TGFβ and LXR signaling in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is one of the most prevalent cancer types in the Western world and in the Asia-pacific regions, with its incidence expected to rise up to 22 million cases till 2020. Hepatocellular carcinoma etiology is mainly due to hepatitis B (HBV) and hepatitis C (HCV) infections, and to a lesser extent it is determined by the development of alcohol-driven cirrhosis and non-alcoholic steatohepatitis (NASH). Furthermore, HCC is characterized by a high mortality rate, with poor prognostic expectance and limited therapeutic options currently available in the clinics. Transforming growth factor beta (TGFβ) is a pleiotropic cytokine with a janus-role in HCC and in other malignancies. TGFβ can in fact elicit either tumor-suppressive and tumor- promoting effects depending on tumor stage, microenvironmental and immunological cues. In HCC specifically, TGFβ determines cytostasis and cellular senescence during the first stages of tumor development, while it enhances HCC malignancy and progression in the later stages due to increased invasiveness, acquired resistance to cytostatic actions and tumor immunotolerance. Liver X receptors (LXRα/NR1H3 and LXRβ/NR1H2) are transcription factors of the nuclear hormone receptor family, which play an important role in oxysterol metabolism and reverse- cholesterol transport to the liver. Their involvement in malignancies has been studied so far to a limited extend, with evidence of both tumor-suppressive -via cytostatic mechanisms- and tumor- immunotolerance activities. Moreover, the potential crosstalk of LXR and TGFβ pathways has not been yet unraveled in the context of hepatocellular carcinoma. We have described (Paper I) a high-content imaging platform for the screening of small molecules able to revert the TGFβ-induced epithelial to mesenchymal transition (EMT) in human keratinocytes. This screening allowed us to identify LXR agonists as epithelial plasticity modulators in established terminally differentiated and mouse embryonic fibroblast, as well as in epithelial and mesenchymal HCC cell lines. We have identified (Paper II) the transcription factor SNAI1 (Snail) as the mediator of the crosstalk between TGFβ and LXRα pathways in epithelial and mesenchymal HCC cell models. LXRα activation diminishes the transcriptional induction of SNAI1 by TGFβ, thus antagonizing the induction of mesenchymal features and the production of reactive oxygen species by TGFβ. However, we have unraveled that LXRα and TGFβ signaling still positively interact in increasing cytostasis in HCC, in order to preserve liver epithelial features. We have described (Paper III) that LXRα activation counteracts the transcriptional induction of α smooth muscle actin (αSMA), a major hallmark of fibroblast activation, elicited by TGFβ in patient-derived primary liver fibroblasts. In conclusion, we herein report that the signaling crosstalk between TGFβ and LXRα pathways results in antagonistic effects either on parenchymal and fibroblast cell lines representative of the HCC disease, suggesting the potential future application of LXR agonists as clinical therapeutic options