UVB light suppresses nitric oxide production by murine keratinocytes and macrophages

Ionizing Radiation Selectively Increases CXC Ligand 10 Level via the DNA-Damage-Induced p38 MAPK-STAT1 Pathway in Murine J774A.1 Macrophages

Ionizing radiation (IR) is an important means of tumor treatment in addition to surgery and drugs. Attempts have been made to improve the efficiency of radiotherapy by identifying the various biological effects of IR on cells. Components of the tumor microenvironment, such as macrophages, fibroblasts, and vascular endothelial cells, influence cancer treatment outcomes through communication with tumor cells. In this study, we found that IR selectively increased the production of CXC motif chemokine ligand 10 (CXCL10), which is emerging as an important biomarker for determining the prognosis of anticancer treatments, without changing the levels of CXCL9 and CXCL11 in murine J774A.1 macrophages. Pretreatment with KU55933, an ataxia telangiectasia mutated (ATM) kinase inhibitor, significantly inhibited IR-induced CXCL10 production. In contrast, pretreatment with N-acetyl-cysteine or glutathione, a reactive oxygen species scavenger, did not inhibit IR-induced CXCL10 production. Further, we attempted to identify the intracellular molecular target associated with the IR-induced increase in CXCL10 secretion by J774A.1 macrophages. IR phosphorylated p38 mitogen-activated protein kinase (MAPK) and signal transducer and activator of transcription 1 (STAT1) in J774A.1 macrophages, and p38 MAPK and STAT1 were involved in CXCL10 via IR using pharmacological inhibitors (SB203580 and fludarabine, respectively) and the siRNA technique.

Ultraviolet B irradiation decreases CXCL10 expression in keratinocytes through endoplasmic reticulum stress

Ultraviolet radiation is one of the standard treatment selections for psoriasis. interferon (IFN)-γ and IFN-γ-induced CXCL10, which are highly expressed by keratinocytes in psoriasis lesion, are therapeutic targets for psoriasis. In this study, we found that ultraviolet B (UVB) irradiation inhibited IFN-γ signaling events, including STAT1 phosphorylation and induction of CXCL10 messenger RNA (mRNA) expression in keratinocytes. IFN-γ-induced expression of CXCL10 mRNA in HaCaT cells, a human keratinocyte cell line, and human epithelial keratinocytes were also inhibited by H2 O2 or endoplasmic reticulum (ER) stress inducers. Conversely, a mixture of antioxidants, Trolox and ascorbic acid, and the ER stress inhibitor salubrinal partially counteracted the inhibitory effect of UVB on IFN-γ-induced CXCL10 mRNA expression in HaCaT cells. We also found that UVB and ER stress reduced IFN-γ receptor 1 protein levels in the plasma membrane fraction of keratinocytes. These observations suggested that ER stress and the generation of reactive oxygen species are essential for the inhibitory effect of UVB on IFN-γ-induced CXCL10 mRNA in keratinocytes.

The regulation of protein kinase casein kinase II by apigenin is involved in the inhibition of ultraviolet B-induced macrophage migration inhibitory factor-mediated hyperpigmentation

Ultraviolet (UV) radiation elicits melanogenesis and pigmentation in the skin. Apigenin (4',5,7-trihydroxyflavone [AGN]) is a plant flavone contained in various herbs, fruits, and vegetables. We herein investigated antimelanogenic properties of AGN and the molecular mechanisms of the action of AGN. In UVB-treated mice, AGN inhibited cutaneous hyperpigmentation and macrophage migration inhibitory factor (MIF) expression as a melanogenesis-related key factor. In mouse keratinocytes, AGN inhibited the expression of MIF and also the related factors (e.g., stem cell factor and proteinase-activated receptor 2) induced by MIF. In addition to ellagic acid as a casein kinase II (CK2) inhibitor, AGN suppressed CK2 enzymatic activity and UVB-induced CK2 expression and subsequent phosphorylation of IκB and MIF expression. These results suggest that AGN inhibits UVB-induced hyperpigmentation through the regulation of CK2-mediated MIF expression in keratinocytes.

S-Allyl Cysteine Alleviates Hydrogen Peroxide Induced Oxidative Injury and Apoptosis through Upregulation of Akt/Nrf-2/HO-1 Signaling Pathway in HepG2 Cells

Hydrogen peroxide (H₂O₂) mediated oxidative stress leading to hepatocyte apoptosis plays a pivotal role in the pathophysiology of several chronic liver diseases. This study demonstrates that S-allyl cysteine (SAC) renders cytoprotective effects on H₂O₂ induced oxidative damage and apoptosis in HepG2 cells. Cell viability assay showed that SAC protected HepG2 cells from H₂O₂ induced cytotoxicity. Further, SAC treatment dose dependently inhibited H₂O₂ induced apoptosis via decreasing the Bax/Bcl-2 ratio, restoring mitochondrial membrane potential (∆Ψ_m), inhibiting mitochondrial cytochrome c release, and inhibiting proteolytic cleavage of caspase-3. SAC protected cells from H₂O₂ induced oxidative damage by inhibiting reactive oxygen species accumulation and lipid peroxidation. The mechanism underlying the antiapoptotic and antioxidative role of SAC is the induction of the heme oxygenase-1 (HO-1) gene in an NF-E2-related factor-2 (Nrf-2) and Akt dependent manner. Specifically SAC was found to induce the phosphorylation of Akt and enhance the nuclear localization of Nrf-2 in cells. Our results were further confirmed by specific HO-1 gene knockdown studies which clearly demonstrated that HO-1 induction indeed played a key role in SAC mediated inhibition of apoptosis and ROS production in HepG2 cells, thus suggesting a hepatoprotective role of SAC in combating oxidative stress mediated liver diseases.

Distinct effects of ultraviolet B light on antioxidant expression in undifferentiated and differentiated mouse keratinocytes

Ultraviolet (UV) B causes oxidative stress, which has been implicated in carcinogenesis. We determined if the sensitivity of keratinocytes to UVB-induced oxidative stress is dependent on their differentiation state. In primary cultures of undifferentiated and differentiated mouse keratinocytes, UVB (25 mJ/cm(2)) stimulated production of reactive oxygen intermediates. This was associated with increased messenger RNA (mRNA) expression of the antioxidant enzymes glutathione peroxidase, heme oxygenase-1 (HO-1) and the glutathione S-transferase (GST), GSTA1-2. The effects of UVB on GSTA1-2 were greater in undifferentiated when compared with differentiated cells. UVB also induced GSTM1, but only in undifferentiated cells. In contrast, UVB reduced expression of manganese superoxide dismutase, metallothionein-2, GSTA3 and microsomal glutathione S-transferase (mGST)3 in both cell types, whereas it had no major effects on catalase, copper-zinc superoxide dismutase, GSTP1, mGST1 or mGST2. Of note, levels of GSTA4 mRNA were 4- to 5-fold greater in differentiated relative to undifferentiated cells. Moreover, whereas GSTA4 was induced by UVB in undifferentiated cells, it was inhibited in differentiated cells. UVB activated p38 and c-jun N-terminal kinase mitogen-activated protein (MAP) kinases in both undifferentiated and differentiated keratinocytes. Whereas inhibition of these kinases blocked UVB-induced HO-1 in both cell types, GSTA1-2 and GST-4 were only suppressed in undifferentiated cells. In differentiated keratinocytes, p38 inhibition also suppressed GSTA1-2. In contrast, MAP kinase inhibition had no major effects on UVB-induced suppression of GSTA4 in differentiated cells. These data indicate that UVB-induced alterations in antioxidant expression are differentiation dependent. Moreover, MAP kinases are critical regulators of this response. Alterations in antioxidants are likely to be important mechanisms for protecting the skin from UVB-induced oxidative stress.

Inhibition of interferon-gamma signaling by a mercurio-substituted dihydropsoralen in murine keratinocytes

Psoralens and ultraviolet light A (PUVA) are used in the treatment of a variety of epidermal proliferative and inflammatory disorders. These compounds are known to intercalate and photo crosslink DNA. Specific receptor proteins for psoralens have also been identified. We describe a novel activity of a thiol reactive derivative, iodomercurio-4',5'-dihydrotrimethylpsoralen (iodomercurio-H2TMP) in keratinocytes. Without UVA, this psoralen was found to be an effective inhibitor of interferon-gamma (IFN-gamma)-signaling as measured by induction of nitric oxide biosynthesis (IC50 = 0.8 microM). This activity was increased (IC50 = 0.1 microM) when the cells were depleted of intracellular glutathione (GSH) with buthionine sulfoximine. In keratinocytes, IFN-gamma stimulates expression of inducible nitric oxide synthase (NOS2). Although iodomercurio-H2TMP did not alter NOS2 enzymatic activity, it blocked IFN-gamma-induced expression of NOS2 mRNA and protein, an effect that was enhanced in GSH-depleted cells. Iodomercurio-H2TMP was found to readily inhibit IFN-gamma signaling in transient transfection assays using NOS2 promoter/luciferase reporter constructs. NOS2 gene expression is known to require a variety of transcription factors including STAT-1, NF-kappaB and AP-1. Using mobility shift assays the psoralen, at concentrations that inhibit nitric oxide biosynthesis, had no effect on the DNA binding activity of STAT-1 or NF-kappaB. However, iodomercurio-H2TMP was found to suppress AP-1. These data indicate that iodomercurio-H2TMP acts at sulfhydryl-sensitive sites to inhibit NOS2. Moreover, this is dependent on early events in the IFN-gamma signal transduction pathway. Inhibition of AP-1 suggests that the psoralen functions by interfering with an important transcription factor that regulates expression of NOS2 in keratinocytes.

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.

Identification of Nitric Oxide Metabolites in Various Honeys: Effects of Intravenous Honey on Plasma and Urinary Nitric Oxide Metabolites Concentrations

Honey has antibacterial activity, promotes healing, and enhances immunity. Its acidity, osmotic effects of its high content of sugar, and hydrogen peroxide are assumed to be responsible for its effects. In this study, various honeys were investigated for the presence of nitrite/nitrate, the stable nitric oxide (NO) metabolites, and the effects of intravenous infusion of honey on urinary and plasma NO end products were studied in healthy sheep. Seven kinds of honey, different in their origin (three from Yemen, two from the United Arab Emirates, one from Germany, and one from India), color, and duration of storage, were investigated for the presence of NO metabolites. The assessment of NO metabolites was performed before and after exposure of the honey samples to heating (80 degrees C for 1 hour) or ultraviolet light (for 24 hours). Seven healthy male sheep were used for the study. Fresh unprocessed yellow honey (2 g/kg of body weight) was infused over a period of 45 minutes to each fasting sheep. Plasma and urinary NO metabolites were measured before and after the infusion. All the honey samples examined had various concentrations of NO metabolites; the highest concentration was in the fresh dark honey collected from Yemen, and the lowest in 1-year-stored dark honey collected from India. Darker or fresh honeys contained more NO metabolites than light or stored honey. After heating, NO metabolites decreased in all the kinds of honey. After ultraviolet exposure, NO metabolites were decreased in four kinds of honey, increased in one kind, and unchanged in two kinds. The darker stored honey had more resistance to heating and ultraviolet exposure. Intravenous infusion of honey elevated urinary NO metabolites from 8.4 +/- 7.4 micromol/L to 14.9 +/- 10 micromol/L during the first 60-90 min after infusion and to 35.2 +/- 34 micromol/L during the next 150-180 min. Plasma NO metabolites were increased during 1, 2, and 3 hours after infusion by 3%, 3.6%, and 17%, respectively. No side effects were reported with the use of intravenous honey. It might be concluded that honey contains various concentrations of NO metabolites. Its intravenous infusion increased plasma and urinary NO metabolites. It is assumed that NO might be responsible, in part, for the biological and therapeutic effects of honey.

UVB light stimulates production of reactive oxygen species: unexpected role for catalase

In keratinocytes, UVB light stimulates the production of reactive oxygen species (ROS). Lysates of these cells were found to possess a non-dialyzable, trypsin- and heat-sensitive material capable of generating ROS in response to UVB light. Using ion exchange, metal affinity, and size exclusion chromatography, a 240-kDa protein was isolated with ROS generating activity. The protein exhibited strong absorption in the 320-360 nm range with additional soret peaks around 400-410 nm, suggesting the presence of heme. Sequencing using liquid chromatography-ion trap mass spectrometry identified the protein as catalase. Using purified catalases from a variety of species, the ROS generating activity was found to be temperature- and O2-dependent, stimulated by inhibitors of the catalatic activity of catalase, including 3-aminotriazole and azide, and inhibited by cyanide. A marked increase in the production of ROS was observed in UVB-treated cells overexpressing catalase and decreased generation of oxidants was found in UVB-treated keratinocytes with reduced levels of catalase. Our data indicate that catalase plays a direct role in generating oxidants in response to UVB light. The finding that catalase mediates the production of ROS following UVB treatment is both novel and highly divergent from the well known antioxidant functions of the enzyme. We hypothesize that, through the actions of catalase, high energy DNA damaging UVB light is absorbed by the enzyme and converted to reactive chemical intermediates that can be detoxified by cellular antioxidant enzymes. Accumulation of excessive ROS, generated through the action of catalase, may lead to oxidative stress, DNA damage, and the development of skin cancer.