CircMALAT1 promotes the proliferation and metastasis of intrahepatic cholangiocarcinoma via the miR-512-5p/VCAM1 axis

Abstract

Circular RNAs play a pivotal role in the progression of various cancers. In our previous study, we observed high expression of the circRNA MALAT1 (cMALAT1) in intrahepatic cholangiocarcinoma (ICC) cells co-incubated with activated hepatic stellate cells. This study is designed to explore the roles of cMALAT1 and the underlying mechanisms in ICC. We find that cMALAT1 significantly facilitates the progression of ICC both in vitro and in vivo. The binding between cMALAT1 and miR-512-5p is subsequently confirmed through RNA pull-down experiments. As anticipated, the application of miR-512-5p mimics noticeably reverses the cMALAT1 overexpression-induced malignant phenotypes of ICC cells. Furthermore, VCAM1 is identified as a downstream gene of the cMALAT1/miR-512-5p axis. Importantly, silencing of VCAM1 not only effectively suppresses the malignant phenotypes of ICC cells but also significantly impairs the functions of cMALAT1. Our study reveals that cMALAT1 promotes the progression of ICC by competitively binding to VCAM1 mRNA with miR-512-5p, leading to the upregulation of VCAM1 expression and the activation of the PI3K/AKT signaling pathway.

1. Introduction

Intrahepatic cholangiocarcinoma (ICC), the second most diagnosed primary liver cancer after hepatocellular carcinoma (HCC), is considered a universally fatal malignancy, accounting for approximately 3% of all gastrointestinal malignancies [1,2]. The 5-year survival rate of ICC is only 10%–40% [3]. Owing to the hidden onset of this cancer, the efficacy of surgery (mainly in the early stages of ICC) is not very promising [4]. The primary reasons for treatment failure in ICC patients undergoing surgical intervention are local recurrence and distant metastasis, which are attributed to the different genetic background-induced individual differences. Therefore, exploring new biomarkers related to the treatment and prognosis of ICC is significantly beneficial for identifying new strategies for the treatment of ICC.

Circular RNAs, a class of non-coding RNAs, have been mistakenly recognized as the by-products of splicing for a long period of time [4,5]. They are abundantly expressed: circRNA expression is much higher than that of their linear host RNAs in fibroblasts [6]; diverse: circRNAs with varying exon-intron combinations are different [7]; stable: circRNAs always lack 5′-3′ polarity and a polyadenylated tail, which contributes to stability [8]; and conserved: circRNAs are reported to be more conserved than other RNA types [9]. These features indicate that there may be a variety of potential bio-functions and clinical settings [10]. In recent years, an increasing number of studies have indicated that circRNAs are closely involved in the regulation of multiple diseases, mainly through the transcriptional or post-transcriptional regulation of genes [11,12]. More importantly, circRNAs are differentially expressed in tumor tissues and are involved in various aspects of tumorigenesis and development [13], at least partially. CircRNAs can function as microRNA (miRNA) sponges, enabling them to bind target miRNAs and inhibit their activity via a mechanism named “miRNA sponging”. As reported, miRNAs are involved in the transcription of mRNAs and contribute to cell proliferation, mobility, death, and differentiation [14]. For example, ciRS-7 (also known as CDR1), the most prominent circRNA, can serve as a miR-7 sponge and bind to miR-7 to inhibit its activity, resulting in upregulated expression level of miR-7-targeted downstream transcripts [15]. In papillary thyroid carcinoma (PTC), CircFAT1 is highly expressed and can strongly decrease the expression of miR-873, thus promoting zinc finger E-box binding homeobox 1 (ZEB1) expression and eventually facilitating the malignant progression of PTC [16]. However, few studies have investigated the role of circRNAs in ICC.

It has been reported that there is apparent mesenchymal and fibrous tissue deposition in the malignant process of ICC [17], and activated hepatic stellate cells (aHSCs) are major contributors to mesenchymal and fiber formation in ICC [18]. Moreover, our previous work indicated that aHSCs regulate the RORα/Wnt/β-catenin axis via miR-1246 to promote cell proliferation, metastasis, and process of epithelial-mesenchymal transition (EMT) in hepatocellular carcinoma (HCC) [19]. More interestingly, our research demonstrated that circMALAT1 (cMALAT1) is highly expressed in ICC cells co-incubated with aHSCs and that highly expressed cMALAT1 promotes the proliferation and metastasis of ICC cells, indicating that cMALAT1 may play a role in the regulation of ICC progression.