Quantitative Detection of G-Quadruplex DNA in Live Cells Based on Photon Counts and Complex Structure Discrimination

G-quadruplex DNA show structural polymorphism, leading to challenges in the use of selective recognition probes for the accurate detection of G-quadruplexes in vivo. Herein, we present a tripodal cationic fluorescent probe, NBTE , which showed distinguishable fluorescence lifetime responses between G-quadruplexes and other DNA topologies, and fluorescence quantum yield (Φ_f) enhancement upon G-quadruplex binding. We determined two NBTE -G-quadruplex complex structures with high Φ_f values by NMR spectroscopy. The structures indicated NBTE interacted with G-quadruplexes using three arms through π–π stacking, differing from that with duplex DNA using two arms, which rationalized the higher Φ_f values and lifetime response of NBTE upon G-quadruplex binding. Based on photon counts of FLIM, we detected the percentage of G-quadruplex DNA in live cells with NBTE and found G-quadruplex DNA content in cancer cells is 4-fold that in normal cells, suggesting the potential applications of this probe in cancer cell detection.     

Solution NMR Structure of a Ligand/Hybrid-2-G-Quadruplex Complex Reveals Rearrangements that Affect Ligand Binding

Telomeric G-quadruplexes have recently emerged as drug targets in cancer research. Herein, we present the first NMR structure of a telomeric DNA G-quadruplex that adopts the biologically relevant hybrid-2 conformation in a ligand-bound state. We solved the complex with a metalorganic gold(III) ligand that stabilizes G-quadruplexes. Analysis of the free and bound structures reveals structural changes in the capping region of the G-quadruplex. The ligand is sandwiched between one terminal G-tetrad and a flanking nucleotide. This complex structure involves a major structural rearrangement compared to the free G-quadruplex structure as observed for other G-quadruplexes in different conformations, invalidating simple docking approaches to ligand–G-quadruplex structure determination.     

DNA G-四链体识别探针研究进展

G-四链体是一种由富含鸟嘌呤核酸序列形成的独特的二级结构,广泛分布于真核生物基因组,如端粒DNA、r DNA和一系列基因中的启动子区域。G-四链体结构对很多重要的生理过程如基因的转录、复制、重组以及保持染色体的稳定性方面具有重要作用。G-四链体的特异、高灵敏检测将为进一步了解G-四链体结构在人类细胞基因组中的分布、功能和机制奠定基础,也可能为靶向G-四链体的肿瘤治疗方法提供新的思路。因而过去几十年人们一直致力于开发设计具有高选择性和高灵敏度的G-四链体识别探针,这些探针已经广泛应用于溶液中G-四链体的识别,而且具有良好的选择性。目前也有少数探针能够直接用于检测活体G-四链体结构。本文综述了一些常见的靶向G-四链体的小分子配体,以及它们在染色体和活体细胞G-四链体检测中的应用。笔者希冀本文能为设计识别G-四链体的高性能探针,进一步实现活细胞内G-四链体的检测提供借鉴。     

G-quadruplex DNA and ligand interaction in living cells using NMR spectroscopy

Gathering structural information from biologically relevant molecules inside living cells has always been a challenging task. In this work, we have used multidimensional NMR spectroscopy to probe DNA G-quadruplexes inside living Xenopus laevis oocytes. Some of these structures can be found in key regions of chromosomes. G-quadruplexes are considered potential anticancer therapeutic targets and several lines of evidence indirectly point out roles in key biological processes, such as cell proliferation, genomic instability or replication initiation. However, direct demonstrations of the existence of G-quadruplexes in vivo are scarce. Using SOFAST-HMQC type spectra, we probed a tetramolecular G-quadruplex model made of d(TG₄T)₄ inside living Xenopus laevis oocytes. Our observations lead us to conclude that the quadruplex structure is formed within the cell and that the intracellular environment preferentially selects a conformation that most resembles the one found in vitro under KCl conditions. We also show for the first time that specific ligands targeting G-quadruplexes can be studied using high resolution NMR directly inside living cells, opening new avenues to study ligand binding discrimination under physiologically relevant conditions with atomic detail.     

One Terminal Guanosine Flip of Intramolecular Parallel G-Quadruplex: Catalytic Enhancement of G-Quadruplex/Hemin DNAzymes

Numerous studies have shown compelling evidence that incorporation of an inversion of polarity site (IPS) in G-rich sequences can affect the topological and structural characteristics of G-quadruplexes (G4s). Herein, the influence of IPS on the formation of a previously studied intramolecular parallel G4 of d(G₃TG₃TG₃TG₃) (TTT) and its stacked higher-order structures is explored. Insertion of 3′–3′ or 5′–5′ IPS did not change the parallel folding pattern of TTT. However, both the species and position of the IPS in TTT have a significant impact on the G4 stability and end-stacking through the alteration of G4–G4 interfaces properties. The data demonstrate that one base flip in each terminal G-tetrad can stabilize parallel G4s and facilitate intermolecular packing of monomeric G4s. Such modifications can also enhance the fluorescence and enzymatic performances by promoting interactions between parallel G4s with N-methyl mesoporphyrin IX (NMM) and hemin, respectively.     

Oxazine750对平行四股螺旋结构的识别与稳定作用

以d[TG₄T]₄、d[G₄T₄G₄]₂和c-myc序列为代表,采用竞争平衡透析、紫外光谱和圆二色光谱研究了oxazine750对四股螺旋结构的识别与稳定作用。 结果表明,oxazine750能够更好的结合在平行四股螺旋上,而对反平行四股螺旋结合较少。 热变性实验表明,oxazine750使d[TG₄T]₄形成的平行结构的四股螺旋的解旋温度提高了11 ℃,对c-myc形成的平行四股螺旋的解旋温度甚至提高了40 ℃。 而对d[G₄T₄G₄]₂形成的反平行结构解旋温度降低了近8 ℃。     

Selective interactions of cationic porphyrins with G-quadruplex structures

G-quadruplex DNA presents a potential target for the design and development of novel anticancer drugs. Because G-quadruplex DNA exhibits structural polymorphism, different G-quadruplex typologies may be associated with different cellular processes. Therefore, to achieve therapeutic selectivity using G-quadruplexes as targets for drug design, it will be necessary to differentiate between different types of G-quadruplexes using G-quadruplex-interactive agents. In this study, we compare the interactions of three cationic porphyrins, TMPyP2, TMPyP3, and TMPyP4, with parallel and antiparallel types of G-quadruplexes using gel mobility shift experiments and a helicase assay. Gel mobility shift experiments indicate that TMPyP3 specifically promotes the formation of parallel G-quadruplex structures. A G-quadruplex helicase unwinding assay reveals that the three porphyrins vary dramatically in their abilities to prevent the unwinding of both the parallel tetrameric G-quadruplex and the antiparallel hairpin dimer G-quadruplex DNA by yeast Sgs1 helicase (Sgs1p). For the parallel G-quadruplex, TMPyP3 has the strongest inhibitory effect on Sgs1p, followed by TMPyP4, but the reverse is true for the antiparallel G-quadruplex. TMPyP2 does not appear to have any effect on the helicase-catalyzed unwinding of either type of G-quadruplex. Photocleavage experiments were carried out to investigate the binding modes of all three porphyrins with parallel G-quadruplexes. The results reveal that TMPyP3 and TMPyP4 appear to bind to parallel G-quadruplex structures through external stacking at the ends rather than through intercalation between the G-tetrads. Since intercalation between G-tetrads has been previously proposed as an alternative binding mode for TMPyP4 to G-quadruplexes, this mode of binding, versus that determined by a photocleavage assay described here (external stacking), was subjected to molecular dynamics calculations to identify the relative stabilities of the complexes and the factors that contribute to these differences. The DeltaG(o) for the external binding mode was found to be driven by DeltaH(o) with a small unfavorable TDeltaS(o) term. The DeltaG(o) for the intercalation binding model was driven by a large TDeltaS(o) term and complemented by a small DeltaH(o) term. One of the main stabilizing components of the external binding model is the energy of solvation, which favors the external model over the intercalation model by -67.94 kcal/mol. Finally, we propose that intercalative binding, although less favored than external binding, may occur, but because of the nature of the intercalative binding, it is invisible to the photocleavage assay. This study provides the first experimental insight into how selectivity might be achieved for different G-quadruplexes by using structural variants within a single group of G-quadruplex-interactive drugs.     

Sequence-specific detection of nucleic acids utilizing isothermal enrichment of G-quadruplex DNAzymes

G-quadruplex DNAzymes are peroxidase-like complexes formed by nucleic acid G-quadruplexes and hemin. Various chemical sensors and biosensors have been developed, based on such DNAzymes. Here we report a novel, specific nucleic acid detection method utilizing the isothermal amplification strategy of G-quadruplex DNAzymes. In this method, an unlabeled oligonucleotide probe was used. The probing sequence of the oligonucleotide was in the form of a stem-loop structure. A G-rich sequence, containing three GGG repeats, was linked to the 5′-end of the stem-loop structure. In the presence of target, the probing sequence hybridized to the target, and a G_n (n ≥ 2) repeat was extended from its 3′-end. This G_n repeat, together with the three GGG repeats at the 5′-end, folded into a G-quadruplex, and displayed enhanced peroxidase acitivity upon hemin binding. Utilizing the dynamic binding interaction between the probe and its target, the enrichment of G-quadruplex DNAzymes was achieved. Using this method, simple, rapid and cost-effective nucleic acid detection could be achieved. This method displayed high target-length tolerance and good detection specificity; one-base mismatch could be judged easily, even by visual inspection. This method may be used as an auxiliary tool for amplified detection of specific DNA targets in some situations, in which isothermal detection is desirable.     

Theoretical Encapsulation of Fluorouracil (5-FU) Anti-Cancer Chemotherapy Drug into Carbon Nanotubes (CNT) and Boron Nitride Nanotubes (BNNT)

Introduction: Chemotherapy with anti-cancer drugs is considered the most common approach for killing cancer cells in the human body. However, some barriers such as toxicity and side effects would limit its usage. In this regard, nano-based drug delivery systems have emerged as cost-effective and efficient for sustained and targeted drug delivery. Nanotubes such as carbon nanotubes (CNT) and boron nitride nanotubes (BNNT) are promising nanocarriers that provide the cargo with a large inner volume for encapsulation. However, understanding the insertion process of the anti-cancer drugs into the nanotubes and demonstrating drug-nanotube interactions starts with theoretical analysis. Methods: First, interactions parameters of the atoms of 5-FU were quantified from the DREIDING force field. Second, the storage capacity of BNNT (8,8) was simulated to count the number of drugs 5-FU encapsulated inside the cavity of the nanotubes. In terms of the encapsulation process of the one drug 5-FU into nanotubes, it was clarified that the drug 5-FU was more rapidly adsorbed into the cavity of the BNNT compared with the CNT due to the higher van der Waals (vdW) interaction energy between the drug and the BNNT. Results: The obtained values of free energy confirmed that the encapsulation process of the drug inside the CNT and BNNT occurred spontaneously with the free energies of −14 and −25 kcal·mol⁻¹, respectively. Discussion: However, the lower value of the free energy in the system containing the BNNT unraveled more stability of the encapsulated drug inside the cavity of the BNNT comparing the system having CNT. The encapsulation of Fluorouracil (5-FU) anti-cancer chemotherapy drug (commercial name: Adrucil^®) into CNT (8,8) and BNNT (8,8) with the length of 20 Å in an aqueous solution was discussed herein applying molecular dynamics (MD) simulation.     

Design,Synthesis, Molecular Modeling and Antitumor Evaluation of Novel Indolyl-Pyrimidine Derivatives with EGFR Inhibitory Activity

Scaffolds hybridization is a well-known drug design strategy for antitumor agents. Herein, series of novel indolyl-pyrimidine hybrids were synthesized and evaluated in vitro and in vivo for their antitumor activity. The in vitro antiproliferative activity of all compounds was obtained against MCF-7, HepG2, and HCT-116 cancer cell lines, as well as against WI38 normal cells using the resazurin assay. Compounds 1–4 showed broad spectrum cytotoxic activity against all these cancer cell lines compared to normal cells. Compound 4g showed potent antiproliferative activity against these cell lines (IC₅₀ = 5.1, 5.02, and 6.6 μM, respectively) comparable to the standard treatment (5-FU and erlotinib). In addition, the most promising group of compounds was further evaluated for their in vivo antitumor efficacy against EAC tumor bearing mice. Notably, compound 4g showed the most potent in vivo antitumor activity. The most active compounds were evaluated for their EGFR inhibitory (range 53–79%) activity. Compound 4g was found to be the most active compound against EGFR (IC₅₀ = 0.25 µM) showing equipotency as the reference treatment (erlotinib). Molecular modeling study was performed on compound 4g revealed a proper binding of this compound inside the EGFR active site comparable to erlotinib. The data suggest that compound 4g could be used as a potential anticancer agent.     

Anti-Cancer Effects of Zotarolimus Combined with 5-Fluorouracil Treatment in HCT-116 Colorectal Cancer-Bearing BALB/c Nude Mice

Zotarolimus is a semi-synthetic derivative of rapamycin and an inhibitor of mammalian target of rapamycin (mTOR) signaling. Currently, zotarolimus is used to prolong the survival time of organ grafts, but it is also a novel immunosuppressive agent with potent anti-proliferative activity. Here, we examine the anti-tumor effect of zotarolimus, alone and in combination with 5-fluorouracil, on HCT-116 colorectal adenocarcinoma cells implanted in BALB/c nude mice. Compared with the control mice, mice treated with zotarolimus or zotarolimus combined with 5-FU showed retarded tumor growth; increased tumor apoptosis through the enhanced expression of cleaved caspase 3 and extracellular signal-regulated kinase (ERK) phosphorylation; reduced inflammation-related factors such as IL-1β, TNF-α, and cyclooxygenase-2 (COX-2) protein; and inhibited metastasis-related factors such as CD44, epidermal growth factor receptor (EGFR), transforming growth factor β (TGF-β), and vascular endothelial growth factor (VEGF). Notably, mice treated with a combination of zotarolimus and 5-FU showed significantly retarded tumor growth, reduced tumor size, and increased tumor inhibition compared with mice treated with 5-FU or zotarolimus alone, indicating a strong synergistic effect. This in vivo study confirms that zotarolimus or zotarolimus combined with 5-FU can be used to retard colorectal adenocarcinoma growth and inhibit tumorigenesis. Our results suggest that zotarolimus may increase the chemo-sensitization of tumor cells. Therefore, zotarolimus alone and zotarolimus combined with 5-FU may be potential anti-tumor agents in the treatment of human colon adenocarcinoma. Future research on zotarolimus may lead to the development of new therapeutic strategies.     

The natural product prodigiosin binds G-quadruplex DNA

Prodigiosin, a metabolite isolated from Serratia bacteria, is a potent anti-cancer agent. The goal of this research was to determine if prodigiosin could bind to G-quadruplex DNA. We found by using UV–vis spectroscopy that the G-quadruplex K⁺√d[TG₄T]₄, but not the single-stranded Li⁺√d[TG₄T], increased the water solubility of prodigiosin. In addition, we found that prodigiosin's fluorescence was strongly enhanced in the presence of K⁺√d[TG₄T]₄, further evidence for quadruplex-ligand binding. Using ¹H and ³¹P NMR experiments, we confirmed that prodigiosin binds at the 3′-end of K⁺√d[TG₄T]₄. Saturation transfer difference NMR experiments, done by irradiating the G5 H8 signal in K⁺√d[TG₄T]₄, indicated specific interactions between the G-quadruplex and prodigiosin's A and B rings. This study raises the possibility that G-quadruplex DNA may well be a target for the anti-cancer agent prodigiosin.     

Newly Synthesized Thymol Derivative and Its Effect on Colorectal Cancer Cells

Thymol affects various types of tumor cell lines, including colorectal cancer cells. However, the hydrophobic properties of thymol prevent its wider use. Therefore, new derivatives (acetic acid thymol ester, thymol β-D-glucoside) have been synthesized with respect to hydrophilic properties. The cytotoxic effect of the new derivatives on the colorectal cancer cell lines HT-29 and HCT-116 was assessed via MTT assay. The genotoxic effect was determined by comet assay and micronucleus analysis. ROS production was evaluated using ROS-Glo™ H₂O₂ Assay. We confirmed that one of the thymol derivatives (acetic acid thymol ester) has the potential to have a cyto/genotoxic effect on colorectal cancer cells, even at much lower (IC₅₀~0.08 μg/mL) concentrations than standard thymol (IC₅₀~60 μg/mL) after 24 h of treatment. On the other side, the genotoxic effect of the second studied derivative—thymol β-D-glucoside was observed at a concentration of about 1000 μg/mL. The antiproliferative effect of studied derivatives of thymol on the colorectal cancer cell lines was found to be both dose- and time-dependent at 100 h. Moreover, thymol derivative-treated cells did not show any significantly increased rate of micronuclei formation. New derivatives of thymol significantly increased ROS production too. The results confirmed that the effect of the derivative on tumor cells depends on its chemical structure, but further detailed research is needed. However, thymol and its derivatives have great potential in the prevention and treatment of colorectal cancer, which remains one of the most common cancers in the world.     

A Bioactive Compound from Sanguisorba officinalis L. Inhibits Cell Proliferation and Induces Cell Death in 5-Fluorouracil-Sensitive/Resistant Colorectal Cancer Cells

Colorectal cancer (CRC) is one of the most common cancer in the world. The first line chemotherapeutic agent, 5-fluorouracil (5-FU), plays a predominant role in the clinical treatment of CRC. However, with the wide use of 5-FU, more and more CRC patients have been obtaining drug resistance to 5-FU, which leads to a large amount of treatment failures. One of the effective strategies to overcome this obstacle is to find bioactive natural products from traditional medicine. In our previous work, Sanguisorba officinalis L. was found to exert a strong anti-proliferative activity against 5-FU-senstive/resistant CRC cells. Therefore, several compounds were isolated from this herb and screened for their anti-CRC effects to find promising compounds. Among them, a triterpenoid compound named 3β-[(α-l-arabinopyranosyl) oxy]-urs-12,18(19)-dien-28-oic acid β-d-glucopyranosyl ester (AGE), showed strong activity against both 5-FU-senstive and resistant CRC cells. In order to further study the mechanism of AGE on CRC cells, flow cytometer analysis, mitochondrial membrane potential (MMP) measurement, Western blotting, and RT-PCR assays were performed. Results demonstrated that AGE induced cell death by apoptosis pathway and autophagy, and inhibited cell proliferation via cell cycle arrest in G0-G1 phase mediated by Wnt signaling pathway. Therefore, AGE may be a potential bioactive compound for CRC treatment in clinic.     

Recognize three different human telomeric G-quadruplex conformations by quinacrine

Recognition of different human telomeric G-quadruplex structures has been a very important task for developing anti-cancer drug design. However, it also is a very challenging question since multiple conformational isomers of telomeric G-quadruplexes coexist under some conditions. Here, three different conformations including parallel, antiparallel, and mixed-type telomeric G-quadruplex structures have been well recognized by quinacrine (QNA) through monitoring its absorption, fluorescence, and fluorescence lifetime spectra. The multiple structures of H22 G-quadruplexes under physiological K(+) conditions could also be easily determined to coexist as mixed-type and antiparallel G-quadruplexes. The recognition mechanism based on the different binding affinity and binding sites has been further elucidated by association with the nuclear magnetic resonance (NMR) results.     

Specific Binding of Anionic Porphyrin and Phthalocyanine to the G-Quadruplex with a Variety of <em>i</em><em>n Vitro</em> and<em> </em><em>i</em><em>n Vivo</em> Applications

The G-quadruplex, a four-stranded DNA structure with stacked guanine tetrads (G-quartets), has recently been attracting attention because of its critical roles in vitro and in vivo. In particular, the G-quadruplex functions as ligands for metal ions and aptamers for various molecules. Interestingly, the G-quadruplex can show peroxidase-like activity with an anionic porphyrin, iron (III) protoporphyrin IX (hemin). Importantly, hemin binds to G-quadruplexes with high selectivity over single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA), which is attributable to an electrostatic repulsion of phosphate groups in ssDNA and dsDNA. The G-quadruplex and hemin-G-quadruplex complex allow development of sensing techniques to detect DNA, metal ions and proteins. In addition to hemin, anionic phthalocyanines also bind to the G-quadruplex formed by human telomere DNA, specifically over ssDNA and dsDNA. Since the binding of anionic phthalocyanines to the G-quadruplex causes an inhibition of telomerase activity, which plays a role in the immortal growth of cancer cells, anionic phthalocyanines are promising as novel anticancer drug candidates. This review focuses on the specific binding of hemin and anionic phthalocyanines to G-quadruplexes and the applications in vitro and in vivo of this binding property.     

5-Phenoxy Primaquine Analogs and the Tetraoxane Hybrid as Antimalarial Agents

The rapid emergence of drug resistance to the current antimalarial agents has led to the urgent need for the discovery of new and effective compounds. In this work, a series of 5-phenoxy primaquine analogs with 8-aminoquinoline core (7a–7h) was synthesized and investigated for their antimalarial activity against Plasmodium falciparum. Most analogs showed improved blood antimalarial activity compared to the original primaquine. To further explore a drug hybrid strategy, a conjugate compound between tetraoxane and the representative 5-phenoxy-primaquine analog 7a was synthesized. In our work, the hybrid compound 12 exhibited almost a 30-fold increase in the blood antimalarial activity (IC₅₀ = 0.38 ± 0.11 μM) compared to that of primaquine, with relatively low toxicity against mammalian cells (SI = 45.61). Furthermore, we found that these 5-phenoxy primaquine analogs and the hybrid exhibit significant heme polymerization inhibition, an activity similar to that of chloroquine, which could contribute to their improved antimalarial activity. The 5-phenoxy primaquine analogs and the tetraoxane hybrid could serve as promising candidates for the further development of antimalarial agents.     

Use of Half-Generation PAMAM Dendrimers (G0.5–G3.5) with Carboxylate End-Groups to Improve the DACHPtCl2 and 5-FU Efficacy as Anticancer Drugs

The DACHPtCl₂ compound (trans-(R,R)-1,2-diaminocyclohexanedichloroplatinum(II)) is a potent anticancer drug with a broad spectrum of activity and is less toxic than oxaliplatin (trans-l-diaminocyclohexane oxalate platinum II), with which it shares the active metal fragment DACHPt. Nevertheless, due to poor water solubility, its use as a chemotherapeutic drug is limited. Here, DACHPtCl₂ was conjugated, in a bidentate form, with half-generation PAMAM dendrimers (G0.5–G3.5) with carboxylate end-groups, and the resulting conjugates were evaluated against various types of cancer cell lines. In this way, we aimed at increasing the solubility and availability at the target site of DACHPt while potentially reducing the adverse side effects. DNA binding assays showed a hyperchromic effect compatible with DNA helix’s disruption upon the interaction of the metallodendrimers and/or the released active metallic fragments with DNA. Furthermore, the prepared DACHPt metallodendrimers presented cytotoxicity in a wide set of cancer cell lines used (the relative potency regarding oxaliplatin was in general high) and were not hemotoxic. Importantly, their selectivity for A2780 and CACO-2 cancer cells with respect to non-cancer cells was particularly high. Subsequently, the anticancer drug 5-FU was loaded in a selected metallodendrimer (the G2.5COO(DACHPt)₁₆) to investigate a possible synergistic effect between the two drugs carried by the same dendrimer scaffold and tested for cytotoxicity in A2780cisR and CACO-2 cancer cell lines. This combination resulted in IC₅₀ values much lower than the IC₅₀ for 5-FU but higher than those found for the metallodendrimers without 5-FU. It seems, thus, that the metallic fragment-induced cytotoxicity dominates over the cytotoxicity of 5-FU in the set of considered cell lines.