The transforming growth factor-beta (TGF-β ) belongs to a superfamily of cytokines that act on protein kinase receptors at the plasma membrane to induce a plethora of biological signals that regulate cell growth and death, differentiation, immune response, angiogenesis and inflammation. Dysregulation of its pathway contributes to a broad variety of pathologies, including cancer. TGF-β is an important regulatory tumor suppressor factor in epithelial cells, where it early inhibits proliferation and induces apoptosis. However, tumor cells develop mechanisms to overcome the TGF-β -induced suppressor effects. Once this occurs, cells may respond to this cytokine inducing other effects that contribute to tumor progression. Indeed, TGF-β induces epithelial-mesenchymal transition (EMT), a process that is favored in tumor cells and facilitates migration and invasion. Furthermore, TGF-β mediates production of mitogenic growth factors, which stimulate tumor proliferation and survival. Finally, TGF-β is a well known immunosuppressor and pro-angiogenic factor. Many studies have identified the overexpression of TGF-β 1 in various types of human cancer, which correlates with tumor progression, metastasis, angiogenesis and poor prognostic outcome. For these reasons, different strategies to block TGF-β pathway in cancer have been developed and they can be classified in: (1) blocking antibodies and ligand traps; (2) antisense oligos; (3) TβRII and/or ALK5 inhibitors; (4) immune response-based strategies; (5) other inhibitors of the TGF-β pathway. In this review we will overview the two faces of TGF-β signaling in the regulation of tumorigenesis and we will dissect how targeting the TGF-β pathway may contribute to fight against cancer.

A role for the NADPH oxidases NOX1 and NOX2 in liver fibrosis has been proposed, but the implication of NOX4 is poorly understood yet. The aim of this work was to study the functional role of NOX4 in different cell populations implicated in liver fibrosis: hepatic stellate cells (HSC), myofibroblats (MFBs) and hepatocytes. Two different mice models that develop spontaneous fibrosis (Mdr2−/−/p19ARF−/−, Stat3Δhc/Mdr2−/−) and a model of experimental induced fibrosis (CCl4) were used. In addition, gene expression in biopsies from chronic hepatitis C virus (HCV) patients or non-fibrotic liver samples was analyzed. Results have indicated that NOX4 expression was increased in the livers of all animal models, concomitantly with fibrosis development and TGF-β pathway activation. In vitro TGF-β-treated HSC increased NOX4 expression correlating with transdifferentiation to MFBs. Knockdown experiments revealed that NOX4 downstream TGF-β is necessary for HSC activation as well as for the maintenance of the MFB phenotype. NOX4 was not necessary for TGF-β-induced epithelial-mesenchymal transition (EMT), but was required for TGF-β-induced apoptosis in hepatocytes. Finally, NOX4 expression was elevated in patients with hepatitis C virus (HCV)-derived fibrosis, increasing along the fibrosis degree. In summary, fibrosis progression both in vitro and in vivo (animal models and patients) is accompanied by increased NOX4 expression, which mediates acquisition and maintenance of the MFB phenotype, as well as TGF-β-induced death of hepatocytes.

SummaryAlthough TGF- suppresses early stages of tumour development, it later contributes to tumour progression when cells become resistant to its suppressive effects. In addition to circumventing TGF--induced growth arrest and apoptosis, malignant tumour cells become capable of undergoing epithelial-to-mesenchymal transition (EMT), favouring invasion and metastasis. Therefore, defining the mechanisms that allow cancer cells to escape from the suppressive effects of TGF- is fundamental to understand tumour progression and to design specific therapies. Here, we have examined the role of Snail1 as a suppressor of TGF--induced apoptosis in murine non-transformed hepatocytes, rat and human hepatocarcinoma cell lines and transgenic mice. We show that Snail1 confers resistance to TGF--induced cell death and that it is sufficient to induce EMT in adult hepatocytes, cells otherwise refractory to this transition upon exposure to TGF-. Furthermore, we show that Snail1 silencing prevents EMT and restores the cell death response induced by TGF-. As Snail1 is a known target of TGF- signalling, our data indicate that Snail1 might transduce the tumour-promoting effects of TGF-, namely the EMT concomitant with the resistance to cell death.

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Transforming growth factor-beta (TGF-β) plays a dual role in hepatocytes, inducing both pro- and anti-apoptotic responses, whose balance decides cell fate. Survival signals are mediated by the epidermal growth factor receptor (EGFR) pathway, which is activated by TGF-β in these cells. Caveolin-1 (Cav1) is a structural protein of caveolae linked to TGF-β receptors trafficking and signaling. Previous results have indicated that in hepatocytes, Cav1 is required for TGF-β-induced anti-apoptotic signals, but the molecular mechanism is not fully understood yet. In this work, we show that immortalized Cav1−/− hepatocytes were more sensitive to the pro-apoptotic effects induced by TGF-β, showing a higher activation of caspase-3, higher decrease in cell viability and prolonged increase through time of intracellular reactive oxygen species (ROS). These results were coincident with attenuation of TGF-β-induced survival signals in Cav1−/− hepatocytes, such as AKT and ERK1/2 phosphorylation and NFκ-B activation. Transactivation of the EGFR pathway by TGF-β was impaired in Cav1−/− hepatocytes, which correlated with lack of activation of TACE/ADAM17, the metalloprotease responsible for the shedding of EGFR ligands. Reconstitution of Cav1 in Cav1−/− hepatocytes rescued wild-type phenotype features, both in terms of EGFR transactivation and TACE/ADAM17 activation. TACE/ADAM17 was localized in detergent-resistant membrane (DRM) fractions in Cav1+/+ cells, which was not the case in Cav1−/− cells. Disorganization of lipid rafts after treatment with cholesterol-binding agents caused loss of TACE/ADAM17 activation after TGF-β treatment. In conclusion, in hepatocytes, Cav1 is required for TGF-β-mediated activation of the metalloprotease TACE/ADAM17 that is responsible for shedding of EGFR ligands and activation of the EGFR pathway, which counteracts the TGF-β pro-apoptotic effects. Therefore, Cav1 contributes to the pro-tumorigenic effects of TGF-β in liver cancer cells.