Identification of G-Quadruplex-Binding Protein from the Exploration of RGG Motif/G-Quadruplex Interactions.

The arginine/glycine-rich region termed the RGG domain is usually found in G-quadruplex (G4)-binding proteins and is important in G4-protein interactions. Studies on the binding mechanism of RGG domains found that small segments (RGG motif) inside the domain contribute greatly to the G4 binding affinity. However, unlike the entire RGG domains that have been broadly explored, the role of the RGG motif remains obscure, with very limited study. Herein, to clarify the role of the RGG motif in G4-protein interactions, we systematically investigated the binding affinity and mode between RGG-motif peptides and G4s. The internal arrangement of RGG repeats and gap amino acids played a more crucial role in the G4-binding mechanism than a critical number of RGG repeats. Arginines and phenylalanines at the exact position of the RGG motif might enable additional hydrogen bonding and π-stacking interaction with nucleobases and strengthen the binding of G4. Impressively, proceeding from a G4-binding RGG peptide, 12, discovered above, we identified the cold-inducible RNA-binding protein (CIRBP) as a new G4 DNA-binding protein both in vitro and in cells. In addition, we found that the key amino acids for G4 binding in peptide 12 and CIRBP were highly similar, and peptide 12 clearly played a key role in the G4 binding of CIRBP. This report is the first in which a G4-binding protein was identified from exploration of the G4-binding RGG motif. Our findings suggest a novel strategy for discovering new G4-binding proteins by exploring key peptide segments.

Natural Alkaloids and Heterocycles as G-Quadruplex Ligands and Potential Anticancer Agents.

G-quadruplexes are four-stranded nucleic acid secondary structures that are formed in guanine-rich sequences. G-quadruplexes are widely distributed in functional regions of the human genome and transcriptome, such as human telomeres, oncogene promoter regions, replication initiation sites, and untranslated regions. Many G-quadruplex-forming sequences are found to be associated with cancer, and thus, these non-canonical nucleic acid structures are considered to be attractive molecular targets for cancer therapeutics with novel mechanisms of action. In this mini review, we summarize recent advances made by our lab in the study of G-quadruplex-targeted natural alkaloids and their derivatives toward the development of potential anticancer agents.

Design, Synthesis, and Evaluation of Isaindigotone Derivatives To Downregulate c-myc Transcription via Disrupting the Interaction of NM23-H2 with G-Quadruplex.

Transcriptional control of c-myc oncogene is an important strategy for antitumor drug design. G-quadruplexes in the promoter region have been proven to be the transcriptional down-regulator of this gene. The transcriptional factor NM23-H2 can reactivate c-myc transcription by unwinding the G-quadruplex structure. Thus, down-regulation of c-myc transcription via disrupting G-quadruplex-NM23-H2 interaction might be a potential approach for cancer therapy. Here, a series of new isaindigotone derivatives were designed and synthesized based on our previous study. The abilities of these derivatives on interacting with G-quadruplexes or NM23-H2, and disrupting G-quadruplex-NM23-H2 interaction were evaluated. Among these derivatives, 19d and 22d showed remarkable abilities on disrupting G-quadruplex-NM23-H2 interaction. They exhibited significant effects on c-myc-relating processes in SiHa cells, including inhibiting the transcription and translation, inhibiting cellular proliferation, inducing apoptosis, and regulating cell cycle. Our findings provided the basis for the anticancer strategy based on c-myc transcriptional regulation via small molecules disrupting G-quadruplex-protein interaction.

Design, Synthesis, and Evaluation of New Selective NM23-H2 Binders as c-MYC Transcription Inhibitors via Disruption of the NM23-H2/G-Quadruplex Interaction.

c-MYC is one of the important human proto-oncogenes, and transcriptional factor NM23-H2 can activate c-MYC transcription by recognizing the G-quadruplex in the promoter of the gene. Small molecules that inhibit c-MYC transcription by disrupting the NM23-H2/G-quadruplex interaction might be a promising strategy for developing selective anticancer agents. In recent studies, we developed a series of isaindigotone derivatives, which can bind to G-quadruplex and NM23-H2, thus down-regulating c-MYC ( J. Med. Chem. 2017 , 60 , 1292 - 1308 ). Herein, a series of novel isaindigotone derivatives were designed, synthesized, and screened for NM23-H2 selective binding ligands. Among them, compound 37 showed a high specific binding affinity to NM23-H2, effectively disrupting the interaction of NM23-H2 with G-quadruplex, and it strongly down-regulated c-MYC transcription. Furthermore, 37 induced cell cycle arrest and apoptosis, and it exhibited good tumor growth inhibition in a mouse xenograft model. This work provides a new strategy to modulate c-MYC transcription for the development of selective anticancer drugs.