Emerging role of nitric oxide in cancer

Induction of Nitric Oxide and Its Role in Facial Nerve Regeneration According to the Method of Facial Nerve Injury

Nitric oxide (NO) is an important molecule in cell communication that also plays an important role in many biological processes. Given the dual role of NO in nerve degeneration and regeneration after facial nerve injury, we sought to delve deeper into its role through a systematic literature review. A comprehensive review of the literature employing SCOPUS, PubMed, Cochrane Library, EMBASE, and Google Scholar databases was conducted to evaluate the induction and role of NO in neurodegeneration and regeneration after facial nerve injury. From the 20 papers ultimately reviewed, the central findings were that neuronal nitric oxide synthase(nNOS), endothelial nitric oxide synthase (eNOS), and induced nitric oxide synthase (iNOS) increased or decreased depending on the method of facial nerve damage, damaged area, harvested area, and animal age, and were correlated with degeneration and regeneration of the facial nerve. Research conducted on rats and mice demonstrated that NO, nNOS, eNOS, and iNOS play significant roles in nerve regeneration and degeneration. However, the relationship between nerve damage and NO could not be defined by a simple causal relationship. Instead, the involvement of NOS depends on the type of nerve cell, source of NO, timing, and location of expression, age of the target animal, and proximity of the damage location to the brainstem. Consequently, nNOS, eNOS, and iNOS expression levels and functions may vary significantly.

Protium javanicum Burm. Methanol Extract Attenuates LPS-Induced Inflammatory Activities in Macrophage-Like RAW264.7 Cells

Protium javanicum Burm. f. is a medicinal plant used in traditional medicine. Gum and oleoresins from this plant have been used as anti-inflammatory agents for treating ulcers, headaches, eyelid inflammation, and rheumatic pain. However, its anti-inflammatory mechanism of action is still unknown. To better understand the mechanism, we used lipopolysaccharide- (LPS-) treated RAW264.7 cells to measure inflammatory mediators with the Griess assay and to identify target signaling molecules by immunoblot analysis. In this study, we report that the Protium javanicum methanol extract (Pj-ME) plays an important role in suppressing nitric oxide (NO) levels without cytotoxicity. The effect of Pj-ME in LPS-induced expression leads to reduced inflammatory cytokine expression, specifically inducible nitric oxide synthase (iNOS), cyclooxygenase (COX-2), and tumor necrosis factor (TNF-α). Pj-ME significantly inhibited LPS-induced protein expression of the nuclear factor-kappa B (NF-κB) signaling pathway in a time-dependent manner. Syk and Src were identified as putative signaling molecules of Pj-ME-mediated anti-inflammatory activity, which were inhibited by Pj-ME. We demonstrated that Pj-ME controls the STAT3 signaling pathway by suppressing STAT3 and JAK phosphorylation and also downregulates the gene expression of IL-6. Therefore, these results elucidate Pj-ME as a novel anti-inflammatory naturally derived drug with anti-inflammatory and antioxidant properties which may be subject to therapeutic and prognostic relevance.

Suppression of Src and Syk in the NF-κB signaling pathway by Olea europaea methanol extract is leading to its anti-inflammatory effects

ETHNOPHARMACOLOGICAL RELEVANCE

Olea europaea L., (Oleaceae) has been used widely in folk medicine in the European Mediterranean islands, India, Asia, and other parts of the world. Although this plant has high ethnopharmacological value for treating inflammatory diseases, the molecular mechanisms of how it inhibits the inflammatory response are not fully understood. In this study, we sought to identify the anti-inflammatory mechanisms of this plant.

MATERIALS AND METHODS

Using macrophages, we investigated the effects of O. europaea L. methanol extract (Oe-ME) and ethanol extract (Oe-EE) on the production of inflammatory mediator nitric oxide (NO) and prostaglandin E₂ (PGE₂), the expression levels of pro-inflammatory genes and intracellular inflammatory signaling activities.

RESULTS

Oe-ME and Oe-EE suppressed the production of NO in lipopolysaccharide-(LPS-), Pam3CSK4-, and poly (I:C)-stimulated RAW264.7 cells; importantly, no cytotoxicity was observed. Oe-ME and Oe-EE reduced production of PGE₂ without exhibiting cytotoxicity. The mRNA expression levels of cyclooxygenase-2 (COX-2), inducible NO synthase (iNOS), IL-6, IL-1β, and tumor necrosis factor (TNF)-α were down-regulated by Oe-ME and Oe-EE. Nuclear fraction and whole lysate immunoblotting analyses and overexpression experiments strongly suggested that Oe-ME decreased the translocation of p65 and p50 (nuclear factors of the NF-κB subunit) as well as Src and Syk.

CONCLUSION

These results suggest that Oe-ME exerts its anti-inflammatory effects by targeting Src and Syk in the NF-κB signaling pathway.

Anti-Inflammatory Effect of Piper attenuatum Methanol Extract in LPS-Stimulated Inflammatory Responses

Piper attenuatum is used as a traditional medicinal plant in India. One of the substances in P. attenuatum has been suggested to have anti-inflammatory effects. However, there is insufficient research about the anti-inflammatory mechanisms of action of P. attenuatum. The effects of P. attenuatum methanol extract (Pa-ME) on the production of inflammatory mediators nitric oxide (NO) and prostaglandin E₂ (PGE₂), the expression of proinflammatory genes, the translocation level of transcription factors, and intracellular signaling activities were investigated using macrophages. Pa-ME suppressed the production of NO and PGE₂ in lipopolysaccharide- (LPS-), pam3CSK4-, and poly(I:C)-stimulated RAW264.7 cells without displaying cytotoxicity. The mRNA expression levels of inducible NO synthase (iNOS) and cyclooxygenase 2 (COX-2) were decreased by Pa-ME. P-ME reduced the translocation of p50/NF-κB and AP-1 (c-Jun and c-Fos), as well as the activity of their upstream enzymes Src, Syk, and TAK1. Immunoprecipitation analysis showed failure of binding between their substrates, phospho- (p-) p85 and p-MKK3/6. p-p85 and p-MKK3/6, which were induced by overexpression of Src, Syk, and TAK1, were also reduced by Pa-ME. Therefore, these results suggest that Pa-ME exerts its anti-inflammatory effects by targeting Src and Syk in the NF-κB signaling pathway and TAK1 in the AP-1 signaling pathway.

Various UVB doses affect change of raf kinase inhibitor protein, nitric oxide and proliferation in keratinocytes

UVB is a potent modulator of cell growth and differentiation in the skin. The UVB irradiation has been used in treating hyperproliferative dermatoses. Otherwise, UVB radiation is also the major risk factor for developing skin cancer. Nitric oxide (NO) has been suggested to be a physiological modulator of cell proliferation. Raf-1 kinase inhibitory protein (RKIP) was involved in cell growth, transformation, and differentiation. The purpose of this study was to search for the possible cause of UVB-inhibited hyperplasia and UVB-resulted hyperproliferation. We evaluated various UVB dose whether affect the expression of RKIP, iNOS, NO and proliferation in keratinocyte. Normal human keratinocytes were treated with UVB dose of 40mJ/cm², 80mJ/cm², 120mJ/cm², 160mJ/cm² and 0mJ/cm² (control group) respectively. The results showed that RKIP, iNOS and NO of keratinocytes with doses of 40mJ/cm² and 80mJ/cm² UVB treatment significantly higher than control group (P<0.01). The proliferation of keratinocyte with doses of 40mJ/cm² and 80mJ/cm² UVB treatment was significantly lower than control group (P<0.01). However, RKIP, iNOS and NO of keratinocytes with doses of 120mJ/cm² and 160mJ/cm² UVB treatment significantly lower than control group (P<0.01). The proliferation of keratinocyte with doses of 120mJ/cm² and 160mJ/cm² UVB treatment was significantly higher than control group (P<0.01). In conclusion, these results showed that the different UVB dosages induced various alteration of RKIP, NO, iNOS and proliferation may provide important information on the therapeutic molecular mechanism of UVB-inhibited hyperplasia and UVB resulted hyperproliferation.

Nitrosothiol signaling and protein nitrosation in cell death

Nitric oxide, a reactive free radical, is an important signaling molecule that can lead to a plethora of cellular effects affecting homeostasis. A well-established mechanism by which NO manifests its effect on cellular functions is the post-translational chemical modification of cysteine thiols in substrate proteins by a process known as S-nitrosation. Studies that investigate regulation of cellular functions through NO have increasingly established S-nitrosation as the primary modulatory mechanism in their respective systems. There has been a substantial increase in the number of reports citing various candidate proteins undergoing S-nitrosation, which affects cell-death and -survival pathways in a number of tissues including heart, lung, brain and blood. With an exponentially growing list of proteins being identified as substrates for S-nitrosation, it is important to assimilate this information in different cell/tissue systems in order to gain an overall view of protein regulation of both individual proteins and a class of protein substrates. This will allow for broad mapping of proteins as a function of S-nitrosation, and help delineate their global effects on pathophysiological responses including cell death and survival. This information will not only provide a much better understanding of overall functional relevance of NO in the context of various disease states, it will also facilitate the generation of novel therapeutics to combat specific diseases that are driven by NO-mediated S-nitrosation.

Development of methods for the bioanalysis of RRx-001 and metabolites

Background: Bioanalytical methods were required to study the novel anticancer drug, RRx-001 preclinically and for clinical pharmacokinetic analysis; however, RRx-001 quickly and completely disappeared on intravenous administration in preclinical species. Results: Quantification of RRx-001 directly or by derivatization was unsuccessful. On exposure to whole blood, RRx-001 formed the glutathione (GSH) adduct very rapidly, suggesting this metabolite as the bioanalyte. However, rapid enzymatic degradation in the blood matrix of RRx-001-GSH posed significant technical problems. Herein, we describe a novel and broadly applicable solution to stabilize GSH conjugates in blood samples by inhibiting the degrading enzyme. Liquid chromatography–tandem mass spectrometry methods for analysis of RRx-001-GSH in rat, dog and human plasma were developed and successfully validated to good laboratory practice standards. Conclusion: Extensive breakdown of RRx-001-GSH was effectively stopped by addition of the enzyme inhibitor, acivicin. The developed liquid chromatography–tandem mass spectrometry assay for RRx-001-GSH was validated for use in preclinical toxicology studies and the Phase I first-in-human clinical trial.

Local lymphocytes and nitric oxide synthase in the uterine cervical stroma of patients with grade III cervical intraepithelial neoplasia

OBJECTIVES

Precancerous and cancerous cells can trigger an immune response that may limit tumor development and can be used as a prognostic marker. The aims of the present study were to quantify the presence of B and T lymphocytes, macrophages and cells expressing inducible nitric oxide synthase (iNOS) in the cervical stroma of women with grade III cervical intraepithelial neoplasia (CIN III) or in the intratumoral and peritumoral tissue of women with stage I invasive carcinoma.

METHODS

Cervical tissue specimens were obtained from 60 women (20 each from control tissues, CIN III and invasive carcinomas). The average ages in the control, CIN III and invasive groups were 43.9 (+/- 4.3), 35.5 (+/- 9.5), and 50 (+/- 11.2) years, respectively. The specimens were immunohistochemically labeled with antibodies to identify T lymphocytes (CD3), cytotoxic lymphocytes (CD8), B lymphocytes (CD20), macrophages (CD68) and iNOS. We evaluated the markers in the stroma above the squamocolumnar junction (control), at the intraepithelial lesion (CIN cases), and in the infiltrating tumor. Two independent observers performed the immunohistochemical analysis.

RESULTS

T lymphocytes, B lymphocytes, macrophages and iNOS were present more frequently (P<0.05) in the stroma of peritumoral invasive tumors compared to the controls and intratumoral invasive cancer samples. CD3+ and CD20+ lymphocytes were present more frequently in CIN III patients compared to samples from patients with intratumoral invasive cancer (P<0.05).

CONCLUSION

High numbers of T and B lymphocytes, macrophages and iNOS-expressing cells in the peritumoral stroma of the invasive tumors were observed. Cell migration appeared to be proportional to the progression of the lesion.

HIF-1α and VEGF are associated with disease progression in esophageal carcinoma

BACKGROUND

Hypoxia inducible factor-1alpha (HIF-1α) is a transcription factor that regulates the transcription of genes associated with cell proliferation and angiogenesis. The purpose of this study is to clarify the correlation of HIF-1α protein expression with vascular endothelial growth factor (VEGF) and inducible (iNOS), endothelial (eNOS), and neuronal nitric oxide synthase (nNOS) expression in esophageal tumors. Additionally, vascular density in tumor tissue was assessed.

MATERIALS AND METHODS

Eighty-eight esophageal carcinomas were analyzed by immunohistochemistry in paraffin embedded sections.

RESULTS

HIF-1α immunoreactivity was seen in 71.2 % of the tumors. Squamous cell carcinomas expressed more often HIF-1α than adenocarcinomas (P = 0.009). HIF-1α immunoreactivity was associated with iNOS (P = 0.049), and iNOS positivity was also more commonly seen in squamous cell carcinomas than adenocarcinomas (P = 0.016). VEGF immunoreaction tended to associate with HIF-1α (P = 0.073) and iNOS (P = 0.08). ENOS did not associate with HIF-1α, but tended to associate with VEGF (P = 0.072). T1-T2 tumors were more often VEGF negative than T3-T4 tumors (P = 0.063). In the subgroup of 78 operatively treated ECs patients with HIF-1α positivity (> +) had more often distant metastases (P = 0.036). There was no association between iNOS, eNOS, nNOS, or VEGF, and microvessel density in tumor tissue, tumor marginal zone, or in peripheral tissue.

CONCLUSIONS

These results show that there is a link in expression between HIF-1α, iNOS, (eNOS), and VEGF in esophageal cancer. This is in line with the fact of HIF-1α's function as a transcriptional factor for these angiogenic factors. Results also show that squamous cell and adenocarcinomas differ in their expression of HIF-1α and iNOS. VEGF appear to have association with depth of invasion in esophageal carcinomas. In our material HIF-1α positivity was associated with distant metastases, but not with patient survival.

Nitric oxide metabolism in cancerous and non-cancerous oral gingivomucosal tissues: possible implications of nitric oxide in cancer process

PURPOSE

Nitric oxide (NO) is a molecule that plays various roles in the body tissues. NO plays important roles in vasodilatation, platelet aggregation, cytokine stimulation, neurotransmission, immune function, etc. NO also exerts dual functions as an oxidant and antioxidant substance depending on its concentrations and environmental conditions. In this study, we aimed to examine possible correlation between NO levels and NO synthase (NOS) activity in the patients with oral cancer.

MATERIALS AND METHODS

The study included 19 tissues from human subjects (11 malign and eight benign lesions).

RESULTS

NO level and NOS activity were found decreased in the malign lesions compared with those of the benign ones.

CONCLUSIONS

In conclusion, two suggestions can be made; first, decreased NO synthesis may be an attempt to suppress angiogenesis, which is known to provide more essential nutrients to malign lesions and/or second, malign lesions may suppress NO production to be capable of creating more rapid proliferation as it has been known that NO is also a powerful free radical inducing apoptosis.

Nitric oxide enhances expression of raf kinase inhibitor protein in keratinocytes

Several reports have focused on the potential of nitric oxide (NO) to influence the proliferation and differentiation cascade in a number of mammalian cells. The purpose of this study was to determine the relationship between expression of raf kinase inhibitor protein (RKIP) and proliferation in keratinocyte with NO treatment. Normal human keratinocytes were treated with SNAP (NO donor) doses of 10(-7), 10(-6), 10(-5), 10(-4) and 0 m (control group) separately. Expression of protein and mRNA of RKIP, cell proliferation and apoptosis have been measured. These results showed that elevated expression of RKIP in keratinocyte with NO treatment may contribute to the pathological and physiological features of NO-inhibited proliferation.

A multifaceted molecule, nitric oxide in oral and periodontal diseases

Nitric oxide (NO) is a molecule with multiple effects on different tissues. NO takes important roles in vasodilatation, bacterial challenge and cytokine stimulation, regulation of mineralized tissue function, neurotransmission, and platelet aggregation, etc. However, under pathological conditions, NO has damaging effects. NO is synthesized by NO synthases (NOS) and inducible isoform of NOS (iNOS) is closely related to the pathophysiological characteristics of inflammatory diseases such as periodontal diseases. The expression of iNOS has been investigated in salivary gland-related diseases, temporomandibular joint disorders and oral cancer as well. The beneficial and damaging effects of NO in diseases related with periodontal, dental and maxillofacial area are discussed in this review. The biological pathways involved with NO and NO inhibitors may be good drug targets to have a role in the future management of patients with diseases in orofacial region.

In vitro effects of nitric oxide synthase inhibitor L-NAME on oral squamous cell carcinoma: a preliminary study

It has been reported that increased nitric oxide synthase (NOS) expression and nitric oxide (NO) production may play an important role in cancer biology. The aim of this study was to determine the roles of NO in tumour cellular proliferation and DNA or RNA synthesis, and to investigate the therapeutic potential of NOS inhibitors in oral cancer. After exposure to different concentrations of the NOS inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), the growth of TSCCa cells, established from a patient with squamous cell carcinoma of the tongue, was evaluated using MTT and crystal violet assay. DNA or RNA synthesis, inducible/endothelial NOS (iNOS/eNOS) mRNA expression and NO production were then examined to determine the possible mechanisms of inhibitory effects of L-NAME on TSCCa cells. L-NAME had an inhibitory effect on TSCCa cell growth in both a concentration- and time-dependent manner. Acridine orange staining revealed that DNA and/or RNA synthesis of TSCCa cells was reduced after treatment with L-NAME. An in situ hybridisation (ISH) study showed clearly that L-NAME down-regulated eNOS and iNOS mRNA expression and this was followed by a decrease in NO production. It is postulated that the NOS/NO pathway may be implicated in cellular proliferation and DNA or RNA synthesis of cancer cells, apart from promoting tumour angiogenesis. Further studies have provided with new insight into the mechanisms by which NOS/NO takes part in oral carcinogenesis, and possible therapeutic interventions based on the NOS/NO pathway for tumour progression control.

Expression of endothelial nitric oxide synthase and vascular endothelial growth factor in oral squamous cell carcinoma: its correlation with angiogenesis and disease progression

BACKGROUND

Angiogenesis is a crucial step in the successful growth, invasion, and metastasis of a tumor. It has been popularly accepted that vascular endothelial growth factor (VEGF) is the most potent angiogenic factor in tumor angiogenesis. As another possible star molecule responsible for tumor angiogenesis, the role of nitric oxide (NO) in tumor biology has gained much attention in recent years. The aim of this study was to investigate whether the expression of endothelial nitric oxide synthase (eNOS) and VEGF in oral squamous cell carcinoma (OSCC) is associated with angiogenesis. The present study also made a preliminary exploration of the possible cross-talking existing between eNOS and VEGF during tumor angiogenesis.

METHODS

In this study, expression of eNOS and VEGF were studied immunohistochemically in tissue sections from 40 patients with OSCC and 20 normal controls. To exclude eNOS antibody cross-reactivity with inducible or neuronal nitric oxide (iNOS or nNOS), eNOS expression was confirmed by using an eNOS mRNA in situ hybridization kit. The intratumoral microvessels were highlighted by immunostaining with anti-factor VIII-related antigen monoclonal antibody and counted as well-established methods. Then, chi-square test or Student's t-test was performed to study the correlations between the expression of eNOS and VEGF, microvessel density (MVD), and various clinicopathologic factors.

RESULTS

Both eNOS and VEGF expression significantly increased in OSCC tissues, with a positive rate of 47.5% and 50%, respectively. The average MVD in OSCC tissues was 23.45 per high-power field (HPF), showing an obvious association with lymph node metastasis and clinical stages of patients with OSCC. Either eNOS or VEGF positivity was correlated with vessel involvement and OSCC progression. The mean MVD was significantly higher in eNOS- or VEGF-positive tumors than in eNOS- or VEGF-negative tumors. An obvious, correlation was also seen between eNOS and VEGF expression in OSCC tissues in this study.

CONCLUSIONS

Overexpression of eNOS and VEGF might make an important contribution to the tumor angiogenesis in OSCC. NO generation by eNOS might be implicated in the VEGF-associated angiogenic process. Further investigation of the possible cross-talking between eNOS and VEGF with respect to tumor angiogenesis is necessary.

Nitric oxide signalling in salivary glands

Nitric oxide (NO) plays multiple roles in both intracellular and extracellular signalling mechanisms with implications for health and disease. This review focuses on the role of NO signalling in salivary secretion. Attention will be paid primarily to endogenous NO production in acinar cells resulting from specific receptor stimulation and to NO-regulated Ca2+ homeostasis. Due to the fact that NO readily crosses membranes by simple diffusion, endogenous NO may play a physiological role in processes as diverse as modifying the secretory output, controlling blood supply to the gland, modulating transmitter output from nerve endings, participating in the host defence barrier, and affecting growth and differentiation of surrounding tissue. Furthermore, the role of NO in the pathogenesis of human oral diseases will be considered.

Effects of exogenous nitric oxide on oral squamous cell carcinoma: an in vitro study

PURPOSE

Nitric oxide (NO) is a newly found unstable free radical gas, serving as an important mediator, messenger, and signal transduction molecule and involved in a variety of pathophysiologic processes. Recently, NO has been reported to have cytotoxic effects on several tumor cells as an effector molecule of activated macrophage. The objective of this study was to investigate the effects of exogenous NO on oral squamous cell carcinoma cell line and to try to clarify the possible mechanisms by which it kills tumor cells.

METHODS

TSCCa cell line, established from a patient with oral squamous cell carcinoma of the tongue, was exposed to various concentrations of exogenous NO that were released from an NO donor, sodium nitroprusside (SNP), for 48 hours. Nitrite/nitrate levels in the culture supernatant were determined with a commercial available NO kit. Both morphologic and ultrastructural changes were evaluated by reverse phase contrast microscopy or transmission electron microscopy. The DNA was harvested from SNP-treated or untreated TSCCa cells and assessed by agarose gel electrophoresis.

RESULTS

SNP released NO into medium in a dose-dependent manner. NO had a concentration-dependent cytotoxicity against TSCCa cells. NO induced tumor cell death through apoptosis, which was characterized by incompleteness of nuclear membrane, disappearance of nucleole and nuclear condensation, chromatin margination, or chromatin homogenization. Agarose gel electrophoresis showed a typical internucleosomal DNA cleavage pattern (DNA ladder), a reliable indicator of apoptosis.

CONCLUSIONS

Our results suggested that NO had a tumoricidal potential against oral cancer cells. NO might exert its cytotoxicity as an effector molecule of activated microphage through at least apoptosis.

Increased expression of inducible nitric oxide synthase for human oral submucous fibrosis, verrucous hyperplasia, and verrucous carcinoma

Three isoforms of nitric oxide synthase (NOS), endothelial, neuronal and inducible NOS (iNOS), have been identified in humans. Enhanced expression of iNOS protein has been previously reported for human oral epithelial dysplasias, a human oral premalignant epithelial lesion; however, this expression has not been demonstrated for other premalignant epithelial lesions, namely, oral submucous fibrosis (SF) and verrucous hyperplasia (VH). On the other hand, iNOS protein expression has not been reported in human oral verrucous carcinoma (VCa). The aim of this current study was to determine whether iNOS protein also occurs for oral SF, VH and VCa lesions. We found that membranous stainings were observed chiefly in oral SF lesions (17/20, 85%), whereas cytoplasmic stainings were mainly found in the VH variants (16/20, 80%). By contrast, cytoplasmic and/or nuclear stainings were observed in the specimens of verrucous carcinoma (17/20, 85%). Since no iNOS activity could be detected for any of our specimens of normal buccal mucosa in the present immunohistochemical study, this suggests that an NOS-dependent mechanism may be involved in the malignant transformation of these two premalignant oral epithelial lesions.

Correlation between type II nitric oxide synthase and p53 expression in oral squamous cell carcinoma

Prolonged nitric oxide (NO) production by the enzyme type II nitric oxide synthase (NOS2) has been implicated in angiogenesis and metastasis of human cancers. In animal models, wild-type p53 (but not mutant) protein results in down-regulation of NOS2 expression, which reduces both tumour growth and dissemination. In the current study, we aimed to find out whether a correlation was present in oral squamous cell carcinoma. Fifty-six cases of squamous cell carcinoma were assessed immunohistochemically using antibodies to NOS2 and p53 (clone DO-7). We also confirmed NOS2 protein expression in selected cases using immunoblotting. The results were correlated with clinicopathological findings. Statistical analysis showed a significant relationship between p53 and NOS2 expression (P= 0.001). No relationship was found between size of tumour or histological degree of differentiation, and NOS2 expression in the primary tumour.

Does expression of inducible nitric oxide synthase correlate with severity of oral epithelial dysplasia?

The small molecule nitric oxide (NO) has generated an exponential amount of research since its discovery as a biological messenger in 1987. It has a vast number of actions, many of which are poorly understood. It has been studied in a variety of human cancers and has been implicated both in tumour promotion and inhibition. Although NO is produced by three distinct isoforms of the enzyme nitric oxide synthase (NOS), most cancer research is directed towards the calcium-independent form, iNOS which following induction, produces much higher quantities of NO than the other two. In this study the expression of iNOS is assessed by immunohistochemistry in 26 cases of oral epithelial dysplasia ranging in severity from mild to severe. iNOS staining was found in all 26 cases of dysplasia with the degree of staining correlating to the severity of dysplasia (p < 0.001). There was no iNOS staining seen in adjacent normal epithelium. The possible role of iNOS in the complex transformation from dysplasia to invasive oral cancer and the clinical applications are discussed.