Dual effect of nitric oxide in immortalized and malignant human oral keratinocytes: induction of apoptosis and differentiation

Role of nitric oxide in regulating epidermal permeability barrier function

Nitric oxide (NO), a free radical molecule synthesized by nitric oxide synthases (NOS), regulates multiple cellular functions in a variety of cell types. These NOS, including endothelial NOS (eNOS), inducible NOS (iNOS) and neural NOS (nNOS), are expressed in keratinocytes. Expression levels of both iNOS and nNOS decrease with ageing, and insufficient NO has been linked to the development of a number of disorders such as diabetes and hypertension, and to the severity of atherosclerosis. Conversely, excessive NO levels can induce cellular oxidative stress, but physiological levels of NO are required to maintain the normal functioning of cells, including keratinocytes. NO also regulates cutaneous functions, including epidermal permeability barrier homeostasis and wound healing, through its stimulation of keratinocyte proliferation, differentiation and lipid metabolism. Topical applications of a diverse group of agents which generate nitric oxide (called NO donors) such as S-nitroso-N-acetyl-D,L-penicillamine (SNAP) can delay permeability barrier recovery in barrier-disrupted skin, but iNOS is still required for epidermal permeability barrier homeostasis. This review summarizes the regulatory role that NO plays in epidermal permeability barrier functions and the underlying mechanisms involved.

Inducible nitric oxide synthase is required for epidermal permeability barrier homeostasis in mice

Nitric oxide (NO) regulates a variety of epidermal functions, including epidermal proliferation, differentiation and cutaneous wound healing. However, whether nitric oxide (NO) and its synthetic enzymes regulate epidermal permeability barrier homeostasis is not clear. In the present study, we employed inducible nitric oxide synthase (iNOS) KO mice to explore the role of iNOS in epidermal permeability barrier homeostasis. Our results showed that iNOS mice displayed a comparable levels of basal transepidermal water loss rates, stratum corneum hydration and skin surface pH to their wild-type mice, but epidermal permeability barrier recovery was significantly delayed both 2 and 4 hours after acute barrier disruption by tape stripping. In parallel, expression levels of mRNA for epidermal differentiation-related proteins and lipid synthetic enzymes were lower in iNOS KO mice versus wild-type controls. Topical applications of two structurally unrelated NO donors to iNOS KO mice improved permeability barrier recovery kinetics and upregulated expression levels of mRNA for epidermal differentiation-related proteins and lipid synthetic enzymes. Together, these results indicate that iNOS and its product regulate epidermal permeability barrier homeostasis in mice.

Regulation of mitochondrial function and endoplasmic reticulum stress by nitric oxide in pluripotent stem cells

Mitochondrial dysfunction and endoplasmic reticulum stress (ERS) are global processes that are interrelated and regulated by several stress factors. Nitric oxide (NO) is a multifunctional biomolecule with many varieties of physiological and pathological functions, such as the regulation of cytochrome c inhibition and activation of the immune response, ERS and DNA damage; these actions are dose-dependent. It has been reported that in embryonic stem cells, NO has a dual role, controlling differentiation, survival and pluripotency, but the molecular mechanisms by which it modulates these functions are not yet known. Low levels of NO maintain pluripotency and induce mitochondrial biogenesis. It is well established that NO disrupts the mitochondrial respiratory chain and causes changes in mitochondrial Ca²⁺ flux that induce ERS. Thus, at high concentrations, NO becomes a potential differentiation agent due to the relationship between ERS and the unfolded protein response in many differentiated cell lines. Nevertheless, many studies have demonstrated the need for physiological levels of NO for a proper ERS response. In this review, we stress the importance of the relationships between NO levels, ERS and mitochondrial dysfunction that control stem cell fate as a new approach to possible cell therapy strategies.

Role of nitric oxide in the maintenance of pluripotency and regulation of the hypoxia response in stem cells

Stem cell pluripotency and differentiation are global processes regulated by several pathways that have been studied intensively over recent years. Nitric oxide (NO) is an important molecule that affects gene expression at the level of transcription and translation and regulates cell survival and proliferation in diverse cell types. In embryonic stem cells NO has a dual role, controlling differentiation and survival, but the molecular mechanisms by which it modulates these functions are not completely defined. NO is a physiological regulator of cell respiration through the inhibition of cytochrome c oxidase. Many researchers have been examining the role that NO plays in other aspects of metabolism such as the cellular bioenergetics state, the hypoxia response and the relationship of these areas to stem cell stemness.

Sodium nitroprusside (SNP) sensitizes human gastric cancer cells to TRAIL-induced apoptosis

AIM

To investigate the effects of the nitrous oxide (NO)-donor sodium nitroprusside (SNP) on tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-mediated apoptosis in human gastric cancer cells.

METHODS

The MTT assay and flow cytometry were used to detect cellular proliferation and markers of apoptosis, respectively. Expression levels of caspases-8, and 9 were determined by Western blot. Changes in Nitric Oxide Synthase (NOS) activity, NO production, and caspase activation were also evaluated.

RESULTS

We found that TRAIL induced apoptosis and cell cycle arrest in human gastric cancer cell lines, and that this effect was mediated by NO production, and activation of both the extrinsic and intrinsic signaling pathways of apoptosis. In addition, we found that the NO-donor SNP sensitizes gastric cancer cells to TRAIL-mediated apoptosis. Treatment of cells with both TRAIL and SNP resulted in increased activation of caspase-8 and caspase-9 and NO release. Inhibition of caspase-8 blocked cell TRAIL-induced apoptosis, while a selective caspase-9 inhibitor was unable to prevent apoptosis induced by either TRAIL or TRAIL plus SNP. Inhibition of NOS could block the activation of caspase-9, but had no obvious effect on cell apoptosis.

CONCLUSIONS

SNP-sensitized gastric cancer cells to TRAIL-induced cytotoxicity by stimulating the release of NO, in turn facilitating the mitochondria-mediated signal transduction pathway. The engagement of the mitochondria signaling pathways along with the TRAIL death receptor signaling pathway synergistically increase levels of apoptosis in these cells.

TNF-α and IFN-γ are potential inducers of Fas-mediated keratinocyte apoptosis through activation of inducible nitric oxide synthase in toxic epidermal necrolysis

Toxic epidermal necrolysis (TEN) is a severe immune-mediated adverse cutaneous drug eruption characterized by rapid and extensive epithelial cell death in the epidermis and mucosae. The molecular events leading to this often fatal condition are only partially understood, but evidence suggests a dual mechanism implicating a "drug"-specific immune response on one side and the onset of target cell death by proapoptotic molecules including FasL on the other side. Herein, we describe a potential molecular bridge between these two events that involves inducible nitric oxide synthase (iNOS), which is highly upregulated in the skin of TEN patients. We show that activated T cells secrete high amounts of tumor necrosis factor-α (TNF-α) and IFN-γ, and that both cytokines lead to increased expression and activity of keratinocyte iNOS. A similar observation has been made with drug-specific T lymphocytes from a TEN patient exposed to the culprit drug. The resulting increase in nitric oxide significantly upregulates keratinocyte FasL expression, resulting in Fas- and caspase-8-mediated keratinocyte cell death. Taken together, our data suggest that T-lymphocyte activation by drugs in TEN patients may indirectly lead to FasL-mediated keratinocyte apoptosis, via a molecular bridge involving TNF-α, IFN-γ, and iNOS.

Dual effect of nitric oxide on uterine prostaglandin synthesis in a murine model of preterm labour

BACKGROUND AND PURPOSE

Maternal infections are one of the main causes of adverse developmental outcomes including embryonic resorption and preterm labour. In this study a mouse model of inflammation-associated preterm delivery was developed, and used to study the relationship between nitric oxide (NO) and prostaglandins (PGs).

EXPERIMENTAL APPROACH

The murine model of preterm labour was achieved by assaying different doses of bacterial lipopolysaccharides (LPS). Once established, it was used to analyse uterine levels of prostaglandins E(2) and F(2α) (by radioimmunoassay), cyclooxygenases (COX) and NOS proteins (by Western blot) and NO synthase (NOS) activity. Effects of inhibitors of COX and NOS on LPS-induced preterm labour were also studied. In vitro assays with a nitric oxide donor (SNAP) were performed to analyse the modulation of prostaglandin production by NO.

KEY RESULTS

Lipopolysaccharide increased uterine NO and PG synthesis and induced preterm delivery. Co-administration of meloxicam, a cyclooxygenase-2 inhibitor, or aminoguanidine, an inducible NOS inhibitor, prevented LPS-induced preterm delivery and blocked the increase in PGs and NO. Notably, the levels of NO were found to determine its effect on PG synthesis; low concentrations of NO reduced PG synthesis whereas high concentrations augmented them.

CONCLUSIONS AND IMPLICATIONS

An infection-associated model of preterm labour showed that preterm delivery can be prevented by decreasing PG or NO production. NO was found to have a dual effect on PG synthesis depending on its concentration. These data contribute to the understanding of the interaction between NO and PGs in pregnancy and parturition, and could help to improve neonatal outcomes.

IGF-I protects human oral buccal mucosal epithelial cells from sodium nitroprusside-induced apoptosis via PI3-kinase

OBJECTIVE

Cancers of the head and neck account for the vast majority of all malignancies of the oral cavity. The insulin-like growth factor (IGF) family of proteins is well documented to have an important role in rescuing cells from apoptosis. While it is known the IGF proteins are present in normal oral epithelial and cancer cells its role is not fully understood. Our aim was to study the ability of IGFs to rescue sodium nitroprusside (SNP)-induced apoptotic normal oral epithelial cells in vitro.

DESIGN

Cultured normal human oral keratinocytes (NOKs) or epithelial cells were used. Apoptosis was induced by SNP then cells were exposed to IGF-I or IGF-II to rescue them. Cell viability was assessed by ELISA (for cell death and caspase 3) and FACS analysis; post receptor effects of IGF-I or IGF-II were assessed by [(3)H] thymidine incorporation. Cell signaling events were measured by western blotting using antibodies against phosphorylated Akt or p42/p44 MAPK, and measuring PI3-K activity by ELISA.

RESULTS

SNP induced apoptosis of NOKs and activated the PI3-K/Akt survival pathway. Exposing cells to IGF proteins prevented their apoptosis. IGF-I and -II caused significant increases in PI3-K, but not MAPK, activity. SNP and LY294002, a PI3-K inhibitor, both caused a significant rise in caspase 3 release from NOKs which was reduced in the presence of IGFs.

CONCLUSIONS

The data establishes the importance of IGF-activated PI3-K in rescuing cells from apoptosis. It lends further evidence to the significance of IGF proteins in the possible development of oral cancer.

Effects of endogenous nitric oxide induced by 5-fluorouracil and L-Arg on liver carcinoma in nude mice

AIM: To study the effects of endogeous nitric oxide induced by 5-fluorouracil (5-FU) and L-arginine (L-Arg) on the human liver carcinoma model in nude mice.

METHODS: The human liver carcinoma model in nude mice was established with BEL-7402 cells and normal saline (NS), 5-FU and 5-FU + L-Arg injected intraperitoneally. The tumor size was measured. The necrotic degree and range were observed under microscope. The apoptosis of cancer cell was detected by turmina deoxynucleotidyl transferanse mediated dUTP nick end labeling (TUNEL) method. Immunohistochemical method was performed to determine the expression of iNOS, P16, BAX. The chemical colorimetry was used to test the activity and nitrate reductase method was adopted to test the concentration of nitric oxide (NO) in the tumor tissue. The BI2000 pathological image analyzer was used to analyze the result of immunohistochemistry.

RESULTS: 5-FU combined with L-Arg could inhibit the tumor growth apparently. In NS, 5-FU and 5-FU+L-Arg groups, the changes of tumor volumes were 257.978 ± 59.0, 172.232 ± 66.0 and 91.523 ± 26.7 mm³, respectively (P < 0.05 5-FU vs 5-FU + L-Arg group; P < 0.05 NS vs 5-FU + L-Arg group; P < 0.05, NS vs 5-FU group). The necrotic range and apoptosis index were significantly increased after the drug injection. The necrotic range was biggest in 5-FU + L-Arg group (χ² = 15.963, P < 0.05).

The apoptosis indexes were as follows: NS, 17.4% ± 6.19%; 5-FU, 31.3% ± 12.3%; and 5-FU + L-Arg, 46% ± 15.24% (P < 0.05, 5-FU vs 5-FU + L-Arg; P < 0.05, NS vs 5-FU + L-Arg; P < 0.05, NS vs 5-FU). The expression and activity of iNOS were increased in the tumor tissue. The concentration of NO was also increased. F of optical density of iNOS, iNOS activity and NO concentration are 31.693, 21.949, and 33.909, respectively, P < 0.05. The concentration of NO was related to the expression of P16 and BAX. The correlation coefficient was 0.764 and 0.554.

CONCLUSION: 5-FU combined with L-Arg can inhibit the growth of tumor in nude mice. The effect may be related to inducing the synthesis and increasing the activity of iNOS. The production of NO is increased, and it can enhance the expression of apoptosis-related gene and antioncogene.

Anti-cancer activity of highly purified sulfur in immortalized and malignant human oral keratinocytes

Sulfur is commonly used in Asia as an herbal medicine to treat inflammation and cancer, and potent chemopreventive effects have been demonstrated in various in vivo and in vitro models for sulfur-containing compounds found in naturally occurring products. Here, we report the growth inhibitory and apoptosis-related effects of a newly developed highly purified sulfur (HPS) on immortalized human oral keratinocytes (IHOKs) and on oral cancer cells representing two stages of oral cancer (HN4, HN12) based on a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, Western blotting, cell cycle analysis, and nuclear staining. The purity of the sulfur preparation was verified by high-performance liquid chromatography. HPS inhibited the proliferation of immortalized and malignant oral keratinocytes in a dose- and time-dependent manner. FITC-annexin V staining, DNA fragmentation testing, and Hoechst 33258 staining revealed that HPS inhibited cell growth via apoptosis. HPS increased the sub-G1 cell cycle fraction, with decreased expression of cyclins D1, D2, and E and their activating partners cdk2, cdk4, and cdk6, and a concomitant induction of p53 and p21/WAF1. Furthermore, HPS treatment increased the cytosolic level of cytochrome c and resulted in caspase-3 activation; this effect was correlated with Bax up-regulation and Bcl-2 down-regulation. Thus, these data suggest that HPS is a potential candidate for anti-cancer therapy in oral cancer.

Functional interaction between nitric oxide-induced iron homeostasis and heme oxygenase-1 in immortalized and malignant oral keratinocytes

Heme oxygenase-1 (HO-1) is involved in a variety of regulatory and protective cellular mechanisms as a stress-responsive protein. Whether HO-1 plays a protective role against NO-induced cytotoxicity in oral cancer cells has not yet been established. We used sodium nitroprusside (SNP) as a source of exogenous NO in studies of NO-induced cytotoxicity in immortalized (IHOK) and malignant oral keratinocytes (HN12). The roles of the caspase pathway, of regulatory proteins of iron metabolism (iron regulatory protein (IRP)1, IRP2, transferrin receptor (TfR), and ferritin), and of HO-1 in protection against NO-induced cytotoxicity were assessed. The SNP-induced growth inhibition and apoptosis of IHOK and HN12 cells was reduced by addition of ferric citrate (FC). At low concentrations (< 1 mM), SNP up-regulated cellular iron metabolism by increasing expression of IRP1, IRP2, and TfR, whereas at high concentrations (> 2 mM), SNP down-regulated expression of these proteins. A consistent correlation between decreased levels of IRP1, IRP2, and TfR and increased NO-induced cytotoxicity and apoptosis was observed. Addition of FC inhibited the NO-induced decrease in IRP1, IRP2, and TfR levels. Moreover, SNP increased the expression of HO-1 and ferritin in IHOK and HN12 cells in a concentration-dependent manner. NO-induced cytotoxicity was also inhibited by hemin (an HO-1 agonist) and was enhanced by zinc protoporphyrin IX (an HO-1 inhibitor). Based on these results, we conclude that HO-1 plays a major role in mediating cytoprotection and iron homeostasis against NO toxicity in immortalized and malignant oral keratinocytes.