as natural 3' termini. Finally, we observed inhibition of exonuclease activity by concentrations of AZT-monophosphate known to occur in cells. Thus, although their greatest inhibitory effects are through incorporation and chain termination, persistence of these analogs in DNA and inhibition of exonucleolytic proofreading may also contribute to mitochondrial toxicity.

Phosphorylation of nucleoside analog antiretrovirals: a review for clinicians

Nucleoside analogs (zidovudine, didanosine, zalcitabine, stavudine, abacavir, lamivudine) have been administered as antiretroviral agents for more than a decade. They undergo anabolic phosphorylation by intracellular kinases to form triphosphates, which inhibit human immunodeficiency virus replication by competitively inhibiting viral reverse transcriptase. Numerous methods are used to elucidate the intracellular metabolic pathways of these agents. Intracellular and extracellular factors affect intracellular phosphorylation. Lack of standardization and complexity of methods used to study phosphorylation in patients limit interpretation of study results and comparability of findings across studies. However, in vitro and in vivo studies give important insights into mechanisms of action, metabolic feedback mechanisms, antiviral effects, and mechanisms of toxicity, and have influenced dosing regimens of nucleoside analogs.

Development of a novel (32)P-postlabeling method for the analysis of 3'-azido-3'-deoxythymidine

3'-Azido-3'-deoxythymidine (AZT) was the first anti-retroviral nucleoside analog to be used in the treatment and prevention of acquired immunodeficiency syndrome (AIDS). A novel (32)P-postlabeling assay, based upon thymidine kinase (TK) instead of the conventional T(4) polynucleotide kinase, has been developed for the detection of the levels of AZT incorporated into DNA. After enzymatic digestion of DNA to deoxynucleoside 3'-monophosphates, AZT was isolated by ethyl acetate extraction. The ethyl acetate was evaporated and the AZT was postlabeled by 5'-phosphorylation with [gamma-(32)P]ATP and TK. AZT was detected at a level of 51.5+/-6.3 (mean+/-SD; n=4) AZT molecules/10(5) nucleotides following in vitro incorporation of the drug into high-molecular-weight rat-liver DNA. The (32)P-postlabeling method was further validated by the detection of AZT in lung and liver DNA from neonatal mice treated with AZT. The levels of AZT in lung and liver DNA were proportional to the dose, with the levels in lung DNA being two-fold higher than those for liver DNA. The limit of detection for the assay was 8 AZT molecules/10(7) nucleotides using 10 microg of DNA.

Effects of zidovudine (AZT) and dideoxyinosine (ddI) on human trophoblast cells

The anti-HIV agents AZT (zidovudine) and ddl (dideoxyinosine) are being used clinically during pregnancy. The toxicity of these agents to the fetus and placenta remains a concern because few human pregnancy exposure data are available, and pregnant rodent studies with AZT indicate increased embryonic resorptions and developmental arrest. The current study used a human choriocarcinoma cell line (JAr), which exhibits many characteristics of the early placenta, to assess the effects of a single 24 h exposure of 7.6 or 0.076 mM AZT, and the effects of a single 24 h exposure of 7.6 or 0.076 mM ddI upon cell proliferation and hormone production of human chorionic gonadotropin (hCG), estradiol (E2), and progesterone (P4). The higher concentration of AZT and ddI produced significant (P < 0.025) reductions in cell numbers and growth rate while producing significant increases in hormone production (hCG, E2, and P4). The lower concentration of AZT and ddI produced significant increases in E2 production, but no changes in cell numbers, hCG, or P4. Because placental cells require androgen precursor for E2 synthesis, exogenous androstenedione was added to confirm observations of increased estradiol synthesis after AZT or ddl exposure. These results demonstrate that single 24 h high dose exposures of AZT or ddI produce significant inhibition of cell proliferation and alterations in hormone production in this paradigm of human placental cells.

A mechanism of AZT resistance: an increase in nucleotide-dependent primer unblocking by mutant HIV-1 reverse transcriptase

Mutations in HIV-1 reverse transcriptase (RT) give rise to 3'-azido-3'-deoxythymidine (AZT) resistance by a mechanism that has not been previously reproduced in vitro. We show that mutant RT has increased ability to remove AZTMP from blocked primers through a nucleotide-dependent reaction, producing dinucleoside polyphosphate and extendible primer. In the presence of physiological concentrations of ATP, mutant RT extended 12% to 15% of primers past multiple AZTMP termination sites versus less than 0.5% for wild type. Although mutant RT also unblocked ddAMP-terminated primers more efficiently than wild-type RT, the removal of ddAMP was effectively inhibited by the next complementary dNTP (IC50 approximately equal to 12 microM). In contrast, the removal of AZTMP was not inhibited by dNTPs except at nonphysiological concentrations (IC50 > 200 microM).

Unblocking of chain-terminated primer by HIV-1 reverse transcriptase through a nucleotide-dependent mechanism

HIV-1 replication is inhibited by the incorporation of chain-terminating nucleotides at the 3' end of the growing DNA chain. Here we show a nucleotide-dependent reaction catalyzed by HIV-1 reverse transcriptase that can efficiently remove the chain-terminating residue, yielding an extendible primer terminus. Radioactively labeled 3'-terminal residue from the primer can be transferred into a product that is resistant to calf intestinal alkaline phosphatase and sensitive to cleavage by snake venom phosphodiesterase. The products formed from different nucleotide substrates have unique electrophoretic migrations and have been identified as dinucleoside tri- or tetraphosphates. The reaction is inhibited by dNTPs that are complementary to the next position on the template (Ki approximately 5 microM), suggesting competition between dinucleoside polyphosphate synthesis and DNA polymerization. Dinucleoside polyphosphate synthesis was inhibited by an HIV-1 specific non-nucleoside inhibitor and was absent in mutant HIV-1 reverse transcriptase deficient in polymerase activity, indicating that this activity requires a functional polymerase active site. We suggest that dinucleoside polyphosphate synthesis occurs by transfer of the 3' nucleotide from the primer to the pyrophosphate moiety in the nucleoside di- or triphosphate substrate through a mechanism analogous to pyrophosphorolysis. Unlike pyrophosphorolysis, however, the reaction is nucleotide-dependent, is resistant to pyrophosphatase, and produces dinucleoside polyphosphates. Because it occurs at physiological concentrations of ribonucleoside triphosphates, this reaction may determine the in vivo activity of many nucleoside antiretroviral drugs.

The bottleneck in AZT activation

Nucleoside-based inhibitors of reverse transcriptase were the first drugs to be used in the chemotherapy of AIDS. After entering the cell, these substances are activated to their triphosphate form by cellular kinases, after which they are potent chain terminators for the growing viral DNA. The two main factors limiting their efficacy are probably interrelated. These are the insufficient degree of reduction of viral load at the commencement of treatment and the emergence of resistant variants of the virus. The reason for the relatively poor suppression of viral replication appears to be inefficient metabolic activation. Thus, for the most extensively used drug, 3'-azido-3'-deoxythymidine (AZT), whereas phosphorylation to the monophosphate is facile, the product is a very poor substrate for the next kinase in the cascade, thymidylate kinase. Because of this, although high concentrations of the monophosphate can be reached in the cell, the achievable concentration of the active triphosphate is several orders of magnitude lower. Determination of the structure of thymidylate kinase as a complex with AZT monophosphate (AZTMP) together with studies on the kinetics of its phosphorylation have now led to a detailed understanding of the reasons for and consequences of the poor substrate properties.

Genotoxicity and mitochondrial damage in human lymphocytic cells chronically exposed to 3'-azido-2',3'-dideoxythymidine

AZT (3'-azido-2',3'-dideoxythymidine), the first nucleoside analog approved for the treatment of AIDS (acquired immunodeficiency syndrome), induces significant toxic effects in humans exposed to therapeutic doses. As an inhibitor of the HIV-1 (human immunodeficiency virus 1) reverse transcriptase, AZT blocks the incorporation of nucleotides into the host's newly synthesized DNA. Incorporation of AZT into mammalian DNA as well as specific localization of the drug into telomeric DNA, has been previously documented by immunohistochemistry. As with other nucleoside analogs, AZT has affinity for polymerase-gamma, the enzyme responsible for the replication of mitochondrial DNA. In order to examine the mechanisms of toxic events induced by long-term AZT exposure, human T-lymphocytic H9 cells were cultured with 25 microM AZT for 7 months. In the resulting H9-AZT cells, incorporation of AZT into DNA was demonstrated by radioimmunoassay and immunohistochemistry, chromosomal aberrations and micronuclei were scored and intracellular lipid distribution was determined. Two pmol of AZT per microgram of DNA were detected by radioimmunoassay in H9-AZT cells. Control cells showed negative values in the radioimmunoassay. Cytogenetic observations on H9-AZT cells showed an increase in chromosomal aberrations and nuclear fragmentation when compared with unexposed H9 cells. Electron microscopy revealed mitochondrial damage and an elevated accumulation of neutral intracellular lipid deposits probably as a consequence of a distortion in the beta-oxidation of fatty acids normally carried out by this organelle. The toxicities explored here suggest that the mechanisms of AZT induced cytotoxicity in bone marrow of the patients chronically exposed to the drug in vivo may involve both chromosomal and mitochondrial DNA damage.

3'-Azido-3'-deoxythmidine uptake into isolated rat liver mitochondria and impairment of ADP/ATP translocator

To gain some insight into the mechanism by which 3'-azido-3'-deoxythymidine (AZT) impairs mitochondrial metabolism, [14C]AZT uptake by rat liver mitochondria (RLM) in vitro was investigated. AZT accumulated in mitochondria in a time-dependent manner and entered the mitochondrial matrix. The rate of AZT uptake into mitochondria showed a hyperbolic dependence on the drug concentration and was inhibited by mersalyl, a thiol reagent that cannot enter mitochondria, thus showing that a membrane protein is involved in AZT transport. Investigation into the capability of AZT to affect certain mitochondrial carriers demonstrated that AZT was able to impair the ADP/ATP translocator, but had no effect on Pi, dicarboxylate, tricarboxylate, or oxodicarboxylate carriers. AZT inhibited ADP/ATP antiport in either mitochondria or mitoplasts in a competitive manner with different sensitivity (Ki values were 18.3 +/- 2.9 and 70.2 +/- 5.8 microM, respectively). Consistent with this were isotopic measurements showing that AZT accumulates in the intermembrane space. AZT does not use ADP/ATP carrier to enter mitochondria, as shown by the failure of both carboxyatractyloside (CAT) to inhibit AZT transport into mitochondria and AZT to induce ATP efflux from ATP-loaded mitochondria. ADP/ATP translocator impairment by AZT as one of the biochemical processes responsible for the ATP deficiency syndrome is discussed.

Stavudine selectively induces apoptosis in HIV type 1-infected cells

We evaluated the cytotoxic effects of various human immunodeficiency virus (HIV-1) reverse transcriptase inhibitors (zidovudine, didanosine, zalcitabine, stavudine, and nevirapine) on HIV-1-infected and uninfected T cell lines. Among the compounds, only stavudine (not the others) proved to be more cytotoxic to MOLT-4/IIIB cells (MOLT-4 cells chronically infected with HIV-1) than to uninfected MOLT-4 cells. Its 50% cytotoxic concentrations were 59.8 and 2.2 microM for MOLT-4 and MOLT-4/IIIB cells, respectively. Stavudine was also more cytotoxic to CEM/ROD (CEM cells chronically infected with HIV type 2) than to uninfected CEM cells. Microscopic analysis revealed that stavudine induced apoptosis in MOLT-4/IIIB cells. Apparent chromatin condensation in the nucleus was observed by electron microscopy. Furthermore, a DNA fragmentation ladder was detected by agarose gel electrophoresis. Addition of thymidine to the culture medium could rescue the cells from stavudine-induced apoptosis. The expression of anti-apoptotic protein Bcl-2 was partially downregulated in MOLT-4/IIIB cells after treatment with stavudine. This downregulation was not identified in MOLT-4 cells. These results indicate that stavudine selectively induces apoptosis in HIV-1-infected T cells and may have potential as a novel strategy for effective chemotherapy of the acquired immune deficiency syndrome (AIDS).

Inhibition of murine leukemia virus with poly-2'-O-(2,4-dinitrophenyl)poly[A]

Poly-2'-O-(2,4-dinitrophenyl)poly[A] (DNP-poly[A] is a potent inhibitor of reverse transcriptases from a variety of sources (I. Kang and J. H. Wang, J. Biol. Chem. 269:12024-12031, 1994). In the present study, its inhibitory effect on the reverse transcriptase (RT) from Moloney murine leukemia virus (MuLV) was investigated. DNP-poly[A] was found to enter the virus spontaneously and to completely inhibit the RT within 30 min at 0 degree C. The inhibitor was also spontaneously transported into isolated human lymphocytes and leukocytes at 37 degrees C. Animal studies have demonstrated the effectiveness of DNP-poly[A] as an antiviral drug when administered intraperitoneally at various doses from 1 to 100 mg/kg of body weight. MuLV-infected mice show the presence of RT in their blood as well as increased numbers of leukocytes. After the administration of DNP-poly[A] at a dosage of 100 mg/kg of body weight three times a week over a 3-week period, RT could no longer be detected by an ultrasensitive RT-PCR assay. Autopsy showed that the spleens of infected but untreated mice were enlarged 2- to 10-fold, with fused nodules and the proliferation of large abnormal lymphocytes, whereas the spleens of infected but treated mice resembled the normal spleens of uninfected control mice. These observations indicate that further study of DNP-poly[A] as a general antiretroviral agent is desirable.

3'-Azidothymidine (zidovudine) inhibits glycosylation and dramatically alters glycosphingolipid synthesis in whole cells at clinically relevant concentrations

Recent in vitro work with Golgi-enriched membranes showed that 3'-azidothymidine-5'-monophosphate (AZTMP), the primary intracellular metabolite of 3'-azidothymidine (AZT), is a potent inhibitor of glycosylation reactions (Hall et al. (1994) J. Biol. Chem. 269, 14355-14358) and predicted that AZT treatment of whole cells should cause similar inhibition. In this report, we verify this prediction by showing that treatment of K562 cells with AZT inhibits lipid and protein glycosylation. AZT treatment dramatically alters the pattern of glycosphingolipid biosynthesis, nearly abolishing ganglioside synthesis at clinically relevant concentrations (1-5 microM), and suppresses the incorporation of both sialic acid and galactose into proteins. Control experiments demonstrate that these changes do not result from nonspecific effects on either the secretory apparatus or protein synthesis. On the other hand, studies using isolated nuclei as a model system for chromosomal DNA replication show that AZTTP is a very weak inhibitor of DNA synthesis. These observations strongly suggest that the myelosuppressive effects of AZT in vivo are due to inhibition of protein and/or lipid glycosylation and not to effects on chromosomal DNA replication.