Circular RNAs: Emerging Regulators of the Major Signaling Pathways Involved in Cancer Progression

Abstract

Signal transduction is an essential process that regulates and coordinates fundamental cellular processes, such as development, immunity, energy metabolism, and apoptosis. Through signaling, cells are capable of perceiving their environment and adjusting to changes, and most signaling cascades ultimately lead to alterations in gene expression. Circular RNAs (circRNAs) constitute an emerging type of endogenous transcripts with regulatory roles and unique properties. They are stable and expressed in a tissue-, cell-, and developmental stage-specific manner, while they are involved in the pathogenesis of several diseases, including cancer. Aberrantly expressed circRNAs can mediate cancer progression through regulation of the activity of major signaling cascades, such as the VEGF, WNT/β-catenin, MAPK, PI3K/AKT, and Notch signaling pathways, as well as by interfering with signaling crosstalk. Deregulated signaling can then function to induce angiogenesis, promote invasion, migration, and metastasis, and, generally, modulate the hallmarks of cancer. In this review article, we summarize the most recently described and intriguing cases of circRNA-mediated signaling regulation that are involved in cancer progression, and discuss the biomarker potential of circRNAs, as well as future therapeutic applications.

Keywords: MAPK/ERK; PI3K/AKT; WNT/β-Catenin; biomarkers; circRNA; metastasis; miRNAs; signal transduction; signaling cascades; targeted therapy.

Figure 1

Biogenesis and types of circRNAs. Several types of circRNAs may derive from the back-splicing of a primary transcript. Exonic circRNAs (EcircRNAs) consist of one or more exons and intronic circRNAs (ciRNAs) are composed of a single intron. Moreover, an exonic–intronic circRNA (EIcircRNA) is produced, when both exons and introns are retained during circularization. A circRNA may also derive from the circularization of a pre-tRNA intron, when this latter is spliced out during the maturation process of a tRNA; such circRNAs are called tricRNAs. The tRNA-splicing endonuclease (TSEN) complex catalyzes the cleavage of an intron from an intron-containing pre-tRNA, whereas the enzyme that is responsible for the intron circularization has not been identified yet.

Figure 5

Two examples of a positive feedback regulation of circRNAs expression, as seen in hepatocellular cancer. circ-LRIG3 and circ-SOD2 promote tumor progression through the activation of JAK/STAT signaling. More specifically, circ-LRIG3 acts as a protein scaffold, assisting the methylation of STAT3 by EZH2 and its subsequent phosphorylation, while circ-SOD2 acts as a competitive endogenous RNA and upregulates DNMT3A, which in turn suppresses the inhibitory role of SOCS3 in STAT3 activation. In both cases, activated STAT3 binds to the respective gene promoters and enhances their transcription, thus forming a positive feedback loop. Green arrows denote induction of expression, whereas the red “reverse tau” symbol (⊥) indicates attenuation of expression. Vertical green and red arrows indicate increased or decreased expression levels, respectively. Black arrows signify increased transcription rates, while dark blue arrows indicate the sequence of events in the feedback loops.

Figure 6

The onco-suppressive role of two circRNAs in bladder cancer. circ-FNDC3B sponges miR-1178-3p, preventing its binding to the 5′ untranslated region (UTR) of G3BP2, hence reducing G3BP2 translation and, consequently, inhibiting the activation of FAK/SRC-mediated signaling. Similarly, circ-PICALM sponges miR-1265 and prevents it from binding to the 3′ UTR of STEAP4, this way mediating its upregulation. Thus, STEAP4 binds to FAK and inhibits its phosphorylation, leading to cancer repression. Red “reverse tau” symbols (⊥) indicate attenuation of expression, whereas green arrows denote induction of expression. Blue arrows indicate the sequence of events, while vertical grey arrows indicate decreased expression levels. Τhe parallel red lines (//) indicate the inhibition of an effect.

Figure 7

Overview of the most promising areas for circRNA research. circRNAs hold great potential as non-invasive biomarkers for the prognosis, diagnosis, and monitoring of the disease, as targets for anticancer therapy, and vehicles for drug delivery as well. Additionally, circRNAs play a crucial role in the reprogramming of cancer cells to gain and maintain stem-like characteristics, often mediating in treatment failure. Viral circRNAs emerge as intriguing regulators of tumorigenesis and cancer progression that render further investigation, since they restrain the onco-suppressive functions of signaling pathways.

Figure 2

The main functions of circular RNAs (circRNAs). The most frequently observed mechanism of circRNA action is the sponging of microRNA molecules (A), thereby preventing them from binding to their mRNA targets. circRNAs restrict the function of proteins as well, by acting as protein sponges/decoys (B), which can lead to their retainment and stabilization in a specific cellular compartment. Such proteins are depicted here in various shapes of pink, yellow, green, and red color. The interaction between circRNAs and RNA-binding proteins also facilitates the formation of complexes (C) since an enzyme and its substrate(s) are brought to proximity through a circRNA scaffold. Interestingly, several circRNAs can encode peptides in a Cap-independent manner (D), and they possess at least one internal ribosome entry site (IRES). Regarding circRNAs that are predominantly located in the nucleus, they usually regulate the transcription rates of their parental gene (E).

Figure 3

A proposed mode of action of circ-MYO10 regarding the regulation of the WNT/β-catenin pathway in osteosarcoma. circ-MYO10 sponges miR-370-3p, and, therefore, the levels of the latter are not sufficient for reducing the expression of RUVBL1. Then, RUVBL1 is transferred to the nucleus, where it interacts with β-catenin and KAT5 and forms a complex that is recruited to the promoter region of MYC. This interaction leads to increased acetylation of H4K16 and expression of WNT-targeted genes, including MYC. Consequently, enhanced osteosarcoma cell proliferation, epithelial to mesenchymal transition (EMT), and metastasis are attained. The red “reverse tau” symbol (⊥) indicates attenuation of expression, while the parallel black lines (//) indicate the inhibition of an effect. Vertical green and red arrows indicate increased or decreased expression levels, respectively. Vertical yellow arrows denote the increased rate of an event, and blue arrows indicate the sequence of events.

Figure 4

The onco-suppressive role of circ-ITGA7 in colorectal cancer. circ-ITGA7 sponges miR-370-3p, which leads to the increased expression of NF1, one of the target genes of miR-370-3p. NF1 promotes the hydrolysis of GTP by RAS GTPase, and therefore inhibits RAS signaling, which consequently obstructs the binding of the transcription factor RREB1 to the promoter of ITGA7. This results in enhanced expression of the linear mRNA of ITGA7. The inactivation of RAS signaling, accompanied by the increased expression of ITGA7, leads to the suppression of cancer progression and metastasis. The red “reverse tau” symbol (⊥) indicates attenuation of expression, while the parallel black and red lines (//) indicate the inhibition of an effect. Vertical red arrows indicate decreased expression levels. Green arrows indicate increased expression levels, and black arrows indicate the sequence of events. The blue arrow indicates the transition from one state to another.