When good transforming growth factor‐β turns bad in hepatocellular carcinoma: Axl takes the stage
Authors: Diego F. Calvisi
Journal: Hepatology (2014)

Potential conflict of interest: Nothing to report. See Article on Page 930 Hepatocellular carcinoma (HCC) is one of the most frequent and lethal cancers worldwide, with limited treatment options when the tumor is not resectable.1 Indeed, treatments with chemotherapeutic agents have proven to be ineffective in the advanced disease, and the benefits of sorafenib administration, the only Food and Drug Administration–approved drug for liver cancer, are almost negligible in HCC patients.2 To overcome this gloomy scenario, a more detailed understanding of the molecular mechanisms responsible for HCC development and progression is imperative. Such an effort will be of great help to develop new and more effective therapeutic approaches against liver cancer. The highly conserved evolutionary transforming growth factor beta (TGF‐β) signaling cascade has been identified as one of the crucial pathways involved in rodent and human hepatocarcinogenesis.3 The pivotal players of this pathway, the TGF‐βs, are multifunctional cytokines that regulate a plethora of cellular processes, such as proliferation, differentiation, death, invasion, and metastasis.3 In this cascade, binding of one of the three TGF‐β isoforms (TGF‐β1, 2, and 3) to the TGF‐β receptor II (TGF‐βRII) triggers heterodimerization with the TGF‐β‐receptor I (TGF‐βRI) and, subsequently, activation of mothers against decapentaplegic homolog (SMAD) proteins via phosphorylation. Once phosphorylated, SMAD2/SMAD3 complexes translocate to the nucleus, where the transcription of TGF‐β target genes is promoted.3 Although a large body of investigation has taken place in the last two decades, the role of the TGF‐β pathway in many tumor types, including HCC, seems conflicting and remains only partially understood. On the one hand, TGF‐β has been demonstrated to determine growth restraint of a variety of tumor cells in vitro and in vivo via inhibition of cell cycle progression, induction of apoptosis, and preservation of tissue architecture.3 On the other hand, TGF‐β mediates disruption of cell adhesion, induces migration and invasion, and stimulates immune suppression and angiogenesis, thus acting as a potent tumor promoter.3 In particular, the process leading to loss of cell polarity and acquisition of motility, invasiveness, and a mesenchymal phenotype of cancer cells is denominated epithelial–mesenchymal transition (EMT) and is mainly regulated by the TGF‐β axis in cancer.6 In hepatocytes, TGF‐β is a major suppressor factor due to its ability of inhibiting proliferation and triggering apoptosis.7 Accordingly, a marked increase in HCC incidence was detected in TβRII heterozygous knockout mice subjected to diethylnitrosamine‐induced hepatocarcinogenesis when compared with wild‐type mice.9 However, in apparent contrast with the latter findings, up‐regulation of TGF‐β1 has been found to often occur in human HCC.10 Similarly, transgenic overexpression of TGF‐β1 in the liver predisposes to spontaneous and chemically induced murine hepatocarcinogenesis.11 Moreover, TGF‐β1 overexpression significantly accelerates c‐Myc‐driven liver tumor development in mice.11 Furthermore, treatment with the TGF‐βRI inhibitor LY2157299 has shown a strong antitumor activity in both in vitro and in vivo models of liver cancer.12 To add further complexity, microarray analysis of a human HCC collection showed the existence of two distinct subsets of TGF‐β‐responsive genes in liver cancer, reflecting both the suppressive and oncogenic properties of TGF‐β. In particular, the early TGF‐β response gene expression pattern was found to be associated with longer survival length of HCC patients, whereas the late response pattern characterized the biologically most aggressive tumors.14 Based on this body of evidence, it appears that the TGF‐β pathway plays both pro‐ and antitumorigenic roles in the cancerous liver. Although it is widely accepted that the resulting effect of TGF‐β in the liver might depend on the cell context, the microenvironment, the tumor stage, and the molecular crosstalk with other signaling pathways, the mechanisms promoting the shift of the TGF‐β activity toward malignancy remain uncertain.3 In the present issue of hepatology, Reichl et al. investigated the molecular mechanisms whereby the TGF‐β cascade collaborates with receptor tyrosine kinases to induce a mesenchymal gene expression program driving tumor progression and metastasis in liver cancer.15 Through a series of elegant and well‐designed experiments, the authors demonstrated the functional collaboration of Axl and TGF‐β signaling pathways along HCC progression.15 The Axl kinase is a member of the TAM receptor family, whose overexpression has been described in various tumor types and correlates with poor patient survival.16 In human HCC, Axl has been shown to be a crucial mediator of YAP‐dependent oncogenic activities.17 Once activated by its ligand, growth arrest–specific 6 (Gas6), Axl promotes proliferation, survival, invasion, and metastasis of cancer cells.16 Importantly, Reich et al. found that Axl is up‐regulated and activated in EMT‐transformed hepatoma cells. Subsequent investigations showed that Axl knockdown abolished invasive and migratory properties of mesenchymal HCC cells in vitro, whereas Axl overexpression promoted metastasis of HCC cells in vivo.15 Importantly, Axl suppression severely impaired the resistance to TGF‐β‐mediated growth inhibition. At the molecular level, the scaffold protein 14‐3‐3ζ was identified as the crucial interactor of Axl responsible for invasion, migration, and resistance against TGF‐β. In particular, the physical interaction between Axl and 14‐3‐3ζ resulted in the phosphorylation of Smad3 linker region (Smad3L) at the Ser213 residue in a c‐Jun‐N‐terminal kinase–dependent manner, leading to the up‐regulation of protumorigenic TGF‐β target genes, including PAI1, MMP9, and Snail, as well as in the increased secretion of TGF‐β1 in mesenchymal HCC cells.15 Of note, SmadL phosphorylation at Ser213 is considered a major molecular event determining the shift of TGF‐β properties from the tumor‐suppressive to the oncogenic, including induction of the EMT program.19 The importance of these experimental data was further substantiated in a human HCC collection, where elevated levels of Axl were directly associated with vessel invasion of HCC cells, higher risk of tumor recurrence after liver transplantation, strong phosphorylation of Smad3L, and shorter patient survival after tumor resection.15 The intriguing study by Reichl et al. provides novel insights of prime relevance to understand the role of TGF‐β in hepatocarcinogenesis. In particular, the authors unraveled a molecular link between the TGF‐β and Axl axes that might explain how liver cancer cells shift TGF‐β responses from tumor suppression toward tumor promotion (Figure 1). Due to the pathogenic role of deregulated TGF‐β and Axl cascades in various types of cancer,3 the present findings might have a broad impact not limited to HCC, in terms of both prognostic and therapeutic implications. On the one hand, based on the observation of Reichl et al., a stricter follow‐up and more aggressive adjuvant approaches (when available) should be implemented in HCC cases harboring elevated levels of Axl, 14‐3‐3ζ, and Smad3L. On the other hand, the present investigation strongly supports the development of therapeutic strategies aimed at suppressing the functional interaction between the TGF‐β and Axl/14‐3‐3ζ cascades, whose disruption might be deleterious for the progression and dissemination of HCC. It is important to underline that Axl alone was unable to determine the EMT program in mesenchymal HCC cells.15 This observation suggests that inhibition of TGF‐β might be sufficient to impair the oncogenic potential of the TGF‐β/Axl axis. Nonetheless, suppression of Axl rather than TGF‐β might be an even more valuable therapeutic strategy as it would presumably result in the abolition of TGF‐β oncogenic, but not tumor‐suppressive, properties. In this regard, a number of specific Axl inhibitors have been recently developed, including SGI‐7079, BGB324, DP3975, and NA80x1, and are currently being tested at the preclinical level or in phase I clinical trials.18 The results from the latter studies will be of great help to understand the eventual usefulness of Axl inhibitors for the treatment of human liver cancer. Concerning the effectiveness of treatments aimed at inhibiting TGF‐β in human HCC, encouraging results come from an ongoing phase II clinical trial of patients who either did not respond to sorafenib administration or were ineligible to receive sorafenib (http://clinicaltrials.gov; identifier NCT01246986). In a subset of these individuals, treatment with the TGF‐βRI inhibitor LY2157299 resulted in a significant decrease of serum alpha‐fetoprotein levels.20 These patients exhibited increased time‐to‐tumor progression and overall survival when compared with patients who had no alpha‐fetoprotein response.20 Although the latter findings are still preliminary and require additional validation, they strongly support the use of TGF‐β inhibitors for the treatment of human HCC.Figure 1: Schematic representation of the molecular crosstalk between the TGF‐β and Axl/14‐3‐3ζ cascades in human hepatocellular carcinoma. (A) In normal hepatocytes, the TGF‐β ligand binds to TGF‐βRII, which phosphorylates TGF‐βRI, forming a heteromeric complex. The activated TGF‐βRII–RI complex induces phosphorylation of the COOH terminus of the Smad3 protein, leading to its nuclear translocation. Once in the nucleus, Smad 3 triggers cell growth arrest and apoptosis. (B) In hepatocellular carcinoma cells, binding of the Gas6 ligand to Axl induces the activation of the Axl/14‐3‐3ζ axis and promotes the phosphorylation of Smad3 at a noncanonical site (pL), known as the linker region (Smad3L), in a JNK‐dependent manner. As a consequence of this noncanonical phosphorylation event, Smad3L induces the expression of selected TGF‐β target genes involved in tumor progression and the epithelial–mesenchymal transition program, thus conferring invasive and migratory properties as well as resistance to TGF‐β‐mediated growth inhibition on malignant hepatocellular carcinoma cells. EMT, epithelial–mesenchymal transition; JNK, c‐Jun‐N‐terminal kinase; Smad, mothers against decapentaplegic homolog; TGF‐β, transforming growth factor‐β; RI/RII, TGF‐β receptors I/II; p, phosphorylation.