Mixed signals at tumor margins: The Hippo signaling pathway has been implicated in tumor growth, sparking interest in the pathway as a potential therapeutic target. In a study of liver cancer in genetically manipulated mice, Moya et al. discovered that the role of this pathway in tumorigenesis is more complex than previously appreciated. They confirmed that activation of the Hippo pathway within tumor cells drives tumor growth; however, they also found that activation of the pathway in adjacent healthy cells has the opposite effect, suppressing tumor growth. Whether tumor cells survive or are eliminated thus appears to depend on competing signals produced by the tumor and surrounding tissue. Science , this issue p. 1029

Cancer stem cells (CSCs) are thought to be persistent in tumours due to their chemoresistance and to cause relapse and metastasis. Hepatic carcinomas displaying hepatic progenitor cell (HPC) features have been associated with a poor prognosis, though it remains unclear how CSCs relate to these different histological subtypes. Candidate CSCs were isolated using the side population (SP) technique from primary tissue samples diagnosed as keratin(K)19-negative or -positive hepatocellular carcinoma (HCC) or as combined hepatocellular/cholangiocarcinoma and analysed for gene and protein expression. The effect of laminin-332 was analysed in vitro by using HCC cell lines and in vivo using a xenograft mouse model. The size of the SP correlated with the degree of HPC features found in human hepatic cancer, and also showed an elevated mRNA expression of biliary/HPC markers and the extracellular matrix marker LAMC2, the gene encoding the laminin γ2-chain. Immunopositivity for the γ2-chain of laminin-332 was seen in the extracellular matrix surrounding small HPC-like tumour cells with a low proliferation rate. In vitro, laminin-332 increased K19 expression, phosphorylated mTOR and decreased phospho-histone H3 expression, indicating reduced cell mitosis. The effect of laminin-332 was enhanced upon mTORC1 inhibition and diminished when inhibiting mTORC1+C2. Resistance to doxorubicin and sorafenib treatment, and the SP fraction increased in the coated condition. In vivo, laminin-332 reduced tumour growth and sustained K19 expression. In this study we identified a prominent role for laminin-332 as part of the specialised CSC niche in maintaining and supporting cell 'stemness', which leads to chemoresistance and quiescence.

The Hippo signaling pathway is widely considered a master regulator of organ growth because of the prominent overgrowth phenotypes caused by experimental manipulation of its activity. Contrary to this model, we show here that removing Hippo transcriptional output did not impair the ability of the mouse liver and Drosophila eyes to grow to their normal size. Moreover, the transcriptional activity of the Hippo pathway effectors Yap/Taz/Yki did not correlate with cell proliferation, and hyperactivation of these effectors induced gene expression programs that did not recapitulate normal development. Concordantly, a functional screen in Drosophila identified several Hippo pathway target genes that were required for ectopic overgrowth but not normal growth. Thus, Hippo signaling does not instruct normal growth, and the Hippo-induced overgrowth phenotypes are caused by the activation of abnormal genetic programs.

Activated hepatic stellate cells (aHSC) are the main source of extra cellular matrix in liver fibrosis. Activation is classically divided in two phases: initiation and perpetuation. Currently, HSC-based therapeutic candidates largely focus on targeting the aHSCs in the perpetuation phase. However, the importance of HSC initiation during chronic liver disease (CLD) remains unclear. Here, we identified transcriptional programs of initiating and activated HSCs by RNA sequencing, using in vitro and in vivo mouse models of fibrosis. Importantly, we show that both programs are active in HSCs during murine and human CLD. In human cirrhotic livers, scar associated mesenchymal cells employ both transcriptional programs at the single cell level. Our results indicate that the transcriptional programs that drive the initiation of HSCs are still active in humans suffering from CLD. We conclude that molecules involved in the initiation of HSC activation, or in the maintenance of aHSCs can be considered equally important in the search for druggable targets of chronic liver disease.