A correlation of polymorphic G-quadruplex formation in vitro and in the lysosomes of live cancer cells

Guanine-rich oligonucleotides (GROs) can fold into G-quadruplex (G4) structures. The diverse roles of G4 structures, particularly as targets for drug design, anticancer agents, and drug delivery systems, highlight their critical significance in cancer research. However, the formation of G4 structures is highly dependent on the specific nucleotide sequences and the number of G-tracts within each GRO. In vitro studies using circular dichroism (CD), nuclear magnetic resonance (NMR), and polyacrylamide gel electrophoresis (PAGE) demonstrated that GROs with fewer than four G-tracts can form intermolecular G4 structures in K⁺ solution at 37 °C. In fluorescence lifetime imaging microscopy study, intermolecular parallel G4 structures, formed by single-stranded GROs containing three G-tracts with three guanines each, were observed to be detectable in the lysosomes of live CL1-0 cancer cells. In contrast, a mutated sequence with only two G-tracts was rarely detected in the lysosomes of CL1-0 cancer cells, highlighting its incapability of forming intermolecular parallel G4s. Furthermore, polymorphic G4 formation in vitro and in-cell studies revealed a potential correlation. Our findings demonstrate that exogenous GROs can be introduced to explore the structural dynamics of G4 formation in live cancer cells, as well as their potential as anticancer agents and drug delivery carriers targeting lysosomes.