BACKGROUND & AIMS Human primary liver cancer (PLC) is classified into biologically distinct subgroups, based on cellular origin. Liver cancer stem cells (CSCs) have been recently described. We investigated the ability of distinct lineages of hepatic cells to become liver CSCs and the phenotypic and genetic heterogeneity of PLC. METHODS We transduced mouse primary hepatic progenitor cells (HPC), lineage-committed hepatoblasts, and differentiated adult hepatocytes with transgenes encoding oncogenic H-Ras and simian virus 40 large-T antigen. The CSC properties of transduced cells and their ability to form tumors were tested by standard in vitro and in vivo assays and transcriptome profiling. RESULTS Irrespective of origin, all transduced cells acquired markers of CSC/progenitor cells, side populations, and self-renewal capacity in vitro. They also formed a broad spectrum of liver tumors, ranging from cholangiocarcinoma to hepatocellular carcinoma, which resembled human liver tumors, based on genomic and histologic analyses. The tumor cells co-expressed hepatocyte (HNF4A), biliary progenitor cell (keratin 19, EpCAM, A6), and mesenchyme (vimentin) markers and showed disregulation of genes that control the epithelial–mesenchymal transition. Gene expression analyses could distinguish tumors of different cellular origin, indicating the contribution of lineage-stage dependent genetic changes to malignant transformation. Activation of c-Myc and its target genes was required to reprogram adult hepatocytes into CSC and for tumors to develop. Stable knockdown of c-Myc in transformed adult hepatocytes reduced their CSC properties in vitro and suppressed growth of tumors in immunodeficient mice. CONCLUSIONS Any cell type in the mouse hepatic lineage can undergo oncogenic reprogramming into a CSC, by activating different cell type-specific pathways. Identification of common and cell-of-origin specific phenotypic and genetic changes could provide new therapeutic targets for liver cancer.

The relative contribution of hepatocyte growth factor (HGF)/MET and epidermal growth factor (EGF)/EGF receptor (EGFR), two key signal transduction systems in the normal and diseased liver, to fate decisions of adult hepatic progenitor cells (HPCs) has not been resolved. Here, we developed a robust culture system that permitted expansion and genetic manipulation of cells capable of multilineage differentiation in vitro and in vivo to examine the individual roles of HGF/MET and EGF/EGFR in HPC self-renewal and binary cell fate decision. By employing loss-of-function and rescue experiments in vitro, we showed that both receptors collaborate to increase the selfrenewal of HPCs through activation of the extracellular signal-regulated kinase (ERK) pathway. MET was a strong inducer of hepatocyte differentiation by activating AKT and signal transducer and activator of transcription (STAT3). Conversely, EGFR selectively induced NOTCH1 to promote cholangiocyte specification and branching morphogenesis while concomitantly suppressing hepatocyte commitment. Furthermore, unlike the deleterious effects of MET deletion, the liver-specific conditional loss of Egfr facilitated rather than suppressed progenitormediated liver regeneration by switching progenitor cell differentiation toward hepatocyte lineage. These data provide new insight into the mechanisms regulating the stemness properties of adult HPCs and reveal a previously unrecognized link between EGFR and NOTCH1 in directing cholangiocyte differentiation.