Investigation of G-quadruplex formation in the FGFR2 promoter region and its transcriptional regulation by liensinine

G-quadruplexes in cancer-related gene promoters: from identification to therapeutic targeting

INTRODUCTION

Guanine-rich DNA sequences can fold into four-stranded noncanonical secondary structures called G-quadruplexes (G4s) which are widely distributed in functional regions of the human genome, such as telomeres and gene promoter regions. Compelling evidence suggests their involvement in key genome functions such as gene expression and genome stability. Notably, the abundance of G4-forming sequences near transcription start sites suggests their potential involvement in regulating oncogenes.

AREAS COVERED

This review provides an overview of current knowledge on G4s in human oncogene promoters. The most representative G4-binding ligands have also been documented. The objective of this work is to present a comprehensive overview of the most promising targets for the development of novel and highly specific anticancer drugs capable of selectively impacting the expression of individual or a limited number of genes.

EXPERT OPINION

Modulation of G4 formation by specific ligands has been proposed as a powerful new tool to treat cancer through the control of oncogene expression. Actually, most of G4-binding small molecules seem to simultaneously target a range of gene promoter G4s, potentially influencing several critical driver genes in cancer, thus producing significant therapeutic benefits.

The potential interplay between G-quadruplex and p53: their roles in regulation of ferroptosis in cancer

Ferroptosis is a novel form of regulated cell death trigged by various biological processes, and p53 is involved in different ferroptosis regulations and functions as a crucial regulator. Both DNA and RNA can fold into G-quadruplex in GC-rich regions and increasing shreds of evidence demonstrate that G-quadruplexes have been associated with some important cellular events. Investigation of G-quadruplexes is thus vital to revealing their biological functions. Specific G-quadruplexes are investigated to discover new effective anticancer drugs. Multiple modulations have been discovered between the secondary structure G-quadruplex and p53, probably further influencing the ferroptosis in cancer. G-quadruplex binds to ferric iron-related structures directly and may affect the p53 pathways as well as ferroptosis in cancer. In addition, G-quadruplex also interacts with p53 indirectly, including iron-sulfur cluster metabolism, telomere homeostasis, lipid peroxidation, and glycolysis. In this review, we summarized the latent interplay between G-quadruplex and p53 which focused mainly on ferroptosis in cancer to provide the potential understanding and encourage future studies.

Long promoter sequences form higher-order G-quadruplexes: an integrative structural biology study of c-Myc, k-Ras and c-Kit promoter sequences

We report on higher-order G-quadruplex structures adopted by long promoter sequences obtained by an iterative integrated structural biology approach. Our approach uses quantitative biophysical tools (analytical ultracentrifugation, small-angle X-ray scattering, and circular dichroism spectroscopy) combined with modeling and molecular dynamics simulations, to derive self-consistent structural models. The formal resolution of our approach is 18 angstroms, but in some cases structural features of only a few nucleotides can be discerned. We report here five structures of long (34-70 nt) wild-type sequences selected from three cancer-related promoters: c-Myc, c-Kit and k-Ras. Each sequence studied has a unique structure. Three sequences form structures with two contiguous, stacked, G-quadruplex units. One longer sequence from c-Myc forms a structure with three contiguous stacked quadruplexes. A longer c-Kit sequence forms a quadruplex-hairpin structure. Each structure exhibits interfacial regions between stacked quadruplexes or novel loop geometries that are possible druggable targets. We also report methodological advances in our integrated structural biology approach, which now includes quantitative CD for counting stacked G-tetrads, DNaseI cleavage for hairpin detection and SAXS model refinement. Our results suggest that higher-order quadruplex assemblies may be a common feature within the genome, rather than simple single quadruplex structures.

Quadruplex Ligands in Cancer Therapy

Nucleic acids can adopt alternative secondary conformations including four-stranded structures known as quadruplexes. To date, quadruplexes have been demonstrated to exist both in human chromatin DNA and RNA. In particular, quadruplexes are found in guanine-rich sequences constituting G-quadruplexes, and in cytosine-rich sequences forming i-Motifs as a counterpart. Quadruplexes are associated with key biological processes ranging from transcription and translation of several oncogenes and tumor suppressors to telomeres maintenance and genome instability. In this context, quadruplexes have prompted investigations on their possible role in cancer biology and the evaluation of small-molecule ligands as potential therapeutic agents. This review aims to provide an updated close-up view of the literature on quadruplex ligands in cancer therapy, by grouping together ligands for DNA and RNA G-quadruplexes and DNA i-Motifs.

An Updated Focus on Quadruplex Structures as Potential Therapeutic Targets in Cancer

Non-canonical, four-stranded nucleic acids secondary structures are present within regulatory regions in the human genome and transcriptome. To date, these quadruplex structures include both DNA and RNA G-quadruplexes, formed in guanine-rich sequences, and i-Motifs, found in cytosine-rich sequences, as their counterparts. Quadruplexes have been extensively associated with cancer, playing an important role in telomere maintenance and control of genetic expression of several oncogenes and tumor suppressors. Therefore, quadruplex structures are considered attractive molecular targets for cancer therapeutics with novel mechanisms of action. In this review, we provide a general overview about recent research on the implications of quadruplex structures in cancer, firstly gathering together DNA G-quadruplexes, RNA G-quadruplexes as well as DNA i-Motifs.

Liensinine inhibited gastric cancer cell growth through ROS generation and the PI3K/AKT pathway

Liensinine, an isoquinoline alkaloid extracted from the seed embryo of Nelumbo nucifera Gaertn, has been shown to exhibit various phrenological effects, including anti‑cancer activity. The aim of this study is to investigate the effects and mechanisms of liensinine in human gastric cancer cells. In this study, we found liensinine can significantly inhibit gastric cancer cell proliferation in vitro and in vivo. Liensinine inducedgastric cancer cell apoptosis by increasing cleaved PARP, caspased 3 and caspased 9. Moreover, liensinine induced cycle arrest by downregulatingcyclinD1/cyclin‑dependent kinase4 and phosphorylated protein kinase B. Furthermore, we found liensinine increases ROS levels and inhibits the PI3K/AKT pathway. These data suggested that liensinine might represent a novel and effective agent against gastric cancer.

Small-Molecule-Targeting Hairpin Loop of hTERT Promoter G-Quadruplex Induces Cancer Cell Death

Increased telomerase activity is associated with malignancy and poor prognosis in human cancer, but the development of targeted agents has not yet provided clinical benefit. Here we report that, instead of targeting the telomerase enzyme directly, small molecules that bind to the G-hairpin of the hTERT G-quadruplex-forming sequence kill selectively malignant cells without altering the function of normal cells. RG260 targets the hTERT G-quadruplex stem-loop folding but not tetrad DNAs, leading to downregulation of hTERT expression. To improve physicochemical and pharmacokinetic properties, we derived a small-molecule analog, RG1603, from the parent compound. RG1603 induces mitochondrial defects including PGC1α and NRF2 inhibition and increases oxidative stress, followed by DNA damage and apoptosis. RG1603 injected as a single agent has tolerable toxicity while achieving strong anticancer efficacy in a tumor xenograft mouse model. These results demonstrate a unique approach to inhibiting the hTERT that functions by impairing mitochondrial activity, inducing cell death.

Liensinine perchlorate inhibits colorectal cancer tumorigenesis by inducing mitochondrial dysfunction and apoptosis

SCOPE

Colorectal cancer (CRC) is one of the most common cancers worldwide with poor survival and limited therapeutic options, and there is an urgent need to develop novel therapeutic agents with good treatment efficiency and low toxicity. This study aims to examine the anticancer bioactivity of liensinine, a constituent of Nelumbo nucifera Gaertn, in CRC and investigate the action mechanisms involved.

METHODS AND RESULTS

Liensinine was found to induce apoptosis and exert a significant inhibitory effect on the proliferation and colony-forming ability of CRC cells in a dose-dependent manner without any observed cytotoxicity on normal colorectal epithelial cells. Mechanistically, our data from quantitative proteomics, western blot analysis and flow cytometry analyses demonstrated that exposure of CRC cells to liensinine caused cell cycle arrest, mitochondrial dysfunction and apoptosis, accompanied by the activation of the JNK signaling pathway. Furthermore, animal experiments showed that liensinine markedly suppressed the growth of CRC tumor xenografts in nude mice by reducing the Ki-67 proliferation index, but did not damage the vital organs of the animals.

CONCLUSION

This study demonstrated for the first time that liensinine, a food-source natural product, could be a novel therapeutic strategy for treating CRC without obvious side effects.

Recent Progress of Targeted G-Quadruplex-Preferred Ligands Toward Cancer Therapy

A G-quadruplex (G4) is a well-known nucleic acid secondary structure comprising guanine-rich sequences, and has profound implications for various pharmacological and biological events, including cancers. Therefore, ligands interacting with G4s have attracted great attention as potential anticancer therapies or in molecular probe applications. To date, a large variety of DNA/RNA G4 ligands have been developed by a number of laboratories. As protein-targeting drugs face similar situations, G-quadruplex-interacting drugs displayed low selectivity to the targeted G-quadruplex structure. This low selectivity could cause unexpected effects that are usually reasons to halt the drug development process. In this review, we address the recent research on synthetic G4 DNA-interacting ligands that allow targeting of selected G4s as an approach toward the discovery of highly effective anticancer drugs.

Identification and characterization of G-quadruplex formation within the EP0 promoter of pseudorabies virus

EP0 is an important early gene that modulates the life cycle of pseudorabies virus (PRV). A guanine-rich sequence overlapping with three Sp1 binding sites is located upstream of the transcription start site (TSS) in the EP0 promoter. Using native polyacrylamide gel electrophoresis (PAGE) and circular dichroism (CD), we verified that the G-rich region in the EP0 promoter forms an intramolecular parallel G-quadruplex (G4) in the presence of K⁺ ions. Further dimethyl sulphate (DMS) footprinting and Taq polymerase stop assays indicates the potential polymorphic folding of G4. In addition, a small chemical ligand, pyridostatin (PDS), promotes and stabilizes the formation of G4. Interestingly, based on the results of electrophoretic mobility shift assays (EMSA), the Sp1 protein bound to G4-bearing DNA with more affinity than DNA lacking the G4 structure. According to the luciferase reporter assay, G4 negatively regulates the EP0 promoter activity. These results demonstrate that Sp1 and G4 cooperate to regulate EP0 promoter activity.