P53 in cytoplasm may enhance the accuracy of DNA synthesis by human immunodeficiency virus type 1 reverse transcriptase

The Role of Tumor Suppressor p53 Protein in HIV-Host Cell Interactions

The virus-host relationship is indispensable for executing successful viral infection. The pathogenesis of HIV is determined by an intricate interaction between the host and the virus for the regulation of HIV infection, thereby influencing various aspects, including the regulation of signaling pathways. High mutation rates and population heterogeneity characterize HIV with consequences for viral pathogenesis and the potential to escape the immune system and anti-viral inhibitors used in therapy. The origin of the high mutation rates exhibited by HIV may be attributed to a limited template-copied fidelity that likely operates in the cytoplasm. HIV-1 infection induces upregulation and activation of tumor suppressor p53 protein in the early stages of HIV-1 infection. p53 plays a multifaceted role in the context of HIV infection, thereby affecting viral replication. p53 is involved in maintaining genetic integrity, actively participating in various DNA repair processes through its various biochemical activities and via its ability to interact with components of the repair machinery. This report focuses on the impact of the p53 protein on the HIV-1 reverse transcription process while incorporating various incorrect and non-canonical nucleotides. The presence of functional host-coded p53 protein with proofreading-repair activities in the cytoplasm may lead to various biological outcomes.

The Role of p53 in HIV Infection

PURPOSE OF REVIEW

This review aims to elucidate the multifaceted role of the tumor suppressor protein p53 in the context of HIV infection. We explore how p53, a pivotal regulator of cellular processes, interacts with various facets of the HIV life cycle. Understanding these interactions could provide valuable insights into potential therapeutic interventions and the broader implications of p53 in viral infections.

RECENT FINDINGS

Recent research has unveiled a complex interplay between p53 and HIV. Several reports have highlighted the involvement of p53 in restricting the replication of HIV within both immune and nonimmune cells. Various mechanisms have been suggested to unveil how p53 enforces this restriction on HIV replication. However, HIV has developed strategies to manipulate p53, benefiting its replication and evading host defenses. In summary, p53 plays a multifaceted role in HIV infection, impacting viral replication and disease progression. Recent findings underscore the importance of understanding the intricate interactions between p53 and HIV for the development of innovative therapeutic approaches. Manipulating p53 pathways may offer potential avenues to suppress viral replication and ameliorate immune dysfunction, ultimately contributing to the management of HIV/AIDS. Further research is warranted to fully exploit the therapeutic potential of p53 in the context of HIV infection.

HIV-1-related factors interact with p53 to influence cellular processes

Human immunodeficiency virus type 1 (HIV-1) is the primary epidemic strain in China. Its genome contains two regulatory genes (tat and rev), three structural genes (gag, pol, and env), and four accessory genes (nef, vpr, vpu, and vif). Long terminal repeats (LTRs) in thegenome regulate integration, duplication, and expression of viral gene. The permissibility of HIV-1 infection hinges on the host cell cycle status. HIV-1 replicates by exploiting various cellular processes via upregulation or downregulation of specific cellular proteins that also control viral pathogenesis. For example, HIV-1 regulates the life cycle of p53, which in turn contributes significantly to HIV-1 pathogenesis. In this article, we review the interaction between HIV-1-associated factors and p53, providing information on their regulatory and molecular mechanisms, hinting possible directions for further research.

Host cell p53 associates with the feline calicivirus major viral capsid protein VP1, the protease-polymerase NS6/7, and the double-stranded RNA playing a role in virus replication

p53 is implicated in several cellular pathways such as induction of cell-cycle arrest, differentiation, senescence, and apoptosis. p53 is activated by a broad range of stress signals, including viral infections. While some viruses activate p53, others induce its inactivation, and occasionally p53 is differentially modulated during the replicative cycle. During calicivirus infections, apoptosis is required for virus exit and spread into the host; yet, the role of p53 during infection is unknown. By confocal microscopy, we found that p53 associates with FCV VP1, the protease-polymerase NS6/7, and the dsRNA. This interaction was further confirmed by proximity ligation assays, suggesting that p53 participates in the FCV replication. Knocked-down of p53 expression in CrFK cells before infection, resulted in a strong reduction of the non-structural protein levels and a decrease of the viral progeny production. These results indicate that p53 is associated with the viral replication complex and is required for an efficient FCV replication.

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Host cell p53 protein levels and subcellular localization do not change during FCV infection.

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Host cell p53 associates with FCV major viral capsid protein VP1, protease-polymerase NS6/7, and the dsRNA in FCV infected cells.

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Host cell p53 is required for a FCV replication.

Inhibition of HIV early replication by the p53 and its downstream gene p21

Background

The tumor suppressor gene p53 has been found to suppress HIV infection by various mechanisms, but the inhibition of HIV at an early stage of replication by host cell p53 and its downstream gene p21 has not been well studied.

Method

VSV-G pseudotyped HIV-1 or HIV-2 viruses with GFP or luciferase reporter gene were used to infect HCT116 p53^+/+ cells, HCT116 p53^−/− cells and hMDMs. The infections were detected by flow cytometry or measured by luciferase assay. Reverse transcription products were quantified by a TaqMan real time PCR. siRNA knockdown experiments were applied to study potential roles of p53 and p21 genes in their restriction to HIV infection. Western blot experiments were used to analyze changes in gene expression.

Results

The infection of HIV-1 was inhibited in HCT116 p53^+/+ cells in comparison to HCT116 p53^−/− cells. The fold of inhibition was largely increased when cell cycle switched from cycling to non-cycling status. Further analysis showed that both p53 and p21 expressions were upregulated in non-cycling HCT116 p53^+/+ cells and HIV-1 reverse transcription was subsequently inhibited. siRNA knockdown of either p53 or p21 rescued HIV-1 reverse transcription from the inhibition in non-cycling HCT116 p53^+/+ cells. It was identified that the observed restrictions by p53 and p21 were associated with the suppression of RNR2 expression and phosphorylation of SAMHD1. These observations were confirmed by using siRNA knockdown experiments. In addition, p53 also inhibited HIV-2 infection in HCT116 p53^+/+ cells and siRNA knockdown of p21 increased HIV-2 infection in hMDMs. Finally the expressions of p53 and p21 were found to be induced in hMDMs shortly after HIV-1 infection.

Conclusions

The p53 and its downstream gene p21 interfere with HIV early stage of replication in non-cycling cells and hMDMs.

Cytoplasmic p53 contributes to the removal of uracils misincorporated by HIV-1 reverse transcriptase

HIV-1 reverse transcriptase (RT) in the cytoplasm of HIV-infected cells efficiently inserts the non-canonical dUTP into the proviral DNA, and extends the dU-terminated DNA. The misincorporation of dUTP leads to mutagenesis, and uracils can down-regulate viral gene expression. However, uracilation might also protect HIV DNA from auto-integration in the cytoplasm. Tumor suppressor p53 protein, exhibiting inherent 3'→5' exonuclease activity, provides a potential host-derived repair mechanism during HIV reverse transcription for the misincorporation of various wrong nucleotides, leading to both base-base mismatches and incorporated non-canonical ribonucleotides. Since the presence of proofreading activity is essential for DNA synthesis accuracy, we elucidated the potential involvement of cytoplasmic p53 in the U-editing activities during insertion of dUTP into DNA by recombinant HIV-1 RT (using isogenic p53-proficient and -deficient HCT116 cells). The biochemical data show that p53 in cytoplasm can participate through the intermolecular pathway in a dU-damage-associated repair mechanism by its ability to remove preformed 3'-terminal dUs, thus preventing further extension of 3' dU-terminated primer during DNA synthesis by HIV-1 RT. The specific depletion of p53 from cytoplasmic lysates of repair-proficient p53-harboring cells reduced this negative effect. Accordingly, the increased abundance of p53 in nutlin-treated cells correlates with enhanced error-correction functions, namely, removal of incorporated uracil. The data substantiate the significance of p53 as a potential proofreader for removal of non-canonical dUTP from HIV DNA, thus preventing the consequences of dUTP misincorporation in cell-type specific infectivity of HIV.

The impact of p53 on the early stage replication of retrovirus

Background

The function of p53 in cancer biology has been studied extensively, but its role in anti-retrovirus infection has been elusive for many years. The restriction of retrovirus early stage replication by p53 was investigated in this study.

Method

VSV-G pseudotyped retrovirus with GFP reporter gene was used to infect both HCT116 p53^+/+ cells and its isogenic p53 knockout HCT116 p53^−/− cells. The infection was detected by flow cytometry. Reverse transcription products were quantified by real time PCR. Mutation analysis was performed after 1-LTR cycle and 2-LTR cycle DNA were amplified and PCR products were sequenced. Transcription and translation of cyclin-dependent kinase inhibitor 1 (p21^Cip1) and SAM domain and HD domain-containing protein 1 (SAMHD1) were analyzed by TaqMan PCR and Western blot experiments. siRNA experiment was applied to study the role of p53 downstream gene p21^Cip1 in the restriction of retrovirus infection.

Results

It was found that the block of retrovirus infection in non-cycling cells was significantly attenuated in HCT116 p53^−/− cells when compared to HCT116 p53^+/+ cells. It was found that both late reverse transcription products and viral 2-LTR cycle DNA were significantly increased in infected non-cycling HCT116 p53^−/− cells. Furthermore, the mutation frequency detected in 1-LTR DNA from HCT116 p53^+/+ cells were significantly decreased in comparison to HCT116 p53^−/− cells. A higher number of insertion and deletion mutations were detected in the joint region of 2-LTR cycle DNA in infected p53^+/+ cells. Cell cycle analysis showed retrovirus infection promoted host cell replication. Higher levels of mRNA and protein of p21^Cip1 were found in HCT116 p53^+/+ cells in comparison to the HCT116 p53^−/− cells. Furthermore, knockdown of p21^Cip1 in non-cycling HCT116 p53^+/+ cells significantly increased the infection.

Conclusions

The results of this study showed that p53 is an important restriction factor that interferes with retrovirus infection in its early stage of replication. Our results suggested that p53 mediates the inhibition of retrovirus infection in non-cycling cells through it downstream gene p21^Cip1, and p53 also functions to influence formation of 1-LTR cycle and 2-LTR cycle DNA.

Removal of ribonucleotides by p53 protein incorporated during DNA synthesis by HIV-1 reverse transcriptase

OBJECTIVE(S)

HIV-1 reverse transcriptase frequently incorporates ribonucleotides into the proviral DNA in macrophages, but not in lymphocytes. The enzyme exerts an efficient ribonucleotide-terminated primer extension capacity. Furthermore, ribonucleotide-editing repair is attenuated in macrophages. Tumor suppressor p53 protein, displaying an intrinsic 3'→5' exonuclease activity, was found to be involved in efficient proofreading of base-base mismatches produced during DNA synthesis. As the presence of proofreading activity is cardinal for the DNA synthesis accuracy, it was of interest to assess whether p53 can serve as a trans-acting proofreader for HIV-1 reverse transcriptase during ribonucleotide incorporation.

DESIGN

We investigated the potential involvement of cytoplasmic p53 in error correction during insertion of ribonucleotides into DNA by recombinant HIV-1 reverse transcriptase in a p53-proficient and deficient background.

METHODS

Primer extension reactions were carried out to elucidate the incorporation and removal of ribonucleotides.

RESULTS

The biochemical studies suggest that p53 is involved in a ribonucleotide damage-associated repair mechanism through its capacity to remove preformed 3'-terminal ribonucleotides, to decrease ribonucleotide incorporation and to prevent the 3'-ribo-terminated primer extension during ongoing DNA synthesis by HIV-1 reverse transcriptase. A positive correlation exists between the presence of endogenous p53 and decrease in stable incorporation of ribonucleotides into DNA with p53-harboring lysates of HCT116 cells. p53, by preferential removal of purine over pyrimidine ribonucleotides, may affect the ribonucleotide mutation spectra produced by HIV-1 reverse transcriptase.

CONCLUSION

The data implies that p53 can excise incorrect sugar in addition to base mispairs, thereby expanding the role of p53 in the repair of nucleic acids replication errors.

Transcriptomic Analysis Implicates the p53 Signaling Pathway in the Establishment of HIV-1 Latency in Central Memory CD4 T Cells in an In Vitro Model

The search for an HIV-1 cure has been greatly hindered by the presence of a viral reservoir that persists despite antiretroviral therapy (ART). Studies of HIV-1 latency in vivo are also complicated by the low proportion of latently infected cells in HIV-1 infected individuals. A number of models of HIV-1 latency have been developed to examine the signaling pathways and viral determinants of latency and reactivation. A primary cell model of HIV-1 latency, which incorporates the generation of primary central memory CD4 T cells (TCM), full-length virus infection (HIVNL4-3) and ART to suppress virus replication, was used to investigate the establishment of HIV latency using RNA-Seq. Initially, an investigation of host and viral gene expression in the resting and activated states of this model indicated that the resting condition was reflective of a latent state. Then, a comparison of the host transcriptome between the uninfected and latently infected conditions of this model identified 826 differentially expressed genes, many of which were related to p53 signaling. Inhibition of the transcriptional activity of p53 by pifithrin-α during HIV-1 infection reduced the ability of HIV-1 to be reactivated from its latent state by an unknown mechanism. In conclusion, this model may be used to screen latency reversing agents utilized in shock and kill approaches to cure HIV, to search for cellular markers of latency, and to understand the mechanisms by which HIV-1 establishes latency.

Ribonuclease activity of p53 in cytoplasm in response to various stress signals

The tumor suppressor p53 protein is expressed at low levels under normal conditions. The subcellular localization and functional activation of p53 are influenced by diverse stress signals. p53 in cytoplasm exerts intrinsic 3'→5' exonuclease activity with various RNA and DNA substrates. ssRNAs containing an adenosine and uridine-rich (ARE) element are permissive targets for p53-mediated degradation. The analysis of the exonuclease activity in cytoplasm with activated p53 induced by various drug treatments or following γ-irradiation revealed that the expression of p53 exonuclease activity in response to stress signals is heterogeneous. Various genotoxic and non-genotoxic agents upregulate p53 yet have different effects on expression of exonuclease activity with ARE RNA but not with DNA substrate. Ribonuclease activity is enhanced in cytoplasmic extracts of HCT116 (p53+/+) cells exposed to γ-irradiation or treated by the non-genotoxic drug AS101 but decreased following treatment by genotoxic (e.g., doxorubicin) or non-genotoxic (e.g., DFMO) agents, thus indicating that p53 exonuclease activity is dependent on the specific stress and nature of the substrate. Apparently, the disparity in expression of p53 ribonuclease activity after each treatment is attributable to the different post-treatment response and to two posttranscriptional events: the interaction of RNA-binding HuR protein with ARE RNA protects the substrate from degradation by p53 and/or decrease in p53 ARE RNA binding capacity due to phosphorylation at Ser392 leads to reduction in p5 ribonuclease activity. Our results provide new insights into p53 exonuclease function and into the mechanisms behind the regulation ARE-RNA degradation by p53 under different cellular conditions.

Role of p53 in neurodegenerative diseases

BACKGROUND

p53 plays an important role in many areas of cellular physiology and biology, ranging from cellular development and differentiation to cell cycle arrest and apoptosis. Many of its functions are attributed to its role in assuring proper cellular division. However, since the establishment of its role in cell cycle arrest, damage repair, and apoptosis (thus also establishing its importance in cancer development), numerous reports have demonstrated additional functions of p53 in various cells. In particular, p53 appears to have important functions as it relates to neurodegeneration and synaptic plasticity.

OBJECTIVE

In this review, we will address p53 functions as it relates to various neurodegenerative diseases, mainly its implications in the development of HIV-associated neurocognitive disorders.

CONCLUSION

p53 plays a pivotal role in the development of neurodegenerative diseases through its interaction with cellular factors, viral factors, and/or small RNAs that have the ability to promote the development of these diseases. Hence, inhibition of p53 may present an ideal target to restore neuronal functions.

Activity-based selection of HIV-1 reverse transcriptase variants with decreased polymerization fidelity

HIV-1 reverse transcriptase (HIV-1 RT) copies the RNA genome of HIV-1 into DNA, thereby committing errors at an exceptionally high frequency. Viral offspring evolve rapidly and consequently are capable of evading the immune response as well as antiviral treatment. However, error-prone viral replication could drive HIV close to extinction owing to an intolerable load of deleterious mutations. We applied a genetic selection scheme to identify variants of HIV-1 RT with a further increased error rate to study the relationship between error rate and viral replication. Using this approach, we identified 16 mutator candidates, two of which were purified and further studiedin vitro. One of these variant enzymes showed a generally increased mutation frequency as compared with the reference enzyme. A single amino acid residue, R448, is probably responsible for the observed effect. Mutation of this residue, which is located within the RNase H domain of HIV-1 RT, seems to perturb the interaction with template RNA and consequently affects polymerase activity and fidelity.

Mutation rates and intrinsic fidelity of retroviral reverse transcriptases

Retroviruses are RNA viruses that replicate through a DNA intermediate, in a process catalyzed by the viral reverse transcriptase (RT). Although cellular polymerases and host factors contribute to retroviral mutagenesis, the RT errors play a major role in retroviral mutation. RT mutations that affect the accuracy of the viral polymerase have been identified by in vitro analysis of the fidelity of DNA synthesis, by using enzymological (gel-based) and genetic assays (e.g., M13mp2 lacZ forward mutation assays). For several amino acid substitutions, these observations have been confirmed in cell culture using viral vectors. This review provides an update on studies leading to the identification of the major components of the fidelity center in retroviral RTs.

Excision of nucleoside analogs in mitochondria by p53 protein

OBJECTIVE

Nucleoside analogs, used against HIV, can be incorporated into a mitochondrial DNA by DNA polymerase gamma. Both the decrease in mitochondrial DNA and increased mutations of mitochondrial DNA may lead to mitochondrial diseases. The tumor suppressor protein p53 exhibits 3' --> 5' exonuclease activity and can provide a proofreading function for DNA polymerases. In the present study, we investigated the ability of p53 to excise incorporated nucleoside analogs from DNA in mitochondria.

DESIGN

The functional interaction of p53 and DNA polymerase gamma during the incorporation of nucleoside analog was examined in mitochondrial fractions of p53-null H1299 cells, as the source of DNA polymerase gamma.

METHODS

Primer extension reactions were carried out to elucidate the incorporation and removal of nucleoside analogs.

RESULTS

The results demonstrate that the excision of incorporated nucleoside analogs in mitochondrial fractions of H1299 cells increased in the presence of purified recombinant p53, or cytoplasmic extracts of large cell carcinoma 2 cells expressing endogenous wild-type p53 (but not specifically predepleted extracts) or cytoplasmic extracts of H1299 cells overexpressing wild-type p53, but not exonuclease-deficient mutant p53-R175H. The amount of nucleoside analogs incorporated into the elongated DNA with mitochondrial fractions of human colon carcinoma 116 (HCT116)(p53+/+) cells was lower than that of HCT116(p53-/-) cells. Furthermore, mitochondrion-localized elevation of p53 in HCT116(p53+/+) cells, following the irradiation-stress stimuli, correlates with the reduction in incorporation of nucleoside analogs and wrong nucleotides.

CONCLUSION

p53 in mitochondria may functionally interact with DNA polymerase gamma, thus providing a proofreading function during mitochondrial DNA replication for excision of nucleoside analogs and polymerization errors.

Influence of vector design and host cell on the mechanism of recombination and emergence of mutant subpopulations of replicating retroviral vectors

Background

The recent advent of murine leukaemia virus (MLV)-based replication-competent retroviral (RCR) vector technology has provided exciting new tools for gene delivery, albeit the advances in vector efficiency which have been realized are also accompanied by a set of fresh challenges. The expression of additional transgene sequences, for example, increases the length of the viral genome, which can lead to reductions in replication efficiency and in turn to vector genome instability. This necessitates efforts to analyse the rate and mechanism of recombinant emergence during the replication of such vectors to provide data which should contribute to improvements in RCR vector design.

Results

In this study, we have performed detailed molecular analyses on packaged vector genomes and proviral DNA following propagation of MLV-based RCR vectors both in cell culture and in pre-formed subcutaneous tumours in vivo. The effects of strain of MLV, transgene position and host cell type on the rate of emergence of vector recombinants were quantitatively analysed by applying real-time PCR and real-time RT-PCR assays. Individual mutants were further characterized by PCR, and nucleotide sequence and structural motifs associated with these mutants were determined by sequencing. Our data indicate that virus strain, vector design and host cell influence the rate of emergence of predominating vector mutants, but not the underlying recombination mechanisms in vitro. In contrast, however, differences in the RNA secondary structural motifs associated with sequenced mutants emerging in cell culture and in solid tumours in vivo were observed.

Conclusion

Our data provide further evidence that MLV-based RCR vectors based on the Moloney strain of MLV and containing the transgene cassette in the 3' UTR region are superior to those based on Akv-MLV and/or containing the transgene cassette in the U3 region of the LTR. The observed discrepancies between the data obtained in solid tumours in vivo and our own and previously published data from infected cells in vitro demonstrates the importance of evaluating vectors designed for use in cancer gene therapy in vivo as well as in vitro.

p53 in mitochondria enhances the accuracy of DNA synthesis

Mitochondrial localization of p53 was observed in stressed and unstressed cells. p53 is involved in DNA repair and apoptosis. It exerts physical and functional interactions with mitochondrial DNA and DNA polymerase gamma (pol gamma). The functional cooperation of p53 and pol gamma during DNA synthesis was examined in the mitochondrial fraction of p53-null H1299 cells, as the source of pol gamma. The results show that p53 may affect the accuracy of DNA synthesis in mitochondria: (1) the excision of a misincorporated nucleotide increases in the presence of (a) recombinant wild-type p53 (wtp53); (b) cytoplasmic fraction of LCC2 cells expressing endogenous wtp53 (but not specifically pre-depleted fraction); (c) cytoplasmic extract of H1299 cells overexpressing wtp53, but not exonuclease-deficient mutant p53-R175H. (2) Mitochondrial extracts of HCT116(p53+/+) cells display higher exonuclease activity compared with that of HCT116(p53-/-) cells. Addition of exogenous p53 complements the HCT116(p53-/-) mitochondrial extract mispair excision. Furthermore, the misincorporation was lower in the mitochondrial fraction of HCT116(p53+/+) cells as compared with that of HCT116(p53-/-) cells. (3) Irradiation-induced mitochondrial translocation of endogenous p53 in HCT116(p53+/+) cells correlates with the enhancement of error-correction activities. Taken together, the data suggest that p53 in mitochondria may be a component of an error-repair pathway and serve as guardian of the mitochondrial genome. The function of p53 in DNA repair and apoptosis is discussed.

P53 represses human papillomavirus type 16 DNA replication via the viral E2 protein

BACKGROUND

Human papillomavirus (HPV) DNA replication can be inhibited by the cellular tumour suppressor protein p53. However, the mechanism through which p53 inhibits viral replication and the role that this might play in the HPV life cycle are not known. The papillomavirus E2 protein is required for efficient HPV DNA replication and also regulates viral gene expression. E2 represses transcription of the HPV E6 and E7 oncogenes and can thereby modulate indirectly host cell proliferation and survival. In addition, the E2 protein from HPV 16 has been shown to bind p53 and to be capable of inducing apoptosis independently of E6 and E7.

RESULTS

Here we use a panel of E2 mutants to confirm that mutations which block the induction of apoptosis via this E6/E7-independent pathway, have little or no effect on the induction of apoptosis by the E6/E7-dependent pathway. Although these mutations in E2 do not affect the ability of the protein to mediate HPV DNA replication, they do abrogate the repressive effects of p53 on the transcriptional activity of E2 and prevent the inhibition of E2-dependent HPV DNA replication by p53.

CONCLUSION

These data suggest that p53 down-regulates HPV 16 DNA replication via the E2 protein.

Exonucleolytic degradation of RNA by p53 protein in cytoplasm

p53 in cytoplasm displays an intrinsic 3'-->5' exonuclease activity. To understand the significance of p53 exonuclease activity in cytoplasm, cytoplasmic extracts of various cell lines were examined for exonuclease activity with different single-stranded RNA (ssRNA) substrates. Using an in vitro RNA degradation assay, we observed in cytoplasmic extracts of LCC2 cells, expressing high levels of endogenous wtp53, an efficient 3'-->5' exonuclease activity with RNA substrates, removing the 3'-terminal nucleotides. Interestingly, RNA containing AU-rich sequences (ARE) is the permissive substrate for exonucleolytic degradation. Evidence that exonuclease function with RNA detected in cytoplasmic extracts is attributed to the p53 is supported by several facts: (1) this activity closely parallels with status and levels of endogenous cytoplasmic p53; (2) the endogenous exonuclease exerts identical RNA substrate specificity and excision profile characteristic for purified baculovirus-or bacterially-expressed wtp53s; (3) the exonuclease activity with ARE RNA is competed out by the presence of ss or double-stranded DNA substrate utilized by p53 protein in cytoplasm; (4) immunoprecipitation by specific anti-p53 antibodies markedly reduced the exonuclease activity with both RNA and DNA substrates; and (5) transfection of the wtp53, but not exonuclease-deficient mutant p53-R175H, into p53-null H1299 or HCT116 cells induced high levels of exonuclease activity with ARE RNA substrate in cytoplasm with characteristic excision profile. The efficient ARE RNA degradation correlates with the efficient binding of p53 to ARE RNA in cytoplasm. The possible role of p53 exonuclease activity in ARE-mRNA destabilization in cytoplasm, which may be important for expression of proteins that control cell growth and/or apoptosis is discussed.

The incorporation of nucleoside analogs by human immunodeficiency virus type 1 reverse transcriptase decreases in the presence of polyamines

Nucleoside analogs (NAs) are an important class of anti-retroviral compounds used against human immunodeficiency virus (HIV). We have analyzed the potential effect of polyamines on the incorporation of NAs during DNA synthesis by HIV type-1 (HIV-1) reverse transcriptase (RT). The polyamines exert the ability to decrease the incorporation of various dideoxynucleoside triphosphates (ddATP, ddTTP or ddCTP) with both RNA/DNA and DNA/DNA substrates in the following order: spermine>spermidine>putrescine. The reduction is a sequence-independent effect, taking place at different sequence context. The results suggest that polyamines might affect the inhibition of reverse transcription by nucleoside analogs HIV-1 RT directed.

Replicative fidelity of lentiviral vectors produced by transient transfection

Previous investigations have estimated the human immunodeficiency virus type 1 (HIV) base pair substitution rate to be approximately 10(-4) to 10(-5) per round of viral replication, and HIV has been hypothesized to be more error-prone than other retroviruses. Using a single cycle reversion assay, we unexpectedly found that the reversion rates of HIV, avian leukosis virus and Moloney murine leukemia virus were the same, within statistical error. Because both the viral enzyme reverse transcriptase (RT) and cellular RNA polymerase II (RNAP) are required for viral replication, we hypothesized that the similar reversion rates actually reflect the intrinsic error rate of RNAP, which is the enzyme common to all three retroviruses in the reversion assay. To address this possibility, HIV vectors with the U3 region replaced by a reporter reversion cassette were constructed and vector supernatant produced by transient transfection. All single integrant revertant cell lines showed the identical mutations at both long terminal repeats. This indicates that either RNAP or another cellular enzyme is responsible for these reversions, or that HIV RT only makes errors during first strand synthesis. Additionally, when HIV particles were rescued from an integrated vector as opposed to being produced by transient transfection, the reversion rate was significantly lower, suggesting that one or more factors in the virus-producing cells plays a role in the fidelity of retroviral replication. These results have implications regarding the fidelity of the transgene after transient transfection production of lentiviral vector supernatants.

Excision of nucleoside analogs from DNA by p53 protein, a potential cellular mechanism of resistance to inhibitors of human immunodeficiency virus type 1 reverse transcriptase

We investigated the ability of p53 in cytoplasm to excise nucleoside analogs (NAs). A decrease in incorporation of NAs by human immunodeficiency virus type 1 reverse transcriptase and their excision from DNA by p53, provided by the cytoplasmic fraction of LCC2 cells, suggest that p53 in cytoplasm may act as an external proofreader for NA incorporation.

The fidelity of DNA synthesis by human immunodeficiency virus type 1 reverse transcriptase increases in the presence of polyamines

The high error rates characteristic of human immunodeficiency virus type-1 reverse transcriptase (HIV-1 RT) are a presumptive source of the viral hypervariability that substantially affect viral pathogenesis and resistance to drug therapy. We have analyzed the potential role of polyamines in the fidelity of DNA synthesis by HIV-1 RT. The current study suggest that polyamines tested has the potential to be "antimutator". The polyamines exert the ability to reduce the misincorporation and mispair extension with both RNA/DNA and DNA/DNA template-primers in the following order: spermine > spermidine > putrescine. In view of the significance of mutations of HIV, the possible roles of polyamines in the accuracy of DNA synthesis could be of particular importance; polyamines may affect the mutation rate of the virus.