Endocytic pathways and biological effects induced by UVB‐dependent or ligand‐dependent activation of the keratinocyte growth factor receptor

FGFR2c Upregulation Contributes to Cancer-Associated Fibroblast Program Activation and to Enhanced Autophagy in Actinic Keratosis-Derived Dermal Fibroblasts: A Possible Role in Precancerous Cell/Stromal Cell Crosstalk

Actinic keratosis (AK) is a preneoplastic skin disorder which can rapidly progress to cutaneous squamous cell carcinomas (SCCs). In light of our previous findings, indicating a possible oncogenic role of the mesenchymal isoform of FGFR2 (FGFR2c) aberrantly expressed in AK keratinocytes, we analyzed the possible tumor-promoting role of this receptor in the stromal AK counterpart in this work. Molecular analysis showed that, particularly in early AK lesions, FGFR2c dermal upregulation is accompanied by the downregulation of the cancer-associated fibroblasts (CAF) transcription repressor CSL, the upregulation of the CAF activator ULK3, and the consequent CAF gene induction. Immunofluorescence and molecular analysis, coupled with silencing approaches by siRNA, applied on primary cultures of KIN I-derived fibroblasts, indicated that FGFR2c upregulation contribute to CAF signature and the increased autophagy in response to FGF2. Magnetic bead-based multiplex assay, combined with FGFR2 signaling shut-off approaches, indicated that, especially in response to FGF2, IL-6 secretion could depend on FGFR2c high expression and signaling, suggesting the possible establishment of FGFR2c-dependent secretory autophagy, contributing to tumor-promoting factor release. Overall, our results identified FGFR2c as a signaling molecule involved in controlling precancerous/stromal cell oncogenic crosstalk, pointing to this receptor as a possible early molecular marker predictive for AK’s rapid malignant progression.

Expression Profile of Fibroblast Growth Factor Receptors, Keratinocyte Differentiation Markers, and Epithelial Mesenchymal Transition-Related Genes in Actinic Keratosis: A Possible Predictive Factor for Malignant Progression?

Simple Summary

In this work, we checked the modulation of Fibroblast Growth Factor Receptors (FGFRs) along with differentiation-related and epithelial-to-mesenchymal transition (EMT)-related markers to identify expression profiles that could be predictive for actinic keratosis (AK) progression through the “differentiated” pathway. We found that the downregulation of the analyzed differentiation markers, but not the modulation of the EMT-related markers, correlated with the canonical progression of AK. In addition, the observed modulation of FGFR2 mesenchymal/epithelial isoforms compatible with FGFR2 isoform switch, as well as the upregulation of FGFR4 suggested their correlation with early steps of AK pathogenesis. In contrast, the increase of mesenchymal FGFR3c isoform expression appeared to suggest that this event correlated with late steps of AK progression. In addition, the strong modulation of filaggrin (FIL), Snail1, as well as of FGFR2c, FGFR4, and their ligand Fibroblast Growth Factor 2 (FGF2), observed in some of the keratinocytic intraepithelial neoplasia grade I (KIN I) samples, may indicate that they could be molecular markers predictive for those KIN I lesions destined to a direct progression to squamous cell carcinoma (SCC) through the “differentiated” pathway.

Abstract

Actinic keratosis (AK) is the ultra violet (UV)-induced preneoplastic skin lesion clinically classified in low (KIN I), intermediate (KIN II), and high (KIN III) grade lesions. In this work we analyzed the expression of Fibroblast Growth Factor Receptors (FGFRs), as well as of keratinocyte differentiation and epithelial-to-mesenchymal transition (EMT)-related markers in differentially graded AK lesions, in order to identify specific expression profiles that could be predictive for direct progression of some KIN I lesions towards squamous cell carcinoma (SCC). Our molecular analysis showed that the keratinocyte differentiation markers keratin 1 (K1), desmoglein-1 (DSG1), and filaggrin (FIL) were progressively downregulated in KIN I, II, and III lesions, while the modulation of epithelial/mesenchymal markers and the induction of the transcription factors Snail1 and Zinc finger E-box-binding homeobox 1 (ZEB1) compatible with pathological EMT, even if observable, did not appear to correlate with AK progression. Concerning FGFRs, a modulation of epithelial isoform of FGFR2 (FGFR2b) and the mesenchymal FGFR2c isoform compatible with an FGFR2 isoform switch, as well as FGFR4 upregulation were observed starting from KIN I lesions, suggesting that they could be events involved in early steps of AK pathogenesis. In contrast, the increase of FGFR3c expression, mainly appreciable in KIN II and KIN III lesions, suggested a correlation with AK late progression. Interestingly, the strong modulation of FIL, Snail1, as well as of FGFR2c, FGFR4, and of their ligand FGF2, observed in some of the KIN I samples, may indicate that they could be molecular markers predictive for those low graded lesions destined to a direct progression to SCC. In conclusion, our data point on the identification of molecular markers predictive for AK rapid progression through the “differentiated” pathway. Our results also represent an important step that, in future, will help to clarify the molecular mechanisms underlying FGFR signaling deregulation in epithelial tissues during the switch from the pre-neoplastic to the oncogenic malignant phenotype.

Insights into autophagy machinery in cells related to skin diseases and strategies for therapeutic modulation

Autophagy, literally meaning "self-eating," is a highly conserved process that is part of the eukaryotic cell cycle. Morphologically, the double membrane contains vesicles with phagocytic components known as autophagosomes. Autophagy is often used as a cellular stress response and quality control mechanisms are used to maintain cell survival. Survival is facilitated by providing energy and metabolic precursors as well as removing damaged proteins or organelles. Moreover, autophagy refers to organelles fused together with part of the cell cytoplasm with a double or multi-membrane structure called phagosome. Research has demonstrated that autophagy is an important mediator of cell fate and has effects on inflammation, pathogen clearance, and antigen presentation. In recent years, studies discussing autophagy have increased in number. Nevertheless, only a small amount of research has considered the impact of autophagy on the pathogenesis of skin diseases. The skin is the largest organ of the body, with a surface area of around two square metre; it is the first line of defense against numerous environmental insults, including ultraviolet radiation, pathogens, mechanical stresses, and toxic chemicals. Autophagy is thought to be a vital modality for endogenous defenses against environmental derangements. This review provides an overview of autophagy machinery in keratinocytes, skin fibroblasts, melanocytes related to skin diseases as well as strategies for therapeutic modulation, for the future development of treatment for skin diseases.

Role of FGFR2b expression and signaling in keratinocyte differentiation: sequential involvement of PKCδ and PKCα

The tumor suppressor epithelial isoform of the fibroblast growth factor receptor 2 (FGFR2b) induces human keratinocyte early differentiation. Moreover, protein kinases C (PKCs) are known to regulate the differentiation program in several cellular contexts, including keratinocytes. Therefore, in this paper we propose to clarify if FGFR2b could play a role also in the late steps of keratinocyte differentiation and to assess if this receptor-induced process would sequentially involve PKCδ and PKCα isoforms. Immunofluorescence, biochemical, and molecular approaches, performed on 2D cultures or 3D organotypic rafts of human keratinocytes overexpressing FGFR2b by stable transduction, showed that receptor signaling induced the precocious onset and an accelerated progression of keratinocyte differentiation, indicating that FGFR2b is a crucial regulator of the entire program of keratinocyte differentiation. In addition, the use of specific inhibitors and gene silencing approaches through specific siRNA demonstrated that PKCδ controls the onset of FGFR2b-triggered differentiation, while PKCα plays a role restricted to the terminal stages of the process. Molecular analysis revealed that the two PKC isoforms sequentially act via induction of KLF4 and DLX3, two transcription factors linked by negative loops to p63, suggesting that p63 would represent the hub molecule at the crossroad of an intricate signaling network downstream FGFR2b, involving multiple PKC-induced transcription factors.

The signaling involved in autophagy machinery in keratinocytes and therapeutic approaches for skin diseases

Autophagy is responsible for the lysosomal degradation of proteins, organelles, microorganisms and exogenous particles. Epidermis primarily consists of keratinocytes which functions as an extremely important barrier. Investigation on autophagy in keratinocytes has been continuously renewing, but is not so systematic due to the complexity of the autophagy machinery. Here we reviewed recent studies on the autophagy in keratinocyte with a focus on interplay between autophagy machinery and keratinocytes biology, and novel autophagy regulators identified in keratinocytes. In this review, we discussed the roles of autophagy in apoptosis, differentiation, immune response, survival and melanin metabolism, trying to reveal the possible involvement of autophagy in skin aging, skin disorders and skin color formation. Since autophagy routinely plays a double-edged sword role in various conditions, its functions in skin homeostasis and potential application as a therapeutic target for skin diseases remains to be clarified. Furthermore, more investigations are needed on optimizing designed strategies to inhibit or enhance autophagy for clinical efficacy.

Interplay between FGFR2b-induced autophagy and phagocytosis: role of PLCγ-mediated signalling

Signalling of the epithelial splicing variant of the fibroblast growth factor receptor 2 (FGFR2b) induces both autophagy and phagocytosis in human keratinocytes. Here, we investigated, in the cell model of HaCaT keratinocytes, whether the two processes might be related and the possible involvement of PLCγ signalling. Using fluorescence and electron microscopy, we demonstrated that the FGFR2b-induced phagocytosis and autophagy involve converging autophagosomal and phagosomal compartments. Moreover, the forced expression of FGFR2b signalling mutants and the use of specific inhibitors of FGFR2b substrates showed that the receptor-triggered autophagy requires PLCγ signalling, which in turn activates JNK1 via PKCδ. Finally, we found that in primary human keratinocytes derived from light or dark pigmented skin and expressing different levels of FGFR2b, the rate of phagocytosis and autophagy and the convergence of the two intracellular pathways are dependent on the level of receptor expression, suggesting that FGFR2b signalling would control in vivo the number of melanosomes in keratinocytes, determining skin pigmentation.

Expression of the FGFR2 mesenchymal splicing variant in epithelial cells drives epithelial-mesenchymal transition

The FGFRs are receptor tyrosine kinases expressed by tissue-specific alternative splicing in epithelial IIIb or mesenchymal IIIc isoforms. Deregulation of FGF/FGFR signaling unbalances the epithelial-stromal homeostasis and may lead to cancer development. In the epithelial-context, while FGFR2b/KGFR acts as tumor suppressor, FGFR2c appears to play an oncogenic role. Based on our recent observation that the switching of FGFR2b versus FGFR2c induces EMT, here we investigated the biological outcome of the ectopic expression of FGFR2c in normal human keratinocytes. Morphological analysis showed that, differently from FGFR2b overexpression, the forced expression and activation of FGFR2c drive the epithelial cells to acquire a mesenchymal-like shape and actin reorganization. Moreover, the appearance of invasiveness and anchorage-independent growth ability in FGFR2c transfected keratinocytes was consistent with the potential tumorigenic role proposed for this receptor variant. Biochemical and molecular approaches revealed that the observed phenotypic changes were accompanied by modulation of EMT biomarkers and indicated the involvement of EMT transcription factors and miRs. Finally, the analysis of the expression pattern of discriminating markers strongly suggested that activation of FGFR2c triggers a process corresponding to the initiation of the pathological type III EMT, but not to the more physiological type II EMT occurring during FGFR2b-mediated wound healing.

Photodynamic therapy inhibit Fibroblast Growth Factor-10 induced keratinocyte differentiation and proliferation through ROS in Fibroblast Growth Factor Receptor-2b pathway

5-aminolevulinic acid-photodynamic therapy (ALA-PDT) is known to be effective in several skin diseases such as acne, actinic keratoses, condyloma acuminata. However, some detailed mechanisms of ALA-PDT to treat these skin diseases still remain elusive. In this study, we aimed to investigate mechanism of ALA-PDT in in-vitro and in-vivo models. For in vitro, we use human keratinocyte cell line (HaCaT) cells. CCK-8 was used to detect cell proliferation activity, immunofluorescence and western blotting method to detect the content of keratin (K)1, K6, K16, protein kinase C (PKC), fibroblast growth factor receptor-2b (FGFR2b) protein, ELISA and RT-PCR to detect expression of interleukin (IL) 1α in the cell supernatant, and detect reactive oxygen species (ROS). For in vivo, we use 20 rabbits to induce hyperkeratosis acne model in their ear. Dermatoscope was used to see follicle hyperkeratosis and skin biopsy to analyze histology and immunohistochemical of PKC, FGFR2b, K1, K6 and K16. Results from this study suggest that ROS stimulated by ALA-PDT lead to inhibition of FGFR2b pathway in PKC downstream to cause reduction of IL1α expression, and eventually, keratinocytes differentiation and proliferation. Our data thus reveal a treatment mechanism of ALA-PDT underlying hyperkeratosis related dermatoses.

FGF7/KGF regulates autophagy in keratinocytes: A novel dual role in the induction of both assembly and turnover of autophagosomes

Autophagy is a degradative pathway through which cells overcome stressful conditions and rapidly change their phenotype during differentiation. Despite its protective role, when exacerbated, autophagy may lead to cell death. Several growth factors involved in cell survival and in preventing differentiation are able to inhibit autophagy. Here we investigated the autophagic role of FGF7/KGF, an important player in epithelial cell protection and differentiation. Biochemical and quantitative fluorescence approaches showed that FGF7 and its signaling induce autophagy in human keratinocytes and the use of specific inhibitors indicated that this effect is independent of the PI3K-AKT-MTOR pathway. The selective block of autophagosome-to-lysosome fusion clarified that FGF7 induces autophagy stimulating autophagosome formation. However, quantitative fluorescence approaches also indicated that, upon a prolonged autophagic stimulus, FGF7 is able to accelerate autophagosome turnover. Moreover, in differentiating keratinocytes, the use of the autophagic inhibitor 3-MA as well as the depletion of BECN1 and ATG5, 2 essential regulators of the process, counteracted the FGF7-induced increase of the differentiation marker KRT1/K1, suggesting that autophagy is required for the FGF7-mediated early differentiation. These results provide the first evidence of a role of FGF7 in the regulation of sequential steps of the autophagic process and strengthen the hypothesis of a direct interplay between autophagy and differentiation. On the other hand, the ability of FGF7 to accelerate autophagosome turnover, preventing their dangerous accumulation, is consistent with the well-established protective role played by the growth factor in epithelial cells.

HPV16 E5 and KGFR/FGFR2b interplay in differentiating epithelial cells

The E5 oncogenic protein of the human papillomavirus type 16 (HPV16 E5) cooperates in epithelial transformation perturbing the behaviour of differentiating suprabasal cells. Among the receptor tyrosine kinases deregulated by 16E5 expression, the key paracrine mediator of epithelial homeostasis keratinocyte growth factor receptor (KGFR/FGFR2b) is altered in its signaling and endocytic traffic in undifferentiated keratinocytes expressing 16E5 and it would represent a major target of the viral protein in differentiated cells. With the aim to specifically address the possible interplay of 16E5 with KGFR/FGFR2b in cells already committed to differentiation, we took advantage of an in vitro model for forced overexpression or depletion of KGFR in E5 expressing human keratinocytes under synchronous waves of differentiation. Quantitative RT-PCR, biochemical and immunofluorescence analysis showed that KGFR down-modulation is responsible for a E5-mediated decrease of the early differentiation marker K1 and that the receptor re-expression as well as triggering of its kinase activity and signaling are able to efficiently counteract the impairment of differentiation, providing a further demonstration of the tumor-suppressive role of KGFR in the new unexplored context of HPV16 E5-mediated carcinogenesis. In addition, KGFR induced a ligand-dependent decrease of p63 through a miR-203 independent mechanism and this effect was blocked by inhibition of the PI3K/Akt signaling, which is the main pathway involved in KGFR-dependent keratinocyte differentiation, suggesting that alterations of the KGFR/p63 crosstalk are responsible for the impairment of keratinocyte differentiation induced by 16E5 and that the opposite tumor-suppressive action of KGFR and oncogenic role of E5 might both involve p63.

Engineering cytochrome-modified silica nanoparticles to induce programmed cell death

A low native membrane permeability and ineffective access to the cellular cytosol, together with aggressive proteolytic degradation, often severely hampers the practical application of any therapeutic protein or antibody. Through engineering the charging profile of mesoporous silica nanoparticles, cellular uptake and subsequent subcellular distribution can be controlled. We show herein that programmed cell death can subsequently be induced across a population of cancer cells with remarkable efficacy on conjugating a specific caspase-cascade-activating cytochrome to such cytosol-accessing particles.

Polarized endocytosis of the keratinocyte growth factor receptor in migrating cells: role of SRC-signaling and cortactin

Cell migration is a physiological process that requires endocytic trafficking and polarization of adhesion molecules and receptor tyrosine kinases (RTKs) to the leading edge. Many growth factors are able to induce motility by binding to specific RTK on target cells. Among them, keratinocyte growth factor (KGF or FGF7) and fibroblast growth factor 10 (FGF10), members of the FGF family, are motogenic for keratinocytes, and exert their action by binding to the keratinocyte growth factor receptor (KGFR), a splicing variant of FGFR2, exclusively expressed on epithelial cells. Here we analyzed the possible role of cortactin, an F-actin binding protein which is tyrosine phosphorylated by Src and is involved in KGFR-mediated cell migration, in the KGFR endocytosis and polarization to the leading edge of migrating cells upon ligand-induced stimulation. Biochemical phosphorylation study revealed that both KGF and FGF10 were able to induce tyrosine phosphorylation of Src and in turn of cortactin, as demonstrated by using the specific pharmacological Src-inhibitor SU6656, although FGF10 effect was delayed with respect to that promoted by KGF. Immunofluorescence analysis demonstrated the polarized localization of KGFR upon ligand stimulation to the leading edge of migrating keratinocytes, process that was regulated by Src. Moreover, we showed that the colocalization of cortactin with KGFR at the plasma membrane protrusions and on early endosomes after KGF and FGF10 treatment was Src-dependent. Further, by using a RNA interference approach through microinjection, we showed that cortactin is required for KGFR endocytosis and that the clathrin-dependent internalization of the receptor is a critical event for its polarization. Finally, KGFR expression and polarization enhanced cell migration in a scratch assay. Our results indicate that both Src and cortactin play a key role in the KGFR endocytosis and polarization at the leading edge of migrating keratinocytes, supporting the crucial involvement of RTK trafficking in cell motility.

The receptor tyrosine kinase FGFR2b/KGFR controls early differentiation of human keratinocytes

The FGFRs trigger divergent responses, such as proliferation and differentiation, and the cell type as well as the context-dependent signaling are crucial for the functional outcome. The FGFR2b/KGFR is expressed exclusively on epithelial cells and plays a key role in skin homeostasis. Here we analyzed in vitro the role of KGFR in the early differentiation of keratinocytes modulating its expression by KGFR cDNA transient transfection or KGFR siRNA microinjection and inducing a synchronous wave of differentiation in pre-confluent cells. Immunofluorescence, biochemical and molecular approaches demonstrated that KGFR overexpression increased the early differentiation marker keratin 1 at both transcriptional and translational levels, while receptor depletion reduced it. Ligand-dependent receptor activation and signaling were required for this differentiative effect. Overexpression of kinase negative KGFR mutant or Tyr769 KGFR signaling mutant, which is not able to recruit and activate PLC-γ, showed that the receptor kinase activity, but not its PLCγ-mediated signaling, is required for differentiation. Reduction of K1 expression, obtained by AKT inhibition, demonstrated that the PI3K/Akt signaling pathway is involved in the control of KGFR-mediated keratinocyte differentiation. This in vitro experimental model indicates that FGFR2b/KGFR expression represents a key event regulating keratinocyte early differentiation during the switch from undifferentiated to differentiating cells.

HPV16 E5 affects the KGFR/FGFR2b-mediated epithelial growth through alteration of the receptor expression, signaling and endocytic traffic

The E5 oncoprotein of the human papillomavirus type 16 (HPV16 E5) cooperates in cervical carcinogenesis and in epithelial transformation deregulating cell growth, survival and differentiation through the modulation of growth factor receptors. Among the epithelial receptor tyrosine kinases, the keratinocyte growth factor receptor/fibroblast growth factor receptor 2b (KGFR/FGFR2b) is a major paracrine mediator of epithelial homeostasis and appears to have an unique and unusual role in epithelial tissues, exerting a tumor-suppressive function in vitro and in vivo. With the aim to better elucidate the molecular events involved in the pathological activity of 16E5, we investigated if the viral protein would be able to affect the KGFR expression, signaling and turnover by interference with its degradative and recycling endocytic pathways. Quantitative reverse transcriptase-PCR and biochemical approaches on human keratinocytes transfected with 16E5-HA showed that E5 protein is able to induce KGFR down-modulation at both transcript and protein levels. Immunofluorescence microscopy in double-transfected cells expressing both E5 and KGFR revealed that the viral protein alters the receptor endocytic trafficking and triggers its endosomal sorting to the indirect juxtanuclear recycling pathway. The shift from lysosomal degradation to recycling at the plasma membrane correlates with a reduced phosphorylation of the fibroblast growth factor receptor substrate-2α tyrosine 196, the major docking site for Grb2-Cbl complexes responsible for receptor ubiquitination and degradation. 5'-Bromo-deoxyuridine incorporation assay demonstrated that expression of 16E5 induces a decrease in the growth response to the receptor ligands as a consequence of KGFR down-modulation, suggesting that 16E5 might have a role on HPV infection in perturbing the KGFR-mediated physiological behavior of confluent keratinocytes committed to differentiation.

Expression and signaling of the tyrosine kinase FGFR2b/KGFR regulates phagocytosis and melanosome uptake in human keratinocytes

Membrane and actin cytoskeleton dynamics during phagocytosis can be triggered and amplified by the signal transduction of receptor tyrosine kinases. The epidermal keratinocytes appear to use the phagocytic mechanism of uptake to ingest melanosomes released by the melanocytes and play a pivotal role in the transfer process. We have previously demonstrated that the keratinocyte growth factor KGF/FGF7 promotes the melanosome uptake through activation of its receptor tyrosine kinase FGFR2b/KGFR. The aim of the present study was to investigate the contribution of KGFR expression, activation, and signaling in regulating the phagocytic process and the melanosome transfer. Phagocytosis was analyzed in vitro using fluorescent latex beads on human keratinocytes induced to differentiate. Melanosome transfer was investigated in keratinocyte-melanocyte cocultures. KGFR depletion by small interfering RNA microinjection and overexpression by transfection of wild type or defective mutant KGFR were performed to demonstrate the direct effect of the receptor on phagocytosis and melanosome transfer. Colocalization of the phagocytosed beads with the internalized receptors in phagolysosomes was analyzed by optical sectioning and 3-dimensional reconstruction. KGFR ligands triggered phagocytosis and melanosome transfer in differentiated keratinocytes, and receptor kinase activity and signaling were required for these effects, suggesting that FGFR2b/KGFR expression/activity and PLCγ signaling pathway play crucial roles in phagocytosis.

Steroid-induced periorificial dermatitis in children--clinical features and response to azelaic acid

Periorificial dermatitis, a common skin disease in young women, has been occasionally reported in children. This study elaborates the clinical features of periorificial dermatitis in children as well as possible pathogenetic factors and the response to 20% azelaic acid cream. A total of 10 children aged 3 to 12 (mean 7.7) years suffering from nongranulomatous periorificial dermatitis for 3 to 7 (mean 4.9) months were evaluated, and dermatologic examination was carried out. Pretreatment was documented. Skin prick tests with a panel of six common inhalative allergens and patch tests with the European Standard Series were performed. An association between atopy and periorificial dermatitis was evaluated, and patients were screened for skin colonization by fungi, bacteria, and Demodex mites. They were treated with 20% azelaic acid cream, which was topically applied twice daily on all affected areas, until complete resolution was achieved. Treatment period was followed by an individual observation period. Periorificial dermatitis had developed in typical distribution and morphology. In all patients, low- to high-potency topical corticosteroids had been used on the face prior to manifestation. Atopy was found in half of the patients. Allergological, bacteriological, and mycological examinations did not reveal pathologic results. Demodex mites could not be demonstrated by skin surface biopsy. Treatment with 20% azelaic acid cream led to complete resolution of skin lesions after 4 to 8 (mean 5.4) weeks in all patients. Transient exacerbation of skin condition with a peak between the 2nd and 6th day of treatment could be observed in three patients. Side effects of 20% azelaic acid cream were registered in six patients and were predominantly present in the first 2 weeks of treatment. Side effects were minimal and became rarer with ongoing treatment. No recurrences were seen within a follow-up period of 2 to 8 (mean 4.4) months. Treatment with 20% azelaic acid cream could provide an effective and safe alternative therapeutic option in children with nongranulomatous periorificial dermatitis.

Role of FGFR2-signaling in the pathogenesis of acne

It is the purpose of this review to extend our understanding of the fibroblast growth factor (FGF) receptor-2b-signaling network in the pathogenesis of acne. A new concept of the role of FGFR2b-signaling in dermal-epithelial interaction for skin appendage formation, pilosebaceous follicle homeostasis, comedogenesis, sebaceous gland proliferation and lipogenesis is presented. The FGFR2-gain-of-function mutations in Apert syndrome and unilateral acneiform nevus are most helpful model diseases pointing the way to androgen-dependent dermalepithelial FGFR2-signaling in acne. Androgen-mediated upregulation of FGFR2b-signaling in acne-prone skin appears to be involved in the pathogenesis of acne vulgaris. In organotypic skin cultures, keratinocyte-derived interleukin-1alpha stimulated fibroblasts to secrete FGF7 which stimulated FGFR2b-mediated keratinocyte proliferation. Postnatal deletion of FGFR2b in mice resulted in severe sebaceous gland atrophy. The importance of FGFR2b in sebaceous gland physiology is further supported by the mode of action of anti-acne agents which have been proposed to attenuate FGFR2b-signaling. Downregulation of FGFR2b-signaling by isotretinoin explains its therapeutic effect in acne. Downregulation of FGFR2b-signaling during the first trimester of pregnancy disturbs branched morphogenesis and explains retinoid embryotoxicity. Insulin-like growth factor-1 (IGF-1), the mediator of growth hormone during puberty, intracts with androgen-dependent FGFR2b-signaling and links androgen- and FGF-mediated signal transduction important in sebaceous gland homeostasis. The search for a follicular defect in the dermalepithelial regulation of growth factor-signaling in acne-prone skin appears to be a most promising approach to clarify the pathogenesis of acne.

Akt1-mediated intracellular oxidation after UVB irradiation suppresses apoptotic cell death induced by cell detachment and serum starvation

Apoptosis is an important cell death system that deletes damaged and mutated cells to prevent cancer. We have previously reported that a certain dose of UVB irradiation inhibited the apoptosis induced by serum starvation and cell detachment, leading to cell transformation. This antiapoptotic effect was partially inhibited by phosphatidylinositol 3-kinase (PI3-kinase) inhibitors. UVB irradiation is known to cause the phosphorylation of Akt via the activation of PI3-kinase; however, the Akt isoform-specific relationship has not yet been clarified. Notably, the role in antiapoptotic effect of UVB has yet to be elucidated. In this study, the role of Akt1 in the UVB-induced inhibition of apoptosis was examined by Akt1 knockdown using small interfering RNA (siRNA). NIH3T3 cells showed typical apoptotic cell death by serum starvation and cell detachment, which was significantly inhibited by UVB irradiation. Akt1 knockdown decreased the antiapoptotic effect of UVB. Hydrogen peroxide-induced suppression of cell death was also decreased in Akt1 knockdown cells. An antioxidant, N-acetylcysteine, inhibited the antiapoptotic effect by UVB irradiation, whereas no inhibition was observed in Akt1 knockdown cells. Furthermore, UVB-induced intracellular peroxidation was not observed in the knockdown cells, indicating that Akt1 played an important role in mediating the intracellular redox status. Treatment with insulin had a similar antiapoptotic effect as UVB irradiation involving intracellular peroxidation, which was also attenuated in Akt1 knockdown cells. These findings suggest that appropriate intracellular oxidation after UVB irradiation prevented apoptosis, a process which might be partially regulated by the production of reactive oxygen species mediated by Akt1.

Keratinocyte growth factor receptors

Modulation of the number of functional growth factor receptors on the epithelial cell surface that is exposed to the action of cognate ligands represents a key strategy in cellular physiology to regulate the proliferation rate and the differentiation process. The keratinocyte growth factor receptor (KGFR) and the epidermal growth factor receptor (EGFR), among the growth factor receptors expressed on keratinocytes, are believed to play a unique crucial role in controlling epithelial proliferation. KGFR and EGFR appear to also contribute to the cell differentiation process. Modulation of KGFR and EGFR on the proliferation rate and differentiation process has been reported either in in vivo or in vitro conditions. This article reviews the architecture, the ligand binding activated-signaling pathways, and the biologic effects of KGFR and EGFR on keratinocytes.

Keratinocyte growth factor receptor ligands target the receptor to different intracellular pathways

The keratinocyte growth factor receptor (KGFR)/fibroblast growth factor receptor 2b is activated by high-affinity-specific interaction with two different ligands, keratinocyte growth factor (KGF)/fibroblast growth factor (FGF)7 and FGF10/KGF2, which are characterized by an opposite requirement of heparan sulfate proteoglycans and heparin for binding to the receptor. We investigated here the possible different endocytic trafficking of KGFR, induced by the two ligands. Immunofluorescence and immunoelectron microscopy analysis showed that KGFR internalization triggered by either KGF or FGF10 occurs through clathrin-coated pits. Immunofluorescence confocal microscopy using endocytic markers as well as tumor susceptibility gene 101 (TSG101) silencing demonstrated that KGF drives KGFR to the degradative pathway, while FGF10 targets the receptor to the recycling endosomes. Biochemical analysis showed that KGFR is ubiquitinated and degraded after KGF treatment but not after FGF10 treatment, and that the alternative fate of KGFR might depend on the different ability of the receptor to phosphorylate the fibroblast growth factor receptor substrate 2 (FRS2) substrate and to recruit the ubiquitin ligase c-Cbl. The recycling endocytic pathway followed by KGFR upon FGF10 stimulation correlates with the higher mitogenic activity exerted by this ligand on epithelial cells compared with KGF, suggesting that the two ligands may play different functional roles through the regulation of the receptor endocytic transport.

AKT and MAPK signaling in KGF-treated and UVB-exposed human epidermal cells

Regulation of proliferation and differentiation in keratinocyte is a complex and dynamic process that involves activation of multiple signaling pathways triggered by different growth factors. Keratinocyte growth factor (KGF) is not only a potent mitogen, but differently from other growth factors, is a potent inducer of differentiation. The MAP kinase and AKT pathways are involved in proliferation and differentiation of many cell types including keratinocytes. We investigated here the role of KGF in modulating AKT and MAPK activity during differentiation of human keratinocytes. Our results show that the mechanisms of action of KGF are dose-dependent and that a sustained activation of the MAPK signaling cascade causes a negative regulation of AKT. We also demostrated increasing expression of KGFR substrates, such as PAK4 during keratinocyte differentiation parallel to the receptor upregulation.

Melanosome transfer promoted by keratinocyte growth factor in light and dark skin-derived keratinocytes

The transfer of melanin from melanocytes to keratinocytes is upregulated by UV radiation and modulated by autocrine and paracrine factors. Among them, the keratinocyte growth factor (KGF/FGF7) promotes melanosome transfer acting on the recipient keratinocytes through stimulation of the phagocytic process. To search for possible differences in the melanosome uptake of keratinocytes from different skin color, we analyzed the uptake kinetics and distribution pattern of fluorescent latex beads in primary cultures of light and dark skin-derived keratinocytes stimulated with KGF and we compared the direct effect of KGF on the melanosome transfer in co-cultures of human primary melanocytes with light and dark keratinocytes. KGF-promoted melanosome transfer was more significant in light keratinocytes compared to dark, due to an increased expression of KGF receptor in light skin keratinocytes. Colocalization studies performed by confocal microscopy using FITC-dextran as a phagocytic marker and fluorescent beads as well as inhibition of particle uptake by cytochalasin D, revealed that beads internalization induced by KGF occurs via actin-dependent phagocytosis. 3D image reconstruction by fluorescence microscopy and ultrastructural analysis through transmission electron microscopy showed differences in the distribution pattern of the beads in light and dark keratinocytes, consistent with the different melanosome distribution in human skin.

The Aryl hydrocarbon receptor: an illuminating effector of the UVB response

The aryl hydrocarbon receptor (AhR) is a cytosolic ligand-activated transcription factor that mediates most of the toxic and carcinogenic effects of drugs and environmental toxins collectively known as xenobiotics. Ligand activation of the AhR stimulates the transcription of genes that encode several xenobiotic-metabolizing enzymes. The molecular mechanisms and signaling pathways evoked by the activation of the AhR are becoming increasingly understood and underscore the participation of the AhR in crucial processes, including cellular stress response, proliferation, differentiation, inflammation, and carcinogenesis. Studies now implicate the AhR as an integral part of the multifaceted signal transduction pathway initiated by the exposure of keratinocytes to ultraviolet B radiation (UVB), which is the most ubiquitous hazard to human skin and the principal risk factor for skin cancer. Ligand-dependent activation of the AhR in the cytosol provides a molecular bridge that links cytoplasmic events to nuclear signals, thus unmasking a previously unknown role for this transcription factor in the complex cellular response to UVB.

Agonist-stimulated reactive oxygen species formation regulates beta2-adrenergic receptor signal transduction

Generation of reactive oxygen species (ROS) can occur upon agonist stimulation of surface receptors to modulate downstream signaling processes. Here, we show that activation of the beta2 adrenergic receptor (beta2AR) by stimulation with the agonist isoproterenol leads to generation of ROS that is required for beta2AR signal transduction. Specifically, we show that inhibition of NADPH oxidase with diphenyliodonium chloride, inhibition of the small GTPase Rac1 with NSC23766, and inhibition of formed ROS with the antioxidant N-acetyl-L-cysteine decreases beta2AR-mediated cAMP formation, protein kinase A activation, and receptor phosphorylation and internalization, but does not impact ligand binding. The results also show that inhibition of ROS attenuates active beta2AR-mediated binding of GTP to alpha subunits of heterotrimeric G proteins. Based on these results, we propose that agonist-dependent ROS formation is needed for beta2AR signal transduction, perhaps through stabilization of active receptor conformers by redox-mediated modification of receptor and/or Galpha proteins cysteine residues.