JNK proteins have been shown to be involved in liver carcinogenesis in mice, but the extent of their involvement in the development of human liver cancers is unknown. Here, we show that activation of JNK1 but not JNK2 was increased in human primary hepatocellular carcinomas (HCCs).

The AP-1 transcription factor c-Jun is a key regulator of hepatocyte proliferation. Mice lacking c-Jun in the liver (c-jun ⌬li* ) display impaired liver regeneration after partial hepatectomy (PH). This phenotype correlates with increased protein levels of the cdk-inhibitor p21 in the liver. We performed PH experiments in several double-knockout mouse models to genetically identify the signaling events regulated by c-Jun. Inactivation of p53 in c-jun ⌬li* mice abrogated both hepatocyte cell cycle block and increased p21 protein expression. Consistently, liver regeneration was rescued in c-jun ⌬li* p21 −/− double-mutant mice. This indicated that c-Jun controls hepatocyte proliferation by a p53/p21-dependent mechanism. Analyses of p21 mRNA and protein expression in livers of c-jun ⌬li* mice after PH revealed that the accumulation of p21 protein is due to a post-transcriptional/post-translational mechanism. We have investigated several candidate pathways implicated in the regulation of p21 expression, and observed increased activity of the stress kinase p38 in regenerating livers of c-jun ⌬li* mice. Importantly, conditional deletion of p38␣ in livers of c-jun ⌬li* mice fully restored hepatocyte proliferation and attenuated increased p21 protein levels after PH. These data demonstrate that c-Jun/AP-1 regulates liver regeneration through a novel molecular pathway that involves p53, p21, and the stress kinase p38␣.[Keywords: c-Jun; p53; p21; p38/liver regeneration; partial hepatectomy] Supplemental material is available at http://www.genesdev.org. Liver regeneration triggered by two-third partial hepatectomy (PH) is a well-established model system in rodents for studying the molecular mechanisms of cell cycle control. Hepatocytes that are normally quiescent and highly differentiated cells enter the S-phase rapidly after surgery and undergo one to two rounds of replication in order to fully restore liver mass (Diehl 2002;Fausto 2004;Taub 2004). Importantly, abnormal regeneration contributes to the pathogenesis of fulminant liver failure, cirrhosis, and primary liver cancer. Initiation of liver regeneration occurs when hepatocytes are primed to synchronously escape quiescence and enter the prereplicative phase of the cell cycle (G1) after PH (Fausto 2004). The priming phase is controlled by several cytokines such as tumor necrosis factor ␣ (TNF␣) and Interleukin-6 (IL-6) (Akerman et al. 1992;Cressman et al. 1996;Yamada et al. 1997). Cytokines activate a variety of transcription factors important during the initial stages of liver regeneration, including nuclear factor-B (NF-B), signal transducer and activator of transcription 3 (STAT3), CCAAT enhancer-binding protein ␤ (C/EBP␤), and activator protein 1 (AP-1) (Cressman et al. 1995;FitzGerald et al. 1995;Heim et al. 1997;Greenbaum et al. 1998). At later stages hepatocyte growth factor (HGF), transforming growth factor ␣ (TGF␣), and heparin-binding epidermal growth factor (HB-EGF) stimulate S-phase entry of hepatocytes (Mead and Fausto 1989;Borowiak et al. 2004;Huh et al....

Properly regulated intercellular adhesion is critical for normal development of all metazoan organisms. Adherens junctions play an especially prominent role in development because they link the adhesive function of cadherin-catenin protein complexes to the dynamic forces of the actin cytoskeleton, which helps to orchestrate a spatially confined and very dynamic assembly of intercellular connections. Intriguingly, in addition to maintaining intercellular adhesion, cadherin-catenin proteins are linked to several major developmental signaling pathways crucial for normal morphogenesis. In this article we will highlight the key genetic studies that uncovered the role of cadherin-catenin proteins in vertebrate development and discuss the potential role of these proteins as molecular biosensors of external cellular microenvironment that may spatially confine signaling molecules and polarity cues to orchestrate cellular behavior throughout the complex process of normal morphogenesis.

The mitogen-activated protein kinase (MAPK) p38α is involved in numerous biological processes and is a drug target for inflammation-associated diseases. Genetic analysis in mice demonstrated that fetuses lacking p38α are embryonic lethal owing to impaired placental development. The function of p38α in mice after birth remained unclear until conditional alleles of p38α were used. It was found that p38α is essential for lung function in both neonatal and adult mice. Increased proliferation and impaired differentiation are the hallmarks of p38α-deficient cells. Moreover, mice deficient in p38α are prone to cancer development using carcinogen or oncogene-induced cancer models. p38α can suppress cell proliferation by antagonizing the JNK/c-Jun pathway, which is an important regulator of proliferation and apoptosis. These findings suggest that therapeutic inhibition of p38 might lead to unwanted proliferation. Therefore, a combined inhibition of p38 and other pathways, such as the JNK pathway, should be considered for targeting cancer inflammation.

The mitogen-inducible gene-6 (mig-6) is a multi-adaptor protein implicated in the regulation of the HER family of receptor tyrosine kinases. We have reported recently that mig-6 is a negative regulator of epidermal growth factor receptor (EGFR)-dependent skin morphogenesis and tumor formation in vivo. In the liver, ablation of mig-6 leads to an increase in EGFR protein levels, suggesting that mig-6 is a negative regulator of EGFR function. In line with this observation, primary hepatocytes isolated from mig-6 knockout and wild-type control mice display sustained mitogenic signaling in response to EGF. In order to explore the role of mig-6 in the liver in vivo, we analyzed liver regeneration in mig-6 knockout and wild-type control mice. Interestingly, mig-6 knockout mice display enhanced hepatocyte proliferation in the initial phases after partial hepatectomy. This phenotype correlates with activation of endogenous EGFR signaling, predominantly through the protein kinase B pathway. In addition, mig-6 is an endogenous inhibitor of EGFR signaling and EGFinduced tumor cell migration in human liver cancer cell lines. Moreover, mig-6 is downregulated in human hepatocellular carcinoma and this correlates with increased EGFR expression. Conclusion: Our data implicate mig-6 as a regulator of EGFR activity in hepatocytes and as a suppressor of EGFR signaling in human liver cancer. (HEPATOLOGY 2010;51: 1383-1390.)I n rodents, the liver has a tremendous capacity to regenerate. Normally, hepatocytes are in a quiescent state and rarely divide. In response to two-thirds partial hepatectomy (PH), however, 95% of all hepatocytes synchronously re-enter the cell cycle and restore the liver to its full size. 1 This rapid growth provides an excellent model to study the molecular mechanisms of cell proliferation in vivo. Importantly, defects in liver regeneration can contribute to the development of severe diseases such as liver cirrhosis and liver cancer. 2 The process of liver regeneration is dependent on various growth factors, cytokines, and metabolic processes. Cytokines, like tumor necrosis factor-␣ or interleukin-6, act as the priming factors of liver regeneration, driving quiescent hepatocytes into the cell cycle. 3 Several growth factors and receptor tyrosine kinases control cell cycle progression by stimulating the S-phase entry of hepatocytes. The epidermal growth factor receptor (EGFR), heparin-binding EGFlike growth factor (HB-EGF), transforming growth factor-␣ (TGF␣), amphiregulin, and the MET receptor have been described to be potent regulators of liver regeneration. [4][5][6][7][8] Mitogen-inducible gene-6 (mig-6)-also known as RALT, gene33, or Errfi1-is an adaptor protein implicated in the regulation of receptor tyrosine kinases. 9,10 Mig-6 can be induced by a variety of external stimuli including growth factors, hypoxia, and stress. 11 Overexpression or small interfering RNA (siRNA)-mediated knockdown studies have shown that mig-6 is a negative regulator of EGF receptor signaling. Mig-6 is induced upon EGFR activati...