The mitogen-activated protein kinase (MAPK) p38alpha controls inflammatory responses and cell proliferation. Using mice carrying conditional Mapk14 (also known as p38alpha) alleles, we investigated its function in postnatal development and tumorigenesis. When we specifically deleted Mapk14 in the mouse embryo, fetuses developed to term but died shortly after birth, probably owing to lung dysfunction. Fetal hematopoietic cells and embryonic fibroblasts deficient in p38alpha showed increased proliferation resulting from sustained activation of the c-Jun N-terminal kinase (JNK)-c-Jun pathway. Notably, in chemical-induced liver cancer development, mice with liver-specific deletion of Mapk14 showed enhanced hepatocyte proliferation and tumor development that correlated with upregulation of the JNK-c-Jun pathway. Furthermore, inactivation of JNK or c-Jun suppressed the increased proliferation of Mapk14-deficient hepatocytes and tumor cells. These results demonstrate a new mechanism whereby p38alpha negatively regulates cell proliferation by antagonizing the JNK-c-Jun pathway in multiple cell types and in liver cancer development.

contributed equally to this workThe transcription factor AP-1, composed of Jun and Fos proteins, is a major target of mitogen-activated signal transduction pathways. However, little is known about AP-1 function in normal cycling cells. Here we report that the quantity and the phosphorylation state of the c-Jun and JunB proteins vary at the M±G 1 transition. Phosphorylation of JunB by the p34 cdc2 ±cyclin B kinase is associated with lower JunB protein levels in mitotic and early G 1 cells. In contrast, c-Jun levels remain constant while the protein undergoes N-terminal phosphorylation, increasing its transactivation potential. Since JunB represses and c-Jun activates the cyclin D1 promoter, these modi®cations of AP-1 activity during the M±G 1 transition could provide an impetus for G 1 progression by a temporal increase in cyclin D1 transcription. These ®ndings constitute a novel example of a reciprocal connection between transcription factors and the cell cycle machinery.

Molecular mechanisms responsible for hepatocellular carcinoma (HCC) remain largely unknown. Using genetically engineered mouse models, we show that hepatocyte-specific expression of unconventional prefoldin RPB5 interactor (URI) leads to a multistep process of HCC development, whereas its genetic reduction in hepatocytes protects against diethylnitrosamine (DEN)-induced HCC. URI inhibits aryl hydrocarbon (AhR)- and estrogen receptor (ER)-mediated transcription of enzymes implicated in L-tryptophan/kynurenine/nicotinamide adenine dinucleotide (NAD(+)) metabolism, thereby causing DNA damage at early stages of tumorigenesis. Restoring NAD(+) pools with nicotinamide riboside (NR) prevents DNA damage and tumor formation. Consistently, URI expression in human HCC is associated with poor survival and correlates negatively with L-tryptophan catabolism pathway. Our results suggest that boosting NAD(+) can be prophylactic or therapeutic in HCC.

Understanding stage-dependent oncogenic mechanisms is critical to develop not only targeted therapies, but also diagnostic markers and preventive strategies. The mechanisms acting during cancer initiation remain elusive, largely owing to a lack of suitable animal models and limited availability of human precancerous lesions. Here we show using genetic mouse models specific for liver cancer initiation, that survival of initiated cancer cells is controlled by c-Jun, independently of p53, through suppressing c-Fos-mediated apoptosis. Mechanistically, c-Fos induces SIRT6 transcription, which represses survivin by reducing histone H3K9 acetylation and NF-κB activation. Importantly, increasing the level of SIRT6 or targeting the anti-apoptotic activity of survivin at the initiation stage markedly impairs cancer development. Moreover, in human dysplastic liver nodules, but not in malignant tumours, a specific expression pattern with increased c-Jun-survivin and attenuated c-Fos-SIRT6 levels was identified. These results reveal a regulatory network connecting stress response and histone modification in liver tumour initiation, which could be targeted to prevent liver tumorigenesis.