ATG9A prevents TNF cytotoxicity by an unconventional lysosomal targeting pathway.

Cell death induced by tumor necrosis factor (TNF) can be beneficial during infection by helping to mount proper immune responses. However, TNF-induced death can also drive a variety of inflammatory pathologies. Protectives brakes, or cell-death checkpoints, normally repress TNF cytotoxicity to protect the organism from its potential detrimental consequences. Thus, although TNF can kill, this only occurs when one of the checkpoints is inactivated. Here, we describe a checkpoint that prevents apoptosis through the detoxification of the cytotoxic complex IIa that forms upon TNF sensing. We found that autophagy-related 9A (ATG9A) and 200kD FAK family kinase-interacting protein (FIP200) promote the degradation of this complex through a light chain 3 (LC3)-independent lysosomal targeting pathway. This detoxification mechanism was found to counteract TNF receptor 1 (TNFR1)-mediated embryonic lethality and inflammatory skin disease in mouse models.

Point mutation I634A in the glucocorticoid receptor causes embryonic lethality by reduced ligand binding.

The glucocorticoid (GC) receptor (GR) is essential for normal development and in the initiation of inflammation. Healthy GRdim/dim mice with reduced dimerization propensity due to a point mutation (A465T) at the dimer interface of the GR DNA-binding domain (DBD) (here GRD/D) have previously helped to define the functions of GR monomers and dimers. Since GRD/D retains residual dimerization capacity, here we generated the dimer-nullifying double mutant GRD+L/D+L mice, featuring an additional mutation (I634A) in the ligand-binding domain (LBD) of GR. These mice are perinatally lethal, as are GRL/L mice (these mice have the I634A mutation but not the A465T mutation), displaying improper lung and skin formation. Using embryonic fibroblasts, high and low doses of dexamethasone (Dex), nuclear translocation assays, RNAseq, dimerization assays, and ligand-binding assays (and Kd values), we found that the lethal phenotype in these mice is due to insufficient ligand binding. These data suggest there is some correlation between GR dimerization potential and ligand affinity. We conclude that even a mutation as subtle as I634A, at a position not directly involved in ligand interactions sensu stricto, can still influence ligand binding and have a lethal outcome.

Antioxidant and food additive BHA prevents TNF cytotoxicity by acting as a direct RIPK1 inhibitor.

Butylate hydroxyanisole (BHA) is a synthetic phenol that is widely utilized as a preservative by the food and cosmetic industries. The antioxidant properties of BHA are also frequently used by scientists to claim the implication of reactive oxygen species (ROS) in various cellular processes, including cell death. We report on the surprising finding that BHA functions as a direct inhibitor of RIPK1, a major signaling hub downstream of several immune receptors. Our in silico analysis predicts binding of 3-BHA, but not 2-BHA, to RIPK1 in an inactive DLG-out/Glu-out conformation, similar to the binding of the type III inhibitor Nec-1s to RIPK1. This predicted superior inhibitory capacity of 3-BHA over 2-BHA was confirmed in cells and using in vitro kinase assays. We demonstrate that the reported protective effect of BHA against tumor necrosis factor (TNF)-induced necroptotic death does not originate from ROS scavenging but instead from direct RIPK1 enzymatic inhibition, a finding that most probably extends to other reported effects of BHA. Accordingly, we show that BHA not only protects cells against RIPK1-mediated necroptosis but also against RIPK1 kinase-dependent apoptosis. We found that BHA treatment completely inhibits basal and induced RIPK1 enzymatic activity in cells, monitored at the level of TNFR1 complex I under apoptotic conditions or in the cytosol under necroptosis. Finally, we show that oral administration of BHA protects mice from RIPK1 kinase-dependent lethality caused by TNF injection, a model of systemic inflammatory response syndrome. In conclusion, our results demonstrate that BHA can no longer be used as a strict antioxidant and that new functions of RIPK1 may emerge from previously reported effects of BHA.

Sensing of endogenous nucleic acids by ZBP1 induces keratinocyte necroptosis and skin inflammation.

Aberrant detection of endogenous nucleic acids by the immune system can cause inflammatory disease. The scaffold function of the signaling kinase RIPK1 limits spontaneous activation of the nucleic acid sensor ZBP1. Consequently, loss of RIPK1 in keratinocytes induces ZBP1-dependent necroptosis and skin inflammation. Whether nucleic acid sensing is required to activate ZBP1 in RIPK1-deficient conditions and which immune pathways are associated with skin disease remained open questions. Using knock-in mice with disrupted ZBP1 nucleic acid-binding activity, we report that sensing of endogenous nucleic acids by ZBP1 is critical in driving skin pathology characterized by antiviral and IL-17 immune responses. Inducing ZBP1 expression by interferons triggers necroptosis in RIPK1-deficient keratinocytes, and epidermis-specific deletion of MLKL prevents disease, demonstrating that cell-intrinsic events cause inflammation. These findings indicate that dysregulated sensing of endogenous nucleic acid by ZBP1 can drive inflammation and may contribute to the pathogenesis of IL-17-driven inflammatory skin conditions such as psoriasis.

Keratinocyte Expression of A20/TNFAIP3 Controls Skin Inflammation Associated with Atopic Dermatitis and Psoriasis.

Keratinocytes are key players in chronic inflammatory skin diseases. A20 regulates NF-κB-dependent expression of proinflammatory genes and cell death, but the impact of its expression in keratinocytes on systemic inflammation and skin disorders has not been determined. Comparative transcriptomic analysis of microdissected epidermis showed that A20 is down-regulated in involved epidermis, but not in dermis, of psoriasis and atopic dermatitis patients, suggesting that loss of A20 expression in keratinocytes increases the vulnerability for psoriasis/atopic dermatitis induction. We have previously shown that epidermis-specific A20 knockout mice (A20EKO) develop mild epidermal hyperplasia but no macroscopic skin inflammation. We now show that various cytokines and chemokines are up-regulated in A20EKO mouse skin. A20EKO mice also display systemic proinflammatory changes, even in the absence of skin immune cell infiltration, and an exacerbated disease severity upon induction of experimental psoriasis, atopic dermatitis, or skin barrier disruption. Keratinocytes showed increased proinflammatory gene expression in the absence of A20 in unstimulated and IL-17A-stimulated conditions, in part resulting from uncontrolled MyD88-dependent signaling. Our findings indicate that absence of A20 in keratinocytes leads to systemic inflammation at homeostatic conditions and is sufficient to exacerbate inflammatory skin disorders associated with different immune profiles by increasing cytokine and chemokine expression.

Keratinocyte-Specific Ablation of RIPK4 Allows Epidermal Cornification but Impairs Skin Barrier Formation.

In humans, receptor-interacting protein kinase 4 (RIPK4) mutations can lead to the autosomal recessive Bartsocas-Papas and popliteal pterygium syndromes, which are characterized by severe skin defects, pterygia, as well as clefting. We show here that the epithelial fusions observed in RIPK4 full knockout (KO) mice are E-cadherin dependent, as keratinocyte-specific deletion of E-cadherin in RIPK4 full KO mice rescued the tail-to-body fusion and fusion of oral epithelia. To elucidate RIPK4 function in epidermal differentiation and development, we generated epidermis-specific RIPK4 KO mice (RIPK4EKO). In contrast to RIPK4 full KO epidermis, RIPK4EKO epidermis was normally stratified and the outside-in skin barrier in RIPK4EKO mice was largely intact at the trunk, in contrast to the skin covering the head and the outer end of the extremities. However, RIPK4EKO mice die shortly after birth due to excessive water loss because of loss of tight junction protein claudin-1 localization at the cell membrane, which results in tight junction leakiness. In contrast, mice with keratinocyte-specific RIPK4 deletion during adult life remain viable. Furthermore, our data indicate that epidermis-specific deletion of RIPK4 results in delayed keratinization and stratum corneum maturation and altered lipid organization and is thus indispensable during embryonic development for the formation of a functional inside-out epidermal barrier.

Elevated ΔNp63α Levels Facilitate Epidermal and Biliary Oncogenic Transformation.

Unlike its family member p53, TP63 is rarely mutated in human cancer. However, ΔNp63α protein levels are often elevated in tumors of epithelial origin, such as squamous cell carcinoma and cholangiocarcinoma. To study the oncogenic properties of ΔNp63α in vivo, we generated transgenic mice overexpressing ΔNp63α from the Rosa26 locus promoter controlled by keratin 5-Cre. We found that these mice spontaneously develop epidermal cysts and ectopic ΔNp63α expression in the bile duct epithelium that leads to dilatation of the intrahepatic biliary ducts, to hepatic cyst formation and bile duct adenoma. Moreover, when subjected to models of 7,12-dimethylbenz[a]anthracene-based carcinogenesis, tumor initiation was increased in ΔNp63α transgenic mice in a gene dosage-dependent manner although ΔNp63α overexpression did not alter the sensitivity to 7,12-dimethylbenz[a]anthracene-induced cytotoxicity in vivo. However, keratinocytes isolated from ΔNp63α transgenic mice displayed increased survival and delayed cellular senescence compared with wild-type keratinocytes, marked by decreased p16Ink4a and p19Arf expression. Taken together, we show that increased ΔNp63α protein levels facilitate oncogenic transformation in the epidermis as well as in the bile duct.

RIPK1 ensures intestinal homeostasis by protecting the epithelium against apoptosis.

Receptor interacting protein kinase 1 (RIPK1) has an essential role in the signalling triggered by death receptors and pattern recognition receptors. RIPK1 is believed to function as a node driving NF-κB-mediated cell survival and inflammation as well as caspase-8 (CASP8)-dependent apoptotic or RIPK3/MLKL-dependent necroptotic cell death. The physiological relevance of this dual function has remained elusive because of the perinatal death of RIPK1 full knockout mice. To circumvent this problem, we generated RIPK1 conditional knockout mice, and show that mice lacking RIPK1 in intestinal epithelial cells (IECs) spontaneously develop severe intestinal inflammation associated with IEC apoptosis leading to early death. This early lethality was rescued by antibiotic treatment, MYD88 deficiency or tumour-necrosis factor (TNF) receptor 1 deficiency, demonstrating the importance of commensal bacteria and TNF in the IEC Ripk1 knockout phenotype. CASP8 deficiency, but not RIPK3 deficiency, rescued the inflammatory phenotype completely, indicating the indispensable role of RIPK1 in suppressing CASP8-dependent apoptosis but not RIPK3-dependent necroptosis in the intestine. RIPK1 kinase-dead knock-in mice did not exhibit any sign of inflammation, suggesting that RIPK1-mediated protection resides in its kinase-independent platform function. Depletion of RIPK1 in intestinal organoid cultures sensitized them to TNF-induced apoptosis, confirming the in vivo observations. Unexpectedly, TNF-mediated NF-κB activation remained intact in these organoids. Our results demonstrate that RIPK1 is essential for survival of IECs, ensuring epithelial homeostasis by protecting the epithelium from CASP8-mediated IEC apoptosis independently of its kinase activity and NF-κB activation.

Caspase-14 overexpression in hairless mice is not involved in utricle formation.

Loss of functional hairless (HR) transcriptional repressor leads to utricle formation and congenital hair loss both in mice and men. Studies in mice have shown that this is preceded by overexpression of caspase-14 at the infundibulum in the hair follicle before conversion to utricle occurs. In this report, we show that HR regulates caspase-14 expression dependent on its interaction with histone deacetylases, implicating chromatin remodelling in the transcriptional regulation of caspase-14. However, crossing hairless mutant mice with caspase-14-deficient mice revealed that caspase-14 overexpression is not the cause of utricle formation.

Caspase-14-deficient mice are more prone to the development of parakeratosis.

Caspase-14 is an important protease in the proper formation of a fully functional skin barrier. Newborn mice that are deficient in caspase-14 exhibit increased transepidermal water loss and are highly sensitive to UVB-induced photodamage. Decreased caspase-14 expression and incomplete caspase-14 processing in lesional psoriatic parakeratotic stratum corneum has been reported previously. In this study, we show that caspase-14-deficient skin frequently displays incompletely cornified cells in the transitional zone between the granular and the cornified layers, pointing to a delay in cornification. We also demonstrate that after challenge of epidermal permeability barrier function by repetitive acetone treatment, a higher incidence of large parakeratotic plaques was observed in caspase-14-deficient skin. Furthermore, caspase-14-deficient mice are more prone than control mice to the development of parakeratosis upon induction of psoriasis-like dermatitis by imiquimod treatment. These results show that lack of caspase-14 expression predisposes to the development of parakeratosis and that caspase-14 has an important role in keratinocyte terminal differentiation and the maintenance of normal stratum corneum, especially in conditions causing epidermal hyperproliferation.

cIAP1/2 are direct E3 ligases conjugating diverse types of ubiquitin chains to receptor interacting proteins kinases 1 to 4 (RIP1-4).

The RIP kinases have emerged as essential mediators of cellular stress that integrate both extracellular stimuli emanating from various cell-surface receptors and signals coming from intracellular pattern recognition receptors. The molecular mechanisms regulating the ability of the RIP proteins to transduce the stress signals remain poorly understood, but seem to rely only partially on their kinase activities. Recent studies on RIP1 and RIP2 have highlighted the importance of ubiquitination as a key process regulating their capacity to activate downstream signaling pathways. In this study, we found that XIAP, cIAP1 and cIAP2 not only directly bind to RIP1 and RIP2 but also to RIP3 and RIP4. We show that cIAP1 and cIAP2 are direct E3 ubiquitin ligases for all four RIP proteins and that cIAP1 is capable of conjugating the RIPs with diverse types of ubiquitin chains, including linear chains. Consistently, we show that repressing cIAP1/2 levels affects the activation of NF-κB that is dependent on RIP1, -2, -3 and -4. Finally, we identified Lys51 and Lys145 of RIP4 as two critical residues for cIAP1-mediated ubiquitination and NF-κB activation.

Caspase-14 is required for filaggrin degradation to natural moisturizing factors in the skin.

Caspase-14 is a protease that is mainly expressed in suprabasal epidermal layers and activated during keratinocyte cornification. Caspase-14-deficient mice display reduced epidermal barrier function and increased sensitivity to UVB radiation. In these mice, profilaggrin, a protein with a pivotal role in skin barrier function, is processed correctly to its functional filaggrin (FLG) repeat unit, but proteolytic FLG fragments accumulate in the epidermis. In wild-type stratum corneum, FLG is degraded into free amino acids, some of which contribute to generation of the natural moisturizing factors (NMFs) that maintain epidermal hydration. We found that caspase-14 cleaves the FLG repeat unit and identified two caspase-14 cleavage sites. These results indicate that accumulation of FLG fragments in caspase-14(-/-) mice is due to a defect in the terminal FLG degradation pathway. Consequently, we show that the defective FLG degradation in caspase-14-deficient skin results in substantial reduction in the amount of NMFs, such as urocanic acid and pyrrolidone carboxylic acid. Taken together, we identified caspase-14 as a crucial protease in FLG catabolism.

Caspase-14 protects against epidermal UVB photodamage and water loss.

Caspase-14 belongs to a conserved family of aspartate-specific proteinases. Its expression is restricted almost exclusively to the suprabasal layers of the epidermis and the hair follicles. Moreover, the proteolytic activation of caspase-14 is associated with stratum corneum formation, implicating caspase-14 in terminal keratinocyte differentiation and cornification. Here, we show that the skin of caspase-14-deficient mice was shiny and lichenified, indicating an altered stratum-corneum composition. Caspase-14-deficient epidermis contained significantly more alveolar keratohyalin F-granules, the profilaggrin stores. Accordingly, caspase-14-deficient epidermis is characterized by an altered profilaggrin processing pattern and we show that recombinant caspase-14 can directly cleave profilaggrin in vitro. Caspase-14-deficient epidermis is characterized by reduced skin-hydration levels and increased water loss. In view of the important role of filaggrin in the structure and moisturization of the skin, the knockout phenotype could be explained by an aberrant processing of filaggrin. Importantly, the skin of caspase-14-deficient mice was highly sensitive to the formation of cyclobutane pyrimidine dimers after UVB irradiation, leading to increased levels of UVB-induced apoptosis. Removal of the stratum corneum indicate that caspase-14 controls the UVB scavenging capacity of the stratum corneum.

The transcription factor snail induces tumor cell invasion through modulation of the epithelial cell differentiation program.

Abberant activation of the process of epithelial-mesenchymal transition in cancer cells is a late event in tumor progression. A key inducer of this transition is the transcription factor Snail, which represses E-cadherin. We report that conditional expression of the human transcriptional repressor Snail in colorectal cancer cells induces an epithelial dedifferentiation program that coincides with a drastic change in cell morphology. Snail target genes control the establishment of several junctional complexes, intermediate filament networks, and the actin cytoskeleton. Modulation of the expression of these genes is associated with loss of cell aggregation and induction of invasion. Chromatin immunoprecipitation experiments showed that repression of selected target genes is associated with increased binding of Snail to their promoters, which contain consensus Snail-binding sites. Thus, Snail constitutes a master switch that directly represses the epithelial phenotype, resulting in malignant carcinoma cells.

GATA-4 and MEF2C transcription factors control the tissue-specific expression of the alphaT-catenin gene CTNNA3.

AlphaT-catenin is a recently identified member of the alpha-catenin family of cell-cell adhesion molecules. Its expression is restricted mainly to cardiomyocytes, although it is also expressed in skeletal muscle, testis and brain. Like other alpha-catenins, alphaT-catenin provides an indispensable link between a cadherin-based adhesion complex and the actin cytoskeleton, resulting in strong cell-cell adhesion. We show here that the tissue-specificity of alphaT-catenin expression is controlled by its promoter region. By in silico analysis, we found that the alphaT-catenin promoter contains several binding sites for cardiac and muscle-specific transcription factors. By co-transfection studies in P19 embryonal carcinoma cells, we demonstrated that MEF2C and GATA-4 each have an activating effect on the alphaT-catenin promoter. Transfections with wild-type and mutant promoter constructs in cardiac HL-1 cells indicated that one GATA box is absolutely required for high alphaT-catenin promoter activity in these cells. Furthermore, we showed that the GATA-4 transcription factor specifically binds and activates the alphaT-catenin promoter in vivo in cardiac HL-1 cells. In vivo promoter analysis in transgenic mice revealed that the isolated alphaT-catenin promoter region could direct the tissue-specific expression of a LacZ reporter gene in concordance with endogenous alphaT-catenin expression.

Defining desmosomal plakophilin-3 interactions.

Plakophilin 3 (PKP3) is a recently described armadillo protein of the desmosomal plaque, which is synthesized in simple and stratified epithelia. We investigated the localization pattern of endogenous and exogenous PKP3 and fragments thereof. The desmosomal binding properties of PKP3 were determined using yeast two-hybrid, coimmunoprecipitation and colocalization experiments. To this end, novel mouse anti-PKP3 mAbs were generated. We found that PKP3 binds all three desmogleins, desmocollin (Dsc) 3a and -3b, and possibly also Dsc1a and -2a. As such, this is the first protein interaction ever observed with a Dsc-b isoform. Moreover, we determined that PKP3 interacts with plakoglobin, desmoplakin (DP) and the epithelial keratin 18. Evidence was found for the presence of at least two DP-PKP3 interaction sites. This finding might explain how lateral DP-PKP interactions are established in the upper layers of stratified epithelia, increasing the size of the desmosome and the number of anchoring points available for keratins. Together, these results show that PKP3, whose epithelial and epidermal desmosomal expression pattern and protein interaction repertoire are broader than those of PKP1 and -2, is a unique multiprotein binding element in the basic architecture of a vast majority of epithelial desmosomes.

A novel role for p120 catenin in E-cadherin function.

Indirect evidence suggests that p120-catenin (p120) can both positively and negatively affect cadherin adhesiveness. Here we show that the p120 gene is mutated in SW48 cells, and that the cadherin adhesion system is impaired as a direct consequence of p120 insufficiency. Restoring normal levels of p120 caused a striking reversion from poorly differentiated to cobblestone-like epithelial morphology, indicating a crucial role for p120 in reactivation of E-cadherin function. The rescue efficiency was enhanced by increased levels of p120, and reduced by the presence of the phosphorylation domain, a region previously postulated to confer negative regulation. Surprisingly, the rescue was associated with substantially increased levels of E-cadherin. E-cadherin mRNA levels were unaffected by p120 expression, but E-cadherin half-life was more than doubled. Direct p120-E-cadherin interaction was crucial, as p120 deletion analysis revealed a perfect correlation between E-cadherin binding and rescue of epithelial morphology. Interestingly, the epithelial morphology could also be rescued by forced expression of either WT E-cadherin or a p120-uncoupled mutant. Thus, the effects of uncoupling p120 from E-cadherin can be at least partially overcome by artificially maintaining high levels of cadherin expression. These data reveal a cooperative interaction between p120 and E-cadherin and a novel role for p120 that is likely indispensable in normal cells.