circRNAs and Exosomes: A Mysterious Frontier for Human Cancer

Exosomes are nano-sized membrane-bound vesicles and contain active substances (DNA, noncoding RNA [ncRNA], protein), which provide a novel method of transferring effector messages between cells. Circular RNAs (circRNAs), a kind of ncRNA, have attracted increasing attention over the last decade given advances in whole-genome and transcriptome sequencing technologies. It has become increasingly clear that circRNAs regulate gene expression through various actions and play diverse roles in many fields of human cancer biology. Notably, several studies reported that circRNAs are enriched in exosomes and that exosomal circRNAs play an important role in cancer biology. Exosomal circRNAs can be taken up by neighboring or distant cells and affect many aspects of physiological and pathological conditions of the recipient cells, potentially promoting cell communication and tumor metastasis. Herein, we briefly review the molecular mechanisms of circRNAs and recent findings regarding exosomal circRNAs, and highlight the specific roles of exosomal circRNAs in human cancer.