Jennifer Huynh

Receptor-interacting Protein Kinase 4 and Interferon Regulatory Factor 6 Function as a Signaling Axis to Regulate Keratinocyte Differentiation

Background: RIPK4 and IRF6 are important for epidermal development. However, whether they function together to regulate keratinocyte differentiation has not been addressed. Results: RIPK4 directly activates IRF6, resulting in expression of the transcriptional regulators GRHL3 and OVOL1. Conclusion: RIPK4 and IRF6 promote keratinocyte differentiation by functioning as a signaling axis. Significance: This study reveals how mutations in RIPK4 may cause epidermal disorders. Receptor-interacting protein kinase 4 (RIPK4) and interferon regulatory factor 6 (IRF6) are critical regulators of keratinocyte differentiation, and their mutation causes the related developmental epidermal disorders Bartsocas-Papas syndrome and popliteal pterygium syndrome, respectively. However, the signaling pathways in which RIPK4 and IRF6 operate to regulate keratinocyte differentiation are poorly defined. Here we identify and mechanistically define a direct functional relationship between RIPK4 and IRF6. Gene promoter reporter and in vitro kinase assays, coimmunoprecipitation experiments, and confocal microscopy demonstrated that RIPK4 directly regulates IRF6 trans-activator activity and nuclear translocation. Gene knockdown and overexpression studies indicated that the RIPK4-IRF6 signaling axis controls the expression of key transcriptional regulators of keratinocyte differentiation, including Grainyhead-like 3 and OVO-like 1. Additionally, we demonstrate that the p.Ile121Asn missense mutation in RIPK4, which has been identified recently in Bartsocas-Papas syndrome, inhibits its kinase activity, thereby preventing RIPK4-mediated IRF6 activation and nuclear translocation. We show, through mutagenesis-based experiments, that Ser-413 and Ser-424 in IRF6 are important for its activation by RIPK4. RIPK4 is also important for the regulation of IRF6 expression by the protein kinase C pathway. Therefore, our findings not only provide important mechanistic insights into the regulation of keratinocyte differentiation by RIPK4 and IRF6, but they also suggest one mechanism by which mutations in RIPK4 may cause epidermal disorders (e.g. Bartsocas-Papas syndrome), namely by the impaired activation of IRF6 by RIPK4.

The JAK/STAT3 axis: A comprehensive drug target for solid malignancies

Intercellular communication between tumor cells, immune cells and the stroma characterises the tumor microenvironment, which is instrumental for establishing the ecological niche that fosters tumor growth and metastasis. While tumor cell intrinsic STAT3 signaling provides a crucial axis to support cell proliferation and survival, it also regulates many activities of the non-transformed cells that collectively make up the tumor microenvironment. Accordingly, excessive activation of STAT3 is a hallmark of many malignancies, and often occurs in response to cytokines of the IL-6 and IL-10 families. However, tumor extrinsic STAT3 signaling also regulates the effector function of tumor-associated immune and stromal cells, which support the growth of tumors by suppressing the hosts anti-tumor immune response. Given that STAT3 mediates tumorigenic effects in many cell types, the molecular players of STAT3 signaling and its upstream JAK kinases provide viable therapeutic targets for the treatment of cancer. Here we provide an update on novel insights into the role of STAT3 in immune suppression and describe current therapeutic strategies that target the JAK/STAT3 signaling axis for the treatment of malignancies.

CSF-1 receptor signalling from endosomes mediates the sustained activation of Erk1/2 and Akt in macrophages.

Colony stimulating factor-1 (CSF-1) mediates its pleiotropic effects on macrophages through the CSF-1 receptor (CSF-1R), a receptor tyrosine kinase. Current models of CSF-1 signalling imply that the CSF-1R activates signalling pathways exclusively at the plasma membrane and the subsequent internalisation of the CSF-1R simply facilitates its lysosomal degradation in order to prevent on-going signalling. Here, we sought to establish if the CSF-1R may in fact continue to signal following its internalisation. Erk1/2, Akt and Stat3 activation were abrogated when the internalisation of the CSF-1R was impaired, with the effects on Stat3 distinct from those for Erk1/2 and Akt. Pharmacologic inhibition of the CSF-1R following its internalisation resulted in less sustained Erk1/2 and Akt activity, whereas Stat3 activity was unaffected. Significantly, the suppressive effects of the CSF-1R inhibitor on the up-regulation of gene expression by CSF-1 (e.g. cyclin D1 and Bcl-xL gene expression) were comparable irrespective of whether the inhibitor was added prior to CSF-1 stimulation or following the internalisation of the CSF-1R. Similarly, pharmacologic inhibition of Erk1/2 (or Akt) activity either prior to CSF-1 stimulation or subsequent to CSF-1R internalisation had comparable effects on the regulation of gene expression by CSF-1. Together, our data argue that key signalling responses to CSF-1 depend on the ability of the CSF-1R to signal from endosomes following its internalisation, thus adding an important spatiotemporal aspect to CSF-1R signalling.

Interferon Regulatory Factor 6 Differentially Regulates Toll-like Receptor 2-dependent Chemokine Gene Expression in Epithelial Cells

Background: The IRF6 transcription factor is critical for epithelial barrier function; however, a role for IRF6 in signaling by Toll-like receptors has not been addressed. Results: The IRAK1-mediated activation of IRF6 promotes TLR2-dependent CCL5 chemokine gene expression in epithelial cells. Conclusion: IRF6 differentially regulates TLR2 inflammatory responses in epithelial cells. Significance: Our results reveal an additional immune-related function for IRF6. Epidermal and mucosal epithelial cells are integral to host defense. They not only act as a physical barrier but also utilize pattern recognition receptors, such as the Toll-like receptors (TLRs), to detect and respond to pathogens. Members of the interferon regulatory factor (IRF) family of transcription factors are key components of TLR signaling as they impart specificity to downstream responses. Although IRF6 is a critical regulator of epithelial cell proliferation and differentiation, its role in TLR signaling has not previously been addressed. We show here that IRF6 is activated by IRAK1 as well as by MyD88 but not by TRIF or TBK1. Co-immunoprecipitation experiments further demonstrated that IRF6 can interact with IRAK1. Gene silencing in epithelial cells along with gene promoter reporter assays showed that IRAK1 mediates TLR2-inducible CCL5 gene expression at least in part by promoting IRF6 activation. Conversely, IRAK1 regulated CXCL8 gene expression independently of IRF6, thus identifying a molecular mechanism by which TLR2 signaling differentially regulates the expression of specific chemokines in epithelial cells. Bioinformatics analysis and mutagenesis-based experiments identified Ser-413 and Ser-424 as key regulatory sites in IRF6. Phosphomimetic mutation of these residues resulted in greatly enhanced IRF6 dimerization and trans-activator function. Collectively, our findings suggest that, in addition to its importance for epithelial barrier function, IRF6 also contributes to host defense by providing specificity to the regulation of inflammatory chemokine expression by TLR2 in epithelial cells.

IRF6 Regulates the Expression of IL-36γ by Human Oral Epithelial Cells in Response to Porphyromonas gingivalis

IFN regulatory factors (IRFs) help to shape the immune response to pathogens by imparting signaling specificity to individual TLRs. We recently demonstrated that IRF6 provides specificity to TLR2 signaling in oral epithelial cells. TLR2 plays an important role in eliciting inflammation to Porphyromonas gingivalis, a keystone pathogen in periodontitis. Therefore, we investigated a role for IRF6 in mediating the inflammatory cytokine response of oral epithelial cells to P. gingivalis. IRF6 expression was strongly upregulated when human oral epithelial cells were challenged with P. gingivalis. Moreover, gene silencing and gene promoter experiments indicated that IRF6 acts downstream of IL-1R–associated kinase 1 to stimulate the expression of the IL-1 family cytokine IL-36γ in response to P. gingivalis. IRF6 and IL-1R–associated kinase 1 also regulated the stimulation of IL-36γ expression by a TLR2 agonist. IL-36γ was shown to elicit inflammatory responses by human monocyte-derived dendritic cells and macrophages, including the expression of the neutrophil chemokines IL-8 and CXCL1, as well as the Th17 chemokine CCL20. IL-36γ similarly stimulated their expression by human oral epithelial cells. Significantly, the Th17 cytokine IL-17 not only stimulated the expression of important regulators of neutrophil recruitment and survival by oral epithelial cells, but IL-17 also stimulated them to express IL-36γ. Thus, our findings suggest that IRF6 is likely to promote inflammation to P. gingivalis through its regulation of IL-36γ.

Disease-associated mutations in IRF6 and RIPK4 dysregulate their signalling functions.

IRF6 and RIPK4 are critical regulators of keratinocyte differentiation and their mutation cause the developmental syndromes Van der Woude syndrome (VWS) and Bartsocas-Papas syndrome (BPS), respectively. RIPK4 promotes keratinocyte differentiation, in part, by inducing IRF6 transactivator function through the phosphorylation of its C-terminal domain at Ser413 and Ser424. Although more than 200 IRF6 mutations have been identified in VWS, a p.Arg412X nonsense mutation is particularly prevalent. A RIPK4 p.Ser376X nonsense mutation in BPS was also recently identified. Here, we demonstrated for the first time that the truncation of IRF6 at Arg412 causes its rapid proteasome-dependent degradation. The truncation of IRF6 also prevented the induction of its transactivator function by RIPK4. Similarly, the p.Ser376X mutation in RIPK4 impaired its induction of IRF6 transactivator function. The mutation also inhibited the stabilisation of β-catenin by RIPK4, and thus may additionally impair Wnt signalling. Collectively, our findings provide important mechanistic insight into how the p.Arg412X and p.Ser376X mutations may cause VWS and BPS, respectively.

Hypoxia Enhances the Proliferative Response of Macrophages to CSF-1 and Their Pro-Survival Response to TNF

In chronic inflammatory lesions there are increased numbers of macrophages with a possible contribution of enhanced survival/proliferation due, for example, to cytokine action; such lesions are often hypoxic. Prior studies have found that culture in low oxygen can promote monocyte/macrophage survival. We show here, using pharmacologic inhibitors, that the hypoxia-induced pro-survival response of macrophages exhibits a dependence on PI3-kinase and mTOR activities but surprisingly is suppressed by Akt and p38 MAPK activities. It was also found that in hypoxia at CSF-1 concentrations, which under normoxic conditions are suboptimal for macrophage proliferation, macrophages can proliferate more strongly with no evidence for alteration in CSF-1 receptor degradation kinetics. TNF promoted macrophage survival in normoxic conditions with an additive effect in hypoxia. The enhanced hypoxia-dependent survival and/or proliferation of macrophages in the presence of CSF-1 or TNF may contribute to their elevated numbers at a site of chronic inflammation.

ADAM17 is required for EGF-R-induced intestinal tumors via IL-6 trans-signaling.

Colorectal cancer is treated with antibodies blocking epidermal growth factor receptor (EGF-R), but therapeutic success is limited. EGF-R is stimulated by soluble ligands, which are derived from transmembrane precursors by ADAM17-mediated proteolytic cleavage. In mouse intestinal cancer models in the absence of ADAM17, tumorigenesis was almost completely inhibited, and the few remaining tumors were of low-grade dysplasia. RNA sequencing analysis demonstrated down-regulation of STAT3 and Wnt pathway components. Because EGF-R on myeloid cells, but not on intestinal epithelial cells, is required for intestinal cancer and because IL-6 is induced via EGF-R stimulation, we analyzed the role of IL-6 signaling. Tumor formation was equally impaired in IL-6−/− mice and sgp130Fc transgenic mice, in which only trans-signaling via soluble IL-6R is abrogated. ADAM17 is needed for EGF-R–mediated induction of IL-6 synthesis, which via IL-6 trans-signaling induces &bgr;-catenin–dependent tumorigenesis. Our data reveal the possibility of a novel strategy for treatment of colorectal cancer that could circumvent intrinsic and acquired resistance to EGF-R blockade.

Interferon Regulatory Factor 6 Promotes Keratinocyte Differentiation in Response to Porphyromonas gingivalis

ABSTRACT We recently demonstrated that the expression of the interferon regulatory factor 6 (IRF6) transcription factor in oral keratinocytes was stimulated by the periodontal pathogen Porphyromonas gingivalis. Here, we have established that IRF6 promotes the differentiation of oral keratinocytes in response to P. gingivalis. This was evidenced by the IRF6-dependent upregulation of specific markers of keratinocyte terminal differentiation (e.g., involucrin [IVL] and keratin 13 [KRT13]), together with additional transcriptional regulators of keratinocyte differentiation, including Grainyhead-like 3 (GRHL3) and Ovo-like zinc finger 1 (OVOL1). We have previously established that the transactivator function of IRF6 is activated by receptor-interacting protein kinase 4 (RIPK4). Consistently, the silencing of RIPK4 inhibited the stimulation of IVL, KRT13, GRHL3, and OVOL1 gene expression. IRF6 was shown to also regulate the stimulation of transglutaminase-1 (TGM1) gene expression by P. gingivalis, as well as that of small proline-rich proteins (e.g., SPRR1), which are covalently cross-linked by TGM1 to other proteins, including IVL, during cornification. The expression of the tight junction protein occludin (OCLN) was found to also be upregulated in an IRF6-dependent manner. IRF6 was demonstrated to be important for the barrier function of oral keratinocytes; specifically, silencing of IRF6 increased P. gingivalis-induced intercellular permeability and cell invasion. Taken together, our findings potentially position IRF6 as an important mediator of barrier defense against P. gingivalis.

Interplay between Porphyromonas gingivalis and EGF signalling in the regulation of CXCL14

Porphyromonas gingivalis is a keystone pathogen in chronic periodontitis. Its expression of gingipain proteases (Kgp and RgpA/B) is central to the stimulation of chronic inflammation. In this study, we investigated the inflammatory response of oral epithelial cells to P. gingivalis. The cells responded by upregulating the expression of the orphan chemokine CXCL14. The stimulation of CXCL14 expression was largely triggered by the gingipain proteases and was dependent on the host protease‐activated receptor PAR‐3. Significantly, CXCL14 expression was transcriptionally repressed in response to epidermal growth factor (EGF)‐induced activation of the MEK‐ERK1/2 pathway. P. gingivalis overcomes the repression of CXCL14 via the gingipain protease‐mediated degradation of EGF. Therefore, P. gingivalis not only directly stimulates CXCL14 expression via PAR‐3 but also promotes its expression by antagonising EGF signalling. In addition to chemotactic activity, some chemokines also have antimicrobial activities. CXCL14 was demonstrated to have bactericidal activity, against commensal oral streptococci associated with health. Notably though, P. gingivalis was not susceptible to killing by CXCL14, potentially because the gingipain proteases can degrade CXCL14. This suggests that the stimulation of dysregulated CXCL14 expression by P. gingivalis may help promote dysbiosis and the development of chronic periodontitis.