BRACO19 analog dimers with improved inhibition of telomerase and hPot 1

Research progress of small-molecule drugs in targeting telomerase in human cancer and aging

Telomeres are unique structures located at the ends of linear chromosomes, responsible for stabilizing chromosomal structures. They are synthesized by telomerase, a reverse transcriptase ribonucleoprotein complex. Telomerase activity is generally absent in human somatic cells, except in stem cells and germ cells. Every time a cell divides, the telomere sequence is shortened, eventually leading to replicative senescence and cell apoptosis when the telomeres reach a critical limit. However, most human cancer cells exhibit increased telomerase activity, allowing them to divide continuously. The importance of telomerase in cancer and aging has made developing drugs targeting telomerase a focus of research. Such drugs can inhibit cancer cell growth and delay aging by enhancing telomerase activity in telomere-related syndromes or diseases. This review provides an overview of telomeres, telomerase, and their regulation in cancer and aging, and highlights small-molecule drugs targeting telomerase in these fields.

Telomere Targeting Approaches in Cancer: Beyond Length Maintenance

Telomeres are crucial structures that preserve genome stability. Their progressive erosion over numerous DNA duplications determines the senescence of cells and organisms. As telomere length homeostasis is critical for cancer development, nowadays, telomere maintenance mechanisms are established targets in cancer treatment. Besides telomere elongation, telomere dysfunction impinges on intracellular signaling pathways, in particular DNA damage signaling and repair, affecting cancer cell survival and proliferation. This review summarizes and discusses recent findings in anticancer drug development targeting different “telosome” components.

TERRA G-quadruplex RNA interaction with TRF2 GAR domain is required for telomere integrity

Telomere dysfunction causes chromosomal instability which is associated with many cancers and age-related diseases. The non-coding telomeric repeat-containing RNA (TERRA) forms a structural and regulatory component of the telomere that is implicated in telomere maintenance and chromosomal end protection. The basic N-terminal Gly/Arg-rich (GAR) domain of telomeric repeat-binding factor 2 (TRF2) can bind TERRA but the structural basis and significance of this interaction remains poorly understood. Here, we show that TRF2 GAR recognizes G-quadruplex features of TERRA. We show that small molecules that disrupt the TERRA-TRF2 GAR complex, such as N-methyl mesoporphyrin IX (NMM) or genetic deletion of TRF2 GAR domain, result in the loss of TERRA, and the induction of γH2AX-associated telomeric DNA damage associated with decreased telomere length, and increased telomere aberrations, including telomere fragility. Taken together, our data indicates that the G-quadruplex structure of TERRA is an important recognition element for TRF2 GAR domain and this interaction between TRF2 GAR and TERRA is essential to maintain telomere stability.

Synthesis and evaluation of 7-substituted-5,6-dihydrobenzo[c]acridine derivatives as new c-KIT promoter G-quadruplex binding ligands

It has been shown that treatment of cancer cells with c-KIT G-quadruplex binding ligands can reduce their c-KIT expression levels thus inhibiting cell proliferation and inducing cell apoptosis. Herein, a series of new 7-substituted-5,6-dihydrobenzo[c]acridine derivatives were designed and synthesized. Subsequent biophysical evaluation demonstrated that the derivatives could effectively bind to and stabilize c-KIT G-quadruplex with good selectivity against duplex DNA. It was found that 12-N-methylated derivatives with a positive charge introduced at 12-position of 5,6-dihydrobenzo[c]acridine ring had similar binding affinity but lower stabilizing ability to c-KIT G-quadruplex DNA, compared with those of nonmethylated derivatives. Further molecular modeling studies showed possible binding modes of G-quadruplex with the ligands. RT-PCR assay and Western blot showed that compound 2b suppressed transcription and translation of c-KIT gene in K562 cells, which was consistent with the property of an effective G-quadruplex binding ligand targeting c-KIT oncogene promoter. Further biological evaluation showed that compound 2b could induce apoptosis through activation of the caspase-3 cascade pathway.

New dimeric carbazole-benzimidazole mixed ligands for the stabilization of human telomeric G-quadruplex DNA and as telomerase inhibitors. A remarkable influence of the spacer

The development of G-quadruplex (G4) DNA binding small molecules has become an important strategy for selectively targeting cancer cells. Herein, we report the design and evolution of a new kind of carbazole-based benzimidazole dimers for their efficient telomerase inhibition activity. Spectroscopic titrations reveal the ligands high affinity toward the G4 DNA with significantly higher selectivity over duplex-DNA. The electrophoretic mobility shift assay shows that the ligands efficiently promote the formation of G4 DNA even at a lower concentration of the stabilizing K(+) ions. The TRAP-LIG assay demonstrates the ligand's potential telomerase inhibition activity and also establishes that the activity proceeds via G4 DNA stabilization. An efficient nuclear internalization of the ligands in several common cancer cells (HeLa, HT1080, and A549) also enabled differentiation between normal HFF cells in co-cultures of cancer and normal ones. The ligands induce significant apoptotic response and antiproliferative activity toward cancer cells selectively when compared to the normal cells.

Mitochondrial dysfunction and nuclear-mitochondrial shuttling of TERT are involved in cell proliferation arrest induced by G-quadruplex ligands

G-quadruplex ligands DODC and TMPyP4 have different binding modes to quadruplex structure and cause cell proliferation arrest. Here we showed that DODC was more efficient in cell growth inhibition than TMPyP4. Both G-quadruplex ligands induced nuclear-cytoplasmic shuttling and accumulation of TERT in mitochondria. This effect was not fully dependent on cellular oxidative stress. DODC induced robust cell apoptosis by perturbing mitochondrial function intensively. Overexpression of TERT could not counteract the effects of DODC on mitochondrial respiratory function. Taken together, our results suggest that interference of mitochondrial function by DODC is one of main targets for its anti-tumor ability.

Identification of G-Quadruplex Inducers Usinga Simple, Inexpensiveand Rapid High Throughput Assay, and TheirInhibition of Human Telomerase

Telomeres are protein and DNA complexes located atchromosome ends. Telomeric DNA is composed of a double stranded region of repetitive DNA followed by single-stranded 3' extension of aG-rich sequence. Single-stranded G-rich sequencescan fold into G-quadruplex structures,and molecules that stabilize G-quadruplexes are known to inhibit the enzyme telomerase and disrupt telomere maintenance. Because telomere maintenance is required for proliferation of cancer cells, G-quadruplex stabilizers have become attractive prospects for anticancer drug discovery.However, telomere-targeting G-quadruplex ligands have yet to enter the clinic owing in part to poor pharmacokinetics and target selectivity. Increasing the pharmacophore diversity of G-quadruplex and specifically telomeric-DNA targeting agents should assist in overcoming these shortcomings. In this work, we report the identification and validation ofligands that bind telomeric DNA and induce G-quadruplex formationusing the NCI Diversity Set I, providing validation of anextremely simple, rapid and high-throughput screen using FRET technology. Hits from the screen were validated by examining telomerase inhibition and G-quadruplex inductionusing CD spectroscopy and DNA polymerase stop assays. We show that two known DNA binding molecules, ellipticine derivativeNSC 176327 (apyridocarbazole) and NSC 305831 (an antiparasitic hetero-cyclediamidine referred to as furamidine and DB75),are selective induceG-quadruplex formation in the human telomeric sequence and bind telomeric DNA quadruplexes in the absence of stabilizing monovalent cations with molar ratios(molecule: DNA)of 4:1and 1.5:1, respectively.

Heterocyclic dications as a new class of telomeric G-quadruplex targeting agents

Small molecules that can induce and stabilize G-quadruplex DNA structures represent a novel approach for anti-cancer and anti-parasitic therapy and extensive efforts have been directed towards discovering lead compounds that are capable of stabilizing quadruplexes. The purpose of this study is to explore conformational modifications in a series of heterocyclic dications to discover structural motifs that can selectively bind and stabilize specific G-quadruplexes, such as those present in the human telomere. The G-quadruplex has various potential recognition sites for small molecules; however, the primary interaction site of most of these ligands is the terminal tetrads. Similar to duplex-DNA groove recognition, quadruplex groove recognition by small molecules offers the potential for enhanced selectivity that can be developed into a viable therapeutic strategy. The compounds investigated were selected based on preliminary studies with DB832, a bifuryl-phenyl diamidine with a unique telomere interaction. This compound provides a paradigm that can help in understanding the optimum compound-DNA interactions that lead to quadruplex groove recognition. DNA recognition by the DB832 derivatives was investigated by biophysical experiments such as thermal melting, circular dichroism, mass spectrometry and NMR. Biological studies were also performed to complement the biophysical data. The results suggest a complex binding mechanism which involves the recognition of grooves for some ligands as well as stacking at the terminal tetrads of the human telomeric G-quadruplex for most of the ligands. These molecules represent an excellent starting point for further SAR analysis for diverse modes of quadruplex recognition and subsequent structure optimization for drug development.

Binding of gemini bisbenzimidazole drugs with human telomeric G-quadruplex dimers: effect of the spacer in the design of potent telomerase inhibitors

The study of anticancer agents that act via stabilization of telomeric G-quadruplex DNA (G4DNA) is important because such agents often inhibit telomerase activity. Several types of G4DNA binding ligands are known. In these studies, the target structures often involve a single G4 DNA unit formed by short DNA telomeric sequences. However, the 3′-terminal single-stranded human telomeric DNA can form higher-order structures by clustering consecutive quadruplex units (dimers or n-mers). Herein, we present new synthetic gemini (twin) bisbenzimidazole ligands, in which the oligo-oxyethylene spacers join the two bisbenzimidazole units for the recognition of both monomeric and dimeric G4DNA, derived from d(T2AG3)4 and d(T2AG3)8 human telomeric DNA, respectively. The spacer between the two bisbenzimidazoles in the geminis plays a critical role in the G4DNA stability. We report here (i) synthesis of new effective gemini anticancer agents that are selectively more toxic towards the cancer cells than the corresponding normal cells; (ii) formation and characterization of G4DNA dimers in solution as well as computational construction of the dimeric G4DNA structures. The gemini ligands direct the folding of the single-stranded DNA into an unusually stable parallel-stranded G4DNA when it was formed in presence of the ligands in KCl solution and the gemini ligands show spacer length dependent potent telomerase inhibition properties.

12-N-Methylated 5,6-dihydrobenzo[c]acridine derivatives: a new class of highly selective ligands for c-myc G-quadruplex DNA

12-N-Methylated and non-methylated 5,6-dihydrobenzo[c]acridine derivatives were designed and synthesized as new series of c-myc G-quadruplex binding ligands. Their interactions with c-myc G-quadruplex were evaluated using fluorescence resonance energy transfer (FRET) melting assay, circular dichroism (CD) spectroscopy, surface plasmon resonance (SPR), polymerase chain reaction (PCR) stop assay, and molecular modeling. Compared with the non-methylated derivatives, 12-N-methylated derivatives had stronger binding affinity and stabilizing ability to c-myc G-quadruplex structure, and could more effectively stack on the G-quartet surface. All these derivatives had high selectivity for c-myc G-quadruplex DNA over duplex DNA. The reverse transcription (RT) PCR assay showed that compound 21c could down-regulate transcription of c-myc gene in Ramos cell line containing NHE III(1) element, but had no effect in CA46 cell line with NHE III(1) element removed.

The catalytic subunit of human telomerase is a unique caspase-6 and caspase-7 substrate

Telomerase is a ribonucleoprotein complex that is essential for persistent cellular proliferation. The catalytic subunit of human telomerase, hTERT, functions as a reverse transcriptase and promotes vitality by maintaining telomeric DNA length. hTERT is tightly regulated with complex but poorly understood positive and negative regulation at several levels including transcription, protein-protein interactions, and post-translation modifications. Because evidence implicates hTERT as an apoptosis inhibitor and because telomerase activity tends to decrease during apoptosis, we hypothesized that hTERT is a caspase substrate leading to down regulation during apoptosis. Caspases are proteases that initiate and execute apoptosis by cleaving target proteins. Indeed, we found that caspases-6 and -7 cleave hTERT during apoptosis in cultured cells. Caspase-6 cleaves at residues D129 and D637, and caspase-7 cleaves at E286 and D628. Three of the caspase cleavage sites are unique motifs. All four caspase motifs appear conserved in TERTs from Old World monkeys and apes, and the caspase-6 sites appear conserved in all primates. The caspase site that cleaves at D129 appears conserved in amniotes. hTERT fragments generated by cleavage were remarkably persistent, lasting hours after caspase activation. These results reveal a new biologically relevant mechanism for telomerase down regulation through caspase-mediated cleavage of hTERT and expand the list of known caspase motifs.

Rational design of acridine-based ligands with selectivity for human telomeric quadruplexes

Structure-based modeling methods have been used to design a series of disubstituted triazole-linked acridine compounds with selectivity for human telomeric quadruplex DNAs. A focused library of these compounds was prepared using click chemistry and the selectivity concept was validated against two promoter quadruplexes from the c-kit gene with known molecular structures, as well as with duplex DNA using a FRET-based melting method. Lead compounds were found to have reduced effects on the thermal stability of the c-kit quadruplexes and duplex DNA structures. These effects were further explored with a series of competition experiments, which confirmed that binding to duplex DNA is very low even at high duplex:telomeric quadruplex ratios. Selectivity to the c-kit quadruplexes is more complex, with some evidence of their stabilization at increasing excess over human telomeric quadruplex DNA. Selectivity is a result of the dimensions of the triazole-acridine compounds, and in particular the separation of the two alkyl-amino terminal groups. Both lead compounds also have selective inhibitory effects on the proliferation of cancer cell lines compared to a normal cell line, and one has been shown to inhibit the activity of the telomerase enzyme, which is selectively expressed in tumor cells, where it plays a role in maintaining telomere integrity and cellular immortalization.

Synthesis and evaluation of fused bispyrimidinoacridines as novel pentacyclic analogues of quadruplex-binder BRACO-19

The present article reports on the design and the synthesis of a series of mono- and bis-pyrimidinoacridines and their evaluation as a novel family of quadruplex-binders. It is shown that bispyrimidinoacridines represent an interesting compromise between easy synthetic access and efficiency in terms of quadruplex interaction (both affinic and selective), as judged by G4-FID assay and molecular modelling. The present study also highlights that control of the pi-stacking interactions taking place between the ligand and the accessible G-tetrad of a quadruplex-DNA is indeed essential for good recognition but not exclusively (key role of direct and water-mediated H-bonds). The introduction of additional amino side chains, valuable in the acridine series, results here in steric perturbations of the ligand/quadruplex recognition and lowers the quadruplex/duplex selectivity.