The molecular signals that control the maintenance and activation of liver stem/progenitor cells are poorly understood, and the role of liver progenitor cells in hepatic tumorigenesis is unclear. We report here that liverspecific deletion of the neurofibromatosis type 2 (Nf2) tumor suppressor gene in the developing or adult mouse specifically yields a dramatic, progressive expansion of progenitor cells throughout the liver without affecting differentiated hepatocytes. All surviving mice eventually developed both cholangiocellular and hepatocellular carcinoma, suggesting that Nf2 À/À progenitors can be a cell of origin for these tumors. Despite the suggested link between Nf2 and the Hpo/Wts/Yki signaling pathway in Drosophila, and recent studies linking the corresponding Mst/Lats/Yap pathway to mammalian liver tumorigenesis, our molecular studies suggest that Merlin is not a major regulator of YAP in liver progenitors, and that the overproliferation of Nf2 À/À liver progenitors is instead driven by aberrant epidermal growth factor receptor (EGFR) activity. Indeed, pharmacologic inhibition of EGFR blocks the proliferation of Nf2 À/À liver progenitors in vitro and in vivo, consistent with recent studies indicating that the Nf2-encoded protein Merlin can control the abundance and signaling of membrane receptors such as EGFR. Together, our findings uncover a critical role for Nf2/Merlin in controlling homeostasis of the liver stem cell niche. The remarkable regenerative capacity of the mammalian liver is well known (Fausto et al. 2006;Michalopoulos 2007). In response to liver cell loss, existing hepatocytes and cholangiocytes (bile duct cells) re-enter the cell cycle to maintain or restore the original liver volume and biliary tree. When pathological or experimental conditions that block hepatocyte proliferation also exist, facultative liver progenitor cells, known in rodents as ''oval cells'' (OCs) for their morphological appearance (Farber 1956), emerge and expand from the most terminal branches of the biliary tree (Fausto and Campbell 2003;Fausto 2004;Roskams et al. 2004;Alison 2005;Theise 2006). Like embryonic hepatoblasts (HBs), OCs are considered to be bipotential, and can give rise to both hepatocytes and cholangiocytes (Evarts et al. 1987;Sell 2001). Cells that are thought to be equivalent to OCs have been identified in humans, and are presumed to also be liver progenitors (Roskams et al. 2004). However, it has not been possible to define the origin, potential, or molecular features of human liver progenitor cells.Chemically induced liver tumors in mice often feature an initial expansion of OCs, suggesting that they can be the cell of origin of at least some liver tumors (Sell 2001;Roskams 2006). However, genetically defined animal models that feature primary OC expansion are rare (Jakubowski et al. 2005). Instead, OCs appear in some genetically engineered models of liver tumorigenesis, but only secondary to hepatocellular dysplasia/neoplasia and inflammation (Sandgren et al. 1989;Santoni-Rugiu et al. 1996;Lu...

The neurofibromatosis type 2 (NF2) tumor suppressor, Merlin, is a FERM (Four point one, Ezrin, Radixin, Moesin) domain-containing protein whose loss results in defective morphogenesis and tumorigenesis in multiple tissues. Like the closely related ERM proteins (Ezrin, Radixin and Moesin), Merlin may organize the plasma membrane by assembling membrane protein complexes and linking them to the cortical actin cytoskeleton. We previously found that Merlin is a critical mediator of contact-dependent inhibition of proliferation and is required for the establishment of stable adherens junctions (AJs) in cultured cells. Here we delineate the molecular function of Merlin in AJ establishment in epidermal keratinocytes in vitro and confirm that a role in AJ establishment is an essential function of Merlin in vivo. Our studies reveal that Merlin can associate directly with α-catenin and link it to Par3, thereby providing an essential link between the AJ and the Par3 polarity complex during junctional maturation.

There is increasing evidence that p21Cip1 and p27 Kip1are requisite positive regulators of cyclin D1⅐CDK4 assembly and nuclear accumulation. Both Cip and Kip proteins can promote nuclear accumulation of cyclin D1, but the underlying mechanism has not been elucidated. We now provide evidence that p21 Cip1 promotes the nuclear accumulation of cyclin D1 complexes via inhibition of cyclin D1 nuclear export. In vivo, we demonstrate that p21Cip1 can inhibit glycogen synthase kinase 3␤-triggered cyclin D1 nuclear export and phosphorylation-dependent nucleocytoplasmic shuttling. Furthermore, we find that cyclin D1 nuclear accumulation in p21/p27 null cells can be restored through inhibition of CRM1-depenendent nuclear export. The ability of p21Cip1 to inhibit cyclin D1 nuclear export correlates with its ability to bind to Thr-286-phosphorylated cyclin D1 and thereby prevents cyclin D1⅐CRM1 association.Cell cycle progression requires the sequential and ordered activation of the cyclin-dependent kinases (CDKs) 1 and inactivation of CDK inhibitors. D-type cyclins (D1, D2, D3), the regulatory subunit of the CDK4/6 kinase, function as critical mitogenic sensors that integrate growth factor-initiated signals with G 1 -phase progression (1). Mitogenic stimuli trigger the accumulation of active cyclin D1⅐CDK4 complexes through both increased cyclin expression and decreased cyclin proteolysis and through the promotion of cyclin D⅐CDK4 assembly (1). Mitogen-dependent expression of cyclin D1 depends upon growth factor-mediated activation of a signal transduction cascade consisting of Ras, Raf-1, and the extracellular signalregulated protein kinases (ERK1 and 2) (2-8). Accumulation of cyclin D1 during G 1 also relies upon mitogen-dependent inhibition of glycogen synthase kinase 3␤ (GSK-3␤) via activation of phosphatidylinositol 3-kinase and Akt (protein kinase B) (9). The subcellular localization of cyclin D1 complexes also oscillates during the cell cycle, being nuclear throughout G 1 -phase and cytoplasmic during the remainder of interphase (9 -11). Nuclear export of cyclin D1 is a major determinant of cyclin D1⅐CDK4 localization (12). Phosphorylation of cyclin D1 at a single threonine residue, Thr-286, by GSK-3␤ facilitates the binding of cyclin D1 with the nuclear exportin, CRM1, and thereby promotes cyclin D1 nuclear export (12). Because neither cyclin D1 nor CDK4 has a recognizable nuclear localization signal, the mechanisms governing cyclin D1 nuclear import remain undefined.