Original ArticlesLncTIC1 interacts with β-catenin to drive liver TIC self-renewal and liver tumorigenesis
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.
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Samples and reagents
Human liver cancer clinical samples were obtained from the first affiliated hospital of Zhengzhou University with informed consent, according to the Institutional Review Board approval. HCC samples were ranked according to obtained time, and several samples with sphere formation capacity (#1, #3 and #4) were used for sphere formation. The details for these sample were: #1, advanced hepatocellular carcinoma, 58 years old, male, tumor size, 7.8 × 5.2 × 4.9 mm, non-metastasis.#3, advanced
LncTIC1 was highly expressed in liver cancer and liver TICs
LncRNAs play critical roles in tumorigenesis, but their roles in liver TICs and liver tumorigenesis remain elusive. To explore the “driver” lncRNAs in liver tumorigenesis, we analyzed the lncRNA expression profiles using online datasets. Considering there are more than 400 samples with detailed clinical information in GSE14520 [28,29], we selected GSE14520 for lncRNA analysis and found that LINC00273 (hereafter termed lncTIC1) was the most highly expressed lncRNA in liver cancer samples (Fig. 1
Discussion
Liver TICs, a small subset of cells within the tumor bulk, are considered the drivers for liver tumorigenesis, metastasis and relapse [27]. Generally, these cells are in a quiescent state and highly express multiple drug pumps, so they can survive drug-induced cell death [6,7]. Upon withdrawal of drugs, these cells can renew and differentiate into new tumors. Because of their critical role in drug resistance and relapse, liver TIC targeting is a promising strategy for liver propagation [17].
Competing financial interests
All authors read and approved the final version of the manuscript, and the authors declare no competing financial interests.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (81601450) and Development Fund for Outstanding Young Teachers of Zhengzhou University (1521311059).
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