Background and Aims: Although the growth suppressor Hippo pathway has been implicated in hepatocellular carcinoma (HCC) pathogenesis, it is unknown at which stage of hepatocarcinogenesis its dysregulation occurs. We investigated in early rat and human preneoplastic lesions whether overexpression of the transcriptional co-activator Yesassociated protein (YAP) is an early event.Methods: The experimental model used is the Resistant-Hepatocyte (R-H) rat model.

The establishment of the role of MET in human cancer has led to the development of small-molecule inhibitors, many of which are currently in clinical trials. Thus far, nothing is known about their therapeutic efficacy and the possible emergence of resistance to treatment, a problem that has been often observed with other receptor tyrosine kinase (RTK) inhibitors. To predict mechanisms of acquired resistance, we generated resistant cells by treating MET-addicted cells with increasing concentrations of the MET small-molecule inhibitors PHA-665752 or JNJ38877605. Resistant cells displayed MET gene amplification, leading to increased expression and constitutive phosphorylation of MET, followed by subsequent amplification and overexpression of wild-type (wt) KRAS. Cells harboring KRAS amplification progressively lost their MET dependence and acquired KRAS dependence. Our results suggest that MET and KRAS amplification is a general mechanism of resistance to specific MET inhibitors given that similar results were observed with two small inhibitors and in different cell lines of different histotypes. To our knowledge, this is the first report showing that overexpression of wt KRAS can overcome the inhibitory effect of a RTK inhibitor. In view of the fact that cellular models of resistance to inhibitors targeting other tyrosine kinases have predicted and corroborated clinical findings, our results provide insights into strategies for preventing and/or overcoming drug resistance.

MicroRNAs (miRNA) are a recently identified class of noncoding, endogenous, small RNAs that regulate gene expression, mainly at the translational level. These molecules play critical roles in several biological processes, such as cell proliferation and differentiation, development, and aging. It is also known that miRNAs play a role in human cancers where they can act either as oncogenes, down-regulating tumor suppressor genes, or as onco-suppressors, targeting molecules critically involved in promotion of tumor growth. One of such molecules is the tyrosine kinase receptor for hepatocyte growth factor, encoded by the MET oncogene. The MET receptor promotes a complex biological program named ''invasive growth'' that results from stimulation of cell motility, invasion, and protection from apoptosis. This oncogene is deregulated in many human tumors, where its most frequent alteration is overexpression. In this work, we have identified three miRNAs (miR-34b, miR-34c, and miR199a*) that negatively regulate MET expression. Inhibition of these endogenous miRNAs, by use of antagomiRs, resulted in increased expression of MET protein, whereas their exogenous expression in cancer cells blocked MET-induced signal transduction and the execution of the invasive growth program, both in cells expressing normal levels of MET and in cancer cells overexpressing a constitutively active MET. Moreover, we show that these same miRNAs play a role in regulating the MET-induced migratory ability of melanomaderived primary cells. In conclusion, we have identified miRNAs that behave as oncosuppressors by negatively targeting MET and might thus provide an additional option to inhibit this oncogene in tumors displaying its deregulation.

In spite of the established knowledge of the genetic alterations responsible for cancer onset, the genes promoting and maintaining the invasive/metastatic phenotype are still elusive. The MET proto-oncogene, encoding the tyrosine kinase receptor for hepatocyte growth factor (HGF), senses unfavorable micro-environmental conditions and drives cell invasion and metastasis. MET overexpression, often induced by tumor hypoxia, leads to constitutive activation of the receptor and correlates with poor prognosis. To establish the role of MET in different phases of tumor progression, we developed an inducible lentiviral delivery system of RNA interference. Silencing the endogenous MET gene, overexpressed in tumor cells, resulted in (i) impairment of the execution of the full invasive growth program in vitro, (ii) lack of tumor growth and (iii) decreased generation of experimental metastases in vivo. Notably, silencing MET in already established metastases led to their almost complete regression. This indicates that persistent expression of the MET oncogene is mandatory until the advanced phases of cancer progression.