Telomerase is expressed in more than 85% of cancer cells. Tumor cells with metastatic potential may have a high telomerase activity, allowing cells to escape from the inhibition of cell proliferation due to shortened telomeres. Human telomerase primarily consists of two main components: hTERT, a catalytic subunit, and hTR, an RNA template whose sequence is complimentary to the telomeric 5′-dTTAGGG-3′ repeat. In humans, telomerase activity is typically restricted to renewing tissues, such as germ cells and stem cells, and is generally absent in normal cells. While hTR is constitutively expressed in most tissue types, hTERT expression levels are low enough that telomere length cannot be maintained, which sets a proliferative lifespan on normal cells. However, in the majority of cancers, telomerase maintains stable telomere length, thereby conferring cell immortality. Levels of hTERT mRNA are directly related to telomerase activity, thereby making it a more suitable therapeutic target than hTR. Recent data suggests that stabilization of telomeric G-quadruplexes may act to indirectly inhibit telomerase action by blocking hTR binding. Telomeric DNA has the propensity to spontaneously form intramolecular G-quadruplexes, four-stranded DNA secondary structures that are stabilized by the stacking of guanine residues in a planar arrangement. The functional roles of telomeric G-quadruplexes are not completely understood, but recent evidence suggests that they can stall the replication fork during DNA synthesis and inhibit telomere replication by preventing telomerase and related proteins from binding to the telomere. Long-term treatment with G-quadruplex stabilizers induces a gradual reduction in the length of the G-rich 3’ end of the telomere without a reduction of the total telomere length, suggesting that telomerase activity is inhibited. However, inhibition of telomerase, either directly or indirectly, has shown only moderate success in cancer patients. Another promising approach of targeting the telomere is the use of guanine-rich oligonucleotides (GROs) homologous to the 3’ telomere overhang sequence (T-oligos). T-oligos, particularly a specific 11-base oligonucleotide (5’-dGTTAGGGTTAG-3’) called T11, have been shown to induce DNA damage responses (DDRs) such as senescence, apoptosis, and cell cycle arrest in numerous cancer cell types with minimal or no cytostatic effects in normal, non-transformed cells. As a result, T-oligos and other GROs are being investigated as prospective anticancer therapeutics. Interestingly, the DDRs induced by T-oligos in cancer cells are similar to the effects seen after progressive telomere degradation in normal cells. The loss of telomeres is an important tumor suppressor mechanism that is commonly absent in transformed malignant cells, and hence, T-oligos have garnered significant interest as a novel strategy to combat cancer. However, little is known about their mechanism of action. In this review, we discuss the current understanding of how T-oligos exert their antiproliferative effe...

T-oligo, a guanine-rich oligonucleotide homologous to the 3'-telomeric overhang of telomeres, elicits potent DNA-damage responses in melanoma cells; however, its mechanism of action is largely unknown. Guanine-rich oligonucleotides can form G-quadruplexes (G4), which are stabilized by the hydrogen bonding of guanine residues. In this study, we confirmed the G4-forming capabilities of T-oligo using nondenaturing PAGE, nuclear magnetic resonance, and immunofluorescence. Using an anti-G-quadruplex antibody, we showed that T-oligo can form G4 in the nuclei of melanoma cells. Furthermore, using DNase I in a nuclease degradation assay, G4-T-oligo was found to be more stable than single-stranded T-oligo. G4-T-oligo had decreased antiproliferative effects compared with single-stranded T-oligo. However, G4-T-oligo has similar cellular uptake as single-stranded T-oligo, as shown by FACS analysis. Inhibition of JNK, which causes DNA damage-induced apoptosis, partially reversed the antiproliferative activity of T-oligo. T-oligo also inhibited mRNA expression of human telomerase reverse transcriptase, a catalytic subunit of telomerase that was reversed by JNK inhibition. Furthermore, two shelterin complex proteins TRF2/POT1 were found to be up-regulated and bound by T-oligo, suggesting that T-oligo may mediate dissociation of these proteins from the telomere overhang. These studies show that T-oligo can form a G-quadruplex and that the antitumor effects of T-oligo may be mediated through POT1/TRF2 and via human telomerase reverse transcriptase inhibition through JNK activation.

Photodynamic therapy is a non-invasive method where light activates a photosensitizer bound to cancer cells, generating reactive oxygen species and resulting in cell death. This study assessed the oncolytic potential of photodynamic therapy, comparing European Medicines Agency and United States Food and Drug Administration-approved 5-aminolevulinic acid (5-ALA) to a metalloporphyrin, Pd(T4), against a highly invasive uveal melanoma cell line (C918) in two- and three-dimensional models in vitro. Epithelial monolayer studies displayed strong oncolytic effects (>70%) when utilizing Pd(T4) at a fraction of the concentration, and reduced pre-illumination time compared to 5-ALA post-405 nm irradiance. When analyzed at sub-optimal concentrations, application of Pd(T4) and 5-ALA with 405 nm displayed cumulative effects. Lethality from Pd(T4)-photodynamic therapy was maintained within a three-dimensional model, including the more resilient vasculogenic mimicry-forming cells, though at lower rates. At high concentrations, modality of cell death exhibited necrosis partially dependent on reactive oxygen species. However, sub-optimal concentrations of photosensitizer exhibited an apoptotic protein expression profile characterized by increased Bax/Bcl-2 ratio and endoplasmic stress-related proteins, along with downregulation of apoptotic inhibitors CIAP-1 and -2. Together, our results indicate Pd(T4) as a strong photosensitizer alone and in combination with 5-ALA against C918 cells.