Identification of specific gene expression signatures characteristic of oncogenic pathways is an important step toward molecular classification of human malignancies. Aberrant activation of the Met signaling pathway is frequently associated with tumor progression and metastasis. We defined the Met-dependent gene expression signature using global gene expression profiling of WT and Met-deficient primary mouse hepatocytes. Newly identified transcriptional targets of the Met pathway included genes involved in the regulation of oxidative stress responses as well as cell motility, cytoskeletal organization, and angiogenesis. To assess the importance of a Met-regulated gene expression signature, a comparative functional genomic approach was applied to 242 human hepatocellular carcinomas (HCCs) and 7 metastatic liver lesions. Cluster analysis revealed that a subset of human HCCs and all liver metastases shared the Met-induced expression signature. Furthermore, the presence of the Met signature showed significant correlation with increased vascular invasion rate and microvessel density as well as with decreased mean survival time of HCC patients. We conclude that the genetically defined gene expression signatures in combination with comparative functional genomics constitute an attractive paradigm for defining both the function of oncogenic pathways and the clinically relevant subgroups of human cancers. We have used c-Met conditional knockout mice in which c-met gene is inactivated in postnatal hepatocytes by Alb-Cre recombinase (MetLivKO) to directly evaluate the biological outcome of a selective loss of c-Met in hepatocytes on liver regeneration. The priming events appear to be intact in MetLivKO livers. Up-regulation of stress response (e.g MAFK, IKBZ, SOCS3) and early growth response (e.g. MYC, DUSP 1 and 6) genes as judged by microarray profiling was similar in MetLivKO regenerating livers as compared to Alb-Cre controls. This was consistent with an early induction of NF-kB, STAT3, and MAPK/ERK. Nevertheless, in the absence of c-Met signaling in the hepatocytes, ERK phosporylation rapidly declined although it remained high in Cre-Ctrl livers. Also, MetLivKO mice displayed impaired liver regeneration as determined by a decrease in BrdU incorporation and a delay in timely progression into mitosis. Upstream signaling pathways involved in the block of G2-M transition included lack of EGR1 transcription factor induction, and inability to up-regulate the levels of cdc2, aurora B and Mad2 followed by decreased histone 3 phosphorylation and lag in chromatin condensation. However, after a delayed passage through G2 phase, c-Met deficient cells eventually entered mitosis. In culture, EGF treatment increased proliferation of MetLivKO hepatocytes and restored expression levels of cell cycle regulators aurora B and Mad2 albeit to a lesser degree than in similarly treated Cre-Ctrl hepatocytes. In conclusion, our results assign a novel function for HGF/c-Met signaling in regulation of G2/M transition during liver regeneration and implicate EGR1 as a potential G2/M target of HGF/c-Met pathway. HGF has been reported to have both positive and negative effects on carcinogenesis. Here we show that the loss of c-Met signaling in hepatocytes enhanced rather than suppressed the early stages of chemical hepatocarcinogenesis. c-Met conditional knockout mice (c-metfl/fl, AlbCre+/-; MetLivKO) treated with N-nitrosodiethylamine (DEN) developed significantly more and bigger tumors and with a shorter latency as compared with control (wt/wt, AlbCre+/-; Cre-Ctrl) mice. Accelerated tumor development was associated with increased rate of cell proliferation and prolonged activation of epidermal growth factor receptor (EGFR) signaling. MetLivKO livers treated with DEN also displayed elevated lipid peroxidation, decreased ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG), and upregulation of superoxide dismutase 1 (Sod1) and heat shock protein 70 (Hsp70), all consistent with increased oxidative stress. Likewise, gene expression profiling performed at 3 and 5 months after DEN treatment revealed upregulation of genes associated with cell proliferation and stress responses in c-Met mutant livers. The negative effects of c-Met-deficiency were reversed by chronic oral administration of anti-oxidant N-acetylcysteine (NAC). NAC blocked the EGFR activation and reduced the DEN-initiated hepatocarcinogenesis to the levels of Cre-Ctrl mice. These results argue that intact HGF/c-Met signaling is essential for maintaining normal redox homeostasis in the liver and has tumor suppressor effect(s) during the early stages of DEN-induced hepatocarcinogenesis
| Kaposi-Novak, Pal; Lee, Ju-Seog; Gomez-Quiroz, Luis et al. (2006) Met-regulated expression signature defines a subset of human hepatocellular carcinomas with poor prognosis and aggressive phenotype. J Clin Invest 116:1582-95 |
