CCA is a malignant cancer with high incident rates worldwide, whose underlying molecular mechanism is yet to be clarified. Great advances have been made in CCA, but the tumor recurrence rate for CCA remains high after surgery. Therefore, developing a potentially effective approach against CCA is urgent. Recently, multiple lines of evidence have supported that Agrin plays an oncogenic role in promoting tumorigenesis[15]. Herein, the function of Agrin in CCA-related processes was investigated for the first time. The results demonstrated that ectopic Agrin expression promoted proliferation, colony formation, migration, invasion and tumorigenesis in CCA cells. The frequently increased expression of Agrin in clinical CCA specimens and its correlation with poorer tumor characteristics and higher postoperative tumor recurrence rates was also verified. Since cytomembrane proteins are the most well-known therapeutic targets in multiple types of malignancies, the present results suggested that targeting Agrin may be a promising strategy against CCA.
The underlying molecular mechanism of the function of Agrin was first defined in cultured myotubes, Wallace et al showed the important role of Agrin in promoting AChRs aggregation by inducing tyrosine phosphorylation of the AChRs β-subunit[16]. Several proteins act as Agrin receptor, including the receptor tyrosine kinase MuSK, which can form a primary structural scaffold for recruiting synaptic components. In addition, Agrin activated MuSK with the Z-site consisting of 8, 11, or 19 amino acids to promote the organization of the AChRs and other neuromuscular junction components[17]. Another study demonstrated that Agrin administration was could potentially treat myasthenia gravis and other neuromuscular disorders by increasing the number of AChRs and enhancing the signal transduction of the neuromuscular junction[18]. These results provided solid evidence that Agrin induces the activation of the AChRs signaling pathway. Recent studies have revealed a close relationship between AChRs and tumor development. Following AChRs agonist treatment, multiple cancer cells underwent epithelial-mesenchymal transition (EMT) and the metastatic ability of cells was increased through the activation of M2 muscarinic receptor (M2R); however, blocking M2R signaling reversed this phenomenon[19, 20]. It is well known that EMT is necessary for cancer cells to breach through the underlying basement membrane and extracellular matrix. However, Chakraborty et al revealed that the molecular mechanism underlying the regulation of EMT by Agrin does not work through AChRs. The study also showed that Agrin promoted invadopodia formation and activated the integrin-FAK pathway to regulate extracellular matrix degradation and drive EMT, and suggested that the biofunction of Agrin is not limited to the neuromuscular junction[13].
Furthermore, Dasgupta et al defined the role of Agrin in increasing intracellular Ca2+ concentration after KCl or caffeine therapy to promote the development of excitation-contraction coupling and myotube maturation[21]. Pirkmajer et al revealed that neural Agrin raised both the expression and activity of Na+/K+-ATPase to regulate skeletal muscle function in response to extrinsic stimuli[22]. Moll et al demonstrated that Agrin binds to α-dystroglycan and promotes the stabilization of laminin α5 chain to attenuate dystrophic symptoms[23]. These findings suggested that the mechanism of Agrin is far from being clarified. Membrane α-dystroglycan serves as a mechanical bridge between cytoskeleton and extracellular matrix, which binds dystrophin with other associated proteins, known as the dystrophin-glycoprotein complex[24], promoting cardiomyocyte differentiation and regeneration[25]. The genes of the dystrophin-glycoprotein complex were recently shown to be a target of the Hippo signaling pathway, and the inhibition of the link between the dystrophin-glycoprotein complex and the musculoskeletal system reduced Hippo signaling[26] and matrix rigidity[27]. This evidence hinted at a close correlation between Hippo signaling and Agrin. Therefore, in the present study, we investigated whether the effect of Agrin on tumorigenesis depends on the Hippo signaling pathway. After upregulating Agrin, a significantly increased YAP and decreased YAP phosphorylation was observed in CCA cells, which was reversed following Agrin-shRNA transfection. Consistent with these findings, Chakraborty et al reported that Agrin relied on the Hippo pathway to enhance oncogenic activities[28].
The nuclear YAP/TAZ combines multiple transcription factors to promote a series of oncogenic transcriptions and increase the malignant ability of the tumor[14]. Its function and expression level was tightly regulated by two major upstream kinases (Mst1/2 and LATS1/2), and acted as converging effectors of the Hippo pathway[29, 30]. The mechanisms through which Agrin regulates YAP activity are considered the relay of mechanosignalling from the extracellular matrix to intracellular YAP/TAZ, since Agrin is secreted and enriched in the basement membrane. Previous data has verified that extracellular matrix stiffness is one of the main stimulants for YAP travelling into the nucleus and facilitating the transcription of target genes[31]. Chakraborty built a model where extracellular matrix stiffness was manipulated with enhancing collagen matrix concentration. Using this model, they observed Agrin inhibition in cells cultured in stiff extracellular matrix reduced YAP’s nuclear localization and transcriptional activity, weakened the local extracellular matrix and provided considerable contractile strength to the cancer cells[32]. The other potential mechanism through which Agrin regulated YAP activity was increasing cell spreading and cytoskeletal tension by manipulating F-actin distributional change[33].
In conclusion, the present study demonstrated that Agrin is an important promoter of the activation and coordination of proliferation, migration and invasion in CCA cells, and that Agrin overexpression might be a prognostic marker. More therapeutic strategies against CCA could be developed by targeting Agrin in the future.