LncTIC1 interacts with β-catenin to drive liver TIC self-renewal and liver tumorigenesis

Highlights

• LncTIC1 is highly expressed in liver TICs.

• LncTIC1 plays a critical role in the self-renewal of liver TICs.

• LncTIC1 exerts its role through Wnt/β-catenin signaling.

• LncTIC1-β-Catenin signaling can be used for liver TIC targeting.

Abstract

Liver tumor-initiating cells (TICs) are drivers of liver tumorigenesis, and Wnt/β-catenin activation plays a principal role in the self-renewal of liver TICs. Despite a deep understanding of Wnt/β-catenin regulation, the roles of long noncoding RNAs (lncRNAs) in Wnt/β-catenin activation and liver TIC self-renewal are largely unknown. Here, we performed unbiased screening of lncRNAs in liver tumorigenesis and found lncTIC1 was highly expressed with liver tumorigenesis. LncTIC1 was also highly expressed in liver TICs and required for the self-renewal of liver TICs. LncTIC1 drove liver TIC self-renewal through Wnt/β-catenin signaling. LncTIC1 interacted with the N terminal of β-catenin and inhibited the phosphorylation of β-catenin, finally maintaining the stability of β-catenin to drive the activation of Wnt/β-catenin signaling. Through β-catenin maintenance and Wnt/β-catenin regulation, lncTIC1 participated in liver TIC self-renewal, liver tumorigenesis and tumor propagation. Moreover, blockade of lncTIC1 signaling greatly inhibited the propagation of liver cancer and liver TICs.

Introduction

Liver cancer is one of the deadliest cancers in the world. Hepatocellular carcinoma (HCC) and cholangiocarcinoma (CC) are two common types of liver cancer [4]. There are heterogeneous cells in liver tumors. Among the various cell types in the liver tumor bulk, liver tumor initiating cells (TICs) are considered the driver cells for liver tumor initiation, metastasis and drug resistance [20]. They generate all cell types in the liver tumor bulk along with tumorigenesis. They are resistant to many drugs because they are in a quiescent state [16]. Upon withdrawal of drug, liver TICs refuel their population and cause relapse. Recently, several functional assays of liver TICs were developed, including sphere formation assays for self-renewal, gradient xenograft for tumor initiation, xenograft for propagation, and side population for drug resistance [8,14,24]. Several surface markers of liver TICs were also developed, including CD133, CD13, CD24, EPCAM and others [11,21,37].

Liver TICs have self-renewal and differentiation capacities, and their activities are finely regulated. Several transcription factors and signaling pathways are involved in the self-renewal of liver TICs. We found Zic2, Oct4 and Sox4 were critical modulators of liver TICs [5,40]. Wnt/β-catenin, Notch, Yap1 and Hedgehog signaling pathways are self-renewal-associated pathways in liver TICs, and Wnt/β-catenin is considered as the most important signaling pathway in the self-renewal of liver TICs [36,39,42,43]. The activation of Wnt/β-catenin signaling is under precise regulation [22]. In Wnt-OFF cells, β-catenin associates with APC, Axin2 and β-TrCP to form the APC degradation complex. β-catenin in the APC complex was ubiquitinated and degraded [3]. In Wnt-ON cells, β-catenin is released from the APC complex, followed by nuclear translocation, where β-catenin interacts with LEF/TCF to trigger the expression of Wnt/β-catenin target genes [15]. Actually, the β-catenin interactome (in the APC degradation complex or in the TCF/LEF activation complex) plays a critical role in Wnt/β-catenin activation [9]. Recently, we found β-catenin interacted with EZH2 and could be methylated by EZH2. β-catenin methylation inhibited its ubiquitination and drove Wnt/β-catenin activation [41]. Although the regulation of β-catenin has been intensively explored, the mechanism of Wnt/β-catenin activation remains largely unknown.

Long noncoding RNAs (LncRNAs) are RNA molecules that are longer than 200 nt but have no protein-coding potential. For a long time, lncRNAs were considered the byproducts of RNA polymerase II, and recently, many biological functions of lncRNAs have been found [2]. LncRNAs exert their roles through multiple mechanisms [32]. As a common mechanism, lncRNAs can function as scaffold molecules and recruit chromatin remodeling complex to the promoters of lncRNA target genes [25,34]. Moreover, increasing numbers of lncRNAs are found to interact with various proteins to modify their stability or activities [19,33]. Through these mechanisms, lncRNAs play critical roles in many physiological and pathological processes, including tumorigenesis [26]. Regarding tumor biology, lncRNAs participate in tumor initiation, metastasis, and colony formation [31,35,38]. Here, we took advantage of online datasets, explored the role of lncRNAs in liver tumorigenesis through unbiased screening and found lncTIC1 was highly expressed in liver cancers and associated with liver cancer severity and prognosis. Moreover, lncTIC1 was also highly expressed in liver TICs and required for the self-renewal of liver TICs. LncTIC1 is associated with β-catenin, inhibited the assembly of the APC complex and finally, primed the activation of Wnt/β-catenin signaling. Through modulation of Wnt/β-catenin activation, lncTIC1 can also serve as a therapeutic target for liver TIC propagation.