Modulating the activation of hepatic stellate cells: A cunning way for metastatic cells to create a permissive soil for seeding in the liver?
Authors: Cédric Coulouarn
Journal: Hepatology (2014)

Potential conflict of interest: Nothing to report. Personal work was supported by Inserm, Université de Rennes 1, Association Française pour l'Étude du Foie, Institut National du Cancer, and Cancéropôle Île‐de‐France. See Article on Page 361 Hepatocellular carcinoma (HCC) is the most frequent type of primary cancer in the liver. However, far more common than HCC are liver metastases. Indeed, the liver is one of the most frequent sites of implantation of metastatic tumor cells originating from skin, colon, and breast primary cancers. This nonrandom profile of metastatic dissemination constitutes the basis of the "seed and soil" hypothesis proposed more than 100 years ago by Stephen Paget, who suggested that metastasis depends on the crosstalk between selected cancer cells (the "seeds") and specific organ microenvironments (the "soil").1 Over the last few decades, considerable efforts have been made to characterize the cellular and molecular determinants of cancer cell metastasis, including the genetic alterations allowing cancer cells to acquire malignant traits together with the remodelling of the microenvironment, the role of which in cancer onset and progression has been increasingly appreciated.2 The metastatic cascade is a complex process made of sequential and inter‐related steps.3 At the primary site, selected tumor cells acquire an invasive phenotype, which is required for local invasion and intravasation into the lymph and blood vessels. Then, tumor cells transit within the circulation to reach a distant organ. At the site of colonization, tumor cells stop, extravasate, and invade into the parenchyma of the foreign tissue, where they face a separate environment from which they originate. To form metastases, tumor cells may corrupt this new microenvironment in order to evade immune cell surveillance and provide growth and angiogenic factors needed for cell survival, tumor growth, and metastatic spread.3 Accordingly, profiling of the liver microenvironment clearly demonstrated that the presence of liver metastases correlated with specific genomic changes, such as a switch toward an increase in T‐helper (Th)2 cytokines and a decrease in Th1 cytokines.4 It is now well accepted that microenvironment remodeling is a critical issue to create a permissive metastatic niche.2 The tumor microenvironment or stroma is a multifaceted biological system made of cellular and noncellular components. In the liver, hepatic stellate cells (HSCs), endothelial cells (ECs), and immune cells are the major cell types of the microenvironment. Noncellular components include the extracellular matrix (ECM) and soluble mediators (e.g., cytokines and growth factors), which play a crucial role in orchestrating the crosstalk between tumor cells and the microenvironment.6 Among these soluble mediators, transforming growth factor beta (TGF‐β) takes a singular place. TGF‐β is a pleiotropic cytokine that contributes to several aspects of liver carcinogenesis from early stages of liver fibrosis, cirrhosis, and hepatic dysplasia to later stages associated with tumor cell growth, invasion, and metastasis.8 The canonical TGF‐β pathway signals through the TGFBR1 and TGFBR2 serine‐threonine kinase receptors and the mothers against decapentaplegic (SMAD) family of transcriptional regulators. Subsequent to a cascade of phosphorylations, activated SMAD factors translocate into the nucleus to form complexes that regulate gene expression in target cells.9 In the tumor microenvironment, TGF‐β modulates the activity of both tumor and stromal cells. As an example, TGF‐β is a well‐known inducer of epithelial‐to‐mesenchymal transition, which contributes to the acquisition of a motile phenotype in cancer cells. TGF‐β is also an important regulator of HSCs in the liver microenvironment. After TGF‐β activation, quiescent HSCs (qHSCs) transdifferentiate into highly motile myofibroblasts, which notably express genes involved in ECM remodelling. Activated HSCs (aHSCs) greatly contribute to liver fibrosis and portal hypertension, but they play also an important role in liver carcinogenesis and metastasis.10 Numerous evidences highlighted the protumorigenic potential of aHSCs (Fig. 1). Thus, cytokines and growth factors produced by aHSCs (e.g., interleukin‐6 and hepatocyte growth factor) have been shown to promote proliferation and invasion of tumor cells and generate proangiogenic signals (e.g., vascular endothelial growth factor A) in favor of tumor growth and metastasis. The properties of tumor cells are also modulated by ECM components produced by aHSCs, including basement membrane components.11 According to their multifunctional role in carcinogenesis, HSCs have been proposed as relevant targets for new therapeutic strategies in liver cancer.12 Thus, a better characterization of HSC metabolism may help to design such targeted therapies.Figure 1: HSC activation and metastasis in the liver. TGF‐β is a potent activator of HSCs in the liver microenvironment. Once activated, qHSCs transdifferentiate into highly motile myofibroblasts. The bidirectional crosstalk between metastatic tumor cells and aHSCs is an important feature of tumor growth and progression. On one side, aHSCs/myofibroblasts modulate the properties of tumor cells toward the acquisition of an aggressive phenotype. On the other side, aHSCs induce changes in the microenvironment, which contribute to create a permissive niche for tumor cell implantation (e.g., ECM remodeling, angiogenesis, and immune suppression). In the current issue of Hepatology, Tu et al. identified VASP as a new critical actor involved in the activation of HSCs mediated by TGF‐β. Together with RAB11, VASP favor HSC activation by increasing the recycling of TGFBR2 to the membrane of HSCs.In the current issue of Hepatology, Tu et al. report on the identification of vasodilator‐stimulated phosphoprotein (VASP) as a critical molecule required for HSC activation.13 Mechanistically, VASP forms complexes with RAB11 and TGFBRII to increase the recycling of TGF‐β receptors to the plasma membrane of HSCs, thus favoring TGF‐β‐mediated activation.13 The study not only highlights an interesting mechanism related to HSC activation, which could have a clinical relevance for designing antifibrotic strategies, but it also provides an elegant illustration of the bidirectional crosstalk that exists between metastatic cells and the liver microenvironment. After a portal vein injection, they show that implantation of colorectal cancer cells in liver of immunodeficient mice induces expression of VASP in adjacent HSCs.13 In addition to this mouse model, which is presented as an experimental model of liver metastasis, they show that HSCs adjacent to cancer cells in liver of patients with colon cancer are also positive for VASP and alpha‐smooth muscle actin, a marker of aHSCs. Using tumorigenic HT‐29 cells, they report that VASP up‐regulation is at least mediated by TGF‐β secreted by tumor cells.13 However, from these observations, it remains to be determined whether up‐regulation of VASP in HSCs depends on the intrinsic nature of the metastazing tumor cells. In this context, it would be important to study other cell lines with different phenotypes, including genetically modified cells with an impaired TGF‐β signaling. Interestingly, hypoxia inducible factor‐1α has been shown to mediate repression of VASP by tumor necrosis factor alpha in breast cancer cells,14 suggesting that the nature of the microenvironment (i.e., inflammatory or hypoxic) may also modulate VASP expression. From these data,14 one can assume that TGF‐β‐mediated up‐regulation of VASP in HSCs may not occur in a hypoxic environment. More generally, it would be interesting to determine whether VASP is modulated by other paracrine factors liberated by stromal cells (SCs). The researchers report that VASP expression in HSCs promotes tumor cell proliferation and migration in vitro.13 More interestingly, evidences are provided for a role of VASP in favoring tumor cell implantation and growth in vivo, although the exact mechanism remains to be elucidated. Obviously, VASP may favor the formation of a prometastatic niche by mechanisms linked to activation of HSCs (Fig. 1). Induction of VASP by TGF‐β in SCs other than HSCs may also contribute to create a favorable environment for metastasis. Supporting this hypothesis, VASP activation has been shown to enhance EC, endothelial cell migration.15 Although the crosstalk between metastatic cancer cells and the microenvironment is now a well‐recognized feature of carcinogenesis,6 elucidating the nature of this crosstalk, its functional consequences and clinical relevance is a real challenge. Recently, by using coculture experiments, we demonstrated that the crosstalk between tumor hepatocytes and aHSCs resulted in the creation of a permissive proangiogenic and ‐fibrogenic microenvironment and a change in hepatocyte behavior toward motility.7 Importantly, by integrative genomics based on HCC initially profiled in the laboratory of Dr Thorgeirsson (National Cancer Institute, Bethesda, MD), we demonstrated that the signature of this crosstalk was predictive of a poor prognosis and was associated with metastasis propensity.7 Interestingly, by using the same microarray data set, Tu et al. demonstrated that VASP was specifically overexpressed in a set of HCC from patients with a poor prognosis.13 Consistent with these findings, HCCs from this subset were shown to exhibit gene signatures associated with progenitor cell traits,16 protumorigenic properties of TGF‐β,17 and with the crosstalk between tumor hepatocytes and HSCs linked to an invasive phenotype.7 Altogether, these data suggest that targeting VASP or, more broadly, the crosstalk between aHSCs and the stroma may represent a relevant strategy in liver primary cancers and metastases.12 However, targeting aHSCs/myofibroblasts should be approached with caution, the role of these cells being probably much more complex than expected and perhaps organ specific. As an example, in pancreatic ductal adenocarcinoma, which is associated with an extensive stromal and fibrotic reaction, myofibroblasts were recently reported to exhibit a protective role.18 Thus, the depletion of myofibroblasts in genetically engineered mice prone to develop pancreatic tumors resulted in a reduced animal survival associated with invasive tumors and immunosuppression.18 From the recent advances in the area of systemic biology, it becomes evident that cancer cells and their microenvironment form a unique entity and a multifaceted biological system. Hence, it is expected that efficient anticancer therapies will have to consider the genetic heterogeneity of tumor cells together with the dynamic nature of the microenvironment. As illustrated by the study of Tu et al.,13 a better knowledge of the crosstalk between tumor cells and their microenvironment may help to design new combined therapies to efficiently target both the tumor cells and their microenvironment. Author names in bold designate shared co‐first authorship.