Emerging Roles of CircRNAs in Tumor Microenvironment

The tumor microenvironment (TME) is an integral part of cancer that serves as a harbor where tumor cells com-municate with neighboring cells and non-cancerous components to determine the progression of the tumor. Researchers are increasingly turning their focus away from tumor cells alone and toward the dynamic and intricate tumor microenvironment to acquire a stronger insight into malignancies. The active crosstalk on TME and its heterogeneity make it challenging to reveal its characteristics, while details of the regulatory mechanisms remain unknown. A deep understanding of TME remodeling may provide potential biomarkers and treatment targets to enhance tumor therapy. Circular RNAs (circRNAs) are subpopulations of noncoding RNAs with unique characteristics and a wide range of biological properties involved in tumorigenesis and metastasis. Accumulating evidence has shown that circRNAs have abnormal expression and mediate signaling pathways. They play various roles in human malignancy events, such as angiogenesis, immune escape, and others. The role of circRNAs in the TME cannot be ignored, which may provide a novel path to elucidate the underlying regulatory mechanisms of TME remodeling. This review summarizes the history of TME and circRNAs and their roles and activities in the TME.