Depletion of uracil-DNA glycosylase activity is associated with decreased cell proliferation

Is Uracil-DNA Glycosylase UNG2 a New Cellular Weapon Against HIV-1?

Uracil-DNA glycosylase-2 (UNG2) is a DNA repair protein that removes uracil from single and double-stranded DNA through a basic excision repair process. UNG2 is packaged into new virions by interaction with integrase (IN) and is needed during the early stages of the replication cycle. UNG2 appears to play both a positive and negative role during HIV-1 replication; UNG2 improves the fidelity of reverse transcription but the nuclear isoform of UNG2 participates in the degradation of cDNA and the persistence of the cellular genome by repairing its uracil mismatches. In addition, UNG2 is neutralized by Vpr, which redirects it to the proteasome for degradation, suggesting that UNG2 may be a new cellular restriction factor. So far, we have not understood why HIV-1 imports UNG2 via its IN and why it causes degradation of endogenous UNG2 by redirecting it to the proteasome via Vpr. In this review, we propose to discuss the ambiguous role of UNG2 during the HIV-1 replication cycle.

Hydrogen peroxide mediated mitochondrial UNG1-PRDX3 interaction and UNG1 degradation

Isoform 1 of uracil-DNA glycosylase (UNG1) is the major protein for initiating base-excision repair in mitochondria and is in close proximity to the respiratory chain that generates reactive oxygen species (ROS). Effects of ROS on the stability of UNG1 have not been well characterized. In the present study, we found that overexpression of UNG1 enhanced cells' resistance to oxidative stress and protected mitochondrial DNA (mtDNA) from oxidation. Proteomics analysis showed that UNG1 bound to eight proteins in the mitochondria, including PAPSS2, CD70 antigen, and AGR2 under normal growth conditions, whereas UNG1 mainly bound to Peroxiredoxin 3 (PRDX3) via a disulfide linkage under oxidative stress. We further demonstrated that the UNG1-PRDX3 interaction protected UNG1 from ROS-mediated degradation and prevented mtDNA oxidation. Moreover, our results show that ROS-mediated UNG1 degradation was Lon protease 1 (LonP1)-dependent and mitochondrial UNG1 degradation was aggravated by knockdown of PRDX3 expression. Taken together, these results reveal a novel function of UNG1 in the recruitment of PRDX3 to mtDNA under oxidative stress, enabling protection of UNG1 and UNG1-bound DNA from ROS damage and enhancing cell resistance to oxidative stress.

Vpr expression abolishes the capacity of HIV-1 infected cells to repair uracilated DNA

The human immunodeficiency virus type 1 (HIV-1) Vpr protein binds to the cellular uracil–DNA glycosylase UNG2 and induces its degradation through the assembly with the DDB1-CUL4 ubiquitin ligase complex. This interaction counteracts the antiviral activity exerted by UNG2 on HIV-1 gene transcription, as previously reported by us. In this work, we show that Vpr expression in the context of HIV-1 infection markedly decreases UNG2 expression in transformed or primary CD4⁺ T lymphocytes. We demonstrate for the first time that Vpr-UNG2 interaction significantly impairs the uracil excision activity of infected cells. The loss of uracil excision activity coincides with a significant accumulation of uracilated bases in the genome of infected cells without changes in cell division. Although UNG2 expression and uracil–DNA glycosylase activity are recovered after the peak of retroviral replication, the mutagenic effect of transient DNA uracilation in cycling cells should be taken into account. Therefore, the possible consequences of Vpr-mediated temporary depletion of endogenous nuclear UNG2 and subsequent alteration of the genomic integrity of infected cells need to be evaluated in the physiopathogenesis of HIV infection.

A human endogenous retrovirus K dUTPase triggers a TH1, TH17 cytokine response: does it have a role in psoriasis?

Psoriasis is a chronic inflammatory immune disease of the skin characterized by a complex interplay between multiple risk genes and their interactions with environmental factors. Recent haplotype analyses have suggested that deoxyuridine triphosphate nucleotidohydrolase (dUTPase) encoded by a human endogenous retrovirus K (HERV-K) may be a candidate gene for the psoriasis susceptibility 1 locus. However, no functional studies have been conducted to determine the role of HERV-K dUTPase in psoriasis. For this purpose, we constructed an HERV-K dUTPase wild-type sequence, as well as specific mutations reflecting the genotype characteristic of high- and low-risk haplotypes, purified the recombinant proteins, and evaluated whether they could modulate innate and/or adaptive immune responses. In this study, we demonstrate that wild-type and mutant HERV-K dUTPase proteins induce the activation of NF-κB through Toll-like receptor 2, independent of enzymatic activity. Proteome array studies revealed that treatment of human primary cells with wild-type and mutant HERV-K dUTPase proteins triggered the secretion of T(H)1 and T(H)17 cytokines involved in the formation of psoriatic plaques, including IL-12p40, IL-23, IL-17, tumor necrosis factor-α, IL-8, and CCL20, in dendritic/Langerhans-like cells and to a lesser extent in keratinocytes. These data support HERV-K dUTPase as a potential contributor to psoriasis pathophysiology.

Uracil DNA N-glycosylase promotes assembly of human centromere protein A

Uracil is removed from DNA by the conserved enzyme Uracil DNA N-glycosylase (UNG). Previously, we observed that inhibiting UNG in Xenopus egg extracts blocked assembly of CENP-A, a histone H3 variant. CENP-A is an essential protein in all species, since it is required for chromosome segregation during mitosis. Thus, the implication of UNG in CENP-A assembly implies that UNG would also be essential, but UNG mutants lacking catalytic activity are viable in all species. In this paper, we present evidence that UNG2 colocalizes with CENP-A and H2AX phosphorylation at centromeres in normally cycling cells. Reduction of UNG2 in human cells blocks CENP-A assembly, and results in reduced cell proliferation, associated with increased frequencies of mitotic abnormalities and rapid cell death. Overexpression of UNG2 induces high levels of CENP-A assembly in human cells. Using a multiphoton laser approach, we demonstrate that UNG2 is rapidly recruited to sites of DNA damage. Taken together, our data are consistent with a model in which the N-terminus of UNG2 interacts with the active site of the enzyme and with chromatin.

Phenotypic knockout of CXCR4 by a novel recombinant protein TAT/54R/KDEL inhibits tumors metastasis

The chemokine receptor, CXCR4, and its specific ligand, CXCL12, have been proven to regulate the directional trafficking and invasion of breast cancer cells to sites of metastases, and similar phenomena have also been identified in many malignant tumors that aberrantly overexpress CXCR4. Therefore, blocking the interaction between CXCR4 and CXCL12 is considered a possible approach to efficiently prevent cancer metastasis. Employing a cellular phenotypic knockout strategy based on intrakines, we developed a novel recombinant chimeric protein, TAT/54R/KDEL, which contains three distinct functional domains: CXCL12/54R, a mutant of CXCL12 with CXCR4 antagonism, as well as HIV-derived TAT (47-57) and an endoplasmic reticulum retention four-peptide sequence KDEL that links at its NH(2) and COOH termini, respectively. Using the MOLT-4 cell line, which expressed CXCR4 highly and stably in vitro, we determined that TAT/54R/KDEL was able to efficiently transfer into the endoplasmic reticulum of tumor cells, where it specifically binds to the newly synthesized CXCR4 and prevents the latter from reaching the surface. Chemotaxis assays showed that the cells treated with TAT/54R/KDEL failed to migrate toward CXCL12. Furthermore, we observed that the systemic treatment of TAT/54R/KDEL could impair lung metastasis in a highly metastatic mammary cancer cell line, 4T1 cells, with the decrease of CXCR4 on their membrane. Our results suggest that the phenotypic knockout strategy of CXCR4 using a novel recombinant protein TAT/54R/KDEL might be a possible approach for inhibiting relative tumor metastasis mediated by CXCR4/CXCL12 interaction.

Small interfering RNA-directed knockdown of uracil DNA glycosylase induces apoptosis and sensitizes human prostate cancer cells to genotoxic stress

Uracil DNA glycosylase (UNG) is the primary enzyme responsible for removing uracil residues from DNA. Although a substantial body of evidence suggests that DNA damage plays a role in cancer cell apoptosis, the underlying mechanisms are poorly understood. In particular, very little is known about the role of base excision repair of misincorporated uracil in cell survival. To test the hypothesis that the repair of DNA damage associated with uracil misincorporation is critical for cancer cell survival, we used small interfering RNA (siRNA) to target the human UNG gene. In a dose-dependent and time-dependent manner, siRNA specifically inhibited UNG expression and modified the expression of several genes at both mRNA and protein levels. In LNCaP cells, p53, p21, and Bax protein levels increased, whereas Bcl2 levels decreased. In DU145 cells, p21 levels were elevated, although mutant p53 and Bax levels remained unchanged. In PC3 cells, UNG inhibition resulted in elevated p21 and Bax levels. In all three cell lines, UNG inhibition reduced cell proliferation, induced apoptosis, and increased cellular sensitivity to genotoxic stress. Furthermore, an in vitro cleavage experiment using uracil-containing double-stranded DNA as a template has shown that siRNA-mediated knockdown of UNG expression significantly reduced the uracil-excising activity of UNG in human prostate cancer cells, which was associated with DNA damage analyzed by comet assay. Taken together, these findings indicate that RNA interference-directed targeting of UNG is a convenient, novel tool for studying the biological role of UNG and raises the potential of its application for prostate cancer therapy.

Real-time monitoring of uracil removal by uracil-DNA glycosylase using fluorescent resonance energy transfer probes

As a highly conserved damage repair protein, uracil-DNA glycosylase (UDG) mainly catalyzes the excision of uracil from DNA to sustain the genome integrity. Here a novel method for monitoring the uracil removal in real time is introduced. Double-stranded DNA probes modified with uracil residues that can occur in fluorescent resonance energy transfer (FRET) were used as substrates and detecting probes in a homogeneous solution. This method not only overcame the drawbacks of traditional radioactive assays, such as discontinuity and being time-consuming and complicated, but also was used to accurately determine the kinetic constant of UDG. The limit of detection of UDG was 0.033 U/ml. The KM and Kcat were 0.11 microM and 4 s(-1), respectively. In addition, the method was applied to investigate the influence of chemical drugs on UDG activity. The results showed that 10 mM fluorouracil (5-FU) and gentamicin are inhibitors to UDG. The in vitro detection of UDG in A549 cells showed that the activity of UDG was four times greater after the cells were treated with cisplatin. These results showed that this method can monitor uracil removal in real time and conveniently assay UDG activity with ultrasensitivity and excellent specificity in the homogeneous solution. This method is also amenable to high-throughput drug screening in vitro.

Discovery of activation-induced cytidine deaminase, the engraver of antibody memory

Discovery of activation-induced cytidine deaminase (AID) paved a new path to unite two genetic alterations induced by antigen stimulation; class switch recombination (CSR) and somatic hypermutation (SHM). AID is now established to cleave specific target DNA and to serve as engraver of these genetic alterations. AID of a 198-residue protein has four important domains: nuclear localization signal and SHM-specific region at the N-terminus; the alpha-helical segment (residue 47-54) responsible for dimerization; catalytic domain (residues 56-94) shared by all the other cytidine deaminase family members; and nuclear export signal overlapping with class switch-specific domain at the C-terminus. Two alternative models have been proposed for the mode of AID action; whether AID directly attacks DNA or indirectly through RNA editing. Lines of evidence supporting RNA editing hypothesis include homology in various aspects with APOBEC1, a bona fide RNA editing enzyme as well as requirement of de novo protein synthesis for DNA cleavage by AID in CSR and SHM. This chapter critically evaluates DNA deamination hypothesis and describes evidence to indicate UNG is involved not in DNA cleavage but in DNA repair of CSR. In addition, UNG appears to have a noncanonical function through interaction with an HIV Vpr-like protein at the WXXF motif. Taken together, RNA editing hypothesis is gaining the ground.