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Telomeric G-quadruplex-forming DNA fragments induce TLR9-mediated and LL-37-regulated invasion in breast cancer cells in vitro.

Authors: Johanna M JM. Tuomela, Jouko A JA. Sandholm, Mika M. Kaakinen, Katherine L KL. Hayden, Kirsi-Maria KM. Haapasaari, Arja A. Jukkola-Vuorinen, Joonas H JH. Kauppila, Petri P PP. Lehenkari, Kevin W KW. Harris, David E DE. Graves, Katri S KS. Selander
Published: 01/18/2016, Breast cancer research and treatment


Toll-like receptor 9 (TLR9) is a cellular DNA-receptor widely expressed in cancers. We previously showed that synthetic and self-derived DNA fragments induce TLR9-mediated breast cancer cell invasion in vitro. We investigated here the invasive effects of two nuclease-resistant DNA fragments, a 9-mer hairpin, and a G-quadruplex DNA based on the human telomere sequence, both having native phosphodiester backbone. Cellular uptake of DNAs was investigated with immunofluorescence, invasion was studied with Matrigel-assays, and mRNA and protein expression were studied with qPCR and Western blotting and protease activity with zymograms. TLR9 expression was suppressed through siRNA. Although both DNAs induced TLR9-mediated changes in pro-invasive mRNA expression, only the telomeric G-quadruplex DNA significantly increased cellular invasion. This was inhibited with GM6001 and aprotinin, suggesting MMP- and serine protease mediation. Furthermore, complexing with LL-37, a cathelicidin-peptide present in breast cancers, increased 9-mer hairpin and G-quadruplex DNA uptake into the cancer cells. However, DNA/LL-37 complexes decreased invasion, as compared with DNA-treatment alone. Invasion studies were conducted also with DNA fragments isolated from neoadjuvant chemotherapy-treated breast tumors. Also such DNA induced breast cancer cell invasion in vitro. As with the synthetic DNAs, this invasive effect was reduced by complexing the neoadjuvant tumor-derived DNAs with LL-37. We conclude that 9-mer hairpin and G-quadruplex DNA fragments are nuclease-resistant DNA structures that can act as invasion-inducing TLR9 ligands. Their cellular uptake and the invasive effects are regulated via LL-37. Although such structures may be present in chemotherapy-treated tumors, the clinical significance of this finding requires further studying.

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