Mechanisms of immune checkpoint inhibitors: insights into the regulation of circular RNAS involved in cancer hallmarks

Abstract

Current treatment strategies for cancer, especially advanced cancer, are limited and unsatisfactory. One of the most substantial advances in cancer therapy, in the last decades, was the discovery of a new layer of immunotherapy approach, immune checkpoint inhibitors (ICIs), which can specifically activate immune cells by targeting immune checkpoints. Immune checkpoints are a type of immunosuppressive molecules expressed on immune cells, which can regulate the degree of immune activation and avoid autoimmune responses. ICIs, such as anti-PD-1/PD-L1 drugs, has shown inspiring efficacy and broad applicability across various cancers. Unfortunately, not all cancer patients benefit remarkably from ICIs, and the overall response rates to ICIs remain relatively low for most cancer types. Moreover, the primary and acquired resistance to ICIs pose serious challenges to the clinical application of cancer immunotherapy. Thus, a deeper understanding of the molecular biological properties and regulatory mechanisms of immune checkpoints is urgently needed to improve clinical options for current therapies. Recently, circular RNAs (circRNAs) have attracted increasing attention, not only due to their involvement in various aspects of cancer hallmarks, but also for their impact on immune checkpoints in shaping the tumor immune microenvironment. In this review, we systematically summarize the current status of immune checkpoints in cancer and the existing regulatory roles of circRNAs on immune checkpoints. Meanwhile, we also aim to settle the issue in an evidence-oriented manner that circRNAs involved in cancer hallmarks regulate the effects and resistance of ICIs by targeting immune checkpoints.

Fig. 1. PD-1/PD-L1 pathway contributes to tumor immune escape, enabling tumors resistant to immune response.

When PD-1 binds to PD-L1 on the surface of immune effector cells such as T cells, T cell receptor (TCR) signaling transduction was suppressed. Monoclonal antibodies (mAbs) blocking the PD-1/PD-L1 pathway have been widely applied for clinical immunotherapy to fight against a fraction of advanced cancer.

Fig. 2. CTLA-4 negative regulates T cell responses through several ways, such as attenuating T cell receptor (TCR) and CD28 signaling by competively binding to B7 with CD28.

The application of mAbs to block CTLA-4 can relieve its inhibitory effects on T cells, reactivate T cell proliferation and differentiation into cytotoxic T lymphocytes (CTLs), thereby exerting anti-tumor immune effects.

Fig. 3. The typical ligand of LAG-3 is MHC class II (MHC-II), which interacts stably with LAG-3 through the D1 domain and has a significantly higher affinity than CD4.

The high-affinity LAG-3Ig/MHC-II binding competes with CD4/MHC-II interaction to mediate the suppressed T cell proliferation and inhibitory immune response. The second ligand of LAG-3 is Gal-3, which has been found to be expressed in multiple cancer cells and activated T cells. Gal-3 can interact with LAG-3 and this binding is essential for the inhibitory cytotoxicity of CD8 + T lymphocytes. Other newly identified LAG-3 functional ligands include FGL1 and LSECtin. The blockade of LAG-3 with mAbs can relieve the T cell inhibitory immune response triggered by the interaction between LAG-3 and its ligands.

Fig. 4. TIM-3 has multiple ligands, such as Gal-9, CEACAM1, PtdSer, and HMGB1, and each of them binds to different regions of the extracellular IgV domain.

When TIM-3 is not bound to the ligand, it interacts with BAT3 and maintains T cell activation through Tyrosine kinase LCK recruitment. Once TIM-3 binds to the ligand, BAT3 is released from TIM-3 and Fyn binds to it, allowing TIM-3 to exert its inhibitory function. Most mAbs targeting TIM-3 hinder the binding of ligands to TIM-3, thereby maintaining the binding state between TIM-3 and BAT3.

Fig. 5. The agonistic antibodies targeting stimulatory receptors and the blocking antibodies targeting inhibitory receptors applied in anti-tumor immunity.

The common stimulatory and inhibitory immune checkpoint receptors with positive and negative regulatory effects on anti-tumor immunity, and the blocking antibodies targeting inhibitory receptors, and the agonistic antibodies targeting stimulatory receptors.

Fig. 6. CircRNAs target PI3K/AKT axis to regulate tumor cell proliferation and metastasis.

The circRNAs target PI3K/AKT axis to regulate cell proliferation and metastasis in the context of various cancers, such as circVRK1/miR-624-3p/PTEN, circLARP4/miR-1323/PTEN, and circ_0007624/miR-224-5p/CPEB3 pathway targeting PI3K/AKT axis in esophageal cancer; ciRS-7/miR-7/PTEN, circPVT1/miR-152-3p/HDGF, and circPIP5K1A/miR-671-5p/KRT80 pathway targeting PI3K/AKT axis in gastric cancer; circIL4R/miR-761/TRIM29/PHLPP1, circ_0008285/miR-382-5p/PTEN, and circ-0001313/miRNA-510-5p/AKT2 pathway targeting PI3K/AKT axis in colorectal cancer; circCDYL/miR-892a and miR-328-3p, circCDK13/JAK/STAT, and circEPHB4/HIF-1ɑ pathway targeting PI3K/AKT axis in liver cancer; circBFAR/miR-34b-5p/MET, circEIF6/miR-557/SLC7A11, and circNFIB1/miR-486-5p/PIK3R1/VEGF-C pathway targeting PI3K/AKT axis in pancreatic cancer; circ0014359/miR-153, circHIPK3/miR-524-5p/KIF2A, circPIP5K1A/miR-515-5p/TCF12, and circ0000215/miR-495-3p/CXCR2 pathway targeting PI3K/AKT axis in glioma.

Fig. 7. CircRNAs target NF-κB signaling to regulate tumor cell proliferation.

The circRNAs target NF-κB signaling to regulate cell proliferation in the context of various cancers, such as circ_0001821/miR-423-5p/BTRC/IKBA, circ-CD44/miR-23b-5p/TAB1, ciRS-7/miR-7/HOXB13, circCYP24A1/PKM2 axis to regulate NF-κB signal in ESCC; circZFR/STAT3/NF-κB, circLIFR/TBK1/NF-κB, circCORO1C/NF-κB/PD-L1 in hepatocellular carcinoma; circADAMTS6/ANXA2/NF-κB in glioblastoma; circ_0002019/TNFAIP6/NF-κB in gastric cancer; circGLIS2/miR-671/NF-κB in colorectal cancer; circINTS4/miR-146b/CARMA3/NF-κB in bladder cancer. circARHGEF28/miR-671-5p/LGALS3BP/NF-κB in prostate cancer; circ_0001461/miR-145/TLR4/NF-κB axis in oral squamous cell carcinoma.

Fig. 8. The circRNAs involving in Wnt/β-catenin signaling to regulate cell proliferation in the context of various cancers.

In lung cancer, circ-EIF3I/miR-1253/NOVA2, circVAPA/miR-876-5p/WNT5A, circ-ZNF124/miR-498/YES1, and cir-ITCH/miR-7, miR-214/ ITCH pathway have been confirmed to affect the proliferation phenotype by targeting Wnt/β-catenin signaling. In hepatocellular carcinoma, circMTO1/miR-541-5p/ZIC1, hsa_circRNA_104348/miR-187-3p/RTKN2, circRNA-SORE/miR-103a-2-5p and miR-660-3p, circZFR/miR-3619-5p/CTNNB1 pathways have been reported to modulate cell proliferation by affecting Wnt/β-catenin signaling. In gastric cancer, circAXIN1-encoded protein AXIN1-295aa/APC, circ_SMAD4/miR-1276/TCF4/CTNNB1, cir-ITCH/miR-17/ITCH, circCNIH4/DKK2/FRZB axis have been confirmed to regulate proliferation by targeting Wnt/β-catenin signaling pathway. In colorectal cancer, circ-ACAP2/miR-143-3p/FZD4 axis, circAGFG1/miR-4262 and miR-185-5p/YY1 and CTNNB1, circ5615/miR-149-5p/TNKS, hsa_circ_0009361/miR-582/APC2 pathway were reported to influence proliferation via modulating Wnt/β-catenin signaling.

Fig. 9. CircRNAs target MAPK signaling to regulate tumor cell proliferation and metastasis.

The circRNAs involving in MAPK signaling pathway to regulate cell proliferation under the settings of various tumors, such as circ-MAPK4/miR-125a-3p/p38/MAPK, circ_PTN/miR-122/SOX6/MAPK/ERK, and circ-TLK1/miR-17-5p/PANX1/MAPK/ERK axis in glioma; circDHPR/miR-3194-5p/RASGEF1B/RAS/MAPK, circASAP1/miR-326 and miR-532-5p/MAPK1, circSETD3/miR-421/MAPK14 pathway in hepatocellular carcinoma; circDNAJC11/TAF15/MAPK6, circBACH2/hsa-miR-944/HNRNPC/MAPK, and circ_0006528/miR-7-5p/Raf1/MAPK/ERK pathway in breast cancer; circRNA C190/EGFR/MAPK/ERK, circ-ZKSCAN1/miR-330-5p/FAM83A/MAPK, and circ0001313/microRNA-452/HMGB3/ERK/MAPK axis in NSCLC; circMAPK1- encoded protein MAPK1-109aa/MEK1/MAPK1, hsa_circ_0044301/hsa-miR-188-5p/DAXX, and hsa-circ-0052001/hsa-miR-608/MAPK pathway in gastric cancer.