Aberrant timing in epidermal expression of inducible nitric oxide synthase after UV irradiation in cutaneous lupus erythematosus

Effects of melanogenesis-inducing nitric oxide and histamine on the production of eumelanin and pheomelanin in cultured human melanocytes

Melanin pigments produced in human melanocytes are classified into two categories; black coloured eumelanin and reddish-yellow pheomelanin. Stimulation of melanocytes with alpha-melanocyte-stimulating hormone (alpha-MSH), one of several melanogenic factors, has been reported to enhance eumelanogenesis to a greater degree than pheomelanogenesis, which contributes to hyperpigmentation in skin. Nitric oxide (NO) and histamine are also melanogenesis-stimulating factors that are released from cells surrounding melanocytes following ultraviolet (UV) irradiation. In this study, the effects of NO and histamine on the ratio of eumelanin and pheomelanin were examined in human melanocytes, and then compared with that of alpha-MSH. The amounts of eumelanin and pheomelanin were quantified using high-performance liquid chromatography analysis after oxidation and hydrolysis of melanin. Melanogenesis was induced by the addition of alpha-MSH, NO, or histamine to melanocytes. The amount of eumelanin production significantly increased with independent stimulation by these melanogenic factors, especially histamine, while that of pheomelanin significantly increased with alpha-MSH and NO, but only slightly with histamine. As a result, the ratio of eumelanin and pheomelanin increased significantly with the addition of NO or histamine. These results suggest that NO and histamine, as in the case of alpha-MSH, may contribute to UV-induced hyperpigmentation by enhancing eumelanogenesis.

Nitric oxide in the human hair follicle: constitutive and dihydrotestosterone-induced nitric oxide synthase expression and NO production in dermal papilla cells

The free radical nitric oxide, generated by different types of epidermal and dermal cells, has been identified as an important mediator in various physiological and pathophysiological processes of the skin, such as regulation of blood flow, melanogenesis, wound healing, and hyperproliferative skin diseases. However, little is known about the role of NO in the human hair follicle and in hair cycling processes. Here we demonstrate for the first time that dermal papilla cells derived from human hair follicles spontaneously produce NO by measuring nitrate and nitrite levels in culture supernatants. This biomolecule is apparently formed by the endothelial isoform of nitric oxide synthase, which was detected at the mRNA and protein levels. Remarkably, basal NO level was enhanced threefold by stimulating dermal papilla cells with 5alpha-dihydrotestosterone (DHT) but not with testosterone. Addition of N-[3-(aminomethyl)benzyl]acetamidine (1400W), a highly selective inhibitor of inducible nitric oxide synthase, restrained the elevation in NO level induced by DHT. Analyses of DHT-stimulated cells at the mRNA and protein levels confirmed the expression of inducible nitric oxide synthase. These findings suggest NO as a signaling molecule in human dermal papilla cells and implicate basal and androgen-mediated NO production to be involved in the regulation of hair follicle activity.

Expression of endomucin, a novel endothelial sialomucin, in normal and diseased human skin

Endomucin is an endothelial sialomucin that was recently identified with the help of monoclonal antibodies raised against mouse endothelial cells. Cloning of human endomucin allowed us to generate monoclonal antibodies against soluble recombinant forms of human endomucin. In this study, we investigated the expression of this novel molecule in human skin under different conditions, using the monoclonal antibodies. In normal human skin, endomucin was detected for the monoclonal antibody L6H10 by immunoblotting, and immunohistologic analysis of wax-embedded sections revealed that this glycoprotein is expressed on capillaries, venules, and lymphatic vessels. Interestingly, staining of arterial endothelium was either weak or focal using the monoclonal antibodies against endomucin. In situ hybridization of normal human skin confirmed the expression pattern on the messenger RNA level obtained above. We further analyzed the expression of endomucin in skin biopsy specimens from patients with inflammatory skin diseases, such as atopic dermatitis, psoriasis, lichen planus, cutaneous lupus erythematosus, and T cell lymphoma as well as with vascular skin tumors, such as hemangioma, pyogenic granuloma, angiolipoma, Kaposi's sarcoma, and angiosarcoma. We found endomucin expressed on the endothelium of each tissue, concluding that this novel molecule is a new endothelial-specific marker in the study of normal and diseased human skin.

The effect of amniotic membrane extract on the expression of iNOS mRNA and generation of NO in HaCaT cell by ultraviolet B irradiation

BACKGROUND/PURPOSE

Amniotic membrane (AM) is the innermost fetal membrane, which contains several proteinase inhibitors and expresses several growth factors. Nitric oxide (NO) has been implicated in the pathogenesis of various inflammatory diseases including sunburn and ultraviolet induced erythema. The expression of inducible nitric oxide synthase (iNOS) is up regulated by UVB irradiation and inhibited by TGF-beta and EGF-beta. We evaluated the effect of AM extract on the expression of iNOS mRNA by UV irradiation in HaCaT cell (immortalized human keratinocyte cell line).

METHODS

HaCaT cells were irradiated UVB 30 mJ/cm2 and AM extract was added. The iNOS mRNA was isolated by RT-PCR and NO production was assessed by spectrophotometric method based on Griess reaction.

RESULTS

The expression of iNOS mRNA was induced by UVB irradiation in HaCaT cell and the expression of iNOS mRNA was higher at 48 h than that at 24 h. AM extract down regulated the induction of iNOS mRNA in HaCaT cell by UVB irradiation. NO generation was increased by UVB irradiation, but down regulated by AM extract treatment in HaCaT cells.

CONCLUSION

These results assured that the expression of iNOS mRNA and generation of NO are up regulated by UVB irradiation and showed that AM extract down regulated the induction of iNOS mRNA and decreased generation of NO by UVB irradiation.

Photoprovocation test and immunohistochemical analysis of inducible nitric oxide synthase expression in patients with Sjögren's syndrome associated with photosensitivity

BACKGROUND

Annular erythema (AE) in Sjögren's syndrome (SS) usually develops on areas of sun-exposed skin and is exacerbated during summer.

OBJECTIVES

To evaluate photosensitivity in SS and to investigate the involvement of ultraviolet (UV) radiation in the development of AE in SS.

METHODS

Phototesting with UVA and UVB was performed on 14 SS patients, including 10 with primary SS. Clinical and histological features as well as expression of inducible nitric oxide synthase (iNOS) in the evoked skin lesions were compared with those of lupus erythematosus (LE). Eleven SS patients had a history of photosensitive AE (n = 4), papules (n = 3) or other types (n = 4) of lesions on their sun-exposed skin that were induced or aggravated by sunlight exposure.

RESULTS

Phototesting induced a prolonged erythematous response (n = 8), infiltrated erythema (IE) (n = 4) and/or papules (n = 3) in 11 of 14 SS patients, including one with primary SS without a history of photosensitivity. Histologically, the induced IE and papules showed coat-sleeve-like or sparse perivascular infiltration of lymphocytes similar to that in primary skin lesions of AE in SS. No epidermal changes characteristic for LE were found except for partial and mild liquefaction degeneration in three cases. In contrast, two cases were indistinguishable from the papular type of polymorphic light eruption in several aspects, including their primary skin lesions and early response to a photoprovocation test. Immunohistochemistry revealed diffuse expression of iNOS throughout the epidermis, which is characteristic for LE, in the three SS patients with minimal liquefaction degeneration, while the remaining seven SS patients examined exhibited no iNOS staining or a normal expression pattern.

CONCLUSIONS

Our results indicate that photosensitivity exists in certain primary SS patients, and that UV is critical to the development of AE in SS, probably through a pathological mechanism distinct from that in LE.

UVB light suppresses nitric oxide production by murine keratinocytes and macrophages

Nitric oxide is an important mediator of excessive cell growth and inflammation associated with many epidermal proliferative disorders. It is a highly reactive oxidant generated in keratinocytes and macrophages via the inducible form of the enzyme nitric oxide synthase (NOS2). In the present studies, we examined the effects of ultraviolet light (UVB, 2.5-25mJ/cm(2)) on interferon-gamma (IFN-gamma)-induced expression of NOS2 in these cells. Transient transfection assays using wild-type and mutant NOS2 promoter/luciferase reporter constructs showed that DNA binding of the transcription factors Stat1 and NF-kappaB was essential for optimal expression of the NOS2 gene. Whereas NF-kappaB was constitutively expressed in both cell types, Stat1 phosphorylation and nuclear binding activity were dependent upon IFN-gamma. UVB light, which is used therapeutically to treat inflammatory dermatosis, was found to suppress IFN-gamma-induced expression of NOS2 mRNA and protein, and nitric oxide production in both keratinocytes and macrophages. In macrophages, this was associated with complete inhibition of NF-kappaB nuclear binding activity and partial (approximately 20-25%) reduction of Stat1 activity. In keratinocytes, both responses were partially reduced at the highest doses of UVB light (15-25mJ/cm(2)). Whereas in macrophages UVB light suppressed NOS2 wild-type promoter-luciferase reporter activity, this activity was stimulated in keratinocytes. These data suggest that UVB light functions to suppress NOS2 gene expression in macrophages by inhibiting the activity of key regulatory transcription factors. In contrast, in keratinocytes, inhibition occurs downstream of NOS2 promoter activity.

Time course of expression of mRNA of inducible nitric oxide synthase and generation of nitric oxide by ultraviolet B in keratinocyte cell lines

BACKGROUND

Nitric oxide (NO), the ubiquitous free radical, has been implicated in the pathogenesis of various inflammatory diseases, including sunburn and ultraviolet (UV) radiation-induced pigmentation, and it also seems to play an important part in host defence against bacterial infection.

OBJECTIVES

The purpose of this study was to determine the time course of production of NO and time course of expression of inducible NO synthase (iNOS) by UVB irradiation and lipopolysaccharide (LPS) stimulation in keratinocyte cell lines. Furthermore, we intended to elucidate the relationship between iNOS and NO in various stimulated conditions.

METHODS

Normal human keratinocytes (NHK), HaCaT cells and PAM212 cells were irradiated with UVB at a dose of 50 mJ cm-2 and 100 mJ cm-2. Separately, the cell lines were stimulated with 20 micro g of LPS. NO was measured by the Griess assay and iNOS mRNA was isolated by reverse transcriptase-polymerase chain reaction at 12, 24, 48 and 72 h after stimulation.

RESULTS

The generation of NO was induced by UVB irradiation and LPS stimulation. NO production was significantly increased at 72 h after irradiation of UVB 100 mJ cm-2 in NHK, and at 48 and 72 h in HaCaT cells. In PAM212 cells, NO production was significantly increased at 12, 24, 48 and 72 h by UVB 100 mJ cm-2 and at 72 h by LPS. Induction of iNOS mRNA peaked at 48 h and then decreased to basal level at 72 h when treated with UVB irradiation. The time course of production of NO was approximately correlated with the timing of induction of iNOS mRNA.

CONCLUSIONS

These results suggest that the expression of iNOS mRNA is upregulated by UVB irradiation, and that NO produced by this inducible enzyme may play a part as a mediator or an immunomodulator in UV-induced skin reactions such as sunburn reaction and photo-induced immune alterations.

Pro- and anti-apoptotic effects of nitric oxide in irradiated keratinocytes: the role of superoxide

Ultraviolet radiation (UV) induces apoptosis in keratinocytes by both p53- and death receptor-dependent pathways. It also generates free radicals in keratinocytes, including the synthesis of nitric oxide (NO) by constitutive and inducible NO synthases (NOS). NO has both pro- and anti-apoptotic effects. We wished to determine which of these was predominant in keratinocytes. Human CCD1106 keratinocytes were irradiated with UVB in the presence and absence of several NOS antagonists. Apoptosis was measured by flow cytometry with annexin V binding. NOS antagonism consistently altered UVB-induced apoptosis measured 18 h after irradiation. In 9 of 13 experiments, NOS antagonism increased apoptosis. However, in 4 of 13 experiments, NOS antagonism reduced apoptosis. We postulated that the variable effects of NO might be due to a critical balance between UVB-induced NO and superoxide production. We predicted that NO would be anti-apoptotic in the presence of low O(-)(2), but pro-apoptotic when NO combined with O(-)(2) to form peroxynitrite. Though superoxide dismutase reduced apoptosis after UVB, addition of peroxynitrite did not affect apoptosis. We conclude that NO released by UV irradiation is anti-apoptotic; however, the levels of O(-)(2) may be a determinant of NO action.

Nitric oxide and wound repair: role of cytokines?

Wound healing involves platelets, inflammatory cells, fibroblasts, and epithelial cells. All of these cell types are capable of producing nitric oxide (NO), either constitutively or in response to inflammatory cytokines, through the activity of nitric oxide synthases (NOSs): eNOS (NOS3; endothelial NOS) and iNOS (NOS2; inducible NOS), respectively. Indeed, pharmacological inhibition or gene deletion of these enzymes impairs wound healing. The wound healing mechanisms that are triggered by NO appear to be diverse, involving inflammation, angiogenesis, and cell proliferation. All of these processes are controlled by defined cytokine cascades; in many cases, NO appears to modulate these cytokines. In this review, we summarize the history and present state of research on the role of NO in wound healing within the framework of modulation of cytokines.

Nitric oxide-peroxynitrite-poly(ADP-ribose) polymerase pathway in the skin

In the last decade it has become well established that in the skin, nitric oxide (NO), a diffusable gas, mediates various physiologic functions ranging from the regulation of cutaneous blood flow to melanogenesis. If produced in excess, NO combines with superoxide anion to form peroxynitrite (ONOO-), a cytotoxic oxidant that has been made responsible for tissue injury during shock, inflammation and ischemia-reperfusion. The opposite effects of NO and ONOO- on various cellular processes may explain the 'double-edged sword' nature of NO depending on whether or not cellular conditions favour peroxynitrite formation. Peroxynitrite has been shown to activate the nuclear nick sensor enzyme, poly(ADP-ribose) polymerase (PARP). Overactivation of PARP depletes the cellular stores of NAD+, the substrate of PARP, and the ensuing 'cellular energetic catastrophy' results in necrotic cell death. Whereas the role of NO in numerous skin diseases including wound healing, burn injury, psoriasis, irritant and allergic contact dermatitis, ultraviolet (UV) light-induced sunburn erythema and the control of skin infections has been extensively documented, the intracutaneous role of peroxynitrite and PARP has not been fully explored. We have recently demonstrated peroxynitrite production, DNA breakage and PARP activation in a murine model of contact hypersensitivity, and propose that the peroxynitrite-PARP route represents a common pathway in the pathomechanism of inflammatory skin diseases. Here we briefly review the role of NO in skin pathology and focus on the possible roles played by peroxynitrite and PARP in various skin diseases.

Evidence for a role of nitric oxide in the mediation of antiproliferative UVA effects in keratinocytes

Using cultured human keratinocytes, the present study investigates the role of nitric oxide (NO) in the mediation of the antiproliferative effects of ultraviolet light A (UVA). UVA treatment of cells (3-21 J cm (-2)) caused a time- and dose-dependent increase in nitrite formation in a micromolar range. This effect was accompanied by a decrease in DNA synthesis by 53.5%. Moreover, UVA treatment slightly reduced cell viability by 23.8%. Preincubation of keratinocytes with the NO scavenger 4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (PTIO, 10-100 microM) or the NO synthase inhibitor N(G)-monomethyl-l-arginine (l-NMMA, 30-300 microM) significantly diminished the UVA-induced increase in nitrite. PTIO as well as l-NMMA partially protected keratinocytes from UVA-induced antiproliferative effects and increased DNA synthesis by 67 or 49% of the control. The co-application of UVA irradiation (10 J cm (-2)) and the essential cofactor of NO synthases tetrahydrobiopetrin (BH4, 500 microM) led to an overadditive increase in the release of nitrite as well as to a decrease in DNA synthesis. These results imply that NO is involved in the antiproliferative UVA effects in keratinocytes.

Regulation of eNOS in normal and diabetes-impaired skin repair: implications for tissue regeneration

An important role of inducible nitric oxide (NO) synthase for epithelial action during skin repair has been well established. Although a delayed healing of skin wounds has been recently described for eNOS-deficient mice, a participation of endothelial-type NO synthase (eNOS) in skin repair largely remains unclear. In this study we determined the expression pattern of eNOS during wound healing in healthy and in diabetic mice. Remarkably, normal repair in healthy animals was characterized by a moderate induction of eNOS at the mRNA and protein level, whereas diabetes-impaired healing was associated with a clearly reduced eNOS protein expression. Immunohistochemistry revealed the endothelial lining of blood vessels within the granulation tissue, and also keratinocytes of the wound margins, the developing neo-epithelium, and the hair follicles to express eNOS protein. Keratinocyte-derived expression of eNOS could be confirmed at the mRNA level in vitro for human primary keratinocytes and the keratinocyte cell line HaCaT. Furthermore, eNOS enzymatic activity most likely contributes to epithelial regeneration, as eNOS-deficient (eNOS -/-) animals exhibited reduced wound margin epithelia associated with reduced keratinocyte proliferation.

Nitric oxide drives skin repair: novel functions of an established mediator

Wound healing of the skin represents a highly ordered process of important tissue movements that aims for a rapid closure of the wound site and a subsequent regeneration of the injured tissue. The factors ensuring the intercellular communication during repair are only known in part. However, although protein-type mediators are well-established players in this process, it has become evident that the diffusible, gaseous molecule nitric oxide (NO) participates in the orchestration of wound healing. The role of wound-derived NO that critically influences macrophage, fibroblast, and keratinocyte behaviour within the intercellular communication network during repair is subject of this review. Thus, cutaneous wound healing prototypically reflects processes that generally occur also in kidney injury and regeneration.

Cell death and proliferation in Nd-YAG laser, electrocautery, and scalpel wounds on mice skin

The purpose of this study is to compare cell death and proliferation in laser, electrocautery and scalpel wounds on the mice epidermis. Wounds were examined by transmission electron microscopy, the detection of free 3'-OH DNA ends and immunohistochemistry of proliferating cell nuclear antigen (PCNA), inducible nitric oxide synthase (iNOS), keratinocyte growth factor (KGF) and keratinocyte growth factor receptor (KGFR). Reepithelization was first observed 5 days after scalpel and laser incisions and 7 days after electrocautery incision. Ultrastructurally, keratinocytes in both electrocautery and laser wounds showed similar post-apoptotic necrotic changes. Interestingly, dividing cells were often observed 3 days after laser incision. Apoptotic index in electrocautery wounds was higher than in laser wounds, although there was no significant difference in the PCNA expression level between them. The expression of iNOS, KGF and KGFR in laser wounds was more intense than in electrocautery wounds. In scalpel wounds, keratinocytes did not show significant changes in morphology or of markers of cell death and proliferation during the observation period. Therefore, the increase in the number of dividing cells and in the expression level of iNOS, KGF and KGFR may induce earlier and thicker reepithelization in laser wounds than in electrocautery and scalpel wounds.

Immunohistochemical study of inducible nitric oxide synthase in skin cancers

BACKGROUND

Nitric oxide (NO) is synthesized from the amino acid L-arginine by NO synthase (NOS). Experimental evidence suggests that increased express of inducible NOS (iNOS), which is an NOS isoform and calcium independent, is related to various pathological processes, such as inflammation and cancer.

METHODS

In this study, we used immunohistochemistry to investigate iNOS expression in a series of basal cell carcinomas (BCC), Bowen's disease, squamous cell carcinomas (SCC), extramammary Paget's disease (EPD) and metastatic tumors of the skin.

RESULTS

Only 1 of 16 BCC cases was positive for iNOS and the intensity of staining was weak. In most of the 10 cases of Bowen's disease, iNOS was weakly expressed and there was a wide range in the percentage of positive tumor cells. Twelve of the 16 cases of SCC were positive for iNOS and the extent of positivity was greater than in Bowen's disease. Two of the 7 cases of EPD were positive for iNOS, and 12 of the 15 cases of metastatic cancer were positive. Well-differentiated adenocarcinomas were diffusely positive, whereas poorly-differentiated ones showed strong and heterogeneous staining.

CONCLUSIONS

These results indicated that the expression of iNOS may reflect the proliferation of tumor cells and that a heterogeneous distribution of iNOS may correlate with a wide variety of biological behavior of tumor cells.

Inducible nitric oxide synthase-derived nitric oxide in gene regulation, cell death and cell survival

Studies from many laboratories have demonstrated the complex role of NO in inflammatory processes. Prolonged exposure to NO shifts the cellular redox potential to a more oxidized state and this is critically regulated by intracellular levels of reduced glutathione. NO-mediated stress will alter gene expression patterns, and the number of genes known to be involved is steadily increasing. Indeed, due to its S-nitrosating activity in the presence of oxygen, NO can modify the activity of transcription factors containing zinc finger motifs or cysteines within the DNA-binding domain. In addition, we are faced with not only NO acting as a powerful inducer of apoptosis or of necrosis in some cells, but also representing an equally powerful protection from cell death in many instances. Some of these apparent discrepancies may be explained by different capacities of cells to cope with the stress of NO exposure. Here, we review our findings on the complex impact of NO on transcriptional regulation of genes, cell death and cell survival. These NO-mediated actions will contribute to a better understanding of the impact of inducible nitric oxide synthase (iNOS) enzyme activity during inflammatory reactions.

Even after UVA-exposure will nitric oxide protect cells from reactive oxygen intermediate-mediated apoptosis and necrosis

Reactive oxygen species (ROS) play a pivotal role in UVA-induced cell damage. As expression of the inducible nitric oxide synthase (iNOS) is a normal response of human skin to UV radiation we examined the role of nitric oxide (NO) as a protective agent during or even after UVA1- or ROS-exposure against apoptosis or necrosis of rat endothelial cells. When added during or up to 2 h subsequent to UVA1 or ROS exposure the NO-donor S-nitroso-cysteine (SNOC) at concentrations from 100-1000 microM significantly protects from both apoptosis as well as necrosis. The NO-mediated protection strongly correlates with complete inhibition of lipid peroxidation (sixfold increase of malonedialdehyde formation in untreated versus 1.2-fold with 1 mM SNOC). NO-mediated protection of membrane function was also shown by the inhibition of cytochrome c leakage in UVA1 treated cells, a process not accompanied by alterations in Bax and Bcl-2 protein levels. Thus, the experiments presented demonstrate that NO exposure during or even after a ROS-mediated toxic insult fully protects from apoptosis or necrosis by maintaining membrane integrity and function.

Expression of nitric oxide synthase isoforms and arginase in normal human skin and chronic venous leg ulcers

Chronic venous ulcers, an example of abnormal wound healing, show chronic inflammation with defective matrix deposition which together with the underlying vascular pathology, result in delayed healing. L-arginine is known to be metabolized by one of two pathways: nitric oxide synthase (NOS), producing nitric oxide (NO), or arginase, producing ornithine. NO is involved in many pathological conditions including vascular and inflammatory disorders. This study therefore investigated the distribution, level and activity of NOS and arginase in chronic venous ulcers in comparison with normal skin, using immunocytochemistry, western blotting, and enzyme assays. The results demonstrated an increased distribution of both NOS and arginase in chronic venous ulcer tissue compared with normal skin, with inflammatory cells and vascular endothelial cells as the main sources. These data were confirmed by western blot analysis, which showed increased levels of both enzymes in chronic venous ulcers. Moreover, there was significantly increased activity of both total NOS (p<0.04) and inducible NOS (p<0.05) in chronic venous ulcer tissue compared with normal skin, and significantly increased activity of arginase (p<0.01) in chronic venous ulcer tissue in comparison with normal skin. NO is known to combine with hydroxyl free radicals forming peroxynitrite, a potent free radical which causes tissue destruction. NO overexpression in chronic venous ulcers may be involved directly or indirectly (through production of peroxynitrite) in the pathogenesis and delayed healing of chronic venous ulcers, through its effects on vasculature, inflammation, and collagen deposition. Arginase is known to enhance matrix deposition. Thus, increased levels of arginase in chronic venous ulcers could contribute to the pathogenesis of lipodermatosclerosis associated with chronic venous insufficiency, predisposing to the formation of chronic venous ulcers and also to matrix cuff formation around blood vessels.

Implications of inducible nitric oxide synthase expression and enzyme activity

We summarize here our current knowledge about inducible nitric oxide synthase (NOS) activity in human diseases and disorders. As basic research discovers more and more effects of low or high concentrations of NO toward molecular and cellular targets, successful therapies involving inhibition of NO synthesis or application of NO to treat human diseases are still lacking. This is in part due to the fact that the impact of NO on cell function or death are complex and often even appear to be contradictory. NO may be cytotoxic but may also protect cells from a toxic insult; it is apoptosis-inducing but also exhibits prominent anti-apoptotic activity. NO is an antioxidant but may also compromise the cellular redox state via oxidation of thiols like glutathione. NO may activate specific signal transduction pathways but is also reported to inhibit exactly these, and NO may activate or inhibit gene transcription. The situation may even be more complicated, because NO, depending on its concentration, may react with oxygen or the superoxide anion radical to yield reactive species with a much broader chemical reaction spectrum than NO itself. Thus, the action of NO during inflammatory reactions has to be considered in the context of timing and duration of its synthesis as well as stages and specific events in inflammation.

The function of nitric oxide in wound repair: inhibition of inducible nitric oxide-synthase severely impairs wound reepithelialization

Recently, we demonstrated a large induction of inducible nitric oxide synthase (iNOS) during cutaneous wound repair. In this study, we established an in vivo model in mice to investigate the role of NO during the wound healing process. During excisional repair, mice were treated with L-N6-(1-iminoethyl)lysine (L-NIL), a selective inhibitor of iNOS enzymatic activity. Compared with control mice, L-NIL-treated animals were characterized by a severely impaired reepithelialization process, as the hyperproliferative epithelia at the wound edges appeared to be delayed and characterized by an atrophied morphology. Immunohistochemical labeling for detection of proliferating cells (BrdU-, Ki67-staining) revealed a strong reduction in proliferating keratinocyte cell numbers during the process of re-epithelialization after inhibition of iNOS activity during repair. Western blot analysis of total wound lysates from PBS- and L-NIL-treated mice clearly demonstrated a reduction in proliferating cell nuclear antigen, representing a marker for cell proliferation, in lysates isolated from L-NIL-treated mice. The dependency between keratinocyte proliferation and NO availability observed during wound repair in vivo is further supported by the observation that proliferation of the keratinocyte cell line (HaCaT) is stimulated by low concentrations of NO-donors also in vitro. In summary, our data demonstrate that the presence of a functionally active iNOS is a crucial prerequisite for normal wound reepithelialization.

Nitric oxide, skin growth and differentiation: more questions than answers?

Nitric oxide has varied effects on the skin. In this review the role of nitric oxide in cutaneous wound healing, apoptosis, carcinogenesis and psoriasis is discussed.

Nitric oxide fully protects against UVA-induced apoptosis in tight correlation with Bcl-2 up-regulation

A variety of toxic and modulating events induced by UVA exposure are described to cause cell death via apoptosis. Recently, we found that UV irradiation of human skin leads to inducible nitric-oxide synthase (iNOS) expression in keratinocytes and endothelial cells (ECs). We have now searched for the role of iNOS expression and nitric oxide (NO) synthesis in UVA-induced apoptosis as detected by DNA-specific fluorochrome labeling and in DNA fragmentation visualized by in situ nick translation in ECs. Activation with proinflammatory cytokines 24 h before UVA exposure leading to iNOS expression and endogenous NO synthesis fully protects ECs from the onset of apoptosis. This protection was completely abolished in the presence of the iNOS inhibitor L-N5-(1-iminoethyl)-ornithine (0.25 mM). Additionally, preincubation of cells with the NO donor (Z)-1-[N(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-i um-1, 2-diolate at concentrations from 10 to 1000 microM as an exogenous NO-generating source before UVA irradiation led to a dose-dependent inhibition of both DNA strand breaks and apoptosis. In search of the molecular mechanism responsible for the protective effect, we find that protection from UVA-induced apoptosis is tightly correlated with NO-mediated increases in Bcl-2 expression and a concomitant inhibition of UVA-induced overexpression of Bax protein. In conclusion, we present evidence for a protective role of iNOS-derived NO in skin biology, because NO either endogenously produced or exogenously applied fully protects against UVA-induced cell damage and death. We also show that the NO-mediated expression modulation of proteins of the Bcl-2 family, an event upstream of caspase activation, appears to be the molecular mechanism underlying this protection.