Exosomal circular RNAs as drivers of temozolomide resistance in glioblastoma: Mechanisms and implications

Glioblastoma (GBM) continues to be 1 of the most malignant tumors with limited success in therapy, primarily owing to the emergence of resistance towards temozolomide TMZ. Resistance to TMZ is a multifactorial phenomenon in GBM, depending on the interactions between genetic, epigenetic, and microenvironmental factors. Noncoding RNAs, most significantly circRNAs, have recently been highlighted as playing important roles in the pathogenesis of GBM and drug resistance against TMZ. These stable, circular RNA molecules can act as microRNA sponges or encode functional peptides; hence, they modulate functions relating to different aspects of tumor development. Furthermore, circRNAs can be carried within exosomes, promoting intercellular communication and propagation of drug resistance. Exosomes serve as sophisticated delivery vehicles that harbor varying bioactive molecules like proteins, lipids, mRNAs, miRNAs, lncRNAs, and many others, including circular RNAs. This review elucidates the specific functions held by exosomal circRNAs, such as circASAP1 and circ-HIPK3, in the modulation of TMZ resistance through different molecular pathways. It also reflects the biomarker potential of exosomal circRNAs as diagnostics and prognostics, allowing their dynamic application in liquid biopsies to monitor the progression of GBM over time. This review tries to develop an understanding of the complex nature of mechanisms conferring resistance to TMZ, with a particular focus on new insights regarding the potential roles of exosomal circular RNAs.