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The incidence of inflammatory bowel diseases (IBD) emerged with Westernisation of dietary habits worldwide. Crohn’s disease and ulcerative colitis are chronic debilitating conditions that afflict individuals with substantial morbidity and challenge healthcare systems across the globe. Since identification and characterisation of calprotectin (CP) in the 1980s, faecal CP emerged as significantly validated, non-invasive biomarker that allows evaluation of gut inflammation. Faecal CP discriminates between inflammatory and non-inflammatory diseases of the gut and portraits the disease course of human IBD. Recent studies revealed insights into biological functions of the CP subunits S100A8 and S100A9 during orchestration of an inflammatory response at mucosal surfaces across organ systems. In this review, we summarise longitudinal evidence for the evolution of CP from biomarker to rheostat of mucosal inflammation and suggest an algorithm for the interpretation of faecal CP in daily clinical practice. We propose that mechanistic insights into the biological function of CP in the gut and beyond may facilitate interpretation of current assays and guide patient-tailored medical therapy in IBD, a concept warranting controlled clinical trials.

Key Points Jun N-terminal kinases (JNKs) and p38 mitogen-activated protein kinases (MAPKs) have important roles in the signalling mechanisms that orchestrate cellular responses to many types of stresses, but also control the proliferation, differentiation, survival and migration of specific cell types. JNKs and p38 MAPKs can exert antagonistic effects on cell proliferation and survival, which depend on cell type-specific differences, as well as on the intensity and duration of the signal and the crosstalk between other signalling pathways. Crosstalk between the JNK and p38 MAPK pathways is emerging as an important regulatory mechanism in many cellular responses. The JNK and p38 MAPK pathways regulate the activity and expression of key inflammatory mediators, including cytokines and proteases, which may function as potent cancer promoters. The specific role of individual JNK and p38 MAPK family members in particular cellular processes in vivo has been addressed by gene-targeting experiments in mice. Genetically engineered mouse models have confirmed the importance of these pathways for tumorigenesis in various organs. The expression or activity of JNK and p38 MAPK pathway components is often altered in human tumours and cancer cell lines. Given the many tumorigenesis-related functions that these kinases can control, both in the cancer cell and in the tumour microenvironment, it is important to carefully consider the type of tumour before attempting to modulate these pathways for cancer therapy. Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) signalling pathways are involved in a vast array of cellular events and are often altered in human cancers. This Review discusses how studies in mouse models have contributed to our understanding of the functions of these pathways in different cancers, and the implications for therapeutic approaches. Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK) family members function in a cell context-specific and cell type-specific manner to integrate signals that affect proliferation, differentiation, survival and migration. Consistent with the importance of these events in tumorigenesis, JNK and p38 MAPK signalling is associated with cancers in humans and mice. Studies in mouse models have been essential to better understand how these MAPKs control cancer development, and these models are expected to provide new strategies for the design of improved therapeutic approaches. In this Review we highlight the recent progress made in defining the functions of the JNK and p38 MAPK pathways in different cancers.

To date, the dogma in the field has been that RANKL, an essential cytokine in osteoclast maturation, is released by osteoblasts as a way to coordinate bone growth and bone loss during adult bone remodeling. Now, Hiroshi Takayanagi and colleagues, as well as Charles O'Brien and colleagues, have independently found that osteocytes are the predominant source of RANKL in the adult mouse. As RANKL signaling is a key target in treating osteoporosis, these results have potentially important implications for disease management. Osteocytes embedded in bone have been postulated to orchestrate bone homeostasis by regulating both bone-forming osteoblasts and bone-resorbing osteoclasts. We find here that purified osteocytes express a much higher amount of receptor activator of nuclear factor-κB ligand (RANKL) and have a greater capacity to support osteoclastogenesis in vitro than osteoblasts and bone marrow stromal cells. Furthermore, the severe osteopetrotic phenotype that we observe in mice lacking RANKL specifically in osteocytes indicates that osteocytes are the major source of RANKL in bone remodeling in vivo .

Hepatocellular carcinoma (HCC), the most common form of primary liver cancer is the third leading cause of cancer-related cell death in human and the fifth in women worldwide. The incidence of HCC is increasing despite progress in identifying risk factors, understanding disease etiology and developing anti-viral strategies. Therapeutic options are limited and survival after diagnosis is poor. Therefore, better preventive, diagnostic and therapeutic tools are urgently needed, in particular given the increased contribution from systemic metabolic disease to HCC incidence worldwide. In the last three decades, technological advances have facilitated the generation of genetically engineered mouse models (GEMMs) to mimic the alterations frequently observed in human cancers or to conduct intervention studies and assess the relevance of candidate gene networks in tumor establishment, progression and maintenance. Because these studies allow molecular and cellular manipulations impossible to perform in patients, GEMMs have improved our understanding of this complex disease and represent a source of great potential for mechanism-based therapy development. In this review, we provide an overview of the current state of HCC modeling in the mouse, highlighting successes, current challenges and future opportunities. ► The treatment of HCC requires effective preventive, diagnostic and therapeutic tools. ► We review state-of-the-art mouse models for HCC highlighting successes and challenges. ► We emphasize tumor–host interactions and organ/systemic crosstalks. ► Cross-species comparative studies will lead to knowledge-based novel therapies.

Inflammation is a physiological response of the body to tissue injury, pathogen invasion and irritants. In the course of inflammation, immune cells of the innate and/or adaptive immune system are activated and recruited to the site of inflammation. Attraction and activation of immune cells is regulated by a variety of different cytokines and chemokines, which are predominantly regulated by transcription factors such as AP-1, NF-κB, NFATs and STATs. The evidence that Jun/AP-1 proteins control inflammation in the skin is summarised in this article. Genetic mouse models have demonstrated that a loss of Jun/AP-1 expression in epidermal cells controls cytokine expression through transcriptional and post-transcriptional pathways. The absence of JunB in epithelial K5-expressing tissues leads to a multiorgan disease, which is characterised by increased levels of granulocyte colony-stimulating factor and interleukin 6. Deletion of both JunB and c-Jun, in a constitutive or inducible manner, leads to perinatal death of newborn pups and to a psoriasis-like disease in adults, in which tumour necrosis factor α and the TIMP-3/TACE pathway have central roles. The loss or reduction of Jun expression in the epidermis relieves a block on cytokine expression. As a consequence, the increased levels of cytokines in mice lead to diseases reminiscent of psoriasis and systemic lupus erythematosus in human patients. New targets identified in mouse models, together with investigations on human samples, will provide important new avenues for therapeutic interventions in psoriasis and other inflammatory skin diseases.

Osteosarcoma (OS) is the most frequent primary malignant bone tumor in urgent need of better therapies. Using genetically modified mouse models (GEMMs), we demonstrate that Wnt signaling promotes c-Fos-induced OS formation via the actions of the collagen-modifying enzyme Loxl2. c-Fos/AP-1 directly regulates the expression of the Wnt ligands Wnt7b and Wnt9a in OS cells through promoter binding, and Wnt7b and Wnt9a in turn promote Loxl2 expression in murine and human OS cells through the transcription factors Zeb1 and Zeb2. Concordantly, inhibition of Wnt ligand secretion by inactivating the Wnt-less ( Wls) gene in osteoblasts in c-Fos GEMMs either early or in a therapeutic setting reduces Loxl2 expression and progression of OS. Wls-deficient osteosarcomas proliferate less, are less mineralized and are enriched in fibroblastic cells surrounded by collagen fibers. Importantly, Loxl2 inhibition using either the pan-Lox inhibitor BAPN or a specific inducible shRNA reduces OS cell proliferation in vitro and decreases tumor growth and lung colonization in murine and human orthotopic OS transplantation models. Finally, OS development is delayed in c-Fos GEMMs treated with BAPN or with specific Loxl2 blocking antibodies. Congruently, a strong correlation between c-FOS, LOXL2 and WNT7B/WNT9A expression is observed in human OS samples, and c-FOS/LOXL2 co-expression correlates with OS aggressiveness and decreased patient survival. Therefore, therapeutic targeting of Wnt and/or Loxl2 should be considered to potentiate the inadequate current treatments for pediatric, recurrent, and metastatic OS.

Idiopathic pulmonary fibrosis (IPF) is a deadly disease with limited therapies. Tissue fibrosis is associated with type 2 immune response, although the causal contribution of immune cells is not defined. The AP-1 transcription factor Fra-2 is upregulated in IPF lung sections, and Fra-2 transgenic mice (Fra-[2.sup.Tg]) exhibit spontaneous lung fibrosis. Here, we show that bleomycin-induced lung fibrosis is attenuated upon myeloid inactivation of Fra-2 and aggravated in Fra-[2.sup.Tg] bone marrow chimeras. Type VI collagen (ColVI), a Fra-2 transcriptional target, is upregulated in 3 lung fibrosis models, and macrophages promote myofibroblast activation in vitro in a ColVI- and Fra-2-dependent manner. Fra-2 or ColVI inactivation does not affect macrophage recruitment and alternative activation, suggesting that Fra-2/ColVI specifically controls the paracrine profibrotic activity of macrophages. Importantly, ColVI-KO mice and ColVI-KO bone marrow chimeras are protected from bleomycin-induced lung fibrosis. Therapeutic administration of a Fra-2/AP-1 inhibitor reduces ColVI expression and ameliorates fibrosis in Fra-[2.sup.Tg] mice and in the bleomycin model. Finally, Fra-2 and ColVI positively correlate in IPF patient samples and colocalize in lung macrophages. Therefore, the Fra-2/ColVI profibrotic axis is a promising biomarker and therapeutic target for lung fibrosis and possibly other fibrotic diseases.

Chronic inflammation is associated with a variety of pathological conditions in epithelial tissues, including cancer, metaplasia and aberrant wound healing. In relation to this, a significant body of evidence suggests that aberration of epithelial stem and progenitor cell function is a contributing factor in inflammation-related disease, although the underlying cellular and molecular mechanisms remain to be fully elucidated. In this study, we have delineated the effect of chronic inflammation on epithelial stem/progenitor cells using the corneal epithelium as a model tissue. Using a combination of mouse genetics, pharmacological approaches and in vitro assays, we demonstrate that chronic inflammation elicits aberrant mechanotransduction in the regenerating corneal epithelium. As a consequence, a YAP–TAZ/β-catenin cascade is triggered, resulting in the induction of epidermal differentiation on the ocular surface. Collectively, the results of this study demonstrate that chronic inflammation and mechanotransduction are linked and act to elicit pathological responses in regenerating epithelia. Nowell et al.  report that chronic inflammation of the corneal epithelium activates β-catenin signalling through YAP/TAZ-dependent mechanotransduction, leading to epidermal differentiation on the ocular surface and corneal squamous cell metaplasia.

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). Further, JNK1 was required for human HCC cell proliferation in vitro and tumorigenesis after xenotransplantation. Importantly, mice lacking JNK1 displayed decreased tumor cell proliferation in a mouse model of liver carcinogenesis and decreased hepatocyte proliferation in a mouse model of liver regeneration. In both cases, impaired proliferation was caused by increased expression of p21, a cell-cycle inhibitor, and reduced expression of c-Myc, a negative regulator of p21. Genetic inactivation of p21 in JNK1-/- mice restored hepatocyte proliferation in models of both liver carcinogenesis and liver regeneration, and overexpression of c-Myc increased proliferation of JNK1-/- liver cells. Similarly, JNK1 was found to control the proliferation of human HCC cells by affecting p21 and c-Myc expression. Pharmacologic inhibition of JNK reduced the growth of both xenografted human HCC cells and chemically induced mouse liver cancers. These findings provide a mechanistic link between JNK activity and liver cell proliferation via p21 and c-Myc and suggest JNK targeting can be considered as a new therapeutic approach for HCC treatment.

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. Wagner and colleagues show that cancer cell survival during liver cancer initiation depends on inhibition of c-Fos-induced apoptosis, through the repression of survivin expression by c-Jun and SIRT6.