Harald Scheuch

Psoriasis-like skin disease and arthritis caused by inducible epidermal deletion of Jun proteins

Psoriasis is a frequent, inflammatory disease of skin and joints with considerable morbidity. Here we report that in psoriatic lesions, epidermal keratinocytes have decreased expression of JunB, a gene localized in the psoriasis susceptibility region PSORS6. Likewise, inducible epidermal deletion of JunB and its functional companion c-Jun in adult mice leads (within two weeks) to a phenotype resembling the histological and molecular hallmarks of psoriasis, including arthritic lesions. In contrast to the skin phenotype, the development of arthritic lesions requires T and B cells and signalling through tumour necrosis factor receptor 1 (TNFR1). Prior to the disease onset, two chemotactic proteins (S100A8 and S100A9) previously mapped to the psoriasis susceptibility region PSORS4, are strongly induced in mutant keratinocytes in vivo and in vitro. We propose that the abrogation of JunB/activator protein 1 (AP-1) in keratinocytes triggers chemokine/cytokine expression, which recruits neutrophils and macrophages to the epidermis thereby contributing to the phenotypic changes observed in psoriasis. Thus, these data support the hypothesis that epidermal alterations are sufficient to initiate both skin lesions and arthritis in psoriasis.

p38α suppresses normal and cancer cell proliferation by antagonizing the JNK–c-Jun pathway

The mitogen-activated protein kinase (MAPK) p38α controls inflammatory responses and cell proliferation. Using mice carrying conditional Mapk14 (also known as p38α) 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 p38α 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 p38α negatively regulates cell proliferation by antagonizing the JNK–c-Jun pathway in multiple cell types and in liver cancer development.

c-Jun Regulates Eyelid Closure and Skin Tumor Development through EGFR Signaling

To investigate the function of c-Jun during skin development and skin tumor formation, we conditionally inactivated c-jun in the epidermis. Mice lacking c-jun in keratinocytes (c-jun(Deltaep)) develop normal skin but express reduced levels of EGFR in the eyelids, leading to open eyes at birth, as observed in EGFR null mice. Primary keratinocytes from c-jun(Deltaep) mice proliferate poorly, show increased differentiation, and form prominent cortical actin bundles, most likely because of decreased expression of EGFR and its ligand HB-EGF. In the absence of c-Jun, tumor-prone K5-SOS-F transgenic mice develop smaller papillomas, with reduced expression of EGFR in basal keratinocytes. Thus, using three experimental systems, we show that EGFR and HB-EGF are regulated by c-Jun, which controls eyelid development, keratinocyte proliferation, and skin tumor formation.

Proliferation of human HCC cells and chemically induced mouse liver cancers requires JNK1-dependent p21 downregulation

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.

Mice lacking JunB are osteopenic due to cell-autonomous osteoblast and osteoclast defects

Because JunB is an essential gene for placentation, it was conditionally deleted in the embryo proper. JunB Δ/Δ mice are born viable, but develop severe low turnover osteopenia caused by apparent cell-autonomous osteoblast and osteoclast defects before a chronic myeloid leukemia-like disease. Although JunB was reported to be a negative regulator of cell proliferation, junB Δ/Δ osteoclast precursors and osteoblasts show reduced proliferation along with a differentiation defect in vivo and in vitro. Mutant osteoblasts express elevated p16INK4a levels, but exhibit decreased cyclin D1 and cyclin A expression. Runx2 is transiently increased during osteoblast differentiation in vitro, whereas mature osteoblast markers such as osteocalcin and bone sialoprotein are strongly reduced. To support a cell-autonomous function of JunB in osteoclasts, junB was inactivated specifically in the macrophage–osteoclast lineage. Mutant mice develop an osteopetrosis-like phenotype with increased bone mass and reduced numbers of osteoclasts. Thus, these data reveal a novel function of JunB as a positive regulator controlling primarily osteoblast as well as osteoclast activity.

Osteoclast size is controlled by Fra-2 through LIF/LIF-receptor signalling and hypoxia

Osteoclasts are multinucleated haematopoietic cells that resorb bone. Increased osteoclast activity causes osteoporosis, a disorder resulting in a low bone mass and a high risk of fractures. Increased osteoclast size and numbers are also a hallmark of other disorders, such as Paget’s disease and multiple myeloma. The protein c-Fos, a component of the AP-1 transcription factor complex, is essential for osteoclast differentiation. Here we show that the Fos-related protein Fra-2 controls osteoclast survival and size. The bones of Fra-2-deficient newborn mice have giant osteoclasts, and signalling through leukaemia inhibitory factor (LIF) and its receptor is impaired. Similarly, newborn animals lacking LIF have giant osteoclasts, and we show that LIF is a direct transcriptional target of Fra-2 and c-Jun. Moreover, bones deficient in Fra-2 and LIF are hypoxic and express increased levels of hypoxia-induced factor 1α (HIF1α) and Bcl-2. Overexpression of Bcl-2 is sufficient to induce giant osteoclasts in vivo, whereas Fra-2 and LIF affect HIF1α through transcriptional modulation of the HIF prolyl hydroxylase PHD2. This pathway is operative in the placenta, because specific inactivation of Fra-2 in the embryo alone does not cause hypoxia or the giant osteoclast phenotype. Thus placenta-induced hypoxia during embryogenesis leads to the formation of giant osteoclasts in young pups. These findings offer potential targets for the treatment of syndromes associated with increased osteoclastogenesis.

Liver cancer initiation is controlled by AP-1 through SIRT6-dependent inhibition of survivin

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.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.

Differentiation-induced skin cancer suppression by FOS, p53, and TACE/ADAM17

Squamous cell carcinomas (SCCs) are heterogeneous and aggressive skin tumors for which innovative, targeted therapies are needed. Here, we identify a p53/TACE pathway that is negatively regulated by FOS and show that the FOS/p53/TACE axis suppresses SCC by inducing differentiation. We found that epidermal Fos deletion in mouse tumor models or pharmacological FOS/AP-1 inhibition in human SCC cell lines induced p53 expression. Epidermal cell differentiation and skin tumor suppression were caused by a p53-dependent transcriptional activation of the metalloprotease TACE/ADAM17 (TNF-α-converting enzyme), a previously unknown p53 target gene that was required for NOTCH1 activation. Although half of cutaneous human SCCs display p53-inactivating mutations, restoring p53/TACE activity in mouse and human skin SCCs induced tumor cell differentiation independently of the p53 status. We propose FOS/AP-1 inhibition or p53/TACE reactivating strategies as differentiation-inducing therapies for SCCs.

Epidermal JunB represses G-CSF transcription and affects haematopoiesis and bone formation

Mice that lack JunB in epidermal cells are born with normal skin; however, keratinocytes hyperproliferate in vitro and on TPA treatment in vivo. Loss of JunB expression in the epidermis of adult mice affects the skin, the proliferation of haematopoietic cells and bone formation. G-CSF is a direct transcriptional target of JunB and mutant epidermis releases large amounts of G-CSF that reach high systemic levels and cause skin ulcerations, myeloproliferative disease and low bone mass. The absence of G-CSF significantly improves hyperkeratosis and prevents the development of myeloproliferative disease, but does not affect bone loss. This study describes a mechanism by which the absence of JunB in epithelial cells causes multi-organ disease, suggesting that the epidermis can act as an endocrine-like organ.

Corrigendum: Psoriasis-like skin disease and arthritis caused by inducible epidermal deletion of Jun proteins

This corrects the article DOI: 10.1038/nature03963