CircRNAs: functions and emerging roles in cancer and immunotherapy

Abstract

Background: Circular RNAs (circRNAs) are emerging as promising tools in cancer and immunotherapy, with unique characteristics that offer potential therapeutic applications.

Main body: This review outlines the discovery, biogenesis, and mechanisms of circRNAs, emphasizing their roles in cancer and immune regulation. CircRNAs modulate immune responses by acting as miRNA sponges, binding RNA-binding proteins, or serving as translation templates. These interactions influence T cells, NK cells, macrophages, and immune checkpoints. The review also explores circRNAs' roles in different cancers, focusing on target identification, immune effects, and mechanisms of action. Additionally, it examines circRNA-based therapies, including vaccines, CAR-T cell therapy, and database applications.

Conclusion: Despite their potential, technical hurdles must be overcome to advance circRNAs' clinical use in cancer immunotherapy. Future research should focus on addressing these challenges to fully realize the therapeutic potential of circRNAs.

Keywords: Biomarkers; Cancer; CircRNAs; Immunotherapy; Tumor vaccines.

Fig. 1

Mechanism of circRNAs biogenesis. This schematic illustrates the molecular mechanisms underlying the biogenesis of circRNAs from precursor mRNA (pre-mRNA). The synthesis of messenger RNA (mRNA) occurs through typical splicing, where introns are removed, and exons are ligated to form a linear mRNA transcript. CircRNAs are transcribed by RNA Pol II and formed through back-splicing of the pre-mRNA. In this process, the 3′ splice site of a downstream exon is joined to the 5′ splice site of an upstream exon, resulting in a covalently closed circular RNA. Exons, except for the first and last, can be incorporated into the circRNAs. This event is facilitated by specific trans-acting factors, such as RNA-binding proteins, and intronic complementary sequences that enhance the proximity between the 5′ and 3′ splice sites, thus driving the back-splicing reaction. Long circRNAs (~ 1300 nt) interact with the spliceosome RNA helicase DDX39B, while short circRNAs (~ 400 nt) associate with the ATP-dependent RNA helicase DDX39A. These complexes are then recruited by the NTF2-related export protein 1 (NXT1) and the nuclear RNA export factor 1 (NXF1) heterodimer, which facilitate their export through the nuclear pore complex (NPC) into the cytoplasm. This figure provides a comprehensive overview of the critical molecular steps involved in circRNAs generation and regulation. Alu: Short interspersed repeats with reverse complementary features, facilitating back-splicing by forming secondary structures. Exonic circRNAs: Composed of exon sequences, devoid of introns, formed through back-splicing, and primarily present in the cytoplasm. Intronic circRNAs: Composed of intron sequences, formed and located in the nucleus. Exonic-intronic circRNAs: Composed of both exon and intron sequences, retaining partial introns during formation

Fig. 2

Functions of circRNAs in immunotherapy. a As miRNA Sponge: CircRNAs can bind to specific miRNAs and act as “sponges” to sequester miRNAs, thereby inhibiting the miRNA-mediated suppression of target genes, which in turn suppresses tumor growth. b RBP Interaction: RBPs interact with specific sequences or structural elements (e.g., AU-rich elements or QKI) of circRNAs to regulate their stability, nuclear retention, and back-splicing. c CircRNAs, such as circFoxo3, can bind to multiple proteins, including p21 and CDK2, forming complexes that inhibit G1 phase progression in the cell cycle or regulate apoptosis. d Translation Template: CircRNAs, such as circZNF609, contain IRES elements, thereby encoding functional proteins

Fig. 3

CircRNAs in immune modulation. a circRNA-encoded cryptic peptides are presented on the surface of tumor cells via MHC class I molecules, where they are recognized by CD8 + T cells. This recognition activates CD8 + T cells, promoting their differentiation into cytotoxic effector T cells, which specifically kill tumor cells and release cytokines (such as IFN-γ and TNF-α) to enhance the immune response, forming a synergistic anti-tumor effect. b circPHLPP2 binds to ILF3, promoting IL36γ transcription and secretion, which inhibits NK cell infiltration and reduces granzyme B and IFN-γ production, thereby decreasing NK cell numbers and function, ultimately promoting tumor growth. c circZNF451, delivered to macrophages via exosomes, interacts with TRIM56 and FXR1 to promote M2 macrophage polarization, leading to the secretion of immunosuppressive cytokines and growth factors (VEGF), establishing an immunosuppressive TME that impairs immune cell function, thereby facilitating tumor growth and immune evasion. d circRNAs act as miRNA sponges, competitively binding miRNAs to prevent their inhibitory effects on PD-L1 and CTLA-4 mRNA, thereby upregulating PD-L1 and CTLA-4 expression and promoting tumor growth