Biphasic effect of exogenous nitric oxide on proliferation and differentiation in skin derived keratinocytes but not fibroblasts

Nitric oxide (NO) is known to exert cytotoxic and cytostatic effects in various cells and tissues. Although NO formation in human skin has been convincingly demonstrated, little is known about the NO-mediated effects in skin physiology and pathology. Here, we investigate the influence of NO on proliferation, differentiation, and apoptosis of primary cultures of normal human keratinocytes and fibroblasts. Four different NO donors at concentrations ranging from 0.01 to 5 mM were added every 12 h or 24 h to primary cultures of human keratinocytes and fibroblasts, and cells cultured for up to 3 d in the presence of these compounds. Cultures were examined for necrosis or apoptosis using trypan blue exclusion and in situ nick-translation. Cultures were also screened for the expression of the proliferation marker Ki67 and for an increase in cell numbers using neutral red staining. In addition, keratinocytes were stained for cytokeratin 6 expression to assess differentiation. We find that both keratinocytes and fibroblasts are highly resistant towards necrosis- or apoptosis-inducing effects of NO. In both cell types NO modulates cell growth, albeit in a cell-type specific pattern: cytostasis becomes significant in fibroblasts at concentrations of > or = 0.25 mM of the NO donor. In keratinocytes a biphasic effect is found with increased proliferation at low concentrations ranging from 0.01 to 0.25 mM and cytostasis at concentrations of > or = 0.5 mM. Conversely, expression of cytokeratin 6 is decreased at the lower NO donor concentrations and increased at higher concentrations as an indication of induction of differentiation at higher NO concentrations. Collectively, our results demonstrate that NO modulates proliferation and differentiation in human skin cells, a finding that will help to explain the pathophysiology of human skin diseases. Moreover, these findings suggest that NO generation in human skin diseases is not directly associated with local cell destruction, in contrast to findings in several other human diseases.

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

Photosensitivity is a main criterion for the diagnosis of systemic lupus erythematosus (LE), and ultraviolet (UV) irradiation plays a key role in the pathogenesis of cutaneous LE. Patients with a tentative diagnosis of LE are routinely tested for skin lesion development after experimental UV irradiation, providing an ideal opportunity to evaluate early, preclinical events involved in the pathogenesis of LE. Several reports have shown expression of the cytokine-inducible nitric oxide synthase (iNOS) in autoimmune diseases. Therefore, we investigated the role of iNOS expression at mRNA and protein level in the pathogenesis of LE lesions. Skin biopsies from patients with different subtypes of LE were examined, and iNOS expression was found in six of 18 biopsies from cutaneous LE patients and two of three biopsies from systemic LE patients. In biopsies taken 4-20 d after UV irradiation, epidermal iNOS expression was seen in all patients (n = 10) after UVB and in four of 10 patients provoked by UVA. In healthy controls (n = 8) epidermal iNOS expression was detected 24 h after UV irradiation, persisting for another day before subsiding on day 3. In LE patients (n = 8) the exact reverse situation was seen: an iNOS-specific signal was undetectable in keratinocytes for 2 d after UV irradiation, but became positive on day 3 and persisted for up to 25 d in the evolving skin lesions. Our findings demonstrate a time-restricted, UV-induced iNOS expression in human skin; moreover, the results indicate that both the kinetics of iNOS induction as well as the time span of local iNOS expression may be critical to the development of cutaneous LE lesions.

Nitric oxide in human skin: current status and future prospects

The gaseous free radical nitric oxide is an important biologic mediator with physiologic and pathophysiologic roles in nearly every organ system. Because of its unique biologic activity, unusual chemical structure, and unprecedented mechanisms of action, nitric oxide, arguably more than any other natural product, has opened new avenues to investigate cellular processes. Nitric oxide is generated in biologic tissues by specific nitric oxide synthases that metabolize arginine and molecular oxygen to citrulline and nitric oxide. Besides its function as a diffusible messenger in the vasculature and in neurons, nitric oxide also plays a key role in innate immunity and inflammation. Recent progress has allowed the identification of the nitric oxide pathway in several cell types that reside in the skin, including keratinocytes, melanocytes, Langerhans cells, fibroblasts, and endothelial cells. Convincing evidence suggests that nitric oxide synthesis in these cells can be modulated by calcium-mobilizing agonists as well as diverse inflammatory and immune stimuli, and thereby contributes to the pathogenesis of several human skin diseases. Characterization of these intrinsic and extrinsic regulatory stimuli of nitric oxide synthesis has afforded substantial insights into the role of nitric oxide in inflammatory, hyperproliferative, and autoimmune skin diseases, as well as skin cancer, and may ultimately form the basis for future therapeutic intervention. The demonstrable and potential roles of nitric oxide in skin disease pathogenesis and treatment are the subjects of this review.

Dobesilate enhances endothelial nitric oxide synthase-activity in macro- and microvascular endothelial cells

1. Dobesilate is used for normalizing vascular dysfunction in a number of diseases. In search for an effect on endothelial NO production, macrovascular endothelial cells from rat aorta, microvascular endothelial cells from rat exocrine pancreatic tissue, and capillary endothelial cells from rat islets, were cultured in the presence or absence of Mg-Dobesilate. The activity of constitutive nitric oxide synthase (ecNOS) in resident cells as well as of inducible nitric oxide synthase (iNOS) in cytokine-activated cells was measured indirectly by recording the citrulline concentrations in culture supernatants. 2. In each of the different endothelial cells Mg-Dobesilate incubation (0.25-1 mM) for 24 h led to a significant and concentration-dependent increase in ecNOS-activities. With cytokine-activated endothelial cell cultures only moderate effects were seen with little or no concentration-dependency. Addition of the NOS-inhibitor N(G)-monomethyl-L-arginine led to a significant suppression of citrulline formation in all cultures as an evidence for the enzyme specificity of these effects. 3. iNOS- and ecNOS-specific reverse transcription and semi-quantitative polymerase chain reaction (RT-PCR) with RNA from resident or cytokine-activated endothelial cells gave no evidence for an increase in NOS-specific mRNA after Mg-Dobesilate-treatment. Furthermore, Dobesilate-mediated enhancement of NO synthesis in resting endothelial cells was not due to iNOS induction in these cells, as no iNOS-specific signal was found by RT-PCR.

The dominant role of exogenous or endogenous interleukin-1 beta on expression and activity of inducible nitric oxide synthase in rat microvascular brain endothelial cells

In the brain large amounts of nitric oxide are produced in response to various pathological stimuli such as infectious agents, ischemia and trauma. Although it is known that endothelial cells can express the inducible isoform of nitric oxide synthase (iNOS) upon activation, the impact of different cytokines on iNOS expression in rat microvascular endothelial cells remains unclear. We now investigated iNOS mRNA expression and enzyme activity in primary cell cultures of rat microvascular brain endothelial cells after treatment with the proinflammatory cytokines interleukin-1beta (IL-1beta), Tumor necrosis factor-alpha (TNF-alpha), and interferon-gamma (IFN-gamma) alone or in combination. Cells were characterized by immunocytochemistry staining for von-Willebrand-factor and the rat brain endothelial antigen recognized by monoclonal antibody Ox2. iNOS-enzyme activity was determined by measurement of nitrite in the supernatants of cell culture using the Griess-reaction. In addition mRNA expression was analysed by RT-PCR with iNOS and IL-1beta specific primers. All cells in the endothelial cell culture were found to express the antigenic phenotype vWF+/Ox2+/Ox43-, thus identifying the cells as rat brain endothelial cells of microvascular origin. IL-1beta was the only cytokine that as a single stimulus induced iNOS mRNA expression and iNOS-enzyme activity in these endothelial cells. All combinations of two cytokines, including that of TNF-alpha and IFN-gamma--or the triple combination led to expression of iNOS-mRNA and active protein. Cell activation by the combination of TNF-alpha + IFN-gamma led to an early expression of IL-1beta by the endothelial cells suggesting iNOS induction as a consequence of endogenous IL-1beta production under this challenge. The experiments prove that rat brain microvascular endothelial cells express iNOS and produce large amounts of NO under inflammatory conditions. Furthermore, our results indicate a decisive role of IL-1beta in iNOS expression and NO generation.

Nitric oxide-induced expression of C-reactive protein in islet cells as a very early marker for islet stress in the rat pancreas

In searches for marker molecules specifically expressed in nitric oxide-treated islet cells as a means to recognize early events in islet destruction, we now establish the presence of neo-C-reactive protein (neoCRP) in rat islet cells as early as 2 hr after treatment. We detected this altered molecular form of the acute-phase-reactant C-reactive protein (CRP) using immunocytochemistry with an anti-neoCRP-specific monoclonal antibody as well as reverse transcription-polymerase chain reaction with CRP-specific primers and in situ hybridization to demonstrate the presence of CRP-specific mRNA. After induction of a generalized inflammatory reaction in rats with heat-inactivated Corynebacterium parvum in vivo, neoCRP expression in islets is also found and within the pancreas restricted to pancreatic islet cells only. Our findings suggest an early heat-shock-like expression of this molecule in response to local nitrite oxide production or to exogeneously added nitric oxide in islet cells.

Nitric oxide: cytotoxicity versus cytoprotection--how, why, when, and where?

Nitric oxide (NO) has been found to play an important role as a signal molecule in many parts of the organism as well as a cytotoxic effector molecule of the nonspecific immune response. It appears paradoxical that NO on one side acts as a physiological intercellular messenger and on the other side may display cytotoxic activity in vivo. To make things even more complicated, cytoprotective properties of NO are also described. We here review the current understanding of cytotoxic versus cytoprotective effects of NO in mammalian cells and try to highlight the janus-faced properties of this important small molecule.

Nitric oxide mediates intracytoplasmic and intranuclear zinc release

We previously described that NO. leads to destruction of ZnS clusters and release of Zn2+ from various proteins including zinc finger transcription factors. To assess the relevance in living cells, we investigated, whether exogenous NO. leads to an increase of cytoplasmic and nuclear free Zn2+. L929 cells, mouse splenocytes, or rat aorta endothelial cells were labeled with Zinquin-E, a Zn2+-specific fluorophore, and were treated with two different spontaneous NO donors, S-nitrosocysteine or DETA/NO. Both NO donors strongly increased the Zn2+-dependent fluorescence in the cellular cytosol and also in nuclei as compared to controls. NO-dependent Zn2+ release in splenocytes was quantitated by flow cytometry. These results show for the first time, that nitrosative stress mediates intracellular and intranuclear Zn2+ release which may be relevant in altering gene expression patterns.

Endothelial cells as cytotoxic effector cells: cytokine-activated rat islet endothelial cells lyse syngeneic islet cells via nitric oxide

In vivo, each beta cell is located in proximity to at least one capillary islet endothelial cell. Rat aorta and islet endothelial cells can be activated in vitro to express inducible nitric oxide synthase by a cytokine mixture of tumour necrosis factor-alpha, gamma-interferon, and interleukin-1 beta and to produce high concentrations of nitric oxide. We have performed co-culture experiments with rat islet endothelial cells together with isolated syngeneic islet cells at low target:effector ratios with or without previous cytokine challenge of endothelial cultures. Co-cultures were always free of exogenous cytokines, which were removed prior to addition of islet cells. We found that pre-activated, in contrast to resident islet endothelial cells, at a target:effector ratio as low as 1:1 almost completely lysed syngeneic beta and non-beta cells with 24 h of co-culture. Lysis by pre-activated islet endothelial cells was found to be preceded by DNA damage found in 46% of islet cells after 8 h of co-culture with pre-activated vs 7% with resting islet endothelial cells. Lysis was blocked to control levels in the presence of the nitric oxide synthase inhibitor NG-methyl-L-arginine. With the results presented here, we demonstrate for the first time, that activated endothelial lining cells can express effector cell activity and thus can contribute to local tissue destruction, especially in organs that are densely capillarized such as pancreatic islets.

A proinflammatory activity of interleukin 8 in human skin: expression of the inducible nitric oxide synthase in psoriatic lesions and cultured keratinocytes

Psoriasis is a common chronic skin disease mediated by cellular immune mechanisms and characterized by an intense neutrophil cell infiltrate and proliferative activation of epidermal keratinocytes. We have previously described the expression of the inducible nitric oxide synthase (iNOS) in epidermal keratinocytes of psoriatic skin lesions. In this study, the role of iNOS in psoriatic inflammation was explored ex vivo in psoriatic skin biopsies and in vitro in primary cultures of human keratinocytes. Messenger RNA for the iNOS enzyme (iNOS mRNA) was detected by reverse transcriptase polymerase chain reaction in skin biopsies from patients with psoriasis, but not in skin specimens from patients with atopic eczema or from healthy volunteers. As demonstrated by in situ hybridization and immunohistochemistry, expression of iNOS mRNA and its gene product was localized to the epidermal keratinocytes of psoriatic skin lesions. In situ hybridization further revealed a complete colocalization of mRNA expression for iNOS with interleukin (IL) 8 receptor-specific mRNA either in the basal germinative cell layer or at focal sites of ongoing neutrophil inflammation in suprabasal cell layers. Because psoriatic keratinocytes have previously been shown to express mRNA transcripts for IL-8, it seemed reasonable to hypothesize that iNOS expression could be induced in an autocrine loop by IL-8. This hypothesis was substantiated by our in vitro experiments showing that a combination of IL-8 and interferon gamma induces the expression of iNOS-specific mRNA and of the functional enzyme in cultured human keratinocytes. These results suggest an important role for iNOS in concert with IL-8 and its receptor early during the formation of psoriatic lesions.

Streptozotocin is not a spontaneous NO donor

The reaction of streptozotocin with oxymyoglobin was analyzed and compared with results using various compounds that spontaneously generate nitric oxide in solution.

Independent modulation of galactose-specific receptor expression in rat liver cells

The expression of galactose-specific receptors on liver cells from rats at the end of pregnancy and from estrogen-treated animals was studied. The number and distribution of binding sites were estimated on hepatocytes and Kupffer and endothelial cells in vitro as well as in situ by means of protein-gold complexes. Hepatocytes and endothelial cells from pregnant rats showed an increased binding activity of at least three times for hepatocytes and one and a half times for endothelial cells with respect to normal rat livers. The increase in the hepatocyte receptor expression was paralleled by an increase in the level of its specific messenger RNA (mRNA). On Kupffer cells, a decreased number of binding sites, at least three times less than control values, was measured. The correlation between the altered hormonal level during pregnancy and the expression of galactose binding sites was examined in hepatocytes and Kupffer cells isolated from virgin rats treated with the synthetic estrogen diethylstilbesterol. In estrogen-treated rats both the binding sites and the specific mRNA of hepatocytes increased as compared with vehicle-treated or untreated animals. In contrast, in Kupffer cells both the estrogen treatment as well as vehicle-only injection led to a significant reduction in the expression of binding sites as compared with virgin untreated animals. To establish whether the decrease of galactose binding sites in Kupffer cells was related to the activation of macrophages or to the removal of plasma membrane caused by enhanced nonspecific phagocytosis, in situ binding experiments were performed after lipopolysaccharide (LPS)-stimulation or latex-bead phagocytosis. Nonspecific phagocytosis does not affect the binding activity, which instead appears strongly reduced after LPS injection. These findings suggest an independent response of galactose-specific receptor expression systems in the different types of liver cells to modulating agents.

Insulitis and islet-cell antibody formation in rats with experimentally reduced beta-cell mass

We studied the effect of severe reduction of beta-cell mass by 90% pancreatectomy on the immune tolerance to the endocrine pancreas. Four months after subtotal pancreatectomy all LEW.Han rats had developed mononuclear infiltration of islets and 9 of 14 rats were positive for islet-cell antibodies. Electron microscopy revealed lymphocytic invasion of endocrine tissue, lysis of beta cells and phagocytotic macrophages. None of these changes were seen 2 weeks after 90% pancreatectomy or 4 months after 10% pancreatectomy. Weekly substitution of islet antigens in the form of a homogenate of 100 islets into 90% pancreatectomized LEW.Han rats almost completely prevented the development of insulitis and autoantibodies. The dependence of insulitis on T cells was shown when 90% pancreatectomy in LEW.rnu rats (i.e., the congenic athymic nude strain), did not result in islet infiltration. The exocrine tissue remained normal in all experimental groups. During the observation period insulitis was not associated with overt diabetes but was accompanied by substantial enlargement of islets and of beta-cell mass, as shown by morphometry. Suppression of islet inflammation by injection of islet antigens abolished beta-cell regeneration, despite continuing metabolic stress in rats with 90% pancreatectomy. The findings indicate induction of islet autoimmunity in response to 90% but not to 10% pancreatectomy. We conclude that severe reduction of the islet-antigen mass allows the development of T-cell-dependent islet autoimmunity which indicates a loss of immune tolerance. In addition, the data suggest the existence of islet-antigen autoreactive immune cells in rats not genetically predisposed to autoimmune diabetes. Finally, we conclude that selective beta-cell regeneration occurs in association with insulitis.

The content of glutathione and glutathione S-transferases and the glutathione peroxidase activity in rat liver nuclei determined by a non-aqueous technique of cell fractionation

Hepatocellular nuclei require glutathione, glutathione S-transferases (GSTs) and Se-dependent glutathione peroxidase (GPx) for intranuclear protection against damage from electrophiles or products of active oxygen. Data so far available from the literature on nuclei isolated in aqueous systems range from glutathione, GSTs and GPx either being absent altogether to being present in quantities in excess of those in the cytoplasm. This paper describes a small-scale preparation of a nuclear fraction from rat liver by a non-aqueous technique, designed to retain nuclear water-soluble molecules in situ, since low-molecular-mass compounds can diffuse freely into other compartments during aqueous separation. This non-aqueous procedure shows the nucleus to contain glutathione at 8.4 mM and soluble GSTs at 38 micrograms/mg of protein, the enrichment over the homogenate being 1.2-1.4-fold. Se-dependent GPx activity was also present in the nucleus (56 m-units/mg), although with slightly lower activity than in the homogenate (0.7-fold).

Nitric oxide induces apoptosis in mouse thymocytes

Nitric oxide (NO) produced at high concentrations by the inducible NO synthase is an important effector molecule involved in immune regulation and defense. We have examined whether NO represents a signal for triggering apoptosis in thymocytes. Freshly isolated thymocytes were incubated with different chemical NO donors for various intervals. Apoptosis was determined by detection of DNA strand breaks with in situ nick translation. All NO donors induced thymocyte apoptosis with 30% positive thymocytes vs 10% in controls after 8 h. Apoptosis was prevented by addition of ZnSO4. Short-term pre-exposure to NO resulted in protection from apoptosis induced by glucocorticoids comparable with the protective effect of heat shock. Flow cytometry revealed that NO treatment as well as heat shock or dexamethasone incubation is accompanied by reduction in the CD4+ CD8+ thymocyte subpopulation. Apoptosis induction was accompanied by increased expression of p53, as detected by PCR analysis 2 h after NO donor addition. In vivo treatment of mice with endotoxin results in increased thymic apoptosis. Focal apoptosis was found to occur in close proximity to blood vessels 18 h after LPS treatment. Capillary endothelium and dendritic cells adjacent to apoptotic foci were found to stain strongly for inducible NO synthase expression. Furthermore, in an in vitro experiment using cocultures of thymocytes with LPS/cytokine-activated endothelial cells expressing inducible NO synthase, a significantly increased rate of thymocyte apoptosis was found, and this could be prevented completely by inhibiting NO production. Addition of dexamethasone to these cocultures did not lead to a further increase in the percentage of apoptotic thymocytes, underlining the protective effect of NO on dexamethasone-induced apoptosis.

Nitric oxide induces apoptosis in mouse thymocytes

Nitric oxide (NO) produced at high concentrations by the inducible NO synthase is an important effector molecule involved in immune regulation and defense. We have examined whether NO represents a signal for triggering apoptosis in thymocytes. Freshly isolated thymocytes were incubated with different chemical NO donors for various intervals. Apoptosis was determined by detection of DNA strand breaks with in situ nick translation. All NO donors induced thymocyte apoptosis with 30% positive thymocytes vs 10% in controls after 8 h. Apoptosis was prevented by addition of ZnSO4. Short-term pre-exposure to NO resulted in protection from apoptosis induced by glucocorticoids comparable with the protective effect of heat shock. Flow cytometry revealed that NO treatment as well as heat shock or dexamethasone incubation is accompanied by reduction in the CD4+ CD8+ thymocyte subpopulation. Apoptosis induction was accompanied by increased expression of p53, as detected by PCR analysis 2 h after NO donor addition. In vivo treatment of mice with endotoxin results in increased thymic apoptosis. Focal apoptosis was found to occur in close proximity to blood vessels 18 h after LPS treatment. Capillary endothelium and dendritic cells adjacent to apoptotic foci were found to stain strongly for inducible NO synthase expression. Furthermore, in an in vitro experiment using cocultures of thymocytes with LPS/cytokine-activated endothelial cells expressing inducible NO synthase, a significantly increased rate of thymocyte apoptosis was found, and this could be prevented completely by inhibiting NO production. Addition of dexamethasone to these cocultures did not lead to a further increase in the percentage of apoptotic thymocytes, underlining the protective effect of NO on dexamethasone-induced apoptosis.

Inducible nitric oxide synthase and its product nitric oxide, a small molecule with complex biological activities

More and more attention is paid to the radical nitric oxide which is now known to be part of the mammalian physiology and immune system. Nitric oxide is synthesized by one of the most complicated and fascinating enzyme families identified so far. Inducible nitric oxide synthesis after appropriate stimuli has regulatory, cytostatic and/or toxic consequences and may play an important role in the pathophysiology of several diseases.

Glucagon effect on intracellular proteolysis and pericanalicular location of hepatocyte lysosomes in isolated perfused guinea pig livers

In guinea pigs, glucagon choleresis is accompanied by a significant, but transient, stimulation of biliary protein secretion, which can be accounted for mainly by biliary discharge of lysosomal enzymes. To clarify whether intracellular proteolysis--a process regulated by glucagon and taking place predominantly in the lysosomes--may interact with biliary protein secretion, we determined hepatic proteolytic activity and bile secretory function during substrate deprivation, amino acid supplementation, and glucagon administration in isolated perfused guinea pig livers. To further elucidate the nature of transient lysosomal enzyme release into bile during glucagon infusion, we analyzed pericanalicular distribution of lysosomes by quantitative electron microscopy. The results demonstrate that intracellular proteolysis is accompanied by biliary excretion of lysosomal enzymes. Glucagon-induced secretion of these enzymes as well as labeled proteins into bile occurs independent of protein breakdown and cannot be modulated by addition of amino acids as potent inhibitors of intracellular proteolysis. During glucagon administration, bile canalicular area and pericanalicular distribution of secondary lysosomes show a rapid increase, which persists during the entire infusion period and thus does not explain the transient biliary release of lysosomal enzymes. We therefore postulate that regulation of this process must be located beyond the lysosomal compartment, either involving transport processes or intracellular kinetics of lysosome formation or altered fusion kinetics at the bile canalicular membrane compartment. Metabolic and biliary effects of glucagon seem to occur independent of each other and to underly different regulatory mechanisms.