UVB-induced activation and internalization of keratinocyte growth factor receptor

Recent advances in reactive oxygen species scavenging nanomaterials for wound healing

Reactive oxygen species play a crucial role in cell signaling pathways during wound healing phases. Treatment strategies to balance the redox level in the deep wound tissue are emerging for wound management. In recent years, reactive oxygen species scavenging agents including natural antioxidants, reactive oxygen species (ROS) scavenging nanozymes, and antioxidant delivery systems have been widely employed to inhibit oxidative stress and promote skin regeneration. Here, the importance of reactive oxygen species in different wound healing phases is critically analyzed. Various cutting-edge bioactive ROS nanoscavengers and antioxidant delivery platforms are discussed. This review also highlights the future directions for wound therapies via reactive oxygen species scavenging. This comprehensive review offers a map of the research on ROS scavengers with redox balancing mechanisms of action in the wound healing process, which benefits development and clinical applications of next-generation ROS scavenging-based nanomaterials in skin regeneration.

Melanin's Journey from Melanocytes to Keratinocytes: Uncovering the Molecular Mechanisms of Melanin Transfer and Processing

Skin pigmentation ensures efficient photoprotection and relies on the pigment melanin, which is produced by epidermal melanocytes and transferred to surrounding keratinocytes. While the molecular mechanisms of melanin synthesis and transport in melanocytes are now well characterized, much less is known about melanin transfer and processing within keratinocytes. Over the past few decades, distinct models have been proposed to explain how melanin transfer occurs at the cellular and molecular levels. However, this remains a debated topic, as up to four different models have been proposed, with evidence presented supporting each. Here, we review the current knowledge on the regulation of melanin exocytosis, internalization, processing, and polarization. Regarding the different transfer models, we discuss how these might co-exist to regulate skin pigmentation under different conditions, i.e., constitutive and facultative skin pigmentation or physiological and pathological conditions. Moreover, we discuss recent evidence that sheds light on the regulation of melanin exocytosis by melanocytes and internalization by keratinocytes, as well as how melanin is stored within these cells in a compartment that we propose be named the melanokerasome. Finally, we review the state of the art on the molecular mechanisms that lead to melanokerasome positioning above the nuclei of keratinocytes, forming supranuclear caps that shield the nuclear DNA from UV radiation. Thus, we provide a comprehensive overview of the current knowledge on the molecular mechanisms regulating skin pigmentation, from melanin exocytosis by melanocytes and internalization by keratinocytes to processing and polarization within keratinocytes. A better knowledge of these molecular mechanisms will clarify long-lasting questions in the field that are crucial for the understanding of skin pigmentation and can shed light on fundamental aspects of organelle biology. Ultimately, this knowledge can lead to novel therapeutic strategies to treat hypo- or hyper-pigmentation disorders, which have a high socio-economic burden on patients and healthcare systems worldwide, as well as cosmetic applications.

Quantitative Assessment of Low-Dose Photodynamic Therapy Effects on Diabetic Wound Healing Using Raman Spectroscopy

One of challenges that faces diabetes is the wound healing process. The delayed diabetic wound healing is caused by a complicated molecular mechanism involving numerous physiological variables. Low-dose photodynamic therapy (LDPDT) provides excellent results in rejuvenation and wound healing. In this study, the LDPDT effect on diabetic wounds in mice was studied using two photosensitizers, 5-aminolevulinic acid and methylene blue, and two laser dose expositions of 1 J/cm² and 4 J/cm² by Raman spectroscopy (RS). The latter was used as a noninvasive method, providing specific information about tissue state based on the fundamental vibrational modes of its molecular components. RS allows high spatial resolution acquisition of biochemical and structural information through the generation of point spectra or spectral images. An approach to in vivo quantitative assessment of diabetic wound healing state was developed. This approach is based on an application of the principal component analysis combined with the Mahalanobis metrics to skin Raman spectra, in particular, intensities of the amide I and CH₂ bands.

Accelerating skin regeneration and wound healing by controlled ROS from photodynamic treatment

Cellular metabolisms produce reactive oxygen species (ROS) which are essential for cellular signaling pathways and physiological functions. Nevertheless, ROS act as “double-edged swords” that have an unstable redox balance between ROS production and removal. A little raise of ROS results in cell proliferation enhancement, survival, and soft immune responses, while a high level of ROS could lead to cellular damage consequently protein, nucleic acid, and lipid damages and finally cell death. ROS play an important role in various pathological circumstances. On the contrary, ROS can show selective toxicity which is used against cancer cells and pathogens. Photodynamic therapy (PDT) is based on three important components including a photosensitizer (PS), oxygen, and light. Upon excitation of the PS at a specific wavelength, the PDT process begins which leads to ROS generation. ROS produced during PDT could induce two different pathways. If PDT produces control and low ROS, it can lead to cell proliferation and differentiation. However, excess production of ROS by PDT causes cellular photo damage which is the main mechanism used in cancer treatment. This review summarizes the functions of ROS in living systems and describes role of PDT in production of controllable ROS and finally a special focus on current ROS-generating therapeutic protocols for regeneration and wound healing.

Homotaurine ameriolates the core ASD symptomatology in VPA rats through GABAergic signalling: Role of GAD67

Dysregulated GABAergic signaling is reported in Autism Spectrum disorder (ASD). In the present study, we evaluated a GABA structural mimicker homotaurine (HT) via in-silico docking and investigated the therapeutic efficacy of this drug to ameliorate ASD symptoms in the valproic acid (VPA) rat model of ASD. For the in-vivo study, animals were divided into two groups [Normal control (NC, 0.9% saline; i.p) and disease control (VPA 600mg/kg; i.p)] on gestational day (GD) 12.5. Male pups from VPA-exposed mothers were further divided into five groups (n=6 in each group): disease control (DC, no-further treatment), standard treatment (risperidone (RES) 2.5mg/kg; i.p, consecutively from PND 23-43), HT (10, 25 and 50mg/kg; i.p, consecutively from PND 23-43). In in-silico studies, the binding pattern of homotaurine to GABA-A receptor was found similar to GABA with Tyr205, Glu155, Tyr157, Arg6, and Thr 130 as shared residues. In the in-vivo phase, the early developmental parameters (from PND 7-23) and behavioral parameters (from PND 43-54) were assessed. The offspring of the VPA exposed group exhibited significant (p<0.05) developmental delays, behavioral deficits [decreased sociability and social novelty (three-chamber sociability test), spatial memory (Morris water maze), increased stereotypy (self-grooming)], increased oxidative stress (decreased GSH, SOD, Catalase, and increased MDA), increased pro-inflammatory (IL-1β, 6, TNF-α) and decreased anti-inflammatory (IL-10) cytokines, Purkinje cell loss in the cerebellum and pyknosis in PFC (H/E, Nissil staining) and decreased GAD67 expression in the cerebellum (RT-PCR & immunohistochemistry). Compared to the DC, HT treatment (50mg/kg) was able to ameliorate the aberrant core behavioral deficits, decreased oxidative stress, decreased pro-inflammatory and increased anti-inflammatory cytokine profile with preservation of the Purkinje cell density in the cerebellum, decreased pyknosis in the prefrontal cortex and normalised the expression of GAD67. Thus, HT can be a useful therapeutic agent in ASD and requires further clinical evaluation.

Single-cell RNA-seq of UVB-radiated skin reveals landscape of photoaging-related inflammation and protection by vitamin D

Ultraviolet rays are a potential threat to nature. It can accelerate skin aging by causing skin damage, cell infiltration, and inflammation. The present study investigated UV-irradiated mouse skin through single-cell sequencing. We observed that UV-irradiated mouse skin mainly induced inflammation of fibroblasts and demonstrated differential gene expression. Cell prediction revealed the significance of macrophages in tissue repair. Furthermore, cell culture studies substantiated vitamin D-induced inhibitory effect on skin inflammation. These findings thus indicate some references for skin photo-protection.

The innate immune system and fever under redox control: A Narrative Review

ABSTRACT:

In living cells, redox potential is vitally important for normal physiological processes that are closely regulated by antioxidants, free amino acids and proteins that either have reactive oxygen and nitrogen species capture capability or can be compartmentalized. Although hundreds of experiments support the regulatory role of free radicals and their derivatives, several authors continue to claim that these perform only harmful and non-regulatory functions. In this paper we show that countless intracellular and extracellular signal pathways are directly or indirectly linked to regulated redox processes. We also briefly discuss how artificial oxidative stress can have important therapeutic potential and the possible negative effects of popular antioxidant supplements.

Keratinocyte Growth Factor 2 Ameliorates UVB-Induced Skin Damage via Activating the AhR/Nrf2 Signaling Pathway

Objective: Exposure to ultraviolet B (UVB) can cause skin damage through oxidative stress, DNA damage, and apoptosis. Keratinocyte growth factor (KGF) has been shown to reduce the content of intracellular reactive oxygen species (ROS) following UVB exposure, a role that is crucial for the efficient photoprotection of skin. The present study evaluated the photoprotective effect of KGF-2 on UVB-induced skin damage and explored its potential molecular mechanism.

Methods: To evaluate the effect of KGF-2 on UVB-induced damage ex vivo, a human epidermal full-thickness skin equivalent was pretreated without or with KGF-2 and then exposed to UVB and the levels of histopathological changes, DNA damage, inflammation, and apoptosis were then evaluated. The ability of KGF-2 to protect the cells against UVB-inflicted damage and its effect on ROS production, apoptosis, and mitochondrial dysfunction were determined in HaCaT cells.

Results: Pretreatment of the epidermis with KGF-2 ameliorated the extent of photodamage. At the cellular level, KGF-2 could attenuate ROS production, apoptosis, DNA damage, and mitochondrial dysfunction caused by UVB exposure. KGF-2 could also activate the aryl hydrocarbon receptor (AhR) to trigger the Nrf2 signaling pathway.

Conclusion: Taken together, our findings suggested that KGF-2 could ameliorate UVB-induced skin damage through inhibiting apoptosis, reducing oxidative stress, and preventing DNA damage and mitochondrial dysfunction via regulating AhR/Nrf2 signaling pathway.

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.

Sprayable hydrogel dressing accelerates wound healing with combined reactive oxygen species-scavenging and antibacterial abilities

Wound management poses a considerable economic burden on the global healthcare system, considering the impacts of wound infection, delayed healing and scar formation. To this end, multifunctional dressings based on hydrogels have been developed to stimulate skin healing. Herein, we describe the design, fabrication, and characterization of a sprayable hydrogel-based wound dressing loaded with cerium oxide nanoparticles (CeONs) and an antimicrobial peptide (AMP), for combined reactive oxygen species (ROS)-scavenging and antibacterial properties. We adopted a mussel-inspired strategy to chemically conjugate gelatin with dopamine motifs and prepared a hydrogel dressing with improved binding affinity to wet skin surfaces. Additionally, the release of AMP from the hydrogel demonstrated rapid release ablation and contact ablation against four representative bacterial strains, confirming the desired antimicrobial activities. Moreover, the CeONs-loaded hydrogel dressing exhibited favorable ROS-scavenging abilities. The biocompatibility of the multifunctional hydrogel dressing was further proven in vitro by culturing with HaCaT cells. Overall, the benefits of the developed hydrogel wound dressing, including sprayability, adhesiveness, antimicrobial activity, as well as ROS-scavenging and skin-remodeling ability, highlight its promissing translational potentials in wound management. STATEMENT OF SIGNIFICANCE: Various hydrogel-based wound-dressing materials have been developed to stimulate wound healing. However, from the clinical perspective, few of the current wound dressings meet all the intended multifunctional requirements of preventing infection, promoting rapid wound closure, and minimizing scar formation, while simultaneously offering the convenience of application. In the current study, we adopted a mussel-inspired strategy to functionalize the GelMA hydrogels with DOPA to fabricate GelMA-DOPA hydrogel which exhibited an enhanced binding affinity for wound surfaces, AMP HHC-36 and CeONs are further encapsulated into the GelMA-DOPA hydrogel to confer the hydrogel wound dressing with antimicrobial and ROS-scavenging abilities. The GelMA-DOPA-AMP-CeONs dressing offered the benefits of sprayability, adhesiveness, antimicrobial activity, as well as ROS-scavenging and skin-remodeling ability, which might address the therapeutic and economic burdens associated with chronic wound treatment and management.

Redox dysregulation in the pathogenesis of chronic venous ulceration

In chronic venous ulcers (CVUs), which account for up to 75% of leg ulcers, the inflammatory stage of wound healing fails to down-regulate, preventing progression to proliferation, remodeling and eventual epithelialisation. The roles of reactive oxygen species (ROS) in the oxidative burst and pathogen killing are well known, but ROS also have important functions in extra-cellular and intra-cellular signalling. Iron deposition, resulting from venous reflux, primes macrophages towards a persistent inflammatory response, with ongoing stimulation by bacteria potentially playing a role. Generation of excessive ROS by activated inflammatory cells causes tissue destruction and disintegration of the dermis, and then at later stages, a failure to heal. Here, we review the evidence for ROS in CVU formation and in normal and delayed healing. We also discuss how ROS modulation might be used to influence the healing of these complex wounds, which cause long-term morbidity and are associated with a significant financial burden to healthcare systems.

FADD-deficient mouse embryonic fibroblasts undergo RIPK1-dependent apoptosis and autophagy after NB-UVB irradiation

Sun or therapy-related ultraviolet B (UVB) irradiation induces different cell death modalities such as apoptosis, necrosis/necroptosis and autophagy. Understanding of mechanisms implicated in regulation and execution of cell death program is imperative for prevention and treatment of skin diseases. An essential component of death-inducing complex is Fas-associated protein with death domain (FADD), involved in conduction of death signals of different death modalities. The purpose of this study was to enlighten the role of FADD in the selection of cell death mode after narrow-band UVB (NB-UVB) irradiation using specific cell death inhibitors (carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]- fluoromethylketone (zVAD-fmk), Necrostatin-1 and 3-Methyladenine) and FADD-deficient (FADD^-/-) mouse embryonic fibroblasts (MEFs) and their wild type (wt) counterparts. The results imply that lack of FADD sensitized MEFs to induction of receptor-interacting protein 1 (RIPK1)-dependent apoptosis by the generation of reactive oxygen species (ROS), but without activation of the proteins p53, Bax and Bcl-2 as well as without the enrolment of calpain-2. Autophagy was established as a contributing factor to NB-UVB-induced death execution. By contrast, wt cells triggered intrinsic apoptotic pathway that was resistant to the inhibition by zVAD-fmk and Necrostatin-1 pointing to the mechanism overcoming the cell survival. These findings support the role of FADD in prevention of autophagy-dependent apoptosis.

Antioxidative study of Cerium Oxide nanoparticle functionalised PCL-Gelatin electrospun fibers for wound healing application

Skin wound healing involves a coordinated cellular response to achieve complete reepithelialisation. Elevated levels of reactive oxygen species (ROS) in the wound environment often pose a hindrance in wound healing resulting in impaired wound healing process. Cerium oxide nanoparticles (CeNPs) have the ability to protect the cells from oxidative damage by actively scavenging the ROS. Furthermore, matrices like nanofibers have also been explored for enhancing wound healing. In the current study CeNP functionalised polycaprolactone (PCL)-gelatin nanofiber (PGNPNF) mesh was fabricated by electrospinning and evaluated for its antioxidative potential. Wide angle XRD analysis of randomly oriented nanofibers revealed ∼2.6 times reduced crystallinity than pristine PCL which aided in rapid degradation of nanofibers and release of CeNP. However, bioactive composite made between nanoparticles and PCL-gelatin maintained the fibrous morphology of PGNPNF upto 14 days. The PGNPNF mesh exhibited a superoxide dismutase (SOD) mimetic activity due to the incorporated CeNPs. The PGNPNF mesh enhanced proliferation of 3T3-L1 cells by ∼48% as confirmed by alamar blue assay and SEM micrographs of cells grown on the nanofibrous mesh. Furthermore, the PGNPNF mesh scavenged ROS, which was measured by relative DCF intensity and fluorescence microscopy; and subsequently increased the viability and proliferation of cells by three folds as it alleviated the oxidative stress. Overall, the results of this study suggest the potential of CeNP functionalised PCL-gelatin nanofibrous mesh for wound healing applications.

Reactive Oxygen Species and NOX Enzymes Are Emerging as Key Players in Cutaneous Wound Repair

Our understanding of the role of oxygen in cell physiology has evolved from its long-recognized importance as an essential factor in oxidative metabolism to its recognition as an important player in cell signaling. With regard to the latter, oxygen is needed for the generation of reactive oxygen species (ROS), which regulate a number of different cellular functions including differentiation, proliferation, apoptosis, migration, and contraction. Data specifically concerning the role of ROS-dependent signaling in cutaneous wound repair are very limited, especially regarding wound contraction. In this review we provide an overview of the current literature on the role of molecular and reactive oxygen in the physiology of wound repair as well as in the pathophysiology and therapy of chronic wounds, especially under ischemic and hyperglycemic conditions.

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.

Melatonin, mitochondria, and the skin

The skin being a protective barrier between external and internal (body) environments has the sensory and adaptive capacity to maintain local and global body homeostasis in response to noxious factors. An important part of the skin response to stress is its ability for melatonin synthesis and subsequent metabolism through the indolic and kynuric pathways. Indeed, melatonin and its metabolites have emerged as indispensable for physiological skin functions and for effective protection of a cutaneous homeostasis from hostile environmental factors. Moreover, they attenuate the pathological processes including carcinogenesis and other hyperproliferative/inflammatory conditions. Interestingly, mitochondria appear to be a central hub of melatonin metabolism in the skin cells. Furthermore, substantial evidence has accumulated on the protective role of the melatonin against ultraviolet radiation and the attendant mitochondrial dysfunction. Melatonin and its metabolites appear to have a modulatory impact on mitochondrion redox and bioenergetic homeostasis, as well as the anti-apoptotic effects. Of note, some metabolites exhibit even greater impact than melatonin alone. Herein, we emphasize that melatonin-mitochondria axis would control integumental functions designed to protect local and perhaps global homeostasis. Given the phylogenetic origin and primordial actions of melatonin, we propose that the melatonin-related mitochondrial functions represent an evolutionary conserved mechanism involved in cellular adaptive response to skin injury and repair.

Toward the Design of Less Hazardous Chemicals: Exploring Comparative Oxidative Stress in Two Common Animal Models

Sustainable molecular design of less hazardous chemicals presents a potentially transformative approach to protect public health and the environment. Relationships between molecular descriptors and toxicity thresholds previously identified the octanol-water distribution coefficient, log D, and the HOMO-LUMO energy gap, ΔE, as two useful properties in the identification of reduced aquatic toxicity. To determine whether these two property-based guidelines are applicable to sublethal oxidative stress (OS) responses, two common aquatic in vivo models, the fathead minnow (Pimephales promelas) and zebrafish (Danio rerio), were employed to examine traditional biochemical biomarkers (lipid peroxidation, DNA damage, and total glutathione) and antioxidant gene activation following exposure to eight structurally diverse industrial chemicals (bisphenol A, cumene hydroperoxide, dinoseb, hydroquinone, indene, perfluorooctanoic acid, R-(-)-carvone, and tert-butyl hydroperoxide). Bisphenol A, cumene hydroperoxide, dinoseb, and hydroquinone were consistent inducers of OS. Glutathione was the most consistently affected biomarker, suggesting its utility as a sensitivity response to support the design of less hazardous chemicals. Antioxidant gene expression (changes in nrf2, gclc, gst, and sod) was most significantly (p < 0.05) altered by R-(-)-carvone, cumene hydroperoxide, and bisphenol A. Results from the present study indicate that metabolism of parent chemicals and the role of their metabolites in molecular initiating events should be considered during the design of less hazardous chemicals. Current empirical and computational findings identify the need for future derivation of sustainable molecular design guidelines for electrophilic reactive chemicals (e.g., SN2 nucleophilic substitution and Michael addition reactivity) to reduce OS related adverse outcomes in vivo.

Hyperbaric oxygen therapy promotes wound repair in ischemic and hyperglycemic conditions, increasing tissue perfusion and collagen deposition

The treatment of chronic wounds remains inconsistent and empirical. Hyperbaric oxygen therapy (HBOT) is a promising method to improve wound repair but there is still a lack of understanding of its mechanisms of action and its indications are not yet clearly defined. We studied the effects of HBOT in four different wound conditions by inflicting bilateral wounds on the dorsal aspect of the feet of nonischemic or ischemic limbs in normoglycemic or hyperglycemic rats. To create an ischemic condition, arterial resection was performed unilaterally. Forty-four animals received HBOT five times a week until complete wound closure. Wound repair was compared with 44 rats receiving standard dressing only. HBOT increased blood flow and accelerated wound closure in ischemic and hyperglycemic wounds, most significantly when the two conditions were combined. Wound contraction and reepithelialization were similarly stimulated by HBOT. The acceleration of wound contraction was not associated with increased myofibroblasts expression, nor fibroblast recruitment or higher cell count in the granulation tissue. Of note, we observed a significant increase in collagen deposition in early time points in ischemic wounds receiving HBOT. This data emphasizes that an early application of HBOT might be crucial to its efficacy. We concluded that wounds where ischemia and hyperglycemia are combined, as it is often the case in diabetic patients, have the best chance to benefit from HBOT.

YES1 activation elicited by heat stress is anti-apoptotic in mouse pachytene spermatocytes

Deregulated expression of protein tyrosine phosphorylation has been implicated in testicular response to different stimuli. Herein, YES1, a nonreceptor protein tyrosine kinase, was found to be significantly up-regulated in pachytene spermatocytes (PS) during early recovery from a transient testicular heat stress. Coculture of PS with Sertoli cells (SCs) could enhance the hyperthermia-induced YES1 activation, indicative of a positive regulation of the paracrine signaling. Moreover, SU6656, a selective YES1 inhibitor, was shown to effectively block YES1 activity, thereafter resulting in a dramatic increase of heat stress-induced apoptosis in primary cultured PS. Mechanistically, the antiapoptotic effect of YES1 activation in response to testicular heat insult may mediate via the regulation of extracellular signal-regulated kinase (ERK)/metastasis-associated 1 (MTA1) cascade. From a clinical standpoint, a notably higher level of YES1 expression was observed in the pathological testis from varicocele patients as compared to a negligible staining in the control group. Taken together, our present results provide the first evidence that the YES1/ERK/MTA1/p53 cascade may serve as a naturally occurring, indispensable self-defensive mechanism maintaining apoptotic balance during meiotic heat stress. Our study may have also partially answered the question of how activation of signal pathways at the cell membrane surface interacts with the key regulatory events occurring in the nucleus during testicular heat shock.

UV, stress and aging

Skin is a model of choice in studies on aging. Indeed, skin aging can be modulated by internal and external factors, reflecting its complexity. Two types of skin aging have been identified: intrinsic, mainly genetically determined and extrinsic—also called "photo-aging"—resulting on the impact of environmental stress and more precisely of UV rays. Simplified in vitro models, based on cellular senescence, have been developed to study the relationship between UV and aging. These models vary on the cell type (fibroblasts or keratinocytes, normal or immortalized) and the type of UV used (UVA or UVB).

The role of the E2F transcription factor family in UV-induced apoptosis

The E2F transcription factor family is traditionally associated with cell cycle control. However, recent data has shown that activating E2Fs (E2F1-3a) are potent activators of apoptosis. In contrast, the recently cloned inhibitory E2Fs (E2F7 and 8) appear to antagonize E2F-induced cell death. In this review we will discuss (i) the potential role of E2Fs in UV-induced cell death and (ii) the implications of this to the development of UV-induced cutaneous malignancies.

Human papillomavirus-16 E7 interacts with glutathione S-transferase P1 and enhances its role in cell survival

Background

Human Papillomavirus (HPV)-16 is a paradigm for “high-risk” HPVs, the causative agents of virtually all cervical carcinomas. HPV E6 and E7 viral genes are usually expressed in these tumors, suggesting key roles for their gene products, the E6 and E7 oncoproteins, in inducing malignant transformation.

Methodology/Principal Findings

By protein-protein interaction analysis, using mass spectrometry, we identified glutathione S-transferase P1-1 (GSTP1) as a novel cellular partner of the HPV-16 E7 oncoprotein. Following mapping of the region in the HPV-16 E7 sequence that is involved in the interaction, we generated a three-dimensional molecular model of the complex between HPV-16 E7 and GSTP1, and used this to engineer a mutant molecule of HPV-16 E7 with strongly reduced affinity for GSTP1.When expressed in HaCaT human keratinocytes, HPV-16 E7 modified the equilibrium between the oxidized and reduced forms of GSTP1, thereby inhibiting JNK phosphorylation and its ability to induce apoptosis. Using GSTP1-deficient MCF-7 cancer cells and siRNA interference targeting GSTP1 in HaCaT keratinocytes expressing either wild-type or mutant HPV-16 E7, we uncovered a pivotal role for GSTP1 in the pro-survival program elicited by its binding with HPV-16 E7.

Conclusions/Significance

This study provides further evidence of the transforming abilities of this oncoprotein, setting the groundwork for devising unique molecular tools that can both interfere with the interaction between HPV-16 E7 and GSTP1 and minimize the survival of HPV-16 E7-expressing cancer cells.

Cell death in the skin

The skin is the largest organ of the body and protects the organism against external physical, chemical and biological insults, such as wounding, ultraviolet radiation and micro-organisms. The epidermis is the upper part of the skin that is continuously renewed. The keratinocytes are the major cell type in the epidermis and undergo a specialized form of programmed cell death, called cornification, which is different from classical apoptosis. In keep with this view, several lines of evidence indicate that NF-kB is an important factor providing protection against keratinocyte apoptosis in homeostatic and inflammatory conditions. In contrast, the hair follicle is an epidermal appendage that shows cyclic apoptosis-driven involution, as part of the normal hair cycle. The different cell death programs need to be well orchestrated to maintain skin homeostasis. One of the major environmental insults to the skin is UVB radiation, causing the occurrence of apoptotic sunburn cells. Deregulation of cell death mechanisms in the skin can lead to diseases such as cancer, necrolysis and graft-versus-host disease. Here we review the apoptotic and the anti-apoptotic mechanisms in skin homeostasis and disease.

A plaidoyer for cutaneous enzymology: our view of some important unanswered questions on the contributions of selected key enzymes to epidermal homeostasis

This review highlights the importance of enzymology, a field of great neglect in current cutaneous biology research. It was therefore the aim by using selected examples of epidermal enzymes and their action including some open questions to demonstrate the importance of this area. Clearly a thorough understanding of basic knowledge in this field is needed which in turn offers a plethora of innovative research projects for a curious mind. Moreover, in order to gain the closest understanding to the truth instead of generating esoteric results, emphasis is put forward on a holistic view utilizing a combination of modern and sometimes old methods to get the answer. Last but not least the bench work is only useful for the welfare of our patients if we can apply our basic knowledge.

Redox signals in wound healing

Physical trauma represents one of the most primitive challenges that threatened survival. Healing a problem wound requires a multi-faceted comprehensive approach. First and foremost, the wound environment will have to be made receptive to therapies. Second, the appropriate therapeutic regimen needs to be identified and provided while managing systemic limitations that could secondarily limit the healing response. Unfortunately, most current solutions seem to aim at designing therapeutic regimen with little or no consideration of the specific details of the wound environment and systemic limitations. One factor that is centrally important in making the wound environment receptive is correction of wound hypoxia. Recent work have identified that oxygen is not only required to disinfect wounds and fuel healing but that oxygen-dependent redox-sensitive signaling processes represent an integral component of the healing cascade. Over a decade ago, it was proposed that in biological systems oxidants are not necessarily always the triggers for oxidative damage and that oxidants such as H2O2 could actually serve as signaling messengers and drive several aspects of cellular signaling. Today, that concept is much more developed and mature. Evidence supporting the role of oxidants such as H2O2 as signaling messenger is compelling. A complete understanding of the continuum between the classical and emergent roles of oxygen requires a thorough consideration of current concepts in redox biology. The objective of this review is to describe our current understanding of how redox-sensitive processes may drive dermal tissue repair.

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.

Altered expressions of fibroblast growth factor receptors and alveolarization in neonatal mice exposed to 85% oxygen

In the present study, we tested the hypothesis that exposure of newborn mice to sublethal hyperoxia would alter lung development and expressions of fibroblast growth factor receptors (FGFRs)-3 and FGFR-4. Newborn FVB mice were exposed to 85% O2 or maintained in room air for up to 14 d. No animal mortality was observed, and body weight gains were not affected by hyperoxia. At postnatal d 7 and 14 (P7, P14), lungs of mice exposed to 85% O2 showed fewer alveolar secondary crests and larger alveoli or terminal air spaces than did mice in room air. In pups kept in room air, lung levels of FGFR-3 and FGFR-4 mRNA were greater at P3 than at P1, but similar increases were not observed in hyperoxic mice. Immunoreactivity of FGFR-3 and FGFR-4 was lower in lungs of hyperoxic mice than in controls at P14. In pups kept in room air, lung fibroblast growth factor (FGF)-7 mRNA levels were greater at P14 than at P1, but similar changes were not observed in hyperoxic mice. The temporally and spatially specific alterations in the expressions of FGFR-3, FGFR-4, and FGF-7 in the mice exposed to hyperoxia may contribute to aberrant lung development.

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.

Multiple mechanisms of Erbb2 action after ultraviolet irradiation of the skin

Ultraviolet (UV) irradiation causes multiple pathologic changes in the skin including inflammation, immune suppression, photoaging, and cancer. Effects of UV irradiation include the activation of numerous signal transduction pathways, including the mitogen-activated protein kinases (MAPK), and the activation of transcription factors such as nuclear factor kappa B (NFkappaB). These responses alter gene expression in a manner that resembles the response to growth factors known as the "UV response". The UV response alters the kinetics of cell division and cell death allowing the skin to recover from the DNA damage caused by UV exposure. UV irradiation also rapidly activates epidermal growth factor receptor (EGFR) family members, including Erbb2 (human epithelial growth factor receptor 2 (HER2)/neu), through the generation of reactive oxygen species. Erbb2, a protooncogene that is activated in many types of cancer and associated with aggressive and chemotherapeutic-resistant disease, is expressed in both follicular and epidermal keratinocytes within the skin. However, the physiological functions of Erbb2 in the skin and its role in the UV response are largely unknown. In this review, evidence that Erbb2 is influential in modulating the response of the skin to UV will be presented. Erbb2 alters the expression of regulatory genes controlling inflammation, angiogenesis, cell division, apoptosis, cell adhesion, and migration following UV irradiation. In addition, Erbb2 dampens UV-induced S-phase arrest, augments inflammation in response to UV irradiation, and suppresses UV-induced apoptosis. In summary, the evidence presented herein links UV-induced Erbb2 activation to many of the effects of UV and implicates Erbb2 in UV-induced carcinogenesis.

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.

Keratinocyte growth factor protects epidermis and hair follicles from cell death induced by UV irradiation, chemotherapeutic or cytotoxic agents

Owing to its potent cytoprotective properties for epithelial cells, keratinocyte growth factor (KGF) is successfully used for the treatment of chemotherapy- and radiotherapy-induced oral mucositis in cancer patients. It is therefore of major interest to determine possible clinical applications of KGF in other organs and in different stress situations and to unravel common and organ-specific mechanisms of KGF action. Here we show that KGF protects human keratinocytes from the toxicity of xenobiotics with electrophilic and oxidative properties and reduces the cell death induced by UV irradiation. In contrast to other cell types, cytoprotection of keratinocytes by KGF is not a direct anti-apoptotic effect but requires de novo protein synthesis. The in vitro findings are clinically relevant because KGF protected keratinocytes in organ-cultured human scalp hair follicles from the toxicity of the xenobiotic menadione. Moreover, injection of KGF into murine back skin markedly reduced cell death in the epidermis after UVB irradiation. This activity is dependent on FGF receptor signaling because it was abrogated in transgenic mice expressing a dominant-negative FGF receptor mutant in keratinocytes. Taken together, our results encourage the use of KGF for skin protection from chemical and physical insults.

Apoptosis signal regulating kinase-1 connects reactive oxygen species to p38 MAPK-induced mitochondrial apoptosis in UVB-irradiated human keratinocytes

The p38 MAPK pathway controls critical premitochondrial events culminating in apoptosis of UVB-irradiated human keratinocytes, but the upstream mediators of this stress signal are not completely defined. This study shows that in human keratinocytes exposed to UVB the generation of reactive oxygen species (ROS) acts as a mediator of apoptosis signal regulating kinase-1 (Ask-1), a redox-sensitive mitogen-activated protein kinase kinase kinase (MAP3K) regulating p38 MAPK and JNK cascades. The NADPH oxidase antagonist diphenylene iodonium chloride and the EGFR inhibitor AG1487 prevent UVB-mediated ROS generation, the activation of the Ask-1-p38 MAPK stress response pathway, and apoptosis, evidencing the link existing between the early plasma membrane-generated ROS and the activation of a lethal cascade initiated by Ask-1. Consistent with this, Ask-1 overexpression considerably sensitizes keratinocytes to UVB-induced mitochondrial apoptosis. Although the JNK pathway is also stimulated after UVB, the killing effect of Ask-1 overexpression is reverted by p38 MAPK inhibition, suggesting that Ask-1 exerts its lethal effects mainly through the p38 MAPK pathway. Moreover, p38alpha(-/-) murine embryonic fibroblasts are protected from UVB-induced apoptosis even if JNK activation is fully preserved. These results argue for an important role of the UVB-generated ROS as mediators of the Ask-1-p38 MAPK pathway that, by culminating in apoptosis, restrains the propagation of potentially mutagenic keratinocytes.

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

UVB exposure of epidermal cells is known to trigger early and late molecular pathways dependent on receptor tyrosine kinases and reactive oxygen species (ROS). We have recently reported that UVB irradiation induces tyrosine phosphorylation, kinase activation, and internalization of the receptor for the keratinocyte growth factor (KGFR), a paracrine mediator of epithelial growth, differentiation, and survival. Here we analyzed in more detail the UVB-induced endocytic pathway of KGFR and the role of KGFR activation and internalization in regulating UVB-promoted apoptosis and cell cycle arrest. Immunogold electron microscopy and confocal analysis revealed that the UVB-induced endocytosis of KGFR occurs through clathrin-coated pits and that the internalized receptors are sorted to the degradative route and reach the lysosomal compartment with a timing similar to that induced by their ligand KGF. Treatment with the anti-oxidant N-acetylcysteine inhibited KGFR endocytosis, suggesting that the receptor internalization is mediated by the intracellular production of ROS. The ligand-independent KGFR endocytic pathway induced by UVB requires receptor kinase activity and tyrosine phosphorylation and involves transient receptor ubiquitination. Inhibition of KGFR activity reduces both the KGF-mediated proliferative response and the UVB-promoted apoptotic cell death, indicating a different effect of ligand-induced and UVB-induced KGFR triggering. In addition, receptor internalization leads to protection from apoptosis caused by UVB exposure. Finally, we compared directly the behavior of KGFR with that of the epidermal growth factor receptor (EGFR) upon UVB exposure. Surprisingly, biochemical and immunofluorescence analysis showed that EGFR, differently from KGFR, does not undergo UVB-induced tyrosine phosphorylation and internalization. Taken together, our results suggest a differential role of KGFR and EGFR in the response of epidermal cells to UVB possibly because KGFR endocytosis could be crucial for attenuation of survival signals in the suprabasal layers of human skin.

Keratinocyte Growth Factor Promotes Melanosome Transfer to Keratinocytes

Melanogenesis and melanosome transfer from the melanocytes to the neighboring keratinocytes are induced by ultraviolet radiation and modulated by autocrine and paracrine factors. Keratinocyte growth factor (KGF/fibroblast growth factor (FGF)7) is a paracrine mediator of human keratinocyte growth and differentiation. We evaluated the influence of KGF on melanosome transfer in co-cultures of keratinocytes and melanocytes. Immunofluorescence analysis using anti-tyrosinase and anti-human cytokeratin antibodies, phagocytic assays using fluorescent latex beads, and ultrastructural analysis indicated that KGF is able to induce melanosome transfer acting only on the recipient keratinocytes and as a consequence of a general role of KGF in the promotion of the phagocytic process. Inhibition of proteinase-activated receptor-2, to block the Rho-dependent phagocytic pathway, or of the Src family tyrosine kinases, to inhibit the Rac-dependent pathway, showed that KGF promotes phagocytosis through both mechanisms. Increased expression of the KGF receptor (KGFR) on the keratinocytes by transfection led to increased phagocytosis of latex beads following KGF treatment, suggesting that the KGF effect is directly mediated by KGFR expression and activation. Moreover, confocal microscopic analysis revealed that KGFR localize in phagosomes during KGF-induced phagocytosis, suggesting a direct role of the receptor in regulating both the early steps of uptake and the intracellular traffic of the phagosomes.

Epidermal growth factor receptor signaling is partially responsible for the increased matrix metalloproteinase-1 expression in ocular epithelial cells after UVB radiation

Pterygia are inflammatory, invasive, and proliferative lesions of the human ocular surface in which the matrix metalloproteinase (MMP) collagenase-1 (MMP-1) is highly expressed. Pterygia development may involve MMP-1 activity against interstitial fibrillar collagen, an abundant extracellular matrix component of the cornea, and its induction by ultraviolet light (UVB). We examined the pathways responsible for enhanced expression of MMP-1 in pterygium epithelial cells after UVB exposure and/or treatment with chemical inhibitors of mitogen-activated protein kinases or epidermal growth factor receptor. The induction of MMP-1 by UVB was comparable to that mediated by heparin-binding epidermal growth factor-like growth factor and epidermal growth factor. The epidermal growth factor receptor inhibitor PD153035 partially blocked the UVB-mediated induction of MMP-1 and totally abrogated its production after stimulation with either heparin-binding epidermal growth factor-like growth factor or epidermal growth factor. UVB exposure enhanced the phosphorylated form of ERK1/2 in a time-dependent manner whereas the ERK1/2 inhibitor PD98059 decreased this induction by at least fivefold. Transcripts for c-jun and c-fos were detected as early as 2 hours after UVB exposure and were suppressed by PD98059. The identification of a specific intracellular signaling pathway responsible for the enhanced production of a key enzyme that denatures intact fibrillar collagen has important implications for understanding the pathophysiology and future therapy for pterygia.

Ultraviolet radiation-induced apoptosis in keratinocytes: on the role of cytosolic factors

Epidemiological and experimental evidences have established solar ultraviolet (UV) radiation as the leading cause of skin cancers. Specifically, the frequency of non-melanoma skin cancer, one of the malignancies with the most rapidly increasing incidence, is directly related to the total exposure to solar UV light. As part of a general effort to elucidate the components of cellular signal transduction pathways, the mechanisms of cellular responses to UV radiation have received considerable attention over the last few years. These efforts were driven mainly by the conviction that understanding how normal cells respond to extracellular stimuli such as exposure to UV radiation will undoubtedly help in deciphering what goes wrong in a variety of clinical disorders including skin cancers and will assist in the development of novel therapeutic strategies. Studies over the last decade have established that UV radiation induces a bewildering array of signal transduction pathways, some of which could lead to apoptotic cell death. UV-induced cell death by apoptosis is considered to be a natural protective mechanism that removes damaged keratinocytes and circumvents the risk of malignant transformation. In this review, we summarize some of the most important findings regarding the response and role of mitogen-activated protein kinases in UVA and UVB radiation-induced signaling to apoptosis in keratinocytes. We will also briefly discuss what is known about the role of the BCL-2 family of proteins, the emerging role of lysosomal proteases and other important cytosolic signaling proteins in UV-induced apoptosis.

Ultraviolet B radiation-induced immunosuppression: molecular mechanisms and cellular alterations

About 30 years ago, the discovery of the connection between UV radiation and the immune system triggered the field of photoimmunology. In that time, many aspects were studied, and a complex picture emerged. UV absorption results in multi-tiered molecular and cellular UV radiation-induced events, eventually affecting the immune system. The shorter wavelengths of the UV spectrum, i.e. UVB appear to be the most critical players for impairing immune reactions. This review summarizes and discusses UVB radiation-induced effects on the skin, considering the primary efferent molecular events following energy absorption of UVB radiation, ending with the various afferent cellular changes, such as induction of regulatory T cells.

Nitric oxide function in the skin

Endogenously produced nitric oxide (NO) has a remarkably diverse range of biological functions, including a role in neurotransmission, smooth muscle relaxation, and the response to immunogens. Over the last 10 years, it has become clear that this extraordinary molecular messenger also plays a vital role in the skin, orchestrating normal regulatory processes and underlying some of the pathophysiological ones. We thought it pertinent to review the current literature concerning the possible function of NO in normal skin, its clinical and pathological significance, and the potential for therapeutic advances. The keratinocytes, which make up the bulk of the epidermis, constitutively express the neuronal isoform of NO synthase (NOS1), whereas the fibroblasts in the dermis and other cell types in the skin express the endothelial isoform (NOS3). Under certain conditions, virtually all skin cells appear to be capable of expressing the inducible NOS isoform (NOS2). The expression of NOS2 is also strongly implicated in psoriasis and other inflammatory skin conditions. Constitutive, low level NO production in the skin seems to play a role in the maintenance of barrier function and in determining blood flow rate in the microvasculature. Higher levels of NOS activity, stimulated by ultraviolet (UV) light or skin wounding, initiate other more complex reactions that require the orchestration of various cell types in a variety of spatially and temporally coordinated sets of responses. The NO liberated following UV irradiation plays a significant role in initiating melanogenesis, erythema, and immunosuppression. New evidence suggests that it may also be involved in protecting the keratinocytes against UV-induced apoptosis. The enhanced NOS activity in skin wounding (reviewed recently in this journal [Nitric oxide 7 (2002) 1]) appears to be important in guiding the infiltrating white blood cells and initiating the inflammation. In response to both insults, UV irradiation and skin wounding, the activation of constitutive NOS proceeds and overlaps with the expression of NOS2. Thus, at a macro-level, at least three different rates of NO production can occur in the skin, which seem to play an important part in organizing the skin's unique adaptability and function.

Solar ultraviolet radiation as a trigger of cell signal transduction

Ultraviolet light radiation in sunlight is known to cause major alterations in growth and differentiation patterns of exposed human tissues. The specific effects depend on the wavelengths and doses of the light, and the nature of the exposed tissue. Both growth inhibition and proliferation are observed, as well as inflammation and immune suppression. Whereas in the clinical setting, these responses may be beneficial, for example, in the treatment of psoriasis and atopic dermatitis, as an environmental toxicant, ultraviolet light can induce significant tissue damage. Thus, in the eye, ultraviolet light causes cataracts, while in the skin, it induces premature aging and the development of cancer. Although ultraviolet light can damage many tissue components including membrane phospholipids, proteins, and nucleic acids, it is now recognized that many of its cellular effects are due to alterations in growth factor- and cytokine-mediated signal transduction pathways leading to aberrant gene expression. It is generally thought that reactive oxygen intermediates are mediators of some of the damage induced by ultraviolet light. Generated when ultraviolet light is absorbed by endogenous photosensitizers in the presence of molecular oxygen, reactive oxygen intermediates and their metabolites induce damage by reacting with cellular electrophiles, some of which can directly initiate cell signaling processes. In an additional layer of complexity, ultraviolet light-damaged nucleic acids initiate signaling during the activation of repair processes. Thus, mechanisms by which solar ultraviolet radiation triggers cell signal transduction are multifactorial. The present review summarizes some of the mechanisms by which ultraviolet light alters signaling pathways as well as the genes important in the beneficial and toxic effects of ultraviolet light.